Skip to content
Organic photovoltaic cell with 17% efficiency and superior processability. Opv solar cell

Organic photovoltaic cell with 17% efficiency and superior processability. Opv solar cell

    Recent progress in solution-processed flexible organic photovoltaics

    The certified power conversion efficiency (PCE) of organic photovoltaics (OPV) fabricated in laboratories has improved dramatically to over 19% owing to the Rapid development of narrow-bandgap small-molecule acceptors and wide bandgap polymer donor materials. The next pivotal question is how to translate small-area laboratory devices into large-scale commercial applications. This requires the OPV to be solution-processed and flexible to satisfy the requirements of high-throughput and large-scale production such as roll-to-roll printing. This review summarizes and analyzes recent progress in solution-processed flexible OPV. After a detailed discussion from the perspective of the behavior of the narrow bandgap small-molecule acceptor and wide bandgap polymer donor active layer in solution-processed flexible devices, the existing challenges and future directions are discussed.


    As a potential sustainable energy technology, organic photovoltaics (OPV) have attracted significant attention from both academia and industry 1,2. OPV have been developed for over three decades. Their power conversion efficiency (PCE) has improved from less than 1% to approximately 19% at present 3,4,5,6,7. The key driving force for the increase in PCE is the development of photoactive materials. In particular, in recent years, the Rapid development of narrow bandgap small-molecule acceptors, also called non-fullerene acceptors, has substantially improved the efficiency of OPV 8,9,10,11. The complex chemical structure of small-molecule acceptors compared with that of fullerene-based acceptors causes the bulk heterojunction (BHJ) active layer to display stronger absorption in the near-infrared and tunable energy levels 12,13,14. These cause OPV to have higher photocurrent and open-circuit voltage. Narrow bandgap small-molecule acceptors have strongly improved the properties of OPV because of their high efficiency, stability, indoor performance, and semitransparency toward commercial applications owing to their effective molecular designs 15,16,17. Therefore, OPV are being used for wider applications.

    The most attractive advantages of OPV are their solution-processability and remarkable mechanical flexibility 18,19,20,21,22,23,24. These can be compatible with large-scale production such as roll-to-roll printing and thereby, reduce production costs and realize commercial applications 25,26,27. Solution-processed and flexible OPV based on fullerene have been widely reported 28,29,30,31,32,33,34,35,36,37,38. By screening materials and optimizing the device structure, certain solution-processed devices can achieve efficiency comparable to those of evaporated devices. This further promotes large-scale printing 34. However, the efficiency of solution-processed flexible OPV is limited by fullerene system materials and remains at a low level.

    The development of narrow bandgap small-molecule acceptors can overcome the bottleneck of solution-processed flexible OPV. The introduction of high-efficiency active layers causes a substantial increase in the efficiency of solution-processed flexible OPV 39. However, the reported conventional solution-processed flexible device structure cannot effectively maintain the performance of the high-performance active layer. The relatively complex chemical structure of the active layers causes the photoactive layer to be more sensitive to the solution-processed interface layers and electrodes. The substitution of fluorine atoms (F substitution) in narrow-bandgap small-molecule acceptors results in low surface energy while improving the performance of the active layer. Such low surface energy would cause wetting issues and the diffusion of the interlayer materials into the top active layers 40,41. The experience in solution-processed OPV needs to be reconsidered and studied 42.

    This paper reviews solution-processed flexible OPV. After a brief introduction to active layer materials and solution-processing techniques, we discuss in detail the requirements for obtaining solution-processed flexible high-efficiency OPV. We FOCUS on processing strategies for solution-based electrodes and interfaces to maintain the high efficiency of the active layer. Finally, certain challenges and recommendations are presented based on our comprehension of solution-processed flexible OPV.

    Photoactive layer

    Material development

    The active layer of OPV is generally composed of a blend of donor and acceptor materials. In the past decades, blends with polymers as donors and fullerenes as acceptors have been studied extensively 43.

    P3HT and PCBM are classic materials in OPV (Fig. 1a) 44. In the early years, researchers focused on optimizing the crystallization behavior of polymer P3HT through various approaches to achieve optimal morphology and improve charge separation efficiency. Miller et al. improved the performance by solvent annealing, increasing the PCE from 0.8% to 3.0% 45. Ma et al. effectively modified the morphology of the active layer film by thermal annealing of the P3HT:PCBM active layer film. The device efficiency attained 5% 46. Zhao et al. developed a new fullerene derivative material, ICBA, which is ~0.2 eV higher in lowest unoccupied molecular orbital (LUMO) energy level than that of PCBM. The open-circuit voltage of the OPV based on P3HT:ICBA can reach 0.84 V (the open-circuit voltage of the device based on P3HT:PCBM is usually ~0.6 V), and the PCE also reached 6.5% 47. Zhang et al. synthesized PDCBT by introducing thiophene units into the main chain. The electron-absorbing functional group on the thiophene unit reduces the highest occupied molecular orbital (HOMO) energy level of the polymer and increases the open-circuit voltage to 0.91 V. The PCE of the device reached 7.2% 48. Subsequently, He et al. prepared an OPV based on a PCE10:PCBM active layer system with an efficiency of over 10% 49. Although fullerenes have strong electron affinity, their weak absorption in the visible and infrared wavelength ranges has limited the development of device performance.

    Recently the reported OPV displays an attractive PCE of over 19% owing to the development of narrow bandgap small-molecule acceptor and wide bandgap polymer donor materials 50. Compared with fullerene-based active layer materials, the recent developed active layer materials can achieve better light absorption, lower energy loss, and a more stable device lifetime. This is because of their conveniently tunable chemical structure and energy levels 51. The recent progress in achieving high-performance OPV originates mainly from three aspects:

    • (1) Narrow bandgap small-molecule acceptors. The most typical and successful small-molecules are ITIC and Y6 11,52. In 2015, Lin et al. reported high-performance OPV based on the non-fullerene acceptor ITIC, achieving high efficiency comparable to fullerene-based OPV devices 52. ITIC has an acceptor–donor–acceptor (A-D-A) chemical structure. Indacenodithieno [3,2-b] thiophene (IDTT) and 1, 1-dicyanomethylene-3-indano (IC) are used as the donor and acceptor units, respectively. Such chemical structures can produce strong electron intramolecular push–pull effects and show a higher absorption capability. Several high-performance OPV based on ITIC acceptors have been reported previously. Gao et al. 53 reported ITIC-based OPV, which used J51 (benzodithiophene-altfluorobenzotriazole copolymer) as a donor. They achieved a PCE of 9.07% with VOC = 0.81 V, JSC = 16.33 mA cm −2. and FF = 0.68. Zhao et al. reported a PCE of 10.68% based on a blend of (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-bʹ]dithiophene))-alt-(5,5-(1ʹ,3ʹ-di-2-thienyl-5ʹ,7ʹ-bis(2-ethylhexyl)benzo[1ʹ,2ʹ-c:4ʹ,5ʹ-cʹ]dithiophene-4,8-dione))]) PBDB-T:ITIC 9. ITIC is an important milestone in the development of OPV. In 2019, Yuan et al. 11 reported a new non-fullerene acceptor, Y6, with an efficiency of up to 15.7%. The Y-series of small-molecule acceptors (represented by Y6) further promotes the development of OPV. The core unit of Y6 is different from that of ITIC. The chemical structure of Y6 is also called A-DA′D-A type structure acceptor. This is because the center is an electron-deficient benzothiadiazole fragment. This design imparts a higher degree of conformational rigidity and uniformity to Y6, thereby reducing the energy losses in the device 54.
    • (2) Wide bandgap polymer donor for non-fullerene acceptors. New requirements for polymer donor materials have also been proposed owing to the development of non-fullerene acceptors. The crystallinity and energy levels should be controlled in the design of polymers for non-fullerene acceptors. A few studies have reported that the driving force for exciton dissociation in non-fullerene-based BHJ differs from that in fullerene-based BHJ 55. The driving force is almost negligible in non-fullerene based OPV devices. Furthermore, certain new copolymer donor materials based on benzo-[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD), 2,1,3-benzothiadiazole (BT), and benzo[d][1,2,3]triazoles (BTz) have been developed 56,57,58. Among these, PBDB-T is one of the most important and widely used polymers because it has led to an impressive breakthrough in the development of OPV. PBDB-T was synthesized by Hou et al. in 2012 59. When it was combined with ITIC as a blend of OPV, it delivered an ambitious PCE of 11.21% and excellent thermal stability 9.
    • (3) Side-chain engineering of polymer donors and small-molecules acceptors materials. The side chain groups of active layer materials affect the solubility, energy levels, absorption, and charge transport properties. For example, F substitution is generally applied in previous donor and acceptor materials to reduce voltage loss, increase the absorption coefficient, and achieve a more significant molecule polarity 60. Li et al. 61 introduced fluorine atoms into the terminal groups of ITIC to synthesize IT-4F. Intramolecular electron push-pull effects are enhanced in IT-4F compared to ITIC. At the same time, the PBDB-T was also modified via fluorination of the thiophene side chain to obtain PBDB-T-SF with a deeper HOMO level 60. The two materials (PBDB-T-SF and IT-4F) were blended to obtain an OPV with a fill factor as high as 75%, achieving an efficiency of 13.3%. In addition, the alkyl chain plays an important role in small-molecule acceptors. Jiang et al. 62 optimized the branched position of the alkyl chain on the pyrrole motif in Y6 to synthesize the N3 acceptors. Changes in the position of the alkyl side chain branching have little effect on the optical and electrochemical properties but result in better packing, optimized crystallinity, and thus improved charge transport properties. The mobility of N3 was 3.94 × 10 –4 cm 2 V −1 s −1. which is higher than that of Y6 (3.12 × 10 –4 cm 2 V −1 s −1 ). Finally, compared with PM6:Y6 (15.04%), the PM6:N3-based OPV device achieved a PCE of 15.79%.

    Figure 1a also shows the chemical structure of the typical wide bandgap polymers and narrow bandgap small molecules of the active layers (PM6, D18, ITIC, and Y6).

    Solvents of active layer solution

    The solutions of the devices with the highest reported efficiency were prepared from chlorobenzene (CB) or chloroform (CF). In particular, recently reported high-efficiency materials such as Y6 need to be dissolved in chloroform owing to their solubility and aggregation state 63. However, continuous solution outflow is generally required during the large-scale printing process. This implies that solvents with low boiling points, such as CF, may be unsuitable. The Rapid evaporation of CF can increase the concentration of the solution and block the ink outlet of the coating head during the coating process. In addition, because large-scale printing is generally carried out in an ambient air environment, halogenic organic solvents cannot be used in the production process because of adverse health and environmental impacts 64. Therefore, it is necessary to examine high-efficiency organic active layers processed in non-halogenated solvents 65. Toluene, o-xylene (o-XY), and metetrahydrofuran (MTHF) are the green solvents most commonly used in OPV. Figure 1b shows the solvents generally used to process the organic active layers. Recently, Wan et al. 66 fabricated all-green solvent-processed OPV with an efficiency of 16%. They used o-XY and THF as orthogonal solvents to prepare PM6 and BO-4F double-layer junctions. This structure exhibits a better morphology and more efficient charge transport channels. There is generally a loss of performance when organic active layers are processed with a green solvent. The different solubilities of conjugated polymers and small molecules in green solvents cause significant phase separation. Therefore, green solvents should afford sufficient solubility for the organic active layer and exhibit suitable boiling points and Hansen solubility parameters 64.

    Chen et al. introduced the third component BTO into binary systems (PM6:Y6). The small molecule BTO assisted the crystallization of Y6 in the non-halogenated solvent paraxylene (PX) 67. The ternary active layer processed with the green solvent show a certificated PCE of over 17%. In addition, because of the high boiling point of the green solvent PX, the active layer solution can be combined with a large-scale blade-coating method. A large-area solar module (36 cm 2 ) was obtained. It achieved an efficiency of over 14%, which is the highest reported PCE for a solar module with an active area exceeding 20 cm 2.

    Solution-processed techniques

    Conventional techniques

    High-efficiency OPV devices reported recently have been fabricated by spin-coating with a small area (2-glovebox. Although spin-coating is a coating process that is widely applied on a laboratory scale, it is not applicable to roll-to-roll methods. Spin-coating is generally accompanied by Rapid evaporation of the solvent and substantial wastage of the solution owing to the high rotational speed. Therefore, other large-scale production techniques including slot-die coating, blade coating, spray coating, and inkjet printing should be introduced into the film formation process (Fig. 2a). These are compatible with the roll-to-roll process.

    Blade coating is generally used as a scale-testing printing method in laboratories owing to its simple processability. The solution is dropped directly onto the substrate in front of the blade. Then, a thin wet film is formed after the blade moves linearly across the substrate 68. The thickness of the film is fixed by the distance between the blade and substrate. The film quality can be affected by the viscosity, concentration of the inks, surface energy, and temperature of the substrates 69. This technique can yield a large-area film using marginal amounts of materials.

    Slot-die coating also produces a large-area homogeneous film by utilizing the meniscus between the slot-die head and the substrates. In contrast with the blade coating method, ink is injected continuously into the slot-die head through the pressure of the pump. This is compatible with the roll-to-roll process owing to the continuous film deposition. Slot-die printing has been demonstrated to be effective for printing the electron transport layer (ETL), hole transport layer (HTL), active layer, and anode 33,70,71,72. This technique has been combined with the roll-to-roll process to prepare OPV by Lee et al. 73.

    Spray coating is a droplet-based noncontact coating technique. The ink is atomized by a pressurized gas and ejected from the nozzle. It then reaches the substrate. The quality of the prepared films can be affected by the pressure of the gas, size of the nozzle, viscosity of the ink, substrate temperature, and the distance between the substrate and nozzle 73,74,75. Spray coating is also effective for printing all the functional layers in OPV devices 76,77,78,79.

    Inkjet printing is also a noncontact coating technique that expels the ink from the nozzle 80,81. However, unlike spray coating, the quantity of droplets ejected from the nozzle with a piezoelectric stage or a thermal unit is quantitative. The droplets are then charged by the charging electrodes and accelerated toward the substrates through deflection plates with an electric field. Inkjet printing is effective for producing high-resolution images without material loss. It is useful for on-demand patterning because it uses digital data. Eggenhuisen et al. 82. prepared large-area OPV with different artistic shapes by full inkjet printing. A 2 cm 2 inkjet-printed device with an efficiency of 6% was reported 83. However, the shape could not be controlled after the droplet was ejected from the nozzle. Therefore, the uniformity of the obtained film was typically less than that obtained with the other processes.

    Novel techniques

    However, conventional printing methods continue to display certain problems. For example, the fluidic lines and grooves of the slot-die head should be filled before continuous printing. This amount of “dead volume” would significantly increase the cost of academic research. In addition, a liquid meniscus should be formed between the slot-die or doctor-blading head and the substrates, and the coating speed would be reduced to retain the liquid meniscus during the coating process. A few impressive printing methods have been reported to compensate for the disadvantages of conventional printing methods 10,84,85.

    Zhong et al. recently used filter paper to fabricate a soft porous blade printing (SPBP) head (Fig. 2b) 84. The ink fills off the paper owing to the porous microstructure of the filter paper and remains on the substrate when the paper overcomes the surface of the substrates. The printed film thickness can be controlled conveniently by tuning the concentration of the ink and coating speed. Higher lamellar packing and stronger face-on orientation of the blend film were observed (by 2D GIWAXS) when this printing method was used. Finally, the printed PM6:Y6 cells fabricated using this method showed a remarkable PCE of 14.75%. Mao et al. developed a Maobi-coating technique inspired by Chinese calligraphy 10. A particular micro squamae structure of the hair of the Maobi would steadily hold and store the ink inside. Maobi can perform complex pattern-printing (such as solar modules) owing to its advantages in writing and painting. The authors realized a fully Maobi-coated solar module containing eight subcells by using this method. The module exhibited a high FF (close to 70%) and Voc of over 6.30 V. These results indicate the reliability and controllability of Maobi coating. Finally, a computer-controlled automatic Maobi-coating setup was constructed. Furthermore, a large-area (18 cm 2 ) solar module with a PCE of 6.3% was fabricated using this technique. Maobi coating has advantages in patterned coating. However, it is challenging to achieve film uniformity while coating a large-area film owing to the hairiness of the Maobi structure. In addition, how to achieve continuous ink storage and high throughput requires further consideration. Sun et al. used a water transfer printing technique to prepare a uniform active layer film (Fig. 2c) 85. The organic active layer solution was dropped onto the water surface. The active layer solution can spread spontaneously on the aqueous surface because of the difference in surface energy. The device fabricated by water transfer printing displayed a performance similar to that of the spin-coated reference. This method prevents the penetration of solvents into the underlying layer. However, the problems of fragile blend film breakage during large-area transfer need to be overcome. This is highly important for large-scale continuous production yield.

    Morphology control

    During the spin-coating process, the Rapid evaporation of the solvent can yield a good morphology to improve the charge extraction efficiency 86. However, the morphology of active layer film would be affected by certain different large-scale printing techniques 87. Controlling the morphology of the large-scale solution-processed active layer film is a key requirement for realizing high-performance OPV compared with the spin-coated reference. 87,88. The addition of the additive into the solution is an effective strategy. In 2018, Lin et al. 89. reported a PCE of 9.54% using a blade-coated PTB7-Th:ITIC active layer. The PCE of the blade-coated devices was higher than that of spin-coated devices. Zhang et al. 90. also prepared highly efficient OPV (PBDB-T:ITIC) with different chemical additives (including chloronaphthalene (CN), 1,8-diiodooctane (DIO), and 1,8-octanedithiol (ODT)) via blade coating. They observed that the addition of ODT improved the performance of the devices and enhanced the device stability during the blading process. In addition, large-area (90 mm 2 ) devices based on ODT showed a high PCE of 8.59%. Although the additive could control the morphology and improve the performance of the devices, the introduction of the additive decreased the stability of the OPV 91. Wang et al. 92. reported the chemical interaction between DIO and PEDOT:PSS. When PEDOT:PSS was deposited on top of the active layer, DIO could straightforwardly produce hydrogen iodide (HI) and chemically reduce the PEDOT:PSS owing to the acid of the PEDOT:PSS solution. It could be inferred that the introduction of additives resulted in many unstable factors.

    Therefore, Vak et al. developed a hot slot-die coating method with a thermal slot-die head and substrate to optimize device performance without additives 93,94. Unlike the conventional slot-die coating method (which has only a temperature-controlled substrate), they added a temperature-controlled module unit on the slot head to tune the temperature of the active layer solution. There are reports on controlling the morphology of the film by tuning the temperature of the active layer solution 95. This coating method achieved BHJ films with optimum morphology and high-performance devices by controlling substrate and solution temperatures without chemical additives. The authors first deposited active-layer films at room temperature (neither the substrate nor the slot head was heated; denoted as RT/RT). The devices yielded an S-shaped curve and exhibited low performance. FF and Jsc improved significantly when the substrate was heated to 120 °C. The PCE of the devices attained 9.36% after the active layer solution was heated to 90 °C. This was better than that of spin-coated devices without DIO. The storage stability of the devices prepared by hot slot-die deposition was also improved compared with that of the samples fabricated at room temperature. A smoother surface and finer phase separation of the hot-blade-coated BHJ films were observed using atomic force microscopy (AFM). This indicated enhanced stability. Then, hot slot-die deposition was combined with the roll-to-roll process to produce flexible OPV based on PBDB-T:ITIC with a PCE of 8.77%.

    Another strategy to control the morphology of BHJ films is to apply a shear impulse during the coating process 96,97. Meng et al. reported a general approach to print flexible OPV by tuning the ratio of the inking speed to the striping speed of the slot-die coating process (denoted as C) 97. They first calculated the impulse accumulation of spin coatings of different durations. The PCEs of the OPV devices varied over the spin-coating time. The performance parameters remained stable after 17 s. The optimized morphology of BHJ films was an important factor that caused the performance variations that occurred after those caused by the reduction in film thickness at the beginning of the process. AFM, TEM, and GIWAXS measurements showed large differences in the surface roughness and phase separation of the BHJ films with various spin-coating times. Next, the concept of impulse accumulation was introduced into the slot-die coating process to explain the morphology evolution. They observed that an optimized morphology was formed when the C value increased to 1.30 ml m −1. GIWAXS measurements indicated that the BHJ films fabricated by slot-die coating with 1.30 ml m −1 exhibited face-on orientation. This was similar to the spin-coated films with a spinning time of 11 s. The well-controlled morphology of the slot-die coating produced large-area (15 cm 2 ) flexible OPV based on both fullerene (PTB7-Th:PC71BM) and non-fullerene (PBDB-T:ITIC) systems with PCE of 7.25% and 8.64%, respectively.

    We also summarized recent work on fabricating OPV devices based on narrow bandgap small-molecule acceptors using printing and coating methods. The performance and details of this study are summarized in Table 1.

    Solution-processed electrodes

    Solution-processed electrodes are required urgently to fabricate flexible OPV for large-scale production because of their high throughput and low cost. The solution-processed electrodes should satisfy the following requirements for different device applications: (1) High electrical conductivity. Both top and bottom electrodes in the device require high conductivity to reduce the series resistance (Rs) of the device. In particular, in large-area devices, high sheet resistance can significantly reduce the fill factor of the device 106. (2) High optical transmittance. To achieve better absorption and improve the photocurrent, transparent electrodes are required to have good transmittance in the visible light range (400–1000 nm). This is because of the better light-absorption properties of the active-layer system 107. However, there is a trade-off between the conductivity and optical transmittance of transparent electrodes. Therefore, the figure of merit (FoM) was introduced to evaluate the quality of the transparent electrodes 108. (3) Uniformity. For the bottom electrodes, uniformity plays an important role in the achievement of a high fabrication yield and good device performance. Owing to the multilayer-stacked structure of OPV, the high roughness of the bottom electrode directly affects the uniformity and morphology of the upper layers and causes a severe short circuit 109. (4) Good solution processability. Electrodes can be deposited by various printing and coating techniques. Conducting polymers, metal nanowires, and metal grids are potential candidates for solution-processed electrodes 108.

    For bottom transparent electrodes, Magnetron-sputtering metal oxide electrodes such as indium tin oxide (ITO) are used to prepare OPV in the laboratory. Although this type of electrode has remarkable photoelectric properties (the square resistance is approximately 10 Ω sq −1 when the light transmittance is 88%), it has a high energy consumption and low material utilization in the preparation process. Meanwhile, the extensive use of rare metals such as indium further increases the preparation cost 110.

    Conducting polymers are a type of transparent electrodes prepared by the solution method. Among these, PEDOT:PSS is the most widely used. It displays good solution processing and mechanical flexibility 111. Song et al. fabricated flexible ITO-free OPV by all-solution processing at low temperatures 98. Figure 4 shows the device structure of a flexible OPV based on PEDOT:PSS electrodes. Treatment with methanesulfonic acid at room temperature prevented damage to the flexible plastic substrates. The devices showed good flexibility and retained ~ 94% of their initial PCE after 1000 bending cycles. However, most post-treatment PEDOT:PSS electrodes used in OPV devices display a low conductivity (

    Metal nanowires such as silver nanowires (AgNWs) are widely used in solution-processed flexible OPV 113. However, the large surface roughness of AgNWs can cause severe short circuits in devices because the networks of AgNWs can conveniently penetrate the thin active layer 114. The embedment of the AgNW network into the substrate and flattening of the AgNW surface by applying an additional layer are the two primary methods for solving this problem 115. For example, Dong et al. 116 reported a flexible bottom electrode obtained by embedding AgNWs into polyimide (PI) substrates. The surface roughness (root mean square (RMS)) of the AgNW electrodes decreased to 1.5 nm. The random alignment of AgNWs hinders the balancing of the transmission and square resistance during preparation. Sun et al. 117 used water-processed AgNWs to prepare a highly ordered AgNW network structure owing to ionic electrostatic charge repulsion. In water-processed AgNWs, poly(sodium 4-styrenesulfonate) (PSSNa) was added to the AgNW solution as a polyelectrolyte. The films prepared with PSSNa exhibited a more regular arrangement than those prepared without PSSNa. Using this electrode, the flexible OPV achieved an efficiency of 16.5% with a tandem device structure. Zeng et al. 118 developed a controllable reduction and chemical welding strategy to improve the conductivity of the AgNW electrode. The fabricated AgNW electrode exhibited a low sheet resistance of 12 Ω sq −1 and high transmittance of 95% at 550 nm. A flexible OPV with a PCE of 17.52% was obtained using this strategy.

    Metal grids such as Ag grids are also widely used in large-area OPV owing to their low sheet resistances. Jiang et al. reported the fabrication of printed transparent Ag mesh electrodes by reverse-offset printing 119. The printed flexible electrode can achieve both high conductivity and remarkable mechanical durability through effective control. The electrodes displayed 17 Ω sq −1 Rsh at a transmittance of 93.2%, thereby surpassing the performance of the sputtered ITO. Ultrathin OPV with a PCE of 8.3% were prepared using the Ag mesh electrodes printed.

    For top electrodes, These differ from bottom solution-processed electrodes. Two important factors should be considered when the top electrode is deposited by the solution method: First, the prepared layers should not be damaged when the top electrode is deposited. Unlike vacuum-evaporated metal electrodes, solution-processed electrodes generally require harsh post-treatment conditions such as high-temperature annealing and penetrating other layers 120. Second, high reflectivity. The thickness of the organic photoactive layer is generally 100 nm because of the short exciton diffusion length of the organic photoactive layer material. This prevents sunlight from being absorbed completely when it shines on the active layer. Commonly used vacuum-evaporated electrodes such as silver have good reflectivity. This enables the active layer to absorb secondary light and thereby, increases the photocurrent. However, conventional solution-processed electrodes such as AgNWs and PEDOT:PSS have high transmittance 121. Recently, He et al. 122 used Ag nanoparticles as the top electrodes. They introduced a hydrogen-intercalated molybdenum oxide (HMO) layer to induce the formation of a smooth solution-processed Ag film. Furthermore, there was no heating or other harsh treatment. However, this process cannot be applied to flexible devices.

    Liquid metals such as eutectic gallium–indium (EGaIn) show high conductivity and opacity 123. importantly, the liquid state of EGaIn at room temperature allows for solution processing by spray coating, etc 124. Wang et al. dropped EGaIn directly on the surface of active layer in an N2-glovebox 125. Pure EGaIn can form a clear and uniform interface without penetration or voids. The ETL would no longer be required in this device because of the matching energy level of pure EGaIn and non-fullerene acceptors. The all-solution-processed devices achieved performance comparable to that of the evaporated reference.

    Bihar et al. developed a metal-free device structure with PEDOT:PSS electrodes to achieve all-solution processed flexible OPV 126. The PEDOT:PSS electrodes were used as both bottom and top electrodes by inkjet printing. In this work, PEDOT:PSS was treated using various methods to improve processability and device performance. Finally, a fully inkjet-printed ultrathin OPV with a PCE of 3.6% was fabricated successfully. It showed high power-per-weight values and good stability in moist environments.

    Solution-processed interface layers

    The first and most important requirement of a solution-processed interface layer is the matched energy level (Fig. 5a). Interface layers with suitable energy levels were introduced between the electrodes and active layers to improve charge extraction. The narrow bandgap small-molecule acceptor materials generally have lower HOMO energy levels. These pose new challenges to conventional interface layer materials. Second, the solution-processed interface layers should be robust to protect the active layers from penetration by the solution-processed electrodes. Various interface layers including electron and hole transport layers have been used widely in solution-processed fullerene-based OPV 127. However, unlike conventional fullerene-based acceptors, high-efficiency narrow bandgap small-molecules acceptors have more complex chemical structures, e.g., ITIC, IT-4F, and Y6 128. These display different chemical activities at the interface 129.

    Electron transport layer

    Sol-gel ZnO and ZnO nanoparticle solutions have been introduced successfully into solution-processed fullerene-based OPV owing to their good solution processability and matched energy levels 130. However, in 2019, Jiang et al. 131. reported a photocatalytic reaction between ZnO and a non-fullerene acceptor (IT-4F) under UV illumination. Mass spectrometry (MS) and Fourier transform infrared (FT-IR) measurements showed that the chemical structure of IT-4F was destroyed owing to the photocatalytic activity of the ZnO film. The unstable interface between ZnO and non-fullerene acceptors restricts the performance and stability of these cells. The authors replaced ZnO with SnO2, which has been reported to be an effective ETL in perovskite solar cells. The wide bandgap of SnO2 causes the device to display higher performance and photostability than ZnO. They also fabricated 1 cm 2 OPV device with a slot-die-coated SnO2 layer. SnO2 also exhibited remarkable thickness tolerance. Bai et al. 132 fabricated OPV based on PM6:Y6 by increasing the thickness of the SnO2 films from 10 nm to 160 nm. The PCEs of the cells range from 16.10% to 13.07%. It is noteworthy that the efficiency of the devices remained at 12% with a 530 nm thick blade-coated SnO2 layer. Recently, Hoff et al. successfully deposited SnO2 nanoparticles (NPs) on top of the active layers in air by slot-die coating 133. Ethanol was added to a commercial SnO2 solution to improve the film quality. To evaluate the suitability of SnO2 NPs as solution-processed ETL, the device based FBT:PC61BM and PTQ10:IDIC active layers with a conventional device structure ITO/PEDOT:PSS/FBT:PC61BM/SnO2 NPs/Ag were fabricated. All the devices showed significantly higher performance than the devices without ETL. Then, all the slot-die-coated flexible OPV with PET/ITO substrates was fabricated. The interface and active layers were coated sequentially onto the electrodes.

    Polyethylenimine (PEI) is a successful polymer-containing amine group for reducing the work functions of various electrodes 134. In a previous report, PEI was introduced successfully into fullerene-based blends and assembled at the bottom of the blends 135. Lee et al. printed PEI and BHJ simultaneously by blade coating and achieved vertical self-organization of the interface layer 136. This new printing method yielded simple tandem OPV devices with four printed layers. However, the amine-containing PEI tends to react with high-efficiency acceptors 137,138. Qin et al. 19 developed a novel interlayer of Zn 2.chelated polyethylenimine (PEI-Zn) to prevent the reaction between PEI and high-efficiency active layers (Fig. 5b). The chemical reaction activity of PEI was suppressed by chelation of Zn 2 with the PEI. The interlayer exhibits a lower work function and higher conductivity than those of conventional ZnO. They fabricated ultraflexible OPV-based PEDOT:PSS and AgNWs electrodes using PEI-Zn. In addition, PEI-Zn exhibits higher mechanical flexibility than ZnO. It is evident that no cracks appeared on the PEI-Zn film. However, a few cracks appeared after the bending tests. These cracks increased the resistance of the ZnO films. The PEI-Zn film exhibited a more robust bending resistance when the bending radius was

    Hole transport layer

    To achieve solution-processed OPV, silver ink and nanowires used as top electrodes would permeate the layer underneath and reduce the performance. It is necessary to construct a robust interlayer to prevent infiltration of the top electrodes. For an inverted-structure OPV, the hole-transport layers should perform such roles. PEDOT:PSS is a practical material owing to its solution processability, good film-forming capability, and high work function. There have been a series of reports on using PEDOT:PSS as the HTL on top of devices to achieve good performance in solution-processed OPV 139,140,141. However, certain problems that occur while depositing PEDOT:PSS films remain to be solved.

    The most important problem is the wettability of the aqueous PEDOT:PSS solution deposited on the surface of the active layer. Li et al. 105. reported a nonionic surfactant (PEG-TmDD) that enhanced the conductivity and wettability of PEDOT:PSS. The vacuum-free cells based on P3HT:ICBA with the PEDOT:PSS (PH1000 mixed with 4 wt% PEG-TmDD) electrode exhibited good performance, with an FF of 0.6 and PCE of 4.1%. Maisch et al. 142 introduced an effective strategy for preparing PEDOT:PSS films on top of the active layer without additives. They improved the wettability of the PEDOT:PSS solution by placing pinning centers on the active-layer film via inkjet printing. This method produced a homogeneous PEDOT:PSS layer on the active layer (P3HT:O-IDTBR) in devices with the structure of ITO/SnO2/P3HT:O-IDTBR/PEDOT:PSS/Ag. It showed performance comparable to those of the reference ones. Recently, Jiang et al. 143 synthesized an alcohol-dispersed conducting polymer complex (PEDOT:F). The perfluorinated sulfonic acid (PFSA) ionomers replaced the poly(styrenesulfonic acid) (PSS) counterion during PEDOT polymerization. The produced PEDOT:F formulations can be dispersed both in alcohols and water owing to the special soluble behavior of PFSA. This enables good solution processability on the surface of the organic active layer.

    However, the chemical activity of PEDOT:PSS is also problematic. We have mentioned earlier that Zhou and their co-workers determined that the acid of PEDOT:PSS would cause a chemical reaction between PEDOT:PSS and additives in the active layer such as DIO 92. This chemical reduction of the PEDOT:PSS resulted in low performance of the devices. Wang et al. 144 also reported vertical phase separation in a non-fullerene BHJ system based on PBDB-T:ITIC. After the aging test, the vertical stratification of the PBDB-T:ITIC BHJ was investigated by X-ray photoelectron spectroscopy (XPS) measurements. The results showed that PBDB-T was enriched at the upper surface and that ITIC was enriched at the bottom surface of the BHJ. The Raman spectra show an apparent shift in the quinoidal PEDOT configuration on the PEDOT:PSS/ITIC surface compared with the pristine PEDOT:PSS surface. This shift was attributed to the chemical reaction between ITIC and PEDOT:PSS.

    Another issue with the use of PEDOT: PSS as an HTL in OPV is the mismatch in energy level. The high-efficiency acceptors such as Y6 and IT-4F have deep HOMO of.5.62 and.5.71 eV, respectively 145. The polymer PM6 containing a F substitution with a HOMO of.5.54 eV is generally selected as a donor to match the energy level. However, the Fermi level of PEDOT:PSS was significantly higher than that of PM6. This large gap in energy level (~0.5 eV) between the donor and PEDOT:PSS causes severe charge extraction. Han et al. 146. reported a reduction in the VOC of the OPV owing to the mismatch in energy level between the HTL solar cell and the polymer donor. A bilayer HTL structure (WO3/PEDOT:PSS) was introduced into a printed OPV for VOC recovery to solve this issue. The work function of the bilayer HTLs (-5.27 eV) is significantly closer to the HOMO of the donor (SMD2; −5.44 eV) than that of the HTL solar (−5.05 eV). Significant improvements in device performance can be achieved using JSC, VOC, and FF. The bilayer HTL structure can also enhance the photostability of the devices under UV light. Flexible OPV modules containing 10 subcells were fabricated by slot-die coating and screen printing owing to the high solution processability of bilayer HTLs. The device exhibited a PCE of 5.25% with an active area of 80 cm 2. Sun et al. observed a similar phenomenon of reduction in Voc when they fabricated all-solution-processed OPV using PEDOT:PSS PH1000 as the printable electrode and PM6:Y6:IDIC as the active layer 39. The researchers attributed the decrease in performance to physical and chemical compatibility issues between the active layer and polymer electrodes. Therefore, solution-processed HxMoO3 was employed to solve the issues of hole extraction and wetting between the high-efficiency active layer and PEDOT:PSS. Compared with PEDOT:PSS, the deeper Fermi level of HxMoO3 (-5.44 eV) enhanced charge extraction in the devices. The solution-processed HxMoO3 had good wettability on the surface of the active layer because of the ethanol solvent. Most importantly, the hydrophilic surface of HxMoO3 facilitated the deposition of PEDOT:PSS on top of it. The HxMoO3 composition was controlled finely by tuning the reaction conditions to obtain processing orthogonality between HxMoO3 and PEDOT:PSS. With the introduction of the HxMoO3 layer, the flexible all-solution processed OPV (all the layers from the bottom substrate to the top electrode) were fabricated effectively with an efficiency of 11.9% for small-area devices (0.04 cm 2 ) and 10.3% for 1 cm 2 devices. The emerging high-performance non-fullerene acceptors (such as Y6) were adopted successfully in all-solution processed OPV device.

    Summary and outlook

    This review summarizes the recent work on solution-processed flexible OPV. Large-scale printing is a prerequisite for the commercial application of OPV. The efficiency of OPV has been a significant breakthrough in the laboratory with the Rapid development of narrow bandgap small-molecule acceptor and wide bandgap polymer donor materials. Researchers need to consider how to translate lab cells into commercial cells using the printing process. In this review, we presented and evaluated studies on solution-processed flexible OPV in terms of various aspects, including the solution process requirements and different solution-processed functional layers.

    Although many researchers have reported remarkable work on solution-processed flexible OPV, the following aspects need to be examined further for commercial applications.

    • (1) Stability of the OPV. The efficiency of OPV is higher (19%) than the market viability that is generally assumed (15%). Stability is the main limitation that needs to be examined in commercial applications. Loo et al. 147 recently published a comment in the journal Nature Energy to induce researchers to pay more attention to the material stability and device longevity of OPV. The complex chemical structure of the active layer materials can generate more unstable factors in solution-processed OPV devices. The next step in the development of OPV should FOCUS on enhancing the stability of the devices. Developing more stable materials and robust device structures is an effective strategy.
    • (2) Thicker and less expensive active layers. The high-efficiency OPV reported in the literature is mostly fabricated in the laboratory by spin coating with a film thickness of ~100 nm. Such a small film thickness causes issues in printing large-area OPV. Thin films are susceptible to defects on the substrates. However, it is difficult to homogenously control such thin films during large-scale fabrication. Thick active layers ( 300 nm) can be better matched for roll-to-roll large-scale printing and enhance photon harvesting. A few groups have reported OPV based on the printed thick-high-efficiency active layer with remarkable PCE 148,149. However, the efficiency of these devices with thick active layers differs significantly from the highest reported efficiency. The low charge carrier mobility and charge carrier recombination decrease the device performance with improved thickness of the active layer. This limits the number of materials suitable for thicker layers. Therefore, the development of a high-efficiency active layer with thickness tolerability is desirable. The cost of high-efficiency active layer materials also needs to be reduced further to satisfy the demand for the scale test of printed OPV in the laboratory 150.
    • (3) Simple device structure. The reported high-efficiency OPV typically have a multilayer structure (more than five layers). A larger number of layers implies a larger number of interfaces. This multilayer structure has high requirements for the interface layer. Interlayers need to be stable, convenient to process, and sufficiently robust to prevent penetration from the deposition of the upper layer. This complex interface would hinder large-scale production because a few of the interface layers have a size of only several nanometers. Therefore, it is highly important to develop a simple device structure while maintaining device performance, for large-scale production.

    To summarize, the development of high-efficiency active layer materials has further promoted the large-scale production of OPV. However, it also introduces new challenges. The preparation of high-efficiency and stable solution-processed flexible OPV requires more extensive research on the interface, printing methods, and materials development. We consider that this review would inspire readers to promote the commercialization of OPV.

    Data availability

    All data are available in the main text.


    This study was financially supported by the Japan Science and Technology Agency (JST) A-STEP under Grant No. AS3015021R.

    Organic photovoltaic cell with 17% efficiency and superior processability

    State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences

    State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences

    State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences

    State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences

    State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences

    State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences

    State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences

    State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences

    State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences

    National Science Review, Volume 7, Issue 7, July 2020, Pages 1239–1246,


    Yong Cui and others. Organic photovoltaic cell with 17% efficiency and superior processability, National Science Review, Volume 7, Issue 7, July 2020, Pages 1239–1246,


    The development of organic photoactive materials, especially the newly emerging non-fullerene electron acceptors (NFAs), has enabled Rapid progress in organic photovoltaic (OPV) cells in recent years. Although the power conversion efficiencies (PCEs) of the top-performance OPV cells have surpassed 16%, the devices are usually fabricated via a spin-coating method and are not suitable for large-area production. Here, we demonstrate that the fine-modification of the flexible side chains of NFAs can yield 17% PCE for OPV cells. crucially, as the optimal NFA has a suitable solubility and thus a desirable morphology, the high efficiencies of spin-coated devices can be maintained when using scalable blade-coating processing technology. Our results suggest that optimization of the chemical structures of the OPV materials can improve device performance. This has great significance in larger-area production technologies that provide important scientific insights for the commercialization of OPV cells.


    Organic photovoltaic (OPV) technology is a promising candidate in use of sustainable solar energy; the power conversion efficiency (PCE) is growing very fast with great potential in practical applications [ 1–5]. In the last 30 years, development of new materials, optimization of device processing methods and blend morphology [ 6–12], and an improved understanding of device physics have greatly contributed to progress in OPV cells [ 13–15]. One of the biggest advantages of OPV cells is solution processability, facilitating large-area production at low-cost via scalable printing technologies [ 16–19]. Although the PCEs of single-junction OPV cells have surpassed 16% [ 20–22], most of the devices with cutting-edge performance were fabricated by spin-coating methods at small areas below 0.1 cm 2. which is far away from practical applications. Furthermore, the spin-coating method is highly wasteful of solution, and is not suitable for large-scale production. Therefore, when designing highly efficient OPV materials, their applicability in scalable fabrication technologies over relatively large active areas must be investigated.

    Recent achievements in OPV cells are dominated by development and application of non-fullerene acceptors (NFAs) [ 23–25]. High-performance NFAs show broad absorption from 400 to 900 nm [ 5], leading to efficient harvesting of solar photons and thus a high output current density. NFA-based devices show both reduced radiative and non-radiative energy losses (Elosss), having the benefit of obtaining high voltages [ 26–28]. PCEs of over 16% were obtained with NFA-based OPV cells. We note that most NFAs consist of fused five- or six-membered heterocycles. For instance, highly efficient NFAs such as ITIC [ 29], Y6 [ 5] and their derivatives have highly fused ladder-type structures. The large conjugated structure is beneficial to form ordered intermolecular π–π stacking and improve the charge transport [ 30–32]. However, the same feature results in poor solubility of the NFAs, making solution-processing procedures difficult. To solve this issue, fine-tuning the flexible side chains of NFAs is crucial in balancing the charge transport and solution processability. This is particularly important when scaling up the active area of the OPV cells because the device performance relies strongly on a uniform morphology [ 33–35]. The best large-area OPV cells using printing methods have a PCE of only 13% [ 36], which is far behind that of small-area spin-coated devices.

    Here, we conduct side-chain engineering on a highly efficient NFA BTP-4Cl and study the applications of the OPV materials under different processing conditions. This approach shows improved photovoltaic performance for OPV cells with large-area fabrication. Impressively, the best device yields a maximum PCE of 17.0% at an active area of 0.09 cm 2. This is among the top efficiencies for OPV cells, and the result has been certified by an independent institution. Importantly, when a blade-coating method was used to extend the active area of the active layer, a high PCE of 15.5% was maintained because of the balanced solution processability and charge transport. In comparison, the high efficiencies of the spin-coated OPV cells based on two other NFAs with shorter or longer alkyl chains suffered significant decreases when fabricating large-area devices using the blade-coating method.


    In our recent work, we designed the chlorinated NFA BTP-4Cl and achieved superior photovoltaic efficiencies over Y6 in OPV cells, where PCEs of 16.1 ± 0.2% and 10.7 ± 0.5% were recorded using a spin-coating method at device areas of 0.09 and 1 cm 2. respectively [ 21]. The high efficiencies of this material make it a good model to investigate the adaptability of scalable production technology in OPV cells. However, when we adopted the doctor blade-coating method to fabricate 0.81 cm 2 devices, the PCE dropped dramatically to 10.7 ± 0.5% (Table 1 and Supplementary Fig. 1 ). This was mainly ascribed to the poor blend morphology caused by limited solubility of BTP-4Cl, as discussed below. As displayed in Fig. 1a, to improve the processability of BTP-4Cl (here named as BTP-4Cl-8 for comparison), we replaced the 2-ethylhexyl with longer side chains of 2-butyloctyl or 2-hexyldecyl and synthesized new NFAs BTP-4Cl-12 and BTP-4Cl-16, respectively. Detailed synthetic procedures and structural characterizations are provided in the Supplementary Data.

    (a) Chemical structures of BTP-4Cl-X, where X represents 8, 12 or 16. (b) Normalized absorption spectra of BTP-4Cl-X in diluted (solid line) and concentrated (dashed line) chlorobenzene solutions. (c) Normalized absorption spectra of the neat donor and acceptors in thin films. (d) J–V curves of the best devices. The inset is a statistical diagram of PCEs for PBDB-TF:BTP-4Cl-12-based cells. (e) J–V curve of the OPV cell certified in the NIM. (f) EQE curves of the corresponding OPV cells.

    Detailed photovoltaic parameters of the OPV cells.

    Active layer. Coating method. VOC (V). JSC (mA/cm 2 ). FF. PCE (%) a. Area (cm 2 ) b. .
    PBDB-TF:BTP-4Cl-8 Spin-coating 0.872 25.2 0.743 16.3 (16.1 ± 0.2) 0.06
    Spin-coating 0.863 24.9 0.711 15.3 (14.8 ± 0.3) 0.81
    Blade-coating 0.838 21.7 0.635 11.5 (10.7 ± 0.5) 0.81
    PBDB-TF:BTP-4Cl-12 Spin-coating 0.858 25.6 0.776 17.0 (16.6 ± 0.2) 0.06
    Spin-coating c 0.853 25.4 0.772 16.7 0.06
    Spin-coating 0.849 25.5 0.738 16.0 (15.5 ± 0.3) 0.81
    Blade-coating 0.833 26.0 0.716 15.5 (14.9 ± 0.4) 0.81
    PBDB-TF:BTP-4Cl-16 Spin-coating 0.862 24.2 0.748 15.6 (15.2 ± 0.2) 0.06
    Spin-coating 0.854 24.0 0.718 14.7 (14.2 ± 0.3) 0.81
    Blade-coating 0.807 19.4 0.689 10.8 (9.81 ± 0.6) 0.81
    Active layer. Coating method. VOC (V). JSC (mA/cm 2 ). FF. PCE (%) a. Area (cm 2 ) b. .
    PBDB-TF:BTP-4Cl-8 Spin-coating 0.872 25.2 0.743 16.3 (16.1 ± 0.2) 0.06
    Spin-coating 0.863 24.9 0.711 15.3 (14.8 ± 0.3) 0.81
    Blade-coating 0.838 21.7 0.635 11.5 (10.7 ± 0.5) 0.81
    PBDB-TF:BTP-4Cl-12 Spin-coating 0.858 25.6 0.776 17.0 (16.6 ± 0.2) 0.06
    Spin-coating c 0.853 25.4 0.772 16.7 0.06
    Spin-coating 0.849 25.5 0.738 16.0 (15.5 ± 0.3) 0.81
    Blade-coating 0.833 26.0 0.716 15.5 (14.9 ± 0.4) 0.81
    PBDB-TF:BTP-4Cl-16 Spin-coating 0.862 24.2 0.748 15.6 (15.2 ± 0.2) 0.06
    Spin-coating 0.854 24.0 0.718 14.7 (14.2 ± 0.3) 0.81
    Blade-coating 0.807 19.4 0.689 10.8 (9.81 ± 0.6) 0.81

    a The average parameters are calculated from more than 20 independent cells.

    b The area of the mask; the device areas of small- and large-area OPV cells are 0.09 and 1.07 cm 2. respectively.

    c The result is obtained from NIM.

    Detailed photovoltaic parameters of the OPV cells.

    Active layer. Coating method. VOC (V). JSC (mA/cm 2 ). FF. PCE (%) a. Area (cm 2 ) b. .
    PBDB-TF:BTP-4Cl-8 Spin-coating 0.872 25.2 0.743 16.3 (16.1 ± 0.2) 0.06
    Spin-coating 0.863 24.9 0.711 15.3 (14.8 ± 0.3) 0.81
    Blade-coating 0.838 21.7 0.635 11.5 (10.7 ± 0.5) 0.81
    PBDB-TF:BTP-4Cl-12 Spin-coating 0.858 25.6 0.776 17.0 (16.6 ± 0.2) 0.06
    Spin-coating c 0.853 25.4 0.772 16.7 0.06
    Spin-coating 0.849 25.5 0.738 16.0 (15.5 ± 0.3) 0.81
    Blade-coating 0.833 26.0 0.716 15.5 (14.9 ± 0.4) 0.81
    PBDB-TF:BTP-4Cl-16 Spin-coating 0.862 24.2 0.748 15.6 (15.2 ± 0.2) 0.06
    Spin-coating 0.854 24.0 0.718 14.7 (14.2 ± 0.3) 0.81
    Blade-coating 0.807 19.4 0.689 10.8 (9.81 ± 0.6) 0.81
    Active layer. Coating method. VOC (V). JSC (mA/cm 2 ). FF. PCE (%) a. Area (cm 2 ) b. .
    PBDB-TF:BTP-4Cl-8 Spin-coating 0.872 25.2 0.743 16.3 (16.1 ± 0.2) 0.06
    Spin-coating 0.863 24.9 0.711 15.3 (14.8 ± 0.3) 0.81
    Blade-coating 0.838 21.7 0.635 11.5 (10.7 ± 0.5) 0.81
    PBDB-TF:BTP-4Cl-12 Spin-coating 0.858 25.6 0.776 17.0 (16.6 ± 0.2) 0.06
    Spin-coating c 0.853 25.4 0.772 16.7 0.06
    Spin-coating 0.849 25.5 0.738 16.0 (15.5 ± 0.3) 0.81
    Blade-coating 0.833 26.0 0.716 15.5 (14.9 ± 0.4) 0.81
    PBDB-TF:BTP-4Cl-16 Spin-coating 0.862 24.2 0.748 15.6 (15.2 ± 0.2) 0.06
    Spin-coating 0.854 24.0 0.718 14.7 (14.2 ± 0.3) 0.81
    Blade-coating 0.807 19.4 0.689 10.8 (9.81 ± 0.6) 0.81

    a The average parameters are calculated from more than 20 independent cells.

    b The area of the mask; the device areas of small- and large-area OPV cells are 0.09 and 1.07 cm 2. respectively.

    c The result is obtained from NIM.

    To investigate the molecular stacking properties, we measured the ultraviolet-visible (UV–vis) absorption spectra of the three NFAs in diluted and concentrated chlorobenzene solutions (Fig. 1b). In the dilute solution (∼ 5 μg/mL), the peak at 740 nm is highly determined by intramolecular charge transfer [ 37, 38], and the change of alkyl chains has no significant effect. The absorption coefficients of the three NFAs were measured and the results are provided in Supplementary Fig. 2. With longer side chains, the NFAs show some increases in absorption coefficient, which may be related to enhanced intermolecular packing properties. When the concentration increases (∼10 mg/mL), the absorption is affected more by intermolecular charge transfer of the aggregators [ 37]. For the three NFAs, the absorption edges redshift with increasing alkyl chain length, which may imply enhanced aggregation properties in BTP-4Cl-12 and BTP-4Cl-16. Figure 1c shows the absorption spectra of the NFAs as thin films. We found that the main peaks of the three NFAs highly overlapped at 836 nm, a redshift of 90 nm over that in solution states. We measured the molecular energy levels of the three NFAs via electrochemical cyclic voltammetry measurements. As shown in Supplementary Fig. 3. the results suggest that modification of the side chains has little impact on the energy levels of the NFAs.

    The crystalline properties of the NFAs were investigated by grazing-incidence wide-angle X-ray scattering (GIWAXS). Supplementary Fig. 4a shows the 2D GIWAXS patterns of the neat NFA films. The clear (010) diffraction peaks in the out-of-plane direction suggest that they have a preferential face-on orientation. Supplementary Fig. 4b presents the 1D profiles along the out-of-plane and in-plane directions. In the out-of-plane direction, the (010) diffraction peaks of BTP-4CL-8, BTP-4Cl-12 and BTP-4Cl-16 are located at 1.81, 1.84 and 1.74 Å −1. respectively, implying that BTP-4Cl-12 has the shortest π–π stacking distance. In the in-plane direction, we found that the lamellar packing distance increases with the longer alkyl chains. In addition, we also conducted the GIWAXS measurements on blend films based on PBDB-TF as donor ( Supplementary Fig. 4c and d ). The calculated (010) coherence length values are 2.18, 1.76 and 1.92 nm for BTP-4Cl-8-, BTP-4Cl-12- and BTP-4Cl-16-based devices, respectively. These results indicate that the PBDB-TF:BTP-4Cl-12-based blend film has the lowest crystalline property. The differences in crystalline properties may lead to varied microscopic morphologies.

    To investigate the photovoltaic performance of BTP-4Cl-12 and BTP-4Cl-16, we first fabricated small area (0.09 cm 2 ) spin-coated OPV cells, in which a conventional device structure of ITO/PEDOT:PSS/PBDB-TF [ 39]:NFA blend/PDINO/Al was adopted (ITO: indium tin oxide; PEDOT:PSS: poly(3,4-ethylenedioxythiophene): poly-(styrenesulfonate); PDINO [ 40]: perylene diimide functionalized with amino N-oxide). The device based on BTP-4Cl-8 was also prepared in parallel for clear comparison. The optimal device fabrication conditions based on the three NFAs are provided in the Supplementary Data.

    Figure 1d shows the current density−voltage (J−V) curves of the optimized OPV cells, and the detailed photovoltaic parameters are collected in Table 1. In comparison, the variances in open-circuit voltages (VOCs) are very small. We carried out highly sensitive EQE and electroluminescence (EL) quantum efficiency (EQEEL) measurements, and found that the three OPV cells have similar Band gaps and Elosss ( Supplementary Fig. 5 and Supplementary Table 1 ). The PBDB-TF:BTP-4Cl-8-based OPV cell shows a maximum PCE of 16.3% with a VOC of 0.872 V, a short-circuit current (JSC) of 25.2 mA/cm 2 and a fill factor (FF) of 0.743, which are consistent with previous results [ 21]. The PCE of the OPV cell based on PBDB-TF:BTP-4Cl-16 is lower than that of the PBDB-TF:BTP-4Cl-8-based device because of the decreased JSC. The BTP-4Cl-12-containing device shows improved JSC and FF values relative to the other two devices, leading to the highest PCE of 17.0%. To the best of our knowledge, this is the highest value for the published single-junction OPV cells so far. The inset in Fig. 1d shows a PCE histogram of 80 devices based on PBDB-TF:BTP-4Cl-12 from eight batches, with an average value of 16.6 ± 0.2%. We then sent the best cell to the National Institute of Metrology (NIM, China) for certification. As shown in Fig. 1e and Supplementary Fig. 6. the optimal PCE obtained from NIM is 16.7%. After 500 h in the nitrogen atmosphere, the encapsulated devices maintain ∼85–90% of the initial efficiencies ( Supplementary Fig. 7 ). Figure 1f shows the EQE curves of the optimal devices. It can be seen that the BTP-4Cl-12-based device shows higher EQE values than the other devices in most regions of 450–850 nm. The integrated current densities are 25.1, 25.4 and 24.0 mA/cm 2 for BTP-4Cl-8-, BTP-4Cl-12- and BTP-4Cl-16-based devices, respectively, which show good consistency with the J–V measurements.

    In addition to high efficiency, low sensitivity to thickness variation is important for practical production. As depicted in Fig. 2a and b, we studied the effect of active layer thickness on the photovoltaic characteristics (VOC, JSC, FF and PCE). The optimal thickness of the active layer is about 100 nm. As the active layer thickness is increased from 80 to 300 nm,

    the VOC and FF decrease. All the devices show some increase in JSCs for enhanced light absorption. As a result, all the devices can maintain 85% of optimal PCEs when the active layer thicknesses increase to 300 nm, which is beneficial to fabrication of large-area modules. In addition, there is no apparent difference in the three devices.

    (a) VOC and JSC versus active layer thickness. (b) FF and PCE versus active layer thickness. (c) Statistics of OPV cell under different preparation conditions; the areas of the masks are shown in the panel. (d) Photo-CELIV curves of the devices. (e) VOC of the devices as a function of light intensity. (f) JSC of the devices against light intensity.

    To explore the applicability of the OPV cells in large-area fabrication, we next adopted a blade-coating method to fabricate 1 cm 2 devices. Fabrication procedures for the devices are described in the experimental part of the Supplementary Data. To better compare the spin-coating and blade-coating methods, we first fabricated the 1 cm 2 devices using the spin-coating method. As shown in Fig. 2c and Supplementary Fig. 1. when extending the active area from 0.09 to 1 cm 2. although all three devices show some decreases in photovoltaic parameters especially FF values, the PCEs are still above 14.5% (the detailed photovoltaic parameters are collected in Table 1). Impressively, a high PCE of 16.0% is recorded for the BTP-4Cl-12-containing OPV cell. For the blade-coated device based on PBDB-TF:BTP-4Cl-12, a maximum PCE of 15.5% was obtained, which is comparable to the spin-coated cell. It should be pointed out that the PCEs of both the spin-coated and blade-coated devices based on PBDB-TF:BTP-4Cl-12 are very pronounced results for OPV cells. In contrast, the BTP-4Cl-8- and BTP-4Cl-16-based cells suffer significant decreases in PCEs, with the best PCEs only around 11%. It is necessary to understand the reasons for the decline of photovoltaic performance for the blade-coating devices.

    We studied the charge transport and recombination in the 1 cm 2 devices fabricated by varied processing methods. First, we measured the mobilities of the fast carrier component by performing photo-CELIV measurements on the working devices (photo-CELIV: the photoinduced charge-carrier extraction in a linearly increasing voltage) [ 41]. As shown in Fig. 2d, when the spin-coating method was used, all the devices had similar mobilities: the calculated mobilities were 2.86 × 10 –5. 2.92 × 10 –5 and 3.10 × 10 –5 cm 2 /V/s for BTP-4Cl-8-, BTP-4Cl-12- and BTP-4Cl-16-based devices, respectively. When the blade-coating technology was used, all the devices showed decreased mobilities to varying extents: the BTP-4Cl-12-based device showed a slight decrease (1.92 × 10 −5 cm 2 /V/s), whereas remarkable decreases were observed in the devices based on BTP-4Cl-8 (9.23 × 10 −6 cm 2 /V/s) and BTP-4Cl-16 (8.31 × 10 −6 cm 2 /V/s). The lower mobilities will cause more charge recombination and thus decrease the JSC and FF [ 42, 43].

    We then measured the VOC and JSC dependence on the incident light intensity (Plight) for the different devices. The VOC as a function of the light intensity is plotted in Fig. 2e. All the spin-coated devices show a weak dependence of VOC on Plight. The slope of ΔVOCvs Δln(Plight) was used to investigate the trap-assisted recombination, where k is the Boltzmann constant, T is the absolute temperature and q is the electric charge [ 44–46]. The slopes were 1.09, 1.10 and 1.13 kT/q for the devices based on BTP-4Cl-8, BTP-4Cl-12 and BTP-4Cl-16, respectively. When the blade-coating method replaced the spin-coating method to fabricate the devices, all the devices showed increased slopes. The BTP-4Cl-12-based device showed a slightly higher slope of 1.19 kT/q, whereas much higher slopes of 1.38 and 1.31 kT/q were calculated for the BTP-4Cl-8- and BTP-4Cl-16-based devices. Under the same processing conditions, the lower slope of the BTP-4Cl-12-based device implies a more suppressed trap-assisted recombination in the devices. The significantly increased slopes are one of the main reasons for the decreased PCEs of the devices based on BTP-4Cl-8 and BTP-4Cl-16 [ 45].

    The relationship between JSC and Plight is plotted in Fig. 2f, where the exponential factor (s) of the power-law equation JSC ∝ Plight s can reflect the degree of bimolecular recombination. For the 1 cm 2 devices made by spin-coating method, we found that the JSC exhibits almost linear dependence on the Plight, implying a negligible bimolecular recombination in these devices [ 47]. When the blade-coating technology was used to fabricate the BTP-4Cl-12-based device, the s value decreased slightly to 0.961. In contrast, the BTP-4Cl-8- and BTP-4Cl-16-based devices yielded much lower S values of 0.911 and 0.899, respectively. These results suggest that bimolecular recombination is more pronounced in the blade-coated devices, which is associated with the lower charge mobilities.

    From the above results, it can be reasonably concluded that higher charge transport and more suppressed charge recombination in the BTP-4Cl-12-based devices are the main reasons for the enhanced JSCs and FFs over the BTP-4Cl-8- and BTP-4Cl-16-based devices. To better understand how the processing technology affects the device performance, we first scanned the entire working area (1 cm 2 ) via a 520 nm laser and mapped the EQE values, which can give a clear view of how the morphology affects the photon-response of the OPV cells. As presented in Fig. 3a–c, the EQE maps for the spin-coated devices are very uniform, which suggests that the whole regions have highly efficient charge generation, transport and collection. The high EQE values are consistent with their high JSCs in the J–V measurements.

    (a–c) The EQE mapping images of the OPV cells fabricated via spin-coating method. (d–f) The EQE mapping images of the OPV cells fabricated via blade-coating method.

    Unlike the spin-coating method, drying wet film is difficult by using the blade-coating method. The solubility and aggregation properties of the active materials have a great impact on the blend morphology of the resulting films. For the blade-coated 1 cm 2 OPV cells, the uniformity of the EQE maps is not as good as that of the spin-coated devices (Fig. 3d–f). For the BTP-4Cl-8-based blend film (Fig. 3d), the relatively low solubility of BTP-4Cl-8 makes it easily dissolve out from the solution, leading to a non-uniform film. For the blend film based on BTP-4Cl-16, good solubility and strong aggregation feature (Fig. 1b) may result in overlarge clusters. The BTP-4Cl-12-based device shows a relatively uniform EQE map without many low EQE regions.

    Furthermore, to get a more microscopic view, we studied differences in the surface morphology between the spin-coated and blade-coated photoactive layers using the atomic force microscopy (AFM). As shown in Fig. 4, the blend films based on PBDB-TF:BTP-4Cl-X fabricated by the different methods present remarkably different surface roughness and phase separation features. For the spin-coated films, the BTP-4Cl-8- and BTP-4Cl-12-based blend show a smooth surface and good phase separation features, and the mean-square surface roughness (Rq) is 1.85 and 1.31 nm, respectively. In contrast, the Rq of the BTP-4Cl-16-based film is as large as 7.92 nm, which could be ascribed to its relatively low photovoltaic performance. The volatilization rate of the solvent decreased significantly when the

    blade-coating method was used [ 48–50], leading to a longer time for ordered molecular alignment and aggregation. As illustrated in Fig. 4b, the Rq values and domain sizes increase for all the blade-coated films. For the blade-coated BTP-4Cl-12-based film, suitable phase separation with appropriate domain size is maintained, which may be attributed to the lower crystalline property. In comparison, larger domains are obtained for the BTP-4Cl-8- and BTP-4Cl-16-based blend films.

    AFM height images and phase images of PBDB-TF:BTP-4Cl-X blend films prepared by (a) spin-coating process and (b) blade-coating method.


    In summary, aiming to improve the photovoltaic performance and processability of OPV cells, we performed side-chain engineering on the highly efficient NFA material and synthesized BTP-4Cl-X (X = 8, 12 or 16). By employing the polymer donor PBDB-TF, and the NFA BTP-4Cl-12, we successfully demonstrated a high PCE of 17% in single-junction OPV cells. As a result of the balanced solution processability and aggregation feature of BTP-4Cl-12, the blend film based on PBDB-TF:BTP-4Cl-12 showed very good morphology when the blade-coating method was used, contributing to high carrier transport, and suppressed charge recombination in the resulting OPV cell. Therefore, 1 cm 2 OPV cells based on the blade-coating method yield a high PCE of 15.5%. These results are among the top values for OPV cells. This work provides important guidelines for developing highly efficient OPV materials by considering their applications in large-scale production.


    This work was supported by the National Key Research and Development Program of China (2019YFA0705900) funded by MOST, and the Basic and Applied Basic Research Major Program of Guangdong Province (2019B030302007). H.Y. acknowledges the financial support from the National Natural Science Foundation of China (NSFC) (21805287) and the Youth Innovation Promotion Association, CAS (2018043). J. H. would like to acknowledge the financial support from the NSFC (21835006, 51961135103 and 51673201). Y. C. thanks China Postdoctoral Science Foundation (2019M660800). This work was also supported by the Beijing National Laboratory for Molecular Sciences (BNLMS-CXXM-201903). W.M. thanks the Fundamental Research Funds for the Central Universities.

    Conflict of interest statement. None declared.

    Organic Solar Cells and Photovoltaics: Structure, Functions, Price

    Scientists all around the world are developing new technologies to make efficient use of solar energy. From roof-top solar panels to solar lights, there are numerous devices to help people generate electricity from sun rays.

    Organic photovoltaics is the most recent development in this sector. over, these high-potential solar cells are the game-changers in how solar electricity is generated.

    Although organic solar cells offer numerous benefits, many people are unaware of this innovative technology. In this blog, you will get familiar with the organic solar photovoltaic cells, their function, pricing, and various aspects of this recent solar technology.

    What are Organic Photovoltaics Solar Cells?

    Organic solar cells use organic electronics and carbon-based materials as semiconductors to generate electricity from solar energy.

    They are also referred to as polymer solar cells or plastic solar cells. Although they follow the same process as traditional silicon solar panels for generic electricity, there is one significant difference between the two.

    Unlike silicon solar panels, organic photovoltaics have a flexible structure. Therefore, they can easily fit multiple spaces and are also suitable for making solar power Windows.

    Although this new technology requires more research and development, the wide range of applications is increasing its popularity and reliability already.

    over, organic solar cells are less costly to produce. It means that more people will be able to install them without worrying about their budget.

    Structure of Organic Photovoltaics Solar Cells

    OPV or organic photovoltaics have a flexible structure due to carbon-rich compounds. As a result, they enhance PV cell functions like bandgap, colour, and transparency.

    To create an OPV structure, organic compounds that easily dissolve in ink are printed on thin plastic layers.

    These solar power generic cells are relatively less efficient and durable than conventional solar panels. However, they are less expensive too. It certainly results in their high production volume.

    In addition, the plastic-based material of the OPV is easily applicable to a variety of surfaces and areas.

    How do Organic Photovoltaics Solar Cells Function?

    The function of organic photovoltaics is similar to polycrystalline and monocrystalline silicon solar cells.

    They generate solar electricity with the photovoltaic effect. It means, they directly convert the sun’s rays into electricity at the atomic level.

    • In the first step of the photovoltaic effect, the organic solar cells absorb sunlight in the form of energy known as photons.
    • The cells then break the photons to loosen electrons.
    • These free electrons flow to electron acceptors to create a direct current.
    • The electrical current then transfers to the solar inverter.
    • The solar inverter converts DC power into AC power for residential usage.

    Different Types of Organic Photovoltaic Solar Cells Available in the Market

    Polymer-based organic solar cells are categorised into three groups according to their production method. Have a look!

    • Single-layer organic cells: In this type, the external circuit connects to two electrodes through a conductor. The difference in the functions creates an electrical field in the layers of the organic cells.
    • Bilayer organic cells: This type of organic photovoltaics consists of multiple layers of cathode, acceptor, ITO, donor, and substrate. Bilayer organic solar cells split excitons for increased efficiency.
    • Bulk heterojunction organic cells: In this type of organic solar cell, there are two transparent electrodes and one active layer to trap the solar energy.

    Power Generation From Organic Photovoltaics Cells

    OPV and PV follow a similar process of power generation. However, the low efficiency of OPV results in less power generation due to insufficient absorption of sun rays.

    Nonetheless, they’re cheaper than other conventional solar cells. Hence, there’s definite future scope.

    What is the Pricing of Organic Photovoltaic Solar Cells?

    Organic photovoltaics technology is a revolutionary development in the sector of solar power generation.

    The OPV harnesses solar energy to domestic power establishments at a highly affordable price. Although this technology is new and requires extensive research for development, the average cost of organic solar cells varies between INR 2,485/m2 to INR 7,456/m2.

    Pros and Cons of Organic Photovoltaics Solar Cells

    Organic photovoltaics offer the following benefits:

    • The soluble organic molecules of organic solar cells facilitate an easy and less costly manufacturing process.
    • The organic solar cells have adaptive and flexible structures, resulting in a large area of application. over, these lightweight structures are appropriate for use in Windows and doors that receive abundant sunlight.
    • Manufacturers have a vast supply of building block materials for organic photovoltaics.

    Cons of Organic solar cells:

    • The efficiency of organic photovoltaics is comparatively lower than a conventional silicon solar cell. Generally, silicon solar cells offer 18-20% efficiency in the conversion of sun rays into usable electricity. On the other hand, an organic cell’s efficiency is estimated at around 8-12%.
    • The organic materials of OPV degrade much faster than silicon. Therefore, OPVs are a little less durable.

    Why Isn’t Solar Energy Popular?

    Sun offers a sustainable source of energy to every part of the world. However, solar energy is not as widespread a process of generating electricity as it should be.

    The following factors play a vital role in limiting the use of organic photovoltaics and PV system for solar power generation:

    • Solar panels require a huge investment. Therefore, many economically backward groups find difficulty in investing in solar panels.
    • The efficiency of organic photovoltaics is low. People need to install multiple organic solar panels to generate sufficient units of electricity. This requires enormous space.
    • The generation of solar power directly depends upon the availability of bright sunlight. Conditions like storms and clouds can rob people’s access to continual electricity from the Sun.

    That being said, applying for a solar subsidy provided by the government and EMI solutions provided by solar companies can nullify the financial constraints.


    Organic photovoltaics is a promising system for generating sustainable energy. Many researchers are developing new ways to increase its efficiency.

    It is highly expected that in the coming days, this technology will gain popularity.

    over, the abundant material availability of organic photovoltaics makes it affordable. Almost everyone will be able to harness the benefits of sunlight to fulfil their daily power requirement.


    Q. How long do organic photovoltaics Solar Cells last?

    Organic photovoltaics solar cells generally show less than 30% degradation in two months when exposed to harsh climatic conditions.

    However, multiple searches are underway to increase the durability of these organic cells.

    Q. What leads to the low efficiency of organic solar cells?

    Organic cells are highly prone to recombination due to the increased attraction between carbon-based materials and electrons. This results in low efficiency of the organic solar compounds.

    Q. What is the meaning of efficiency in organic photovoltaics?

    The efficiency of solar cells means the amount of solar energy they can convert into usable electricity.

    Therefore, an 8% efficiency means that the solar panel can convert 8% of the total sunlight it receives into an electric current.

    Plastic Solar Cells: OPV power conversion efficiency now reaches 18%

    Don’t just think plastics are insulators. Two Nobel prizes were won by showing they can be semiconductors. That celebrated research opened the door to polymer-based solar cells. They are made in a lab from common elements, using processes established decades ago for ordinary household items like plastic wrap. A design strategy breakthrough led by NREL has now created an organic photovoltaic (OPV) solar cell with a record-breaking 18.07% power conversion efficiency. That’s not far behind conventional silicon-based cells. The research challenge, as is so often the case, is to optimise the results when changing one component’s characteristics at the molecular level negatively affects the performance of another. As solar power moves centre stage, new and cheaper cell designs will be needed. OPVs look promising, particularly as in liquid form they can be sprayed onto rigid or flexible surfaces, meaning OPV modules could be printed in much the same way as newspapers are. They’re recyclable, too.

    National Renewable Energy Laboratory (NREL) Research Scientist Bryon Larson, as part of an international research team, has achieved a record-breaking 18.07% power conversion efficiency from an organic photovoltaic (OPV) solar cell—or as such materials are better known: plastic. Historically, OPV cells have mostly improved through an iterative process. However, Larson and his collaborators have hit upon an “aha!” moment with a new design strategy that simultaneously improves the cell’s open-circuit voltage, short-circuit voltage, and fill factor.

    Concurrently improving all three of these primary metrics—which represent how efficiently a solar cell converts sunlight into electricity—is difficult due to the traditional constraints of only using two components to create the binary donor-acceptor blend in an OPV cell’s light absorbing layer. Changing a single component to improve one metric can negatively affect another, leading to performance trade-offs.

    Optimising four parameters

    “The name of the game for advancing OPV is new materials,” Larson said. “But because of the way that OPVs work, every time you introduce a new material into the absorber blend, you have to reoptimise everything about the cell design. It’s a time-consuming, haphazard process, like trying to find a needle in a haystack. Our strategy demonstrates a quaternary, four-component, approach where each component works in synergy to avoid performance trade-offs and to produce a high power conversion efficiency.”

    The absorber layer in an OPV cell is responsible for light absorption, exciton splitting to generate positive and negative charges, and effective transport of charges to the contacts to produce photocurrent (see image below). The design of a traditional OPV cell struggles to balance the electronic and morphological characteristics of just two sets of molecules or polymers for all these functions.

    Larson, working with researchers from Shanghai Jiao Tong University and the University of Massachusetts Amherst, showed that using four components in an OPV device’s active layer could better balance the microstructure and electronic function of the cell’s absorber layer. This holistic strategy is described in the Nature Communications article “Single-layered organic photovoltaics with double cascading charge transport pathways: 18% efficiencies.”

    Organic photovoltaic devices (OPV) use a unique process to convert sunlight into electricity. This graphic depicts a cross section of an OPV device that has an active layer only 100 nanometers thick and explains the basic operating physics that are unique to OPV. Their low cost and flexible form factor could enable new applications for solar energy. / Image by Bryon Larson, NREL

    Quaternary blends improve power conversion efficiency

    The research article describes numerous strategies to optimise light absorption, carrier transport, and charge-transfer-state energy levels when the chemical structures and excited-state properties of the additional donor and acceptor components are considered. On a molecular level, each of the four components has unique absorption, transport, and electrical properties that can contribute to realising the optimal OPV configuration.

    The quaternary donor-acceptor mixture makes double cascading energy level alignment possible in the active layer, enabling efficient exciton splitting and long carrier lifetimes. The second donor component generates cascading charge-hopping channels that allow manipulation of multiple charge-transfer pathways to maximise the carrier transport and reduce recombination. The second acceptor component improves electron transport and promotes extraction efficiency. The electronic structure and morphology of each of the four components of the quaternary blend mixture are fine-tuned through molecular design to maximise light absorption, exciton splitting, and carrier extraction, ultimately improving power conversion efficiency in OPV devices.

    The potential of Organic Photovoltaics

    OPV materials are semiconductors made up of common organic elements like carbon, hydrogen, nitrogen, fluorine, oxygen, and sulfur. They are man-made molecules and polymers that can be synthesised in a chemistry lab. These non-toxic ‘plastics’ can even be easily recycled back into their constituent building blocks. Their raw materials are cheap, infinitely abundant, and the manufacturing processes for creating them have been used for decades in industries that fabricate household items like plastic wrap and Tupperware.

    OPV blends are handled in liquid form and sprayed or coated onto rigid or flexible surfaces, meaning OPV modules could be printed in the same way as newspapers. The number of potential applications is broad, yet OPV’s have only been intensively researched since the early 2000s, after a relatively new molecule called a fullerene (its discovery led to a Nobel Prize in Chemistry in 1996) was incorporated as the acceptor component in OPV cells. A second Nobel Prize in 2000 recognised the discovery and development of conductive polymers.

    And that sparked the attention of Bryon Larson. “Both of those Nobel Prizes were a really big deal in the history of OPV,” Larson said. “Everyone thinks of plastics as insulators, but to have a polymer-based electronic device that acts as a semiconductor—that opened up a world of new options.”

    Leave a Reply

    Your email address will not be published. Required fields are marked *