Achievements and Challenges of CdS/CdTe Solar Cells
Thin film CdS/CdTe has long been regarded as one promising choice for the development of cost-effective and reliable solar cells. Efficiency as high as 16.5% has been achieved in CdS/CdTe heterojunction structure in laboratory in 2001, and current techniques for CdS/CdTe solar cells gradually step toward commercialization. This paper reviews some novel techniques mainly within two years to solve this problem from aspects of promotion of fabrication technology, structural modification, and choice of back contact materials.
As the world is suffering from impending death of fossil fuels and serious pollution resulted from the fuels, solar energy is now regarded as one promising solution to the global energy crisis. Among various means for generating energy from the sun, solar cells are an effective approach to convert solar energy into practical electrical energy. In 2009, the global production of photovoltaic cells and modules in 2009 was 12.3 GW , and it increased to over 20 GW one year later . Many kinds of solar cells based on Si , thin film [4, 5], or even organic materials [6, 7] are gradually developed these years. According to the US Department of Energy, solar energy should only be economically viable for large-scale production if the cost can be reduced to 0.33/Wp (Wp = wart peak) .
Thin-film cadmium telluride (CdTe) is now regarded as one leading material for the development of cost-effective photovoltaics (PV), and it is also the first PV technology with the price for Wp below 1 (0.85) . CdTe has a Band gap of ~1.5 eV, which is close to the ideal value for photovoltaic conversion efficiency. Meanwhile, high optical absorption coefficient and high chemical stability also appear in CdTe. All of them make CdTe a very attractive material for thin-film solar cells. The theoretical efficiency of CdTe thin-film solar cells is expected to be 28%–30% [10, 11]. Currently, First Solar has announced a new world record this year for CdTe PV solar cell efficiency of 17.3% with a test cell constructed using commercial-scale manufacturing equipment and materials, and its average efficiency of modules produced in the first quarter of 2011 was 11.7% .
One of best choices for CdTe cells are heterojunction structures with n-type cadmium sulfide (CdS) as a transparent window layer, and they are generally fabricated in a superstrate configuration (Figure 1). Despite the lattice mismatch of 10% between CdTe and CdS, the formed heterojunction has an excellent electrical behavior, leading to a high fill factor of 0.77 in produced solar cells . Therefore, this structure is favored by a variety of world-leading corporates. For example, First Solar has launched one project to double its manufacturing capacity of CdS/CdTe solar cells from 1.5 GW at the beginning of 2011 to nearly 3 GW by the end of 2012 . Calyxo also expended their capacity up to 25 MWp in 2008 and expects to finish their second production line with capacity of 110 MWp in 2011 . In CdS/CdTe heterojunction structure, efficiency up to 16.5% has been achieved in lab as early as 2001 , while the best commercial modules are approximately 10%-11% . However, these are still much lower than the theoretical value. There are currently several challenges for further making CdS/CdTe thin-film solar cells more competitive: (1) short minority carrier lifetime due to the recombination of electron-hole pairs at the defect centers in CdTe layers and at the interface between CdS and CdTe, (2) insufficient transparency of transparent conductive oxide (TCO) and CdS window layers, (3) lack of good ohmic contact between CdTe layers and back contacts, and (4) possibility in doping p-type CdTe films in a stable way. Techniques coming up in recent years are mainly concentrated on the first three challenges above.
Instead of summarizing the history, this paper aims to review the progress of CdS/CdTe photovoltaics by evaluating some current techniques to deal with first three challenges above from three aspects: fabrication technology, structural design, and choice of back contact, respectively.
Promotion in Fabrication Techniques
Fabrication methods for CdS/CdTe layers could have a significant effect on cell efficiency and cost. Samples have been obtained successfully by several common techniques: radio frequency sputtering (R. F. Sputtering) , close-spaced sublimation (CSS) , and chemical bath deposition (CBD)  for CdS preparation, while electrodeposition (ED) , screen printing (SP) , and CSS [23–25] for CdTe thin-film formation. However, these methods are still not sufficient good due to their inherent drawbacks. For example, R. F. Sputtering for preparing CdS window layer can be rather fast but with poor quality that reduces the cell efficiency, while CBD could provide a dense and smooth CdS layer but the solution waste recycling and management cause extra cost. Therefore, the techniques should be carefully chosen with the consideration of high quality thin-film structure (high efficiency), proper pollution control, and commercial production prospect. Currently, the emerging problems related to fabrication mainly FOCUS on (1) the short circuit of TCO and the CdTe layer caused by the partial grain covering and pinholes in CdS with extremely small thickness and (2) short minority carrier life time due to the defects inside the CdTe layer. Fortunately, several modifications to conventional methods and new techniques have been explored to improve the problems above these years [26–36].
2.1. Sputtering in Ar CHF3 Atmosphere
One excellent improvement to the fabrication of CdS layers is employing R. F. sputtering in the atmosphere of argon (Ar) containing ~3% of CHF3 [11, 26–29]. This gas is decomposed and ionized during the sputtering discharge, delivering electronegative fluorine (F) to the substrate. Although it has been proved that F does not reduce the resistance of CdS layers [11, 27, 29], compared with undoped CdS, F-doped CdS still exhibits a larger forbidden gap , a stronger photoconductivity, and, most importantly, gives higher-efficiency solar cells . This phenomenon probably results from the presence of F. On one hand, it strongly reduces the growth rate of CdS and bombards the CdS films during the growth, which can eliminate the excess of Cd and S, hence producing a dense film with excellent thickness and quality control [28, 29]. On the other hand, it also promotes the formation of CdF2, which could passivate the grain boundaries and forming good CdS/CdTe junctions [11, 28, 29]. Efficiency as high as 15%–15.8% has been obtained by Bosio et al.  in this approach. Therefore, this technique can be treated as a good choice for the fabrication of CdS layers in industrial production of photovoltaic modules.
2.2. Bilayer CdS Thin-Film Preparation
The structure of the bilayer CdS thin-film solar cell is shown in Figure 2. In this technology, the first CdS layer is made by standard CBD process with reduced time (compared with standard structure shown in Figure 1), and then the second CdS layer is deposited by CBD at lower temperature (55°C). Approximately 14.6 nm’s roughness was observed in atomic force microscopy (AFM) for standard CBD of CdS thin film, while it could be reduced to 7.2 nm in bilayer case (see in Figures 3(a) and 3(b), resp.). Then smaller grain size would be obtained since the bilayer structure could provide the compact and uniform CdS layers without pinholes and cracks among grain boundaries. The dense and smooth CdS thin films could generate isolation between TCO and CdTe layers, which could solve the shunt problem between these two layers. over, thinner CdS film results in higher short-circuit current (Figure 4) and improved the efficiency by around 6.1% with less demand of material (only 80 nm thickness of CdS layer shown in Figure 2) compared with standard CBD process [30, 31].
Flexible cadmium telluride solar cell with 12.6% efficiency via lift-off method
A U.S. research team has developed a cadmium telluride (CdTe) solar cell through a lift-off method that reportedly ensures higher crystallinity of the cadmium sulfide film. The device has a power conversion efficiency of 12.60%, an open-circuit voltage of 0.829 V, a short-circuit current density of 23.64 mA/cm2, and a fill factor of 64.30%.
Image: University of Toledo
Researchers at the University of Toledo in the United States have developed a flexible CdTe solar cell through a water-assisted lift-off approach, including an additional cadmium chloride treatment for the cell’s cadmium sulfide (CdS) film.
“These are lightweight and flexible solar cells that can especially install on any curve surface or electric device and, most importantly, they are good for space applications,” researcher Sandip Singh told pv magazine.
The scientists described the water-assisted lift-off method as a low-cost fabrication process to offer advantages such as lower stress in CdTe films and ambient operation temperature. The lift-off approach consists of different photolithography techniques for creating a photoresist profile that ensures separation between the thin film coating in desired and undesired areas of a give pattern. It includes washing away the photoresist to leave behind the film in the patterned area.
They used cleaved mica sheets as a mediator substrate for the solar cell and a CdS buffer layer with thickness of 100 nm. They also applied a cadmium chloride (CdCl 2 ) vapor treatment to the cadmium sulfide (CdS) film before CdTe deposition and a second CdCl 2 treatment after CdTe deposition.
“A 3.5 μm CdTe absorber layer was deposited by the closed space sublimation method with source temperature of 580 C and a substrate temperature of 520 C in a 1% oxygen and 99% argon environment,” they explained.
The academics dried the peeled film with a nitrogen jet and deposited a transparent conduction oxide indium zinc oxide (IZO) layer as a front contact.
“In this process, the mica sheet serves only to grow CdS and CdTe films, after the delamination mica sheet is not a part of the solar cell,” they said.
They said the process results in a higher cristallinity of the films. They found that the average grain size of the CdS film without additional CdCl 2 treatment is much smaller than that with additional CdCl 2 treatment. This means that the CdS film has a higher c onductivity.
Tested under standard illumination conditions, the cell achieved a power conversion efficiency of 12.60%, an open-circuit voltage of 0.829 V. a short-circuit current density of 23.64 mA/cm 2. and a fill factor of 64.30%. The group described this efficiency value as highest reported among all flexible CdTe solar cells fabricated by the lift-off technique.
“The additional CdCl2 treatment on the CdS layer significantly improves the open-circuit voltage and fill while the improvement of short-circuit current density is negligible,” the researchers said. They described the novel cell technology in “ Water-Assisted Lift-Off Process for Flexible CdTe Solar Cells ,” which was recently published in ACS Publications.
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Emiliano joined pv magazine in March 2017. He has been reporting on solar and renewable energy since 2009.
Highly improved light harvesting and photovoltaic performance in CdTe solar cell with functional designed 1D-photonic crystal via light management engineering
Photonic-based functional designs and integrations for advanced optoelectronic devices are regarded as promising candidates considering the enhancement of efficiency and tunability. With the aim to improve photovoltaic performance by increasing photon harvesting, the study presents the prominent findings of experimental and theoretical comparison of optical and electrical evaluation integrating a functionally designed one-dimension photonic crystal (1D-PC) into CdTe solar cells. Since transparency of the CdS/CdTe heterojunction based solar cell (SC) is reduced by a photonic Band gap formed by (MgF2/MoO3) N 1D-PC; namely, re-harvesting is improved by increasing absorbance. The period number at resonance wavelength of 850 nm and photocurrent density ( \(_\) ) have remarkable influence on the investigation. For four periods, the reflectance in the region of photonic Band gap is sufficient for photon harvesting and saturation occurs. The photovoltaic performances are comparatively analysed for SCs with and without 1D-PC produced at optimal values. The open-circuit voltage does not change, besides, short-circuit current density and maximum-current density vary between 15.86–17.23 mA cm −2 and, 13.08–15.41 mA cm −2. Having integrated the 1D-PC into the structure, it is concluded that the FF and power conversion efficiency increase from 55.27 to 63.35% with an improvement of 15.91% and, from 8.26 to 10.47% with an improvement of 21.10%.
In today’s world, where the increasing energy demand is high, the importance of the energy need for abundant, low-cost, and clean energy sources is increasing day by day. Therefore, renewable energy, an alternative to fossil fuel-containing sources, offers a solution to overcome this energy need. In solar cells (SC), which are the most promising alternative among renewable energy sources, photon energy from the sun can be collected cheaply, efficiently, and simply 1. Especially Cadmium Tellurium (CdTe) based solar cells have recently attracted attention in academic and industrial studies due to their ability to reach 22.1% efficiencies 2,3. high thermal cell stability, low cost, and long-term stable photovoltaic performance 4,5,6,7.
CdTe has a direct optical Band gap around 1.45 eV, which is the convenient Band gap for SC’s. Also, due to the high absorbency of CdTe, CdTe-based SCs generally consist of a thin n-type Cadmium Sulfide (CdS) and a relatively thick p-type CdTe heterojunction. The efficiency of CdS/CdTe heterojunction SCs has been increased by 1.5% in the last 20 years under laboratory conditions, with cell efficiency exceeding 20% and module efficiency close to 15% 8,9,10. Through the absorber CdTe, which provides high absorption at the joint, all photons with energies greater than the Band gap can be absorbed even with a very thin p-type material. SCs based on CdS/CdTe heterojunctions with a CdTe absorber layer about 2 μm thick can absorb almost all photons from AM 1.5G. In addition, the flexibility of the CdS/CdTe heterojunction at this thickness is quite high. For this reason, the use of CdS/CdTe-based SCs in technology is very advantageous in terms of production, mechanical functionality, and lightness. The n-type CdS layer is a suitable thin window layer in the heterojunction and can form a suitable heterojunction with p-type CdTe. In cases where thin CdS films produce higher short-circuit current densities, it affects the cell’s conversion efficiency 11. In addition to the advantages, there are fundamental and significant challenges for high-efficiency CdS/CdTe-based systems for long-term thermal and mechanical stability. The most important of these is the compatibility of p-type CdTe with the bottom contact metal, which has a stable, non-rectifying, and low-resistance configuration. Metals used as bottom contact metals can diffuse into the cell over time, and this causes a critical decrease in cell efficiency. Therefore, after initial generation, systems based on heterojunction CdS/CdTe exhibit good photovoltaic performance, but the efficiency decreases over time. Therefore, this problem can be overcome by adding a Back Surface Area (BSF) with a high Band gap material between the absorbent layer CdTe and the metal contact 12. BSF also limits the movement of minority carriers around the active region and improves photovoltaic performance.
Even if the efficiency is high in SC consisting of CdTe/CdS heterojunction, a significant increase in efficiency has not been observed in recent years. Therefore, using multiple absorbers with different Band gaps in tandem or a bifacial illuminated configuration may be viable to increase sunlight utilization 13. As a result of the low absorption of CdTe acting as the absorber layer in the long-wavelength region of air mass 1.5 global (AM 1.5G) distribution, studies on appropriate Band alignment have been carried out with Band engineering to increase the absorption in the long-wavelength region 4,14,15,16,17,18. Apart from the modifications to be made with different material alloying, doping, or Band alignment in CdS and CdTe, light management engineering-based approaches can be tried to adjust the optical properties of the cell by modifying the propagation of the electromagnetic wave inside the cell without making any modifications directly to the CdS/CdTe heterojunction 19,20,21,22. Among these approaches, the integration of a periodic one-dimension (1D) dielectric mirror (DM) layer with photonic crystal (PC) properties into the structure for improved and modifiable optical properties is an effective approach 20,23. With this feature, 1D-PC systems are also included in the literature as Distributed Bragg Reflector (DBR) and are used functionally to increase the photovoltaic performance consisting of various material systems 21,24,25,26. In the SC, which consists of 1D-PC and CdS/CdTe heterojunction, the unabsorbed photons—especially those with long wavelengths—can be sent back to the heterojunction by creating a suitable reflection, thus increasing the absorption.
1D-PCs consist of structures with two or more dielectric constants grown or deposited in a single direction, and therefore the dielectric constant and refractive index periodically change in one direction in these structures 27. Thence, with a suitable 1D-PC design, a photonic Band gap formed in the wavelength range where reflection will occur for the propagation of photons in a single direction can be created. By optimizing the thickness and number of layers of 1D-PC system, the width and reflection intensity of the photonic Band gap can be adjusted. Thus, the absorption characteristics can be modified by especially adjusting the reflection characteristics without changing the properties of the active layer as a result of the integration of 1D-PCs in CdS/CdTe SC. By sending the photons that are not absorbed in the CdS/CdTe heterojunction to the active region by internal reflection, the photocurrent density ( \(_\) ) in the SC can be increased. Thus, the photovoltaic performance of the cell can be improved.
In the literature, there are studies in which an increase in photovoltaic performance is observed with the integration of Si/SiO2 DBR system into CdS/CdTe-based SCs 9,26. However, in these studies, physical interpretation was made only on the optical spectra of the DBR layers and not on the optical properties of the SC. In addition, the bandwidths are kept quite wide in the related studies. Because in the Si/SiO2 DBR system, the refractive index contrast is high, and a wide photonic Band gap is obtained. So it is not possible for SCs to work with bifacial illumination. For bifacial working condition, it should be aimed that the reflection Band is not wide for both top and bottom illumination, that is, material systems with low refractive index contrast.
In the present study, we focused on an effective photonic Band gap design for only the wavelength region with low absorption, based on the optical properties of each layer in the CdS/CdTe SC and 1D-PC integration. The FOCUS is on improving the photovoltaic performance by integrating the appropriate design N period (MgF2/MoO3) N 1D-PC into the SC with CdS/CdTe heterojunction. For this purpose, the optical properties of CdS/CdTe heterojunction SC were investigated in detail, both experimentally and theoretically, and it was integrated into the (MgF2/MoO3) N 1D-PC SC with a functionally fine-tuned photonic Band gap property to improve absorption by photon harvesting. In the production of SCs, RF sputter technique was used for the deposition of materials. Methodologically, we designed photonic Band gap in the near-infrared region (NIR) and in different periods for the transparent wavelength region in the absorption Band of CdTe, which is the absorber layer. We calculated the optical characteristics of the designed (MgF2/MoO3) N 1D-PCs and the SCs formed by integrating these 1D-PCs into the SC with CdS/CdTe heterojunction using the Transfer Matrix Method (TMM). We produced CdTe-based SCs with and without 1D-PC, whose optimal period was determined based on the theoretically calculated \(_\). and we presented the photovoltaic performance comparatively. We observed a significant increase in photovoltaic performance by improving the NIR absorption characteristic of 1D-PC and CdTe-based SCs.
In the study, the changes in photovoltaic performance result from the integration of the (MgF2/MoO3) N 1D-PC system designed inappropriate parameters based on the optical properties of the SC formed in the FTO/SnO2/CdS/CdTe/MoO3 SC were investigated. The investigated FTO/SnO2/CdS/CdTe/MoO3 and FTO/SnO2/CdS/CdTe/MoO3/(MgF2/MoO3) N SCs are given in Fig. 1a,b, respectively. Fluorine Tin Oxide (FTO) coated on glass substrate was used as the bottom contact in the produced SCs. FTO is electrically conductive and optically highly transparent in the wavelength range for which AM 1.5G is responsible. 100 nm thick SnO2 was used on FTO due to its wide optical Band gap, high optical and electrical properties to prevent recombination of photogenerated carriers. At this thickness, SnO2 is transparent and at the same time, it provides Band arrangement and prevents the leaking of the holes formed by photoproduction to the bottom contact. In this respect, SnO2 also acts as an electron transport layer (ETL). A 50 nm thick CdS window layer is added to the structure to absorb electromagnetic waves and transmit photogenerated electrons to the SnO2 layer. CdS is a promising n-type material from group II-VI and has a wide direct Band gap (2.42 eV) 28,29. In order to increase the photovoltaic performance by creating a large number of electron–hole pairs, CdTe material with an optical Band gap of 1.5 eV was chosen as the active layer in the structures produced. CdTe with high material quality can be achieved with the RF sputter technique, especially for submicron thicknesses 30. In addition to the majority carriers formed as a result of photogeneration, 100 nm thick MoO3 is used as the BSF layer to localize the minority carriers around the pn junction to collect them more efficiently under the effect of the internal electric field and to reduce recombination. Due to its convenient work function and high p-type doping capability, the formation of ohmic contacts with MoO3 is both easier and selective transport is provided by preventing photogenerated electrons from reaching the top contact in the structure 13. In this respect, MoO3 also acts as a hole transport layer (HTL) in SC. Also, in our previous studies, we have structural and morphological investigations for MoO3 thin film 31,32. In order to improve the absorption by reducing the transparency in the FTO/SnO2/CdS/CdTe/MoO3 SC, the photonic Band gap in the structure is designed with 1D-PC, which consist of MgF2 and MoO3 have different dielectric constants and therefore different refractive indices.
The materials excluding the metallization parts were deposited using the RF sputter technique in the SCs examined in the study. The RF sputter technique provides homogeneous and thickness-controlled deposition for CdTe, CdS, and other metal-oxide alloys at the desired stoichiometric ratio 19,30,31,33,34,35. In addition, in our previous study, we found that the optical calculations we made with TMM on the devices we produced by deposition with the RF sputter technique showed a nearly perfect agreement with the experiment 19.
In the examined SCs, the absorber layer is CdTe, which is relatively thicker and has a higher absorption coefficient than the other layers. Therefore, to achieve an increase in photovoltaic performance as a result of only 1D-PC integration without making any modifications to the heterojunction or the SC, first of all, the optical characteristic of the entire SC should be known. Therefore, we started the investigation based on the experimental and calculated transmittance spectra with TMM for the 1D-PC-free SC (FTO/SnO2/CdS/CdTe/MoO3). Since the transmittance of the SC will increase in the region where the absorption decreases, we first focused on the longer wavelength region –NIR–, starting from about 700 nm, which is the wavelength region where the transmittance starts and increases. In Fig. 2, the SC’s experimental and calculated transmittance spectra, the variation of the absorption coefficient of CdTe and AM 1.5G spectral irradiance depending on the wavelength are given.
The fact that the trends are very close depending on the wavelength in the experimental and theoretical transmittance spectra calculated with TMM in Fig. 2 shows that TMM is a powerful method. CdTe-based SC’s were deposited by RF sputter technique and the layers may be deposited with imperfect flatness and slight deviations from the desired thicknesses in terms of nm. This small difference may have occurred because the calculations made with TMM were calculated for the ideal condition of the layers with perfect flatness over the thicknesses in terms of nm. Accordingly, a slight difference is observed in some wavelength regions. The absorption coefficient of the absorber layer CdTe decreases, especially after 800 nm. However, the absorption characteristic is still observed in the NIR region, albeit slightly. The fact that the transparency of the FTO/SnO2/CdS/CdTe/MoO3 SC also increases after 700 means that photons with wavelengths longer than 700 nm pass through the SC without being absorbed. Increasing the absorption by reducing the transparency in this region, i.e., reflecting unabsorbed photons back into the active region and obtaining an efficient harvest, can positively affect the photovoltaic performance of the cell.
In studies on DBR integration into CdS/CdTe-based SCs, only physical interpretation was made on the optical spectra of the DBR layers, and no evaluation was made on the optical properties of the entire SC 9,26. In addition, the bandwidths were kept quite wide in the related studies and therefore, it was not possible for SCs to work with bifacial illumination. In principle, the refractive indices of the materials that make up the photonic crystal and the contrast of these refractive indices determine the characteristics of the photonic Band gap or stop-Band to be created with DBR. Increasing the refractive index contrast between materials increases both the reflection intensity and the photonic bandwidth. Therefore, a narrower photonic Band gap can be obtained with materials whose refractive indices are closer to each other (less contrast) for a given central wavelength. We offer a methodologically more effective CdS/CdTe SC design that can be improved in the wavelength region where the absorption is low and can operate under bifacial illumination. For this purpose, we try to reduce the permeability of the with a 1D-PC design with photonic Band gap only in the NIR region. Therefore, we calculated the reflectance spectra of the (MgF2/MoO3) N 1D-PC system with a central wavelength of \(\lambda _\) =850 nm at different periods by TMM (Fig. 3). For \(\lambda _\) =850 nm, the thicknesses of MgF2 and MoO3 are 155 and 100 nm, respectively. When Fig. 3 is examined, 1D-PC is designed to act as a mirror in the desired wavelength range according to the mentioned methodology.
The (MoO3/MgF2) N 1D-PC system’s refractive index contrast is not much, and thus a narrower photonic Band gap can be obtained. Increasing the number of periods in the (MgF2/MoO3) N 1D-PC system narrows the photonic bandwidth and increases reflection intensity. The rate of increase in the reflection intensity decreases as the number of periods increases. For N = 5 and 6 periods, accumulation starts at 90% reflectance. As expected, the period does not affect the center wavelength. In addition, the short wavelength tail of the photonic Band gap formed by the (MgF2/MoO3) N 1D-PC system shifts from 657 to 700 nm as the period increases from 1 to 6. Therefore, by integrating the (MgF2/MoO3) N 1D-PC system into the FTO/SnO2/CdS/CdTe/MoO3 SC, it is necessary to examine how it affects the increasing SC transparency after 700 nm. The transmittance and absorption spectra were calculated for different periods of the FTO/SnO2/CdS/CdTe/MoO3/(MgF2/MoO3) N SC at \(\lambda _\) =850 nm are given in Fig. 4a,b, respectively.
As intended, the transmittance spectrum of the FTO/SnO2/CdS/CdTe/MoO3/(MgF2/MoO3) N SC was decreased with the increase of the period number after 700 nm with the (MgF2/MoO3) N 1D-PC. Increasing the number of periods up to N = 4 in the SC reduces the transmittance up to 1000 nm to almost zero. At around 700 nm, the intersection of the transparency of the FTO/SnO2/CdS/CdTe/MoO3 SC and the transparency of the (MgF2/MoO3) N 1D-PC (Fig. 4a inset) is evident. Therefore, the (MgF2/MoO3) N 1D-PC system designed for \(\lambda _\) =850 nm is a suitable design for CdTe-based SC. In high periods (N 4), the photonic Band gap having sharp lines and decreasing width tends to increase the transmittance around 1000 nm. In order to interpret the effect of this critical change in the optical characteristic of the SC on the photovoltaic performance, it is necessary to examine the absorbance spectrum.
In the FTO/SnO2/CdS/CdTe/MoO3/(MgF2/MoO3) N SC, decreasing the transmittance after 700 nm with the period number directly affects the absorption spectrum, and an increase in absorption was observed after 700 nm. Similar to the trend in the transmittance spectrum, there is no significant increase in absorption in the NIR region with a period number greater than N = 4. In order to examine how this improvement in absorption with 1D-PC in CdTe-based SCs affects photovoltaic performance, AM 1.5G spectral irradiance distribution should also be examined. Therefore, for a more effective evaluation, we calculated the absorption characteristic of SC and \(_\) over \(_\) using Eq. (16). Here, when calculating the \(_\). it is assumed that each photon creates an electron and a hole in the SC 19. This situation provides a relative evaluation and allows us to understand whether the flow mechanisms in the SC have improved relatively or not. The variation of \(_\) in FTO/SnO2/CdS/CdTe/MoO3/(MgF2/MoO3) N SC according to the number of periods is given in Fig. 5.
\(_\) increases significantly in SC up to N = 4 periods and becomes saturated for higher periods. Increasing the reflection in (MgF2/MoO3) N 1D-PC up to N = 4 periods significantly reduces the transmittance of SC, especially by sending photons with wavelengths greater than 700 nm back into the active region. The improvement in absorption by sending photons that were not absorbed in the active region into back with 1D-PC led to re-harvest and increased \(_\). Therefore, the integration of (MgF2/MoO3) N 1D-PC produced in 4 periods into CdTe-based SC is sufficient for the required re-harvesting of photons. This examination shows that the structural parameters of 1D-PC can be determined experimentally with TMM and an effective methodology without excessive material consumption and fabrication process.
In order to determine how the improvement was brought about by the (MgF2/MoO3) N 1D-PC system in \(_\) affects the photovoltaic performance of the cell and the cell output parameters, we produced the FTO/SnO2/CdS/CdTe/MoO3/(MgF2/MoO3) 4 SC, which we determined as the optimal N = 4 period. We fabricated SCs with and without 1D-PC at the same parameters and deposition conditions to compare their photovoltaic performances for a quantitative comparison. The \(J-V\) and \(P-V\) characteristics of FTO/SnO2/CdS/CdTe/MoO3 and FTO/SnO2/CdS/CdTe/MoO3/(MgF2/MoO3) 4 SC are given comparatively in Fig. 6a,b, respectively. Photovoltaic performance outputs obtained and calculated from \(J-V\) and \(P-V\) characteristics are given in Table 1.
The fact that MoO3 with a thickness of 40 nm, which acts as the BSF layer, is very transparent in the visible and IR region allows photons of wavelengths not reflected by the photonic Band gap of 1D-PC to enter the active region under top illumination. Therefore, the presence of SnO2 on the bottom side of the SC and MoO3 on the top side provides symmetry for the optical path that light will follow in the SC, since the optical characteristics of these materials are the same, especially for the IR region. Therefore, the transmittance characteristic of the SC does not change under the top and bottom illumination, and the absorption-reflection is in exchange with each other. Modification for the optical trace is done with photonic Band gap. This shows that the functional photonic Band gap design targeted in the study is methodologically suitable and light management is provided for the SC to work bifacially. As mentioned above, photon harvesting is increased with internal reflection under low illumination, allowing both an improvement in photovoltaic performance and an efficient photon harvest under top illumination.
The study was focused on the increment of photon harvesting and improvement of photovoltaic performance with a modifying optical characteristic due to functionally designed 1D-PC integration into CdS/CdTe-based SCs. The absorbance of the SC is enhanced by reducing the transparency with an appropriate photonic Band gap which corresponds to the long-wavelength part of the CdTe absorbance Band. A photonic Band gap was designed with (MgF2/MoO3) N 1D-PC, and the optical characteristics of the CdTe-based SC were calculated by TMM. The optimal period number of the PC was determined based on the calculated photocurrent density. In the (MgF2/MoO3) N 1D-PC system, which is functionally designed at 850 nm resonance wavelength, the number of periods supports the absorbance by reducing the transparency in the NIR region of the CdTe-based SC. For the period numbers higher than four, accumulation started around 90% reflection and saturation was observed in the \(_\). The small-wavelength tail of the photonic Band gap formed by 1D-PC system shifted from 657 to 700 nm as the period increases to six. This variation range covered the exact intersection of the transmittance spectrum of the CdTe-based SC and the photonic Band gap of the 1D-PC.
It has been determined that an optimally four period (MgF2/MoO3) 4 1D-PC system is sufficient to increase photon harvesting and improve photovoltaic performance in CdTe-based SCs. CdTe-based SCs with and without 1D-PCs at optimal values were produced under the same deposition conditions and their photovoltaic performances were presented comparatively. It was determined that the (MgF2/MoO3) N 1D-PC system did not change the Band arrangement in the CdTe-based SC and only affected the optical characteristics. As a result of 1D-PC integration, there was no significant change in \(_\). \(_\) improved from 15.86 to 17.23 mA cm −2 and \(_\) from 13.08 to 15.41 mA cm −2. These increments in current densities indicated that photons absorbed were reflected into the active region, and, re-harvesting occurred thanks to the photonic Band gap designed in NIR. Consequently, \(FF\) was improved by 15.91% from 55.27 to 63.35%, and \(PCE\) was increased by 21.10% from 8.26 to 10.47%, dependent on the functionally designed 1D-PC integration.
It remains essential to increase photon harvesting and efficiency with light management engineering in 1D-PC and SCs without different material alloying, doping or Band alignment. Unlike the studies conducted in the literature for this purpose, designing a functional photonic Band gap only in the wavelength region where photon harvesting is weak shows the originality and potential to be a pioneer for future studies of the present study. In addition, the photonic Band gap in the study is fine-tuned and functional, as well as not having a conventional wide Band gap, allowing light to enter the SC from the top side.
In an overview of the study, it is verified that for the bifacial CdS/CdTe design with 1D-PC, very efficient photon harvesting is provided from the bottom side. Furthermore, an appropriate absorbance characteristic is provided for sufficient photon harvesting from the top side.
Material and methods
Optical characteristics such as reflection, absorption and transmittance of the (MgF2/MoO3) N 1D-PC system and FTO/SnO2/CdTe/CdS/MoO3/(MgF2/MoO3) N SC designed within the scope of the study were calculated using the Transfer Matrix Method (TMM), which is a very effective method used in the simulations of various multilayer optoelectronic devices. TMM is a method that examines the propagation of electromagnetic waves in the structure and theoretically determines the optical properties of the structure 36,37,38. The electromagnetic wave’s electric and magnetic field components are interconnected in each layer by a transfer matrix. The propagation field components in the layers are connected with the propagation matrix at the interface of each layer.
With TMM, calculations can be made on structures designed by integrating metal layers with different conductivity and dielectric materials. In the most general case, this provides a general framework that allows the calculation of optical characteristics due to the integration of both dielectric, metal, and semiconductor materials with each other. Therefore, we took the most general form as a basis for explaining the calculations and performed the calculations by making the necessary reductions for the structures designed in the study. The plane of a metal layer sandwiched by two dielectrics can be taken as parallel to the \(z\) =0 planes. We can examine the propagation of electromagnetic waves on the interface of conductor and dielectric with the configuration and orientation in Fig. 8.
It can be thought that the electromagnetic wave is propagated in the \(z\) direction and polarized in the \(y\) direction by choosing a special orientation, so that the \(s\) and \(p\) polarizations can be studied. The magnetic field polarization \(p\) can be written in the following form:
Here, \(c\) is the propagation speed of the light in space, \(\omega\) is the angular frequency, \(_ (i=1, 2)\) is the dielectric constant of the \(i\) medium, \(_\) and \(_ (i=\mathrm)\) are the coefficients. \(\vec_ = \sqrt \varepsilon_ \omega /c\;\left( \right)\) is the wave vector of the electromagnetic wave. The wavevectors do not change at the material interfaces in the \(x\) direction, that is, the \(x\) components of the wave vectors at the interface are equal ( \(_=_\) ). According to Snell’s law, if this situation is applied for the magnetic field and electric field at the interfaces, the following equations are obtained:
\(\overrightarrow\) is the surface current density and \(_\) is the normal unit vector of the surface. According to Ohm’s law, the following set of equations can be derived at \(z\) =0:
New Perovskite Solar Cells: How Low (And How Fast) Can Solar Go?
The long wait for low-cost, high-performance perovskite solar cells is coming to a close. Now the fun begins.
The cost of solar power has been dropping like a rock, and apparently we ain’t seen nothing yet. New low-cost perovskite solar cells are finally beginning to bump their way into the solar market. One promising pathway involves a piggyback with silicon technology, and thin film is also in play.
“Transformational” Perovskite Solar Cells Are Coming
The long wait for low-cost, high-performance perovskite solar cells is coming to a close. Now the fun begins. Photo courtesy of Qcells.
Researchers at the US Department of Energy’s National Renewable Energy Laboratory (NREL) have been cheerleading for perovskite solar cells, and with good reason. Perovskite is a mineral with superior optical properties, and synthetic variations can be lab-grown at relatively low cost (more details here).
There being no such thing as a free lunch, synthetic perovskites require some tweaking before they can beat silicon solar cells to the clean power punch.
“Perovskite materials offer excellent light absorption, charge-carrier mobilities, and lifetimes, resulting in high device efficiencies with opportunities to realize a low-cost, industry-scalable technology,” NREL enthuses, while cautioning that “[a]chieving this potential will require us to overcome barriers related to stability and environmental compatibility.”
If those kinks can be worked out, perovskite solar cells will have “transformational potential for Rapid terawatt-scale solar deployment,” NREL added.
If You Can’t Beat ‘Em, Join ‘Em
The rap on perovskite solar cells used to be their fragility and relatively low solar conversion efficiency. There are different ways to resolve those issues, and one of them involves combining perovskite solar cells with silicon. The tandem relationship is mutual, with silicon providing the stability and perovskites kicking up overall solar cell efficiency without adding excess expense.
NREL is a longtime fan of perovskites and the lab has picked up on the tandem perovskite-silicon angle. In 2020, NREL took note of the potential for boosting the efficiency and reducing the cost of silicon solar cells by adding perovskites. They achieved a conversion efficiency of 27% with their perovskite-silicon solar cell, compared to just 21% for a silicon-only version.
The South Korean firm Qcells (an offshoot of Hanwha Solutions) has also gotten the memo, and the company is taking it on the road. On May 17, Qcells announced that it has invested a cool 100 million in a pilot line to bring tandem perovskite cells with silicon to market.
“The investment will pave the way for Qcells to mass-produce perovskite tandem cells, which have a much higher efficiency rate than silicon-based solar cells that utilize TOPCon (Tunnel Oxide Passivated Contact) or heterojunction technology,” Qcells explained.
Don’t just take their word for it. Earlier this year, NREL verified a solar conversion efficiency rate of up to 29.3% for the tandem perovskite-silicon solar cell developed by Qcells and its research partners in Germany. That surpasses the theoretical maximum of 29.1% for silicon-only solar cells.
Hanwha will host the new tandem line at its Jincheon factory in South Korea, and it is not letting the grass grow under its feet. The line should be up and running by late 2024, with commercialization expected 2026.
The Perovskite Solar Cell Game Is Afoot
That’s just the tip of the perovskite-silicon iceberg. Qcells is part of a European tandem solar cell commercialization project called PEPPERONI, which currently includes 17 partners from 12 different European countries. Korea also has a foot in the door by way of the Qcells team in Germany, which is hosting the PEPPERONI project at its facility in Thalheim.
PEPPERONI launched in November last year, and that may have caught the eye of the US company First Solar, which has been scouting for opportunities to boost the conversion efficiency of its thin-film solar technology.
Thin-film technology can’t match the solar conversion efficiency of conventional silicon solar cells, but it does have two advantages. Thin-film solar cells lend themselves to Rapid, high-volume, low-cost manufacturing processes, and they are lightweight and flexible, which means they have a far greater range of application.
Perovskites could help narrow the conversion gap for thin film. That may be what First Solar had up its sleeve when it recently acquired the Swedish perovskite startup Evolar AB (more First Solar coverage here). The deal is a two-phase arrangement, with the second phase depending on technology advancements, so stay tuned for more on that.
Perovskite Solar Cells, Thin-Film Edition
First Solar’s thin-film formula is based on cadmium telluride solar cell technology, or CdTe for short, which the company describes as “lower cost, superior scalability, and a higher theoretical efficiency limit” than conventional silicon solar cells, along with a laundry list of other advantages.
The company is still tweaking its formula, and its website currently anticipates achieving 25% solar conversion efficiency by 2025, with 28% in the company’s sights by 2030.
Perovskite solar cells could boost First Solar ahead of those goals. Back in 2019, a team of researchers at the University of Texas at Austin and Colorado State University made the case for adding perovskites to thin-film CdTe solar cells. They indicated that the perovskite formula of methylammonium-lead-bromide would be a good candidate, partly because it is amenable to energy-efficient, low-temperature fabrication.
“The approach of increasing CdTe PV efficiency by the addition of a wide bandgap perovskite layer in a 4T tandem device configuration appears to be a plausible way to increase efficiency without significantly increasing manufacturing cost,” they concluded.
Many Paths To A Perovskite Future
The tandem approach is just one angle on the potential for next-generation perovskite technology to keep driving the cost of solar power down. Another angle involves eliminating the need to use an expensive gold anode to juice the performance of perovskite solar cells.
A research team in China has just published a study that indicates how a buffer layer can prevent a low-cost carbon paste anode from corroding the perovskite. They reached a conversion rate of 20.8% for their new carbon-perovskite solar cell. That doesn’t hit the high mark set by perovskite-silicon solar cells, but it does offer a pathway towards driving down the cost of reasonably efficient, affordable solar technology.
In another cost-cutting development for solar power, earlier this year, NREL introduced a self-assembling formula for trimming down the number of heating and coating steps needed to fabricate perovskite solar cells. Xiaopeng Zheng, a postdoctoral researcher in the Chemistry and Nanoscience Center at NREL, estimates that the process could be cut down by one-third.
Those of you following the perovskite saga may also be wondering whatever happened to the firm Oxford PV. The Oxford University spinoff first sailed across the CleanTechnica radar about 10 years ago with a perovskite-silicon solar cell (complete covereage here). It’s been a long journey, but it looks like all that hard work is about to pay off. Earlier this year, Reuters reported that Oxford PV’s tandem solar cells are expected to launch on the market sometime this year, sporting a conversion efficiency of 27%.
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Photo: Fabricating solar cells with perovskites, courtesy of Qcells.
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