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    Tunnel Junctions for III-V Multijunction Solar Cells Review

    Tunnel Junctions, as addressed in this review, are conductive, optically transparent semiconductor layers used to join different semiconductor materials in order to increase overall device efficiency. The first monolithic multi-junction solar cell was grown in 1980 at NCSU and utilized an AlGaAs/AlGaAs tunnel junction. In the last 4 decades both the development and analysis of tunnel junction structures and their application to multi-junction solar cells has resulted in significant performance gains. In this review we will first make note of significant studies of III-V tunnel junction materials and performance, then discuss their incorporation into cells and modeling of their characteristics. A Recent study implicating thermally activated compensation of highly doped semiconductors by native defects rather than dopant diffusion in tunnel junction thermal degradation will be discussed. AlGaAs/InGaP tunnel junctions, showing both high current capability and high transparency (high bandgap), are the current standard for space applications. Of significant note is a variant of this structure containing a quantum well interface showing the best performance to date. This has been studied by several groups and will be discussed at length in order to show a path to future improvements.

    Introduction

    This review will be a discussion of both development and analysis of tunnel junction structures and their application to multi-junction solar cells. Solar energy is abundant and environmentally friendly. Efforts to generate power from solar energy have benefited from the higher efficiency of solar cell technology. The highest efficiency devices incorporate multiple solar cells in a vertically connected stack for peak efficiency at various wavelengths within the solar spectrum. These multi-junction devices require a transparent and conductive layer to join them, most commonly in the form of tunneling junctions. The first monolithic multi-junction solar cell was grown in 1980 by Bedair et al. at NCSU [1]. This solar cell used gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) materials which consisted of an AlGaAs-GaAs tandem cell structure utilizing a very thick (due to The liquid phase epitaxy (LPE) growth method) AlGaAs/AlGaAs tunnel junction. A further evaluation of this tunnel junction was later published [2]. The first tandem cell to achieve higher efficiency than any single cell was described in 1990 at the Solar Energy Research Institute (now NREL) [3]. This cell consisted of an InGaP-GaAs tandem cell structure utilizing a GaAs/GaAs tunnel junction. The dopants used in this second structure consisted of carbon doping for the p-type side (very common in modern structures) and selenium on the n-type side (tellurium is more commonly used today). This tunnel junction showed the advantages of newer growth methods by utilizing metal–organic chemical vapor deposition (MOCVD) to grow considerably thinner layers resulting in lower optical absorption in the tunnel junction and reduced series resistance. This type of tunnel junction was used in the first mass-produced tandem cells. Continuing advances in growth and fabrication methods have led to the use of materials and structures which improve the conductivity and transparency of modern tunnel junctions as well as industry ability to produce materials in volume at lower cost. In this review we will first make note of significant studies of III-V tunnel junction materials and performance, then discuss their incorporation into cells and modeling of their characteristics. Table 1 at the end of this review shows a timeline of the most pertinent publications.

    Studies of Various Tunnel Junctions

    In parallel with and independently of the development of tandem solar cells there have been several studies of the fabrication of tunnel junctions (TJs) which we will now discuss.

    These include several studies of various doping schemes for producing GaAs TJs ([4,5,6,7,8,9,10] inc. ref. therein). The tellurium-carbon doping combination in GaAs TJs has shown the best performance to such an extent that there has been little reason for further development of other dopant material within GaAs TJ systems. Despite this much work has been aimed at using this relatively simple structure to understand the mechanisms involved in practical (for solar applications) tunnel diodes. This approach has been chosen because GaAs is comparatively well characterized as compared to AlGaAs or InGaP materials [11].

    Studies initially done at NTT ECL showed that a considerable reduction in diffusion of dopants in TJ structures could be achieved by using aluminum containing cladding layers [12]. It must be noted that this effect is most significant with rapidly diffusing dopants such as zinc [13]. Such highly diffusing dopants have not generally been used in later work. TJs embedded in solar cell structures have generally been clad in aluminum containing layers for other reasons such as transparent window and back surface field (BSF) layers (generally on the outside of the solar cell structure rather than the TJ). These layers provide passivation of surface or interface trap states and potential barriers for minority carriers.

    The AlGaAs TJs used in the first cells grown by NCSU were quite transparent in spite of the thick layers having 35% aluminum, the same as the upper cell. When concentrator cell applications began to be considered the relatively low maximum doping that could be achieved in n-type AlGaAs worked against the use of the structure. While GaAs/GaAs TJs could provide the peak current needed for concentrator cells the optical losses, particularly in the p-type GaAs, are excessive. Since high carbon doping could be easily achieved in p-type AlGaAs grown using metal organics a TJ using a p type AlGaAs layer and an n type GaAs layer was developed at NCSU using Atomic Layer Epitaxy (ALE) [14]. This structure takes advantage of the fact that most of the absorption occurs on the p-type material in a GaAs/GaAs TJ. The n type side of the TJ displays a higher effective bandgap due to the Moss Burstein effect and therefore has considerably less absorption.

    A relatively early attempt by TTI to produce a highly transparent structure used indium gallium phosphide (InGaP) to fabricate an InGaP/InGaP TJ [15]. Obtaining high performance for this structure has proven challenging due to a lack of any convenient way to produce highly doped p-type material in InGaP MOCVD growth with a low diffusion dopant. Carbon does not act as an acceptor in InGaP and zinc has a relatively low saturation concentration and a high diffusion coefficient at growth temperatures.

    The first structure to provide a high tunneling current combined with highly transparent layers on both sides of the TJ is the n-type InGaP/p-type AlGaAs TJ. This structure was originally fabricated at NCSU by ALE using selenium as the n-type dopant in the top GaAs and InGaP layers which were grown over carbon doped p-type AlGaAs [16]. The performance of this junction is shown in Figure 1. The peak tunneling current is equal to that obtained in the lower bandgap gallium arsenide system. This is due to the high doping concentration that can be obtained in n-type InGaP and the bandgap offset between InGaP and AlGaAs which reduces the tunneling distance. Additional studies discussed later in this paper use MOCVD growth and investigated optical absorption, higher tunneling current, and incorporation into cells. Recent studies include modeling and development of very high-performance structures [17,18,19].

    Unfortunately, there are relatively few studies that compare different types of TJs side by side. An exception is a study from the UO which compared AlGaAs/AlGaAs TJs with AlGaAs/GaAs and AlGaAs/InGaP TJs grown under similar conditions. In addition to experimental comparisons this study includes numerical modeling using parameters based on experimental results. While this study concluded that the AlGaAs/AlGaAs diodes were satisfactory high bandgap TJs it was noted that the AlGaAs/GaAs TJs required much lower effective doping [20]. The AlGaAs junctions fabricated in this study had sufficiently low aluminum content (20%) that they were not fully transparent. Additionally, the study was unable to duplicate the performance other groups achieved with AlGaAs/InGaP TJs. These results are consistent with results from other groups showing AlGaAs/GaAs TJs being easier to fabricate reliably when compared to AlGaAs/InGaP TJs which show a much higher sensitivity to growth conditions [21]. These details will be discussed later in this paper.

    A 2013 report by 3IT-US achieved a p-type carbon doped AlGaAs/GaAs TJ with both a high aluminum composition 40% and a doping concentration reaching up to the 10 20 cm −3 range using CBE [22]. This report clarified that the tunneling current is not significantly affected by aluminum concentration due to high doping levels decreasing the tunneling barrier width.

    Several of the proposed high-efficiency tandem cell structures, both upright and inverted, use metamorphic layers to grow material with a larger lattice constant on gallium arsenide or germanium substrates. High concentrations of carbon doping in alloys containing high indium concentrations have proven difficult to achieve. Larger lattice constant TJs lattice matched to InGaAs layers composed of higher indium concentrations can use p-type layers consisting of gallium antimony arsenide to produce highly carbon doped material [23,24].

    Another specialized TJ development is the use of erbium arsenide inclusions to produce layers which are capable of withstanding higher annealing temperatures. This has been used in MBE grown cells where low bandgap nitride containing layers require TJs which can withstand higher temperature annealing conditions [25].

    Discussion of the Integration of Tunnel Junctions in Cells

    The first tandem cells using AlGaAs TJs needed high aluminum content (35%) layers for transparency due to the LPE process producing relatively thick layers. The next notable development in tandem cells was the InGaP/GaAs tandem using GaAs tunnel junctions. This structure was feasible with the MOCVD process which allowed the growth of much thinner layers. This design was adequate for the first mass production of tandem cells. By the time a second-generation of tandem space cells was developed TJs composed of materials with lower absorption were available.

    An early manufacturing [21] study at Spectrolab showed that AlGaAs/InGaP TJs produced the highest efficiency cells while also showing the cells had greater variability due to growth conditions. This report compared the performance of otherwise similar cells which used AlGaAs/InGaP TJs or GaAs/GaAs TJs. The comparison indicates about 1% absolute efficiency improvement with the higher bandgap AlGaAs/InGaP TJ. Another report [26] by the same group compared the measured and calculated absorptions of these two TJ structures. The measured and calculated absorption agreed to a value of 1.4 mA/cm 2 and accounted for the 3% increase (relative to GaAs TJ) or about 1% (absolute) increase in efficiency between cells containing one of the two TJ structures. The high bandgap AlGaAs/InGaP TJ showed negligible absorption in the spectral region of importance. They also reported separately [27] an MOCVD grown AlGaAs/InGaP tunnel junction with high peak tunneling current. While these studies mention that most of the optical absorption in a GaAs/GaAs tunnel junction occurs in the p-type layer due to the Moss Burstein effect. This value actually translates to 55–60% of light absorbed in the p-type layer.This is consistent with a later report by IES-UPM of a gain of 0.56 mA/cm 2 when replacing an AlGaAs/GaAs TJ with an AlGaAs/InGaP TJ [28]. Inside The IES-UPM report [28] Figure 4 shows the improvement in spectral response with the AlGaAs/InGaP TJ in the higher energy part of the GaAs cell response. This is as might be expected from the transparency of the near bandgap region seen in the report from NUB [29] on a detector based on the Burstein-Moss shift. Another report from The RIERC, which concluded that the absorption in GaAs/GaAs TJs was typically less than 1%, was concerned mostly with absorption below the GaAs bandgap energy [30].

    The AlGaAs/GaAs tunnel junction is a widely used structure in concentrator cells. This structure has been used in a number of record breaking concentrator cells [31,32,33]. This is mostly due to two significant advantages: AlGaAs can be more easily doped with carbon than GaAs (which at least partially compensates for the higher bandgap) and it also seems possible that the diffusion suppressing effect of a high aluminum content layer is operative in this structure. Since the high aluminum content layer has negligible absorption in the relevant spectral region this structure eliminates somewhat over one half of the absorption of a GaAs/GaAs tunnel junction. The principal means of optimization of the AlGaAs/GaAs tunnel junction has been the minimization of the thickness of the GaAs layer. Experiments to achieve this have been described in both stand-alone tunnel junction papers [5] and in reports on the fabrication of some record holding multi-junction cells [33]. Tellurium is the most commonly used dopant since it will usually produce the highest n-type carrier concentrations, however, Te has the characteristic that during the doping growth there is some surface segregation. It therefore takes some thickness for the doping concentration to reach its full value which tends to limit the minimum thickness that can be achieved using highly Te doped GaAs layers. This effect is stronger in InGaP as will be discussed later.

    Sharp Corporation likewise recognized the advantages of the AlGaAs/InGaP design for high-efficiency cells and considered it part of the later generation [34] of cell design. A modified version of this tunnel junction with a higher indium content to match 17% InGaAs was used in the record Fraunhofer ISE 41.1% cell [35]. It is worth noting that this same epitaxial structure with a different grid pattern was able to operate with high efficiency at 1700× solar concentration. Since inverted metamorphic cells have become important for record efficiency cells [31] even while using AlGaAs/GaAs TJs, it is significant that an inverted version of the AlGaAs/InGaP junction was used in the relatively recent inverted metamorphic cell of 44% efficiency developed by Sharp [36]. This demonstrated that the fabrication of high performance inverted or n on p versions of this structure are feasible.

    There are two recent studies available explicitly comparing an AlGaAs/InGaP TJ to the commonly used AlGaAs/GaAs TJ in cells [28,37]. Recently NREL has developed some new tunnel junctions using p-type 60% AlGaAs and both 60% AlGaAs and InGaP n-type layers with a 12-nm GaAs quantum well in between. Both of these designs give much better performance than their previous design using 30% aluminum AlGaAs. However, in their final structure they used a quantum well thickness of 60 Å which is similar to other high transparency designs [17,28]. Figure 2 from the NREL study [37] shows how the higher transparency tunnel junction affects the spectral response of the multijunction cell. The improved high aluminum content tunnel junction produced a current improvement of about 0.6 mA in the 1.7 eV AlGaAs cell (the improvement in the AlGaInP cell is due to lower Se in the window) as is shown in Figure 2, which is approximately the same increase as was seen by IES-UPM [28]. Similarly to the report of IES-UPM, the improvement is seen only in the higher energy part of this cells spectral response. It is also about the same improvement that would be expected based on the results of the earlier Spectrolab study [21] which had demonstrated that the AlGaAs/InGaP TJ would produce higher efficiency cells than other available designs but initially was not as reliable a process.

    Detailed Discussion of AlGaAs/InGaP Tunnel Junction Fabrication

    Because the AlGaAs/InGaP tunnel junction provides the highest-performance of the high transparency tunnel junctions it will be discussed in more detail. As was previously mentioned achieving reproducible fabrication has been more difficult than for other tunnel junctions. This is due to several factors, probably the most important factor being tellurium segregation on the growing surface under high doping conditions [38,39]. This makes fabrication of this basic structure very sensitive to the exact growth procedures followed at the interface of the junction. One report by VII [38] found that pausing growth and raising the temperature of the InGaP surface before starting growth of the AlGaAs layer, thus preventing carryover of tellurium into the AlGaAs layer, was necessary to fabricate good tunnel junctions.

    recently, higher performance [17] has been achieved by the Bedair group at NCSU in AlGaAs/InGaP tunnel junctions by including a very thin GaAs layer in between the InGaP and the AlGaAs layers. This is consistent with tellurium accumulation on growing GaAs surfaces being less than on InGaP surfaces in agreement with an earlier report from IES-UPM [39]. This interlayer is of quantum well thickness at around 50 Å or less and has a twofold effect on the tunnel junction characteristics: it serves to reduce the carry-over of tellurium into the AlGaAs layer and the quantum well energy level produces increased tunneling current. The effect is large as can be seen in Figure 3a, which shows the characteristics of otherwise similar TJs with and without the GaAs interlayer. Figure 3b illustrates the effect of tellurium segregation on the growth surface.

    solar, cells

    The stability under annealing conditions was drastically improved by using a slower growth rate and by cutting off the flow of tellurium early to allow the tellurium accumulation on the surface to dissipate. Both of these procedures would be expected to reduce the defect concentration in the as grown film and presumably even more so in the annealed structure. Figure 4 shows the much greater stability of a structure grown under these conditions as compared to the structure shown in Figure 3a.

    A report by IES-UPM on the doping of InGaP with diethyl-tellurium [39] confirms that there is a surface accumulation phenomena large enough to seriously affect the indium/gallium incorporation ratio. This makes the effects obtained by the early cut off of tellurium in tunnel junction growth seem reasonable. The importance of tellurium surface segregation is emphasized by the sensitivity of the tunnel junction characteristics to the details of the tellurium cutoff procedure during the growth. The effect has been reported by a number of different laboratories [19,28,38,39,40].

    There is also a report from TSAR on the doping of InGaP with diisopropyl-telluride (DIPTe) and the growth of TJs with this dopant [40]. It is worth noting that in this work, there is no large effect on the indium to gallium ratio as seen from the diethyl-telluride (DETe). The accumulation of tellurium on the surface of the InGaP was removed from the metallurgical tunnel junction by holding at a growth temperature of 580 °C for 15 min. under an arsenic atmosphere. It seems reasonable that this step will remove the adsorbed tellurium on the surface. Additionally this will also probably produce a monolayer or two of InGaAs at the surface which may act as a quantum well at the junction as has been used in other structures [17,19,28]. It is also noteworthy that in this investigation the same procedure had to be used at the interface between the low doped buffer layer and the highly doped TJ layer. It seems possible that this procedure also removed grown in defects, perhaps in a similar way to the lower defect concentration produced by reduced growth rates in other laboratories [19]. Unfortunately there is no high temperature annealing data reported in this work. An interesting aspect of this work is the growth of TJs both n-up and p-up, high (though not record) performance was obtained in both cases. This is important since much of the high-efficiency work is proceeding on inverted structures which require the junctions to be grown n-up rather than the p-up junctions which are more widely studied for upright cell use.

    Modeling

    While empirical methods have been developed for producing satisfactory tunnel junctions for concentrator cell applications, a deeper understanding of the TJs is still elusive.

    The GaAs/GaAs TJ has probably had the most analysis. This is important because GaAs is a much better characterized material than ternary III-V [11,41] compounds used in high performance TJs and thus the GaAs structures can provide validation of proposed models for the less well understood materials. The development of TJ modeling has been somewhat complicated. Several studies have concluded that the properties of the GaAs/GaAs TJ are adequately explained in the region of peak tunneling current by direct Band to Band tunneling generally following the analytical methods of Kane at HRL [42,43] and numerical calculations developed from them [19,44] at NCSU using existing methods [45,46]. however, one study from PUM [8] has been cited repeatedly [28,47] by IES-UPM to support a conclusion that trap assisted tunneling dominated by nonlocal resonant tunneling through defects is necessary to explain the data in high performance devices. The reason in the PUM study for this contention is that their calculation of the Band to Band tunneling is much lower than that computed by other groups and thus will not account for the observed tunneling current. A probable reason for this discrepancy has been found in an analysis from UT [48] that considers the Band to Band coupling (mixing) of the conduction Band and the light hole valence Band which will reasonably account for the observed tunneling current at doping densities below the level at which Band narrowing effects may dominate. A subsequent analysis [49] refined this calculation and found agreement with a more complete non-equilibrium Green’s function formalism (NEGF) [50,51] calculation. However, quantitative agreement with experimental results was achieved only by using bandgap narrowing as a fitting parameter as there is no complete theory for this effect. One point worth noting is that earlier experiments on germanium tunnel diodes showed that when a large number of defects were added by irradiation (neutrons) the valley current increased but the tunneling current peak was not effected very much [52,53,54]. Also gold doped silicon tunnel diodes exhibited mainly increased valley current with increasing gold concentration [55]. This argues against the peak tunneling current being due to traps. Numerical analysis utilizing commercial software (Synopsis Device) [20,56] have typically concluded that Band-to-Band tunneling explains the peak currents although these numerical models contain a fair number of fitting parameters. Some of the experimental data contain fairly high valley currents which almost all models regard as coming from trap assisted tunneling effects. However, a recent study of GaAs/GaAs tunnel diodes [6] which used Band gap narrowing data from photoluminescence as well as the methodology used for the initial AlGaAs/InGaP analyses [9,44] concluded that Band to Band tunneling will account for the current when the effective tunneling barrier thickness of the depletion layer is properly accounted for. It is also worth noting that the temperature dependence of the peak current as discussed in [28] is not straightforward in other systems. It is controlled by a balance between the temperature dependence of the bandgap and that of the density of states [53] and can thus vary with the doping densities.

    The only detailed modeling of the AlGaAs/GaAs structure uses numerical analysis from the Synopsis Device commercial software package [57] and concluded that this structure needed a much lower effective doping than other structures for the same peak current. It even suggested that this structure might have a peak current of 10,000 A/cm 2 at attainable doping levels.

    The first thorough analytic analysis of the empirically developed AlGaAs/InGaP structure was published by the Bedair group at NCSU in 2010 [44]. The approach used the methodology of Kane [58] which was extended by numeric integration taking into account published bandgap narrowing models and Band offsets. A diagram of the bandgap lineup and tunneling distances is shown in Figure 5a and the expected peak tunneling currents calculated in this way are shown in Figure 5b. An approach which was broadly similar was later used by HRL [43] to get good agreement with the experimental data in silicon tunnel diodes [59]. This study showed that very high tunneling currents were predicted when the bandgap narrowing effects of the high doping levels attainable in n-type InGaP and the favorable bandgap offset of InGaP with AlGaAs were taken into account. From Figure 5b, it is clear that a reasonable Band offset is responsible for about an order of magnitude higher tunneling current in this heterojunction.

    After it was discovered that the inclusion of a quantum well layer led to greatly improved performance [17], a modeling effort was initiated to understand this phenomena. Figure 6a shows the Band diagram for a heterojunction with a GaAs quantum well at the interface. After the improved annealing performance with low growth rate and early Te cut-off was discovered [18] a more sophisticated modeling effort was undertaken. Since the constant field approximation from the HRL paper would not be applicable to the structure with the quantum well, the tunneling current was calculated using the Esaki expression for tunneling in a given field and numerically integrating Poisson’s equation across the junction using a transfer matrix approach taking the expected Band narrowing and Band offsets into account [19]. The results shown in Figure 6b indicate that there is an optimal doping level for the quantum well that is lower than the maximum that can be attained in junctions of this type. This result is helpful in explaining the favorable result of the early tellurium cutoff as well as the effectiveness of a nominally carbon doped 30 Å layer in other experiments [28]. It is worth noting that improved performance has been achieved in InP based systems with a double quantum well structure [60] and successfully modeled with nonequilibrium Green’s function formalism (NEGF) [61]. This modeling approach has also been applied to other tunnel junctions [51,62].

    Thermal Stability

    There are two major problems in the fabrication of TJs for use in multi-junction solar cells: The initial growth of the TJ with the necessary characteristics and its stability while the rest of the cell structure is being grown. This latter condition is significant since high-performance cell structures are typically grown at higher temperatures than optimal TJs. In many of the experimental TJ structures large annealing effects have been seen. In general there are several competing explanations for these effects. Modeling of this process and the consequent change in tunnel junction characteristics is in a much less developed state than the modeling of the as grown characteristics.

    The obvious assumption is that the deterioration is due to inter-diffusion at the junction. While this might have been important with some early junctions using fast diffusing dopants (i.e., Zn) [13,15,63] the reports of studies searching for this effect in later junctions grown with slow diffusers (Te, Se ,C) have been negative [28]. (Admittedly there are no studies with the resolution comparable to that seen in studies of carryover phenomenon in superlattice growth [64]).

    There are several other possibilities. If the tunneling current proceeds through traps then it is reasonable to think that the traps might be annealed out by high temperature annealing. However, it is established [53,55] that trap related processes primarily contribute to the valley current and thus a trap reduction would be expected to reduce current more than proportionately which does not generally appear to be the case. It also seems likely that the TJ structures grown at low growth rates or annealed at higher growth temperature have fewer defects than the high growth rate structures. The higher growth rate structures show more current loss than the lower defect structures after annealing.

    The mechanism that is the most likely to dominate the annealing effects is one recently discussed by IES-UPM [65]. Their proposal is that annealing causes a reduction in net carrier concentration via compensating dopants (more favorable to donors) by changing native amphoteric defect sites (either thermally generated or already present, vacancies in this report). The driving potential for defect compensation is stated as the difference between the Fermi level and a Fermi Level Stabilization Energy which is determined by the energy level of stoichiometric defects. This behavior would be in accord with the explanation from LBNL of the maximum donor concentrations that are stable in III-V materials given that it has been found possible to incorporate more tellurium (and other donor dopants) than are electrically active [66]. This limit is based on compensation by complexing with stoichiometric defects. They have verified experimentally that under annealing conditions a reduction in carrier concentration is seen in highly doped low growth temperature GaAs layers which, using an analysis similar to that of Hauser at NCSU [44], is sufficient to account for the reduction of tunneling current seen in tunnel junctions using similar n-type GaAs layers. Additional support for this explanation is provided by the reported behavior of highly doped n-type InGaAs [67]. In the InGaAs case layers with several times 10 19 n-type carriers can be grown at low temperatures. However, when these are annealed at higher temperatures the concentration converges to about 1.5 × 10 19 which is the same carrier concentration that can be achieved by ion implantation in the layers. This behavior is believed to be related to defect chemistry [66]. However, we have not been able to find any direct reports in the literature discussing the annealing of highly doped InGaP layers.

    Another factor to be remembered is that knowledge of the details of the Band tails producing Band gap narrowing is only approximate. There is no detailed study that we are aware of that investigates the effect of annealing on the details of the edge of the bandgap narrowing in these materials. The reduction of the peak voltage range seen in Figure 4 would suggest a reduction of Band gap narrowing after annealing if the analysis of peak broadening is similar to that in silicon [59].

    Conclusions

    The first proof of concept monolithic tandem cell used an AlGaAs/AlGaAs tunnel junction due to factors involved in liquid phase epitaxy, significantly the high thickness required more transparency. The first widely produced cells used GaAs/GaAs tunnel junctions with much thinner layers grown by MOCVD. This empirically developed junction was satisfactory for first-generation cells. Subsequent work on GaAs/GaAs TJs was aimed largely at understanding the junctions since a GaAs homo-junction could be more easily modeled. Most [6,9,10], though not all [8], of the reports on modeling concluded that direct tunneling (particularly if bandgap narrowing effects were added) adequately explained the tunneling currents observed. This discrepancy was resolved when it was shown [48] that the current in lower doped structures could be explained by direct tunneling if Band coupling (mixing) effects were included. Once the carbon doping procedure for GaAs was developed the p AlGaAs/ n GaAs tunnel junction was easy to implement and eliminated somewhat more than half of the absorption of GaAs/GaAs junctions. This structure was widely used in a number of record setting concentrator cells but was not as extensively studied or modeled. However, there was one modeling study (using a commercial software package) comparing this structure with other structures. This study showed that the AlGaAs/GaAs structure provided the highest conductance with equivalent doping compared to other structures and predicted that a peak tunneling current of 10,000 A/cm 2 was possible with plausible doping levels. Later experimental work achieved this current [47]. Recent work has shown that highly doped p-type AlGaAs containing a high aluminum content will produce junctions of equal electrical performance to those fabricated with lower aluminum content while providing higher transparency [22]. The first all high bandgap TJ with a high peak tunneling current was The AlGaAs/InGaP junction which was fabricated by ALE [16]. When the MOCVD grown version was incorporated into tandem cells it provided about 1% absolute efficiency increase over the GaAs/GaAs junctions previously used [21]. It became the standard Junction for high-efficiency one sun cells and was also used in some record-breaking concentrator cells [35]. however, the AlGaAs/InGaP TJ was found to be relatively difficult to fabricate with high yields and high-performance. The high-performance of this junction is attributed to the relative ease of doping InGaP highly n-type as compared to GaAs and to the favorable Band offset between InGaP and AlGaAs. While this junction began to be used circa year 2000, detailed modeling demonstrating this explanation was not reported until 2010 [44]. The modeling followed the approach of Kane at HRL [42] and additionally took both bandgap narrowing and the bandgap offset between materials into account. The resulting approach is thus analogous to later work which provided a good analysis of silicon tunnel junctions [59]. Later experimental work with the structure showed that the addition of a thin GaAs quantum well at the junction provided a record peak tunneling current for a junction of this bandgap. Other work suggested that tellurium segregation on the surface of the growing InGaP might be the cause of the difficulties that are found in growing the structure reliably. A later experiment with an early tellurium cut off showed record performance for an annealed tunnel junction.

    Structures of similar design with a quantum well in-between the InGaP and the AlGaAs have been fabricated in several laboratories and found to have high performance. When incorporated into multi-junction cells these QW containing devices have provided the predicted 0.6 mA one sun current increase. It thus appears that this design or a variant using AlGaAs instead of InGaP will become the standard for new high-performance concentrator cell designs.

    Detailed modeling of GaAs tunnel junctions [49] using non-equilibrium Green’s function formalism (NEGF) [50,51] combined with the previous success of this approach on more complex structures [61] provides a promising approach for future modeling of high performance tunneling structures.

    Recent work by IES-UPM [65], which had previously argued [35,37] for the dominance of trap related tunneling in the peak current, now supports the direct tunneling model and provides an important advance in the understanding of annealing effects. This study states that degradation of tunneling current has been observed in tunneling junctions regardless of the material system. This reduced current is explained by compensation of dopants via generation of amphoteric native defects becoming vigorously energetically favorable in highly doped material. This mechanism is basically the same mechanism which makes it impossible to grow highly doped uncompensated films at the higher annealing temperatures.

    Funding

    This research was funded by The National Science Foundation research grant DMI-1102060 “GOALI: Cooperative Integration of High Efficiency Multijunction Solar Cell Structures” and by the US Department of Energy EERE SETP CSP subprogram grant DE-EE000540 “Technology Enabling Ultra High Concentration Multi-Junction Cells”.

    Acknowledgments

    Our work in this area of research has been supported by both the National Science Foundation and the Department of Energy.

    Conflicts of Interest

    The authors declare no conflict of interest influencing The representation or interpretation of reported research results in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript, or in the decision to publish the results must be declared in this section. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

    Abbreviations

    The following abbreviations are used in this manuscript:

    3IT-US Interdisciplinary Institute for Technological Innovation University of Sherbrooke
    ALE atomic layer epitaxy
    AlGaAs aluminum gallium arsenide
    BSF back surface field
    CBE chemical beam epitaxy
    DETe diethyl-telluride
    DIPTe diisopropyl-telluride
    EQE External Quantum Efficiency
    Fraunhofer ISE Fraunhofer Institute for Solar Energy Systems
    GaAs gallium arsenide
    IES-UPM Solar Energy Institute of The Universidad Politécnica de Madrid
    InGaAs indium gallium arsenide
    InGaP indium gallium phosphide
    LPE liquid phase epitaxy
    LBNL Lawrence Berkeley National Laboratory
    MBE molecular beam epitaxy
    MJSC Multi-Junction Solar Cell
    MOCVD metalorganic chemical vapor deposition
    NCSU North Carolina State University
    NEGF None-Equilibrium Green’s Function
    NREL National Renewable Energy Laboratory
    NTT ECL NTT Electrical Communications Laboratories
    NUB Northeastern University Boston
    PUM Philipps University Marburg
    RIERC Rockwell1 International Electronics Research Center
    TJ tunnel junction
    TSAR Total S.A. Renewables
    TTI Toyota Technological Institute
    UO University of Ottawa
    UT University of Toulouse
    VII Veeco Instruments Inc.

    References

    Figure 1. I-V characteristics of the tunnel diodes (a) as-grown and (b) annealed at 650 °C for 30 min. (a) was measured at room temperature and at 150° K. (b) was measured at room temperature [16].

    Figure 1. I-V characteristics of the tunnel diodes (a) as-grown and (b) annealed at 650 °C for 30 min. (a) was measured at room temperature and at 150° K. (b) was measured at room temperature [16].

    Figure 2. EQE of inverted triple-junction solar cells that will form the top junctions of a 6J cell. The dashed lines use the old nontransparent TJ that consists of n and p Al0.3Gal0.7As layers with a 12-nm GaAs QW while the solid lines use a more transparent TJ [37].

    Figure 2. EQE of inverted triple-junction solar cells that will form the top junctions of a 6J cell. The dashed lines use the old nontransparent TJ that consists of n and p Al0.3Gal0.7As layers with a 12-nm GaAs QW while the solid lines use a more transparent TJ [37].

    Figure 3. (a) Junction grown with a 30-Å quantum well at high growth rate. The annealing occurs for 15 min at 625 °C [17] and (b) Te segregation at surface and its effect on grown structure [19].

    Figure 3. (a) Junction grown with a 30-Å quantum well at high growth rate. The annealing occurs for 15 min at 625 °C [17] and (b) Te segregation at surface and its effect on grown structure [19].

    Figure 4. The J-V characteristics of an InGaP/GaAs (50 Å)/AlGaAs TJ, both as-grown and annealed for 30 min at 650 °C for the low growth rate structure coupled with the early Te source shut-off [19].

    Figure 4. The J-V characteristics of an InGaP/GaAs (50 Å)/AlGaAs TJ, both as-grown and annealed for 30 min at 650 °C for the low growth rate structure coupled with the early Te source shut-off [19].

    Figure 5. (a) Example of junction tunneling width and depletion layer width and (b) Peak tunneling current for model 1.91 eV Band gap tunneling junction [45].

    Figure 5. (a) Example of junction tunneling width and depletion layer width and (b) Peak tunneling current for model 1.91 eV Band gap tunneling junction [45].

    Figure 6. (a) Band diagram for structure incorporating a GaAs quantum well at the junction [18] and (b) Peak tunneling current range for various In x Ga1− x P:Te/GaAs:Te/Al0.6Ga0.4As:C tunnel junction architectures with GaAs:Te interfacial layer thickness ranging from 15 Å to 50 Å [19].

    Figure 6. (a) Band diagram for structure incorporating a GaAs quantum well at the junction [18] and (b) Peak tunneling current range for various In x Ga1− x P:Te/GaAs:Te/Al0.6Ga0.4As:C tunnel junction architectures with GaAs:Te interfacial layer thickness ranging from 15 Å to 50 Å [19].

    1961 ⍿ Interband Tunneling Model [42] Hughes Research Laboratories
    1980 ⍿ First monolithic MJSC [1] AlGaAs/AlGaAs TJ (ALE) 9% EQE NCSU
    1990 ⍿ First record setting MJSC [3] GaAs/GaAs TJ 27.3% EQE Solar Energy Research Institute
    1993 ⍿ New TJ Structure [16] AlGaAs/InGaP (ALE) NCSU
    2001 ⍿ Production Study of MJSC design [21] AlGaAs/InGaP TJ Spectrolab
    2007 ⍿ Comprehensive study of Tellurium dopant memory effects [39] InGaP IES-UPM
    2009 ⍿ Record concentrator MJSC [35] Al(In)GaAs/InGaP TJ 41.1% EQE454 suns Fraunhofer Institute for Solar Energy Systems
    2010 ⍿ First InGaP/AlGaAs TJ Model [44] NCSU
    2013 ⍿ Effects of QW GaAs interfacial layer [17] InGaP/GaAs/AlGaAs TJ NCSU
    2017 ⍿ New model describing thermal degredation of TJ structures [65]-Stolle IES-UPM

    © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

    Share and Cite

    Colter, P.; Hagar, B.; Bedair, S. Tunnel Junctions for III-V Multijunction Solar Cells Review. Crystals 2018, 8, 445. https://doi.org/10.3390/cryst8120445

    Colter P, Hagar B, Bedair S. Tunnel Junctions for III-V Multijunction Solar Cells Review. Crystals. 2018; 8(12):445. https://doi.org/10.3390/cryst8120445

    Chicago/Turabian Style

    Colter, Peter, Brandon Hagar, and Salah Bedair. 2018. Tunnel Junctions for III-V Multijunction Solar Cells Review Crystals 8, no. 12: 445. https://doi.org/10.3390/cryst8120445

    Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

    Iii v solar cells

    Special Issue on ‘WCPEC-8: State of the Art and Developments in Photovoltaics’, edited by Alessandra Scognamiglio, Robert Kenny, Shuzi Hayase and Arno Smets

    • Top
    • Abstract
    • 1 Introduction
    • 2 Overview for.
    • 3 Analytical procedure.
    • 4 Analysis for.
    • 5 Summary
    • Conflict of interest
    • Supporting Information
    • Data availability
    • Author contribution statement
    • References
    • List of tables
    • List of figures

    Overview and loss analysis of III–V single-junction and multi-junction solar cells

    Masafumi Yamaguchi 1. Frank Dimroth 2. Nicholas J. Ekins-Daukes 3. Nobuaki Kojima 1 and Yoshio Ohshita 1

    1 Toyota Technological Institute, Nagoya 468-8511, Japan 2 Fraunhofer Institute for Solar Energy Systems ISE, Freiburg 79110, Germany 3 University of New South Wales, Sydney 2052, Australia

    Received: 2 June 2022 Received in final form: 26 July 2022 Accepted: 29 August 2022 Published online: 14 October 2022

    The development of high-performance solar cells offers a promising pathway toward achieving high power per unit cost for many applications. Because state-of-the-art efficiencies of single-junction solar cells are approaching the Shockley-Queisser limit, the multi-junction (MJ) solar cells are very attractive for high-efficiency solar cells. This paper reviews progress in III–V compound single-junction and MJ solar cells. In addition, analytical results for efficiency potential and non-radiative recombination and resistance losses in III–V compound single-junction and MJ solar cells are presented for further understanding and decreasing major losses in III–V compound materials and MJ solar cells. GaAs single-junction, III–V 2-junction and III–V 3-junction solar cells are shown to have potential efficiencies of 30%, 37% and 47%, respectively. Although in initial stage of developments, GaAs single-junction and III–V MJ solar cells have shown low ERE values, ERE values have been improved as a result of several technology development such as device structure and material quality developments. In the case of III–V MJ solar cells, improvements in ERE of sub-cells are shown to be necessary for further improvements in efficiencies of MJ solar cells.

    Key words: High-efficiency / singe-junction solar cells / multi-junction solar cells / loss analysis

    © M. Yamaguchi et al., Published by EDP Sciences, 2022

    This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

    Introduction

    The development of high-performance solar cells offers a promising pathway toward achieving high power per unit cost for many applications. Because state-of-the-art efficiencies of single- junction solar cells are approaching the Shockley-Queisser limit [ 1], the multi-junction (MJ) solar cells [ 2] are very attractive for high-efficiency solar cells as shown in Figure 1. Although the concept of MJ solar cells was first and most successfully implemented using III–V compound materials, there is a need to further improve their conversion efficiency of III–V MJ solar cells and to show guidelines for realizing high-efficiency MJ solar cells composed of other materials like perovskite, II-VI compounds and chalcogenides.

    This paper reviews progress in III–V compound single-junction and multi-junction solar cells. In addition, analytical results for efficiency potential and non-radiative recombination and resistance losses in III–V compound single-junction and multi-junction solar cells are presented for further understanding and decreasing major losses in III–V compound materials and multi- junction solar cells.

    Calculated efficiencies of III–V compound MJ solar cells under 1-sun condition as a function of the number of junction and average external radiative efficiency (ERE) [2] in comparison with efficiency data (best laboratory efficiencies [ 3]). (Reproduced with permission from Ref. [2]. Updated).

    Overview for III–V single-junction and multi-junction solar cells

    Figure 2 summarizes chronological improvements in conversion efficiencies of Si, GaAs, CIGS and perovskite single-junction solar cells and III–V compound multi-junction solar cells under 1-sun operation [3] and future efficiency predictions of those solar cells (original idea by Professor A. Goetzberger et al. [ 4] and modified by M. Yamaguchi et al. [ 5]).

    The function chosen here (Eq. (1)) is derived from the diode equation:

    (1)

    where η(t) is the time-dependent efficiency, η L limiting asymptotic maximum efficiency, a 0 is the year for which η(t) is zero, a is the calendar year and c is a characteristic development time. Fitting of the curve is done with three parameters which are given in Table 1. For example, 43% for η L , 17 for a 0 and 1975 for c were used in the case of III–V compound multi-junction solar cells. The function can be fitted relatively well to the past development of best laboratory efficiencies of various solar cells under 1-sun condition.

    Chronological efficiency improvements of crystalline Si, GaAs, CIGS, and perovskite single-junction solar cells and III–V compound multi-junction (MJ) solar cells under 1-sun condition.

    Fitting parameters for different technologies.

    Analytical procedure for estimating efficiency potential of various solar cells

    One of the problems to attain the higher efficiency MJ and Si tandem solar cells is to reduce non-radiative recombination loss. The open-circuit voltage Voc drop compared to bandgap energy (Eg/qVoc) is dependent upon non-radiative voltage loss (Voc,nrad) that is expressed by external radiative efficiency (ERE). Open-circuit voltage is expressed by [ 6– 10]

    (2)

    where the second term on the right-hand side of equation (2) is denoted as V oc,nrad because it is associated to the voltage-loss due to non-radiative recombination and V oc,rad is radiative open-circuit voltage and is given by [6–10]

    (3)

    where JL (Voc,rad) is photo-current at open-circuit in the case of only radiative recombination and J0,rad is saturation current density in the case of only radiative recombination. 0.28 V [ 8–10] for III–V compounds and perovskite, and 0.26 V [8–10] for Si solar cells were used as ΔVoc, rad (= Eg/qVoc,rad) in this study. In the case of multi-junction tandem solar cells, we define average ERE (EREave) by using average Voc loss [ 11]:

    (4)

    where n is the number of junctions.

    The resistance loss of a solar cell is estimated solely from the measured fill factor. Fill factor is dependent upon V oc and ideal fill factor FF0. defined as the fill factor without any resistance loss, used in the calculation is empirically expressed by [ 12],

    (5)

    where v oc is normalized open-circuit voltage and is given by

    (6)

    The measured fill factor FF is decreased as increase in series resistance R s and decrease in shunt resistance R sh of solar cell and approximated by

    (7)

    where rs and rsh are normalized series resistance and normalized shunt resistance, respectively and are given by

    solar, cells

    (8)

    (9)

    The characteristic resistance R CH is expressed by [12]

    (10)

    In the calculation, highest values obtained were used as J sc. V oc and FF were calculated by equations (2)–(10) and conversion efficiency potential of various solar cells were calculated as a function of ERE.

    Analysis for non-radiative recombination loss and efficiency potential of III–V single-junction and multi-junction solar cells

    Figure 3 shows calculated efficiencies of GaAs single-junction, III–V 2-junction, III–V/Ge 3-junction and III–V/InGaAs 3-junction solar cells as a function of average external radiative efficiency (ERE) in comparison with the state-of-the art efficiencies of those solar cells [3] including chronological efficiency improvements [5]. GaAs single-junction, III–V 2-junction and III–V 3-junction solar cells have potential efficiencies of 30%, 37% and 47%, respectively. Highest efficiency of 39.5% [3, 13] has been demonstrated with III–V 3-junction solar cells under 1-sun by NREL, further efficiency improvements in III–V multi-junction solar cells by improving ERE and reducing resistance loss are expected. For this end, reduction on non-radiative recombination and resistance losses is necessary.

    Figure 4 shows chronological ERE improvements of GaAs single-junction, III–V 2-junction, III–V/Ge 3-junction and III–V/InGaAs 3-junction solar cells.

    Although in initial stage of developments, GaAs single-junction and III–V MJ solar cells have shown low ERE values, ERE values have been improved as a result of several technology development. For example, in the case of GaAs single-junction solar cells, hetero-face and double hetero junction solar cells have been developed from homo junction solar cells. Recently, high ERE values have been realized by photon recycling [ 14, 15].

    solar, cells

    In the case of III–V MJ solar cells, improvements in ERE of sub-cells are necessary for further improvements in efficiencies of MJ solar cells. For example, in the case of 6-junction solar cell [ 16], ERE (1.3 × 10 −4 %) for the 1st 2.19 eV AlGaInP cell 0.044% for the 2nd 1.76 eV AlGaAs cell are lower than 1.4% for the 3rd 1.42eV GaAs cell and 2.1% for 4th 1.19 eV GaInAs cell and 5th 0.97 eV GaInAs cell. Lower ERE values for Al-related cells are thought to be due to oxygen-related non-radiative recombination center [ 17].

    Figure 5 shows chronological fill factor FF improvements of GaAs single-junction, III–V 2-junction, III–V/Ge 3-junction and III–V/InGaAs 3-junction solar cells.

    Although in initial stage of developments, GaAs single-junction and III–V MJ solar cells have shown low fill factor values, FF values have been improved as a results of improvements in series resistance and shunt resistance.

    Figure 6 shows changes in average external radiative efficiency ERE for III–V multi-junction, III–V/Si tandem solar cells and perovskite/Si tandem solar cells as a function of number of junctions. In Figure 6, estimated average ERE values for GaAs single-junction [3,14], III–V 2-junction [3, 18], III–V 3-junction [3,13], III–V 5-junction [3, 19], III–V 6-junction [3,16], III–V/Si 2-junction [3, 20], III–V/Si 3-junction [3, 21], perovskite single-junction [3] and perovskite/Si 2-junction [3, 22] solar cells were plotted. It is clear in Figure 6 that ERE for III–V MJ and Si tandem solar cells decreases as increase in number of junctions. Further improvements in ERE of sub-cells are necessary for further improvements in efficiencies of MJ and Si tandem solar cells. Especially, in the case of III–V based MJ solar cells, Al contained wide-bandgap sub-cell layer is suggested to be lower ERE due to oxygen-related non-radiative recombination center [17].

    In this paper, non-recombination center behavior of AlGaInP top cell and AlGaAs 2nd layer solar cells was analyzed by using data for molecular-beam epitaxy grown All0.3Ga0.7As solar cells reported by one of the authors [ 23]. In this analysis, we assumed that the external radiative efficiency IRE is equal to the internal radiative efficiency IRE as follows:

    (11)

    where τ rad is the radiative recombination lifetime and expressed by

    (12)

    where N is carrier concentration and B is radiative recombination probability (2 × 10 −10 cm 3 /s for GaAs [ 24]).

    Effective lifetime τ eff is expressed by

    (13)

    where τ nonrad is non-radiative recombination lifetime and given by

    (14)

    where σν is minority-carrier thermal velocity, σ is capture cross section of non-radiative recombination centers, and N r is density of non-radiative recombination centers. In this analysis, 10 −10 cm 2 was used as capture cross section of oxygen related non-radiative recombination center (E c – 0.86 eV in AlGaAs) by fitting correlation curve between ERE and N r as shown as dotted line in Figure 7. ERE values were determined by using voltage loss expressed by equation (2). Table 2 shows activation energies and capture cross sections in AlGaAs [ 25– 27] and our result. As shown in Table 2, it is clear that oxygen-related defect in AlGaAs acts as very active defect center because it has higher capture cross section. In addition, the oxygen related defect in AlGaAs has been confirmed to act as a non-radiative recombination center by using double carrier pulse DLTS (Deep Level Transient Spectroscopy) in our previous study [17]. Density of non-recombination center density Nr in 2.19 eV AlGaInP top cell and 1.76eV AlGaAs 2nd layer cell of 6-junction solar cell [16] was estimated by equations (11)–(14) and was compared with data for MBE-grown 1.77 eV AlGaAs single-junction solar cell [23].

    Figure 7 shows changes in external radiative efficiency ERE for 2.1 eV AlGaInP top cell and 1.76 eV AlGaAs 2nd layer cell of 6-junction solar cell [16] and MBE-grown 1.77 eV AlGaAs single-junction solar cell [23] as a function of non-radiative recombination centers estimated by using equations (11)–(14). Because ERE (1.3 × 10 −4 %) for the 1st 2.19 eV AlGaInP cell 0.044% for the 2nd 1.76 eV AlGaAs cell are lower than 1.4% for the 3rd 1.45 eV (Al)GaAs cell and 2.1% for 4th 1.19 eV GaInAs cell and 5th 0.97 eV GaInAs cell in the 6-junction solar cell [16], reduction in density of non-recombination center in Al-contained wide-bandgap layer is necessary as pointed out by one of co-authors [17]. Lower ERE values for Al-contained solar cells are thought to be due to oxygen-related non-radiative recombination center [17].

    Figure 8 shows changes in fill factor FF for III–V multi-junction, III–V/Si tandem solar cells and perovskite/Si tandem solar cells as a function of number of junctions. In Figure 8, reported fill factor values for GaAs single-junction [3,14], III–V 2-junction [3,18], III–V 3-junction [3,22], III–V 5-junction [3,19], III–V 6-junction [3,16], III–V/Si 2-junction [3,20], III–V/Si 3-junction [3,21], perovskite single-junction [3] and perovskite/Si 2-junction [3,22] solar cells were plotted. It is clear in Figure 8 that FF for III–V MJ and Si tandem solar cells decreases as increase in number of junctions. Further improvements in FF for III–V MJ solar cells and Si tandem solar cells are necessary by improvements in series resistance and shunt resistance.

    Although resistance loss is composed of absorber, interface, contact, interconnection and grid of solar cells, especially, fill factor of perovskite/Si tandem solar cells is dependent on contact resistance of interconnection of sub-cells such as transparent conductive oxide layer, recombination junction and so forth. Figure 9 shows correlation between fill factor and series resistance of perovskite single-junction solar cell [28] and perovskite/Si 2-junction solar cell [ 29]. Calculated results for series resistance of perovskite single-junction solar cell were estimated by using equations (5)–(10). Difference between measured values and calculated results for perovskite single-junction solar cells are thought to be attributed from shunt resistance. Further improvements in FF for perovskite/Si tandem solar cells are necessary by improvements in series resistance and shunt resistance.

    Calculated efficiencies of GaAs single-junction, III–V 2-junction, III–V/Ge 3-junction and III–V/InGaAs 3-junction solar cells as a function of average ERE in comparison with the state-of- the-art efficiencies of those solar cells including chronological efficiency improvements.

    Chronological ERE improvements of GaAs single-junction, III–V 2-junction, III–V/Ge 3-junction and III–V/InGaAs 3-junction solar cells.

    Chronological fill factor improvements of GaAs single-junction, III–V 2-junction, III–V/Ge 3-junction and III–V/InGaAs 3-junction solar cells.

    Changes in average external radiative efficiency ERE for III–V multi-junction, III–V/Si tandem solar cells and perovskite/Si tandem solar cells as a function of number of junctions.

    Changes in external radiative efficiency ERE for AlGaInP top cell and AlGaAs 2nd layer cell of 6-junction solar cell [16] and MBE-grown AlGaAs single-junction solar cell [18] as a function of non-radiative recombination centers estimated by using equations (11)–(14).

    Activation energies and capture cross sections in AlGaAs [25– 27] and our result.

    Changes in fill factor for III–V multi-junction, III–V/Si tandem solar cells and perovskite/Si tandem solar cells as a function of number of junctions.

    Correlation between fill factor and contact resistivity of perovskite single-junction and perovskite/Si 2-junction solar cells.

    Summary

    This paper overviewed progress in III–V multi-junction (MJ) solar cells. In addition, analytical results for efficiency potential of III–V MJ solar cells were presented and non- radiative recombination and resistance losses of III–V MJ solar cells were discussed in this paper. GaAs single-junction, III–V 2-junction and III–V 3-junction solar cells have potential efficiencies of 30%, 37% and 47%, respectively. Although in initial stage of developments, GaAs single-junction and III–V MJ solar cells have shown low external radiative efficiency ERE values, ERE values have been improved as a result of several technology development such as device structure and material quality developments. It was shown in this study that ERE and fill factor FF for III–V MJ and Si tandem solar cells decreases as increase in number of junctions. Further improvements in FF for III–V MJ solar cells and Si tandem solar cells are necessary by improvements in series resistance and shunt resistance. Improvements in ERE of sub-cells were shown to be necessary for further improvements in efficiencies of MJ solar cells. Especially, Al contained wide-bandgap sub-cell layer showed lower ERE due to oxygen-related non-radiative recombination center.

    Conflict of interest

    The authors declare no conflict of interest.

    Supporting Information

    Supporting Information is available from the Wiley Online Library or from the authors.

    Data availability

    The data that support the findings of this study are available upon reasonable request from the authors.

    Author contribution statement

    The supervision of the project was ensured by M. Yamaguchi. The experiments were conducted by M. Yamaguchi, N. Ekins-Daukes and N. Kojima. The analysis was conducted by M. Yamaguchi, F. Dimroth and N. Ohshita. Discussion on results was conducted by all co-authors. The writing of the manuscript and proof reading were done by all co-authors.

    The authors would like to express sincere thanks to Dr. J.F. Geisz, Dr. M.A. Steiner and Dr. R. France, NREL for their fruitful discussion, to the NEDO for their support and to Dr. T. Takamoto, Sharp, Dr. K. Araki and Dr. K-H. Lee, former Toyota Tech. Inst., Prof. A. Yamamoto, Dr. H. Sugiura, Prof. K. Ando, Dr. C. Amano, Prof. S. Katsumoto and Dr. M. Sugo, Former NTT Labs., Dr. M. Al-Jassim, Dr. R. Ahrienkiel, Dr. J. Olson, Dr. S.R. Kurtz and Dr. D.J. Friedman, NREL, Prof. A. Luque and Prof. G. Sala, UPM, Dr. A. Bett and Dr. G. Siefer, FhG-ISE, Dr. Y. Hishikawa, AIST, Prof. Y. Okada and Prof. M. Sugiyama, Univ. Tokyo, Prof K. Nishioka and Prof. Y. Ota, Univ. Miyazaki for their helpful discussion and collaboration.

    References

    Cite this article as: Masafumi Yamaguchi, Frank Dimroth, Nicholas J. Ekins-Daukes, Nobuaki Kojima, Yoshio Ohshita, Overview and loss analysis of III–V single-junction and multi-junction solar cells, EPJ Photovoltaics 13, 22 (2022)

    All Tables

    Fitting parameters for different technologies.

    Micro III-V solar cell with 33.8% efficiency

    Developed by a French-Canadian research group, the triple-junction cell is based on indium gallium phosphide (InGaP), indium gallium arsenide (InGaAs) and germanium (Ge) and has an active area of only 0.089 mm2. It can be used for applications in micro-concentrator photovoltaics (CPV).

    The micro‐scale multijunction solar cells designed for micro‐CPV applications

    Image: Université de Sherbrooke/i

    Share

    A French-Canadian research team has recently developed a micrometer-scale triple-junction III-V solar cell for applications in concentrated photovoltaics (CPV).

    The device is based on indium gallium phosphide (InGaP), indium gallium arsenide (InGaAs) and germanium (Ge) and has an active area of only 0.089 mm2. “We have not included these cells into real modules yet,” Sherbrooke Professor, Maxime Darnon, told pv magazine. “We would be happy to collaborate with any group that would need such cells.”

    According to him, the processes used to fabricate the cells are almost identical to the processes used for conventional III-V cells, such as those produced by German specialist Azur Space, except for the isolation and dicing process that is plasma-based, while in other cells for CPV applications saw dicing in commonly used. “It is hard to anticipate the exact cost in production conditions, but we can say that mm2 cells fabricated with plasma etching would be approximately the same price as cells fabricated with saw dicing, and for micrometer-scale solar cells, plasma etching is the only economically viable solution since we waste much less valuable material with plasma etching than with saw dicing,” he further explained.

    The cell was designed with rectangular, circular, and hexagonal active areas and built with a commercial epitaxial germanium wafer and titanium-aluminum (Ti/Al) metallization deposited on the backside of the wafer. An anti-reflective coating (ARC) made of silicon nitride hydrogen (SiNxHy) and hydrogenated silicon oxide (SiOxHy) was then deposited by plasma-enhanced chemical vapor deposition (PECVD), which was key to minimize surface recombinations.

    Saw dicing was discarded to avoid defect generation due to the fragility of the germanium wafer and the targeted small area of the cell. “In addition, saw dicing generates linear channels that force the fabricated cells to be rectangular, which may be inadequate depending on the incoming light profile,” the researchers explained.

    The performance of the micro-scale cell was analyzed under standard AM1.5G sunlight conditions and the device showed an open-circuit voltage of 2.350 V, a short-circuit current density of 12.40 mA cm − 2, and a fill factor of 82.7%. “However, a degradation of the electrical performance was observed when reducing cell areas with up to 10.2% drop in open-circuit voltage for cells without passivating ARC,” the research team noted.

    The scientists achieved the maximum efficiency of 34.4% for a 1-mm 2 cell under concentrated light of 450 suns and of 33.8% under 584 suns for a 0.25-mm 2 device. “Lower current and lower series resistance in smaller cells are expected to shift the maximum efficiency to higher concentration,” they further explained.

    Popular content

    The micrometer-scale cell is presented in the paper Miniaturization of InGaP/InGaAs/Ge solar cells for micro‐concentrator photovoltaics, published in Progress in Photovoltaics. The research group includes scientists from the Université de Sherbrooke and the University of Ottawa in Canada, the French National Centre for Scientific Research (CNRS), the University of Bordeaux in France.

    The wrok was performed conjointly at the Laboratoire Nanotechnologies Nanosystèmes ([labn2.ca]LN2) and Laboratoire d’Intégration du Matériau au Système (IMS). The research project was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC), Prompt, a Canadian non-profit organization dedicated to facilitating partnerships and R D financing between businesses and the public research sector, and Canada-based industrial partner Saint-Augustin Canada Electric inc. (STACE).

    The cost of producing solar cells based on compounds of III-V element materials–named according to the groups of the periodic table that they belong to–has confined such devices to niche applications including drones and satellites, where low weight and high efficiency are more pressing concerns than costs, in relation to the energy produced.

    This content is protected by copyright and may not be reused. If you want to cooperate with us and would like to reuse some of our content, please contact: editors@pv-magazine.com.

    Emiliano Bellini

    Emiliano joined pv magazine in March 2017. He has been reporting on solar and renewable energy since 2009.

    PHOTO : Materials. III-V semiconductor solar cells

    The multijunction solar cells based on III-V semiconductors have led the path towards the 3rd generation of photovoltaic cells, and can now reach up to 47 % conversion efficiency. Our objective is to demonstrate novel junctions with high efficiency for solar cells grown on GaAs. Our approach is to use the remarquable properties of low bandgap lattice-matched materials such as GaInAsN, GaAsSbN and GaInBiN fabricated by molecular beam epitaxy in order to improve the performances of the 1eV subcell and more prospectively for new conversion concepts. An increase of performances will then have a direct impact for photovoltaic applications both terrestrial and in space.

    For space applications, III-V solar cells have demonstrated for several decades their primacy with an interesting ratio between delivered power to weight, and a better radiation withstand. Nevertheless, the use of solar panels in space based on new diluted III-V materials requires the full characterization of their behavior with respect to radiations. The reliability and robustness of solar panels remain the key point for satellites. These requirements of radiation withstand are even more critical with new satellite in-orbit techniques.

    InGaAsN solar cells for space applications: (a) structure of the fabricated cells, (b) spectral response of a 0.25 cm 2 cell.

    Collaborations:

    LNE Trappes, ONERA, CNES, IPVF, IES, PROMES

    Projects:

    EURAMET SolCELL project, CIFRE LNE, PhD co-supervised by CNES/ONERA

    Related publications:

    Levillayer, M. et al., As-Grown InGaAsN Subcells for Multijunction Solar Cells by Molecular Beam Epitaxy, IEEE Journal of Photovoltaics 2021, doi: 10.1109/JPHOTOV.2021.3093048. Levillayer, M. et al., Degradation study of InGaAsN PIN solar cell under 1 MeV electrons irradiation, IEEE Transactions on Nuclear Science 2021, doi: 10.1109/TNS.2021.3068044. Louarn, K. et al. Thickness limitation of Band-to-Band tunneling process in GaAsSb/InGaAs type-II tunnel junctions designed for multijunction solar cells. ACS Applied Energy Materials 2, 1149-1154, 2019. Louarn, K. (2018). Etude et réalisation de jonctions tunnel à base d’hétérostructures à semiconducteurs III-V pour les cellules solaires multi-jonction à très haut rendement. Doctoral dissertation, Université de Toulouse 3 Paul Sabatier.

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