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. 2024 Jul 2;16(28):36667–36677. doi: 10.1021/acsami.4c03446

Photodegradation of Organic Solar Cells under Visible Light and the Crucial Influence of Its Spectral Composition

Paul Weitz †,*, Jonas Wortmann †,‡, Chao Liu †,‡, Tian-Jiao Wen §, Chang-Zhi Li §, Thomas Heumüller †,‡,*, Christoph J Brabec †,‡,*
PMCID: PMC11262306  PMID: 38955357

Abstract

graphic file with name am4c03446_0008.jpg

While wavelength-dependent photodegradation of organic solar cells (OSCs) under visible light is typically discussed in terms of UV/blue light-activated phenomena, we recently demonstrated wavelength-dependent degradation rates up to 660 nm for PM6:Y6. In this study, we systematically investigated this phenomenon for a broad variety of devices based on different donor:acceptor combinations. We found that the spectral composition of the light used for degradation, tuned in a spectral range from 457 to 740 nm and under high irradiances of up to 30 suns, has a crucial influence on the device stability of almost all tested semiconductors. The relevance of this phenomenon was investigated in the context of simulated AM1.5 illumination with metal halide lamps and white LEDs. It is concluded that the current stability testing protocols in OSC research have to be adjusted to account for this effect to reveal the underlying physics of this still poorly understood mechanism.

Keywords: organic solar cells, stability, acceleration, wavelength dependence, high irradiances

1. Introduction

Organic solar cells (OSCs) are on a promising path to becoming a next-generation photovoltaic technology, with power conversion efficiencies (PCEs) already closing in on the 20% milestone.1−4 Although the reported PCEs are steadily increasing, the long-term stability of the devices is still not suitable for the commercialization of the technology.4−7 While combining the right semiconductors, interface materials, and testing conditions has led to very stable devices in the past,4,8 these devices were not necessarily the ones with the best PCE. Furthermore, there is still a limited understanding on the scatter of stability values reported, especially when comparing benchmark devices to the majority of publications.9,10 One reason is that the best-performing devices today are built from bulk-heterojunction (BHJ) composites that offer a rather broad variability in microstructures even if all processing parameters are nominally kept constant. Given that the film morphology has a great influence on the device performance and stability,11,12 it is not surprising that efficiency and stability are not necessarily directly related – a phenomenon that is frequently related to fast burn-in degradation.12−14 Interestingly, recent data suggest that such trends are in contrast to microcrystalline semiconductors.15 This urgently requires a change in strategy for assessing the performance potential of a semiconductor blend by testing in parallel the efficiency and stability of as many as possible variations for one semiconductor composition. Lifetime investigations are further complexed by the massive number of parameters that influence the device stability, ranging from the material purity and material combinations to the process variables, the testing conditions or the choice of interface materials, and the inherent statistical variance of and during each stability testing experiment.16,17 Considering the testing conditions, multiple environmental factors, which can be categorized into extrinsic and intrinsic degradation factors, influence the stability of an OSC.5 While extrinsic degradation can, in principle, be overcome by perfect encapsulation, intrinsic degradation processes caused by the absorption of light and the generation of carriers (photodegradation) cannot be compensated by extrinsic concepts such as packaging.

To develop a device that is resilient against photodegradation, all photodegradation pathways need to be known. Given the need for statistical relevance of the observed effects and the inherently long experimental measurement time of thousands of hours for long-term lifetime experiments, methods to accelerate stability testing become highly attractive.5 These accelerated lifetime testing techniques have in common that they increase the stress on the device by either harshening the test conditions, such as increasing the light intensity for photodegradation18−20 tests or adding additional stress factors, such as air exposure or elevated temperatures, that might lead to additional degradation pathways.

It is well-known that UV light is one of these stress parameters that causes rapid degradation of OSCs. Protecting the device from UV radiation with an optical 380 nm cutoff filter has become standard for both outdoor and indoor photostability testing. The wavelength of 380 nm was chosen with the rationale of minimizing Jsc losses and maximizing protection against highly energetic photons. However, we recently reported that specific OSC composites exhibit a wavelength-dependent degradation behavior that is sensitive far beyond the UV regime up to wavelengths beyond 500 nm.19,21 Furthermore, these most recent findings also pointed out that degradation studies with white light sources of different spectral compositions cannot be used to isolate degradation pathways and to compare photodegradation rates among different laboratories using different light sources. An extension of the degradation approaches commonly used today and recommended in the ISOS protocols may be necessary. However, if the wavelength-dependent degradation of OSCs is better understood, the usage of filters with a cutoff wavelength that is adapted to the spectral degradation behavior of the semiconductor composite is a technically sound option. Unfortunately, a cutoff wavelength beyond 500 nm is not an attractive option as it can lead to a significant power loss due to the reduced short circuit current (Jsc). It is therefore interesting to precisely understand the minimum cutoff wavelength required to stabilize the solar cell. Moreover, advanced spectral light management concepts such as photon down-conversion or even photon-shifting offer concepts to maximize stability gain and minimize performance losses.22 In that context, hybrid tandem cells with a perovskite wide bandgap and an organic narrow bandgap subcell also provide a lever to stabilize OSCs without a loss in power.23

In this study, we report a broad systematic investigation of the spectrally dependent degradation behavior of OSCs and build the scientific foundation necessary to develop solutions for this phenomenon. In detail, we investigated BHJ composites from 5 donors and 6 acceptors, ranging from traditional fullerene-based devices to modern state of art composites based on non-fullerene acceptor (NFA) materials. For future applications, not only performance and stability are of interest but also cost efficiency and scalability. PTVT-T is a cost-effective and high-performance donor material in OPV with reported efficiencies above 16% and relevance for industrial applications.24,25 On this background, PTQ10 can also be highlighted as a system of great importance as it exhibits attractive scalability potential.26,27 In our study, these two donor materials were benchmarked to the most commonly used state-of-the-art material PM6. In addition, we added WF3, a polymer that is known for its excellent stability.4,21 These donors were paired with state-of-the-art acceptors like Y12, a representative of the very successful Y-series acceptors as well as with ITIC and its more stable derivative ITIC-4F as representatives of the first generation of modern NFAs. In addition, PTB4CL and PTIC as representatives of the emerging class of nonfused ring electron acceptors (NFREAs) were added because of their unique inherent photostability.28,29

High-intensity monochromatic LED lights of different colors were focused onto the devices to provide accelerated spectral degradation with up to 30 sun equivalents. The results were compared to stability tests under white LED illumination and metal-halide lamp (MHL) illumination with different filters, and the acceleration factors as well as the spectral effects of the LEDs compared to one sun illumination are discussed. Schematic drawings of the device architectures, the materials, and the degradation and measurement routines and setups are shown in Figure 1.

Figure 1.

Figure 1

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Top: device architecture as well as donor and acceptor molecules used in this manuscript. Middle: schematic drawing of the experimental degradation and measurement routine under high-intensity LED illumination that was used in this manuscript. Bottom: schematic drawing of the experimental degradation and measurement routine under MHL illumination that was used in this manuscript.

2. OSC Photodegradation under Visible Light Is Strongly Wavelength-Dependent

To compare the degradation of the different devices under various illumination conditions, their behavior was monitored over photon dose rather than time to account for the different light intensities of the lamps and external quantum efficiencies (EQEs) of the devices. Sun equivalent hours (SE*h) were used as a measure of photon dose, which has been proven to be a good measure in previous studies.18,19 In this study, SE is determined by the OSC’s Jsc to account for the individual EQE of a device, while h is the total time t[h] under illumination. Details on how the SE value was determined in case of high light intensities can be found in the Supporting Information. By using SE*h, it can easily be seen if the acceleration of the stability tests with high light intensities adds additional degradation mechanisms since the degradation curves for different intensities at the same wavelength would overlap on this scale if no additional mechanisms were present.

Figure 2 shows the normalized PCE, fill factor (FF), Jsc, and open-circuit voltage (Voc) of a series of OSCs based on the donor polymer PTQ10 paired with different NFAs over SE*h under different illumination conditions. The PTQ10:Y12-based samples remained stable over 500 SE*h when illuminated with red light (660 nm) while they degraded linearly when illuminated with green light, leaving 91% of its initial efficiency after 500 SE*h. In contrast, the devices aged under blue light (457 nm) only maintained 68% of their initial efficiency after 500 SE*h, which corresponds to a much faster degradation than that under green light (523 nm). Furthermore, the result indicates the existence of an additional degradation mechanism that was present in the device under blue light conditions as can be seen when looking at the behavior of Voc over time. While Voc stayed constant under green and red light, it degraded linearly when the device was degraded with blue light. FF and Jsc stayed constant under red light throughout the experiment, while degradation was visible under blue and green light with blue light being more severe.

Figure 2.

Figure 2

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Normalized PCE, FF, Jsc, and Voc over SE*h for PTQ10:Y12, PTQ10:ITIC4F, and PTQ10:ITIC devices under different illumination conditions. The devices were illuminated with monochromatic LEDs (lines colored by the respective LED’s central wavelength) at different intensities (intensities distinguished by point shape, values given in SE). Intensities differed, depending on the device’s EQE and the relative position of the light spot during the experiment (for more details see the Supporting Information). A clear wavelength dependence of the degradation was seen for PTQ10:Y12 and PTQ10:ITIC4F. PTQ10:ITIC suffered from inherent instability. Only the first and the last data points for a given series are displayed for improved visibility. The plot with all points displayed can be found in the Supporting Information.

PTQ10:ITIC4F-based devices showed a qualitatively similar behavior under these conditions with red light being the least and blue light being the most harmful to the devices. However, none of the devices was stable under any conditions and a burn-in-like degradation under blue light was apparent. The burn-in-like behavior for blue light arises mainly from Jsc burn-in along with a small fraction of FF burn-in. A slight change in the slope of the PCE degradation curve was also observed for red and green light and might point toward a burn-in effect that occurs on longer time scales. However, as the Jsc burn-in type behavior is absent for green and red light, blue light can be identified as the main driving force for the burn-in-like degradation behavior. Furthermore, it is worth noting that Voc remained constant under all these conditions.

PTQ10:ITIC-based samples did not provide a clear wavelength dependence but showed a stronger degradation behavior with increasing the light intensity. An exception to this rule was the device that had been degraded at 6 SE with red light. The Jsc burn-in visible for the PTQ10:ITIC4F samples as well as the constant Voc were also observed for the PTQ10:ITIC samples, but the burn-in did not seem to be triggered by the illuminating light’s wavelength. However, in contrast to the other devices, PTQ10:ITIC devices also showed performance losses after storing them in N2 in the dark for several days, which showed that these devices are inherently unstable and the observed degradation is not only due to photodegradation. Consequently, it is plausible that wavelength-dependent degradation effects were masked by the inherent device instability. Interestingly, it could also be seen that even the inherent degradation of these devices seemed to have negligible influence on the Voc of the devices.

Up to this point, the results might be characteristic of PTQ10 since all the devices presented in Figure 2 have been based on PTQ10 as a donor. To eliminate PTQ10-specific degradation effects from consideration, the experiments were repeated under comparable conditions using the same NFAs, but with PTVT as a different donor material (Figure 3).

Figure 3.

Figure 3

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Normalized PCE, FF, Jsc, and Voc over SE*h for PTVT:Y12, PTVT:ITIC4F, and PTVT:ITIC devices under different illumination conditions. The devices were illuminated using monochromatic LEDs (lines colored by the respective LED’s central wavelength) at different intensities (intensities distinguished by point shape, values given in SE). Intensities differed, depending on the device’s EQE and the relative position of the light spot during the experiment (for more details, see the Supporting Information). A clear wavelength dependence of the degradation was seen for PTVT:Y12 and PTVT:ITIC4F. PTVT:ITIC suffered from inherent instability. Only the first and the last data points for a given series are displayed for improved visibility. The plot with all points displayed can be found in the Supporting Information.

Unlike the PTQ10:Y12 cells, the PCE of the PTVT:Y12 cells increased over 500 SE*h when illuminated with red light and, during the course of the experiment, it also increased under green light. However, over 8000 SE*h, the PCE of the device degraded with red light showed a Jsc-induced burn-in-like degradation after the initial PCE increase. This Jsc degradation was not observed in PTQ10:Y12 cells, suggesting that PTVT as the donor material is likely responsible for this effect. The initial increase in PCE for these cases is attributed to an increase in FF, which might be due to an unoptimized layer microstructure or device architecture in combination with light soaking of the device during its initial operating hours. Besides, PTVT:Y12-based devices also showed a Voc degradation behavior under blue light illumination similar to those of the PTQ10:Y12 devices. More generally, the behavior of Voc was the same for both donors and a given NFA, which indicates that the selected NFA was responsible for the Voc degradation while it seems to have been unaffected by the choice of the donors.

The PTVT: ITIC4F-based and PTQ10:ITIC4F-based samples have in common that the burn-in like PCE and Jsc degradation only occurred under blue light illumination while devices monitored under red light exclusively experienced linear degradation. Additionally, an increase in PCE was visible for the PTVT:ITIC4F-based samples which is again attributed to an unoptimized microstructure/device architecture and light soaking of the device during its initial operating hours.

The similar overall degradation trend in performance parameters observed for PTVT:ITIC-based and PTQ10:ITIC-based devices underlines the inherent instability of ITIC-based OSCs. It further shows that the fluorination of ITIC into ITIC4F does mitigate the probably intrinsic instability of ITIC. However, the ITIC-family-based devices still suffered from PCE degradation even under red light illumination, which is absent for the Y12-based devices. Furthermore, it is worth pointing out that the PCE of the PTVT:Y12-based devices that were illuminated with blue light went below the 80% threshold within 100 SE*h while the PCE of the PTVT:Y12 devices illuminated with red light comfortably stayed above this threshold even after 8000 SE*h.

To check whether spectrally dependent degradation behavior is a general phenomenon or NFA-specific, additional experiments were carried out based on NFREA composites with the commonly used donor polymer PM6 as well as WF3, which had previously been used to fabricate some of the most stable polymer-based OSCs that have been reported up to this day.4 The results of these experiments are shown in Figure 4.

Figure 4.

Figure 4

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Normalized PCE, FF, Jsc, and Voc over SE*h for WF3:PTB4CL, PM6:PTB4CL, and PM6:PTIC devices under different illumination conditions. The devices were illuminated by monochromatic LEDs (lines colored by the respective LED’s central wavelength) at different intensities (intensities distinguished by point shape, values given in SE). Intensities differed, depending on the device’s EQE and the relative position of the light spot during the experiment (for more details, see the Supporting Information). A clear wavelength dependence of the degradation was seen for PM6:PTB4CL, and PM6:PTIC. WF3:PTB4CL remained unaffected by the illumination over 500 SE*h because of the drop in performance for the 457 nm, 10.6 SE device is believed to be a fabrication-related effect, and the device just degraded over time (see the reference experiment in the Supporting Information). Only the first and the last data points for a given series are displayed for improved visibility. The plot with all points displayed can be found in the Supporting Information.

The degradation trends observed for NFREAs were found to be quite comparable to the ones for NFAs. Figure 4 shows that light of shorter wavelengths was more harmful to the NFREA-based devices than light of longer wavelengths. Nevertheless, all devices could be stabilized with red light illumination and remained comfortably above the 80% threshold during 500 SE*h under 660 nm. WF3 was unique in terms of a massively reduced spectral dependence. The WF3:PTB4CL samples showed no obvious degradation during the first 200 SE*h, irrespective of whether illuminated with blue light or red light. However, after 200 SE*h under blue light, devices started to slowly degrade with linear kinetics. A control experiment at lower light intensity showed the same behavior when plotted over time rather than SE*h (Figure S14). The degradation of WF3:PTB4CL devices was not reported before, to the best of our knowledge. The fact that degradation is independent of light intensity suggests a degradation mechanism different than photodegradation, such as thermal instability, and will require additional studies.

PM6-based OSCs have recently proven to be stable under red light illumination when paired with Y6.19 Likewise, devices showed excellent stability when PM6 is paired with the NFREA PTB4CL or PTIC (Figure 4). Interestingly, the wavelength-dependent burn-in under blue light that had been reported earlier was not observed in these cases, if the sharp drop in intensity after SE*h = 0 for PM6:PTIC was interpreted as an annealing effect rather than a burn-in effect. Keeping the first drop out of consideration, the degradation of both the PM6:PTB4CL and PM6:PTIC was very comparable to that of each other except for a slightly more expressed Voc degradation of the PM6:PTIC samples. As a reference, P3HT:PCBM devices were fabricated that experienced wavelength-dependent degradation as well (Figure S16).

These findings further emphasize the interpretation of multiple mechanisms that were happening at once under illumination with white light and show that these mechanisms could be isolated with the help of spectrally resolved stability testing. To solidify this argument, multiple experiments were carried out with different white light sources and in different experimental setups. The results are compared with the results from the monochromatic experiments in the following section.

3. White Light Degradation Can Be Mimicked with Monochromatic Illumination

Figure 5 shows the results of photostability tests with PTQ10-based devices under MHL illumination. Additionally, long-pass (LP) 515 nm cutoff filters and luminescent down-shifting (DS) layers based on lumogen Y0853 have been used to investigate wavelength-dependent effects under white light illumination and evaluate the potential of luminescent DS layers for OSC stabilization. Furthermore, the cells were covered using a UV filter to exclude UV radiation as a possible degradation trigger (absorption spectra in Figure S15).

Figure 5.

Figure 5

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Normalized PCE, FF, Jsc, and Voc over time for PTQ10:Y12, PTQ10:ITIC4F, and PTQ10:ITIC devices under MHL illumination with UV filter, UV filter + DC layer, and UV filter + 515 nm LP filter. The average intensity during the experiment was estimated to be 0.4 SE, making 800 h equivalent to 360 SE*h.

For stabilization purposes, it could be seen that the LP filter functioned best, followed by the DS layer with just the UV filter coming in last. This proves two things: first, wavelength-dependent degradation was present under white light illumination, as also demonstrated earlier for different OSC systems.21 However, we stress that the wavelength dependence was correlated not only to UV light as previously thought but also to the visible part of the spectrum. Second, a DS layer can improve OSC stability which opens up an interesting pathway for future engineering via optimizing DS layers to balance high PCE and stability in parallel and to maximize the lifetime energy yield of future devices. From the first measurements of this series, it was estimated that the intensity during the experiment was around 0.4SE for the unfiltered device. This makes t = 800 h shown in Figure 5 comparable to SE*h = 360 shown in Figure 2. We observed a remarkable qualitative similarity under MHL conditions when compared to monochromatic illumination, with the unfiltered device showing similar degradation kinetics as under blue light. That was further corroborated by the degradation behavior of LP- or DS-filtered devices that mimiced the one under monochromatic green or red light. We highlight two exceptions from that trend, which may require further detailed investigations. They are the burn-in behavior of Voc for the PTQ10: ITIC devices and the absence of the Jsc burn-in for the PTQ10:ITIC4F devices. However, due to the overall high comparability of the degradation trends reported in Figures 2 and 5, it is worth pointing out that filtered white light degradation phenomena can be very well mimicked by monochromatic degradation under LED illumination. This is highly attractive, as all the insight from the 800 h white light experiment reported in Figure 5 could have been obtained at least an order of magnitude faster by a monochromatic high-intensity experiment with the lamp intensity and the cooling capabilities of the experimental setup being the limiting factors for acceleration. Finally, it is worth highlighting that the data presented in Figure 5 was generated on an entirely different experimental setup than the rest of the data presented in this manuscript, which further underlines the generality of the observed effects.

Another method for observing and distinguishing multiple degradation mechanisms is to irradiate the same sample with different light sources after each other, as can be seen in Figure 6. Here, a PTQ10:ITIC4F-based sample was illuminated with red light at first, showing its characteristic linear degradation, which was immediately followed by a burn-in once blue light illumination was added.

Figure 6.

Figure 6

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Normalized PCE over SE*h for PTQ10:ITIC4F devices under different illumination conditions. Left: the sample was degraded with 660 nm red light, and 457 nm blue light was added at a later stage. It shows that additional wavelength-dependent effects can be triggered even after an initial illumination step with less harmful light. Right: comparison of the degradation under monochromatic blue LED and a white LED illumination. The overall great similarity shows the dominant influence of high energy photons in the white light LED’s spectrum on the degradation of the device. In the right plot, only the first and the last data points for a given series are displayed for improved visibility. The plot with all points displayed can be found in the Supporting Information.

In the tests, blue light was almost always sufficient to capture all the degradation kinetics of a device under white light illumination, as can be seen from the comparison of the PTQ10:ITIC4F devices’ degradation under blue and white LED light in Figure 6. Small deviations in the curve shape, especially during the burn-in period do not diminish the validity of the statement – simulated AM1.5 white light degradation behavior can be mimicked by blue LED illumination.

This is further corroborated by analyzing the photon flux of several LEDs at 1 sun condition for a PTQ10:ITIC4F cell in relation to the AM1.5G spectrum in different energy intervals (Figure 7). It clearly shows that the acceleration factor for degradation experiments can be much higher than the SE value if a certain energy interval is of interest and if the device degradation is independent of the illumination intensity. For example, PTVT:Y12 stayed above the 80% relative PCE threshold for the tested 8000 SE*h, which would correspond to over 50 000 h under real-world conditions if the illuminating light’s spectrum was restrained to >660 nm since the SE*h value does not account for the relative emission strength in a given energy interval.

Figure 7.

Figure 7

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Photon flux of different LEDs in relation to AM1.5 g in different energy intervals at 1 SE condition for a PTQ10:ITIC4F OSC. Photon fluxes were calculated as a convolution of the relative LED spectra and the sample’s EQE, weighted by the total amount of photons converted into current at one sun condition (details in the Supporting Information). Under the assumption that the degradation scales approximately linear with photon dose, the similarity of the curves from Figure 6 can only be explained if the highest energy photons dominate the degradation kinetics.

When comparing the spectra of the blue and the white LED, it becomes apparent that the photon flux of the blue LED is around five times higher around its peak wavelength than the photon flux from the white LED, while the photon flux of both LEDs is very similar to the highest energy photons they emit. As this five-time difference is not reflected in the degradation curve, it can be concluded that the degradation behavior of OSCs is mainly determined by the highest energy photons in the illuminating light’s spectrum. In particular, the burn-in-like degradation seems to be strongly photon energy dependent as it requires a minimum energy to occur in the studied systems. A possible reason for such burn-in-like photodegradation was recently brought into discussion with the observation of triplet states in Y6 that originate from a singlet-fission process once photons approach an energy of twice the bandgap of the device (in the case of Figure 7: Eg = 1.59 eV, calculated from EQE (Supporting Information)).30

However, the PTVT:Y12 samples showed that lower energy photons can also trigger a burn-in-like behavior although this only was recognized after several thousands of operating hours. In any case, the burn-in-like degradation was always due to a burn-in either in Jsc or FF. Voc remained stable for the ITIC family acceptors and could also be prevented in OSCs with Y12-acceptor by using low-energy photons for degradation. Furthermore, the stability of Voc was predominantly determined by the choice of the acceptors and was independent of the paired donor. The ITIC family showed promising results in this regard, but due to the Jsc degradation in these devices and the fact that the Voc degradation in Y12-based devices could be prevented by blocking high energy photons, Y12 has proven to be superior to the ITIC family if photostability is of interest.

The experiments showed that stability tests with blue LEDs under high irradiances were able to reproduce the leading degradation kinetics of multiple state-of-the-art OSC composites under white light illumination. This finding is of great importance for future stability tests as it enables a new pathway of accelerated stability testing of pristine photodegradation. Additionally, the usage of blue LEDs makes the comparison of results between different laboratories much more reliable, because the spectral composition of a white light source is more difficult to compare and of utmost importance in determining the degradation kinetics as is demonstrated in this paper. Furthermore, all the systems (except ITIC-based systems, which were inherently unstable and experienced performance losses even in N2 atmosphere in the dark) degraded significantly more slowly and showed different degradation behavior when illuminated with red light compared to blue light, with some of them remaining completely stable throughout the experiments. These findings prove that the photodegradation of OSCs is generally wavelength-dependent, even in the visible light spectrum, and severe degradation can be avoided in many systems by restraining the incoming light’s spectrum, making it a powerful tool to greatly improve the photostability of OSCs in the future.

Another aspect of photostability that needs to be mentioned is its dependence on device architecture. In general, OSC stability and performance are not only material dependent but also depend on processing conditions and architecture. For example, a previous study showed that the ETL can significantly influence the stability by comparing the ETLs ZnO and SnO2 paired with PM6:Y6 as the active layer.19 There, it was also shown that ZnO does not absorb light of the wavelengths in question, which is why it is believed that the effects are not pristine ZnO effects. While a pristine ZnO degradation seems unlikely, an interaction between different layers of the OSCs that leads to the degradation cannot be excluded and needs to be investigated in the future. However, there are multiple reasons why the findings in this manuscript are considered valid for OSCs in general and not only for OSCs in the tested architecture. First, if the stack was intrinsically unstable, the cells would have also degraded under an inert atmosphere in the dark. Except for the ITIC-based samples, this was not observed. Second, assuming the architecture was the leading cause of the degradation, the degradation still would have been wavelength-dependent beyond the typical UV-induced degradation and the results would have shown that even an OSC with an unstable architecture can be stabilized by restraining the light spectrum. Furthermore, this would raise the question of why the WF3 cells were initially stable, independent of the light source. Finally, the overall variety of the observed degradation phenomena showed a significant influence of the active layer material and the illumination wavelength and therefore showed the relevance to OPV in general without being restrained to the architecture used.

4. Conclusion

Wavelength-dependent in situ photostability tests of ten different OSC systems were performed. The systems were chosen in a way to represent a broad variety of OPV materials to investigate the nature of wavelength-dependent photodegradation and whether it occurs in only some OSCs or if it is a characteristic intrinsic behavior in OPV. The results proved that the photostability of OSCs is wavelength-dependent across the visible light’s spectrum, revealing different degradation mechanisms related to different wavelengths. We report material composites such as those based on WF3 where wavelength-dependent degradation is significantly suppressed. Great improvements in stability can be achieved by restraining the visible light spectrum to longer wavelengths above 550 nm or even 600 nm as well. The magnitude of the stability gain is generally dependent on the cutoff wavelength in a non-step function type way. This increase in photostability was demonstrated not only for monochromatic illumination under high irradiances but also for one sun MHL illumination with a LP filter and a luminescent DS layer respectively, as well as for white LED light and across different experimental setups. It was found that the degradation kinetics of an OSC under white light illumination are mostly determined by the highest energy photons in the spectrum. Furthermore, it was shown that the degradation kinetics under white light LED illumination could be reproduced with a blue LED. This finding is of great importance for future stability testing studies as it enables a new pathway of accelerated stability testing, better comparison between laboratories, and higher acceleration factors due to the lack of low energy photons that contribute little to the photodegradation but still increase heat, which is the limiting factor for high-intensity stability testing. In addition, the usage of blue LEDs for stability tests is expected to reduce setup building and maintenance costs.

5. Materials and Methods

One factor that influences the stability of an OSC is the production process. Therefore, the stability of OSCs from different production processes must be considered when conclusions on OSC stability in general are of interest. In this paper, some cells were built by the automated OSC processing platform AMANDA31 after an internal optimization routine, while others were built manually either with optimized processing conditions or by using educated guesses for the processing parameters for jet unoptimized systems. Using this approach, the data set presented in this paper offers a broad range of processing conditions and OSC PCEs ranging from around 3% to just below 12%, which, in combination with the ten different material systems, makes this data set representative of OSC stability in general.

Combinations of different donor- and acceptor materials have been used in this study to represent different families of OSC active layer materials (full names and formulas can be found in the Supporting Information). PM6 serves as one state-of-the-art high-performance donor material and has already shown wavelength-dependent degradation behavior when paired with Y6.19,32 PTQ10 and PTVT (PTVT-T) are state-of-the-art low-cost donor polymers with performance comparable to PM6 when paired with a matching acceptor,24,33,34 which is why they were used as representatives of donor materials for future industry applications. WF3 was chosen as another donor material because it has produced some of the most stable devices up until today.4 P3HT in combination with PCBM is one of the most studied systems from the early stages of OPV research and is served as a representative of the fullerene-based OSC compounds. Being one of the first reported nonfullerene acceptors (NFAs) for OSCs, ITIC serves as the traditional representative of the NFA class, with ITIC4F (IT-4F) being a fluorinated derivative of ITIC lower HOMO and LUMO levels.35 Y12 (BTP-4F-12) was used as a representative of the state-of-the-art NFA-Y family, from which some of the highest PCEs up to date have been reported.2 Finally, PTIC and PTB4CL (PTB-4CL) are part of the recently developed nonfused-ring-acceptor (NFRA) family, which have proven to be cheaper alternatives to state-of-the-art NFAs with decent performance and serve as representative acceptors for future OSCs.28

Data Availability Statement

All data shown in the graphs of this article and the Supporting Information can be provided by the author Paul Weitz upon reasonable request.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.4c03446.

  • Calculation of acceleration factors, details on the experimental setup and thermal management, molecular structures and full names of the used semiconductors, degradation of WF3:PTB4CL samples over time, transmission spectra of the used filters, wavelength-dependent degradation series of P3HT:PCBM as a reference, Supporting Information Figures 1,2,3, and 5 with all the data points displayed, table of initial device performances (PDF)

The authors want to thank the Deutsche Forschungsgemeinschaft (DFG) for financial support in the scope of the SFB 953 (182849149) and the Research Unit FOR 5387 POPULAR, project no. 461909888. We gratefully acknowledge financial support through “ELF-PV—Design and development of solution-processed functional materials for the next generations of PV technologies” (no. 44–6521a/20/4) and the “Aufbruch Bayern” initiative of the state of Bavaria (EnCN and SFF). C.J.B. gratefully acknowledges the financial support through the Bavarian Initiative “Solar Technologies go Hybrid” (SolTech). This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement no. 952911 (BOOSTER) and no. 101007084 (CITYSOLAR).

The authors declare no competing financial interest.

Supplementary Material

am4c03446_si_001.pdf (1.7MB, pdf)

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

am4c03446_si_001.pdf (1.7MB, pdf)

Data Availability Statement

All data shown in the graphs of this article and the Supporting Information can be provided by the author Paul Weitz upon reasonable request.

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