• Stability comparison of OPVs encapsulated in glass–glass or plastic–plastic.

Organic solar cells (OSCs) have received great research attention due to their potential low cost, light weight, and flexibility, making them a promising emerging technology. In recent years, their power conversion efficiency (PCE) has increased significantly, reaching up to 18% for full organic cells and up to 12% for tandem architectures (You et al., 2013; Liu et al., 2020), which are recent record efficiencies in the respective technologies (Best-Research-Cell-Efficiencies, 2019). In order to achieve these efficiencies, the active layer materials used in the bulk heterojunction play a fundamental role: a polymer or small molecule acts as an electron donor (P3HT is the benchmark donor) and fullerene derivatives, such as PCBM, or non-fullerene acceptors (NFAs), such as O-IDTBR, act as the acceptors (Wadsworth et al., 2019). Ternary blends combine fullerene and non-fullerene acceptors, both in single junction and in tandem monolithic cells (Xu and Gao, 2018).

After several years in the research stage, organic technologies have reached the market, being viable and attractive for many applications, such as mobile applications, IoT, and building-integrated photovoltaics (BIPV). OSCs can be processed by roll-to-roll (R2R) techniques, easily scalable toward industrial manufacturing (Pascual-San-José et al., 2019), with some examples reaching module areas of 64 cm² with PCE of 5% on ITO glass substrates using glass encapsulation (Strohm et al., 2018). In many cases, the use of ITO has been avoided and flexible substrates have been preferred, in order to bring costs down and further improve the industrial attractiveness of non–fullerene-based OSCs. Additionally, laboratory roll-processed OSCs with active areas of 1 cm² have reached an impressive 12.16% PCE, reinforcing the potential of this processing method for OPV (Wang et al., 2020).

NFAs have attracted the attention of researchers because they combine stability, good light absorption, easy synthesis at low cost, and compatibility with the use of solvents that are non-harmful for the environment (Nielsen et al., 2015; Pascual-San-José et al., 2019). One successful family of NFAs is indacenodithiophene (IDT) and indacenodithienothiophene-based acceptors, particularly those benefiting from the planarization obtained by the IDT unit, which allows better aggregation (Holliday et al., 2016). OSC devices with the NFAs O-IDTBR in blend with P3HT have reached efficiencies of up to 6.7% on ITO-coated glass substrates (Pascual-San-José et al., 2019; Holliday et al., 2016; Gertsen et al., 2020; Jørgensen, Norrman, Gevorgyan, Tromholt, Andreasen, Krebs). Additionally, this type of material, as well as the non-chlorinated solvents used for solution processing, enables serial R2R manufacturing and allows OSC processing to be scaled up to an industrial manufacturing level with low environmental impact (Pascual-San-José et al., 2019).

However, non–encapsulated OSCs have some stability limitations when compared with crystalline silicon or inorganic thin-film technologies, which highlights the importance of studying the encapsulant materials and methods (Gertsen et al., 2020). Organic materials degrade quickly due to external environmental agents, mainly oxygen and moisture, as well as UV radiation (Jørgensen, Norrman, Gevorgyan, Tromholt, Andreasen, Krebs; Balderrama et al., 2014; Córcoles et al., 2015). The degradation causes an alteration of the electrical properties of the cells such as PCE, open circuit voltage (V_OC), short-circuit current density (J_SC), and fill factor (FF). In addition, the burn-in degradation effect (when the solar cell rapidly reduces its PCE by degradation of several electrical parameters, mainly photocurrent, in the first hours of work) is strongly reduced when NFAs are used in the solar cells, with just around 5% relative PCE loss during the first 2,000 h compared to almost 34% in only a few hundred hours in fullerene-acceptor solar cells (Jørgensen, Norrman, Gevorgyan, Tromholt, Andreasen, Krebs).

In recent years, the study of stability and degradation in OSCs has been crucial and focused mainly on increasing their stability and consequently on improving their PCE throughout their lifetime (Jørgensen, Norrman, Gevorgyan, Tromholt, Andreasen, Krebs). A set of protocols called ISOS (Reese et al., 2011) was proposed in order to create standard methodologies, delivering results that could be comparable between different laboratories around the world. These protocols, although not mandatory, are recommended covering a wide range of different variables and environmental conditions to be measured such as oxygen, humidity, ultraviolet radiation, and temperature.

Meitzner et al. (2019) have compared different commercial P3HT batches to study their properties and layer architecture in stress conditions using different ISOS protocols, such as ISOS-L-2 (laboratory weathering), ISOS-O-1 (outdoor aging), and ISOS-D-2 (dark aging). Urban et al. (2019) investigated the degradation of the OSC contacts under very stressful thermal conditions, from −40 to 85°C. Their results show the evolution of the series and shunt resistances due to mechanical stress applied on the contacts. In order to show the effect of UV illumination, mechanical stress, and encapsulation, Weerasinghe et al. (2016) carried out several tests on large-area R2R-processed OSCs under continuous illumination, also including fracture tests. Their results demonstrated better lifetime and better mechanical stability for the encapsulated devices. Encapsulation is therefore a critical component in photovoltaic devices, with strong impact on their lifetime, because it is responsible for protecting the devices from external environmental conditions, including UV radiation. Uddin et al. (2019) enumerated a list of the most important encapsulants in OSCs and their influence on the degradation process. Although no encapsulation procedure is declared as the best one, it is noted that good temperature control to avoid damaging the organic components, as well as an R2R solution process deposition of thin multilayers can be beneficial, also providing a low-cost solution to isolate and protect the device.

In this article, we report a detailed stability study of fully R2R processed ITO-free OSCs using three fast aging tests comparing plastic and glass encapsulation. The devices were tested under different conditions: a weathering chamber with ISOS-LT-1, ISOS-T-3 using a thermal chamber, and ISOS-L-1 (Table 1). The results were compared with previously reported stability data for similar cells (Gevorgyan et al., 2008; Reese et al., 2011; Dos Reis Benatto et al., 2016; Barreiro-Argüelles et al., 2018; Uddin et al., 2019). The main purpose of the study presented in this article is the comparative impact of encapsulating P3HT:O-IDTBR OSCs in glass–glass or plastic–plastic envelopes. Whereas the R2R-compatible plastic–plastic encapsulation preserves the flexibility of the solar cells, the glass–glass encapsulation is widely used by the research community mainly because most of the reported devices are built on ITO substrates, for which a glass–glass encapsulation is straightforward. Therefore, the results presented here are attractive for both research laboratories working on a small scale and research teams aiming to scale-up the production of solar cells regardless of the use of organic, inorganic, or hybrid (e.g., perovskite) photoactive materials. The study on the lifetime of ITO-free, R2R-compatible open-air–produced OSCs with PCEs >3% is reported in this article, where the lifetime essays have reached more than 1,000 h in the devices under ISOS-L-1. Each of the different accelerated degradation systems has been tested using procedures as recommended by the ISOS protocols (Reese et al., 2011).

Three different accelerated degradation systems have been used to perform a detailed degradation experiment to test the impact of different encapsulation techniques on the performance and lifetime of the devices. We used weathering chambers to simulate accurate irradiances, extreme temperatures, humidity, and storage conditions in darkness. These chambers are used by test centers to produce reliable and reproducible performance reports depending on the storage and operating environment, which are mandatory to bring this kind of OSC closer to commercialization. For the continuous lighting system, a solar simulator calibrated to standard AM1.5G irradiance has been used. Illuminated and dark current density vs. voltage curves (J-V), external quantum efficiency (EQE) of the OSCs, and UV-vis transmittance data of the encapsulants are reported, in order to study the performance of the OSCs and compare their optical aging.

The key electrical parameters of the solar cells are presented in Table 4. V_OC, J_SC, FF, and PCE were analyzed as a function of time during the degradation process. The list of electrical parameters, depending on the kind of encapsulation and time T₈₀, when a drop of 20% of the initial PCE is measured, is also included in the last column of Table 4.

The solar cells showed lifetimes reaching up to T₈₀ = 86 h (ISOS-T-3) in an accelerated degradation process. The evolution of the J-V curves of different tests is summarized in Figure 2. For the ISOS-LT-1 test, Figure 2A and Table 4, both plastic and glass encapsulation J-V curves show similar behavior with similar T₈₀ at 12 and 14.5 h, respectively. In the thermal stress experiment (ISOS-T-3), the plastic encapsulation has a better long-term behavior (>700 h) than the glass encapsulation (400 h) as shown in Figure 2C, although T₈₀ is slightly better for glass (Table 4). It might be because the coefficient of thermal expansion of the plastic encapsulants is closer to that of the OSCs, with a stronger influence during mid-life of the cells (after T₈₀ has been reached). This makes plastic encapsulants the material of choice when the OSCs might experience below zero temperatures when installed in the field. In the continuous light illumination test (ISOS-L-1), the main difference between both encapsulations is that T₈₀ is almost three times larger for glass than for plastic encapsulation, with a similar long-term degradation behavior of the J-V curves (Figure 2B).

In general, the open circuit voltage (V_oc) does not follow a clear tendency in the different degradation experiments (Figures 3A, 4A, 5A), since V_oc is not a reliable parameter to study the degradation processes. The decrease of the PCE and current density (Figures 3–5) is strong for all OSCs, the latter indicates a loss of photogenerated carriers (considering that absorption and EQE are similar in all cases as shown below). The FF had also decreased (Figures 3B, 4B, 5B), in coincidence with a simultaneous increment in the series resistance (Table 5). This is due to contact degradation and reduction of shunt resistance (indicative of increased bulk recombination) produced by the degradation of the active layer by exposure to oxygen due to poorer encapsulation (Urban et al., 2019). The loss of photocurrent between the measurements is more pronounced in the ISOS-LT-1 test (weathering chamber) than in the ISOS-T-3 test (thermal chamber), probably due to harsher lighting and humidity stress cycles applied to the solar cells there.

Figure 3 shows the evolution of the key photovoltaic parameters obtained for the ISOS-LT-1 test (solar weathering chamber). They provide detailed information about the degradation process of the OSCs and the influence of the different encapsulation materials. The PCE is dependent on the evolution of the other parameters such as J_SC and FF. The values shown in the graphs have been normalized to the initial value, and exponential fits to the J_sc and PCE data have been included. The FF behavior is linked to the series and shunt resistances evolution induced by the degradation experiment. The drop in PCE suffers the so-called “burn-in” effect, which makes it rapidly and drastically decrease, only to stabilize later. This is more pronounced in the glass encapsulation device, meaning that this might be caused by ingress of oxygen and moisture through the edges of the samples where the two glass encapsulants are glued by epoxy. In fact, the rigidity of the glass combined with the manual processing may explain a different sealing along the four edges due to unbalanced pressure. This technique results in a poorer sealing in one of the edges. The plastic encapsulation is carried out using a laminating machine, thus providing a more uniform sealing along all the edges of the sample. In all the cells, the degradation curves of the PCE and J_sc are fitted using exponential functions (single or biexponential); however, only the fittings of J_sc are analyzed in detail, since this parameter gives better information of the degradation mechanism than PCE. We want to highlight that linear functions have been also tested, but the fit with the greater coefficient of determination (COD) or R-square has been obtained for exponential functions (De Bettignies et al., 2006).

The single exponential is: J S C = A 1 e - k 1 t ,

and the biexponential is: J S C = A 1 e - k 1 t + A 2 e - k 2 t , where, k is the so-called degradation constant. For the ISOS-LT-1 test (Figures 3C,D), a single exponential behavior is observed for both encapsulations. This same behavior points out to temperature as the main degradation mechanism (Schuller et al., 2004; Gevorgyan et al., 2008; Djeddaoui et al., 2019).

The ISOS-T-3 test (thermal degradation process) results are shown in Figure 4, and we again observe the variations in V_OC and FF, with the drastic drop in V_OC for the glass device. For J_SC, a biexponential fit for plastic and a single exponential fit for glass (Figure 4C) is used for this system, showing, in this case, two different mechanisms. According to the literature (Schuller et al., 2004; Gevorgyan et al., 2008; Djeddaoui et al., 2019), these two mechanisms are closely linked to temperature and humidity. The plastic-encapsulated device shows a better stability than the glass-encapsulated device, reaching up to 700 h. This behavior seems to indicate that the plastic encapsulation provides better isolation from ambient conditions (less oxygen and moisture ingress into the device), better thermal cycling (from −40 to 85°C), and mainly a superior manipulation resilience. In this case, two exponential degradation processes are present. The second one is much slower than the first one. This one can be attributed to the effect of UV light, in contrast to the first process, which was due to ambient conditions (ingress of moisture and oxygen into the cell).

The evolution of the normalized key photovoltaic parameters of the solar cells subjected to the ISOS-L-1 degradation test (continuous illumination) is shown in Figure 5, showing the difference between the plastic and glass encapsulation. When considering the degradation of V_OC, the difference between glass- and plastic-encapsulated devices increases strongly as the process evolves in time. Figure 5 shows a similarity in the evolution of J_SC and PCE. J_SC decays faster at the beginning and then slows down its decay to almost zero current for both plastic and glass. In the case of Figure 5A, it is observed that the trend is inverse to the other two, where plastic shows better behavior compared to glass. The possible cause of this is due to the breakage or poor sealing of the plastic during the encapsulating process.

It should be emphasized that both cases of exponential fit indicating double or single processes provide, respectively, a good fit for J_SC and PCE for glass or plastic encapsulation in the ISOS-L-1 tests. In every degradation test that was carried out, glass encapsulation achieved longer lifetimes for the OSCs except for the thermal chamber (ISOS-T3) where plastic encapsulation was a better choice to extend the working life of the OSCs due to the extreme temperatures of the test. The degradation constants k₁ and k₂ obtained from the J_SC fits for all tests are shown in Table 6.

These degradation constants are used to compare different degradation tests, since each of them is closely related to a fundamental parameter in stability processes. k₁ has been related mainly to temperature, whether a linear, simple exponential, or biexponential fit is carried out. This constant is usually mainly responsible for the degradation; k₂, on the other hand, is usually related to humidity, another important parameter, but one that is usually present in some specific conditions. The degradation rates included in Table 6 are similar to those reported by Gevorgyan et al. (2008) when the cells are encapsulated, and the slower degradation process is attributed to the effect of UV light. Only in the ISOS-T3 essay for the plastic-encapsulated cell, a biexponential behavior was observed, with a second degradation rate even lower, pointing to a second slower UV-driven degradation process triggered by the effect of temperature.

According to Table 6, it can be observed that even though ISOS-LT1 has a stage of high humidity in darkness, it does not seem decisive for the rapid degradation of the samples, since it is more strongly dependent on the temperature reached because the different irradiances tested are predominant.

On the other hand, in the ISOS-T3, where the thermal stress is very strong, k₁ is lower and also shows a very small k₂ in the plastic encapsulation system, which indicates that it is probable that some moisture from the stages of extreme cold and heat has penetrated in to the device due to a break in the encapsulation. From then on, it becomes too difficult or unrealistic to fit the data resulting in a fit with low R ².

Finally, when continuous lighting ISOS-L1 test is carried out (Figure 6), where the temperature is controlled but humidity is not (room conditions), a single exponential behavior is recorded. On the other hand, the plastic encapsulation sample shows the highest k₁.

To analyze in detail the different behaviors of the encapsulations, further EQE measurements have been carried out to explore photoelectronic properties of the OSCs and compare the effects of extrinsic degradation agents (Meitzner et al., 2019), such as photo oxidation and possible oxygen and moisture ingress when using two different encapsulants. Figures 6, 7 show the evolution of the EQE at different stages of the degradation ISOS-LT-1 and ISOS-T-3 tests. Considering that the thickness of the absorber layer is the same in all cases during the tests, P3HT degrades faster than O-IDTBR. As seen in Figure 6A, for glass encapsulation and ISOS LT-1 test, the EQE photocurrent of the spectral region where P3HT is expected to contribute (450–650 nm) decreased faster than the EQE photocurrent at the O-IDTBR main contribution region (650–800 nm) (Pascual-San-José et al., 2019). It indicates that in glass encapsulation, P3HT degrades faster than O-IDTBR during the ISOS LT-1 test, which simulates illumination of day cycles with above-zero temperatures. In plastic encapsulation (Figure 6B), the photocurrent decreases more uniformly over the spectral range and time. In Figures 6, 7 the local maximum around 420 nm corresponds mainly to the photocurrent contribution of the secondary absorption peak of O-IDTBR located at 400 nm that in blend with P3HT shifts to 420 nm (Fernández Castro et al., 2020). In Figure 7, the EQE photocurrent at the O-IDTBR contribution regions decreases at a slower pace than the photocurrent at the P3HT region of contribution. It might be indicative of microstructural modifications and possible phase separation for which further studies are necessary (see Supplementary Figure S2). As shown before during the ISOS-T-3 experiments, where the solar cells experience cycles from −40 to 85°C, the plastic encapsulation protects better the solar cells from the environment, allowing the continuation of the experiment for more than 700 h (Figure 2) with a stable photocurrent (∼5 mA/cm²) and V_OC (∼0.55 V). Figures 6, 7 also verify that materials such as O-IDTBR can reduce the burn-in effect produced at the beginning of the cell operation, when compared to cells with materials such as PCBM in the active layer. Additionally, the PEDOT:PSS is also degraded by moisture since it is a highly hygroscopic material. This ingress of moisture in the cell might have happened, most probably through the edges of the sample, thus contributing to the degradation of the charge extraction layer.

Losses in photocurrent are also possibly due to degradation of the electrodes or the encapsulant, which will affect the measured photogenerated current. The series resistance of the devices increases during the degradation experiments in almost all combinations of encapsulation and degradation test protocols. The increment in R_S is an indication of internal deterioration of the layers and interfaces, complemented by a probable surface recombination in the electrode/transport layer interfaces, which in this case contributes to the lowering of the shunt resistance. Also, an increase in the reverse saturation current of the solar cells is observed, as seen in the dark current before and after degradation of the solar cells shown in Supplementary Figure S3.

Figures 8, 9 show the transmittance evolution over time for both kinds of encapsulation of the OSCs. For the ISOS-LT-1 degradation test (Figure 8), external factors influence the degradation of the encapsulation. Different irradiation times produce incremental photon losses in the transmittance of the glass/epoxy/glass encapsulation, especially in the visible region. This photo degradation of epoxy glue turns it yellowish and possibly reducing its capability as a moisture barrier. For the plastic encapsulation in Figure 8B, after a minor change in transmittance in the first hours of illumination, the transmittance remains constant up to more than 700 h. In the thermal chamber experiment (ISOS-T-3), shown in Figure 9, the differences between the glass and plastic are less noticeable. Specifically, the plastic is practically intact (Figure 9B) while the epoxy in the glass–glass encapsulation stops acting as a UV barrier (transmittance increase around 300 nm). This allows the epoxy to also turn yellowish as a consequence of extreme temperature cycles in dark. Nevertheless, both experiments indicate that the plastic encapsulation provides better protection against environmental degradation agents. This fact may be mainly due to the encapsulation method. For the case of plastic, both sides are laminated, passing the OSCs between two pressing rolls. In contrast, for glass, the sealing is done with tweezers holding the two glass slides, resulting in a probably less homogeneous sealing with a bigger amount of epoxy trapped between the two glasses.

Bulk heterojunction OSCs based on P3HT:O-IDTBR have been fabricated to study the effect of different materials (glass or plastic) as encapsulants. The OSCs have been characterized by J-V, EQE, and UV-vis transmittance measurements during the degradation tests carried out according to recommended ISOS protocols. An exponential degradation rate has been found, with bimodal behavior for plastic in ISOS-T3. The J-V parameters and EQE and UV-vis spectra evolution point to a worsening of the carrier transport and to an increased series resistance as the main causes for the degradation of the solar cell PCE. The longest achieved T₈₀ lifetime is 86 h for glass–glass encapsulated devices under an accelerated ISOS-T3 protocol. The spectral evaluation of the degradation tests indicates that P3HT degrades faster than O-IDTBR during tests with simulated sunlight. We also found that plastic is a better long-term encapsulant, likely reducing ingress of oxygen and humidity into the cell when compared to glass encapsulants carried out, respectively, by hand. The ingress of moisture might affect strongly the PEDOT:PSS/Ag and Ag:PEDOT:ZnO transporting layers, and electrodes as PEDOT:PSS is highly hygroscopic that when degraded, the transport properties are worsened. It is reflected in an increment in the series resistance of the equivalent circuit of the solar cells. Finally, the optimization of the encapsulation process and the use of alternative donors seem to be more important than the encapsulant material itself in order to provide longer lifetimes. The use of NFAs in the active layer together with an improvement of the encapsulation techniques open the possibility of longer lifetimes for bulk heterojunction solar cells.