The limited operational lifetime of organic solar cells remains an obstacle to their commercial development and is largely due to the poor intrinsic photostability of the conjugated molecules that constitute the photoactive layer. Here, we selected a series of state-of-the-art donor and acceptor materials including PBDB-T, Y5, PF5-Y5, and PYT to study their photostability under AM1.5 simulated sunlight in ambient conditions. Their properties are monitored over time, using various spectroscopy techniques, including UV-Vis absorption, Fourier-transform infrared (FTIR), and X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). We found that the absorption spectra of Y5 and PYT films remain almost intact even after 30 hours of light exposure in air, while the PF5-Y5 and PBDB-T films undergo rapid photobleaching. The absorption losses observed in blend films of PBDB-T with Y5 and with PF5-Y5 can be understood as composed of contributions from the separate blend components that are similar to the absorption losses in neat films. The new peaks emerging in the FTIR spectra of PBDB-T, PF5-Y5, and their blend films witness the formation of new carbonyl groups, while these are absent in the spectra of the Y5 and PYT films. The XPS C 1s spectra of the PF5-Y5 and PBDB-T films confirm this carbonyl formation and the S 2p spectra reveal that sulphone groups are formed after 30 hours of exposure of these films. These results confirm that films of Y5 and the copolymer PYT are significantly more resistant to photooxidation, compared to the copolymer PF5-Y5. The comparison of these results suggests that the benzo[1,2-b:4,5-b ']dithiophene moiety with alkylated thiophenes as side chains (BDT-T) accelerates the photodegradation of PBDB-T and PF5-Y5. The replacement of the BDT-T unit by thiophene contributes to the enhanced stability of PYT, demonstrating that the nature of the co-monomer has a significant effect on the intrinsic photostability of Y5-based copolymers. These new insights are expected to stimulate the design of stable donors and acceptor polymers for the development of long-lived OPV devices. Absorption spectra show the photobleaching of acceptor copolymer PF5-Y5. The replacement of BDT-T by thiophene strongly improves the photostability.

The lifetime of organic solar cells critically depends on the photochemical stability of the materials. To shed light on the photostability of novel Y-series electron acceptors, we investigate the evolution of optical properties and composition during one-sun illumination in ambient atmosphere of thin films of the small-molecule acceptor Y5 and its copolymers PF5-Y5 and PYT. We employ UV-vis, Fourier-transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS), to assess changes in these properties as a function of illumination time. UV-Vis spectra show that PF5-Y5 undergoes rapid photobleaching, while the Y5 spectrum remains essentially unaffected even after 30 hours of exposure. The absorption spectrum of PYT, which contains a different co-mer than PF5-Y5, is only weakly affected. XPS C1s spectra of the PF5- Y5 film show a decreasing main peak and the development of a new component after 30 hours exposure, while the Y5 film surface composition remained intact. The photodegradation products of PF5-Y5 are characterized by the presence of new carbonyl groups, emerging as absorption bands in the FTIR spectra, while such spectral changes are absent for the Y5 film, indicating that Y5 is resistant to photooxidation, while PF5-Y5 undergoes photochemical reactions. The faster photodegradation of PF5-Y5 compared to Y5 and PYT raises the question about the role of the copolymer’s BDT moiety in the photooxidation. These new insights on the dependence of the photostability of acceptor molecules on their molecular structure are expected to contribute to the design of stable acceptor copolymers for organic solar cells with long operational lifetimes. 

Despite the rapid advancements in the performance of organic solar cells (OSCs), improving their operational lifetime remains a significant challenge. The photodegradation of donor polymer PM6, small molecule non-fullerene acceptor (NFA) Y6, and their blend was investigated under ambient conditions. To photodegrade the spin-coated thin films, samples were exposed to AM 1.5 illumination, as well as UV-filtered and long-wavelength-filtered light. The evolution of their properties upon increasing the exposure time up to 45 h was monitored using UV-vis absorption, Fourier transform infrared (FTIR), and photoemission spectroscopy. The results demonstrate that neat PM6 films exhibit faster absorbance loss than the neat Y6 films. This is accompanied by the formation of new carbonyl groups on PM6, while only minor indications of photooxidation were observed in degraded Y6 films. The valence band spectra of Y6 remain unchanged upon photodegradation. Interestingly, the photobleaching rate of Y6 in PM6:Y6 blend films was found to be higher than that of neat Y6 films. XPS spectra of C 1s and S 2p confirm that photooxidation products formed in PM6 and PM6:Y6 films, evidenced by new oxidized carbonyl C 1s and oxidized sulfur S 2p peaks. Under AM 1.5 illumination, several photooxidation pathways can be active, involving the formation of both superoxide radicals and singlet oxygen species and their subsequent oxidation reactions with conjugated molecules. Using filtered light conditions, these different degradation pathways could be separated. Upon exposure to long-wavelength-filtered light, which is predominantly absorbed by the Y6 acceptor, the generation of superoxide radicals is significantly suppressed, resulting in enhanced photostability of the blend compared to illumination with unfiltered light. The remaining photodegradation of the blend components under these illumination conditions can therefore be ascribed to energy transfer from the photosensitizing acceptor, feeding into the singlet oxygen formation. These insights could inspire the design of new donor and acceptor materials with improved photostability by tuning the positions of their singlet and triplet states to minimize the formation of oxygen-mediated reactive species.

Despite significant advancements in performance, the market integration of organic solar cells(OSCs) is slow due to their limited operational lifetime, the causes of which are still not fullyunderstood. In this study, we investigate the impact of the processing solvent (chloroform andchlorobenzene) on the light-induced degradation of the photoactive layers and the photovoltaicperformance of solar cells comprising the state-of-the-art donor polymer PM6 and nonfullereneacceptor (NFA) Y6. We examine the photobleaching of the photoactive layersdegraded under AM 1.5 solar simulator in air. Our results show that films of pristine PM6degrade much faster than those of Y6 for both solvents, and that pristine Y6 films prepared inchlorobenzene degrade relatively faster than those prepared from chloroform solutions. Wefound that PM6 photobleaches more rapidly under continuous AM 1.5 illumination in air, inPM6:Y6 blend films processed from chlorobenzene than in those processed from chloroform,whereas Y6 degrades at approximately the same rate in blend films processed from both solvents. Infrared spectra reveal the formation of new carbonyl groups in the PM6:Y6 blendupon photodegradation. To investigate whether morphological differences could account forthis difference in photostability, we examined the sample morphology using atomic forcemicroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Thesignificantly higher surface roughness of PM6:Y6 films processed from chlorobenzenecompared with those processed from chloroform suggests that the difference may be related tofilm morphology. The nitrogen K-edge NEXAFS spectra indicates preferential face-onorientation of the Y6 molecules in PM6:Y6 films processed from chloroform, while they showa mixed orientation in films processed from chlorobenzene. This suggests that the choice ofsolvents influences the degradation process on PM6:Y6 films. The OSCs made fromchloroform-processed films demonstrate superior power conversion efficiency, which isattributed to more efficient charge transport and extraction in the films with preferential faceonorientation. The photo-CELIV measurements show that PM6:Y6 devices processed fromchloroform show higher charge carrier mobility. Despite these differences, both solventprocesseddevices degrade at a similar rate, suggesting that solvent choice for the active layerdeposition does not influence the degradation rate of the OSCs.

Here, we study the photostability of thin films and solar cells based on a binary blend of the low‑cost donor polymer PTQ10 and the small‑molecule non‑fullerene acceptor (NFA) Y6 under exposure to air, as well as the effect of incorporating PC70BM as a third component. UV-vis spectra show that spin-coated thin films of pristine PTQ10 and Y6 remained highly stable after up to 45 hours of continuous AM 1.5 illumination in air, while both PTQ10 and Y6 in films of the binary PTQ10:Y6 blend exhibit rapid photobleaching. To unravel the degradation pathways of the donor and acceptor, the PTQ10:Y6 blend film was exposed to AM 1.5 light filtered through a 400 nm long-pass filter, revealing a significantly reduced degradation rate, particularly for the Y6 acceptor. Incorporating 20 vol% PC70BM into the PTQ10:Y6 blend was found to suppress the formation of new carbonyl groups, suggesting reduced photo-oxidation. Steady-state photoluminescence (PL) spectra of the Y6 component confirmed improved exciton dissociation in the ternary PTQ10:Y6:PC70BM blend compared to the binary PTQ10:Y6 blend. After 45 hours of degradation, the PL intensity increases for both the binary and ternary blends, suggesting an increase in radiative recombination from the Y6 components. Moreover, the addition of PC70BM enhances both the photovoltaic performance and photostability of these organic solar cells (OSCs). The power conversion efficiency (PCE) of the binary PTQ10:Y6 OSCs degraded by 82% after 80 minutes of continuous AM 1.5 light illumination in air, and by 72% under 400 nm-filtered light, while the ternary PTQ10:Y6:PC70BM devices exhibited a 74% degradation. These findings demonstrate that spectral management and the incorporation of PC70BM as a third component can effectively reduce photodegradation processes in PTQ10:Y6-based OSCs.