Recently, the advent of non-fullerene acceptors (NFAs) made it possible for organic solar cells (OSCs) to break the 10% efficiency barrier hardly attained by fullerene acceptors (FAs). In the past five years alone, more than hundreds of NFAs with applications in organic photovoltaics (OPVs) have been synthesized, enabling a notable current record efficiency of above 15%. Hence, there is a shift in interest toward the use of NFAs in OPVs. However, there has been little work on the stability of these new materials in devices. More importantly, there is very little comparative work on the photostability of FA versus NFA solar cells to ascertain the pros and cons of the two systems. Here, we show the photostability of solar cells based on two workhorse acceptors, in both conventional and inverted structures, namely, ITIC (as NFA) and [70]PCBM (as FA), blended with either PBDB-T or PTB7-Th polymer. We found that, irrespective of the polymer, the cell structure, or the initial efficiency, the [70]PCBM devices are more photostable than the ITIC ones. This observation, however, opposes the assumption that NFA solar cells are more photochemically stable. These findings suggest that complementary absorption should not take precedence in the design rules for the synthesis of new molecules and there is still work left to be done to achieve stable and efficient OSCs.

The last decade saw myriad new donor polymers, among which benzodithiophene-co-thienothiophene polymers are attractive due to their relatively high power conversion efficiency in bulk heterojunction solar cells. We examine the effect of UV-light on the stability of these polymers. The relationship between the polymer chemical structure and the UV-stability of the cells is explored on the one hand, and on the other hand, the effect of additives on their UV-stability: 1,8-diiodooctane against 1-chloronaphthalene in the cells and 1,8-octanedithiol in solution. For example, PBDTTT-E with 18% efficiency loss is more stable than PBDTTT-ET with 36% loss throughout the exposure. While 1,8-diiodooctane acts as photo-acid and leads to accelerated degradation of the solar cells, 1-chloronaphthalene does not. Acidity is known to be detrimental to the efficiency and stability of organic solar cells. The degradation is initiated upon UV-irradiation by the cleavage of the side chains, resulting in more electron traps and by the formation of iodine, dissolved HI and carbon-centered radicals from 1,8-diiodooctane as revealed by ¹H NMR spectrum. The 1,8-octanedithiol spectra do not show such species. Finally, the mechanisms behind the effect of 1,8-diiodooctane are explained, paving the way for the design of new, efficient as well as stable materials and additives.

The design of novel polymers has brought more attention to bulk heterojunction polymer:fullerene solar cells in the past decade. A typical example is the synthesis, through chemical structure engineering, of the benzodithiophene-co-thieno[3,4-b]thiophene (BDT-TT) polymers leading to power conversion efficiency of over 10%. In this work, we study the stability for a set of PBDT-TT polymers. We conduct a systematic UV-degradation study on the solar cells. Most importantly, the paper shows clearly the effect of polymer chemical structure on the UV-degradation pathway of the solar cells. We find that based on the polymer chemical structure, solar cells of polymers with alkoxy side chains are more stable (<20% loss in PCE) than those with alkylthienyl side chains (∼48% loss in PCE) over the period of study. These findings pave the way for new materials that yield efficient as well as stable organic solar cells.

Polymer solar cells are potentially key contributors to the next-generation organic photovoltaics for sustainable green sources of energy. In the past few years, ternary organic solar cells have emerged with promising characteristics. They have proven to yield high efficiency at about 15% for single junction donor:acceptor (D:A) solar cells. However, the low stability of organic solar cells is a hindrance to the commercialisation of this technology, and thus, needs more attention. Here, we show that with the right ratio of D : A₁ : A₂, ternary blend solar cells can be more efficient and more photostable than their D:A binary blend solar cells. We add [70]PCBM to PBDB-T:ITIC and PTB7-Th:ITIC binary blend solar cells in various ratios to fabricate ternary solar cells. The ternary solar cells outperform all binary cells in terms of efficiency and photostability with only a 10% average loss in efficiency under continuous illumination irrespective of the device structure. We identify changes in the molecular structure of the active layer blends as the main reason behind the observed degradation behaviour of the solar cells. The ternary blends are the most resilient to photo-induced molecular structural changes. This finding suggests that ternary organic solar cells could be a way to achieve photostable devices.

Fabrication of efficient halide perovskite solar cells under ambient conditions and their stability remain a challenge due to the sensitivity of halide perovskites to moisture, oxygen, light, and temperature.