The role of spin in the degradation of organic photovoltaics

Abstract

Stability is now a critical factor in the commercialization of organic photovoltaic (OPV) devices. Both extrinsic stability to oxygen and water and intrinsic stability to light and heat in inert conditions must be achieved. Triplet states are known to be problematic in both cases, leading to singlet oxygen production or fullerene dimerization. The latter is thought to proceed from unquenched singlet excitons that have undergone intersystem crossing (ISC). Instead, we show that in bulk heterojunction (BHJ) solar cells the photo-degradation of C60 via photo-oligomerization occurs primarily via back-hole transfer (BHT) from a charge-transfer state to a C60 excited triplet state. We demonstrate this to be the principal pathway from a combination of steady-state optoelectronic measurements, time-resolved electron paramagnetic resonance, and temperature-dependent transient absorption spectroscopy on model systems. BHT is a much more serious concern than ISC because it cannot be mitigated by improved exciton quenching, obtained for example by a finer BHJ morphology. As BHT is not specific to fullerenes, our results suggest that the role of electron and hole back transfer in the degradation of BHJs should also be carefully considered when designing stable OPV devices.

Organic Solar Cells—The Path to Commercial Success

“…[ 57–60 ] Raman and mass spectroscopy have shown the formation of C 60 oligomers, upon irradiation of neat C 60 films. [ 61 ]…”

Traps and transport resistance: the next frontier for stable state-of-the-art non-fullerene acceptor solar cells

“…2,3 Despite this promising increase in performance, rooftop stability remains arguably the most pressing challenge. Device stability can be affected by many factors including material system, 4 device architecture, 5 deposition method, 6,7 morphology, 8,9 environmental conditions, 10 and additives. 11 Devices exposed to the atmosphere are vulnerable to processes involving oxygen and water.…”

“…13,14 In an inert atmosphere the main degradation pathway in fullerene-based devices is the photo-induced dimerization of fullerene molecules, leading to a severely limited short-circuit current J sc due to photobleaching. 8,15,16 In order to avoid photo-induced dimerization, considerable attention has recently been focused on non-fullerene acceptor (NFA)-based devices. Dimerization also occurs in NFAs if the energetic alignment between donor and acceptor allows for back-hole transfer, back-electron transfer, or inter-system crossing.…”

“…Dimerization also occurs in NFAs if the energetic alignment between donor and acceptor allows for back-hole transfer, back-electron transfer, or inter-system crossing. 8,17 However, the dominant degradation processes in NFAs include intermixing and demixing of the heterojunction components, which can be controlled to some extent by tuning crystallinity, miscibility, and the diffusion coefficient of the NFA. 12,16,18,19 Structural decomposition of acceptors such as IT-4F 14,20 occurs near to zinc oxide surfaces and under the influence of oxygen.…”

Resist or Oxidize: Identifying Molecular Structure–Photostability Relationships for Conjugated Polymers Used in Organic Solar Cells

“…[7] It has been shown that organic semiconductor materials, both p-and n-type, have certain issues with intrinsic photostability and undergo light-induced dimerization, cross-linking, reactions with other components of the active layer or adjacent charge-transport interlayers. [8][9][10][11][12][13][14] Hence, the design of intrinsically photostable absorber materials is a crucially important task for achieving long lifetimes in organic photovoltaics.…”