The fullerene derivatives PC₆₀BM and PC₇₀BM are widely used as electron accepting components in the active layer of polymer solar cells. Here we compare their photochemical stability by exposing thin films of PC₆₀BM and PC₇₀BM to simulated sunlight in ambient air for up to 47 h, and study changes in their UV–vis and FT-IR spectra. We quantify the photo-degradation by tracking the development of oxidation products in the transmission FT-IR spectra. Results indicate that PC₆₀BM photodegrades faster than PC₇₀BM. The rate of photo-oxidation of the thin films is dependent on the rate of oxygen diffusion in to the film and on the photo-oxidation rate of a single molecule. Both factors are dependent on the nature of the fullerene cage. The faster photo-oxidation of PC₆₀BM than of PC₇₀BM is in agreement with its slightly lower density and its higher reactivity. The use of PC₇₀BM in solar cells is advantageous not only because of its absorption spectrum, but also because of its higher stability.

A short lifetime is the main factor hindering the wider implementation of low-cost organic photovoltaics in large-area and outdoor applications. Ingress of oxygen and water vapour through non-ideal encapsulation layers is a known cause of degradation for polymer/fullerene based solar cells. To better understand the origin of this performance degradation, we study the effect of intentional exposure of the photo-active layer to simulated sunlight (AM1.5) in air both on the solar cell performance and on the molecular semiconductor materials. Cathode-free thin films of a blend of the electron donor polymer poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) and the electron acceptor fullerene derivative [6,6]-phenyl-C₇₀-butyric acid methyl ester (PC₇₀BM) were exposed to simulated sunlight in air. Fourier-transform infrared spectra demonstrate the formation of carbonyl photo-oxidation products in the blend films, as well as in the pristine polymer and fullerene films. Solar cells prepared with photo-oxidized active layers show increasingly degraded electrical performance (lower short circuit current, open circuit voltage and fill factor) with increasing exposure time. The increased diode ideality factor indicates that trap-assisted recombination hinders device operation after exposure. The external quantum efficiency decreases drastically with increasing exposure time over the whole photon energy range, while the UV-vis absorption spectra of the blend films only show a mild photo-induced bleaching. This demonstrates that not only the photo-induced degradation of the solar cell performance is not predominantly caused by the loss in light absorption, but charge transport and collection are also hampered. This is explained by the fact that photo-oxidation of PC₇₀BM causes bonds in its conjugated cage to break, as evidenced by the decreased ∏* intensity in C1s-NEXAFS spectra of PC₇₀BM films. This degradation of unoccupied states of PC₇₀BM will hinder the transport of photo-generated electrons to the electrode. Surface photovoltage spectroscopy gives direct evidence for gap states at the surface of a PC₇₀BM film, formed after 2 hours of exposure and resulting in upward band bending at the PC₇₀BM/air surface. These observations indicate that the photo-oxidation of PC₇₀BM is likely to be the main cause of the performance degradation observed when the photoactive layer of a TQ1:PC₇₀BM solar cell is intentionally exposed to light in air.

In the quest towards more durable solution-processed solar cells, the stability of the active layer materials under operation conditions is important. While lifetimes of several years have been demonstrated for encapsulated organic solar cells, it is generally known that degradation events can be accounted for by air components (O₂ and/or water vapour) leaking into the cell through a non-ideal sealing. Here we present a fundamental study of intentional photo-degradation of the electron-acceptor PC₆₀BM ([6,6]-phenyl-C₆₁-butyric acid methyl ester) in air, with the purpose of improving the understanding of the electronic effects of fullerene photo-oxidation. We have studied spincoated thin films of PC₆₀BM by X-ray Photoelectron Spectroscopy, Near-edge X-ray Absorption Fine Structure spectroscopy, and Fourier Transform Infrared Spectroscopy, before and after exposing them to simulated sunlight in air. The changes observed in the spectra obtained by these complementary methods were compared with calculated spectra of a large set of possible oxidation products of PC₆₀BM where oxygen atoms have been attached to the C₆₀ cage. The best fit with experimental IR spectra of photodegraded PC₆₀BM films was obtained for a linear combination of calculated spectra for two degradation products, a dicarbonyl and an anhydride, both with open cages with 58 carbon atoms, and the pristine PC₆₀BM molecule. From this comparison, we conclude that the conjugation of the fullerene cage is disturbed by the formation of several carbonyl-based derivatives on the C₆₀ cage, accompanied by a transition from sp² to sp³-hybridized carbon. The π* resonance in the C1s NEXAFS spectrum was found to be a very sensitive probe for small changes to the fullerene cage, and FT-IR was needed in combination with O1s NEXAFS, to identify the oxidation products.