Photo-degradation in air of the active layer components in a thiophene–quinoxaline copolymer:fullerene solar cell

Abstract

We have studied the photo-degradation in air of a blend of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1), and how the photo-degradation affects the solar cell performance. Using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, changes to the electronic structure of TQ1 and PCBM caused by illumination in ambient air are investigated and compared between the pristine materials and the blend. The NEXAFS spectra show that the unoccupied molecular orbitals of TQ1 are not significantly changed by the exposure of pristine TQ1 to light in air, whereas those of PCBM are severely affected as a result of photo-induced degradation of PCBM. Furthermore, the photo-degradation of PCBM is accelerated by blending it with TQ1. While the NEXAFS spectrum of TQ1 remains unchanged upon illumination in air, its valence band spectrum shows that the occupied molecular orbitals are weakly affected. Yet, UV-Vis absorption spectra demonstrate photo-bleaching of TQ1, which is attenuated in the presence of PCBM in blend films. Illumination of the active layer of TQ1:PCBM solar cells prior to cathode deposition causes severe losses in electrical performance.

Impact of intentional photo-oxidation of a donor polymer and PC70BM on solar cell performance

“…An exception is the exposure time of 2 hours, where there is no significant development of photo-oxidation products in the blend spectra. This means that the presence of the TQ1 does not accelerate the photodegradation of PC 70 BM, which is different from the case of TQ1:PC 60 BM blends, where Hansson et al 27 found by NEXAFS that the PC 60 BM in the TQ1:PC 60 BM blend films degraded more rapidly than a pure PC 60 BM film. After exposure times longer than 2 hours, the blend spectrum develops increased absorption in the carbonyl region, to which both the photo-oxidation of TQ1 and the photo-oxidation of PC 70 BM contribute.…”

“…31,45 The pure TQ1 films photobleach rapidly under simulated sunlight in ambient air, which is seen by the decrease in intensity of both absorption bands. 27,46 However, in Fig. 5b, it can be seen that the photobleaching of the TQ1 : PC 70 BM (1 : 3) blend film in the 500-700 nm range is moderate compared to that of the pure TQ1 film.…”

“…By a comparative FT-IR study we recently demonstrated that the rate of carbonyl growth on PC 70 BM is lower than that on PC 60 BM, when thin films are exposed to simulated sunlight and ambient air. 26 Several studies of polymer/fullerene blends have demonstrated that the presence of PC 60 BM in a blend slows down the photo-bleaching of the donor polymer, e.g., MDMO-PPV, 19 P3HT 23 and TQ1, 27 explaining this by the oxygen scavenger role of the fullerene derivative. On the other hand, the photooxidation of the PC 60 BM itself was found to be accelerated by its dispersion in a TQ1 matrix, which Hansson et al 27 assigned to the enhanced absorption range.…”

“…26 Several studies of polymer/fullerene blends have demonstrated that the presence of PC 60 BM in a blend slows down the photo-bleaching of the donor polymer, e.g., MDMO-PPV, 19 P3HT 23 and TQ1, 27 explaining this by the oxygen scavenger role of the fullerene derivative. On the other hand, the photooxidation of the PC 60 BM itself was found to be accelerated by its dispersion in a TQ1 matrix, which Hansson et al 27 assigned to the enhanced absorption range. Interestingly, Speller et al 28 showed a similar acceleration of the PC 60 BM photodegradation when PC 60 BM is dispersed in a transparent non-conjugated polymer (polystyrene) matrix.…”

“…A few studies have addressed the effect of the PC 60 BM and PC 70 BM degradation on the device performance. 24,27,29 Very recently, Doumon et al 30 conducted a study on addition of PC 70 BM to PBDB-T:ITIC and PTB7-Th:ITIC active layer blends. Upon exposure to simulated sunlight and air, the complete devices with ternary blends have shown increased photostability, regardless of device architecture (i.e., conventional or inverted), further indicating that the major contribution to the loss of device performance comes from the active layer.…”