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

Supramolecular interaction of PCBM with porphyrins in solution: Photophysical insights

This work reports the self-assembly between [6,6]-phenyl C71 butyric acid methyl ester (PCBM) and 2,3,7,8,12,13,17,18-octaethyl-21H,23H porphyrin (1)(and/2,3,7,8,12,13,17,18-octaethyl-21H,23H porphyrin Zn(II) (2) in toluene. Ground state intermolecular interaction is evidenced from absorption spectrophotometric measurements. New absorption bands are observed in the visible region which may be identified due to charge transfer (CT). Several important physicochemical factors are enumerated for PCBM-1 and PCBM-2 systems. Fluorescence investigations elicit complex formation of PCBM with porphyrins (with both 1 and 2) and reveal considerable magnitude of binding constant (K) for PCBM-2 system, i.e., KPCBM-2 = 80,435 dm3⋅mol-1 compared to PCBM-1 system, i.e., KPCM-1 = 12,600 dm3·mol-1 as well as highly ratio of selectivity in binding (KPCBM-2/KPCBM-1 ∼ 6.4). Time resolved fluorescence experiments reveal that photoexcited decay from the excited singlet state of porphyrins (i.e., 1* and 2*) by PCBM is statically controlled compared to dynamic path. Magnitude of solvent reorganization energy indicates possibility of faster charge recombination in case of PCBM-2 system. Both 1H and 13C NMR measurements provide substantial support behind complexation of PCBM with porphyrins (both 1 and 2) in solution. Ab initio calculations in vacuo support the trend in K for PCBM-1 and PCBM-2 systems and establish the proper orientation of PCBM towards 1 (and/ 2) during complexation. Transient absorption measurements establish two different mode of energy transfer pathway from porphyrin to PCBM in toluene.

Photooxidation of PC60BM: new insights from spectroscopy

This joint experimental-theoretical spectroscopy study of the fullerene derivative PC₆₀BM ([6,6]-phenyl-C₆₀-butyric acid methyl ester) aims to improve the understanding of the effect of photooxidation on its electronic structure. We have studied spin-coated thin films of PC₆₀BM by X-ray Photoelectron Spectroscopy (XPS), Near-edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, and Fourier Transform Infrared Spectroscopy (FTIR), before and after intentional exposure to simulated sunlight in air for different lengths of time. The π* resonance in the C1s NEXAFS spectrum was found to be a very sensitive probe for the early changes to the fullerene cage, while FTIR spectra, in combination with O1s NEXAFS spectra, enabled the identification of the oxidation products. The changes observed in the spectra obtained by these complementary methods were compared with the corresponding Density Functional Theory (DFT) calculated single-molecule spectra of a large set of in silico generated oxidation products of PC₆₀BM where oxygen atoms were attached to the C₆₀ cage. This comparison confirms that photooxidation of PC₆₀BM disrupts the conjugation of the fullerene cage by a transition from sp² to sp³-hybridized carbon and causes the formation of several oxidation products, earlier proposed for C₆₀. The agreement between experimental and calculated IR spectra suggests moreover the presence of dicarbonyl and anhydride structures on the fullerene cage, in combination with cage opening at the adsorption site. By including PC₆₀BM with physisorbed O₂ molecules on the cage in our theoretical description in order to model oxygen diffused through the film, the experimental O1s XPS and O1s NEXAFS spectra could be reproduced.

An experimental setup for dip-coating of thin films for organic solar cells under microgravity conditions

We report the design and testing of a custom-built experimental setup for dip-coating from volatile solutions under microgravity conditions onboard an aircraft. Function and safety considerations for the equipment are described. The equipment proved to work well, both concerning the safety and the preparation of thin films. No leakage of the solvents, nor the solvent vapors, was detected, not even in a situation with a fluctuating gravitational field due to bad weather conditions. We have shown that the equipment can be used to prepare thin films of polymer blends, relevant for organic solar cells, from solution in a feasible procedure under microgravity conditions. The prepared films are similar to the corresponding films prepared under 1 g conditions, but with differences that can be related to the absence of a gravitational field during drying of the applied liquid coating. We report on some introductory results from the characterization of the thin films that show differences in film morphology and structure sizes.