Polaron Pair Dissociation and Polaron Recombination in Polymer: Fullerene Solar Cells
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1031-H09-21
Polaron Pair Dissociation and Polaron Recombination in Polymer: Fullerene Solar Cells Carsten Deibel, Andreas Baumann, Jens Lorrmann, and Vladimir Dyakonov Experimental Physics VI, Julius-Maximilians-University of Wurzburg, Am Hubland, Wurzburg, 97074, Germany ABSTRACT In polymer:fullerene solar cells, the field-dependent photocurrent is due to a combination of polaron pair dissociation, competing with monomolecular recombination, and bimolecular recombination. We compare of the experimental photocurrent of P3HT:PCBM bulk heterojunction solar cells to Monte Carlo simulations of bilayer and bulk heterojunction devices. The shape of the photocurrent can be reproduced, the high quantum yield, however, cannot be explained with help of the simulations. In order to analyse the dominant experimental recombination mechanism, we apply the photo-CELIV technique. Our data can be fitted with bimolecular recombination, but only if a reduced Langevin recombination factor is assumed. Thermalisation is accounted for by measuring the time-dependent charge carrier mobility. INTRODUCTION Organic bulk heterojunction solar cells have received a growing interest of the scientiÞc community during the last years. A lot of research has been dedicated to the optimisation as well as the gaining of an enhanced physical understanding of these devices; signiÞcant progress has been made in both directions [1, 2]. Despite these efforts, the elementary processes of carrier generation, transport and extraction are not fully understood yet. Unresolved issues concern the high polaron pair dissociation yield and the dominant recombination mechanism of polarons in bulk heterojunction solar cells. For bilayer solar cells, the strong influence of the monomolecular process of polaron pair dissociation on the photocurrent as well as the solar cell fill factor has been shown by Peumans et al. [3], highlighting its relevance for organic photovoltaics. In bulk heterojunction polymer solar cells, the disorder induced by the two material components and their mixture leads to favorable exciton dissociation, but more difficult polaron pair dissociation and polaron transport. Usually, bimolecular processes are seen experimentally by charge extraction and photoinduced absorption [4, 5], highlighting the polaron pair dissociation yield. In this contribution, we address the mechanism and efficiency of geminate and nongeminate recombination of polymer–fullerene solar cells by comparing experiments and Monte Carlo simulations of photocurrent and time-resolved charge extraction. EXPERIMENTAL DETAILS We prepared organic bulk heterojunction solar cells by spin coating 1:1 blends of poly[3hexyl thiophene-2,5-diyl] (P3HT) with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), 2wt% dissolved in Chlorobenzene, on PEDOT:PSS covered ITO/glass substrates. The active layer was around 300nm thick, and was annealed after spin coating. Al anodes were thermally evaporated. We obtained P3HT from Rieke Metals and PCBM from Solenne.
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