​Fluorinated copolymer PCPDTBT with enhanced open-circuit voltage and reduced recombination for highly efficient polymer solar cells

by S. Albrecht, S. Janietz, W. Schindler, J. Frisch, J. Kurpiers, J. Kniepert, S. Inal, P. Pingel, K. Fostiropoulos, N. Koch, D. Neher
Year: 2012 DOI: DOI: 10.1021/ja305039j

Bibliography

​Fluorinated copolymer PCPDTBT with enhanced open-circuit voltage and reduced recombination for highly efficient polymer solar cells
S. Albrecht, S. Janietz, W. Schindler, J. Frisch, J. Kurpiers, J. Kniepert, S. Inal, P. Pingel, K. Fostiropoulos, N. Koch, D. Neher
Journal of the American Chemical Society 134 (36), 14932-14944, 262, 2012

Abstract

Pub_FCP_2012A novel fluorinated copolymer (F-PCPDTBT) is introduced and shown to exhibit significantly higher power conversion efficiency in bulk heterojunction solar cells with PC 70BM compared to the well-known low-band-gap polymer PCPDTBT. Fluorination lowers the polymer HOMO level, resulting in high open-circuit voltages well exceeding 0.7 V. Optical spectroscopy and morphological studies with energy-resolved transmission electron microscopy reveal that the fluorinated polymer aggregates more strongly in pristine and blended layers, with a smaller amount of additives needed to achieve optimum device performance. Time-delayed collection field and charge extraction by linearly increasing voltage are used to gain insight into the effect of fluorination on the field dependence of free charge-carrier generation and recombination. F-PCPDTBT is shown to exhibit a significantly weaker field dependence of free charge-carrier generation combined with an overall larger amount of free charges, meaning that geminate recombination is greatly reduced. Additionally, a 3-fold reduction in non-geminate recombination is measured compared to optimized PCPDTBT blends. As a consequence of reduced non-geminate recombination, the performance of optimized blends of fluorinated PCPDTBT with PC 70BM is largely determined by the field dependence of free-carrier generation, and this field dependence is considerably weaker compared to that of blends comprising the non-fluorinated polymer. For these optimized blends, a short-circuit current of 14 mA/cm 2, an open-circuit voltage of 0.74 V, and a fill factor of 58% are achieved, giving a highest energy conversion efficiency of 6.16%. The superior device performance and the low band-gap render this new polymer highly promising for the construction of efficient polymer-based tandem solar cells. 




Keywords

Bulk heterojunction solar cells Charge extraction Field dependence Fill factor Fluorinated copolymers Gain insight Geminate recombination Morphological study