Abstract
Time-delayed collection field (TDCF), bias-assisted charge extraction (BACE), and space charge-limited current (SCLC) measurements are combined with complete numerical device simulations to unveil the effect of the solvent additive 1,8-diiodooctane (DIO) on the performance of PTB7:PCBM bulk heterojunction solar cells. DIO is shown to increase the charge generation rate, reduce geminate and bimolecular recombination, and increase the electron mobility. In total, the reduction of loss currents by processing with the additive raises the power conversion efficiency of the PTB7:PCBM blend by a factor of almost three. The lower generation rates and higher geminate recombination losses in devices without DIO are consistent with a blend morphology comprising large fullerene clusters embedded within a PTB7-rich matrix, while the low electron mobility suggests that these fullerene clusters are themselves composed of smaller pure fullerene aggregates separated by disordered areas. Our device simulations show unambiguously that the effect of the additive on the shape of the currentvoltage curve (J-V) cannot be ascribed to the variation of only the mobility, the recombination, or the field dependence of generation. It is only when the changes of all three parameters are taken into account that the simulation matches the experimental J-V characteristics under all illumination conditions and for a wide range of voltages.
Highlights
Organic solar cells have experienced significant efficiency improvements in recent years, mainly due to the development of new low-band-gap copolymers with optimized absorption spectra
We presented a conclusive and comprehensive study of the charge carrier dynamics in high efficiency PTB7:phenyl C71 butyric acid methyl ester (PCBM) solar cells prepared with and without the solvent additive DIO
For the devices prepared with DIO all data can be consistently explained by the combination of a field-independent free charge carrier generation, a moderate bimolecular recombination with a carrier density-independent recombination coefficient, and efficient extraction of both electron and holes
Summary
Organic solar cells have experienced significant efficiency improvements in recent years, mainly due to the development of new low-band-gap copolymers with optimized absorption spectra. 4 μs for both blends at Voc and 1 sun illumination but a 2-fold larger steady state charge carrier concentration for the device with DIO, from which a smaller nongeminate recombination rate coefficient would be concluded These data failed to explain the J−V characteristics of the devices without additive and led to a strong overestimation of the performance. In this paper we combine TDCF, bias-assisted charge extraction (BACE), and space charge-limited current (SCLC) measurements together with complete numerical device simulations to investigate charge carrier generation, recombination, and extraction From this we gain an unprecedented deep understanding of the dynamic quantities determining the steady state J−V characteristics of PTB7:PCBM solar cells prepared with and without the solvent additive DIO. The results of experiments and simulations are discussed in terms of the blend morphology based on extensive studies present in the literature
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
