Charge carrier mobility of the organic photovoltaic materials PTB7 and PC71BM and its influence on device performance

Bernd Ebenhoch,Stuart A.J Thomson,Kristijonas Genevičius,Gytis Juška,Ifor D.W Samuel

doi:10.1016/j.orgel.2015.03.013

Bernd Ebenhoch, Stuart A.J Thomson + Show 3 more

Open Access

https://doi.org/10.1016/j.orgel.2015.03.013

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Abstract

The mobility is an important parameter for organic solar cell materials as it influences the charge extraction and recombination dynamics. In this study, the time of flight technique is used to investigate the charge mobility of the important organic photovoltaic materials PC71BM, PTB7 and their blend. The electron mobility of PC71BM is in the region of 1×10−3cm2/Vs for the neat fullerene film, and has a positive electric field dependence. At room temperature the hole mobility of PTB7 is 1×10−3cm2/Vs for the neat film and 2×10−4cm2/Vs for their blend. The hole mobility of the blend reduces by a factor of a thousand when the sample is cooled from room temperature to 77K. This finding is compared with the device performance of efficient PTB7:PC71BM solar cells for varying temperature. At 77K the solar cell efficiency halved, due to losses in fill factor and short circuit current. Bimolecular and trap-assisted recombination increase at low mobility (low temperature) conditions, whereas at high mobility conditions the open circuit voltage reduces. The power conversion efficiency as a function of temperature has a maximum between 260K and 295K, revealing an optimized mobility at room temperature.

Highlights

The polymeric donor material PTB7 (poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl][3-fluoro-2-(2ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl]]) has attracted great interest in recent years due to its high performance in bulk heterojunction solar cells
High hole mobility of about 1 Â 10-3 cm2/Vs was observed in neat PTB7 and the same value was observed for the electron mobility of PC71BM
The temperature dependence of PTB7:PC71BM revealed a fast drop of the hole mobility in PTB7:PC71BM by three orders of magnitude when cooled from room temperature to 77 K

Summary

Introduction

The polymeric donor material PTB7 (poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl][3-fluoro-2-(2ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl]]) has attracted great interest in recent years due to its high performance in bulk heterojunction solar cells. Devices of PTB7 blended with PC71BM ([6,6]phenyl C71 butyric acid methyl ester) exhibit a high fill factor (FF) of up to 72% and external quantum efficiency (EQE) of 80% [1]. Both of these parameters are strongly influenced by the charge transport properties of the active layer. The first is to report mobility measurements of these important and widely used solar cell materials. The second is to explore the influence of the mobility on device operation and charge recombination

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