Subnanosecond charge photogeneration and recombination in polyfluorene copolymer-fullerene solar cell: Effects of electric field

Wei Zhang,Yan Jing,Xi-Cheng Ai,Long Ye,Jian-Hui Hou,Ye Huang,Jian-Ping Zhang,Li-Min Fu,Ya-Dong Xing

doi:10.1364/oe.21.00a241

Abstract

Influence of electric field on the subnanosecond charge photogeneration dynamics in the polymer solar cell based on polyfluorene copolymer BisDMO-PFDTBT blended with PC(61)BM was examined with transient absorption spectroscopy. The charge dynamics showed no difference under short- or open-circuit conditions and under a forward bias of 0.79 V (1.6 × 10(5) V/cm), implying negligible field effects on the subnanosecond dynamics of charge photogeneration/recombination. However, under the reverse biases of -2 V (4.0 × 10(5) V/cm) and -5 V (1.0 × 10(6) V/cm), significant enhancement of charge photogeneration and apparent suppression of polaron pair recombination were observed, which agrees with the field-assisted enhancement of external quantum efficiency of the solar cell devices.

Highlights

The primary charge photogeneration in the bulk heterojunction (BHJ) polymer solar cells involves the formation and diffusion of excitons in the polymer phases, and the subsequent dissociation of excitons at the BHJ interfaces yielding free charge carriers and Coulombically bound polaron pairs (PPs)
Since the filling factor (FF) of an operating solar cell is determined by the comprehensive exciton and charge dynamics under the internal electric field, it is important to examine the influence of electric field on the elementary light conversion processes, especially the PP dissociation and the charge recombination
Our results have revealed significantly enhanced charge photogeneration and substantially suppressed PP recombination under reverse but not forward potential biases, which agrees with the field dependence of FF and EQE of the BisDMO-PFDTBT/PC61BM device

Summary

Introduction

The primary charge photogeneration in the bulk heterojunction (BHJ) polymer solar cells involves the formation and diffusion of excitons in the polymer phases, and the subsequent dissociation of excitons at the BHJ interfaces yielding free charge carriers and Coulombically bound polaron pairs (PPs). Note that the BisDMO-PFDTBT/PC70BM device exhibits moderate FF (~50%) and external quantum efficiency (EQE, ~50%) in the visible spectral region, implying significant dependence of the photocurrent on the internal electric field. Such device is suitable for investigating the possible field dependence of the dynamics of PP and polymer radical cation (P +, referred to as polaron in literature). Our results have revealed significantly enhanced charge photogeneration and substantially suppressed PP recombination under reverse but not forward potential biases, which agrees with the field dependence of FF and EQE of the BisDMO-PFDTBT/PC61BM device

Device fabrication

Time-resolved near infrared absorption spectroscopy

Results and discussion

Conclusions