Light-induced EPR study of charge transfer in P3HT/bis-PCBM bulk heterojunctions

Victor I Krinichnyi,Eugenia I Yudanova

doi:10.1063/1.3599411

Abstract

Radical pairs, polarons and fullerene anion radicals photoinduced by photons with energy of 1.98 – 2.73 eV in bulk heterojunctions formed by poly(3-hexylthiophene) (P3HT) with bis(1-[3-(methoxycarbonyl)propyl]-1-phenyl)-[6.6]C62 (bis-PCBM) fullerene derivative have been studied by direct light-induced EPR (LEPR) method in a wide temperature range. A part of photoinduced polarons are pinned in trap sites which number and depth are governed by an ordering of the polymer/fullerene system and energy of initiating photons. It was shown that dynamics and recombination of mobile polarons and counter fullerene anion radicals are governed by their exchange- and multi-trap assisted diffusion. Relaxation and dynamics parameters of both the charge carriers were determined separately by the steady-state saturation method. These parameters are governed by structure and conformation of the carriers’ microenvironment as well as by the energy of irradiating photons. Longitudinal diffusion of polarons was shown to depend on lattice phonons of crystalline domains embedded into an amorphous polymer matrix. The energy barrier required for polaron interchain hopping is higher than that its intrachain diffusion. Pseudorotation of fullerene derivatives in a polymer matrix was shown to follow the activation Pike model.

Highlights

In bulk heterojunction solar cells,[1,2] in which power conversion efficiency have already attained about 6%,3 light photons generate singlet excitons, which are strongly bound due to weak screening in polymer semiconductor.[4]
Detached regioregular P3HT and bis-PCBM shown in Fig. 1 are characterized by the absence of both “dark” and photoinduced light-induced electron paramagnetic resonance (EPR) (LEPR) signals over the entire range of temperatures studied
Low- and high-field lines were attributed to positively charged polarons P+ with isotropic giPso and negatively charged methanofullerene with effective giFso background photoinduced in the P3HT/bis-PCBM bulk heterojunctions

Summary

INTRODUCTION

In bulk heterojunction solar cells,[1,2] in which power conversion efficiency have already attained about 6%,3 light photons generate singlet excitons, which are strongly bound due to weak screening in polymer semiconductor.[4]. That is a reason why spins in radical pairs can be considered as non-interacting; they are long-lived and demonstrate separate LEPR spectra This accounts for a wide use of the light-induced EPR (LEPR) as direct method for investigation of fullerene-modified conjugated polymers.[1,19] LEPR measurements revealed the photoinducing under light irradiation of organic polymer/fullerene composite of polarons and fullerene anion radicals with different line shapes, magnetic resonance parameters and saturation properties. The system inhomogeneity leads to the arising of traps in polymer matrix occupied by some carriers, so that changes their magnetic resonance parameters and complicates effective LEPR spectrum By using such direct method it appears to be possible to control the texture and other structural and electronic properties of photovoltaic devices for the further increase in their efficiency. Such substitution increases the planarity of a polymer matrix, decreases the number of traps, and, accelerates charge transfer through bulk heterojunctions

EXPERIMENTAL

Spectral composition and g-factor

Paramagnetic susceptibility

Linewidth

Spin relaxation and dynamics

SUMMARY