Tail state limited photocurrent collection of thick photoactive layers in organic solar cells

Jiaying Wu,Hyojung Cha,James R Durrant,Martin Heeney,Ji-Seon Kim,Hongkyu Kang,Harrison Ka Hin Lee,Pabitra Shakya Tuladhar,Kwanghee Lee,Joel Luke,Soo-Young Jang,Wing Chung Tsoi,Thomas Kirchartz

doi:10.1038/s41467-019-12951-7

Abstract

We analyse organic solar cells with four different photoactive blends exhibiting differing dependencies of short-circuit current upon photoactive layer thickness. These blends and devices are analysed by transient optoelectronic techniques of carrier kinetics and densities, air photoemission spectroscopy of material energetics, Kelvin probe measurements of work function, Mott-Schottky analyses of apparent doping density and by device modelling. We conclude that, for the device series studied, the photocurrent loss with thick active layers is primarily associated with the accumulation of photo-generated charge carriers in intra-bandgap tail states. This charge accumulation screens the device internal electrical field, preventing efficient charge collection. Purification of one studied donor polymer is observed to reduce tail state distribution and density and increase the maximal photoactive thickness for efficient operation. Our work suggests that selecting organic photoactive layers with a narrow distribution of tail states is a key requirement for the fabrication of efficient, high photocurrent, thick organic solar cells.

Highlights

We analyse organic solar cells with four different photoactive blends exhibiting differing dependencies of short-circuit current upon photoactive layer thickness
We have investigated the dependence of photovoltaic performance on the absorber layer thickness for four different series of organic bulk heterojunction (BHJ) solar cells
While the loss of FF we observe is consistent with this conclusion, when we quantify the photocurrent loss resulting from the bimolecular recombination flux at short circuit, we find this recombination loss to be too small to explain the loss of Jsc for thick devices under one sun irradiation

Summary

Introduction

We analyse organic solar cells with four different photoactive blends exhibiting differing dependencies of short-circuit current upon photoactive layer thickness. A few material systems, including most notably the lower efficiency P3HT:PC61BM system[6,7], have been shown to work effectively with photoactive layer larger than 300 nm[8,9,10] This thickness limitation for OPV has typically been associated with non-geminate charge recombination losses during charge transport to the device electrodes, often quantified regarding the charge carrier mobility-lifetime (μτ) product[11], or analogous combinations of the bimolecular recombination coefficient and the mobility[12]. The charge carrier diffusion lengths are typically thought to be relatively small (16 nm for typical values for μτ of 10−10 cm[2] V−1 as reported in the literature)[11,15,16], such that purely diffusive transport does not lead to efficient charge extraction (CE) To address this limitation, organic solar cells typically comprise a thin photoactive layer sandwiched between two electrodes with different work functions. As such, obtaining a high value for drift length Ldr versus photoactive layer thickness d, and efficient charge collection for thick devices, requires a high value for the mobility-lifetime product, with several studies in the literature addressing this issue[15,17,18]

Methods

Results

Conclusion