Carrier Formation Dynamics in Prototypical Organic Solar Cells as Investigated by Transient Absorption Spectroscopy

Yutaka Moritomo,Kouhei Yonezawa,Takeshi Yasuda

doi:10.1155/2016/9105460

Abstract

Subpicosecond transient absorption spectroscopy is a powerful tool used to clarify the exciton and carrier dynamics within the organic solar cells (OSCs). In this review article, we introduce a method to determine the absolute numbers of the excitons and carriers against delay time (t) only from the photoinduced absorption (PIA) and electrochemically induced absorption (EIA) spectra. Application of this method to rr-P3HT-, PTB7-, and SMDPPEH-based OSCs revealed common aspects of the carrier formation dynamics. First, the temporal evolution of the numbers of the excitons and carriers indicates that the late decay component of exciton does not contribute to the carrier formation process. This is probably because the late component has not enough excess energy to separate into the electron and hole across the donor/acceptor (D/A) interface. Secondly, the spectroscopy revealed that the exciton-to-carrier conversion process is insensitive to temperature. This observation, together with the fast carrier formation time in OSCs, is consistent with the hot exciton picture.

Highlights

The late decay component of exciton does not contribute to the carrier formation process, as observed in (i) PTB7/phenyl C71-butyric acid methyl ester (PC71BM) blend, (ii) rr-P3HT/phenyl C61-butyric acid methyl ester (PCBM) blend, (iii) PTB7/C70 bilayer, and (iv) SMDPPEH/PC71BM blend films
We applied a new method, which determines the absolute numbers of the excitons and carriers only from the photoinduced absorption (PIA) and electrochemically induced absorption (EIA) spectra, to (i) PTB7/PC71BM blend, (ii) rr-P3HT/PCBM blend, (iii) PTB7/C70 bilayer, and (iv) SMDPPEH/PC71BM blend films
The analyses revealed important common features on the carrier formation dynamics in the Organic solar cells (OSCs)

Summary

Introduction

By means of atomic force microscopy (AFM) coupled with plasma-ashing technique, Hedley et al [76] observed a substructure of ∼10 nm inside the fullerene domain (∼100 nm) of the PTB7/PC71BM blend film prepared without additive Such a complexity of the domain structure of the BHJ layer may prevent a true understanding of the carrier formation dynamics. The late decay component of exciton does not contribute to the carrier formation process, as observed in (i) PTB7/PC71BM blend, (ii) rr-P3HT/PCBM blend, (iii) PTB7/C70 bilayer, and (iv) SMDPPEH/PC71BM blend films This is probably because the late component has not enough excess energy to separate into electron and hole at D/A interface. The exciton-carrier conversion process is insensitive to temperature, as observed in (iii) PTB7/C70 bilayer and (iv) SMDPPEH/PC71BM blend films This observation, together with the fast carrier formation time in OSCs, is consistent with the hot exciton picture

Experimental Technique

Findings

Summary