Spectroscopic Investigation of the Effect of Microstructure and Energetic Offset on the Nature of Interfacial Charge Transfer States in Polymer: Fullerene Blends.

S D Dimitrov,J R Durrant,P Shakya Tuladhar,M Azzouzi,Y Dong,J Yao,J Wu,I Mcculloch,E R Bittner,J Nelson,B C Schroeder

doi:10.1021/jacs.8b11484

Abstract

Despite performance improvements of organic photovoltaics, the mechanism of photoinduced electron–hole separation at organic donor–acceptor interfaces remains poorly understood. Inconclusive experimental and theoretical results have produced contradictory models for electron–hole separation in which the role of interfacial charge-transfer (CT) states is unclear, with one model identifying them as limiting separation and another as readily dissociating. Here, polymer–fullerene blends with contrasting photocurrent properties and enthalpic offsets driving separation were studied. By modifying composition, film structures were varied from consisting of molecularly mixed polymer–fullerene domains to consisting of both molecularly mixed and fullerene domains. Transient absorption spectroscopy revealed that CT state dissociation generating separated electron–hole pairs is only efficient in the high energy offset blend with fullerene domains. In all other blends (with low offset or predominantly molecularly mixed domains), nanosecond geminate electron–hole recombination is observed revealing the importance of spatially localized electron–hole pairs (bound CT states) in the electron–hole dynamics. A two-dimensional lattice exciton model was used to simulate the excited state spectrum of a model system as a function of microstructure and energy offset. The results could reproduce the main features of experimental electroluminescence spectra indicating that electron–hole pairs become less bound and more spatially separated upon increasing energy offset and fullerene domain density. Differences between electroluminescence and photoluminescence spectra could be explained by CT photoluminescence being dominated by more-bound states, reflecting geminate recombination processes, while CT electroluminescence preferentially probes less-bound CT states that escape geminate recombination. These results suggest that apparently contradictory studies on electron–hole separation can be explained by the presence of both bound and unbound CT states in the same film, as a result of a range of interface structures.

Highlights

Recent technological advances in organic photovoltaics (OPV)have resulted in the development of devices with power conversion efficiencies of >17%.1 An unresolved scientific challenge for this technology is understanding−and controlling−the mechanism of photoinduced electron−hole (e−h) separation
These PL results reflect the structural differences between the four PCDTBT:PCBM films and are consistent with direct morphological studies reported in the literature on these and analogous blends.[27−32] We consider that two main phases are present in these blend films: an intermixed polymer:fullerene phase in which the molecular D:A interface is dominant and a pure fullerene phase
We implement a two-dimensional lattice exciton model to simulate the excited state spectrum of a polymer:fullerene blend system as a function of both microstructure and LUMO energy offset.[26,55−57] We model the microstructure as a combination of a “mixed” phase containing PCBM molecules and small PCBM domains mixed into polymer, and a “clustered” phase consisting of a large fullerene cluster, with increasing fraction of clustered phase as the volume fraction of PCBM increases (Figure 5a,b)

Summary

Introduction

Recent technological advances in organic photovoltaics (OPV)have resulted in the development of devices with power conversion efficiencies of >17%.1 An unresolved scientific challenge for this technology is understanding−and controlling−the mechanism of photoinduced electron−hole (e−h) separation. Recent technological advances in organic photovoltaics (OPV). Have resulted in the development of devices with power conversion efficiencies of >17%.1. An unresolved scientific challenge for this technology is understanding−and controlling−the mechanism of photoinduced electron−hole (e−h) separation. The extent to which Coulombic interactions between the electron and hole directly after exciton dissociation at a donor:acceptor (D:A) interface limit photocurrent generation has remained controversial,[2] because of inconclusive evidence from experimental and theoretical studies producing contradictory e−h separation models.[3−8] There is extensive evidence from transient absorption and photoluminescence (PL) spectroscopic studies for the existence of photogenerated e−h pairs that are spatially localized and subject to stronger Coulombic binding interaction which, once formed, can limit photocurrent. Received: October 24, 2018 Published: February 26, 2019.

Results

Discussion

Conclusion