Reducing recombination losses in planar organic photovoltaic cells using multiple step charge separation

T D Heidel,D Hochbaum,I Hiromi,V Singh,M E Bahlke,J Lee,J M Sussman,M A Baldo

doi:10.1063/1.3585863

Abstract

We enhance the efficiency of heterojunction organic solar cells by introducing a thin interfacial layer between the acceptor and donor layers. The interfacial layer energy levels are chosen to provide a gradient for charges crossing the interface, approximating a conventional p-n junction with three organic semiconductors. Devices with interfacial layers exhibit increased open circuit voltage (VOC) and increased short circuit current (JSC). The increase in VOC is due to a reduction in dark current and charge recombination. The increase in JSC is correlated with an increase in the conversion efficiency of excitons originating in the donor or acceptor layers. The interfacial layer destabilizes charge transfer states at the donor-acceptor interface, yielding reduced exciton recombination. The introduction of thin interfacial layers may prove to be an important probe of the physics of exciton separation in organic photovoltaic cells.

Highlights

The introduction of thin interfacial layers may prove to be an important probe of the physics of exciton separation in organic photovoltaic cells
It is known that organic solar cells require an interface to efficiently dissociate tightly bound excitons,7 but it is evident that charge transfer states at the donor-acceptor interface can be strongly bound
When a 10-nm-thick interfacial layer is introduced, the External quantum efficiency (EQE) associated with perylenetetracarboxylic bisbenzimidazole (PTCBI) returns to the same magnitude found for devices with no interfacial layer, while the EQE for excitons generated in m-MTDATA falls below that of the control device

Summary

INTRODUCTION

Organic semiconductor photovoltaics have the potential to become a scalable, inexpensive source of solar energy generation. The technology has advanced through various device structures in recent years, including donor/acceptor heterojunctions, bulk heterojunctions, tandem structures, and the use of electrically doped transport layers. The efficiency of all devices, could benefit from improved understanding of the physics of exciton separation at the donor-acceptor interface. The efficiency of all devices, could benefit from improved understanding of the physics of exciton separation at the donor-acceptor interface. It is known that organic solar cells require an interface to efficiently dissociate tightly bound excitons, but it is evident that charge transfer states at the donor-acceptor interface can be strongly bound.. The open circuit voltage (VOC) of an organic solar cell is believed to be fundamentally limited by the energy of the bound charge transfer (CT) state, parameters like interface morphology are significant.. The VOC of organic solar cells is typically far below the fundamental limit set by the energy of the bound CT state..

EXPERIMENT

Findings

CONCLUSION