Factors Limiting Device Efficiency in Organic Photovoltaics

TLDR

Organic photovoltaic cells have recently achieved power conversion efficiencies exceeding 10 %. This review seeks to identify the factors limiting OPV efficiency, explain the operational mechanism and detailed balance limit, and highlight how closing this gap will guide future materials research. The authors discuss empirical and fundamental approaches—based on donor–acceptor semiconductor physics, thermodynamics, and kinetics—to estimate the maximum practically achievable efficiency in OPV cells. Empirical models predict 10–12 % limits, whereas fundamental descriptions indicate 20–24 % limits are attainable in single‑junction OPVs, comparable to crystalline silicon.

Abstract

The power conversion efficiency of the most efficient organic photovoltaic (OPV) cells has recently increased to over 10%. To enable further increases, the factors limiting the device efficiency in OPV must be identified. In this review, the operational mechanism of OPV cells is explained and the detailed balance limit to photovoltaic energy conversion, as developed by Shockley and Queisser, is outlined. The various approaches that have been developed to estimate the maximum practically achievable efficiency in OPV are then discussed, based on empirical knowledge of organic semiconductor materials. Subsequently, approaches made to adapt the detailed balance theory to incorporate some of the fundamentally different processes in organic solar cells that originate from using a combination of two complementary, donor and acceptor, organic semiconductors using thermodynamic and kinetic approaches are described. The more empirical formulations to the efficiency limits provide estimates of 10-12%, but the more fundamental descriptions suggest limits of 20-24% to be reachable in single junctions, similar to the highest efficiencies obtained for crystalline silicon p-n junction solar cells. Closing this gap sets the stage for future materials research and development of OPV.