Public Abstract

DE-SC0012458: Exploring the nature of charge transfer states at molecular and hybrid organic-inorganic heterojunctions

Award Status: Active

Institution: The Trustees of Princeton University, Princeton, NJ
UEI: NJ1YPQXQG7U5
DUNS: 002484665

Most Recent Award Date: 06/17/2024
Number of Support Periods: 11
PM: Mewes, Tim

Current Budget Period: 08/15/2024 - 08/14/2025
Current Project Period: 08/15/2023 - 08/14/2026
PI: Rand, Barry

Supplement Budget Period: N/A

Public Abstract

The field of organic photovoltaics (OPVs) has progressed over the past several years and has now reached a stage where further advances in the power conversion efficiency (current state-of-the-art = 18.2%) require a fundamental understanding of all possible sources of energy loss in the conversion process and the development of strategies to eliminate them systematically. Charge transfer (CT) states at the donor-acceptor (DA) interface of OPVs are a key intermediary in both the charge photogeneration and recombination processes, and have been shown to underlie the thermodynamic limiting efficiency of OPV cells. The presence of CT states is the primary reason why OPV cells exhibit a larger open-circuit voltage loss than their inorganic counterparts, and is why understanding their role in charge separation, energetic relaxation, and non-radiative recombination is so crucial. Over the past several years, it has become clear that a substantial portion of this voltage loss stems from the energetically-disordered distribution of CT states in a typical OPV cell based on the assumption that they exist in quasi-equilibrium with one another (i.e. they are described by a Boltzmann distribution with a well-defined chemical potential). Recent work suggests, however, that this assumption (a bedrock for most photovoltaic material systems) may not hold for OPV owing to inefficient relaxation within the CT density of states. Perhaps this could be expected, but not yet described, as hopping transport implies little correlation between too-distant sites. A key objective of the proposed work is therefore to explore the breakdown of quasi-equilibrium in OPV cells, understand its implications for OPV operation and modeling, and determine whether non-thermal CT state and free carrier distributions can be harnessed to improve performance. If successful, this work will provide a scientific basis for reducing voltage loss in future organic solar cells and strengthen the technological leadership of the U.S. in the development of materials for solar energy conversion. In addition to organic-organic heterojunctions, there is an emphasis on exploring new CT state physics in hybrid systems consisting of organic molecules paired with two-dimensional perovskite and transition metal dichalcogenide semiconductors. These systems provide a unique platform to study the factors that influence CT state binding energy by controlling the dimensionality and spatial extent of the underlying electronic states. To address these questions, the three PIs in this project bring together a broad range of complementary expertise and measurement techniques including ultraviolet and inverse photoelectron spectroscopy, voltage- and temperature-dependent quantum efficiency measurements, transient photoluminescence, modulation electroluminescence spectroscopy, X-ray scattering, and a variety of microscopy techniques. Coupled with the ability to grow heterojunction morphologies ranging from single crystal donor-acceptor interfaces to hybrid and disordered bulk heterojunctions, the PIs in this proposal bring together the expertise and record of successful collaboration that are needed to advance the fundamental understanding of CT states that is required to advance OPV performance.

The field of organic photovoltaics (OPVs) has progressed over the past several years and has now reached a stage where further advances in the power conversion efficiency (current state-of-the-art = 18.2%) require a fundamental understanding of all possible sources of energy loss in the conversion process and the development of strategies to eliminate them systematically. Charge transfer (CT) states at the donor-acceptor (DA) interface of OPVs are a key intermediary in both the charge photogeneration and recombination processes, and have been shown to underlie the thermodynamic limiting efficiency of OPV cells. The presence of CT states is the primary reason why OPV cells exhibit a larger open-circuit voltage loss than their inorganic counterparts, and is why understanding their role in charge separation, energetic relaxation, and non-radiative recombination is so crucial.

Over the past several years, it has become clear that a substantial portion of this voltage loss stems from the energetically-disordered distribution of CT states in a typical OPV cell based on the assumption that they exist in quasi-equilibrium with one another (i.e. they are described by a Boltzmann distribution with a well-defined chemical potential). Recent work suggests, however, that this assumption (a bedrock for most photovoltaic material systems) may not hold for OPV owing to inefficient relaxation within the CT density of states. Perhaps this could be expected, but not yet described, as hopping transport implies little correlation between too-distant sites. A key objective of the proposed work is therefore to explore the breakdown of quasi-equilibrium in OPV cells, understand its implications for OPV operation and modeling, and determine whether non-thermal CT state and free carrier distributions can be harnessed to improve performance. If successful, this work will provide a scientific basis for reducing voltage loss in future organic solar cells and strengthen the technological leadership of the U.S. in the development of materials for solar energy conversion.

In addition to organic-organic heterojunctions, there is an emphasis on exploring new CT state physics in hybrid systems consisting of organic molecules paired with two-dimensional perovskite and transition metal dichalcogenide semiconductors. These systems provide a unique platform to study the factors that influence CT state binding energy by controlling the dimensionality and spatial extent of the underlying electronic states. To address these questions, the three PIs in this project bring together a broad range of complementary expertise and measurement techniques including ultraviolet and inverse photoelectron spectroscopy, voltage- and temperature-dependent quantum efficiency measurements, transient photoluminescence, modulation electroluminescence spectroscopy, X-ray scattering, and a variety of microscopy techniques. Coupled with the ability to grow heterojunction morphologies ranging from single crystal donor-acceptor interfaces to hybrid and disordered bulk heterojunctions, the PIs in this proposal bring together the expertise and record of successful collaboration that are needed to advance the fundamental understanding of CT states that is required to advance OPV performance.