Abstract
In organic photovoltaics, the energetic characteristics of the charge-transfer state determine the device properties, and are defined by molecular interactions at the donor-acceptor interface. This study uses sensitive spectroscopy to investigate the roles of morphological disorder and molecular dynamics in charge-transfer states, which ultimately affect the open-circuit voltage. The authors model temperature-dependent data and find that dynamic, rather than static, disorder determines the optical properties of the charge-transfer states. Thus, to develop highly efficient organic solar cells, the intramolecular properties of the working materials should receive more attention.
Highlights
Organic solar cells (OSCs) have gained renewed interest since their power conversion efficiency increased rapidly over the last few years and are nowadays approaching 20% [1,2,3]
This study provides an in-depth analysis of temperature-dependent broadening of the spectroscopic absorption and emission features of CT states in devices with small molecule-fullerene blends
Absorption and ηPV are related by the internal photovoltaic quantum efficiency for conversion of absorbed photons to extracted charge carriers [24]
Summary
Organic solar cells (OSCs) have gained renewed interest since their power conversion efficiency increased rapidly over the last few years and are nowadays approaching 20% [1,2,3]. All efficient approaches favor a combination of at least two materials, namely an electron donor and acceptor, forming a heterogeneously blended layer sandwiched between selective electrodes. To facilitate the dissociation of strongly bound photogenerated excitons, free energy is usually sacrificed. This is realized at the donor-acceptor interface by the formation of energetically more favorable and less strongly bound charge-transfer (CT) states. CT states are responsible for more efficient charge carrier generation. As optical absorption of CT states is usually very weak in OSCs, it is often measured by sensitive detected ηPV spectroscopy. Absorption and ηPV are related by the internal photovoltaic quantum efficiency (ηIQE) for conversion of absorbed photons to extracted charge carriers [24]. The effect was shown to be negligible for absorbing layer thicknesses below 80 nm [14,25,26], and to be independent of the device temperature [18]
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