Abstract
Strong light-matter coupling can re-arrange the exciton energies in organic semiconductors. Here, we exploit strong coupling by embedding a fullerene-free organic solar cell (OSC) photo-active layer into an optical microcavity, leading to the formation of polariton peaks and a red-shift of the optical gap. At the same time, the open-circuit voltage of the device remains unaffected. This leads to reduced photon energy losses for the low-energy polaritons and a steepening of the absorption edge. While strong coupling reduces the optical gap, the energy of the charge-transfer state is not affected for large driving force donor-acceptor systems. Interestingly, this implies that strong coupling can be exploited in OSCs to reduce the driving force for electron transfer, without chemical or microstructural modifications of the photo-active layer. Our work demonstrates that the processes determining voltage losses in OSCs can now be tuned, and reduced to unprecedented values, simply by manipulating the device architecture.
Highlights
Strong light-matter coupling can re-arrange the exciton energies in organic semiconductors
As organic solar cell (OSC) efficiencies are currently limited by the large voltage losses, there is an urgent need for strategies to reduce the Eopt–qVOC losses and steepen the absorption edge to improve the power conversion efficiencies (PCE) of organic photovoltaic devices
To support strong exciton–photon coupling, organic materials need to exhibit strong absorption and a significant overlap between absorption and emission, which is most achieved in materials with a sharp absorption edge and small Stokes shift
Summary
Strong light-matter coupling can re-arrange the exciton energies in organic semiconductors. The open-circuit voltage of the device remains unaffected This leads to reduced photon energy losses for the low-energy polaritons and a steepening of the absorption edge. While strong coupling reduces the optical gap, the energy of the charge-transfer state is not affected for large driving force donor-acceptor systems. VOC is the open-circuit voltage of the device under 1 sun illumination and q the elementary charge This difference is often more than 0.6 eV for OSCs, which is 0.2 to 0.3 eV higher than for silicon, gallium arsenide, or perovskite-based solar cells[4]. As OSC efficiencies are currently limited by the large voltage losses, there is an urgent need for strategies to reduce the Eopt–qVOC losses and steepen the absorption edge to improve the PCE of organic photovoltaic devices. Strong coupling in organic semiconductors has been reported to lead to longer exciton diffusion lengths[21,22,23], higher charge carrier mobilities[24], control of photoisomerization[25], extended responsivity[26], and enhanced intersystem crossing between singlet and triplet states[27]
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