Molecular vibrations reduce the maximum achievable photovoltage in organic solar cells

The low-energy edge of optical absorption spectra is important for solar cells but is not well understood in the case of organic solar cells (OSCs) where excitonic effects are important. Here we study​ the microscopic origin of exciton bands in molecular blends and investigate their role in OSCs.
Molecular vibrations reduce the maximum achievable photovoltage in organic solar cells

In our recent publication, we investigate the temperature dependence of the excitonic density of states and low-energy absorption features including low-frequency molecular vibrations and the hybridization between excitons.  For model donor-acceptor blends, featuring charge-transfer excitons, our simulations agree very well with temperature-dependent experimental absorption spectra. We unveil that the quantum effect of zero-point vibrations, mediated by electron-phonon interaction, causes a substantial exciton bandwidth and reduces the open-circuit voltage, which is predicted from electronic and vibronic molecular parameters. This effect is surprisingly strong at room temperature and can substantially limit the efficiency of OSCs.

To investigate the impact of microscopic molecular parameters on lineshape, linewidth and thermal disorder, as well as their influence on OSC device parameters, we first study theoretically and experimentally model systems of electron donating (donor) and electron accepting (acceptor) molecules. This is based on the simulation of excitonic properties (including molecular vibrations and exciton hybridisation) and sensitively measured external quantum efficiency (EQE) spectra for temperatures between 80 and 350 K. We find that absorption tails can be dominated by zero-point vibrations even at room temperature. This fundamental quantum effect is responsible for lowering the performance of OSCs, thus rendering vibrations and their electron-phonon coupling an important subject for future OSC research. 

We model the low-energy optical absorption of OSC materials, which is strongly influenced by the electron–electron and the electron–phonon interaction that lead to excitonic states that are spatially more localized than Wannier excitons in traditional semiconductors. To include excitonic effects, we need to simulate the excitation spectrum (i.e. the excitonic density of states (EDOS)), taking into account both the electronic and vibrational properties of the organic systems. For the prediction of the EDOS the molecular electronic and vibrational material parameters of the material systems are obtained from density functional theory calculations. The EDOS is calculated numerically with a Lanczos approach that has been extended to calculate excitonic properties. The new approach allows large-scale calculations of the EDOS of donor-acceptor blends composed of up to 40,000 molecules as depicted in figure 1. Thanks to the supercomputing centres of the Technische Universität Dresden (ZIH) and the Leibniz Supercomputing Centre in Garching (LRZ) the excitation spectra of these large molecular blends could be obtained which has not been available so far.

The firm connection between molecular material parameters and characteristics of the OSC blends, allows us to discuss guidelines to reduce vibration-induced radiative voltage losses based on molecular parameters. The proposed strategies to minimise such voltage losses (of up to several hundred meV, depending on the system), i.e. reduction or compensation of electron-phonon coupling and exciton delocalisation, are supported by the excellent agreement between theory and experiment. These guidelines should be helpful to consider at the stage of materials development.

Illustration of the generation of charge pairs (excitons), the precursors of free charge carriers in the active layer of an organic solar cell. Credit: M. Panhans

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