A considerable portion of photonics based research to enhance thin film solar cell performance considers the incorporation of plasmonic nano-particles to enhance light absorption and consequently increase the short circuit current. This approach, which has produced some interesting results, suffers from, essentially, two shortcomings: It has a positive effect in only one of the three photovoltaic parameters that determine the power conversion efficiency (PEC), while it may also have a negative impact on the electronic performance of the device. In the work we present, we will discuss several novel photonic approaches, having a minimal impact on electronic aspects of the solar cell not directly linked to light absorption or emission, capable of enhancing either the short circuit current or open circuit voltage for organic and perovskite solar cells.

In one configuration the standard ITO based transparent electrode is replaced by a 1-dimensional multilayer containing, at least, a TiO2 and a Ag layers [1]. This combination forms with the back metal electrode in thin film cells two coupled optical cavities with a resonance degeneracy which can be broken to produce a broadband light trapping. When applied to non-fullerene acceptor based single junction polymer cells we show that PECs close to 14% can be reached.

In a perovskite cell, by naturally transferring the random nano-texturing inherent of the perovskite layer to the back semiconductor/metal interface, where the contrast in the imaginary part of the refractive index is very large, we demonstrate that backscattering reduces light escape leading to an optimal light absorption bringing the PCE from 19.3% to 19.8%. Such path towards an ergodic behavior for maximum light absorption in perovskite cells may lead to the most effective light absorption in such cells [2].

To bring perovskite solar cells towards the Shockley-Queisser limit requires lowering the bandgap while simultaneously increasing the open circuit voltage. This, to some extent divergent objective, may demand the use of large cations to obtain a perovskite with larger lattice parameter together with a large crystal size to minimize interface non-radiative recombination. We successfully incorporated such large cations in larger than 1 μm perovskite crystals and fabricated cells that exhibited a largely increased fluorescence quantum yield and an open circuit voltage equivalent to 93% of the corresponding radiative limit one.

1 Quan Liu et al., Adv. Energy Mater. 7, Art. No. 1700356 (2017).

2 Hui Zhang et al., ACS Photonics (2018).