Investigation of reducing parasitic absorption losses in perovskite solar cells with carbon back electrode
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Perovskite solar cells offer a sustainable pathway towards multi-terawatt photovoltaics, but their poor long-term stability remains a challenge for commercialization. The use of carbon rear electrodes shows improved stability compared to metal electrodes due to the chemical inertness of the material. However, carbon electrodes introduce an additional optical loss as parasitic absorption in the non-reflective electrode reduces the current generation and thus device efficiency.
This work investigates the reduction of parasitic absorption losses in the carbon electrodes and enhance useful absorption in the perovskite layer. Two approaches are explored: First increasing the absorber layer thickness and implementing a reflective rear electrode to redirect light back into the absorber, effectively doubling the optical path length. Optical simulations using the transfer matrix method (TMM) and a Monte Carlo approach for thickness variation were conducted, followed by experimental fabrication and characterisation of perovskite solar cells. An optical model for the TMM was established and validated with UV-VIS spectrometry measurements.
Second the absorber thickness was systematically increased by raising the perovskite precursor molarity from 1.1 M to 1.5 M in three steps. Thickness changes were characterised using SEM, and absorption and current enhancement were quantified via UV-VIS spectrometry, external quantum efficiency (EQE), and stabilised current measurements. Higher molarity led to increased thickness and current enhancement. Substituting the carbon electrode with a reflective gold electrode resulted in a significant decrease in device performance due to electrical issues at the gold/PEDOT interface. However, normalised EQE measurements demonstrated the optical benefits of the reflective material. Additional experiments with Spiro-OMeTAD instead of PEDOT as hole transport layer and gold electrodes yielded good performance and reduction of the parasitic absorption in the 650-800 nm range.
The results show that parasitic absorption losses in the carbon electrode account for approximately 6-12% of potential photocurrent loss. The reflection-based approach showed more promising results than thickness enhancement. Future work should focus on implementing optical structures in front of the carbon electrode that provide reflective properties while maintaining electrical contact.
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This item has been published with the following license: In Copyright