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Back-contact architectures for perovskite solar cells eliminate parasitic-absorption losses caused by the electrode and charge collection layers but increase surface reflection due to the high refractive index mismatch at the air/perovskite interface. To mitigate this, a ∼85 nm thick layer of poly(methyl methacrylate) (PMMA), with a refractive index between those of air and perovskite, has been applied as an antireflective coating. Transfer matrix modelling is used to determine the ideal PMMA layer thickness, with UV-Vis spectroscopy measurements used to confirm the increase in absorption that arises through the application of the antireflective coating. The deposition of a thin film of PMMA via spin coating onto a solar cell results in a 20–30% relative increase in short circuit current density and stable power output density.

The fabrication of back-contact electrodes with micron-sized features by microsphere lithography is implemented via a modified "natural lithography" approach. The solution-based assembly of microsphere beads on a substrate occurs via the electrostatic attraction between the molecular monolayer-functionalized substrate and the micron-sized polystyrene microbeads with carboxyl surface groups. Through a modification of the original "natural lithography" method, the density of the microbeads used as a lithographic mask can be increased 5-fold. The resulting back-contact electrodes are used for the fabrication of perovskite solar cell devices and the examination of their potential. Devices with electrodes fabricated using a modified "natural lithography" approach showed a 3.5fold increase in performance compared to the devices with electrodes made using the original method.

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