Metal halide perovskite solar cell has demonstrated a rapid growth in power conversion efficiency (PCE) in the last decade attracting a high level of research and development (R&D) and commercialisation activities. As the bandgap of perovskite can be tuned over a wide range (1.2 eV to 2.3 eV) by simply tuning its composition, perovskites are promising candidates for low-cost, solution-processable multi-junction tandem solar cells with efficiency potential over 40%.[1] The certified efficiency of perovskite-based tandem solar cells has reached over 31% in just 5 years, while it took over a decade for perovskite single-junction solar cells to attain efficiency over 25%. For the fabrication of monolithic perovskite-perovskite tandem solar cells, generally, a high bandgap (~1.8 eV) Pb based perovskite subcell is first fabricated on a transparent substrate followed by the deposition of interconnecting-layer(-stack) and lastly by the fabrication a low bandgap (~1.25 eV) Sn-Pb based perovskite subcell.

Water-based poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has been used as the hole transport layer (HTL) for low bandgap Sn-Pb based perovskite subcell in state-of-the-art perovskite-perovskite tandems demonstrations[2]-[7] due to its low-lying highest occupied molecular orbital (HOMO) level for favourable energy band alignment with Sn-Pb perovskite. However, when used in a tandem, the presence of water in the PEDOT:PSS albeit small will be exposed to the underlying interconnecting layer stack and the high bandgap cell that are water-sensitive resulting in pinholes and film non-uniformity. In addition, the presence of water in the PEDOT:PSS  “contaminates” the inert atmosphere of gloveboxes and the like requiring part of the process sequence to be conducted in ambient exacerbating water and oxygen exposure during the fabrication of perovskite-perovskite tandem solar cell.

Another drawback of the conventional water-based PEDOT:PSS is its limited conductivity. Spin-casted PEDOT:PSS has a core-shell structure consisting of a highly conductive, positively charged conjugated PEDOT core, and an insulating, negatively charged polyelectrolyte counterions of PSS shell.[8] The conductivity of PEDOT:PSS is limited by the latter, but its presence is essential for preparing an aqueous dispersion of PEDOT, which is generally insoluble.[9] To increase the conductivity of PEDOT:PSS layer, the partial removal of PSS from the PEDOT:PSS surface has been demonstrated in a number of prior works by additive incorporation [10]-[12] or post-solvent treatment.[13]-[14] In principle, a completely PSS-free PEDOT with alternative counter-ions  can further enhance the conductivity and hole transport property of perovskite solar cell.

In our work we demonstrate that the use of water-free and PSS-free PEDOT increases the ratio of the conductive component to the non-conductive component in the HTL by ~5 times. The hydrophobic surface of water-free PEDOT also facilitates large-grain-perovskite-growth and exhibits enhanced electron blocking capability resulting in suppressed Shockley–Read–Hall recombination in the low bandgap subcell. This leads to improved device performance. Our champion perovskite-perovskite tandem cell achieved 85.8% fill factor and 21.5% efficiency under reverse scan. The fill factor achieved in our work is the highest among any perovskite-based double junction tandem solar cell.

The absence of water also improves the stability of the underlying high bandgap cell with its interconnecting layer stack and the overall reproducibility of tandem cells. All photovoltaic parameters of our tandem cells based on water-free and PSS-free PEDOT have narrower distributions than those reported for perovskite-perovskite tandems reported to date showing high reproducibility. The demonstrated results will be highly relevant to the future development of high-efficiency perovskite-perovskite tandem solar cells.