  Perovskite solar cells (PSCs) with transparent electrodes can be integrated
with existing solar panels in tandem configurations to increase the power
conversion efficiency. A critical layer in semi-transparent PSCs is the
inorganic buffer layer, which protects the PSC against damage when the
transparent electrode is sputtered on top. The development of n-i-p structured
semi-transparent PSCs has been hampered by the lack of suitable p-type buffer
layers. In this work we develop a p-type CuOx buffer layer, which can be grown
uniformly over the perovskite device without damaging the perovskite or organic
charge transport layers, can be grown using industrially scalable techniques
and has high hole mobility (4.3 +/- 2 cm2 V-1 s-1), high transmittance (>95%),
and a suitable ionisation potential for hole extraction (5.3 +/- 0.2 eV).
Semi-transparent PSCs with efficiencies up to 16.7% are achieved using the CuOx
buffer layer. Our work demonstrates a new approach to integrate PSCs into
tandem configurations, as well as enable the development of other devices that
need high quality p-type layers.
