The development of high efficiency semi-transparent perovskite solar cells is necessary for application in integrated photovoltaics and tandem solar cells. However, perovskite's sensitivity to temperature and solvents impose a restriction on following processes, thus favouring physical vapor deposition for the transparent contacts. Protection may be necessary especially for high energy sputtering and a transparent buffer layer providing good electrode adhesion and conductivity is desired. Here we evaluate Ag and MoOx buffer layers in pursuit of high efficiency tandem solar cells. The usage of thin Ag as a buffer layer demonstrated Indium Tin Oxide (ITO) contacts that were resistant to delamination and yielded a 16.0% efficiency of semi-transparent perovskite solar cell with average transparency of 12% in visible range and > 50% in near infrared. Further application in tandem with Cu(In,Ga)Se showed an overall efficiency of 20.7% in a 4-terminal (4T) configuration exceeding the subcells individual efficiencies.

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The promising potential of achieving high efficiency organic tandem cells due to the widened absorption spectrum range has resulted in significant research in novel device concepts such as the modification of the recombination layer. In this study, a typically used charge transport interlayer in the recombination layer, MoO3, is modified by tuning the energy band and work function, with the help of an energy band model, to achieve energetic alignment without additional metallic layers. The energy level tuning is demonstrated by the co-evaporation of aluminum, which results in a doping effect. This shifts the work function of pristine MoO3 from 5.8[Formula: see text]eV to 4.3[Formula: see text]eV. The shift is proposed to be due to the formation of oxidized Al3+ and reduced MoO6+ that generates further mid-gap defect states and this shift increases with respect to Al concentration. The energy band shift changes the transport mechanism from hole-transporting to electron-transporting and the modification is modeled in this work. This suggests that recombination layer of MoO3/Al-MoO3 may have better energetic alignment and integrating this modified recombination layer results in improvement in the stacked device efficiency from 1.38% to 3.38% with enhancement in [Formula: see text], [Formula: see text] and fill factor. This enhancement is contributed from better aligned energy level and higher transparency of the recombination layer. The effective transparent recombination allows greater photon distribution to the top cell and increasing maximum matched photocurrent to the limiting subcell, thus improving photocurrent by 25% and overall efficiency up to 4.6%.