Energy band and optical modeling of charge transport mechanism and photo-distribution of MoO₃/Al-doped MoO₃ in organic tandem cells

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 5.8eV to 4.3eV. 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 Jsc, Voc 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%.