Coupling two solar cells in series (in “tandem”) allows one to split the solar spectrum into separate blue and red absorbers, which enables to match the bandgap at which solar photons are absorbed, to the energy of the photons. This recipe is able in principle to notably enhance the efficiency of solar cells with respect to single absorber devices. However, the construction of a monolithic tandem device on a substrate, with adequate intermediate layer to add up the voltages of the two layers, is challenging. There has been a lot of talk about the prospect of highly efficient tandem solar cells using the organometal halide perovskite as front absorber. It is believed that the metal halide perovskite, due to the properties of tunable bandgap and low temperature processing, is a good complement to enhance the efficiency of inorganic high efficiency solar cells as crystal silicon or CIGS. Another important target is to develop tandem solar cells entirely by solution process routes, that have the potential of lower production cost, albeit at a cost of lower conversion efficiency. A high voltage monolithic solar cell is also a good candidate to drive fuel production reaction as water splitting, if combined with suitable catalysis layers.
Progress in this direction is reported now in Applied Physics Letters by our good friend Teo Todorov and his coworkers at IBM T. J. Watson Research Center. In the past Teo contributed with David Mitzi the highest efficiency inorganic Cu2ZnSn(S,Se)4 (CZTSSe) kesterite solar cells. In this work the IBM researchers present a monolithic cell CZTSSe/NH3CH3PbI3 with PEDOT:PSS intermediate recombination layer and aluminium front contact. A nice result is that the obtained photovoltage 1353 mV is practically the addition of the separate voltages of the components. On the other hand the current is relatively low and the tandem device does not improve the separate efficiencies. This is explained in terms of the drastically reduced illumination in all layers and high sheet resistance of the Al contact. This is nevertheless and important result that shows further the versatility of perovskite solar cells and solution processed photovoltaics.