Perovskite solar cells and their application in tandem devices with crystalline silicon

As the world longs for a solution to the energy problem, a new photovoltaic (PV) technology might have come about to turn things around. Even though commercialization might still take a while (and it may as well be not too long), Perovskites have shaken things up in the world of solar energy conversion, and are not done yet. The possible applications are countless, ranging from flexible solar cells to building integration (BIPV) or from stand-alone large scale energy harvesting applications to tandem devices with other technologies. We do not know what the future holds, but this technology is surely paving its way to success. This thesis deals with some of the hot topics in the field and after a brief introduction on the working principles of photovoltaics and of the various technologies, it presents lead halide based perovskites and their use as energy harvesting material. The final scope is to identify the ideal strategy to couple the solar cells with silicon PV in a tandem configuration. For this reason, a multi-directional approach is necessary to investigate which possibilities the field of perovskites has to offer. Two different solar cell configurations are presented: the mesoscopic n-i-p structure and the planar p-i-n. For the n-i-p architecture, the two main topics discussed are the doping of the mesoporous layer with crystalline silicon nanoparticles in order to enhance the performance of the devices, and the substitution of the expensive spiro-OMeTAD hole selective material with a less expensive alternative, in the attempt to ease the path toward large-scale industrial production. The p-i-n structure is relatively new and will require more development to reach the performance of its counterpart, but it offers higher stability. One of the issues this work addresses is the scarce wettability of the commonly used polymeric hole selective layer, solved by an easy, yet effective, light treatment. Then, the substitution of said polymeric material with the more stable inorganic nickel oxide, also considering its dependence on ambient relative humidity. The heart of the thesis is two-terminal perovskite/silicon tandem solar cells. A novel and efficient design is presented with the two-terminal mechanically stacked configuration, yielding a 26.3% Power Conversion Efficiency on a 1 cm2 active area by means of graphene doping of the mesoporous layer of the n-i-p perovskite top-cell. Lastly, a first attempt at a monolithic tandem is shown with an innovative strategy to avoid parasitic absorption at the interface between the two sub-cells. 5 Finally, an outlook on the perovskite and tandem technologies is provided, with the aim to draw a strategy for future developments of this very promising approach.