Perovskite Silicon Tandem Photovoltaics

Perovskite silicon tandem solar cells promise to reduce electricity generation costs and lower resource consumption compared to conventional silicon solar cells. The perovskite solar cell is deposited directly onto the silicon solar cell in a tandem stack. Since the perovskite semiconductor has a higher energy band gap than silicon, the perovskite solar cell can utilize high-energy photons better than the silicon solar cell. In turn, the silicon solar cell can efficiently use the low-energy photons, which are transmitted through the perovskite solar cell. The overall efficiency increases and efficiencies of over 30% are possible.

As part of the »Presto« project funded by the German Federal Ministry of Economics and Climate Protection (BMWK), Fraunhofer ISE was able to achieve high efficiencies using scalable methods for perovskite silicon tandem solar cells. The silicon solar cell was an a-Si/c-Si heterojunction solar cell textured on both sides. A layer of indium-doped tin oxide (ITO) served as the electrical connection to the perovskite solar cell. A very thin layer of an organic hole conductor was deposited on this, followed by the perovskite absorber. With the absorber compound FA0.75Cs0.25Pb(I0.8Br0.2)3 we could achieve an optimal band gap of 1.68 eV and a high stability. 

In the Fraunhofer lead project »MaNiTU« we are now further developing this technology with the aim of achieving even higher efficiencies. In addition, we are researching lead-free alternatives for the absorber and for optimizing the contact and intermediate layers. Initial analyses of the life cycle already indicate major ecological added value. Among other things, this is due to the fact that the layers of the perovskite solar cell are very thin and the higher efficiency significantly reduces the CO2 footprint for generating one kilowatt hour of electricity.

The situation is similar for the electricity generation costs: The production of the additional layers for the perovskite solar cell is prospectively possible at very low costs. Overall, the efficiency increases significantly and the cost per kWh of electricity generation decreases. In a detailed cost analysis, we found that perovskite silicon tandem solar cells are particularly promising for rooftop and other applications with area constraints. Aside from this, they promise a significant cost advantage over single-junction silicon solar cells, assuming that perovskite technology stays on track and reaches a level where the efficiency of industrial-scale tandem solar cells exceeds 30% in the near future. The aim is to implement cost-effective processes and make the lifetime of the modules comparable to that of silicon.