Rohit Prasanna1 Tomas Leijtens1 Michael McGehee1

1, Stanford University, Stanford, California, United States

As photovoltaic module manufacturing costs have plummeted, improving the power conversion efficiency has emerged as the best way to reduce the cost of installed solar power. Metal halide ABX3 perovskite solar cells benefit from versatile processing - they have been demonstrated by roll-to-roll-compatible processes using both solution and vapor phase deposition. They can be compositionally tuned to have band gaps appropriate to both top and bottom subcells in a tandem. All-perovskite tandem solar cells, therefore, can surpass the Shockley-Queisser efficiency limit without requiring cost-prohibitive epitaxial growth methods needed for III-V solar cells.

We recently demonstrated the first monolithic perovskite tandems based on a tin lead perovskite as the bottom cell. With 50% tin and 50% lead at the B-site, a band gap of 1.27 eV is attained. A 1.8 eV band gap lead-based mixed halide perovskite is used as the top cell. A sputtered ITO interlayer functions as the recombination layer connecting the subcells and serves to protect the first cell from solvents used to deposit the second cell. This proof-of-concept device shows a stabilized efficiency of 17%. We present a roadmap for development of all-perovskite tandems to reach significantly higher efficiencies than single junction devices.

Perovskite tandems so far have been hampered by poor quantum efficiency in the infrared. Depositing high quality optically thick tin lead perovskite films has proved challenging. In addition, processing the second cell by methods that do not damage the first cell is necessary for high yield over large areas. There is room for improvement in open circuit voltage of the top cell - larger voltage losses have typically been observed for wider band gaps that are needed to match high-performing low gap perovskites.

Tin-lead perovskites for the bottom cell already attain excellent open circuit voltage relative to band gap. We show that there is further room for improvement - the photoluminescence quantum efficiency is far below typical values for lead perovskites, and improving this should yield voltage losses smaller than in the best crystalline-Si solar cells.

Optical modeling combined with device simulations fit to experimental JV curves shows that perovskite tandems can feasibly reach 32% efficiency. Energy yield modeling shows they can achieve over 42% improvement over a single junction silicon solar cell in both sunny and overcast climates. Cost models of thin-film solar manufacturing show that substrate costs outweigh materials costs for the absorber. Depositing two absorber stacks to make a tandem is not expected to add much cost. Perovskite tandems, therefore, show great promise for low-cost high-efficiency photovoltaics. This presentation aims to identify the potential of all-perovskite tandems and discuss strategies to tackle the challenges that must be overcome in order to realize this promise.