Bifacial Four-Terminal Perovskite/Silicon Tandem Solar Cells and Modules

Ten years after the first paper(1) on the properties of metal halide perovskite solar cells, their efficiency and stability have increased tremendously.(2) It was quickly realized that their application goes beyond the single-junction use. Indeed, perovskite cell technology, by virtue of its tunable bandgap and low sub-bandgap absorption, offers new opportunities for stacking solar cells of different bandgap in a multijunction device to overcome the fundamental Shockley–Queisser (SQ) efficiency limit of a single-junction device. Under AM1.5 irradiation, this limit is 33.7% for the optimal bandgap, and for perovskite with a normally somewhat higher bandgap of 1.55 eV it drops to 31%.(3,4) It is not expected that perovskite will exceed 26% single-junction efficiency.(5) For crystalline silicon solar cells (c-Si), including Auger recombination, the theoretical SQ limit is 29.4%.(6,7) Currently, single-junction silicon solar cells reached an efficiency in the lab of 26.7%;(8,9) while in mass production, solar cells are produced with efficiencies up to about 25%,(10) with main stream efficiencies of about 22%. The latter have been increasing by 0.4%/year, and this trend is expected to continue for a number of years, but it will likely become overly costly to go beyond 24–25%. This efficiency increase has contributed significantly to the steep learning rate, which is the average reduction of Si PV module selling price for every doubling of cumulative shipment, of 39.8%(11) that has been experienced since 2006. Although the manufacturing cost reduction also plays a major role, we expect that when the practical efficiency limits are being approached, the silicon PV industry will not be able anymore to maintain such a learning rate. Aside from module price, the further PV system costs (like installation) to a large extent depend on area and are reduced per unit of power output simply by higher module efficiency. It is because of the possibility that it can help overcome both these performance and cost limitations that metal halide perovskite-on-silicon tandem devices have been under development since 2015(12) and today lead to power conversion efficiencies of over 29%.(13,14