The multiple ways of making perovskite/silicon tandem solar cells: Which way to go?

Abstract

Monolithic perovskite/silicon tandem solar cells are of interest in the photovoltaic community thanks to their potential to combine high power conversion efficiency (PCE) with affordable cost. In the last decade, significant advancements have been reported towards this goal. However, to make perovskite/silicon tandems fully industry-relevant, exclusively scalable fabrication methods and materials need to be employed. Vacuum-based processing techniques can provide a conformal coverage on the pyramidal texture, typical for single-junction silicon solar cells. For such tandems, we reported 25% certified PCE with record current densities of 19.8 mA cm-2. Specifically, we used the vacuum/solution hybrid technique for the perovskite layer, combined with nanocrystalline recombination junctions to keep possible electrical micro shunts localized.[1] Solution-based techniques, specifically one-step perovskite spin-casting, have shown rapid advancements for single-junction perovskite solar cells. However, fully covering perovskite films on micron-scale textured interfaces with this technique requires process sophistication. To achieve end-to-end coverage, we reduced the pyramid size to 1-2 mm and adjusted the perovskite precursor solution concentration. Combining this with 1-butanethiol surface passivation enabled a certified PCE of 25.7% with negligible losses after 400 hours of operation.[2] Next, to translate the solution-based method to large-scale deposition, we adopted slot-die-coated perovskite top cells on textured surfaces since it offers significant advantages in throughput and material utilization. With this approach, we reported 23.7% PCE for the first proof-of-concept device.[3]

Beyond the requirement towards the use of industry-compatible silicon bottom cells (avoiding mirror-polished surfaces), which dictates appropriate perovskite processing techniques, the best choice for the device polarity is still to be settled as well. The initial perovskite/silicon tandems were in the n-i-p configuration but were limited by a high parasitic absorption in the hole-collecting contact stacks at the front (as well as the non-ideal optical design of the bottom cells, using double-side polished wafers). Global tandem research refocused, therefore, onto the p-i-n configuration. However, as a result, perovskite/silicon tandem research no longer stood to benefit from impressive progress made for efficient n-i-p perovskite single-junction solar cells. Nevertheless, adopting these advancements to tandem solar cells may be key towards perovskite/silicon tandems with PCEs well over 30%. Therefore, in this contribution, we will also discuss the existing challenges and our recent advancement on the n-i-p configuration tandems. Overall, this talk will give insight into the future directions to be taken to push the PCE of the perovskite/silicon tandem solar cells beyond 30%.