Future of n-type PV

Abstract

In this chapter, we have reviewed candidates for further enhancement of cell efficiencies beyond those of today's mainstream PERC cells, with a focus on technological aspects rather than, e.g. cost. Regarding silicon single junctions, the prevalent theme is the use of carrier-selective passivating contacts, CSPCs. Of these, silicon heterojunction and polysilicon-on-silicon oxide (TOPCon/POLO) are most advanced and have enabled record high efficiencies above and close to 26%, respectively, on n-type silicon wafers. Further important topics are bifacial cell designs, which can be applied to different PV technologies. Single-side efficiencies above 25% have been achieved on bifacial TOPCon and bifacial SHJ solar cells. With proven bankability, bifacial PV products can be expected to gain more momentum in future development. In contrast, contacts based on metal compounds have yielded remarkable results in the last decade, yet failing to clearly evidence a significant advantage compared to the ones based on silicon. Further research is needed to unravel the material combination that would enable the long-awaited ultimate passivating contact for Si solar cells.

The second major topic are tandem and multijunction cells. This is the technology to move beyond the ultimate efficiency barrier of 29.4% for silicon PV and indeed, efficiencies well above 29% have been demonstrated in the lab for Si-based tandems. We have reviewed the current state of the art in lead halide perovskite-silicon tandems as well as III-V/silicon tandems. The former have reached a record PCE of 32.5% in monolithically integrated 2-terminal tandems, while III-V/Si 2T tandems currently stand at 23.4%. However, in III-V-Si devices, the number of absorbers has already been increased further, to three: in triple junction III-V/III-V/Si cells, PCEs of 35.9% have been realized with both 2T and 4T architectures. With a substantially higher cost for the III-V technology as compared to perovskites, but still inferior long-term stability in perovskites, as well as challenges in upscaling for both technologies, it remains to be seen which one of these technologies will gain an advantage. It should be mentioned that an important difference between reported silicon single junction and tandem/multijunction record devices is the cell area: while the single junction Si record devices have "industrial-size" active areas of several tens of cm2 or even full wafers, record tandem cells are lab-scale 1-4 cm2. Thus, up-scaling of tandem cells will remain an important topic in the near future.

At any rate, it can be expected that the exponential growth of PV as well as the diversity of applications (utility, rooftop and BIPV, agri-PV, etc.) will create ample opportunity for the market entry of quite a few of the mentioned technologies, and even for entirely new concepts such as three-terminal tandems or, at the module level, integrated PV and storage systems.