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Over the past decade, the global cumulative installed photovoltaic (PV) capacity has grown exponentially, reaching 591 GW in 2019. Rapid progress was driven in large part by improvements in solar cell and module efficiencies, reduction in manufacturing costs and the realization of levelized costs of electricity that are now generally less than other energy sources and approaching similar costs with storage included. Given this success, it is a particularly fitting time to assess the state of the photovoltaics field and the technology milestones that must be achieved to maximize future impact and forward momentum. This roadmap outlines the critical areas of development in all of the major PV conversion technologies, advances needed to enable terawatt-scale PV installation, and cross-cutting topics on reliability, characterization, and applications. Each perspective provides a status update, summarizes the limiting immediate and long-term technical challenges and highlights breakthroughs that are needed to address them. In total, this roadmap is intended to guide researchers, funding agencies and industry in identifying the areas of development that will have the most impact on PV technology in the upcoming years.

. Broader ContextThin film photovoltaics (PV), based on materials that absorb light 10-100 times more efficiently than crystalline silicon, has the potential to drive down PV module cost by increasing efficiency and requiring less raw material. Currently, the market for thin film PV is dominated by CdTe and CuIn x Ga 1-x Se 2 technologies, which suffer from concerns of toxicity (Cd) or rarity (Te, In) when considering terawatt-scale deployment. As an alternative to these technologies, researchers have turned to studying new absorber materials that exhibit equivalent light absorbing properties but are comprised of Earth-abundant elements. In this work, we explore the properties of a thin film III-N analog, ZnSnN 2 , that until recently has received little attention in the literature. Classification as a III-N analog is advantageous for PV applications, considering that III-N materials are well-known for their stability. ZnSnN 2 possesses a direct bandgap and large absorption coefficient in a PV-relevant energy range, in addition to being composed of abundant and non-toxic elements. However, a few key optoelectronic properties (e.g. doping control and exact bandgap) of this material are not well understood. If this challenge is addressed, ZnSnN 2 has excellent potential as an absorber material for Earth-abundant thin film PV. AbstractZnSnN 2 is an Earth-abundant semiconductor analogous to the III-Nitrides with potential as a solar absorber due to its direct bandgap, steep absorption onset, and disorder-driven bandgap tunability. Despite these desirable properties, discrepancies in the fundamental bandgap and degenerate n-type carrier density have been prevalent issues in the limited amount of literature available on this material. Using a combinatorial RF co-sputtering approach, we have been able to explore a growth-temperature-composition space for Zn 1+x Sn 1-x N 2 over the ranges 35-340• C and 0.30-0.75 Zn/(Zn+Sn). In this way, we were able to identify an optimal set of deposition parameters for obtaining as-deposited films with wurtzite crystal structure and carrier density as low as 1.8 x 10 18 cm -3 . Films grown at 230• C with Zn/(Zn+Sn) = 0.60 were found to have the largest grain size overall (70 nm diameter on average) while also exhibiting low carrier density (3 x 10 18 cm -3 ) and high mobility (8.3 cm 2