This work shows a bimolecular additive engineering approach to prepare highly efficient wide-band-gap perovskite solar cells. The coupling of PEA + and SCN À provides a synergistic effect that overcomes growth challenges with either additive individually and improves perovskite quality with enhanced crystallinity, reduced defect density, and improved carrier mobility and lifetime. When coupling a semitransparent wide-band-gap perovskite top cell with a low-band-gap CIGS bottom cell, we achieve a 25.9%-efficient polycrystalline perovskite/CIGS 4-terminal thinfilm tandem solar cell.

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.

We report on improvements to the energy conversion efficiency of wide bandgap (Eg > 1.2 eV) solar cells on the basis of CuIn1−xGaxSe2. Historically, attaining high efficiency (>16%) from these types of compound semiconductor thin films has been difficult. Nevertheless, by using (a) the alkaline‐containing high‐temperature EtaMax glass substrates from Schott AG, (b) elevated substrate temperatures of 600–650 °C, and (c) high vacuum evaporation from elemental sources following National Renewable Energy Laboratory's three‐stage process, we have been able to improve the performance of wider bandgap solar cells with 1.2 < Eg < 1.45 eV. The current density–voltage (J–V) data we present includes efficiencies >18% for absorber bandgaps of ~1.30 eV and efficiencies of ~16% for bandgaps up to ~1.45 eV. In comparing J–V parameters in similar materials, we establish gains in the open‐circuit voltage and, to a lesser degree, the fill factor value, as the reason for the improved performance. The higher voltages seen in these wide gap materials grown at high substrate temperatures are due to reduced recombination. We establish the existence of random and discrete grains within the CIGS absorbers that yield limited or no generation/collection of minority carriers. We also show that interfacial recombination is the main mechanism limiting additional enhancements to open‐circuit voltage and therefore performance. Solar cell results, absorber materials characterization, and experimental details and discussion are presented. Copyright © 2012 John Wiley & Sons, Ltd.

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Analysis of steady-state and transient photoconductivity measurements at room temperature performed on c-axis oriented GaN nanowires yielded estimates of free carrier concentration, drift mobility, surface band bending, and surface capture coefficient for electrons. Samples grown ͑unintentionally n-type͒ by nitrogen-plasma-assisted molecular beam epitaxy primarily from two separate growth runs were examined. The results revealed carrier concentration in the range of ͑3-6͒ ϫ 10 16 cm −3 for one growth run, roughly 5 ϫ 10 14 -1ϫ 10 15 cm −3 for the second, and drift mobility in the range of 500-700 cm 2 / ͑V s͒ for both. Nanowires were dispersed onto insulating substrates and contacted forming single-wire, two-terminal structures with typical electrode gaps of Ϸ3-5 m. When biased at 1 V bias and illuminated at 360 nm ͑3.6 mW/ cm 2 ͒ the thinner ͑Ϸ100 nm diameter͒ nanowires with the higher background doping showed an abrupt increase in photocurrent from 5 pA ͑noise level͒ to 0.1-1 A. Under the same conditions, thicker ͑151-320 nm͒ nanowires showed roughly ten times more photocurrent, with dark currents ranging from 2 nA to 1 A. With the light blocked, the dark current was restored in a few minutes for the thinner samples and an hour or more for the thicker ones. The samples with lower carrier concentration showed similar trends. Excitation in the 360-550 nm range produced substantially weaker photocurrent with comparable decay rates. Nanowire photoconductivity arises from a reduction in the depletion layer via photogenerated holes drifting to the surface and compensating ionized surface acceptors. Simulations yielded ͑dark͒ surface band bending in the vicinity of 0.2-0.3 V and capture coefficient in the range of 10 −23 -10 −19 cm 2 . Atomic layer deposition ͑ALD͒ was used to conformally deposit Ϸ10 nm of Al 2 O 3 on several devices. Photoconductivity, persistent photoconductivity, and subgap photoconductivity of the coated nanowires were increased in all cases. TaN ALD coatings showed a reduced effect compared to the Al 2 O 3 coated samples.

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We improved the efficiency of ultra‐thin (0.49‐μm‐thick) Cu(In,Ga)Se2 solar cells to 15.2% (officially measured). To achieve these results, we modified growth conditions from the 3‐stage process but did not add post‐deposition treatments or additional material layers. The increase in device efficiency is attributed to a steeper Ga gradient in the CIGS with higher Ga content near the Mo back contact, which can hinder electron‐hole recombination at the interface. We discuss device measurements and film characterization for ultra‐thin CIGS. Modeling is presented that shows the route to even higher efficiencies for devices with CIGS thicknesses of 0.5 μm.

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