Ultrathin solar cells with thicknesses at least 10 times lower than conventional solar cells could have the unique potential to efficiently convert solar energy into electricity while enabling material savings, shorter deposition times and improved carrier collection in defective absorber materials. Efficient light absorption and hence high power conversion efficiency could be retained in ultrathin absorbers using light-trapping structures that enhance the optical path. Nevertheless, several technical challenges prevent the realization of a practical device. Here we review the state-of-the-art of c-Si, GaAs and Cu(In,Ga)(S,Se)2 ultrathin solar cells and compare their optical performances against theoretical light-trapping models. We then address challenges in the fabrication of ultrathin absorber layers and in nanoscale patterning of light-trapping structures and discuss strategies to ensure efficient charge collection. Finally, we propose practical architectures for ultrathin solar cells that combine photonic and electrical constraints, and identify future research directions and potential applications of ultrathin photovoltaic technologies. Ultrathin solar cells attract interest for their relatively low cost and potential novel applications. Here, Massiot et al. discuss their performance and the challenges in the fabrication of ultrathin absorbers, patterning of light trapping structures and ensuring efficient charge-carrier collection.

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Progress and prospects for ultrathin solar cells

Conventional photovoltaic devices are currently made from relatively thick semiconductor layers, ~150 µm for silicon and 2–4 µm for Cu(In,Ga)(S,Se)2, CdTe or III–V direct bandgap semiconductors. Ultrathin solar cells using 10 times thinner absorbers could lead to considerable savings in material and processing time. Theoretical models suggest that light trapping can compensate for the reduced single-pass absorption, but optical and electrical losses have greatly limited the performances of previous attempts. Here, we propose a strategy based on multi-resonant absorption in planar active layers, and we report a 205-nm-thick GaAs solar cell with a certified efficiency of 19.9%. It uses a nanostructured silver back mirror fabricated by soft nanoimprint lithography. Broadband light trapping is achieved with multiple overlapping resonances induced by the grating and identified as Fabry–Perot and guided-mode resonances. A comprehensive optical and electrical analysis of the complete solar cell architecture provides a pathway for further improvements and shows that 25% efficiency is a realistic short-term target. Ultrathin solar cells having thicknesses below 1 µm can still reach efficiencies comparable to their thicker counterparts, but require less material to manufacture. By exploiting light-trapping nanostructures, Chen and colleagues achieve GaAs solar cells with 20% efficiency at just 205 nm thicknesses.

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A 19.9%-efficient ultrathin solar cell based on a 205-nm-thick GaAs absorber and a silver nanostructured back mirror

We report on the fabrication and characterization of ultrathin GaAs solar cells with a silver back mirror and absorber thicknesses of only $t = 120$ nm and $t = 220$ nm. The silver back mirror is combined with localized ohmic contacts. Without antireflection coating, Fabry–Perot resonances lead to strong enhancement over single-pass absorption (up to 4), and external quantum efficiency reaches 0.8 at resonance wavelengths. An analytical model is used to determine the resonance wavelengths and the absorption maxima. A short-circuit improvement of 27% results from the enhanced absorption induced by the Fabry-Perot resonances. By implementing an additional antireflection coating, short-circuit currents reach 16.3 mA/cm2 for $t = 120$ nm and 20.7 mA/cm2 for $t = 220$ nm, corresponding to efficiencies of 8.7 % and 12.9 %, respectively.

Ultrathin GaAs Solar Cells With a Silver Back Mirror

Due to their favorable materials properties including direct bandgap and high electron mobilities, epitaxially grown III–V compound semiconductors such as gallium arsenide (GaAs) provide unmatched performance over silicon in solar energy harvesting. Nonetheless, their large-scale deployment in terrestrial photovoltaics remains challenging mainly due to the high cost of growing device quality epitaxial materials. In this regard, reducing the thickness of constituent active materials under appropriate light management schemes is a conceptually viable option to lower the cost of GaAs solar cells. Here, we present a type of high efficiency, ultrathin GaAs solar cell that incorporates bifacial photon management enabled by techniques of transfer printing to maximize the absorption and photovoltaic performance without compromising the optimized electronic configuration of planar devices. Nanoimprint lithography and dry etching of titanium dioxide (TiO2) deposited directly on the window layer of GaAs solar cells ...

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High Performance Ultrathin GaAs Solar Cells Enabled with Heterogeneously Integrated Dielectric Periodic Nanostructures

The peculiar nature of light-matter interaction in atomically thin transition metal dichalcogenides is recently under examination for application in novel optoelectronic devices. Here, we show that heterostructures composed of two or more such layers can be used for solar energy harvesting. The strong absorption in these atomically thin layers makes it possible to achieve an efficient power conversion with a minimal amount of active material. We describe in detail two different fabrication techniques that allow to realize heterostructures with clean, atomically sharp interfaces. The observed electrical and photovoltaic properties are analyzed. Our findings suggest that, accompanied by the advances in large area fabrication of atomically thin transition metal dichalcogenides, van der Waals heterostructures are promising candidates for a new generation of excitonic solar cells.

Photovoltaics in Van der Waals Heterostructures

This paper reports the proposal, design, and demonstration of ultra-thin GaAs single-junction solar cells integrated with a reflective back scattering layer to optimize light management and minimize non-radiative recombination. According to our recently developed semi-analytical model, this design offers one of the highest potential achievable efficiencies for GaAs solar cells possessing typical non-radiative recombination rates found among commercially available III-V arsenide and phosphide materials. The structure of the demonstrated solar cells consists of an In0.49Ga0.51P/GaAs/In0.49Ga0.51P double-heterostructure PN junction with an ultra-thin 300 nm thick GaAs absorber, combined with a 5 μm thick Al0.52In0.48P layer with a textured as-grown surface coated with Au used as a reflective back scattering layer. The final devices were fabricated using a substrate-removal and flip-chip bonding process. Solar cells with a top metal contact coverage of 9.7%, and a MgF2/ZnS anti-reflective coating demonstrated...

Ultra-thin GaAs single-junction solar cells integrated with a reflective back scattering layer

This paper proposes and demonstrates the use of a textured and lattice-matched semiconductor layer coated with a Au reflector for reflective back scattering to enhance the efficiency of single-junction solar cells with ultra-thin absorbers. The device structure studied in this work consists of an In0.49Ga0.51P/GaAs/In0.49Ga0.51P double-heterostructure single-junction solar cell with a GaAs absorber of either 300 nm or 1000 nm thick, as well as a textured Al0.52In0.48P layer coated with a highly reflective Au film. The devices, areas ranging from 0.3×0.3 mm2 to 1×1 mm2, are flip-chip bonded onto Si carrier substrates, covered by a contact metal grid (with a 9.7% or 10.7% shadow area for the 300 nm and 1000 nm devices, respectively) and a MgF2/ZnS anti-reflective coating. Both types of device designs demonstrate an open-circuit voltage of 1.00 V, short-circuit current densities of 24.5 and 26.1 mA/cm2, and power conversion efficiencies of 19.1% and 20.6%, respectively; these measurements are carried out under 1 sun solar radiation (AM 1.5G, 0.1 W/cm2). It is reasonable to expect that the short-circuit current densities and conversion efficiencies of these devices could reach 26.6 mA/cm2 and 28.6 mA/cm2, and 20.7% and 22.6%, respectively, when a typical contact grid layout with 2% surface coverage is used. These short-circuit current densities are only 3.6 and 1.6 mA/cm2 lower than the maximum theoretical value 30.2 mA/cm2, respectively.

Ken Campman

Papers

Ultra-thin GaAs single-junction solar cells integrated with a reflective back scattering layer

Ultra-thin GaAs single-junction solar cells integrated with an AlInP layer for reflective back scattering