Solar Panel Technologies for Light-to-Chemical Conversion

Abstract

The sustainable synthesis of fuels and chemicals is key to attaining a carbon-neutral economy. This can be achieved by mimicking the light-harvesting and catalytic processes occurring in plants. Solar fuel production is commonly performed via established approaches, including photovoltaic-electrochemical (PV-EC), photoelectrochemical (PEC), and photocatalytic (PC) systems. A recent shift saw these systems evolve into integrated, compact panels, which suit practical applications through their simplicity, scalability, and ease of operation. This advance has resulted in a suite of apparently similar technologies, including the so-called artificial leaves and PC sheets. In this Account, we compare these different thin film technologies based on their micro- and nanostructure (i.e., layered vs particulate), operation principle (products occurring on the same or different sides of the panel), and product/reaction scope (overall water splitting and CO₂ reduction, or organics, biomass, and waste conversion).For this purpose, we give an overview of developments established over the past few years in our laboratory. Two light absorbers are generally required to overcome the thermodynamic challenges of coupling water oxidation to proton or CO₂ reduction with good efficiency. Hence, tandem artificial leaves combine a lead halide perovskite photocathode with a BiVO₄ photoanode to generate syngas (a mixture of H₂ and CO), whereas PC sheets involve metal-ion-doped SrTiO₃ and BiVO₄ particles for selective formate synthesis from CO₂ and water. On the other hand, only a single light absorber is needed for coupling H₂ evolution to organics oxidation in the thermodynamically less demanding photoreforming process. This can be performed by immobilized carbon nitride (CN_x) in the case of PC sheets or by a single perovskite light absorber in the case of PEC reforming leaves. Such systems can be integrated with a range of inorganic, molecular, and biological catalysts, including metal alloys, molecular cobalt complexes, enzymes, and bacteria, with low overpotentials and high catalytic activities toward selective product formation.This wide reaction scope introduces new challenges toward quantifying and comparing the performance of different systems. To this end, we propose new metrics to evaluate the performance of solar fuel panels based on the areal product rates and commercial product value. We further explore the key opportunities and challenges facing the commercialization of thin film technologies for solar fuels research, including performance losses over larger areas and catalyst/device recyclability. Finally, we identify emerging applications beyond fuels, where such light-driven panels can make a difference, including the waste management, chemical synthesis, and pharmaceutical industries. In the long term, these aspects may facilitate a transition toward a light-driven circular economy.

Figure 1

Examples of standalone thin film technologies described in this Account: (a, b) PEC artificial leaves, (c, d) PC sheets. (a) A tandem BiVO₄–perovskite device can produce O₂ and syngas (H₂ + CO) or formate from water and CO₂. (b) A single perovskite light absorber simultaneously performs proton reduction and organics oxidation. The perovskite PV device structure corresponds to the one in (a), but the BiVO₄ photoanode is replaced by a metal alloy electrocatalyst. (c) Doped BiVO₄ and SrTiO₃ powders are interfaced through a solid gold layer, achieving hydrogen or selective formate production. (d) Immobilized semiconductor particles combine H₂ evolution with the reforming of organics. Abbreviations: OEC, oxygen evolution catalyst; FTO, fluorine-doped tin oxide; HTL/ETL, hole/electron transport layer; GE, graphite epoxy paste (conductive encapsulant); and CN_x, carbon nitride.

Figure 2

Comparison between the product rates of suspension and thin film technologies for photoreforming. Waste conversion can be performed with semiconductor particles (that is, PC reforming) or with integrated artificial leaves (PEC reforming). The star indicates a PC sheet. CDs, carbon dots; PET, poly(ethylene terephthalate). Adapted with permission from ref (4). Copyright 2022 the Authors. Published by Wiley under a Creative Commons Attribution 4.0 International (CC BY 4.0) License.

Figure 3

Upscaling. (a) Image of a 5 × 5 cm² PC sheet. (b) Same PC sheet under operation in a 3D-printed reactor. (c) 2 × 2 cm² PEC leaf in the 3D-printed reactor. (d) PC sheet in a machined flow reactor for photoreforming (5 × 5 cm² window). Adapted with permission from ref (3). Copyright 2020 Wiley-VCH GmbH. (b, c) Batch reactors. (d) Flow reactor.