Spectrally robust series/parallel-connected triple-junction photovoltaic cells used for artificial photosynthesis

Abstract

We have overcome two drawbacks involved in series-connected double-junction (S-2J) and triple-junction (S-3J) photovoltaic cells to compose monolithic modules of artificial photosynthesis consisting of directly connected photovoltaic cells and electrolyzers of the same size. One is current mismatching among the subcells under solar spectrum variation. The other is inefficient utilization of high-energy photons that can generate sufficiently high voltage to promote the target reaction, caused by consumption of two or three photons for extracting a single electron regardless of the photon energies. This arises from the predetermined operating voltage, contrasting to no restriction of the output voltage for solar cells combined with power conditioners. In a series/parallel-connected triple-junction (S/P-3J) photovoltaic cell, the series-connected middle and bottom cells are connected with the top cell in parallel. High-energy photons absorbed in the top cell are efficiently utilized because the photoexcited electrons are directly extracted. Although relative intensities of high-energy photons in the measured solar spectra changes remarkably, the top cell is free from the current matching restriction. On the other hand, current matching between the middle and bottom cells approximately holds, because solar spectrum in the relevant range changes only slightly. Consequently, the S/P-3J significantly improves solar-to-chemical conversion efficiency (ηSTC) and is more advantageous on overcast days owing to the spectral robustness. Using the state-of-the-art electrolyzers, ηSTC = 32%−26% of H2 production by water splitting is estimated on fine and overcast days. CO2 reduction to CO proceeds with ηSTC = 30%−25%. These values are higher than those for the S-2J by 3%−9%.