Efficient n-GaAs Photoelectrodes Grown by Close-Spaced Vapor Transport from a Solid Source

Abstract

n-GaAs films were grown epitaxially on n(+)-GaAs substrates by a close-spaced vapor transport method and their photoelectrochemical energy conversion properties studied. Under 100 mW cm(-2) of ELH solar simulation, conversion efficiencies up to 9.3% for CSVT n-GaAs photoanodes were measured in an unoptimized ferrocene/ferrocenium test cell. This value was significantly higher than the 5.7% measured for similarly doped commercial n-GaAs wafers. Spectral response experiments showed that the higher performance of CSVT n-GaAs films relative to the commercial wafers was due to longer minority carrier diffusion lengths (L(D)), up to 1,020 nm in the CSVT films compared to 260 nm in the commercial n-GaAs wafers. Routes to improve the performance of CSVT GaAs and the implications of these results for the development of scalable GaAs-based solar energy conversion devices are discussed.

Towards high-efficiency GaAs thin-film solar cells grown via close space vapor transport from a solid source

“…The solution was rapidly stirred during current-potential (J-E) measurements to aid mass transport. In contrast to our previous study in which PEC measurements were performed in an inert gas glove box, [5] these measurements were performed in a sealed electrochemical cell under positive N 2 pressure on the bench top. This allowed for minimal time (~5 s) between aqueous HCl etching and measurement, which we found to increase photovoltages by 50-100 mV relative to the previous measurements ( Fig.…”

“…5 panel A). N D for the CSVT GaAs was determined using PEC MottSchottky impedance analysis as described previously, [5] and found to range from 2×10 16 cm -3 to 2×10 17 cm -3 . Several of the best films were grown at both the high and low range of N D and no clear correlation was found between N D and PEC performance.…”

“…Panels A and B in each figure show the J-E data before and after the typical corrections for series resistance (R ~ 130 Ω) and concentration overpotential. [5] Panels C and D show the measured external quantum efficiencies Φ ext and internal quantum efficiencies Φ int , which were calculated by correcting for reflection losses at the air/glass and ACN/GaAs interfaces (there is negligible loss at the glass/ACN interface). [5] Optical measurements in an integrating sphere showed that the reflectivity of the GaAs wafers and CSVT samples were similar and consistent with that predicted for a smooth GaAs surface.…”

“…[5] Panels C and D show the measured external quantum efficiencies Φ ext and internal quantum efficiencies Φ int , which were calculated by correcting for reflection losses at the air/glass and ACN/GaAs interfaces (there is negligible loss at the glass/ACN interface). [5] Optical measurements in an integrating sphere showed that the reflectivity of the GaAs wafers and CSVT samples were similar and consistent with that predicted for a smooth GaAs surface. Table 1 summarizes the measured photoelectrode parameters for the range of growth conditions studied here.…”

“…[4] We recently published the first analysis of CSVT GaAs photovoltaic material. [5] In that work, we grew n-GaAs films epitaxially on n + -GaAs substrates in order to study the properties of the CSVT GaAs in the absence of defects associated with heteroepitaxy on non-latticematched substrates. We used non-aqueous photoelectrochemical (PEC) methods to measure standard PV device parameters without fabricating solid-state pn junctions, top-contact grids, or addressing other device-related issues.…”