Analysis of performance-limiting defects in pn junction GaAs solar cells grown by water-mediated close-spaced vapor transport epitaxy

Abstract

Precursor and substrate costs currently limit the adoption of III-V photovoltaics for large scale manufacturing. Here, we use water-mediated close-spaced vapor transport (CSVT) to produce homojunction GaAs devices with pressed GaAs powder as an alternative to expensive gas-phase precursors. These unpassivated devices reach V oc > 910 mV, demonstrating the plausibility of CSVT as an alternative method for growth of III-V epitaxial films for photovoltaic devices. We find that Zn-doping of the absorber films decreases after a number of growths cycles using a single source, which suggests an alternative transport agent should be investigated for p-type doping. Performance of these solar cells is largely limited by formation of macroscopic surface defects which we find to be caused by particulate transfer from the source material and the formation of oxide phases during growth. We present strategies for mitigating these defects and improving device performance.

Carrier-Transport Study of Gallium Arsenide Hillock Defects

“…Beyond the physical location of the hillocks, the composition is uniform. This observation differs from the results in Boucher et al (2017), where the composition of the oval defects of GaAs films deposited by water-mediated, close-spaced vapor transport was found to be identical to the rest of the epitaxial film.

Fig.

…”

“…The formation of hillocks depends on the growth rate, film thickness, and substrate contamination. The size of hillocks, as observed by scanning electron microscopy (SEM), can be up to tens of microns, and hillocks can significantly alter the electrical and optical properties of GaAs thin films (Shinohara et al, 1984; Mehta et al, 1992; Lazzarini et al, 2001; Kaniewska & Klima, 2002; Szerling et al, 2006; Boucher et al, 2017). However, to date, there are no reports on direct mapping of carrier transport and quantitative measurement of diffusion lengths around these defects.…”

Low-Cost Approaches to III–V Semiconductor Growth for Photovoltaic Applications

“…(C,D) Φ int for n - and p -type GaAs films grown by CSVT and measured using nonaqueous PEC; adapted from ref with permission from the Royal Society of Chemistry. (E) Φ int of a CSVT-grown homojunction, with the inset schematic of the device; adapted with permission from ref . Copyright 2017 Elsevier.…”

“…Initial devices used a VGF wafer absorber, enabling characterization of CSVT-grown emitters compared to a fixed absorber quality; these devices reached V OC > 900 mV . Despite the high growth temperatures, cross-diffusion was limited, and dopant profiles were similarly abrupt to those grown by the Ptak group using HVPE, occurring over ∼20 nm . Work on solar cells grown entirely by CSVT yielded improved devices, the best having J SC = 13.9 mA cm –2 , V OC = 916 mV, ff = 75%, and η = 9.5% for an unpassivated device with no AR coating (Figure E) .…”

“…This is consistent with conversion of Zn to ZnO over multiple exposures to the H 2 O transport agent, which prevents Zn transport as ZnO has limited volatility. Additionally, oxide-related defects were observed on emitter films grown at high [H 2 O], as well as particulate-related defects consisting of GaAs crystallites that originated at the powder source that cross to the substrate during growth . Elimination of these defects to improve consistency of device properties is a focus of future work.…”