BaCd2P2: a promising impurity-tolerant counterpart of GaAs for photovoltaics

TL;DR

BaCd2P2 (BCP) is proposed as an impurity-tolerant GaAs-like solar absorber with a direct band gap $E_g\approx1.51\ \mathrm{eV}$. The study combines solid-state synthesis and Sn-flux crystal growth, PL-based quasi-Fermi level analysis, and first-principles defect modeling to compare BCP with GaAs. It finds dominant deep intrinsic defects in BCP have a lower SRH recombination rate than GaAs under typical growth conditions, and many extrinsic impurities do not form deep nonradiative centers, indicating robust impurity tolerance. Collectively, these results suggest BCP can achieve high open-circuit voltages and long carrier lifetimes at lower material-purity costs, potentially improving PV cost-performance, i.e., a more favorable balance of $E_g$, $\tau$, and reduced sensitivity to impurities.

Abstract

BaCd2P2 (BCP) has been recently identified as a new solar absorber with promising optoelectronic properties. This work demonstrates that, despite having a low precursor purity (98.90% to 99.95%), synthesized BCP samples exhibit promising photoconductive carrier lifetime up to 300 ns and an implied open circuit voltage exceeding 1 V, comparable to a high-purity single-crystalline GaAs wafer. To better understand the underlying mechanisms of the promising properties of BCP, its tolerance to intrinsic defects and extrinsic impurities is investigated using first-principles defect modeling and compared with the well-studied GaAs. The results show that the nonradiative recombination rates induced by dominant deep-level intrinsic antisite defects are lower in BCP than in GaAs under typical growth conditions. Further exploration of the impact of transition metal impurities in the raw materials used to make BCP and impurities introduced during its synthesis shows that most of these do not form deep-level nonradiative recombination centers. As an impurity-tolerant counterpart of GaAs, BCP demonstrates great potentials to improve the cost to performance ratio of photovoltaics.

Figures (13)

• Figure 1: (a) PL spectra at $298\ \mathrm{K}$ for BCP-Powder, GaAs-Powder synthesized using solid state reactions, and GaAs-GW — a commercial 2-inch GaAs (001) single-crystalline prime wafer that has been ground into a powder. All PL spectra were collected using a $532\ \mathrm{nm}$ laser with an excitation power of $30\ \mathrm{\mu W}$ and a focal spot diameter of 3 $\mu$m incident on the surface of the samples. The broad sub-bandgap peaks near $1.3\ \mathrm{eV}$ arise from shallow defect emissions Klingshirn2007SemiconductorOpticsBhattacharya2011ComprehensiveTechnology. (b) Temperature dependent PL tracking the shift in the band-to-band emission peak position of BCP-Powder and GaAs-GW; the shift is fit using Varshni’s equation Varshni1967TemperatureSemiconductors to determine the 0 K band gap as discussed in \ref{['sec:s2']}. The inserts in (b) are the optical microscope images of BCP-Power and GaAs-GW (the scale bar represents $20\ \mathrm{\mu m}$).
• Figure 2: Fitting the 298 K PL spectra of (a) BCP-Crystal and (b) GaAs-Wafer to the spontaneous emission equation for nonequilibrium conditions (\ref{['eq:1']}) using an incident excitation power density of $3,537\ \mathrm{kW/m^2}$ and $1,591\ \mathrm{kW/m^2}$ respectively. The black dashed lines in both (a) and (b) represent the overall spectra obtained from the fitting and agree well with the measured PL. The GaAs-Wafer peak is a result of a single direct gap transition, while that of BCP-Crystal is the result of two band-to-band transitions. The brown dashed lines in (a) illustrate the deconvolution of the band-to-band transitions.
• Figure 3: Implied V$_\mathrm{{OC}}$ vs incident optical power density of BCP-Crystal and GaAs-Wafer. The dashed lines correspond to linear fits. Note that the error bars are smaller than the size of the symbols for both materials. The measurements were preformed at 298 K.
• Figure 4: (a) Magnitude of photoconductive current for BCP-Crystal at three illumination levels of a $780\ \mathrm{nm}$ laser diode compared with the dark current ($0\ \mathrm{kW/m^2}$). The dashed lines correspond to linear fits. The minimum of the dark current at $0\ \mathrm{V}$ bias was $0.1\ \mathrm{pA}$, which is the noise floor of the system. Accordingly, the current value at $0\ \mathrm{V}$ has been set to $0\ \mathrm{A}$. The inset shows a BCP-Crystal placed on a ruler with $1\ \mathrm{mm}$ tick spacing. (b) The effective carrier lifetime of BCP-Crystal from $0.5$ to $1.6\ \mathrm{kW/m^2}$ extracted from photoconductive current measurements (left) and from $354$ to $3,537\ \mathrm{kW/m^2}$ extracted from the PL fit of \ref{['eq:2']} using a 532 nm laser (right). The error bars are smaller than the size of the symbols for the PL extracted carrier lifetime (right). All the measurements were carried out at $298\ \mathrm{K}$.
• Figure 5: (a) The evolution of $\mathrm{P_{Cd}}$ and $\mathrm{As_{Ga}}$ deep defect concentrations assuming they were frozen at temperatures ranging from 500 K to the respective synthesis temperatures of BCP and GaAs (left), and the resulting SRH recombination rate (right). We note that GaAs is prepared in As-rich conditions and Ga-rich results are not realized. (b) The charge transition levels of point defects in BCP (left) and GaAs (right). Sn and C impurity defects are explored in BCP in addition to the intrinsic P on Cd antisite. Aside from As on Ga antisite, As vacancy, Ga vacancy, and Ga on As antisite defects are presented for GaAs.