With a high absorption coefficient of 10⁴-10⁵ cm^-1 over the visible wavelength range, the earth-abundant, low cost and non-toxic material zinc phosphide (Zn₃P₂) is a promising absorber candidate for efficient photovoltaics (PVs)¹. Nevertheless, some optoelectronic properties of Zn₃P₂ still require more in-depth understanding, such as dielectric function, carrier lifetime and mobility. In this project, the charge carrier dynamics of a Zn₃P₂/InP thin film sample were characterized via contact-free and non-invasive terahertz spectroscopy with sub-picosecond temporal resolution. Specifically, by applying optical-pump terahertz-probe (OPTP) spectroscopy in reflection mode, the photoconductivity and carrier dynamic properties of Zn₃P₂ are investigated, offering insight into the underlying physical mechanisms governing PV application efficiency. 

Fig.1 shows the change in carrier density for a Zn₃P₂ thin film on an InP substrate when photoexcited at a centre wavelength of 750 nm that is above the bandgap energy of Zn₃P₂ but below the one for InP at photoexcitation fluences (12, 28, 57 and 128 µJ/cm²). As the photoexcitation wavelength is above the bandgap energy of the thin film, it generates free charge carriers upon photoexcitation within the Zn3P2 thin film. Additionally, the over three magnitudes differences in simulation based on absorption coefficients suggest the photoconductivity response is dominated by Zn₃P₂ other than InP. The change in carrier density is therefore proportional to the measured THz reflection difference and decays as a function of time after photoexcitation. We therefore globally fit the carrier rate equation to the measured photoconductivity decays to obtain more carrier dynamic information: dn/dt = -αn - βn² - γn³, where α, β and γ are the monomolecular, bimolecular and Auger recombination coefficients, respectively. We obtained the following values for the carrier recombination coefficients of α = (1.264±0.162)×10⁸ /s, β = (1.642±0.256)×10^-11 cm³/s and γ = 2.701×10^-31 /s, indicating that the decay was dominated by monomolecular recombination. Additionally, by modifying the time delay between the THz probe and pump bean thus to vary the delay time, from the spectra we extracted the carrier mobility of the sample spans from 2644 to 176 cm²/V.s for photoexcitation fluences of 12, 28, 57 and 128 µJ/cm² at 0.5 ps after photoexcitation. Furthermore, the results from 128 µJ/cm² can be referenced to the ideal carrier mobility of the physical device as the excitation energy is comparable to AM1.5G.

[1] Stutz, E. Z. et al. Showcasing the optical properties of monocrystalline zinc phosphide thin films as an earth-abundant photovoltaic absorber. Mater. Adv. 3, 1295–1303 (2022).