Impact of dopant-induced band tails on optical spectra, charge carrier transport, and dynamics in single-crystal CdTe

Abstract

Cadmium telluride (CdTe) semiconductors are used in thin-film photovoltaics, detectors, and other optoelectronic applications. For all technologies, higher efficiency and sensitivity are achieved with reduced charge carrier recombination. In this study, we use state-of-the-art CdTe single crystals and electro-optical measurements to develop a detailed understanding of recombination rate dependence on excitation and temperature in CdTe. We study recombination and carrier dynamics in high-resistivity (undoped) and arsenic (As)-doped CdTe by employing absorption, the Hall effect, time-resolved photoluminescence, and pump-probe in the 80–600 K temperature range. We report extraordinarily long lifetimes (30 µs) at low temperatures in bulk undoped CdTe. Temperature dependencies of carrier density and mobility reveal ionization of the main acceptors and donors as well as dominant scattering by ionized impurities. We also distinguish different recombination defects. In particular, shallow AsTe and deep VCd−AsCd acceptors were responsible for p-type conductivity. AX donors were responsible for electron capture, while nonradiative recombination centers (VCd−AsTe, As2 precipitates), and native defects (VCd−TeCd) were found to be dominant in p-type and n-type CdTe, respectively. Bimolecular and surface recombination rate temperature dependencies were also revealed, with bimolecular coefficient T−3/2 temperature dependence and 170 meV effective surface barrier, leading to an increase in surface recombination velocity at high temperatures and excitations. The results of this study allowed us to conclude that enhanced crucible rotation growth of As-doped CdTe is advantageous to As activation, leading to longer lifetimes and larger mobilities and open-circuit voltages due to lower absorption and trapping.

Band Bending at CdTe Solar Cell Contacts: Correlating Electro‐Optical and X‐Ray Photoelectron Spectroscopy Analyses of Thin Film Solar Cells

“…For example, in As-doped single crystal CdTe, unactivated As (As Te activation in devices studied here ≤2%) [10,11] was suggested to reduce charge carrier mobility from 800 to 10-40 cm 2 (Vs) À1 . [33] Using the value from the middle of this range, μ = 20 cm 2 (Vs) À1 , estimated φ bi ≥0.35 V is consistent with the surface analysis results (Table 1).…”

Why Increased CdSeTe Charge Carrier Lifetimes and Radiative Efficiencies did not Result in Voltage Boost for CdTe Solar Cells

“…We note that band tails were also identified in As‐doped single crystal CdTe [ 80 ] with low As activation. [ 65 ] As a result, mobility was reduced >50 times and trapped carriers had >35 µs lifetimes at low temperature, [ 80 ] similar to polycrystalline absorbers in this study. In contrast, single crystals with high (50%) As activation did not have band tails.…”

Long-lived excitons in thermally annealed hydrothermal ZnO

“…With a similar approach the S = 450 cm/s can be obtained at 100 ns delay after excitation. The S value may be larger at a higher initial exciton density due to the effect of surface potential screening [ 40 ].

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