Excitation-dependent carrier lifetime and diffusion length in bulk CdTe determined by time-resolved optical pump-probe techniques

Abstract

We applied time-resolved pump-probe spectroscopy based on free carrier absorption and light diffraction on a transient grating for direct measurements of the carrier lifetime and diffusion coefficient D in high-resistivity single crystal CdTe (codoped with In and Er). The bulk carrier lifetime τ decreased from 670 ± 50 ns to 60 ± 10 ns with increase of excess carrier density N from 1016 to 5 × 1018 cm−3 due to the excitation-dependent radiative recombination rate. In this N range, the carrier diffusion length dropped from 14 μm to 6 μm due to lifetime decrease. Modeling of in-depth (axial) and in-plane (lateral) carrier diffusion provided the value of surface recombination velocity S = 6 × 105 cm/s for the untreated surface. At even higher excitations, in the 1019–3 × 1020 cm−3 density range, D increase from 5 to 20 cm2/s due to carrier degeneracy was observed.

Identification of Recombination Losses in CdSe/CdTe Solar Cells from Spectroscopic and Microscopic Time‐Resolved Photoluminescence

“…The estimated μ ≈ 50 cm 2 V −1 s −1 is similar to mobility measured with light‐induced transient grating for polycrystalline CdSeTe heterostructures [ 24 ] and for compensated CdTe single crystals. [ 33 ] We also estimated similar mobility for earlier‐generation CdTe solar cells with CdS buffer. [ 29 ] Therefore, ternary CdSeTe alloy has similar intragrain mobility as polycrystalline CdTe.…”

Radiative Efficiency and Charge‐Carrier Lifetimes and Diffusion Length in Polycrystalline CdSeTe Heterostructures

“…Impact of carrier localization to D e reduction is also not excluded . Regardless of uncertainty in scattering mechanisms, we find comparable D e and μ in polycrystalline CdSe 0.2 Te 0.8 DHs and in CdTe single crystals, which indicates the low density of scattering centers. With τ TRPL ≈ 750 ns and μ ≈ 100 cm 2 (Vs) −1 , L d ≈ 14 μm, which exceeds the thickness of our samples and also typical CdSeTe/CdTe solar cell thickness (2–4 μm).…”

“…Because solar cells require carrier collection, it is important to know diffusion length L d = ( D e × τ TRPL ) 1/2 (charge‐carrier diffusivity D e = k B T / q μ , where μ is mobility). Light‐induced transient grating (LITG) with 2PE at 1053 nm was used for optical D e measurements . From the grating decay times at grating periods Λ = 2.5, 3.4, and 7.3 μm, we determined excitation‐dependent diffusivity D e .…”

“…Light‐induced transient grating (LITG) with 2PE at 1053 nm was used for optical D e measurements . From the grating decay times at grating periods Λ = 2.5, 3.4, and 7.3 μm, we determined excitation‐dependent diffusivity D e . Results are shown in Figure b, where D e ranges from 2.5 cm 2 s −1 ( μ ≈ 100 cm 2 (Vs) −1 ) to 4.5 cm 2 s −1 ( μ ≈ 180 cm 2 (Vs) −1 ).…”

“…Because grating periods Λ = 2.5–7.3 μm are similar to the grain size (Figure ), GB potential barriers can be a factor that reduces diffusivity from D e = 5 cm 2 s −1 (experimental value for compensated CdTe single crystals) to D e = 2.5 cm 2 s −1 . Similar GB effects were reported for perovskites .…”