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

Abstract

Cadmium Telluride is the lowest‐cost commercial photovoltaic technology, but CdTe solar cell efficiency is still limited by charge carrier recombination. To understand and reduce recombination losses, scientists at the National Renewable Energy Laboratory (NREL, Golden, Colorado, USA) developed polycrystalline double heterostructures as model systems for thin film solar cells. Using spectroscopic and microscopic electro‐optical characterization, researchers at NREL and Vilnius University (Lithuania) showed that heterostructures have high external radiative efficiency, which was attributed to near‐microsecond charge carrier lifetimes and high mobilities. Results show that polycrystalline CdTe photovoltaic technology has overcome long‐standing recombination lifetime limitation, which will enable higher voltages in solar cells. For further details see article number http://doi.wiley.com/10.1002/pssr.201900606.

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

“…𝜇 0 = 10 cm 2 V −1 s −1 is reasonable for the hole conductivity mobility given that the electron mobility 𝜇 n = 100 cm 2 V −1 s −1 was measured in similar Cd(Se,Te) absorbers. [12] To further test the low 𝜇 p hypothesis, we simulated variation in doping, N A , minority carrier mobility, 𝜇 n , and the back contact metal work function, 𝜑 M . Bulk non-radiative lifetimes were assumed to be 1000 ns, based on the room temperature TRPL measurements (Figure 3b; Figure S2, Supporting Information) and published results.…”

“…Bulk non-radiative lifetimes were assumed to be 1000 ns, based on the room temperature TRPL measurements (Figure 3b; Figure S2, Supporting Information) and published results. [12,13,28] Model parameter values are listed in Table S2 and Table S3 (Supporting Information). Figure 4a shows that the JV data for both thicknesses can be accounted for with baseline parameter values 𝜇 p = 0.07 cm 2 V −1 s −1 , N A = 0 cm −3 (intrinsic), 𝜇 n = 320 cm 2 V −1 s −1 , and 𝜑 M = 5.4 eV.…”

“…[6,7] Slow progress is surprising, because material and device improvement metrics (lifetime, doping, interface passivation) identified by modeling for 25% efficient CdTe solar cells [8][9][10] have been largely reached. [11][12][13][14][15] It is likely that device models used to guide CdTe solar cell development do not include some important aspects. Here, we propose a hypothesis of what may be limiting voltage in CdTe solar cells and hope to contribute to overcoming this limitation.…”

“…[5,17] First, absorber composition changed from a binary CdTe to ternary CdSeTe alloy. [18] Use of ternary Cd-SeTe (<40% CdSe) has increased minority carrier lifetimes [13] and radiative efficiencies [12] (typically, by a factor of 100x), correspondingly increasing electron (minority charge carrier) quasi-Fermi level. Second, the change from cation (Cu Cd substitution) to anion site (As Te substitution) doping enabled higher acceptor density-also by a factor of 100x-favorably changing hole quasi-Fermi level.…”