Dependence of the Minority-Carrier Lifetime on the Stoichiometry of CdTe Using Time-Resolved Photoluminescence and First-Principles Calculations Jie Ma, Darius Kuciauskas, David Albin, Raghu Bhattacharya, Matthew Reese, Teresa Barnes, Jian V. Li, Timothy Gessert, and Su-Huai Wei National Renewable Energy Laboratory, Golden, Colorado 80401, USA (Received 4 February 2013; published 7 August 2013) CdTe is one of the most promising materials for thin-film solar cells. However, further improvement of its performance is hindered by its relatively short minority-carrier lifetime. Combining theoretical calculations and experimental measurements, we find that for both intrinsic CdTe and CdTe solar cell devices, longer minority-carrier lifetimes can be achieved under Cd-rich conditions, in contrast to the previous belief that Te-rich conditions are more beneficial. First-principles calculations suggest that the dominant recombination centers limiting the minority-carrier lifetime are the Te antisite and Te interstitial. Therefore, we propose that to optimize the solar cell performance, extrinsic p-type doping (e.g., N, P, or As substitution on Te sites) in CdTe under Cd-rich conditions should be a good approach to simultaneously increase both the minority-carrier lifetime and hole concentration. DOI: 10.1103/PhysRevLett.111.067402 PACS numbers: 78.47.jd, 61.72.J, 71.15.Mb, 88.40.jm CdTe is one of the most promising photovoltaic absorb- ers for thin-film solar cell devices, due to its near-optimum band gap (1:5 eV), high absorption coefficient, and low cost. Its theoretical efficiency for converting terrestrial sunlight can be as high as 29% [1]. A critical problem for CdTe solar cells is that the open-circuit voltage (V OC ) and fill factor (FF) remain lower than expected for a 1.5 eV band gap material. The experimental efficiency has so far only reached 19.6% [2]. One of the main reasons for the low V OC and FF is the relatively short minority-carrier lifetime. Unless the minority-carrier lifetime can be increased, the open-circuit voltage will still be low. Furthermore, although we may gain improved open-circuit voltage by increasing net acceptor doping, the current density and FF will both decrease if the minority-carrier lifetime remains unchanged, because the photon-generated carriers cannot be collected efficiently [3,4]. The main process that limits the minority-carrier lifetime in CdTe is defect-mediated recombination. According to the Shockley-Read-Hall model [5,6], the most efficient recombination centers are defects with deep levels in the middle of the band gap. In order to improve the minority-carrier lifetime, the recom- bination process has to be suppressed, and therefore the concentrations of deep-level defects have to be reduced. Determining the main deep-level defects in CdTe and con- trolling their concentrations are crucial issues for making high-efficiency CdTe solar cells. It is well known that in semiconductors the stoichiome- try determines defect concentrations, which affect material properties, including free-carrier concentration, mobility, optical absorption or emission, and minority-carrier life- time [7,8]. However, there are surprisingly few reports discussing the effect of stoichiometry on the minority- carrier lifetime in CdTe. A previous theoretical model based on local density approximation (LDA) suggests that the Te vacancy (V Te ) could be the main deep-level defect in CdTe [9]. Therefore, reducing the Te vacancy concentration through Te-rich conditions could minimize the recombination and thus improve the minority-carrier lifetime. Many experimental groups rely on the assumption that higher CdTe growth temperatures would yield better device performance, because high growth temperatures should make CdTe more Te rich based on the phase diagram [10]. However, because LDA underestimates the band gap, the defect-level predictions, especially for anion vacancies, may need reexamination. Moreover, a recent experiment [11] showed that when extra Te was introduced through the back surface of CdTe devices, which should make the material more Te rich and passivate the Te vacancies, both the minority-carrier lifetime and solar cell efficiency dropped significantly. This experiment sug- gests that the Te vacancy may not be the most important detrimental defect in CdTe solar cells. In this Letter, combining advanced theoretical calcula- tions and experimental measurements, we show that longer minority-carrier lifetimes are actually achieved under Cd-rich conditions. Time-resolved photoluminescence (TRPL) is employed to measure the minority-carrier life- time in intrinsic CdTe. While traditional one-photon exci- tation (1PE) measurements could be limited by fast surface recombination, the two-photon excitation (2PE) TRPL technique allows for the determination of minority-carrier lifetimes. We find that the minority-carrier lifetimes are significantly greater in Cd-rich CdTe (20 ns) compared to Te-rich CdTe (3 ns). We also find the same behavior in CdTe solar cell devices: the minority-carrier lifetime increases as the CdTe layer is more Cd rich. First- principles calculations using hybrid functional [12] are employed to calculate the intrinsic defects in CdTe. We find that the Te vacancy is a shallow donor rather than a PRL 111, 067402 (2013) PHYSICAL REVIEW LETTERS week ending 9 AUGUST 2013 0031-9007= 13=111(6)=067402(5) 067402-1 Ó 2013 American Physical Society