The electrical and optical properties of kesterites

Abstract

Kesterite Cu 2 ZnSn(S x Se 1-x ) 4 (CZTSSe) semiconductor materials have been extensively studied over the past decade, however despite significant efforts, the open circuit voltage remains below 60% of the theoretical maximum. Understanding the optical and electrical properties is critical to explaining and solving the voltage deficit. This review aims to summarize the present knowledge of optical and electrical properties of kesterites and specifically focuses on experimental data of intrinsic defects, charge carrier density and transport, and minority carrier lifetime and related rate-limiting recombination mechanisms. It concludes with suggestions for further investigation of the electrical and optical properties of kesterite materials. of the (001) cationic planes are randomly occupied by Cu and Zn atoms [6,7]. While theoretical calculations by Chen et al [8] find the lowest formation energy for the kesterite crystal structure, cation disorder is present in CZTSSe due to the low energy difference between the stannite-and kesterite-related structures. It has been suggested that the Cu-Zn disorder in Cu 2 ZnSnS 4 (CZTS) follows a second order phase transition with a critical temperature of ∼260°C for CZTS, which was first observed and introduced to describe the modifications in the Raman spectra of CZTS annealed at different temperatures [9]. A similar phenomenon has also been found in Cu 2 ZnSnSe 4 (CZTSe), with a critical temperature of 200°C [10], where the measured increase in band gap is explained by thermally induced ordering of Cu and Zn cations. While the remarkable effect of the orderdisorder transition on the band gap and vibrational spectra is explained by Vineyard's theory [11], the disorder parameter itself is not experimentally determined in the probed samples. A recent direct comparison between resonant x-ray diffraction and photoluminescence (PL) data in CZTSe has confirmed a relationship between the Cu-Zn disorder and the band gap modification [12]. A change in the order parameter from 0 to 0.7 leads to a OPEN ACCESS RECEIVED significant increase in the band gap on the order of ∼0.11 eV and ∼0.20 eV for CZTSe and CZTS, respectively [10,13].The absorption coefficient for CZTS and CZTSe is about ∼2-3 × 10 4 cm −1 at 1.6 eV and at 1.1 eV, respectively, as determined by the normal reflectance and transmittance [2, 13] spectroscopic ellipsometry (SE) [14-16] and external quantum efficiency [17] methods. In the recent review by Choi et al [18] the main theoretical and experimental results on the energy band structure and SE data were discussed and summarized. Recently, Zamulko et al [19] re-examined first principle theory at different levels in order to improve the description of the electronic structure and optical properties of CZTSSe. Nishiwaki et al [20] performed density functional calculation to study absorption band tail of the kesterites. In particular, a theoretical estimate for the tail energy of ∼30 meV for both CZTS and CZTSe was obtained. It was suggested that quite large Urbach energi...

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“…Copper vacancies are a well-known benecial point defect in kesterite and chalcopyrite based solar cells, which leads to the formation of a shallow acceptor level and has a strong inuence on the electrical conductivity of the absorber layer. 44,45 In this way, a decit of this benecial defect leads to a decrease of the charge carrier concentration. However, an excess results in the formation of a high amount of scattering centres which significantly decreases the mobility of the charge carriers.…”

Assessing the defect tolerance of kesterite-inspired solar absorbers

“…PL features of CZTS have been extensively discussed in the literature [39][40][41][42][43] and were recently summarized. 44 Briefly, the PL peak of CZTS thin films at room temperature (RT) is broad and significantly red-shifted with respect to the band gap (FWHM and Stokes shift of more than 150 meV). Analysis of temperature-dependent and excitation power-dependent PL indicates strong spatial fluctuations in the band gap or the electrostatic potential of CZTS, or both.…”

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“…9,10 Several candidate defects are proposed, including S vacancies stabilised by Sn(II), Sn Zn point defects and Cu-Zn disorder 11,12 Therefore, many reports point out the need for strategies to suppress detrimental defects in the material that are responsible for bulk recombination. 13,14 Disregarding their specic nature, these defects must form during material growthas a polycrystalline thin lmtypically at temperatures in the 400À600 C range, and their populations will be inuenced by both kinetic and thermodynamic factors. These factors are affected strongly by non-stoichiometry (it is well-known that CZTS can adopt an appreciable range of offstoichiometric compositions, due to the formation of numerous neutral defect complexes 15,16 ).…”