Abstract
Cu2ZnGeSe4 (CZGSe) is a promising earth-abundant and non-toxic semiconductor material for large-scale thin-film solar cell applications. Herein, we have employed a joint computational and experimental approach to characterize and assess the structural, optoelectronic, and heterojunction band offset and alignment properties of a CZGSe solar absorber. The CZGSe films were successfully prepared using DC-sputtering and e-beam evaporation systems and confirmed by XRD and Raman spectroscopy analyses. The CZGSe films exhibit a bandgap of 1.35 eV, as estimated from electrochemical cyclic voltammetry (CV) measurements and validated by first-principles density functional theory (DFT) calculations, which predicts a bandgap of 1.38 eV. A fabricated device based on the CZGSe as a light absorber and CdS as a buffer layer yields power conversion efficiency (PCE) of 4.4% with VOC of 0.69 V, FF of 37.15, and Jsc of 17.12 mA cm-2. Therefore, we suggest that interface and band offset engineering represent promising approaches to improve the performance of CZGSe devices by predicting a type-II staggered band alignment with a small conduction band offset of 0.18 eV at the CZGSe/CdS interface.
Highlights
IntroductionShockley–Queisser limit for CZTSSe of 32.8%.4 The lower efficiency of CZTSSe compared to the chalcogenide Cu(In,Ga)(S,Se)[2] (CIGS) counterparts (420% efficiency) has been attributed to several factors, including but not limited to, large open-circuit voltage (Voc) deficits,[5,6,7] high-defect states,[8] co-existence of secondary phases,[9,10] short carrier lifetime,[11] and unfavorable band offset and alignment at the CZTSSe/buffer interface.[12,13] In addition to these issues, the kesterite based photovoltaic devices suffer from severe band tailing, which reduces the open circuit voltage (Voc) of solar cells.[14]
To provide an atomic-level insight into the structure and composition of the CZGSe/CdS interface and to understand better the energy band offset and alignment derived from the cyclic voltammetry (CV) characterizations, we have carried out electronic structure density functional theory (DFT) calculations as implemented in the Vienna ab initio simulation package (VASP).[37,38]
We report the successful deposition of CZGSe thin films by DC-sputtering and e-beam evaporation and confirmed by XRD and Raman spectroscopy analyses
Summary
Shockley–Queisser limit for CZTSSe of 32.8%.4 The lower efficiency of CZTSSe compared to the chalcogenide Cu(In,Ga)(S,Se)[2] (CIGS) counterparts (420% efficiency) has been attributed to several factors, including but not limited to, large open-circuit voltage (Voc) deficits,[5,6,7] high-defect states,[8] co-existence of secondary phases,[9,10] short carrier lifetime,[11] and unfavorable band offset and alignment at the CZTSSe/buffer interface.[12,13] In addition to these issues, the kesterite based photovoltaic devices suffer from severe band tailing, which reduces the open circuit voltage (Voc) of solar cells.[14]. The lower efficiency of CZTSSe compared to the chalcogenide Cu(In,Ga)(S,Se)[2] (CIGS) counterparts (420% efficiency) has been attributed to several factors, including but not limited to, large open-circuit voltage (Voc) deficits,[5,6,7] high-defect states,[8] co-existence of secondary phases,[9,10] short carrier lifetime,[11] and unfavorable band offset and alignment at the CZTSSe/buffer interface.[12,13] In addition to these issues, the kesterite based photovoltaic devices suffer from severe band tailing, which reduces the open circuit voltage (Voc) of solar cells.[14] Rey et al reported the origin of band-tails in kesterite materials and showed that the large band tailing causes voltage losses that limit the efficiency of kesterite-based devices.[3,15].
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