Cation Substitution in Earth‐Abundant Kesterite Photovoltaic Materials

Abstract

As a promising candidate for low‐cost and environmentally friendly thin‐film photovoltaics, the emerging kesterite‐based Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have experienced rapid advances over the past decade. However, the record efficiency of CZTSSe solar cells (12.6%) is still significantly lower than those of its predecessors Cu(In,Ga)Se2 (CIGS) and CdTe thin‐film solar cells. This record has remained for several years. The main obstacle for this stagnation is unanimously attributed to the large open‐circuit voltage (V OC) deficit. In addition to cation disordering and the associated band tailing, unpassivated interface defects and undesirable energy band alignment are two other culprits that account for the large V OC deficit in kesterite solar cells. To capture the great potential of kesterite solar cells as prospective earth‐abundant photovoltaic technology, current research focuses on cation substitution for CZTSSe‐based materials. The aim here is to examine recent efforts to overcome the V OC limit of kesterite solar cells by cation substitution and to further illuminate several emerging prospective strategies, including: i) suppressing the cation disordering by distant isoelectronic cation substitution, ii) optimizing the junction band alignment and constructing a graded bandgap in absorber, and iii) engineering the interface defects and enhancing the junction band bending.

Suppressed Deep Traps and Bandgap Fluctuations in Cu2CdSnS4 Solar Cells with ≈8% Efficiency

“…[6,13,20] However, the formation energy of Cu Zn +Zn Cu antisite (solid gray line in Figure 2a) remains unaffected by the complete substitution of Zn with the larger Cd cation, i.e., the formation energy of Cu Cd +Cd Cu in stannite-Cu 2 CdSnS 4 (0.22 eV, solid gray line in Figure 2b) is similar to Cu Zn +Zn Cu in kesterite-Cu 2 ZnSnS 4 (0.22-0.25 eV, solid gray lines in Figure 2a). [6,13,20] However, the formation energy of Cu Zn +Zn Cu antisite (solid gray line in Figure 2a) remains unaffected by the complete substitution of Zn with the larger Cd cation, i.e., the formation energy of Cu Cd +Cd Cu in stannite-Cu 2 CdSnS 4 (0.22 eV, solid gray line in Figure 2b) is similar to Cu Zn +Zn Cu in kesterite-Cu 2 ZnSnS 4 (0.22-0.25 eV, solid gray lines in Figure 2a).…”

“…In 2012, Chen et al reported a low formation energy for Cu Zn +Sn Zn and 2Cu Zn +Sn Zn in kesterites and also proposed their deleterious role of introducing deep defects and bandgap narrowing. [6,9,11,[20][21][22][23] Here, we study the role of the proposed performance-limiting defects Cu Zn +Zn Cu and 2Cu Zn +Sn Zn by systematically substituting cations in Cu 2 ZnSnS 4 as characterized by both experimental and theoretical methods. [19] Hence, based on theoretical calculations, the possible performance-limiting point defects in kesterites are proposed to be the Cu-Zn antisite Cu Zn +Zn Cu , and the deep-trap-level-inducing Sn-antisite 2Cu Zn +Sn Zn .…”

“…[19] Hence, based on theoretical calculations, the possible performance-limiting point defects in kesterites are proposed to be the Cu-Zn antisite Cu Zn +Zn Cu , and the deep-trap-level-inducing Sn-antisite 2Cu Zn +Sn Zn . [6,13,20,25] ii) Cu 2 ZnSnS 4 (kesterite-type structure) has Cu-Zn planes and Cu-Sn planes ( Figure S1, Supporting Information) along the c-axis, with the Cu-Zn plane showing the highest degree of cationic disorder; [26] Cu 2 CdSnS 4 (expected to be in a stannitetype structure [15] ) has Cu-only and Cd-Sn planes ( Figure S1, Supporting Information) along the c-axis, and the lack of a Cu-Cd plane may mitigate the Cu-Cd disorder. [6] Altering the point-defect characteristics using cation substitution is reported to enhance the photovoltaic performance of kesterites, showing especially promising results for the substitution of Zn 2+ by isovalent ions such as Ba 2+ , Mn 2+ , and Cd 2+ .…”

Efficient Optimization of the Performance of Mn2+‐Doped Kesterite Solar Cell: Machine Learning Aided Synthesis of High Efficient Cu2(Mn,Zn)Sn(S,Se)4 Solar Cells

“…Photovoltaic (PV) attracts much more attention as an inexhaustible source of clean energy nowadays. Among them, thin film PV technology is an important branch applied in building integrated photovoltaics (BIPV), wearable power supply, etc . Presently, Cu(In,Ga)Se 2 (CIGS) and CdTe thin film solar cells have achieved 22.9% and 22.1% power conversion efficiency (PCE), respectively .…”

“…Thin film solar cells Cu 2 ZnSnS 4 (CZTS) and Cu 2 ZnSnSe 4 (CZTSe) are prospective substitutes for Cu(In,Ga)Se 2 (CIGS) due to the composition elements of their absorber materials are non‐toxic and earth abundant. Up to now, the champion efficiency of CZTSSe solar cells is still 12.6%, which is mainly because of the large V OC deficit ( E g / q ‐ V OC ) . One of the important factors for low efficiency in CZTSSe is cationic disorder .…”