Research progress of kesterite solar cells

Photovoltaic technology offers a sustainable solution to the challenge of increasing energy demand. Nowadays, various high-performance solar cells are emerging. Thin-film solar cells made from inorganic materials have become one of the major categories of solar cells, showing potential in the fast-growing photovoltaic (PV) market. The production technology of Cu(In,Ga)Se-2 (CIGS) solar cells and CdTe solar cells has reached a mature level. The earth-abundant and environmentally friendly kerterite Cu2ZnSn(S,Se)(4) (CZTSSe) is a promising alternative to chalcopyrite CIGS and CdTe for PV applications and is considered to be a cost-effective next-generation solar cell material. The crystal structure of the CZTSSe absorber material is derived from CIGS, in which In and Ga are replaced by one group II (Zn) cation and one group IV (Sn) cation, and has a similar lattice and energy band structure with CIGS. Therefore, CZTSSe inherits the advantages of high absorption coefficient, adjustable band gap, and inherent P-type conductivity, and has the new advantages of non-toxicity and abundant element reserves. It is a new generation of thin film photovoltaic technology with high efficiency, stability, safety, environmental protection, and low price. CZTSSe PV technology has made significant progress in the past few years, reaching a maximum efficiency of 14.9%, but still far below CIGS (23.6%) and CdTe (22.1%). The undesirable back/front interface is one of the main reasons for the difficulty in improving the fill factor. The detrimental interface reaction results in a large number of secondary phases, voids and defects in absorbers, which can form abundant recombination centers and limit the minority carrier diffusion length. The thicker Mo(S,Se)(2) layer at the back interface leads to carrier transport barriers and has a negative impact on the crystalline quality of the absorber; high density of interface defects, unfavorable band alignment, and structural inhomogeneity across the front interface are the main factors leading to heterojunction recombination. Meanwhile, kesterite, as one of the most complex compound semiconductors, has a more complex defect chemistry than CIGS and CdTe, making the control of intrinsic defects a major challenge. Deep limit defects, such as deep defect SnZn and associated [Cu-Zn+Sn-Zn] clusters, act as deep recombination centers, leading to short carrier lifetimes. In addition, a large number of defect clusters like [2Cu(Zn)+Sn-Zn] introduce considerable potential (i.e., band or electrostatic) fluctuations. As a result, the performance of kerterite-based solar cells is currently stagnant due to low fill factor and large open-circuit voltage (VOC) deficits. In this review, the state-of-the-art strategies to improve the device performance are provided, with a particular focus on back and front-interface engineering, cation substitution, and selenization annealing, post-annealing processes and so on. These strategies have led to step-wise improvements in the power conversion efficiency (PCE) of the corresponding kesterite solar cells and are the most promising approaches to achieve further efficiency breakthroughs in kesterite solar cells. This paper reviews the recent research progress around these pathways in kesterite solar cells and, more importantly, provides a comprehensive understanding of the mechanisms at play and an outlook on the future development of kesterite solar cells.