Cu₂ZnSnS(e)₄ (CZTS(e)) solar cells have attracted much attention due to the elemental abundance and the non-toxicity. However, the record efficiency of 12.6% for Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells is much lower than that of Cu(In,Ga)Se₂ (CIGS) solar cells. One crucial reason is the recombination at interfaces. In recent years, large amount investigations have been done to analyze the interfacial problems and improve the interfacial properties via a variety of methods. This paper gives a review of progresses on interfaces of CZTS(e) solar cells, including:(i) the band alignment optimization at buffer/CZTS(e) interface, (ii) tailoring the thickness of MoS(e)₂ interfacial layers between CZTS(e) absorber and Mo back contact, (iii) the passivation of rear interface, (iv) the passivation of front interface, and (v) the etching of secondary phases.

Colloidal quantum dot (CQD) solar cells have attracted great interest due to their low cost and superior photo-electric properties. Remarkable improvements in cell performances of both quantum dot sensitized solar cells (QDSCs) and PbX (X=S, Se) based CQD solar cells have been achieved in recent years, and the power conversion efficiencies (PCEs) exceeding 12% were reported so far. In this review, we will focus on the recent progress in CQD solar cells. We firstly summarize the advance of CQD sensitizer materials and the strategies for enhancing carrier collection efficiency in QDSCs, including developing multi-component alloyed CQDs and core-shell structured CQDs, as well as various methods to suppress interfacial carrier recombination. Then, we discuss the device architecture development of PbX CQD based solar cells and surface/interface passivation methods to increase light absorption and carrier extraction efficiencies. Finally, a short summary, challenge, and perspective are given.

The kesterite thin film solar cells based on the quaternary Cu₂ZnSnS₄ and Cu₂ZnSnSe₄ and their alloys Cu₂ZnSn(S,Se)₄ have been considered as environment-friendly and non-toxic alternatives to the currently commercialized CdTe and Cu(In,Ga)Se₂ thin film solar cells. From the theoretical point of view, we will review how the group I₂-Ⅱ-IV-VI₄ quaternary compound semiconductors are derived from the binary CdTe and the ternary CuInSe₂ or CuGaSe₂ through the cation mutation, and how the crystal structure and electronic band structure evolve as the component elements change. The increased structural and chemical freedom in these quaternary semiconductors opens up new possibility for the tailoring of material properties and design of new light-absorber semiconductors. However, the increased freedom also makes the development of high-efficiency solar cells more challenging because much more intrinsic point defects, secondary phases, surfaces, and grain-boundaries can exist in the thin films and influence the photovoltaic performance in a way different from that in the conventional CdTe and Cu(In,Ga)Se₂ solar cells. The experimental characterization of the properties of defects, secondary phase, and grain-boundaries is currently not very efficient and direct, especially for these quaternary compounds. First-principles calculations have been successfully used in the past decade for studying these properties. Here we will review the theoretical progress in the study of the mixed-cation and mixed-anion alloys of the group I₂-Ⅱ-IV-VI₄ semiconductors, defects, alkaline dopants, and grain boundaries, which provided very important information for the optimization of the kesterite solar cell performance.

Perovskite solar cells (PVSCs) have attracted extensive studies due to their high power conversion efficiency (PCE) with low-cost in both raw material and processes. However, there remain obstacles that hinder the way to their commercialization. Among many drawbacks in PVSCs, we note the problems brought by the use of noble metal counter electrodes (CEs) such as gold and silver. The costly Au and Ag need high energy-consumption thermal evaporation process which can be made only with expensive evaporation equipment under vacuum. All the factors elevate the threshold of PVSCs' commercialization. Carbon material, on the other hand, is a readily available electrode candidate for the application as CE in the PVSCs. In this review, endeavors on PVSCs with low-cost carbon materials will be comprehensively discussed based on different device structures and carbon compositions. We believe that the PVSCs with carbon-based CE hold the promise of commercialization of this new technology.