This paper reviews the development history of alkali element doping on Cu(In,Ga)Se2 (CIGS) solar cells and summarizes important achievements that have been made in this field. The influences of incorporation strategies on CIGS absorbers and device performances are also reviewed. By analyzing CIGS surface structure and electronic property variation induced by alkali fluoride (NaF and KF) post-deposition treatment (PDT), we discuss and interpret the following issues: ① The delamination of CIGS thin films induced by Na incorporation facilitates CuInSe2 formation and inhibits Ga during low-temperature co-evaporation processes. ② The mechanisms of carrier density increase due to defect passivation by Na at grain boundaries and the surface. ③ A thinner buffer layer improves the short-circuit current without open-circuit voltage loss. This is attributed not only to better buffer layer coverage in the early stage of the chemical bath deposition process, but also to higher donor defect (CdCu+) density, which is transferred from the acceptor defect (VCu−) and strengthens the buried homojunction. ④ The KF-PDT-induced lower valence band maximum at the absorber surface reduces the recombination at the absorber/buffer interface, which improves the open-circuit voltage and the fill factor of solar cells.

•HC-Se facilitates the Cu–Se and In–Se reaction at lower temperatures.•“Polygon grains” in HC-Se samples are necessary for selenium diffusion since they make the film incompact.•HC-Se atmosphere delays compact CIS formation at the first heating.•Ga is more uniform throughout CIGS films in HC-Se atmosphere.•Cell performance is improved in HC-Se samples due to shunt conductance reduction.Thermal-cracking system has been adopted to produce cracked selenium and the influence of cracked selenium flux on the structure and reaction pathway during the first-step selenization is investigated. High Cracked-Selenium (HC-Se) may facilitate the Cu–Se and In–Se reaction at lower temperatures. The “Polygon grains” observed in the HC-Se samples play a key role in further selenium diffusion into the film since they make the film more incompact. In addition, activation energy analysis indicates that Cu2−xSe and β-In2Se3 formed in the samples prepared in HC-Se atmosphere may result in different growth pathway of Cu(In1−xGax)Se2 (CIGS) thin film compared with that prepared in Low Cracked-Selenium (LC-Se) atmosphere during the first-step selenization, which restrains lamination in CIGS films effectively so that the distribution of Ga is more uniform throughout CIGS films and no small grains of CuGaSe2 (CGS) accumulate near the Mo back-contact. As a result, the shunt conductance in this CIGS thin film device prepared in HC-Se is reduced, and the fill factor, open-circuit voltage, as well as the cell efficiency are improved.

•The selenized CZTSSe films show poor adhesion to the Mo substrate unless the Cu layer was stacked on the top of the film.•Stacking order of Cu–ZnS–SnS determines the orientation of crystal growth of CZTSSe (bottom-to-top or top-to-bottom).•A thin film solar cell with 3.35% conversion efficiency is obtained using Cu/SnS/ZnS/Mo precursor.Sputtering and subsequent sulfurization (or selenization) is one of the methods that have been extensively employed to fabricate Cu2ZnSn(S,Se)4 (CZTSSe) thin films. However, there are limited reports on the effect of precursor stacking order of the sputtered source materials on the properties of the synthesized CZTSSe films. In this work, the morphology and crystallization process of the CZTSSe films which were prepared by selenizing Cu–ZnS–SnS precursor layers with different stacking sequences and the adhesion property between the as-synthesized CZTSSe layer and Mo substrate have been thoroughly investigated. It has been found that the growth of CZTSSe material and the morphology of the film strongly depend on the location of Cu layer in the precursor film. The formation of CZTSSe starts from the diffusion of Cu–Se to Sn(S,Se) layer to form Cu–Sn–(S,Se) compound, followed by the reaction with Zn(S,Se). The investigation of the morphology of the CZTSSe films has shown that large grains are formed in the film with the precursor stacking order of Mo/SnS/ZnS/Cu, which is attributed to a bottom-to-top growth mechanism. In contrast, the film made from a precursor with a stacking sequence of Mo/ZnS/SnS/Cu is mainly consisted of small grains due to a top-to-bottom growth mechanism. The best CZTSSe solar cell with energy conversion efficiency of 3.35% has been achieved with the selenized Mo/ZnS/SnS/Cu film, which is attributed to a good contact between the absorber layer and the Mo substrate.

With an amorphous silicon (a-Si:H)/crystalline silicon (c-Si) heterojunction structure, the heterojunction with intrinsic thin-layer (HIT) solar cell has become one of the most promising technologies for c-Si based solar cells. By replacing a-Si:H thin films with appropriate compound semiconductors, we propose novel heterojunction structures which allow c-Si heterojunction solar cells to possess higher power conversion efficiencies than HIT solar cells. Several promising heterojunction candidates and hetero-structures have been proposed in this work, and this kind of novel c-Si compound heterojunction solar cell is denominated HCT (heterojunction with a compound thin-layer). The feasibilities of these novel HCT structures are further investigated by theoretical approaches, and the modeling results demonstrate the device performance improvement. Finally, this paper proclaims the compound selection standards and essentials of achieving high-efficiency HCT solar cells, which are guidelines for the real device implementation.

Aluminum doped zinc oxide (ZnO:Al) thin films are suitable for the use as transparent conductive electrode in copper indium gallium selenide Cu(In,Ga)Se2 thin film solar cells. The resistivity and film quality of ZnO:Al deposited on soda lime glass is nonuniform in magnetron sputtering process. According to the measurement results of magnetic field on the top of the target, obvious magnetic field distribution nonuniformity is observed along the vertical and horizontal directions respectively. With the longer distance between target and substrate, the magnetic field intensity becomes lower and flatter between the two magnet poles. Based on the simulation results by finite element analysis, it is verified the nonuniformity of magnetic field distribution influences the probability of Ar+ particles collision and the deposition of zinc oxide (ZnO) particles in different regions on substrate. The higher resistivity of ZnO:Al films is obtained where the magnetic field intensity is stronger.

With an amorphous silicon (a-Si:H)/crystalline silicon (c-Si) heterojunction structure, the heterojunction with intrinsic thin-layer (HIT) solar cell has become one of the most promising technologies for c-Si based solar cells. By replacing a-Si:H thin films with appropriate compound semiconductors, we propose novel heterojunction structures which allow c-Si heterojunction solar cells to possess higher power conversion efficiencies than HIT solar cells. Several promising heterojunction candidates and hetero-structures have been proposed in this work, and this kind of novel c-Si compound heterojunction solar cell is denominated HCT (heterojunction with a compound thin-layer). The feasibilities of these novel HCT structures are further investigated by theoretical approaches, and the modeling results demonstrate the device performance improvement. Finally, this paper proclaims the compound selection standards and essentials of achieving high-efficiency HCT solar cells, which are guidelines for the real device implementation.