Fabrication of high efficient solar cells is critical for photovoltaic application. The bandgap-graded absorber layer can not only drive carriers efficient collection but also improve the light harvesting. However, it...

ment of CZTSSe is a large open-circuit voltage deficit (V OC,def ). Many optimization strategies borrowed from CIGSSe solar cells have been used to break through the current high V OC,def issue of CZTSSe materials, including isovalent cation doping [5][6][7][8][9][10][11] and gradient band-gap design. [12] Ag is equivalent to Cu but its ion radius is larger than that of Cu, which will reduce the recombination caused by the high density of Cu/Zn antisite defects, thereby reducing V oc,def . [5,13] Simultaneously, Ag doping can enlarge the band gap (E g ) of the absorber layer, which will be effective for increasing the open voltage (V OC ) of thin film photovoltaic devices. [12,[14][15][16][17] Based on the above advantages of Ag doping, our group prepared Ag-doped (Ag,Cu) 2 ZnSnSe 4 solar cells through pre-alloying followed by a selenization process, and the V OC of the devices was improved. [12] There is, however, a problem with incorporating Ag into kesterite film, i.e., Ag diffuses very quickly and easily distributes uniformly throughout the whole absorber film during the high temperature annealing process. Therefore it is difficult to control the content along the depth of CZTSSe films. [14,18] Theoretical calculations and experimental results both show that Ag 2 ZnSn(S,Se) 4 is an n-type material and can form a p-n junction with CZTSSe, [13,15,19] but it cannot ensure the existence of n-type (Cu,Ag) 2 ZnSn(S,Se) 4 on the surface due to the quick diffusion of Ag. So far, surface type As a low-cost substitute that uses no expensive rare-earth elements for the high-efficiency Cu(In,Ga)(S,Se) 2 solar cell, the Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cell has borrowed optimization strategies used for its predecessor to improve its device performance, including a profiled band gap and surface inversion. Indeed, there have been few reports of constructing CZTSSe absorber layers with surface inversion to improve efficiency. Here, a strategy that designs the CZTSSe absorber to attain surface modification by using n-type Ag 2 ZnSnS 4 is demonstrated. It has been discovered that Ag plays two major roles in the kesterite thin film devices: surface inversion and front gradient distribution. It has not only an excellent carrier transport effect and reduced probability of electron-hole recombination but also results in increased carrier separation by increasing the width of the depletion region, leading to much improved V OC and J SC . Finally, a champion CZTSSe solar cell renders efficiency as high as 12.55%, one of the highest for its type, with the open-circuit voltage deficit reduced to as low as 0.306 V (63.2% Shockley-Queisser limit). The band engineering for surface modification of the absorber and high efficiency achieved here shine a new light on the future of the CZTSSe solar cell.

Li+-doping strategy is a promising route for achieving high-efficient Cu2ZnSn(S,Se)4 (CZTSSe) photovoltaic devices with large grain absorber layer, high p-type carrier concentration and good band alignment in Cu2ZnSn(S,Se)4/CdS interface. However,...

Cu2ZnSn(S,Se)4 are emerging as promising photovoltaic materials due to their outstanding photoelectrical performances, benign grain boundaries, and Earth‐abundant constituent elements. However, there are largely distributed cation‐disordering defects and defect clusters, which lead to an increase in recombination and a large open‐circuit voltage deficit and thus deteriorate device performance. Herein, defect control for a high‐efficiency Cu2ZnSn(S,Se)4 solar cell by atomic layer deposition of aluminum oxide (ALD‐Al2O3) on the precursor film is reporter. CuZn defects and Sn‐related deep defects are largely suppressed because of the decrease in Sn2+ and the increase in Sn4+ in the film by ALD‐Al2O3 on the precursor are found, and the crystallinity of absorber layer is improved from a double‐layer structure to a completely single‐layer structure. Furthermore, the carrier lifetime and recombination in the bulk and interface are significantly improved for devices with ultrathin Al2O3. Using this approach, the conversion efficiency increases from 8.8% to 11.0% and the open‐circuit voltage deficit decreases from 0.621 to 0.577 V. Herein, a deep understanding of the relationship between Al2O3 incorporation and high‐efficiency Cu2ZnSn(S,Se)4 devices and a new direction for controlling defects to further improve the performance of kesterite solar cells are provided.

Increasing the fill factor (FF) and the open‐circuit voltage (VOC) simultaneously together with non‐decreased short‐circuit current density (JSC) are a challenge for highly efficient Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Aimed at such target in CZTSSe solar cells, a synergistic strategy to tailor the recombination in the bulk and at the heterojunction interface has been developed, consisting of atomic‐layer deposited aluminum oxide (ALD‐Al2O3) and (NH4)2S treatment. With this strategy, deep‐level CuZn defects are converted into shallower VCu defects and improved crystallinity, while the surface of the absorber is optimized by removing Zn‐ and Sn‐related impurities and incorporating S. Consequently, the defects responsible for recombination in the bulk and at the heterojunction interface are effectively passivated, thereby prolonging the minority carrier lifetime and increasing the depletion region width, which promote carrier collection and reduce charge loss. As a consequence, the VOC deficit decreases from 0.607 to 0.547 V, and the average FF increases from 64.2% to 69.7%, especially, JSC does not decrease. Thus, the CZTSSe solar cell with the remarkable efficiency of 13.0% is fabricated. This study highlights the increased FF together with VOC simultaneously to promote the efficiency of CZTSSe solar cells, which could also be applied to other photoelectronic devices.