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...

Wide-bandgap perovskites have attracted substantial attention due to their important role in serving as a top absorber in tandem solar cells (TSCs). However, wide-bandgap perovskite solar cells (PVSCs) typically suffer from severe non-radiative recombination loss and therefore exhibit high open-circuit voltage (V OC ) deficits. To address these issues, a 2D octyl-diammonium lead iodide interlayer is adopted onto the hole-transporting layer to induce the formation of an ultrathin quasi-2D perovskite that is close to the hole-selective interface. This approach not only accelerates hole transfer and retards hole accumulation but also reduces the trap density in the perovskite layer on top, thereby efficiently suppresses non-radiative recombination pathways. Consequently, the champion wide-bandgap device (≈1.66 eV) exhibits a power conversion efficiency (PCE) of 21.05% with a V OC of 1.23 V, where the V OC deficit of 0.43 V is among the lowest values for inverted wide-bandgap PVSCs. Moreover, by stacking a semi-transparent perovskite top cell on a 1.1 eV Cu 2 ZnSn(S,Se) 4 (CZTSSe) bottom cell, a 22.27% PCE was achieved on a perovskite/CZTSSe four-terminal tandem solar cell, paving the way for all-solution-processed, low-cost, and efficient TSCs with mitigated energy loss in the wide-bandgap top cells.

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,...

The Cu 2 ZnSn(S,Se) 4 (CZTSSe) has gained extensive attention in thin film solar cells due to their potential as a nontoxic, low-cost, and earth-abundant absorber material, and a rapid increase in power conversion efficiencies has been demonstrated in laboratory. Compared with the most successful hydrazine-based solution process, the nanocrystalbased ink method and non-hydrazine molecular precursor solution approach are more eco-friendly for fabricating highefficiency CZTSSe solar cells. However, it is hard to obtain a complete large-grain CZTSSe absorber thin film which can facilitate the transport of photogenerated carriers while minimize grain boundary recombination. Here, we present a simple and effective strategy to significantly enhance grain growth of CZTSSe absorber layers by insetting Sb 2 S 3 , CuSbS 2 , and NaSb 5 S 8 thin films. The incorporation of Sb-based thin films can induce grain growth in the selenization process, and did not produce the impurity phase confirmed by XRD patterns and Raman spectra. It was found that the order of the crystal growth promotion ability is Sb 2 S 3 > CuSbS 2 > NaSb 5 S 8 under the same experimental conditions. The presented approach can be extended to other solution processes of fabricating CZTSSe solar cells to enhance their microstructural properties, which are critical for applications in CZTSSe absorbers with fine-grain layers.