Defect Regulation And Band Engineering Of Critical Interfaces For CZTSSe Solar Cells

To achieve carbon neutrality,developing renewable energy technology with a low-carbon footprint is in urgent need.Solar energy is one of the most abundant renewable energy sources that could fully replace traditional energy sources in the future.The development of Cu2 Zn Sn（S,Se）4（CZTSSe）solar cells have achieved fruitful results due to their tunable bandgap,high light absorption coefficient,low cost,low toxicity,and abundance constitute elements.However,the current world record efficiency of CZTSSe solar cells is 13.0%（Voc=529 m V）,which is much lower than that of their predecessor Cu（In,Ga）Se2（CIGS）solar cells of 23.35%（Voc=734 m V）.It is mainly due to the low open-circuit voltage（Voc）of CZTSSe solar cells compared to CIGS solar cells.The low Voc of CZTSSe solar cells is mainly due to the dominated carrier recombination at the interface.The harmful defects at the CZTSSe/CdS and CZTSSe/Mo interface,which have been considered as the main reason to restrict the further improvement of CZTSSe device performance.i)At the CZTSSe/CdS interface,the electrical performance is poor due to the high formation energy of VS vacancy in the CdS buffer layer,resulting a small depletion region width（Wd）,which hinders the separation and transport of carriers at the p-n junction.ii)At the CZTSSe/CdS interface,the Fermi level pinning induced by high CuZn antisite defects on the surface of the CZTSSe layer,resulting a small band bending.The weak back electric field causing insufficient electrons and driving force in the quasi-neutral region.iii)At the CZTSSe/Mo interface,due to the low vacancy formation energy VSe,the weak n-type Mo Se2 is formed at the back interface during the high-temperature selenization of the absorber layer,and the Schottky contact between CZTSSe and Mo Se2 induces the recombination of the back interfaces,raising the interface transport barrier,which hinders the transportation of carriers.Aiming to minimize the interface electrical performance loss caused by the above-mentioned interface defects,researchers have developed two approaches: cation substitution and constructing an absorber gradient.i)At the CZTSSe/CdS interface,the dopants that can be used to tune the electrical properties of the CdS buffer layer,such as In,Al,Ga,etc.However,the secondary phase with low solubility product（Ksp）is easily formed during the preparation of cation-doped CdS films by chemical bath deposition（CBD）,which affects the quality of p-n junctions.ii)At the CZTSSe/CdS interface,the introduction of Ag into the CZTSSe absorber layer can effectively suppress the CuZn antisite defects and introduce ZnAg and SnZn donor defects.Constructing an Ag gradient in the CZTSSe absorber layer not only can suppress the CuZn antisite defect at the front interface,but also reduce the Fermi level pinning,and realize the p-type to n-type conversion at the interface.However,the diffusion of high Ag content into the bulk phase leads to the poor electrical properties of the films,limiting the separation and transport of carriers.iii)At the CZTSSe/Mo interface,introducing an intermediate layer or a passivation layer between the CZTSSe layer and the Mo layer can reduce the structural defects but cannot fundamentally change Mo Se2 semiconducting properties.In order to solve the key problems aused by the above-mentioned harmful defects,such as: the narrow depletion region width of CZTSSe/CdS interface,the decrease of bulk charge transport ability caused by high silver gradient,and the poor electrical contact of CZTSSe/Mo interface.Considering the issues mentioned above,We focus on suppressing the interfacial recombinations from the following three aspects,such as: In doped CdS,absorber gradient and（V,Nb,Ta）doped MoSe₂.In the first part,n-type doping in CdS was adopted to improve the energy band structure of the CZTSSe/CdS interface: To address the narrow depletion region width of the CZTSSe/CdS interface,a distinctive indium-incorporation（DI）strategy was developed to deposit In1-xCdxS buffer layer for optimizing the heterojunction interface.The results reveal that adopting this DI method can effectively inhibit the formation of undesirable secondary phase,and indium（In）can be more easily doped into the bulk lattice of CdS to form additional beneficial shallow donor InCddefects,which significantly improve the electrical properties of the CdS layer and the quality of heterojunction interface.Besides,the energy band alignment was adjusted to facilitate the extraction and transfer of interfacial charges,and thus reducing the nonradiative charge recombination at the front CZTSSe/CdS heterojunction interface.Consequently,the combination of the above enhances the power conversion efficiency（PCE）from 10.2 to 12.4%.In the second part,Ag-Ge double gradient was introduced to modulate the band structure of the front/back interface: To address the poor bulk charge transportation caused by a high Ag content gradient,we demonstrate a dual gradient architecture to increase Voc and cell efficiency by accessing the surface and bottom absorber via Ag and Ge substitution,respectively.The key feature of this double cation synergistic substitution is that they can downshift the VBM and upshift the CBM of the absorber on each side independently,and their benefits can be ascribed in three aspects: The substitution of Cu by Ag at p-n junction could retard Fermi level pinning and enhance interface band bending to accelerate charge separation;The substitution of Sn by Ge near back contact imposes an additional drift field and carrier concentration gradient to aid in continuous carrier transport toward p-n junction and minimize recombination loss at Mo/CZTSSe interface.The diffused Ag and Ge ions into middle absorber would passivate the deleterious CuZn and CuSn deep level defects.With significant gains in Voc,FF and Jsc,the Ag,Ge synergistic substituted device finally increases the conversion efficiency from 9.29% to 12.26%,which is the highest value for amine-thiol system so far.In the thirdly part,p-type doping in MoSe₂ was employed to modulate the band structure of the CZTSSe/Mo interface: To improve the electrical contact of the CZTSSe/Mo interface,the p-type V/Nb/Ta-doped Mo Se2 was prepared by thermally evaporate a thin layer of VB group transition metals on the Mo layer followed by high-temperature selenization.Theoretical calculations show that VMo,NbMo,and TaMo have low defect formation energies and are all acceptor defects.The equilibrium Fermi level of the doped Mo Se2 film is closer to the top of the valence band,and it is a p-type semiconductor.The enhanced electrical properties and back electric field of the doped Mo Se2 films are beneficial to the accelerated separation and transport of electrons and holes in the quasi-neutral region of the device,resulting a higher Voc than that of the device with undoped Mo Se2.A low energy barrier and ohmic contact of the back interface are beneficial for obtaining a high fill factor（FF）and a high Voc.Our results show that the PCE of the CZTSSe solar cell with the Ta-doped Mo Se2 was improved from 10.82 % to 12.72 %.In summary,creating an n-type donor defect environment at the CZTSSe/CdS interface is beneficial for the construction of a p-type to n-type transition and the reduction of the Fermi level pinning caused by CuZn acceptor defects.Hence,under the light,the absorber layer can produce a larger band bending and a wider Wd to accelerate the separation and collection of carriers at the p-n junction,resulting in reduced front interface recombination improving Voc.In addition,a p-type defect environment is constructed at the back interface of the CZTSSe/Mo interface,enabling absorption and Mo Se2 layer forms ohmic contact to reduce the hole transport barrier;The enhanced back electric field accelerates the movement of the carriers in the quasi-neutral region toward the CZTSSe/CdS interface,reducing the back-interface recombination and improving the PCE.