Graded Cu Component Engineering To Achieve Continuous Carrier Transport For Enhanced CZTSSe Solar Cells

In recent years,the chalcogenide thin film solar cells have entered a vigorous development period due to their competitive production cost,component-enabled bandgap tunability and high output performance.As the large-area manufacturing capacity of Cu(In,Ga)Se2(CIGS)solar cells will be hindered in the mid-or long-term by rare In,the relative earth-abundant Cu2 Zn Sn(S,Se)4(CZTSSe)solar cells have been emerged as a promising candidate in next generation photovoltaic(PV)technologies.In spite of the rapid progress,the state-of-the-art efficiency of 12.62% is still far behind the highest value of 23.35% for CIGS.This large efficiency gap leaves tremendous improvement room for kesterite compounds,while one remaining challenge is the inferior electronic properties of CZTSSe absorber originated from the coexistence of impurity phase and the high popular concentration of intrinsic defects/defect clusters.To gain control over these electronic properties,an imposed off-stoichiometric within the restriction of stable thermodynamic phase limit has been identified experimentally effective in suppression of secondary phases.Meanwhile,a more specific Cu-poor condition with Cu/(Zn+Sn)ratio ranging from 0.7 to 0.75 is structural benign in reducing the detrimental antisite defects/defect clusters concentration and increasing the beneficial shallow acceptor concentration of copper vacancies(VCu).While this traditional developed Cu-poor architecture addresses many of the limitations for kesterite PV technology,the device efficiency still encounters in a bottleneck and three features of the inferior bulk properties need to be revealed and highlighted:(1)the low mobility and degraded back electric field at quasineutral region;(2)the generated valence band(EV)notch acting as hole injection blocking barrier at absorber/Mo interface determined by the out-diffusion of Cu;(3)more grain boundaries(GBs)acting as recombination centers induced by smaller grain sizes.All these issues would deteriorate device performance through blocking carrier diffusion and creating shunting or resistive paths.On the contrary,the electronic properties of CZTSSe absorber with higher Cu concentration(Cu/(Zn+Sn)?1)showing no potential fluctuations and higher mobility are superior to Cu-poor materials.More pragmatically,the Cux Sey-assisted grain growth mechanism at higher Cu concentration facilitates the formation of superior polycrystalline film morphology with larger grains and higher preferred orientation,allowing continuous carrier transport within the bulk and minimizing deleterious GBs effects.Nevertheless,the performance of Cu-rich devices is always experimentally inferior compared with that of the Cu-poor ones,primarily arising from the interface dominated recombination mechanism.The Cu-rich phase at buffer layer/absorber interface pins the Fermi level and would act as electrical shunts.Therefore,how to comprehensively utilize the advantages of copper-rich at back interface and copper-poor at surface interface has become the major focus of future research.In this paper,a Cu-graded absorber structure with low copper content on top and high copper content in the bottom was developed to improve CZTSSe device performance and its electrical benefits are in depth investigated.The specific work is divided into two parts and listed as following:Firstly,a solution route by utilizing copper gradient to improve the performance of double-layer structured CZTSSe solar cells is developed.The effects of homogeneous copper composition with different content on absorber morphology and phase structure are initially investigated.With increasing the copper concentration,the grain size is continues increased and the grain boundary and porosity is decreased as well.It is found that the absorption layer produces good crystallization without obvious secondary phase when the content of Cu/(Zn+Sn)is 1.10.However,the relative uniform copper-rich would even degrade device performance due to the large recombination losses induced by current shunts.Afterwards,the localized copper-rich with different incorporation sites,copper-rich layer number and the corresponding Cu/(Zn+Sn)ratio are explored in sequence.Finally,an appropriate copper-rich level was obtained with the structure of two copper-rich layers in the bottom(Cu/(Zn+Sn)=0.95 and five copper-poor layers on top.This structure can produce a denser absorber,reduce current shunting,promote charge transport within the bottom fine grain layer,and improve the ohmic contact at back contact of the device.In return,the photoelectric conversion efficiency of the device was increased to 11.57%.Secondly,a solution route by utilizing copper gradient to improve the performance of tri-layer structured CZTSSe solar cells is also developed.In this work,we explore a newly developed Cu-graded absorber to increase device performance of CZTSSe solar cells by constructing a trilayer large grain/fine grian/large grain architecture and imposing a concentration gradient to drive the diffusion of carriers towards front contact.The principle of this strategy is based on the presence of fine grian layer in the middle,which has intense composition fluctuations to tolerate the excess Cu from bottom layer,retard the mutual Cu ion migration and maintain the Cu concentration difference between upper layer and bottom layer.Based on copper-rich location exploration,copper-rich layer exploration and copper-rich element ratio exploration,the optimal copper graded construction method is found as the bottom three layers of copper-rich Cu/(Zn+Sn)=1.15 and the upper six layers of copper-poor.Upon the redistribution of Cu-graded profile,the hole concentration is abruptly changed at different Cu composition stoichiometric,resulting in the improvements of charge transport within the quasineutral region and hole injection at back contact.After depth characterization and further composition optimization,an unexpected photoelectricity conversion efficiency of 12.54% is achieved.