Improvement of the Open Circuit Voltage of CZTSe Thin-Film Solar Cells by Surface Sulfurization Using SnS

The objective of this study is to find an effective method to improve Voc without Jsc loss for Cu2ZnSnSe4 （CZTSe） thin film solar cells, which have been fabricated by the one step co-evaporation technique. Surface sulfurization of CZTSe thin films is carried out by using one technique that does not utilize toxic H2S gas; a sequential evaporation of SnS after CZTSe deposition and the annealing of CZTSe thin films in selenium vapor. A Cu2ZnSn（S, Se）4 （CZTSSe） thin layer is grown on the surface of the CZTSe thin film after the annealing. The conversion efficiency of the completed device is improved due to the enhancement of Voc, which could be attributed to the formation of a hole-recombination barrier at the surface or the passivation of the surface and grain boundary by S incorporation.

Influence of selenium evaporation temperature on the structure of Cu_2ZnSnSe_4 thin film deposited by a co-evaporation process

Cu2ZnSnSe4 （CZTSe） thin film solar cells have been fabricated using a one-step co-evaporation technique. The structural properties of polycrystalline CZTSe films deposited at different selenium evaporation temperatures （TSe） have been investigated using X-ray diffraction spectra, scanning electron microscopy, and atomic force microscopy. A relationship between TSe and the secondary phases deposited in the initial stage is established to explain the experimental observations. The Se flux is not necessarily increased too much to reduce Sn loss and the consumption of Se during fabrication could also be reduced. The best solar cell, with an efficiency of 2.32%, was obtained at a medium Tse of 230 ℃ （active area 0.34 cm2）.

Influence of growth temperature and thickness on the orientation of Cu(In,Ga)Se_2 film

Cu(In,Ga)Se2(CIGS) films are deposited on the Na-free glass substrate using three-stage co-evaporation process,and the effects of thickness and growth temperature on the orientation of CIGS film are investigated by X-ray diffraction(XRD) and scanning electron microscopy(SEM).When the growth of CIGS film does not experience the Cu-rich process,the increase of the growth temperature at the second stage(Ts2) promotes the(112) orientation of CIGS film,and weakens the(220) orientation.Nevertheless,when the growth of CIGS film experiences Cu-rich process,the increase of Ts2 significantly promotes the(220) orientation.In addition,with the thickness of CIGS film decreasing,the extent of(In,Ga)2Se3(IGS) precursor orientation does not change except for the intensity of Bragg peak,yet the(220) orientation of following CIGS film is hindered,which suggests that(112) plane preferentially grows at the initial growth of CIGS film.

The effects of Na on the growth of Cu_2ZnSnSe_4 thin films using low-temperature evaporation process

Cu_2ZnSnSe_4(CZTSe) absorbers were deposited on borosilicate glass substrate using the low-temperature process, and different Na incorporation methods were applied to investigate the effects of Na on the CZTSe growth.Na was diffused into some of the absorbers after growth, which led to strongly improved device performance compared with Na-free cells.With the post-deposition treatment, the effect of Na on CZTSe growth was excluded, and most of Na was expected to reside at grain boundaries.The conversion efficiency of the completed device was improved due to the enhancement of open circuit voltage and fill factor.The efficiency of 2.85% was achieved at substrate temperature as low as 420℃.

Impact of Cu-rich growth on the Cu2ZnSnSe4 surface morphology and related solar cells behavior

Inorderto study the influence ofCu-rich growth on the performance ofthe Cu2ZnSnSe4 （CZTSe）thin film solar cells, a multi-stage co-evaporation process is applied. The CZTSe films are grown at a lower substrate temperature to reduce the existence time of Cux Sey at the first period caused by the volatility of SnSex. This study examines the surface morphology and device performance in Cu-rich growth and close-to-stoichiometric growth. Although the grain size of Cu-rich growth film increases a little, the difference was not dramatic as the results of CIGS reported previously. A model based on the grain boundary migration theory is proposed to explain the experimental results. The mechanisms of Cu-rich growth between CZTSe and CIGS might be different.

First principles calculation of the electronic-optical properties of Cu2MgSn(SxSe1-x)4

Based on the density functional theory with hybrid functional approach,we calculated the structural,electronic,and the optical properties of Cu2MgSn(S1-xSex)4(CMTSSe),an potential photovoltaic material for thin film solar cells.The calculation reveals a phase transition from kesterite to stannite structure when Zn atoms are substituted by Mg atoms.In particular,the S-to-Se ratio can determine the energy splitting between the electronic states at the top of the valence band.The band gaps of CMTSSe can be tuned in the ranges of 1.01-1.58 eV.Calculated optical properties and tunable band gaps make them beneficial for achieving band-gap-graded solar cells.