CdTe Thin Film Solar Cells With A SnTe Buffer Layer In Back Contact

In recent years,CdTe thin film solar cells have demonstrated promising market application foreground.However,a relatively large gap between the achieved solar cells efficiencies and the theoretically predicted maximum efficiency still exists.In particular,the open circuit voltage(Voc)of the CdTe thin film solar cells could be further increased,which is limited by high-work-function of CdTe.In this work,p-type SnTe thin films are proposed as a buffer layer in the back contact of CdTe solar cells.SnTe is a member of IV-VI group compound semiconductors with the trait of direct and narrow bandgap,and it has the advantages of low resistance,high mobility,and remarkably high hole concentration.X-ray photoelectron spectroscopy(XPS)characterization of the CdTe/SnTe heterojunction indicates that the valence band maximum of SnTe is 0.76 eV higher than CdTe.This band offset makes SnTe very suitable for hole transport from CdTe into SnTe.In this thesis,the thermal evaporation method was used to deposit SnTe thin films.Then,SnTe and other back contact buffer layers were applied on CdTe thin film solar cells,and the performance of these cells was analyzed comprehensively.The innovative research results are as follows:1、With the insertion of a 100 nm SnTe single buffer layer at the back contact,a Cu-free CdTe thin film solar cell has been developed,which displayed the same cell efficiency as the traditional cells using CuxTe or ZnTe:Cu as buffer layer in the back contact.As Cu is responsible for the degradation of CdTe thin film solar cells,the cell with only SnTe buffer layer could present better stability.2、Considering the carrier recombination at the interface of CdTe and SnTe,a more rational structure was designed with ZnTe:Cu(60 nm)-SnTe(40 nm)between CdTe and Ni.ZnTe:Cu was used as a blocking layer for minority carriers(electrons),which could reduce carrier recombination at the interface of CdTe and SnTe.Thus,the cell of highest performance was obtained with the 14.60%efficiency,70.21%FF,841.3 mV Voc,and 24.7 mA/cm2 JSC.Moreover,the ZnTe:Cu/SnTe double buffer layers achieved nearly 40 mV improvement in Voc.3、The diffusion of Sn in the CdTe absorber layer has been demonstrated as an efficient technique to improve hole concentration.This mechanism provides a new method to achieve better p-type doping for CdTe,which could be useful to improve the performance of CdTe thin film solar cells.