Effect of CZTS/CdS interfaces deposited with sputtering and CBD methods on V_oc deficit and efficiency of CZTS solar cells

The mechanism of V_OC deficit in CZTS solar cell has been systematically investigated by depositing the CdS buffer layer with sputtering (SP) and chemical bath deposition (CBD), respectively. It was found that SP-CdS film exhibits larger crystallite size and more densely packed morphology than CBD-CdS film. The average optical transmittance of SP-CdS films deposited at 250 °C is 71.5% (74.6% at 450 °C), evidently higher than 50.2% of CBD-CdS film at 70 °C. However, CZTS solar cell with CBD-CdS shows less V_oc deficit and higher conversion efficiency. By virtue of Hall and TEM measurements, the mechanism is further revealed: i) CdS can diffuse into CZTS layer easily and form a shallow n-type layer by CBD method. ii) CBD-CdS possesses high resistivity and tends to form colloidal, which can block pin-holes and type I voids between the grain boundaries. All of them improve the CZTS/CdS interface and raise the value of R_sh, and reduce the V_oc deficit and boost the conversion efficiency. By optimizing the stoichiometric ratio of Cu/Sn and Zn/Sn, the best conversion efficiency of CZTS solar cells with CBD-CdS is beyond 8.2% (active area) without MgF₂ anti-reflection coating.