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

The mechanism of VOC 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 Voc 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 Rsh, and reduce the Voc 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.