Macroscopic versus microscopic photovoltaic response of heterojunctions based on mechanochemically prepared nanopowders of kesterite and n-type semiconductors

Mechanochemically prepared nanopowder of selenium-free kesterite Cu 2 ZnSnS 4 (CZTS) in combination with n-type semiconductors, i.e., CdS, ZnO and TiO 2 , was tested in planar and bulk-heterojunction solar cells.The samples have been studied by macroscopic current-voltage (I-V) measurements and Kelvin-probe atomicforce microscopy (KPFM).KPFM images taken under light illumination showed the distribution of the potential across the surface, with negative potential on the n-type semiconductor domains and positive potential on the CZTS domains, which indicated charge separation at the interface of the counterparts.The best result was found for the CdS-CZTS composition, which showed a potential difference between the domains up to 250 mV.These results were compared with the planar heterojunctions of CdS/CZTS and TiO 2 /CZTS, where CZTS nanopowder was pressed/deposited directly onto the surface of films of the corresponding n-type semiconductors.Again, I-V characteristics showed that cells based on CdS/CZTS heterojunctions have the best performance, with a photovoltage up to 200 mV and photocurrent densities up to 0.1 mA/cm 2 .However, the carrier generation was found to occur mainly in the CdS semiconductor, while CZTS showed no photo-response and served as the hole-transporting layer only.It is concluded that sensitization of the kesterite powder obtained by mechanochemical method is necessary to improve the performance of the corresponding solar cells.