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The thin film solar cells based on Cu2ZnSnS4 (CZTS) compounds have been considered the potentially substitute to Cu(In,Ga)Se2 and CdTe absorbers due to its low cost, earth-abundant and non-toxic components. The present convention efficiency based on Mo/p-CZTS/n-CdS solar cell is far away from the theoritical value, which originated from the interfacial recombination at the Mo/CZTS heterojunction. In our study, we sputtered a 25nm thickness temporary Ge layer between the Mo electrode layer and the CZTS absorbed layer fabricated by spin coating, which could reduce the recombination centers and the Sn-loss. The partial Ge element would diffuse to CZTS which resulted in the formation of kesterite Cu2ZnSnGeS4 (CZTGS) thin films in which the (112) diffraction peaks shifted towards higher angle. The reason is the decreased lattice constant due to the replacement of large Sn atoms by smaller Ge atoms. The field emission scanning electron microscopy (FESEM) showed that the regularity and uniformity have been improved and the voids decreased after inserting Ge layer. The photovoltaic device (ITO/i-ZnO/CdS/CZTGS/Mo) was fabricated. The illuminated J–V characteristics showed that the open circuit voltage (Voc) increased from 513.42 to 620.71 mV, the short circuit current density (Jsc) increased from 11.57 to 12.44 mA, and the efficiency increased from 2.41 to 3.88% after inserting the Ge layer, which attribute to the decreased recombination at the Mo/CZTS contact and the broaden absorption spectra.
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