Abstract
This thesis demonstrates the deposition and growth of earth abundant kesterite (i.e., Cu2ZnSnS4, Cu2ZnSnSxSe4–x) absorber layers by using non-toxic sol-gel spin coating approach and their solar cells device engineering. In chapter 2, we have introduced 1,3-dimethyl-2-Imadazolidinone (DMI) as a solvent for the preparation of high viscosity homogeneous nontoxic Cu2ZnSnS4 (CZTS) ink. Annealing the spin coated CZTS thin film in diluted H2S (6% N2) gas with externally supplied tin and sulfur environment suppresses the tin loss from the thin-film surface and enhanced the device performance. Grain size of CZTS has been achieved to > 0.7 to 1.5 µm with no carbon rich or small grain layer at the Mo/CZTS interface. The fabricated champion device achieved 5.67% efficiency with open circuit voltage of 0.58 V, short circuit current density of 18.48 mA/cm2, and a fill factor of 53.14%. In chapter 3 – reactive gas selenization – we demonstrate the synthesis of CZTSSe absorber layers with desired bandgap by tuning the composition of S to Se ratio. Annealing the spin coated CZTS thin film in diluted H2S (10% Ar) gas with externally supplied tin and selenium; we successfully have obtained CZTSSe absorber layers of different bandgaps. By changing Sn+Se amount (x= 274 mg, x/5, x/10), we successfully tuned the CZTSSe absorber layers bandgaps from 1.14 to 1.34 eV. Finally, by optimizing the Sn+Se (=54.8 mg) amount, the best CZTSSe (Eg = 1.22 eV) device efficiency was achieved to be 4.72% with Voc = 0.48 V, Jsc = 22.2 mA/cm2 and FF = 44.3 %. In second part, H2+Ar-assisted selenization, when annealed in H2 (diluted in Ar) and Se, we have observed formation of completely grown CZTSe absorber layer. In 100sccm H2, the device efficiency was achieved to 5.19%, Voc = 0.38 V, Jsc = 27.6 mA/cm2, FF=49.5 %. In order to obtain “stable CZTS” solution, we have changed copper source to copper formate to get preferred oxidation Cu2+1Zn+2Sn+4S4-2. H2-assisted selenization of the spin-coated film with such a sol-gel, gave an improved solar cell performance. The champion cell efficiency found to be 5.19%, Voc = 0.38 V, Jsc = 27.6 mA/cm2, FF=49.5 %. Eg = 1.06 eV. In our previous studies on CZTS/CZTSSe devices, cadmium sulfate (CdSO4) has been used as the cadmium source for depositing CdS layer (80±20 nm) via CBD (hereafter referred to as standard procedure). Devices fabricated with the standard procedure show poor charge collection at shorter wavelengths of the visible spectrum due to absorption by the CdS buffer layer. In chapter 4, a Cadmium sulfide (CdS) layer with a thickness of 37±5 nm is deposited onto a Cu2ZnSn(SSe)4 absorber layer using Cd(NO3)2 precursor at pH 11.8 via CBD process. Here the absorber is grown by sputtering process. Full devices fabricated with the thin CdS layer show improved champion efficiency of 6.97%, compared with 5.91% control device due to increased current density from optimized hetero-junction interface and enhanced charge collection in the external quantum efficiency spectrum. In chapter 5, preparation of solution processed earth abundant p-n junction, Cu2ZnSnS4/Zn(S,O,OH), is presented here. A thin, n-type Zn(S,O,OH) buffer layer of 40±5 nm thickness is chemical bath deposited (CBD) on Cu2ZnSnS4 absorber layer with 13 min deposition time. The chemical composition of the film is determined by energy dispersive spectroscopy and found gradient distribution of S and O across the film. After CBD, the p-n junction is rinsed in NH4OH and subsequently heated at 200 C for 10 min. The solar cell performance of CBD-ZnS buffer layer reached up to 4.1% efficiency compared with 5.67% of standard CdS buffer layer solar cell.