Efficient Superstrate Solar Cells with Electrodeposited Cu2ZnSnS4
Authors: Di Iorio Yesica , Berruet Mariana , Pereyra Javier , Marotti Ricardo , Vazquez Marcela 
Published: February 2020

Cu2ZnSnS4 (CZTS) is a quaternary semiconductor that has emerged as a potential absorber substitute for CuInGaSe2 in photovoltaic devices. This material has excellent optical properties (α ≥ 105 cm-1) and a direct bandgap energy value (E
GAP) close to 1.5 eV 1. Furthermore, it does not contain toxic elements such as selenium or expensive and scarce ones as indium and gallium. Due to these properties, CZTS films are being intensively studied and are excellent candidates for developing low-cost, highly efficient and environmentally friendly solar cells 2-6.

In order to achieve real cost reductions, the deposition of thin films should involve inexpensive equipment and be easily transferable to industrial scale. Electrodeposition and spray pyrolysis techniques meet all these requirements and have been chosen in the present investigation.

Solar cells were prepared combining TiO2 as n-type transparent window, In2S3 as buffer layer and Cu2ZnSnS4 as p-type absorbing layer in superstrate configuration using conductive glass (FTO) as substrate. A thin film of TiO2 and an ultrathin film of In2S3 were deposited on FTO by spray pyrolysis. CZTS was electrodeposited on top of this duplex layer, from a single bath, at room temperature. The electrodeposition was carried out using a standard three-electrode cell; a saturated calomel electrode (SCE) and a Pt mesh of big area were used as reference and counter electrodes respectively. A constant potential (E = -1.05 V) was applied during 15 minutes. The electrolytic bath consisted of an aqueous solution containing 0.02 mol L-1 CuSO4, 0.01mol L-1 ZnSO4, 0.02mol L-1 SnSO4, 0.02mol L-1 Na2S2O3, 0.2 mol L-1 sodium citrate, and 0.1mol L-1 tartaric acid. Then, an annealing step was undertaken in sulfur vapor atmosphere (sulfur powder at 580 º C) for 90 minutes using a purpose-built reactor consisting of a quartz tube furnace. Post treatments such as soft annealing (at 150 °C during 30 minutes in air) and/or chemical etching (in 0.25 mol L-1 KCN solutions during 30 s) were performed and evaluated. These post treatments were meant to introduce oxygen into grain boundaries and dissolve Cu(I) and Cu(II) sulfides that could have formed as secondary phases.

Morphology, thickness, crystalline structure and chemical composition were analyzed by electronic microscopy, profilometry, X-Ray diffraction and Raman spectroscopy. The band gap value of each semiconductor was determined by UV-Vis spectroscopy, resulting in values close to those expected for the bulk materials. The photoresponse of the different solar cells was analyzed by current-voltage (I-V) curves under simulated solar irradiation, quantum efficiency (QE), intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy (IMPS). The results proved that the solution-based and vacuum-free deposition of these materials has promising photovoltaic applications. No significant improvements were found if a soft-annealing or etching treatment were performed after the regular annealing stage in sulfur vapor. The best solar cell performance showed an efficiency equal to 3.6 % with a Voc= 0.59 V, Jsc= 13.9 mA cm-2, FF = 0.44. These numbers mark a clear improvement when compared with our previous results 7-9. Although the efficiency is not as high as high as those reported for substrate configuration (record of 9.6% 10) , these results represent an improvement when compared to those previously published for superstrate CZTS solar cells prepared using solution-based methods 11.

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