Date of Award

8-9-2014

Document Type

Open Access Dissertation

First Advisor

Krishna C. Mandal

Abstract

Current thin-film solar cell technologies based on CuInxGa1-xSe2 (CIGS) and CdTe photo-absorber materials use rare and expensive elements, such as In, Te, Ga, and toxic Cd which severely limit the mass production and deployment of these solar cells. Thus, a major research effort is focused toward the development of new photovoltaic (PV) absorber materials comprising of earth-abundant, low-cost, and environmentally benign constituent elements that can support terawatt (TW)-scale PV generation in the near future and be economically sustainable. Cu-based I2-II-IV-VI4 quaternary kesterite compound Cu2ZnSn(SxSe1-x)4 (CZTSSe) have recently emerged as a potential photo-absorber material for thin-film solar cells. All constituent elements in CZTSSe are abundant in earth’s crust, are much cheaper and possess no acute toxicity. CZTSSe is an intrinsically p-type material with a large optical absorption coefficient (α>104 cm-1) and exhibits a tunable direct optical bandgap in the range of 1.0 eV ≤ Eg ≤ 1.5 eV corresponding to chalcogen ratios of 0 ≤ x ≤ 1. The theoretical Shockley-Queisser efficiency limit for a single junction CZTSSe solar cell is estimated to be ~32% – similar to that of CIGS solar cells. All these merits make CZTSSe an ideal photo-absorber material for thin-film solar cells.

In this dissertation, a comprehensive investigation is undertaken on the growth and characterization of Cu2ZnSn(SxSe1-x)4 photovoltaic absorber material followed by thin-film solar cell fabrication and cell characterization. CZTSSe films were fabricated by a vacuum-based two-step process of ZnS/Cu/Sn stacked precursor layer deposition on bi-layer molybdenum (Mo)-coated soda-lime glass (SLG) substrates via thermal evaporation and successive annealing of the precursor stacks under a mixed sulfur and selenium vapor at 550°C. The heterojunction was formed by deposition of n-CdS layer on top of p-CZTSSe absorber film via a low-cost chemical bath deposition (CBD) technique. The structural, compositional and morphological characterization of the CZTSSe films were carried out by Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The best solar cell was obtained with a Cu-poor and slightly Zn-rich absorber composition corresponding to elemental ratios of Cu/(Zn+Sn) = 0.773, Zn/Sn = 1.13 and S/(S+Se) = x = 0.58. Thickness of the CZTSSe film was measured to be 1.3-1.4 μm with an estimated bandgap of ~1.3 eV. The photovoltaic performance of the fabricated cells were evaluated under simulated AM1.5G (100 mW/cm2) solar radiation. The champion cell exhibited an open-circuit voltage (VOC) of 506 mV, a short-circuit current density (JSC) of 22.92 mA/cm2, and a fill-factor (FF) of 35% resulting in a total area efficiency (η) of 4.06% without any antireflection coating.

Performance of the fabricated solar cells were found to be limited by high series resistance (RS), low shunt resistance (RSh), and poor fill factor (FF). The sources of high series resistance were attributed to the small multi-grain microstructure of the polycrystalline CZTSSe film, presence of micro air-voids, and a Mo(SSe)x interfacial layer at the Mo back contact. AFM studies revealed micro-pores on the film surface which act as low resistance shunt paths and are attributed to the source of low shunt resistance. Impedance spectroscopy were performed on the solar cells to model and extract the equivalent AC circuit parameters. Fitting of the experimental results showed the presence of a blocking barrier at the back contact and a recombination center resembling a constant phase element (CPE). Temperature dependent illuminated current-voltage (J-V) studies indicated a major recombination phenomena occurring at the heterojunction interface corresponding to an activation energy of 1.12 eV. Further investigation of the electronic defect levels in the fabricated solar cells have been carried out by current-mode deep level transient spectroscopy (I-DLTS). Two dominant deep acceptor defects at Ev+0.12 eV, and Ev+0.32 eV have been observed and were identified as the CuZn(-/0) and CuSn(2-/-) antisites respectively.

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