Towards Better Understanding of High Efficiency Cd-free CIGS Solar Cells Using Atomic Layer Deposited Indium Sulfide Buffer Layers

Abstract

This paper presents optimization studies on the formation of cadmium free buffer layers for high efficiency copper indium diselenide (CIGS) thin film solar cells using a vapor phase route. Indium sulfide layers have been deposited on CIGS substrates by Atomic Layer Deposition (ALD) at substrate temperatures between 140 and 260 °C using indium acetylacetonate and hydrogen sulfide precursors. The parametric study of the deposition temperature shows an optimal value at about 220°C, leading to an efficiency of 16.4 % which is a technological breakthrough. The analysis of the device shows that indium sulfide layers give an improvement of the blue response of the cells as compared a standard CdS processed cell, due to a high apparent band gap (2.7-2.8 eV), higher open circuit voltages (up to 665 mV) and fill factor (78 %) . This denotes high interface quality of the system. Atomic diffusion processes of sodium and copper in the buffer layer are evidenced.

Buffer layers and transparent conducting oxides for chalcopyrite Cu(In,Ga)(S,Se)2 based thin film photovoltaics: present status and current developments

“…The reactants are fed into the reactor separately and sequentially making ALD a pulsed process where chemical reactions on or in the vicinity of the substrate surface control the film growth. Ideally the chemical reactions during each precursor pulse are self‐terminating, making the thickness control in ALD very good (Figure 5) 84. Other advantages are that the method produces films with excellent step coverage and uniformity.…”

“…Deposition of Zn(O,S) by ALD is reported in Reference 84 and as buffer layer on CIGS for the first time in Reference 92. The most common precursors are diethyl zinc, H 2 S, and H 2 O where the film composition can be varied from ZnO to ZnS depending on the pulsing ratio.…”

Defect analysis of sprayed β-In2S3thin films using photoluminescence studies

“…Junctions necessary for photovoltaic action in most of the cells were fabricated using CdS as the buffer layer [3]. The replacement of CdS with In x (OH,S) y /In 2 S 3 in the solar cells as the buffer layer had given good results [4][5][6][7][8]. Presently there are serious efforts to prepare In 2 S 3 -based thin film solar cells using simple techniques [9].…”

“…In 2 S 3 is a III-VI compound which by birth is an n-type semiconductor. It can be prepared using different chemical techniques such as chemical spray pyrolysis (CSP) [6,7,9], chemical vapour deposition (CVD) [9] and chemical bath deposition (CBD) [10]. It exhibits a band gap in the range of 2-3 eV, depending upon the preparation technique [11][12][13][14] and has a defect spinal structure with ordered vacancies in the III sub-lattice (vac).…”

Solution Processing Route to High Efficiency CuIn(S,Se)₂ Solar Cells

“…Replacing CdS by ZnS for instance could help gain another 2 mA/cm². Early attempts were very encouraging [25].…”