Synthesis and Post-Annealing of Cu2ZnSnS4 Absorber Layers Based on Oleylamine/1-dodecanethiol

Narges Ataollahi,Claudia Malerba,Maurizio Ferrari,Francesca Bazerla,Andrea Chiappini,Paolo Scardi,Rosa Di Maggio

doi:10.3390/ma12203320

Abstract

Cu2ZnSnS4 (CZTS) nanocrystals in oleylamine (OLA) and 1-dodecanethiol (1-DDT) solvents were successfully prepared via hot-injection method, to produce inks for the deposition of absorber layers in photovoltaic cells. In this process, 1-DDT acts as a coordinating ligand to control the nucleation and growth of CZTS nanocrystals, whereas lower amounts of OLA promote a homogeneous growth of the grains in the absorber layer. X-Ray Diffraction (XRD) revealed both tetragonal and hexagonal phases of CTZS in films obtained after soft thermal treatments (labeled TT0). In particular, 1-DDT is responsible for the formation of a greater percentage of the hexagonal phase (ZnS-wurtzite type) than that formed when only OLA is used. The thermal treatments have been varied from 500 °C to 600 °C for improving crystallization and eliminating secondary phases. Both features are known to promote CZTS thin films with band gap values typical of CZTS (1.5–1.6 eV) and suitable resistivity. This study let to compare also the CZTS post-annealing without (TT1) and with sulfur vapor (TT2) in a tubular furnace. Only tetragonal CZTS phase is observed in the XRD pattern of CZTS thin films after TT2. A small presence of localized residues of secondary phases on the same samples was revealed by μRaman measurements. The best values of band gap (1.50 eV) and resistivity (1.05 ohm.cm) were obtained after thermal treatment at 500 °C, which is suitable for absorber layer in photovoltaic application.

Highlights

The Cu2 ZnSnS4 (CZTS) solar cells recently reached efficiency higher than 11% [1]
The results show that there is no significant loss of any elements during the synthesis of nanoparticles, and the final composition of the CZTS film is, within the experimental error, the desired one, and their ratios are close to the values expected for high-performance solar cell applications
CZTS NCs were synthesized by a hot-injection process with metal chlorides, using oleylamine (OLA) and dodecanethiol (1-DDT) as solvents

Summary

Introduction

The Cu2 ZnSnS4 (CZTS) solar cells recently reached efficiency higher than 11% [1]. The best results reported in the literature are usually reached using high cost vacuum-based processes or hard/hazardous chemical preparation procedures [2]. Non-vacuum based chemical methods employing non-toxic or non-hazardous agents seem preferable for high throughput, large production and promising conversion efficiency [5,6]. Cu2 ZnSnS4 (CZTS) quaternary nanocrystals (NCs) have been recently drawing significant research interest to form active-layer semiconductor. CZTS NCs have been mainly reported in two different crystallographic forms: tetragonal (kesterite), thermodynamically stable in ambient conditions [11], and hexagonal (wurtzite). One of the methods to obtain kesterite NCs is high temperature synthesis, mixing metal salts with elemental sulphur in oleylamine (OLA) [12,13]

Methods

Results

Conclusion