Kesterite CZTS Quantum Dot Based Photovoltaic cells: From the Prospective of Material Characterization and Photovoltaic Performance

Abstract

Increasing worldwide clean energy crisis calls urgently for the development of harvesting the renewable solar energy through photovoltaic device. Thin film photovoltaic cells (TFPVCs) belong to the second generation, where the absorber layer attracts the attention of research community due to large versatility in deposition, low-cost device design and easy fabrication process. In contrast to well-known absorber materials such as CdTe, CIGS, Copper-based Multinary I2–II–IV–VI4 quaternary chalcogenide Cu2ZnSnS4also known as CZTS has developed as one of the promising candidates for the absorber layer of photovoltaic cell (PVC) due to its non - toxic nature, cost effective and earth abundance. As a direct p-type semiconductor CZTS offers the advantage of tuneable band gap ~ 1.45–1.65 eV favourably matches with solar spectrum having a high absorption coefficient of 104 cm-1. Effective and promising efficiency improvement of kesterite CZTS photovoltaic cell draws the attention of scientific community for its further development. Inclusiveness of all required materials for thin film photovoltaic cell, specifically absorber material has a major responsibility to establish its futuristic evolutionary footprint. The highest achieved power conversion efficiency (PCE) is at 12.6%, considerably lower than the Schokley-Quassier limit i.e., 32.4% for single a junction CZTS photovoltaic cell device. Therefore, a consistent effort has been focused to develop highly efficient earth abundant, non-toxic, and most importantly cost-effective kesterite CZTS photovoltaic device. To track the unexploited UV photons, CZTS quantum dots (QDs) are highly fascinated due to their wide band gap range, band gap tunability results from quantum confinement, multiple exciton generation (MEG) capability, size dependent photoluminescence, hot electron injection and possibility of solution processed device fabrication.The demonstration of charge carrier multiplication phenomenon in QDs unwraps the possibility to obtain higher quantum efficiency.All the comprehensive and strategic attempts motivate us to fabricate size controlled kesterite CZTS QDs photovoltaic device to use the unexploited UV photons. Dynamic versatility of CZTS QDs structure is the new exploration towards energy harvesting applications. Still, now there are no reports on the application of kesterite CZTS QDs in photovoltaic device and no proper device performance study. Promoting p-type semiconducting quaternary chalcogenide kesterite CZTS QDs as promising absorber; the aim is to synthesize and characterize size controlled CZTS QDs and to estimate their performance in photovoltaic device. To produce high quality, monodisperse CZTS QDs, we followed a very simple, non-toxic solvothermal approach. A regular photovoltaic device with the configuration of Mo/CZTS/CdS/ZnO/Al has been fabricated without post sulphurization, where instead of CZTS thin films; synthesized CZTS QDs were applied as absorber. This PV device resulted open circuit voltage (Voc) of 0.67 V and 15.8 mA/cm2 short circuit current density (Jsc) along with fill factor (FF) of 56.2%, PCE of 5.94%. The stability of fabricated photovoltaic device was investigated over keeping the device in inert atmosphere for long 60 days. However, the high performance has not yet been achieved, motivated the scientific community to study various aspects of kesterite intensively. The short carrier lifetime, poor carrier mobility, truncated charge transference and assortment are the hitches for improved photovoltaic performance. Several used efforts for improving performance mostly stem from the point that semiconducting QDs have enormous surface defects result in substantial charge carrier recombination and consequently in Voc deficit. Most likely Voc arises from non-optimal band alignment of absorber and buffer. Recently, an III-VI n-type semiconductor Indium sulphide (In2S3) has emerged as the replacement of Cd supported by few experimental works. To show the feasibility of In2S3, ZnS, ZnO buffer layers with CZTS QDs absorber, the photovoltaic devices were fabricated and studied. Our results show that alternative buffers (In2S3/ZnS/ZnO) can effectively boost the Voc. Photovoltaic devices with the structure of Mo/CZTS QDs/ In2S3, ZnS, ZnO/ZnO/Al and the fabricated devices achieved efficiency of 5.76%, 4.92% and 4.46% respectively. However, the PCE of fabricated photovoltaic devices with alternative non-toxic and non-carcinogenic buffer does not improve up to our expectations. Moreover, Anion/cation substitution inside CZTS structure facilitates to improve PCE of photovoltaic cell through the enhancement in Voc. Here, in this work we have studied two systems CZTSe and AZTS and used as absorber in the replacement of CZTS QDs in the photovoltaic device fabrication procedure. CZTSe QDs were used successfully in the active layer of a photovoltaic device with the structure of Mo/CZTSe QDs/CdS/ZnO/Al and the fabricated device achieved PCE of 6.23%. The as synthesized AZTS QDs were used successfully in the active layer of a photovoltaic device with the structure of Mo/AZTS QDs/CdS/ZnO/Al and the fabricated device achieved PCE of 6.28%. Their anchoring platform to improve the photo generated charge carrier collection is an alternative promising approach for high performance device without deteriorating their properties. Carbon nano structured materials like reduced graphene oxide (rGO), multi-walled carbon nanotubes (MWCNTs) and few layer graphene sheet (FLGS) are eminent ultrafast charge transporters. Hence, improvement in the charge transfer behaviour has been carried out by incorporating rGO or MWCNTs or FLGS. The active area PCE has seen to be enhanced by adding rGO or MWCNTs or FLGS. CZTS QDs-rGO absorber-based device showed 6.25% PCE and CZTS QDs-MWCNTs showed further enhanced PCE of 6.77% and CZTS QDs-FLGS showed further enhanced PCE of 7.64% due to the establishment of conducting network in active layer which facilitates fast charge carrier transfer process.The stability of fabricated photovoltaic device was investigated over keeping the device in inert atmosphere for long 60 days. Our study can help to provide a practical approach of a low cost, scalable fabrication of CZTS QDs based photovoltaic devices.