CZTS (Cu2ZnSnS4) photocathode for solar energy conversion and storage

Abstract

Sunlight provides abundant, renewable energy on earth’s surface. Photoelectrochemical photovoltaic cells (PEPC) are one of the most convenient ways to capture this solar energy and convert into electricity. Cu2ZnSnS4 (CZTS) is a p-type semiconductor which holds great opto-electronic and economic advantages over traditional solar absorbing materials such as silicon. The elements in CZTS are earth abundant and environment friendly and CZTS has an energy band gap of ~1.5 eV which is considered to be optimum for solar absorption. The high absorption coefficient ~ 104 cm-1 of CZTS thin films allow for 95% absorption of the incident energy within 1µm. However, there are significant challenges with CZTS in separating the photogenerated charge carriers. These challenges results from the narrow thermodynamic window for chemical stability and the high density of intrinsic defects within the film. During the deposition of this quaternary material, domains of binary compounds develop within the thin film. The interface between these binary impurities and quaternary CZTS induces recombination of the charge carriers that are generated through the absorption of light. To minimize the impure phases, we developed a facile and reproducible CZTS thin film deposition protocol using an air stable precursor solution. Initially, the solution was sprayed onto a preheated FTO substrate for instantaneous film deposition. The film was then annealed under S and SnS atmosphere at 470 oC with inert gas (N2) flow. The precursor composition was varied to study evolution of impure phases with compositional changes in the film. The thin film was employed as a photocathode in a three-electrode photoelectrochemical cell (PEC) to study the electrode performance as a solar absorbing layer. The thin film deposited using the precursor solution requires high temperature annealing which is energy intensive. In a different approach, CZTS nanocrystals can be presynthesized in the solution at low temperature (~270oC). Cu2ZnSnS4 (CZTS) nanocrystals are important materials for next generation solar energy capture and conversion strategies. However, progress in utilizing CZTS nanocrystals is impeded by the numerous crystal and morphological defects present without high temperature annealing. These defects commonly result in reduced carrier diffusion length and carrier lifetime. One recent and promising direction for employing CZTS nanocrystals in solar energy capture and conversion strategies is through remediation of defects by doping with alkali Earth metal ions such as Li+, Na+, and/or K+. We focused on the K+ cationic doping since it has demonstrated the highest efficiencies but without fundamental investigation into the underlying drivers. A flexible and reproducible synthesis route was developed to prepare K+-doped CZTS nanocrystals with low polydispersity. We employed undoped and K+-doped nanocrystals to fabricate CZTS photocathodes and evaluated their photoresponsiveness in a photoelectrochemical cell. K+-doped CZTS nanocrystals showed the previously-reported trend of improved photocurrent density. We obtained further insight into the role of K+ dopant using Raman spectroscopy, x-ray photoelectron spectroscopy, and transient absorption spectroscopy. K+-doping of CZTS nanocrystals boosts charge carrier lifetime and enables better charge extraction efficiency to boost photocurrent. Improved carrier lifetime is attributed to remediation of binary/tertiary impurity phases and surface anion vacancies to yield higher degree of phase purity and lower degree of surface electron traps in CZTS. Ligand exchange reactions using Na2S and pyridine have been exploited to reduce the carbon content which serves as the recombination site for the charge carriers.

The selenized CZTS nanocrystal (CZTSSe) provides an opportunity for tuning the band gap energy of the thin film. Achieving a homogenous precursor solution of selenium in organic solvent (high boiling point) is vital to the synthesis of CZTSSe nanocrystals. We dissolved elemental Se in oleylamine at room temperature by using thioacetamide as a reducing agent. The Se precursor was utilized to synthesize selenized CZTS nanocrystals via hot injection route. The characterization of nanocrystals revealed improved grain growth and reduced binary phases in CZTSSe compared to pristine CZTS. The electrodes of CZTSSe nanocrystal showed significant improvement on photocurrent generation over the CZTS counterparts under AM1.5 illumination. Cost-effective industrial-scale energy storage system is a great challenge for accommodating contributions from unpredictable renewable sources (solar, wind, and hydro) to the power grid. Polysulfide (S_n^(2-)/S_(n-1)^(2-)) redox couple has received much attention among researchers as an affordable energy storing system. Sulfur, a by-product in crude oil production, is considered a cheap industrial waste with potential for megawatt scale energy storage systems such as redox flow batteries (RFB). We demonstrate photo-assisted reduction of polysulfide by developing a Cu2ZnZnS4 (CZTS) nanocrystal-based photocathode which provides opportunities for in-situ solar energy conversion and storage in a polysulfide based solar rechargeable RFB. The electrode performance was improved first by doping the nanocrystal with K+ and then by using Cu+ as an electrocatalyst. Electrochemical tools were used to show that Cu+ dramatically reduces electrode overpotential by reducing charge transfer resistance. The electron injection rate was probed using ultrafast laser absorption spectroscopy which reveals that K+-CZTS\Cu+ electrodes show 8 times faster electron injection rates than the bare K+-CZTS electrode.