Zhenghua Su

Origin of Photocarrier Losses in Iron Pyrite (FeS2) Nanocubes

Iron pyrite has received significant attention due to its high optical absorption. However, the loss of open circuit voltage (Voc) prevents its further application in photovoltaics. Herein, we have studied the photophysics of pyrite by ultrafast laser spectroscopy to understand fundamental limitation of low Voc by quantifying photocarrier losses in high quality, stoichiometric, and phase pure {100} faceted pyrite nanocubes. We found that fast carrier localization of photoexcited carriers to indirect band edge and shallow trap states is responsible for major carrier loss. Slow relaxation component reflects high density of defects within the band gap which is consistent with the observed Mott-variable range hopping (VRH) conduction from transport measurements. Magnetic measurements strikingly show the magnetic ordering associated with phase inhomogeneity, such as FeS2-δ (0 ≤ δ ≤ 1). This implies that improvement of iron pyrite solar cell performance lies in mitigating the intrinsic defects (such as sulfur vacancies) by blocking the fast carrier localization process. Photocarrier generation and relaxation model is presented by comprehensive analysis. Our results provide insight into possible defects that induce midgap states and facilitate rapid carrier relaxation before collection.

A 4.92% efficiency Cu2ZnSnS4 solar cell from nanoparticle ink and molecular solution

Quaternary Cu2ZnSnS4 (CZTS) thin films were prepared by a low-cost, simple and environmentally-friendly ink method. By depositing molecular solution on the nanoparticle thin film, the quality of as-prepared CZTS thin films was greatly improved (e.g. reduction of fine grain layer formation and improved crystallinity). The effect of the number of spin-coated layers from molecular solution on solar cell performance was investigated. The results indicated that the CZTS thin film had the best performance when 5 layers were spin-coated from molecular solution on the nanoparticle thin film. The crystallinity of the as-prepared CZTS thin film and the interface at Mo/CZTS was found to be obviously enhanced by addition of a molecular solution layer. Finally, a CZTS thin film solar cell with an efficiency of 4.92% has been fabricated.

Impact of molybdenum out diffusion and interface quality on the performance of sputter grown CZTS based solar cells

We have investigated the impact of Cu2ZnSnS4-Molybdenum (Mo) interface quality on the performance of sputter-grown Cu2ZnSnS4 (CZTS) solar cell. Thin film CZTS was deposited by sputter deposition technique using stoichiometry quaternary CZTS target. Formation of molybdenum sulphide (MoSx) interfacial layer is observed in sputter grown CZTS films after sulphurization. Thickness of MoSx layer is found ~142 nm when CZTS layer (550 nm thick) is sulphurized at 600 °C. Thickness of MoSx layer significantly increased to ~240 nm in case of thicker CZTS layer (650 nm) under similar sulphurization condition. We also observe that high temperature (600 °C) annealing suppress the elemental impurities (Cu, Zn, Sn) at interfacial layer. The amount of out-diffused Mo significantly varies with the change in sulphurization temperature. The out-diffused Mo into CZTS layer and reconstructed interfacial layer remarkably decreases series resistance and increases shunt resistance of the solar cell. The overall efficiency of the solar cell is improved by nearly five times when 600 °C sulphurized CZTS layer is applied in place of 500 °C sulphurized layer. Molybdenum and sulphur diffusion reconstruct the interface layer during heat treatment and play the major role in charge carrier dynamics of a photovoltaic device.

Cation substitution of CZTS solar cell with > 10% efficiency

High efficiency CZTS-based thin film solar cells are achieved by cation substitution and sol-gel method. Two approaches are presented here: 1) the substitution of Zn with Cd and 2) the substitution of Cu with Ag. For Cd-doped CZTS (i.e. CZCTS) the efficiency of CZCTS thin film solar cell can be improved from 9.24% to 10.66% by careful modification of alloys composition and interfaces between TCO/CdS/CZTS. For Ag-doped CZTS thin film solar cells, the improvement in power conversion efficiency can be correlated with the Voc and crystallinity improvements. Thus the limitations in CZTS system, such as Voc deficit, secondary phases, crystallinity and antisite defects, can be alleviated to some degree by using cation substitution.