Zhanhui Ding

Bandgap engineering of Cu2CdxZn1−xSnS4 alloy for photovoltaic applications: A complementary experimental and first-principles study

We report on bandgap engineering of an emerging photovoltaic material of Cu 2Cdx Zn 1−x SnS 4 (CCZTS) alloy. CCZTS alloy thin films with different Cd contents and single kesterite phase were fabricated using the sol-gel method. The optical absorption measurements indicate that the bandgap of the kesterite CCZTS alloy can be continuously tuned in a range of 1.55–1.09 eV as Cd content varied from x = 0 to 1. Hall effect measurements suggest that the hole concentration of CCZTS films decreases with increasing Cd content. The CCZTS-based solar cell with x = 0.47 demonstrates a power conversion efficiency of 1.2%. Our first-principles calculations based on the hybrid functional method demonstrate that the bandgap of the kesterite CCZTS alloy decreases monotonically with increasing Cd content, supporting the experimental results. Furthermore, Cu 2ZnSnS4/Cu2CdSnS4 interface has a type-I band-alignment with a small valence-band offset, explaining the narrowing of the bandgap of CCZTS as the Cd content increases. Our results suggest that CCZTS alloy is a potentially suitable material to fabricate high-efficiency multi-junction tandem solar cells with different bandgap-tailored absorption layers.

Influencing Mechanism of the Selenization Temperature and Time on the Power Conversion Efficiency of Cu2ZnSn(S,Se)4-Based Solar Cells.

Cu2ZnSn(S,Se)4 (CZTSSe) films were deposited on the Mo-coated glass substrates, and the CZTSSe-based solar cells were successfully fabricated by a facile solution method and postselenization technique. The influencing mechanisms of the selenization temperature and time on the power conversion efficiency (PCE), short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) of the solar cell are systematically investigated by studying the change of the shunt conductance (Gsh), series resistance (Rs), diode ideal factor (n), and reversion saturation current density (J0) with structure and crystal quality of the CZTSSe film and CZTSSe/Mo interface selenized at various temperatures and times. It is found that a Mo(S1-x,Sex)2 (MSSe) layer with hexagonal structure exists at the CZTSSe/Mo interface at the temperature of 500 °C, and its thickness increases with increasing selenization temperature and time. The MSSe has a smaller effect on the Rs, but it has a larger influence on the Gsh, n, and J0. The PCE, Voc, and FF change dominantly with Gsh, n, and J0, while Jsc changes with Rs and Gsh, but not Rs. These results suggest that the effect of the selenization temperature and time on the PCE is dominantly contributed to the change of the CZTSSe/CdS p-n junction and CZTSSe/MSSe interface induced by variation of the quality of the CZTSSe film and thickness of MSSe in the selenization process. By optimizing the selenization temperature and time, the highest PCE of 7.48% is obtained.

Alternative Spectral Photoresponse in a p-Cu2ZnSnS4/n-GaN Heterojunction Photodiode by Modulating Applied Voltage.

We report alternative visible and ultraviolet light response spectra in a p-Cu2ZnSnS4 (p-CZTS)/n-GaN heterojunction photodiode. A CZTS film was deposited on an n-GaN/sapphire substrate using a magnetron sputtering method. Current-voltage characteristic of the p-CZTS/n-GaN heterojunction photodiode showed a good rectifying behavior. The spectral response measurements indicate that the response wavelength of the photodiode can be tuned from ultraviolet to visible regions via applying zero and reverse bias. A band alignment at the interface of the p-CZTS/n-GaN heterojunction was proposed to interpret the spectral response of the device.

Significantly enhancing the stability of a Cu2ZnSnS4 aqueous/ethanol-based precursor solution and its application in Cu2ZnSn(S,Se)4 solar cells

The stability of a CZTS precursor solution plays an important role for potential industrial applications, and could provide insight into the understanding of the solubility mechanism of the raw materials. In this work, we report a systematic investigation on the stability of a metal/thiourea aqueous/ethanol-based precursor solution used for fabricating Cu2ZnSnS4 (CZTS) thin films. It is found that the metal/thiourea aqueous/ethanol solution is metastable, and easily produced a mass of precipitation when the solution was prepared for few hours. This could cause an element to be missed out of the film, giving a very bad film-processing ability, and effect the application of such a method. So we studied in detail the crystalline precipitation through SEM, EDS and FTIR, and found that they mainly contain copper ion and thiourea coordination complexes. Furthermore, we demonstrated that the low-toxicity 3-mercaptopropionic acid (MPA) can play an important role as an auxiliary ligand in the precursor solution. Addition of MPA into the precursor solution can avoid the precipitation and noticeably improve the stability of the precursor solution from ∼3 hours to 1 week. Using the precursor solution with added MPA, we fabricated a single phase of a CZTS film, and prepared a large-grain and hole-free Cu2ZnSn(S,Se)4 (CZTSSe) film. More importantly, a CZTSSe film solar cell with a competitive power conversion efficiency of 7.25% has been reported in this work.

A facile route to realize ultraviolet emission in a nano-engineered SnO2-based light-emitting diode

We reported a facile route to fabricate a tin dioxide (SnO2)-based light-emitting diode (LED) and obtain an electrically pumped band-edge ultraviolet (UV) emission. We first investigated the photoluminescence (PL) properties of the SnO2 thin films deposited on quartz substrates annealed at various temperatures. It was found that SnO2 nanocrystals were embedded in the SnO2 amorphous matrix after annealing at 400 °C to form a SnO2 nanoparticle/amorphous hybrid film; the band-edge UV emission was observed from the hybrid film due to the hybrid structure breaking the dipole-forbidden rule of bulk SnO2. This hybrid SnO2 film was then deposited on a p-type GaN substrate to form a SnO2 hybrid film-based LED and a band-edge UV electroluminescence (EL) was observed. Our results suggest that this easy and effective approach may find extensive application in the field of optoelectronics, displays and solid-state lighting.

Conversion mechanism of conductivity of phosphorus-doped ZnO films induced by post-annealing

The effects of post-annealing on conductivity of phosphorus-doped ZnO (PZO) films grown at 500 C by radio frequency magnetron sputtering are investigated in a temperature ranging from 600 C to 900 C. The as-grown PZO exhibits n-type conductivity with an electron concentration of 1.19 1020 cm-3, and keeps n-type conductivity as annealed at 600 C-700 C but electron concentration decreases with increasing temperature. However, it converts to p-type conductivity as annealed at 800 C. Further increasing temperature, it still shows p-type conductivity but the hole concentration decreases. It is found that the P occupies mainly Zn site (PZn) in the as-grown PZO, which accounts for good n-type conductivity of the as-grown PZO. The amount of the PZn decreases with increasing temperature, while the amount of Zn vacancy (V Zn) increases from 600 C to 800 C but decreases greatly at 900 C, resulting in that the amount of PZn-2VZn complex increases with increasing temperature up to 800 C but decreases above 800 C. It is suggested that the PZn-2VZn complex acceptor is responsible for p-type conductivity, and that the conversion of conductivity is due to the change of the amount of the PZn and PZn-2VZn with annealing temperature. 2013 AIP Publishing LLC.

Fabrication of Cu2MSnS4 (M = Co2+, Ni2+) nanocrystal thin films and their application in photodetectors

Cu2CoSnS4 (CCTS) and Cu2NiSnS4 (CNTS) can be regarded as Cu2ZnSnS4 compounds with the Zn2+ replaced by Co2+ and Ni2+, and are alternative absorber materials in thin film solar cells. Here, we fabricated high-quality CCTS and CNTS nanocrystal thin films by employing the ion substitution strategy. We systematically studied the crystal structures, surface morphologies and compositions of the as-prepared and selenized thin films via X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The selenized CCTS and CNTS films showed an obvious phase separation process for the oxidizing reaction of Co2+ and Ni2+ with high-temperature selenium steam, and then formed Cu2Sn(S,Se)3, Co(S,Se)2 and Ni(S,Se)2 compounds. We successfully fabricated photodetector devices using CCTS and CNTS nanocrystal films, and achieved a strong and reversible photoresponse performance.

A versatile strategy for fabricating various Cu2ZnSnS4 precursor solutions

Herein, we successfully prepared dithiocarbamate-based Cu2ZnSnS4 (CZTS) precursor solutions using various amino groups such as methylamine, ethylamine, propylamine, butylamine, amylamine, hexylamine, octylamine, and ethanolamine. These solutions exhibited a distinguishing wetting behavior, viscosity, and stability and could be used to fabricate CZTS thin films by a simple spin-coating and annealing process. We compared the thermogravimetric analysis of the precursor solutions and the XRD patterns of the as-prepared CZTS films and clear differences between the samples were observed. Furthermore, we fabricated high-quality CZTS films using the ethanoldithiocarbamate-based CZTS precursor solution, and the Cu2ZnSn(S,Se)4 solar cell with the best power conversion efficiency (7.66%) under 1.5 AM illumination was achieved.

Band offsets of Ag2ZnSnSe4/CdS heterojunction: An experimental and first-principles study

Band offsets at the interface of the Ag2ZnSnSe4 (AZTSe)/CdS heterojunction were systematically investigated by combining experiments and first-principles calculations. For the AZTSe/CdS interface, a higher conduction-band minimum (CBM) of the CdS than that of the AZTSe was found and the conduction-band offset of 0.31 eV was determined using X-ray photoelectron spectroscopy. Theoretically, we constructed the AZTSe/CdS interface and calculated the band alignments. Two different configurations were adopted in the calculations: the AZTSe/CdS superlattice and the AZTSe/CdS heterojunction with a vacuum layer. The calculated results indicate that CdS has a higher CBM than AZTSe at the AZTSe/CdS interface, well supporting the experimental results. Our results suggest that the AZTSe/CdS heterojunction has an ideal band structure for photovoltaic applications.

Shallow Acceptor State in Mg-Doped CuAlO2 and Its Effect on Electrical and Optical Properties: An Experimental and First-Principles Study

Shallow acceptor states in Mg-doped CuAlO2 and their effect on structural, electrical, and optical properties are investigated by combining first-principles calculations and experiments. First-principles calculations demonstrate that Mg substituting at the Al site in CuAlO2 plays the role of shallow acceptor and has a low formation energy, suggesting that Mg doping can increase hole concentration and improve the conductivity of CuAlO2. Hall effect measurements indicate that the hole concentration of the Mg-doped CuAlO2 thin film is 2 orders of magnitude higher than that of undoped CuAlO2. The best room temperature conductivity of 8.0 × 10-2 S/cm is obtained. A band gap widening is observed in the optical absorption spectra of Mg-doped CuAlO2, which is well supported by the results from first-principles electronic structure calculations.