Jiahua Tao

7.1% efficient co-electroplated Cu2ZnSnS4 thin film solar cells with sputtered CdS buffer layers

Cu2ZnSnS4 (CZTS) thin films with fine control over composition and pure phase were fabricated by sulfurization of co-electroplated Cu–Zn–Sn–S precursors. We have systematically investigated that the concentration of Cu(II) ions can influence the properties of CZTS absorber layers and the photovoltaic performance of the resulting solar cell devices. The results indicate that an increase in Cu(II) concentration almost linearly increases the Cu content in the final CZTS thin films, greatly enhances the (112) preferred orientation, significantly improves the crystallinity of the absorber layer, remarkably reduces the ZnS secondary phase, and hence improves their photovoltaic performance. However, a further increase in the Cu(II) concentration degrades the crystal quality of the absorber layer, and forms the CuSx secondary phase, which is quite detrimental to the device photovoltaic performance. Here we introduce a novel sputtered CdS buffer layer for the CZTS solar cells. For the first time, co-electrodeposited CZTS solar cells exceed the 7% efficiency threshold. These findings offer new research directions for solving persistent challenges of chemical bath deposition of CdS in CZTS solar cells.

Synthesis and characterization of Cu2ZnSnS4 thin films by the sulfurization of co-electrodeposited Cu–Zn–Sn–S precursor layers for solar cell applications

Cu2ZnSnS4 (CZTS) absorbers have been successfully deposited on tin-doped indium oxide coated glass (ITO/glass) substrates by sulfurization process of co-electrodeposited Cu–Zn–Sn–S precursor thin films at various annealing temperatures ranging from 500 to 580 °C for 30 min in an atmosphere of Ar–H2S (6.5%). The effects of sulfurization temperature on the structure, morphology, composition and optical properties of CZTS thin films have been investigated in details. XRD and Raman measurements reveal that the intensity of preferential orientation along the (112) direction becomes relatively more intense and sharp with increasing annealing temperature. The morphological and chemical composition studies indicate the formation of compact and homogenous CZTS thin films with Cu-poor and Zn-rich composition at a sulfurization temperature of 560 °C. And its band gap energy is around 1.50 eV. The AZO/i-ZnO/CdS/CZTS/ITO/glass thin-film solar cell is fabricated with the CZTS absorber layer grown at an optimized sulfurization temperature of 560 °C. It shows a power conversion efficiency of 1.98% for a 0.25 cm2 area with Voc = 490 mV, Jsc = 9.69 mA cm−2 and FF = 40.03%.

Co-electrodeposited Cu2ZnSnS4 thin-film solar cells with over 7% efficiency fabricated via fine-tuning of the Zn content in absorber layers

A simple and cost-effective co-electrodeposition process has been demonstrated to fabricate high-performance Cu2ZnSnS4 (CZTS) photovoltaic materials with composition tunability and phase controllability. Here we report a systematic investigation of the effects of the Zn(II) concentration on the properties of CZTS thin films and thus the performance of the as-resulted solar cells. These results indicate that increasing the concentration of Zn(II) linearly increases the Zn content in the final composition of CZTS thin films, significantly improves the grain size and morphology of the absorber layers, and consequently improves their photovoltaic properties, especially the response to the medium wavelength. In contrast, further increase of the Zn(II) concentration degrades the crystal quality of the absorber layer, and more ZnS phase appears on the surface of the CZTS thin film, forming a rather rough morphology, which is harmful to the photovoltaic performance of the device. When the concentration of Zn(II) is optimized to 30 mM, a power conversion efficiency of 7.23% is achieved, which, to the best of our knowledge, is the highest efficiency for a co-electrodeposited CZTS solar cell with a sputtered CdS buffer layer to date. Our findings offer a promising alternative approach towards the industrialization of CZTS solar cell modules.

Synthesis and characterization of earth-abundant Cu2MnSnS4 thin films using a non-toxic solution-based technique

Earth-abundant Cu2MnSnS4 (CMTS) thin films were fabricated through a non-toxic spin-coating technique. The precursor solution is based on a 2-methoxyethanol solvated thiourea complex with acetyl-acetone used as an additive agent, and the spin-coated films were post-annealed at 570 °C under a N2 atmosphere. The influence of annealing time on the structure, composition, morphology, and optical properties of the processed precursor films has been studied in detail. We found that a longer annealing time during CMTS growth can improve the phase purity, promote the preferred orientation along the (112) direction, and enhance grain growth in the micrometer range. Film annealed for 10 min gives a pure CMTS phase, whereas other films annealed for lower and/or higher than 10 min (especially 13 min) can form secondary phases (i.e., SnS, MnS). The band gap energy is estimated as 1.63–1.18 eV for post-annealed films depending on the heat treatment, compared to 1.69 eV for as-prepared film. An efficiency of 0.49% for the device fabricated here has been achieved with an open-circuit voltage of 308.4 mV, a short-circuit current density of 4.7 mA cm−2, and a fill factor of 33.9%. It offers a new research direction for the application of a CMTS absorber layer in low-cost solar cells.

Long-term reliability of silicon wafer-based traditional backsheet modules and double glass modules

An extensive program of series extended sequential long-term reliability stress including thermal cycling (TC) 600, damp heat (DH) 3000, 600 hours potential induced degradation (PID) and humidity freeze (HF) 50 were performed on silicon wafer-based traditional backsheet modules and double glass photovoltaic (PV) modules. The relative module maximum power (Pmax) degradations of traditional backsheet modules are 3.87%, 7.34%, 13.3%, 33.73% and those of double glass modules are 2.78%, 3.12%, 2.27%, 2.72%, respectively. From all the above results, HF50 has a greater impact on Pmax degradation of traditional backsheet modules, and a strong correlation is thereby found between the Water Vapor Transmission Rate (WVTR) of the backsheet and the Pmax degradation. Traditional backsheet modules have higher WVTR and greater Pmax degradation, while double glass modules are impermeable and have much lower Pmax degradation. The key factor for excellent performance of Si wafer-based double glass PV modules is replacing the polymer backsheet by a glass panel with impermeability to water vapor, which enables double glass modules to offer much higher reliability and longer durability.

Composition control in Cu2ZnSnS4 thin films by a sol–gel technique without sulfurization

Kesterite Cu2ZnSnS4 (CZTS) thin films with a smooth, compact and crack-free morphology are obtained via a sol–gel method without sulfurization process. Non-toxic ethylene glycol is selected as solvent, while Cu(CH3COO)2, Zn(CH3COO)2, SnCl2·2H2O and thiourea are used as raw materials. Chemical composition dependence of CZTS films on pre-annealing and post-annealing process is comprehensively investigated. The analysis of energy dispersive X-ray indicates that composition control of CZTS films can be easily realized by the preparation of precursor solution and varying the annealing conditions.

A large-volume manufacturing of multi-crystalline silicon solar cells with 18.8% efficiency incorporating practical advanced technologies

The optimization processes for the mass-production of high-efficiency multi-crystalline silicon solar cells have been observed in this paper. After incorporating several practical advanced technologies such as grain-size controlled low defect-density mc-Si casting ingot, precisely aligned selective emitter, surface-damage free reactive ion etch texturing on a mass production line, the total-area efficiencies up to 18.84% and a production average efficiency of 18.65% for large size (156 × 156 mm2) multi-crystalline silicon (mc-Si) solar cells have been demonstrated, equating to an absolute efficiency gain of 1.58% compared to a conventional solar cell. The corresponding module power is 270.3 W, a 20 W increase over a conventional 60-cell mc-Si module of the same dimensions. These results demonstrate a successful transfer of advanced techniques from laboratory to large scale industrial production, which promote the mc-Si solar cells taking a big step forward in solar cells applications.