Fangyang Liu

Fabrication of Cu2ZnSnS4 solar cells with 5.1% efficiency via thermal decomposition and reaction using a non-toxic sol–gel route

Earth-abundant Cu2ZnSnS4 (CZTS) has been confirmed as a promising semiconductor material for thin film solar cells. To meet the requirements of high-efficiency and low-cost for photovoltaic technologies, a modified thermal decomposition sol–gel method with low-cost and low-toxicity for CZTS thin film preparation is presented, and the detailed formation mechanism of the thin film is investigated to obtain an optimized process. By introducing non-aqueous thiourea–metal–oxygen sol–gel processing, as well as applying extrinsic dopants and chemical etching, high-quality and phase-controlled CZTS thin films with homogeneous elemental distribution and a low impurity content have been synthesized. Based on the modified sol–gel method, solar cells with a structure of Ni:Al/ZAO/i-ZnO/CdS/CZTS/Mo/glass have been fabricated, and a power conversion efficiency of 5.10% is obtained, indicating its potential for high-throughput and high power conversion efficiency photovoltaic devices.

Enhancing the Cu2ZnSnS4 solar cell efficiency by back contact modification: Inserting a thin TiB2 intermediate layer at Cu2ZnSnS4/Mo interface

In this work, TiB2 thin films have been employed as intermediate layer between absorber and back contact in Cu2ZnSnS4 (CZTS) thin film solar cells for interface optimization. It is found that the TiB2 intermediate layer can significantly inhibit the formation of MoS2 layer at absorber/back contact interface region, greatly reduces the series resistance and thereby increases the device efficiency by short current density (Jsc) and fill factor boost. However, introducing TiB2 degrades the crystal quality of absorber, which is detrimental to device performance especially Voc. The careful control of the thickness of TiB2 intermediate layer is required to ensure both MoS2 with minimal thickness and CZTS absorber with large grain microstructure according to the absorber growth process.

Fabrication of ternary Cu-Sn-S sulfides by a modified successive ionic layer adsorption and reaction (SILAR) method

Ternary Cu–Sn–S chalcogenides, Cu2SnS3, Cu5Sn2S7 and Cu3SnS4, have been successfully synthesized by annealing three precursor film samples deposited via a modified successive ionic layer adsorption and reaction (SILAR) method. The mechanism of ion-exchange and improvement of the rinsing procedure were introduced into the SILAR process for the purpose of achieving the codeposition of different metal sulfides and increasing the growth rate of thin films. The crystal structure, composition, surface morphology, optical and electrical properties of three ternary sulfide samples have been characterized. The temperature dependence of the Seebeck coefficient, electrical conductivity, thermal conductivity and ZT values of the Cu3SnS4 thin film sample have also been measured between 293 K and 573 K. Owing to the intrinsic advantages of the SILAR method and the improvement of the SILAR process, ternary Cu–Sn–S thin films can be deposited on glass substrates at a speed of 400 nm per hour and the surface morphologies of the thin films are comparable with those of thin films prepared by vacuum based methods, which can satisfy the requirements for high quality, high efficiency and low-cost production of thin films. With the appropriate band gap energies (1.0 eV, 1.45 eV and 1.47 eV for Cu2SnS3, Cu5Sn2S7 and Cu3SnS4 respectively) and considerable absorption coefficients (α > 104 cm−1), most importantly with earth-abundant elements, Cu–Sn–S thin films can be used as alternative absorber layer materials in thin film solar cells. Additionally for the Cu5Sn2S7 and Cu3SnS4 film samples, some novel properties (such as strong optical absorption in the NIR band, excellent conductivity and suitable carrier concentration) make them attractive for potential research interest as thermoelectric materials.

Boosting Cu2ZnSnS4 solar cells efficiency by a thin Ag intermediate layer between absorber and back contact

In this work, 20 nm Ag is deposited on Mo coated soda lime glass prior to Cu2ZnSnS4 absorber deposition to improve the back contact and therefore enhance solar cell efficiency. This thin coating is found to inhibit the formation of SnS2, MoS2, and other defects especially voids at the back contact; therefore, reduces the series resistance and recombination leading to substantially higher short circuit current density (JSC), fill factor, open circuit voltage (VOC), and efficiency in comparison to the controlled non-coating Mo, though the former results in lower material crystallinity.

Fabrication of Cu2ZnSnS4 nanowires and nanotubes based on AAO templates

Cu2ZnSnS4 nanowires and nanotubes have been synthesized via a modified sol–gel solution approach with AAO templates. The prepared nanowires and nanotubes have been characterized and show the typical morphology of nanostructure and properties of kesterite CZTS, which can provide the potential application and research for low-dimension solar cells.

Effects of potassium doping on solution processed kesterite Cu2ZnSnS4 thin film solar cells

Alkaline metals doping is one of the approaches for achieving high efficiency Cu(In,Ga)Se2 (CIGS) solar cell. Recently, potassium doping helps to break the record efficiency of CIGS solar cell doped with sodium. In this paper, we have investigated how incorporation of potassium can influence the properties of Cu2ZnSnS4 (CZTS) thin film and the performance of resulting solar cell. Our results showed that K doping can enhance the (112) preferred orientation, increase the grain size and reduce the second phase ZnS of the CZTS thin films. After K doping, despite of some drop of Voc for CZTS thin film solar cells, the Rs is decreased and the Jsc is improved markedly, and the solar cell efficiency is boosted.

Solution-based synthesis of chalcostibite (CuSbS2) nanobricks for solar energy conversion

Chalcostibite brick-like nanoparticles have been synthesized using hot-injection method in coordinating solvents. The CuSbS2 nanobricks possess a band gap of 1.40 eV and the corresponding nanobrick-electrode shows an IPCE of 5%–15% in the visible region. Our work demonstrates CuSbS2 nanobricks have potential in the field of solar energy conversion.

Kesterite Cu2ZnSn(S,Se)4 Solar Cells with beyond 8% Efficiency by a Sol–Gel and Selenization Process

A facile sol-gel and selenization process has been demonstrated to fabricate high-quality single-phase earth abundant kesterite Cu2ZnSn(S,Se)4 (CZTSSe) photovoltaic absorbers. The structure and band gap of the fabricated CZTSSe can be readily tuned by varying the [S]/([S] + [Se]) ratios via selenization condition control. The effects of [S]/([S] + [Se]) ratio on device performance have been presented. The best device shows 8.25% total area efficiency without antireflection coating. Low fill factor is the main limitation for the current device efficiency compared to record efficiency device due to high series resistance and interface recombination. By improving film uniformity, eliminating voids, and reducing the Mo(S,Se)2 interfacial layer, a further boost of the device efficiency is expected, enabling the proposed process for fabricating one of the most promising candidates for kesterite solar cells.

Growth and Characterization of Cu2ZnSnS4 Thin Films by DC Reactive Magnetron Sputtering for Photovoltaic Applications

Cu 2 ZnSnS 4 (CZTS) thin films were grown by a dc reactive magnetron sputtering technique and characterized by studying their composition, structural, optical, and electrical properties. Raman and X-ray diffraction analyses confirm the formation of quaternary Cu 2 ZnSnS 4 phase with strong preferential orientation along the (112) plane and the presence of minor secondary phases Cu 2-x S and Cu 3 SnS 4 . The grown CZTS film with a homogeneous morphology demonstrates an optical absorption coefficient of higher than 10 5 cm -1 and an optical bandgap of 1.50 ± 0.01 eV. All samples are p-type and exhibit high carrier concentration in the order of 10 18 cm -3 and low carrier mobility.

Improving the conversion efficiency of Cu2ZnSnS4 solar cell by low pressure sulfurization

Cu2ZnSnS4 thin films have been prepared by the sol-gel sulfurization method on Mo-coated substrates, and the comparative studies between the atmospheric pressure sulfurization and low pressure sulfurization was carried out. The Cu2ZnSnS4 film sulfurized at low pressure exhibits larger grain size, thinner MoS2 layer, and free of SnS secondary phase, but more ZnS on surface. The device efficiency of 4.1% using Cu2ZnSnS4 absorber from atmospheric pressure sulfurization is improved to 5.7% using that from low pressure sulfurization via the boost of open-circuit and fill factor.