Xiaojing Hao

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.

Si nanocrystal p-i-n diodes fabricated on quartz substrates for third generation solar cell applications

We fabricated p-i-n diodes by sputtering alternating layers of silicon dioxide and silicon rich oxide with a nominal atomic ratio O/Si=0.7 onto quartz substrates with in situ boron for p-type and phosphorus for n-type doping. After crystallization, dark and illuminated I-V characteristics show a diode behavior with an open circuit voltage of 373 mV. Due to the thinness of the layers and their corresponding high resistivity, lateral current flow results in severe current crowding. This effect is taken into account when extracting the electronic bandgap based on temperature dependent diode I-V measurements.

Silicon Quantum Dots in a Dielectric Matrix for All-Silicon Tandem Solar Cells

We report work progress on the growth of Si quantum dots in different matrices for future photovoltaic applications. The work reported here seeks to engineer a wide-bandgap silicon-based thin-film material by using quantum confinement in silicon quantum dots and to utilize this in complete thin-film silicon-based tandem cell, without the constraints of lattice matching, but which nonetheless gives an enhanced efficiency through the increased spectral collection efficiency. Coherent-sized quantum dots, dispersed in a matrix of silicon carbide, nitride, or oxide, were fabricated by precipitation of Si-rich material deposited by reactive sputtering or PECVD. Bandgap opening of Si QDs in nitride is more blue-shifted than that of Si QD in oxide, while clear evidence of quantum confinement in Si quantum dots in carbide was hard to obtain, probably due to many surface and defect states. The PL decay shows that the lifetimes vary from 10 to 70 microseconds for diameter of 3.4 nm dot with increasing detection wavelength.

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.

Effects of Si-rich oxide layer stoichiometry on the structural and optical properties of Si QD/SiO2 multilayer films

The effects of the stoichiometry of the Si-rich oxide (SRO) layer, O/Si ratio, on the structural and optical properties of SRO/SiO2 multilayer films were investigated in this work. SRO/SiO2 multilayer films with different O/Si ratios were grown by a co-sputtering technique, and Si quantum dots (QDs) were formed with post-deposition annealing. By transmission electron microscopy (TEM) and glancing incidence x-ray diffraction (GIXRD), it was found that the Si QD size decreases with increases in O/Si ratio. The photoluminescence (PL) spectrum varies with the O/Si ratio in band position, shape and intensity. In addition, it was observed that the absorption edge blue-shifts with increases in the O/Si ratio. The change in the absorption edge is consistent with strengthening quantum confinement effects in Si QDs, as indicated by TEM and GIXRD. The optical properties were also investigated by 2D photoluminescence excitation (2D-PLE) and lifetime measurements. The origin of emission and absorption is discussed based on the absorption, PL, 2D-PLE and decay time measurements.

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.

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.

Si solid-state quantum dot-based materials for tandem solar cells.

The concept of third-generation photovoltaics is to significantly increase device efficiencies whilst still using thin-film processes and abundant non-toxic materials. A strong potential approach is to fabricate tandem cells using thin-film deposition that can optimise collection of energy in a series of cells with decreasing band gap stacked on top of each other. Quantum dot materials, in which Si quantum dots (QDs) are embedded in a dielectric matrix, offer the potential to tune the effective band gap, through quantum confinement, and allow fabrication of optimised tandem solar cell devices in one growth run in a thin-film process. Such cells can be fabricated by sputtering of thin layers of silicon rich oxide sandwiched between a stoichiometric oxide that on annealing crystallise to form Si QDs of uniform and controllable size. For approximately 2-nm diameter QDs, these result in an effective band gap of 1.8 eV. Introduction of phosphorous or boron during the growth of the multilayers results in doping and a rectifying junction, which demonstrates photovoltaic behaviour with an open circuit voltage (VOC) of almost 500 mV. However, the doping behaviour of P and B in these QD materials is not well understood. A modified modulation doping model for the doping mechanisms in these materials is discussed which relies on doping of a sub-oxide region around the Si QDs.

Four-Terminal Tandem Solar Cells Using CH3NH3PbBr3 by Spectrum Splitting.

In this work, the use of a high bandgap perovskite solar cell in a spectrum splitting system is demonstrated. A remarkable energy conversion efficiency of 23.4% is achieved when a CH3NH3PbBr3 solar cell is coupled with a 22.7% efficient silicon passivated emitter rear locally diffused solar cell. Relative enhancements of >10% are demonstrated by CH3NH3PbBr3/CH3NH3PbI3 and CH3NH3PbBr3/multicrystalline-screen-printed-Si spectral splitting systems with tandem efficiencies of 13.4% and 18.8%, respectively. The former is the first demonstration of an all perovskite split spectrum system. The CH3NH3PbBr3 cell on a mesoporous structure was fabricated by the vapor-assisted method while the planar CH3NH3PbI3 cell was fabricated by the gas-assisted method. This work demonstrates the advantage of the higher voltage output from the high bandgap CH3NH3PbBr3 cell and its suitability in a tandem system.

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.