Shi-Joon Sung

8.01% CuInGaSe2 solar cells fabricated by air-stable low-cost inks

CuInGaSe(2) (CIGS), a promising thin film solar cell material, has gained lots of attention in decades due to its high energy conversion efficiency and potential lower manufacture cost over conventional Si solar cells. As a cheaper processing method compared to vacuum-based techniques, solution-based deposition has been successfully applied to fabricate electronic devices, such as transistors and solar cells. In this paper, we reported CIGS thin film solar cells with an energy conversion efficiency reaching up to 8.01% using air-stable, low-cost inks. The newly developed inks consist of commercially available, low-cost compounds and solvents and can be processed using a variety of printing and coating techniques. More importantly, the inks can produce CIGS films free of copper selenides and amorphous carbon, two common by-products from solution-based CIGS processes. The mechanism for the transformation from metal salt precursor films to CIGS absorber thin films and the influence of selenium vapour pressure on absorber film quality and photovoltaic device performance were investigated and discussed. High-quality CIGS films with micrometer-sized crystals were obtained by using higher selenization partial pressure.

A band-gap-graded CZTSSe solar cell with 12.3% efficiency

Although Cu2ZnSn(S,Se)4 (CZTSSe) has attracted attention as an alternative to CuInGaSe2 (CIGS) as an absorber material in solar cells, its low efficiency is a serious shortcoming preventing its commercialization. To realize a high-efficiency CZTSSe solar cell, improved grain crystallinity, inhibited secondary-phase formation, controlled defect generation, adequate Na content, and band gap grading are required in the absorber layer. Few studies have focused specifically on band gap grading. In this study, a method of using SeS2, a new potential chalcogenization source material, to control the S and Se contents in a CZTSSe absorber and its effects were investigated. Using an appropriate SeS2/Se weight ratio, band gap grading was realized within the depletion region. By increasing the value of VOC through band gap grading in the depletion region, a record VOC deficit of 0.576 V was achieved. Furthermore, the possibility of enhancing JSC through the formation of a type-inverted n-type phase at the absorber surface in response to an appropriate alignment of the conduction-band minimum energy level and the Fermi energy pinning level is discussed. By introducing the chalcogenization source material SeS2 during the annealing process, CZTSSe solar cells with a maximum efficiency of 12.3% were obtained.

Solution-processed Cu2ZnSnS4 absorbers prepared by appropriate inclusion and removal of thiourea for thin film solar cells

We studied how to effectively add/remove organic chemicals to/from CZTS precursor thin films to prepare uniform CZTS thin films that show optimal properties. We used multi-functional thiourea, as both a stabiliser and a source of sulphur, to prepare the precursor solutions. This is because it forms complexes with metal chlorides, which stabilise the CZTS precursor solutions and enable CZTS thin films to be spin-coated onto substrates thereby enabling fabrication of CZTS absorbers. However, the excess thiourea, required to stabilise the CZTS precursor solutions, induced the formation of a ZnS secondary phase in the CZTS thin films, which deteriorated the photovoltaic properties of the CZTS solar cells. We therefore pre-annealed the thin films to inhibit ZnS formation. We used the thiourea-stabilized CZTS precursor solutions and simple solution processing techniques to prepare CZTS precursor thin films, and optimized the pre-annealing temperature to fabricate CZTS solar cells that showed 5.29% efficiency.

A discussion on the origin and solutions of hysteresis in perovskite hybrid solar cells

Although the record efficiencies of perovskite hybrid solar cells are gradually reaching the efficiency of crystalline Si solar cells, perovskite hybrid solar cells often exhibit significant current density–voltage (J–V) hysteresis with respect to the forward and reverse scan direction and scan rate. The origin of the J–V hysteresis of perovskite hybrid solar cells has not, to date, been clearly elucidated. Dielectric polarization by the ferroelectric properties of perovskite (i), the ionic motion/migration of perovskite materials (ii), and charge trapping and detrapping at trap sites by the unbalanced electron and hole flux (iii) are considered the possible origins of J–V hysteresis. Here, we reviewed the origin of the J–V hysteresis of perovskite solar cells from the above three points of view and we then suggest how one may reduce the J–V hysteresis with respect to the scan direction and scan rate.

Low Voltage, High Performance Thin Film Transistor with HfInZnO Channel and HfO2 Gate Dielectric

We fabricated thin film transistors (TFTs) using HfInZnO thin films as active channel layers. The thin films of HfInZnO were deposited by co-sputtering from HfO 2 and InZnO targets. The HfInZnO TFTs were investigated according to the radio-frequency power applied to the HfO 2 target. The transistor on and off currents were greatly influenced by the composition of Hf atoms suppressing the formation of oxygen vacancies. The electrical characteristics of the TFTs show a field-effect mobility of 3.53 cm 2 V ―1 s ―1 , a threshold voltage of 1.28 V, an on/off ratio of 1.4 × 10 ―7 , and a subthreshold swing of 95 mV/dec.

Photo-induced liquid crystal alignment on polyimide containing fluorinated groups

Polyimides were prepared from pyromellitic dianhydride, 4,4′-bis[2-(4-aminophenyl)hexafluoroprop-2-yl]diphenyl ether and 4,4′-diamodiphenyl ether (PMDA-BDAF-ODA) and used for liquid crystal alignment using linearly polarized UV exposure. The alignment properties of a LC on the polyimide films were found to depend on the fluorine content in the PMDA-BDAF-ODA alignment layer and on the UV exposure time. Pretilt angles were obtained in the range 0° to 90° dependent upon the fluorine content in the polyimide film and the UV exposure time. These effects seem to be closely related to the surface energy of the photo-alignment layer.

Relationship between pretilt angle and surface energy of the blended films based on poly(vinyl cinnamate) and alkanoyl cinnamic acid

The photo-reactive polymer was prepared by blending of poly(vinyl cinnamate) (PVCi) with alkanoyl cinnamic acid (ACA) which has been derived from 2-hydroxy cinnamic acid and alkanoyl chloride. The photo-alignment properties such as pretilt angle, azimuthal anchoring energy and the surface energy of the PVCi/ACA films were investigated by changing the alkyl chain length of ACA in the alignment layer. The exposure of polarized ultraviolet (UV) to the PVCi/ACA thin film induces the uniform homogeneous liquid crystal alignment. The pretilt angles increased with the alkyl chain length of ACA in the alignment layer. These changes may be greatly attributed to the polar and dispersion surface energy of the alignment layer.

Efficient hysteresis-less bilayer type CH₃NH₃PbI₃ perovskite hybrid solar cells.

Bilayer type CH3NH3PbI3 (MAPbI3) perovskite hybrid solar cells were fabricated via a one-step spin-coating process by using solubility controlled MAPbI3 solutions of MAPbI3-DMSO (dimethyl sulfoxide) and MAPbI3-DMF (N, N-dimethylformamide)-HI. The best DMSO-bilayer device showed 1.07 ± 0.02 V V(oc) (open-circuit voltage), 20.2 ± 0.1 mA cm(-2) J(sc) (short-circuit current density), 68 ± 2% FF (fill factor), and 15.2 ± 0.3% η (overall power conversion efficiency) under the forward scan direction and 1.07 ± 0.02 V V(oc), 20.4 ± 0.1 mA cm(-2) J(sc), 70 ± 3% FF, and 15.9 ± 0.4% η under the reverse scan direction. The best HI-bilayer device had 1.08 ± 0.02 V V(oc), 20.6 ± 0.1 mA cm(-2) J(sc), 75 ± 1% FF, and 17.2 ± 0.2% η under the forward scan direction and 1.08 ± 0.02 V V(oc), 20.6 ± 0.1 mA cm(-2) J(sc), 76 ± 2% FF, and 17.4 ± 0.3% η under the reverse scan direction. The deviation of average device efficiency (η(avg)) of 20 DMSO samples and 20 HI samples was 14.2 ± 0.95% and 16.2 ± 0.85%, respectively. Therefore, the HI-bilayer devices exhibited better device efficiency and smaller J-V (current density-voltage) hysteresis with respect to the scan direction than the DMSO-bilayer devices due to the reduced recombination and traps by the formation of a purer and larger MAPbI3 perovskite crystalline film.

Azimuthal anchoring energy and pretilt angle of photo-aligned nematic liquid crystals on a polyimide and poly(vinyl cinnamate) blend alignment layer

Abstract The azimuthal anchoring energy and pretilt angle of a nematic liquid crystal (E7) which was aligned on a thin layer of polyimide (PI) and poly(vinyl cinnamate) (PVCi) blend by the irradiation of polarized UV on the surface, were studied. Within a PVCi content of 10–100 wt%, the azimuthal anchoring energy remains unchanged with the decrease of the PVCi content in the blend, which is attributed to the preferential occupation of the surface by cinnamate chromophore. The blend alignment layer revealed a significantly enhanced thermal stability of azimuthal anchoring in comparison with the pure PVCi based alignment layer on account of the stiffening of the layer by the incorporated PI, which caused low thermal relaxation of the structural anisotropy developed from PVCi in the blend. The addition of PI into PVCi was also found to be effective for improving the thermal stability of the pretilt angle.

Pixel-isolation liquid crystals formed by polarization-selective UV-curing of a prepolymer containing cinnamate oligomer

A pixel isolated liquid crystal display was fabricated by polarization-selective anisotropic photoreaction of a prepolymer containing a cinnamate oligomer. The cinnamate oligomer was mainly distributed on the surface region of a UV-cured polymer wall. Anisotropic photo-dimerization of cinnamate moiety was achieved by polarized UV exposure. It was found that the polymer walls containing cinnamate dimers formed by polarized UV exposure showed ordered orientation of LC molecules at the boundary of the polymer walls resulting in electro-optic performance improvement.