Dong-Won Kang

Highly Transparent and High Haze Bilayer Al-Doped ZnO Thin Film Employing Oxygen-Controlled Seed Layer

Al-doped ZnO (AZO) film was continuously deposited by DC magnetron sputtering using pure Ar on a thin AZO seed layer prepared using an approximately 4% dilution of oxygen with Ar. X-ray diffraction measurements showed that the AZO film grown on the seed layer exhibited a much higher crystallinity and larger grain size than that without the seed layer. The electrical properties such as resistivity and Hall mobility were improved. The average visible transmittance was increased from 81.6 to 86.2%, and near infrared (NIR) transmittance was increased from 76.0 to 84.4% by employing the seed layer. The haze value characterizing the light scattering property was significantly increased from 59.4 to 89.5% in the visible region by the seed layer, and it was increased from 15.1 to 50.8% in the NIR region. Surface topography analysis showed that the bilayer AZO film had larger craters allowing for improvement of the light scattering properties than the conventional AZO film without the seed layer.

Effect of Ga Doping on Transparent and Conductive Al-Doped ZnO Films Prepared Using Magnetron Cosputtering

Al-doped ZnO (AZO) films with various Ga contents were prepared by magnetron co-sputtering in order to investigate the effect of Ga additions on the structural and optoelectronic characteristics of AZO films. The appropriate Ga doping level improved the crystallinity of the AZO films, investigated by X-ray diffraction analysis. The resistivity of AZO films decreased from 3.5 ×10-3 to 8.1 ×10-4 Ω cm by Ga doping at 2.1 at. %. The Hall mobility was improved by enhancing the polycrystalline growth of the films. The carrier concentration was increased by Ga doping, which was activated as an extrinsic donor. At a further increase in the Ga content of more than 2.1 at. %, the crystallinity and resistivity of the Ga-doped AZO films deteriorated. The optical band gap was increased, and the transmittance in the visible region was increased from 86.7 to 91.0% using the same level of Ga doping at 2.1 at. %.

Novel application of MgF2 as a back reflector in a-SiOx:H thin-film solar cells

We present high-quality a-SiOx:H solar cells with a very thin i-layer of 100 nm fabricated at a low temperature of 100 °C. To boost the photocurrent with such a thin absorber, we suggested the application of a low-index MgF2 buffer at the n-type nanocrystalline silicon oxide (n-nc-SiOx:H)/Ag nanotextured interface to suppress the absorption loss at the Ag back contact. The introduction of MgF2 of only a few nanometers (~4 nm) thickness enhanced the reflection at the n-nc-SiOx:H/Ag interface, which resulted in the reinforcement of the short-circuit current by about 7.3% from 9.60 to 10.30 mA/cm2 while almost maintaining Voc and FF. We demonstrated the efficiency improvement of up to 7.66% by MgF2 at the back contact.

Improved interface of ZnO/CH3NH3PbI3 by a dynamic spin-coating process for efficient perovskite solar cells

For low-temperature solution-processed perovskite solar cells, ZnO as an electron transport layer (ETL) has been studied instead of TiO2 requiring high sintering temperature. However reports have been scarce about successful fabrication of perovskite solar cells based on the solution-processed ZnO ETL under atmospheric air. In this study, we found a possible mechanism to cause poor performance of the perovskite solar cells, which can be ascribed to chemical reactions between the methylammonium iodide (MAI) of the perovskite precursor (MAPbI3) solution and the ZnO film under humid conditions. The MAI solution could etch out ZnO ETL and promote serious carrier recombination if processed in air, whereas there was no such reaction for processing in a glovebox (H2O < 0.1 ppm). In order to suppress the reactions in air-ambient, we applied a simple and novel dynamic spin-coating process, dripping the perovskite precursor solution on ZnO during the spin-coating process, followed by an anti-solvent washing treatment. Using this approach, for the first time, the interfacial reaction of ZnO/MAPbI3 was significantly suppressed and thus a power conversion efficiency of the perovskite solar cells fully-processed in air-ambient was enhanced from 0% to 11.2%. These experimental results pave the way for the development of perovskite solar cells under low-temperature processing in air-ambient.

PCBM-blended chlorobenzene hybrid anti-solvent engineering for efficient planar perovskite solar cells

We suggest a phenyl-C61-butyric acid methyl ester (PCBM)-blended chlorobenzene (CBZ) hybrid anti-solvent washing technique for the morphological control of perovskite films. Compared to standard CBZ washing, the CBZ + PCBM hybrid washing technique endows perovskite films with a pinhole-free and compact surface morphology. Surface topography combined with phase imaging using atomic force microscopy showed that PCBM could fill up the voids and pinholes on the perovskite surfaces. Inverted planar perovskite solar cells processed using this new approach exhibited an impressive increase in the average fill factor from 0.583 to 0.747 for 40 tested devices by reducing charge recombination through the pinholes, which also contributed to an enhancement of the power conversion efficiency of the best performing cells of about 16.1% (from 9.48 to 11.01%). Furthermore, the fabricated devices showed negligible hysteresis phenomena. The presented results suggest a new and simple approach that reinforces the feasibility of low-temperature-processed inverted planar perovskite photovoltaics with enhanced performances.

Highly Efficient and Stable Sn-Rich Perovskite Solar Cells by Introducing Bromine

Compositional engineering of recently arising methylammonium (MA) lead (Pb) halide based perovskites is an essential approach for finding better perovskite compositions to resolve still remaining issues of toxic Pb, long-term instability, etc. In this work, we carried out crystallographic, morphological, optical, and photovoltaic characterization of compositional MASn0.6Pb0.4I3-xBrx by gradually introducing bromine (Br) into parental Pb-Sn binary perovskite (MASn0.6Pb0.4I3) to elucidate its function in Sn-rich (Sn:Pb = 6:4) perovskites. We found significant advances in crystallinity and dense coverage of the perovskite films by inserting the Br into Sn-rich perovskite lattice. Furthermore, light-intensity-dependent open circuit voltage (Voc) measurement revealed much suppressed trap-assisted recombination for a proper Br-added (x = 0.4) device. These contributed to attaining the unprecedented power conversion efficiency of 12.1% and Voc of 0.78 V, which are, to the best of our knowledge, the highest performance in the Sn-rich (≥60%) perovskite solar cells reported so far. In addition, impressive enhancement of photocurrent-output stability and little hysteresis were found, which paves the way for the development of environmentally benign (Pb reduction), stable monolithic tandem cells using the developed low band gap (1.24-1.26 eV) MASn0.6Pb0.4I3-xBrx with suggested composition (x = 0.2-0.4).

Low-temperature-processed a-SiOx:H/a-Si:H tandem cells for full spectrum solar cells

We developed wide-bandgap amorphous silicon (a-Si:H) and amorphous silicon oxide (a-SiOx:H) absorbers by extremely decreasing deposition temperature to as low as 100 °C. By adjusting hydrogen and carbon dioxide gas flow rates, device-quality absorbers and thus suitable single junction cells were obtained. An a-SiOx:H single-junction cell (i = 100 nm) fabricated employing the absorber we developed showed an open circuit voltage (Voc) of 1.007 V and a fill factor of 0.741, which are better than those of a-Si:H cells. This a-SiOx:H cell was introduced in a-SiOx:H/a-Si:H tandem cells as the top cell, which contributed to the achievement of a markedly high Voc of 1.910 V. This tandem cell with an efficiency of 9.25% showed better Voc and current matching property than the a-Si:H/a-Si:H (8.74%) tandem structure. The low-temperature-gradient a-SiOx:H/a-Si:H tandem cells can be a promising configuration for spectrum splitting applications.

Development of wide band gap p-a-SiOxCy:H using additional trimethylboron as carbon source gas

We report p-type a-SiOxCy:H thin films which were fabricated by introducing additional Trimethylboron (TMB, B(CH3)3) doping gas into conventional standard p-type a-SiOx:H films. The TMB addition into the condition of p-a-SiOx:H improved optical bandgap from 2.14 to 2.20 eV without deterioration of electrical conductivity, which is promising for p-type window layer of thin film solar cells. The suggested p-a-SiOxCy:H films were applied in amorphous silicon solar cells and we found an increase of quantum efficiency at short wavelength regions due to wide bandgap of the new p-layer, and thus efficiency improvement from 10.4 to 10.7% was demonstrated in a-Si:H solar cell by employing the p-a-SiOxCy:H film. In case of a-SiOx:H cell, high open circuit voltage of 1.01 V was confirmed by using the suggested the p-a-SiOxCy:H film as a window layer. This new p-layer can be highly promising as a wide bandgap window layer to improve the performance of thin film silicon solar cells.

Numerical simulation of p-type diamond Schottky barrier diodes for high breakdown voltage

P-type diamond devices have high potential for power semiconductors due to their high critical field, hole mobility, and thermal conductivity. The electrical characteristics of p-type pseudovertical diamond Schottky barrier diodes (SBDs) were investigated by numerical simulation. The impact ionization coefficients were required to obtain the breakdown voltage. They were revised to satisfy a parallel-plane breakdown field of 10 MV/cm. The doping concentration and thickness of a low-doped drift layer were key parameters in determining the parallel-plane breakdown voltage. The p-type pseudovertical diamond SBDs exhibited lower breakdown voltage than the parallel-plane breakdown voltage because field crowding occurred at the edge of the cathode. When the doping concentration and thickness of the p− drift layer were 1016 cm−3 and 4 µm, respectively, the breakdown voltage of the p-type pseudovertical diamond SBD was 961 V, which was considerably less than the parallel-plane breakdown voltage of 3646 V.

Progress in a-SiOx:H thin film solar cells with patterned MgF2 dielectric for top cell of multi-junction system

We successfully designed and experimentally demonstrated an application of patterned MgF2 dielectric material at rear Al-doped ZnO (AZO)/Ag interface in thin film amorphous silicon oxide (a-SiOx:H) solar cells. When it was realized in practical device process, MgF2 coverage with patterned morphology was employed to allow for current flow between the AZO and Ag against highly resistive MgF2 material. On the basis of the suggested structure, we found an improvement in quantum efficiency of the solar cells with the patterned MgF2. In addition, an enhancement of open circuit voltage (Voc) and fill factor (FF) was observed. A remarkable increase in shunt resistance of the cells with the MgF2 would possibly indicate that the highly resistive MgF2 layer can partly suppress physical shunting across top and bottom electrodes caused by very thin absorber thickness of only 100 nm. The approach showed that our best-performing device revealed an essential improvement in conversion efficiency from 7.83 to 8.01% with achieving markedly high Voc (1.013 V) and FF (0.729).