Cheolmin Park

Bias-stability improvement using Al2O3?interfacial dielectrics in a-InSnZnO thin-film transistors

We report amorphous-indium–tin–zinc-oxide (a-ITZO) thin-film transistors (TFTs) obtained using an aluminum-oxide (Al2O3) interfacial dielectric using atomic layer deposition between a silicon-nitride (SiNX) gate dielectric and an a-ITZO active channel layer. The effect of the Al2O3 interfacial layer on the suppression of charge trapping in a-ITZO TFTs is presented. In transparent oxide TFTs, reducing the shift in threshold voltage by stress-including negative-bias stress (NBS) is one of the key issues in improving the stability performance of TFTs. The NBS stability of an a-ITZO TFT using an Al2O3/SiNX double-layered dielectric is superior to that using an SiNX single-gate dielectric, because of the smooth surface with a root-mean-square roughness of 0.147 nm and a low defect density of less than 3×1011 eV−1 cm−2, which increases hydrophobicity. The a-ITZO TFTs using the Al2O3 interfacial dielectric show little change in the threshold voltage (~0 V), and a long trapping time of ~5000 s when a gate voltage of −25 V and drain voltage of 1 V are applied for 10 000 s. We show that the gate dielectric has a profound effect on the electrical stability, and suggest a way of improving the stability of a-ITZO TFTs.

The effect of small pyramid texturing on the enhanced passivation and efficiency of single c-Si solar cells

In this work, a simple method to form small random pyramid texturing (0.5–2 μm size) is proposed to enhance the surface passivation of commercial p-type Cz-Si wafers. Small pyramid texturing was generated with chemical nano-masking for anisotropic etching. The surface recombination velocity obtained after the passivation of the thermal oxide layer reduced from 65 and 10 cm s−1 for the large pyramids (10–15 μm size) and small pyramid (0.5–2 μm) texturing respectively. The solar cell fabricated with large pyramid texturing resulted in an efficiency of 17.82% with a current density (JSC) of 36.91 mA cm−2, an open circuit voltage (VOC) of 620 mV whereas small pyramid texturing resulted in an efficiency of 18.5% with JSC of 37.6 mA cm−2 and VOC of 628 mV. The low surface recombination velocity increases the VOC by 8 mV. The small pyramid textured wafers are found to enhance the quantum efficiency performance in both short and long wavelength regions.

Surface Passivation Schemes for High-Efficiency c-Si Solar Cells - A Review

Copyright ©2015 KIEEME. All rights reserved. This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited. pISSN: 1229-7607 eISSN: 2092-7592 DOI: http://dx.doi.org/10.4313/TEEM.2015.16.5.227 OAK Central: http://central.oak.go.kr TRANSACTIONS ON ELECTRICAL AND ELECTRONIC MATERIALS Vol. 16, No. 5, pp. 227-233, October 25, 2015

Selective emitter using a screen printed etch barrier in crystalline silicon solar cell

The low level doping of a selective emitter by etch back is an easy and low cost process to obtain a better blue response from a solar cell. This work suggests that the contact resistance of the selective emitter can be controlled by wet etching with the commercial acid barrier paste that is commonly applied in screen printing. Wet etching conditions such as acid barrier curing time, etchant concentration, and etching time have been optimized for the process, which is controllable as well as fast. The acid barrier formed by screen printing was etched with HF and HNO3 (1:200) solution for 15 s, resulting in high sheet contact resistance of 90 Ω/sq. Doping concentrations of the electrode contact portion were 2 × 1021 cm−3 in the low sheet resistance (Rs) region and 7 × 1019 cm−3 in the high Rs region. Solar cells of 12.5 × 12.5 cm2 in dimensions with a wet etch back selective emitter Jsc of 37 mAcm−2, open circuit voltage (Voc) of 638.3 mV and efficiency of 18.13% were fabricated. The result showed an improvement of about 13 mV on Voc compared to those of the reference solar cell fabricated with the reactive-ion etching back selective emitter and with Jsc of 36.90 mAcm−2, Voc of 625.7 mV, and efficiency of 17.60%.

Surface passivation of boron emitters on n-type c-Si solar cells using silicon dioxide and a PECVD silicon oxynitride stack

Recombination of charge carriers is a significant loss mechanism in solar cells. To achieve high efficiency, recombination losses should be minimized. The surface passivation technique is used to minimize recombination losses. Also, surface passivation is essential to achieve high conversion efficiency, especially for thin crystalline-Si (c-Si) wafers. We have investigated the passivation properties of a silicon oxynitride (SiOxNy)/silicon dioxide (SiO2) stack for boron doped emitters. SiOxNy single layer properties were optimized using various gases (N2O, NH3 and SiH4) and gas flow ratios. Optimized SiOxNy films resulted in low refractive indices ranging from 1.49 to 1.64. FTIR spectroscopy was used to analyze the chemical composition of the SiOxNy films. As the gas flow ratio increases, the absorbance peaks shift towards higher wave numbers due to an increase in oxygen concentration in the film with a decrease in refractive index. After optimization, SiOxNy film was capped over with a 10 nm thermal SiO2 layer. The effective lifetime of the SiOxNy/SiO2 stack was found to be 0.690 ms. Light I–V results showed an efficiency of 19.58% with Voc, Jsc and fill factor of 644 mV, 37 mA cm−2 and 82.3%, respectively. The role of SiOxNy/SiO2 stack passivation for the improvement of solar cell efficiency is discussed.

Al2O3/SiON stack layers for effective surface passivation and anti-reflection of high efficiency n-type c-Si solar cells

Excellent surface passivation and anti-reflection properties of double-stack layers is a prerequisite for high efficiency of n-type c-Si solar cells. The high positive fixed charge (Q f) density of N-rich hydrogenated amorphous silicon nitride (a-SiNx:H) films plays a poor role in boron emitter passivation. The more the refractive index ( n ) of a-SiNx:H is decreased, the more the positive Q f of a-SiNx:H is increased. Hydrogenated amorphous silicon oxynitride (SiON) films possess the properties of amorphous silicon oxide (a-SiOx) and a-SiNx:H with variable n and less positive Q f compared with a-SiNx:H. In this study, we investigated the passivation and anti-reflection properties of Al2O3/SiON stacks. Initially, a SiON layer was deposited by plasma enhanced chemical vapor deposition with variable n and its chemical composition was analyzed by Fourier transform infrared spectroscopy. Then, the SiON layer was deposited as a capping layer on a 10 nm thick Al2O3 layer, and the electrical and optical properties were analyzed. The SiON capping layer with n = 1.47 and a thickness of 70 nm resulted in an interface trap density of 4.74 = 1010 cm−2 eV−1 and Q f of −2.59 = 1012 cm−2 with a substantial improvement in lifetime of 1.52 ms after industrial firing. The incorporation of an Al2O3/SiON stack on the front side of the n-type solar cells results in an energy conversion efficiency of 18.34% compared to the one with Al2O3/a-SiNx:H showing 17.55% efficiency. The short circuit current density and open circuit voltage increase by up to 0.83 mA cm−2 and 12 mV, respectively, compared to the Al2O3/a-SiNx:H stack on the front side of the n-type solar cells due to the good anti-reflection and front side surface passivation.

Boron diffused layer optimization by metal impurity concentration control using gettering process for n-type c-Si solar cell applications

In this paper, boron diffused layer optimization was described by metal impurity concentration control using gettering process. Boron diffusion was performed using BBr3 diffusion furnace with varying cooling rate and treated Rs with acid etching. The effect of gettering in the diffused layer was analyzed using SIMS and QSS-PC method, where by metal impurity and boron concentration and saturation current density (J0). Metal and boron retained dose was decreased from 2.04 × 10¹⁴ to 6.52 × 10¹³ atoms/cc, 1.2 × 10²⁰ to 3.54 × 10¹⁹ atoms/cc. Metal impurity and boron concentration determined SRV, MCLT, Jo. and solar cell output characteristics.

Rear side passivation characteristics of silicon oxy nitride films for high efficiency silicon solar cell

High quality rear surface passivation is a prerequisite to obtain high conversion efficiency on thin wafers. SiO_xN_y with low absorbance and variable refractive index inherits the properties of both SiN_x and SiO₂ which act as a rear surface reflector. SiO_xN_y layer with low refractive indices ranging from 1.51 to 1.61 was deposited and their chemical composition was analyzed by Fourier transform infrared (FTIR) spectroscopy. With the optimized SiO_xN_y layer the I-V results yielded a short circuit current density (J_sc) of 38.7 mA/cm² and efficiency of 19.34%, with open circuit voltage (V_oc) of 635 mV.

Analysis of Contact Properties by Varying the Firing Condition of AgAl Electrode for n-type Crystalline Silicon Solar Cell

n-type silicon shows the better tolerance towards metal impurities with a higher minority carrier lifetime compared to p-type silicon substrate. Due to better lifetime stability as compared to p-type during illumination made the photovoltaic community to switch toward n-type wafers for high efficiency silicon solar cells. We fabricated the front electrode of the n-type solar cell with AgAl paste. The electrodes characteristics of the AgAl paste depend on the contact junction depth that is closely related to the firing temperature. Metal contact depth with p+ emitter, with optimized depth is important as it influence the resistance. In this study, we optimize the firing condition for the effective formation of the metal depth by varying the firing condition. The firing was carried out at temperatures below 670°C with low contact depth and high contact resistance. It was noted that the contact resistance was reduced with the increase of firing temperature. The contact resistance of 5.99 mΩcm2 was shown for the optimum firing temperature of 865°C. Over 900°C, contact junction is bonded to the Si through the emitter, resulting the contact resistance to shunt. we obtained photovoltaic parameter such as fill factor of 76.68%, short-circuit current of 40.2 mA/cm, open-circuit voltage of 620 mV and convert efficiency of 19.11%.

Optimization of the Phosphorus Doped BSF Doping Profile and Formation Method for N-type Bifacial Solar Cells

n-type PERT (passivated emitter, rear totally diffused) bifacial solar cells with boron and phosphorus diffusion as p+ emitter and n+ BSF (back surface field) have attracted significant research interest recently. In this work, the influences of wafer thickness, bulk lifetime, emitter, BSF on the photovoltaic characteristics of solar cells are discussed. The performance of the solar cell is determined by using one-dimensional solar cell simulation software PC1D. The simulation results show that the key role of the BSF is to decrease the surface doping concentration reducing the recombination and thus, increasing the cell efficiency. A lightly phosphorus doped BSF (LD BSF) was experimentally optimized to get low surface dopant concentration for n type bifacial solar cells. Pre-oxidation combined with a multi-plateau drive-in, using limited source diffusion was carried out before pre-deposition. It could reduce the surface dopant concentration with minimal impact on the sheet resistance.