Junpei Irikawa

High Quality Aluminum Oxide Passivation Layer for Crystalline Silicon Solar Cells Deposited by Parallel-Plate Plasma-Enhanced Chemical Vapor Deposition

We investigated hydrogenated aluminum oxide (a-Al1-xOx:H) as a high quality rear surface passivation layer of crystalline silicon solar cells. The a-Al1-xOx:H films were deposited by plasma-enhanced chemical vapor deposition (PECVD) using a mixture of trimethylaluminum (TMA), carbon dioxide (CO2), and hydrogen (H2) at a low substrate temperature of about 200 °C. The ratio of CO2 to TMA during deposition and thermal annealing after the film deposition are the key factors in achieving high quality passivation. A 28-nm-thick a-Al1-xOx:H film deposited by PECVD showed a low surface recombination velocity of about 10 cm/s.

High-quality nanocrystalline cubic silicon carbide emitter for crystalline silicon heterojunction solar cells

We developed a highly transparent n-type hydrogenated nanocrystalline cubic silicon carbide (nc-3C–SiC:H) emitter for crystalline silicon (c-Si) heterojunction solar cells. A low emitter saturation current density (J0e) of 1.4×101 fA/cm2 was obtained under optimal deposition conditions. A c-Si heterojunction solar cell fabricated on a p-type c-Si wafer without texturing showed an active area efficiency of 17.9% [open-circuit voltage (Voc)=0.668 V, short-circuit current density (Jsc)=36.7 mA/cm2, fill factor=0.731]. The high Jsc value is associated with excellent quantum efficiencies at short wavelengths (<500 nm).

High Efficiency Hydrogenated Nanocrystalline Cubic Silicon Carbide/Crystalline Silicon Heterojunction Solar Cells Using an Optimized Buffer Layer

Heterojunction crystalline silicon solar cells using a nanocrystalline cubic silicon carbide (nc-3C-SiC) emitter were optimized by changing the deposition time of a buffer layer. The implied open circuit voltage (implied-Voc) estimated from quasi-steady state photoconductance measurements strongly depended on the buffer deposition time. The implied-Voc of 0.690 V was achieved with a buffer deposition time of 30 s. The optimized solar cell showed an active area efficiency of 19.1% (Voc=0.680 V, Jsc=36.6 mA/cm2, and FF=0.769). The excellent cell performance is a direct evidence of the potential of the nc-3C-SiC:H emitter.

Modeling and simulation of heterojunction crystalline silicon solar cells with a nanocrystalline cubic silicon carbide emitter

We have developed a simulation model for a heterojunction crystalline silicon (HJ-c-Si) solar cell with an n-type hydrogenated nanocrystalline cubic silicon carbide (nc-3C-SiC:H) emitter and a p-type hydrogenated microcrystalline silicon oxide back surface field layer. Analyses of experimentally obtained solar-cell performance using the simulation model indicate that the conversion efficiency of the solar cell is limited by the rear-surface recombination velocity (Sr) and acceptor concentration (NA) of the p-type c-Si base region. Simulation results indicate that a potential conversion efficiency of HJ-c-Si solar cells using n-type nc-3C-SiC:H emitters is approximately 23% when Sr, NA, and bulk lifetime of the p-type base are 10 cm/s, 2 × 1016 cm−3, and 1.0 × 10−3 s, respectively.

Silicon heterojunction solar cells with high surface passivation quality realized using amorphous silicon oxide films with epitaxial phase

The epitaxial growth of the i-layer of crystalline silicon heterojunction solar cells has been widely accepted as harmful to surface passivation. In our experiments, however, although a very rough epitaxial phase in the intrinsic a-Si1−xOx:H passivation layer was confirmed by transmission electron microscopy and spectroscopic ellipsometry, a high effective lifetime and an implied-VOC of over 720 mV were achieved with lifetime samples. The high passivation quality was confirmed by the obtained open circuit voltages of 728 and 721 mV for n- and p-type solar cells, respectively, with an a-Si1−xOx:H/p-µc-Si1−xOx:H stack rear structure. These results indicate that, contrary to the common knowledge, high surface passivation quality can be achieved even when the epitaxial phase is present.

Development of the Transparent Conductive Oxide Layer for Nanocrystalline Cubic Silicon Carbide/Silicon Heterojunction Solar Cells with Aluminum Oxide Passivation Layers

We developed an In2O3:H/indium–tin oxide (ITO) stack as the front transparent conductive oxide (TCO) layer of nanocrystalline cubic silicon carbide/crystalline silicon heterojunction solar cells with Al2O3 passivation layers. We investigated the solar cell performance and optical and electrical properties of this layer with various annealing temperatures. The solar cells with In2O3:H and In2O3:H/ITO layers show a higher short circuit current density (Jsc) than that with an ITO layer owing to their lower surface reflection and lower free carrier absorption. The solar cell with the In2O3:H/ITO stack shows a higher fill factor (FF) than that with the In2O3:H layer. The solar cell with the In2O3:H/ITO stack shows an aperture area efficiency of 16.8% (Voc = 0.638 V, Jsc = 34.5 mA/cm2, and FF= 0.762). These results indicate that the In2O3:H/ITO stack has good optical and electrical properties after annealing.

Effects of Annealing and Atomic Hydrogen Treatment on Aluminum Oxide Passivation Layers for Crystalline Silicon Solar Cells

of trimethylaluminum (TMA), hydrogen (H2), and carbon dioxide (CO2) was used for the reactant gas. The TMA flow rate was controlled by a vaporizer system equipped with a liquid mass flow controller. The flow rate of TMA used in this study is 1 mg/min, which is equivalent to 0.3 sccm. The CO2/TMA ratio was kept constant at 37. The details of the deposition condition were reported elsewhere. 3) To evaluate the surface passivation quality, identical a-Al1� xOx:H films with thicknesses of 6–28 nm were deposited on both sides of the wafers. The substrates were cleaned by ultrasonic cleaning in acetone and ethanol solvent. A 1-min HF (5%) dip was performed to remove native oxides after ultrasonic cleaning. Between the front and back side deposition, no chemical treatment was carried out. The thicknesses of a-Al1� xOx:H films were determined by spectroscopic ellipsometry using Cauchy model. To investigate the effect of hydrogen in the films, thermal annealing and AHT were carried out. The thermal annealing was carried out at 475 � C in forming gas (3% H2 in N2) for 1 min. The AHT was carried out using a hot-wire technique. 18) The front sides of the samples were exposed to atomic hydrogen produced from H2 gas using heated tungsten wires. The atomic hydrogen density, the pressure, the substrate temperature, the tungsten wire temperature, and the exposure time were 1 � 10 12 cm � 3 , 80 Pa, 185 � C, 1390 � C, and 24 min, respectively. The tungsten wire temperature was measured using a radiation thermometer. The atomic hydrogen density was estimated from the transmittance of the tungsten phosphate glass. 19) The atomic composition and bonding conditions for each state were investigated using Rutherford backscattering (RBS), elastic recoil detection analysis (ERDA), and attenuated total reflection Fourier transform infrared spectroscopy (ATRFTIR) measurement. The effective lifetime (� eff) was measured by microwave photoconductance decay (MW-PCD) and quasi-steady-state

Characterization of laser transferred contact through aluminum oxide passivation layer

The point contact structure of the rear side of crystalline silicon solar cells is used to realize high passivation quality and good electrical contact. A conversion efficiency of 19.8% was achieved on the passivated emitter and rear cell (PERC) structure with a widegap heterojunction emitter. In this solar cell, laser-fired contact (LFC) process is used to make the point contacts. In the LFC process, laser irradiation damage to the surface of crystalline silicon was found to be the big issues for higher passivation quality. We investigated laser-transferred contact (LTC) process as a new contacts formation process. The LTC samples showed higher effective minority carrier lifetime and good contact resistivity compared with the LFC samples.

Improvement of n-type nc-3C-SiC:H heterojunction emitter for c-Si solar cells

We optimized an n-type hydrogenated nanocrystalline cubic silicon carbide (nc-3C-SiC:H) emitter by changing the plasma power density and monomethylsilane (MMS) flow rate during buffer layer deposition. Quasi-steady state photoconductance (QSSPC) method was carried out to measure the effective lifetime and implied open circuit voltage (implied-Voc). The implied-Voc above 0.7 V was achieved with the plasma power density of 1.6 W/cm2 and MMS flow rate of 2.75 sccm. These result indicates that the properties of n-type nc-3C-SiC:H emitter strongly depend on the deposition condition of a-SiC:H buffer layer.

Effect of atomic hydrogen treatment on passivation quality of aluminum oxide for p-type crystalline silicon

We investigated effects of annealing and atomic hydrogen treatment on passivation quality of aluminum oxide (a-Al<inf>1−x</inf>O<inf>x</inf>:H) films deposited by plasma-enhanced chemical vapor deposition (PECVD) for p-type crystalline silicon (c-Si). We found that the surface recombination velocity (S<inf>eff</inf>) decreased from 5.0 ×10² to 2.0 ×10¹ cm/s after 475°C annealing, however the S<inf>eff</inf> increased up to 2.8×10³ cm/s after atomic hydrogen treatment. This change in the passivation quality was caused by the change in the negative fixed charge density. The negative fixed charge density increased from 1.2×10¹² to 2.9×10¹² cm⁻² by thermal annealing and decreased down to 5.9×10¹¹ cm⁻² by atomic hydrogen treatment. This result suggests that hydrogen at the a-Al<inf>1−x</inf>O<inf>x</inf>:H/c-Si interface significantly influences negative fixed charge density.