Diao Hongwei

Texturization of mono-crystalline silicon solar cells in TMAH without the addition of surfactant

Etching was performed on (100) silicon wafers using silicon-dissolved tetramethylammonium hydroxide (TMAH) solutions without the addition of surfactant. Experiments were carried out in different TMAH concentrations at different temperatures for different etching times. The surface phenomena, etching rates, surface morphology and surface reflectance were analyzed. Experimental results show that the resulting surface covered with uniform pyramids can be realized with a small change in etching rates during the etching process. The etching mechanism is explained based on the experimental results and the theoretical considerations. It is suggested that all the components in the TMAH solutions play important roles in the etching process. Moreover, TMA+ ions may increase the wettability of the textured surface. A good textured surface can be obtained in conditions where the absorption of OH-/H2O is in equilibrium with that of TMA+/SiO2 (OH)22-.

Influence of Ring Oxidation-Induced Stack Faults on Efficiency in Silicon Solar Cells

We observe a strong correlation between the ring oxidation-induced stack faults (OISF) formed in the course of phosphor diffusion and the efficiency of Czochralski-grown silicon solar cells. The main reason for ring-OISF formation and growth in substrate is the silicon oxidation and phosphorus diffusion process induced silicon self-interstitial point defect during POCI3 diffusion. The decreasing of minority carrier diffusion length in crystal silicon solar cell induced by ring-OISF defects is identified to be one of the major causes of efficiency loss.

Optical and electrical properties of porous silicon layer formed on the textured surface by electrochemical etching

Porous silicon (PS) layers were formed on textured crystalline silicon by electrochemical etching in HF-based electrolyte. Optical and electrical properties of the TMAH textured surfaces with PS formation are studied. Moreover, the influences of the initial structures and the anodizing time on the optical and electrical properties of the surfaces after PS formation are investigated. The results show that the TMAH textured surfaces with PS formation present a dramatic decrease in reflectance. The longer the anodizing time is, the lower the reflectance. Moreover, an initial surface with bigger pyramids achieved lower reflectance in a short wavelength range. A minimum reflectance of 3.86% at 460 nm is achieved for a short anodizing time of 2 min. Furthermore, the reflectance spectrum of the sample, which was etched in 3 vol.% TMAH for 25 min and then anodized for 20 min, is extremely flat and lies between 3.67% and 6.15% in the wavelength range from 400 to 1040 nm. In addition, for a short anodizing time, a slight increase in the effective carrier lifetime is observed. Our results indicate that PS layers formed on a TMAH textured surface for a short anodization treatment can be used as both broadband antireflection coatings and passivation layers for the application in solar cells.

Excellent Passivation of p-Type Si Surface by Sol-Gel Al2O3 Films

Al2O3 films with a thickness of about 100 nm synthesized by spin coating and thermally treated are applied for field-induced surface passivation of p-type crystalline silicon. The level of surface passivation is determined by techniques based on photoconductance. An effective surface recombination velocity below 100 cm/s is obtained on 10Ω cm p-type c-Si wafers (Cz Si). A high density of negative fixed charges in the order of 1012 cm−2 is detected in the Al2O3 films and its impact on the level of surface passivation is demonstrated experimentally. Furthermore, a comparison between the surface passivation achieved for thermal SiO2 and plasma enhanced chemical vapor deposition SiNx:H films on the same c-Si is presented. The high negative fixed charge density explains the excellent passivation of p-type c-Si by Al2O3.

Emitter of hetero-junction solar cells created using pulsed rapid thermal annealing

In this paper, we use a pulsed rapid thermal processing (RTP) approach to create an emitter layer of hetero-junction solar cell. The process parameters and crystallization behaviour are studied. The structural, optical and electric properties of the crystallized films are also investigated. Both the depth of PN junction and the conductivity of the emitter layer increase with the number of RTP pulses increasing. Simulation results show that efficiencies of such solar cells can exceed 15% with a lower interface recombination rate, but the highest efficiency is 11.65% in our experiments.

The investigation of ZnO:Al2O3/metal composite back reflectors in amorphous silicon germanium thin film solar cells

Different aluminum-doped ZnO (AZO)/metal composite thin films, including AZO/Ag/Al, AZO/Ag/nickel—chromium alloy (NiCr), and AZO/Ag/NiCr/Al, are utilized as the back reflectors of p—i—n amorphous silicon germanium thin film solar cells. NiCr is used as diffusion barrier layer between Ag and Al to prevent mutual diffusion, which increases the short circuit current density of solar cell. NiCr and NiCr/Al layers are used as protective layers of Ag layer against oxidation and sulfurization, the higher efficiency of solar cell is achieved. The experimental results show that the performance of a-SiGe solar cell with AZO/Ag/NiCr/Al back reflector is best. The initial conversion efficiency is achieved to be 8.05%.

The effect of emitter profile on laser doped multicrystalline silicon selective emitter cells

The emitter dopant profile between the metal grid in lase doping selective emitter cells was modified by oxidation the phosphosilicate glass (PSG) film at a higher temperature. This diffusion process contains two steps. Step 1 form the PSG layer on the wafer surface by the reaction of POCl3 and oxygen. Step 2 increase the temperature to a higher value at which the oxidation of PSG layer is proceeded. For the practical cell process, the laser power and front metal-grid were optimized, considering the dependence on the light induced plating nickel-silicon contact and on the emitter sheet resistance. Comparing with the uniform emitter multicrystalline silicon solar cells, significant increase of short circuit current by 0.3 A and open circuit voltage by 6 mV was obtained resulting in an average gain of 0.6%abs. An average efficiency of 17.2% and the largest of 17.42% for the best cell on a large area commercial grade p-type multi-crystalline silicon substrate were achieved.

One-dimensional modeling of BC-BJ silicon soalr cells

In this study, simulation using a simplified one-dimensional (1D) back-junction solar cells structure to describe the effects which occur in the Back-Contact Back-Junction (BC-BJ) solar cell is performed. Our simulation results show that the BC-BJ cell is particularly suited for high lifetimes and thin substrate. Simulation results indicate that achieving a bulk lifetime of around 1 ms will need the requirement of realizing very high-efficiency BC-BJ cells. The effect of different cell parameters (e.g., surface recombination velocities-SRV, bulk lifetime, phosphorus doped front surface-FSF, bulk resistivity, etc.) is explored. The presence of the front FSF improves the front surface passivation and the optimum phosphorus doping profile depending on the SRV.