Zhou Chunlan

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.

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.

Characterization of the nanosized porous structure of black Si solar cells fabricated via a screen printing process

A silicon (Si) surface with a nanosized porous structure was formed via simple wet chemical etching catalyzed by gold (Au) nanoparticles on p-type Cz-Si (100). The average reflectivity from 300 to 1200 nm was less than 1.5%. Black Si solar cells were then fabricated using a conventional production process. The results reflected the output characteristics of the cells fabricated using different etching depths and emitter dopant profiles. Heavier dopants and shallower etching depths should be adopted to optimize the black Si solar cell output characteristics. The efficiency at the optimized etching time and dopant profile was 12.17%. However, surface passivation and electrode contact due to the nanosized porous surface structure are still obstacles to obtaining high conversion efficiency for the black Si solar cells.

Self-Assembled TiO2/PSS Multilayer Films Studied by Slow Positron Spectroscopy

TiO2/PSS nano-structured multilayer films are fabricated by a layer-by-layer self-assembly method, and the deposition process and interface structure of films are studied in detail by slow positron spectroscopy. The results indicate that injection energy of positron at the interface between the substrate and the Elm shows a linear dependence on the number of bilayer, which suggests that the repeatability of the depositing process is good, and the thickness of films shows a linear, increase with the number of bilayers adding. The calculated result of the film thickness shows that there is an overlay between the adjacent TiO2 nanoparticle layers.

Defects in Si-Rich SiO2 Films Prepared by Radio-Frequency Magnetron Co-sputtering Using Variable Energy Positron Annihilation Spectroscopy

Si-rich SiO2 films prepared by rf magnetron co-sputtering method are studied by slow positron beams. The negatively charge point defects (probably Pb centres or peroxy radicals) at the silicon nanocluster (nc-Si)/SiO2 interface are observed by Doppler broadening spectra. Coincidence Doppler-broadening spectra show that positrons have a higher annihilation probability with core electrons nearby oxygen atoms than silicon atoms. The formation of N-related bonds may be the reason for the prevention of the migration reaction of Si and O atoms, hence nc-Si formation is inhibited by annealing in nitrogen compared to in vacuum.

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.

Theoretical simulation and experimental study of phosphorus concentration profiles in laser doped crystalline silicon solar cells

In this paper, a nonlinear numerical model of laser heating on crystalline silicon solar cells has been established based on the thermodynamic method. According to the non-steady temperature field gained by finite difference method, variation characteristics of melt front as a function of time are described during laser melting of crystalline silicon. Considering the features of liquid phase diffusion of phosphorus atoms in melting silicon, theoretical simulation for phosphorus diffusion behavior and the evolution of phosphorus concentration profiles at various times are achieved by the image method. The theoretical phosphorus concentration profile is in good agreement with the corresponding experimental ECV data.

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.

Simulation of laser ablation of crystalline silicon solar cells by ANSYS

By the finite element analysis software ANSYS, we simulate a nonlinear model of laser ablation of crystalline silicon solar cells. Using thermodynamic method, the finite element model of pulsed laser heating crystalline silicon solar cells has been established. Adopting reasonable material thermal physical parameters description, meshing, thermal source model, initial conditions and boundary conditions, the ablation depth, ablation width and ablation morphology of laser ablating crystalline silicon solar cells have been simulated and compared with the experimental observations by scanning electron microscopy. The obtained simulation results are consistent with the experimental measurements.