Ching-hsi Lin

Silicon Solar Cells: Structural Properties of Ag-Contacts/Si-Substrate

The screen-printed silver (Ag) thick-film is the most widely used front side contact in industrial crystalline silicon solar cells. The front contacts have the roles of efficiently contacting with the silicon (Si) and transporting the photogenerated current without adversely affecting the cell properties and without damaging the p-n junction. Although it is rapid, has low cost and is simplicity, high quality screen-printed silver contact is not easy to make due to the complicated composition in the silver paste. Commercially available silver pastes generally consist of silver powders, lead-glass frit powders and an organic vehicle system. The organic constituents of the silver paste are burned out at temperatures below 500°C. Ag particles, which are ~70-85wt% and can be different in shape and size distribution, show good conductivity and minor corrosive characteristics. The concentration of glass frit is usually less than 5wt %; however, the glass frit in the silver paste plays a critical role for achieving good quality contacts to high-doping emitters. The optimization of the glass frit constitution can help achieve adequate photovoltaic properties. The melting characteristics of the glass frit and also of the dissolved silver have significant influence on contact resistance and fill factors (FFs). Glass frit advances sintering of the silver particles, wets and merges the antireflection coating. Moreover, glass frit forms a glass layer between Si and Ag-bulk, and can further react with Si-bulk and forms pin-holes on the Si surface upon high temperature firing. This chapter first describes the Ag-bulk/Si contact structures of the crystalline silicon solar cells. Then, the influences of the Ag-contacts/Si-substrate on performance of the resulted solar cells are investigated. The objective of this chapter was to improve the understanding of front side contact formation by analyzing the Ag-bulk/Si contact structures resulting from different degrees of firing. The observed microscopic contact structure and the resulting solar-cell performance are combined to clarify the mechanism behind the hightemperature contact formation. Samples were fired either at a optimal temperature of ~780°C or at a temperature of over-fired for silver paste to study the effect of firing temperature. The melting characteristics of the glass frit determine the firing condition suitable for low contact resistance and high fill factors. In addition, it was found the post forming gas annealing can help overfired solar cells recover their FF. The results show that after 400°C post forming gas annealing for 25min, the over-fired cells improve their FF. On the other hand, both of the optimally-fired and the under-fired cells did not show similar

Characterization of selective-emitter solar cells consists of laser opened window and subsequently screen-printed electrodes

Selective emitter technique is still an interesting research subject and attracts many attentions in photovoltaic industry. A selective emitter is a doping layer that is heavily doped underneath the electrode while lightly doped in between the electrode grids. It offers good short-wavelength response due to low surface doping concentration and in the mean time maintains low contact resistance. To successfully incorporate the selective-emitter technique into production, one of the requirements for a cost-effective selective emitter is that the efficiency should be increased significantly compare to those conventional solar cells with uniform doped emitter. However, the developments of commercial Ag-pastes, which are suitable for high sheet-resistance silicon, make uniform lightly-doped solar cell possible. In other words, the successfully developed Ag-pastes may limit the demand of selective-emitter techniques. In this study, we try to demonstrate that the selective-emitter is still an attractive technique even in light/light sheet-resistance combination. In comparison to solar cells with uniform lightly-doped 65 Ohm/sq emitter, the results show an efficiency improvement of more than 0.5% absolute can be achieved for selective-emitter solar cells. The process sequence of the selective emitters in this work includes a laser opening through the oxide mask. It was followed by conventional POCl3 diffusion and a subsequent electrode screen printing.

Fast nano-scale texturing using the self-assembly polymer mask and wet chemical etching

An efficient suppression of reflection in a broad spectral range can be achieved when the textured scale is comparable to the wavelength of incident light. The nano-scale texturing method is, therefore, attracting many interests in many fields. In this study, a fast nano-scale texturing method for the crystalline silicon substrate has been provided. By using the self-assembly polymer mask and wet chemical etching, an almost uniform low reflectance can be achieved. The self-assembly polymer mask was fabricated by coating the pre-prepared polymer solution onto silicon substrate followed by suitable heat treatment. In addition to being technologically easy, the process is also inexpensive.

Effect of NdF3 and NdCl3 substitution for Nd2O3 on the reduction–diffusion process of Nd–Fe–B powders

The effect of partial substitution NdX3 (X=F or Cl) for Nd2O3 on the reduction–diffusion (R/D) process of Nd–Fe–B powders has been investigated. NdX3 reacts with Ca in the R/D process to produce CaX2. CaCl2 is distributed uniformly around the Nd2Fe14B grains after reaction. During water washing, CaCl2 reacts quickly with water. This improves the disintegration of compacts and facilitates the removal of Ca. The best results with residual Ca content of 400 ppm and oxygen content of 3000 ppm were obtained. In the case of fluoride, however, a negative result was found. There was no improvement in the disintegration of fluoride compact. This was due to the poor water affinity of CaF2.A method for measuring the pair interaction potential between colloidal particles by extrapolation measurement of collective structure to infinite dilution is presented and explored using simulation and experiment. The method is particularly well suited to systems in which the colloid is fluorescent and refractive index matched with the solvent. The method involves characterizing the potential of mean force between colloidal particles in suspension by measurement of the radial distribution function using 3D direct visualization. The potentials of mean force are extrapolated to infinite dilution to yield an estimate of the pair interaction potential, U(r). We use Monte Carlo (MC) simulation to test and establish our methodology as well as to explore the effects of polydispersity on the accuracy. We use poly-12-hydroxystearic acid-stabilized poly(methyl methacrylate) (PHSA-PMMA) particles dispersed in the solvent dioctyl phthalate (DOP) to test the method and assess its accuracy for three different repulsive systems for which the range has been manipulated by addition of electrolyte.

Hydrogen induced radial anisotropic R‐Fe‐B magnet (R=Pr,Dy)

The effect of alloying concentration of Dy and the absorption amount of hydrogen on the magnetic properties of PrFeB magnets have been studied in this work. Experimental data show that the easy direction of magnetization (EDM) of the Pr‐Fe‐B‐H system is determined by hydrogen concentration. Radial anisotropic magnets can be obtained from hydrogenated Pr15Fe79B6 powders with hydrogen of 4500 ppm. A slight decrease of hydrogen content down to 4250 ppm leads to a uniaxial anisotropic magnet. Partial replacement of Pr by Dy significantly influences the anisotropy of the Pr‐Fe‐B‐H system with saturated hydrogen. A 0.9 at. % Dy substitution for Pr changes the anisotropy of hydrogen decrepitated powders from planer to cone, thus resulting in an isotropic magnet. As the amount of Dy substitution increases to 1.35 at. %, the EDM of magnets persist in the field direction during alignment compaction.

Inverted pyramid texturisation without photolithography for multicrystalline solar cell

New surface texturing method including formation of inverted pyramids has been further investigated in this study. Electroless deposition of Ag particles as metal catalyst in HF/H2O2 etching solution is found to be efficient for drilling nano-size holes into c-Si or m-Si. After etching back by KOH solution, the inverted pyramids structure gradually appears in c-Si (100) surface. Although the m-Si wafers have different orientation grains, the average reflection after texturing demonstrates lower than that of traditional acid treatment. The new texturing wafers were processed into cells with a conventional process including POCl3 diffusion (leading to 65 Ω/□), removal of native oxide by BOE solution. The deviations of sheet resistance on 5″ wafers are controlled below 5% which are consistent with that of acidic or alkaline treatment wafers. The SiNx antireflection coating was deposited uniformly by PECVD deposition. From the results of IV measurement, the conversion efficiency of new texturing has 0.1% higher than that of acidic treatment (texturied by Rena facilities). We expect the optimization of texturisation will lead to more short-circuit current density( Jsc) and gain more conversion efficiencies.

Unified texturization method for mono- and multi-crystalline silicon solar cells

A texturization method suitable for both c- and mc-Si is developed. The method is applied on as-cut wafers and is found to be suitable for combined saw damage removal and texture formation. The texturization with inverted surface structures was obtained using wet chemistry process sequence at room temperatures without using a mask and lithography. Potential for an improvement of the standard screen — printing cells performance by incorporation of the new developed texturization method is demonstrated.

Immersion of silicon solar cells in an oxidation solution

The conventional wafer-type crystalline silicon solar cells have an approximately 80nm thick top SiNx coating which acts as an anti-reflection layer. The SiNx layer also passivates the silicon substrate surface to reduce free electron consumption very near the surface. Besides, it was known that the properties of the SiNx layer itself may, to some extent, affect the performance of the solar cell. The purpose of this study is to provide a low-temperature method that will help to modify the properties of the SiNx film, especially for low-temperature grown SiNx thin layer. This is achieved by immersing the finished silicon solar cells into an oxidation solution containing strong oxidant. Oxidation solution helps to oxidize the low-temperature grown SiNx as well as the weakly passivated silicon surface. By applying the techniques, the properties of the SiNx layer have been modified and an enhancement on cell performance was demonstrated. The results presented in this study show that the properties of the SiNx layer need to be taken with care, because they may affect the performance of the solar cells.