Vichai Meemongkolkiat

Factors Limiting the Formation of Uniform and Thick Aluminum–Back-Surface Field and Its Potential

Theoretical calculations reveal that the quality of an aluminum-back-surface field (BSF) in a silicon solar cell can be improved by either increasing the thickness of the deposited aluminum (Al), peak alloying temperature, or both. However, this study shows that there is a critical temperature for a given screen-printed Al thickness, above which the BSF quality begins to degrade because of nonuniformity triggered by the agglomeration of Al-Si melt in combination with the bandgap narrowing resulting from the high doping effect in the agglomerated regions. It is found that this critical temperature decreases with the increase in the thickness of the deposited Al layer and, therefore, limits the quality and thickness of the Al-BSF that can be achieved before degradation sets in. This nonuniformity of Al-BSF is observed in the form of scattered Al bumps with thick and thin BSF regions. A combination of experimental results and model calculations is used to provide improved understanding and guidelines for choosing the optimal combination of Al thickness and alloying temperature.

Boron Diffusion with Boric Acid for High Efficiency Silicon Solar Cells

A boron diffusion process using boric acid as a low cost, nontoxic spin-on source is introduced. Using dilute solutions of boric acid, sheet resistances ranging from 20 to 200 Ω/□ were achieved, along with saturation current densities as low as 85 fA/cm 2 . These results indicate that boric acid is a suitable source for forming both p + emitters and back surface fields for high efficiency n- and p-type solar cells. The degradation of the minority carrier bulk lifetime, which is a common efficiency-limiting characteristic of low cost boron sources, was also minimized through the use of a high purity boric acid source. The ability to achieve low sheet resistances, high bulk lifetimes and low saturation current densities with boric acid were exploited to achieve a 19.7% efficient screen printed solar cell exhibiting a bulk lifetime >400 μs.

Investigation of Modified Screen-Printing Al Pastes for Local Back Surface Field Formation

This paper reports on a low-cost screen-printing process to form a self-aligned local back surface field (LBSF) through dielectric rear surface passivation. The process involved formation of local openings through a dielectric (SiNx or stacked SiO2/SiNx) prior to full area Al screen-printing and a rapid firing. Conventional Al paste with glass frit degraded the SiNx surface passivation quality because of glass frit induced pinholes and etching of SiNx layer, and led to very thin LBSF regions. The same process with a fritless Al paste maintained the passivation quality of the SiNx, but did not provide an acceptably thick and uniform LBSF. Al pastes containing appropriate additives gave better LBSF because of the formation of a thicker and more uniform Al-BSF region. However, they exhibited somewhat lower internal back surface reflectance (<90%) compared to conventional Al paste on SiNx. More insight on these competing effects is provided by fabrication and analysis of complete solar cells

Effect of material inhomogeneity on the open-circuit voltage of string ribbon Si solar cells

The effect of material inhomogeneity on open-circuit voltage (V/sub OC/) of string ribbon Si solar cells is investigated by a combination of experimental results and a simple analytical model. Light beam-induced current (LBIC) measurements showed that a cell with no detectable defective region gave an efficiency of 15.9% with a V/sub OC/ of 616 mV. However, a neighboring cell with highly defective regions covering 38% of its area, as determined by LBIC measurements, gave an efficiency of 14.1% with V/sub OC/ of 578 mV. Another cell with 19% highly defective regions gave an efficiency of 15.0% with V/sub OC/ of 592 mV. A simple and approximate analytical model was developed to quantify the loss in V/sub OC/ on the basis of recombination intensity and the area fraction of defective regions. This model showed that the majority of the loss in V/sub OC/ is associated with the most defective region, even if its area fraction is relatively small.

2D-Modeling and Development of Interdigitated Back Contact Solar Cells on Low-Cost Substrates

Two-dimensional numerical simulations were performed to derive design rules for low-cost, high-efficiency interdigitated back contact (IBC) solar cells on a low-cost substrate. The IBC solar cells were designed to be fabricated using either the conventional screen printing or photolithography metallization processes. Bulk lifetime, bulk resistivity, contact spacing (pitch), contact opening width, recombination in the gap between the p+ BSF and n+ emitter, and the ratio of emitter width to pitch have been used as key variables in the simulations. It is found that short circuit current density (Jsc) is not only a strong function of the bulk lifetime but also the emitter coverage of the rear surface. Fill factor (FF) decreases as the emitter coverage increases because the majority carriers need to travel a longer distance through the substrate for longer emitter width. The simulated IBC results were compared with those for conventional screen printed solar cells. It was found that the IBC solar cell outperforms the screen printed (SP) solar cell when the bulk lifetime is above 50 mus due to higher Voc and Jsc , which suggests that higher performance can be realized on low-cost substrates with the IBC structure

3D-modeling of a back point contact solar cell structure with a selective emitter

Three dimensional numerical simulations were performed to investigate a novel high efficiency back contact solar cell design with a selective emitter. The effect of several physical parameters (bulk lifetime, substrate doping, emitter fraction and surface recombination velocity in the gap between the emitter and BSF) on solar cell performance is explored using the SENTAURUS DEVICE™ program (formerly DESSIS™). It is found that efficiencies in excess of 22 and 20.8 percent can be achieved on p and n type substrates respectively with a bulk lifetime of 300 microseconds.

Rapid Thermal Processing of High Efficiency N-Type Silicon Solar Cells with Al Back Junction

In this paper we report on the design, fabrication and modeling of 49 cm2, 200-mum thick, 1-5 Omega-cm, n- and p-type lang111rang and lang100rang screen-printed silicon solar cells. A simple process involving RTP front surface phosphorus diffusion, low frequency PECVD silicon nitride deposition, screen-printing of Al metal and Ag front grid followed by co-firing of front and back contacts produced cell efficiencies of 15.4% on n-type lang111rang Si, 15.1% on n-type lang100rang Si, 15.8% on p-type lang111rang Si and 16.1% on p-type lang100rang Si. Open circuit voltage was comparable for n and p type cells and was also independent of wafer orientation. High fill factor values (0.771-0.783) for all the devices ruled out appreciable shunting which has been a problem for the development of co-fired n-type lang100rang silicon solar cells with Al back junction. Model calculations were performed using PC1D to support the experimental results and provide guidelines for achieving >17% n-type silicon solar cells by rapid firing of Al back junction

Fabrication of 20 % efficient cells using spin-on based simultaneous diffusion and dielectric anneal

Low-cost high efficiency solar cells are the key to achieving grid parity with photovoltaic devices. This paper details the fabrication, characterization and analysis of 4 cm2 screen printed cells with efficiency over 20% achieved using a local back surface field cell structure in conjunction with a 75 ohm/sq emitter and efficient back surface reflector. A streamlined process sequence involving a single high temperature step for simultaneous formation of emitter and rear passivation was used. The combination of LBSF and improved rear reflector resulted in a peak efficiency of 20.1% with JSC of 39.4 mA/cm2 and VOC of 652 mV. Detailed characterization and modeling involving IQE and escape reflectance measurements revealed that the increase in VOC and JSC was the result of increased BSR from 67% to 93% and reduced BSRV from 325 cm/s to 125 cm/s. Improved rear passivation was found to be an effective method for controlling charge-induced inversion or parasitic shunting at the rear surface. Future experiments and further optimization are expected to result in efficiencies of over 20% on thin wafers using a similar cell structure.

Production viability of gallium doped mono-crystalline solar cells

Results of efforts at Shell Solar to implement the use of gallium dopant as a commercial solar cell production process are presented. Both small area cell results and production related activities and results are discussed. Many researchers have demonstrated that gallium effectively eliminates light induced degradation (LID) of the bulk lifetime, but less effort has been dedicated to implement gallium dopant into a commercial production process. Shell Solar has worked in this direction and expanded past research activities to demonstrate that the full range of resistivity values produced from a gallium-doped crystal can be used to successfully fabricate high efficiency cells. In addition, Shell has produced significant numbers of gallium-doped cells in their production facility and characterized process results from crystal growth to module build. This paper discusses additional subjects essential to production viability, such as gallium metal availability, silicon feedstock availability and management specific to a gallium process and overall cost effectiveness.

19% efficient screen- printed cells using a passivated transparent boron back surface field

Boron back surface field is a promising replacement for the industry standard screen-printed Aluminum. However, the use of boron back surface fields is largely confined to laboratory scale solar cells. In order to increase its industrial applicability, we present a method for achieving a high quality boron back surface field using a cheap and safe boron source and short diffusion time. Metal contacts are fabricated using screen-printing, and degradation of rear passivation after contact firing is minimized through optimized screen-printing of local (point) contacts on the rear. On p-type silicon wafers, this process technology has been utilized to achieve, on 4 cm2 cells, efficiency of up to 19.1% with open-circuit voltage of 650 mV.