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An integration process was developed for the fabrication of rear passivated point contact solar cells achieving 19.36% conversion efficiency by using 156×156mm, pseudo square, p-type single crystalline silicon wafers. This is a significant improvement when compared to unpassivated, full area aluminum back surface field solar cells, which exhibit only 18.64% conversion efficiency on the same wafer type. The rear surface was passivated with a Al 2 O 3 layer and a SiN X capping layer. The thicknesses of individual films were optimized to obtain maximum minority carrier lifetimes. The rear surface contact pattern was created by laser ablation and the contact geometry was optimized to obtain voids free contact filling resulting in a uniform back surface field. Internal quantum efficiency and reflectance measurement show significant improvement in rear passivated cells in the infrared wavelength region in comparison to reference cells. The rear surface internal reflectivity for the passivated cell was 93% while that for the reference cell was only about 73%. The rear surface recombination velocity for the rear passivated cell was about 52 cm/s while that for the reference cell was about 300 cm/s. The efficiency gain in rear passivated cells over the reference cells is mainly due to improved short circuit current and open circuit voltage. However, rear passivated solar cells show lower fill factors due to increased series resistance.

This work reports on the integration process of rear point contact solar cells with reduced recombination and better light trapping than the conventional cells. Al 2 O 3 /SiN x passivation stacks were used to ensure the backside passivation and the effective lifetime of minority carrier is found to be >100 µs (estimated surface recombination velocity ∼50 cm/s) on solar p-type Cz wafers. The estimated fixed charge associated with the Al 2 O 3 is in the range of −5e12 C/cm2. Laser ablation of Al 2 O 3 /SiN x stack and aluminum alloying are studied in detail to understand the process window for clean ablation and back surface field. It is found that laser energy plays an important role cleanly ablating the Al 2 O 3 /SiN x passivation stack. The solar cell is fabricated using standard processes based on screen-printed aluminum paste onto laser ablated passivation layer consisting of Al 2 O 3 /SiN x . Aluminum alloying is dependent on the firing profile and amount of Al melted into Si. Back point contact cells show improvements in J sc by 1 mA/cm2 and V oc by 10 mV due to better response in infrared spectrum. The best conversion efficiency of back point contact solar cells fabricated with standard industrial emitter and backside passivation is 19.35%.

A novel optical metrology technique has been developed to study textured silicon wafers used to manufacture solar cells. This high efficiency optical design to maximize the signal from surfaces with reflectivity well below 1% was developed. Pyramid dimensions were measured on as sawed p-type Czochralski wafers having a bulk resistivity of 1–5 Ohm-cm. These wafers were subjected to a single step texturization process by using a non-alcoholic chemical etching formulation. Results were compared with SEM imaging. Reflectivity tests and pyramid height measurements were used to study the efficiency of the texturing processes. In a related study [2] it was found that 20μm of material removal was required to attain minimum surface reflectivity. Further removal of material affected pyramid dimensions, but did not improve surface reflectivity.

Optical analysis is performed for mono-crystalline silicon wafers with and without the texture and with and without the POCl doping, the passivating anti-reflective nitride film on front surface, and the screen printed aluminum conductor on the back surface. Reflectance is measured in the wavelength range from 300 nm to 1200 nm. Modeling of the light reflectance, absorbance, and transmittance is done using ray-tracing technique for the regular and the random texture patterns. Good agreement of measured and modeled data is obtained for the sub — 1 micron wavelengths by using standard material optical properties. However, the infrared light above the 1 micron wavelength requires accounting for several mono-layers thick native oxide present on silicon surfaces and adjusting the optical properties of specific nitride and aluminum films used in the solar cell manufacturing. It is found that the random texture exhibits 15% to 20% better light capture than the regular texture. Theoretical analysis provides plausible explanation of this effect and suggests a way to further improve optical performance of the textured surfaces. The optical modeling methodology can be used to find the optimum combination of texture and passivating/contact films for different solar cell designs.

Sawed p-type Czochralski wafers with a bulk resistivity of 1–5ωcm were subjected to single step texturization process by using a non-alcoholic chemical etching formulation. The minimum amount of silicon removal required for achieving optimum solar cell efficiencies was estimated by measuring cell efficiency as a function of silicon loss. The surface damage layer of as cut wafers was found to be approximately 6μm deep as indicated by the minority carrier lifetime measurements. However for attaining minimum surface reflectivity it was required to remove at least 20μm of silicon from the initial wafer. Further removal of silicon offered no additional improvements in surface reflectivity or the solar cell efficiency.