Silicon Heterojunction Solar Cells With Nanocrystalline Silicon Oxide Emitter: Insights Into Charge Carrier Transport
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We recently demonstrated how the short-circuit current density of an a-Si:H/c-Si heterojunction solar cell can be significantly improved to above 40 mA/cm 2 by replacing the standard a-Si:H(p) emitter by a silicon oxide emitter containing p-doped silicon nanocrystallites. While we could obtain a conversion efficiency of 20.3%, the cell suffered from a lower fill factor of 72.9%, compared with 77.0% for our standard process. In this paper, we address this issue both theoretically and experimentally. We found that a thin (~3 nm) highly doped nanocrystalline silicon layer on top of the emitter can greatly improve the fill factor. Using 1-D device simulation, we explain the prevalent loss mechanism, which originates mostly from poor tunnel recombination at the transparent conducting oxide/emitter interface rather than in the bulk of the emitter. We suspect that have their origin in the lower effective dopant concentration of the nanocrystalline silicon oxide emitter. From the model, implications for further developments can be derived.Keywords:
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We employed plasma enhanced chemical vapor deposition technique to fabricate nanocrystalline Si films at a low temperature of 250 degrees C by using SiCl4 and H2 as source gases. The evolution of microstructure of the films with deposition periods shows that nanocrystalline Si can be directly grown on amorphous substrate at the initial growth process, which is in contrast to the growth behavior observed in the SiH4/H2 system. Furthermore, it is interesting to find that the area density of nanocrystalline Si as well as grain size can be controlled by modulating the concentration of SiCl4. By decreasing the SiCl4 concentration, the area density of nanocrystalline Si can be enhanced up to 10(11) cm(-2), while the grain size is shown to decrease down to 10 nm. It is suggested that Cl plays an important role in the low-temperature growth of nanocrystalline Si.
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Two methods of pulsed DC magnetron sputtering deposition have been used to form high rate, hydrogen-free crystalline silicon layers. The first method is in situ crystalline silicon deposition. The second method is high rate amorphous silicon deposition followed by an anneal to induce crystallization. Over 20 μm thick crystalline silicon can be formed on wafers up to 200 mm round or 156 mm square. Two vacuum deposition systems were used for substrate cleaning and deposition. The crystallinity of silicon layers was analyzed by ellipsometry and Raman spectroscopy. Almost fully crystalline silicon is deposited in situ at table temperatures greater and equal to 650 °C. In situ crystalline silicon has been deposited at 40 nm/min and amorphous silicon can be deposited at over 400 nm/min subject to power density limitations for the silicon target. Up to 20 μm thick amorphous silicon deposited at room temperature is fully crystallized by annealing in vacuum on a 1000 °C table for 2 h. This work demonstrates that sputtering offers significant potential for depositing the absorber layer in silicon based photovoltaics.
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A new way of realizing crystalline silicon selective emitter by simultaneous formation of front side selective emitter as well as rear back surface field (BSF) layer in rear side diffusion step has been presented in this work. In this paper, we have demonstrated a novel technique to achieve a selective emitter having a highly diffused region with lower sheet resistance around 30–32 Ω sq−1 along with peak doping concentration 7.34 × 1019 atoms cm−3, junction depth around 0.97 μm and lightly diffused region with higher sheet resistance around 78–80 Ω sq−1 along with peak doping concentration 4.57 × 1019 atoms cm−3, junction depth around 0.64 μm. These results show that selective emitter has been formed in single diffusion process without any extra heat treatment and chemical etching process, thus this process becomes cost effective.
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In this work we study the electronic passivation of crystalline silicon surfaces with thin silicon films deposited by hot-wire chemical vapour deposition. Both intrinsic hydrogenated amorphous silicon and p-doped nanocrystalline silicon films were evaluated on p- and n-type float zone silicon wafers (1-10 /spl Omega//spl middot/cm). The effective surface recombination velocity was measured by the contactless quasi-steady-state photoconductance technique. Heterostructures consisting of a p-doped nanocrystalline silicon layer with a 10 nm thick intrinsic amorphous silicon buffer allowed effective surface recombination velocities of 120 and 170 cm/spl middot/s/sup -1/ on p- and n-type crystalline silicon. Current density-voltage characteristics of rectifying heterojunctions were also measured. These studies are of great interest to evaluate the possibility to obtain high efficiency heterojunction solar cells fully processed at low temperatures.
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Selective emitter in crystalline silicon solar cells improves the cell efficiency by reducing the recombination in the emitter region while maintaining low contact resistance to the front side electrodes. There are many approaches to realize selective emitter solar cells, some more complicated than the others, but all involve creating heavier doping in the region under electrodes. In this paper, we present the effect of selective emitter patterns, with or without heavy doping under busbars, on the solar cell performance. The results showed basically identical electrical characteristics for both types of patterns. Even though the selective emitter structure in this study was made with a printable dopant approach, the same results could apply to other selective emitter methods, including laser doping and ion implantation. This conclusion points to potentially significant savings in materials and/or processing time as heavy doping is needed only to cover the finger area but not the busbars.
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This article reports on an amorphous-crystalline silicon heterojunction photovoltaic cell concept wherein the heterojunction regions are laterally narrow and distributed amidst a backdrop of well-passivated crystalline silicon surface. The localized amorphous-crystalline silicon heterojunctions consisting of the laterally thin emitter and back-surface field regions are precisely aligned under the metal grid-lines and bus-bars while the remaining crystalline silicon surface is passivated using the recently proposed facile grown native oxide–plasma enhanced chemical vapour deposited silicon nitride passivation scheme. The proposed cell concept mitigates parasitic optical absorption losses by relegating amorphous silicon to beneath the shadowed metallized regions and by using optically transparent passivation layer. A photovoltaic conversion efficiency of 13.6% is obtained for an untextured proof-of-concept cell illuminated under AM 1.5 global spectrum; the specific cell performance parameters are VOC of 666 mV, JSC of 29.5 mA-cm−2, and fill-factor of 69.3%. Reduced parasitic absorption, predominantly in the shorter wavelength range, is confirmed with external quantum efficiency measurement.
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