Assessment of Bulk and Interface Quality for Liquid Phase Crystallized Silicon on Glass

Abstract

This work reports on the electrical quality of liquid phase crystallized silicon (LPC-Si) on glass for thin-film solar cell applications. Spatially resolved methods such as light beam induced current (LBIC), microwave photoconductance decay (MWPCD) mapping and electron backscatter diffraction (EBSD) were used to access the overall material quality, intra-grain quality, surface passivation and grain boundary (GB) properties. LBIC line scans across GBs were fitted with a model to characterize the recombination behavior of GBs. According to MWPCD measurement, intra-grain bulk carrier lifetimes were estimated to be larger than 4.5 µs for n-type LPC-Si with a doping concentration in the order of 10 16 cm -3 . Low-angle GBs were found to be strongly recombination active and identified as highly defect-rich regions which spatially extend over a range of 40-60 µm and show a diffusion length of 0.4 µm. Based on absorber quality characterization, the influence of intra-grain quality, heterojunction interface and GBs/dislocations on the cell performance were separately clarified based on 2D-device simulation and a diode model. High back surface recombination velocities of several 10 5 cm/s are needed to get the best match between simulated and measured open circuit voltage (Voc), indicating back surface passivation problem. The results showed that Voc losses are not only due to poor back surface passivation but also due to crystal defects such as GBs and dislocation.

Toward High Solar Cell Efficiency with Low Material Usage: 15% Efficiency with 14 μm Polycrystalline Silicon on Glass

“…LPC‐Si films have grain sizes on the centimeter scale in the scanning direction and millimeter scale in the perpendicular direction. V OC values of up to 661 mV in 0.6 cm 2 cells have been reported, which are comparable with the 674 mV constituting the current record efficiency shown on multicrystalline silicon wafers . This work showcases a new in‐house record efficiency of 15.1% and potential for ≥16% in the near future with an optimization of series resistance.…”

“…Growth of sputtered ITO was imperfect with cracks/voids visible in some valleys between pyramids (Figure b). Simulations corroborated by experimental data have shown that gains of 40 mV V OC and more than 1.2 mA cm −2 J SC can be expected by improving surface passivation at the electron and hole contacts …”

“…An average V OC of 638 mV and a maximum of 658 mV have never been achieved previously for a doping concentration ≤12 × 10 16 cm −3 . This indicates a lowering of V OC deficit due to recombination at the a‐Si:H contacting interface . J SC was observed to be up to 33.1 mA cm −2 which is also higher than any previously observed value for this technology .…”

“…As measured by TEM, the thickness of a‐Si:H(i/p+) layers was 12 nm, whereas the expected thickness was 22 nm. Simulations corroborated by experimental data have also shown that gains of 40 mV V OC and more than 1.2 mA cm −2 J SC can be expected by reduced recombination at the electron and hole contacts . Test solar cell structures were made to compare different deposition times and hydrogen plasma treatment.…”

“…In the test structures, the absorber contact is not passivated for simplified cell processing and is expected to have a minor effect on Suns‐ V OC and pFF. The effective diffusion length of minority carriers have been previously calculated to be up to 79 μm in passivated areas but less than 20 μm in poorly passivated areas . The isolation gap between the hole and electron contact can be assumed to be around 60 μm .…”