Photoconductive decay lifetime and Suns-Voc diagnostics of efficient heterojunction solar cells
Minority carrier lifetime and Suns-Voc measurements are well-accepted methods for characterization of solar cell devices. We use these methods, with an instrument from Sinton Consulting, as we fabricate and optimize state-of-the-art all hot-wire chemical vapor deposition (HWCVD) silicon heterojunction (SHJ) devices. For double-sided SHJ devices, lifetime measurements were performed immediately after hydrogenated amorphous silicon (a-Si:H) deposition of the front emitter and back base contacts on a Silicon wafer, and also after indium tin oxide (ITO) deposition of transparent conducting oxide contacts. We report results of minority carrier lifetime measurements for double-sided p-type Si heterojunction devices and compare Suns-Voc results to Light I–V measurements on 1-cm2 solar cell devices measured on an AM1.5 calibrated XT-10 solar simulator.
Determination of Surface Recombination Velocities for Thermal Oxide and Amorphous Silicon on Float Zone Silicon
Introduction Recombination at the surface of a silicon solar cell is an important factor in determining the cell efficiency. The descriptive parameter is surface recombination velocity, s. Generally, s increases as the doping of the surface increases, just as the lifetime of carriers within the crystal (τ) generally decreases as the bulk doping increases. Thus, in a comparative study of candidate surface treatments, it is important to choose a silicon surface doping level which is typical of common cell designs. Furthermore, because standard measurement techniques cannot distinguish between recombination at the crystal surface and recombination within the crystal bulk, it is desirable to choose a silicon crystal with high bulk lifetime in order to improve the sensitivity of the technique to surface recombination. In this study, 100 mm diameter float zone (FZ) silicon wafers, double side polished, with a (100) surface and 300 μm thickness were chosen: p-type (boron-doped) nominally 3 Ω-cm and n-type (phosphorus-doped) nominally 1.2 Ω-cm. The primary comparison was made between surfaces passivated with a thermally-grown oxide and those passivated with amorphous silicon deposited by the hot wire method. In addition, the degree of surface passivation associated with the thin chemical oxide created by RCA cleaning was measured, as was that associated with a temporary iodine-methanol treatment.
Obtaining a higher Voc in HIT cells
We have achieved a very high conversion efficiency of 21·5% in HIT cells with a size of 100·3 cm2. One of the most striking features of the HIT cell is its high open‐circuit voltage Voc, in excess of 710 mV. This is due to the excellent surface passivation at the a‐Si/c‐Si heterointerface realized by Sanyo's successful technologies for fabricating high‐quality a‐Si films and solar cells with low plasma damage processes. We have studied ways to treat the surface to produce a good interface throughout our fabrication processes. We have also investigated the deposition conditions of a‐Si layers for optimizing the barrier height for the minority carriers in the heterojunction. Our approach for obtaining HIT cells with a high Voc is reviewed here. Copyright © 2005 John Wiley & Sons, Ltd.