High-Efficiency Silicon Heterojunction Solar Cells by HWCVD

We report progresses in the development of silicon heterojunction (SHJ) solar cells by hot-wire chemical vapor deposition (HWCVD). A confirmed 18.2% efficiency on a p-type textured wafer has been achieved based on improvements in surface passivation by a-Si:H emitter and back contact as well as in fill factor. The primary objective of high open-circuit voltage (V_oc) is achieved by front a-Si:H/c-Si heterojunction optimization, by replacing a conventional Al-alloyed or P-diffused back-surface field with a back c-Si/a-Si:H heterojunction, and by maintaining excellent surface passivation on textured silicon wafers. We first obtain a V_oc of 652 mV with a front a-Si:H(n/i) heterojunction emitter on p-type solar cells with an Al back-surface-field (BSF) contact. The high-temperature AI-BSF is then successfully replaced by low-temperature HWCVD-deposited a-Si:H(i/p) layers as the back contact. Lifetime measurement shows the surface recombination velocity (SRV) is reduced to -15 cm/sec. A higher V_oc of 676 mV is obtained with an a-Si:H(n/i) front-emitter and a-Si:H(i/p) back-contact double-heterojunction SHJ solar cell structure, indicating superior back-surface passivation of the textured p-wafer. On n-type silicon wafers, we use an a-Si:H(p/i) front emitter and an a-Si:H(i/n) back contact, to achieve a V _oc of 711 mV, the highest voltage obtained by the HWCVD technique so far. Good fill factors are also obtained using the amorphous-phase materials as the back contacts. S-shaped I-V curves are observed if doping cross-contamination are present among different a-Si:H layers or doping level is not enough in the TCO-contacting p-type a-Si:H layer.