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Based on potential high efficiency, low thermal budget and compatibility with very thin Si wafers, many research groups and industries are considering interdigitated all back contact silicon heterojunction (IBC-SHJ) solar cell technology. Compared to front junction silicon heterojunction (FJ-SHJ) solar cells, IBC-SHJ cells also have the unique advantages for simpler module integration. However, the IBC-SHJ solar cells to date suffer with low fill factors. Both simulation and experimental results have been conducted to understand the cause of the low FF.In this paper, the effects of processing conditions are discussed by comparing FJ-SHJ and IBC-SHJ solar cells. The fabrication of IBC-SHJ cells requires several photolithography steps to form the interdigitated back structure, whereas the FJ-SHJ requires no photolithography steps. The effect of processing temperatures, deposition sequence and photolithography processing are evaluated by using the FJ-SHJ as a test bed and base-line standard.Contamination and/or introduction of defects at intrinsic/doped layer interfaces resulting from photolithography steps is identified as one of the major contributors to reduced performance of IBC-SHJ devices fabricated using a low temperature process.

The interdigitated back contact silicon heterojunction (IBC-SHJ) solar cell requires a low temperature front surface passivation/anti-reflection structure. Conventional silicon surface passivation using SiO2 or a-SiNx is performed at temperature higher than 400°C, which is not suitable for the IBC-SHJ cell. In this paper, we propose a PECVD a-Si:H/a-SiNx:H/a-SiC:H stack structure to passivate the front surface of crystalline silicon at low temperature. The optical properties and passivation quality of this structure are characterized and solar cells using this structure are fabricated. With 2 nm a-Si:H layer, the stack structure exhibits stable passivation with effective minority carrier lifetime higher than 2 ms, and compatible with IBC-SHJ solar cell processing. A critical advantage of this structure is that the SiC allows it to be HF resistant, thus it can be deposited as the first step in the process. This protects the a-Si/c-Si interface and maintains a low surface recombination velocity.

Interdigitated back contact silicon hetero-junction (IBC-SHJ) solar cells using a-Si emitter and contact layers show significant potential advantages over standard hetero-junction devices: higher short-circuit current (Jsc) since there is no grid shading and higher open-circuit voltage (Voc) due to better surface passivation. However, they often suffer from low fill factor (FF). Using twodimensional simulations to model IBC-SHJ devices on FZ n-Si, we found that the FF was nearly independent of the defect concentrations in contact and passivating i-layers but strongly dependent on the defects in emitter and the band gap in the rear i-layer. Voc and Jsc were nearly independent of defects in either doped layer. In a-Si doped layers it is well known that the number of defects increase with doping. We find that the FF is sensitive to either mid-gap or band tail states and that S-shaped JV curves responsible for low FF can be eliminated by a decrease in p-layer mid-gap or band tail defect levels, or by decreasing the rear i-layer's band gap. The insensitivity of FF to defects in the n-layer or in the i-layer suggests the FF is dominated by minority carrier injection/collection from the p-type emitter layer. The dependence of FF on the rear i-layer band gap suggests that increasing the offset in the valence band impedes minority carrier collection.

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