Passivating contacts for homojunction solar cells using a-Si:H/c-Si hetero-interfaces

Crystalline silicon (c-Si) homojunction solar cells account for over 90% of the current photovoltaic market. However, further progress of this technology is limited by recombinative losses occurring at their metal-semiconductor contacts. The goal of this thesis is to develop passivating contacts to resolve this issue. The novel idea presented in this work is to insert an ultra-thin wide bandgap semiconductor-hydrogenated amorphous silicon (a-Si:H)-film underneath the metal to passivate the doped c-Si surface and suppress the recombination of minority charge carriers. Simultaneously, this layer should provide a contact to the metal allowing majority charge carrier transport. A transparent conductive oxide is additionally inserted between the a-Si:H layer and the metal to ensure efficient carrier collection. This concept is inspired by the silicon heterojunction solar cells, a technology characterized by extremely high open-circuit voltages. The development of these new passivating contacts requires two features: a homojunction, for charge separation, and a silicon heterojunction contact for improved passivation. In this thesis, we explicitly focus on large-area thin-film deposition technology for fabrication of our devices, guaranteeing the scalability of our findings. The main results of this thesis are then threefold. First, we show that, using low-temperature plasma enhanced chemical vapor deposition, a doped homo-epitaxial layer can be deposited to form the homojunction. Second, we develop passivating contacts and optimize them in silicon heterojunction solar cells. An in-depth analysis of the contact formation is provided, including a detailed investigation of the relevant interfaces in our proposed structure. Finally, combining these two technologies, we demonstrate a proof-of-concept for these passivating contacts. Highly doped phosphorus- and boron-doped c-Si surfaces are shown to be efficiently passivated by a-Si:H layers and a lower contact resistivity is obtained for our optimized passivating contacts on such doped surfaces compared to a heterojunction contact on lightly doped surfaces. We show that homojunction solar cells on diffused and ion-implanted wafers featuring such passivating contacts (called homo-hetero cells hereinafter) yield improved open-circuit voltages compared to conventional homojunction solar cells, due to reduction of recombination losses. Additionally, the temperature coefficient of such homo-hetero solar cells is lower. With these advantages, the homo-hetero solar cells outperform homojunction solar cells when operating at a cell temperature above 60 °C. This work contributes to the research and development of high-efficiency silicon solar cells by providing new insights on the properties of contact formation and a novel contact-type

Very fast light-induced degradation of a-Si:H/c-Si(100) interfaces

Light-induced degradation (LID) of crystalline silicon (c-Si) surfaces passivated with hydrogenated amorphous silicon (a-Si:H) is investigated. The initial passivation decays on polished c-Si(100) surfaces on a time scale much faster than usually associated with bulk a-Si:H LID. This phenomenon is absent for the a-Si:H/c-Si(111) interface. We attribute these differences to the allowed reconstructions on the respective surfaces. This may point to a link between the presence of so-called "fast" states and (internal) surface reconstruction in bulk a-Si:H

Damage at hydrogenated amorphous/crystalline silicon interfaces by indium tin oxide overlayer sputtering

Damage of the hydrogenated amorphous/crystalline silicon interface passivation during transparent conductive oxide sputtering is reported. This occurs in the fabrication process of silicon heterojunction solar cells. We observe that this damage is at least partially caused by luminescence of the sputter plasma. Following low-temperature annealing, the electronic interface properties are recovered. However, the silicon-hydrogen configuration of the amorphous silicon film is permanently changed, as observed from infra-red absorbance spectra. In silicon heterojunction solar cells, although the as-deposited film’s microstructure cannot be restored after sputtering, no significant losses are observed in their open-circuit voltag

Amorphous silicon passivated contacts for diffused junction silicon solar cells

Carrier recombination at the metal contacts is a major obstacle in the development of high-performance crystalline silicon homojunction solar cells. To address this issue, we insert thin intrinsic hydrogenated amorphous silicon [a-Si: H(i)] passivating films between the dopant-diffused silicon surface and aluminum contacts. We find that with increasing a-Si: H(i) interlayer thickness (from 0 to 16nm) the recombination loss at metal-contacted phosphorus (n(+)) and boron (p(+)) diffused surfaces decreases by factors of similar to 25 and similar to 10, respectively. Conversely, the contact resistivity increases in both cases before saturating to still acceptable values of similar to 50 m Omega cm(2) for n(+) and similar to 100 m Omega cm(2) for p(+) surfaces. Carrier transport towards the contacts likely occurs by a combination of carrier tunneling and aluminum spiking through the a-Si: H(i) layer, as supported by scanning transmission electron microscopy-energy dispersive x-ray maps. We explain the superior contact selectivity obtained on n(+) surfaces by more favorable band offsets and capture cross section ratios of recombination centers at the c-Si/a-Si: H(i) interface. (C) 2014 AIP Publishing LLC

Silicon heterojunction solar cells on n- and p-type wafers with efficiencies above 20%

A systematic comparison of front- and rear-emitter silicon heterojunction solar cells produced on nand p-type wafers was performed, in order to investigate their potential and limitations for high efficiencies. Cells on p-type wafers suffer from reduced minority carrier lifetime in the low-carrier-injection range, mainly due to the asymmetry in interface defect capture cross sections. This leads to slightly lower fill factors than for n-type cells. However, these losses can be minimized by using high-quality passivation layers. High Vocs were obtained on both types of FZ wafers: up to 735 mV on n- and 726 mV on p-type. The best Voc measured on CZ p-type wafers was only 692 mV, whereas it reached 732 mV on CZ n-type. The highest aperture-area certified efficiencies obtained on 4 cm2 cells were 22.14% (Voc=727 mV, FF=78.4%) and 21.38% (Voc=722 mV, FF=77.1%) on n- and p-type FZ wafers, respectively, demonstrating that heterojunction schemes can perform almost as well on high-quality p-type as on ntype wafers. To our knowledge, this is the highest efficiency for a full silicon heterojunction solar cell on a p-type wafer, and the highest Voc on any p-type crystalline silicon device with reasonable FF

21% efficiency silicon heterojunction solar cells produced with very high frequency PECVD

Silicon heterojunction solar cells have high open-circuit voltages thanks to excellent passivation of the wafer surfaces by thin intrinsic amorphous silicon (a-Si:H) layers deposited by plasma-enhanced chemical vapor deposition (PECVD). By using in-situ plasma diagnostics and ex-situ film characterization, we show that the best a-Si:H films for passivation are produced from deposition regimes close to the amorphous-to-crystalline transition. Based upon this finding, layers deposited in a large-area very high frequency (40.68 MHz) PECVD reactor were optimized for heterojunction solar cells. 4 cm2 solar cells were produced with fully industry-compatible processes, yielding open-circuit voltages up to 725 mV and aperture area efficiencies up to 21%

Atomic-Layer-Deposited Transparent Electrodes for Silicon Heterojunction Solar Cells

We examine damage-free transparent-electrode deposition to fabricate high-efficiency amorphous silicon/crystalline silicon heterojunction solar cells. Such solar cells usually feature sputtered transparent electrodes, the deposition of which may damage the layers underneath. Using atomic layer deposition, we insert thin protective films between the amorphous silicon layers and sputtered contacts and investigate their effect on device operation. We find that a 20-nm-thick protective layer suffices to preserve, unchanged, the amorphous silicon layers beneath. Insertion of such protective atomic-layer-deposited layers yields slightly higher internal voltages at low carrier injection levels. However, we identify the presence of a silicon oxide layer, formed during processing, between the amorphous silicon and the atomic-layer-deposited transparent electrode that acts as a barrier, impeding hole and electron collection

Sputtered hydrogenated amorphous silicon for silicon heterojunction solar cell fabrication

This work shows that RF sputter-deposited hydrogenated amorphous silicon (a-Si:H) films are very effective in passivating silicon surfaces. We have previously found that sputter-deposited 45 nm thick intrinsic a-Si:H provides outstanding surface passivation on n-type silicon, similar to that achieved by ‘classic’ plasma enhanced chemical vapour deposition [1]. In this paper, we show that p-type silicon surfaces can be well passivated as well, achieving effective carrier lifetimes of 1.1 ms for a 1 Ω∙cm ptype wafer, compared to 4.5 ms for a 1.5 Ω∙cm n-type sample. Next, on n-type textured surfaces reasonable passivation is also achieved. Post-deposition annealing of our samples shows that sputtered a-Si:H films can perform similarly to PECVD deposited films in terms of thermal stability. Importantly, with stacks of intrinsic and doped (n or p) amorphous silicon effective carrier lifetimes of 1.9 ms and 1.6 ms on 1.5 Ω∙cm n-type wafers were obtained for i/n+ and i/p+ stacks respectively. These results underline the promise of sputter-deposited a-Si:H as an attractive alternative for heterojunction solar cell fabrication. However, dark conductivity measurements show that sputter-deposited doped a-Si:H films feature a relatively low conductivity, so far. We speculate that this may be caused by differences in microstructure compared to PECVD a-Si:H films, as suggested from the extracted optical band gap values for the respective films.This program has been supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). The authors from EPFL thank the Axpo Naturstrom Fonds, the European Commission (FP7 project Hercules), the EuroTech Universities Alliance and the Swiss Commission for Technology and Innovation for their financial support