Bénédicte Demaurex

Improved amorphous/crystalline silicon interface passivation by hydrogen plasma treatment

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. We show a dramatic improvement in passivation when H2 plasma treatments are used during film deposition. Although the bulk of the a-Si:H layers is slightly more disordered after H2 treatment, the hydrogenation of the wafer/film interface is nevertheless improved with as-deposited layers. Employing H2 treatments, 4 cm2 heterojunction solar cells were produced with industry-compatible processes, yielding open-circuit voltages up to 725 mV and aperture area efficiencies up to 21%.

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 films microstructure cannot be restored after sputtering, no significant losses are observed in their open-circuit voltage.

Amorphous/crystalline silicon interface defects induced by hydrogen plasma treatments

Excellent amorphous/crystalline silicon interface passivation is of extreme importance for high-efficiency silicon heterojunction solar cells. This can be obtained by inserting hydrogen-plasma treatments during deposition of the amorphous silicon passivation layers. Prolonged hydrogen-plasmas lead to film etching. We report on the defect creation induced by such treatments: A severe drop in interface-passivation quality is observed when films are etched to a thickness of less than 8 nm. Detailed characterization shows that this decay is due to persistent defects created at the crystalline silicon surface. Pristine interfaces are preserved when the post-etching film thickness exceeds 8 nm, yielding high quality interface passivation.

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 16 nm) the recombination loss at metal-contacted phosphorus (n+) and boron (p+) diffused surfaces decreases by factors of ∼25 and ∼10, respectively. Conversely, the contact resistivity increases in both cases before saturating to still acceptable values of ∼ 50 mΩ cm2 for n+ and ∼100 mΩ cm2 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.

Field-effect passivation on silicon nanowire solar cells

Surface recombination represents a handicap for high-efficiency solar cells. This is especially important for nanowire array solar cells, where the surface-to-volume ratio is greatly enhanced. Here, the effect of different passivation materials on the effective recombination and on the device performance is experimentally analyzed. Our solar cells are large area top-down axial n-p junction silicon nanowires fabricated by means of Near-Field Phase-Shift Lithography (NF-PSL). We report an efficiency of 9.9% for the best cell, passivated with a SiO2/SiNx stack. The impact of the presence of a surface fixed charge density at the silicon/oxide interface is studied.

Amorphous/Crystalline Silicon Interface Passivation: Ambient-Temperature Dependence and Implications for Solar Cell Performance

Silicon heterojunction (SHJ) solar cells feature amorphous silicon passivation films, which enable very high voltages. We report how such passivation increases with operating temperature for amorphous silicon stacks involving doped layers and decreases for intrinsic-layer-only passivation. We discuss the implications of this phenomenon on the solar cells temperature coefficient, which represents an important figure-of-merit for the energy yield of devices deployed in the field. We show evidence that both open-circuit voltage (Voc) and fill factor (FF) are affected by these variations in passivation and quantify these temperature-mediated effects, compared with those expected from standard diode equations. We confirm that devices with high Voc values at 25°C show better high-temperature performance. However, we also argue that the precise device architecture, such as the presence of charge-transport barriers, may affect the temperature-dependent device performance as well.

Amorphous silicon enhanced metal-insulator-semiconductor contacts for silicon solar cells

The authors from the ANU acknowledge financial support by The Australian Solar Institute/Australian Renewable Energy Agency as well as access to equipment at the Australian National Fabrication Facility. 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.

Is light-induced degradation of a-Si:H/c-Si interfaces reversible?

Thin hydrogenated amorphous silicon (a-Si:H) films deposited on crystalline silicon (c-Si) surfaces are sensitive probes for the bulk electronic properties of a-Si:H. Here, we use such samples during repeated low-temperature annealing and visible-light soaking to investigate the long-term stability of a-Si:H films. We observe that during annealing the electronic improvement of the interfaces follows stretched exponentials as long as hydrogen evolution in the films can be detected. Once such evolution is no longer observed, the electronic improvement occurs much faster. Based on these findings, we discuss how the reversibility of light-induced defects depends on (the lack of observable) hydrogen evolution.

Low-temperature plasma-deposited silicon epitaxial films: Growth and properties

Low-temperature (≤200 °C) epitaxial growth yields precise thickness, doping, and thermal-budget control, which enables advanced-design semiconductor devices. In this paper, we use plasma-enhanced chemical vapor deposition to grow homo-epitaxial layers and study the different growth modes on crystalline silicon substrates. In particular, we determine the conditions leading to epitaxial growth in light of a model that depends only on the silane concentration in the plasma and the mean free path length of surface adatoms. For such growth, we show that the presence of a persistent defective interface layer between the crystalline silicon substrate and the epitaxial layer stems not only from the growth conditions but also from unintentional contamination of the reactor. Based on our findings, we determine the plasma conditions to grow high-quality bulk epitaxial films and propose a two-step growth process to obtain device-grade material.

Practical silicon deposition rules derived from silane monitoring during plasma-enhanced chemical vapor deposition

We clarify the difference between the SiH4 consumption efficiency η and the SiH4 depletion fraction D, as measured in the pumping line and the actual reactor of an industrial plasma-enhanced chemical vapor deposition system. In the absence of significant polysilane and powder formation, η is proportional to the film growth rate. Above a certain powder formation threshold, any additional amount of SiH4 consumed translates into increased powder formation rather than into a faster growing Si film. In order to discuss a zero-dimensional analytical model and a two-dimensional numerical model, we measure η as a function of the radio frequency (RF) power density coupled into the plasma, the total gas flow rate, the input SiH4 concentration, and the reactor pressure. The adjunction of a small trimethylboron flow rate increases η and reduces the formation of powder, while the adjunction of a small disilane flow rate decreases η and favors the formation of powder. Unlike η, D is a location-dependent quantity. It is...