Christian Monachon

Modification of textured silicon wafer surface morphology for fabrication of heterojunction solar cell with open circuit voltage over 700 mV

Crystalline silicon wafer (c-Si) can be extremely well passivated by plasma enhanced chemical vapor deposited (PECVD) amorphous silicon (a-Si:H) films. As a result, on flat substrates, solar cells with very high open circuit voltage are readily obtained. On textured substrates however the passivation is more cumbersome, likely due to the presence of localized recombinative paths situated at the pyramid valleys. Here, we show that this issue may be resolved by selecting a silicon substrate morphology featuring large pyramids. Chemical post-texturization treatments can further reduce the surface recombination velocity. This sequence has allowed us to fabricate solar cells with open circuit voltage over 700 mV, demonstrating also on device level the effect of pyramid density and surface micro-roughness on the surface passivation quality.

Qualitative link between work of adhesion and thermal conductance of metal/diamond interfaces

We report Time-Domain ThermoReflectance experiments measuring the Thermal Boundary Conductance (TBC) of interfaces between diamond and metal surfaces, based on samples consisting of [111]-oriented diamond substrates with hydrogen or with sp2 carbon surface terminations created using plasma treatments. In a concurrent theoretical study, we calculate the work of adhesion between Ni, Cu, and diamond interfaces with (111) surface orientation, with or without hydrogen termination of the diamond surface, using first-principles electronic structure calculations based on density functional theory (DFT). We find a positive correlation between the calculated work of adhesion and the measured conductance of these interfaces, suggesting that DFT could be used as a screening tool to identify metal/dielectric systems with high TBC. We also explain the negative effect of hydrogen on the thermal conductance of metal/diamond interfaces.

Influence of diamond surface termination on thermal boundary conductance between Al and diamond

The effect of diamond surface treatment on the Thermal Boundary Conductance (TBC) between Al and diamond is investigated. The treatments consist in either of the following: exposition to a plasma of pure Ar, Ar:H and Ar:O, and HNO3:H2SO4 acid dip for various times. The surface of diamond after treatment is analyzed by X-ray Photoelectron Spectroscopy, revealing hydrogen termination for the as-received and Ar:H plasma treated samples, pure sp2 termination for Ar treated ones and oxygen (keton-like) termination for the other treatments. At ambient, all the specific treatments improve the TBC between Al and diamond from 23 ± 2 MW m–2 K–1 for the as-received to 65 ± 5, 125 ± 20, 150 ± 20, 180 ± 20 MW m–2 K–1 for the ones treated by Ar:H plasma, acid, pure Ar plasma, and Ar:O plasma with an evaporated Al layer on top, respectively. The effect of these treatments on temperature dependence are also observed and compared with the most common models available in the literature as well as experimental values in the same system. The results obtained show that the values measured for an Ar:O plasma treated diamond with Al sputtered on top stay consistently higher than the values existing in the literature over a temperature range from 78 to 290 K, probably due a lower sample surface roughness. Around ambient, the TBC values measured lay close to or even somewhat above the radiation limit, suggesting that inelastic or electronic processes may influence the transfer of heat at this metal/dielectric interface.

TEM characterization of textured silicon heterojunction solar cells

Increasing the efficiency of crystalline silicon (c-Si) solar cells requires the reduction of both bulk and interface recombination. Even if bulk recombination is almost suppressed, the symmetry of the crystal lattice is disturbed at the surface and hence, due in particular to non-saturated bonds (dangling bonds), a large density of defects exists. Thus, in this case, the free-carrier lifetimes are no longer limited by the quality of the bulk c-Si, but by its surface. To keep the recombination losses at the c-Si surface at minimal levels, the surface must be electronically well passivated. An efficient way to obtain passivation is to use low temperature grown (typically 200°C) hydrogenated amorphous silicon (a-Si:H) [1]. In the case of photovoltaic applications, the passivation of both c-Si wafer surfaces (i.e., the emitter and the rear surface) is of crucial importance for good performances. In this work, the 3-7 nm a-Si:H based passivation layers of the device are grown by VHF-PECVD in a single chamber. The solar cells consist of a multilayered structure: Al back contact / DC-sputtered ITO (Indium Tin Oxyde) / n/i a-Si:H back surface field (BSF) / n-type c-Si substrate with a resistivity of 1-3 Ωcm / intrinsic a-Si:H / i/p a-Si:H emitter / and a front contact made out of DC-sputtered ITO via a shadow mask to define the cells (diameter = 4.5 mm). Such devices are called heterojunction (HJ) silicon solar cells. Surface recombination losses are a major concern for all c-Si solar cells. In particular, mastering of HJ emitter and back surface field formation on textured c-Si is crucial for high-performance HJ solar cells. High Resolution Transmission Electron Microscopy (HRTEM) is necessary to identify the key microstructural features of the a-Si:H/c-Si interface, and TEM micrographs of HJ interfaces on flat and textured high-performance devices are shown for the first time and discussed with respect to the resulting solar cell electrical performances. TEM micrographs of our flat high-efficiency HJs show abrupt c-Si/a-Si:H/μc-Si:H interfaces for emitter and BSF formation. Whereas on the pyramidal facets of the textured substrate the growth is identical to the flat substrate interface, we observed unexpected epitaxial growth at the bottom of the pyramid valleys. We have identified these local epitaxial domains as an efficient surface recombination path, and by consequence, as responsible for the observed decreased VOC on textured HJ cells. When minimizing the density of epitaxial domains at the c-Si/a-Si:H interface by adapting the deposition conditions, a solar cell VOC of 660 mV is obtained. An additional modification of the textured c-Si surface morphology leads to VOCs as high as 700 mV.