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In the field of photovoltaics, the term is used to describe a heterojunction between amorphous silicon (a-Si:H) and crystalline silicon (c-Si). The idea of an a-Si:H/c-Si heterojunction (Het) was proposed by Fuhs et al. [[i]] and later industrially developed for photovoltaic applications by Sanyo [[ii]] but with an additional intrinsic a-Si:H layer which is essential for the quality of passivation. This design has been marketed by Sanyo under the HIT® trademark.
Advantages of heterojunction solar cells are numerous: outstanding surface passivation, low temperature process, reasonable number of process steps, high conversion efficiency, and the ability to process ultra-thin wafers (< 100 m). Moreover, Het solar cells exhibit better temperature characteristics than conventional cSi ones, which means that more power is generated in outdoor conditions for the same nominal conversion efficiency. Sanyo has developed solar cells on large area (> 100 cm2) with exceptional efficiencies (more than 23 %) and this also achieved on ultrathin substrates (>21% on 98µm c-Si wafers). Moreover, on 2011, Sanyo holds the record for conversion efficiency of an industrial solar cell (21.6 % in production). In the figures below, one can see a schematic of a heterojunction solar cell as well as a band diagram of the front surface.
[[i]] W. Fuhs, K. Niemann, and J. Stuke, Bull. Am. Phys. Soc. 19, 345 (1974).
The main research topics investigated by the TOTAL/LPICM research team are listed below. The diversity of scientific themes and means at the LPICM (which covers far more than the field of PV) enables us to broaden our scientific studies of heterojunction solar cells.
The laboratory enjoys a wide variety of characterization tools especially useful to study the interface between a-Si:H and c-Si. Only a few a nanometers differentiate high efficiencies from low ones. We carry on in situ and ex situ ellipsometry, FTIR, Hall effect, exo-diffusion measurements, lifetime determination (PCD) and electro-photoluminescence characterization. We have recently demonstrated the minor role of ion bombardment [[i]] in the quality of the passivation as well as the major role of hydrogen diffusion in the quality of passivation. For example, Figure 2 displays the pseudo-dielectric function as a function of photon energy for a p-type c-Si sample after HF dip. Measurements were taken throughout a H2 plasma treatment of the surface.
[[i]] J. Damon-Lacoste and P. Roca i Cabarrocas, J. Appl. Phys. 105, 063712 (2009).
The laboratory is also involved in the determination of band offset values [3,[i]]. Indeed, conduction band offsets and valence band offsets impact the solar cells' output parameters on both sides of the c-Si wafer.
As LPICM is deeply involved in plasma studies, we are implementing some plasma techniques in order to substitute dry process technologies for wet ones. We aim both to clean and texture c-Si wafers with plasma. Promising results have yet been obtained with SF6/O2 plasma in order to texture c-Si surfaces [[ii]] achieving very low reflectivities. In addition, we obtained excellent passivation with in situ cleaning and removal of the native oxide layer of the c-Si wafer [[iii]]. Several other plasma sources are currently being tested to increase the speed and quality of these texturing and/or cleaning steps.
[[i]] A.S. Gudovskikh, S. Ibrahim, J.-P. Kleider, J. Damon-Lacoste, P. Roca i Cabarrocas, Y. Veschetti, and P.-J. Ribeyron, Thin Solid Films 517, 6401 (2009).
Figure 4: c-Si wafer textured by plasma for heterojunction solar cells application
We are also studying the frontier between amorphous silicon deposition and epitaxial growth [3,[i]]. In this direction, we have successfully grown n-doped as well as p-doped epitaxial films on c-Si. The record mobility values of these p-type or n-type epi-Si layers are almost as good as doped c-Si layers. With these doped layers, we fabricated innovative c-Si homojonction solar cells at low temperature (< 250 °C) without any dopant gas diffusion, and with conversion efficiencies above 14 %. Figure 4 shows epitaxial growth on a (100) c-Si surface obtained at 200 °C by PECVD.[[i]] M. Labrune, M. Moreno, and P. Roca i Cabarrocas, Thin Solid Films 518, 2528 (2010).
For information on internships, doctoral studies, post-doctoral positions, or collaborations, please contact Dr Antoine Salomon
Last but not least, optical modeling in this disordered media is developed in order to understand/optimize light aborption and thus increase the short circuit current of the solar cells, while keeping a high carrier collection.