New role of thin films in advanced photovoltaics

Currently, photovoltaics has become an established industrial field with the global installed capacity over 500 GWp and with a perspective of reaching the terawatt installed capacity within the following decade. The field is dominated by silicon wafer-based cells for which previously unforeseen low system prices have been reached. The advantages of photovoltaics based on silicon thin film (lower consumption of energy-costly silicon and thus shorter energy payback time) have not been sufficient to overcome the disadvantage of lower efficiencies (12 % record efficiency vs. ~25 %). Thus, the development and production of silicon thin film photovoltaics has nearly stopped. The other types of thin film-based PV (CdTe, CIS) struggle as well.

Yet, the highest recent efficiencies have been achieved by merging both technologies [1]: Si wafers for photogeneration and thin films for selective passivated contacting scheme. The record efficiency [2] reached 26.7 % by using silicon heterojunction (SHJ) cells with interdigitated back contacts composed of intrinsic and either n or p-type hydrogenated amorphous silicon layers with thicknesses of ~ 10 nm. Many variations of solar cells with carrier-selective passivation contacts (e.g. TOPCON or POLO) are also being explored.

The record SHJ cell above has been fabricated by photolithography, unsuitable for mass production. Thus, simpler stencil mask-based technology is being developed in the NextBase H2020 project [3]. The tunnel-IBC approach further simplifies the process by eliminating the patterning and aligning the hole collector contacts.

Assay of the back-contacting scheme with n and p fingers brings new challenges on how to measure these very thin layers, in particular, when deposited on textured wafers. We will review the principle and use of optical profilometry [4] based on the attenuation of the Raman scattering from the underlying wafer which allowed us to map the layers with a thickness resolution better than 0.5 nm. The optical profilometry also finds its use in other new ways of interface engineering by inserting 2D materials [5] or self-assembled dipolar molecule monolayers. [6,7].

[1] W.C. Sinke, Development of photovoltaic technologies for global impact, Renew. Energy. 138 (2019) 911–914.
[2] K. Yoshikawa, H. Kawasaki, W. Yoshida, T. Irie, K. Konishi, K. Nakano, T. Uto, D. Adachi, M. Kanematsu, H. Uzu, K. Yamamoto, Silicon heterojunction solar cell with interdigitated back contacts for a photoconversion efficiency over 26%, Nat. Energy. 2 (2017) 17032.
[3] NextBase, Web. (2016). (accessed May 3, 2019).
[4] M. Ledinský, B. Paviet-Salomon, A. Vetushka, J. Geissbühler, A. Tomasi, M. Despeisse, S.D. Wolf, C. Ballif, A. Fejfar, Profilometry of thin films on rough substrates by Raman spectroscopy, Sci. Rep. 6 (2016) 37859.
[5] Z. Hájková, M. Ledinský, A. Vetushka, J. Stuchlík, M. Müller, A. Fejfar, M. Bouša, M. Kalbáč, O. Frank, Photovoltaic characterization of graphene/silicon Schottky junctions from local and macroscopic perspectives, Chem. Phys. Lett. 676 (2017) 82–88.
[6] A. Vetushka, L. Bernard, O. Guseva, Z. Bastl, J. Plocek, I. Tomandl, A. Fejfar, T. Baše, P. Schmutz, Adsorption of oriented carborane dipoles on a silver surface, Phys. Status Solidi B. (2015).
[7] M. Hladík, A. Vetushka, A. Fejfar, H. Vázquez, Tuning of the gold work function by carborane films studied using density functional theory, Phys. Chem. Chem. Phys. 21 (2019) 6178–6185.