Perovskite/crystalline silicon tandem solar cells have the potential to reach efficiencies beyond those of silicon single-junction record devices. However, the high-temperature process of 500 °C needed for state-of-the-art mesoscopic perovskite cells has, so far, been limiting their implementation in monolithic tandem devices. Here, we demonstrate the applicability of zinc tin oxide as a recombination layer and show its electrical and optical stability at temperatures up to 500 °C. To prove the concept, we fabricate monolithic tandem cells with mesoscopic top cell with up to 16% efficiency. We then investigate the effect of zinc tin oxide layer thickness variation, showing a strong influence on the optical interference pattern within the tandem device. Finally, we discuss the perspective of mesoscopic perovskite cells for high-efficiency monolithic tandem solar cells.

As highlighted by recent conversion efficiency records, passivating contacts are keys to fully exploit the potential of crystalline silicon as a light absorbing semiconductor. Prime passivating contact technologies include a-Si/c-Si silicon heterojunctions and high temperature tunnel oxide/polysilicon-based contacts. The first has the advantage of a simple fabrication process, but it is incompatible with standard metallization processes and bulk semiconductor defect treatments which take place at temperature > 800°C. The second relies on a buried junction or dopant profile near the tunnel oxide, and requires process times of several minutes at high temperature. In this paper, we solve the scientific question to know whether such a dopant profiles, with the possible the presence of nano-holes, is required to make an efficient contact when using a tunnel oxide. We show that, by leveraging the versatility of plasma deposition processes, it is possible to realize Si-based thin-film doped layers that withstand a short annealing at high temperature (> 800 for typ 10 s, called "firing"), passivate the c-Si interface and foster collection of photo-generated charge carriers by inducing a strong electric field at the Si-surface near the interface with SiOx. The contact has a high-compatibility with existing industrial process: a plasma deposition of a thin-film layer at the rear side followed by a rapid thermal treatment ("firing"), an essential process for metallization formation of industrial cells. With the developed technology, we fabricated proof-of-concept p-type solar cells with conversion efficiency up to 21.9%.

The link between sub-bandgap states and optoelectronic properties is investigated for amorphous zinc tin oxide (a-ZTO) thin films deposited by RF sputtering. a-ZTO samples were annealed up to 500 °C in oxidizing, neutral, and reducing atmospheres before characterizing their structural and optoelectronic properties by photothermal deflection spectroscopy, near-infrared-visible UV spectrophotometry, Hall effect, Rutherford backscattering, hydrogen forward scattering and transmission electron microscopy. By combining the experimental results with density functional theory calculations, oxygen deficiencies and resulting metal atoms clusters are identified as the source of subgap states, some of which act as electron donors but also as free electron scattering centers. The role of hydrogen on the optoelectronic properties is also discussed. Based on this detailed understanding of the different point defects present in a-ZTO, their impact on optoelectronic properties, and how they can be suppressed by postdeposition annealing treatments, an amorphous indium-free transparent conductive oxide, with a high thermal stability and an electron mobility up to 35cm2V−1s−1, is demonstrated by defect passivation

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This is the pre-peer reviewed version of the following article: "Towards annealing-stable molybdenum-oxide-based hole-selective contacts for silicon photovoltaics", which has been published in final form at https://DOI. Sandwiched between a hydrogenated intrinsic amorphous silicon passivation layer and a transparent conductive oxide, this material allows a highly efficient hole-selective front contact stack for crystalline silicon solar cells. However, hole extraction from the Si wafer and transport through this stack degrades upon annealing at 190 °C, which is needed to cure the screen-printed Ag metallization applied to typical Si solar cells. Here, we show that effusion of hydrogen from the adjacent layers is a likely cause for this degradation, highlighting the need for hydrogen-lean passivation layers when using such metal-oxidebased carrier-selective contacts. Pre-MoOX-deposition annealing of the passivating a-Si:H layer is shown to be a straightforward approach to manufacturing MoOX-based devices with high fill factors using screen-printed metallization cured at 190 °C.a Electronic mail: Mathieu.boccard@epfl.ch

Broadband-transparent and highly-conducting electrodes are key to avoid parasitic absorption and electrical losses in solar cells. Here, we propose zirconiumdoped indium oxide (IO:Zr) as a transparent electrode intrinsically meeting both requirements and demonstrate its application as the front electrode in silicon heterojunction (SHJ) solar cells. The exceptional properties of this material rely on the combination of high-doping and high electron mobilities, achieving with this a wide optical band gap (3.5-4 eV), low free carrier absorption and high lateral conductivity. A single film of IO:Zr, has an electron mobility of 100 cm 2 /Vs with a carrier density of 2.5-3×10 20 cm -3 , resulting in a sheet resistance around 25 Ohm/sq for 100 nm-thick films. Their implementation as front electrode in SHJ solar cells results in an important gain in current density as compared to the standardly used Sn-doped indium oxide (ITO). This is due to reduced parasitic absorption in both, the UV and IR, as confirmed by external quantum efficiency (EQE) measurements. Silicon heterojunction devices with the optimized IO:Zr front electrode, resulted in current densities of 40 mA/cm 2 , fill factor of 80%, and a conversion efficiency of 23.4 %.Index Terms-zirconium-doped indium oxide, wide band gap semiconductors, transparent electrodes, heterojunctions, electron mobility, solar cells, silicon. 2002 Neuchatel, Switzerland

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Zr-doped indium oxide (In 2 O 3 :Zr) has been shown to satisfy the requirements of low resistance, wide band gap, and high infrared transmittance for application as a front contact in broadband solar cells. However, the reduction of indium usage in front of transparent electrodes is still an unsatisfied requirement. With the goal of reducing the amount of indium while leveraging its properties, in this work, In 2 O 3 :Zr films with reduced thickness compared to those standardly used in solar cells are studied. 100 to 15-nm-thick films were sputtered at room temperature and annealed in distinct atmospheres to study the links between thickness, microstructure, and optoelectronic properties. As-deposited films exhibit an amorphous microstructure embedding bixbyite In 2 O 3 nanocrystals. Annealing in neutral (N 2 ) or reducing atmosphere (H 2 ) allows a slight growth of these crystallites but the layers remain mostly amorphous. Whereas annealing in air results in polycrystalline films with an average grain lateral size ranging from 350 to 500 nm. The large crystalline grains formed during air annealing lead to increased electron mobility for all thickness: up to 100 cm 2 V −1 s −1 for 100-nm-thick films and up to 50 cm 2 V −1 s −1 for 15-nm-thick films, which is remarkable for such thin polycrystalline films. Conversely, H 2 annealing ensures high free-carrier densities (>1 × 10 20 cm −3 ) but not high mobilities, still achieving conductivities between 1000 and 2000 S cm −1 , with the films less than 50-nm-thick keeping high broadband transmittance. The possibility of thinning down In 2 O 3 :Zr to a few tens of nanometers while keeping both high lateral conductivity and good transparency makes this material a promising candidate to reduce the amount of indium in optoelectronic applications, such as flexible touch screens and solar cells.