WP2 - High efficiency crystalline silicon solar cell technology

Team leader: Assoc.prof. Mgr. Jakub Holovský, Ph.D.

General overview

The research has one main long-term objective that is 25% efficient solar cell based on silicon passivated contacts technology, optimized for ½ sun irradiance and fabricated without use of any kind of chemical vapour or plasma deposition. Only wet chemistry passivation combined with multifunctional transparent oxide layers prepared by Pulsed Laser Deposition or RF Sputtering will be used. The idea of ½ sun optimization is supported by many arguments: 1) within the tandem, cells work only below ½ sun irradiance, 2) energy cost is high when sunshine is low, 3) capping PV modules to ½ of nominal power saves BOS cost and makes system more profitable, 4) reduced current allows for relaxed requirements for contacts. The 25% objective might not be reached by Si cell alone and so tandem with perovskite might be required. For this reason we study hybrid perovskite materials by advanced techniques such as FTPS and in terms of devices we reached 19% efficiency for mixed-halide technology.

Fundamentals of solar cell operation

The idea of passivated contacts to silicon were studied experimentally (Fig. 1) and theoretically (Fig. 2) from the point of view of the device performance, mainly under low illumination. In the collaboration with EPFL Neuchatel special samples of silicon heterojunction (SiHJ) solar cells with crystalline silicon/amorphous silicon contact layers. Level of doping and thickness of doped amorphous silicon was varied and the effect on low-light performance was studied and also theoretically simulated, see below. We confirmed experimentally that reducing contact strength does not have effect on voltage at maximum power point Vmpp at low irradiances (Fig. 1). We simulated required parameter windows for of work function (WF) and thickness of contact layer for different thicknesses (Fig. 2). Relaxed requirement for contacts at low level illumination should principally allow more options for contact materials, mainly with less parasitic absorptance (Conrad et al., 2020). In tandem cells we investigated the fundamental role of tunnelling interface and the consequence on shunts (Ventosinos et al., 2018).

New contact materials

In the second year of the project we successfully started with our thin-film technology of pulsed laser deposition (PLD). Layer quality was characterized by various methods: photothermal deflection spectroscopy (PDS) was used to determine parasitic absorption and defect density (Fig. 3), X-ray diffraction (XRD) was used for determining crystallinity and X-ray photoelectron spectroscopy (XPS) was used to determine stoichiometry (Fig. 4). We tested effects of deposition temperature and oxygen pressure on layers of MoOX, VOX and WOX (Horynová et al., 2020).

Silicon surface

High efficient contacts require smooth and sharp interfaces with silicon. By tuning pH of silicon etching solution it is possible to induce steps on (111) oriented Si surface facing either 〈(11) ̅2〉 or 〈112 ̅ 〉 directions. These are terminated by dihydrides or monohydrides respectively, see Fig. 5. We simulated for the first time vibrational spectra by periodic DFT and we brought new insights into peak assignments, e.g. we confirmed that peak experimentally observed at 2135 cm-1 and simulated at 2142 cm-1 (Fig. 6) is indeed di-hydride under strong stearic interaction and not tri-hydride. Also we explained that satellite peak observed by 0.9 cm-1 above H/Si(111)-(1×1) peak at 2083.6 cm-1 is not defect, but an artifact. In parallel to simulations we develop sensitive FTIR techniques (Holovský et al., 2018).

Hybrid perovskite materials and cells

The record silicon based technology is now tandem with silicon bottom cell and metal-halide hybrid perovskite top cell with efficiency 28% achieved by Oxford PV. Within this work package our activities are therefore also partially oriented towards thin polycrystalline CH3NH3PbI3 perovskite films and solar cells. In last year, we have achieved significant progress in the solar cell technology, reaching up to 17% conversion efficiencies by new type of TiO2 layer doping (Peter Amalathas et al., 2019) and treatment (Peter Amalathas et al., 2020). Such efficiency is already high enough to allow us performing studies on finished devices that give relevant results, more attractive to photovoltaic community.

Perovskites devices and also layers exhibit many reversible and irreversible effects. We studied reversible effect of blue illumination on the I-V curves (Holovský et al., 2019), see Fig 7. Very similar reversible effects were revealed by measurement of sub-bandgap absorption by PDS method and Fourier-transform photocurrent spectroscopy (FTPS) method, see Fig 8. To induce the reversible changes the blue illumination has to be applied from the side of the PbI2-rich side for two step materials. The explanation is the light-induced decomposition of PbI2 into metallic lead and iodine, confirmed by XPS. Because metallic lead is strongly absorbing but does not contribute to photocurrent it can be measured by PDS. On the other hand iodine diffuses and deposit in interstitial positions of CH3NH3PbI3 phase where it creates electronically active deeps states that can be measured by FPTS (Holovský et al., 2017). One of the explanation of the prominence of the perovskite material is its sharp absorption edge. We study in collaboration with Institute of Physics of ASCR different aspects of absorption edge of novel perovskite materials, e.g. temperature dependence (Ledinsky et al., 2019).

Equipment

Publications related to project

2020

2019

2018

2017