Investigation of carrier multiplication in van der Waals heterostructures for highly efficient solar cells

Project Information

CAMPVANS

Grant agreement ID: 885893

DOI

10.3030/885893

Project closed

EC signature date 27 February 2020

Start date 1 March 2021

End date 28 February 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 191 149,44

EU contribution

€ 191 149,44

Coordinated by

EIDGENOESSISCHE TECHNISCHE HOCHSCHULE ZUERICH
Switzerland

Periodic Reporting for period 1 - CAMPVANS (Investigation of carrier multiplication in van der Waals heterostructures for highly efficient solar cells)

Reporting period: 2021-03-01 to 2023-02-28

Summary of the context and overall objectives of the project

To satisfy the always growing energy demand worldwide, new sources of renewable and green energy are urgently needed. This action responds to this societal challenge by designing and predicting novel photovoltaic (PV) cells based on recently discovered two-dimensional (2-D) crystals with the support of beyond state-of-the-art numerical simulator developed as part of this research effort. van der Waals (vdW) heterostructures (vdWHs) fabricated by stacking different 2-D crystals exhibit optoelectronic properties drastically different from those of the constituent materials.

To design efficient PV cells, a deeper understanding of the light-matter interaction in these devices is required. Access to internal physical quantities that cannot be directly measured is of paramount importance to achieve properly working vdWH PV cells. Moreover, experimentally exploring the vast design space is very time consuming and expensive. Advanced physics-based technology computer aided design (TCAD) tools can help address these challenges. By taking advantage of such tools, researchers can explore multiple device configurations in record times and can rapidly provide experimentalists with reliable design guidelines, thus reducing the overall costs.

In this action, our goal was to design highly-efficient PV cells based on vdWH of 2-D materials. Our first objective was to develop a beyond state-of-the-art predictive TCAD tool to support the design of vdWH-based devices and assess their performance. The second objective consisted of identifying material combinations covering a large portion of the light spectrum and of increasing the light conversion efficiency by stacking multiple 2-D layers with different band gaps. The third objective was to carefully engineer vdWHs to enhance the inter-layer carrier multiplication process, thereby substantially improving the light conversion efficiency of the vdWH solar cells and exceeding the theoretical limit of conventional solar cells. A parallel goal of the MSCA Individual Fellowship is to foster the development of the individual researcher.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Work was conducted via 6 work packages (WPs). In WP1, the Fellow further developed the light-matter interaction model available in the OMEN quantum transport solver. This state-of-the-art device simulator is maintained in the Nano-TCAD research group of Prof. Mathieu Luisier at ETH Zurich. Relying on the density-functional theory (DFT) and the NEGF formalism, predictive simulations of the I-V characteristics of vdWHs as well as of their photo-response properties were efficiently calculated on high-performance supercomputers. Light-matter interactions were taken into account from first-principles to enable accurate performance predictions. This fully ab initio simulation framework relies solely on the knowledge of the atomistic structures of the materials and the device configuration. The complex band structure, band edge alignment of vdWHs, electrostatics in 3-D and required quantum mechanical effects such as tunnelling and confinement are automatically captured. WP1 yielded 1 conference publications and 3 journal publication to date, with 1 journal manuscripts underway.

In WP2, the Fellow specifically investigated 2-D transition-metal dichalcogenides (TMDCs) and their vdWHs. From DFT calculations, the MoSe2-WSe2 combination was identified to show strong light-matter interaction. The Fellow investigated light-matter interactions and charge transport in MoSe2-WSe2 vdWH PIN photodiodes under the influence of a monochromatic electromagnetic light. In addition, he examined a carefully engineered defective 2-D crystal, PtSe2, and discovered that Se vacancies in bilayers break the centrosymmetry of the crystal. The defective bilayers exhibit 10 times larger photo-response than the pristine ones. This peculiar effect is known as bulk photo-voltaic effect (BPVE) and has only been observed in some special non-centrosymmetric materials. The BPVE can lead to photogeneration of hot and non-equilibrium electrons and above-gap photovoltage. The simulated photo-response of the defective bilayer PtSe2 agrees very well with the experiment data of the collaborator. In WP2, the Fellow delivered 1 conference publications and 1 journal publication to date, with 1 journal manuscript underway.

WP3 involved developing a self-consistent GW (scGW) method in our quantum transport simulator to include electron-electron interactions in non-equilibrium systems, from first-principles. This scGW method goes beyond the state-of-the-art and allows for many-body simulations of hot-carrier effects in advanced solar cells. By identifying novel 2-D materials and designing vdWH to enhance the CM effect, the power conversion efficiency can be theoretically increased above the Shockley-Queisser limit. The Fellow contributed in the prestigious IEEE International Electron Devices Meeting, with additional 1 conference publication and 3 journal manuscripts underway. In WP5, for researcher training, the Fellow attended 6 intensive training workshops and multi-day conferences. In WP6, for transfer of knowledge, he provided supervision and mentoring for semester projects and Master thesis. He was appointed Guest Editor of Frontiers In Electronics. The project was managed under WP4.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

This action advanced the state-of the-art in the theory and application of the NEGF formalism for optoelectronic devices based on 2-D materials and their vdWHs. The utilization of an ab initio material descriptor calculated with DFT in device simulations removes the difficulty of empirical models that need to be pre-calibrated to available experimental data. The resulting predictive power of simulations can now be used to explore and assess the performance of PV cells based on novel 2-D materials and vdWHs whose experimental data is sparse. The ‘experiments’ can be done via simulations even before these materials and devices have been synthesized and fabricated.

Furthermore, this action made significant progresses in the field of quantum transport simulation by combining the dynamically screened Coulomb interaction in many-electron systems with light-matter interactions on an equal footing within the NEGF formalism. This development will enable more accurate quantum transport investigations of devices that contain highly non-equilibrium and strongly interacting electron gas. Such theoretical advances are especially relevant for the 2-D materials and vdWHs, due to their combination of low-dimensional structures and strong Coulomb interactions. Their strong excitonic effect increases the optical absorption and affects the photoconductivity. The simulation tool developed during this project provides a better understanding of the excitonic physics in the vdWHs.

The simulation framework also promoted the innovative idea of using inter-layer carrier multiplication, which could potentially lead to a significant improvement of the light conversion efficiency of photovoltaic cells. vdWH-based PV cells will be designed and optimized with this simulator to maximize the carrier multiplication efficiency. This can harvest the excess kinetic energy of those hot photo-carriers and utilize the high energy range of the solar spectrum.

Schematic view of a vdWH PIN photodiode with a vertically incident, linearly polarized light.