WP1 - Computational simulations and the design of low-dimensional

materials for solar cells

Team leader: prof. Ing. Tomáš Polcar, Ph.D.

Team members:

Team deals with bottom-up approach in design of novel materials/interfaces using advanced atomistic simulations. Two lines of research are envisaged: study of low dimensional materials and perovskite/oxide interface. First, we explore 2D materials based on monolayer transition metal dichalcogenides (TMD - direct bandgap semiconductors) to find their electronic characteristics. Selective doping of TMD monolayer should yield optimum material for expected application. Then, selected material will be combined into heterostructures (different TMD or TMD/graphene). The second research line is related to optimization of an interface between organic−inorganic hybrid perovskite materials with oxide scaffolds to improve efficiency of a perovskite.

Summary

Low-dimensional structures open new perspective for energy harvesting. We will use advanced atomistic scale simulations to design and optimize low-dimensional scale functional units, which can be later embedded into active conductive matrix.

State-of-the-Art

Ultrathin, low-dimensional (in)organic materials are at the forefront of the burgeoning research field of solid state nanotechnology, kick-started by the synthesis of graphene over a decade ago. It is now a well-established fact that both size and dimensionality have a remarkable influence on the properties of a material, with low-dimensional (0-, 1- or 2-D) crystals exhibiting different electronic and optical properties to the bulk. Two-dimensional materials will inevitably play a vital role in meeting future energy demands, as part of the necessary move away from fossil fuels to renewable energy sources. Key properties of 2D materials include pristine interfaces free of dangling bonds, ultra-thin (uniform) thickness, and compositions that may include a choice of metals, insulators and semiconductors – all of which facilitate the design of the next-generation 'green' electronics, namely solar cells. Generally speaking, these devices must perform two functions: (i) photogeneration of electron-hole pairs (ii) separation of charge carriers to generate a current. Solar energy conversion for photovoltaics or photocatalysis is initiated by light harvesting, or sensitization, of a semiconductor or catalyst as a first step [1]. Rare elements are often used for this purpose, however, they are not suitable for large-scale production cycles. As such, we must design solar energy-converting materials based on the Earth's most abundant elements. One such example includes an iron-nitrogen-heterocyclic-carbene sensitizer, with an excited state lifetime roughly one-thousand-fold longer than traditional iron-polypyridyl complexes [1]. This iron complex has been shown to generate photoelectrons in the conduction band of TiO2 with a quantum yield of 92% from the 3MLCT state. Moreover, metal-free sensitizers have been target of research as well and in recent years their performance have increased significantly. They offer advantages over the metal-based sensitizers in terms of durability issues, as being more environmental-friendly materials and they can also be designed in order to extend the adsorption threshold up to the near infrared region [2]. As an example, one of the best organic sensitizers available so far is the compound named C219 – a cromophore made of a binary π-conjugated spacer, apart from the blocks of a lipophilic alkoxy-substituted triphenylamine electron-donor and a hydrophilic cyanoacrylic acid electron-acceptor – that has been proved to achieve a photoelectric conversion efficiency up to 10.3% [3]. Monolayer MoS2 and other TMDs are direct bandgap semiconductors, rendering them suitable for use in optoelectronic devices such as LEDs and solar cells. In conjunction with their impressive mechanical properties, TMDs are promising materials for future (opto)electronic materials such as flexible electronics [4], combining the mechanical flexibility of organic semiconductors with optimal electronic characteristics of TMDs. Heterostructure-based photovoltaics such as a graphene-WS2-graphene 'sandwich' also show remarkable properties [5]. In such a design, the Van Hove singularities in the electronic DOS of TMDs ensures superior light-matter interactions, yielding improved photon absorption and electron-hole creation. Moreover, TMDs show excellent response to both laser [5] and visible [6] light.

Objective

The final goal is to create new photovoltaic materials starting from the most recent technological achievements. Carbon nanotubes[7], solid state perovskites[8], all-polymer solar cells [9], TMD-based heterostructures[10,11] can be modified and combined in highly stable self-assembling multifunctional materials, to get optimal performance together with high technological flexibility. Using advanced Molecular Dynamics, (TD)DFT, and Green's function-based techniques, it is possible to investigate bandgap tunability, material thickness and self-assembling properties. Optimized optically active centers (functional units derived from few-layers of 2D [in]organic heterostructures), able to convert incoming photons into free electrons, can be thought as embedded into an active conductive matrix that drive the generated photoelectrons towards an external circuit. Functional units and active conductive matrix can then be inserted into self-assembling mixtures. The self-assembling property is exploited to create the photodevice in situ, granting great technological flexibility, since it is the characteristic to form organized hierarchies at the nanoscale with minimal external intervention. There are still many challenges that must be addressed when it comes to 2D crystals. These include the development of “straightforward” and economically viable material synthesis, investigating the roles of dopants, and establish the effects of the nature/quality of 2D crystal interfaces on the electrical properties. Moreover, it is crucial that we develop the wholly ab-initio description of hot carriers (electrons/holes with markedly higher energy than those at thermal equilibrium) in semiconductors[12]. As hot carrier thermalization results in a major loss of efficiency, theoretical modelling of hot carriers in novel materials will be vitally important in the design of solar cells of unprecedented efficiency. 2 Design of optimum oxide as scaffold (for perovskites) Organic−inorganic hybrid perovskite materials have gained much attention in the past years because of their high efficiency, low cost, and the ease to make these materials solution processable. The efficiency of pervskites could be significantly improved by use of a mesoporous oxide (TiO2, ZrO2, ZnO, and Al2O3) scaffold. Nanocrystalline TiO2 (rutile) forms an intimate junction of large interfacial area with the CH3NH3PbI3 film, is much more effective in extracting photogenerated electrons from the perovskite than a conventional planar TiO2 film (anatase; porosity 60%) [12]. Liu et al. observed similar behaviour in case of ZnO electron collection layers [13]. It is evident that further optimisation of oxide scaffold offers significant potential to improve performance of various perovskites. We suggest here low-temperature magnetron sputtering as an alternative route to prepare titanium oxides with extremely well-defined nanostructure and high density. We will focus mainly on interface design and possibility of oxide doping. These oxides will be applied as scaffolds for variety of substrates (e.g. FTO glass) and perovskites. Outreach (beyond photovoltaics): Titanium oxide deposited by magnetron sputtering is known as antibacterial material and is often applied in variety of applications. However, the deposition process is typically high temperature (above 100 C), which eliminate number of substrate, such as flexible polymers. We aim to produce highly crystalline and, above all, dense titanium oxide with selected crystallographic structure by low-temperature magnetron sputtering (up to 50 C), which expands market possibilities in various fields related to antibacterial properties.

References

[1] T. C. B. Harling et al., Nature Chemistry, 7, 883 (2015).
[2] C.-P. Lee et al., RSC Advances, 5, 23810 (2015).
[3] W. Zeng et al., Chem. Mater., 22, 1915 (2010).
[4] M. Bernardi et al., Nano Lett., 13, 3664 (2013).
[5] L. Britnell et al., Science, 340, 1311 (2013).
[6] S. Wi et al., ACS Nano, 8, 5270 (2014).
[7] F. Wang et al., Nat. Comm., 6, 6305 (2015).
[8] L. Sun, Nat. Chem., 7, 684 (2015).
[9] T. Kim et al., Nat. Comm., 6, 8547 (2015).
[10] S. Lin et al., Sci. Rep., 5, 15103 (2015).
[11] A. K. Geim et al., Nature, 499, 419 (2013).
[12] A. Yella, L. Heiniger, P. Gao, M.K. Nazeeruddin, Michael Grätzel, Nano Lett. 2014, 14, 2591−2596
[13] Liu, M.; Johnston, M. B.; Snaith, H. J. Nature 2013, 501, 395−398