The computational design of Pb free and stable hybrid materials for solar cells
Authors: El Mellouhi Fadwa 
Published: July 2019

Fadwa El Mellouhi1, Akinlolu Akande2, Sergey Rashkeev1, Mohamed El-Amine Madjet1, Golib Berdiyorov2, Carlo Motta2, Stefano Sanvito2 and Sabre Kais1, Fahhad Alharbi1

1Qatar Environment and Energy Research Institute, Hamad Bin Khalifa University, Qatar Foundation, Doha, Qatar

2School of Physics and CRANN, Trinity College Dublin, Ireland

In the past four years, the solar cell field has experienced an unprecedented meteoritic emergence of a new family of solar cell technologies; namely, perovskites solar cells (PSC) using (CH3NH3)PbI3 as absorber. [1] However, two main challenges prevent deploying PSC technology: the presence of the toxic element lead (Pb) and their structural instability.

The CRANN-QEERI initiative for Solar Energy Harvesting Materials (CRAQSolar project) aims at establishing a new research protocol for the rational and rapid design of advanced materials for solar energy harvesting. This will drastically accelerate the time necessary for materials discovery and will allow us to produce a new generation of photovoltaic compounds displaying high efficiency, ease of manufacturability, longevity and low costs. Our research protocol is based on the rational design of new compounds by means of state-of-the-art high-throughput electronic structure theory, followed by synthesis, processing and characterization. In particular we will target both chemical synthesis and liquid phase processing, two protocols, which guarantee handling of materials at low temperature and low costs. The grand objective of CRAQSolar is that of revolutionizing such cycle and to provide a way for producing a significant pallet of new promising compounds for further optimization. In particular the project has two targets:

Objective 1: Developing new hybrid organic/inorganic perovskites (HOIPs) with robust solar harvesting efficiency but also useful thermal and mechanical stability. These should be synthesized/processed chemically and have mobility superior to their all-organic counterparts.

Objective 2: Developing a range of compounds made of 2D materials (such as MoS2, TiS2, GaS, etc.) with strong light absorption properties and long-living excitons. These will be processed from the liquid phase so that solar cells devices may be produced by ink-jet printing.

We conducted extensive density functional theory (DFT) calculations on the prototypical light absorbing perovskite, CH3NH3PbI3, in its cubic phase, by taking into consideration the experimentally reported evidence of the fast rotation of CH3NH3 at room temperature. This compound has the standard AMX3 perovskite structure, where the cation position, A, is taken by methyl-ammonium, CH3NH3.

Ground-state total energies and forces were computed using DFT calculations with the FHI-aims all-electron quantum chemistry code [2]. These were used to optimize the crystal geometry, without the use of constraints. The GGA in the Perdew-Burke-Ernzerhof (PBE) parameterization was employed. Long-range van der Waals interactions have been taken into account via the Tkatchenko and Scheffler (TS) scheme [3]. The reciprocal space integration was performed over an 8 × 8 × 8 Monkhorst-Pack grid after performing convergence tests on the energy and forces.

Our pioneering work for this class of material consisted of lifting the limitation imposed in previous studies on the orientation of the organic molecule. This means that we did not limit our analysis to CH3NH3 oriented along the (100) or the (111) direction, for which the high Oh symmetry is maintained, but we also explore cases where the symmetry was lowered. Our main conclusion was that such symmetry lowering has profound consequences on the electronic structure namely that the bandgap changes from direct to indirect depending on the orientation of the CH3NH3 group.[4] Crucially such symmetry-lowering configurations represent local minima in the free energy surface of the crystal and they are stabilized by van der Waals (vdW) interactions. These are the key ingredients not only for obtaining accurate lattice parameters but also for the internal geometry. Our calculations then return a picture of the CH3NH3PbI3 as a “dynamical” bandgap semiconductor, in which the exact position of the conduction band minimum depends on the particular spatial arrangement of the molecules. Importantly our results are robust against bandgap corrections and spin-orbit interaction, and deliver an absorption spectrum in good agreement with experimental data near absorption edge.

These preliminary studies suggest that better efficiency might be achieved by seeking novel molecules maybe combined with different halides in order to enhance the symmetry breaking in these compounds. Our finding thus offers a design strategy to increase the efficiency of hybrid halide perovskites. This also lays a foundation in screening and designing alternative nontoxic lead-free materials.

The two classes of compounds selected for CRAQSolar's exploration encompass some among the most promising materials for developing a solar energy harvesting technology. In particular, since both can be grown and processed chemically from the liquid phase, they offer the possibility of achieving high energy-conversion efficiency, while maintaining low fabrication costs. Furthermore, these are families of compounds comprising a vast number of members with highly tunable properties, so that they are the ideal platform for a large-scale search.

Our efforts to design lead free family of hybrid materials demonstrate that the design of hybrid materials containing organic cations might require careful considerations among them the size of the cell used during the screening process. Our strategy will be presented as well as some resulting promising compounds.

Acknowledgement

Computational resources are provided by research computing at Texas A&M University at Qatar and the Swiss Super Computing Center(CSCS). This work is supported by the Qatar National Research Fund (QNRF) through the National Priorities Research Program (NPRP 8-090-2-047)

References

[1] M. Peplow, “The perovskite revolution [news],” Spectrum, IEEE, vol. 51, no. 7, pp. 16–17, 2014.

[2] V. Blum, R. Gehrke, F. Hanke, P. Havu, V. Havu, X. Ren, K. Reuter and M. Scheffler, Ab initio molecular simulations with numeric atom-centered orbitals, Comp. Phys. Comm. 180, 2175 (2009).

[3] A. Tkatchenko and M. Scheffler, Accurate molecular van der Waals interactions from ground-state electron density and free-atom reference data, Phys. Rev. Lett. 102, 073005 (2009).

[4] C. Motta, F. El Mellouhi, S. Kais, N. Tabet, F. Alharbi, and S. Sanvito, Revealing the role of organic cations in hybrid halide perovskites CH3NH3PbI3, Nature Communications 6, Article number:7026 (2015) (doi:10.1038/ncomms8026)