2nd TYC workshop on energy materials

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Abstract

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This Editorial refers to a web collection of themed papers, and is not an Editorial for this issue of PCCP. In June 2012, the Thomas Young Centre for theory and simulation of materials (TYC, http://www.thomasyoungcentre.org/), organized the second Workshop on Energy Materials. Following the success of the first workshop¹, this meeting focused on charge transfer (CT), which underlies the function of many devices ranging from transistors to light-emitting diodes, and from solar to biofuel cells. The workshop was attended by more than 120 participants from 13 European countries, the US and Japan and demonstrated the strong desire by the experimental and theoretical communities to strengthen their collaboration in the attempt to overcome common problems. All of the papers in this special online collection, have been presented at the workshop and can be divided into three areas: heat transport (He et al.; D'Agosta; Ophale et al.), charge transfer and energy levels in semiconductors (Dabo et al.; Blumberger et al. Beenken et al.); and dye-sensitized solar cells (Aeberhard et al.; Lamberti et al.; Sacco et al.).

One key factor that comes into play in the theoretical investigation of thermoelectric energy conversion is the thermal conductivity of the ions. The techniques often used in these studies are the equilibrium and non-equilibrium molecular dynamics (EMD and NEMD, respectively) and the Boltzmann Transport Equation (BTE). He et al. used both MD and BTE, subject to a time relaxation approximation, and discuss extensively the effects and consequences of these approximations. The authors concluded that EMD is a robust method for the calculation of thermal conductivity of Si, Ge and SiGe alloys. The need for a better treatment of the electronic contribution beyond the linear response theory was pointed out in the perspective article by D'Agosta. In this contribution a possible extension for the calculation of the vibrational thermal contribution was described, where electrons and ions are treated on an equal footing. In a more applied contribution, the workflow of the High-Throughput-Environment (HTE) is discussed by Ophale et al. In this work different chemical compositions of binary Mg–X (X = Si, Ge and Sn) and 4d metallic silicides are scanned in order to predict their phase stability, followed by an investigation of the thermoelectric properties of Ru₂Si₃.

The treatment of charge transfer with density functional theory (DFT) methods is a well known problem. Two methods are investigated here to circumvent the spurious effects of the charge delocalization error of common functionals. In the paper of Dabo et al. the accuracy of an orbital-dependent density functional method is investigated that is based on Koopmans’ condition. It is found that this method gives donor and acceptor levels for organic semiconducting photovoltaic compounds that are in significantly better agreement with experiment than the local spin density approximation and comparable to the performance of many-body perturbation theory methods. In the contribution of Blumberger et al. the constrained DFT (CDFT) method is assessed for calculation of rates for electron tunneling between F-centre oxygen vacancies in MgO. It was found that electronic coupling is very sensitive to the fraction of exact exchange used. The authors conclude that for calculation of ET rates in oxides, the same functional should be used that reproduces the band gap of the material. Turning to organic photovoltaic interfaces, Beenken et al. report on a computational interpretation of some of the unsettled sub-band gap features of P3HT:PCBM.

The theoretical description of dye-sensitized solar cells (DSC), requires developments beyond the adiabatic approximation. A theoretical method, built on a steady-state formulation of the non-equilibrium Greens function (NEFG) theory, is discussed by Aeberhard et al. and then applied to PbSe/PbS core–shell quantum dots. The NEGF is used to compute photo-luminescence spectra, where the photo-generation, relaxation and radiative recombination processes are considered. In two experimental contributions recent advances on the fabrication and characterization of photo-anodes for DSCs are presented. Lamberti et al. describe the morphology of TiO₂ nanotubes based on X-ray diffraction measurements, and report a power conversion close to 8%. In the paper of Sacco et al. ZnO nano-sponges are investigated, which are obtained by radio-frequency sputtering followed by thermal oxidation. This material forms a branched thin film with performances comparable to Ti-compounds.

This collection of articles should be seen as a snapshot of current activities in the area of charge transfer for energy applications ranging from thermoelectrics to organics to dye-sensitised photovoltaics. We hope that this collection provides some stimulation for further work in the field. We wish to thank all the authors for their contributions to this Special Collection as well as Psi-k/ESF, CECAM and NPL for their generous financial support. We are particularly indebted to Philip Earis for offering the opportunity to publish this collection of workshop papers in PCCP.

References

1. J. Blumberger, F. Baletto and A. Shluger, Phys. Chem. Chem. Phys., 2011, 13, 7602.

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