Perovskite solar cells: the quest for a theoretical description
CECAM-HQ-EPFL, Lausanne, Switzerland
Hybrid organic-inorganic halide perovskites have revealed to be very promising materials for photovoltaic (PV) oriented applications as testified by the very high photoconversion efficiencies already achieved [1-3] since their very recent initial employment as light harvesters in solar devices .
The goal of PCEs of ~ 20% seems neither far nor impossible to be achieved in the very next years  and this result would represent a great breakthrough towards a novel concept of cheap and easily accessible PV.
In this context efforts are needed not only at experimental level but, not secondarily, at the theoretical one: state-of-art computational methodologies applied to materials science have shown their powerful predictive capabilities and their employment in the study of hybrid organic-inorganic halide perovskite based solar cells can reduced the lack of theoretical knowledge that still characterizes them.
Theoreticians are expected (1) to find out and suggest viable and optimized procedures to assemble the final devices (2) predict environmentally friendly alternatives to the usage of Pb [6,7].
We have identified the following points of discussion. Each of them includes precise questions to which we would like to give an answer through our workshop.
1) We would like to summarise the latest experimental developments in the field. This, indeed, is changing very rapidly. We will do this thanks to the presence of few key experimental scientists.
2) Similarly, we will, at the beginning of the workshop, have a few presentations in order to summarise the theoretical achievements in the modelling of the perovskite. The aim of this is to make all the participants more familiar also with research topics which are not their own.
3) Focusing on bulk hybrid-perovskites and on their structural and electronic properties, we want to understand which theoretical methods are the most appropriate:
a) standard DFT approaches vs hybrid-functionals;
b) in the case of many body-perturbation theory several levels of approximation are possible: which are the one we need?
4) For some properties (e.g. electronic or optical ones) the inclusion of spin-orbit coupling seems to be mandatory. Where is the origin of such behaviour? Is it still possible to use scalar-relativistic methods in evaluating structural properties? Do we find Rashba-Dresselhaus effects?
5)At which point is the simulation of excitonic effects such as the optical absorption?
6) It seems that dielectric screening plays a fundamental role for the high-efficiency of such devices.
Can we explain experimental data? Can we predict it?
7) Is it possible to realistically simulate interfaces with hybrid perovskites? Does the convergence represent an issue? What do we know about: ferroelectric domains,band alignment, interfaces with TiO2, Hole transport medium
8) How the modelling of defects in these materials compare with experiments? How can we dope them?
9) Charge dynamics represents a topic of paramount difficulty. What can we understand from theory and experiment? Which is the nature of charge carriers (free-carrier vs excitons). What should we do in the next future to fully clarify this point?
10) Finally, do we think that computational predictive screening of new perovskite materials is achievable? If so, which effort should be spent to accomplish such a goal? Can theoreticians find lead-free perovskites for solar cells?
In order to stimulate active discussions and collective working during the workshop, we propose a three-fold strategy:
a) After each presentation enough time ( 15 mins) will be let for discussion. Particular care will be put in the choice of the chairs which will stimulate and moderate the dialogue among the participants.
b) We will envisage each day to have one or two, well focused, presentations during the end of the day aperitif (small wine and cheese session). This will stimulate informal discussion.
c) We will report daily the scientific discussions following each presentation in a website accessible to the participants of the conference. This will highly enhance the networking and the development of future collaborations.
Jacky Even (Institut national des sciences appliquées (INSA), Rennes) - Organiser
Filippo De Angelis (University of Perugia) - Organiser
Edoardo Mosconi (National Research Council) - Organiser
Paolo Umari (University of Padova) - Organiser
Giacomo Giorgi (Department of Chemical System Engineering, School of Engineering, The University of Tokyo) - Organiser