Materials and photovoltaic components
Although the contribution of photovoltaics to the production of renewable energy is still largely due to the traditional crystalline silicon sector, many so-called "emerging" technologies are the subject of research projects around the world and aim to reduce the cost of photovoltaics and widen its field of application. In this context, the development of new thin-film materials with outstanding optical, electronic and mechanical properties plays a key role. Our team participates in these efforts by developing new inorganic and organic materials, studying their fundamental properties, and implementing them in the manufacturing of innovative photovoltaic components.
Organic photovoltaic cells
Organic solar cells are part of the emerging photovoltaic technologies whose very specific characteristics (flexibility, manufacturing at room temperature, semi-transparency, etc.) should make it possible to broaden the integration of photovoltaics in various fields. The team's activities are mainly aimed at improving photovoltaic conversion efficiency, the stability of organic cells and reducing the environmental impact of their manufacture. We also carry out more fundamental studies on the relationships between the molecular structure and the electronic or photovoltaic properties of new molecules.
This work is carried out in close collaboration with the consortium STELORG, which brings together around fifteen researchers in chemistry, physico-chemistry and component physics from four Strasbourg research institutes, complementary skills.
Our current research projects on this theme are illustrated by a few examples below.
Molecular structure and optoelectronic properties
Persons involved: T. Heiser, P. Lévêque, E. Martin, E. Steveler
The efficiency of organic photovoltaic (OPV) devices is currently limited by the short lifetime (< 1 ns) and short diffusion length (a few nm) of the photogenerated excitons. The development of organic materials with long diffusion lengths (typically > 10 nm) is therefore proving to be a particularly interesting way to improve charge transport and should lead to an improvement in OPV performance. In thin films, the dynamics of excitons and charge carriers, crucial for the operation of OPV devices, is controlled by intermolecular interactions and depend in a non-trivial way on the molecular organization in the solid state.
In this context, we are studying families of organic molecules with different side chains and heat treatment conditions, allowing us to obtain molecular structures and various crystalline orders (liquid crystal, needles or crystalline grains...). We are thus studying the influence of molecular organization and self-assembly on the dynamics of excitons in order to improve the performance of OPV devices. [1,2]
 J. Jing, E. Steveler, N. Leclerc, A. D'Aléo, B. Heinrich, W. Uhring, T. Heiser, Proc. SPIE 12149, Organic Electronics and Photonics: Fundamentals and Devices III, 1214904 (2022).
 J. Jing, E. Steveler, N. Leclerc, B. Heinrich, W. Uhring, T. Heiser, Proc. SPIE 11365, Organic Electronics and Photonics: Fundamentals and Devices II; 113650F (2020).
An atomic-scale modelling activity of exciton diffusion in organic semiconductors has recently been initiated, using first-principles molecular dynamics, based on DFT, in order to strenghten the understanding issue from experiments on this phenomenon.
The addition of a structuring platform (TAT) on either side of an effective motif (TB2) makes it possible to act on the molecular arrangement in the solid state to improve the dynamics of charge carriers and ultimately the conversion efficiency of organic solar cells.
 T. Bura, N. Leclerc, R. Bechara, P. Lévêque, T. Heiser, R. Ziessel, Adv. Energy Mater. 3 (2013) 1118.
 T. Bura, N. Leclerc, S. Fall, P. Lévêque, P. Retailleau, S. Rihn, A. Mirloup, R. Ziessel, J. Am. Chem. Soc. 134 (2012) 17404.
 N. Leclerc, I. Bulut, Q. Huaulmé, A. Mirloup, P. Chávez, S. Fall, A. Hébraud, S. Méry, B. Heinrich, T. Heiser, P. Lévêque ChemSusChem. 10 (2017) 1878.
Research of alternative, non-toxic solvents, by reverse engineering
Persons involved: T. Heiser, P. Lévêque
Collaborations: Sophie Thibaud-Roux, Ivonne Rodrigues-Donis et Vincent Gerbaud, ENSIACET (Toulouse)
The toxicity of halogenated solvents usually used to solubilize 𝜋-conjugated materials is a major obstacle to the industrialization of organic photovoltaic modules. As a result, the search for alternative solvents, less toxic and potentially biosourced, is today an important issue for the organic sector. In this context, we have recently shown that reverse molecular engineering, which consists in identifying by numerical means solvents presenting a set of target properties, is a promising approach. In collaboration with the teams of Sophie Thibaud-Roux, Ivonne Rodrigues-Donis and Vincent Gerbaud from ENSIACET in Toulouse, we were able to apply the computer-aided design tool, IBSS®, developed by V. Gerbaud, to the problem solvents.
This methodology allowed us in particular to identify several alternative solvents for the manufacture of solar cells based on poly(3-hexylthiophene), a reference organic polymer, without loss of performance.
Jing Wang, Ivonne Rodriguez-Donis, Sophie Thiebaud-Roux, Olzhas A. Ibraaikulov, Nicolas Leclerc, Patrick Lévêque, Vincent Gervaud, Markus Kohlstädt, Thomas Heiser, Molecular Systems Design & Engineering, 7 (2022) 182
Ternary mixtures for improved yield and/or stability
Persons involved: P. Lévêque, T. Heiser, S. Fall
Study ternary mixtures in the active layer to increase the photovoltaic conversion efficiency and/or device stability. Starting from an electron donor polymer (PF2), synthesized within the Strasbourg consortium STELORG, conversion yields greater than 12% were obtained by using two electron donor polymers (PF2 and J71) and an electron acceptor. underived fullerene (Y6) electrons from complementary absorption spectra.  By using an electron donor polymer (PF2) and two acceptors (PC71BM and EH-IDTBR), good stability under illumination was observed. A better understanding of the influence of ternary mixtures in terms of solid-state structure and tuning of electronic boundary levels is a lock to be lifted in order to jointly obtain high yields and sufficient stability.
 X. Ma , Q. An , O. Ibraikulov, P. Lévêque, T. Heiser, N. Leclerc , X. Zhang , F. Zhang, Journal of Materials Chemistry A, 8 (2020) pages 1265.
Organic photovoltaics for “indoor” applications
Persons involved: P. Lévêque, S. Fall
Organic photovoltaic cells absorb particularly in the wavelength range of artificial lighting and often see their efficiency increase when the illumination decreases. The purpose of this theme is to show the potential of organic solar cells to power connected objects located inside buildings. Electronics allowing sober energy management have been developed specifically for this application.
Photovoltaic optical modulators based on liquid crystals and organic semiconductors
Persons involved: T. Heiser, S. Fall, Y. Lin
The integration of organic semiconductor materials into liquid crystal optical modulators offers new functionalities to these devices. Indeed, these "hybrid" modulators have by construction a behavior sensitive to the incident light intensity and can therefore be used as photorefractive elements  or as dynamic glasses (similar to photochromic or electrochromic glasses, whose tint is adjustable ).
In this context, we have recently proposed a new concept of dynamic glass, called PSLM (for "photovoltaic spatial light modulator")  (see principle diagram). The operation of a PSLM is energy self-sufficient, easily controllable by the user and benefits from a response time of less than one second. Our current work aims to increase the transparency in the "clear" state of PSLMs, to optimize their spectral response according to the targeted applications and to improve their manufacturing method (increase in size, robustness, etc.).
 T. Regrettier, M. Kaczmarek, G. D'Alessandro, T. Heiser, "Integrated organic donor-acceptor bulk heterojunctions for self-activated liquid crystal light modulators.," Proc. SPIE 10735, Liquid Crystals XXII, 1073514 (14 September 2018)
 T. Heiser, T. Regrettier, M. Kaczmarek, « Liquid Crystal Spatial Light Modulator », US 2020/0233248 A1
Photon conversion by downshifting or downconversion for solar cells
Persons involved: T. Fix, G. Ferblantier, A. Slaoui
Several phenomena limit the efficiency of solar cells (c-Si, CIGS…), such as the thermalization of photons with energy higher than the bandgap or the low spectral response in the ultraviolet (window layers…). A possible solution is to better match the bandgaps to the solar spectrum by fabricating tandem solar cells. Another solution is to adapt the solar spectrum to the existing solar cell by converting ultraviolet photons towards the visible or near-infrared before being absorbed by the cell. Downshifting and downconversion consist in converting an ultraviolet photon into respectively 1 or 2 photons in the visible or near-infrared. We study several downshifting and downconversion systems, in the form of oxide thin films or polymers functionalized with coordination complexes. Our functionalized encapsulants with photon conversion by photoluminescence allow an increase of conversion efficiency from 13.5 to 14.3 % in CIGS solar cells.
 Enhancement of silicon solar cells by downshifting with Eu and Tb coordination complexes, T. Fix, A. Nonat, D. Imbert, S. Di Pietro, M. Mazzanti, A. Slaoui and L. J. Charbonnière, Progress in Photovoltaics: Research and Applications 24, 1251 (2016)
 Enhancement of CIGS solar cells using europium complex as photon downshifter, A. Gavriluta, T. Fix, A. Nonat, M. Paire, A. Slaoui, L. J. Charbonnière, J.-F. Guillemoles, Adv. Opt. Mater. 4, 1846 (2016)
Emerging oxides as absorbers or transparent conductive oxides
Persons involved: T. Fix, G. Ferblantier, D. Muller, A. Slaoui
Transparent conductive oxides (TCOs) are present in many solar cell architectures. Research aims to improve these TCOs and avoid the use of Indium, scarce element present in ITO.
As well, emerging oxide materials are developed for the role of photon absorber in solar cells. Inorganic photovoltaic technologies are mainly based on CdTe, amorphous Si and CuInxGa1-xSe2 (CIGS). A recent major breakthrough was demonstrated with perovskite halides, with conversion efficiencies higher than 20% using a small surface and not stabilized. Another path is the use of metal oxides based on abundant elements, generally stable and non-toxic.
We use pulsed laser deposition (PLD) and sputtering to study novel oxide absorbers for solar cells. The oxides studied must have a bandgap low enough to be compatible with the solar spectrum. Examples of oxides investigated are LaVO3, Cu2O, KBiFe2O5, h-TbMnO3 and Bi2FeCrO6. For the latter, ferroelectricity can play an important role in the photovoltaic properties.
 The Role of Dimensionality on the Optoelectronic Properties of Oxide and Halide Perovskites, and their Halide Derivatives, R. Hoye*, J. Hidalgo, R. Jagt, J.-P. Correa-Baena, T. Fix*, J. MacManus-Driscoll*, Advanced Energy Materials, 2100499, pages 1-59 (2021)
Ferroelectric solar cells
Ferroelectric materials are being considered for photovoltaic applications, after the demonstration of a conversion efficiency higher than 8% in a solar cell based on ferroelectrics, while only a limited number of researchers are into this field.
In the case of a ferroelectric solar cell, there is no need of a p-n junction and the electric polarization from ferroelectricity is responsible for the charge separation. In particular, the double perovskite Bi2FeCrO6 presents the best conversion efficiency while BiFeO3 allows to obtain open circuit voltage of a few dozen volts. The current-voltage characteristics present a bistability in open circuit voltage as a function of the initial polarization voltage of the cell, allowing to obtain solar cells that are tunable with a voltage pulse.
 Band-gap tuning in ferroelectric Bi2FeCrO6 double perovskite thin films, A. Quattropani, D. Stoeffler, T. Fix, G. Schmerber, M. Lenertz, G. Versini, J. L. Rehspringer, A. Slaoui, A. Dinia and S. Colis, Journal of Physical Chemistry C 122, 1070 (2018)
 Insights on hexagonal TbMnO3 for optoelectronic applications: From powders to thin films, T. Fix, G. Schmerber, J.-L. Rehspringer, M. Rastei, S. Roques, J. Bartringer, A. Slaoui, Journal of Alloys and Compounds 883, 160922 (2021)
Silicon clathrate films
Persons involved: T. Fix, D. Muller, A. Slaoui
Common forms of elemental silicon (mono-, multi-crystalline and amorphous) play a foundational role in the field of electronics and the underlying technologies are well mastered. Silicon is an element that is abundant, stable and non-toxic. Silicon clathrates are an exotic form of silicon, discovered in 1965, based as in fullerenes on hollow spheres of various size. The synthesis of clathrates in the form of films is not well mastered and presents technological bottlenecks that we aim to solve (integration into functional devices). ICube is one of the few laboratories that can elaborate such material in the form of films. The electronic and optical properties of these clathrates are strongly different to the “standard” silicon as they can provide a direct bandgap (for type II clathrates), paving the way for novel applications in electronics, optoelectronics and photovoltaics. We have demonstrated by Spectroscopic Surface Photovoltage that type II clathrates are a semiconductor in itself, distinct from diamond silicon. Not only the size of the clathrates but also the presence of doping atoms can dramatically modify their properties. Ion implantation available at ICube is used to modify the properties of the clathrates. Applications in sodium-ion batteries are also emerging for these materials.
 Silicon Clathrate Films for Photovoltaic Applications, T. Fix, R. Vollondat, A. Ameur, S. Roques, J.-L. Rehspringer, C. Chevalier, D. Muller, and A. Slaoui, J. Phys. Chem. C 124, 28, 14972–14977 (2020)  Synthesis and characterization of silicon clathrates of type I Na8Si46 and type II NaxSi136 by thermal decomposition, R. Vollondat, S. Roques, C. Chevalier, J. Bartringer, J.-L. Rehspringer, A. Slaoui, T. Fix, Journal of Alloys and Compounds 903, 163967 (2022)
Archives: old topics
Towards the industrialization of organic photovoltaics
Persons involved: P. Lévêque, T. Heiser, J. Wang, S. Fall
An electron-donor polymer designed and synthesized at the Cronenbourg campus (PF2) gives high conversion efficiencies (about 10%) when mixed with the PC71BM electron acceptor. This project aims to demonstrate its industrial potential by developing several approaches:
- Polymer production at the gram scale or more,
- Avoid halogenated solvents for the wet deposition of the active layer,
- Avoid rare materials (e.g. Indium) when making transparent conductive electrodes,
- Go from laboratory scale (12 mm2) to large areas (> 60 cm2).