Organic Photovoltaics from Materials to Device Applications

Organic photovoltaics (OPV) is an emerging technology whose performance has been steadily increasing over the past decades and which has the potential to become a major player in renewable energy production. The synthesis of new molecular compounds has played a key role in the development of highly efficient solar cells and will continue to contribute to the advancement of this technology. Beyond achieving better performances, challenges such as reducing the environmental impact of OPV module production, increasing the operational lifetime of OPV modules or demonstrating economically viable applications of OPV are also essential for the future of this remarkable technology and have to be addressed.

STELORG offers its members access to a wide range of expertise covering all the steps needed to go from the synthesis of new molecular compounds to their integration into devices. By fostering a close collaboration between the disciplines involved, STELORG can contribute effectively to the challenging field of organic photovoltaics. Our main objectives are:

  • the design of new organic compounds and the investigation of the relationship between their molecular structure and properties relevant to OPV devices (such as optical and electrochemical properties, solid state assembly, charge transport, exciton dynamics,...) 
  • the manufacturing, characterization and optimization of small-sized OPV devices based on new organic materials, 
  • exploring ways to avoid or at least minimize the use of toxic and unsustainable solvents, and
  • developing new applications that can take advantage of the specific properties of OPV materials and devices.

STELORG's activities in the field of organic photovoltaics

Planar dumbbell-shaped bi-triazatruxene derivatives used as electron donor small molecule.

Triazatruxene (TAT) is a highly planar discotic molecular unit that promotes intermolecular stacking and can be used as a molecular segment to improve charge transport in organic semiconductors. Furthermore, it can be easily functionalized by solubilizing alkyl chains or other conjugated units. Many of our studies have been devoted to the design, synthesis and characterization of small molecular donor-acceptor-donor types, using TAT as the donor unit. This configuration leads to dumbbell-shaped planar molecules whose self-assembly is controlled by the TAT units, while the band-gap and the related optical absorption properties are mostly determined by the acceptor unit. [I. Bulut et al.] In particular, we found that these TAT-acceptor derivatives can lead to a new columnar bridging phase (see figure), in which efficient charge transport occurs not only along the TAT columns formed by the planar TAT units, but also along a direction perpendicular to the columns, promoted by the presence of bridges formed by the central conjugated acceptor unit. Furthermore, a family of molecules based on TAT units has been used to study the influence of molecular organization and self-assembly on the exciton dynamics, which is crucial for the operation of OPV devices.
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.

Impact of halogenation of the polymer conjugated backbone on properties relevant to OPV.

Over the past decade, halogenated semiconducting polymers have attracted considerable interest due to their outstanding optoelectronic properties. Halogenation influences several material’s properties including the solubility, the frontier molecular orbital energy levels and the self-assembling properties.

Making good use of this synthetic tool to improve the performance of organic semiconducting polymers in OPV requires the development of a comprehensive characterization approach including the structural characterization of the polymers in the solid state. In STELORG, we have recently used a combination of solid-state nuclear magnetic resonance (NMR) under magic angle spinning (MAS) and X-ray scattering to identify the molecular conformation of halogenated polymers in relation to their chemical composition and their charge transport properties.

Applying reverse engineering to the selection of green solvents for solution-processed organic photovoltaic devices.

The use of toxic solvents during the processing of organic semiconductors is not recommended for the production of large area devices. It is therefore crucial to find sustainable solvents with a low environmental and health impact. The selection of alternative solvents is typically based on empirical knowledge and requires a trial-and-error approach that is not very effective.

In collaboration with the University of Toulouse and the University of Freiburg, we have recently used a reverse engineering approach, based on computer-aided molecular design and a genetic algorithm optimisation method, to identify sustainable solvents that can be used to process organic photovoltaic devices without loss of efficiency. [REF]

Photovoltaic spatial light modulators (PSLM): a new class of self-activated dynamic windows combining a donor-acceptor bulk heterojunction with stimuli-responsive liquid crystals.

Dynamic windows can be used for various applications such as controlling solar radiation inside buildings or vehicles, or improving user comfort in open-space offices. The commercial development of dynamic windows based on current technologies is however still hampered by cost issues, slow response times and limited user-control.

We have recently designed a new type of dynamic glazing, labeled PSLM, which combines stimuli-responsive liquid crystals with an organic donor-acceptor bulk heterojunction. The latter is used as liquid crystal alignment layer and as photovoltaic unit. The optical transmittance of a PSLM adapts spontaneously to the intensity of the incident light in sub-second times, without the need for an external power supply and with a sensitivity that can be adjusted by a simple electrical resistor.

Representative publications of STELORG illustrating the results obtained in the field of organic photovoltaics

  • S. Fall, J. Wang, T. Regrettier, N. Brouckaert, O. A. Ibraikulov, N. Leclerc, Y. Lin, M. Ibn Elhaj, L. Komitov, P. Lévêque, Y. Zhong, M. Brinkmann, M. Kaczmarek, and T. Heiser, ACS Appl. Mater. & Interfaces, 2023, 15, 4267;

  • J. Wang , I. Rodriguez-Donis , S. Thiebaud-Roux , O. Ibraikulov , N. Leclerc , P. Lévêque , V. Gerbaud , M. Kohlstädt , T. Heiser, Molecular Systems Design & Engineering, 2022, 7, 182

  • J. Jing , B. Heinrich , A. Prel , E. Steveler , T. Han , I. Bulut , S. Méry , Y. Leroy , N. Leclerc , P. Lévêque , M. Rosenthal , D. Ivanov , T. Heiser,  J. Mat. Chem. A, 2021, 9, 24315; DOI:10.1039/D1TA06300F 

  • X. Ma , Q. An , O.A.Ibraikulov , P. Lévêque , T. Heiser , N. Leclerc , X. Zhang , F. Zhang, J. Mat. Chem. A, 2020, 8, 1265

  • O.A. Ibraikulov,  J. Wang, N. Kamatham, B. Heinrich, S. Mery; M. Kohlstadt, U. Wurfel, S. Ferry, N. Leclerc, T. Heiser, P. Lévêque, Solar RRL, 2019,3, 1900273;  DOI: 10.1002/solr.201900273

  • O A.Ibraikulov, C. Ngov, P. Chavez, I. Bulut, B. Heinrich, O. Boyron, K.L. Gerasimov, D.A. Ivanov,S.  Swaraj,S. Mery, N. Leclerc, P.  Lévêque, T. Heiser, J. Mat. Chem. A, 2018, 6, 12038; DOI:10.1039/c8ta04127j