Printable organic photovoltaic solar cells (OPV), i.e. polymer solar cells, have now reached impressive power conversion efficiencies at lab scale up to 19%. It is one crucial milestone towards the deployment of OPV products in real life. OPV holds many promisses including potential low cost, large scale, low temperature processing, low energy payback time, low carbon footprint for the production of photovoltaic modules exempt of critical raw materials. However, today, not all are yet scientifically achieved. For example, commercially available OPV modules suffer from low PCE, from 3 to 5 % (30-50 Wp/m2) and are made with still costly raw materials mostly processed from organic solvents. It is a matter of time for the industrial players to catch up with recent academic research to push industrial OPV performances further. This presentation will focuss on three of our recent results: (1) a doping strategy to enable a homojunction hole exctration layer for improved efficiency and stability of OPV, (2) the processing of OPV active layer from water based inks as the ultimate non-toxic, responsible printing with record efficiencies thanks to nanoparticules control and surface energy matching, (3) the investigation on the impact of synthesis impurities, such as metal catalyst residues, on the performances of OPV to design a strategy for cost-effective purification of raw materials. The presentation will close on some real life outdoor OPV energy yield considerations. Will be presented the results on recent products from Héole for marine decarbonation, such as the first OPV sail designed full-sized (92 m2) on a 52” catamaran, released in spring 2022, under testing since then.

The fabrication of the active layers in optoelectronic devices involves the use of large amounts of chlorinated organic solvents on the laboratory scale (i.e., chloroform, chlorobenzene, dichlorobenzene) in order to obtain morphology with an optimized interpenetrating network between electron donor and acceptor materials [1]. The ideal industrial production should be highly sustainable dramatically reducing the use of chlorinated solvents, then the environmental impact and the manufacturing cost of the devices [2,3].

Water-processable organic nanoparticles (WPNPs) of semiconducting polymers recently received wide attention for optoelectronic applications due to their simple fabrication and tunable properties. The WPNP-based approach could be appealing to control active layer morphology, and considerably reducing halogenated solvent use.[4]

Here we will report about a series of amphiphilic low band gap rod-coil block copolymers (BCPs), constituted by semiconducting electron donor polymers as the rigid segment (PCPDTBT and PTB7), and differing for the poly-4-vinylpyridine (P4VP)-based flexible blocks with different length and chemical composition.[5-8] These materials were designed in order to prepare blend WPNPs in aqueous suspensions with surfactant-free miniemulsion approach exploiting the interaction of the P4VP-based flexible segments with the non-solvent aqueous phase. Thus, we were able to prepare working OPV devices, exhibiting high short-circuit current density (J_sc=11.5 mA·cm⁻², PCE 2.5%), with a sustainable fabrication process [9].

It is important to underline that the phase separation between the electron acceptor and donor materials leads to complex internal structures in the nanostructures. This is controlled by the inherent material properties, as long as the solvent evaporates. As in standard OPV devices, the surface energy plays a crucial role in the improvement of the miscibility of two components of the blend. Moreover, the surface energy affects the quality of the active layer’s morphology, enhancing the interfacial area between the materials with an efficient charge generation and dissociation, until the efficiency of the device is increased [10]. We prepared semiconducting blend WPNPs by combining the BCPs with suitable fullerene derivatives, showing higher surface energy with respect to the semiconducting polymers. The high surface energy leads to core–shell blend WPNPs with fullerene derivatives in the core and the electron donor polymers in the shell [11].

In order to clarify how the morphology of the nanodomains into the blend WPNPs is related to the features of the different coil molecular structures in the BCPs, and in turn how they led to different device performances, we achieved a complete spectroscopical, electrical and morphological WPNP characterization [12-14].

Organic photovoltaics (OPVs) have emerged as a promising technology for sustainable energy harvesting due to their lightweight, flexibility, and cost-effectiveness. However, realizing their full potential relies heavily on understanding and optimizing the intricate interplay of material properties, especially in the context of optical characteristics.

The optical properties of organic materials, such as absorption and emission spectra, exciton dynamics, and light scattering behavior, play a pivotal role in dictating the overall performance of OPVs. Accurate characterization of these properties provides insights into fundamental processes like exciton generation, dissociation, and charge transport within the active layer. Precise knowledge of absorption spectra enables the selection and design of materials with optimal light-harvesting capabilities, ensuring an absorption range compatible with the source spectrum, whether it is the sun or an artificial source.

Optical characterization techniques provide exceptional sensitivity and precision in probing material properties. Spectroscopic methods, including UV-Vis, Ellipsometry, Photoluminescence and Raman spectroscopy, enable the identification of molecular structures, chemical compositions, and electronic transitions with high accuracy. One of the primary advantages of optical characterization lies in its non-destructive nature. These techniques allow for the examination of materials without altering their intrinsic properties. This non-invasive quality facilitates the continuous monitoring of dynamic processes, making it possible to study real-time changes in materials under varying conditions. Also, as these techniques are contactless and do not require special preparation of the samples in most of the cases are particularly adapted for inline and online quality control on industrial environments.

In this work we present some examples of how the use of techniques such as Ellipsometry, Photoluminescence, optical spectroscopy or raman spectroscopy can help us to better understand the optical properties of the materials that are part of an organic photovoltaic cell, and lead to improvements in the performance on the final device, unveil the mechanism of degradation process es or develop special features on our final device.

Solution processing from nanoparticle dispersions allows the use of eco-friendly processing agents for the deposition of organic semiconductor thin-films for photovoltaic and other optoelectronic applications. Omitting surfactants to stabilize the dispersions is essential to not jeopardize the solar cell performance. So far, solar cells could only be fabricated from surfactant-free P3HT dispersions which show some intrinsic self-stabilization. In this work, the self-stabilization of P3HT nanoparticle dispersions is demystified, and electrostatic effects are identified as the origin of self-stabilization. By application of this gained knowledge, novel surfactant-free nanoparticle dispersions from other, high-performance organic semiconductors are synthesized by nanoprecipitation. Design criteria will be discussed how to select the components of the dispersion. Electrical doping warrants the electrostatic stabilization of the dispersions. The role of the ionization potential of donors, the miscibility of donors and acceptors as well as the properties of the dispersion medium are elucidated. For the first time, the corresponding solar cells achieved power conversion efficiencies of up to 10.6%, demonstrating the general feasibility of this alternate, all-eco-friendly processing route.

Organic photovoltaic (OPV) devices require active layers comprised of molecular heterojunctions to split excitons into free charges. These molecular heterojunctions are comprised of a binary blend of electron donor and electron acceptor material, where the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) offsets are sufficient for exciton dissociation. While optimising the morphology of organic photovoltaic active layers is increasingly important, measuring the morphology accurately has for some time been a challenge for researchers in the discipline. Here we report, for the first time, sub-4 nm mapping of donor : acceptor nanoparticle composition in eco-friendly colloidal dispersions for organic photovoltaics.¹ Low energy scanning transmission electron microscopy (STEM) energy dispersive X-ray spectroscopy (EDX) mapping has revealed the internal morphology of organic semiconductor donor : acceptor blend nanoparticles at the sub-4 nm level. A unique element was available for utilisation as a fingerprint element to differentiate donor from acceptor material in each blend system. Si was used to map the location of donor polymer PTzBI-Si in PTzBI-Si:N2200 nanoparticles, and S (in addition to N) was used to map donor polymer TQ1 in TQ1:PC₇₁BM nanoparticles. For select material blends, synchrotron-based scanning transmission X-ray microscopy (STXM), was demonstrated to remain as the superior chemical contrast technique for mapping organic donor : acceptor morphology, including for material combinations lacking a unique fingerprint element, or systems where the unique element is in a terminal functional group and hence can be easily damaged under the electron beam, e.g. F on PTQ10 donor polymer in the PTQ10:IDIC donor : acceptor blend. We provide both qualitative and quantitative compositional mapping of organic semiconductor nanoparticles with STEM EDX, with sub-domains resolved in nanoparticles as small as 30 nm in diameter. The sub-4 nm mapping technology presented shows great promise for the optimisation of organic semiconductor blends for applications in organic electronics (solar cells and bioelectronics) and photocatalysis, and has further applications in organic core–shell nanomedicines.

The recent developments in organic photovoltaic (OPV) field make this technology highly promising with record power conversion efficiencies (PCE) of 19% in single junction solar cells achieved.¹ This printed and low temperature processes technology is very attractive due to several advantages such as a low energy payback time, light weight, flexibility and transparency. However, one of the drawbacks of OPV is the high toxicity of the solvents used to process the active layer which are mainly aromatic and/or chlorinated.

In order to render this PV technology more environmentally friendly, we have been working on the development of water-based organic semiconductor colloidal dispersions.² In this communication, I propose to present our recent developments in this field, on the control of the size and the morphology during the nanoparticle synthesis to fabricated highly efficiency OPV devices. Two strategies have been investigated. On the one hand, attempts to fabricated pure donor or acceptor nanoparticles with diameter below 30 nm have been done. To do so, we have been working on millifluidic systems to control precisely the size of the nanoparticles synthesized by nanoprecipitation. By increasing the flow rate of water anti-solvent, turbulent regimes were achieved in the mixing chambers, leading to smaller nanoparticles size. Using such kind of continuous flow devices, the effect of different additives such as surfactant on the nanoparticle size have been investigated. On the other hand, composite donor/acceptor nanoparticles were fabricated and control of the nanoparticle internal morphology was targeted. To this aim, we have been studying the influence of the surface energies of the donor and acceptor materials on the morphology of donor/acceptor composite NPs. We showed that matching the surface energies of the donor and the acceptor plays a major role to control the internal morphology of the NPs: a large interfacial energy between the donor and the acceptor leads to core-shell structure while a small one tends to give intermixed morphology. Organic photovoltaic devices were fabricated from water-based inks with varying donor/acceptor combination. Optimal performances were with PTQ10:Y6 NP, a donor/acceptor system presenting low interfacial energy. As a result, a NP-based active layer with optimal intermixed morphology was achieved and high efficiency devices with up to 9.98% PCE were fabricated.³ This work highlights the importance of selecting donor/acceptor combination with matching surface energy to ensure an optimal nanoparticle morphology in miniemulsion processes and, ultimately, reach highly efficient water-processed organic photovoltaic devices.

The scanning transmission soft X-ray spectromicroscopy (STXM) technique allows detailed characterisation of nanoscale materials with strong natural contrast mechanisms. STXM scans a sample across a focused X-ray beam and sequentially measures the intensity of the transmitted beam to produce high resolution images (typically about 30 nm) and scans the photon energy of the incident X-ray beam to produce near-edge X-ray absorption fine structure (NEXAFS) spectra of nanoscale objects.[1] While the nanoscale imaging resolution is advantageous, the real power of the technique comes from using spectroscopic effects to achieve strong natural contrast based on molecular structure (and orientation via linear dichroism), elemental composition or oxidation state, and/or magnetisation (via XMCD).[2] The STXM hyperspectral datasets can be quantitatively analysed (i.e. produce percentage composition maps) either by comparing to the known spectra of the component materials, or by grouping sets of similar pixels (i.e. principal component analysis) to identify the materials present and quantify their proportion in each image pixel.

STXM is an excellent tool for characterising nanoengineered polymer films. The molecular structures present in the materials correspond to resonance peaks in the C K-edge NEXAFS spectra (280 – 350 eV), and so STXM images recorded at these photon energies provide strong, natural contrast to differentiate organic materials according to their molecular structures. This works especially well with polymeric materials since the molecular structures are repeated many times and the resulting resonance peaks tend to be intense, providing very clear contrast between the component materials. Here, we will present the operation principles of STXM and discuss some illustrative STXM characterisations of nanoengineered conjugated polymer materials.

The sun’s photons provide the largest source of energy on earth and its exploitation could help solve the energy crisis without being detrimental for the environment. Organic semiconductors, either polymers (usually electron donor) or small molecules (fullerene or non-fullerene acceptors), can be dispersed in water to provide human and environmentally-friendly processes for photovoltaics as alternative to classical options typically using toxic organic solvents. Concerning solar fuel production, Pinaud et al. have demonstrated by life cycle analysis that a colloidal system, in which the photocatalysts are in the form of nanoparticles dispersed in water, would be the cheapest technology to the production of H₂.[1] Therefore there is an important need to study the formation of organic semiconducting nanoparticles and develop their use in these solar-technologies.[2] 

In this communication, I will show what we have developed the 4 last years on this topic and more precisely the use of the Nanoprecipitation methodology to prepare the nanoparticles.[3] I will discuss its advantages and drawbacks and the Donnor/Acceptor morphologies that can be expected from nanoprecipitation. Through two systems, P3HT/PCBM and PTQ10/Y6 I will describe the incorporation of these dispersions as the medium for photocatalyzed hydrogen generation and as the active layer of photovoltaics cells.

Assembling hydrophobic conjugated polymers into water-treatable nanoparticles is an innovative technology with many potential applications in optoelectronics, biology, and medicine. [1][2]
We demonstrate that it is possible to prepare stable water-processable nanoparticles (WPNPs) with good control of the morphology and the domain shape and distribution, exploiting amphiphilic low band gap rod-coil block copolymers (LBG-BCPs). The WPNPs were synthesized by an adapted miniemulsion approach, without  surfactants. [3]
The LBG-BCPs consist of a rigid hydrophobic p-type semiconductor polymer, like PCPDTBT or PTB7, and 4-vinylpyridine(4VP)-based coil blocks. The obtained WPNPs are stable in water thanks to the presence of the 4VP-based coil, which improve the WPNPs stability in water without the presence of surfactants. The used copolymers have different molecular structure and length for each BCP used. To make the WPNPs a good material for OPV device active layer, we mixed the LBG-BCPs with an electron acceptor fullerene derivative ([6,6]-phenyl-C61-butyricacid methyl ester, PC61BM) to achieve blend WPNPs (b-WPNPs). The WPNPs and b-WPNPs were fully characterized by TEM, STEM-EDX, EFTEM, AFM, DLS, and z-potential. [4] [5] [6]
The Transmission Electron Microscopy and the micro-analytical technique associated provide information about morphology, and elemental intraparticle distribution with nanometric resolution (STEM-EDX analysis), which allows us to understand how the LBG-BCPs auto-assemble to give WPNPs. In fact, using the sulfur as a maker of the rod components in the LBG-BCPs, we were able to identify that the rods, the most hydrophobic components of the LBG-BCP, prefer staying in the inner part of each WPNP and the coils, which are more hydrophilic, lay at the surface. Moreover, by exploiting the correlation between the plasmon peak position and the variation of the electron density [7], we were able to identify the b-WPNPs-rich and the PCBM-rich areas and the shape of domains by the EFTEM, allowing us to correlate the domain shape and distribution to the material efficiency. [6][8][9]

An improved understanding of the fundamental chemistry, morphology, and dynamics of polymers and soft materials necessitates advanced characterization techniques that are suitable for in situ and in operando studies. Small angle scattering methodologies have evolved rapidly over the past few decades in response to the ever-increasing demand for more detailed information on the complex nanostructures of multiphase and multicomponent soft materials, such as polymer assemblies and biomaterials. Currently, element-specific and contrast variation techniques—such as resonant (elastic) soft/tender X-ray scattering, anomalous small angle X-ray scattering, and contrast-matching small angle neutron scattering, or a combination thereof—are routinely employed to extract the chemical composition and spatial arrangement of constituent elements at multiple length scales and to examine electronic ordering phenomena. This presentation will discuss the recent development of resonant soft X-ray scattering (RSoXS) at the Advanced Light Source (ALS), which has enabled its application to various critical areas of materials research. RSoXS, by integrating conventional X-ray scattering with soft X-ray absorption spectroscopy, has emerged as a chemically sensitive structural probe that provides a novel method for unambiguously resolving the complex morphologies of mesoscale materials. Tuning the X-ray photon energies to match the absorption spectra of different chemical components allows for the selective enhancement of the scattering contributions from these components, thus offering a detailed view of their complex morphologies. The applications of RSoXS have broadened to include structured polymer assemblies, organic electronics, functional nanocomposites, liquid crystals, and bio/bio-hybrid materials. The advancement in correlative analysis through multimodality, combined with high-throughput and autonomous experiments, is opening a new paradigm in materials research. The further development of resonant X-ray scattering instrumentation with cross-platform sample environments will facilitate multimodal in-situ and in-operando characterization of system dynamics with significantly improved spatial and temporal resolution.