Niko Van den Brande

High-Permittivity Conjugated Polyelectrolyte Interlayers for High-Performance Bulk Heterojunction Organic Solar Cells

Conjugated polyelectrolyte (CPE) interfacial layers present a powerful way to boost the I-V characteristics of organic photovoltaics. Nevertheless, clear guidelines with respect to the structure of high-performance interlayers are still lacking. In this work, impedance spectroscopy is applied to probe the dielectric permittivity of a series of polythiophene-based CPEs. The presence of ionic pendant groups grants the formation of a capacitive double layer, boosting the charge extraction and device efficiency. A counteracting effect is the diminishing affinity with the underlying photoactive layer. To balance these two effects, we found copolymer structures containing nonionic side chains to be beneficial.

All-conjugated cationic copolythiophene “rod–rod” block copolyelectrolytes: synthesis, optical properties and solvent-dependent assembly

Amphiphilic diblock copolythiophenes were synthesised by an efficient two-step strategy. The diblock copolyelectrolytes were obtained via quasi-living Kumada catalyst-transfer polycondensation followed by quaternisation of the bromohexyl side chains of one of the monomer constituents into N-methylimidazolium, pyridinium, trimethylammonium or trimethylphosphonium units. The effect of the nature of the charged group on the thermal properties was investigated by Rapid Heat–Cool (RHC) calorimetry measurements. The solvent-driven assembly of these block copolyelectrolytes in chloroform (CHCl3), water, methanol (MeOH), water–MeOH mixtures and in subsequently prepared thin films was investigated using a combination of photoluminescence, scattering and microscopic techniques. The rigid rod-structure of the block copolyelectrolytes led to the formation of core–shell cylindrical or disc-like aggregates in solution, with features determined by the nature of the solvent. AFM studies revealed that the aggregates formed in solution can be transferred into thin films allowing for the reliable control of the self-organisation process and the resulting nanoscale architecture.

Synthesis of ester side chain functionalized all-conjugated diblock copolythiophenes via the Rieke method

Ester-functionalized all-conjugated diblock copolythiophenes are synthesized by an optimized Rieke zinc protocol. These light-harvesting semiconducting electron donor polymers are of high interest in the field of organic photovoltaics as both the block architecture and the (functionalized) side chains play a crucial role in determining the bulk heterojunction active layer blend (nano)morphology and thereby the final polymer solar cell efficiency and lifetime.

X-Ray Nanoscopy of a Bulk Heterojunction

Optimizing the morphology of bulk heterojunctions is known to significantly improve the photovoltaic performance of organic solar cells, but available quantitative imaging techniques are few and have severe limitations. We demonstrate X-ray ptychographic coherent diffractive imaging applied to all-organic blends. Specifically, the phase-separated morphology in bulk heterojunction photoactive layers for organic solar cells, prepared from a 50:50 blend of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) and thermally treated for different annealing times is imaged to high resolution. Moreover, using a fast-scanning calorimetry chip setup, the nano-morphological changes caused by repeated thermal annealing applied to the same sample could be monitored. X-ray ptychography resolves to better than 100 nm the phase-segregated domains of electron donor and electron acceptor materials over a large field of view within the active layers. The quantitative phase contrast images further allow us to estimate the local volume fraction of PCBM across the photovoltaically active layers. The volume fraction gradient for different regions provides insight on the PCBM diffusion across the depletion zone surrounding PCBM aggregates. Phase contrast X-ray microscopy is under rapid development, and the results presented here are promising for future studies of organic-organic blends, also under in situ conditions, e.g., for monitoring the structural stability during UV-Vis irradiation.

Structure-property relations of high permittivity cathode interlayer materials toward bulk heterojunction organic solar cells(Conference Presentation)

Organic photovoltaics (OPV) show strong potential for a number of renewable energy applications because of some specifically appealing features (light weight, flexibility, color, …). Over the past decade, the power conversion efficiencies of organic solar cells have strongly risen to values surpassing the 10% threshold, mainly due to strong efforts in chemical engineering of the photoactive components, architectural device optimization and acquisition of fundamental insights in the underlying device physics. As part of the device optimization, the use of conjugated polyelectrolyte (CPE) interfacial layers has been introduced as a popular and powerful way to boost the inherent I-V characteristics. In the presented work, we applied impedance spectroscopy to probe the dielectric permittivity of a series of polythiophene-based CPE interlayer materials as a means to postulate design rules toward novel generation interfacial layers. The presence of ionic pendant groups grants the formation of a capacitive double layer, boosting the charge extraction and device efficiency. A counteracting effect is that the material’s affinity with respect to the underlying photoactive layer diminishes. To enhance the interlayer-photoactive layer compatibility, copolymer structures containing a certain amount of non-ionic side chains are found to be beneficial.

Crystallization kinetics and morphology relations on thermally annealed bulk heterojunction solar cell blends studied by rapid heat cool calorimetry (RHC)

Optimizing the post-production annealing conditions of polymer:fullerene bulk heterojunction solar cells is vitally important, not only for fine-tuning the morphology - thus increasing the efficiency - but also for retaining the desired morphology during long-term operation. However, optimal conditions for annealing temperatures and times can only be chosen, once thermal transition temperatures and annealing kinetics of the blends are well-known. For instance, for systems with glass transition temperatures (Tg) lower than the maximum device operation temperature of 80°C, the mobility needed for morphology coarsening is present, leading to efficiencies decreasing in the course of time. Using advanced fast-scanning thermal analysis techniques, the formation of nuclei and growth of crystals during heating or cooling can be reduced or avoided, and thus, the fast crystallization processes occurring during annealing of the polymer:fullerene blends can be followed. In this study, non-isothermal and isothermal crystallization kinetics of the P3HT:PCBM (poly(3-hexyl thiophene: [6,6] -phenyl C61 - butyric acid methyl ester) and P3HT:bis-PCBM blends are investigated and compared by using Rapid Heating Cooling Calorimetry (RHC).

Looking at bulk-heterojunction organic photovoltaics from two viewpoints: morphology development and charge transfer

In this paper, a combined experimental and theoretical study was performed on the P3HT:PCBM system used in organic photovoltaics. Fast-scanning differential chip calorimetry, an advanced thermal analysis technique, was used to simulate the thermal annealing used in the production of P3HT:PCBM solar cells to increase the degree of crystallinity, and thus efficiency. The main advantage of this technique for stuying the thermal annealing are the very high rates of heating and cooling that can be used, up to 106 K.s-1, permitting one to avoid crystallization during cooling. In parallel with the experimental study, the charge transfer between donor (P3HT) and acceptor (PCBM) at the interface is studied using density functional theory. The charge separation between donor and acceptor present for the ground state of the combined system, diminished when the first triplet was investigated. This was explained by the formation of a bridge state, formed after population by the LUMO with one electron. Such a molecular orbital can facilitate charge transfer.