Deanna L. Pickel

Ternary behavior and systematic nanoscale manipulation of domain structures in P3HT/PCBM/P3HT-b-PEO films

Nanophase separation plays a critical role in the performance of donor–acceptor based organic photovoltaic (OPV) devices. Although post-fabrication annealing is often used to enhance OPV efficiency, the ability to exert precise control over phase separated domains and connectivity remains elusive. In this work, we use a diblock copolymer to systematically manipulate the domain sizes of an organic solar cell active layer at the nanoscale. More specifically, a poly(3-hexylthiophene)-b-poly(ethylene oxide) (P3HT-b-PEO) diblock copolymer with a low polydispersity index (PDI = 1.3) is added to a binary blend of P3HT and 6,6-phenyl C61-butyric acid methyl ester (PCBM) at different concentrations (0–20 wt%). Energy-filtered TEM (EFTEM) results suggest systematic changes of P3HT distribution as a function of block copolymer compatibilizer concentration and thermal annealing. X-ray scattering and microscopy techniques are used to show that prior to annealing, active layer domain sizes do not change substantially as compatibilizer is added; however after thermal annealing, the domain sizes are significantly reduced as the amount of P3HT-b-PEO compatibilizer increases. The impact of compatibilizer is further rationalized through quantum density functional theory calculations. Overall, this work demonstrates the possibility of block copolymers to systematically manipulate the nanoscale domain-structure of blends used for organic photovoltaic devices. If coupled with efficient charge transport and collection (through judicious choice of block copolymer type and composition), this approach may contribute to further optimization of OPV devices.

Correlation of polymeric compatibilizer structure to its impact on the morphology and function of P3HT:PCBM bulk heterojunctions

The impact of various polymeric compatibilizers, including end-functionalized P3HTs and diblock copolymers containing P3HT, on the structure and function of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunctions is presented. Careful analyses of small angle neutron scattering curves provide a measure of the miscibility of PCBM in P3HT, the average PCBM domain size, and the interfacial area between PCBM and the P3HT-rich phase in the uncompatibilized and compatibilized systems. Differential scanning calorimetry (DSC) also provides information regarding the changes in the crystallinity of P3HT due to the presence of the compatibilizer. Results show that most compatibilizers cause the domain sizes to decrease and the P3HT crystallinity to increase; however, some cause an increase in domain size, suggesting that they are not effective interfacial modifiers. The correlation of morphology with photovoltaic activity shows that the decreased domain size, increased crystallinity and increased interfacial area do not always result in improved power conversion efficiency (PCE). It appears that the introduction of an insulating molecule at the PCBM:P3HT interface as a compatibilizer results in a decrease in PCE. Thus, the presence of the compatibilizer at this interface dominates the photovoltaic activity, rather than the morphological control.

Assembly and Organization of Poly(3-hexylthiophene) Brushes and Their Potential Use as Novel Anode Buffer Layers for Organic Photovoltaics

Buffer layers that control electrochemical reactions and physical interactions at electrode/film interfaces are key components of an organic photovoltaic cell. Here the structure and properties of layers of semi-rigid poly(3-hexylthiophene) (P3HT) chains tethered at a surface are investigated, and these functional systems are applied in an organic photovoltaic device. Areal density of P3HT chains is readily tuned through the choice of polymer molecular weight and annealing conditions, and insights from optical absorption spectroscopy and semiempirical quantum calculation methods suggest that tethering causes intrachain defects that affect co-facial π-stacking of brush chains. Because of their ability to modify oxide surfaces, P3HT brushes are utilized as an anode buffer layer in a P3HT-PCBM (phenyl-C₆₁-butyric acid methyl ester) bulk heterojunction device. Current-voltage characterization shows a significant enhancement in short circuit current, suggesting the potential of these novel nanostructured buffer layers to replace the PEDOT:PSS buffer layer typically applied in traditional P3HT-PCBM solar cells.

Hydrodynamics of polystyrene–polyisoprene miktoarm star copolymers in a selective and a non-selective solvent

The hydrodynamics of PSnPIn miktoarm (mixed arm) star copolymers made from polyisoprene (PI) and polystyrene (PS) arms are studied in a selective and a non-selective solvent, n-hexane and THF, respectively. It is found that in n-hexane, the number of arms affects the organization of the miktoarm copolymers: stars with 2 arms (a linear diblock for reference purposes) or 4 arms show aggregation in this selective solvent, whereas no aggregation is observed for stars with 8 and 16 arms in the concentration region studied. This behavior is due to shielding posed by the soluble blocks, which prevents the insoluble blocks from coming together. Interestingly, the contribution from aggregates observed for the two arm star (PS1PI1 diblock) at the highest concentration studied is rather small because the chains predominantly exist as single diblocks in n-hexane. This result may be due to the fact that low molecular weight PS is slightly soluble in linear hydrocarbon solvents. The hydrodynamic sizes found in THF are similar to those in n-hexane for the 2 and 4 arm stars but smaller for the 8 and 16 arms stars. We propose that this is a result of both the limited free space needed for motion of the chains and also because of an increased probability of heterocontacts between the collapsed PS blocks and the swollen PI arms near the stars core.

Controlled End-Group Functionalization; Including Telechelics

This review describes the use of living polymerizations for the synthesis of well-defined polymers with chain-end functional groups. The living polymerizations described include anionic, cationic, as well as reversible-deactivation radical polymerization methods (e.g., atom-transfer radical polymerization (ATRP), nitroxide-mediated radical polymerization (NMP), and reversible addition–fragmentation chain transfer (RAFT) polymerization). The two most general methods involve terminal chain-end functionalization and the use of functionalized initiators. These functional end groups can be converted into other functional groups using functional group transformation reactions. Examples of the use of protection–deprotection schemes are described when the functional group is not stable to the propagating chain-end species. General functionalization methods (GFMs) are described; GFMs are reactions that proceed efficiently to introduce a variety of different functional groups.

Combatting ionic aggregation using dielectric forces—combining modeling/simulation and experimental results to explain end-capping of primary amine functionalized polystyrene

Chain-end functionalization of living poly(styryl)lithium using 1-(3-bromopropyl)-2,2,5,5-tetramethyl-1-aza-2,5-disilacyclo-pentane (BTDP) to generate primary amine end-functionalized polystyrene was investigated using high vacuum anionic polymerization techniques. 13C NMR spectroscopy and Matrix Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) were used to evaluate polymer end-groups and demonstrated that quantitative amine functionalized polymer was attained under appropriate reaction conditions. In general, the polymerization of styrene was conducted in benzene and the end-capping reaction was performed by adding tetrahydrofuran (THF) to the reaction prior to the addition of BTDP in THF at room temperature. Results indicated that approximately 20% THF by volume is required to obtain 100% end-capping free from side reactions. When too little or no THF was present, side reactions such as lithium halogen exchange followed by Wurtz coupling resulted in unfunctionalized head-to-head dimer as well as other byproducts. Modeling and simulation of the solvent effects using hybrid methods (the so-called QM/MM method) suggest that THF effectively dissociated the anionic chain-end aggregation, thereby resulting in the desired primary amine functionalized polymer. Molecular dynamics (MD) simulations were conducted to develop an understanding of the physics of counterions involved in the end-functionalization process.