Investigation Of Interface Modification Effect On Optical, Electrical And Morphology Properties In Organic Photovoltaics

Organic photovoltaic (OPV) cells have attracted extensive research and development due to their low cost and compatibility with large-scale, flexible, and high throughput roll-to-roll production. To achieve high efficiencies and long lifetime, advances in the design of new absorber materials and device structures are required. Moreover, interface engineering also plays an important role in determining the efficiency of an organic photovoltaic cell and determines different device parameters. Interface, between the electron donor and electron acceptor in activelayer, are responsible for the film morphology and the carrier generation, transporting and recombination; while the interface layer, between the activelayer and the electrodes are responsible for forming ohmic contact, the internal electric field, the film morphology, the carrier recombination rate and device stability. In this dissertation, new approaches for controlling donor/acceptor interface are presented and the development of new interfacial materials in charge transporting layers is presented, for high efficiency and air stability OPVs.In the activelayer interface, two self-organization approaches, liquid-crystalline and intermolecular hydrogen bonds, are used to control the molecular packing and develop easy-to-process bulk heteroj unction films with nanoscale morphologies for organic photovoltaics. First, we report a novel donor-acceptor type liquid-crystalline copolymer, PFcbpDTBT, which contains both electron-donating fluorene and electron-accepting benzothiadiazole units. The films with structural anisotropy can endow the PFcbpDTBT with special features, including absorption band red-shift, fluorescence enhancement and lower lying LUMO level. When blended with fullerene PCBM, the polymer enables PCBM to adopt the preferential well-oriented arrangement in the bulk. From the device annealed at200℃, the power conversion efficiency values reaches1.1%without extensive optimization. Then, a regioregular polythiophene (P3HTM) containing imidazole rings and acceptor including carboxylic acids is synthesized for the intermolecular interaction. The results from red-shifted absorption and enhanced quenching photoluminescence of the P3HTM:PCBA in solvents with polar additives indicate the building blocks through hydrogen bonding interactions. Processing of P3HTM and PCBA complexes with heat-annealing, constructed from cooperative self-assembly, show optimized photovoltaic performance, with PCE reached3.2%. Besides, the achieved optimum nanomorphology after annealing can be freeze using photocrosslinking method to preserve long term performance.For the interface materials between activelayer and electrodes, we developed and synthesized a new thiophene polymer for charge transporting layer. Water-soluble HT-poly[3-(6’-N,N,N-trimethylammonium)-hexyl thiophene](P3HTN) is simply inserted between activelayer and cathode as an interfacial dipole layer by spin-coating, the power conversion efficiency (PCE) of the devices annealed in air is enhanced from1.8%to3.3%, resulting from a reduction of the metal work-function and improved electron extraction efficiency. In particularly, the analogue of active layer as buffer layer could improve interchain interactions between the P3HT and the P3HTN to modify interfacial contact, consequently obtaining an unattainable enhancement Jsc, with respect to the interlayer polymer replaced with unanalogous conjugated polymer. In comparison with P3HTN, fluoroalkyl side-chain diblock copolymers,(P3HT-b-P3FAT) were used as hole transporting layer in inverted device. Driven by the low surface energy of fluoro alkyl side chains, the fluorinated polymers can spontaneously segregate on the surface of activelayer (P3HT:PCBM) during spin-coating processes. Required concentrations of fluoro-polymers to form the self-assembled monolayer on surface are related with block ratios. The fluorinated part forms an interfacial dipole that shifts the work function of the anode metal while the P3HT block can interact with the P3HT donor for hole transport. Overall, devices prepared with copolymer PFT-3HT (3:1ratio of P3HT to P3FAT block) in the activelayer solution displayed PCE values of4.6%(50%increase over a PEDOT: PSS control device) and showed a significant long-term stability in air.Moreover, to meet the requirement of different activelayer, we represent interface optimization work by using hybrid materials. For the acceptor system with high position LUMO level (non-fullerene), the electron transporting layer ZnO modified with PEIE can cause a vacuum level shift up to0.7eV. The reduced work function of ZnO can facilitate the electron transfer from PIDSe-DFBT and achieve higher Voc. However, for inverted solar cells using fullerene as electron acceptor, fullerene-SAM modified ZnO showed the best device performance compared with other polyelectrolyte. Because the fullerene based materials not only passivate the inorganic surface traps but also enhance the electronic coupling at the ZnO-organic interface. Meanwhile, we developed a new GO/MoO3bilayer film as the hole transporting layer, which provides a more continuous and high conductive surface. The device therefore showed significantly improved Jsc, Voc, and FF with PCE up to7.3%in optimized devices. These results indicate that hybrid bilayer interfacial material can take the full advantage of each compound to get the best device performance for various activelayer systems.Lastly, interface modification can also enhance light harvesting in devices. We provide a powerful, general, and tunable means to increased light absorption at desired wavelength bands in existing and emerging OPV materials by embedding various types of colloidal silver nanoprisms into both interfacial layers. By permitting incorporation of the plasmonic particles into layers on both sides of the bulk heterojunction, our method enables universal cooperative optical enhancement up to16-18%in the best cases, with maximum power conversion efficiency from7.7%to9.0%in devices based on PIDTT-DFBT:PC71BM. We show that the method can be applied to various bulk heteroj unction systems with various types of nanoparticles (size and shape), all without compromising the active layer morphology.