| With the development of human society, traditional energy, such as petroleum and gas are consumed heavily. At the same time, the consumption of traditional energy brings a lot of pollution, which limits the sustainable development in modern society. Harnessing solar energy is one of the most promising way to tackle today’s energy issues. Although the present dominant commercial photovoltaic technology is based on inorganic materials, high material and manufacture costs limit some areas application. Intensive research has been focused on the development of low cost photovoltaic technologies. With the rapid development of organic electronics, new organic materials are synthesized with outstanding parameters, such as high mobility, high absorption coefficient and high solubility. The new materials have been applied in organic solar cells (OSCs). Over the past decades years, OSCs based on conjugated polymers and fullerene have attracted considerable attention because of their advantageous properties including flexibility, low weight, easy and solution-based process without high temperature and vacuum. To date, best-performance OSCs can be achieved with power conversion efficiency beyond 10%. It shows a bright application in the future life.Briefly, the OSC working process can be explained in four steps:(ⅰ) the excitons (Frenkel exciton) were formed after absorbing photons in the donor material; however, free charge carriers are formed after photoexcitation in inorganic semiconductor; (ⅱ) the excitons diffused to the interface between the donor and acceptor; (ⅲ) the excitons dissociated at the interface to form charge-transfer state; (ⅳ) the electron and hole were transported to the cathode and anode respectively. During these processes, some theory believed that energy offset between the donor and acceptor determines the efficiency of exciton dissociation. However, recent ultrafast spectra and theory calculation demonstrated that driving force for charge separation is the vibrational coherence between electron and nuclear and the molecule delocalization level. Therefore, it is necessary to understand the photophysical processes of the polymer systems, which plays a significant important role in exploring the working mechanism of the OSCs. Generally speaking, the mobility of organic semiconductor is smaller than that of inorganic semiconductor leading to the short diffusion length of exciton (about 10 nm). Therefore, the phase separation and the microstructure of the active layer control the performance of OSCs. The basic component of the donor or acceptor is carbon, which results in the low contrast between donor and acceptor. Besides, the phase separation between donor and acceptor is always small in the nanometer level. It is difficult to investigate the microstructure of OSCs with the conventional techniques. The phase separation of donor and acceptor determines the charge transport and collection. Thus, with a better understanding of the phase separation in the vertical direction, we could optimize the device structure with different structures (conventional or inverted structure) to improve the efficiency of OSCs. The photophysical processes and nanomorphology of OSCs are investigated in the thesis to improve the performance of OSCs.The key research work and results of this thesis are listed as follows:1. First of all, we studied the effect of alkyl side-chain on the photophysical processes of polythiophene, such as exciton generation, energy transfer, charge dissociation and excition recombination with femtosecond fluorescence up-conversion technique. The result showed that the fluorescence lifetime and electron transfer speed is not linear to the length of side chain. While the polymer chain have conformation ’memory’ evolving from solution to solid state. Furthermore, we found that the long alkyl side-chain of polythiophene is beneficial of optoelectronic device fabrication owing to the high and reversible on/off and photostability.2. Secondly, the active layer of OSCs was modified by solvent additive. Solvent additive process is a simple and effect way to optimize the morphology of active layer and can be intergraded with roll-to-roll solution process technique. P3HT and PCBM are selected as the donor and acceptor materials respectively. We compared the different effect of thermal annealing and solvent additive treatments on the nanomorphology of P3HT:PCBM blend films. The result demonstrated that thermal annealing and solvent additive all facilitate P3HT forming more ordered structure. Solvent additive could enhance PCBM aggregation to form interconnected transport network and keep the large interfacial area facilitating excition dissociation. However, thermal annealing would decrease the interfacial area between P3HT and PCBM.3. Thirdly, solvent additive is also suitable in PTB7:PCBM bulk heterojunction OSCs with high power conversion efficiency. We investigated the effect of DIO on the nanomorphology of PTB7.PCBM system. Time-resolved PL was used to explore the structure evolution from solution to solid film. The phase separation between PTB7 and PCBM was quantitatively analyzed by the combination of atomic force microscopy (AFM) and grazing-incidence small angle X-ray scattering (GISAXS) techniques. The vertical phase separation was observed by X-ray photoemission spectroscopy (XPS). The results showed that DIO disperses PCBM aggregation in solvent. During the film formation, PCBM would not aggregate due to the higher boiling temperature of DIO than chlorobenzene and preferential solubility of PCBM in DIO than PTB7. With the solvent evaporation, PTB7 chains solidify to framework initially. The PCBM molecules adsorbed to PTB7 matrix act as nuclei and accumulated along the polymer chains. Meanwhile, the solidified polymer matrix confines the PCBM domains from growing to large scale. The quantitative analyses on the phase separation of PTB7 and PCBM indicated that the domain size of PCBM aggregation decreases from 67 nm to 38 nm with the DIO addition. In addition, DIO could induce the uniform distribution of PTB7 and PCBM in the vertical direction. The working mechanisms of solvent additive in PTB7 and P3HT systems are different. Therefore, the solvent additive is just suitable to a specified polymer system. The effect of vertical phase separation on the device performance of OSCs was investigated with conventional and inverted structures. The OSC with an inverted structure demonstrated a superior performance that that with the conventional structure owing to the acceptor accumulated in the cathode. However, the inverted and conventional structure of OSCs manifested the similar performance with the DIO treatment. These results further verified that DIO induces PTB7 and PCBM homogeneous distribution in the vertical direction.4. Fourthly, it is challenging to obtain broadband coverage with a single polymer owing to the discrete band structure and the narrow molecular orbital energy level of the polymer. It caused a large amount waste of light energy and a narrow band photoresponse. A simple and effective way, adding a third polymer with a complementary absorption region, was adopted to broad the photoresponse region and improve the on/off ratio. The improved device performance was explained by investigating the photophysical processes and nanomorphology in ternary devices. Femtosecond fluorescence up-conversion technique was used to quantitatively analyze the Forster energy transfer efficiency (FRET) between P3HT and PTB7. The result showed that the efficiency reached 94.2% in the blend film with 50% PTB7 addition. However, in the ternary system, large amount of PTB7 will lead to the increased percentage of exciton recombination. The morphology demonstrated large amount of defects and recombination centers with AFM and GISAXS measurements. The effect of PTB7 on the photoresponse of ternary device was studied by external quantum efficiency (EQE). The enhancement of EQE in the region from 600 nm to 750 nm was caused by PTB7 extra absorption. In the region from 450 nm to 600 nm, the high efficiency of energy transfer contributes to the enhancement of EQE. Besides, PTB7 acted as the energy cascade between P3HT and PCBM, leading to the EQE enhancement in the region from 350 nm to 450 nm. We found that the bimolecular recombination was relative high in the ternary system with 50% PTB7 addition by the relationship between short circuit and light power. This may be related with the structural defects and recombination centers. Therefore, it is needed to balance the energy transfer and morphology in the ternary system to achieve high on/off ratio and broadband response.5. Last but not the least, nanodiamond (ND) shows small size, stable physical and chemical property, abundant material, low cost fabrication and semiconductor property. The bandgap of ND is similar with PCBM. We investigate the nanomorphology and photophysical property of P3HT.ND blend films. It is demonstrated that the addition of ND could increase the conjugation length of P3HT and improve the crystallization of P3HT. Charge transfer processes occurred from P3HT to ND with the observation of time-resolved fluorescence spectroscopy. Energy transfer was excluded due to the larger bandgap of ND than P3HT. However, the morphology investigation showed that the solubility of ND is low. So we would increase the solubility and dispersion of ND in solution in the next step.The innovation in this work is focused on the deep understanding of the photophysical processes of polymers and their blend films. Besides, we have concluded a series methods, such as time-resolved fluorescence spectroscopy, AFM, GISAXS and XPS to analyze the microstrucrure and phase separation of the active layer quantitatively. These method could be utilized to observe the morphology evolution from solution to solid film and the vertical phase separation. Ternary bulk heterojunction was used to achieve broadened absorption region, high on/off ratio and power conversion efficiency. We also analyze the sophisticated photophysical processes in the ternary system. The charge transfer dynamics between P3HT and ND was investigated. These results may be helpful to the investigation on the new polymer and fullerene blends, photophysical processes and complicated ternary system. |
