Synthesis and application of pi-conjugated polymers for organic solar cells

Organic photovoltaic cells (OPCs) hold the promise of being a cheap and environmentally benign large scale renewable energy resource. The photoactive layer in OPCs is based on a blend, or bulk heterojunction, of an electron donor (p-type) and an electron acceptor (n-type) material. The operational principle involves the absorption of light to create an exciton that diffuses to the p-n interface and is split into charges that are subsequently transported to the electrodes and collected. All of these processes have to occur with high quantum efficiencies and minimal energy losses to make a high efficiency solar cell. In this thesis new materials have been developed by means of synthetic chemistry to improve the efficiency of existing materials in OPCs by absorbing a larger part of the solar spectrum and minimizing energy losses in the conversion process. The morphologies of the new donor-acceptor bulk heterojunction layers have been optimized for high efficiency by influencing the degree of mixing of the two materials. In designing new small band gap polymers for bulk heterojunctions that can absorb and convert a large part of the solar spectrum various aspects need to be considered. The frontier energy levels of the new polymers must be designed to minimize energy losses by increasing the open-circuit voltage with respect to the optical band gap, while maintaining a high coverage of the absorption with the solar spectrum. Another balance is the amount of interface between the two materials that should be large to create free charge carriers, while maintaining percolating domains of pure material to transfer these charges to the electrodes. To prevent recombination during the transport, free charge carrier mobility in the two materials should be high. In a first approach, alternating copolymers based on cyclopentadithiophene (CPDT) and five different electron deficient aromatic units with reduced optical band gaps have been synthesized via palladium catalyzed coupling. All polymers showed a significant photovoltaic response when mixed with a fullerene (PCBM) as acceptor. The best cells have been obtained for a copolymer of CPDT and benzooxadiazole with a band gap of 1.5 eV. This cell gives a power conversion efficiency (PCE) of about 2.5%. Next, a series of polythiophenes (PTn) based on CPDT units alternating with short oligothiophenes of different length n along the chain has been synthesized to control the morphology of PTn:PCBM blends via the chemical structure of the polymer, rather than via processing conditions. The degree of phase separation in PTn:PCBM blends can be controlled via n, because with increasing n the number of solubilizing side chains per thiophene is reduced. The best cells and most intimately mixed morphology were obtained for PT2 that exhibits a PCE of about 1.5% when mixed with PCBM. Although the final efficiency is moderate, the study represents an example of a rational approach towards morphology control via chemical structure. To maximize the open-circuit voltage of polymer:PCBM cells, a new polymer has been synthesized based on benzothiadiazole and a substituted thienothiophene, to create a material with a relatively deep lying level of the highest occupied molecular orbital. To obtain working and reproducible devices, the poly(ethylenedioxythiophene):poly(styrenesulfonate) hole conducting layer was treated with UV-ozone. This treatment increased the work function of the electrode, facilitating an Ohmic contact with the polymer in the active layer. The open-circuit voltage of 1.15 V corresponds to the highest value obtained for any polymer:PCBM cell to date and matches with the previously predicted ultimate limit for PCBM-based OPCs. In a new approach a semiconducting polymer with alternating diketopyrrolopyrrole (DPP) and terthiophene units has been developed with a small band gap of 1.3 eV. This polymer exhibits high, nearly balanced hole and electron mobilities of 0.04 cm2 V-1 s-1 and 0.01 cm2 V-1 s-1, respectively in field-effect transistors (FETs). When the polymer was combined with [60]PCBM or [70]PCBM, photovoltaic cells were made that provide a photoresponse up to 900 nm and a PCE of 3.8 and 4.7% in sunlight, respectively. The efficiency of the photovoltaic cells was found to be strongly dependent on the molecular weight of the polymer and the use of processing agents during film formation. In a further development, a new easily accessible, alternating DPP and dithienylphenylene co-polymer has been developed, again with high electron and hole mobilities, exceeding 0.01 cm2 V-1 s-1. PCEs of 4.6 and 5.5% were obtained with [60]PCBM and [70]PCBM. The performance of these cells strongly depends on the use of a processing agent, 1,8-diiodooctane (DIO), during film formation. The active layers have been studied with atomic force microscopy and transmission electron microscopy and show vast variations in size of the PCBM clusters upon application of DIO, together with appearance of fiber-like structures. Photoinduced absorption measurements support the generation of more charges in the optimized morphology. The use of thienothiophene (TT) as a co-monomer was further explored by alternation with DPP units in an attempt to combine the successful strategies for improved performance described above. Two polymers, with substituted and unsubstituted thienothiophenes were prepared. No significant difference was observed in charge carrier mobility, but the photovoltaic behavior was better for the polymer with more and shorter side chains (2.3% vs. 1.2%). A further co-polymer with extra thiophene units between TT and DPP in the chain was also synthesized. This polymer has a lower charge carrier mobility in an FET, but outperformed the other polymers in an OPC, with the best cell exhibiting a PCE of 3.4%. One reason for the overall lower PCE in the TT-DPP copolymers is the moderate molecular weight of the materials obtained. In further exploring the electron deficient DPP structural motif for photovoltaic polymers, furan rings were considered as a building block in the main chain. Four small band gap copolymers based on DPP alternating with electron rich trimers of benzene, furan, and thiophene have been synthesized via Suzuki polymerization. The polymers show optical band gaps between 1.4 and 1.6 eV, optimized for solar energy conversion, and exhibit ambipolar charge transport in field-effect transistors with hole and electron mobilities higher than 10-2 cm2 V-1 s-1. In solar cells the polymers are used as electron donor and provide power conversion efficiencies up to of 3.7% in simulated solar light when mixed with [70]PCBM as acceptor. Again the reduced molecular weight of the new materials is possibly limiting the PCE compared to higher efficiencies reached earlier. With the development of DPP type polymers a new, highly successful, class of promising materials for OPCs has been created. The final efficiencies obtained, compare favorably to the state of the art in the field. Several of the materials made have close to optimal energy levels for very efficient OPCs. Hence further improvements can be expected when improved control over molecular weight, morphology, and charge carrier mobility can be obtained.