The semiconducting conjugated polymers are important class of materials for high performance organic electronics including polymer solar cells (PSCs) and organic field-effect transistors (OFET). Especially, the low-bandgap conjugated polymers which are constructed from a polymer framework with an alternating electron-rich donor and an electron-deficient acceptor repeating units, have attracted significant attention because of their potential for generating the high photo-current density of high performance PSCs. Additionally the design of low-bandgap conjugated polymer can offer an opportunity for tuning not only the bandgap but also energy levels which can modulate the open circuit voltage of the PSCs through the appropriate choice of the donor and acceptor building blocks.
In this thesis, we designed and synthesized novel low-bandgap conjugated polymers based on diketopyrrolo[3,4-c]pyrrole (DPP) for photovoltaic application are investigated. First, we synthesized a low-bandgap conjugated polymer, PDTTDPP, which has a D?A structure with dithieno[3,2-b:2’,3’-d]thiophene (DTT) as an electron donating unit and DPP as an electron accepting unit, respectively. The PDTTDPP showed a low-bandgap of 1.22 eV due to strong intra-molecular charge transfer between DTT and DPP, and the strong intermolecular π?π stacking of polymer backbone contributes to high hole mobility of PDTTDPP in OFETs. Consequently, the high hole mobility and broad light absorption of PDTTDPP achieves a high PCE of 6.05% under the optimized morphology of the PDTTDPP-based PSCs.
While the PSCs with the DPP-based low-bandgap polymers have yielded the promising PCE with high JSC, the low VOC around 0.6 V is a main limiting factor for high efficiency PSCs, since the DPP-based conjugated polymers usually shows relatively high-lying highest occupied molecular orbital (HOMO) level around ?5.1 eV. One of the effective methods to lower the HOMO level of the conjugated polymer is the removal of the substituents which usually act as electron-donating groups. Therefore we synthesized the DPP-based low-bandgap conjugated polymer using unsubstituted benzo[1,2-b:4,5-b’]dithiophene as an electron donor and DPP as an electron accptor. This approach affords the low HOMO level of ?5.46 eV of the polymer and then the PSCs exhibited a PCE of 5.2% with a VOC of 0.82 V, which values are much higher than those of alkoxy substituted benzodithiophene-based polymer.
The further enhancement of the PCE of PSCs can be achieved by extending the light absorption range of the conjugated polymer because the limited absorption of the conjugated polymer leads to lower current density. A fascinating approach to extend light absorption range is the synthesis of copolymers composed of at least two different chromophores. For the purpose of broadening the light absorption and enhancing the performance of PSCs, we synthesized novel random conjugated copolymers consisting of DPP and isoindigo as co-electron acceptor units of donor-acceptor type conjugated polymer. The random copolymers exhibited not only intense and broad light absorption range of 500?900 nm but also low-lying HOMO levels. Furthermore, the predominant face-on orientation of the random copolymers on the substrate is beneficial for vertical charge transport in the device. The combination of excellent optoelectrical properties and favorable molecular conformation makes these random copolymers a promising candidate for active layer material in high-performance PSCs.