Low optical gap materials for organic solar cells - research and application

Abstract

In spite of the cost of silicon solar cells decreasing in recent years, there is considerable interest in solar cells with lightweight and flexible forms. Organic semiconductor-based photovoltaic cells (OPVs) have the potential to offer low cost, lightweight and flexible devices. At this stage there is still much to be improved for OPVs as the efficiencies are still lower than their inorganic counterparts. Polymeric materials have been more widely studied than their non-polymeric counterparts. In recent years OPVs polymeric donor materials have emerged with efficiencies reaching 10% with quite a range of materials now providing efficiencies of around 7%. However, the problem with semiconducting polymers is that control of the regioregularity, poly(dispersity), and molecular weight from batch-to-batch is not a simple process. Solution-processable non-polymeric semiconductors are attractive for opto-electronic applications as they are simpler to synthesise, purify, characterise, and the optical properties can be more easily fine-tuned. In this context, diketopyrrolopyrrole (DPP) based small molecules have shown promising results with devices efficiencies reaching around 5-6%. Key properties of the DPP unit are strong p-p interactions, and suitable and tunable energy levels. These properties have been the main drivers in their use in OPVs. This thesis consist of the synthesis, characterisation, and bulk hetrojunction (BHJ) devices properties of novel DPP(ThAr)2 non-polymeric materials. Accordingly, a first series of compounds was synthesized with different electron affinity groups such as fluorenone (Fl) [EhDPP(ThFl)2 and OddDPP(ThFl)2] and benzothiadiazole (Bt) [OddDPP(ThBt)2] attached to the distal ends of a bisthienyl-diketopyrrolopyrrole unit DPP(Th)2 unit. The lDPP(ThAr)2r derivatives were synthesised under standard Suzuki-Miyaura cross-coupling or by direct arylation reaction conditions and the rate of the direct arylation reaction was found to be faster than for the Suzuki-Miyaura cross-coupling reactions. The solubility of the compounds was poor when the 2-ethylhexyl (Eh) moiety was used as the solubilising group for DPP(ThFl)2. 2-Octyldodecyl (Odd) solubilising groups gave better solubility in chlorinated solvents, which are typically used in OPV fabrication. The three non-polymeric DPP materials exhibited bipolar charge transport in organic field effect transistors (OFETs), with hole and electron mobility in the order of 10-2m10-3 cm2 V-1 s-1, and in a diode architecture OddDPP(ThFl)2 and OddDPP(ThBt)2 exhibited mobilities of the order of 10-4 cm2 V-1 s-1 for pristine films. In BHJ devices using [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM) as the acceptor, different solvent mixtures were used for device fabrication to achieve the maximum efficiency. The EQE spectra of the blend film suggested that charge generation due to absorption at wavelengths longer than 750 nm occurred through the Channel I pathway, and at wavelengths shorter than 750 nm, both Channel I and II pathways were in play. OddDPP(ThFl)2 was found to be the best material when acting as an electron donor in BHJ device and achieved maximum PCE of 4.1%. To further study the charge transporting properties of OddDPP(ThFl)2 based materials, its dithiane derivative (SS) [OddDPP(ThFl(SS)2)2)] was prepared. OddDPP(ThFl(SS)2)2 exhibited different thermal and optical properties, which had a direct impact on the OPV device performance and the devices performed poorly. Next, a series of OddDPP(ThBt)2 derivatives were prepared with different electron withdrawing groups, dicyanovinylene (DCV) [OddDPP(ThBt-DCV)2] and n-butyl-2-cyanoacetate (B2A) [OddDPP(ThBt-B2A)2] as end groups. The end groups had a drastic effect on both the thermal and optoelectronic properties. The materials exhibited a similar electron affinity to that of [6,6]-phenyl-C60-butyric acid methyl ester (PC60BM) and hence were used as an electron acceptor with poly(3-n-hexylthiophene) as the electron donor. OddDPP(ThBt-DCV)2 lacked sufficient solubility to be solution processed, but OddDPP(ThBt-B2A)2 could be. The EQE spectra of the OPVs suggested that the Channel II mechanism was responsible for charge generation for wavelengths above 650 nm and both Channels (I and II) wavelengths shorter than 650 nm. However, the best device efficiency was only 0.1%. A final series of materials consisted of 9,9r-bifluorenylidene (BF) and its derivatives as end groups. BF-based materials have been previously reported to give rise to electron accepting materials. In this series the basic structure was kept same, with the difference being simple addition of methoxy groups (OMe) or fluorine (F) atoms to OddDPP(ThBF)2 to form OddDPP(ThBF(OMe)2)2) and (OddDPP(ThBF(F)2)2), respectively. However, only the parent OddDPP(ThBF)2 and the methoxy derivative, OddDPP(ThBF(OMe)2)2 could be purified and characterised. The materials exhibited different thermal phase transition behaviours, but had similar optoelectronic properties. Devices fabricated with BF derivatives and the acceptor (PC70BM) from chloroform and chloroform with 10% dichlorobenzene exhibited similar device performance. However, the device performance was poor. Interestingly, the film fabricated from chloroform with 0.5% 1,8-diiodooctane (DIO) exhibited the best device performance and the OddDPP(ThBF)2:PC70BM OPV had an efficiency of 2.6%.