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 nonpolymeric 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 π-π 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 'DPP(ThAr) 2 ' 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 crosscoupling 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 nonpolymeric DPP materials exhibited bipolar charge transport in organic field effect transistors (OFETs), with hole and electron mobility in the order of 10 -2 −10 -3 cm 2 V -1 s -1 , and in a diode architecture OddDPP(ThFl) 2 and OddDPP(ThBt) 2 exhibited mobilities of the order of 10 -4 cm 2 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 iii wavelengths longer than 750 nm occurred through the Channel I pathway, and at wavelengths shorter than 750 nm, both Channel I and II ...

Development of a Library of Novel Compounds for Use in Organic Solar Cells

“…The organic phase was then dried over anhydrous sodium sulphate and solvents removed to give 2.14 as a pale yellow solid (3.16 g, 39%). The 1 H NMR and TLC retention time were consistent with an authentic sample 278.…”

“…As discussed within the retrosynthetic analysis, previous work has found that the aldehyde derivative, 2.14, is not suitable for direct C-H activated arylations due to functional group interference leading to low yields and unwanted by-products. 278,282,283 Therefore, the aldehyde functional group was protected with ethylene glycol in a refluxing solution containing p-toluene sulfonic acid and toluene fitted with a dean stark condenser. The acetal derivative was isolated in a 91 % yield.…”