Synthesis of novel small molecules for use in bulk heterojunction solar cells
Creators: name: Raynor, Aaron
Publisher: name: RMIT University
Year: 2024

description: Solar power is fast becoming the “go-to” form of renewable energy. The most likely reason for this is the ease at which solar panels can be installed on private residences and other buildings. Government incentives and rebates in countries such as Australia have also spurred along the industry making it an affordable option for the public as well as private business. Although silicon-based panels offer moderate efficiencies and a decent return on investment newer technologies, in particular organic photovoltaics, offer an exciting alternative with the potential to give cheaper power devices with lower impact on the environment. In this body of work, the synthesis of several acceptor and donor compounds used with in the bulk-heterjunction (BHJ) architecture organic photovoltaic device, is presented. The donor compounds, AS-1 and AS-2, were designed in order to investigate the effect that insertion of a thiophene pi-spacer would have of the properties of a triphenyl amine donor-acceptor-donor (D-A-D) compound. Results showed that the thiophene containing compound (AS-2) had far superior optical and electronic properties compaired to AS-1 as well as experiencing no ill effects on solubility or processability despite having no solubilizing alkyl chains. <br><br>The thiophene insertion resulted in a lowering of the band-gap, increased light harvesting ability and superior electronic performance. In simple bulkheterjuction cells using PCBM61 as an electron acceptor AS1 gave an open circuit voltage (Voc) of 0.90 V, a current density (Jsc) of 3.15mA/cm2 and a power conversion efficiency (PCE) of 1.23%. AS2 however greatly outpaced its counterpart with thanks to the thiophene spacer with a Voc of 0.88 V, Jsc of 8.01 mA/cm2 and a PCE of 4.01%. The acceptor compounds consisted of two groups; class-A and class-N compounds. Both class of compounds contained diketopyrrolopyrrole (DPP) as either the central (class-A) or terminal (class-N) moiety. For the class-N compounds N6 and N7 were designed and synthesised. N6 contained a benzothiadiazole central unit with two DPP moieties attached to create an A-D-A structure. The compound exhibited a strong absorption in the visible range and appropriate HOMO/LUMO energy levels to be paired with poly-3-hexyl thiophene (P3HT). When paired with the ever-versatile electron donor P3HT in a 1:1 blend in a bulk-heterojunction device architecture it gave a Voc of 1.08 V, Jsc of 2.06 mA/cm2 and a PCE of 1.08%. At the time of testing this was one of the best performing non-fullerene DPP containing small molecules. N7 was a similar construction to N6, differing in the central moiety. N7 used an N-alkyl substituted carbazole moiety as the central unit. The compound also gave strong absorption in the visible region with appropriately placed HOMO/LUMO levels. When paired with P3HT in a 1:1.2 ratio. A Voc of 1.17 V, Jsc of 3.16 mA/cm2 and a PCE of 2.30% were recorded.<br><br> Using N7 in these devices required no special treatment and the results obtained from them are among the highest values for a single junction bulk-heterojunction cell using a non-fullerene, small molecule acceptor. The class-A compounds consisted of an A-A-A design, in which DPP was to be the central acceptor moiety. The other moieties were to be paranitrophenylacetonitrile (A1), 4-(2-methoxyethoxy)-3-oxobutanenitrile (A2), 3- dibutylbarbituric acid (A3), and 2-ethyl rhodanine (A4). All compounds were synthesised simply by refluxing the bis-carbaldehyde homologue of DPP with the corresponding nucleophile. The synthesis itself appeared to be simple, the desired compound forming as the major product. There was however small amounts (typically 10 – 15%) of the mono-condensed compound. This proved troublesome and practical removal of the byproduct was only achievable for the barbituric homologue. The inability to remove the impurity is in part due to the innate insolubility of both compounds. The optical properties of the class-A compounds however were promising, showing decent absorption in the visible region and it was for this reason A5 and A6 were subjected to photovoltaic testing, despite the poor solubility. Unfortunately they showed little to no activity, which is likely a result of poor solubility leading to inhomogeneity in the blend.