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

Abstract

Currently, the most common method of optimising organic solar cells is to synthesise new materials before fabricating and testing devices under a wide range of processing conditions to empirically determine the optimum conditions, with other measurements supporting the explanation. This is both a laborious and material intensive process. Simplification of device optimisation processes would likely enable devices to be more easily manufactured at a lower cost. In turn, it would be expected for organic solar cells to become more accessible to the consumer, decreasing the need for fossil fuels. Two approaches to simplifying organic solar cell manufacture were investigated.First, a series of triphenylamine (TPA) centred materials were synthesised to act as nonfullerene acceptors. However, as all four synthesised TPA materials showed comparable optoelectronic properties which were complimentary to that of common donor P3HT, no distinction towards the relative device performance could be made based on these parameters alone. As bulk heterojunction (BHJ) morphology is critical to device performance, differential scanning calorimetry (DSC) in conjunction with X-ray diffraction (XRD) provided improved screening capabilities. When blended with P3HT at varying concentrations and deposited from solution, the morphological differences of the resulting blended films were probed by thermal methods to successfully predict the optimum donor:acceptor pair. Application of Flory-Huggins theory and Kyu plots to determine the interaction parameter ( 0 ) reveal that the materials must be miscible to enable low-energy interfaces, while phase separation into pure domains enables charges to be extracted from the device through continuous percolation pathways. The donor:acceptor pair with the highest positive value of 0 demonstrated a PCE of 2.0% and JSC above 4 mA/cm 2 , while the pair with the lowest 0 value lead to poor device performance (<0.4% PCE) due to excessive mixing. DSC methods were also able to provide a calculated critical solvation concentration (wcrit); below which the donor and acceptor were expected to become immiscible and perform poorly due to limited phase separation and non-ordered regimes. Additional insights provided by DSC analysis include the observation of a coldcrystallisation process; where liquidation upon heating nucleates crystallisation. Typically due to the entrapment of disordered phases, the DSC experiment was able to provide insight towards highly variable device performance, which otherwise may have been attributed to manufacturing errors.Secondly, further simplification of organic solar cells to homojunction (i.e., single component) devices was completed. Materials with low exciton binding energies are able to efficiently separate into free charge carriers and limit recombination pathways. The binding energy of an exciton is dependent on a number of factors, such as that described by the Wannier-Mott construct, which states that the exciton binding energy is proportional to the inverse square of the d...