Cyclopentadithiophene and phenothiazine based compounds for emerging photovoltaics
Emerging photovoltaics (PVs), such as dye-sensitised solar cells (DSSCs) and perovskite solar cells (PSCs), have attracted myriad attention in the PV field due to their attractive properties, especially suitable for niche applications, such as wearable PV and indoor light harvesting. Although DSSCs and PSCs share common device architecture, the light absorber units are rather different, as are the working principles. For DSSCs, the most researched dyes are metal-free organic dyes, which are designed based on the-state-of-the-art donor-π-acceptor (D-π-A) structure. The common donor units are triphenylamine derivatives due to appropriate electron-donating ability. Thiophene derivatives are used as π-spacer to link the donor to the acceptor. Pd-cross-coupling reactions are routinely conducted to connect the donor and π-spacer. Frequently found in literature, most dyes also feature modified donor units to tune their electron donating ability. In contrast to DSSCs, the light absorbers in PSCs are organic-inorganic or inorganic perovskite semiconductors. The original PSC device architecture is ETM/perovskite/HTM (ETM and HTM = electron- and hole-transport material, respectively). The well-known HTM, Spiro-OMeTAD, has been employed in PSCs showing high performance of 25.2% which is comparable to silicon PVs. Despite the efficacy, Spiro-OMeTAD has suffered from low conductivity and high synthesis cost and purification, inhibiting the large-scale production, thus, commercialisation. Although numerous novel HTMs have been investigated, most are based on complex structures. In this thesis, we aim to simplify the design of dyes and HTMs together with using cost-effective starting materials to realise commercialisation. Chapter 3 describes the simpler dye design by using only commonly used 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) as both donor and π units. This new series of dyes called T-CPDT-1 to -3 also feature a thiophene unit attached directly to cyanoacrylic acid acceptor moiety. UV-Vis absorption shifts bathochromically together with larger extinction coefficient upon extending the CPDT. T-CPDT dyes were applied in DSSC devices with I-/I3- electrolyte and Spiro-OMeTAD in which T-CPDT-3 shows the highest PCE of 5.88% and 4.38%, respectively. When compared with previously-reported CPDT-3 and 5T dyes, T-CPDT-3 shows slower electron recombination kinetics. Interestingly, the solid-state device with T-CPDT-3 achieves very high Jsc of 11.27 mA cm-2 with unusually thin TiO2 film (960- thick). Chapter 4 describes cost-effective dyes by using low-cost materials, phenyl and phenothiazine (PTZ), as the donor and CPDT as π-spacer. We also highlight a Pd-free synthesis to lower realise sustainability. Two dyes, coded BzC and PTZC, were synthesised in high overall yield (68% for BzC and 55% for PTZC). The optical characterisation revealed BzC and PTZC possessed high extinction coefficient of approximately 60000 M-1 cm-1. The best photovoltaic performance showed 3.00% and 5.92% for BzC and PTZC, respectively. The synthesis costs for BzC and PTZC were estimated to be approximately $50 and $60 per gram, respectively, which are about 7.5-9 times lower than that of LEG-4 (used as a reference dye). The cost performance of BzC and PTZC show approximately $0.160 per area per %PCE (20% of LEG-4). In Chapter 5, a new series of HTMs coded as PZC, were synthesised by simple structural design using alkylated CPDT as a core structure and peripheral PTZ units to realise low synthesis cost. The synthesis cost of PZC was estimated to be approximately $30 per gram (around 30% of Spiro-OMeTAD). The optical and electrochemical properties of PZC are comparable to those of Spiro-OMeTAD, confirming good energy alignment with perovskite absorber. The preliminary PSC results of the PZC HTMs showed that they did not appreciable photovoltaic performances. PZHC (with hexyl chains on CPDT) was chosen to study in-depth as a representative to explain poor performance as HTM. The studies showed that PZHC exhibited large reorganisation energy upon oxidation, leading to slow hole hopping rate. The inhomogeneous film morphology of doped PZHC gave rise to pinhole issue. All of these effects are the main causes of detrimental photovoltaic results of PSC with the PZC HTMs.