A better understanding of the materials' fundamental physical processes is necessary to push hybrid perovskite photovoltaic devices towards their theoretical limits. The role of the perovskite grain boundaries is essential to optimise the system thoroughly. The influence of the perovskite grain size and crystal orientation on physical properties and their resulting photovoltaic performance is examined. We develop a novel, straightforward synthesis approach that yields crystals of a similar size but allows the tuning of their orientation to either the (200) or (002) facet alignment parallel to the substrate by manipulating dimethyl sulfoxide (DMSO) and tetrahydrothiophene-1-oxide (THTO) ratios. This decouples crystal orientation from grain size, allowing the study of charge carrier mobility, found to be improved with larger grain sizes, highlighting the importance of minimising crystal disorder to achieve efficient devices. However, devices incorporating crystals with the (200) facet exhibit an s-shape in the current density-voltage curve when standard scan rates are used, which typically signals an energetic interfacial barrier. Using the drift-diffusion simulations, we attribute this to slower-moving ions (mobility of 0.37 × 10^-10 cm² V^-1 s^-1) in combination with a lower density of mobile ions. This counterintuitive result highlights that reducing ion migration does not necessarily minimise hysteresis.

State-of-the-art perovskite-based solar cells employ expensive, organic hole transporting materials (HTMs) such as Spiro-OMeTAD that, in turn, limits the commercialization of this promising technology. Herein an HTM (EDOT-Amide-TPA) is reported in which a functional amide-based backbone is introduced, which allows this material to be synthesized in a simple condensation reaction with an estimated cost of <$5 g⁻¹. When employed in perovskite solar cells, EDOT-Amide-TPA demonstrates stabilized power conversion efficiencies up to 20.0% and reproducibly outperforms Spiro-OMeTAD in direct comparisons. Time resolved microwave conductivity measurements indicate that the observed improvement originates from a faster hole injection rate from the perovskite to EDOT-Amide-TPA. Additionally, the devices exhibit an improved lifetime, which is assigned to the coordination of the amide bond to the Li-additive, offering a novel strategy to hamper the migration of additives. It is shown that, despite the lack of a conjugated backbone, the amide-based HTM can outperform state-of-the-art HTMs at a fraction of the cost, thereby providing a novel set of design strategies to develop new, low-cost HTMs.

The current state-of-the-art hole transporting materials (HTM) for perovskite solar cells are generally synthesized via cross-coupling reactions that require expensive catalysts, inert reaction conditions and extensive product purification, resulting in high costs and therefore limiting large-scale commercialisation. Here we describe a series of HTMs prepared via simple and clean Schiff-base condensation chemistry with an estimated material cost in the range of 4-54 $ per g. The optoelectronic and thermal properties of the materials are linked to the changes in the chemical structure of the HTMs, which allow us to extract design rules for new materials, supported by density functional theory calculations. Charge transport measurements show hole mobilities in the range of 10^-5 to 10^-7 cm² V^-1 s^-1. Upon addition of LiTFSI the HTMs can be oxidized, resulting in a large increase in the conductivity of the hole transporting layer (HTL). When employed as HTL in perovskite solar cells, power conversion efficiencies close to those of spiro-OMeTAD are obtained. In particular, devices prepared with Diazo-OMeTPA show a higher open-circuit voltage. Furthermore, we show that azomethine-based HTMs can act as effective moisture barriers, resulting in a significant increase in the stability of the underlying perovskite film. We assign the improved properties to the presence of a dipole in our molecules which promotes a close molecular packing and thus leads to a high density of the as-formed HTM films, preventing the ingress of water. This work shows that HTMs prepared via condensation chemistry are not only a low-cost alternative to spiro-OMeTAD, but also act as a functional barrier against moisture-induced degradation in perovskite solar cells.

Nanocrystalline cellulose (NCC) is an abundant biogenic nanomaterial with unique properties that enables the efficient synthesis of mesoporous crystalline titania. We significantly enhance the photocatalytic activity of titania thin films by introducing solvothermally synthesized preformed anatase nanoparticles into a sol-gel based biotemplated titania scaffold. The resulting dual source titania thin films containing different amounts of preformed crystalline species were investigated by time-resolved microwave conductivity (TRMC) measurements and tested in the photocatalytic conversion of 4-chlorophenol. The gradual addition of preformed nanoparticles leads to a consistent increase of the mean size of titania crystalline domains, whereas the porosity of the composite is well-preserved due to the shape-persistent nature of the NCC template. Microwave conductivity studies establish increased photoconductivity of the films containing preformed anatase nanoparticles in comparison to that of films made without the nanoparticles. The synergistic features of the dual source titania, namely the improved crystalline properties brought by the preformed nanocrystals in combination with the high surface area provided by the NCC-templated sol-gel titania, result in a very high photocatalytic activity of the films in the photocatalytic decomposition of 4-chlorophenol. In quantitative terms, the dual source titania films prepared with 75% nanoparticles exhibit a first order degradation rate constant of 0.53 h^-1 (1.47 × 10^-4 sec^-1), which strongly outperforms the activity of commercial P90 nanopowder showing a rate constant of 0.17 h^-1 (0.47 × 10^-4 sec^-1) under the same conditions.