Solution processable semiconductors offer a promising route to flexible, low-cost and scalable optoelectronic devices. Device performance is critically dependent on both the material interfaces present within, as well as active layer molecular conformation. In this thesis, the effects of interface engineering and molecular conformation control are investigated within the context of two contemporary technologies; polymer light emitting diodes (PLEDs) and perovskite solar cells (PSCs).
Firstly, the role of the conjugated backbone structure of conjugated polyelectrolytes (CPEs) when used as the electron injection layer in PLEDs is investigated. It is found that the interface between the CPE and light emitting polymer determines the efficiency and luminance turn-on time of the PLED, allowing general molecular design rules of CPEs to be deduced. Additionally, a novel organic-inorganic hybrid composite material comprised of a CPE and zinc oxide (ZnO) nanoparticles is examined as an electron injection material in inverted hybrid PLEDs. By optimising the ratio of the ZnO:CPE, it is found that this material has great potential as a solution processed electron injection layer in inverted hybrid light emitting diodes.
Next, a novel approach to achieve deep-blue, high-efficiency PLEDs via a simple molecular level conformation change of an emissive conjugated polymer is discussed. Rigid β-phase segments are introduced into a 95% fluorene-5% arylamine copolymer emissive layer, creating intramolecular type II heterointerfaces. The conformational change alters the nature of the dominant luminescence from a broad, charge transfer like emission to a significantly blue-shifted and highly vibronically structured excitonic emission. As a consequence, a significant improvement in the Commission International de L’Eclairage (x, y) coordinates is observed from (0.149, 0.175) to (0.145, 0.123) while maintaining high efficiency (3.60 cd/A, 2.44 lm/W) and improved stability. This approach is further extended to fluorene-benzothiadiazole copolymers.
Finally, the effect of varying the halide composition of mixed halide (bromide-iodide) lead-based perovskite materials on solar cell performance is investigated using a combination of Kelvin probe, air photoemission spectroscopy, surface photovoltage and absorption/emission measurements. It is found that using 25:75 Br:I ratio gives optimum solar cell performance by (i) passivating defect states within the perovskite, (ii) increasing the optical band gap which increases open circuit voltage and (iii) stabilising the perovskite such that no halide phase segregation occurs.
Collectively, these works highlight the critical role material interfaces play in the operation of solution processed optoelectronic devices and how, by suitable interface engineering strategies, device performance can be fully optimised.