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Author Geraghty, Paul Bythell

Title Materials development to achieve thermally robust high performance organic photovoltaic devices

Published 2017


Location Call No. Status
Physical description 1 online resource
Thesis notes Thesis (PhD thesis)-- School of Chemistry 2017
Summary Organic photovoltaic (OPV) devices utilize solution-processable organic semiconductor molecules as the photoactive component. OPVs offer an alternative to inorganic semiconductors (e.g. silicon solar panels) with the dominant advantage of cheap fabrication costs due to roll-to-roll (R2R) printing processes. This is the fundamental driving force behind OPV materials and realising them as a commercial product. The photocurrent generation process relies on precise thin film structure of an interconnected network of an electron rich (donor, D) material and an electron deficient (acceptor, A) material in the active layer. Excitation of an OPV device generates a coulombically bound exciton that requires an energy offset between D and A materials due to their differing electron affinities in close proximity to one another, where the exciton can be split into free charge carriers, a radical electron and radical hole. Optimisation of morphology of the active layer and interfaces between the D and A materials is essential to enable a good charge generation/separation process. Polymeric organic semiconductors are typically used as the light harvesting donor (D) material with fullerene derivatives as the A material. However, batch-to-batch variations limit the reproducibility of these polymeric materials, and a prominent shift has begun towards donor molecular materials (MMs) with electron deficient end groups, Ï€-bridges and an electron rich core, typically denoted as A-Ï€-D-Ï€-A. MMs have advantages due to their discrete structure, relative ease of purification and have shown high power conversion efficiencies (PCEs) on par with polymer- based systems. Unfortunately the discrepancy of PCEs between lab scale devices (0.1mm2, 13%) and R2R devices (3 - 7%) is large, but the gap is closing. This is primarily due to the lack of high performance materials that are suitable for the R2R printing process. There are inherent problems between scaling from small area devices and R2R printed solar cells. Firstly, different active layer deposition processes between small area (spin cast) and R2R (Gravure) result in considerably thicker layers for R2R processes. Secondly, R2R printing requires thermal annealing (TA) treatment to dry the back electrode typically at 120 °C for 10 min. The MM benzodithiophene terthiophene rhodanine, BTR and polymeric donor, p-BDT- BT have demonstrated high PCEs with PC71BM for lab scale OPV devices of 9.3% and 9.4% PCE, respectively. BTR OPV devices also maintain high PCEs for thick active layers (̃8% PCE, 400 nm thick films) making it an excellent candidate for printing application. However, TA treatment of both donor materials results in significant performance degradation of the respective OPV device. In this thesis targeted donor materials design is used to develop thermally robust, high performance OPV devices (>8% PCE) around the known polymeric donor, p-BDT-BT and MM, BTR to facilitate scale up from small area (lab scale) devices to flexible, R2R processed large area devices and industrial applicability. A synthetic optimization of both the p-BDT-BT and BTR materials was conducted to improve the quantity of material to enable further device processing optimisations. Only 30 mg of p- BDT-BT and <1 gram of BTR could easily be isolated per batch under the previous synthetic conditions. The synthesis of p-BDT-BT could not be improved upon, as the polymerisation was anomalous and the polymer weight could not be predicted using the Carothers equation. The synthetic route to obtain BTR was changed from a Stille cross coupling to a Suzuki-Miyaura cross coupling that proceeded with a key intermediate that was synthesised on 0.6 mol scale. The synthesis of two new series of MMs was investigated where the conjugation length of BTR was decreased or increased by 3-hexyl thiophene units to give the BXR series or the rhodanine electron deficient end group was changed for stronger electronegative moieties to give the BTA series. Both the BXR and BTA series were investigated for their chemical and physical properties to improve the PCE of OPV devices with PC71BM and maintain high performances after TA treatment. The conjugation compression/extension for the BXR series focused on changing the molecular size of the MM and its diffusion rates in a blend matrix with PC71BM, as BTR/PC71BM films show increased domain sizes (larger than exciton diffusion limits) upon TA treatment that may account for the loss in OPV device PCE. Furthermore, changing the conjugation length has impacts on the absolute energy levels of the HOMO and LUMO molecular orbitals and therefore its optoelectronic properties affecting charge generation and separation. This design strategy concluded by extending the Ï€-bridge of the BTR MM chromophore by two (BQR) or four (BPR) 3-hexyl thiophene units in the π-bridge an improved device PCE (highest PCE of 9.4% for BQR) was achieved, and this high performance was maintained after TA treatment (8.9% PCE) with little change observed in the morphology of the blend. Increasing the electronegativity of the acceptor end groups of the BTA series directly impacts the HOMO and LUMO energy levels and intramolecular charge transfer state causing a red shift in the absorption profile. This results in increasing the amount of usable light in the spectrum to be converted to electrical current in OPV device operation. The subtle changes at the end of the MM will also impact the crystallinity and morphology of the blend active layer to achieve thermally robust OPV devices. However, only one of the five new BTA derivatives exhibited high PCEs in OPV devices with PC71BM, known as the BTB MM (8.8% PCE). All other BTA derivatives had PCEs of <5% under all annealing conditions tested and no improvement was observed for TA BTB devices, exhibiting the same degree of performance degradation as BTR devices. However, further device processing optimisations is required for these BTA materials as low fill factors (̃60%) were obtained suggesting that they require different fabrication conditions to optimise the blend morphology and/or their electronic nature may be unfavourable for charge generation. Finally, the best performing MM, BQR was investigated using transient absorption spectroscopy to gain a better understanding of the charge generation/separation process of this system. Furthermore, as the MM BTR had been previously investigated under the same conditions this allowed a comprehensive picture to be established of these MM systems and their photogeneration charge behaviour. Whilst BQR OPV devices maintain high PCEs after TA treatment, TAS indicates a highly crystalline thin film is developed with passive solvent vapour annealing treatment (similar to BTR) but is enhanced upon TA treatment. This conclusion is based on the observation of two kinetic profiles of the radical cation (hole polaron) signal that can be spectroscopically probed. The behaviour may arise due to an electro-absorption (EA) signal where, as the formation of free charges proceeds, the exciton diffuses to an interface and the hole and electron remain coulombically bound but are localized on the respective donor and acceptor domains in the blend matrix. If a highly crystalline interface is present a dipole like electric field is generated assisting in the charge separation process. This EA effect is not commonly observed in OPV materials but has been attributed to one of the reasons for efficient charge generation/separation processes in high PCE OPV systems. Interestingly, this phenomenon was not seen in the BTR material but only in BQR and may be further evidence as to why this material maintains a high PCE after TA treatment. Collectively this thesis presents a synthetic design strategy for two classes of MM systems (BXR and BTA) designed around the BTR material.
The importance of the molecular structure and morphological implications of these new MMs provides a greater understanding of the charge generation/separation processes for future materials design considerations to enable high performance large area flexible OPV devices to be fabricated and have commercial applicability.
Subject organic chemistry, materials chemistry, ultrafast spectroscopy, organic photovoltaics, organic synthesis