Author: Bradley Kirk
Kirk, Bradley, 2023 Thermal stability of slot-die coated Organic Photovoltaics, Flinders University, College of Science and Engineering
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There is a large amount of interest towards the commercialisation of flexible solar cells, with several types that have gained traction, one such being organic photovoltaics (OPVs). Improving solar cell efficiency has dominated the OPV research field for several years, however, there has been less research aimed towards device stability, resulting majority of devices having poor lifetime under ambient and operating conditions. There has been even less research focused on the shift from research on lab-based spin-coated OPVs fabricated under protective nitrogen conditions to scalable and large-scale devices fabricated via coating/printing techniques under ambient conditions. This has led to the majority of OPV stability research to focus on reducing the degradation of bulk heterojunction (BHJ) of the active layer, with strategies for improving stability previously aimed at small-scale solar cells fabricated under nitrogen environment.
This thesis work has aimed at understanding the major thermal degradation pathways associated with slot-die polymer:fullerene-based OPV, with two temperatures being investigated, 85 °C and 120 °C. 85 °C is a thermal aging temperature that is accepted by the International Summit on OPV Stability (ISOS) community used for thermal degradation studies under dark storage conditions. Whereas 120 °C is one of several “accelerated” thermal aging temperatures that have been implemented in in previous literature to increase the degradation rate. From our work, however, it was found that the two temperatures led to completely different degradation pathways, with 85 °C resulting in significant phase changes in the active layer, while 120 °C resulted in fullerene crystallisation and migration.
With this important observation in degradation pathway, a range of solid-based additives were extensively investigated, from neat fullerenes, cyclic-based small molecules and an insulating polymer, all having either been speculated to, or shown in previous literature to decrease thermal aging of the active layer. Though additives such as Piperazine (PP), 4,4’-Bipiperidine (BP) and Polyacenaphthylene (PAN) were observed to reduce the crystallisation rate at 120 °C, the additives had a negative impact on degradation at 85 °C as they allowed for the formation of fullerene crystals that would otherwise have been observed at the lower temperature. Whereas for neat C70, it was found to improve thermal stability at 120 °C and 85 °C, due to its ability to influence the thermal behaviour of the active layer itself.
Lastly, the thesis work aimed at demonstrating the scalability and translatability of the previously conducted work, using a device structure that has been demonstrated in roll-to-roll fabrication. For the additives that have been investigated in the thesis, they were found to have a negative impact on thermal degradation of the OPV devices, where it is was suspected that by influencing the thermal behaviour and morphology of the active layer had led to increased interfacial degradation.
The work conducted in the thesis demonstrates a strategy, by implementing several spectroscopy, microscopy, and material analysis methods together, for investigating morphological changes associated with thermal degradation of the active layer of OPVs. The work has also shown the importance of investigating thermal degradation at appropriate annealing temperatures of OPVs, as well as further challenges associated with scalability and translatability between scalable and large-scale OPV fabrication.
Keywords: organic photovoltaics, slot-die coating, thermal stability, scalable fabrication, organic solar cells
Subject: Nanotechnology thesis
Thesis type: Doctor of Philosophy
Completed: 2023
School: College of Science and Engineering
Supervisor: Mats Andersson