On the Relation Between Exciton Dynamics and Nano-Morphology in Organic Semiconductor Blends

Optoelectronic processes in semiconductor-based devices are widely understood through the constructs of highly-symmetric crystalline inorganic systems, where the lattice periodicity allows significant simplification. Emerging technologies, such as organic semiconductor-based devices, share many qualities with crys-talline inorganic semiconductors; however, they diverge in subtle yet important ways. Optical absorption in organic semiconductors gives rise to long-lived and tightly-bound excitons, which migrate in manner often disregarded in the un-derstanding of highly-symmetric crystalline inorganic semiconductors. Further, ‘free’ charge carriers in organic semiconductors ‘hop’ between organic molecules in a disordered film rather than the undergo band transport that delocalised car-riers in periodic lattices do. This hopping transport leads to lower charge carrier mobilities which have far-reaching ramifications to device operation.In the work summarised in this thesis the effect that excitonic and charge transport have on device performance of solar cells based on organic photo-voltaics will be explored. Experimental techniques are designed and developed to gain insight into the efficiency of exciton transport, the nanostructure of organic-semiconductor blends, and the relation between charge injection and extraction. Utilising these techniques, various state-of-the art systems are examined in detail and various pathways for improving device performance are voiced.Specifically, a technique to measure the exciton diffusion length in organic semiconductors is developed and shown to have many advantages over established techniques while improving the accuracy of the measurement. This technique is expanded to blends of organic semiconductors to quantify the efficiency of dif-fusion and quenching occurring between semiconductors in blends. This, along with a developed theoretical understanding, allows for the size of the phase sepa-rated domains to be quantified. Relationships between the excitons generated in organic semiconductors, charge carriers created in the blends, and the transport of charges to and extraction at the electrodes is considered in detail. Finally, a technique to distinguish between the nonradiative recombination occurring within an organic semiconductor blend and at the interface between the blend and the larger device structure is introduced. This technique utilises the well-establish reciprocity theory to reconcile the imbalance between charge injection and ex-traction unique to low-mobility organic semiconductors

Toward More Efficient Organic Solar Cells: A Detailed Study of Loss Pathway and Its Impact on Overall Device Performance in Low‐Offset Organic Solar Cells

Low-offset organic solar cell systems have attracted great interest since nonfullerene acceptors came into the picture. While numerous studies have focused on the charge generation process in these low-offset systems, only a few studies have focused on the details of each loss channel in the charge generation process and their impact on the overall device performance. Here, several nonfullerene acceptors are blended with the same polymer donor to form a series of low-offset organic solar cell systems where significant variation in device performance is observed. Through detailed analyses of loss pathways, it is found that: i) the donor:acceptor interfaces of PM6:Y6 and PM6:TPT10 are close to the optimum energetic condition, ii) energetics at the donor:acceptor interface are the most important factor to the overall device performance, iii) exciton dissociation yield can be field-dependent owing to the sufficiently small energetic offset at the donor:acceptor interface, and iv) the change in substituents in the terminal group of Y-series acceptors in this work mainly affects energetics at the donor:acceptor interface instead of the interface density in the active layer. In general, this work presents a path toward more efficient organic solar cells

Evaluating the Effectiveness of Using Peer-Dialogue Assessment (PDA) for Improving Pre-Service Teachers\u27 Perceived Confidence and Competence to Teach Physical Education

Developing effective methods for improving student learning in higher education is a priority. Recent findings have shown that feedback on student work can effectively facilitate learning if students are engaged as active participants in the feedback cycle; where they seek, generate and use feedback in the form of dialogue. This novel study investigates the use of peer dialogue assessment as an assessment for learning tool used in an existing undergraduate physical education course. Our findings demonstrate that when thirty six undergraduate physical education students were provided with instruction and practice using peer dialogue assessment after consecutive teaching performances, they exhibit significant improvements in perceived teaching confidence and competence, and teaching self-efficacy. Process evaluation results implying thatembedding peer dialogue assessment in higher education courses may be a feasible approach for facilitating learning, and that students were satisfied with using peer dialogue as a feedback method for improving teaching practices

Role of Exciton Diffusion and Lifetime in Organic Solar Cells with a Low Energy Offset

Despite general agreement that the generation of free charges in organic solar cells is driven by an energetic offset, power conversion efficiencies have been improved using low-offset blends. In this work, we explore the interconnected roles that exciton diffusion and lifetime play in the charge generation process under various energetic offsets. A detailed balance approach is used to develop an analytic framework for exciton dissociation and free-charge generation accounting for exciton diffusion to and dissociation at the donor–acceptor interface. For low-offset systems, we find the exciton lifetime to be a pivotal component in the charge generation process, as it influences both the exciton and CT state dissociation. These findings suggest that any novel low-offset material combination must have long diffusion lengths with long exciton lifetimes to achieve optimum charge generation yields

Quasi-Steady-State Measurement of Exciton Diffusion Lengths in Organic Semiconductors

Exciton diffusion plays a decisive role in various organic optoelectronic applications, including lasing, photodiodes, light-emitting diodes, and solar cells. Understanding the role that exciton diffusion plays in organic solar cells is crucial to understanding the recent rise in power conversion efficiencies brought about by nonfullerene acceptor (NFA) molecules. Established methods for quantifying exciton diffusion lengths in organic semiconductors require specialized equipment designed for measuring high-resolution time-resolved photoluminescence (TRPL). In this paper we introduce an approach, named pulsed-photoluminescence quantum yield (PLQY), to determine the diffusion length of excitons in organic semiconductors without any temporal measurements. Using a Monte Carlo model, the dynamics within a thin-film semiconductor are simulated and the results are analyzed using both pulsed-PLQY and TRPL methods. It is found that pulsed-PLQY has a larger operational window and depends less on the excitation fluence than the TRPL approach. The simulated results are validated experimentally on a well-understood organic semiconductor, after which pulsed-PLQY is used to evaluate the diffusion length in a variety of technologically relevant materials. It is found that the diffusion lengths in NFAs are much larger than in the benchmark fullerene and that this increase is driven by an increase in diffusivity. This result helps explain the high charge generation yield in low-offset state-of-the-art NFA solar cells

Efficient Nanoscale Exciton Transport in Non‐Fullerene Organic Solar Cells Enables Reduced Bimolecular Recombination of Free Charges

The highest-efficiency organic photovoltaic (OPV)-based solar cells, made from blends of electron-donating and electron-accepting organic semiconductors, are often characterized by strongly reduced (non-Langevin) bimolecular recombination. Although the origins of the reduced recombination are debated, mechanisms related to the charge-transfer (CT) state and free-carrier encounter dynamics controlled by the size of donor and acceptor domains are proposed as underlying factors. Here, a novel photoluminescence-based probe is reported to accurately quantify the donor–acceptor domain size in OPV blends. Specifically, the domain size is measured in high-efficiency non-fullerene acceptor (NFA) systems and a comparative conventional fullerene system. It is found that the NFA-based blends form larger domains but that the expected reductions in bimolecular recombination attributed to the enhanced domain sizes are too small to account for the observed reduction factors. Further, it is shown that the reduction of bimolecular recombination is correlated to enhanced exciton dynamics within the NFA domains. This indicates that the processes responsible for efficient exciton transport also enable strongly non-Langevin recombination in high-efficiency NFA-based solar cells with low-energy offsets

Organic solar cells with near-unity charge generation yield

The subtle link between photogenerated charge generation yield (CGY) and bimolecular recombination in organic semiconductor-based photovoltaics is relatively well established as a concept but has proven extremely challenging to demonstrate and probe especially under operational conditions. Received wisdom also teaches that charge generation in excitonic systems will always be lower than non-excitonic semiconductors such as GaAs – but this view is being challenged with the advent of organic semiconductor blends based upon non-fullerene acceptors (NFAs) with power conversion efficiencies exceeding 18%. Using a newly developed approach based upon temperature dependent ultra-sensitive external quantum efficiency measurements, we observe near unity CGY in several model NFA-based systems measured with unprecedented accuracy. We find that a relatively small increase in yield from 0.984 to 0.993 leads to a reduction in bimolecular recombination from 400 times to 1000 times relative to the Langevin limit. In turn, this dramatic reduction delivers the best thick junction performance to date in any binary organic solar cell – notably 16.2% at 300 nm. The combination of high efficiency and thick junction is the key for industrial fabrication of these devices via high-throughput deposition processing such as roll-to-roll, and thus central to a viable solar cell technology. These results also clearly reveal and elucidate the relationship between photo-generation and recombination in excitonic semiconductor photovoltaics thus providing an important bridge between basic device physics and practical cell engineering

Mitigating Detrimental Effect of Self‐Doping Near the Anode in Highly Efficient Organic Solar Cells

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has been one of the most established hole transport layers (HTL) in organic solar cells (OSCs) for several decades. However, the presence of PSS− ions is known to deteriorate device performance via a number of mechanisms including diffusion to the HTL-active layer interface and unwanted local chemical reactions. In this study, it is shown that PSS− ions can also result in local p-doping in the high efficiency donor:non-fullerene acceptor blends – resulting in photocurrent loss. To address these issues, a facile and effective approach is reported to improve the OSC performance through a two-component hole transport layer (HTL) consisting of a self-assembled monolayer of 2PACz ([2-(9H-Carbazol-9-yl)ethyl]phosphonic acid) and PEDOT:PSS. The power conversion efficiency (PCE) of 17.1% using devices with PEDOT:PSS HTL improved to 17.7% when the PEDOT:PSS/2PACz two-component HTL is used. The improved performance is attributed to the overlaid 2PACz layer preventing the formation of an intermixed p-doped PSS− ion rich region (≈5–10 nm) at the bulk heterojunction-HTL contact interface, resulting in decreased recombination losses and improved stability. Moreover, the 2PACz monolayer is also found to reduce electrical shunts that ultimately yield improved performance in large area devices with PCE enhanced from 12.3% to 13.3% in 1 cm2 cells

Mitigating Detrimental Effect of Self‐Doping Near the Anode in Highly Efficient Organic Solar Cells

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has been one of the most established hole transport layers (HTL) in organic solar cells (OSCs) for several decades. However, the presence of PSS− ions is known to deteriorate device performance via a number of mechanisms including diffusion to the HTL-active layer interface and unwanted local chemical reactions. In this study, it is shown that PSS− ions can also result in local p-doping in the high efficiency donor:non-fullerene acceptor blends – resulting in photocurrent loss. To address these issues, a facile and effective approach is reported to improve the OSC performance through a two-component hole transport layer (HTL) consisting of a self-assembled monolayer of 2PACz ([2-(9H-Carbazol-9-yl)ethyl]phosphonic acid) and PEDOT:PSS. The power conversion efficiency (PCE) of 17.1% using devices with PEDOT:PSS HTL improved to 17.7% when the PEDOT:PSS/2PACz two-component HTL is used. The improved performance is attributed to the overlaid 2PACz layer preventing the formation of an intermixed p-doped PSS− ion rich region (≈5–10 nm) at the bulk heterojunction-HTL contact interface, resulting in decreased recombination losses and improved stability. Moreover, the 2PACz monolayer is also found to reduce electrical shunts that ultimately yield improved performance in large area devices with PCE enhanced from 12.3% to 13.3% in 1 cm2 cells