University of Groningen
Reducing losses in solution processed organic solar cells
Rahimichatri, Azadeh
DOI:
10.33612/diss.170159026
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Publication date: 2021
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Rahimichatri, A. (2021). Reducing losses in solution processed organic solar cells. University of Groningen. https://doi.org/10.33612/diss.170159026
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Summary
In spite of great progress in recent years, widespread application of organic photovoltaic devices requires further improvement of efficiency and stability. Considering the mul-tilayered structure of organic photovoltaic devices and the multiple parameters playing role in limiting their performance, maintaining their efficiency over time is a complex topic. This thesis aims at reducing losses in organic photovoltaic devices, first by quan-tifying recombination losses, and next, by studying doped charge transport layers.
In chapter 2, we study the impact of induced charges on the recombination in or-ganic solar cells. To this end, the net recombination lifetime of photogenerated charge carriers in the presence of electrodic induced charges (EICs) is measured by means of conventional and newly developed transient photovoltage techniques. Moreover, a new approach is introduced to exclusively measure the bulk recombination lifetime, i.e., in the absence of EICs; this approach is conducted by depositing transparent insulating layers on both sides of the organic semiconducting active layer. We reveal that EICs can only reduce the recombination lifetime of the photogenerated charges in organic solar cells with very weak recombination strength. This work supports that for organic so-lar cells with highly reduced recombination strength, eliminating the recombination of photogenerated charges and EICs is critical for achieving better performance.
Inrecentyears, non-fullereneacceptororganicsolarcellshavedrawnattentionmostly due to their high absorption, tunable energy levels, and high efficiency. However, still a lot is to be understood about the physics of these solar cells. Earlier, our group has shown that the overall influence of different material properties and device parameters can be described by a single parameter θ. In chapter 3, θ is shown to be directly related to the ratio of the rate of recombination to that of extraction. We achieve this by direct measure-ments of the rates of recombination and extraction in non-fullerene acceptor solar cells using ITIC as the acceptor, with varying active layer thicknesses, light intensities, and interfacial layers. We provide a simple experimental tool for understanding how modifi-cation of different parameters can lead to further enhancement of fill factor. The results
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98 Summary support that the competition between charge extraction and recombination determines the fill factor of non-fullerene based organic solar cells.
Doping enhances the conductivity of organic semiconductors. The efficiency of dop-ing is influenced by parameters such as host-dopant interaction, solvent, etc. In chapter 4, we provide a tool to make a distinction between efficient and inefficient doping, and show that the temporal response of the current under an applied bias voltage can be used to identify inefficient doping in doped organic semiconducting devices.
In chapter 5, we study the stability of the electrical conductivity of doped fullerene derivative films using host molecules with different side chain lengths or polarities and tetrabutylammonium fluoride (TBAF) as the dopant. The fullerene derivatives with po-lar ethylene glycol side chains exhibit higher conductivity and better stability under ther-mal stress when doped than those with acetylene moieties in their side chains or PCBM. By decreasing the length of the ethylene glycol side chain, the stability of the fullerene derivatives under the application of bias voltage was enhanced by an order of magni-tude.
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