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Modulating molecular aggregation by facile heteroatom substitution of diketopyrrolopyrrole based small molecules for efficient organic solar cells
Linköping University, Faculty of Science & Engineering. Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics.
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
Linköping University, Department of Science and Technology, Physics and Electronics. Linköping University, Faculty of Science & Engineering.
Stanford University, CA 94305 USA.
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2015 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 3, no 48, 24349-24357 p.Article in journal (Refereed) Published
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Text
Abstract [en]

In conjugated polymers and small molecules of organic solar cells, aggregation induced by intermolecular interactions governs the performance of photovoltaics. However, little attention has been paid to the connection between molecular structure and aggregation within solar cells based on soluble small molecules. Here we demonstrate modulation of intermolecular aggregation of two synthesized molecules through heteroatom substitution to develop an understanding of the role of aggregation in conjugated molecules. Molecule 1 (M1) based on 2-ethylhexyloxy-benzene substituted benzo[1,2-b:4,5-b]dithiophene (BDTP) and diketopyrrolopyrrole (DPP) displays strong aggregation in commonly used organic solvents, which is reduced in molecule 2 (M2) by facile oxygen atom substitution on the BDTP unit confirmed by absorption spectroscopy and optical microscopy, while it successfully maintains molecular planarity and favorable charge transport characteristics. Solar cells based on M2 exhibit more than double the photocurrent of devices based on M1 and yield a power conversion efficiency of 5.5%. A systematic investigation of molecular conformation, optoelectronic properties, molecular packing and crystallinity as well as film morphology reveals structure dependent aggregation responsible for the performance difference between the two conjugated molecules.

Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY , 2015. Vol. 3, no 48, 24349-24357 p.
National Category
Biological Sciences Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:liu:diva-123846DOI: 10.1039/c5ta06501aISI: 000366163000022OAI: oai:DiVA.org:liu-123846DiVA: diva2:892840
Note

Funding Agencies|Swedish Energy Agency; China Scholarship Council (CSC)

Available from: 2016-01-11 Created: 2016-01-11 Last updated: 2017-08-18
In thesis
1. Studies of Voltage Losses in Organic Solar Cells
Open this publication in new window or tab >>Studies of Voltage Losses in Organic Solar Cells
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Organic photovoltaic (OPV) devices based on semiconducting polymers and small molecules are potential alternatives to inorganic solar cells, owing to their advantages of being inexpensive, lightweight, flexible and suitable for roll-to-roll production. The state of art organic solar cells (OSCs) performed power conversion efficiencies (PCEs) over 13%.

The quantum efficiency losses in OSCs have been significantly reduced within the charge generation and extraction processes, resulting in high EQEPV (70-90%) and high FF (70-80%). Whereas, large voltage losses (Δ𝑉 = 𝐸𝑔/𝑞 − 𝑉𝑂𝐶) were observed in conventional fullerene based solar cells, and it has been the main limiting factor for further OPV advancement. Therefore, strategies to reduce the voltage losses are required.

In this thesis, newly designed non-fullerene (NF) acceptors are used to construct novel material systems for high efficiency solar cells. In particular, we studied the hole transfer in these fullerene free systems. We also reported a NF system that exhibit ultrafast and efficient charge separation despite a negligible driving force, as ECT is nearly identical to 𝐸𝑔. Moreover, the small driving force is found to have minimal detrimental effects on charge transfer dynamics of the OSCs. We demonstrate a NF based OSC with efficiency of 9.5% and internal quantum efficiency nearly 90% despite a low voltage loss of 0.61 V. This creates a path towards highly efficient OSCs with a low voltage loss.

CT states in OSCs are also investigated, since VOC is governed by the CT energy (ECT), which is found as 𝑞𝑉𝑂𝐶 = 𝐸𝐶𝑇 − 0.6 in a large set of fullerene based solar cells. In order to reduce these recombination losses from CT states, we explored polymer-diPDI systems which exhibited weakened D-A coupling strength, due to the steric hindrance effect. The radiative recombination losses at D/A interface in these NF devices are all reduced to less than 0.18 eV. In particular, in some cases, the additional emission from pure material is favorable for suppressing the non-radiative CT states decay. Consequently, the recombination losses in these NF systems are reduced to 0.5 eV, while the charge generation is still efficient as confirmed by PL quenching and EQEPV.

Novel material systems based on non-fullerene acceptors are investigated. The systems performed energy offsets (ΔHOMO or ΔLUMO) less than 0.15eV, resulting in the same energy of CT states and bulk excitons. In this regard, the charge transfer energy loss is minimized. We also found that the EL spectra as well as the EQEEL of the blend solar cells are similar with that of lower gap components in blends. Thus the non-radiative voltage losses are reduced to < 0.3V and small voltage loss of 0.5-0.7V are obtained. Meanwhile, the charge generation in systems are still efficient and high EQEPV of 50-70% can be achieved. It confirms that there is no intrinsic limit for the VOC and efficiency of OPVs as compared with other photovoltaic technologies.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2017. 63 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1872
National Category
Condensed Matter Physics Energy Engineering Physical Sciences
Identifiers
urn:nbn:se:liu:diva-139868 (URN)9789176854747 (ISBN)
Public defence
2017-09-15, Schrödinger, Fysikhuset, Campus Valla, Linköping, 10:26 (English)
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Funding: China Scholarship Council (CSC).

Available from: 2017-08-18 Created: 2017-08-18 Last updated: 2017-08-18Bibliographically approved

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