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Fast charge separation in a non-fullerene organic solar cell with a small driving force
Hong Kong University of Science and Technology, Peoples R China; Hong Kong University of Science and Technology, Peoples R China.
Hong Kong University of Science and Technology, Peoples R China; Hong Kong University of Science and Technology, Peoples R China.
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
North Carolina State University, NC 27695 USA.
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2016 (English)In: NATURE ENERGY, ISSN 2058-7546, Vol. 1, 16089Article in journal (Refereed) Published
Abstract [en]

Fast and efficient charge separation is essential to achieve high power conversion efficiency in organic solar cells (OSCs). In state-of-the-art OSCs, this is usually achieved by a significant driving force, defined as the offset between the bandgap (E-gap) of the donor/acceptor materials and the energy of the charge transfer (CT) state (E-CT), which is typically greater than 0.3 eV. The large driving force causes a relatively large voltage loss that hinders performance. Here, we report non-fullerene OSCs that exhibit ultrafast and efficient charge separation despite a negligible driving force, as E-CT is nearly identical to E-gap. Moreover, the small driving force is found to have minimal detrimental effects on charge transfer dynamics of the OSCs. We demonstrate a non-fullerene OSC with 9.5% efficiency and nearly 90% internal quantum efficiency despite a low voltage loss of 0.61V. This creates a path towards highly efficient OSCs with a low voltage loss.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP , 2016. Vol. 1, 16089
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:liu:diva-135409DOI: 10.1038/NENERGY.2016.89ISI: 000394175200001OAI: oai:DiVA.org:liu-135409DiVA: diva2:1081476
Note

Funding Agencies|National Basic Research Program of China (973 Program) [2013CB834701, 2014CB643501]; Hong Kong Research Grants Council [T23-407/13 N, N_HKUST623/13, 606012]; HK JEBN Limited; National Science Foundation of China [21374090, 51361165301]; Office of Naval Research [N000141410531, N000141512322, N000141310526 P00002]; Office of Science, Office of Basic Energy Sciences, of the US Department of Energy [DE-AC02-05CH11231]; Swedish Research Council (VR) [330-2014-6433]; Swedish Research Council (FORMAS) [942-2015-1253]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (faculty grant SFO-Mat-LiU) [2009-00971]; European Commission [691210, INCA 600398]; Wallenberg Scholar grant

Available from: 2017-03-14 Created: 2017-03-14 Last updated: 2017-11-01
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)
Opponent
Supervisors
Note

Funding: China Scholarship Council (CSC).

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

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