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Studies of Voltage Losses in Organic Solar Cells
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
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: urn:nbn:se:liu:diva-139868ISBN: 9789176854747 (print)OAI: oai:DiVA.org:liu-139868DiVA: diva2:1134229
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
List of papers
1. Modulating molecular aggregation by facile heteroatom substitution of diketopyrrolopyrrole based small molecules for efficient organic solar cells
Open this publication in new window or tab >>Modulating molecular aggregation by facile heteroatom substitution of diketopyrrolopyrrole based small molecules for efficient organic solar cells
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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
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
National Category
Biological Sciences Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:liu:diva-123846 (URN)10.1039/c5ta06501a (DOI)000366163000022 ()
Note

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

Available from: 2016-01-11 Created: 2016-01-11 Last updated: 2017-08-18
2. A fused-ring based electron acceptor for efficient non-fullerene polymer solar cells with small HOMO offset
Open this publication in new window or tab >>A fused-ring based electron acceptor for efficient non-fullerene polymer solar cells with small HOMO offset
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2016 (English)In: NANO ENERGY, ISSN 2211-2855, Vol. 27, 430-438 p.Article in journal (Refereed) Published
Abstract [en]

A non-fullerene electron acceptor bearing a novel backbone with fused 10-heterocyclic ring (in-dacenodithiopheno-indacenodiselenophene), denoted by IDTIDSe-IC is developed for fullerene free polymer solar cells. IDTIDSe-IC exhibits a low band gap (E-g=1.52 eV) and strong absorption in the 600850 nm region. Combining with a large band gap polymer J51 (E-g=1.91 eV) as donor, broad absorption coverage from 300 nm to 800 nm is obtained due to complementary absorption of J51 and IDTIDSe-IC, which enables a high PCE of 8.02% with a V-oc of 0.91 V, a J(SC) of 15.16 mA/cm(2) and a FF of 58.0% in the corresponding PSCs. Moreover, the EQE of 50-65% is achieved in the absorption range of IDTIDSe-IC with only about 0.1 eV HOMO difference between J51 and IDTIDSe-IC. (C) 2016 Elsevier Ltd. All rights reserved.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2016
Keyword
Non-fullerene acceptor; Indacenodithiophene; Polymer solar cells
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-132487 (URN)10.1016/j.nanoen.2016.07.019 (DOI)000384910500047 ()
Note

Funding Agencies|National Natural Science Foundation of China [21504062, 21572152]; China Postdoctoral Science Foundation [2015M581853]; Jiangsu Province Postdoctoral Science Foundation [1501024B]; Vinnova [2015-04751]; China Scholarship Council (CSC) [201306730002]; Swedish Research Council (VR) [621-2013-5561]; Swedish Energy Agency [EM 42033-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; Collaborative Innovation Center of Suzhou Nano Science and Technology; Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD)

Available from: 2016-11-13 Created: 2016-11-12 Last updated: 2017-08-18
3. Fast charge separation in a non-fullerene organic solar cell with a small driving force
Open this publication in new window or tab >>Fast charge separation in a non-fullerene organic solar cell with a small driving force
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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
National Category
Other Physics Topics
Identifiers
urn:nbn:se:liu:diva-135409 (URN)10.1038/NENERGY.2016.89 (DOI)000394175200001 ()
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: 2017-08-18

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