| 1. |
- Ma, Zaifei, et al.
(author)
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Influences of Surface Roughness of ZnO Electron Transport Layer on the Photovoltaic Performance of Organic Inverted Solar Cells
- 2012
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 116:46, s. 24462-24468
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Journal article (peer-reviewed)abstract
- Here, we demonstrate the correlation between the surface roughness of the ZnO interlayer used as an electron transporting interlayer (ETL) in organic inverted solar cells (ISCs) and the photovoltaic performance of the ISCs. Three different surfaces of the ZnO ETL are studied in ISCs with the polymer poly[2,3-bis-(3-octyloxyphenyl)-quinoxaline-5,8-diyl-alt-thiophene-2,5-d iyl] (TQ1) mixed with [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as the active layer. The results obtained from these ISCs show that power conversion efficiency increases from 2.7% to 3.9% when the root-mean-square roughness of the ZnO layer decreases from 48 to 1.9 nm. Moreover, it is found that the short-circuit current density is higher in the ISC based on the smoother ZnO interlayer, with a larger donor/acceptor (D/A) interfacial area in the active layer that facilitates exciton dissociation. The reduced effective interfacial area between the photoactive layer and the ZnO interlayer with decreased ZnO surface roughness leads to an observed improvement in both fill factor and open circuit voltage, which is ascribed to a reduced concentration of traps at the interface between the ZnO interlayer and the active layer.
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| 2. |
- Ma, Zaifei, 1986-
(author)
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Studies of Morphology and Charge-Transfer in Bulk-Heterojunction Polymer Solar Cells
- 2013
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Doctoral thesis (other academic/artistic)abstract
- The work presented in this thesis focuses on the two critical issues of bulk-heterojunction polymer solar cells: morphology of active layers and energy loss during charge transfer process at electron donor/acceptor interfaces. Both issues determine the performance of polymer solar cells through governing exciton dissociation, charge carrier recombination and free charge carrier transport.The morphology of active layers (spatial percolation of the donor and acceptor) is crucial for the performance of polymer solar cells due to the limited diffusion length of excitons in organic semiconductors (5-20 nm). Meanwhile, the trade-off between charge generation and transport also needs to be considered. On the one hand, a finely mixed morphology with a large donor/acceptor interface area is preferred for charge generation because efficient exciton dissociation only occurs at the interface, but on the other hand, proper phase separation is needed to reduce charge carrier recombination and facilitate free charge carrier transport to the electrodes. In this thesis, morphologies of the active layers based on different polymeric donors and fullerene acceptors are correlated to the performance of solar cells with various microscopic and spectroscopic techniques including atomic force microscope, transmission electron microscope, grazing incidence x-ray diffraction, photoluminescence, electroluminescence and Fourier transform photocurrent spectroscopy. Furthermore, methods to manipulate the morphologies of solution processed active layers to achieve high performance solar cells are also presented. Processing solvents, chemical structures of the donor and the acceptor materials, and substrate surface properties are found critically important in determining the nanoscale phase separation and performance of polymer solar cells.Optimizing morphology of active layers alone does not guarantee high performance devices. In addition to spatial percolation, energy arrangements of donors and acceptors are also essential due to contrary requests of the photocurrent and the photovoltage: Efficient exciton dissociation or charge transfer at donor/acceptor interfaces requires large enough energetic driving force, which is also known as energy loss for charge transfer. However, the energy loss due to charge transfer will unavoidably reduce the photovoltage. In this thesis the balance between the photocurrent and the photovoltage in polymer solar cells due to charge transfer at donor/acceptor interfaces is investigated for different active material systems. The driving force tuned by synthesizing series of polymers is determined by directly measuring the optical band gap via UV-Vis spectroscopy and probing the charge transfer recombination via electroluminescence measurements. Influences of driving force on the photocurrent and the photovoltage are characterized via field dependent photoluminescence and internal quantum efficiency measurements. The results correlated well with the performance of the solar cells.
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| 3. |
- Ma, Zaifei, et al.
(author)
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Synthesis and characterization of benzodithiophene-isoindigo polymers for solar cells
- 2012
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In: Journal of Materials Chemistry. - : Royal Society of Chemistry (RSC). - 1364-5501 .- 0959-9428. ; 22:5, s. 2306-2314
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Journal article (peer-reviewed)abstract
- Three new alternating polymers with the electron-deficient isoindigo group as the acceptor unit and benzo[1,2-b:4,5-b'] dithiophene (BDT) or BDT flanked by thiophenes (or octylthiophenes) as the donor unit were designed and synthesized. All the polymers have good thermal stability, solubility and broad absorption spectra. Their photophysical, electrochemical and photovoltaic (PV) properties were investigated. To understand their different PV performance in the resulting polymer solar cells (PSCs), the morphology of their blends with fullerene derivatives was investigated by atomic force microscopy, and the molecular geometries as well as the molecular frontier orbitals were simulated by density functional theory calculations (Gaussian 09). The polymer PBDT-TIT, with BDT flanked by thiophenes as the donor unit and isoindigo as the acceptor unit, exhibits quite planar backbones and its blend with fullerene derivatives shows optimal morphology. As a result, the PSCs based on PBDT-TIT with a conventional device configuration of ITO/PEDOT: PSS/PBDT-TIT: PC(61)BM/LiF/Al showed a power conversion efficiency of 4.22%, with a short-circuit current density of 7.87 mA cm(-2), an open-circuit voltage of 0.79 V and a fill factor of 0.68 under the AM 1.5G illumination with an intensity of 100 mW cm(-2) from a solar simulator.
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| 4. |
- Tang, Zheng, et al.
(author)
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Semi-Transparent Tandem Organic Solar Cells with 90% Internal Quantum Efficiency
- 2012
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In: Advanced Energy Materials. - : Wiley. - 1614-6840 .- 1614-6832. ; 2:12, s. 1467-1476
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Journal article (peer-reviewed)abstract
- Semi-transparent (ST) organic solar cells with potential application as power generating windows are studied. The main challenge is to find proper transparent electrodes with desired electrical and optical properties. In this work, this is addressed by employing an amphiphilic conjugated polymer PFPA-1 modified ITO coated glass substrate as the ohmic electron-collecting cathode and PEDOT:PSS PH1000 as the hole-collecting anode. For active layers based on different donor polymers, considerably lower reflection and parasitic absorption are found in the ST solar cells as compared to solar cells in the standard geometry with an ITO/PEDOT:PSS anode and a LiF/Al cathode. The ST solar cells have remarkably high internal quantum efficiency at short circuit condition (similar to 90%) and high transmittance (similar to 50%). Hence, efficient ST tandem solar cells with enhanced power conversion efficiency (PCE) compared to a single ST solar cell can be constructed by connecting the stacked two ST sub-cells in parallel. The total loss of photons by reflection, parasitic absorption and transmission in the ST tandem solar cell can be smaller than the loss in a standard solar cell based on the same active materials. We demonstrate this by stacking five separately prepared ST cells on top of each other, to obtain a higher photocurrent than in an optimized standard solar cell.
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| 5. |
- Vandewal, Koen, et al.
(author)
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Quantification of Quantum Efficiency and Energy Losses in Low Bandgap Polymer:Fullerene Solar Cells with High Open-Circuit Voltage
- 2012
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In: Advanced Functional Materials. - : Wiley-VCH Verlag Berlin. - 1616-301X .- 1616-3028. ; 22:16, s. 3480-3490
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Journal article (peer-reviewed)abstract
- In organic solar cells based on polymer:fullerene blends, energy is lost due to electron transfer from polymer to fullerene. Minimizing the difference between the energy of the polymer exciton (ED*) and the energy of the charge transfer state (ECT) will optimize the open-circuit voltage (Voc). In this work, this energy loss ED*-ECT is measured directly via Fourier-transform photocurrent spectroscopy and electroluminescence measurements. Polymer:fullerene photovoltaic devices comprising two different isoindigo containing polymers: P3TI and PTI-1, are studied. Even though the chemical structures and the optical gaps of P3TI and PTI-1 are similar (1.4 eV1.5 eV), the optimized photovoltaic devices show large differences in Voc and internal quantum efficiency (IQE). For P3TI:PC71BM blends a ED*-ECT of similar to 0.1 eV, a Voc of 0.7 V and an IQE of 87% are found. For PTI-1:PC61BM blends an absence of sub-gap charge transfer absorption and emission bands is found, indicating almost no energy loss in the electron transfer step. Hence a higher Voc of 0.92 V, but low IQE of 45% is obtained. Morphological studies and field dependent photoluminescence quenching indicate that the lower IQE for the PTI-1 system is not due to a too coarse morphology, but is related to interfacial energetics. Losses between ECT and qVoc due to radiative and non-radiative recombination are quantified for both material systems, indicating that for the PTI-1:PC61BM material system, Voc can only be increased by decreasing the non-radiative recombination pathways. This work demonstrates the possibility of obtaining modestly high IQE values for material systems with a small energy offset (andlt;0.1 eV) and a high Voc.
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| 6. |
- Wang, Ergang, 1981, et al.
(author)
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An Easily Accessible Isoindigo-Based Polymer for High-Performance Polymer Solar Cells
- 2011
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 133:36, s. 14244-14247
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Journal article (peer-reviewed)abstract
- A new, low-band-gap alternating copolymer consisting of terthiophene and isoindigo has been designed and synthesized. Solar cells based on this polymer and PC(71)BM show a power conversion efficiency of 6.3%, which is a record for polymer solar cells based on a polymer with an optical band gap below 1.5 eV. This work demonstrates the great potential of isoindigo moieties as electron-deficient units for building donor-acceptor-type polymers for high-performance polymer solar cells.
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| 7. |
- Wang, Ergang, 1981, et al.
(author)
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An isoindigo-based low band gap polymer for efficient polymer solar cells with high photo-voltage
- 2011
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In: Chemical Communications. - : Royal Society of Chemistry (RSC). - 1364-548X .- 1359-7345. ; 47:17, s. 4908-4910
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Journal article (peer-reviewed)abstract
- A new low band gap polymer (E-g = 1.6 eV) with alternating thiophene and isoindigo units was synthesized and characterized. A PCE of 3.0% and high open-circuit voltage of 0.89 V were realized in polymer solar cells, which demonstrated the promise of isoindigo as an electron deficient unit in the design of donor-acceptor conjugated polymers for polymer solar cells.
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| 8. |
- Wang, Ergang, 1981, et al.
(author)
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Conformational Disorder Enhances Solubility and Photovoltaic Performance of a Thiophene–Quinoxaline Copolymer
- 2013
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In: Advanced Energy Materials. - : Wiley. - 1614-6840 .- 1614-6832. ; 3:6, s. 806-814
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Journal article (peer-reviewed)abstract
- The side-chain architecture of alternating copolymers based on thiophene and quinoxaline (TQ) is found to strongly influence the solubility and photovoltaic performance. In particular, TQ polymers with different linear or branched alkyloxy-phenyl side chains on the quinoxaline unit are compared. Attaching the linear alkyloxy side-chain segment at the meta- instead of the para-position of the phenyl ring reduces the planarity of the backbone as well as the ability to order. However, the delocalisation across the backbone is not affected, which permits the design of high-performance TQ polymers that do not aggregate in solution. The use of branched meta-(2-ethylhexyl)oxy-phenyl side-chains results in a TQ polymer with an intermediate degree of order. The reduced tendency for aggregation of TQ polymers with linear meta-alkyloxy-phenyl persists in the solid state. As a result, it is possible to avoid the decrease in charge-transfer state energy that is observed for bulk-heterojunction blends of more ordered TQ polymers and fullerenes. The associated gain in open-circuit voltage of disordered TQ:fullerene solar cells, accompanied by a higher short-circuit current density, leads to a higher power conversion efficiency overall. Thus, in contrast to other donor polymers, for TQ polymers there is no need to compromise between solubility and photovoltaic performance.
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| 9. |
- Wang, Ergang, 1981, et al.
(author)
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Side-Chain Architectures of 2,7-Carbazole and Quinoxaline-Based Polymers for Efficient Polymer Solar Cells
- 2011
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In: Macromolecules. - : American Chemical Society (ACS). - 1520-5835 .- 0024-9297. ; 44:7, s. 2067-2073
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Journal article (peer-reviewed)abstract
- Three polymers bearing a common carbazole thiophene quinoxaline thiophene backbone, but different side chains, were designed and synthesized in order to investigate the effect of side chains on their photovoltaic performance. Their photophysical, electrochemical, and photovoltaic properties were investigated and compared. The polymer EWC3, with the largest amount of side chains, showed the highest power conversion efficiency of 3.7% with an open-circuit voltage (V-oc) of 0.92 V. The atomic force microscopy images of the active layers of the devices showed that the morphology was highly influenced by the choice of the solvent and processing additive. It is worth noting that polymer solar cells (PSCs) fabricated from EWC3, with branched side chains on the carbazole units, gave a much higher V-oc than the devices made from EWC1, which bears the same electron-deficient segment as EWC3 but straight side chains on carbazole units. This study offered a useful and important guideline for designing 2,7-carbazole-based polymers for high-performance PSCs.
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