| 1. |
- Gadisa, Abay, et al.
(author)
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Transparent polymer cathode for organic photovoltaic devices
- 2006
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In: Synthetic metals. - : Elsevier BV. - 0379-6779 .- 1879-3290. ; 156:16-17, s. 1102-1107
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Journal article (peer-reviewed)abstract
- We demonstrate a prototype solar cell with a transparent polymer cathode, and indium-tin-oxide (ITO)/poly (3, 4-ethylene dioxythiophene)-poly (styrene sulphonate) (PEDOT:PSS) anode. As an active layer, thin film of a bulk heterojunction of polyfluorene copolymer poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2thienyl-2′,1′3′-benzothiadiazole)] (APFO-3) and an electron acceptor molecule [6] and [6]-phenyl-C61-butyric acid methyl ester (PCBM) (1:4 wt.) was sandwiched between the two transparent polymer electrodes. The cathode is another form of PEDOT formed by vapor phase polymerised PEDOT (VPP PEDOT) of conductivity 102–103 S/cm. The cathode is supported on an elastomeric substrate, and forms a conformal contact to the APFO-3/PCBM blend. Transparent solar cells are useful for building multilayer and tandem solar cells.
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| 2. |
- Inganäs, Olle, et al.
(author)
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Polymer Photovoltaics with Alternating Copolymer/Fullerene Blends and Novel Device Architectures
- 2010
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In: Advanced Materials. - : Wiley. - 0935-9648 .- 1521-4095. ; 22:20, s. E100-E116
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Journal article (peer-reviewed)abstract
- The synthesis of novel conjugated polymers, designed for the purpose of photovoltaic energy conversion, and their properties in polymer/fullerene materials and photovoltaic devices are reviewed. Two families of main chain polymer donors, based on fluorene or phenylene and donor-acceptor-donor comonomers in alternating copolymers, are used to absorb the high-energy parts of the solar spectrum and to give high photovoltages in combinations with fullerene acceptors in devices. These materials are used in alternative photovoltaic device geometries with enhanced light incoupling to collect larger photocurrents or to enable tandem devices and enhance photovoltage.
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| 3. |
- Müller, Christian, et al.
(author)
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Influence of Molecular Weight on the Performance of Organic Solar Cells Based on a Fluorene Derivative
- 2010
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In: Advanced Materials for Optics and Electronics. - : Wiley. - 1616-301X .- 1616-3028. ; 20:13, s. 2124-2131
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Journal article (peer-reviewed)abstract
- The performance of organic photovoltaic (OPV) bulk-heterojunction blends comprising a liquid-crystalline fluorene derivative and a small-molecular fullerene is found to increase asymptotically with the degree of polymerization of the former. Similar to various thermodynamic transition temperatures as well as the light absorbance of the fluorene moiety, the photocurrent extracted from OPV devices is found to strongly vary with increasing oligomer size up to a number average molecular weight, M-n approximate to 10 kg mol(-1), but is rendered less chain-length dependent for higher M-n as the fluorene derivative gradually adopts polymeric behavior.
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| 4. |
- Tang, Zheng, et al.
(author)
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Interlayer for Modified Cathode in Highly Efficient Inverted ITO-Free Organic Solar Cells
- 2012
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In: Advanced Materials. - : Wiley. - 0935-9648 .- 1521-4095. ; 24:4, s. 554-
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Journal article (peer-reviewed)abstract
- Inverted polymer solar cells with a bottom metal cathode modified by a conjugated polymer interlayer show considerable improvement of photocurrent and fill factor, which is due to hole blocking at the interlayer, and a modified surface energy which affects the nanostructure in the TQ1/[70]PCBM blend.
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| 5. |
- 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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| 6. |
- 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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| 7. |
- Zhang, Fengling, et al.
(author)
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High photovoltage achieved in low band gap polymer solar cells by adjusting energy levels of a polymer with the LUMOs of fullerene derivatives
- 2008
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In: Journal of Materials Chemistry. - : Royal Society of Chemistry (RSC). - 0959-9428 .- 1364-5501. ; 18:45, s. 5468-5474
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Journal article (peer-reviewed)abstract
- Solar cells based on organic molecules or conjugated polymers attract great attention due to their unique advantages, such as low cost, and their use in flexible devices, but are still limited by their low power conversion efficiency (PCE). To improve the PCEs of polymer solar cells, more efforts have been made to increase short-circuit current (J(sc)) or open-circuit voltage (V-oc). However, the trade-off between J(sc) and V-oc in bulk heterojunctions solar cells makes it tricky to find a polymer with a low band gap to efficiently absorb photons in the visible and near infrared region of the solar spectrum, while maintaining a high V-oc in solar cells. Therefore, it is crucial to design and synthesize polymers with energy levels aligning with the LUMO (lowest unoccupied molecular orbital) of an electron acceptor to minimize the LUMO level difference between donor and acceptor to keep enough driving force for charge generation, thereby maximizing photovoltage in solar cells. Here a novel copolymer APFO-Green 9 was synthesized. Polymer solar cells based on APFO-Green 9 blended with a derivative of fullerene demonstrate high photovoltage by fine tuning the HOMO and LUMO level of APFO-Green 9. Solar cells based on APFO-Green 9 and [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) present a photoresponse extended to 900 nm with J(sc) of 6.5 mA cm(-2), V-oc of 0.81 V and PCE of 2.3% under illumination of AM1.5 with light intensity of 100 mW cm(-2). As a low band gap polymer with a V-oc bigger than 0.8 V, APFO-Green 9 is a promising candidate for efficient tandem solar cells.
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| 8. |
- Zhou, Yi, et al.
(author)
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Observation of a Charge Transfer State in Low-Bandgap Polymer/Fullerene Blend Systems by Photoluminescence and Electroluminescence Studies
- 2009
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In: ADVANCED FUNCTIONAL MATERIALS. - : Wiley. - 1616-301X .- 1616-3028. ; 19:20, s. 3293-3299
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Journal article (peer-reviewed)abstract
- The presence of charge transfer states generated by the interaction between the fullerene acceptor PCBM and two alternating copolymers of fluorene with donor-acceptor-donor comonomers are reported; the generation leads to modifications in the polymer bandgap and electronic structure. In one of polymer/fullerene blends, the driving; force for photocurrent generation, i.e., the gap between the lowest unoccupied molecular orbitals of the donor and acceptor, is only 0.1 eV, but photocurrent is generated. It is shown that the presence of a charge transfer state is more important than the driving force. The charge transfer states are visible through new emission peaks in the photoluminescence spectra and through electroluminescence at a forward bias. The photoluminescence can be quenched under reverse bias, and can be directly correlated to the mechanism of photocurrent generation. The excited charge transfer state is easily dissociated into free charge carriers, and is an important intermediate state through which free charge carriers are generated.
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