Fourier-Transform Photocurrent Spectroscopy for a fast and highly sensitive spectral characterization of organic and hybrid solar cells

Vandewal, Koen; Goris, Ludwig; Haeldermans, I.; Nesládek, Miloš; Haenen, Ken; Wagner, Patrick; Manca, Jean

doi:10.1016/j.tsf.2007.12.056

Cited by 62 publications

(57 citation statements)

References 14 publications

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“…[ 16 ] Similar experiments on polyfl uorene-PCBM and polymerpolymer blends were performed. [17][18][19] The CT states could also be excited optically by subgap illumination, [20][21][22] and be observed by electroluminescence as a result of charge injection. [ 23 , 24 ] The commonly used names for CT states and complexes are diverse, either used alternatively or to defi ne special cases.…”

Section: Experimental Evidence For the Charge Transfer Statementioning

confidence: 99%

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Role of the Charge Transfer State in Organic Donor–Acceptor Solar Cells

2010

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Charge transfer complexes are interfacial charge pairs residing at the donor-acceptor heterointerface in organic solar cell. Experimental evidence shows that it is crucial for the photovoltaic performance, as both photocurrent and open circuit voltage directly depend on it. For charge photogeneration, charge transfer complexes represent the intermediate but essential step between exciton dissotiation and charge extraction. Recombination of free charges to the ground state is via the bound charge transfer state before being lost to the ground state. In terms of the open circuit voltage, its maximum achievable value is determined by the energy of the charge transfer state. An important question is whether or not maximum photocurrent and maximum open circuit voltage can be achieved simultaneously. The impact of increasing the CT energy-in order to raise the open circuit voltage, but lowering the kinetic excess energy of the CT complexes at the same time-on the charge photogeneration will accordingly be discussed. Clearly, the fundamental understanding of the processes involving the charge transfer state is essential for an optimisation of the performance of organic solar cells.

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Section: Experimental Evidence For the Charge Transfer Statementioning

confidence: 99%

“…The CT state can alternatively be populated by a direct excitation into a weak CT absorption band [ 21 ] -termed as ground state charge transfer complex [ 32 ] -or via nongeminate recombination of free polarons.…”

Section: Experimental Evidence For the Charge Transfer Statementioning

confidence: 99%

Role of the Charge Transfer State in Organic Donor–Acceptor Solar Cells

2010

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“…Vandewal et al found that a highly sensitive technique Fourier-transform photocurrent spectrum (FTPS) could be used to measure the PV EQE (EQE PV ) spectrum where the low energy CT absorption bands could be resolved. 63,96 This technique allows for the determination of E CT via EQE PV in the absorption range of the charge transfer state:…”

confidence: 99%

Studies of Morphology and Charge-Transfer in Bulk-Heterojunction Polymer Solar Cells

Ma¹

2013

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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 processes 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. VI Populärvetenskaplig...

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“…Study of structural changes induced by annealing has been done ). According to (Vandewal et al 2008), frequency dependence is not an issue in case of organic solar cells.…”

Section: Nanocrystalline Diamond Cis and Organic Semiconductorsmentioning

confidence: 99%