High resolution electrical characterisation of organic photovoltaic blends

Douhéret, Olivier; Swinnen, Ann; Breselge, Martin; Severen, Ineke Van; Lutsen, Laurence; Vanderzande, Dirk; Manca, Jean

doi:10.1016/j.mee.2006.10.056

Cited by 21 publications

(19 citation statements)

References 14 publications

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“…First performed in UHV on MDMO:PPV:PCBM blends by Glatzel et al 58 the discrepancies between the topography and the surface potential images suggested that a polymer rich matrix still covers most of the PCBM clusters protruding at the surface. This was latterly confirmed by the qualitative EFM characterisation of different PPV:PCBM blends reported by Douhéret et al 59 and monitored in air upon illumination (see Figure 6). This study also suggested locally a higher charge density in these thin polymer rich matrix layers covering the PCBM clusters (Figure 6), therefore pointing out the specific role of these interfacial regions in the photovoltaic process.…”

Section: Kpfm/efm: Electrical Characterisation Of Mdmo-ppv:pcbmsupporting

confidence: 81%

High‐resolution morphological and electrical characterisation of organic bulk heterojunction solar cells by scanning probe microscopy

Douhéret

Swinnen

Bertho

et al. 2007

Progress in Photovoltaics

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State-of-the-art organic bulk heterojunction (BH) solar cells, also called excitonic solar cells, are based on intimate mixtures of donor and acceptor organic materials of which the nanoscale morphology strongly influences both the photovoltaic performances and the stability of the device. In particular, the form and the size of the three-dimensional (3D) interpenetrating network of donor/acceptor material is shown to be crucial for the electrical transport properties and the resulting photovoltaic properties. Powerful high-resolution characterisation tools to locally map the morphology of these material systems are Scanning and Transmission Electron Microscopy (SEM/TEM), Atomic Force Microscopy (AFM) and Nuclear Magnetic Resonance (NMR). Yet, to correlate morphology with local electrical properties, significant progress has been made by the recent introduction of advanced Scanning Probe Microscopy (SPM) methods based on electrostatic force microscopy (EFM) and conductive atomic force microscopy (C-AFM). EFM related methods measure the electrostatic interaction between the probe and the surface of the organic thin films, hence to derive the variations in the sample surface potential. C-AFM based methods perform two-dimensional (2D) current mapping of sample conductivity and local spectroscopy to analyse transversal charge transport mechanisms in the blends. In case the space charge limiting current (SCLC) regime is dominating the charge transport mechanisms, carrier mobility can also be determined. Finally, the sensitivity of C-AFM to photovoltaic properties is reported. In this paper a review dealing with the different SPM methods currently used and the respective achievements performed on organic blends for BH solar cells is proposed. INTRODUCTIONPhoto-excitation in organic semiconductors usually hardly results in free electrons and holes--as in inorganic semiconductors-but in Coulomb-bound electron-hole pairs (excitons) with typical binding energies in the order of 0Á5 eV, 1 exceeding the thermal energy at room temperature (kT ¼ 0Á025 eV). To yield free charge carriers, an additional driving force is required and can be accomplished by the formation of donor/acceptor heterojunctions, by means of the combination of a photo-excited material with another material with dissimilar energetic levels (e.g. a larger electron affinity and ionisation potential). In practice, this principle has first been realised by means of an evaporated stack structure of a hole conducting molecule in contact with an electron conducting small molecule layer.2 The currently most efficient systems consist of an intimate mixture between a hole conducting conjugated polymer and an electron conducting small molecule, for example C 60 (bulk heterojunction (BH) solar cell), 3 or providing heterointerfaces between an organic dye molecule and inorganic oxides, for example TiO 2 or SnO 2 (dye-sensitised solar cells). 4 More recently, full polymeric photovoltaic systems have also been reported. [5][6][7] In most of the systems, the dissimilarit...

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Section: Kpfm/efm: Electrical Characterisation Of Mdmo-ppv:pcbmsupporting

confidence: 81%

High‐resolution morphological and electrical characterisation of organic bulk heterojunction solar cells by scanning probe microscopy

Douhéret

Swinnen

Bertho

et al. 2007

Progress in Photovoltaics

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“…Contrast between the two components of bulk heterojunctions in dark SP images of bulk heterojunction blends has been observed before, [13][14][15]26] but was not fully explained. The presence of contrast in the dark images is to some extent remarkable, because in the dark charges are thought to be absent in these pristine, undoped organic semiconductors, and the SP should be www.afm-journal.de with N 0 the density of states in the semiconductor and k B T the thermal energy.…”

Section: Dark Contrastmentioning

confidence: 72%

Scanning Kelvin Probe Microscopy on Bulk Heterojunction Polymer Blends

Maturová

Kemerink

Wienk

et al. 2009

Adv Funct Materials

101

105

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Here, correlated AFM and scanning Kelvin probe microscopy measurements with sub‐100 nm resolution on the phase‐separated active layer of polymer‐fullerene (MDMO‐PPV:PCBM) bulk heterojunction solar cells in the dark and under illumination are described. Using numerical modeling a fully quantitative explanation for the contrast and shifts of the surface potential in dark and light is provided. Under illumination an excess of photogenerated electrons is present in both the donor and acceptor phases. From the time evolution of the surface potential after switching off the light the contributions of free and trapped electrons can be identified. Based on these measurements the relative 3D energy level shifts of the sample are calculated. Moreover, by comparing devices with fine and coarse phase separation, it is found that the inferior performance of the latter devices is, at least partially, due to poor electron transport.

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“…Among the vast number of different scanning probe methods, atomic force microscopy with conducting tips ͑cAFM͒ is especially helpful to investigate nanostructures, 1,2 local properties of dielectric films, 3-6 and recently also organic films. 7 The cAFM technique we want to discuss in this paper is scanning capacitance microscopy ͑SCM͒. In SCM, a conductive AFM tip is used to measure the local capacitance between the tip and the sample.…”

Section: Introductionmentioning

confidence: 99%