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

Abstract

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...

Electrical Modes in Scanning Probe Microscopy

“…[11] The later observation lead to the picture that PSS is transported towards the surface upon applying the high tip sample voltage. Douheret and co-workers reported that the topography of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) was slightly altered upon scanning in contact mode using cantilevers with a spring constant <1 N Á m. [12] However, at a bias potential of À1 V, currents of 10 pA lead to significant degradation of the electrical properties of the scanned area. The internal structure of bulk heterojuntion P3HT:PCBM films of the surface and focused ion beam cross-sections were investigated.…”

Nanoscale Inhomogeneity of Conducting‐Polymer‐Based Materials

“…As was the case with KFM and EFM, there have been a few CS-AFM studies of the polymer blends [67][68][69][70][71] generally showing inhomogeneity due to pronounced phase segregation in these systems. Further details can be found in a recent review [52].…”

Morphology of organic electronic materials imaged via electron tomography

“…Due to the importance of morphology for the function of organic solar cells much effort have been put into examining the phase structure. Commonly this is done with techniques such as atomic force microscopy (AFM) (Roman et al , 1997; Douheret et al , 2007; Dante et al , 2008), including Kelvin probe force microscopy and conductive AFM (Douheret et al , 2007), or scanning electron microscopy (SEM) (Hoppe et al , 2004). These techniques are used to map the surface topography and material differences, as well as electrical properties.…”