Electronic devices nowadays are commonly based on inorganic semiconductors (e.g., silicon or germanium) and, thus, face the problem of high-cost industrial processes (high-vacuum technology, clean rooms, or high-purity source materials). A possible way to reduce costs is the use of organic materials with semiconducting or metallic properties. Although the first report on the conductivity of doped polypyrrole (PPy) was published in 1963 [1
], the breakthrough is attributed to the studies on doped polyacetylene since 1977 by Heeger, Shirakawa, and MacDiarmid [2
], for which they were awarded the Nobel prize in 2000. Since then organic semiconductors have followed similar scientific evolution as inorganic ones (all-polymer field effect transistor in 1994 [3
], organic integrated circuit by the Philips company (1998)) until recently, which have seen application of organic displays in cell phones, and the start of commercial production of organic photovoltaic (PV) cells.
PV effect in organic materials is different from inorganic ones. The binding energy between photo-excited electron-hole pair is strong due to the low dielectric constant, typically in the range of 0.1-1 eV. Therefore, the excitons are not dissociated by thermal energy, which is approximately 26 meV at room temperature. Additional driving force is needed, which can be supplied by introducing a layer of a second organic material (the so-called double-layer cell, Figure ). Typical PV power conversion efficiencies of such devices are not higher than 0.1% [4
] because of the short exciton diffusion length (around 10 nm [6
]) compared to the total film thickness needed for efficient light harvesting (100-200 nm).
Schematic cross-sectional drawings of two main organic PV cell designs: (a) double-layer junction and (b) bulk-heterojunction.
Significant improvement emerged with the bulk-heterojunction design [7
]. In the bulk-heterojunction design both materials (donor and acceptor) are prepared as a microscopic interpenetrating network (Figure ). Therefore, wherever the excitons are generated, an interface with the other material is always closer than the exciton diffusion length (in an ideal case). The remaining task is to ensure efficient charge carrier transport to electrodes. So far the best reported PV efficiency is the attainment of reaching 7.4% [8
For a long time, electronics has been the domain of inorganic materials. Even though electronic behavior of organic materials has been known for several decades, their applications are still limited. So far, the research and development activity of organic and inorganic electronics has been strictly divided, although their combination might be fruitful. An example of promising organic-inorganic (hybrid) systems is the dye-sensitized PV cell. In this so-called Grätzel cell [9
], a photoexcited organic dye gives electrons to the electrode via porous inorganic TiO2
. The missing electron is returned from the surrounding electrolyte, which restores the original state at the counter electrode. Although the PV power conversion of this type of PV cells is relatively high (10-12%), their wider application is limited by the need of an electrolyte, which is commonly in liquid form.
Another example of an organic-inorganic system, which is extensively studied, is diamond in combination with organics, e.g., fullerene [10
]. Such systems are highly promising for bio-sensoric or opto-electronic applications, as a charge transport between the two materials is observed. However, the basic electronic properties at their interface still need to be fully understood.
From the electronic point of view, diamond is a wide bandgap (5.5 eV) semiconductor, which in its intrinsic form is electrically insulating. Apart from the bulk conductivity induced by doping [12
], intrinsic diamond exhibits a special phenomenon of surface conductivity when a thin (10-20 nm) electrically conductive (p
-type) layer is formed at the surface [13
]. It is observed under ambient conditions when the diamond surface is terminated by hydrogen atoms. Various electrically conductive areas or channels can be patterned on the surface by changing the surface termination [16
]. Hydrogen termination can also be used as a starting surface for grafting of more complex organic molecules on diamond [10
In this study, we have chosen PPy because of its wide universality as a model of a chemically and optically sensitive organic dye. Its polymerization can be achieved by electrooxidation from a solvent [21
], chemical vapor deposition [22
], UV irradiation [23
], or chemical polymerization [24
]. PPy is a well known yet still remains under intensive study in many fields of applications, like sensors [26
], biosensors [29
], fuel cells [31
], corrosion protection [33
], or rechargeable batteries [34
Organic-based electronic as well as optoelectronic devices are commonly made from several compounds and their mutual electronic cooperation at microscale is of key importance for the device properties. Therefore, revealing and understanding of microscopic structural, chemical, electronic, and optoelectronic properties is crucial for their further improvements. Such challenging task may be fulfilled by techniques based on local probe scanning. One of the prominent methods employing scanning probe is atomic force microscopy (AFM) with its diverse regimes of operation [36
]. In AFM, a sharp tip mounted on a flexible lever (the so-called can-tilever) can detect both morphologic and electronic information with high lateral resolution.
When the tip is in contact with the studied surface, a bias voltage can be applied between the sample and the AFM tip, and the induced electric current is dependent on the local conductivity at the position of the tip. As a result, maps of both topography and local conductivity are obtained at once in this so-called current-sensing AFM regime (CS-AFM) [37
]. Applying this mode on organic materials is rather difficult, however, as the soft organic materials can be easily modified by the sharp AFM tip (radius typically 10-30 nm) under the applied force of typically several nN or more.
Application of an AC voltage between the sample and the AFM tip when the tip is at a certain distance from the surface creates a modulated electrostatic force that makes the cantilever oscillate because of contact potential difference. These oscillations can be minimized to zero by applying an additional DC voltage to compensate the potential difference. If the tip surface potential is calibrated, then the absolute values of the surface potential (and then the work function) can be obtained. This is the principle of the so-called Kelvin force microscopy (KFM) [16
In spite of being a powerful tool for resolving microscopic structural, mechanical, and electronic properties, AFM had not been able to provide direct chemical contrast until very recently [38
]. Such chemical detection in AFM makes use of adhesion differences on atomic scale and was, thus so far, applicable only to atomically flat surfaces. Spatially resolved chemical information can be obtained in more straightforward way by micro-Raman spectroscopy mapping, where the focused laser beam plays the role of the scanning probe. Although it is an optical method, the resolution can be satisfactory at sub-micrometer scale [39
]. The resolution can be further enhanced using special metal tip in the so-called tip-enhanced Raman spectroscopy [40
]. This method works well on resonant systems such as in the case of carbon nanotubes [41
], but its general application is still highly challenging.
In this study, we employ and combine advanced scanning probe techniques as well as macroscopic characteristics to provide us significant insights into the correlation of microscopic structural, chemical, and opto-electronic properties of organic-based heterojunctions. We demonstrate benefits of such multi-dimensional characterizations on (i) bulk heterojunctions of fully organic composite films using fullerenes as electron acceptors, indicating differences in blend quality and component segregation leading to local shunts of PV cell [42
], and (ii) thin-film heterojunction of PPy electro-polymerized on hydrogen-terminated diamond, indicating covalent bonding and enhanced exciton dissociation in such systems [44