Synthesis, structure, and opto-electronic properties of organic-based nanoscale heterojunctions
© Rezek et al; licensee Springer. 2011
Received: 1 October 2010
Accepted: 18 March 2011
Published: 18 March 2011
Enormous research effort has been put into optimizing organic-based opto-electronic systems for efficient generation of free charge carriers. This optimization is mainly due to typically high dissociation energy (0.1-1 eV) and short diffusion length (10 nm) of excitons in organic materials. Inherently, interplay of microscopic structural, chemical, and opto-electronic properties plays crucial role. We show that employing and combining advanced scanning probe techniques can provide us significant insight into the correlation of these properties. By adjusting parameters of contact- and tapping-mode atomic force microscopy (AFM), we perform morphologic and mechanical characterizations (nanoshaving) of organic layers, measure their electrical conductivity by current-sensing AFM, and deduce work functions and surface photovoltage (SPV) effects by Kelvin force microscopy using high spatial resolution. These data are further correlated with local material composition detected using micro-Raman spectroscopy and with other electronic transport data. We demonstrate benefits of this multi-dimensional characterizations on (i) bulk heterojunction of fully organic composite films, indicating differences in blend quality and component segregation leading to local shunts of photovoltaic cell, and (ii) thin-film heterojunction of polypyrrole (PPy) electropolymerized on hydrogen-terminated diamond, indicating covalent bonding and transfer of charge carriers from PPy to diamond.
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 , the breakthrough is attributed to the studies on doped polyacetylene since 1977 by Heeger, Shirakawa, and MacDiarmid , 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 , 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.
Significant improvement emerged with the bulk-heterojunction design . In the bulk-heterojunction design both materials (donor and acceptor) are prepared as a microscopic interpenetrating network (Figure 1b). 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% .
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 , 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, 11]. 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 , 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 [13–15]. 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, 17]. Hydrogen termination can also be used as a starting surface for grafting of more complex organic molecules on diamond [10, 18–20].
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 , chemical vapor deposition , UV irradiation , or chemical polymerization [24, 25]. PPy is a well known yet still remains under intensive study in many fields of applications, like sensors [26–28], biosensors [29, 30], fuel cells [31, 32], corrosion protection , or rechargeable batteries [34, 35].
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 . 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) . 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) .
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 . 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 . The resolution can be further enhanced using special metal tip in the so-called tip-enhanced Raman spectroscopy . This method works well on resonant systems such as in the case of carbon nanotubes , 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, 43], and (ii) thin-film heterojunction of PPy electro-polymerized on hydrogen-terminated diamond, indicating covalent bonding and enhanced exciton dissociation in such systems [44–47].
Basic organic bulk heterojunction
Basic type of organic bulk heterojunction was based on fullerene C60 as electron acceptor. Composite blends made of poly[(2,7-(9,9-dihexa)fluorence)-co-(1,4-(2,5-didecylaminoketo) phenylene)] (VYP-120, developed at the Institute of Macro-molecular Chemistry, ASCR, v.v.i.) and fullerene C60 (Sigma Aldrich) were spincoated on indium tin oxide (ITO)-covered glass substrates and dried at 50°C under vacuum for 4 h. For characterizing the PV performance of the thin film, top Al electrodes were evaporated through a shadow mask.
This assertion is proven by micro-Raman spectroscopy and mapping. The spectrum collected on the dendrite showed a sharp peak at 1468 cm-1, which was attributed to fullerene C60, as well as a broad band at 1400 cm-1[42, 48]. Similar spectra, although with lower peak intensity, were detected on the small clusters in the surroundings. The map and the spatial profile of C60 characteristic Raman scattering peak at 1468 cm-1 are shown in Figure 2c. The image reveals higher concentration of C60 at the dendrite and low C60 Raman signal in its close surroundings.
Spectrum detected in the dendrite vicinity did not show the sharp peak, only a broad band around 1400 cm-1. Similar spectra were obtained on the smooth surface in the farther surroundings. This band is not present in typical C60 Raman spectrum . Hence, we attribute it to a photoluminescence (PL) background due to Raman laser excitation (note that it cannot be directly compared with usual PL spectra). Spatially resolved map and the profile of Raman intensity at the 1400 cm-1 band are shown in Figure 2d. It is noteworthy that both Raman maps are qualitatively quite similar. Yet, quantitative comparison of the spatial profiles shows differences: the C60 signal is higher on the dendrite and small clusters, while the PL intensity prevails over C60 on the smooth surface and the very vicinity of the dendrite. We suggest that the map of sharp peak at 1458 cm-1 is related with segregated crystallized C60 on the dendrite and on small clusters in the surroundings. The broad band at around 1400 cm-1 may be attributed to C60 that is highly dissolved in the blend. This assertion is supported also by the AFM and KFM data.
In both Raman maps, there is a minimum in the vicinity of the dendrite. This is not effect of thickness as the AFM shows even surface compared to surroundings. On the other hand, there is clear difference in the work function detected by KFM. This indicates that this area consists predominantly of the conductive polymer. In such a case, the dendrite vicinity is not of heterostructural nature, and the conductive polymer may electrically shunt the device.
To resolve the above effect, we characterized local electronic transport properties of the organic film by CS-AFM, so that its integrity was preserved, and its surface morphology was the same as in tapping mode. The image of local conductivity (Figure 2e: note that the applied bias voltage is negative) shows that the dendrite and farther surroundings are electrically insulating. Conductivity in the close vicinity of the dendrite (area lacking C60) is an order of magnitude higher. This observation corroborates the conclusion that such features can cause electrical shunting of the organic film and reduce its power conversion efficiency.
Advanced organic bulk heterojunction
Based on the result with pure fullerene, more advanced bulk heterojunctions were prepared, with fullerene derivatives being used as electron acceptors: [6, 6]-thienylC61 butyric acid methyl ester (ThCBM), and [6, 6]-thienylC71 butyric acid methyl ester(ThCBM). Poly(3-hexylthiophene-2,5-diyl) (P3HT) was used as electron donor. The blends were prepared in an inert atmosphere on ITO-covered glass substrates with a hole-conducting polymer (PEDOT:PSS) by spin coating from a 2.5% chlorobenzene solution . Both blends were annealed at 125°C (the annealing improved the PV efficiency more than twice compared with non-annealed blends). Resulting thickness of the films was 140 nm, as measured by KLA-Tencor P-10 profiler. LiF (2 nm) and Al (60 nm) were thermally evaporated to make the second contact. Active area of the prepared PV cells was 6.5 × 2.5 mm2.
It is noticeable that the initial power conversion efficiency of the P3HT:ThCBM blend is lower but its decrease in time is also slightly slower. Based on AFM topography (shown as inset images in Figure 3a, b), both blend films are smooth (RMS roughness around 2 nm) with shallow pits (10-20 nm deep, 100-200 nm in diameter). These pits do not affect the electronic homogeneity of the films. The films appear electronically uniform as observed by KFM . Therefore, to understand the differences in PV performance, we characterized dynamic opto-electronic response by modifying standard KFM technique. We stopped the physical motion of the AFM tip in one direction so that the corresponding axis in the image rep-resents time axis. During scanning in this modified regime we were switching on and off white light illumination repeatedly.
Resulting data (KFM images and their cross sections) are shown in Figure 3c, d. Under the white light illumination, the surface potentials of both blend films shifted. In the case of P3HT:ThCBM blend, the shift of surface potential is slower. This indicates that, after the illumination is switched off, the film keeps negative charge. This behavior has already been observed also on BH PV cells containing PCBM  as well as all-polymer cells . It was attributed to electron trapping in shallow traps . Persisting negative charge may then limit the surface potential shift under repeated illumination being switched on. In the case of P3HT:ThCBM, the surface potential comes repeatedly back almost to the starting level during the short term, which indicates that the electron trapping in this blend is minimized, and therefore it is beneficial for PV applications.
The absolute values of the overall surface potentials shifted over time independently on the illumination. This effect is attributed to degradation of the blend films, as the AFM/KFM characterization was performed under ambient conditions. Degradation of the blends was also observed as a decrease in photo-response. There was almost no photo-response observed after 250 min in the case of P3HT:ThCBM blend, and the same state is reached after 1000 min in the case of P3HT:ThCBM.
I(V) and KFM characteristics indicate that the P3HT:ThCBM degrades much faster than the other one. The difference between the macroscopic and microscopic performance is most likely caused by the presence of the metal electrode (in the macroscopic characterization), which partially seals the organic films. From this point of view, contact-less KFM characterization revealed different opto-electronic properties of the blends, not detected by the macroscopic I(V) characteristics.
Covalent bonding between PPy and diamond was confirmed also theoretically. Calculated interaction energy of about 6 eV per bond pointed to a covalent character of the bonds formed at the one- and multi-bond contacts between PPy and diamond .
Based on the above experimental and theoretical results, we propose a model that, during the electrochemical synthesis of PPy on diamond, the hydrogen atoms are removed and, instead, PPy molecules establish covalent bonds with carbon atoms of diamond.
Under illumination, the observed positive shifts of surface potentials correspond to a decrease of work function in both PPy and diamond as indicated in Figure 8. These shifts of surface potentials were attributed to the SPV effects. Based on the SPV theory, a model of charge transfer between diamond and PPy was established . This model suggests that under illumination strongly bound excitons are generated in PPy. The excitons can be split at the interface with diamond, and the holes are trapped in diamond surface/interface states.
We can explain this illumination-induced conductivity by the charge transfer of holes from PPy to diamond as suggested above. The electronic transport measurements indicate that the holes accumulated in diamond-trap states are excited to diamond valence band, where they support the in-plane conductivity . Such concept is schematically illustrated in Figure 9b. Relatively quick response to the change of illumination can be explained by a quick re-trapping of photogenerated holes in diamond and their recombination with photo-electrons remaining in the vicinity of the PPy-diamond junction due to downward band bending of PPy LUMO. The response rate is comparable with the organic blend ThCBM:P3HT shown in Figure 3c. Also, we suggest that the photo-electrons are not persistently trapped. Hence they can support the quick changes in potential and current. They may also form another transport channel along the heterojunction but this still remains to be investigated.
The observed enhancement of in-plane conductivity under illumination might be explained also by the bridging effect via the semi-conductive and photosensitive PPy. To resolve this issue, we have measured mobility of charge carriers using Hall effect. Mobility of holes in PPy and diamond differs by at least five orders of magnitude (H-terminated diamond: 100-102 cm2/Vs, PPy: 10-5-10-10 cm2/Vs). We prepared microscopic conductive square (10 × 10 μ m2) with electric leads at the corners by selective hydrogen and oxygen terminations. A resin layer with the opening at the position of the square has been prepared by UV lithography to restrict the active area for electrochemical deposition. PPy was electrochemically synthesized under similar conditions as described in the previous sections. The Hall voltage was detected by two electrometers, and the current of 2 pA was supplied by Keithley 220 source. The magnetic field applied during the experiment was ± 0.2 T. The mobility was evaluated from the Hall voltages to 7 cm2/Vs . However, the Hall voltages were slowly drifting in time, and thus the mobility was obtained with high error bar of 20 cm2/Vs.
The same Hall voltages plotted as a function of magnetic field are shown in Figure 10c. If an arithmetic mean is performed for each applied magnetic field and these values are used for evaluation, then the resulting Hall mobility is 16 ± 10 cm2/Vs. This revised value of mobility with much lower relative error supports the idea of charge transport via diamond and thus transfer of charge from PPy to diamond.
In this article, we showed that employing and combining advanced scanning probe techniques can provide significant insight into the correlation of structural, chemical, and opto-electronic properties of organic-based heterojunctions on the microscopic scale. We presented specific results obtained on fully organic bulk heterostructures and on heterojunction of diamond with electrochemically synthesized organic dye. Although these systems seem to be different, from the optoelectronic point of view, they behave in a similar way. In both systems, strongly bound excitons are created which dissociate at the interface with the other material. Correlation of advanced scanning probe techniques (AFM, KFM, and CS-AFM) with micro-Raman mapping identified the reasons for low PV efficiency and could resolve differences in optoelectronic quality and stability by contact-less monitoring of dynamic photo-response of the organic blends. We demonstrated the applicability of the above methodology also to a hybrid organic-inorganic system. By correlating AFM nanoshaving, KFM surface potentials, as well as I(V) characteristics, we proved that during the electrochemical synthesis a covalent bond between diamond and PPy is formed. Such conclusion is difficult to make based on commonly applied techniques such as X-ray photoelectron spectroscopy because here we have all-carbon systems (polymer-diamond). Furthermore, based on microscopic KFM and SPV measurements, the model of charge transfer from PPy to diamond under illumination was proposed. The model was supported by in-plane I(V) and Hall effect measurements.
atomic force microscopy
current-sensing AFM regime
indium tin oxide
Kelvin force microscopy
The authors appreciate technical assistance of Z. Poláčková, J. Potmĕšil, V. Jurka, K. Jurek, and J. Zemek. The authors also acknowledge the financial support of the Academy of Sciences of the Czech Republic, (project No. KAN400100701, and the Fellowship J.E. Purkynĕ), the Ministry of Education, Youth and Sports of the Czech Republic (grants No. LC06040, LC510, 1M06031, AV0Z10100521), and the Grant Agency of the Czech Republic (grants No. 202/09/H041, P204/10/0212).
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