Monitoring Charge Exchange in P3HT-Nanotube Composites Using Optical and Electrical Characterisation
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Charge exchange at the bulk heterojunctions of composites made by mixing single wall nanotubes (SWNTs) and polymers show potential for use in optoelectronic devices such as solar cells and optical sensors. The density/total area of these heterojunctions is expected to increase with increasing SWNT concentration but the efficiency of solar cell peaks at low SWNT concentrations. Most researchers use current–voltage measurements to determine the evolution of the SWNT percolation network and optical absorption measurements to monitor the spectral response of the composites. However, these methods do not provide a detailed account of carrier transport at the concentrations of interest; i.e., near or below the percolation threshold. In this article, we show that capacitance–voltage (C–V) response of (metal)-(oxide)-(semiconducting composite) devices can be used to fill this gap in studying bulk heterojunctions. In an approach where we combine optical absorption methods withC–V measurements we can acquire a unified optoelectronic response from P3HT-SWNT composites. This methodology can become an important tool for optoelectronic device optimization.
Composites of polymers and nanotubes have attracted much attention lately because they are lightweight, relatively simple to fabricate, and are a low cost alternative to current structural and electronic materials. In mechanical applications nanotubes are used to increase the stiffness and toughness of the host polymer  with much research on dispersion methodologies [2, 3] and the mechanical behavior of nanotubes and their arrays . Polymer–nanotube electronic materials on the other hand are set to explore the charge exchange at the polymer–nanotube heterojunctions within the volume of the composite. Optoelectronic characterization such as photoluminescence [5, 6] optoelectronic memory effect , and photovoltaic response [8–16] suggest that nanotubes act as electron donors to the polymer host. Even though the mechanical properties of composites improve by increasing the nanotube concentration, the electronic response is usually optimum for low concentrations of nanotubes [11, 14, 17]; usually close to the percolation threshold.
In this article we use two different methodologies to probe the interaction between poly (3-hexylthiophene), (P3HT), and single wall nanotubes (SWNTs) and probe the charge exchange at their heterojunctions. Optical linear absorption and femtosecond transient absorption measurements are then used to study P3HT–SWNT composites at high SWNT concentrations. Electrical capacitance–voltage measurements of metal-oxide-semiconductor (MOS) devices are then used to monitor charge exchange at SWNT concentrations near or below the percolation limit. Our results show that this combination of optical and electrical methods provide a useful tool for studying charge exchange in polymer–nanotube composites over a wide range of SWNT concentrations and therefore can help to optimize their optoelectronic response.
Composites of P3HT and SWNTs were prepared by mixing appropriate amounts of the two materials dissolved in 1,2-dichlorobenzene. SWNTs were obtained from CNI (research grade purified Hipco SWNTs) and were used without further functionalization. The composite solution was then sonicated further until it was homogeneous. Layers of the composite were drop cast on the substrate: quartz discs for optical measurements; n+Si with 200 nm thick thermally grown SiO2for capacitance–voltage (C–V) measurements; glass for conductivity measurements. The inset in Fig. 3shows the geometry of the devices used forC–V andI–V measurements. The heavily doped n+Si with evaporated Al back contact serves as the gate duringC–V testing. All samples were dried in air overnight and then were kept in vacuum for at least 12 h to ensure full drying of the solvent. The Au top contacts forC–V andI–V measurements were subsequently deposited by evaporation. The Au contacts forC–V were 1 mm dots while forI–V we used 200 × 200 μm square contacts at 50 μ m from each other. TheC–V,I–V, and optical measurements were performed in air. The ramp rate duringC–V measurements was 1 V/s and the peak-to-peak value of the probing voltage was 50 mV.
For the time resolved measurements we have used a non collinear super-continuum pump probe configuration in conjunction with a regenerative Ti:Sapphire amplifier system producing 100 fs pulses at 800 nm. The temporal resolution of the system has been measured to be better than 150 fs. In this work, optical pumping at a fluence of 2 mJ/cm2 was used to excite the composites and determine their temporal behaviour. Here, we should point out that around this fluence non linear effects such as exciton–exciton annihilation were not observed in our experimental studies. More details on sample preparation and details of our optical system can be found in a recent publication .
Results and Discussion
Figure 4 shows that at a low frequency value, the capacitance increases with increasing SWNT concentration up to a value of SWNT concentration and then remains almost unchanged. This trend is reproducible over several sets of samples with the upper limit in SWNT concentration always being close to the percolation threshold (0.7–1 wt%). As the nanotube concentration increases the total area of P3HT–SWNT junctions should increase so the saturation in the value of the low frequency capacitance is not straightforward to explain. Firstly, as the nanotube concentration increases in the composite solution, SWNTs will tend to form bundles. The total area of the P3HT–SWNT heterojunctions should still increase but at a lower rate above a certain SWNT concentration. Our ultrafast transient absorption measurements also showed a monotonic decrease in exciton life-time with increasing SWNT concentration, so the total area of bulk junctions does increase. Secondly, the increased SWNT concentration above the percolation threshold will make the composite increasingly conductive and the semiconducting response of the composite will diminish. The increased availability of carriers at the interface means that albeit trapping at bulk heterojunctions still taking place as shown by pump–probe experiments, the device will behave as a single capacitor with the n+S and the composite behaving as metals. Therefore the low frequency capacitance will slowly tend to become equal to C ox . These two competing factors will tend to impose an upper limit to the value of accumulation capacitance at low frequencies. The above explanation needs to be verified with more experiments and this is the immediate focus of our work. Our results agree well with the work of Kymakis et al.  who have shown that above a certain concentration of SWNTs, the semiconducting response of the composite and the efficiency of photovoltaic devices deteriorate. Our estimated limit in SWNT concentration of 0.7 wt% is close to the concentration at which these authors have measured their maximum efficiency (1 wt%).
In summary, we have presented a combination of electrical and optical methods for studying the charge exchange at bulk P3HT–SWNT heterojunctions. The optical methods show that at high SWNT concentrations the structure of the polymer is altered as the polymer chains movement is restricted. The change in structure is obvious from the optical absorption spectra. Ultrafast transient absorption measurements have been used here to monitor the population of states at the bottom of P3HT’s LUMO with a temporal resolution of 150 fs. The existence of SWNTs in the composite accelerated exciton dissociation up to SWNT concentrations of 65 wt%. However, the optical methodologies explored could not provide detailed information for very low SWNT loadings near the percolation threshold (0.75 wt%). However, this is an extremely interesting range of SWNT concentrations because, electrically, the composites change from semiconducting to almost metallic very rapidly for SWNTs concentrations above the percolation threshold. Here we show that low frequencyC–V characterization is a methodology which can be used to complement optical characterization and detect charge exchange at P3HT–SWNT heterojunctions. The signature of this interaction is the value of the accumulation capacitance being higher than C ox at low frequencies. In analogy to MOS devices with leaky dielectrics, the higher than C ox value of accumulation capacitance is a measure of the charges trapped by the SWNTs at bulk junctions near the interface.
The authors would like to thank Dr. S. Taylor in the Department of Electrical Engineering and Electronics, University of Liverpool for his critical help withC–V measurements. IA and EM would also like to acknowledge partial funding of this work by the University of Liverpool and grant ACCESS/0308/13 by the Research Promotion Foundation in Cyprus.