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We are exploring the development of low cost alternative solar cells by using conductive polymers mixed with carbon nanotubes. The understanding and the optimization of the electronic and physical interactions in the composite material is a crucial step towards higher efficiency. Mixtures of polymers and multiwall carbon nanotubes have been investigated by scanning tunneling microscopy in ultrahigh vacuum, obtaining stunning images of their architecture. These studies provide crucial information to the improvements of these devices.
Our result highlights the fundamental role played by the CNT chirality in the interaction with rrP3HT. The structural arrangement is also expected to have consequences in the electrical behavior of the local p-n heterojunction.
STM image of a SWNT wrapped by P3HT and model of polymer wrapping
The need for solar energy production is dramatically increasing in these years, making Photovoltaics the fastest-growing energy technology in the world. The diffusion of solar panels is still quite limited though, due to the high cost of photoactive materials and energy-intensive processing technology. Semiconducting polymers, inexpensive to synthetise and easily processable in solution, can replace such inorganic components as silicon compounds. Organic solar cells show tremendous advantages:
However, polymer-based solar cells still need to overcome intrinsic shortcomings as low exciton diffusion lengths and low mobility, in order to reach higher efficiencies and compete with CO2-producing technology. Nanoscale morphology control of these materials is deemed the key for a breakthrough in this field, thus representing the main research interest of our Applied Nanotechnology Group.
"State of the art" OPV cell
The cell is usually made by sandwiching a blend of a conjugated polymer and an electron-acceptor material between two metal contacts with different work functions. Nowadays, the state-of-the-art in the field of organic photovoltaics is represented by the bulk-heterojunction solar cell based on a blend of poly(3-hexylthiophene) (P3HT) and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) . Its reproducible efficiency approaches 5%. In order to attain efficiencies towards 10%, much effort is required to understand the fundamental electronic interactions between the polymeric donors and the acceptor materials as well as the complex interplay of device architecture, morphology, processing and the fundamental electronic processes.
Carbon nanotubes have superior electron transport properties and have been successfully integrated in organic photovoltaic devices to various purposes.
A quite low concentration (~0.1%) of SWNTs mixed with a donor polymer can provide paths of percolation, thus contributing to charge separation and collection, while increasing the conductivity of the polymer . The overall efficiency (up to 0.1%) of a device made with this blend is though significantly limited as compared with the fullerene-based counterparts, mainly due to:
Another approach consist in the introduction of nanotubes only at low concentrations in a P3HT:PCBM blend, in order to promote the dissociation of excitons at the junctions and the charge extraction . The SWNT addition increased both the device short-circuit current and FF, resulting in an efficiency boost of 40%. Further optimization is then necessary to tackle the aforementioned issues and unveil the molecular scale interaction of a polymer-nanotube composite.
MWCNTs can be grown directly onto ITO coated glass via CVD, constituting a 3-dimensional and highly-conductive electrode . The contact resistance results remarkably lowered and the hole-extraction favoured. The improvement is attributed to the increase in the area of the electrode, as compared to planar ITO glass, reducing the average distance holes need to travel before extraction to the external circuit. Unfortunately, the amount of light entering the cell is reduced due to the loss of transparency of the electrode. To this end, the growth of vertical-aligned CNTs driven by pre-patterning of the substrate will restore the necessary transparency of the electrode.
Indium-tin oxide (ITO) has been regularly used as the hole-collecting electrode in organic photovoltaic devices, but if suffers from two major shortcomings: it is an expensive material, due to the low availability of indium; it cannot be used in flexible cells . Recent research has then focussed on the development of thin layers of highly transparent conductive films based on carbon nanotube dispersions. These nanotube films exhibit mechanical reliability and can be formed using low-temperature printing techniques. SWNT dispersions would not only increase the electrode conductivity but also might extend the exciton dissociation area.
Atomic investigation of P3HT-CNTs mixtures
Materials and tools required for the preparation of devices are kept in a glove-box with nitrogen environment, in order to prevent any kind of contamination during the assembly of the cell.
The apparatus consists of a two- zone furnace with temperature controllers and flow meters for gases. It is used for the growth of carbon nanotubes on various substrates.
A FIB is an instrument that can obtain images of a sample by scanning it with a high-energy ion beam (Ga+). The metal ions strongly interact with the matter, thus a FIB provides two functions by atomic sputtering: nano-machining and nano-deposition. Owing to the very small size of the beam, this FIB can mill patterns on any substrate with a resolution of ~10 nm. It is also capable of fabricating nanostructures on a surface by depositing platinum.
The photovoltaic module testing system feature a light source that closely matches the solar spectrum for the analysis of the device performance under sun simulation condition.
The scanning electron microscope is typically used to acquire morphologic information of the device layers and the in-house grown carbon nanotubes. It can also provide chemical information through the embedded Energy Dispersive X-Ray (EDX) detector.
Atomic force and scanning tunnelling microscopes are both important tools for the investigation of the nanotube-polymer mixtures at the nanoscale. Scanning tunnelling microscopes are also capable of spectroscopy analysis.
The transmission electron microscope is required to image the inner structure of the nanotube-polymer assembly.
High-resolution thermogravimetric analyser is suitable for the investigation of the composition of single and multi walled carbon nanotubes.
Vibrational spectroscopy, which comprises the techniques of infrared spectroscopy and Raman spectroscopy, is used to investigate the substances and materials, generally non-destructively, for the purpose of both qualitative and quantitative analysis.
Figure 1: STM image of a SWNT wrapped by P3HT and model of polymer wrapping
Mixtures of regioregular poly(3-hexyl-thiophene) rrP3HT and multiwall carbon nanotubes have been investigated by scanning tunnelling microscopy in ultrahigh vacuum. Carbon nanotubes covered by rrP3HT have been imaged and analysed, providing clear evidence that this polymer self-assembles on the nanotube surface following geometrical constraints and adapting its equilibrium chain-to-chain distance. Largely spaced covered nanotubes have been analysed to investigate the role played by nanotube chirality in the polymer wrapping, evidencing strong rrP3HT interactions along well-defined directions. In particular, we have demonstrated that the coiling angle is affected by the nanotube hexagonal cells alignments, by showing the polymer formations and the underlying nanotube (Fig.1). Our result highlights the fundamental role played by the CNT chirality in the interaction with rrP3HT. The structural arrangement is also expected to have consequences in the electrical behaviour of the local p-n heterojunction.
Figure 2: STS analysis: (a) IV curve of the tube section covered by P3HT; (b) normalized differential conductance curves.
Scanning Tunneling Spectroscopy was performed on a (15,0) single wall carbon nanotube partially wrapped by Poly(3-hexyl-thiophene). On the bare nanotube section, the local density of states is in good agreement with the theoretical model based on local density approximation and remarkably is not perturbed by the polymer wrapping. In Fig. 2 (left) STS analysis of the polymer self-assembly on a SWNT is shown: at point P, the analysis confirmed the metallic nature of the tube. The IV curve in Fig. 2a is performed on the tube section covered by P3HT (in dashed lines) and is compared with the curve of P3HT deposited on Highly Ordered Pyrolytic Graphite (inset). On the coiled section, a rectifying current-voltage characteristic has been observed along with the charge transfer from the polymer to the nanotube, as shown by the normalized differential conductance curves in Fig. 2b. The electron transfer from Poly(3-hexyl-thiophene) to metallic nanotube was theoretically predicted and contributes to the presence of a Schottky barrier at the interface, responsible for the rectifying behaviour.
Current-voltage characteristics of regioregular poly(3-hexylthiophene) diodes at room temperature have been analysed. Experimental curves were fitted to two models, to take into account at low-moderate electric fields Schottky behaviour mixed with space charge limited current (SCLC) regime and, at higher fields, trap-free SCLC. The results provide a description of IV curves over five decades, along with the determination of zero field and effective mobility and the field dependence prefactor. Forward and reverse IV measurements highlighted the presence of shallow and deep localized states inside the band gap. The latter enhance the current over time and have been modelled as an inductor-like element.
A high control on relative distances and diameter of MWNTs has been achieved by using a Focused Ion Beam (FIB) template method. Three-dimensional architecture of self-standing nanotubes can be created by pyrolysis of FePc on pre-patterned silicon substrates. Vertical-aligned CNTs, grown directly on any flat surface, could lead to new designs for organic solar cell electrodes.
FIB patterned sample before and after CNTs growth
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