Iskandar Yahya

Hybrid Carbon Nanotube Networks as Efficient Hole Extraction Layers for Organic Photovoltaics

Transparent, highly percolated networks of regioregular poly(3-hexylthiophene) (rr-P3HT)-wrapped semiconducting single-walled carbon nanotubes (s-SWNTs) are deposited, and the charge transfer processes of these nanohybrids are studied using spectroscopic and electrical measurements. The data disclose hole doping of s-SWNTs by the polymer, challenging the prevalent electron-doping hypothesis. Through controlled fabrication, high- to low-density nanohybrid networks are achieved, with low-density hybrid carbon nanotube networks tested as hole transport layers (HTLs) for bulk heterojunction (BHJ) organic photovoltaics (OPV). OPVs incorporating these rr-P3HT/s-SWNT networks as the HTL demonstrate the best large area (70 mm(2)) carbon nanotube incorporated organic solar cells to date with a power conversion efficiency of 7.6%. This signifies the strong capability of nanohybrids as an efficient hole extraction layer, and we believe that dense nanohybrid networks have the potential to replace expensive and material scarce inorganic transparent electrodes in large area electronics toward the realization of low-cost flexible electronics.

Temperature dependent separation of metallic and semiconducting carbon nanotubes using gel agarose chromatography

Post-synthesis separation of metallic (m-SWNTs) and semiconducting (s-SWNTs) single-wall carbon nanotubes (SWNTs) remains a challenging process. Gel agarose chromatography is emerging as an efficient and large scale separation technique. However, the full (100%) separation has not been achieved yet, mainly due to the lack of understanding of the underlying mechanism. Here, we study the temperature effect on the SWNTs separation via gel agarose chromatography, for four different SWNT sources. Exploiting a gel agarose micro-beads filtration technique we achieve up to 70% m-SWNTs and over 90% s-SWNTs, independent of the source material. The process is temperature dependent, with yields up to 95% for s-SWNT (HiPco) at 6 °C. Temperature affects the sodium dodecyl sulfate surfactant-micelle distribution along the SWNT sidewalls, thus determining the effectiveness of the SWNTs sorting by electronic type. The sorted SWNTs are then used to fabricate transistors with very low OFF-currents (∼10−13 A), high ON/OFF current ratio (>106) and charge carriers mobility ∼ 40 cm2 V−1 s−1.

Carbon nanotube field effect transistor measurements in vacuum

Three terminal measurements on a carbon nanotube field effect transistor (CNTFET) were carried out in high vacuum and the ambient, and its performance compared. The on-off current ratio, ION/IOFF, were 102 and 105 for devices operated in high vacuum and in ambient air, respectively. Here, we show that the conversion of p-type to ambipolar behavior may largely be attributed to the O2 in ambient doping the single walled carbon nanotubes (SWCNTs) in the active channel which consists of bundles of SWCNTs. Switching behaviour of these devices, with respect to constituent types of SWCNTs in the bundles will be discussed.

An investigation on copper doping to CdTe absorber layers in CdTe thin film solar cells

Polycrystalline Cadmium Telluride (CdTe) is the most stable compound that melts congruently. This leads to the feasibility of multiple methods of deposition, which consistently produce stoichiometric thin film CdTe. On the other hand, this has become a disadvantage of CdTe due to the tendency of self-compensation and limiting the doping capability hence limiting the improvement of solar cells efficiency and open-circuit voltage. AMPS 1D simulation of CdTe conventional device structure indicates significant open-circuit voltage improvement with effective acceptor concentration >1015 cm−3. Copper has been utilized commonly as p-type dopant in the R&D of CdTe thin film solar cells. In this work, different copper concentration were examined and introduced to CdTe thin film absorbers. Controlled amount of Cu were applied in solution form prior to the back contact formation process. The solar cells efficiency and open-circuit voltage indicate that doping of Cu in CdTe has an optimum range that limits the effective acceptor concentration. Exceeding the optimum concentration has adverse impact to efficiency and open-circuit voltage due to self-compensation. Short-circuit current density and fill factor saturates at high Cu concentration. This suggests that the method of Cu doping application through CdTe thin film surface has a saturation level.