Christopher Kyle Renshaw

Broad Spectral Response Using Carbon Nanotube/Organic Semiconductor/C60 Photodetectors

We demonstrate that photogenerated excitons in semiconducting carbon nanotubes (CNTs) can be efficiently dissociated by forming a planar heterojunction between CNTs wrapped in semiconducting polymers and the electron acceptor, C(60). Illumination of the CNTs at their near-infrared optical band gap results in the generation of a short-circuit photocurrent with peak external and internal quantum efficiencies of 2.3% and 44%, respectively. Using soft CNT-hybrid materials systems combining semiconducting small molecules and polymers, we have fabricated broad-band photodetectors with a specific detectivity >10(10) cm Hz(1/2) W(1-) from lambda = 400 to 1450 nm and a response time of tau = 7.2 +/- 0.2 ns.

Small-Molecule Photovoltaics Based on Functionalized Squaraine Donor Blends

Two squaraine (SQ) donor molecules with different absorption bands are blended together for better coverage of the solar spectrum. The blend SQ device shows a significant improvement compared with single SQ donor devices. By applying a solvent annealing process and a compound buffer layer, a power-conversion efficiency of 5.9 ± 0.3% is achieved under 1 sun illumination.

Independent Control of Bulk and Interfacial Morphologies of Small Molecular Weight Organic Heterojunction Solar Cells

We demonstrate that solvent vapor annealing of small molecular weight organic heterojunctions can be used to independently control the interface and bulk thin-film morphologies, thereby modifying charge transport and exciton dissociation in these structures. As an example, we anneal diphenyl-functionalized squaraine (DPSQ)/C(60) heterojunctions before or after the deposition of C(60). Solvent vapor annealing of DPSQ before C(60) deposition results in molecular order at the heterointerface. Organic photovoltaics based on this process have reduced open circuit voltages and power conversion efficiencies relative to as-cast devices. In contrast, annealing following C(60) deposition locks in interface disorder found in unannealed junctions while improving order in the thin-film bulk. This results in an increase in short circuit current by >30% while maintaining the open circuit voltage of the as-cast heterojunction device. These results are analyzed in terms of recombination dynamics at excitonic heterojunctions and demonstrate that the optimal organic photovoltaic morphology is characterized by interfacial disorder to minimize polaron-pair recombination, while improved crystallinity in the bulk increases exciton and charge transport efficiency in the active region.

Multiple growths of epitaxial lift-off solar cells from a single InP substrate

We demonstrate multiple growths of flexible, thin-film indium tin oxide-InP Schottky-barrier solar cells on a single InP wafer via epitaxial lift-off (ELO). Layers that protect the InP parent wafer surface during the ELO process are subsequently removed by selective wet-chemical etching, with the active solar cell layers transferred to a thin, flexible plastic host substrate by cold welding at room temperature. The first- and second-growth solar cells exhibit no performance degradation under simulated Atmospheric Mass 1.5 Global (AM 1.5G) illumination, and have a power conversion efficiency of ηp=14.4±0.4% and ηp=14.8±0.2%, respectively. The current-voltage characteristics for the solar cells and atomic force microscope images of the substrate indicate that the parent wafer is undamaged, and is suitable for reuse after ELO and the protection-layer removal processes. X-ray photoelectron spectroscopy, reflection high-energy electron diffraction observation, and three-dimensional surface profiling show a sur...

Improved power conversion efficiency of InP solar cells using organic window layers

We employ the organic semiconductor 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) as a nanometer thick window layer for p-InP/indium tin oxide (ITO) Schottky barrier diode solar cells. The power conversion efficiency is enhanced compared to ITO/InP cells lacking the PTCDA window layer, primarily due to neutralizing InP surface state charges via hole injection from the PTCDA. This leads to an increased ITO/p-InP Schottky barrier height, and hence to an increased open circuit voltage. The power conversion efficiency of the cells increases from 13.2±0.5% for the ITO/InP cell to 15.4±0.4% for the ITO/4 nm PTCDA/p-InP cell under 1 sun, AM1.5G simulated solar illumination. The PTCDA window layer is also shown to contribute to the photocurrent by light absorption followed by exciton dissociation at the organic/inorganic semiconductor interface.

A 10×10 all-organic passive pixel sensor array

We demonstrate a 10 × 10 monolithically integrated organic passive pixel sensor array, and discuss methods to fabricate large scale organic focal plane arrays on hemispherical surfaces to realize a new generation of simple, low noise and high bandwidth imaging systems.