Functionalized Nanoporous Gold Leaf Electrode Films for the Immobilization of Photosystem I

Ciesielski, P. et al. ACS Nano, Article ASAP (2008)

Purpose of the Study
The purpose of this study was to develop a technique for fabricating nanoporous gold leaf electrode films, and functionalize the surface of the electrode with PSI for solar energy conversion.

Methods
Nanoporous gold leaf was fabricated from an initial gold/silver alloy film. Immersion in concentrated nitric acid dissolved the silver, dealloying the film. The porous film was then placed on a gold/silicon support substrate. Characterization was performed using cyclic voltammetry and SEM. Self-assembled monolayers were formed by exposing the film to a variety of ω-terminated alkyl thiols. Presence of SAMs was determined using electrochemical impedance spectroscopy (EIS). Photosystem I was directly attached to the NPGL by functionalizing the electrode surface with terephthaldialdahyde (TPDA). Measurements were made of photocurrent enhancement with PSI adsorption.

Key Findings

• Maximum surface area enhancement occurs for short dealloying times, but maximum photocurrent enhancement occurs for longer dealloying times. This is due to the small pore size associated with short dealloying times.
• Immobilization of PSI on NPGL electrodes provides an increase in PSI-catalyzed photocurrent, compared to planar electrodes.

Definitions

• Cyclic voltammetry – A potentiodynamic electrochemical measurement which sweeps the working electrode potential linearly, and reverses direction when reaching a set potential. This can cycle several times during an experiment, and gives information about the oxidation and reduction of mediator species in solution.
• Electrochemical impedance spectroscopy – Probes energy storage in a device by measuring the impedance of a system over a range of AC frequencies.

Nanostructuring Titania by Embossing with Polymer Molds Made from Anodic Alumina Templates

Goh, C. et al. Nano Letters, 5 (2005)

Purpose of the Study
The purpose of this study was to demonstrate a method of fabricating porous films of titanium dioxide using a polymer mold. The resulting TiO2 films have a variety of potential applications.

Methods
Embossing was carried out using the following procedures:

1. Aluminum on glass was anodized in an electrochemical cell to form a porous anodic alumina template. Average pore size was 45 nm.
2. The AAO template was filled with PMMA by first spin-casting from solution and then heating to infiltrate the polymer into the pores. This serves as the mold.
3. A 1 mm layer of PDMS was coated onto the back of the PMMA, to provide stability and ease of manipulation to the mold.
4. An initial wet etch in a FeCl/HCl solution removes the backing aluminum. A second wet etch in NaOH removes the alumina layer.
5. A TiO2 thin film is fabricated by spin-casting from an initial sol-gel solution. Immediately after spin-casting the mold is pressed onto the film, embossing the titania film. Once the film has dried, the mold is removed and excess PMMA is dissolved in acetonitrile.

Key Findings

• Pore depth of titania film is less than initial alumina film. This is attributed to air trapped in the alumina pore bottoms, preventing complete penetration of the PMMA mold.
• Average pore depth: ~120 nm. Average pore diameter: 50 nm. Defect free replication on the order of 40 mm².
• XPS measurements show no contamination from the precursor materials in the final TiO2 films.

Carrier multiplication in a PbSe nanocrystal and P3HT/PCBM tandem cell

Kim, SJ et al. APPLIED PHYSICS LETTERS 92.19 (2008)

Purpose of the Study
The purpose of this study was to demonstrate the fabrication of a photoactive device exploiting the phenomena of efficient multiple exciton generation in semiconductor nanocrystals.

Methods
A simple tandem solar cell was constructed: ITO/PbSe/PEDOT/P3HT/Al. Polymer and nanocrystal layers were deposited by spin-casting. Illumination was through the ITO coated glass slide. I-V curves were taken under AM1.5 illumination @ 100mW/cm². To measure increased photocurrent due to CM, a light biased spectral response measurement was taken. In this measurement, a 5 mW He-Ne laser excites the P3HT layer, providing an internal bias to help extract photocurrent from the PbSe layer (where CM occurs). An external quantum efficiency measurement was then made using a white light source+monochromater. To isolate photocurrent due to the PbSe layer, a control sample without the nanocrystal film was made and then used to estimate the QE due to the polymer layer. The isolated photocurrent was used to determine carrier multiplication yields of the film.

Key Findings

• Device efficiency of 1.3% under AM1.5.
• Observed increased photocurrent upon selective light biasing, with a threshold at E=3E_g, corresponding to carrier multiplication in the PbSe film.
• Biasing due to polymer layer may be a useful approach to extracting short-lived carrier due to MEG.

Definitions

• Carrier Multiplication: Generation of multiple electron/hole pairs from absorption of a single photon. It’s kind of an inverse Auger process. Also known as Multiple Exciton Generation (MEG)
• Selective Light Bias Activation: Production of a built-in potential by optical excitation of photoactive material

Ripening Kinetics of CdSe/ZnSe Core/Shell Nanocrystals

Yun-Mo Sung, Kyung-Soo Park, and Yong-Ji Lee

J. Phys. Chem. C, 111 (3), 1239 -1242, 2007

Article Link

Purpose of the Study

The purpose of the study was to determine the Ostwald ripening behavior of CdSe and CdSe/ZnSe nanocrystals as a function of ripening temperature and time period. A ripening mechanism is proposed for core/shell nanocrystals, and important kinetic variables are obtained.

Methods

CdSe and CdSe/ZnSe core/shell nanocrystals were synthesized by a controlled nucleation via pyrolysis of organometallic precursors, following standard literature techniques. The synthesized nanocrystals were stabilized in toluene. Ostwald ripening was performed by heating nanocrystal solutions to temperature between 240-290 °C for time lengths between 5 and 480 min. Structural characterization was obtained using X-ray diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM). Ripening kinetics were obtained using a Lifshitz-Slyozov-Wagner (LSW) kinetic analysis. Size variation was determined by correlating with shifts in the band-edge absorption feature.

Key Findings

• Synthesized nanocrystals have a wurtzite structure with high crystallinity and a size-dependent bandgap.
• Ripening experiments show higher ripening kinetics in CdSe nanocrystals, compared with CdSe/ZnSe core/shells
• Observed change in ripening activation energy at 266 °C; attributed to transition from surface diffusion to lattice diffusion mechanisms: Dissociation of surface Zn-Se dominates in low-temperature region; both Zn-Se and Cd-Se dissociation occurs in high-temperature region.
• Low activation energy for ripening attributed to importance of surface diffusion in the ripening reaction.

Important Definitions

• Activation energy: energy that must be overcome in order for a chemical reaction to occur.
• Arrhenius equation: gives the dependence of a reaction rate constant on temperature and activation energy.