Session W36: Focus Session: Materials for Photovoltaics and Photocatalysis III

Hole Mobility Studies on Thiophene-Based Conjugated Polymers Developed for Use in Organic Electronic Devices

In optimizing organic electronic devices, such as solar cells and field effect transistors, the mobility plays a crucial role affecting many aspects of performance, including: charge separation efficiencies, carrier densities, and drain currents. By fabricating hole-dominated devices and fitting the measured current-voltage characteristics to the field-dependent space-charge-limited mobility model we were able to measure hole mobilities in a set of conjugated polymers including p-Pt-BTD-Th, p-Pt-BTD-EDOT, and both regio-regular and regio-randem P3HT. These materials have been shown to exhibit promise as active layers in organic solar cells, light-emitting diodes, and field effect transistors. We present the results of these measurements and the effects induced by thermal annealing.   

Molecular and electronic structure at C$_{60}$:pentacene interfaces

Successful utilization of organic donor-acceptor systems for photovoltaic applications requires understanding factors controlling molecular and electronic structure at interfaces. We have used STM, STS, and photoemission to study the donor- acceptor system C$_{60}$:pentacene. At low coverage, C$_{60}$ deposited on a well-ordered pentacene bilayer structure on Ag (111) adsorbs in between two adjacent pentacene rows. Isolated C$_{60}$ molecules are easily observed at room temperature indicating that the mobility of C$_{60}$ on pentacene is significantly smaller than on metal surfaces. Some images of C$_{60}$ reveal structure that may indicate a preferred C$_{60} $ orientation. Electrostatic contributions to intermolecular interactions are discussed to help explain C$_{60}$ adsorption between pentacene molecules. With increasing coverage, C$_{60} $ forms linear chains, still locked to underlying pentacene rows. A further increase in coverage results in domains of disordered C$_{60}$ that we propose result from competing C$_ {60}$- C$_{60}$ and C$_{60}$-pentacene interactions. Information on nanoscale transport gaps and band alignment was obtained using constant-current distance-voltage spectroscopy. A gap of 4.5 eV is found over the linear C$_{60}$ chains compared with a gap of 3.6 eV for the surounding pentacene bilayer.   

Porous nanocrystalline TiO$_{2}$ thin films for dye-sensitized solar cells

We report a rapid and low cost method to fabricate porous TiO$_{2}$ thin films used as anode electrodes for solid state dye-sensitized solar cells. Polymethylmethacrylate (PMMA) gel was used as template to define a network co-structure with alkali titanium oxide, then spin cast on substrates. After thermally removing polymer, smooth and crack-free large area TiO$_{2}$ thin films with fine pores were generated. Thin film structures were detected by powder {\&} grazing incident X-ray diffraction. Film thickness can be controlled over a range of tens of nanometers to several microns by precursor viscosity, spin coating speed and coating times. The SEM image shows the highest quality porous TiO$_{2}$ film derived from a certain concentration of precursor. The above TiO$_{2}$ thin films were then used to fabricate solid state dye sensitized solar cells. Porphyrine dye and poly(ethylene glycol) electrolyte with I$^{-}$/I$_{3}^{-}$ redox couple were used in the cells. Current-voltage curves were recorded. The open circuit voltage boosts to more than 1.0 V. The reasons for the high open circuit voltage probably will be discussed. Overall photo-electricity conversion efficiency reaches 2.05{\%} under an illumination of one solar unit (AM1.5, 100 mW/cm$^{2})$.   

TiO2 nanowire sensitized by natural dyes for solar cell applications

We investigate the electronic coupling between a semiconductor TiO$_2$ nanowire and a natural dye sensitizer based on time-dependent first-principles calculations. The model dye molecule, cyanidin is found to dissociate into the quinonoidal form upon adsorption, rendering its highest occupied molecular orbitals (HOMO) located in the middle of TiO$_2$ bandgap and its lowest-unoccupied molecular orbital (LUMO) at the bottom of TiO$_2$ conduction band. The visible light absorption is greatly enhanced with two prominent peaks at 460 nm and 650 nm. The excited electrons are injected into the TiO$_2$ conduction within a ultrafast timescale of $<$50 fs, with negligible non-radiative energy dissipation and recombination.   

Electronic structure of N3 DYE molecules on the TiO$_{2}$(110) surface and on anatase nanoparticle films

We have used direct and inverse photoemission to measure the occupied and unoccupied electronic states, and their alignment with the band edges of the substrate, of N3 dye adsorbed on the rutile TiO$_{2}$(110) surface and on anatase TiO$_{2}$ nanoparticle thin films. In dye-sensitized solar cell applications, the HOMO-LUMO gap determines the useful portion of the solar spectrum, and charge transfer of photoexcited electrons to the substrate depends on the alignment of the LUMO to the TiO$_{2}$ conduction band edge. Samples were prepared and passivated with a pivalate layer in UHV, then sensitized in air in a solution of N3 dye in acetonitrile. STM measurements show that the pivalic acid forms an ordered overlayer on the TiO$_{2}$(110) surface and that the N3 dye molecules can be imaged after sensitization. Our spectroscopic measurements show that contamination (presumably from water in the ambient) is significantly reduced and that the N3 HOMO occurs at 1.1 eV above the TiO$_{2}$ valence band edge, and the LUMO is found 0.3 eV above the conduction band edge. Comparison with experimental and theoretical values from the literature will be discussed.   

Electronic structure of N3 DYE molecules on the ZnO single crystal and epitaxial film surfaces

Most dye-sensitized solar cells use TiO$_{2}$ nanoparticle films as the electrode, but ZnO offers an interesting alternative. We have used direct and inverse photoemission to measure the occupied and unoccupied electronic states, and their alignment with the band edges of the substrate, of N3 dye adsorbed on ZnO(0001), ZnO(11-2), epitaxial ZnO a-plane film surfaces, and ZnO nanopillars. As the unoccupied states of ZnO are of \textit{sp}-character and of relatively low cross section, the LUMO of the dye is easily observed. Samples were prepared and passivated with a pivalate layer in UHV, then sensitized in air in a solution of N3 dye in acetonitrile. As opposed to the case of the TiO$_{2}$(110) surface, STM measurements indicate that the pivalic acid does. From UPS, the N3 HOMO is found at $\sim $0.8 eV above the ZnO valence band edge, and the LUMO is found $\sim $1.5 eV above the conduction band edge for the epifilm. Differences in dye adsorption and orbital alignment for these different ZnO surfaces will be discussed.   

Towards visible light activity of wide band gap photocatalysts: Surface functionalization of ZnO with ZnS

We show that at the ZnO/ZnS interface the band alignment is favorable for reducing the photo excitation threshold energy; signifying that the combination of two wide band gap photocatalysts can yield a material with visible light activity. Modification of ZnO with a sub monolayer ZnS is investigated by scanning tunneling microscopy (STM) and photoemission spectroscopy. STM studies indicate that the ZnS grows by nucleation and spreading of 2D clusters of monolayer height ($\sim $ 2.5 {\AA}). Photoemission spectroscopy is used to measure the band alignment between ZnO and ZnS, as well as measure the changes in the surface charge region and work function. An increase in work function by 1.1 eV is observed and a staggered band alignment is found with ZnS states effectively narrowing the band gap for photo excitation from 3.4 to 2.7 eV. We propose that the combination of these structural and electronic properties of the modified ZnO surface result in an improved, visible light active photocatalyst.   

Energy bands and point defects in CuInSe$_{2}$ and CuGaSe$_{2}$ calculated by Quasiparticle Self-Consistent $GW$

CuIn$_{x}$Ga$_{1-x}$Se$_{2}$, or CIGS, is emerging as a leading candidate for second-generation solar cell applications. Here we present the bulk energy band properties and dielectric response of CuInSe$_{2}$ and CuGaSe$_{2}$, computed within the Quasiparticle Self-Consistent \emph{GW} (QS$GW$) approximation. QS$GW$ has been proven to be a very reliable, true \emph{ab initio} predictor of QP levels in a wide variety of materials systems; it is expected to be similarly reliable for chalcopyrite semiconductors. The fundamental gap agrees well with experiment. Also, the electron and hole effective masses are evaluated. Various kinds of point defects were considered using certain approximations to QS$GW$. Of particular interest are low-energy cation defects (antisites and vacancies). Rather unusual properties of these levels are found, owing to the unique role that shallow Cu $d$ states play in CIGS.   

Synthesis of type II core/shell nanowires for photovoltaic application

The core/shell semiconducting nanowires based on II-VI semiconductors, involving with type II band energy alignments, are predicted to be a new kind of nanostructured materials for efficient charge separation for stable and efficient photovoltaic devices. In this talk, we report a successful synthesis of II-VI semiconducting core/shell nanowires by a relatively simple and low-cost approach. The structures and optical properties were characterized by applying a set of comprehensive techniques. A sharp interface and the epitaxial relationship between the core and shell were observed. Two excitonic absorption peaks were clearly found at respective excitonic bandgaps, indicating a good crystallinity of both the core and shell. Compared to the single component nanowires, the PL spectrum of the core/shell nanowires shows a reduction in intensity and a slight blue shift at the band edge emission of the core nanowires, which may partially arise from spatially charge separation between the core and shell. The direct growth of core/shell nanowires represents a major step toward fabricating a low-cost, high efficiency and stable solar cell.   

Nanocoax Solar Cells

A novel architecture for high efficiency solar energy conversion, employing separated photo-- and --voltaic pathways and antenna-based light collection, is described. This \textit{material-independent} architecture solves the ``thick-and-thin'' conundrum of solar photovoltaics, wherein solar cells must be thick enough to absorb light yet thin enough to allow for charge extraction. Our solar cells are comprised of arrays of high aspect ratio, vertically-aligned, nanoscale, metallic coaxial wires (\textit{nanocoax}) which are indeed simultaneously thick (tall) and thin (narrow). Photons captured by nanoscale antennas are channeled axially as TEM-mode radiation in the nanocoax annulus, which is filled with a PV medium. This annulus is unprecedentedly thin radially ($\sim $100 nm), such that exciton lifetimes and subsequent electron and hole diffusion lengths of virtually any PV material are sufficiently long to enable highly efficient solar energy conversion. We discuss results with radial $p-i-n $junctions using $ a$-Si PV and carbon nanofiber coax center conductors, where nanocoax solar cell efficiencies exceed those of comparable planar junctions. Moreover, this nanoscale architecture can be considered a feasible portal to 3$^{rd}$ generation solar power.   

Low temperature pulsed electrically detected magnetic resonance on a-Si:H p-i-n solar cells

Hydrogenated amorphous silicon (a-Si:H) has become one of the most important semiconductor materials, with applications including solar cells and thin film transistors. In spite of this, and more than 30 years of intensive studies of this material, the microscopic nature of various recombination mechanisms in this material are still not well understood. Recently, pulsed electrically and optically detected magnetic resonance (p-EDMR, p-ODMR, respectively) spectroscopy has provided a method for directly and quantitatively observing some of these microscopic processes. Here, we present p-EDMR measurements on a-Si:H p-i-n solar cells at temperatures T $\le $ 40K, with a comparatively low light excitation density. After a short, coherent microwave excitation, we record transients for a range of externally applied magnetic fields. The results show the presence of a number of resonances, which we discuss with regard to previous continuous wave (cw-) ESR and cw-EDMR studies, as well as cw- and p-ODMR measurements.   

Carrier Multiplication in Semiconductor Nanocrystals: Theoretical Screening of Candidate Materials

The process of Direct Carrier Multiplication (DCM) involves the creation of TWO electron-hole pairs as a results of exciting a nanostructure by ONE photon with energy two times larger than the band gap $E_g$. The ratio $R(E)=\rho_{XX}(E)/\rho_X(E)$ between the biexciton ($XX$) and monoexciton ($X$) density of states [Nano Lett. {\bf6}, 2191 (2006)] is a ``figure of merit'' of the DCM process. Using the ``Truncated Crystal'' approximation to the electronic structure of nanocrystals based on the atomistic, semi-empirical pseudopotential approach we calculated $R(E)$ for nanocrystals of GaSb, InAs, InP, GaSb, InSb, Ge, Si, and PbSe. We found that InSb, GaSb, Ge, and PbSe quantum dots have larger DCM figure of merit than the other quantum dots. Our calculations suggest that there are three requirements for high DCM efficiency: (1) Small nanocrystal band gap ($<$ 1.6 eV) to match the solar spectrum, (2) high degeneracy of the band-edge states, and (3) heavy electron and hole effective mass. We conclude that nanocrystal made of HgS, HgTe, HgSe, PbS, PbTe, and Sn are also good candidates for DCM-based solar cells.   

Exploring the Use of Self-Assembled InGaAs/GaAsP Quantum Dots as Intermediate Band Solar Cells (IBSC)

It has been recently proposed that the efficiency of photovoltaic solar cells based on wide-gap III-V absorbing materials can be enhanced if quantum dots are embedded in such matrices, leading to confined electron and hole states that can be excited to the band edges of the wider-gap matrix material, thereby capturing the lower energy (IR) solar photons. Such proposals, however, were not scrutinized so far by modern quantum-dot calculations. We apply our pseudopotential Linear-Combination-of Bloch-Bands (LCBB) approach to this problem. Lens-shaped dots of InGaAs were vertically stacked with varying dot-dot separation. The effects of spin-orbit, multi-band, and multi-valley coupling are included by a direct diagonalization of the atomistic problem. A matrix of GaAsP was chosen so as to strain-balance the system epitaxially on a GaAs(001) substrate. We will discuss the energies of the band edges of the matrix material, and those of the confined dot levels relative to the expected values for ideal IBSC operation, as well as their variation with respect to either the vertical dot-dot separation, or the band gap of the matrix material.