Various temperature dependent optoelectronic properties were measured for GaAs-based p-type / intrinsic / n-type (pin) solar cell devices containing 5-layers of InAs quantum dots (QDs) grown with strain-compensation layers. Curve fitting of the dark diode characteristics allowed for the temperature dependence of the saturation current and the ideality parameter to be determined. The resulting parameter values indicate high material quality. Air mass zero illuminated current density vs. voltage measurements were used to determine the temperature coefficients of the open circuit voltage, short circuit current, maximum power, and fill factor. A strong correlation between the temperature dependent quantum dot electroluminescence peak emission wavelength and the sub-GaAs bandgap spectral response was observed.

In polymer:fullerene solar cells, the field-dependent photocurrent is due to a combination of polaron pair dissociation, competing with monomolecular recombination, and bimolecular recombination. We compare of the experimental photocurrent of P3HT:PCBM bulk heterojunction solar cells to Monte Carlo simulations of bilayer and bulk heterojunction devices. The shape of the photocurrent can be reproduced, the high quantum yield, however, cannot be explained with help of the simulations. In order to analyse the dominant experimental recombination mechanism, we apply the photo-CELIV technique. Our data can be fitted with bimolecular recombination, but only if a reduced Langevin recombination factor is assumed. Thermalisation is accounted for by measuring the time-dependent charge carrier mobility.

We describe the present state of the intermediate band (IB) solar cell research, a cell concept with very high efficiency potential. A comprehensive presentation of the theory is included followed of a description of its implementation using quantum dots and of the experiments performed to prove their principles. Present solar cells do not give today very high efficiencies; the steps to be given towards the real achievement of higher efficiencies is described and the use of alloys, instead of nanostructured materials, to fabricate IB cells is also discussed.

The authors report the study of the dependence of the device performance of polymer solar cells based on single 50-nm heterojunction poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methylester (P3HT/PCBM) layer on annealing process. Annealing before and after cathode deposition were performed for comparison. In the case of post-annealing at 150¢XC for 60 min., the device attains a conversion efficiency of 4.9%, a fill factor of 53 %, and an open-circuit voltage of 0.67 V. These values are comparable with the highest values reported previously. The annealing process is expected to modify the network morphology of the P3HT/PCBM layer. This study demonstrates that it is possible to attain good solar cell performance with the combination of single thin active layer and post-annealing treatment. This may open up an opportunity to fabricate tandem polymer solar cells.

The effects of strain within stacked layers of InAs quantum dots (QDs) were investigated. InAs QD test structures with and without strain compensation (SC) were analyzed using atomic force microscopy, transmission electron microscopy, and X-ray diffraction. The affects of strain compensation on test structure morphology and on GaAs-based QD solar cell performance was studied as a function of the thickness of the SC layer. X-ray diffraction analysis of the QD embedded test structures reveals a relationship between the SC thickness and the observed crystalline quality. Air mass zero illuminated current vs. voltage data and spectral responsivity measurements were used for the solar cell comparison. When SC is employed, QD insertion shows a lower open circuit voltage, in reference to a baseline device without QDs, but leads to an enhancement in the short circuit current of the device.

Several InAsP/InP p-i-n Multi-Quantum Well (MQW) solar cells, only differing by their MQW region composition and geometry, were investigated. For each sample, the Arrhenius plot of the temperature related variation of the photoluminescence intensity was used to deduce the radiative recombination activation energy. The electron and holes confinement energy levels in the quantum wells and the associated effective potential barriers seen by each carrier were theoretically calculated. Carrier escape times were also estimated for each carrier. The fastest escaping carrier is found to display an effective potential energy barrier equal to the experimentally determined photoluminescence activation energy. This not only shows that the temperature related radiative recombination extinction process is driven by the carrier escape mechanism but also that the carriers escape process is sequential. Moreover, a discrepancy in device performance is directly correlated to the nature of the fastest escaping carrier.

Silicon nanoparticles (Si-NPs) < 6 nm have been doped with B and P in nonthermal plasma. The doping efficiency of B is smaller than that of P, consistent with the theoretically predicted larger formation energy of B than P. The effect of doping on the oxidation-induced changes in light emission from Si-NPs is different between B and P. It appears that P is at or near the surface of Si-NPs, and that B is well incorporated inside Si-NPs. Inks based on doped Si-NPs are produced by attaching alkyl ligands to the surface of Si-NPs and dispersing them in organic solvents.

A series of “rigid-rod” dyes with an organic chromophore (pyrene or coumarin) attached through an oligophenylenethynylene (OPE) rigid bridge, linear or branched, to an anchoring isopthalic acid unit (Ipa) were synthesized and studied for solar cells (DSSCs) applications. The new dyes were attached to metal oxide (MOn = TiO2, ZrO2 and ZnO) nanoparticles films via the two COOH binding groups on the Ipa unit to investigate their binding and photophysical properties at the semiconductor surface. FTIR-ATR spectra show that all dyes did bind to the metal oxide films through carboxylate bonds. Fluorescence emission on insulating ZrO2 films was employed to study aggregation of the organic rigid-rods. Studies of the pyrene rigid-rods in solar cells showed near quantitative conversation of absorbed photons into electricity.

Improvement of photovoltaic performance for dye sensitized solar cells (DSC) is discussed in terms of electron-path and ion-path. In order to make electron path, we focused on passivation of TiO2 surface states which are observed by thermally stimulated current (TSC). The TiO2 surface was well-passivated with dye molecules under pressurized CO2 atmosphere. It was found that DSC cells prepared by a CO2 process (Cell-CO2) had higher efficiency than those prepared by a conventional dipping process (Cell-DIP) and the higher efficiency was associated with low TiO2-surface state, high electron diffusion coefficient and long electron life time in TiO2 for the Cell-CO2. In addition, dye-staining under pressurized CO2 atmosphere had advantages over a conventional dipping process on rapid dye-uptake and less dye aggregation. In order to fabricate ion-path in solidified electrolyte, we focused on surface modification of nano-materials. Surface of nano-materials such as TiO2-nanoparticles and porous alumina films were modified with imidazolium iodide moieties consisting of long alkyl chains which render surface-molecules self-organized. Redox-species are concentrated on the self-organized molecules and make ion-path. We propose quasi-solid electrolyte system consisting of two layers having different charge carrier concentration to keep high photoconversion efficiencies even after solidification.

Nowadays nanostructures play a vital part in the rapidly expanding areas of photovoltaics. The ability of nanowires to transfer photo-generated carriers rapidly across a solar cell has lead our interest in growth of nanowires . Currently Vapor Liquid Solid (VLS) epitaxy is the most common method used to grow epitaxial vertical nanowires. A metal particle such as gold is used to form a liquid alloy eutectic with the material of a substrate or with material supplied in the vapor phase. In growing semiconductor wires using metal droplets, it has been shown that the wires grow in the (111) direction and have clean facets . Furthermore these wires generally present an undesirable larger pyramidal base at the bottom and there is also evidence of surface migration of the metal catalyst. Thus far, most of the effort in the development of vertical III-V semiconductor nanowires has been limited to homo-polar combinations (e.g. InAs on InP). The ability to fabricate III-V nanowires on silicon could however pave the way toward the monolithic integration of III-V nanostructured solar cells with Si. Here we demonstrate the growth of GaAs and InP nanowires on silicon (111) using gold as the metal seed particle. An ordered array of gold nano dots is integrated on the surface of a silicon substrate using self-assembled polystyrene nanospheres as the Au evaporation template. The size of the gold dots range from 40 nm to 150 nm and the pitch is about 500 nm. The growth of the wires is done by chemical beam epitaxy under a vapour phase environment. Scanning electron microscopy and photoluminescence are used to characterize these nanowires. Wire exhibit high crystallinity and there is an absence of the pyramidal base at the bottom of the nanowire using this technique. Furthermore the study also shows evidence of pregrowth motion of some of the gold particles causing coalescence of nanowires and leading to the development of nanopods and tilted (off-normal) nanowires. Finally in the light of their optical properties the relevance of these wires to photovoltaic applications is discussed.

Nanostructured semiconductors have received a great deal of attention due to their unusual physical properties. The possibility of controlling these properties by varying the particle size, shape and surface properties is of great interest for nanoscale device applications in microelectronics, non-linear optics and optoelectronics in particular. Copper Indium diselenide (CIS), a p-type semiconductor with a band gap of about 1.04 eV has shown promise as an absorber for photovoltaic cells. Polycrystalline CIS film has been fabricated by various physical and chemical techniques over the past couple of decades. However, the preparation of nanostructured CIS is still challenging due to the fact that CIS tends to agglomerate resulting in large grains. Furthermore, an annealing step is also required to improve the film properties, which results in increased grain size. Here we report a simple, low cost solution to this problem through electrochemical deposition of CIS into the confined nanopores of an anodic aluminum oxide (AAO) template. The nanoporous aluminum oxide template, a few microns thick was prepared by either a one step or a two anodization in oxalic acidic media. CIS was deposited into the nanoporous AAO template by pulsed cathodic electro-deposition from an aqueous mixture. The electrodeposited nanowires had diameter ranging up to 40 nm and length ranging from 600 nm to 5 μm depending on the length of the template. The hybrid nanostructured CIS/AAO was annealed in vacuum at 230ºC for several hours to achieve stoichiometric CuInSe2 phase. The embedded CIS nanowires were characterized by X-ray diffraction and scanning electron microscopy. The optical bandgap was deduced from UV-Vis absorption spectroscopy measurements.

A wide variety of nanomaterials and associated nanomaterial/polymer composites are being developed in an effort to produce higher efficiency organic solar cells. This development requires a fundamental understanding of the energy levels for the individual materials, and their composites, to enable device designs which posess appropriate energy level matching. Cyclic voltammetry (CV) allows for the determination of the band gaps (Eg) and energy levels of these various nanomaterials and composites by measuring their oxidation and reduction potentials. These potentials correspond to a given material's ionization potential (IP) and electron affinity (EA), respectively. The results for the EA, IP, and Eg have been determined by CV for derivatized fullerenes and CdSe quantum dots (QD), measured in isolation, and in conjugated polymer composites with MEH-PPV. In addition, CV measurements conducted under dark and illuminated conditions were used to investigate the relationship between energy levels within the composites.

Two tripod-shaped adamantane derivatives carrying a pyrene chromophore and three carboxylic acid binding groups, and varying in footprint size (∼ 0.7 and 2.7 nm2), were synthesized as models to study how the footprint size can influence the aggregation of organic dyes bound to ZrO2 thin films. Synthetic approaches and binding properties of large footprint tripodal linkers are discussed.

The first step in synthesizing a model film morphology via a surface-driven hierarchical assembly process is presented. The goal of the hierarchical assembly is the control of the morphology of complex molecular layers for the investigation of fundamental processes in organic solar cells. Using a focused ion beam (FIB) with Ga+ ions at 30 keV, the surface of highly oriented pyrolitic graphite (HOPG) is patterned with an array of local amorphous carbon ellipsoid spots (ACES), which provide preferential nucleation lines at their perimeter, and thus are instrumental in the control of fullerene island growth. On the undamaged surface regions outside the ACES pattern the fullerene island growth is unperturbed, and presents the well-known combination of round and fractal island shapes. The fullerene deposition at the periphery of the ACES pattern, which is characterized by single ion impact defects, results in stunted, smaller and irregular islands. Inside the ACES array, the C60 island growth is controlled by the shape of the ACES and is constrained to lobes which form around each ACES spot. The array and C60 lobe morphology and geometry are characterized and a subsequent understanding of the C60 diffusion fields and nucleation lines within the array is discussed.

Photovoltaic devices based on organic semiconductors are of interest because of their potential as flexible, lightweight and inexpensive devices. One of the promising devices, involves the heterojunction between copper phthalocyanine (CuPc) and 3,4,9,10-perylenetetracarboxylic bis-benz-imidazole (PTCBI). Earlier, we reported, the highest Voc (1.125V) in a single organic heterojunction solar cell in an ITO/PEDOT:PSS/CuPc/PTCBI/Al structure. Results were interpreted in terms of a modified CuPc-Al Schottky diode for this thin PTCBI layer case and a CuPc-PTCBI heterojunction for the thick PTCBI case. We also reported the device characteristics of Copper phthalocyanine (CuPc)/Aluminum (Al) Schottky diode solar cells. Here, open circuit voltages (Voc) increased from 220 mV at 15 nm to 907 mV at 140 nm. Analysis of the current-voltage characteristics indicated that tunneling and interface recombination mechanisms are important components of the current transport at the CuPc/Al junction. With the objective of higher short circuit current densities in mind, we have extended our earlier work on organic semiconductors to nanowire based designs. In this paper, we report the fabrication, materials and electrical characterization of CuPc nanowire based solar cells in AAO templates by electro-deposition. The CuPc nanowires were electrochemically deposited into the AAO templates using CHCl3 (Chloroform) containing 10-4 M CuPc with 0.5 ml of CF3COOH (Trifluoro acetic acid). The nanowires were characterized by XRD, UV-Vis absorption spectroscopy, electron microscopy and electrical measurements. CuPc nanowires were also electrodeposited onto ITO/glass substrates and compared with templated nanowires for their structural, optical and electrical properties. Effect of the PEDOT: PSS buffer layer on the nanowire based device characteristics was also investigated.

The use of InGaAsN and GaBiAsN quantum structures in the intrinsic region of conventional III-V p-i-n solar cells, lattice matched to GaAs, presents several advantages for photovoltaic (PV) application. First they allow for very shallow to zero valence band offsets thus permitting the free movement of holes. Second, a wide range of band-gap values are made possible due to the large band gap decrease upon the introduction of minute amounts of N and Bi. Using band structure calculations that include the strain effects, conduction and valence band anti-crossing models describing the large band gap bowing and the transfer matrix method, we present the theoretical investigation of optimum design conditions for enhanced vertical transport. The direct quantum mechanical resonant tunneling of electrons out of the quantum structures and into the continuum of the conduction band of the host semiconductor material can be facilitated provided that an adequate choice of material parameters is made. The high electron transmission probability together with the free movement of quasi-3 D holes is predicted to result in enhanced PV device performance. Furthermore, the increase in electron effective mass due to the incorporation of N translates in enhanced absorptive properties, ideal for PV application.

In the present work, polymer solar cells were fabricated from composite blends of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and poly(3-hexylthiophene)-(P3HT with PCBM[60] and PCBM[70]. The composite blends were used as active layers in an ITO/PEDOT:PSS/active layer/Al device structure. Power conversion efficiencies have been measured from current-voltage (I-V) measurements for each of these different composite blends under simulated AM1.5 illumination. In the case of the MEH-PPV devices, the I-V performance has been measured as a function of polymer molecular weight, type of fullerene derivative (C60 or C70), and PCBM:polymer ratios. The highest efficiencies for the ranges used in this study were obtained using the 150,000 g/mol MEH-PPV molecular weight, the C70 PCBM derivative, and a 1:4 MEH-PPV:PCBM ratio. The effect of thermal cycling on the I-V performance for both MEH-PPV and P3HT devices has also been measured from 77K to 330K. The devices exhibited a positive temperature coefficient for the short-circuit current density (Jsc), which dominated the overall efficiency of the device over this temperature range. Finally, the use of a combination of parylene and polymethylmethacralate for device passivation was shown to provide a dramatic reduction in device degradation under ambient conditions as compared to non-passivated devices.

CdSe nanocrystals chemically linked to nanocrystalline titanium dioxide substrates form a promising material for nanostructured photovoltaic devices. The usual method for attaching the nanocrystals to the titanium dioxide substrate is by means of a linking molecule (such as mercaptopropionic acid) or in-situ growth. In this paper, we report the use of an alternative technique, electrophoretic deposition (EPD), to directly deposit already formed CdSe nanocrystals onto the substrate. In EPD, a voltage is established between two electrodes that are immersed in a solution of nanocrystals. At room temperature, a fraction of the nanocrystals are thermally charged, and these charged nanocrystals migrate to the electrodes and adhere to the surface. A significant advantage of EPD over the use of linking molecules is the speed with which the nanocrystals are deposited: EPD takes only a few minutes, compared to the several hours required for the alternative techniques. Additionally, we have fabricated initial photovoltaic devices based on electrophoretically deposited CdSe nanocrystals on a planar TiO2 thin film.