We review scanning tunneling luminescence (STL) – a photon emission spectroscopy based on scanning tunneling microscopy (STM) – and report on its application to photovoltaics. As part of this exploratory research, we have investigated CuInSe2 thin films, solar cells based on quantum dots, and dilute nitride compounds. STL is very attractive because it is capable of nanometer resolution, which is being demanded by the spectacular advancement of nanoscience and nanotechnology. In addition, STM offers both unipolar and bipolar excitation of the luminescence and, consequently, the transport and recombination of electrons and holes can be investigated independently.

Spectral response and dark current-voltage characteristics of heterojunctions are used to investigate grain boundary degradation in photovoltaic properties of a-Si/mc-Si heterojunction solar cells. Measured spectral response inside the grain and on the grain boundary shows small but consistent QE degradation due to minority carrier recombination at the grain boundaries. No consistent difference is observed in dark current-voltage characteristics because of large diode area and periphery leakage current in the employed heterojunction diodes. Comparing measurement results and results from device modeling using the simulation software Medici, a recombination velocity of 4900 cm/sec is found at the grain boundaries of employed multicrystalline silicon wafer. The modeling and experimental results can also be used to define an effective grain area that serves as a measure of grain boundary recombination and the influence of grain size.

The interplay between phase separation in polymer blends consisting of the electron donating poly(3-hexylthiophene-2, 5-diyl) (P3HT) or poly[2-methoxy-5-(2'-ethylhexyloxy)-, 4-phenylenevinylene] (MEH-PPV), with either polyquinoline or a fluorene-oxadiazole copolymer as the electron accepting polymers is presented herein. The bulk heterojunction photovoltaic performance has been examined for these blends along with a new copoly(aryl ether) containing substituted anthracene and fluorene segments coupled with aromatic 1, 3, 4-oxadiazole moieties. Photoluminescence quenching in the MEH-PPV blends as well as a 100-fold photocurrent enhancement of the bulk heterojunction device P3HT/SDPQ photovoltaic device were observed compared to the single layer P3HT device. Finally, the structure and morphology of these films was investigated using atomic force microscopy and scanning electron microscopy in an attempt to correlate the role of morphology to photovoltaic performance.

A prototype monolithic HgCdTe/CdTe superstrate tandem cell has been fabricated by RF sputtering, comprising a CdTe/CdS top cell, a ZnTe:N/ZnO:Al interconnect junction and a HgCdTe/CdS bottom cell. The Hg1−xCdxTe film as the bottom absorption layer was deposited by RF sputtering with 70% or 85% Cd content in the Hg1−xCdxTe magnetron target. Hg1−xCdxTe films with band gap from 0.98 eV to 1.45 eV were obtained by controlling the deposition temperature. CdCl2 thermal treatments were used to improve the Hg1−xCdxTe film electrical properties. A nitrogen-doped ZnTe film combined with an aluminium (Al) doped ZnO film formed a good interconnect junction. Results of Voc = 0.99 V and Jsc = 2.1 mA/cm2 were obtained in the best such tandem cell at one sun (AM1.5).

Die-casting growth was used for manufacturing the multicrystalline silicon sheet with a size of 100 × 120 × 0.5 mm. During the growth, incorporation of contaminants such as iron, cobalt, nickel and chromium was well suppressed. The average etch-pit density values ranged from 1×104 cm-2 to 4x106 cm-2 for growth rates of 5 to 60 mm/h, respectively. Measurement of minority-carrier lifetime bye microwave-photoconductivity-decay (μ-PCD) method was 0.5 μs for as-grown specimens, suggesting that defects and residual strain exist in the grown sheet. Moreover, post heat treatment at 1473 K reduced the etch-pit density and improved carrier lifetime up to 2.2 μs.

CIGS nanoparticles for the CIGS absorber layer have been synthesized by low temperature colloidal routes. The CIGS absorber layers for solar cells have been prepared by spray deposition of CIGS nanoparticle precursors (∼20 nm) in glove box under inert atmosphere. An automatic air atomizing nozzle spray system with computer controlled X-Y step motor system was used to spray. The nanoparticle precursor CIGS film was deposited onto molybdenum-coated soda-lime glass substrates (2.5 cm × 5.0 cm) heated to 160°C. The film thickness in the range of 2 μm ± 0.3 μm was attained by spraying of 3 mM colloidal over an area of 12.5 cm2. The coalescence between particles was observed in the CIGS absorber layer under post-treatment of over 550 °C. This is related to the reactive sintering among the nanoparticles to reduce surface energy of the particles. The CuxSe thin film, formed on Mo film by evaporation, improved adhesion between CIGS and Mo layers and enhanced the coalescence of the particles in the CIGS layer. These are closely related to the fluxing of Cu2Se phase which has relatively low melting temperature. The CdS buffer layer was deposited on the CIGS/Mo/soda-lime glass substrate by chemical bath deposition. The CIGS nanoparticles-based absorber layers were characterized by using energy dispersive spectroscopy (EDS), x-ray diffraction (XRD) and high-resolution scanning electron microscopy (HRSEM).

Dye sensitized solar cells (DSSC's) are constructed using TiO2 electrodes synthesized by aqueous liquid phase deposition in combination with microcontact printing techniques or porous thin film template methods. Layer-by-layer deposition of polyelectrolytes is used to produce an ionic conducting solid-state electrolyte thin film, which is enhanced by post-processing in oligoethylene glycol diacid (OEGDA). The impact of TiO2 film architecture, as well as the thin film electrolyte, on device performance is discussed.

We report on a novel use of nanorod arrays for organic based solar cell devices. A metal foil with copper nanorods attached to the surface was developed by electrodepositing copper from a copper sulfate solution into an anodic alumina oxide (AAO) template that had been coated with a metal on one side. The AAO membrane was dissolved in NaOH leaving behind an aligned array of copper nanorods. This nanorod array was evaluated to explore the possibility of increasing the power conversion efficiency of organic solar cells. Nanorod array characteristics were investigated by focus ion beam, scanning electron microscopy, and x-ray diffraction spectroscopy. A solar cell device was made by applying a polymer layer of poly(2-methoxy-5-(3', 7'-dimethyloctyloxy)-1, 4-phenylene-vinylene) (MDMO-PPV) mixed with 6, 6 phentl-C61-butyl acid-methylester (PCBM) onto the copper nanorod array and sandwiching it with a film of poly(3, 4-ethylenedioxythiophene): poly(styrene-sulfonate) (PEDOT:PSS) applied onto a indium tin oxide coated glass substrate.

Selenium thin films (350 nm) deposited from a 0.01 M solution of Na2SeSO3 of pH 4.5 maintained at 10 °C for 13 h, have been used as a source of selenium vapour for reaction with vacuum deposited Ag thin film on chemically deposited Sb2S3+Ag layers. When a stack of Sb2S3+Ag is heated in contact with Se film, AgSbSe2 is formed through solid state reaction of Sb2S3 and Ag2Se. The latter is formed at 80°C through the reaction of Ag-film in Se-vapour. This thin film is photoconductive and p-type. The optical band gap is nearly 1 eV and dark conductivity, 10-3 Ω-1cm-1. This thin film has been incorporated to form a photovoltaic structure, SnO2:F-(n)CdS:In-(i)Sb2S3-(p)AgSbSe2-silver print. Voc> 400 mV and Jsc>12 mA/cm2 have been observed in this under an illumination intensity of 1 kWm-2.

We report a nearly twofold increase of short circuit current: from Isc ∼ 10 mA/cm2 to Isc = 16–20 mA/cm2 in P3HT/PCBM solar cells (SC) employing freshly prepared regio-regular poly(3-hexylthiophene) (RR-P3HT) without special purification. The power conversion efficiency is enhanced to η≥ 4% as compared to our best η=3.8% in SC with commercial polymer despite the decreased filling factor (FF= 0.42, as compared to best FF = 0.59). We used our earlier found [1] procedures with optimal post heat treatment temperatures and time for our polymer SC. We also discovered a strong correlation between the device preparation procedures and performance. The optimal phase separation of PCBM and RR-P3HT into a bi-continuous network structure occurs after quite long solution stirring times (enhanced homogenization) and surprisingly very short annealing time at optimal temperature. We also found that the optimal concentration of PCBM in a RR-P3HT matrix is rather low, only c∼35 wt%, contrary to high c∼80 wt% in PPV based SC.

We have explored a large range of experimental space for photoluminescence (PL) and electroluminescence (EL) measurements of CdS/CdTe solar cells. This space includes changes in temperature, injection intensity (laser power for PL, current for EL), electrical bias for PL, and laser energy for PL. Measurements were resolved both spectrally and spatially (2D for EL, 1D for PL). Combination of EL and PL measurements revealed that most spatial inhomogeneity was the result of non-uniform current transport rather than local variations in recombination rate. The greatest spectral resolution was obtained with low temperature EL at low injection rates. High injection EL as well as high forward biased PL suppressed band-edge emission at low temperatures. Spectral structure was found to be greater in EL than in PL. These effects likely originated from preferential current transport along grain boundaries and/or certain grains.

Semiconducting boron carbide overlayers, formed from the decomposition of orthocarborane and metacarborane have been studied by angle resolved photoemission. The incurrence of surface photovoltage and the photovoltaic process, from the photoemission experiment, reveal band offsets in the orthocarborane multilayer configurations that are invereted relative to single layer configurations. Defect induced gap states which trap charge at the heterostructure interface is used as one explanation of these results. The role of defects is also used to help illuminate why opposite semiconducting type materials are formed from the decomposition of isomer carborane molecules.

Multilayer bulk heterojunction photovoltaic cells using chlorotricarbonyl rhenium (I) bis(phenylimino)acenaphthene (Re-DIAN) complex as photosensitizer were studied. The complex is sublimable, has lower band gap, good thermal stability and good processibility. It has broad absorption in UV-visible region. Therefore, Re-DIAN exhibits good photosensitising property for photovoltaic cells. Multilayer bulk heterojunction photovoltaic cells with simple structures were fabricated base on Re-DIAN complex. The active layer consists of a blend of Re-DIAN and fullerene that were co-deposited in the same layer by vacuum deposition. The photovoltaic properties of the devices were studied by irradiaton under AM1.5 simulated solar light. The effects of changing the co-deposition film thickness, amount of Re-DIAN photosensitizer, and hole transporting materials were studied.

Gram quantities of titania (TiO2) nanotubes, with typical outside diameter about 9 nm, wall thickness about 2.5 nm, and length about 600 nm, were synthesized from anatase nano- and micro-powder using the hydrothermal method. The crystallization, structure, and phase stability of the nanotubes at high temperatures were systematically studied. A morphology change from nanotube to nanowire was observed at 650°C. The as-prepared nanotubes were usually contaminated with sodium impurities, other TiO2-derived phases and were poorly crystallized, but under optimized synthesis conditions the impurity phases was completely removed, resulting in highly crystallized pure nanotubes. The volume filling fraction of the autoclave as well as the concentration of the acid treatment were found to be particularly important for controlling the purity and crystallinity of the resulting nanotubes.

We report here on the time evolution of photoconductivity under continuous illumination in nanocrystalline TiO2 samples prepared by a sol gel method, and also on the conductivity decay once the illumination is switched off. We observe strong dependence of the photoconductivity on the illumination intensity for both processes. It is found that the conductivity decay after high-intensity illumination is slower than after low-intensity illumination, and we have attempted to explain these experimental results using a model involving hole trapping-detrapping processes.

Dye sensitized nanocrystalline TiO2 solar cells have been reported with over 11% efficiency and are extremely promising as very low cost and lightweight photovoltaic sources. However, most reports are for cells of low area fabricated on glass, which withstands processing temperatures of ∼450°C. In this paper, we describe the fabrication and performance of cells made on flexible ITO-coated polyethylene terephthalate (PET) substrates with 6” × 3” dimensions. To improve the efficiency in the cells, we enhanced the ITO current collection efficiency with metallization fingers. The fingers resulted in a >10 fold increase in short-circuit current under normal solar illumination compared to cells without metallization. Further improvements were realized by passivating the metallization fingers at the metal/polymer electrolyte interface.

Dye sensitized solar cells (DSSCs) were fabricated using porous thin films of TiO2. These films were deposited by electron beam evaporation and an advanced substrate motion technique called PhiSweep. PhiSweep, an extension of glancing angle deposition (GLAD), allows for greater control over the surface area of nanostructured thin films than is possible with traditional GLAD. The as-deposited films were amorphous, so the films were annealed to improve their crystal structure. The films were sensitized with a photoactive dye and implemented into a DSSC configuration as the electron collecting electrode. It was expected that the higher surface area of the films produced using the PhiSweep method would improve the cell performance compared with cells made using traditional GLAD films of TiO2. However, the performance of the cells prepared using PhiSweep films was likely hindered by higher internal resistance of the films compared to the films prepared by traditional GLAD. The highest photoelectric conversion efficiency of the dye sensitized solar cells produced using the PhiSweep method was 1.5%.

Potentially n-type and p-type semi-conducting discotic liquid crystal dyes are linked together to star-shaped heptamers, which might self-organize into super-columns of separated p-type and n-type columnar stacks. Their synthesis, mesomorphism, and electronic properties will be discussed along with their potential use in photovoltaic devices.

Cd1-xMnxTe alloy films with band gaps of 1.6 ∼ 1.8 eV have been deposited by RF magnetron sputtering for solar-cell applications. The films have been treated by chloride vapors to improve the photovoltaic performance. These as-deposited and chloride-treated CdMnTe films have been investigated by Raman spectroscopy, x-ray diffraction (XRD) and scanning electron microscopy (SEM). Raman results indicate that Te and/or TeO2 exists in the annealed samples depending on anneal conditions.

We report the control of molecular ordering of discotic liquid crystals in thin, blended films used in photovoltaic diodes. The external quantum efficiency (EQE%) of photovoltaic diodes incorporating a crystalline hexabenzocoronene (HBC) derivative is improved by lowering the surface energy of the transparent anode surface with a short alkyl chain. Upon increasing the length of the functionalizing group on the anode, we find that the relative efficiency of the HBC component in the blend improves. Evidence of changed film morphology is also presented.