In this study Ruthenium (II) tris (2, 2’-bipyridyl) complex in nonaqueous solution was studied by cyclic voltammetry and also its kinetic parameters were calculated. The redox reaction of ruthenium (II) trisbipyridyl complex is totally quasi-reversible and it can be described as a diffusion-controlled process. As a result of the redox reaction, the kinetic parameters of the electrode process such as diffusion coefficient (D), and heterogeneous rate constant (ks ) were calculated. In addition, the different thermodynamic parameters such as standard free energy (ΔG# ), enthalpy change (∆H# ), and entropy change (∆S# ) were determined and enthalpy change revealed the exothermic behavior of the electrode reaction. Both oxidation and reduction onset potentials of the ruthenium (II) trisbipyridyl complex was determined under the same experimental conditions to estimate the low ionization potential and electron affinity. The electrochemical and optical band gaps of the ruthenium (II) complex were compared.

The photovoltaic characteristics of all polymer bulk heterojunction solar cells made of P3HT and a perylene diimide-based copolymer (PEK3) have been studied. Thermal annealing is needed to improve the performances. Annealing optimization induces an enhancement of the power conversion efficiency from 0.06 to 1%, Jsc from 0.24 to 2.9 mA/cm2 and FF from 0.32 to 0.59. The origin of such improvements has been investigated by studying the P3HT:PEK3 blend morphology, by means of absorption and emission spectroscopy and charge transport, from single carrier measurements on P3HT:PEK3 diodes. Upon annealing we have observed an increase in phase segregation and a 100-fold enhancement of the hole and electron mobilities, that favor the dissociation of bound electron-hole pairs and their transport to the electrodes. This explains the high FF of the annealed P3HT:PEK3 solar cell.

Since its introduction in early 1990s, bulk-heterojunction organic photovoltaic solar cell (BHJ-OPV) has promised high-efficiency at ultra-low cost and weight, with potential for non-traditional applications such as building-integrated PV. There is a widespread presumption, however, that the complexity of morphology makes carrier transport in OPV irreducibly complicated, and possibly, beyond predictive modeling. In this paper, we use elementary and intuitive arguments to derive the fundamental thermodynamic as well as morphology-specific practical limits of BHJ-OPV efficiency. We find that constraints of the percolation threshold and trade-off among short-circuit current, open circuit voltage, and fill factor make substantial improvement in OPV efficiency difficult. We posit that future improvement in OPV will rely not on morphology engineering, or reducing the polymer bandgap, but on increasing both the effective μ × τ product and the cross-gap between donor/acceptors. Even if the OPV fails to achieve the highest efficiency anticipated by the thermodynamic limit, its novel form factor, lightweight, and transparency can make it a commercially viable option for many applications.

In bulk heterojunction organic solar cells, open-circuit voltage (Voc ) is mainly dependent on the lowest unoccupied molecular orbital and the highest occupied molecular orbital of the donor/acceptor polymer pair in the active layer. However, there are other factors that contribute to considerable reduction in the Voc . The active layer/cathode interface is one of these factors. Previous studies show that e-beam evaporation of the cathode metal contact forms deep interface trap holes in the active layer which increases the Voc of the solar cells. Although these studies show the effect of deeply trapped holes on the Voc , several attempts to elucidate the mechanism behind this effect revealed their subtle and elusive nature. In this work, the effect of cathode contact annealing rate on the overall efficiency is studied. Three different sets of devices were fabricated with varying cathode evaporation rates of 0.1Å/s, 1Å/s and 5Å/s. The results show that at low evaporation rates, atoms in the cathode materials lack adequate energy to form deeply trapped holes. Additionally, above a certain value, the evaporation rate does not have a significant effect on the formation of deeply trapped holes. We also demonstrate that power conversion efficiencies of the devices can be maximized by maintaining the evaporation rate within a specific range.

If graphene is to be incorporated into transistors, solar cells, and capacitors, a large-scale synthesis of graphene must be devised. This research developed an innovative, simple, and cost-efficient synthesis procedure, dispersing graphene oxide in an ethanol-water solvent and reducing slowly with sodium borohydride (NaBH4). Reducing graphene oxide in 75:25 and 50:50 H2O:ethanol solutions with 15 mmolar NaBH4 produced numerous single-layered reduced graphene oxide sheets >1 μm2, and sometimes even >2 μm2. The quality of these sheets was confirmed by Raman spectroscopy, XRD, TGA, TEM, ED, and HRTEM.

Photo-excitation of high surface area semiconductor nanorods decorated with surface catalyst particles are investigated. DFT-based simulation is applied to the charge transfer dynamics at the interface of the supported nanocatalyst by modeling dynamics of photo-excitations. The modeling is performed by reduced density matrix method in the basis of Kohn-Sham orbitals. The energy of photo-excitation is dissipating due to interaction with lattice vibrations, treated through non-adiabatic coupling as the electron/hole pair relaxes to the conduction / valence band edges. The methodology is applied to TiO2 nanorod modeled as a periodic anatase (100) slab functionalized by minimalistic nano-clusters or doping. Simulations of these models demonstrate the formation of charge transfer state in both time and frequency domain. Computed charge dynamics leads to creation of positively charged areas on the nanorod surface that is an important prerequisite for oxidation catalysis. Our computation identifies optimal composition and morphology of nanocatalyst for such applications as water splitting for hydrogen production or solar cells.

As global energy demand is steadily growing, renewable energy generation by solar cells is becoming increasingly important. The use of mono- and polycrystalline silicon solar cells, which nowadays dominate the market, is limited by wafer size, rigidness of substrates and the requirement of large energy amounts for manufacturing. Organic solar cells (OSC) have the potential to overcome these limitations; especially organic vapor phase deposition (OVPD) technology offers the possibility of reproducible, large-scale production at low temperatures and on flexible substrates.

We report on planar heterojunction OSC utilizing an active layer of pentacene/N, N’- ditridecylperylene-3, 4, 9, 10-tetracarboxylic diimide (PTCDI) fabricated by an Aixtron Gen-1 OVPD tool. The influence of substrate temperature was studied using atomic force microscopy (AFM) on single layers and bilayers. In addition electrical characterization with and without illumination of fully processed solar cells which utilize different cathode layers was carried out.

AFM images indicate that crystallization of pentacene layers can be widely influenced by substrate temperature, a PTCDI-C13H27 layer atop of these covers the crystallites. Open-circuit voltage was found to be 0.47 V and short-circuit current densities beyond 0.8 mA/cm2 were measured under a spectrum close to AM 1.5 with 100 mW/cm2. Fill factors were determined to be as high as 44 %.

Polymer solar modules, based on glass or flexible PET-substrates, structured either by laser ablation, mechanical scribing, or by a combination of the two, were prepared and analyzed. The photo-active layer of the solar modules is based on poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester or Poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]:phenyl-C71-butyric acid methyl ester donor-acceptor bulk heterojunctions. Since such polymer-fullerene solar cells and modules are designed in a multilayer architecture, local defects such as shunts or blocking junctions in the device can cause critical losses to the solar module performance. Of special importance for solar module preparation is the structuring process, as it allows the serial interconnection of the cells. The high precision required for removing neither too few nor too much of the thin layers to be structured presents challenges in the processing of polymer solar modules. Herein we demonstrate that laser structuring is a suitable technology to face these challenges. We report about completely or partially laser structured polymer photovoltaic modules. By using highly sensitive dark lock-in thermography we analyze the influence of defects and failures on the performance and operation of solar module devices. Finally, promising results for fully laser structured solar modules on glass and partly laser structured solar modules on PET are presented.

By using electric-field-induced second-harmonic generation (EFISHG) measurement, we analyzed photovoltaic effect of two-layer solar cells (indium zinc oxide/pentacene/C60/Al). Results evidently showed that negative and excessive charges Qs accumulated at the two-layer interface under illumination, e.g., Qs =-1.7×10-9 C/cm2 at 0.05 mW/cm2 and –3.6×10-9 C/cm2 at 0.5 mW/cm2, while a short-circuit current flowed. The open-circuit voltage changed in accordance with accumulation charge Qs , and finally saturated. Modeling that accumulated negative charge is a source of space charge field and directly effects on the electrostatic energy stored in OSCs, dependence of the open voltage on the accumulated charge Qs was explained.

Organic solar cells with PEDOT:PSS hole transport layer or P3HT:PCBM active layer deposited by a novel solution-based mist deposition technique were fabricated and demonstrated. The device with mist-deposited PEDOT:PSS layer showed higher short circuit current density and power conversion efficiency than those of the one with spin-coated PEDOT:PSS layers. The device with mist-deposited P3HT:PCBM layer also performed with higher power conversion efficiency. The results encourage the promising potential of the mist deposition method for device fabrication.

Dye-sensitized solar cells (DSSC) may provide an economical alternative to the present p–n junction photovoltaic devices. Here the relation between chlorophyll purity and photovoltaic performance was examined. Also the commercial grade copper chlorophyll was examined. The performance under simulated sunlight and the quantum efficiency were measured. All samples had large short wavelength quantum efficiency however the high purity chlorophyll had larger quantum efficiency in the visible. The highest purity samples produced DSSC solar cells with the highest open circuit voltage and efficiency while the fill factor and the short circuit current were not strongly correlated with purity. The un-altered short circuit current suggests that chlorophyll attachment and charge transfer at the titanium oxide are not altered by impurities. However the results suggest that impurities (and/or copper in the commercial chlorophyll case) alter the photo-absorption and the electrolyte so as to either change the iodine chemical potential or decrease the diffusivity of iodine ions.

We report the synthesis, structural and optical characterization of PbSexS1-x nanorods with diameter between 2 nm to 4.5 nm and length of 12 nm to 24 nm. Their typical photoluminescence spectra exhibit a split of the band-edge exciton band. The temperature dependence photoluminescence of these nanorods revealed a relatively small band-gap temperature coefficient and a mild extension of the radiative lifetime at cryogenic temperatures - all in comparison with photoluminescence processes in PbSe nanorods, as well as in PbSexS1-x quantum dots, with similar absorption band-edge energy. A theoretical model associates the experimental observations to the occurrence of independent transitions from either degenerate or non-degenerate band-edge valleys in PbSexS1-x nanorods, each of which possessing a relatively small electron-hole exchange interaction.

The effect of phase separation of the donor-acceptor (DA) blend on the dominant recombination mechanism in polymer-fullerene [(poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM)] based bulk heterojunction (BHJ) cells has been investigated. Coarse (70-150 nm) and fine (20-25 nm) phase separated blends and corresponding devices were prepared using chlorobenzene (CB) and ortho-dichlorobenzene (1,2-DCB) as spin casting solvents respectively. Nanoscale mobility measurements indicated highly unbalanced charge transport in coarse morphology based (CB cast) devices. Linear dependence of short circuit current (Jsc) vs. light intensity (I) suggested first order monomolecular (MR) recombination in the fine phase separated devices (1,2-DCB cast) whereas sub-linearity suggested dominant role of bimolecular (BR) recombination in coarse phase separated devices (CB cast). Improved device efficiency of 1,2-DCB based devices (η ≈ 2.54 %) compared to CB (η ≈ 0.9 %) may be attributed to reduced BR recombination as a result of finer phase separation.

We report the synthesis, characterization, and implementation of various naphtho[2,3- b:6,7-b’]dithiophene (NDT)-based donor molecules for organic photovoltaics (OPVs) When NDT(TDPP)2 (TDPP = thiophene-capped diketopyrrolopyrrole) contains all branched 2- ethylhexyl chains and is combined with the electron acceptor PC61BM, a power conversion efficiency (PCE) of 4.06±0.6% is achieved. This respectable PCE is attributed to the broad, high oscillator strength visible absorption, the ordered molecular packing, and a high hole mobility of 0.04 cm2·V-1·s-1. We find utilizing linear C-12 side chains on either the TDPP or NDT framework dramatically increases the d-spacing, which directly correlates with inferior OPV device performance. This leads to the conclusion that the selection of an appropriate side chain plays a key role in determining OPV device performance of a small molecule donor.

Block copolymers are considered to be highly attractive materials with regards to future applications of nanomaterials and nanostructures owing to their self-assembling nature. Block copolymers, when supplied with sufficient energy, phase separate at the nanoscales to form periodically ordered structures in the nanometer-scale range. A diversity of architectures can be accessed via composition control of individual block components. An exciting area of application for block copolymer self assembly is organic photovoltaic devices (OPV‟s) where it is expected that the very high interfacial area of the blocks with ∼10-20 nm domain spacing would be highly advantageous for exciton diffusion and separation. For this purpose BCPs composed of amorphous (non-conjugated) polymers can also serve as a template for directed assembly of nanoparticles. Zone annealing is a well established method predominantly utilized for metallurgical and semi-conductor purification processes, where recrystallization and oriented grain growth occur on the planar front formed by the cooling-edge of the zone. We have previously applied this process to create highly ordered BCP cylinders that are parallel to the substrate with orientational control, long range order and faster ordering kinetics than conventional thermal annealing. In the present paper, we extend this idea to block copolymer - [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend system and report how the presence of PCBM nanoparticles influence the micro-phase separation behavior of cylinder forming poly(styrene-b-2-vinyl pyridine) under a dynamic thermal gradient field. A range of scattering techniques have been on the BCP:PCBM blend system, including grazing incidence small angle x-ray scattering (GISAXS) experiments to characterize in-plane and lateral ordering of BCP-PCBM blend system.

Solid state PbS Quantum Dots (QDs)/TiO2 Nanoparticles heterojunction solar cells were produced by depositing PbS QDs on a 500nm thick Mesoscopic TiO2 films using layer-by-layer deposition. The heterojunction solar cells show photovoltaic response from the visible to the near infra-red region. Importantly, the PbS QDs act here as photosensitizers and at the same time as hole conductors. The PbS QDs/TiO2 device produces a remarkable short circuit photocurrent (Jsc) of 16.3 mA/cm2, an open circuit photovoltage (Voc) of 0.54 V and a fill factor (FF) of 0.41, corresponding to a light to electric power conversion efficiency (η) of 4.04% under 0.9 sun intensity.

Nanoscale heterojunction systems consisting of fullerenes blended with conjugated polymers are promising materials candidates for achieving high performance organic photovoltaic (OPV) devices. In order to understand the phase behaviour in these thin film devices, we have used neutron reflectivity to determine the behavior of model conjugated polymer-fullerene mixtures. Neutron reflectivity is particularly useful for these types of thin film studies since the fullerenes generally have a higher scattering contrast with respect to most polymers. We are studying model bulk heterojunction (BHJ) films based on mixtures of poly(3-hexyl thiophene)s (P3HT), a widely used photoconductive polymer, and different fullerenes (C60, PCBM and bis-PCBM). We have used neutron reflection measurements to determine the film morphology normal to the film surfaces in real device configurations. The novelty of the approach over previous studies is that the BHJ layer is measured with the confining films of PEDOT/PSS and Al in place. Using this model system, we have measured the effect of typical thermal annealing processes on the film development as a function of the polythiophene-fullerene mixtures.

Inkjet printing is a highly material-efficient solution deposition technique that enables the preparation of thin-film libraries using little amounts of materials. As a reproducible and precise patterning technique inkjet printing can be integrated into a combinatorial screening workflow that allowed the systematic characterization of thin-film properties of newly developed materials as well as the methodical investigation of preparation parameter that influence the performance of the inkjet printed layers.

This contribution provides a demonstration of a combinatorial screening workflow that utilizes inkjet printing to evaluate structure-property relationships of polymer/fullerene blends for the application in organic photovoltaics. Using this approach it is shown that optimized blend compositions as well as printing conditions lead to improved performances of organic solar cell devices.

Single-crystalline organic solar cells were investigated. Rubrene single crystals made by train sublimation method were used for the active layer of the solar cells. Typical solar cell characteristics and external quantum efficiency (EQE) were observed with the film thickness of several micrometers. In spite of their large film thickness, the EQE spectra showed no screening effect, which means that absorbed photons efficiently converted to electric charges. This can be attributed to the extended exciton diffusion due to uniform and trap free characteristic of rubrene single crystal.

We have investigated the effect of temperature annealing on bilayer heterojunction solar cells based on poly[9,9’-hexyl-fluorene-alt-bithiophene] as active layer. Film morphology for different temperature annealing was probed by atomic force microscopy (AFM) and the values of roughness range from 0.59 up to 2.15 nm. The best photovoltaic performance was found for devices with active layer annealed at 200°C with power conversion efficiency (η) of 2.8 % while devices without annealing presented only 0.4%. This performance enhancement is attributed to the reduction of traps and increased hole mobility after the thermal annealing.