A study on the individualization of commercially purchased SWCNT and MWCNT were made in an N-N dimethyl tetraformamide solvent by a combination of ultrasonication and centrifugation, and processing of these individualized CNTs are applied as a stacking layer in bulk-heterojunction (BHJ) solar cells. The wt% (mg) of pristine CNTs loading was optimized in respect to volume of solvent (ml). Then as prepared BHJ devices were modified by spin casting at 4000 rpm/30s of these pristine individualized CNTs by incorporating a stacking layer (∼30 nm) for efficiency evaluation. Comparisons of the devices made with known functionalized CNTs (acid treated) and found that stacking of pristine individualized CNTs between the PEDOT:PSS and active (P3HT:PCBM) layer demonstrate a significantly enhanced efficiency of 2.1% (JSC of 9 mA/cm2, VOC of 0.6, FF of 39) from the normal BHJ of 1.2% (JSC of 5.3 mA/cm2, VOC of 0.62, FF of 33). However, SWCNT with acid treated when applied in BHJ shows degrading efficiency (0.51%) which can be attributed to the degradation of corrugated tubular side walls leading to potential loss of opt-oelectric properties. The enhanced efficiency of devices with pristine individualize CNTs can be conjectured due to better opto-electrical properties and undamaged tubes. The microstructures of the heterojunction active layer were examined by using UV-Vis spectra, IV curve and EQE techniques.

Interface physics is at the heart of organic photovoltaics (OPV). Here we reveal for the first time the actual charge generation mechanism in a low-band-gap polymer: fullerene blend as prototypical system for efficient OPV. We demonstrate that the photogenerated excitons dissociate into bound interfacial charge transfer states (CTS) and free charges in 20-50 fs, with a branching ratio that depends on the excess energy. Providing an excess energy, high energy singlet polymer states are excited, giving a direct hot electron transfer into the interfacial hot CTS* before internal energy dissipation occurs. This process ultimately leads to a higher fraction of free charges. Thanks to strong electronic coupling between high-energy-states and hot CTS, we demonstrate the opening of additional paths for charge generation that would otherwise be quenched by internal conversion to the lowest-lying states. Our results provide a new framework to understand charge generation in OPV system, suggesting that hot dissociation is a strategic option to enhance the photovoltaic conversion.

Solutions of individual, unbroken single-walled carbon nanotubes in organic solvent were fabricated in a reductive dissolution process. Transparent conductive films deposited from these organic inks gave a significantly higher conductivity to absorptivity ratio than those cast from an aqueous dispersion of carbon nanotubes. For example, films from the organic ink have achieved a sheet resistance of 250 Ω/□ with transmittance of 92% at 550 nm wavelength, compared to 76% transmittance for a 250 Ω/□ film from the aqueous dispersion. The promise of these films as transparent electrodes has been demonstrated by their incorporation into organic solar cells with power conversion efficiency of 2.3%, comparable to that of solar cells produced using indium tin oxide transparent electrodes.

CdSe quantum dots of hexagonal Wurtzite crystal structure with an average diameter of ∼7 nm were synthesized and processed for bulk heterojunction solar cell applications. The UV-Vis absorption spectrum shows an excitonic peak at 625 nm and at 635 nm in synthesized and dual ligand exchanged samples, respectively. The synthesized quantum dots were successively ligand exchanged by pyridine and 2-propanethiol to remove the TOPO ligands on quantum dot surface and then hybrid solar cell devices were fabricated. Initially the weight ratio was optimized by using pyridine capped CdSe blend with P3HT polymer as an active layer in chloroform as a solvent on the patterned ITO glass. Then dual ligand exchanged CdSe was compared with pyridine optimized samples. The maximum solar cell conversion efficiency of 1.21% was achieved with Jsc of 4.1 mA/cm-2, VOC of 0.51 and FF of 44 compared to the optimized pyridine capped CdSe quantum dots where efficiency of 0.74% with Jsc of 2.15 mA/cm-2, VOC of 0.53 was observed. The increase in solar cell efficiency was attributed to the better ligand exchanged and additional treatment with 2-propanethiol at ambient temperature. Such an exchange of organic ligands by successive ligand exchanger will open new domain for hybrid solar cell research. The morphology of QDs and microstructures of the heterojunction active layer (P3HT:CdSe) were examined by using TEM, XRD, UV-Vis spectra, and IV curve techniques.

We explore theoretically the effect of incorporating a thin tunnel barrier between the electron and hole transport layers of organic heterostructure photovoltaic devices. The device efficiency can be improved significantly by controlling the rates of microscopic processes associated with exciton dissociation and recombination at the interface between the electron and hole transport layers. The effects of different parameters are examined, and conclusions for organic photovoltaic device design are discussed.

We have investigated the counter intuitive phenomenon of inserting a metal oxide layer to improve hole injection or extraction in organic semiconductor devices using ultraviolet photoemission, x-ray photoemission, and inverse photoemission spectroscopy (UPS, XPS and IPES). We observed that metal oxides, such as MoO3, substantially increase the work function when deposited on indium-tin-oxide (ITO). The increase lifts up the highest occupied molecular orbital (HOMO) of the hole transport layer, therefore reduces the energy barrier between the HOMO and the Fermi level of the anode. The uplift creates an interface band bending region that results in a drift electric field that encourages the collection of holes at the anode. The optimum thickness for the oxide layer is estimated to be 2 nm. We have also investigated the effects of ambient or O2 exposure of MoO3. We observed that while most of the electronic energy levels of the oxide remained largely intact, the work function reduction was significant. This opens a way for optimal energy level alignment by modifying the work function through exposure. Furthermore, we observed that the work function reduction by exposure could be reversed by proper annealing of the sample in vacuum. The investigations therefore point to manipulate the interface electronic structure and charge injection/extraction by thin metal oxide films.

Dye sensitized solar cells (DSSCs) belong to an innovative third generation photovoltaic technology, which is demonstrating tremendous potential to become a revolutionary technology due to recent breakthroughs in fabrication cost. A seven-step quality improvement method is implemented to enhance process efficiency and effectiveness of the DSSCs. Lean Manufacturing’s 5S method successfully increased efficiency in all of the processes. Six Sigma’s DMAIC methodology is used to identify and eliminate each of the root causes of defects in the critical titanium dioxide deposition process. These optimizations resulted with the following significant improvements in the production process: 1. fabrication time of the DSSCs is reduced by 54 %; 2. fabrication procedures improved to the extent eliminated all critical defects 3. The yield of good cells increased from 17% to 90%.

Organic photovoltaics has attracted much effort and many research groups during the past decade, because of low-cost and easy fabrication techniques. Despite the great progress that has been achieved in increasing the conversion efficiencies of the devices, there are still several problems to be solved to make the solar cells commercially viable, especially for cells based on bulk heterojunctions.

The purpose of this work is to supply techniques for predicting the order of magnitude of the charge carrier mobilities of bulk heterojunction devices, on the basis of easy-to-perform measurements for experimentalists. A one dimensional model of a bulk heterojunction cell was used, and then simulations were performed in order to obtain the photocurrent as a function of an effective applied voltage. Plotted in a double logarithmic scale, the resulting curves exhibit different signatures depending on the mobilities of the charge carriers. These signatures could be helpful for experimentalists in order to predict an order of magnitude for both the electron mobility and the hole mobility.

The lifetime performance and reliability of photovoltaic (PV) modules are critical factors in their successful deployment. Interfaces in thin film PV, such as that between the transparent conductive oxide (TCO) electrode and the absorber layer, are frequently an avenue for degradation; this degradation is promoted by exposure to environmental stressors such as irradiance, heat and humidity. Understanding and suppressing TCO degradation is critical to improving stability and extending the lifetime. Commercially available indium tin oxide (ITO), fluorine doped tin oxide (FTO) and aluminum doped zinc oxide (AZO) were exposed to damp heat (DH), ASTM G154 cycle 4, and modified ASTM G154 for up to 1000 hours. The TCOs’ electrical and optical properties and surface energies were determined before and after each exposure and their relative degradation classified. Data demonstrate that AZO degraded most rapidly of all the TCOs, whereas ITO and FTO degraded at lower to non-quantifiable rates. One approach to suppress degradation could be to use interfacial layers (IFLs), including organofunctional silane layers, to modify the TCO. We modified the TCO surfaces using a variety of organofunctional silanes, and determined a range of surface energies could be obtained without affecting the electrical and optical properties of the TCO. Degradation studies of TCOs with a silane layer were also conducted. We found that an inhomogeneous silane layer was able to delay the resistivity increase for ITO in DH.

The present work investigates the UV stability of the dye-sensitized solar cell (DSC) by parametrical investigation of the material influence on UV stability. UV illumination has been observed to cause degradation by slow photocatalysis in the DSC. Photooxidized impurities represent an unwanted side reaction with the redox pair of the electrolyte as the released electron will deplete the triiodide concentration. A study on the DSC cell was carried out with intermediate electrical characterization by cyclic voltammetry (CV) and electrical impedance spectroscopy (EIS) to map the influence of UV illumination as a function of the H2O concentration in the electrolyte, the plate distance and the triiodide concentration. The results show that the H2O content has a detrimental influence on the DSC stability during UV illumination. A higher concentration of triiodide can buffer the reaction with impurities, so that a longer-term stability is achieved. A recovery of triiodide in UV aged cells with either no remaining triiodide or with such a low concentration that the cell current has been diffusionlimited, was seen during CV to -0.75 V under illumination. The reappearance of triiodide was accompanied with a production of hydrogen bubbles, which was related to the H2O content in the electrolyte and the exposure to UV. Our approach can be used to test the purity and the UV stability of various electrolytes.

The dye-sensitized solar cells (DSSC) are a technological and economical alternative to conventional p-n junction solar cells. The DSSC is composed of a transparent conducting electrode (SnO2:F) coated by a porous, nanocrystalline film of n-ZnO to which dye molecules are attached, an organic electrolyte containing a reduction-oxidation couple, and finally a counter-electrode (glass/SnO2:F) coated by a thin film of platinum. The most efficient dyes for DSSCs are based on Ruthenium polypyridyl complexes, related to the high absorption coefficient in the entire visible range and the efficient injection of electrons into the conduction band of ZnO. However, the ruthenium polypyridyl complex contains a heavy metal of relatively high cost, and synthetic routes are complicated with low yields. Moreover, natural dyes in addition to their availability, are cost-effective, non-toxic and biodegradable materials, and can be extracted by simple procedures. In this paper we report the extraction of natural dyes from the stems of mangrove (D1) and tinto (D2) trees as well as from walnut (D3) shell. First, it was necessary to dehydrate the materials, after which extraction was performed using ethanol, water and sodium hydroxide solution. The dyes were characterized using UV-visible and infrared spectroscopy. The analysis of the infrared spectra shows an intense and broad band related to OH bond stretching vibration at 3393, 3442 and 3390 cm-1 for the mangrove tree, tinto tree and walnut shell, respectively. At 1051, 1123 and 1050 cm-1, there was a very strong absorption due to the stretching vibration of CO group, for the mangrove tree, tinto tree and walnut shell, respectively. These results indicate that the functional group for bonding to the ZnO is -OH for these dyes. The results of the U-Vis spectroscopy show that the strongest absorption in the visible region is provided by dyes of the tinto and mangrove trees. The current - voltage curve of a preliminary ZnO-DSSC sensitized with the natural dye of the mangrove tree bark is presented.

An increase in molecular weight of the polymer generally impedes solubility in common solvents and may influence the polymer optoelectronic properties as well. Indeed, higher molecular weights are expected to increase charge carrier mobilities and therefore give rise to better photovoltaic performances of bulk heterojunction solar cells. In this work, we use copolymers based on 2,1,3-benzothiadiazole, thiophene and thieno[3,2-b]thiophene units of various fractions differing in molecular weights almost by a factor of 4 with a fullerene based acceptor material Indene-C60 Bisadduct (IC[60]BA) to elaborate bulk heterojunction solar cells. We investigate the influence of post-deposition annealing temperatures and polymer:fullerene ratios on the final cell performances. We use IC[60]BA as an acceptor to enhance the open circuit voltage due to its high lying LUMO level [1]. Additionally, charge carrier mobilities were probed using bottom contact organic field-effect transistors. As expected, higher molecular weights (as long as homogeneity was maintained) resulted in an increase of the hole field-effect mobility (up to 7x10-3 cm2V-1s-1). Consequently, the power conversion efficiencies of bulk heterojunction solar cells could be improved by increasing the copolymer molecular weight. A power conversion efficiency of 2.4% with an open circuit voltage of 0.82V was reached in a standard device configuration with aluminum as a cathode after post-deposition thermal annealing.

Dye sensitized solar cells (DSSCs) are currently being explored as a cheaper alternative to the more common silicon (Si) solar cell technology with improved performance in low light conditions and less sensitivity to varying angles of incident light. One of the major challenges facing DSSCs is loss of the liquid electrolyte, through evaporation or leakage, which lowers stability and leads to increased degradation. To address this, batches of gel electrolyte cells are fabricated with 7 wt% nanoclay gel electrolyte and liquid electrolyte and were evaluated at standard test conditions over time. The gel cells achieved efficiencies as high as 9.18% compared to the 9.65% achieved by the liquid cells. Over a period of 10 days, the liquid cells degraded less than 20% of its maximum efficiency. By contrast, the gel cell's efficiency did not decrease to 20% of its maximum efficiency until 45 days. After several measurements, the liquid cells showed visible signs of leakage through the sealant, whereas the gel cells did not. This resistance to leakage likely contributed to the improved performance of the quasi-solid cells over liquid electrolyte DSSCs.

Poly(3-hexylthiophene)/Titania (P3HT/TiO2) heterojunction has been widely studied in the field of hybrid solar cells. Usually, organic dyes shift the neat TiO2 absorption edge toward the visible range improving the conversion efficiency or/and the TiO2 surface is modified with ligands in order to increase the electron transport. On the other hand, copper sulfide, non-toxic semiconductor, has been included in bulk organic P3HT based solar cell, increasing the photocurrent density of devices. Therefore, we propose the use of copper sulfide in the hybrid TiO2/P3HT heterojunction to determine its effect in the performance of TiO2/P3HT solar cell. Copper sulfide nanocrystals (CuxS) were synthesized at 230 °C, 240 °C and 260 °C and, they were mixed with P3HT in order to form P3HT:CuxS bulk heterojunctions. Scattered grains and irregular morphology in the final topography of the reference device (P3HT/TiO2 heterojunction) were observed by AFM, while a granular morphology and a few pores like craters were observed in the devices containing P3HT:CuxS bulk heterojunctions. Chalcocite phase (Cu2S) was obtained at 230 and 240°C and, digenite (Cu1.8S) phase at 260°C, both copper sulfide phases are very promising for solar cells. Despite this, poor rectifications in the devices were found in the current-voltage curves of the devices containing copper sulfide nanocrystals in contrast to the P3HT/TiO2 cell (device without nanocrystals), it could be due to the current leakage or recombination process in the copper sulfide/TiO2 interface. It suggests future work in order to improve the devices.