Dye-sensitized solar cells (DSCs) are one of the most promising photovoltaic technologies for production of renewable, clean, and affordable energy. They are generally made from cheap and nontoxic components and can be designed in a variety of different colors and transparencies, which distinguishes them as an ideal photovoltaic concept for integrated â?ogreenâ? architecture (1,2). In DSCs, charge carrier generation takes place in a monolayer of photoactive dye chemisorbed on a mesoscopic anatase TiO2. This high surface area semiconductor is necessary to ensure high dye loading and efficient light harvesting in the visible part of the solar spectrum because of the relatively low absorption cross-section of the molecular sensitizer,. This implies that the DSC has an exceedingly large, heterogeneous interface through which electrons may be (parasitically) intercepted as a result of the slow electron transport in the semiconductor. The latter is governed by an ambipolar diffusion mechanism controlled by trap-limited hopping through a relatively long and tortuous path to the transparent conductive electrode. It is further hindered by the low electron mobility in anatase TiO2 nanoparticles and the multiple grain boundaries in the mesoporous film (3). We will present novel bottom-up 3D host-passivation-guest electrode concepts based on inverse transparent conductive oxide opals that enables structural control on the electron extraction and recombination dynamics as well as on the optical scattering in photovoltaic devices. To ensure high dye loading the macropores contained in the passivated 3D host are infiltrated with anatase TiO2 nanoparticles or ZnO nanowires grown in-situ enabling optimized sensitization and electron injection. Using this novel architecture, an increase in the photovoltage of up to 110 mV over state-of-the-art TiO2 and ZnO-based DSCs is obtained (4). Finally, we will report on the use of atomic layer deposition to passivate the TiO2 and ZnO surfaces found in the 3D structures and mesoporous particle-based photoanodes using thin conformal metal oxide layers. Passivation layers have been shown to affect the TiO2 conduction band and improve both electron transport by filling electron traps and photovoltage by slowing down recombination at the TiO2-electrolyte interface.. 1. O'Regan B, Grätzel M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature News. 1991;353(6346):737â?"740. 2. Graetzel M. The advent of mesoscopic injection solar cells. Prog Photovoltaics. 2006;14(5):429â?"442. 3. Tetreault N, Horvath E, Moehl T, et al. High-Efficiency Solid-State Dye-Sensitized Solar Cells: Fast Charge Extraction through Self-Assembled 3D Fibrous Network of Crystalline TiO2 Nanowires. ACS Nano. 2010;4(12):7644â?"7650. 4. Tetreault N, Arsenault Ã?, Heiniger L-P, et al. High-Efficiency Dye-Sensitized Solar Cell with Three-Dimensional Photoanode. Nano Lett. 2011.

Quantum dots (QDs), having unique opto-electronic properties which can be tailored by changing their size appear as a good candidate for third generation photo-voltaics. Quantum dots (QDs), having unique opto-electronic properties which can be tailored by changing their size appear as a good candidate for third generation photovoltaics. Many architectures for QD-based solar cells were proposed, including QD layers sandwiched between electron and hole conductors, QD/polymer blends, QD/nanocrystalline wide-bandgap semiconductor hetrojunctions and the photoelectrochemical approach involving QD-sensitized solar cells (QDSSCs) and QD films immersed in electrolyte. Here we report on a QD based tandem solar cell utilizing two QD materials. The QDs deposited on conducting electrodes in a photo-electrochemical solar cell (PESC) are tuned to match the solar spectrum. A thin layer of CdSe quantum dots (QDs) deposited on a conductive glass was used as a photo-anode in photo-electrochemical solar cells yielding photovoltage of 675mV and 2mA/cm2 short circuit photocurrent. The response of the QDs based photoactive electrodes correlated with the absorption spectra of the QDs which is easily tuned by size modification. Similar geometry based on acid treated CdS quantum dots, provided a photo-cathode with complimentary photovoltage. A tandem photo-electrochemical solar cell consisting of the two photoactive electrodes exhibit Voc of 816mV, the highest photo voltage reported up to date for QDs based photo-electrochemical solar cells. However, current matching still requires optimization of the spectral response and charge collection efficiencies of the two photoactive electrodes. The results demonstrate a new approach for beneficial utilization of QDs in photo-electrochemical solar cells towards efficient, low cost photovoltaics. 1) Shalom, M.; Hod, I.; Tachan, Z.; Buhbut, S.; Tirosh, S.; Zaban, A., Energ Environ Sci, 2011, 4, 1874.

The use of inorganic semiconductors as effective light sensitizers in a dye sensitized solar cell (DSC) configuration have awaken a great interest in the last few years. Semiconductors present some advantages with respect to conventional dyes, such as high extinction coefficient and large dipole intrinsic moment. In addition, within the quantum confinement regime, semiconductor nano-crystals or quantum dots (QDs) allow tailoring the light absorption range by the control of QD band gap, achieved by changing their size and/or shape. However, Dye and quantum dot (QD) sensitized solar cells are currently studied assuming that quantum dots behave merely as an alternative inorganic dye. Here we show that this interpretation is not accurate. There is a fundamental difference between dye and QD sensitized solar cells, due to the fact that QDs play a direct role in the recombination process. This fact has been determined by a fingerprint of QDs in the capacitance of the device. This result highlights the necessity of treating (and optimizing) QD sensitized solar cells from another perspective than dye sensitized solar cells, considering the fundamental differences in their behavior.

Extremely thin absorber (ETA) solar cells consisting of a thin inorganic layer sandwiched between a semi-transparent nanoporous n-type material and a p-type semiconductor, offer several key advantages that may lead to high efficiency devices. Among these advantages, we can cite the partial decoupling between light absorption and charge carrier transport, the non excitonic nature of the excited species in the absorber, and the compatibility of the device structure with low cost processes based on abundant materials. However, as a matter of fact, the research in the field of ETA cells did not yet deliver solar cell performances at the level of the expectations of the scientific community. The current record of all inorganic ETA cells made of a stack of TiO2 / Sb2S3 / CuSCN is about 4.0 % while the hybrid version of this device based on an organic p-type semiconductor was measured up to 6 %. During this talk, we will present an overview of the ETA cell research, describe the working mechanisms of this type of devices, and discuss their main limiting factors. Finally, we will propose new direction to push forward the performances of this elegant photovoltaic concept.

Recent years many efforts have been paid to ZnO based photoanode preparation for dye sensitized solar cells (DSSCs) and improved efficiency of over 5% using ruthenium based sensitizer was achieved. However, synthesis of size confined ZnO nanostructures with uniform distribution has several difficulties like agglomeration, Ostwald ripening and oriented attachment growth of nanoparticles. In order to obtain monodispersed nanostructures with confined size, surface passivation is required. Recent years, organic molecule passivation to the surface of inorganic semiconductors has received much attention due to lone pair electrons present in their valance orbital. In our research work, organic ligands such as amine molecules have been used to prepare the monodispersed ZnO nanostructures by wet chemical and hydrothermal growth methods. In detail, Ethylenediaminetetraacetic acid,Triethylamine, N-Methylaniline and were used as organic ligands and the morphology of the obtained ZnO nanostructures were nanosheets, spherical nanoparticles and thin nanodisks, respectively. The growth of ZnO nanostructures is as follows: 0.2 M of zinc acetate and 0.3 M of sodium hydroxide were dissolved in 50 ml of deâ?"ionized water under magnetic stirring at 460 rpm at room temperature. An appropriate amount of EDTA was added to the mixture. The reaction was continued for 10 hours, and then precipitates were dried at 150 degree celsius for 5 hours. The similar growth procedure was maintained for other organic ligands. In addition, the similar reaction procedure was adopted and the growth was maintained under hydrothermal conditions using an autoclave at the growth temperature of 180 degree celsius. The fabrication method of dye sensitized solar cell is as follows. Synthesized ZnO nanostructures were dispersed in 50 ml of ethanol. Cleaned FTO substrates were used as a conducting electrode and it was placed on the hot plate with temperature of 150 degree celsius. ZnO colloidal solution was sprayed using the spray gun. Finally, the ZnO coated FTO substrates were annealed at 450 degree celsius for three hours. ZnO nanostructures coated FTO substrates were immersed in N719 Ruthenium dye for 90 min at70 degree celsius. Solar cell characteristics of the device were studied under 1.5 sun illuminations. The results indicated that the size and shape of ZnO nanostructures highly depended on the addition of organic ligand during the growth. ZnO nanostructures exhibited the enhanced band edge luminescence and suppressed defect level emission as compared to that of uncapped counterparts. The photoanodes for DSSCs have been prepared by spray pyrolysis deposition using 3 gram of synthesized ZnO nanostructures dispersed in ethanol. Photoanodes were annealed and immersed in N-719 ruthenium sensitizer. Among the three kinds of nanostructures, ZnO nanodisks coated photoanodes exhibited the remarkable efficiency of 5.1 %. The detailed investigations and results will be presented in the conference.

Hybrid nanostructured organic/inorganic heterojunction solar cell architectures are a promising route to efficient, low cost photovoltaic devices because the nanostructured inorganic allows a decoupling of absorption and charge collection. The use of an inorganic component also often allows the use of low-cost, solution based deposition methods that give better control of the nanostructure and improved mobilities over organic-only devices. Unfortunately hybrid architectures haven't yet reached their expected potential and in fact work worse than their organic-only counterparts. In zinc oxide (ZnO)/poly-3-hexylthiophene (P3HT) devices poor performance has been traced to interfacial issues associated with exciton dissociation, P3HT ordering, and P3HT intercalation into ZnO nanostructures. These devices have benefitted from the attachment of self-assembled monolayers (SAMs) of functional molecules at the ZnO/P3HT interface. These SAMs can be used to control interfacial energy alignment through the introduction of surface dipoles, while wetting and ordering behavior can be controlled through surface energy changes. By varying both attachment chemistry and functional group, we demonstrate that a large range of tunability is possible in both surface potential and energy. We find the surface potential can be changed by 500mV by simply varying the attachment chemistry between thiol, triethoxysilane and carboxylic acid while keeping the same functional group. Conversely, using the triethoxysilane attachment, and varying the functional group between several different electron donating and accepting groups, we achieve a surface potential tunability of 475mV. Overall, we find that surface potential can be tuned by as much as 700mV and contact angle by 60°, and that we have the ability to independently vary these two parameters by using the attachment chemistry to vary surface potential, while reserving the functional group for surface energy changes. While motivated by the idea of improving the nanostructured ZnO/P3HT interface, the tunability of these molecules makes them broadly applicable as surface modifiers across all of hybrid electronics; in electron injectors in LEDs, electron acceptors in photovoltaic devices, and electron transport layers in bulk heterojunction devices. This work is supported by the NSF through DMR-0907409 and the Renewable Energy Materials Research Science and Engineering Center (REMRSEC).

Ultrafast spectroscopy is a powerful tool for understanding nanostructured solar cells because it probes photophysical phenomena possessing characteristic time scales from hundreds of femtoseconds to nanoseconds. Photoexcitation, carrier cooling, trapping, recombination, and interfacial charge transfer in nanostructured solar cells all take place on ultrafast time scales. Visible transient absorption (TA) spectroscopy probes changes to the visible absorption spectrum of a material after photoexcitation with a tunable pump pulse. TA has been widely used to evaluate excited state lifetimes and charge transfer rates in dye-sensitized solar cells, as well as multiple exciton generation in semiconductor quantum dots. We will discuss the application of TA to extremely thin absorber (ETA) solar cells. Planar solar cells require thick absorbers to increase light harvesting, while thin absorbers to improve charge collection. To balance these competing constraints, ETA cells use a thin absorber at the high-surface-area interface between nanostructured n- and p-type layers. By matching the absorber thickness to the carrier collection length, this architecture facilitates charge separation and enables use of lower-grade absorber materials with shorter lifetimes and smaller mobilities. The ETA cells studied here consist of a thin CdSe absorber electrodeposited onto an n-type ZnO nanowire array with a CuSCN hole transport layer or a liquid electrolyte containing [Fe(CN)6]^3+/4+. TA is used to investigate interfacial charge transfer, trapping, and bulk and interfacial recombination, all of which significantly impact ETA cell performance. For instance, a bleach of the CdSe band edge appears almost instantaneously upon photoexcitation, and then decays as photoexcited electrons leave the conduction band. The decay is on the order of 50-100 ps in thick CdSe films where bulk recombination is the major loss mechanism. However, the decay occurs within a few picoseconds in thin CdSe films interfaced with ZnO, indicating very rapid interfacial electron transfer that is necessary for efficient charge separation. We will further describe the use of TA on nanowire arrays, as well as model planar geometries, to determine charge transfer and recombination kinetics at the interface of CdSe with ZnO, and both hole conductors; and we will discuss their implementations for the design of efficient ETA solar cells.

The dye-sensitized solar cell (DSC) is a promising alternative to conventional solar cells. DSCs with a solid state hole-conductor material may have practical benefits compared to DSCs with a liquid electrolyte. The dominating solid state hole-conductor material in DSCs is 2,2',7,7'-tetrakis(N,N-di-p-methoxypheny-amine)-9,9'-spirobifluorene (Spiro-OMeTAD), which has resulted in DSCs with efficiencies in the order of 5 percent in the most efficient cases. Hole-conducting polymers that are usually used in the polymer/fullerene organic solar cells have also shown to be rather effective in the dye-sensitized solar cells. In this case the polymer in addition to the hole conduction may also be used to absorb the light and inject electrons into the nanoparticles, i.e. the polymer act both as light absorber and hole-conductor. Two organic dyes with high extinction coefficients, have been designed and synthesized in collaboration with KTH. Solid-state DSCs employing these two dyes have been tested and optimized in cooperation with nanoporous TiO2, the hole transport material Spiro-OMeTAD, LiTFSI and 4-t-butylpyridine. IPCE above 80% has been obtained by using the organic dyes on the surface of the TiO2 nanoporous structure. Furthermore above 4.5% energy conversion efficiencies have been attained in solid-state DSCs with the two organic dye molecules. In further comparison, the dyes were tested in solid-state DSCs with polymer P3HT as hole conductor instead of Spiro-OMeTAD. The energy conversion efficiency of the systems based on the P3HT polymer was low without any additives in the polymer hole conductor. However, the efficiency for the system with one of the organic dyes was significantly improved from 0.1% up to above 3% by introducing LiTFSI and 4-t-butylpyridine additives in the polymer. The P3HT polymer is therefore a promising alternative to the standard Spiro-OMeTAD hole-conductor.

Mesorporous TiO2 beads were synthesized using both a conventional hydrothermal process and a microwave-assisted hydrothermal process. The major difference in the resulting TiO2 beads is the phase purity. Pure anatase TiO2 beads were obtained through the microwave-assisted process. In both cases, TiO2 beads having various sizes (250 to 750 nm) and characteristics were obtained. The use of the obtained mesoporous TiO2 beads in all-plastic flexible dye-sensitized solar cell (FDSC) is demonstrated. The concept of chemical sintering, eliminating the step of additive removal, was used to prepare bead-containing paste at room temperature fabrication of photoanodes having good adhesion to the substrate. The photoanodes consist of pure commercial TiO2 powders, pure beads, or mixture of the two. The dye loading and light absorbance properties of the obtained photoanodes were first investigated. Various plastic substrate FDSCs were then fabricated. The resulting cells were evaluated for the J-V characteristics, electron diffusion time, electron lifetime, charge-collection efficiency, electron-injection efficiency and incident photon-to-electron conversion efficiency. The bead-containing and bead-only cells not only have better efficiencies but also exhibit ultra-fast electron diffusion rates, for example, 0.37 ms. The best efficiency and electron diffusion rates are respectively 15% higher and two-order of magnitude faster than the P25-only cell. The effects of the bead characteristics on the cell performance is presented and discussed.

In this work, three dimensional nanostructure of polymer/(6,6)-phenyl-C71-butyric acid methyl ester (PCBM) blending bulk heterojunction (BHJ) was systematically investigated with respect to the processing additive, 1, 8-diiodooctane (DIO). Polymer photovoltaic device based on the BHJ of poly[4,8-bis-substituted-benzo[1,2-b:4,5-bâ?T]dithiophene-2,6-diyl-alt-4-substituted-thieno[3, 4-b]thio-phene-2,6-diyl] -derived polymers (PBDTTT-C) /PC71BM and poly{[4,4-bis(2-ethylhexyl)-cyclopenta-(2,1-b;3,4-bâ?T)dithiophene]-2,6-diyl-alt-(2,1,3- benzothiadiazole)-4,7-diyl} (PCPDTBT)/PC71BM exhibit considerably improved power conversion efficiency (PCE) by 40% (PCE from 3.0% to 4.2%) and 53% (from 4.0% to 6.1%) respectively by including 3% DIO as the processing additive. The significant enhancement can be attributed to the controlled nanostructure of the BHJ layer during the deposition procedure. The present study adopted simultaneous synchrotron incidence grazing incidence wide angle X-ray diffraction (GIWAXD) and small angle X-ray scattering (GISAXS) to quantitatively analyze the nanostructural variation. Dense-packed PCBM clusters, fractal-like PCBM cluster, and network of PCBM molecules intercalated with amorphous polymer chains and polymer crystallites were carried out from the GISAXS and GIWAXD profiles and compared between the BHJ with and without DIO. Moreover, simultaneous mapping of topography and surface potential obtained from Kelvin probe atomic force microscopy (KPFM) further provide supportive results of the nanomorphology variation from microphase separation to nanophase separation with DIO additive. The detailed structural characterization can be correlated to the promising device performance (PCPDTBT/PCBM:4.2%, PBDTTT-C/PCBM:6.1%) which extends the current knowledge and provides a rational guild for the polymer solar cell processing.

Graphene aerogels were prepared following an organic sol-gel process developed by Worsley et al. [Ref. 1] with some minor modifications. Under preferred reaction conditions, the graphene aerogels obtained were with a high specific surface area of 814 m2/g and a high electrical conductivity of 850 S/m, and were applied as a counter electrode material for dye-sensitized solar cells. The performance of the graphene aerogel in dye sensitized solar cells was found to be dependent on its thickness, with thicker films offering more surface areas for the involved catalytic reaction but at the same time increasing the charge transport resistance. At a graphene aerogel thickness of 2.9 μm, a power conversion efficiency of 5.96% was achieved, which was 96% of that obtained with the Pt based counter electrode. From the electrochemical impedance spectroscopy analysis, the resistance associated with the counter electrode was 1.69 ohm for the graphene aerogel case and 3.09 ohm for the Pt case, indicating the excellent electrode characteristics of the present graphene aerogel for DSSC applications. Ref. 1: M.A. Worsley, P.J. Pauzauskie, T.Y. Olson, J. Biener, J.H. Satcher, Jr., T.F. Baumann, J. Am. Chem. Soc., 132, 14067 (2010).

Dye sensitized solar cell (DSSC) are the third generation solar cells, which are expected to outperform the first two generations of solar cells with its advantages of comparable higher efficiency and lower cost of manufacturing. The photo conversion efficiency of DSSC is highly dependent on the roughness of counter electrode as well as transparency of working electrode. The research work presented in this paper focuses on applicability of SWCNT (Single-Walled carbon Nanotubes) in the working electrode and using the sputtered platinum counter electrode to improve surface roughness of the electrode. In addition, modifying the existing DSSC sealing process resulted in improved efficiencies in the range of 9.5 to 11.8%. The TiO2 layer thickness on the working electrode of the cell was optimized to 40 μm with nanoparticle size of 20 nm. The DSSCs with improved sealing process showed higher open circuit voltage (0.68-0.74 V) and short circuit current (25-35 mA/cm2) compared to our standard procedure. Additionally, cell stability is also improved by minimizing the electrolyte evaporation. This improved process enhanced the repeatability of the cell fabrication with higher yield around 90%. Since the DSSC efficiency is highly dependent on the surface roughness of the counter electrode, is the reason for using the sputtered platinum counter electrode in this study. However, the cell fabricated with the sputtered platinum showed three times higher series resistance compared to the standard cell and consequently degraded the cell efficiency based on the EIS (Electrochemical Impedance Spectroscopy) data. The possible explanation for this degradation could be caused by damaged conductive surface of the electrode due to heavy bombardment of the platinum ions during the sputtering process. The aim behind integration of the Single wall carbon nanotubes into TiO2 coating on the working electrode is to enhance surface area of the working electrode to achieve better absorption of the dye and subsequently increase the photocurrent generated by the cell to achieve higher cell efficiency. The integration of SWCNTs into TiO2 layer was carried out using different weight percent of 0.1%, 0.2% and 0.4% into the TiO2 layer. Comparing the efficiency of DSSC for different weight percent, 0.1 wt. % showed better performance in terms of efficiency, while 0.2wt% and 0.4 wt% showed gradual decrease in efficiency. It was observed that both open circuit voltage and short circuit current were found to have measurable dependence on the TiO2 layer loading. Open circuit voltage ranged from ~0.73 V to ~0.43 V and correspondingly short circuit current ranged from ~8 to ~33 mA depending on weight percent loading. Based on the cell impedance plots it was observed that with increased SWCNT loading, the series resistance increased and consequently decreased the cell efficiency.

Organic photovoltaic (OPV) cells are attractive devices due to their potential to offer low-cost power sources, not only as solar energy converters but also as energy harvesters in low-irradiance indoor environments. Herein, we report on OPV cells suitable for indoor light sources such as fluorescent lamps. The active layers of the OPV cells are bulk heterojunction (BHJ) films comprising poly [N-9â?-hepta-decanyl-2,7-carbazole-alt-5,5- (4â?T,7â?T-di-2-thienyl-2â?T,1â?T,3â?T-benzothiadiazole)] (PCDTBT) and PC71BM. The relatively deep HOMO energy of PCDTBT easily enables a high open-circuit voltage, Voc. Additionally, BHJ film comprising PCDTBT and PC71BM indicates high incident photon-to-current efficiency and flat spectra over wavelengths of 400 â?" 650 nm [1]. The latter feature is especially suitable for indoor lights, since mostly the larger parts of the spectral distribution fall into the visible irradiance range. We found that in PCDTBT : PC71BM BHJ film fabricated on MoO3 buffer with vertical composition gradient, the relative composition of PCDTBT is higher at the bottom, enabling high short-circuit current Jsc and fill factor FF under fluorescent lamps. The power conversion efficiency of the above-mentioned PV cell is 13% with FF = 0.61, Voc = 0.74 V and Jsc = 25 μA cm-2, under irradiation of 380 lux (88 μW cm-2 as irradiance) with a commercially available white-color fluorescent lamp. As a Jsc value under monochromatic irradiation (λ = 532 nm, 20 mW cm-2), 6.0 mA cm-2 has been reported as being a 95% internal quantum efficiency device using PCDTBT: PC71BM active layer [1]. The above-mentioned Jsc of our OPV cell is comparable to 27 μA cm-2 of the converted value of the referenced device at λ = 532 nm and 88 μW cm-2. This result thus indicates excellent suitability of our OPV cells for fluorescent lamps. [1] Sung Heum Park, et al. Nature Photonics, 3, 297 (2009).

In our previous paper, enhanced performance was obtained in dye-sensitized solar cells (DSSCs) with nanoporous TiO2 photoanodes post-annealed by 11200 shots of 80 mJ/cm² KrF excimer laser irradiations [1]. This paper reports the results of electrochemical impedance spectroscopy (EIS) analyses on the dye-sensitized solar cells with KrF laser irradiated nanoporous TiO₂ photoanodes. The nanoporous TiO₂ paste was administered onto a fluorine-doped tin oxide (FTO) glass by the screen-printing technique and subsequently calcined at 510°C for 15 min. The coating procedure was repeated three times to thicken the coating to approximately 9 μm. After calcination, 1000 pulses of KrF laser irradiation (λ = 248 nm) at various fluence were applied to the TiO₂ layers, which were then assembled into DSSCs. The charge transport processes were analyzed by EIS. The exact resistance, capacitance and electron recombination lifetime were extracted from the Nyquist plots. Raising the laser fluence from 0 to 160 mJ/cm² resulted in increase of open circuit voltage, implying that the surface states of the porous TiO₂ were modified by the laser irradiation. The cell efficiency was strongly correlated with the photocurrent of the cell, which increased first and then decreased as the laser fluence increased. Similar trend was observed in the carrier lifetime estimated from the Nyquist plot. The preliminary results showed that the enhancement of cell performance might attribute to both the increase of light trapping by the textured surface and the reduction of electron recombination in the laser-irradiated TiO₂ photoanode. Further results will be reported in the symposium. [1] M.-Y. Pu and J. Z. Chen , â?oImproved performance of dye-sensitized solar cells with laser-textured nanoporous TiO₂ photoanodes,â? Material Letters, vol. 66, pp. 162-164, 2011.

Dye-sensitized solar cells (DSSCs), which are one of the most potential photovoltaic devices, have attracted considerable attention as an alternative platform to the future renewable energy production owing to their high solar energy conversion efficiency as well as relatively low fabrication cost and simple process. Power conversion efficiencies exceeding 11% have been attained in DSSCs based on semiconductor nanoparticles (NPs) networks. To date, diverse research efforts to improve the overall conversion efficiency have been intensively taken, including; 1) the rational design of sensitizers for increasing the light harvesting ranging from visible to near IR, 2) optimization of semiconducting titanium dioxide (TiO₂ ) nanostructures, which strongly depends on their dimensional (e.g., size and shape) and morphological features, for facial dye loading and electron injection into the conduction band and electrode with a quantum yield of unit, 3) the utilization of electrolytes with suitable ground- and excited- state redox potential for effective hole transport, 4) the replacement of Pt counter electrodes with less-expensive and electro-chemically stable elements, 5) extending to a tandem cell for improving the spectral response of solar cells, 6) finally, guarantee of long-term stability of device performance. Recently, unconventional approaches for enhancing performance have been actively demonstrated. An increase in the charge carrier generation can be achieved by introducing novel materials such as metal NPs, periodic nanostructures, and carbon structures into conventional solar cells. In this work, we introduce distinctly different and creative two strategies for improving the efficiency of TiO₂ NPs based DSSCs by incorporation of tailored hybrid nanostrcutures. Firstly, carbonized TiO₂ thin layer was incorporated into at the interface either between the transparent electrode and TiO₂ NP layers or between the electrolyte and TiO₂ NP layers. Massively-ordered arrays of TiO₂ dots embedded in carbon matrix were fabricated via direct carbonization of UV-stabilized polystyrene-block -poly(4-vinylpyridine) (PS-b -P4VP) diblock copolymer films containing TiO₂ sol-gel precursors. Dye-sensitized TiO₂ NPs based solar cells containing carbon/TiO₂ thin layers exhibited remarkably enhanced overall power conversion efficiency compared with neat TiO₂ NPs based DSSCs. Secondly, various types of noble metal NPs having unique surface plasmon resonance property were anchored with TiO₂ NPs, and applied as tailor-designed metal/TiO₂ photoanodes in DSSCs. Improved performance was achieved in DSSCs coupled with shape-controlled gold nanostructures. The simple yet novel approaches presented in this work can be utilized as a generalized protocol and applied to other energy conversion devices to improve the device capability.

Dye-sensitized solar cells (DSSCs) are considered as a promising alternative to silicon-based photovoltaics due to their low cost and environmental friendliness. One way to improve the efficiency of DSSCs is to utilize localized surface plasmon resonance (LSPR) to enhance the light absorption by incorporating metallic nanoparticles in the photoanode. LSPRs exist at the surface of metallic nanoparticles (especially Ag and Au) and can enhance the surrounding electromagnetic field. In additional to the phenomenon of LSPR, the metallic nanoparticles can also act as scattering centers to enhance the light trapping. In this study, we develop two different methods to incorporate silver nanoparticles into the TiO₂ photoanodes. (1) The TiO₂ and the Ag-TiO₂ mixed pastes with Ag particle size of 20~40 nm are prepared. We then screen print the Ag-TiO₂ mixed paste as the first layer, and the TiO₂ paste as the second and third layers onto a FTO substrate. The photoanode is then annealed at 510°C for 15 minute. (2) The TiO₂ paste is screen printed as the first layer and sintered in advance. A 10 nm Ag thin film is subsequently deposited on by electron-beam evaporation and annealed to form silver nanoparticle aggregation, followed by the deposition of a 10 nm TiO₂ compact thin film as the buffer layer. Then the TiO₂ paste is screen printed twice as the second and third layers. Finally, the whole photoanode is annealed at 510°C for 15 minute to form a TiO₂/Ag/TiO₂ sandwiched structure. In the method (1), SEM images show that the silver nanoparticles are randomly distributed in the Ag-TiO₂ mixed paste. For Ag nanoparticle-embedded TiO₂ photoanodes, the optical absorption triples at the wavelength ranged from 400 to 500 nm, and remains almost double around the wavelength of 600 nm. The striking increase in optical absorption indicates that the silver nanoparticles do raise the LSPR phenomenon. The photoelectric conversion efficiency and the fill factor of the DSSCs with Ag nanoparticle-embedded TiO₂ photoanodes increase by 15% and 4%, respectively. In the method (2), we observe a notable increase in optical absorption for the TiO₂/Ag/TiO₂ sandwiched structure. However, the efficiency does not increase prominently as expected. It is probably due to the spectral overlap between the N719 dye and the LSPR. The next task is to develop a better structure for suitable LSPR wavelengths to compensate the low absorption regions of the dyes. Further results will be presented in the symposium.

Dye-sensitized solar cell is thin and possible to fabricate flexible devices. Also, it can be dyed various color so it has lots of possibilities in photovoltaic industry field. To enhance the energy conversion efficiency many efforts have been tried. In our previous research newly designed Dssc which has polyfluorene spin coated layer showed the enhanced energy conversion efficiency by conversing the band of light from uv-infrared to visible light. To use Polyfluroene in other way, we changed the method; electrospinning to insert Polyfluorene in Dssc. Polyfluorene added TiO2 precursor is electro-spun and sintered fiber grinded and mixed with the TiO2 paste. In the photo-electrode the fiber provides the path of electron instead of hopping in TiO2 nano-particle and also the ability of Polyfluorene as a light convertor appeared. To recognize if polyfluorene remains after the electro-spinning and sintering procedure we checked XRD data and to get electro-chemical information we analyzed i-v curve data. We could see the role of Polyfluorene in TiO2 nanofiber and it helped increasing the efficiency of dye sensitized solar cell.

Dye-sensitized solar cells (DSCs) have been highly spotlighted as an alternative device for conventional photovoltaic devices due to their low cost and easy fabrication. Highly ordered TiO2 nanotube arrays as a photoelectrode have higher charge collection efficiencies than a nanoparticle-based structure due to their faster charge percolation and slower recombination of electrons. Highly ordered TiO2 nanotube arrays were grown by anodic oxidation of 0.25mm-thick titanium foil. To increase the conversion efficiency of DSCs with TiO2 nanotube arrays, the several oxide thin films such as MgO and ZnO was deposited on the surface of TiO2 nanotube arrays by sol-gel method. The oxide thin films improved photovoltage and photocurrent due to increasing specific area and blocking backward electrons. The conversion efficiency of DSCs with MgO layer on 4.5µm-thick TiO2 nanotube arrays was 1.06%. It was 3 times the conversion efficiency of DSCs without oxide layer. Microstructure and phase were observed by SEM and TEM.

Highly ordered TiO2 nanotube arrays fabricated by anodization are very attractive to dye-sensitized solar cells (DSCs) and heterojunction hybrid solar cell due to their superior charge percolation and slower charge recombination. We have fabricated the heterojunction hybrid solar cell with P3HT(poly[3-hexylthiophene-2,5-diyl])-PCBM([6,6]-phenyl-C61-butyric acid methyl ester) and TiO2 nanotube arrays. Heterojunction of P3HT-PCBM was modified by new p-type dominate substance. The conversion efficiency of heterojunction hybrid solar cells with p-type dominant substance was measured. The organic materials were coated at the surface of 3µm-thick TiO2 nanotube arrays by sol-gel method. Heterojunction hybrid solar cell with new p-type dominant substance showed the improved conversion efficiency. Microstructure and phase of hybrid solar cells were observed by SEM and TEM.

Indiumâ?"tinâ?"oxide (ITO) has properties of high conductivity and transparency in the visible spectral region. Therefore, ITO is widely used as electrode in flat displays, organic photovoltaic (PV) cells and organic light emitting diodes (OLEDs). ITO is a highly degenerate n type semiconductor with a wide band gap and relatively high work function. The effect of various surface treatments and chemical modification is studied to improve PV cells and OLEDs. Surface modifications and treatments have an effect on ITO parameters such as the work function, surface roughness, carrier concentration, mobility, and surface sheet resistance, so that with appropriate surface treatment significant improvement in the solar cells and OLED performance can be achieved. In this work, we investigated the morphologies of chemical modified ITO by electron-donor organic acids which are depend on the modification time and concentration of the acids in aqueous solution. And we performed to confirm the surface roughness of chemical modified ITO using molecular dynamics simulation.

A series of spin cast, polymer:fullerene blend, thin films representative of the active layer in polymer:fullerene solar cells have been fabricated on Si substrates. The morphologies of these films have been characterised using neutron reflectivity (NR) and neutron spin-echo resolved grazing incidence scattering (SERGIS) both before and after being subjected to a series of increasingly harsh thermal annealing steps. Characterisation and analysis of these samples has highlighted various different processes that occur as the annealing conditions become increasingly harsh. The materials used in this study were P3HT and PCBM, blended with a composition ratio of 1:0.7 P3HT:PCBM which has been shown to produce optimal device performance for this material system. The NR results demonstrate that the vertical depth profile of the thin films changes quickly under mild annealing conditions. Initially (before any annealing) there is a region rich in P3HT and hence depleted of PCBM at the top surface of the film. This is undesirable because this top surface in a full device is typically coated in metal which functions as the cathode. A relative depletion of the electron conductor PCBM next to the cathode will be detrimental to device performance as it will hinder electron transport. Upon gentle annealing there is a migration of the PCBM into this depletion region. The composition depth profile of the thin film goes from being unfavourable to being a favourable, more uniform composition profile. Under these gentle annealing conditions no significant changes are observed in our SERGIS experiments. However, upon exposure to more harsh annealing conditions the SERGIS experiments do reveal significant changes in the film morphology. These changes we attribute to formation of relatively large PCBM crystallites at the exposed top surface of the film. The presence of large PCBM crystallites in the harshly annealed samples is confirmed by atomic force microscopy.

In a society concerned with environmental, economic, and geopolitical consequences of energy consumption, organic photovoltaics (OPV) are emerging as viable alternatives to traditional energy sources. In typical OPV devices, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is used as the hole transport layer. However, the use of PEDOT:PSS invites numerous problems; among them is its extremely low pH, insufficient electron blocking capability, and tendency to absorb atmospheric water; all factors which contribute to significant degradation or performance reduction of the device. An emerging alternative to PEDOT:PSS is nickel (II) oxide (NiO). Non-stoichiometric NiO is a wide bandgap semiconductor with natural p-type conduction from the formation of Ni2+ vacancies and accompanying charge-balance holes. Recently, solution-processed NiO (sNiO) from a protected chemical recipe was employed in high efficiency polymer solar cells (PSCs), but needed high temperatures above 250 oC, making it incompatible with plastic substrates commonly used in roll-to-roll (R2R) processing. In this work, we demonstrate a novel approach to fabricating sNiO from commercially available precursors, available at temperatures compatible with R2R plastic substrates. We then compare devices fabricated from this approach to ones from high temperature processing or with PEDOT:PSS. With high temperature NiO processing, combined with a dithieonogermole-based donor-acceptor copolymer, we fabricated PSCs with average PCE of 7.3% - a 22% improvement over PEDOT:PSS for the highest performing NiO devices. Unencapsulated NiO-based devices show a 300% improvement over PEDOT:PSS-based devices in the maintenance of >50% of the original PCE. Utilizing our low temperature technique, we fabricated high efficiency SCs with comparable efficiency to PEDOT:PSS-based devices with a PCE of 6%. Detailed studies on the chemical composition and physical properties of the NiO thin films will be discussed along with their impact on the device performance. As a final demonstration of this robust low temperature technique, devices fabricated on R2R-compatible flexible substrates will be presented.

have succeeded in the synthesis, optimization and characterization of a new genre of photoanode to enhance the efficiency of DSSC. Since 1993 the efficiency of these cells has undergone a slow improvement from 10% to 11.5%. Kinetics and dynamics study of charge transport between different interfaces revealed that large diffusion length and small diffusion coefficient of TiO2 is responsible for the slow charge transport. To overcome this bottleneck we have prepared a porous, electrically conductive and optically transparent nanocrystalline antimony doped tin oxide to function as an integrated charge collecting photo anode. The short charge transfer diffusion length combined with the multiple light scattering properties of the new electrode is envisioned to improve the overall efficiency of the DSSC, a pivotal advance towards low-cost photovoltaic cells for commercial applications.

Dye-sensitized solar cells (DSSCs) have attracted great interest as an alternative to inorganic solar cell because of low cost and high conversion efficiency. Particularly, the DSSCs with liquid electrolytes show the solar-to-electricity conversion efficiency up to 11.5%. Despite such high efficiency of the DSSCs, the high-voltage applications for various electronic devices are restricted due to the difficulty of the serial integration of individual solar cells into a single substrate. In order to achieve the high-voltage in a small device area, a low-temperature patterning method with high-resolution is required for patterning a TiO₂ layer which is usually processed at a high temperature of 450°C. In addition, the DSSCs with liquid electrolytes cannot be easily integrated into an array due to confinement problems resulting from the leakage and the evaporation of the liquid. Here, we fabricated high-voltage DSSCs using a new type of a patterning technique that can be used for simultaneously patterning both the TiO₂ layer and the solid polymer electrolyte at a low temperature of about 120°C. A patterned fluorous-polymer on a substrate was used as a sacrificial layer to lift-off the unnecessary regions of the TiO₂ and solid polymer electrolyte layers. An example of a high-voltage source constructed with an array of the DSSCs for the operation of a liquid crystal display was shown. A monolithic integration of electronic devices together with a power source provides a versatile platform to devise a variety of compact electric and optoelectronic systems. Acknowledgements This research was supported by the Ministry of Knowledge Economy of Korea through the Information Technology Research Center program (NIPA-2011-C1090-1121-0004) supervised by the National IT Industry Promotion Agency.

Solution processable technologies have the potential to lower the production cost of solar cells by eliminating need for vacuum processing. In this study, we explore the applications of a solution processable TiOx thin film of few tens of nanometer for both organic and inorganic solar cells. In our previous study, the photochemically activated protection mechanism has been demonstrated for TiOx as spacer and protection layer in organic solar cells (OSCs). However, the resistance of such films is still high and may affect the conversion efficiency. Hence, we explore the applications of this novel TiOx thin films doped with Cs2CO3 or other dopants for OSC applications. We report the variation of electrical resistivity of TiOx material with different Cs2CO3 doping concentration, annealing temperature, etc. We also report the effect of doped TiOx films on OSC device parameters like efficiency, open circuit voltage, short circuit current and fill factor. Finally, we report the results of TiOx thin films as anti-reflection coating (ARC) and surface passivation layers for silicon solar cells. Surface passivation quality is evaluated by near-surface lifetime technique and we have observed a maximum value of 546 μs for n-type Cz-Si wafers. Silicon solar cells fabricated with TiOx thin film for ARC and surface passivation showed an efficiency of 15.7 %.

Titanium dioxide (TiO2) powders were deposited on fluorine doped tin oxide (FTO) glass using Nano Particle Deposition System (NPDS) for the fabrication of DSSC. Conventionally, TiO2 layer for DSSC has been fabricated using paste type by applying doctor blade and screen printing method. However, using those methods, TiO2 powders are mixed with several binders and solutions. Moreover, during thermal sintering process, cracks are induced and this thermal treatment can not be applied to flexible substrate. To overcome these problems, Laser-assisted NPDS (LaNPDS) was used to deposit nano particles on flexible substrate by directly depositing TiO2 powders on FTO substrate using NPDS followed by laser treatment. NPDS is a novel method to deposit both metallic and ceramic nano particles by means of accelerating particles to supersonic velocity. Moreover, since powders can be deposited at supersonic velocity at room temperature, various substrates can be used. 15-nm sized TiO2 powders on FTO substrate were deposited via NPDS. Here, TiO2 powders affected by UV laser are directly sprayed on substrate through supersonic nozzle. LaNPDS was used to minimize thermal damage so that flexible substrate can be used for deposition. For comparison, thermal sintering process at 500 °C was done for 1 hr to compare with the sample fabricated by LaNPDS. Solar simulator for DSSC was used to measure cell conversion efficiency. And impedance of each cell was measured through electrochemical impedance spectroscopy (EIS). As a result, photovoltaic efficiency of DSSC cell treated using LaNPDS showed about 4.2% while another DSSC cell sintered thermally showed about 4.0%. Therefore, DSSC fabricated by LaNPDS showed promising results to replace thermal sintering method.

Since the first introduction of the dye sensitized solar cell (DSSC), lots of strategies have been tried to increase its efficiency, e.g., developing new dyes or optimizing nanostructures. A different approach is to insert optical elements tino DSSCâ?Ts for enhanced light absorption. However, due to the complex multistep processes required to form an inverse opal photonic crystal layer on the DSSCâ?Ts, it has been challenging to clarify photonic crystal effects on the efficiency of DSSCâ?Ts. In this study, a simple 2-D surface patterning is proposed to form a photonic crystal structure on a DSSC surface with reliable continuity. Mesoporous patterned oxide coatings were fabricated into a photonic crystal structure in order to accommodate more dye catalyst while enhancing the absorption of photons. A soft fluorocur (PFPE) mold was adopted to make 2-D patterns on the surface of titanium oxide and niobium oxide. Self-assembly of latex beads was also used to form a 3-D inverse opal mesoporous photonic crystal film. The mesoporous patterned oxide electrodes revealed different absorption and transmission spectra relative to non-patterned references that depend on the geometry of the surface patterns. Incident-photon-to-current conversion efficiency (IPCE) and global efficiency of planar and patterned mesoporous electrodes were compared. Optical simulations were performed to model and optimize the effects of 2-D surface patterning.

In the past decade, dye-sensitized solar cells based on nano-porous TiO2 and quasi-solid polymer (or gel) electrolytes have drawn the interest of many research groups. These cells have become potential, low cost alternatives to conventional inorganic photovoltaic devices. However, a major drawback of these solar cells with gel electrolytes is their relatively low power conversion efficiencies compared to their liquid electrolyte counterparts. Several attempts have been made to enhance the efficiencies of these cells. Using gel electrolytes with polymer hybrids, incorporating better plasticizers, optimizing the electrolyte compositions and incorporating ionic liquids are among some of these methods. In this paper, we report the effect of using a mixed iodide salt system with two cations to enhance the power conversion efficiency of dye sensitized solar cells made with polyacrylonitrile (PAN) based gel electrolyte and nano-porous TiO2 electrode. Instead of a single iodide salt, a mixture of two salts, namely MgI2 and tetrapropylammonium iodide (Pr4NI) were used to provide iodide ion conductivity. Solar cells of configuration Glass/FTO/TiO2/Dye/electrolyte/Pt/FTO/glass were fabricated using nano-porous TiO2 electrode sensitized with Ruthenium dye (N719). The composition of the gel electrolyte was: PAN(10.36 wt %), EC (41.43 wt%), PC (41.43 wt%), salt (6.21 wt%) and I2(0.57wt%). With identical electrolyte compositions, the cell with MgI2 alone gave an efficiency of 2.5 % and the cell with Pr4NI alone gave an efficiency of 3.3 %. The cell with the mixed iodide system, MgI2: Pr4NI = 36:64(molar ratio) however, showed an enhanced efficiency of 3.7 % with a short circuit current density (Jsc) of 8.61 mA cm-2, open circuit voltage (Voc) of 743 mV and a fill factor of 58.2 %.

Dye sensitized solar cell (DSC) also called Grätzel cell, represents an attractive alternative to solid state photovoltaics due to high efficiency, low cost and ease of fabrication. The generic device is a photoelectrochemical DSC. Its key components are dye-sensitized TiO2 photoanode, electrolyte solution with a redox mediator and the cathode material. The latter is typically a film of Pt nanoparticles on F-doped SnO2 (Pt-FTO) and the former is the I3-/I- in aprotic electrolyte medium. Graphene nanoplatelets (GNP) in the form of optically transparent thin films on FTO are useful as cathode material to replace platinum. They exhibit good electrocatalytic activity towards I3-/I-, particularly in electrolyte based on ionic liquids. Semitransparent (>85%) film of graphene nanoplatelets presented no barrier to drain photocurrents at 1 Sun illumination and potentials between 0 to ca. 0.3 V. Recent progress in the field has been highlighted by replacing the traditional I3-/I- mediator by organic or organometalic couples with more positive redox potentials. The obvious motivation consists in enhancing the voltage of DSC. GNP exhibit high electrocatalytic activity for a mediator Co(L)2; where L is 6-(1H-pyrazol-1-yl)-2,2â?T-bipyridine. The exchange current densities for the Co2+/3+(L)2 redox reaction scaled linearly with the GNP filmâ?Ts optical absorbance, and they were by 1-2 orders of magnitude larger than those for the I3-/I- couple on the same electrode. The electrocatalytic activity of GNP films with optical transmission below 88% is outperforming the activity of Pt-FTO for the Co2+/3+(L)2 redox reaction. Dye-sensitized solar cells achieved energy conversion efficiencies between 8 to 10 % for both GNP and Pt-based cathodes. However, the cell with GNP cathode is superior to that with Pt-FTO cathode particularly in fill factors and in the efficiency at higher illumination intensities. In both cases, open circuit voltage exceeding 1 V is accessible, which represents a clear challenge for optimization of DSC.

Dye-sensitized solar cells (DSSCs) are part of photoelectrochemical cells, which are able to convert solar light into electric energy. DSSCs are expected to be used in mobile applications for which inexpensive, non rare and non toxic materials are required. The current optimized DSSC structure was invented in 1991 by Grätzel and Oâ?TReagan [1] through the use of a nanoporous TiO2-based photoanode having a high specific surface area. This type of DSSCs can achieve conversion efficiency up to 11% [2]. Owing to its very similar energy band structure and its high electronic mobility, ZnO has been extensively explored as an alternative to TiO2. However, the best conversion efficiency for ZnO does not exceed so far 7%. ZnO is regardless promising due to its ability to grow in a wide variety of nanostructures presenting unique properties for optics and electronics. In this paper, the potentiality of ZnO nanocomposite photoanodes is investigated [3, 4] by hybridizing ZnO nanowires (NWs) with ZnO nanoparticles (NPs). The key point is to combine a large specific surface area as given by NPs with direct conduction paths as allowed by NWs. Our work focuses on two different aspects. First, we present a simple and low-cost wet chemical route to grow nanocomposite ZnO photoanodes on transparent electrodes. The analysis of the structural and optical properties of such ZnO nanostructures is carried out and thoroughly discussed. In particular, the beneficial effects of thermal heat treatments are investigated in terms of ZnO NPs formation and of the interface properties with the dye. Second, these nanocomposite ZnO photoanodes are integrated in DSSCs, from which typical short-circuit photocurrent density of more than 10 mA/cm2 and solar energy conversion efficiency closed to 5% are reached. The DSSC photovoltaic performances are also determined by impedance spectroscopy measurements. [1] B. Oâ?TReagan & M. Grätzel, Nature, 353, 737 (1991) [2] Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide and L. Han, â?oDye-sensitized solar cells with conversion efficiency of 11,1%â?, Japanese Journal of Applied Physics, 45, 638 (2006) [3] J. Baxter & E.S. Aydil, Solar Energy Materials & Solar Cells 90, 607 (2006) [4] V.-M. Guerin & T. Pauporte, Energy & Environmental Science 4, 2971 (2011)

Comparison between graphene oxide (GO) and reduced graphene oxide (rGO) as hole extraction layers (HEL) in organic photovoltaic (OPV) cells with poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester were performed. Hydrazine hydrate (HYD) and thermal method (Thermal) were adopted to change GO to rGO. The GO HEL was deposited on indium tin oxide electrode by spin coating and followed by a reduction process to form rGO HELs. The reduction processes were confirmed by x-ray diffraction, Raman spectroscopy, x-ray photoemission spectroscopy, transmittance, and 2-point probe method. The OPV cell with GO (~ 3 nm) HEL exhibits increased power conversion efficiency (PCE) as high as 2.5 % under 100 mW/cm2 illumination with an air mass condition, which is higher than PCE of OPV cell without HEL, 1.78 %. However, PCE improvement of OPV cell with rGO HEL is not high as 1.8 % for HYD-rGO and 1.9 % for Thermal-rGO. Ultraviolet photoemission spectroscopy results showed that the work function of GO was 4.6 eV, but that of HYD-rGO and Thermal-rGO were 3.2 eV and 3.4 eV. Therefore, it is considered that GO is adequate to extract the holes from the active layer, but HYD-rGO and Thermal-rGO are not appropriate to HEL in OPV cells in the point of energy alignment. [Acknowledgements] This research was supported in part by a Seoul R&BD program (ST10004M093171) and in part by Basic Science Research Program and (2011-0008994) and Mid-career Research Program (2011-0028752) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology.

Bulk heterojunction inverted OSCs (BHJ IOSCs) have been intensively studied due to their high potential interface stability, which results from the replacement of the low work function metallic cathode with an air-stable, high work function metallic anode (e.g., Au, Ag) and the introduction of n-type metal oxide semiconductors as a desirable path that can effectively transport electrons from the polymer layer to the cathode electrode. And graphene is one of the most promising next-generation transparent conducting materials for replacing traditional transparent conducting oxides (TCOs) such as indium tin oxide (ITO) and fluorine tin oxide (FTO) in optoelectronic devices including solar cells and light-emitting diodes. To the best of our knowledge, no reports have described graphene cathode-based flexible BHJ IOSCs, possibly due to technical difficulties associated with the low-temperature direct growth of high quality n-type metal oxide semiconductors on graphene/plastic substrates. Numerous efforts have been made to prepare high-quality metal oxide such as ZnO and TiOx, thin films or nanostructures on graphene using various methods including sol-gel, aqueous solutions, metal organic chemical vapor deposition (CVD), sputtering, and atomic layer deposition (ALD) with a plasma surface treatment, a seeding layer, or an interfacial self-assembled monolayer. However, problems such as plasma damage to graphene, non-uniform formation of metal oxides, and poor contact between metal oxides and graphene still have not been solved due to the hydrophobic and chemically inert nature of the basal plane of graphene. Here we report on the first demonstration of the direct growth of ZnO thin films as an electron transport layer on graphene transferred onto plastic substrates using a mist pyrolysis (MPCVD) process based on a non-vacuum process at the low temperature of 160 °C. ZnO thin films with no damage to the structural and electrical properties of graphene were successfully grown on graphene. The MPCVD-grown ZnO thin films revealed uniform surface morphology, high crystallinity, and high transparency. In addition, we successfully realized flexible bulk heterojunction inverted organic solar cells using the MPCVD-grown ZnO thin film with superior interfacial properties on the flexible graphene cathode.

Silicon nanowires (SiNWs) combined with a conducting polymer are studied to constitute a hybrid organic / inorganic solar cell. This type of cell shows a particularly high interfacial area between SiNWs and the polymer so that interfacial control and interface optimization are required. For that purpose we terminated the SiNW surfaces with well selected functional groups (molecules) such as native oxide (hereinafter SiO2-SiNW), hydrogen (hereinafter H-SiNW) and methyl (hereinafter CH3-SiNW). A radial heterojunction solar cell is formed and the cell parameters with and without interface control by functionalization with molecules are compared. Electronically, the three surfaces were close to the flat-band condition. The CH3-SiNW, H-SiNW and SiO2-SiNW produced a surface dipole of -0.12 eV, +0.07 eV and 0.2 eV and band bending of 50meV, 100 meV and 170meV respectively. The surface properties of functionalized SiNWs are investigated by photoelectron yield (PY) and photoemission spectroscopy. PY studies on functionalized SiNWs are presented for the first time and our results show that this type of measurement is an excellent option to carry out interface optimization of NWs for envisaged nano-electronic and photonic applications. The PY and thus the solar cell efficiency are increased dramatically after terminating the surface with CH3 molecules due to a decrease of the surface state density. The differently functionalized SiNW surfaces showed identical absorbance, reflectance and transmission so that an increase in PY can be attributed to the Si-C bonds at the surfaces. This finding permits the design of new solar cell concepts.

Spiro-MeOTAD(2, 2â?T,7,7â?T-tetrakis(N,N-di-p methoxyphenylamine)-9,9â?T-spirobifluorene) is widely used as the hole conductor in solid-state dye-sensitized solar cells (ss-DSSC). While spiro-MeOTAD remains the material of choice for obtaining high efficiency in ss-DSCSs, lower cost alternatives to spiro-MeOTAD are needed. Polymers are attractive as solid state conductors, but unlike small molecules, the large size of high molecular weight polymers prevents complete pore-filling in photoanodes. Here we report a triarylamine-based polymer for ss-DSSCs, which is transparent in the visible, has a proper energy level, and has good conductivity. We also describe an in-situ polymerization method that overcomes incomplete pore-filing, which should enable high photon-to-electron conversion.

Two new methods for measuring the rate of adsorption of a sensitizing dye to TiO₂ for the production of dye-sensitized solar cells (DSC) are introduced. When a TiO₂ film is immersed in a dye solution it will adsorb the dye molecules over time. The time required to achieve maximum sensitization is a key parameter in the continuous manufacture of DSC. The ability to easily quantify this parameter is essential in developing more efficient and rapid sensitizing techniques and is aligned to the scalability of DSC technology. The traditional method for measuring dye adsorption is to desorb the dye at set intervals into a solution that can be measured using UV-Vis, which is a laborious process. The first method presented here entails visually monitoring the underside of a TiO₂ film using time lapse photography and subsequently performing image analysis to extract the red, green and blue (RGB) values of the TiO₂ film enabling rapid quantification of dye uptake over periods from seconds to hours. These can then be plotted against time to provide data on time evolved film colouration. The second method uses UV-Visible reflectance spectroscopy to analysis the TiO₂ surface during dyeing to derive highly resolved wavelength data. This method is important as DSC co-sensitisation becomes increasingly prevalent since it allows simultaneous kinetic data to be obtained for cocktail dyes (multiple dyes used to maximise photon capture). Both these methods have been shown to follow the same adsorption kinetics as traditional dye desorption experiments. The key difference however is that the new methods allow accurate analysis of fast dyeing processes. These are particularly important in order to maximise manufacturing effectiveness by dyeing for the shortest period possible.

Well-aligned M@TiO2 (M = Ag, Au, Pt) nanocomposite nanorod arrays with high aspect ratio have been grown on FTO substrate by hydrothermal process. Metal nanoparticles were loaded onto the surface of TiO2 nanorod via Rapid thermal analysis and a deposition approach based on a photochemical reduction process under ultraviolet irradiation. Metal nanoparticles can be obtained in different size-morphological regimes as a function of the irradiation time, due to light-induced photo fragmentation and ripening processes. Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) were applied to characterize the as-prepared M/TiO2 nanorod composites. The photoelectrochemical experiments were performed in a sandwich type two-electrode cell and Iâ?"V curves were obtained. The presence of metallic nanoparticle can help the electron-hole separation by attracting photoelectrons. Result of solar cell testing showed that addition of metal nano particle to the TiO2 nanorod significantly increased the JSC (short circut current density), VOC (open circuit Voltage), fill factor and efficiency relative to devices without metal nano particle. The results demonstrated that it is possible to fabricate M/TiO2 cells of higher efficiency by using nanorod arrays and other morphologies with increased surface area.

In this presentation, we demonstrate the fabrication of dye sensitized solar cells (DSCs) using differently processed multiwall carbon nanotube (MWNT) sheets as a counter electrode (CE) with catalytic activity optimized by layering and/or UV treatment as an alternative to platinum CE deposited on fluorinated tin oxide (FTO). The sheets are drawn directly from an especially CVD synthesized highly aligned forest of MWNTs grown on silicon. We used 10, 15 and 20 layers of the oriented MWNT sheet on the FTO-free glass and for comparison also on the FTO coated glass. Cell performance was found to vary with the number of MWNT sheets with optimal results given by 20 layers for UV non-treated CE and twice smaller for UV-ozone modified case . The device (glass/FTO) showed an operating current density of 17.6 mA/cm2 / 12.7 mA/cm2, an open circuit voltage of 670 mV / 700mV, and fill factor of 0.56 / 0.63 with a power conversion efficiency of 6.6% / 5.66%. The relatively high obtained efficiency of DSCs (6.6%) cell is determined by the high generated photocurrent, which is comparable to the reference DSCs made by same method using the standard Pt catalyst on FTO CE. We also demonstrate the UV treatment improves charge transfer resistance (Rct) of CE while deteriorating Rsh of single layers. Thus the optimal multilayering and UV-ozone effect are found to optimally enhance DSCs performance by decreasing both resistances. This work was supported through the Rice/AFRL grant of CONTACT of Texas and the Welch Foundation grant AT-1617. We acknowledge the help with MWNT forests made in Chi Huynh of the CSIRO, Australia.

Nanocrystalline solids have become the subject of intense study due to their unique optical properties and their capacity to form self-assembled superlattices. These properties make them suitable for use in a variety of applications such as solar cells, light emitting devices, transistors, etc. The self-assembly of these nanoparticles is governed by interactions at the molecular level, and hence, understanding the nature of these interactions could be instrumental in achieving precise control over the self-assembly process. The nanocrystals are "capped" by organic molecules which are believed to drive the self-assembly and stabilize the final superlattice and which are responsible for most of the interparticle forces. Despite the importance of the ligand-ligand interactions, there is very little fundamental understanding of these hybrid systems. In our work, we use atomically and molecularly explicit Molecular Dynamics simulations to understand the interactions between the ligands on the surface of the nanocrystals. Specifically, we observe the energetic interactions of experimentally sized nanocrystals capped with ligands in various lattices including face-centered cubic (fcc) and body-centered cubic (bcc) structures in collaboration with the Hanrath group at Cornell. These results form the first report of computer simulations of experimentally relevant sized ligand-capped nanocrystals (3-6 nm) in contrast to prior simulation work in literature. Our results show the clear dependence of nanocrystal interactions on the ratio of ligand length to nanocrystal size. Our simulations have also shown that these interactions are dependent on key parameters like ligand grafting density and ligand coverage on various facets of the nanocrystals which ultimately govern the self assembly process. We are also investigating the orientational alignment of nanocrystals in superlattices and the effect of local asymmetries in ligand density on the selection of preferred morphologies. The alignment of nanocrystals in the superlattice is primarily due to the orientations of the capping ligands which is governed by the ligand coverage on nanocrystal facets. And this anisotropy of ligand coverage is responsible for orientational alignment of the nanocrystals, which, in turn, drives superlattice symmetry. We have also undertaken simulations to examine energetic interactions between nanocrystal bilayers under rotational and translational displacement to understand the preferred alignment patterns of one layer over another. We are also investigating the effect of shape and size of nanocrystals on electronic energy states. This kind of study provides key insights into molecular-scale information about ligand interactions which can be leveraged into more coarse-grained mesoscale simulations of nucleation and growth phenomena which may to be used to predict and guide future experimental studies.

Charge transport layers have been shown to improve the performance of bulk heterojunction organic solar cells. Various mechanisms have been proposed for the improved performance, including band bending due to the Fermi level offset between the transport layer and the active layer, reduction of recombination at the electrode by selective blocking of carriers, and formation of Ohmic contacts. However, other than changing the electrode work function, the exact mechanism(s) for the improved performance remains unknown at this point. To study the transport layer effect on device performance, we utilize an inverted poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction organic solar cell with hole-collecting metal electrodes of different work functions as a model system and compare the impact of several frequently used hole transport layers (HTLs) inserted between the active layer and the electrode, including poly(ethylenedioxythiohene):polystyrene sulfonate (PEDOT:PSS) and thermally evaporated MoO3. We also fabricate room temperature processed hole transport layers by spin coating thin films of MoO3 and WO3 nanoparticles synthesized using microwave-assisted solvothermal reaction. We find that devices without a HTL exhibit an anomalous spectral change in the external quantum efficiency (EQE) spectra as a function of the light bias illumination intensity, which depends on the metal hole-collecting electrode work function. We also find a sub-linear dependence of the open-circuit current density (Jsc) versus illumination intensity for devices without an HTL. The insertion of a HTL eliminates the spectral change in EQE and restores the linearity between Jsc and illumination intensity. We explain these observations in terms of the effects of the HTL on the metal work function determined using Kelvin probe measurement, on the mobility and carrier concentration within the active layer, and on the recombination rate at the hole-collecting electrode interface.

Recently, Si nanowire array hybrid solar cell has attracted a lot of attention due to their simple and lower processing cost and the combined advantage of efficient light harvesting and carrier collection in Si wire arrays. However, the high surface area of the wires and resulting interfacial recombination of charge carriers is a primary limit on device performance. In this report, we explore nanowire arrays produced on n-type Si wafers through Ag-assisted chemical etching. Following published approaches., this technique creates vertical arrays of wires approximately 300nm in diameter and with lengths than can easily be varied depending on etch time. In Field Emission Scanning Electron Microscopy (FESEM) studies the wire surfaces exhibit a micro to nano roughness which increases the surface area. Such high surface area is undesirable for low surface recombination. To smooth the surfaces, the wires were thermally oxidized at 1100 deg in flowing O2 followed by an HF dipping. FESEM cross sectional imaging and Electron Paramagnetic Resonance (EPR) spectra, show this treatment effectively removes the micro-roughness and reduces the surface defect (dangling bond) density. Devices were prepared from smoothed and as-etched wire arrays by intercalating a PEDOT:PSS conducting organic layer into the arrays to create both a conformal heterojunction interface and contact. Compared to as-grown Si wire devicse, the smoothed arrays had higher short-circuit current density (JSC ) but lower open-circuit voltage (VOC). We attribute these changes to a reduction in charge carrier recombination at the smoothed surface in conjunction with diffusion of residual Ag from the etching process into the wires during thermal treatment. The later is detected in infrared photoluminescence (PL) measurements and confirmed. This work was support by the NSF sponsored Renewable Energy Materials Research Science and Engineering Center.

Thin films of electronically coupled nanoparticles have a great potential for use in optoelectronic devices due to their size-tunable bandgap and ligand-tunable thermal and electronic transport properties. Great strides have been made towards the realization of this potential through the careful study of the effects surface ligand length and type on transport properties. While these advances have led to an increase in film mobilities, many optoelectronic devices also require controlled doping to implement semiconductor homojunctions. However, control of doping levels in nanoparticle films has been difficult to achieve as traditional approaches towards impurity doping are made difficult by nanoparticle self-purification processes during synthesis. Additionally, a large density of surface states leads to a diverse collection of mid-gap states which effect equilibrium carrier concentrations. One advantage of having such surface access in porous nanoparticle films is that simple chemical treatments have the ability to penetrate the film and control bulk carrier concentration values. Here we detail methods to chemically control doping concentrations in nanoparticle films using a series of organometallic dopants with known reduction / oxidation potential.

Our study investigates the effect of metal nanoparticles sandwiched in graphene as electrodes for plasmonically enhanced P3HT:PCBM bulk-heterojunction solar cells. Metal nanoparticles on graphene tune the work function, decrease the sheet resistance, and act as scattering centers for incoming light. We investigate the effect of this graphene encapsulated nanoparticle electrode on organic solar cell performance.

Hybrid approaches to solar cells offer the combined advantage of high mobilities of inorganics with tailored design of organic / polymeric materials in one device. A well designed inorganic scaffold is desired in which the organic material can be embedded. This scaffold requires on one hand, a dense inorganic layer as blocking layer at the interface to the cathode, on the other hand, rods of a high aspect ratio on top to offer a large interface to the organic material. The latter acts as a direct high mobility pathway for created charges to the cathode. Such well designed hybrid solar cells are expected to outperform fully organic photovoltaics by minimizing tortuous pathways for charges to the appropriate electrodes, typical for organic devices. In this context, we report on the morphological control of crystalline ZnO deposited by electrochemistry at low temperatures (< 100 °C). In the first part, we show a controlled lateral growth for the blocking layer by a properly timed pulsing of the growth voltage. The resulting rather neutral pH on the cathode surface leads to a more lateral oriented growth of the ZnO compared to the rod-like growth in the case of an applied dc voltage. This demonstrates, that controlling the surface pH, i. e. by pulsing the applied voltage, is a key parameter in electrodeposition of ZnO to create a dense hole blocking layer for hybrid photovoltaics. Secondly, we report on well-aligned ZnO nanorod arrays which represent direct high mobility pathways to the cathode. We show that a control of the rod spacing by varying deposition parameter might be less reliable, since the resulting rod spacing distribution is rather broad, while a guided growth through a nano-structured template results in well-aligned ZnO rods with equidistant spacing. The latter was achieved by electrodeposition through an electron beam structured PMMA template. Being able to control the distance and position of the ZnO rods with extreme accuracy by guided template growth opens the possibility to systematically study the effect of rod spacing on hybrid photovoltaic cell performance and gain fundamental understanding in this system. The high quality of the electrodeposited ZnO crystals is confirmed by SEM, Raman Spectroscopy and HR-TEM measurements. Moreover, the compatibility of the electrodeposition of ZnO with PET foil is demonstrated.

We study the evolution of structure and optoelectronic properties of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) photovoltaic thin film blends upon thermal annealing using spectroscopic ellipsometry. Four distinct processes are identified: the evaporation of residual solvent above the glass transition temperature of the blend, the relaxation of non-equilibrium molecular conformation formed through spin-casting, the crystallization of both P3HT and PCBM components and the phase separation of the P3HT and PCBM domains. Devices annealed at 150 oC for between 10 and 60 mins exhibit an average power conversion efficiency of around 4.0%. We find that the rate at which the P3HT/PCBM is returned to room temperature is important in determining device efficiency. The rapid quenching of a solar device from the annealing temperature to room temperature hampers the crystallization of the P3HT and can trap non-equilibrium morphological states. Such states apparently impact on device short circuit current, fill factor and thus operational efficiency.

As components of the solar energy harvesting and conversion devices, nanostructured assemblies with well-defined geometrical shapes have emerged as high efficiency and economically viable alternate over planar junction thin film architectures. However, fabrication of inorganic nanostructures generally requires complicated and multiple step processing techniques, making them less suitable for large-scale manufacturing. By exploiting phase-separated self-assembly, we present a new approach towards nanostructured thin film solar cells. Through a single step process, we demonstrate growth of a composite film matrix formed as self-assembled, well ordered, phase segregated, oriented p-n type interfacial nanopillars of Cu₂O and TiO₂, which are fully epitaxial and single crystalline in both phases. The composite films were structurally characterized to atomic resolution by a variety of analytical tools, and evaluated for preliminary optical properties using absorption measurements. We show nearly complete atomic order at the Cu₂O-TiO₂ interface (i.e., p-n junction), and an absorption profile that captures a wide range of the solar spectrum extending from ultraviolet to visible wavelengths. This work opens a novel avenue for development of simple and cost-effective optically active thin film architectures, and offers promise for significantly increased photovoltaic device efficiencies by orthogonalizing incident light absorption and carrier collection.

Detailed characterization of polymer based organic photovoltaic (OPV) devices has guided chemical design, leading to recent OPV efficiencies of up to 10%. As device efficiencies have climbed, the importance of lifetime and reliability in OPV systems has increased as well. For OPV to be successful in the long term, lifetimes approaching 25 years are critical. Many factors cause OPV devices to degrade. Extrinsic degradation pathways, like encapsulation packaging failure and electrode degradation, are common to many different OPV devices, and steps are being taken to improve their design. Intrinsic degradation pathways of the active polymer layer itself, however, are not as well understood. It is not currently known if they are universal or polymer specific. Characterizing intrinsic degradation pathways in high efficiency polymer OPVs is important to establish an upper limit of device lifetimes, as well as to inform chemical design to not only improve device efficiencies, but to increase OPV lifetimes. We have previously reported a lifetime approaching 7 years for OPV devices made with the high efficiency polymer poly[N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT). Characterizing the intrinsic degradation of the photoactive layer showed an initial burn-in period in which the open circuit voltage dropped substantially, and then leveled off. In depth characterization methods attributed the loss of open circuit voltage during burn-in to an increase in energetic disorder in the band gap, most likely caused by photochemical reactions in the active layer. It is not known if these effects dominate the degradation of other polymer systems. Similar characterization methods are used to investigate the intrinsic degradation pathways of high efficiency OPV devices made with a poly-benzodithiophene, thieno[3,4-c]pyrrole-4,6-dione (PBDTTPD) copolymer. Recently, the chemical moiety TPD has become a common constituent of high efficiency copolymers in OPVs, so investigating device lifetimes is important. When blended with PC61BM or PC71BM, devices with efficiencies of up to 7.3% have been produced. Like PCDTBT, initial degradation levels off after a burn-in period. However, it behaves differently than PCDTBT, in that burn-in in PBDTTPD devices is dominated by a loss in short circuit current. However, this current loss in PBDTTPD is not universal across all devices. In this discussion, we will present the methods used to determine the cause of the initial drop in short circuit current. We will then show how eliminating this effect increases the lifetimes of devices made with PBDTTPD, and discuss the upper limit of PBDTTPD device lifetimes.

Solid-state dye-sensitized solar cells (ss-DSSCs) have record power conversion efficiencies now at 6.1% [1]. Composed of abundant, cheap materials, ss-DSSCs meet the needs of large-area solar plants as well as disposable solar-powered devices. By using a solid-state organic hole-conductor, ss-DSSCs avoid the problems of liquid electrolyte stability faced by the standard DSSC design. However, recombination at the TiO2/hole-conductor interface is much higher than that with the liquid electrolyte. This limits the solid-state device active layer to 2 μm; a thickness of 10 μm is necessary to absorb all incident light. In this work, we (1) replaced the dye in ss-DSSCs with inorganic quantum dots as the active light absorber, creating solid-state quantum dot-sensitized solar cells (ss-QDSSCs). Quantum dots (QDs) show favorable absorption properties due to their size-dependent band gap in addition to higher molar extinction coefficients than commonly-used dyes. Increased absorption is critical in as the active layer thickness is limited by recombination. In addition, we (2) deposited recombination barriers of metal oxides or self-assembled organic monolayers (SAMs) at the TiO2/hole-conductor interface. The use of QDs allows for new barrier configurations not available to the dye-sensitized devices. If the SAM is deposited after the QDs, the organic molecules in the SAM will not compete with QDs for surface space on the TiO2, as can occur with the dye molecules. For the metal oxide barriers, the higher temperature stability of QDs allows these barriers to be deposited on TiO2 after the QDs. We successfully fabricated ss-QDSSCs, using CdS as a proof-of-concept material. CdS is easy to deposit by successive ion layer absorption and reaction. These devices had fill factors and open-circuit voltages comparable to ss-DSSCs, but lower short circuit current, most likely due to the non-ideal band gap CdS (2.4 eV). The deposition of CdS quantum dots was confirmed with transmission electron microscopy, x-ray photoelectron spectroscopy, and UV-vis spectroscopy. Next, we deposited SAMs of various phosphonic acids as a barrier layer. These devices showed increased open-circuit voltages, which could be due to the decreased recombination. Finally, we deposited metal oxides, such as Al2O3, by atomic layer deposition on the QD-sensitized TiO2. Device characterization showed that the barrier layer must be about a monolayer thick, or less, to avoid slowing down the current-generating charge collection. Transient photovoltage measurements were used to determine carrier lifetimes. [1]. Cai, N., et al., Nano Lett., 11, (2011): 1452.

For more than 16 years, dye sensitized solar cells (DSSC) have been under extensive research. DSSCs are especially attractive for Building Integrated Photovoltaics (BIPV). The cell concept is believed to reduce the production costs and energy payback time significantly compared to standard silicon cells or other thin film cells. The main problem to get a high conversion efficiency was that a monolayer of dye molecules on a flat surface can only absorb up to 1 % of the incident light. By introducing nanoporous TiO2 electrodes with a roughness dramatically increased the light harvesting efficiency to 11.2 % [1]. Currently, to increase the surface area, which maximize dye absorption and efficient electron transort that delivers the electron to the collection electrode without recombination, replaced the TiO2 nanoparticles with a dense network of wide-band-gap semiconductor nanowires [2]. It may be advantageous because the nanowire morphology provides direct conduction paths for the electrons from the point of injection to the collection electrode while maintaining high surface area for dye adsorption. In this study, we applied SnO2 covered ZnO nanowires hydrothermally synthesized on FTO glass for the DSSCs. We approached new method to make SnO2/ZnO nanowire composite. After the growth of ZnO nanowire hydrothermally, we covered the ZnO nanowire structure with SnO2 by using chemical vapor deposition. The DSSCs with the novel composite structures were fabricated and characterized by solar simulator. The fabricated SnO2/ZnO nanowire composite were analyzed by XRD, TEM and SEM. References 1. M. K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru, M. Gratzel, J. Am. chem. Soc., 127, 16835-16847 (2005) 2. J. B. Baxter and E. S. Aydil, Applied Physics Letters 86, 053114 (2005)

We study hybrid bulk heterojunction (BHJ) solar cells with active areas consisting of blends of low-bandgap polymers (PDTPQx, PPEHTT, and PSOTT) and IR absorbing PbS nanocrystal quantum dots. We observe significant improvment in the device performance for devices that underwent a post-film-deposition thiol treatment. The formation of long-lived photogenerated carrriers in the BHJ blends is probed via photoinduced absorption (PIA) spectroscopy and provides insight into the mechanisms behind the improved device performance. We further characterize the performance enhancements with improved local charge generation measured locally via time-resolved electrostatic force microscopy (trEFM).

Organic materials used for photovoltaic generally have poor charge-carrier mobility comparing with inorganic materials. This fact leads to a controversial issue lies in the thickness of organic absorber layer in organic solar cell: when itâ?Ts too thin, it cannot absorb the entire incident light, and on the contrary, poor charge-carrier mobility will dramatically increase the recombination rate when the film is thick. Here, we report a brand new architecture for organic solar cell to solve this problem. Meanwhile, this structure not only avoids the problem stated above, but it also avoid the electrodes reflect incident light. Besides, it can provide a roll-to-roll method which has huge commercial potential. In this work, we calculated the prime distance between adjacent "teeth" on combs and devices are fabricated and investigated.

Semiconducting polymers offer a low cost solution process for large area, light weight, and mechanically flexible organic photovoltaics (OPVs). Buckminster-fullerene C60 and its derivatives as electron acceptors in combination with P3HT or other p-type semiconducting polymers have been dominated as the highest performance nanopattern materials. However, this type of OPVs still suffers lifetime and mechanical and chemical stability problems. Our motivation was to replace the fullerene family with single-walled carbon nanotubes (SWCNTs) to improve the photoactive film stability, and to explore its nanocomposite PV response. SWCNT has high electron affinity, mechanical strength and Youngâ?Ts modules, as well as good thermal conductivities. The high electron affinity leads to a strong dissociation of excitons generated in P3HT phase, which results from the interaction of SWCNTs through its graphitic layer with this conjugated polymer according to our early research. The high electrical conductivity increases the polymer thin film conductivity by increasing the charge carrier mobility. All these advantages would improve the power conversion efficiency and mechanical properties for polymer solar cells, In this report, a novel P3HT/SWNTs donor-acceptor nanohybrid organic photovoltaic system was constructed based on the bulk heterojunction concept. CdSe quantum dots modification to this system has been performed, and the single device's PV response was studied. Comparing with that of the pristine P3HT/SWNTs device, the PV performance of the modified hybrid system is significantly improved. A dramatically quenching P3HT fluorescent emission via the nanostructures of QDs on SWNTs was observed, indicating an efficient charge/energy transfer from excited P3HT to SWNTs. CdSe QDs may functional as both a photo-sensitizer and an electron collector as well in this system. This research was partially supported by NSF grant #0520789. The corresponding author's current address: Center for Excitonics, Research Laboratory for Electronics, MIT, 77 Massachussetts Ave, Cambridge, MA 02139. Email: [email protected]

Semiconductor organic-inorganic nanocomposites were synthesized by directly grafting conjugated polymer poly(3-hexylthiophene) onto cadmium selenide nanorods surface (i.e., P3HT-CdSe NR nanocomposites). The direct grafting was accomplished by two simple yet robust coupling reactions: Heck coupling of vinyl-terminated P3HT with bromobenzylphosphonic acid functionalized CdSe NRs (i.e., BBPA-CdSe), and a newly developed catalyst-free click reaction of ethynyl-terminated P3HT with azide functionalized CdSe NRs. Such rationally designed nanocomposites possessed a well-defined interface between P3HT and CdSe NRs, thereby promoting the effective dispersion of CdSe NRs within the nanocomposites and facilitating their electronic interaction. The success of grafting was confirmed by nuclear magnetic resonance spectroscopy and dynamic light scattering. The occurrence of charge transfer at the P3HT/CdSe interface was evidenced by UV-Vis absorption, photoluminescence (PL), and time-resolved PL studies. Two coupling reactions yielded similar grafting density of nanocomposites. Notably, the nanocomposites prepared by the catalyst-free click reaction exhibited a faster charge transfer from P3HT to CdSe. To the best of our knowledge, this is the first study of directly placing conjugated copolymers in intimate contact with semiconductor NRs, dispensing with the need for ligand exchange chemistry as in copious past work. These nanocomposites offer a maximum interfacial area between the constituents for efficient exciton dissociation. As such, it represents a significant advance in rational design and fabrication of organic-inorganic hybrid solar cells with improved power conversion efficiency.

Organic photovoltaics (OPVs) provide an avenue for inexpensive, efficient devices for converting sunlight into electricity. These devices usually consist of a mixture of phase-separated donor (usually polymers) and acceptor materials (usually fullerenes) called a bulk-heterojunction (BHJ). The BHJ structure allows for efficient charge separation and photocurrent production. The length scales at which these materials phase-separate, as well as their mixing properties, have a large impact on the performance of the devices. Therefore, understanding the nanoscale morphology of BHJs is of great importance to the OPV field. In this work we present a rather unique sample/technique combination to reveal the nanoscale morphology of a BHJ with unprecedented detail. Here we use the scanning transmission electron microscope (STEM) to perform electron tomography (ET) with unprecedented contrast between the donor/acceptor components. To achieve a high contrast between the two materials, we fabricated BHJ devices using P3HT and a novel endohedral fullerene which contains a Lu3N group in the center of the fullerene cage. Because the STEM has the ability to form images in which the contrast depends (approximately) on the atomic number squared (Z2), the contrast between the donor and acceptor components is quite high. To achieve even better resolution and detail within the reconstructed volume than the conventionally used weighted back projection (WBP) or simultaneous iterative reconstruction technique (SIRT), a relatively new technique to the field of ET was used called the discrete algebraic reconstruction technique (DART). This allowed for automatic segmentation of the volume and the use of multiple grey levels to represent both the pure and mixed phases of the materials. Using this combination of a unique sample and technique, we studied the effect of annealing on the nanoscale structure of BHJ. We show that the two materials are de-mixing into larger domains upon an annealing cycle, as seen by an increase and decrease in the size of both pure phase domains and mixed phases, respectively. This is consistent with what previous authors have suggested, however, for the first time, it can fully be visualized in a real sample. This technique reveals the nanoscale structure which occurs in a BHJ, as well as the processes which occur upon heating. It can be used to study the nanostructure of other polymer/fullerene systems as well, allowing unprecedented resolution and detail.

Electroluminescence (EL) spectroscopy and imaging have emerged in recent years as simple yet powerful characterization methods for inorganic solar cells and modules. For these systems, a theory based on the principle of detailed balance has been developed, which connects the solar cell quantum efficiency to the shape of the EL spectrum and the open circuit voltage to the absolute amount of EL emission. In organic bulk heterojunction solar cells, however, the description of EL emission is complicated due to disorder, the existence of different phases in the blend and due to non-linear recombination mechanisms. Here we present a series of measurements that indicate that the detailed balance theory is a good approximation for organic solar cells as well. We show that the shape of the EL spectrum closely resembles the theoretical prediction. In addition, we show that the absolute intensity is linked to the non-radiative recombination mechanisms dominant during the EL measurement. Interestingly, relatively high mobility solar cells have a lower absolute EL emission than materials with lower mobilities. To rationalize this behavior, we propose that the EL emission is affected by the transport of carriers to the opposite electrode and subsequent non-radiative recombination. This loss process is more efficient if the material has a higher mobility thereby facilitating the transport of carriers to the electrode. Relative to other loss processes, electrode recombination becomes more effective at higher injection currents. Thus, to study the influence of electrode recombination, we compare EL measurements at different injection currents in bulk heterojunction devices with those in bilayer devices where electrode recombination is effectively suppressed. With raising injection current the emission from the bilayers increases above the level for the blends especially for high mobility materials. This indicates that the studied blends are limited by surface recombination at the injection currents typically used in EL experiments. For the analysis of absolute EL intensities, these findings imply that low EL emission can be an indication of either efficient transport with subsequent electrode recombination or large non-radiative recombination losses at the donor-acceptor interface. Thus, in combination with complementary measurements, EL can yield valuable information on the recombination in the solar cell. However, the interpretation of the data is not straightforward as in crystalline silicon where high emission is correlated with good electronic quality and vice versa.

Dye-sensitized solar cells (DSSC) are one of the most promising technologies in the field of solar energy. Their efficiency is controlled by the rates of electron transfer processes at the dye-semiconductor interface, in particular by the electron injection process. The attachment chemistry of the dye on the semiconductor surface is one of the main factors that influence the efficiency of electron injection in DSSC. We calculate injection times for a large set of organic dyes that bind to the TiO2 surfaces via the phosphonic acid anchoring group, using a model based on partitioning the semiconductor-chromophore system into fragments. We study the effect of different binding modes on the injection time for TiO2 rutile (110) and anatase (101) surfaces. We analyze the influence of the size and nature of the anchoring group on the injection times, performing calculations with larger models of the anchoring group (e.g. phenyl-phosphonic acid). Through the partitioning procedure we are able to separate the effect of the binding geometry from other effects influencing the efficiency of the electron injection. The results show that dissociative bidentate adsorption modes generally lead to faster injection, compared to monodentate and molecular ones, similarly to the results obtained earlier for analogous carboxylated dyes. Our results are in good agreement with experiments (where available), showing that our model is capable of predicting the effects of anchoring groups and of spacer groups on the injection times, and is therefore suitable for designing new and more efficient chromophores.

We describe the synthesis and application to the photoanode of dye sensitized solar cells of spherical colloidal aggregates of ZnO nanoparticles and nanorods. The initial ZnO particles were synthesized in the solutions of alkanols, resulting in controlled aspect ratio, the latter being increased with lowering the polarity of the solvent. The surface of synthesized nanoparticles initially comprised the acetate and tetramethylammonium functional groups originating from synthesis precursors and lead to the stabilization of the particles in polar dispersion media. A ligand exchange reaction was then carried out with oleic acid, rendering the nanoparticles hydrophobic and dispersible in ethanol or octane. Following emulsification of such dispersions with an aqueous solution and evaporation of the solvent we obtained spherical colloidal aggregates of nanoparticles. The arrangement of nanoparticles with low aspect ratio within the aggregates was found to be random, whereas nanoparticles with aspect ratios of 2.5 (rods) undergo self-organization, forming a hexagonal close packed arrangement normal to the aggregate surface. Analysis of the cross sections of aggregates large compared to the length of the rods revealed that this order existed only in the outermost layer, with rods having a random orientation inside the sphere. On the other hand, aggregates with a diameter only a few times the length of the rods showed the formation of domains of regular order. We also found that nanoparticles initially dispersed in octane show the tendency for weak agglomeration, suggesting the presence of hydrophilic sites on the surface. Therefore, during emulsification, particle-droplet interface interactions dominate over particle-particle interactions resulting in the rods arranging at the surface of the droplets and in this way decreasing the interface energy. During the slow evaporation of the hydrocarbon, as the droplet radius decreases, the rods try to remain at the interface by forming a densely packed layer. Due to the possibility of arranging ZnO nanoparticles in domains with close-packed arrangement within a larger colloidal structure, we recognize these materials as promising building blocks for the photoanode of dye sensitized solar cells. Furthermore, their dimensions make them a suitable light scattering layer to improve the light harvesting of the solar cell. Using spin-coating, we formed a film of spherical ZnO aggregates over the traditional transparent layer of ZnO nanoparticles. Heat treatment of the layer to remove all organic material and sinter the ZnO particles to improve electron transport was followed by dye adsorption. Comparative tests were carried out on cells using electrodes based on the colloidal ZnO aggregates as well as controls using non-aggregated ZnO particles.

Dye Solar Cells (DSCs) are interesting as a promising low cost photovoltaic technology primed for migration from lab scale to large area devices. In order to increase output voltages in applications, cells are connected in series in modules. Cells can be interconnected effectively between two conductive substrates utilizing the â?oZâ? and â?oWâ? architecture schemes. We present a vis-a-vis experimental and PSPICE circuital comparison and optimization of W and Z-type dye solar cell modules. Important design and technological aspects which can at the same time promote and hamper each scheme were identified, including differing photocurrents (by a factor 1.6) between front and back-illuminated cells, sheet resistance-induced halfing of fill factor when increasing cell width (2mm - 20mm) and vertical connections with resistances that can affect fill factor. Modules fabricated showed efficiencies of 4.7% and 4.2% over active area for Z and W modules and 3.4% over total area for both (due to different aperture ratios). Our simulations showed that geometrically-optimized front-illuminated dye solar cell Z modules would yield higher efficiencies than W modules built with same materials if ideal vertical connections can be achieved. Pointers for further differing optimization of each scheme are laid out[1]. We have also designed and realized dye solar cell â?oZâ? connected modules with optimized geometries and processes. Optimization was achieved by varying the materials, multilayer combination of the TiO2 active layers and the fabrication processes. With the best combination of TiO2 multilayers, titanium tetrachloride (TiCl4) treatment, a reflective counterlectrode and optimized layout of cells and interconnections, a DSC module with a conversion efficiency of 6.9% on total area (43 cm2) (9% on active area) was fabricated. This result confirms that an effective scale-up of high performance Z-series connected DSC modules can be achieved[2].

One key in making polymer:fullerene photovoltaic devices more economically viable is in improving their overall, and especially morphological, stability. Taken into account the typically short exciton diffusion lengths, domain sizes of fullerene and polymer phases have to be kept small, in order to allow for efficient photoinduced charge transfer across the heterojunction. Here we present a simple, fast and reliable method of monitoring degradation within polymer:fullerene blend films. To accelerate the aging effects, we exposed the films to certain thermal stress. Our results tracked down the formation of fullerene aggregates on characteristic nano- and micro-meter scales corresponding to distinct stages of phase separation using simple spectroscopic measurements. These results were correlated with microscopic and more complicated measurements for improved correlation of optical effects.

In the last years the development of new designs for the next generation solar energy conversion devices with higher efficiencies, using less expensive materials and prepared by low cost processes is in intensive research. Aligned one dimensional nanowire arrays are beneficial for the nano-photovoltaic device concept providing a direct pathway for charge transport as well as a high junction area. The all-inorganic extremely thin absorber (eta) solar cell appears as a promising candidate to reduce the high cost and massive use of materials in their single-crystalline and thin film counterparts. The proposed nanowires-version of eta-solar cell, is composed by a thin semiconductor absorber (Eg ~ 1.5 eV) sandwiched between transparent electron and hole conductors (n and p-type semiconductors, respectively with Eg > 3 eV. ZnO possesses very interesting optical and electrical properties suitable for eta-solar cells. The sensitization of ZnO nanowires with various narrow band gap semiconductor quantum dots would allow the generation of electron-hole pairs trough visible light excitation. Thus only a few nanometer thick absorber layer could fully absorb the available solar energy radiation. In this work, we are reporting on sensitization of electrodeposited ZnO nanowires with quantum dots of different absorbing materials as, CdS, CuInS2 and Cu2ZnSnS4. Firstly the electrodeposition of ZnO 2D layers and nanowires on rigid and flexible substrates (glass/SnO2:F and PET/ITO) will be discussed. Electrodeposited 2D films are used as a buffer layers and consequently ZnO nanowires are grown on them. Nanowires with diameter between 120 and 350 nm and length until 2 µm could be obtained by tailoring the deposition parameters. In the second part, the photosensitization of the electrodeposited ZnO nanowire arrays with different light absorbing materials, as CdS, CdTe, CuInS2 and Cu2ZnSnS4 will be presented. These semiconductors are prepared by Successive Ionic Layer Adsorption and Reaction (SILAR) techniques and electrodeposition. The thin films (between 20 and 50 nm) prepared by both method are conformal along the entire nanowire surface. The layer thickness for the SILAR method is determined by the cycle numbers whereas for the electrodeposition it is controlled by the passed charge density. As prepared quantum dots films are with good optical properties and only few nm are enough to reach full light absorption in the large range of visible solar spectrum. Further an investigation of different annealing temperatures on physico-chemical properties of these materials has been done. With the aim to obtain more efficient light absorbtion from the available solar spectrum a double shell structure is proposed, where the first shell is composed of CdS (Eg ~ 2.1 eV) and the second of CuInS2 or Cu2ZnSnS4 (Eg ~ 1.5 eV). Finally the integration of these ZnO/Quantum dots (CdS, CuInS2, Cu2ZnSnS4) core/shell nanowires in eta-solar cell will be discussed.

A new class of photovoltaic (PV) material, crystalline nanoporous frameworks (CNFs), allows for detailed control over key interactions at the nanoscale so that the disorder and limited synthetic control inherent in conventional excitonic heterojunction (HJ) PV material can be overcome. CNF based solar cells will create new materials wherein: (1) an exciton only travels at most 0.5-3 nm before meeting a charge separating HJ; (2) the donor-acceptor offset is tunable; and (3) carrier mobility is maximized by eliminating disorder and defects that inhibit charge transport. We synthesized both p and n-type CNF and characterized them to determine bandgap and work function. These results are compared with predictions from density functional theory and allow us to determine band alignment with donor or acceptor guest molecules. A CNF donor-acceptor composite was synthesized by infiltrating these luminescent materials with molecules such as PCBM. Luminescence quenching is observed, suggesting the possibility of exciton formation.

Dye-sensitized solar cells (DSCs) have potential to be a low cost alternative to silicon based solar cells due to their lower material and fabrication cost. A DSC comprises photoelectrode, which consists of a wide bandgap semiconductor sensitized by dye molecules, and counter electrode separated by electrolyte solution containing an iodide/triiodide (I-/I3-) redox couple. Platinum is very widely used in DSCs due to its efficient catalytic nature. Carbon nanofiber (CNF), in recent years, has also shown comparable catalytic action for reduction of I3- ions [1,2]. Low cost electrospun carbon nanofibers (ECN) were used with solution processed platinum to fabricate composite carbon nanofiber/platinum counter electrodes for dye-sensitized solar cells. These cells showed better performance in terms of power conversion efficiency, charge transfer resistance, and catalytic action as compared with DSCs using solution processed platinum as counter electrode and DSCâ?Ts with ECN as counter electrodes. Efficiency of 7.18% was reached with the composite counter electrode whereas solution processed platinum and ECN used separately as counter electrodes showed efficiencies of 6.13% and 6.11% respectively (under 100mW/cm2 illumination with an AM 1.5 filter). The percentage concentration of platinum and carbon in the composite counter electrode was 53% and 47% respectively according to EDX analysis. References: [1] Journal of Photochemistry and Photobiology C: Photochemistry Reviews 4 (2003) 145â?"153 [2] ACS Appl. Mater. Interfaces, 2010, 2 (12), pp 3572â?"3577

Nanostructured composites are attracting intense interest for electronic and optoelectronic device applications. The material properties of composites based on nanoscale semiconductors are uniquely influenced by the strong surface interactions and, under appropriate conditions, quantum confinement effects. These systems offer considerable flexibility in the manipulation of device-relevant properties through the control of the interplay between the nanostructure and the optoelectronic response. Such capabilities are essential if these systems are to be incorporated as active elements in thin film photovoltaic (PV) cell architectures. In the present work, nanocomposite CdTe-ZnO and CdTe- ZnMgO thin films were produced by sequential RF sputtering. We show that, by controlling the design of these nanocomposites, we can tailor the resulting optical absorption, photoconductive response and charge transport properties. In this context, these nanocomposites were incorporated into PV devices as photoactive, thin-film heterojunction elements. The control of the solar cell performance, through the design of the nanocomposite incorporated into an established PV cell, was demonstrated in thin-film, inorganic (Cu(In,Ga)Se2 ) and hybrid (P3HT) device architectures. The opportunity to tune the spectral response of these cells, via control of semiconductor phase assembly in the nanocomposite, directly impacts the potential for enhanced solar cell conversion efficiency and, in a more general context, the tailored spectral sensitivity of other devices in which such composite films were introduced.

The morphology of bulk-heterojunction is critically important for conjugated polymer and fullerene blend solar cells. To alter the morphology, high pressure carbon dioxide treatment is applied for the blend film under ambient temperature. It is realized that vertical phase separation is induced by this process. In P3HT:PCBM blend film, carbon dioxide selectively plasticize and draw PCBM towards the surface, which results an unique composition distribution with PCBM-rich layer around the surface. Although P3HT tend to be abundant around a film surface because of lower surface energy in conventional process, it is suggested that PCBM surface energy is depressed with carbon dioxide in this process. This stratified morphology is observed by SIMS and cross-sectional SEM analysis. The performance of regular solar cell can be significantly improved with this process. This process can be applied for various compound films with different solubility of carbon dioxide.

Dye sensitized solar cell (DSC) has been extensively studied and developed in terms of commercialization of DSC. In order to fabricate DSC modules with more than 10% solar energy conversion efficiency, it is necessary to obtain at least over 11% efficiency in a laboratory-scale mini cell such as a 5mm-sqaure cell. We have obtained 10.5% to 10.7% efficiency so far. [1, 2,] The best efficiency of Black-dye-sensitized solar cell in the world is 11.1% (cell area:0.21cm2), which was reported by Sharp Group in 2006.[3] Further increases of not only photovoltage (Voc) but also photocurrent (Jsc) are requested to improve the solar cell efficiency of DSC over 11.1%. The optimization of light-confining effect of TiO2 photoelectrode is one of ways to improve Jsc. In addition, dye-cocktail system is also effective approach to improve Jsc. Recently, Ogura et al reported an interesting multiple dye system which improved efficiency from 10.0% to 11.0% by the combination of Black dye and D131 dye.[4] Therefore, we conducted above-mentioned two approaches. One is a detailed research with an influence of light-confining effect on cell performance of Black dye-sensitized solar cell in terms of optical properties of multi-layered TiO2 photoelectrode, which includes light-scattering TiO2 particles with different mixing ratio. As a result, by utilizing the optimized TiO2 photoelectrode, we could improve solar cell efficiency from 10.7% to 11.2% in a 5mm-square Black-dye-sensitized solar cell with an AR film and a black mask under simulated solar light (AM1.5, 100mW/cm2). Furthermore, by applying dye-cocktail of Black dye and D131 dye to the optimized TiO2 photoelectrode, solar cell efficiency was improved up to 11.8% (Jsc=25.1mA/cm2. Voc=0.67, ff=0.70, cell area=0.15cm2), which is the best record in Black dye based DSCs, under the same conditions. We report here these new research results. ACKNOWLEDGEMENT This work was supported by the NEDO, Japan. REFERENCES [1] Z-S. Wang, T. Yamaguchi, H. Sugihara and H. Arakawa, Langmuir, 21, 4272(2005). [2] H. Ozawa, M. Awa, T. Ono and H. Arakawa, Chem. Asian J., in press. [3] Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide and L. Han, JJAP Express Letter, 45, L638(2006). [4] R. Ogura, S. Nakane, M. Morooaka, M. Orihashi, Y. Suzuki and K. Noda, Applied Physics Letters, 94, 073308(2009).

Among all the organic and hybrid organic-inorganic solar cell technologies, dye solar cells (DSCs) have demonstrated the highest conversion efficiencies and a mature research and development plan. Compared to traditional photovoltaics, DSCs have several advantages, such as low dependence on angle of light, colors and transparency, which make DSCs very appealing for building integration photovoltaics (BIPV). In this presentation, the efforts made to scale-up this technology from small area cells to modules and panels suited for BIPV applications will be discussed. Optimization of DSCs for building integration is quite different with respect to conventional optimization strategies where efficiency is the main factor. In fact, in BIPV, transparency, color coordinates, color stability are as important as the photovoltaic efficiency. The relation between transparency and efficiency will be shown for several dyes and for co-sensitized devices. Results will outline the strong interplay between TiO2 thickness, dye/dyes dipping time and the absorbance profile of the dye/dyes. Finally, results of module and panel stability under indoor and outdoor tests will be shown and the acceleration factor for indoor light soaking stress tests will be carried out. A brief discussion of the industrialization activities for DSC-BIPV will conclude the presentation

We present a joined experimental and first-principle investigation of the TiO₂ polymorphism effects on dye-sensitized solar cells (DSSC) photovoltaic properties. TiO₂ building blocks based on pure anatase, pure rutile and pure brookite stabilized phases with various sizes and shapes have been prepared by growth techniques in solution in order to evaluate their properties in photovoltaic devices. For a valuable comparison, these various nanoparticles have been used to construct identical solar cells, i.e. fabricated in a similar manner. Their properties have been finely estimated and analyzed by impedance spectroscopy measurements and computed data using an ab-initio density functional theory (DFT) approach. We show that the open circuit voltage (V_OC) of the solar cells ranges in the following order rutile â?¤ anatase â?¤ brookite which is explained in the light of DFT calculations by the conduction band edge position depending on the TiO₂ phase. Also important are the quantifications of electron lifetimes, transfer times, diffusion coefficient in the various TiO₂ photoanodes. For the best cells the polymorph conductivity order is rutile â?¤ brookite â?¤ anatase and the present study illustrates that for similar dye loadings, the porous layer conductivity is a key parameter for explaining the cell performances.

Transient optoelectronic and charge extraction measurements are a very useful tool for understanding processes occurring in dye sensitised solar cells (DSSCs) and similar devices. Small perturbation current and voltage transient measurements provide information about the transport and recombination of charge carriers in a device. Charge extraction allows the energetic distribution of electronic trapping states of the semiconducting phase to be estimated. Together they provide a powerful combination for diagnosing and comparing the performance of different devices. We have developed a number of new methods to analyse these transients. For example by measuring the â?~time of flightâ?T of a small photovoltage perturbation the transport and recombination properties of a device can be simultaneously determined. The measurements can be used to understand factors influencing the performance of DSSCs. To illustrate this we present practical examples related to variation in cell performance due to process of device fabrication and aging. Additionally non-ideality is explored; this quantity influences the fill factor and thus performance of devices. Ideality in DSSCs is normally measured by examining the variation in photovoltage with light intensity. Non-ideality is typically ascribed to non-linear recombination of free electrons. Here we examine DSSCs constructed from a number of metal oxides (TiO2, SnO2, ZnO and WO¬3). The transient measurements show that the transport of free electrons is generally also non-linear implying the same effect may govern both processes.

Dye Sensitized Solar Cells (DSC) [1] are very promising photovoltaic devices reaching up to 11% efficiency, relatively recently discovered and with wide room for improvement. A DSC is essentially an electrochemical cell based on a thin nanoporous Titanium Dioxide layer (10-15 μm) where a monolayer of molecular organic dye (generally Ru-based) is chemisorbed. This hybrid organic-inorganic structure is dip in a liquid electrolyte where a redox couple is present (generally triodide/iodide). The whole system is sandwiched between two conductive glasses and encapsulated by a sealant. Despite an intense investigation a final model to simulate these photovoltaic devices is still missing. We have implemented a model of DSC using drift diffusion model solved on a generic mesh within Finite Element Method. The model is implemented within multiscale TiberCAD simulation tool [2] and includes all the component of the device consistently. Our complete model of DSC requires a very careful selection of the parameters to be implemented in the code. In order to achieve the most reliable comparison between simulations and real cells, we derive the most important parameters from experimental results. We extract the electron mobility and the recombination constant ranges starting from a set of experimental cells with different efficiencies (3-8%): by means of a IV characteristic fitting procedure, where the non-electronic simulation parameters are fixed by the experiment (i.e. dye, ions diffusion coefficient), the mobility and recombination rate values are recursively varied until the complete experimental curve is reproduced. This allows to carefully fit the physical parameters of the cell. We applied this procedure to a wide samples of cells, fabricated with different geometries and treatments. In this work we summarize the results obtained in the experimental comparison, the impact of the different parameters to the final efficiency. The fine tuning of the light absorption, transport parameters and the geometry of the active layer, allow us to define a consistent parameterization of the simulator which is then used as a predictive tool to calculate maps of efficiency for different working conditions and different directions of incoming light. References: [1] B. O'Regan and M. Gratzel. 1991, Nature, pp. 737 - 740. [2] M. Auf der Maur, G. Penazzi, G. Romano, F. Sacconi, A. Pecchia and A. Di Carlo, IEEE Transaction on Electronic Devices, vol. 58, no. 5, 1425 (2011).

Mesoporous anatase thin films are very promising materials to act as electrode in dye-sensitized solar cells. Randomly oriented nanocrystalline TiO2 particles are usually used to prepare photoelectrodes with a thickness of 10-15 µm. Templated-assisted dip-coating techniques are used to obtain thin films with ordered porosity. However, monolayer films prepared by dip-coating from a solution suffer from a low quantity of active material with a limited surface area, leading to poor photovoltaic performances. Therefore a multilayer deposition process is needed to increase the film thickness along with surface area. Multilayer dip-coating procedures have already been reported but are usually characterized by a lack of linearity in the evolution of parameters (roughness, surface area, PV performances) as the number of layer increases. In this study, we investigate a dip-coating-based multilayer deposition technique delaying these limitations. First, the influence of the template on the film organization and porosity is studied in terms of long-range order, percentage of porosity, pore size, surface area and pores connectivity. Different techniques such as transmission electron microscopy (TEM), atmospheric poroellipsometry (AEP) and UV-visible absorption spectroscopy (UV-vis.) have been used to describe the microstructural features of the films. The film exhibiting the highest dye loading was selected and its thickness gradually increased up to 4 µm. Finally, the photovoltaic performances of the thick films (1 to 4 µm) have been evaluated in combination with the N719 dye and a liquid electrolyte and show excellent efficiency (6.1%) when compared to values reported in the literature. Such mesostructured films were compared in terms of photovoltaic performance with TiO2 nanoparticles films, generally used in DSSC. Films were further evaluated as high performance photoelectrode in solid-state DSSCs, in combination with Z907 dye and Spiro-OMeTAD as solid electrolyte.

Light absorption and charge separation in dye sensitized solar cell (DSC) occurs in a dye monolayer chemisorbed to a meso-porous wide-bandgap semiconducting metal oxide. Solid state dye sensitized solar cells utilize a solid hole conducting material for dye regeneration. The interpenetration of the hole conductor into the inorganic framework currently limits the viable thickness of the meso-porous metal oxide layer in these type of devices. Consequently, panchromatic light harvesting efficiencies reaching 100% are difficult to achieve when the titania is sensitized with a dye monolayer. This bottleneck can be circumvented by employing a larger fraction of the pore volume for light harvesting. Using dyes as bulk absorbers and hole transporting media in solid state dye sensitized solar cells the light harvesting efficiency can be increased without augmenting the complexity of the meso-porous metal oxide layer. We have investigated triphenylamine-based hole transporting dyes (HTD) in conjunction with titanium dioxide in nanostructured hybrid solar cells. Power conversion efficiencies above 1% were attained. In these devices light is absorbed in the bulk of the dye layer generating excitons which diffuse to the hetero-interface where they can be charge separated. Excitons generated within the exciton diffusion length from the interface can contribute to the photocurrent. Furthermore we investigated triphenylamine-based HTDs in combination with titania-bound injection dyes (ID). Using dyes with a complementary absorption the ID and HTD can harvest different parts of the solar spectrum. The HTD contributes to the photocurrent either by direct injection into the titania or energy transfer of the excitation energy to the ID followed by charge injection by the latter.

We present a type of poly(oxyethylene)-segmented polyamide-imide copolymer (POE-PAI), prepared from the reaction by poly(oxyethylene)-diamine and 4,4�-oxydiphthalic anhydride at a proper molar ratio. The cross-linking mechanism involving the reaction of amine termini and the amidoacid intermediate was elucidated by Fourier Transform Infrared Spectroscope. The POE-PAI exhibited unusual swelling properties and was beneficial to entrap a large volume of liquid electrolyte in quasi-solid-state dye-sensitized solar cells (DSSC). When soaking electrolyte 1 h, the PGE containing 23.15±0.49 wt% POE-PAI copolymer and 76.85±0.49 wt% electrolyte (0.5M LiI, 0.05M I2, 0.5M 4-tert-butylpyridine in 3-methoxypropionitrile) shows the short-circuit photocurrent density of 19.60±0.05 mA/cm2, open-circuit voltage of 0.76±0.01 V, fill factor of 0.64±0.01 and overall conversion efficiency of 9.50±0.03% which was better than liquid electrolyte (8.53±0.13 %) under AM1.5 irradiation (100 mW/cm2). To our best knowledge, the efficiencies of gel-state DSSC reported in the literature are yet than 9%. These outstanding photoelectric properties of polymer gel electrolyte were explored by using incident-photo-to-current efficiency and electrochemical impedance spectra.

Dye-sensitized solar cells (DSCs) based on mesoporous semiconductor oxides have emerged as a prominent candidate for efficient solar energy conversion, both from an environmental and fabrication cost perspective as well as from its efficiency of 13%.[1,2] We report on a new high-surface metal oxide photoanode incorporating a 3D TCO host filled by in-situ hydrothermal growth of single-crystal ZnO nanowires (NWs) for fast charge extraction coupled to a passivation layer that reduces charge recombination and increases photovoltage. Structural advantages to increase the efficiency of DSCs are threefold. Firstly, NWs offer direct and high conductive transport pathways for photoexcited electrons.[3] They enable better charge extraction and injection from the dyeâ?Ts excited state to ZnOâ?Ts conduction band. Secondly, the high surface area 3D structure enables a higher dye loading than in the case of nanowire carpets, which translate into higher photocurrent. Thirdly, the disordered nature of the mesoporous backbone macropores improves light harvesting through light scattering in the visible part of the spectrum. Furthermore, ZnO can be synthesized in a highly crystallized form at low temperature making it compatible with roll-to-roll, low-cost bottom-up self-assembly techniques. However, the chemical instability of ZnO films in conventional iodide/triiodide electrolytes has been a major impediment to its widespread use in DSCs. This obstacle can be overcome by using various metal oxide thin films deposited via atomic layer deposition to increase material stability and control the recombination rate.[4] In particular, new results using a blocking layer together with a high extinction coefficient organic dye and a Co(II)/Co(III) redox couple have enabled photovoltages as high as 1.1V. We will present the state-of-the-art DSC photovoltaic characteristics using this novel morphology. [1] O'Regan B., and Graetzel M., Nature 1991, 353, 737-740. [2] Yella, A., et al., Porphyrin-sensitized solar cells with cobalt (II/III) based redox electrolyte exceed 12% efficiency, Science 2011, accepted. [3] Zhang Q., Dandeneau C. S., Zhou X., and Cao G., Adv Mater 2009, 21, 4087-4108. [4] Tetreault, N.; Arsenault, E.; Heiniger, L.-P.; Solheinia, N.; Brillet, J.; Moehl, T.; Zakeeruddin, S. M.; Ozin, G. A.; Graetzel, M., High-Efficiency Dye-Sensitized Solar Cell with Three-Dimensional Photoanode, Nano Lett 2011, accepted.

In dye-sensitized solar cells (DSSCs) with cobalt complex electrolyte, mass transport problem of cobalt complexes is one of the most important issues to be overcome. Although cobalt complex-based redox mediator is a promising alternative to the iodide/triiodide redox couple, cobalt complex electrolytes still show limited performance due to rapid recombination of electrons in the TiO₂ conduction band with oxidized Co^III species and mass transport problems in the mesoporous TiO₂ electrode. In this report, relation between mass transport of the [Co(bpy)₃]^II/III redox electrolyte and the mesoporous structure of TiO₂ film has been investigated in terms of the light-on photocurrent (J_max) and the saturated photocurrent (J_sat) using a transient photocurrent decay measurement. Porosity and pore size of TiO₂ film is found to have significant influence on mass transport of cobalt redox electrolyte. For the case of the relatively small pore size (18 nm) and low porosity (52%), regeneration efficiency (J_sat/J_max) decreases from 0.95 to 0.55, corresponding to 42% decrease, as the light intensity increases from 0.2 to 1 sun, which is indicative of strong hindrance of mass transport. On the other hand, only 6% decrease is estimated for the porosity of 59% and the pore size of 24 nm. As the result of the improved mass transport, photocurrent density is substantially improved by nearly two times with increasing the porosity from 52% to 59% despite the 23% decreased dye loading. As the TiO₂ film thickness increases, photocurrent density is significantly decreased for the small pore and low porosity film. However, little change in photocurrent density is observed for the larger pore and higher porosity film. The porosity of about 60% and pore size of ca. 24 nm are found to be a suitable structure for the bulky cobalt redox couple to be easily transported.

Semiconductor nanorods have attracted considerable interest as building blocks for a variety of applications ranging from biomedical diagnostics to next generation electronics. In nanorod, the inherent shape and length dependent properties of discrete nanorods such as linear polarized emission and total photon absorption can be collectively harnessed for scaleable application as light emitting diodes or low-cost nanorod based solar cells. In particular nanorod based solar cells; the significant charge recombination due to random nanorod distribution in these devices limits the power conversion efficiencies. Control over the direction and orientation of the rods in these devices â?" the ideal being perpendicular alignment of nanorods would greatly improves these properties. Here we describe the organization of II-VI nanorods into perpendicular assemblies by a novel electrophoretic deposition approach. Multilayer assemblies are obtained by completely immersing parallel electrodes in a nanorod solution. The positively charged nanorods deposit on the negative ITO electrode in ordered arrays which completely coat the electrode surface. The nanorods are hexagonally close packed with an interparticle separation of <3 nm occupied by interdigitated ligands. The precise control on number of layers on the substrate is also achievable. The nanorods are characterized by TEM, HRTEM, HRSEM and I/V nanoprobe measurements. This approach allows deposition of CdSe and CdS nanorods in over large areas optimal for perpendicularly aligned nanorod solar-cell application.

Recent researches in third generation solar cells aim at developing multi-bandgap structures. Quantum dots present interesting optical properties and an adaptable bandgap according to dot size. In this context, the synthesis of Si or Ge nano-crystals (NCs) and their microstructural and optical characterizations were widely studied. For that purpose, several multilayers are deposited successively and, by high temperature annealing (1100°C for Si and 850°C for Ge), the NCs are precipitated inside a dielectric matrix [1]. For quantum confinement tuning, the size distribution of the NCs must be sharp and the mean size well controlled. In the case of Si NCs, the nanocomposite presents optical properties consistent with the quantum confinement effect. In contrast, efficient electrical transport is harder to obtain and require a close control of NC inter-distance in order to ensure tunnelling conduction between the dots. In this work, we report the synthesis of Ge NCs using a nanocluster source [2]. This physical vapour deposition method is based on the condensation of a sputtered vapour. This setup provides the fabrication of NCs at room temperature, the size distribution of the NCs and their surface density being well managed. Furthermore, thanks to a separated embedding matrix deposition, the choice of the matrix is completely free and the properties of the NCs and the matrix are uncorrelated. Microstructural characterizations (TEM, Raman spectroscopy, XRD) indicate a nearly complete crystallization of the nanoparticles even on as-deposited samples. Photoluminescence characterizations seem to indicate the presence of Ge-O bonds at the interface NC/matrix, whereas spectrophotometry measurements allow to estimate the optical bandgap of the nanocomposites. A successful management of the quantum confinement effect is brought out by modifying the NCs size and the embedding matrix. Preliminary electrical measurements show that the microstructure of nanocomposite plays a crucial role. The electrical properties of various nanocomposites with different microstructures are investigated. Quantum efficiency measurements are leaded to bring out a photogenerated current produced by the NCs. The transport properties of nanocomposites are also studied by means of temperature-dependant Hall Effect measurements. [1] G. Conibeer et al., Thin Solid Films 516 (2008) 6748. [2] E. Quesnel et al., J. Appl. Phys., vol.107, issue 4 (2010).

The radial junction geometry enables more efficient photogenerated carrier collection in silicon solar cells while circumventing the need for expensive high purity single crystal silicon. For the fabrication of the radial junction, thermal diffusion of dopants and chemical vapor deposition of poly-Si on wire and pillar-array structures have been investigated while amorphous silicon nitride has been employed for surface passivation. In this study, plasma-enhanced chemical vapor deposition (PECVD) was used to deposit hydrogenated amorphous silicon (a-Si:H) to fabricate heterojunction intrinsic thin layer (HIT) on n-type silicon pillar arrays. The HIT cell structure is of interest as it allows for improved surface passivation while maintaining a low processing temperature around 200-300°C. For the fabrication of the radial junction HIT solar cells, silicon pillar arrays were etched into single crystal n-type silicon wafers using deep reactive ion etching (DRIE) via the Bosch process. The pillars in the 3.3 Ã- 3.3 mm^2 array had a diameter of 8 µm and height of 10 - 25 µm with a filling ratio of 0.5. Conformal layers of intrinsic (50 - 100 Ã.) and p-type (30 - 100 Ã.) a-Si:H were subsequently deposited via PECVD. Radio-frequency (RF) sputtered indium tin oxide (ITO) was employed to serve as the transparent top contact. Conformality of the a-Si:H and ITO was evaluated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Substrate doping densities ranging from 1Ã-10^16 cm^-3 to 1Ã-10^18 cm^-3 were used to investigate the effect of minority carrier diffusion length on solar cell performance. The fabricated radial junction HIT solar cells exhibited an energy conversion efficiency of 4.1% when illuminated using a Newport class A solar simulator (Air Mass 1.5 Global). The solar cells had an open-circuit voltage of 500 mV, short-circuit current density of 21 mA/cm^2, and a fill factor around 40%. The low fill factor arises from high series resistance due to the ITO contact layer. We expect significant improvement of the device performance with improved front surface contacts as the saturated reverse bias photogenerated current density was 34 mA/cm^2. We also investigated the effect of the minority carrier diffusion length in the radial junction geometry. There was no obvious change in the energy conversion efficiency with an increase in the substrate doping density from 1Ã-10^16 cm^-3 to 1Ã-10^18 cm^-3, Therefore, although the minority carrier diffusion length decreased from approximately 100 µm to 10 µm, charge-carrier collection in these radial junction devices was relatively unaffected. The results experimentally demonstrate the feasibility of utilizing the HIT structure in radial junction devices.

Graphene oxide (GO) is emerging as a highly appealing, solution-processable material for a variety of large-area thin film electronic and optoelectronic applications. Although an insulator, chemical reduction of GO transforms it to a semiconductor possessing unique optoelectronic properties. Owing to its excellent electronic conductivity, tunable optical properties and large-scale manufacturability, reduced GO (rGO) shows great promise in entering next-generation carbon-based PV devices, which have already received attention due to material abundance and stability. The fact that these properties can be tuned to our interest depending on the degree of reduction makes GO even more appealing. In this work, we explore the possibility of employing rGO in photovoltaic (PV) devices composed solely of all-carbon materials. We use ab-initio density functional theory calculations to study the interfaces between rGO of varying oxygen atom concentration and other carbon-based materials including fullerenes and carbon nanotubes. Results for several interface combinations will be presented and the key role played by rGO in such all-carbon heterostructures will be discussed. Our focus is on the accurate prediction of fundamental properties such as the band offsets, formation of schottky junctions and optical absorption that hold key in designing efficient GO-based all-carbon PV systems. In addition, possible ways of optimizing the optoelectronic properties and engineering rGO fragments for practical device-scale applications will be presented.

Recently, colloidal quantum dots (CQD) have been actively investigated since they offer the benefits of solution-based processing and wide spectral responses. Lead sulfide (PbS) CQD is one of the most promising materials in QD photovoltaics applications, although devices made from them have shown to date only a relatively low 3-4% power conversion efficiency. A key bottleneck in improving efficiencies lies in our lack of a detailed microscopic understanding of the role of morphology, shape, and ligand chemistry on the properties of the CQD film. Although QD shapes and film morphology have been examined through TEM techniques to some extent, many aspects such as surface reconstructions or ligand-ligand interactions, which cannot be clearly observed in TEM figures, still need to be elucidated. In this work, we use molecular dynamics simulations to model the mechanically stable shapes of PbS nanocrystals (NC) as a function of the size of the NC and importantly the extent of faceting. The effects of temperature, impurities/defects and solvents on the equilibrium NC shape will be investigated. In addition, the role of the ligand chemistry will be explored, in order to address questions regarding the effects of faceting on ligand attachment, the equilibrium ligand coverage, and the ligand interactions from different NCs.

A study of core-shell nanowire and planar structures for flexible solar cells is presented, together with preliminary comparative device results. The structure of these devices is the â?~substrateâ?T design i.e. ITO/CdS/CdTe(planar and nanotextured)/Mo foil, rather than the ubiquitous â?~superstrateâ?T configuration on glass. In this work we explore a) the processing features of planar substrate devices and b) a comparison of the performance of preliminary planar and nanotextured devices. The growth methods used were; sputtering and close space sublimation for the CdTe layers/nanowires and sputtering and chemical bath deposition for the ITO and CdS. Planar device structures were fabricated on Mo foil with particular attention being paid to two issues: firstly the metal contact to CdTe, which is a known problem. Both CdTe/Mo and CdTe/Sb2Te3/Mo junctions were compared, with the barrier heights being measured by temperature dependent J-V methods. Secondly the sequencing of the CdCl2 post-growth doping step of the CdTe was explored for these substrate devices. This can be introduced either before or after CdS deposition and a further consideration for nano-devices is that of preserving the integrity of the nanowires. Nanowire fabrication was demonstrated using the Au-catalysed vapour-liquid-solid method on CdTe (500 nm)/Mo hybrid substrates i.e. metamorphic growth. High density nanowire arrays (10^7 â?" 10^8 cm^2) were generated, with individual wires having diameters in the range 100 â?" 200 nm and lengths from 1 â?" 20 μm. Conformal coverage of the wires was achieved using a CdS (100 nm) window layer to complete the radial heterojunctions. Devices were finished by sputtering ITO (200 nm). A comparison of both planar and nano-textured â?~substrateâ?T ITO/CdS/CdTe/Mo PV device performance shall be presented. Characterisation of the devices and structures was performed by J-V (AM1.5) efficiency measurement, external quantum efficiency measurement, J-V-T current transport and barrier height analysis and SEM/TEM for structural analysis. Prospects for producing competitive nanowire core-shell CdS/CdTe PV devices shall be discussed.

Films of Zinc Sulfide (ZnS), Copper(I) Sulfide (Cu2S), and an alloy of CuZnS were created by Atomic Layer Deposition (ALD) on a nanoporous matrix of Titanium Dioxide (TiO2), to form a three-dimensional heterostructure photovoltaic device. The metal source for ZnS was Zn(TMHD)2, and the source for Cu2S was KI5 (a direct descendant of CupraSelect) while the sulfur precursur was H2S, generated in situ via a reaction between Aluminum Sulfide (Al2S3) and water. Energy-dispersive x-ray spectroscopy (EDX) and extended x-ray absorption fine structure (EXAFS) data show that the Copper Sulfide films are a Copper-rich phase of CuxS with x~2. The films have low surface roughness and penetrate fully into the porous TiO2 matrix, indicating conformal ALD. Optical absorption measurements show that the Copper(I) Sulfide has an indirect band gap of 1.1 eV, making these films promising for nanostructured extremely-thin absorber solar cells. In the future, this technique could be applied to other phases including Copper Zinc Tin Sulfide (CZTS) photovltaic device.

A hierarchical structure consisting of micropyramids and nanowires (NWs) was fabricated on the mono-crystalline Si (mc-Si) using a KOH anisotropic etching method followed by an galvanic wet etching process. The hierarchical structure shows excellent photon trapping properties in the wavelength region of 300â?"1000 nm, with the average reflectance of 3.96 %. Upon the application of the hierarchical Si surfaces, the current density of solar cells was increased from 20.33 mA/cm2 to 29.23 mA/cm2 and the conversion efficiency could be improved from 6.7 % to 10.47 % under 1.5 AM illumination. The concept and technique presented in this study should benefit the development of next generation of Si-based solar cells.

Porous silicon (PS) offers a wide range of physical properties that can lead to high efficiency photovoltaic cells by tuning the PS material geometry during fabrication and the modification of its surface chemical composition. We propose to use PS modified with Ge quantum dots (QDs) for third generation solar cells. The modification is intended to increase the IR absorption right in the near surface zone of the PS thin film. The nanomaterial structure and composition is prone for making solar cells with Energy Intermediate Band (IB). We previously reported on ordered PS hexagonal columnar cavities. High spatial resolution confocal Raman spectroscopy and imaging correlated with cross-sectional FE-SEM showed that PS comprises nanoparticles rigidly connected to pore walls. Confocal Raman spectra suggested strained Si nanocrystals are embedded in an oxide layer. Luminescence images exhibited at least three decay mechanisms associated with surface relaxation and surface composition change. Induced IB in PS modified with Ge QD is a new idea being studied by the author. The Ge QDs can be incorporated in the columnar pore structure with adequate porosity using various methods. Both PS and QDs exhibit very specific electronic properties that useful for the two-photon absorption fundamental process in IB PS solar cells. The electronic states induced by Ge QD attached to the cavity walls and the electron wave functions are presented. A model of cylindrical cavity whose walls are coated with oxide and covered with nanoparticles can simply describe this material system. Schrodinger equation was solved for with appropriate boundary conditions; it resulted in two secular equations. Two quantum numbers appeared to fully describe this system and Ge QDs appeared to bring-in new electronic states to the material system. These states are localized in Ge nanoparticles and their energy levels are lined up with the band gap of the Si host material. The energy spectrum of the system appeared to depend on the coupling of the QD with the cavity wall through two key parameters called transparency of QD/Si and QD-QD interfaces. When the interface is totally impermeable to electrons, the electron energy spectrum becomes discrete. Remarkably, we found that for non-null transparency electrons from the cavity walls spill over the QDs, which results in a continuum energy spectrum. While the majority of wave functions vanish in QDs, some resonant modes localized in the QDs grow large, even for low interface transparency. Depending on geometrical factors, another resonance arises between localized electronic states of neighboring QDs attached to PS cavities. The resonance can arise even between distant QDs and in the case of opaque interface between QDs. As the amplitude of electron wavefunctions determines photon absorption in QDs, large amplitudes in resonance modes between neighboring QDs are likely to increase two photon absorption in QDs attached to PS cavity walls.

Multiple exciton generation (MEG) has the potential to enhance solar cell efficiencies through the harvesting of multiple charge carriers per absorbed photon. In traditional solar cells, the excess energy borne by photons with energy greater than the band gap is lost to heat by thermalization of the excited carrier. For a given bandgap, the multiplication process has been observed to be enhanced in quantum dots, prompting attempts to demonstrate the collection of multiple carriers per incident photon, or an external quantum efficiency greater than 100%. In this work we will discuss a PbSe QD heterojunction solar cell that demonstrates internal quantum efficiencies as high as 130% and external quantum efficiencies as high as 112 ± 0.7%. We will also explore the secondary chemical treatment that is essential to the function of these solar cells and that leads to the enhancement of a number of other kinds of QD solar cells.

PbX (X = S, Se, Te) nanocrystal (NC) thin films typically suffer from poor air stability due to their large surface-to-volume ratios and tendency to exhibit major changes in electronic properties upon oxidization. Commercial prospects of NC-based solar cells will be improved if NC surface oxidation, surface diffusion and physicochemical changes at the interfaces are understood and controlled. It is not yet clear why exposing PbSe NC cells to air cause them to quickly lose efficiency, while similar PbS NC based devices improve in efficiency upon air soaking. This suggests a need to study the mechanisms of degradation common to NCs within a typical solar cell device stack. In this talk, I will describe a method of investigating the effects of oxidation in PbS NC solar cells one interface at a time by using atomic layer deposition (ALD) to systematically infill and protect different layers of the device stack with amorphous Al2O3. We have recently demonstrated the ability of ALD to increase the efficiency of PbSe NC-based Schottky solar cells and to stabilize these devices in air for at least months. Infilling PbS/ZnO NC heterojunction solar cells with Al2O3 also passivates surface traps, resulting in a larger increase of the short-circuit current with air exposure. The fact that both ALD infilling and air exposure improve device performance suggests the possibility of multiple treatments (i.e., short organic ligands, air exposure and ALD infilling) are able to work in concert to remove barriers of carrier transport through PbX NC devices. This ALD approach to studying PbS/ZnO NC heterojunction solar cells helps clarify which interfaces are responsible for the evolution of device performance and provides guidance in the design of higher-performance PbX NC solar cells.

We study the basic photophysics and possible photovoltaic applications of bulk heterojunctions made from mass-produced silicon nanoparticles in combination with the model organic semiconductors P3HT and PCBM. Bulk heterojunctions are prepared by spin-coating of suitably prepared constituent solutions. Spectrally resolved photocurrent measurements, electron spin resonance in the dark and under illumination, and femtosecond pump-probe spectroscopy over a wide sprectral range are employed to investigate and quantify charge separation, recombination and transport in these structures. Both, the electronic quality of the Si particles in terms of surface termination and defect density as well as the structural order of P3HT have a strong influence on the collection efficiency of photogenerated carriers. Optimized junctions are obtained for post-growth treated Si nanoparticles with a strongly reduced defect density in combination with highly aggregated regioregular P3HT. Other material parameters relevant for the photovoltaic performance of Si-nanocrystal/P3HT heterojunctions are the nanocrystal size and the Si/P3HT mixing ratio. The influence of these parameters on the short circuit current, open circuit voltage, fill factor , and the overall efficiency of prototypical solar cells will be discussed.

The most efficient fully organic solar cell is made of a conjugated polymer as electron donor material and fullerene derivatives as electron acceptor materials. Different advantages could be brought by the introduction of a vast library of inorganic compounds as alternative to fullerenes, such as high dielectric constant, high electron mobility and better stability, however, hybrid solar cells have not express their full potential yet. Why they are so inefficient respect to the polymer/fullerene system is still not fully understood and the issue needs to be investigated. Metal oxides represent one of the most appealing solutions thanks to their low cost and easy synthesis. A large number of investigations have been carried out on devices based on prototypical materials such as TiO2 and Poly(3-hexylthiophene). These show poor photocurrents, though the energetics should be ideal for charge generation. In addition there is a lack of control in the three-dimensional morphology, especially in the bulk-heterojunctions configuration when simple solutions, such as colloidal nanocrystals, are used, maintaining the power conversion efficiency as low as 0.5% Inspired by the successful concept of DSSCs where the metal oxide works as electron acceptor and transporting layer while providing an optimal scaffold for meeting the right compromise between high density of donor/acceptor interfaces and controlled percolation paths for good charge transport and collection, we have made hybrid solar cells by using mesoporous films of TiO2, about 1µm thick, infiltrated by P3HT. We demonstrate that by careful engineering of the TiO2/polymer interface, without using any optical and electrical active interlayer, we are able to dramatically enhance the photocurrent extracted from the nanostructured solar cell, reaching a power conversion efficiency exceeding the 1%; this represents a step change in performances for polymer-based hybrid solar cells. Here we investigate the device physics by combining time-resolved optical spectroscopy, XPS, and, for the first time, positron spectroscopy. The later provides a detailed mapping on the nanometer-scale of the oxide pores filling over the entire device thickness. We obtain a clear picture of the device physics, decoupling the influence of energetics, sovramolecular interactions and morphology on the device performances. Our results fully unveil the potential of a new generation of solid state devices where a new interface engineering, which goes from a molecular to a mesoscopic level, is the handle for optimizing the performances.

Photovoltaic properties of solid state hybrid solar cells, comprising of nanostructured TiO2 in contact with organic light harvesting polymers (P3HT), depend critically on the interface between these two inherently different materials. This interface has been widely studied and modified through a plethora of morphological and chemical tailoring. Recently, advanced solar cell designs incorporating solution processed extra thin absorber (ETA) layer of Sb2S3 has demonstrated remarkable enhancements in solar energy conversion efficiency, but the exact role of this layer on interfacial charge separation is not known. We find that replacing the Sb3S3 in this ETA layer by sulfides of other elements like Cd, In, and Sn in the same row of the periodic table results in devices that exhibit up to 40% decrease in efficiency compared to Sb3S3. Among the various elements investigated, Sb has the highest electronegativity and the ionicity of the metal-sulfur bond decreases in the order CdS (18%) > In2S3 (15%) > SnS (9%) > Sb2S3 (7%). Thus, with a high covalent character of the Sb-S bond, Sb can bond to the S atoms of the P3HT polymer with relative ease compared to Cd, In, or Sn. Sb could also interact easily with the oxide ions of TiO2. Morphologically, this interaction of Sb towards either side would reduce the interfacial incompatibility between the TiO2 and P3HT layers as revealed by atomic force microscopy (AFM) studies. Energetically, this is indicated by short circuit current density (Jsc) values > 8 mA/cm2 with Sb2S3-based devices compared to as low as 0.2 mA/cm2 with others. The observed differences are further explained by combining the data from photovoltaic device performance tests under simulated AM 1.5G solar spectrum with surface characterization data collected with X-ray photoelectron spectroscopy (XPS) and interfacial charge separation analysis data obtained with ultra-fast laser spectroscopy.

Since the first report of all inorganic solution processed solar cells in 2005 with power conversion efficiency (PCE) of 2.9% [1] tremendous progress has been made in this field and recently PCE of 6% [2] has been reported. PbS Colloidal quantum dot solar cells have thus far relied either on Schottky junctions [3] or p-n heterojunctions formed between p-type PbS quantum dots and n-type titania electrodes [4]. High performance devices based on titania require high temperature processing (> 400 oC) which is a rather costly step in large scale manufacturing. In addition titania is a high bandgap semiconductor which does not offer solar harnessing. We introduce Bi2S3 nanocrystals as a novel n-type solution processed nanomaterial and demonstrate its successful incorporation, employing low temperature solution process, with p-type PbS quantum dots [5]. We will report the first solution processed colloidal quantum dot p-n heterojunction where both phases contribute to solar harnessing. Bi2S3 is a non toxic material with absorption coefficient of ~ 105 cm-1 and band gap of ~ 1.3 eV desirable for PV application. We achieved PCE of 1.6% and EQE of 25% in a bilayer configuration. We also confirm the contribution of Bi2S3 in solar energy harnessing from spectral EQE data of our devices using ultra small PbS of band gap 1.9 eV as p-type layer. We will provide further insights into the underlying physics of our devices with respect to carrier density, depletion width and carrier lifetime. We show that the best performing bilayer devices consist of fully depleted structures in view of the short carrier lifetime in the Bi2S3 nanocrystals. We will then demonstrate the importance of having a bulk nano-heterojunction architecture where charge separation in the p and n phases takes place at the nanoscale. We will show the first solution processed bulk nano-heterojunction solar cell based on inorganic semiconductors with PCE of 4.8% a three-fold improvement over the bilayer configuration. We achieve this improvement by exploiting the charge separation of the carriers in the two different phases at the nanoscale which leads to an increase of the carrier lifetime in the Bi2S3 phase. This study opens the way towards the employment of novel non-toxic nanomaterials whose inherent poor physical properties that limit their performance in conventional device architectures can be surpassed by careful device engineering at the nanoscale. [1] I. Gur et al. Science 2005 , 310 , 462 â?" 465 [2] J. Tang et al. Nat. Mater. 2011, 10 , 765 â?" 771 [3] J. Luther et al. Nanolett. 2008, 8, 3488-3492 [4] A. G. Pattantyus-Abraham et al. ACS Nano, 4, 3374-3380 [5] A. K. Rath et al. Adv. Mater. 2011, 23 , 3712 â?" 3717

Metal nanoparticle and graphene based structures are presented as an optically active electrode for photovoltaic devices. We present surface potential measurements to demonstrate the tuning of the work function of this layer via metal nanoparticles, and the effect of an encapsulating graphene layer on the work function and sheet resistance. Additionally, we discuss graphene as an embedding dielectric to tune the resonant frequency of the nanoparticles, and the effect of the graphene layers on light trapping via plasmonic scattering. Furthermore, we discuss graphene encapsulation of metal nanoparticles as a method to potentially reduce recombination at the metal surface.

Nanostructured materials attract considerable attention as candidates for practical photovoltaic (PV) devices. Many of current PV devices are based on charge transfer schemes and frequently suffer from bad interface quality and poor carrier transport resulting in much lower light conversion efficiencies than in inorganic crystalline devices. An alternative is offered by non-contact energy transfer-based hybrid nanostructures combining strongly absorbing components, such as inorganic nanocrystal quantum dots (NQDs), with high-mobility semiconductor (SC) layers. It is envisioned that in hybrid systems, the excitonic energy is transferred via non-radiative energy transfer (NRET) and radiative (RET) waveguide coupling across the interface with the subsequent separation and transport of charge carriers entirely within the SC-based component. We have studied hybrid structures consisting of monolayers (ML) of the colloidal CdSe/ZnS NQDs attached to oxide-free Si surfaces via self-assembled layer of amine-modified carboxy-alkyl chain linkers. Using hydrosilylation, H-terminated Si surfaces can be fully functionalized through Siâ?"C bonding (no interface oxide), providing functional headgroups for NQD attachment. This approach results in the creation of single, tightly controlled ML of NQDs at a well-determined distance to the Si surface, ideally suited for energy transfer studies. Time-resolved PL studies of NQDs grafted on Si show considerable (~7 fold) acceleration of PL decay as compared to NQDs on glass (used as a reference). Taking into account local field effects, we find NRET efficiency from NQDs into Si at 65%. This result is consistent with theoretical calculations based on the Forster energy transfer mechanism and is further supported by a specific (~1/R3) distance dependence of NRET rates. To increase absorption in NQD films, we made multilayered structures with NQDs of different sizes arranged in monolayers and resulting in size-gradient NQD structures. The results are consistent with an exciton funneling mechanism involving interdot NRET between layers and into Si substrate, thus confirming the directional energy flow. In addition to NRET coupling, the effects of radiative waveguide coupling into Si substrate have been observed. We prepared ultrathin (100 nm) Si films on glass support produced from SOI samples and observed further acceleration of PL decays (~10 fold). To further increase NQD coverage and produce optimized multilayer structures, we produced 3D Si nanopillar large area arrays that were covered with dense NQD monolayers. The spectroscopy data indicate ~40 fold increase in NQD surface coverage, while PL dynamics exhibit further reduction (~15 fold) indicating efficient NRET and RET energy transfer into underlying Si substrate. Such new approach would considerably boost the efficiency of the Si-based photovoltaics, especially of thin, flexible cells where direct Si absorption is strongly reduced

Core-shell nano hetero or homo structures are the essential brick for many of the new third generation photovoltaic devices, for advanced photo electrochemistry elements or, even, for a more complex and promising artificial photosynthesis systems. However, all their outstanding properties depend on the adequate capability for photon capture and the consequent control of the charge separation. Under these conditions, doping of the inner part of the structure becomes basic for the charge extraction associated with a high transport facility, low internal resistance, as well as the surface conditions are determining for the charge transfer of the other type of carriers. Therefore, doping management becomes an essential point for energy band engineering and, so, a fundamental key for controlling the overall nanostructure performances. In this contribution, we demonstrate how the control of the coaxial doping profile in nanowires allows improving their surface charge carrier transfer while conserving their potentially excellent transport properties. As demonstration examples arrays of vertically aligned ZnO:Cl/ZnO and ZnO:Cl/ZnS core/shell nanowires grown by a facile and low cost electrodeposition two-step process have been used to support a general model that will be presented and discussed. In this way, photovoltaic and photoelectrochemical properties of ZnO nanowires have been found to be highly enhanced with the presence of these shell layers and a study as function of their thicknesses will be presented. From these features, it is shown and modelized how the presence of this thin shell can promote the surface-related radiative recombination processes. The enhancement factor is proved to depend on the shell thickness. These experimental evidences are associated with the improvement of the photogenerated charge carrier separation and surface to neutral inner part transfer capability achieved when increasing the space charge area within the nanowires with a built-in electric field introduced by the doping profile.

A variety of both nano-architectures and morphologies can be produced through design and material processing in solar cells. This provides great opportunities for improving the device quality and performance. The knowledge of correlation between morphology and electrical properties is essential both to understand fundamental physics and to facilitate device optimization. We use conductive atomic force microscopy (c-AFM) to simultaneously map the surface morphology and collect the electrical current. In-situ annealing is employed to study the influence of temperature on the morphology and thus the electrical transport mechanism. Cross-section AFM is attempted to get the internal 3D morphology so as to establish its correlation with the electrical properties. In all measurements, nanometer-resolved information is obtained and the heterogeneous properties are investigated. Results on the following two systems will be reported: the organic blend of P3HT fibers and PCBM particles, and the hybrid cell of ZnO nanowires infiltrated with P3HT.

Impressive developments have been made in the field of polymer-based, bulk-heterojunction photovoltaics (OPV) in recent years. Single junction power conversion efficiencies have reached 10%. As efficiencies have risen, the questions of lifetime and reliability of OPV have gained in importance. We recently observed a record lifetime approaching 7 years in encapsulated solar cells employing the high-efficiency polymer poly(-heptadecanyl-2,7-carbazole-alt-5,5-(-di-2-thienyl-benzothadiazole) (PCDTBT). We showed that PCDTBT degrades very differently than the well-studied polymer poly(3-hexylthiophene). PCDTBT devices experience an abrupt, exponential decay in efficiency, which lasts about 500 hours. Although the devices are stable after the burn-in period; the burn-in reduces the initial efficiency by 25%. We performed a systematic study to find the cause of burn-in in PCDTBT. Changing electrodes at the anode and the cathode do not effect degradation. Running current through the device or heating the device does not degrade it as long as the device is in the dark. The root cause appears to be the photochemical creation of hole traps in PCDTBT that occur on the same timescale as burn-in. To ultimately eliminate burn-in in PCDTBT solar cells, the cause of the traps must be identified and eliminated. Previously, it was not clear what role the electron acceptor, the fullerene derivative PC(70)BM, has during burn-in. To show the role of PC(70)BM in burn-in, we monitored the internal and external quantum efficiency of these solar cells. Although PC(70)BM has a greater ionization potential than PCDTBT, the role of the fullerene in polymer-based photovoltaic degradation cannot be ignored. Weak, heteronuclear bonds, impurities, and cross-linking have been hypothesized as potentially causing traps on the polymer. We used Raman spectroscopy and 2D liquid NMR to investigate the degradation of weak, C-N single bonds on PCDTBT. Mechanical cohesion testing was used to monitor cross-linking. One would expect to see an increase in mechanical cohesion as the material cross-links. To investigate the effect of impurities, we tested polymer batches of different purity levels and looked at the severity and duration of burn-in.

Currently, the worldwide energy supply is mostly coming from fossil fuels like coal, oil and gas. The transformation of these resources into usable energy induces CO₂ emissions, the main cause of the greenhouse effect which is directly related to climate changes. Taking into account the potential damages that could induce climate changes and the growing demand for energy throughout the world, the exploitation of renewable energy resources becomes urgent. Among them, solar energy is certainly one of the most promising long-term solutions for clean, safe and relatively cheap renewable energy. Organic bulk heterojunction (oBHJ) photovoltaic devices whose photoactive layer is composed of an electro-donating Ï?-conjugated polymer and an electro-accepting fullerene material have attracted considerable attention over the last years due to their fascinating potential for low-cost and large-area production through solution processing and roll-to-roll coating. However promising this technology is, the commercial viability of oBHJ solar cells depends on the level of efficiency and stability that can be reached.

The morphology of the donorâ?"acceptor heterostructure controls several crucial aspects of exciton dissociation, charge separation and carrier collection in organic solar cells, and their overall power conversion efficiencies (PCE). While much recent progress has been made in donor and acceptor materials, the formation of the central donorâ?"acceptor nanostructure still relies on a simple one-step demixing. This is inherently sensitive to the drying rate and hence difficult to implement uniformly on large areas or in thicker films. We report here a versatile polymer network methodology that overcomes these limitations by first crosslinking a polymer donor network to a critical density and then infiltrating or â?odopingâ? with the molecular acceptor. This achieves separate control of the donor and acceptor phases and their nanomorphology. Using the conventional regioregular poly(3-hexylthiophene) (rrP3HT) : phenyl-C61-butyrate methyl ester (PCBM) system as model, we show that this results surprisingly in an ultrafine rrP3HT donor network that possesses the elusive combination of crystallinity and connectivity which leads to a 30% higher PCE than that in spin-cast blend films, pushing the internal photon to electron efficiency to near unity. Based on this optimized nanomorphology, we have surveyed the PCE, fill-factor and short-circuit current landscape as a function of the amount of rrP3HT and PCBM in photoactive layer, which reveals the spearate role of nanomorphology and film thickness on space-charge effects and overall carrier generation efficiencies. Because the donorâ?"acceptor length scales are defined separately from the demixing length scale, these infiltrated polymer network solar cells allow a wider combination of materials to be used and have better compatibility with roll-to-roll processing.

Eight19 develops and manufactures flexible organic solar modules, with a specific focus on off-grid markets. We will address the challenges in the technology development of low-cost, efficient solar modules for these applications. The Organic Photovoltaic field has recently seen reports of encouraging results with power conversion efficiencies approaching 10% and extrapolated life-times of several years. In many cases, these results are obtained on glass substrates with optimized cell geometry prepared with minor constraints on the processing time. Up-scaling of this technology to large area solar modules requires a radical change of processing methods, conditions and materials. We will present aspects of our two core technology development activities, the optimization of the solar cell architecture in sheet-fed devices and the transfer to a roll-to-roll production process. The development of cell architectures is done entirely on flexible substrates utilizing roll-to-roll compatible methods. Larger cell areas, the solar module geometry instead of an optimized cell geometry and the use of flexible substrates are some of the factors that affect the electrical characteristics of the solar cell. Using complementary characterisation techniques supported by device simulations allows separation some of these effects and provides both a better understanding of the optimization potential and also the effects of degradation pathways. Roll-to-roll process development is carried out on a combination of small scale lab printers and coaters and a large, multiple station printing machine. We will give an overview state of the art at Eight19 and address the challenges in the technology development that lie ahead.

Increasing the efficiency in organic solar cells to levels which make them competitive with other solar energy conversion platforms may require the successful implementation of tandem cell configurations, where thin metal interlayers, combinations of n-doped and p-doped molecular semiconductors, or combinations of thin p-type and n-type charge selective oxides will be required as recombination layers between two current-matched OPVs. In all cases the interface chemistry and electronic properties of these interlayers are poorly understood, and require significant optimization prior to their implementation. This talk will focus on the formation and characterization of two types of potential interlayer films: a) ultra-thin metal films based on vacuum deposited Ag and Au nanoparticles and, b) ultra-thin oxide films based on combinations of ZnO and NiOx. We focus on the types of compositional and frontier orbital energy changes which occur between the metal nanoparticles and common donor (e.g. CuPc) or acceptor (e.g. C60), as revealed by X-ray and UV-photoemission spectroscopies (XPS/UPS), and by AFM and conducting tip AFM studies to probe local changes in conductivity at sub-micron to nanometer length scales â?" correlating differences in metal type, coverage and structure with changes in device performance in prototype tandem cell OPVs. In the case of ZnO/NiOx interlayers we demonstrate the formation of rectifying heterojunctions, where the electrical properties appear to be dictated by the band edge offsets between the two oxides (revealed by UPS), and by the processing conditions previously optimized to make these useful charge selective interlayers in single junction OPV platforms.

The transport length of charge carriers in organic photovoltaic materials is usually short, which is limiting the thickness of them and hence light absorption. Herein, nanostructures have been applied to reduce carrier collection distance without losing the light absorption. Besides, light management via nano-structures gives access to enhance light absorption in organic solar cells. In this work, the nanostructures are designed by optical molds to have light trapping effect while kept in the scale of charge carrier transport length. We also develop a simple and easy controlled method of directly imprinting SU-8 on glass substrate. Then it is coated by ITO(Indium tin oxide) thin film by pulsed laser deposition. These patterned ITO substrates are used to replace regular flat ones in the fabrication of bulk heterojunction organic solar cell. To demonstrate the effects of the patterned substrates, we choose P3HT and MDMO:PPV blended with PCBM as absorption material to build solar cells on the nanostructured substrates. Application to other kinds of solar cells is also indicated.

Organic bulk heterojunction solar cells use donor/acceptor junctions to create carriers at the interface. Due to the low exciton diffusion length, donor/acceptor blends have to be used for efficient solar cells. However, if they are too thick, free carriers cannot reach the electrodes by percolation paths and are stuck inside the absorber, thereby decreasing device efficiency and fill factor. Thus organic photovoltaics greatly benefit from light management and optical concentration in the absorber layer. We successfully use direct laser interference patterning (DLIP) to generate periodic sinusoidal surface structures. DLIP uses high power laser pulses to ablate the target material directly, without need for classical lithography, resist, and transfer steps. The structures can be processed on PET substrates or on TCO electrodes. DLIP allows scalable structuring at reasonable costs and throughput, compatible with roll to roll processing of organic solar cells. The DLIP generated line and hexagonal periodic patterns are successfully integrated with organic solar cells made by vacuum evaporation of small molecules, using the pin concept. For periods below a few micrometer incident light gets refracted at angles large enough to allow internal total reflection at the interface and thereby effectively trap the light inside the device and increase the absorption. As a side effect, the surface pattern folds the absorber layer along the surface and thereby improves the charge collection of the solar cell. PV cells with DLIP structures show increased absorption with the laser pattern serving as broadband antireflection coating. Compared to flat, unstructured surfaces the relative efficiency for cells on PET, could be improved by 5%, using periodic lines as DLIP patterned surfaces, and by 21% using a hexagonal surface pattern.

Recently, there has been significant research focused on plasmonic enhancement strategies for increasing absorption and therefore photocurrent in organic-based solar cells. The two plasmonic strategies with the best chance of enhancing solar cell performance are either to exploit near-field absorption enhancement, or to utilize scattering of incoming light to increase the path-length of light within the absorbing medium. The most promissing approach to exploit the near-field absorption enhancement is to embed metal nanoparticles (NPs) in the solar cell organic active layer, such that the interactions between the localized surface plasmons excited in the NPs and the excitons generated in the organic medium are maximized. The distance dependence of these interactions is critical, as one needs to isolate the nanoparticles sufficiently to prevent charge or exciton quenching, while still allowing for absorption enhancement. Finally, the issue of uniformly dispersing NPs within organic thin films is also not trivial. Here, we combine absorption with steady-state and transient photoluminescence (PL) measurements to probe the near-field interactions of Ag NPs embedded in organic thin-films. We routinely observed an increase in light absorption in the presence of Ag NPs, and by introducing a spacer layer, the range of this absorption enhancement was found to be ~5 nm. The PL intensity and decay-time of these organic semiconductors were strongly affected by the presence of the Ag NPs, with a distance-dependence that heavily depended on the spectral overlap between the NP surface plasmon resonance and the PL, the quantum yield, and the refractive index of the embedding medium. We found, both experimentally and analytically, situations where PL could be enhanced by 25% or reduced by 80%. Based on these results and our work on uniformly dispersing NPs within polymer layers, we conclude that this technique appears quite challenging. Regarding scattering, it is desirable to structure the rear electrode, which acts as a back-reflector. This is mainly because metal structures at the front of the cell would reflect and absorb a significant amount of the light before it reaches the active layer. By applying hole-mask colloidal lithography (HCL), we were successful in making nano-patterned electrodes which efficiently scatter light. For the HCL, we used polystyrene beads with sizes between 60 and 200 nm, which led to resonances between 470 and 760 nm. By adjusting the plasmon resonance to 560 nm using 110 nm diameter beads, we observed a quantum efficiency enhancement of up to 36% in the red absorption tail of the solar cell, which we could directly attribute to plasmonic light scattering. We therefore see this approach as being universally promising to produce plasmonically-enhanced organic solar cells.

Polymer:fullerene bulk heterojunction (BHJ) solar cells, which have achieved power conversion efficiencies in excess of 8%, are attracting a great deal of attention as a potential low-cost alternative to traditional inorganic photovoltaics. Some polymer:fullerene blends, such as blends of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTT) with phenyl-c71-butyric acid methyl ester (PC₇₁BM), form bimolecular crystals due to fullerene intercalation between the polymer side chains. We will present the determination of the detailed molecular structure of the PBTTT:PC₇₁BM bimolecular crystal using specular x-ray diffraction, two-dimensional grazing incidence x-ray scattering, solid-state nuclear magnetic resonance, and molecular mechanics simulations. The unit cell reveals interesting structural features including twists in the polymer backbone that allow increased backbone-fullerene interactions, curved polymer side chains, and one-dimensional fullerene channels for electron transport. The results explain previous observations regarding mobility and absorptionand open the door for improved design of polymer:fullerene bulk heterojunction blends. We will also present the determination and application of the PBTTT:PC₇₁BM phase diagram, demonstrate the ability to control intercalation by tuning the size of the fullerene derivative, and show how fullerene intercalation affects absorption, exciton splitting, and solar-cell performance. We will show that molecular mixing with fullerenes occurs for a wide range of polymers and discuss how many observations that have been made in labs around the world can be explained by analyzing how molecules pack in the cells.

In the last few years, organic solar cells based on oligomer (small molecule) substances have made major progress and deliver comparable efficiencies like cells prepared from polymers: The best results for small area cells are beyond 10%, certified results on areas >1 sq cm are over 8%. Combined with the ability to easily deposit multilayer structures e.g. for tandem cells and the excellent possibilities to purify the materials, small molecule solar cells are therefore a promising materials alternative. One key issue of small molecule organic solar cells is the morphology of the donor-acceptor bulk heterojunction. Compared to polymer solar cells, there are less alternatives to control morphology. I will discuss recent results which show that the morphology depends in a sensitive way on the molecular structure of the donor material: Even small changes of the molecules can cause significant changes in morphology, leading to modified exciton separation properties and thus different solar cell parameters.

Recombination orders higher than two have been reported for organic solar cells in various publications which provide different explanations. The two most popular are a delayed recombination by trapping of charge carriers in electronic trap states and by a charge carrier dependent mobility. To clarify which mechanism is dominant we performed temperature and illumination dependent charge extraction measurements under open circuit as well as short circuit conditions at poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61butyric acid methyl ester (P3HT:PC61BM) and PTB7:PC71BM (PTB7 is a thieno[3,4-b]thiophene and benzodithiophene polymer) solar cells in com- bination with currentâ?"voltage characteristics. We show that the charge carrier density n dependence of the mobility μ is different from the charge carrier density dependence of the recombination pref- actor. Therefore μ(n) alone cannot explain recombination orders higher than two, and an additional mechanism has to be taken into account. We interpret this as a process of delayed recombination which becomes more and more dominant at lower temperatures. In order to rationalise our experimental findings, we apply a multiple trapping and release model to show the separate influences of mobility and carrier decay on the nongeminate recombination dynamics in organic solar cells.

Previously we have used neutron reflectivity to observe self-stratification in a device relevant model P3HT/PCBM blend. The as-spun and solvent-annealed films show a depletion of PCBM near the top surface and enrichment of PCBM at the substrate. Depletion of PCBM at the cathode interface in a photovoltaic device could act as a barrier to efficient electron extraction. On thermal annealing, the PCBM depleted region is eliminated; an effect that partially explains the improvement of P3HT/PCBM devices on thermal annealing. We have since examined the solar cell blend system of PCDTBT/PCBM and observed that the vertical profile exhibits the optimal profile with the right components at the right electrodes. I will also show some recent results of SAXS experiments that show an intermixing of PCBM within the amorphous regions of P3HT upon thermal annealing.

We have studied thin films as well as polymer solar cells based on PPE-PPV copolymers in combination with a collection of fullerene derivatives with respect to their optical, electrical, structural, morphological and photovoltaic properties. A large variety of resulting bulk heterojunction blend morphologies as well as photovoltaic performances was observed. Complementary combinations of different characterization methods are employed in order to correlate the results with underlying morphologies. We show that the fullerene component in a polymer-fullerene derivative blend has a major impact on the polymer conformation and the in-depth organization of the donor-acceptor blend.

Organic solar cells are a promising alternative to traditional silicon solar cells. These cells can be produced inexpensively via roll-to-roll printing, however they have a theoretical efficiency lower than their silicon-based counterparts, and they also suffer in actual efficiency. Proposed device improvements utilizing metallic nanoparticles have been theorized to mitigate some of this deficiency. However, merely blending the nanoparticles (NPs) and polymers together does not show the large projected improvement due to the generation of additional recombination sites, which prevent the full theoretical improvement from being realized in practice. Overall efficiency can be drastically increased by employing tailored, anchored nanoparticle structures as sites for exciton generation and electron conduction. Due to the highly preferential scattering of visible photons in a direction transverse to the incident illumination, un-absorbed photons are scattered into the plane of the structure, further enhancing absorption probability. By densely packing these structures the exciton diffusion length can be accommodated in a thinner device. The possibility of incorporating ordered plasmonic nanostructures in roll-to-roll printing to manufacture devices exhibiting an increased efficiency could lead to a competitive alternative solar cell design. Herein, I will present our approach toward the creation of ordered nanostructures embedded in conjugated polymer thin films. Gold substrates are patterned to produce a surface suitable for separated, individual NP chain growth. A chain of gold nanoparticles is then grown upwards from the surface. The surface of the NPs is sensitized to bind to a photoactive polymer. The entire structure, including the space between the chains, is then filled with the same active polymer. These devices are monitored to determine their performance relative to both a simple planar device and a dispersed nanoparticle device.

A semiconducting polymer, PBDTTPD (TPD), was synthesized in 2010 and shown to have an exceptional power conversion efficiency of 6.8% when mixed with PCBM in bulk heterojunction organic solar cells (BHJs). We have performed a thorough investigation into the morphology and device physics of TPD BHJs in order to elucidate why this polymer works so well and to push the efficiency of TPD-based BHJs towards 8%. Our investigation is aimed at answering three key questions about what is limiting the performance of TPD BHJs: 1) How does the unique morphology of TPD affect its charge transport properties? 2) Why does the power conversion efficiency of TPD BHJs degrade when the active layer thickness is increased above 100 nm? 3) What causes the efficiency of TPD BHJs to degrade during annealing? To date we have made much headway in investigating these areas. We have also achieved efficiencies of 7.3% with TPD BHJs placing it among the highest efficiency polymers for BHJs. One of the most unique properties of TPD is that it packs in a face-on orientation with both its backbone and side branches parallel to the device substrate. We have determined that the hole mobility is high in pure TPD in both hole-only diodes (~2X10^-4 cm^2/Vs) and field effect transistors (~1X10^-4 cm^2/Vs). This result is exciting because the charge mobility in most other polymers is highly anisotropic, with mobility in the perpendicular direction being one to two orders of magnitude lower than parallel to the cell substrate. Additionally, we have determined that the hole mobility in TPD:PCBM blends is substantially lower than in pure TPD. TPD BHJs optimize at a thickness of 100 nm which is too thin to absorb all incident light. We have shown that increasing the thickness of TPD BHJs allows the cells to absorb more light and produce more current. Unfortunately, this increase in current is offset by a large decrease in fill factor resulting in an overall drop in device efficiency. We believe that the low hole mobility in TPD:PCBM blends may be responsible for this drop in fill factor. BHJs made with many semiconducting polymers must be annealed at high temperatures or in a solvent environment in order to reach their optimal efficiency. We found that TPD BHJs perform worse after annealing because the morphology of the BHJ changes in such a way that a portion of the charge carriers become â?otrappedâ? in the device, decreasing the amount of current that can be extracted. Evidence shows that these traps in TPD are morphological in nature and are caused by morphological coarsening. These results further the understanding of the morphology and device physics of BHJs based on new high efficiency polymers.

We present three different theoretical approaches to identify pathways to organic solar cells with power conversion efficiencies in excess of 20%. A radiation limit for organic solar cells is introduced that elucidates the role of charge-transfer (CT) state absorption. Provided this CT action is sufficiently weak, organic solar cells can be as efficient as their inorganic counterparts. Next, a model based on Marcus theory of electronic transfer that also considers exciton generation by both the electron donor and electron acceptor is used to show how reduction of the reorganization energies can lead to substantial efficiency gains. Finally, we introduce the dielectric constant as a central parameter for efficient solar cells. By using a drift-diffusion model it is found that efficiencies of more than 20% are within reach.