Perovskite solar cells are very attractive for multijunction applications because the bandgap of perovskite semiconductors can be easily tuned in the range of 1.55 to 2.2 eV and the open circuit voltage of the cells is large. We have made highly efficient semitransparent perovskite solar cells using silver nanowire meshes as the top electrode. These cells can be used in combination with either silicon or copper indium gallium diselenide solar cells to make four-terminal tandems. We will also present detailed characterization of perovskite semiconductors made with different processing conditions to show what needs to be done to minimize recombination.

Finding viable alternatives to silicon-based photovoltaics, through low-cost solution processable materials, is crucial, facing as we are, a complex transition out of the fossil fuelled civilization. In this scenario, the utilization in nanostructured solid-state solar cells of an underexplored eclectic class of materials, the hybrid halide perovskites, has represented a field breakthrough, allowing novel device architectures leading to record device performances up to 15%,[2] thus holding the promise of cost effective solar energy production. Among the first results of the pioneer reports on perovskites-based solar cells, probably the most intriguing discover concerned the application of a iodide/chloride mixed-halide perovskite CH3NH3PbI3-xClx in a so called “meso-superstructured” Solid State Solar Cell, where the perovskite is concomitantly capable of both absorbing light and transporting charge within a mesopourus network.[3] This mixed system has recently been compared to the I-based perovskite CH3NH3PbI3 via spectroscopical investigation and interesting differences have been reported, some of them leading to hypothesizing a better charge transport within CH3NH3PbI3-xClx.[4] However, an investigation on the exact materials composition and structure is still missing.
Here[1] we report a detailed investigation on Cl/I mixed halide self-assembling perovskites, and study the relation between the I:Cl ratio in the material and the solar cell characteristics, aiming at optimizing device performances through composition tuning. We found out that the exact composition of the final compound does not reproduce the stoichiometry of the precursor solution. We demonstrated, both experimentally and theoretically, that the formation of continuous solid phase MAPbI3-xClx is actually not allowed and that chloride incorporation into MAPbI3 is possible only at relatively low concentration (3-4%), so that it could be classified as a dopant agent. However, even if the material band-gap remains substantially unchanged, the Cl doping dramatically improves the charge transport within the perovskite layer, explaining the outstanding performances of meso-superstructured solar cells based on this material.
[1] Colella S., Mosconi E., Fedeli P., Listorti A., Gazza F., Orlandi F., Ferro P., Besagni T., Rizzo A., Calestani G., Gigli G., De Angelis F., Mosca R.; Chemistry of Materials, in press.
[2] Liu M., Johnston M. B., Snaith H. J.; Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395.
[3] Lee M.M., Teuscher J., Miyasaka T., Murakami T.N., Snaith H.J.; Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643-647.
[4] Stranks S.D., Eperon G.E., Grancini G., Menelaou C., Alcocer M.J.P, Leijtens T., Herz L.M., Petrozza A., Snaith H.J.; Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341.

Dye-sensitized solar cell (DSC) is an electrochemical device that has two electrodes with electrolyte between them. Recently the energy conversion efficiency of DSCs using a zinc porphyrin as a sensitizer has reached >12%. However, most porphyrin dyes exhibit narrow absorption in the visible region and relatively weak absorption in the red and near-infrared region; as a result, photons in these regions are not sufficiently converted to electrons, which has been a bottle-neck for high efficiency solar cells. Additionally, the widely used benzoic acid for attaching porphyrin dyes to titanium dioxide nanoparticles tends to dissociate into electrolyte, leading to poor long-term stability. In this presentation, synthesis, characterization, photophysical properties and photovoltaic properties of several novel porphyrin dyes with enhanced binding strength and broader light absorption capability will be described. The results demonstrate the potential of these dyes for achieving high efficiency in DSCs.

Our report is on all solid dye-sensitized solar cell (hybrid thin film solar cells) consisting of a transparent conductive oxide layered glass/a dense TiO2 layer working as a hole blocking layer/a porous TiO2 layer working as a electron collector layer/ self-organized monolayer anchored on porous titania/a perovskite layer (CH3NH3/PbI3) as light harvesting layer /2,2',7,7'-tetrakis(N,N-di-p-methoxyphenilamine) -9,9'-spirobifluorene (Spiro) working as a hole collection layer/Ag/Au layers, where photovoltaic performances are affected by perovskite crystal structures. In this paper, we report control of the crystal structure by monomolecular layer anchored on the porous titania, and the relationship between the crystal structure and the solar cell performance. In addition, the reason why the perovskite sensitized solar cells realized high efficiency from the view point of trap distribution of charge generation layers. I-NH3+-(CH2)n-COOH were absorbed on the porous titania surfaces to prepare I-NH3+-(CH2)n-COO-(porous titania) (A). The layer acts as anchoring groups to bond perovskite crystal to porous titania surface as well as seeds to grow perovskite crystals on the porous titania layer. Therefore this layer is termed “anchoring and seeding layer”. Alanine, glycine, or gamma-aminobutyric acid (GABA) was employed. PbI2 crystal was grown on I-NH3+-(CH2)n-COO-(porous titania) (A) as the seed of the perovskite crystals. It was proved that the PbI2 crystal on (A) had better controlled structure than that on bare porous titania (no anchoring layer). Among them, GABA gave the best results and the photovoltaic performance increased drastically from 8% to 10.3% after the GABA layer was inserted. After optimization, 12% efficiency was observed. Transition spectroscopic experiment strongly demonstrated that the increase in the efficiency is due to retardation of charge recombinations after “the anchoring and seeding layer” was inserted. In addition, the high efficiency was explained by extremely low trap density (4 digit) of perovskite/porous titania composite (10(13)/cm3), compared with that of porous titania layer (10(17)/cm3), where trap density and trap depth were evaluated by thermally stimulated current.

In the past few years, organometallic halide perovskite materials deposited on titanium oxide have gained a lot of attention as promising photovoltaic absorbers due to rapidly increasing device efficiencies. However, there has been little focus on the molecular composition and the electronic properties of the perovskite. In this presentation, we show our recent photoelectron spectroscopy studies on the interaction between CH3NH3PbI3 perovskite and the TiO2 substrate. CH3NH3PbI3 is spin coated on TiO2 from a γ-butyrolactone solution of stoichiometric CH3NH3I and PbI2. We will discuss the impact of perovskite-TiO2 interaction on the valence band position, chemical composition, and chemical configuration of the perovskite. These results are compared to reference perovskite spectra on FTO or gold. We find that the interaction between the perovskite and the substrate affects the structure of perovskite, which in turn influences the electronic properties of the perovskite. The implications of these studies to the understanding of the operation of perovskite solar cells will be discussed.

In recent years, wide interesting was attracted by using two or more dyes with complementary absorbance wavelength to enhance light harvesting in dye-sensitized solar cells (DSCs). In this presentation, the effort of developing co-sensitizers for Ru dye will be discussed. In the incident-photon-to-current efficiency (IPCE) spectrum of black dye based DSC, there is a dip induced by the absorption of triiodide at around 380 nm and the IPCE value at wavelength range from 450 to 550 nm is relatively lower than the high platform at lambda; = 600-700 nm. Thus we aimed to improve the lower photoresponse at these regions by developing complementary co-sensitizers for black dye. The ideal co-sensitizer should have high molar extinction coefficient than that of triiodide at the near UV region and have a moderate molecular size to co-adsorb with black dye on TiO₂ surfaces, and in the meantime effectively suppress the electron from TiO₂ recombination with I₃^- in the electrolyte as well as dyes aggregation. Accordingly, we developed a simple donor-π-acceptor (D-π-A) structured organic dyes Y1 and HC5 with dibutoxyphenyl or N,N-dioctylaminophenyl group as the donor moiety, thiophene as a π-spacer, and cyanoacetic acid as the acceptor/anchor, which successfully enhanced the IPCE of black dye at UV region in a cocktail DSC and helped achieve the highest certified conversion efficiency of 11.6%.

A series of benzophenoxazine dyes and their potential application in low-cost dye-sensitised solar cells (DSC) are analysed by establishing structure-property relationships. The 3-D crystalline molecular structures of four dyes are determined in single-crystal X-ray diffraction (XRD) experiments. Using UV-vis absorption measurements as references, density-functional theory (DFT) and time-dependent density-functional theory (TDDFT) calculations with the SMD solvent model are performed to analyse molecular orbitals, ground state and excitation properties. Ab initio calculations on 13 carboxyl substituted crystal structures show that torsions of donating or accepting groups against the molecular plane weaken their push-pull effect and the intramolecular charge transfer (ICT). The charge transfer onto the anchoring group is found to be strongest at positions with a symmetric environment with respect to the molecular plane improving stability against torsions. By applying the HOSE model, a linear relationship between the maximum peak absorption wavelength and the contribution of the para-quinoidal state in ring 1 is confirmed within series of structures with same anchoring positions. In combination with an investigation of the orbitals involved into the first excitation, principles for a future successful engineering of dyes are derived. By comparing dye energy levels with photoanode conduction band energy and electrolyte redox potential, the investigated dyes, with high absorptivities in the red spectrum, are found to be interesting candidates as co-sensitisers in DSCs.

Solid-state dye-sensitized solar cells has become one of the most promising alternative to the silicon-based solar cells, especially after the recent development of perovskite-based solar cells. However, pristine spiro-OMeTAD possesses low conductivity which limits the photovoltaic performance. Therefore, chemical doping in spiro-OMeTAD is crucial. In this work, we synthesized a new p-dopant, tris[2-(1H-pyrazol-1-yl)pyrimidine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (MY11), consisting of pyrimidine moiety which shows electron-withdrawing effect and consequently tune the redox potential of the dopant more positively. With deeper redox potential, there is a larger driving force for spiro-OMeTAD one-electron oxidation reaction and reduce the amount of dopant used during device fabrication. An overall power conversion efficiency of 12% was accomplished by using this newly developed Co-dopant. We believe this dopant can also be used for other HTM with deeper HOMO energy level due to its positively shifted redox potential.

Reduction of recombination losses in dye-sensitized solar cells (DSC) is vital to fabricate efficient devices. The electron recombination lifetime depends on the relative energetics of the semiconductor and the redox pair and on the chemical nature of the electrolyte (hole conductor). In this work the behavior of the electron lifetime in DSC devices prepared with various solvents (acetonitrile, valeronitrile, propylene carbonate, water, pure ionic liquids), additives (lithium ions, TBP) and redox pairs (iodide/iodine, Co(II)/Co(III)) is thoroughly studied. Lifetimes were extracted by means of small-perturbation electrochemical techniques (electrochemical impedance spectroscopy, intensity-modulated photovoltage spectroscopy) and open circuit voltage decays. To ensure a safe inner comparison and a proper interpretation, all devices were constructed using the same type of TiO2 electrode and the same dye (except for the cobalt-based electrolytes). Furthermore, small-perturbation techniques and voltage decay provided consistent results. It is observed that organic solvents with the iodide/iodine redox couple are characterized by relatively small reorganization energies, in agreement with numerical simulations based on the multiple-trapping model and the Marcus-Gerischer theory. Relatively small reorganization energies lead to a curvature in the lifetime-voltage semilogarithmic plot. In contrast, solar cells made with water or pure ionic liquid electrolytes, and those with cobalt-based electrolytes, do no exhibit such curvature, hence suggesting much larger reorganization energy. As a general rule, larger reorganization energies produce shorter lifetimes. This observation suggests that a chemical environment that interacts strongly with the redox mediator leads to a wider overlap in energies between donor and acceptor states and favors extra routes for electron recombination.

The recent employment of self-assembling hybrid halide perovskites, as key component of solid-state solar cells, has signified a substantial field advancement, allowing the obtainment of record efficiencies, up to 15%, for easy to prepares and stable devices.[2,3] Many advantages are related to the utilization of these hybrid materials, as they possess of the organic compounds an easy solution processability and a straightforward optical properties tunability, and of the inorganic semiconductors a high charge mobility and large absorption coefficients. This class of materials has been firstly investigated many years ago,[4] but they have, until very recently, never been employed as active components of solar converting devices. For this reason, despite the widening literature on the subject, many questions, concerning their peculiar structural chemistry and the physics of light induced processes have to be addressed, foreseeing further advancements on this emerging research front. Here we propose a picture for charge generation and recombination in (CH3NH3)PbI3-xClx perovskite-based solar cells, employing TiO2 as mesoporous substrate and spiro-OMeTAD as solid state hole transporting material. Time Correlated Single Photon Counting, Photoinduced Absorption and Transient PhotoVoltage measurements, were selected as direct investigative tools for the determination of charge generation and transport in model systems and working devices. The collection of our results depicts a peculiar behavior for the charge dynamics, as the presence of two n-type materials, TiO2 and perovskite, generates alternative paths for electron transport. In fact, an active role is played by the perovskite/TiO2 interface as additive electron transporting channel besides the TiO2 mesostructure, compatible with the formation of a charge accumulation layer in the perovskite, recently proved by Kim et al..[5] This important new insight would help the comprehension of the innovative perovskite-based devices, suggesting potential design improvements.
References
1. Roiati V.; Colella S.; Lerario G.; De Marco L.; Rizzo A.; Listorti A.*; Gigli G. Investigating charge dynamics in halide perovskite sensitized mesostructured solar cells. Submitted 2013.
2. Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-8.
3. Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 3-7.
4. Mitzi, D. B.; Watson, I. B. M. T. J.; Box, P. O.; Heights, Y.; Mitzi, D. Solution-Processed Inorganic Semiconductors. J. Mater. Chem. 2004, 14, 2355-2365.
5. Kim, H.-S.; Mora-Sero, I.; Gonzalez-Pedro, V.; Fabregat-Santiago, F.; Juarez-Perez, E. J.; Park, N.-G.; Bisquert, J. Mechanism of Carrier Accumulation in Perovskite Thin-Absorber Solar Cells. Nat. Commun. 2013, 4, 2242.

Dye-sensitized solar cells are characterized by complex, coupled kinetics. As such, design changes that are intended to improve the dynamics or energetics of a specific process can often have unintended consequences for the kinetics of other processes. This talk will focus on some of these entanglements, specifically as they relate to new designs for redox-shuttles, electron-tunneling-inhibition layers built in the gaps surrounding light harvesters, atomic-layer deposited (ALDed) metal-oxide layers for chromophore chemical and sorptive stabilization, and/or other new developments.

High aspect ratio metal oxide nanostructures promise to provide increased electron transport and light scattering over nanoparticle films in dye-sensitized solar cells (DSSCs).^{1, 2} We have investigated the growth of ZnO nanowires (NWs) on zinc substrates using anodization, a method that benefits from rapid growth, relatively simple apparatus and the ability to control the morphology of the metal oxide nanostructures produced. Through a systematic study of the synthesis conditions, we have been able to tune the morphology of the NW arrays and optimise growth rates to over 3 µm min^-1 with NW film thicknesses varying from 1 to 100 mu;m. The ZnO NW arrays produced by anodization have been applied as working electrodes in back-illuminated DSSCs, linking cell performance to NW structural characteristics and anodization conditions. Nanostructured metal oxides on conductive metallic substrates have previously been reported as an interesting alternative to conventional working electrodes for DSSCs due to their flexibility, which could allow fabrication by low-cost roll-to-roll production.^{3, 4}
1. M. Law et al., Nature Mater., 2005, 4, 455-459.
2. J.-Y. Liao et al., Energy Environ. Sci., 2011, 4, 4079-4085.
3. S. Ito et al., Chem. Comm., 2006, 4004-4006.
4. Y.-H. Lai et al., J. Mater. Chem., 2010, 20, 9379-9385.

Efficient solid state dye-sensitized solar cells (sDSCs) were obtained using a small hole transport material with inexpensive synthetic scheme, high solubility and hole mobility, MeO-TPD,1 (N,N,N',N'-tetrakis(4-methoxyphenyl)benzidine) after an initial light soaking treatment. It was discovered that the light soaking treatment for the MeO-TPD based solar cells is essential in order to achieve the high efficiency (4.9%), which outperforms spiro-OMeTAD based sDSCs using the same dye and device preparation parameters. A mechanism based on Li+ ion migration is suggested to explain the light soaking effect. It was observed that the electron lifetime for the MeO-TPD based sDSC strongly increases after the light soaking treatment, which explains the higher efficiency. After the initial light soaking treatment the device efficiency remains considerably stable with only 0.2 % decrease after around one month. We also describe how a light soaking treatment is essential for devices based on MeO-TPD to obtain high efficiency. By treating the devices under simulated illumination (AM 1.5G) at open-circuit condition for 30 minutes, the efficiency is increased more than 4 times. After light soaking treatment sDSCs based on MeO-TPD outperform spiro-OMeTAD based devices in spite of the poor initial device performance. We have obtained a record power conversion efficiency (eta;) of 4.9% in a 2.2 mu;m thick film of mesoporous TiO2 device by utilizing an organic dye coded LEG4 together with MeO-TPD. Thus MeO-TPD is one of the best organic small-molecule HTMs in sDSCs ever reported. After the light soaking treatment maximal efficiency retains at a nearly similar level for at least one month, which shows that the process occurring during the light-soaking treatment improved the device performance to a stable level. The specific nature of the HTM is essential, which requires in-depth characterizations and analysis to be fundamentally understood. The discovery is very important for future development of sDSC with different HTMs. Therefore we further discuss this phenomenon in terms of Li+ migration towards the TiO2 surface in presence of the different HTMs. It might be the case that many of the inapplicable HTMs previously tested in sDSC could probably exhibit better performance if treated by light soaking in combination with Li+ salts. According to these results a mechanism of device performance evolution depending on Li+ ion migration towards the surface of TiO2 nanoparticles under light soaking was suggested.2 These results provide a promising pathway for developing new small-molecule HTMs alternative to spiro-OMeTAD in sDSCs.
[1] K. Walzer, B. Maennig, M. Pfeiffer, K. Leo, Chem. Rev., 2007, 107, 1233minus;1271.
[2] L. Yang, B. Xu, D. Bi, H. Tian, G. Boschloo, L. Sun, A. Hagfeldt, E. M. J. Johansson, J. Am. Chem. Soc., 2013, 135, 7378minus;7385.

Organometallic perovskite- based solar cells have recently shown a breakthrough in power conversion efficiency, with devices exceeding 15 percent. However, the fundamental mechanisms of charge generation and recombination in these systems have not yet been fully investigated. Therefore, we have performed a series of ultrafast transient absorption measurements on working photovoltaic devices to more clearly elucidate the optoelectronic processes occurring in these systems.
We performed transient absorption and photoluminescence measurements on working mixed halide perovskite devices under different electrical biases. Additionally, we conducted a study into the effect of different inert mesoporous oxide scaffolds (SiO2, TiO2 and Al2O3) on charge formation and recombination. Using photocurrent extraction along with transient absorption allows us to correlate spectral signatures with charge populations in the devices. We find that there are significant differences in decay lifetime depending on different processing conditions. The choice of inert scaffold also has an effect on charge lifetime and device efficiency. Interestingly, the effect of varying the electrical bias on the device affects charge lifetimes less than expected.
Our findings add to a fuller understanding of the photo-physics of these devices, which is crucial for further improvements in photovoltaic performance.

Solution processed deposition, i.e. electrodeposition and chemical bath deposition (CBD), are utilized to grow a ZnO compact layer and ZnO nanorods respectively. The low thermal budget deposition open possibilities for them to be used for flexible solid state perovskite CH3NH3PbI3 solar cells. Power conversion efficiency of 8.90 % were achieved on rigid substrates while the flexible ones yielded 2.62 %.

Organometal halide perovskite-based solar cells have recently realized large conversion efficiency over 15% showing great promise for a new large scale cost-competitive photovoltaic technology. The knowledge of physical electronic mechanisms that govern carrier separation, transport, extraction, and their recombination in these solar cells is required to assess the possibilities and properties of alternative materials, configurations, and electrode contacts. We describe the analysis of solar cells of different materials, CH3NH3PbI3-xClx, and CH3NH3PbI3 on nanostructured TiO2, and morphologies, either thin film or nanoheterojunction, using impedance spectroscopy. We show that the separation of transport and recombination allows to better understand the performance of the solar cells.

Solid state dye-sensitized solar cells (ss-DSCs) have received considerable attention due to potential for high solar power conversion efficiency.1 Optimization for the hole transport materials (HTMs) are under intense investigation. 2,2&’,7,7&’-tetrakis(N,N-di-p-methoxy phenylamine)-9,9&’-spirobifluorene (spiro-MeOTAD) is the most widely used HTM in ss-DSCs. In this work, we report on the fundamental interaction between pristine spiro-MeOTAD prepared under ultrahigh vacuum conditions (UHV) and different gas atmosphere (O2, H2O, N2, air) typically present during the device fabrication steps and storage. The films prepared under UHV conditions allowed us to systematically quantify in a controlled way the amounts of diffused gas molecules and study the changes in the electronic properties of the neat spiro-MeOTAD films. Such UHV studies provided a direct connection with the real fabrication steps, where the device is air-exposed for different periods of time. X-ray photoelectron spectroscopy (XPS) measurements revealed that O2 and H2O molecules from the gas phase diffused into the film within 33 hours and affect the electronic structure of pristine spiro-MeOTAD. Ultraviolet photoelectron spectroscopy (UPS) results showed features similar to the p-type doping effect. However, spiro-MeOTAD based organic field-effect transistor devices showed deterioration of the hole-mobility value after air exposure, suggesting that incorporated species possibly act as hole-traps. UV-visible spectroscopy and Fourier transform infrared spectroscopy measurements show no indication of oxidized spiro-MeOTAD+ species. We have shown how in situ surface science tools such as UPS and XPS can be used to provide valuable insights on ss-DSCs.
(1) Kim, H. S.; Lee, J. W.; Yantara, N.; Boix, P. P.; Kulkarni, S. A.; Mhaisalkar, S.; Gratzel, M.; Park, N. G. Nano Lett. 2013, 13, 2412.
(2) Ono L.K.; Schulz, P.; Endres, J.J.; Kato, Y.; Nikiforov, G.; Roy, M.C.; Kahn, A.; Qi, Yabing*, J. Am. Chem. Soc. (submitted).

To provide a lower-cost and simple production method for counter electrode of dye-sensitized solar cells (DSSCs), in this work we developed polyaniline (PANi)/graphene/multi-walled carbon nanotubes (MWCNTs) composite films growing on glass substrates by using chemical/electrochemical deposition method and on fluorine-doped tin oxide (FTO)/glass substrates by using electrochemical deposition method respectively. First of all, we exclusively found that aniline sulfate is an efficient dispersing agent to debundle MWCNTs and to avoid graphene nanoplatelets aggregation in aqueous solution. The aromatic ring of aniline and its positive charge provided by sulfuric acid dopant are capable of enhancing the physical interaction with MWCNT and graphene nanoplatelets probably through π-π and cation-π interactions,1,2 resulting a well dispersed solution for electrochemical deposition process. A proper weight ratio of PANi/graphene/MWCNTs (1/0.0030/0.0045) composite film deposited on FTO substrate as counter electrode with sheet resistance of 7.65±0.11 Omega;/sq showed its potential to replace high-cost platinum (Pt). This Pt-free solar cell yielded power conversion efficiency (PCE) up to 7.67±0.05%, which is able to compete with the original Pt cell (7.62±0.07%).
In addition, we also fabricated the DSSCs composed of a proper weight ratio of PANi/graphene/MWCNTs (1/0.0045/0.0060) composite film deposited on glass substrate without Pt and FTO as counter electrode. The sheet resistance of resulting composite film was 45.19±14.25 Omega;/sq. This Pt/FTO-free solar cells exhibited a PCE of 3.58±0.06%. With further optimizing the performance, we believe it would have high potential to be used as a low cost counter electrode for flexible DSSC.
References:
1. Y. H. Chang, P. Y. Lin, M. S. Wu, K. F. Lin, Polymer 2012, 53, 2008.
2. Y. H. Chang, P. Y. Lin, S. R. Huang, K. Y. Liu, K. F. Lin, J. Mater. Chem. 2012, 22, 15592.

Methylammonium lead iodide perovskite materials look set to revolutionize the field of solution processed solar cells - after only four years since the first report, greater than 15% power conversion efficiency has been achieved. Reports in the literature show that device performance and the optoelectronic properties of methylammonium iodide perovskites strongly depend on the fabrication methods in ways that are not fully understood.
Here we report the structure of methylamonium lead iodide supported by mesoporous titania, the most common active layer used in high performance solar cells to date. Characterizing the structure in this composite material is difficult because of its complex heterogeneity and disordered nanostructures. To solve this problem we have applied total X-ray scattering pair distribution function analysis and discovered that only thirty percent of the perovskite has a long-range ordered tetragonal structure similar to the bulk material, while the remaining seventy percent forms as a highly disordered phase with the same local perovskite structure with a coherence range of only 1.4 nm. To the best of our knowledge, this is the first observation and quantitative analysis of disordered species in this material system. Furthermore, we demonstrate that the presence of disordered phase influences the absorption spectrum and photoluminescence and distinguishes this material from the bulk. This suggests that disordered and amorphous phases, which are not visible in conventional XRD measurements, is likely important to device efficiency. Our results underscore the importance of understanding the effects of various processing methods on the crystallization process and the optoelectronic properties of the disordered species in order to control the device performance of lead iodide perovskites.

Semiconductor quantum dots offer unique opportunities to harvest light energy and convert it into electricity. The size dependent electronic structure of quantum dots enables the design of photovoltaic devices with tunable electronic properties. Our initial work which focused on quantum dot solar cells employed metal chalcogenides (CdS and CdSe) as light harvesters. Basic understanding of the charge transfer processes of quantum dot solar cells has enabled the development of thin film solar cells. Organometal halide perovskite are new semiconductor materials that can deliver relatively high photoconversion efficiency. One of the major factors that dictate the overall power conversion efficiency in these solar cells is the hole transport across a hole conducting film. By employing transient absorption and impedance spectroscopic measurements we have elucidated the hole transport properties of copper iodide and copper thiocyanate in thin film solid state solar cells. Recent advances in the development of high efficiency perovskite solar cell will be discussed.

Neutral-coloured semi-transparent solar cells are commercially desired to integrate solar cells into the windows and cladding of buildings and for automotive applications. Current technologies for achieving semi-transparent solar cells include very thin silicon solar cells or organic photovoltaics. However, these both have problems when it comes to achieving high efficiency and colour-neutral semi-transparency. Thin silicon or other crystalline semiconductors show a reddish-brown tint due to the absorption coefficient increasing from the bandgap, and although impressive progress has been made recently with organic photovoltaics,[1,2] to attain a colour-neutral solar cell absorbers have to very carefully selected, often at a loss to overall efficiency.[3]
Hybrid organic-inorganic semiconducting perovskites have recently emerged as a new and promising class of photovoltaic materials. They have properties similar to bulk inorganic semiconductors, but can be solution processed using inexpensive and abundant materials. After only a couple of years of research, they have now demonstrated impressively high power conversion efficiencies of over 15% in a range of device configurations.[4,5] Here, we report the use of morphological control of perovskite thin films to form semi-transparent planar heterojunction solar cells with neutral colour and high efficiencies. We take advantage of spontaneous dewetting to create micro-structured arrays of perovskite “islands”, on a length-scale small enough to appear continuous to the eye yet large enough to enable unattenuated transmission of light between the islands. The “islands” are thick enough to absorb most visible light and the combination of completely absorbing and completely transparent regions results in neutral transmission of light. Using these films, we fabricate thin-film solar cells with high power conversion efficiencies, and highlight the potential for further advances. Additionally, we demonstrate the ease of “colour-tinting” such micro-structured perovksite solar cells with no reduction in performance, by incorporation of a dye within the hole transport medium.
1. Colsmann, A. et al. Efficient Semi-Transparent Organic Solar Cells with Good Transparency Color Perception and Rendering Properties. Adv. Energy Mater. 1, 599-603 (2011).
2. Chen, C.-C. et al. High-performance semi-transparent polymer solar cells possessing tandem structures. Energy Environ. Sci. (2013). doi:10.1039/c3ee40860d
3. Ameri, T. et al. Fabrication, Optical Modeling, and Color Characterization of Semitransparent Bulk-Heterojunction Organic Solar Cells in an Inverted Structure. Adv. Funct. Mater. 20, 1592-1598 (2010).
4. Liu, M., Johnston, M. B. & Snaith, H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature (2013). doi:10.1038/nature12509
5. Burschka, J. et al. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 499, 316-319 (2013).

Availability of small organic dyes with strong solar absorbance would go a long way towards the development of economically viable dye-sensitized solar cells. We first show that small thiophene-containing molecules can achieve a stronger solar absorbance when the conjugation order is changed and show that this is due to the interaction of the thiophene with an electron withdrawing group. We establish a correlation between the change in the BLA (bond length alternation) and the amount of redshift of the absorption spectrum achieved by changing either functional groups or the conjugation order. The strongest BLA change (from about -0.03 to about +0.1) from aromatic to quinoid character of the thiophene unit is achieved by changing the position of the methine unit separating the thiophene from the cyanoacrilyc anchoring group. We show that it is possible to achieve the quinoidization and a similar magnitude of the redshift by sidechain functionalizations.
We then present rational design of phenothiazine dyes by controlled quinoidization of the thiophene unit by choosing the electron withdrawing group. We systematically study the effect of several functional groups including pseudo- and (for the first time) super- halogens. A super-halogen unit induced the strongest quinoidization (in terms of BLA) of all functional groups tried.
We propose a new dye where a fumaronitrile unit induces an increase in the bond length alternation and a concurrent red shift in the absorption spectrum vs. the parent dye 3-(5-(3-(4- (Diphenylamino) phenyl) -10-octyl-10H-phenothiazin-7-yl) thiophen-2-yl)-2-cyanoacrylic acid. The visible absorption peak is predicted at 520 nm, in CH2Cl2 vs. 450 nm for the parent dye. The LUMO and HOMO levels of the new dye are suitable for injection into TiO2 and regeneration by available redox shuttles, respectively.

Organometallic halide perovskites (e.g., (CH3NH3)PbI3 and (CH3NH3)PbI3-xClx) have recently emerged as a new class of light absorbers that have demonstrated a rapid progress and impressive efficiencies (15%) for solar conversion applications. These absorbers have strong light absorption properties compared to other traditional thin film light absorbers and can be produced by a low cost solution approach. Despite the rapid progress demonstrated by these light absorbers, there is a lack of understanding of some fundamental physical and chemical properties of these materials. In this presentation, we report on our investigation on charge transport, recombination, and device characteristics of perovskite (CH3NH3)PbI3 sensitized solar cells. Charge transport and recombination properties were studied by frequency-resolved modulated photocurrent/photovoltage spectroscopies. The impact of device composition and fabrication conditions on the solar cell characteristics will be discussed. Charge transport, recombination, and device characteristics of perovskite-sensitized solar cells will be compared to those of dye-sensitized solar cells. These results and others are discussed.

A cross-linked copolymer was designed and synthesized by the imidation of poly(oxyethylene)-diamine and 4,4&’-oxydiphthalic anhydride, and followed by a late-stage curing to generate the cross-linked gels. The copolymers consisting of crosslinking sites and multiple functionalities such as poly(oxyethylene)-segments, amido-acids, imides, and amine termini, characterized by Fourier Transform Infrared Spectroscopy. After the self-curing at ambient temperature, the gel-like material enabled to absorb liquid form of electrolytes in the medium of propylene carbonate (PC), dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP). By using a field emission scanning electronic microscope, we observed a 3D interconnected nanochannel microstructure, within which, the liquid electrolytes were absorbed. When the novel polymer gel electrolyte (PGE) was fabricated into a dye-sensitized solar cell (DSSC), an extremely high photovoltaic performance was demonstrated. The PGE, absorbed 76.7 wt% of the liquid electrolyte (soaking in the PC solution) based on the polymer&’s weight gave rise to a power conversion efficiency of 8.31%, superior to that (7.89%) of the DSSC with liquid electrolytes. It was further demonstrated that the cell had a long-term stability during the test of 1000h at-rest at room temperature or only slightly decreasing in efficiency of 5%. This is the first time demonstration for a PGE exhibiting a higher performance than its liquid counterpart cell. The observation is ascribed to the suppression of the back electron transfer through the unique morphology of the polymer microstructures.

Tungsten oxides WO3-x (0le;xle;1) have tunable chemical constitutions and can also be developed into various functional materials due to their notable performances in gas sensors, electrochromic windows, optical devices, photocatalysts, etc In the range of WO2.625minus;WO3, WO2.72 (or W18O49) has the largest oxygen deficiency and in the meantime, it is the only one that can be isolated as a stable form. The unusual defect structure of WO2.72 may bring about extraordinary properties. In this study, nonstoichiometric WO2.72 was used as a counter electrode (CE) in dye-sensitized solar cells (DSSCs). Oxygen-vacancy-rich WO2.72 nanorod bundles with notable catalytic activity for triiodide and thiolate reduction were prepared. The photovoltaic parameters of dye-sensitized solar cells (DSSCs) with WO2.72 nanorod bundles as CEs are superior compared with those of the WO3-based cells, and nearly the same as those of the precious metal Pt-based cells.

Organic-inorganic lead halide perovskite solar cells show remarkable development in efficiency, but just as remarkable, they exhibit high VOC/Egap values up to 70% with mA/cm2 currents. Thus, there appears a possibility to find an affordable high voltage PV cell, that can be used in conjunction with Si PV for better use of the solar spectrum, instead of the highly efficient, but very expensive GaInP2.
The perovskite materials used for the cells, even though deposited by spin-coating from solution, form highly-crystalline materials, which makes it possible for them to have excellent electronic transport characteristics. Their simple synthesis, along with high chemical versatility, allows tuning their electronic and optical properties. By judicious selection of a combination of perovskite lead halide-based absorber (CH3NH3PbBr3-based, with a 2.2 eV band gap), matching organic hole conductor and contacts, we already reported a cell with a ~ 1.3 V open circuit voltage (VOC/Egap = 56%).
There is a dire need for low-cost cells of this type, which as the high photon energy cell in a PV system with spectral splitting (as noted above), can also help to drive electrochemical reactions, needed for decentralized solar fuel production. However, if we extrapolate the VOC\Egap from the high efficiency system (CH3NH3PbI3-xClx, Egap = 1.55 eV), we would expect a VOC of ca. 1.8 V. Re-evaluation of the materials selection resulted in an improved VOC (up to ~ 1.5 V) and initial JSC of ~ 3 mA/cm2 (three times that of the previously reported 1.3 V cell), resulting in VOC/Egap values reaching ~ 65%. With further improvements in terms of interface engineering and materials modifications we can thus expect the dream of affordable high-voltage PV.

Next generation dye-sensitized solar cells may include species that are capable of producing more than one electron-hole pair per absorbed photon. When coupled with a conventional absorbing layer, these singlet fission dyes may enhance the overall power conversion efficiency significantly by increasing the photocurrent in the higher energy region of the solar spectrum. In order to achieve such gains, dyes must be designed appropriately and charges extracted after singlet fission occurs. We have employed the organic compound 1,3-diphenylisobenzofuran (DPIBF) and derivatives as effective singlet fission sensitizers in conventional mesoporous TiO2 devices. Under optimized deposition conditions, internal quantum efficiencies exceed 50%, and electron injection with a direct connection of DPIBF to TiO2 occurs in < 200 fs, far in advance of singlet fission, which requires at least a few ps to produce a greater than unity yield of triplet excitons. We have measured cell quantum efficiencies after adding varying thicknesses of a ZrO2 coating to the TiO2 particles, finding a distinct rise in photocurrent at a specific ZrO2 thickness. This rise is not due to changes in transport properties but is most likely the result of the need for a waiting time of several ps for singlet fission to first occur before electrons are subsequently injected from the triplet state.

Organometallic mixed halide perovskite-based solar cells have shown a breakthrough in power conversion efficiency. Solution-processed devices with power conversion efficiencies of 10-12%[1-3] were reported in 2012, and have more recently exceeded 15% in devices processed by evaporation[4] and sequential deposition[5]. The fundamental questions are whether the photoexcitations in the perovskite remain as bound excitons and are later ionised at the electron and/or hole accepting interface or whether charge generation occurs in the pristine perovskite material.
We report the transient photoluminescence and absorption of devices and spin-coated perovskite films in the range from 30 fs to several µs. We find that charge generation in the pristine CH3NH3PbI3-xClx perovskite results from exciton ionisation within 1ps, and that these free charge carriers undergo bimolecular recombination on timescales of 10s to 100s of nsecs, to give photoluminescence with a quantum efficiency as high as 70%. We note that these long carrier lifetimes together with exceptionally high luminescence yield are unprecedented in such simply prepared inorganic semiconductors and that these properties are ideally suited for photovoltaic diode operation[6].
1 Ball, J. M., Lee, M. M., Hey, A. & Snaith, H. J. Low-temperature processed meso-superstructured to thin-film perovskite solar cells. Energy & Environmental Science, doi:10.1039/c3ee40810h (2013).
2 Kim, H.-S. et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Scientific Reports 2, doi:10.1038/srep00591 (2012).
3 Lee, M. M., Teuscher, J., Miyasaka, T., Murakami, T. N. & Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 338, 643-647, doi:10.1126/science.1228604 (2012).
4 Liu, M., Johnston, M. B. & Snaith, H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 501, 395-398, doi:10.1038/nature12509 (2013).
5 Burschka, J. et al. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 499, 316-319, doi:10.1038/nature12340 (2013).
6 Miller, O. D., Yablonovitch, E. & Kurtz, S. R. Strong Internal and External Luminescence as Solar Cells Approach the Shockley-Queisser Limit. Photovoltaics, IEEE Journal of 2, 303-311, doi:10.1109/JPHOTOV.2012.2198434 (2012).

Three novel heteroleptic amphiphilic polypyridyl Ru-complexes, MH01, MH03, and MH05, with oxygen-containing-electron-donor stilbazole-based ancillary ligands were synthesized to study the influence of cyclic-electron-donor (MH01), presence of cyclic electron donor coupled with acyclic electron-donor auxochromes (MH03) ortho to the CH=CH bridge of stilbazole, and presence of only acyclic electron-donor methoxy (MH05) on molar extinction coefficient, light harvesting efficiency (LHE), ground and excited state oxidation potentials, and photovoltaic performance for DSSCs. Although MH05 has three electron donor methoxy groups, it achieved the lowest molar extinction coefficient of 18250M-1cm-1 and exhibited the lowest photocurrent. The highest photocurrent density (Jsc) was observed for the longest interatomic distance between the CH=CH bridge of stilbazole moiety and cyclic-electron-donor auxochrome (MH01). It was also shown that while incorporation of acyclic electron-donor auxochrome ortho to the CH=CH (MH03) has little effect on the ground and excited state oxidation potentials, lambda;max of the low energy MLCT, and molar absorptivity, the lowest photovoltage and %eta; were observed. When compared under the same experimental device conditions using 0.3M t-butylpyridine (TBP), only MH01-TBA achieved 18% more in Jsc and 8.6% greater in eta; than the benchmark dye N719. To probe the interrelationship between the cyclic-vs-acyclic oxygen-containing electron donor of the ancillary ligands, and photocurrent and photovoltage of these dyes, the equilibrium molecular geometries of the ancillary ligands were calculated using DFT. The HOMO distribution on cyclic-vs-acyclic electron donor and the position of OMe in the ancillary ligands rationalized the fundamental science behind the photovoltaic performance and photostability of these dyes.
Keywords: dye solar cells, IPCE, auxochromes, photocurrent, photovoltage, photostability, electron donor, solar-to-electric conversion, molecular modeling, DFT and TD-DFT.
Corresponding author: [email protected]

Indigo as a very stable chromophore is of outstanding interest as building block for conjugated polymers for organic electronics applications. Because of its stability it is used to dye the famous ‘blue jeans&’.1 It&’s strong blue absorption is based on the crossed assembly of electron-accepting and electron-donating building blocks. High chemical stability of indigo in solid-state and solution is observed. Due to these properties, polymers containing indigo units have been proposed for application in organic solar cells.2
We have investigated novel donor-acceptor-copolymers with indigo as acceptor component combined with donor units as dialkylfluorene or dialkylcyclopentadithiophene. Poly{[(2,2-biindoinylidene)-3,3‘-dion-6,6‘-diyl]-2,7-(9,9-didodecylfluorene)} as example shows absorption maxima at 330, 420 and 610 nm. Its application as additive material of organic solar cells will be tested in further experiments.
1) Rondao, R.; Seixas de Melo, J. S.; Melo, M.J.; Parola, A.J. J. Phys. Chem. A. 2012, 116, 2826.
2) Seixas de Melo, J. S.; Burrows, H. D.; Serpa, C.; Arnaut, L. G. Angew. Chem. Int. Ed. 2007, 46, 2094.

Optimization of hole transport materials (HTMs) are important in solid state dye-sensitized solar cells (ss-DSCs) for enhancing solar power conversion efficiency.1,2 2,2&’,7,7&’-tetrakis(N,N-di-p-methoxy phenylamine)-9,9&’-spirobifluorene (spiro-MeOTAD) is the most widely used HTM in ss-DSCs. In this work, LiTFSI doped spiro-MeOTAD film samples with different doping concentrations were prepared by co-evaporation under ultrahigh vacuum (UHV) conditions. The fundamental interaction between LiTFSI and spiro-MeOTAD as well as the influences on the electronic structure of co-evaporated films by exposure to different gas atmosphere (O2, H2O, N2), commonly present during the device fabrication steps and storage, were studied by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Preliminary scanning tunneling microscopy and scanning tunneling spectroscopy (STM/STS) measurements will be presented showing the fundamental interaction of spiro-MeOTAD and LiTFSI at the nanoscopic level. The films prepared under UHV conditions allowed us to systematically study in a controlled way the influences of diffused gas molecules on the changes in the electronic properties of the LiTFSI doped spiro-MeOTAD films. Such UHV studies provided a direct connection with the real device fabrication steps. LiTFSI doped spiro-MeOTAD based devices were fabricated for conductivity and mobility property characterization. UV-visible spectroscopy measurements showed new features due to presence of oxidized spiro-MeOTAD+ species and/or Li-salt.
(1) Cappel, U.B.; Daeneke, T.; Bach, U. Nano Lett. 2012, 12, 4925.
(2) Ono L.K.; Schulz, P.; Endres, J.J.; Kato, Y.; Nikiforov, G.; Roy, M.C.; Kahn, A.; Qi, Yabing*, J. Am. Chem. Soc. (in preparation).

Dye-sensitized solar cells (DSSC) are a potential competitor to silicon-based photovoltaics, since they have undergone gradual progress with power conversion efficiencies (PCE) ready up to 13%.1 However, further improvements are highly desirable in terms of eliminating highly expensive materials and developing highly efficient solid-state DSSCs. To this end, the implementation of graphene and single-walled carbon nanotubes (SWCNT) in all the parts of the device architecture has recently led to significant breakthroughs.2
In this contribution, we present the benefits of implementing single-walled carbon nanohorns (SWCNH) in DSSCs. This type of nanocarbon shows interesting incentives. For instance, they have a strong semiconductor character, high porosity, and large surface areas. In addition, they are easily handled in solution and are produced in high yields and excellent purities.
In light of the above-mentioned features, we have developed an easy-to-implement strategy to utilize SWCNHs as either interlayers or as dopant agents.3-4 A full-fledge characterization of the electrodes as well as the device performance and mechanism lead us to conclude that the presence of SWCNHs strongly enhances the charge collection efficiency, that is, up to 50% under operation conditions. The latter is mainly responsible for the increase of the efficiency in doped DSSCs. Furthermore, we provided a direct comparison with devices doped with graphene and SWCNTs species in the current contribution.
1. A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W.-G. Diau, C. Y. Yeh, S. M. Zakeeruddin and M. Grätzel, Science, 2011, 334, 629.
2. L. J. Brennan, M. T. Byrne, M. Bari and Y. K. Gunko, Adv. Ener. Mater., 2011, 1, 472.
3. R. D. Costa, S. Feihl, A. Kahnt, S. Gambhir, D. L. Officer, M. I. Lucio, M. A. Herrero, E. Vázquez, Z. Syrgiannis, M. Prato and D. M. Guldi, Adv. Mater., 2013, (in press).
4. R. Casillas, F. Lodermeyer, M. Prato, R. D. Costa and D. M. Guldi, Adv. Mater., 2013, (submitted).

Exfoliated montmorillonite (exMMT) nanoplatelets are a two-dimensional electrolyte carrying ~1.78 dissociable monovalent cations per nanometer square [1]. Because of possessing anions fixing on the nanoplatelets, they can be used as a gelator to gelatinize the ionic liquid electrolyte for dye-sensitized solar cell (DSSC). According to our previous studies [2,3], we surprisingly found that they were not only capable of gelatinizing 1-methyl-3-propylimidazolium iodide (PMII) ionic liquid-based electrolyte, but also increased the power conversion efficiency of resulting DSSC from 6 to 7.77%. Recently, we investigated the ionic conductive mechanism of exMMT-gelled PMII ionic liquid-based electrolyte and found that the exMMT nanoplatelets acted like a reduction agent for iodide ions (I-). As exMMT nanoplatelets were mixed with PMII, I- ions readily reduced to I3- and even to I5- ions. Consequently, the ionic conductivity was significantly increased due to the fact that I-, I3-, I5- can form redox couples and the charges transport faster by way of the Grothus/exchange reaction process. Therefore, in this presentation, we will discuss how to prepare the exMMT nanoplatelets and their role for the reduction of I- to I3- and I5- ions.
References:
1. C.W. Tu, K.Y. Liu, A.T. Chien, M.H. Yen, T.H. Weng, K.C. Ho, K.F. Lin, 2008, “Enhancement of Photocurrent of Polymer-Gelled Dye-Sensitized Solar Cell by Incorporation of Exfoliated Montmorillonite Nanoplatelets”, J. Polym. Sci. Part A: Polym. Chem., 46, 47-53.
2. C.H. Lee, K.Y. Liu, S.H. Chang, K.J. Lin, J.J. Lin, K.C. Ho, K.F. Lin, 2011, “Gelation of Ionic Liquid by Exfoliated Montmorillonite Nanoplatelets and its Application for Quasi-Solid-State Dye-Sensitized Solar Cells”, Journal of Colloid and Interface Science 363, 635-639.
3. K.F. Lin, C.H. Lee, K.J. Lin, K.Y. Liu, “Use of exfoliated clay nanoplatelets and method for encapsulating cations”, USPTO Applicaton #20110031429-Class: 252 622. (US Patent already approved)

Solid-state hole-conducting spiro molecules, such as the investigated 2,2 prime; ,7,7 prime; -tetrakis(N,N-di-p-methoxyphenylamine)- 9,9 prime; -spirobifluorene (Spiro-MeOTAD), are currently under investigation as possible solid-state replacement of the liquid electrolyte hole conductor triiodide/iodide applied in mesoscopic and dye sensitized solar cells (DSSC). Li-salts and other additives are added to the Spiro-MeOTAD solution to enhance the poor conductivity of the intrinsic material and thus the efficiency of DSSC. Another approach of p-type doping is the codeposition of transition metal oxides (TMO), such as WO3 and MoO3 to organic semiconductors processed via vapor deposition. In addition TMO layers may be used as hole extraction layers on the Spiro hole conductor.
In this paper we systematically investigate the electronic interaction of Spiro-MeOTAD and WO3 on coevaporated Spiro-MeOTAD:WO3 samples and Spiro-MeOTAD/WO3 interfaces. Charge transfer between WO3 and Spiro-MeOTAD are investigated using synchrotron induced photoelectron spectroscopy on in-situ coevaporation and interface experiments. Both interfaces, Spiro-MeOTAD deposited onto WO3 and WO3 onto Spiro-MeOTAD are investigated. Band diagrams of the pristine materials and interface band diagrams including electronic state alignment, band bending, and interface dipole formation at the heterocontacts are derived. In addition core levels of Spiro-MeOTAD and WO3 codeposited films and interfaces are analysed. Also solution doped Spiro-MeOTAD:Li-TFSI samples are analyzed using synchrotron radiation and the results are compared to WO3 doping.
As a prerequisite, the equivalency of the electronic structure of drop-cast and vacuum-deposited Spiro-MeOTAD films is demonstrated. Almost identical valence band spectra and electronic-state energies are obtained for Spiro-MeOTAD films evaporated in UHV or prepared by drop-casting from cyclohexanone solution. With increasing amounts of WO3 p-doping is indicated by a shift of the Spiro HOMO and core level binding energy by up to 0.98 eV toward the Fermi level in coevaporated Spiro-MeOTAD:WO3 films. Electronic shifts of similar values are induced in Spiro-MeOTAD space charge regions at Spiro-MeOTAD/WO3 and WO3/Spiro-MeOTAD interfaces. In addition, interface dipole potentials around 1eV are induced in the two deposition sequences to compensate for the large work function difference. The formation of charge transfer complexes at the interface is derived from W4f and C1s core orbital analysis. A clear correlation of the interface charge transfer to the charge transfer in the doped samples is derived and is described in the internal interface charge transfer doping model. This doping model in general describes the doping mechanism for systems of precipitating dopants in semiconductor matrices as observed for CuPc:WO3, CBP:MoO3, and also CuPc:TCNQ.

Solid-state dye-sensitized solar cells and, more recently, perovskite solar cells, have begun to challenge not only liquid dye-sensitized solar cells but established thin film solar cell technologies as competitive, low-cost, alternative energy solutions. Though perovskite solar cells have reached certified efficiencies over 14%, they suffer from large variations in device performance which can be partly attributed to the use of 2,2&’,7,7&’-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9&’-spirobifluorene (SpiroOMeTAD) as the hole transport material (HTM).
The low hole-mobility and -conductivity inherent of SpiroOMeTAD necessitates p-doping the HTM via chemical oxidation to improve its conductivity. The current state-of-the-art uses chemical dopants such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), cobalt complexes, or protic ionic liquids in the presence of ambient oxygen. Reported methods to circumvent the use of chemical dopants and directly incorporate an oxidized form of SpiroOMeTAD into the HTM have also been demonstrated, yet such devices suffered from fast recombination and high series resistances or a significantly lower fill factor as compared to the control devices.
The ability to effectively introduce an oxidized form of SpiroOMeTAD directly to the HTM under an inert, oxygen-free atmosphere holds a number of potential advantages. Consistent control of the precise amount of oxidized SpiroOMeTAD present in the HTM enables the facile tuning of device properties and enhances reproducibility. Additionally, as oxygen is no longer necessary to oxidize the HTM, it can be excluded from the atmosphere during device fabrication and operation. This is a significant advantage in terms of device reliability, as it has been well documented that organic molecules readily degrade in the presence of oxygen and light.

This work presents the synthesis and use of 2,2&’,7,7&’-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9&’-spirobifluorene di[bis(trifluoromethanesulfonyl) imide], a.k.a. Spiro(TFSI)2, as an effective means of introducing oxidized SpiroOMeTAD to the HTM and demonstrates the first solid-state dye-sensitized solar cells fabricated with the complete exclusion of oxygen after deposition of the sensitizer with no performance loss compared to fabrication under ambient oxygen conditions.

Hybrid solar cells with metal-organic perovskite absorbers are of major interest due to their remarkable power conversion efficiencies of up to 15%[1]. It has been shown that the morphology of the perovskite itself and the interplay between the absorber and the mesostructured electron acceptor strongly affects the electrical properties and the performance of the device[2]. Therefore, revealing the morphology is crucial for the improvement of material and device design, which will ultimately lead to enhanced power conversion efficiencies.
We present a combined study of the structure-function relationship of solution processed solar cells based on mesostructured perovskites. For this purpose we used po-rous TiO2 as electron transport layer and 2,2,7,7-tetrakis-(N,N-dip-methoxyphenylamine)9,9-spirobifluorene (spiro-OMeTAD) as hole transport layer. The absorber is the organometal perovskite CH3NH3PbI3.
The morphology of the solar cells was studied by analytical transmission electron microscopy (ATEM). In ATEM electron energy loss spectroscopy (EELS) and electron spectroscopic imaging (ESI) are applied in order to gain material contrast[3]. We determined the excitation energies of TiO2 and Pb by EELS in the low loss regime. Subsequently a series of monochromatic images in the same energy range was acquired. As the TiO2 and the perovskite exhibit excitations at different energies it is possible to distinguish the distribution of both materials.
Given that solar cells are vertical devices cross-sections were prepared by focused ion beam milling. We compared devices prepared with different annealing conditions after the deposition of the perovskite. After the analysis of the ESI series of the cross-sections we were able to classify TiO2 and perovskite rich areas. We observed significant changes in pore size, pore filling and pore distribution of the mesostructured layer depending on the annealing ramp. Our results were also correlated to the I-V characteristics of the solar cells. The device with the less homogenously distributed mesostructure exhibits a decrease in the fill factor and the current density.
[1] M. Liu et al., Nature 501, 397 (2013)
[2] M. M.Lee et al., Science 338, 643 (2012)
[3] M. Pfannmöller et al., Nano Lett. 2011, 11, 3099-3107

Dye sensitized solar cells (DSSCs) have been developed into one of the most attractive third generation photovoltaic devices, with easy fabrication and relatively high conversion efficiency. Generally, platinum (Pt) with high conductivity and good electrocatalytic activity, is used as a counter electrode in DSSCs. However, its scarcity and high price highly favoured its replacement with low cost, low toxicity and environmentally abundant materials. Cu₂ZnSnS₄ (CZTS) kesterite phase is a promising alternative to Pt. The p-type semiconductor contains earth abundant and non-toxic inorganic materials. Moreover, its ideal direct band gap of sim; 1.5 eV and its high absorption coefficient ( > 1 x 10^-4 cm^-1), make it suitable for a wide range of applications such as light absorber material for thin film photovoltaics. Here, we report a ligand free, one step synthesis of vertically oriented CZTS nanoplate array directly grown on fluorine doped tin oxide (FTO) glass substrate by a simple pulsed laser deposition (PLD) technique. The array follows a two-step growth by first forming a CZTS thin film (sim; 100 nm), followed by a vertical nanoplate formation. The nanoplates are about 20 nm thick and 300 nm high with a petal-like shape. The purity of the nanoplates was determined by X-rays diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and energy diffraction spectroscopy (EDS). Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) were used to characterize the morphology of the nanoplates. Furthermore, the nanoplate array was integrated in a DSSC as a counter electrode with a power conversion efficiency of 3.65%, which is comparable to that of a conventional sputtered Pt counter electrode (3.33%) and higher than that fabricated with a “classical” CZTS thin film (2.83%). The CZTS nanoplate array is proved to be suitable for counter electrode fabrication to achieve Pt-free DSSCs, which could significantly cut down the cell cost and provide environmentally friendly photovoltaic devices.

Dye-sensitized solar cells (DSSCs) are considered as a promising photovoltaic technology because they are ecofriendly and cost-effective. In conventional fabrication processes of DSSCs, thermal processes are required for the preparation of TiO₂ photoanode and Pt counter-electrode. The thermal processes take place in a timescale ranging from several minutes to hours with large thermal budgets. To reduce the cost and energy payback time for DSSCs, several alternative methods have been developed, for example, atmospheric pressure plasma jet (APPJ) technology, which consists of highly reactive plasmas that affords rapid processing capability.
The fabrication process of reference sample is described as follows. For a photoanode, titanium isopropoxide solution was first spin-coated on a fluorine-doped tin oxide coated (FTO) glass and baked. Next, TiO₂ pastes layer was screen-printed and calcined at 510 °C for 15 min using a tube-furnace, followed by immersion in a TiCl₄ solution. After rinsing with ethanol, the photoanode was calcined at 510 °C again for 15 min using a tube-furnace. Then the sintered TiO₂ films were immersed in a solution containing N719 dye. For a Pt counter-electrode, chloroplatinic acid hydrate (H₂PtCl₆) solution was spin-coated onto a FTO glass and heated by furnace in air at 400 °C for 15 min. Finally, a commercial liquid iodine/iodide electrolyte was then injected into the assembled cell. In this study, APPJs were used to replace all the tube-furnace calcination processes. The APPJ system was operated with a voltage of 275 V; an on/off duty cycle of 7/33 mu;s; N₂ carrier flow rates of 31 and 30 slm for TiO₂ and Pt conversion processes, respectively.
The photovoltaic performance of DSSCs using APPJs shows efficiencies comparable to the reference cell. The open circuit voltage, short circuit current density, fill factor, efficiency of the reference cell are 0.72 V, 10.71 mA/cm², 68.86%, and 5.31%, respectively. APPJs can reduce the processing durations from 30 to 4 min for TiO₂ calcination and from 15 to 1 min for Pt counter-electrode conversion. The ultra-short processes of DSSCs are made possible by the reactive nitrogen molecules and the jet temperature of APPJs.

Dye-Sensitized solar cells (DSSCs) have attracted interest as one of the attractive photovoltaic cells due energy. DSSCs generate photocurrent via ultrafast electron transfer (< 100 fs) from the photo-excited dye to the nanoparticle semiconductor, TiO2. The oxidized dye is reduced by redox couple, typically iodide/triiodide (I-/I3 ), in the electrolyte. Although the rapid regeneration of oxidized dye is one of the important factors for the energy conversion efficiency of DSSC, this elementary step remains still controversial area. Due to the mismatch of standard reduction potentials, mostly many researches have been proposing the Two Iodide Process, (TIP) [(D+hellip;I-) + I- -> (Dhellip;I2-) -> D0 +I2-)], where I-I bond formation precedes the electron transfer step. However, by using first principle quantum mechanical (QM) studies, we discuss the new possibility of the Inner-Sphere Electron Transfer Single Iodide Process (ISET-SIP) [D+ + I- -> D0 + I].1 We discuss that I- is attracted to the oxidized dye positioning I- next to the NCS within the same solvation shell. At this equilibrium position, the desolvation effect adjusts the reduction potential of I- to match with the dye, resulting in a fast electron transfer to dye without involving I-I bond formation. We then scrutinize the kinetics of ISET and OSET (outer-sphere ET) within the most realistic descriptions (all ionic constituents and explicit solvents with TiO2 surface are considered) at the DSSC operation temperature by using molecular dynamic (MD) simulations.
References
[1] Jeon, J.; Goddard, W. A.; Kim, H. J. Am. Chem. Soc. 2013, 135, 2431.

Among the ternary oxides, Zn2SnO4 (ZSO) has received notice as an electron transporting material in dye-sensitized solar cells (DSSCs) because of its wide bandgap, high optical transmittance, and high electrical conductivity. However, ZSO-based DSSCs have a poor performance record owing largely to the absence of systematic efforts to enhance their performance. In this study, we propose general strategies for improving the performance of ZSO-based DSSCs involving the interfacial engineering/modification of the photoanode, the effects of which were investigated from the perspective of electron dynamics (injection and transport). A conformal ZSO thin film (blocking layer) deposited at the FTO/electrolyte interface by pulsed laser deposition effectively suppressed the back electron transfer while maintaining high optical transmittance, resulting in a 22% improvement in the short-circuit photocurrent density. The surface modification of ZSO nanoparticles (NPs) through a wet chemical route resulted in an ultrathin ZnO shell layer and resulted in a 9% improvement in the open-circuit voltage and a 4% improvement in the fill factor owing to the reduced electron recombination at the ZSO NPs/electrolyte interface. Each interfacial engineering strategy could be applied to the ZSO-based DSSC independently, leading to an improved conversion efficiency of 6% (an improvement of 26%), a very high conversion efficiency for a non-TiO2 based DSSC. Some of the recent results regarding the use of ZSO NO films for the perovskite solar cells will also be introduced.

Dye-sensitized solar cells (DSSCs) are being investigated extensively for their use in renewable energy technologies because of their low cost and high light-to-electrical energy conversion efficiency. Since their initial report in 1991 by Grätzel, DSSCs have attracted much attention from researchers, resulting in the preparation of hundreds of ruthenium complexes, non-ruthenium organometallic dyes and metal-free organic sensitizers. Due to the high cost of ruthenium, great efforts have been made toward replacing ruthenium complexes with organic dyes or ruthenium free organic metal-hybrid sensitizers. Recently different organic dyes architectured with coumarins, indolines, cyanoacrylic acids, merocynaines, polyenes and tetrahydroquinolines showed good performance. Herein, we report new types of sensitizers bearing halogen substituted oxindole acceptors, instead of the traditional cyanoacrylic acid, for dye-sensitized solar cell applications. These acceptors provide an additional route for electron injection through the chelation of their amide carbonyl groups to the TiO2 surface. In addition, we manifest the first comparative study of the acceptors with various halogen substituents and their effects on overall performances of the dyes. The electronegative F element is well known in the field of organic light-emitting diodes and artificial amino acid syntheses. Recently, its photovoltaic applications have received particular attention for improving the device performances. The F substituent in the acceptor of the dye was reported to affect the Fermi levels and increase the open-circuit voltage of the dye by reducing the dark current. However, our results revealed that, relative to the dye with the most common halogen substituent, F, the brominated sensitizer showed superior photo-physical properties. The cell based on the Br-substituted oxindole exhibited the highest efficiency of 6.35% with photovoltaic parameters of Jsc = 12.46 mA cm-2, Voc = 720 mV, and FF = 0.708. This finding will shed light on further exploration of Br incorporated sensitizers and provides a new approach for future rational dye design for the application in DSSCs. Evidently, absorption profiles of oxindole sensitizers can be further enriched by incorporating more efficient donors and fine tuning of their acceptors by adding or changing different electron withdrawing substituents. Such molecular and energy-level engineering of oxindole dyes to explore their roles in developing superior DSSCs are currently underway in our laboratory and will also be reported in the meeting.

In this work a step-wise cosensitization process was used to improve the power conversion efficiency (PCE) of dye sensitized solar cells employing N719 and metal free dye TA-St-CA. The DSSC sensitized with N719/TA-ST-CA showed PCE of 8.27% which is higher than that for the DSSCs sensitized with N719 (5.78 %) and TA-St-CA (4.45%). The improved PCE is attributed to the enhanced overall dye loading as well the reduced dye aggregation that resulted from the usage of dye with different anchoring units. The enhancement in the PCE has also been attributed to increases in both short circuit photocurrent and open circuit voltage. This may be due to the reduced dark current and suppression of back recombination of electrons with the ions in the electrolyte.

The 15% efficiency organmetal halide perovskite CH3NH3PbI3 and CH3NH3PbI3-xClx solar cell with unbelievable combined advantages of low cost, high efficiency and facile process has attracted the attention and focus of many solar energy researchers and become a superstar in the arena of next generation solar cells. Although CH3NH3PbI3 pervoskite has been proven to be one of most promising light absorber materials, there is a lack of study of chemical and physical properties of CH3NH3PbI3 or similar materials. Here we reported on the optical properties of CH3NH3PbI3 changed in presence of some gas molecules. This gas sensitive optical property can be utilized in sensors and it is also helpful for the further fundamental understanding of CH3NH3PbI3 pervoskite halides. The gas induced optical variation of CH3NH3PbI3 perovskite is presented and the plausible mechanism is discussed.

Development of multi-layer, nanostructured devices requires a unique combination of fabrication and characterization techniques. The research presented here analyzes photovoltaic response and material characterization to determine the reliance of photovoltaic performance on material integration and interfacial interactions within an interdigitated photovoltaic device containing a never-before utilized combination of TiO₂ nanotubes, PbS quantum dots (QDs), and an conformal ITO film. Our devices utilize the inorganic oxides as separate charge-transfer media for the extraction of electrons and holes generated by the photo-excitation of PbS QDs. The impact that the quantum dot surface environment has on the infiltration of ITO during electrochemically-assisted deposition into quantum dot-functionalized nanotubes is correlated to photovoltaic performance. Specifically, the photovoltaic performance, materials integration, and interfacial interaction between materials during various stages of fabrication of a solid-state photovoltaic are assessed using SEM-EDX, TEM, optical spectroscopy, advancing contact angle measurements, inductively-coupled plasma optical emission spectroscopy, glancing angle XRD, and I-V measurements under simulated solar illumination. The results of the analyses indicate that electrochemically-assisted deposition of ITO generates deleterious side reactions with the quantum dots; the nature and severity of which are dictated by the quantum dot surface environment.

The photophysical behavior of the polycyclic aromatic hydrocarbon peropyrene is studied in both dilute solution and in the solid-state, with the goal of evaluating this molecule as a singlet fission material. In solution, the fluorescence quantum yield is consistently in the range 0.90-0.95, while the fluorescence lifetime changes from 3.2 ns to 5.5 ns. Analysis of the solvent dependence of the radiative rate provides evidence that bright 1Bu singlet state mixes with a second, optically dark state. The presence of a dark state slightly above the 1Bu state in energy is confirmed using two-photon fluorescence excitation spectroscopy. The crystal structure of solid peropyrene consists of a herringbone arrangement of pi-stacked molecular pairs, similar to the alpha -polymorph of perylene. There are two emitting species, centered at approximately 550 nm and 650 nm, both of which are formed within the 15 ps time resolution of the experiment, and which relax independently via biexponential decays. We find no evidence for rapid SF in the peropyrene crystals, most likely due to the large shift of the singlet state to lower energy where it no longer fulfills the energy condition for singlet fission. These results demonstrate how both energetics and crystal packing influence the ability of a molecule to function as a singlet fission material.

The effect of tethering group acidity upon the coupling of porphyrinic dyes to zinc oxide nanorod and nanotube photoanodes was examined. A homologous series of zinc porphyrin dyes with varying pKa of the carboxylic acid tether was used to sensitize zinc oxide nanorod, nanotube, and, as a control, titanium dioxide nanoparticle photoanodes. Higher cell efficiencies were observed when the tethering group pKa was below 3 on zinc oxide, an effect not seen on titanium dioxide nanoparticles. This is evidence that efficient electron injection into zinc oxide photoanodes requires that (a) the zinc oxide must be etched by the dye and/or (b) the subsequent electron injection is coupled to interfacial proton adsorption or intercalation.

Polymer based quasi-solid electrolytes are a very good choice to improve properties of dye-sensitized solar cells (DSSCs), due to its high ionic conductivity, long-term stability, good interfacial filling properties and inhibiting leakage. In this paper, a new composite gel polymer electrolyte was prepared by adding layered Mg-Al hydrotalcite, a two-dimensional layered materials with good interchangeability of interlayer anions, into iodide-based liquid electrolyte with the addition of 4-tert-butylpyridine (TBP), propylene carbonate (PC) and poly (ethylene oxide) (PEO-600,000). A typical DSSC was packaged with the new polymer gel composite electrolyte to measure photovoltaic conversion efficiency. Then we found the photovoltaic properties of the quasi-solid DSSCs including hydrotalcite were obviously improved. When adding 5wt% hydrotalcite, the maximum photovoltaic conversion efficiency of DSSC is obtained, which reaches almost three times as large as that without hydrotalcite. After that, we used ion exchange method to prepare modified hydrotalcite, in which the layer spacing increases obviously. It was found that the photovoltaic conversion efficiency of the DSSC with composite electrolytes including modified hydrotalcite was improved further. When adding 5wt% modified hydrotalcite, the photovoltaic conversion efficiency of the quasi-solid DSSC of 2.84% was obtained under 100 mWcm-2 irradiation, which reaches more than four times as large as that without hydrotalcite. This enhancement was primarily explained by studying X-ray diffraction (XRD) spectra, and analyzing the electrochemical properties of the composite electrolyte including the charge transfer resistance, limiting current density, the dark current density, and and diffusion coefficient.

A series of molecular dyes and donor-type conjugated polymers and the composites of the dye/polymer pairs have been investigated for potential high efficiency sunlight harvesting and photoelectric energy conversion applications. Specifically, molecular dyes are designed to minimize the photo-generated charge carrier recombination and improve the photon capture as their frontier orbitals lie between the donor-type polymers and certain electron acceptors or n-type semiconductors. This study also reveals that photoluminence (PL) quenching of certain dye/polymer composites could be due to either photo-induced charge transfer or molecular aggregation in solid state or solution. In the case of aggregation, an increase in the PL emission could be caused by the de-aggregation of molecular aggregates, which may be induced by either solvents or polymers. For composites of three particularly dyes (TCPP, Hemin and protoporphyrin) with P3HT, Stern-Volmer plots revealed PL quenching with Ksv values of three P3HT/dye composite pairs are 15300M-1, 9610M-1 and 3400M-1 corresponding to Hemin, protoporphyrin and TCPP, respectively. However, this Ksv descending order is an opposite trend to that of the peak absorption coefficients of the three dyes. Preliminary solar cell device of P3HT/Dyes/PC60BM at AM 1.5G one Sun intensity exhibit photoelectric power conversion efficiencies (eta;) decreases following the Ksv but not the absorption trends.

The thermal stability of dye sensitized solar (DSS) module depends on the density of electrolyte. In this research, the density of electrolyte in a DSS module was varied to investigate the effects of density on the degradation. High temperature test was conducted to derive the relation between degradation rate and density with 3 types of dye sensitized solar modules (DSMs).
The efficiency of DSM with high-density electrolyte decreased over 13% as compared to initial efficiency under thermal stress, while the efficiency of DSM with medium density electrolyte decreased about 5%.
To better understand the degradation mechanism, degradation analysis was carried out with UV-vis absorption, electrochemical impedance, and Raman spectroscopy.

Nowadays, the exploration of new photoelectrode architectures to improve the light-harvesting and charge-collection properties of sensitized solar cells and related devices has been regarded as a challenging work. In particular, the inverse opal film consisting of 3-dimensional pores can provide the functional structures as well as the photonic crystal effect. Here, we demonstrate the synthesis of the inverse opal SnO2 film for sensitized solar cells by gravimetric sedimentation method, subsequently followed by the spin-coating or drop casting method. Based on the 300nm, 430 nm and 520 nm poly styrene (PS) beads, the SnO2 inverse opal structure was well developed, confirmed by field-emission scanning electron microscopy, X-ray diffraction and ultraviolet-visible spectrophotometer. Also, SnO2 inverse opal film was applied to sensitized solar cell and photoelectrochemical water splitting cell to identify their unique properties and evaluated exploring the photocurrent-voltage (J-V) measurement.

Dye-sensitized solar cells (DSSCs) have been widely attracted due to low manufacturing cost, environmental friendliness, and high efficiency. However, there are still several drawbacks to attain the conversion efficiency above 15 % for the commercialization. In particular, in the photoanode comprised of TiO2 nanoparticles, the trap-limited diffusion process leads to the high charge recombination with a reduction of electron collection efficiency, even though it shows large surface area ensuring a high dye loading for light harvesting. One-dimensional TiO2 nanotube (TONT) by electrochemical two-step anodization in 0.25 wt% NH4F of ethylene glycol was suggested to overcome this problem. Herein, the overcoating of TiO2 nanoparticle (TONP) was introduced to support the surface area because nanotubular structure has low surface area resulted from large inner diameter (approx. 100 nm) of nanotube. The double layered TONT/NP electrodes provide a high short-circuit current density (Jsc) and fill factor (FF) while maintaining a constant open-circuit voltage (Voc), finally disclosing a higher conversion efficiency (eta;), attributed to the high loading of dye molecules, light scattering ability, low resistance in the photoanode/electrolyte interface, and longer electron life-times. In particular, the optimal efficiency was attained with the TONT/NP (18 mu;m) electrode having a Voc of 0.80 V, a Jsc of 7.80 mA/cm2, a FF of 62.7%, and a eta; of 3.93%. Incidentally, when using a longer TONT film (28 mu;m), eta; decreases steadily due to the fast charge recombination process. A more detailed discussionis included in the presentation.

Dye-sensitized solar cells (DSCs) generate the electricity from the sun light at the interfaces between the dye-sensitized semiconductor oxides and the redox couples. The dye cation formed by injection of electron into TiO2 should be regenerated by the redox couples for the photoelectrochemical cycles. In this system, the redox couples can interact with injected electrons and dye cations for the recombination and regeneration, respectively. Therefore, it is essential to understand the reaction kinetics of the I3-/I- redox couples in the device. Here, we propose that the introduction of a sodium bis (2-ethylhexyl) sulfosuccinate (AOT)/water system to the I3-/I- electrolyte enables the control of the total and local concentration of the redox couples.
We have demonstrated that the AOT/water system coupled with the I3-/I- electrolyte enables the control of the concentration of the redox couples without diminishing the surface concentration of the chemisorbed dyes and can be used in the implementation of high-efficiency DSCs. The easy regeneration and difficult recombination by the hydrogen bonding between the SO3- group in the AOT and the COOH group in the commercial dye (N719) and the accelerated hydrolysis of I3- primarily contribute to enhancing the power conversion efficiencies without deteriorating the long-term stability of DSCs. This is important information on the concentration variation effects of the I-/I3- redox couple in the electrolyte and can assist in understanding and enhancement of the regeneration and recombination kinetics, achieving a high power conversion efficiency of ~11% for ~1000 h of DSCs.

We report on rutile TiO₂-based perovskite solar cell showing efficiency exceeding 9%. Rod shaped nanocrystalline rutile TiO₂ with aspect ratio of about 0.3 was prepared by hydrolysis of TiCl₄ at ambient temperature. Anatase TiO₂ with diameter of about 50 nm was also synthesized for comparison. Submicrometer-thick mesoporous rutile and anatase TiO₂ layers were prepared on FTO conductive substrates by spin-coating method. CH₃NH₃PbI₃ perovskite light harvester was infiltrated in porous TiO₂ layer by spinning CH₃NH₃PbI₃ precursor solution. Perovskite capping layer formed on TiO₂ film by the spin coating procedure as confirmed by the cross-sectional scanning electron microscopy. Spiro-MeOTAD and Au were used as hole transporting layer and counter electrode, respectively. Rutile TiO₂ based CH₃NH₃PbI₃ solar cells showed average power conversion efficiency (PCE) of 8.19% obtained from 48 cells, which was 13% higher than the average PCE of anatase TiO₂ based solar cells (7.23%). Rutile-based cell showed higher photocurrent, but lower voltage than anatase-based one. Higher photovoltaic performance was thus mainly due to an improved fill factor by 18% (from 0.55 to 0.65). Longer electron life time was observed for the rutile-based perovskite solar cell despite lower voltage, which might be the consequence of electron injection from the perovskite to rutile TiO₂, whereas more than one order of magnitude shorter electron life time for anatase-based cell was likely to be ascribed to non-injected electrons in the perovskite. A PCE of 9.54% at one sun illumination was obtained by optimizing rutile-based perovskite solar cell.

ZnO anodes composed of nanoparticles (NPs) with various diameters were chemically integrated on ITO-plastic substrates at room temperature (RT) for use in flexible dye-sensitized solar cells (DSSCs).Two layers of ZnO NPs with diameters of ~5 nm and ~20 nm followed by a layer of ZnO porous aggregates were coated on the ITO-plastic substrate in sequence, which are supposed to perform as the blocking layer, conventional anode and light scattering layer, respectively. Internecking of these NPs were conducted using a RT chemical bath deposition (CBD) method. The RT-grown ZnO nanospines on the surfaces of NPs connect the NPs to form the efficient electron pathways. Dynamics of recombination and electron transport measurements indicate that the electron collection in the NPs film is significantly enhanced by the facile RTCBD method. In the absence of high temperature annealing and mechanical compression, an efficiency of ~4 % can be simply achieved in the flexible ZnO DSSC using this process.

Earth abundant metal chalcogenides, such as cobalt sulfide and iron sulfide, could be platinum (Pt) alternative as counter electrodes for dye-sensitized solar cells (DSSCs). We have synthesized high quality colloid Co9S8 and FeS2 nanocrystals by facile solution-based synthetic approach that are more proper for massive particle production. The as-prepared products were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy dispersive spectrometer (EDS). Nanoscale metal chalcogenides showed good stability with respect to I-/I3- redox reactions and with excellent electrocatalytic activity. Both Co9S8 and FeS2 nanocrystals can serve as inks to fabricate uniform thin films on conductive substrate through non-vacuum deposition and had superior electrocatalytic activity as a counter electrode for DSSCs. Different thickness of metal chalcogenide thin films were obtained by tuning the experimental parameters, including deposition time and precursor concentration. With a 2 cm2 working area, the counter electrodes made of metal chalcogenide nanomaterials showed comparable photon-to-electron converting efficiency to Pt. These results demonstrate that metal chalcogenide nanomatreials are promising substitutes for the conventional Pt counter electrode for DSSCs.

CdS nanoparticles (NPs) and CdS nanorods (NRs) were deposited on fluorine-doped tin-oxide (FTO) glass by using the chemical bath deposition (CBD) method for CdS NPs and CdS NRs by a one-step thermal process. The FTO glass was coated with Aluminum doped Zinc Oxide (AZO) by atomic layer deposition (ALD). We controlled the morphology of CdS NPs and NRs and characterized them with multiple techniques including scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), x-ray diffraction (XRD), photoluminescence (PL), Raman scattering, and optical absorption. SEM and XRD analysis reveal the different crystal structures between the CdS NRs and the CdS NPs. The PL peak at ~ 710 nm for the CdS NPs is shifted to 728 nm for the CdS NRs. Besides a common Raman peak at ~ 300 cm-1, CdS NRs exhibit a second peak around 468 cm-1 while CdS NPs show a second peak at ~ 600 cm-1. Similarly the optical absorption peak position for CdS NPs (~450 nm) is higher than that of the NRs (~ 400 nm), indicating that the CdS NPs has a larger energy gap. The diversity in the optoelectric properties between CdS NPs and NRs also reflect in the different photovoltaic performance of the CdS-decorated ZnO/PANI hybrid solar cell devices. The CdS NRs device without ZnO NPs, and ZnO NPs without CdS NRs yielding energy conversion efficiencies of 1.8% and 0.125% respectively. However, the incorporation of the CdS NRs-ZnO NPs-PANI-FTO solar cell device gives an energy conversion efficiency of 2.44% while CdS NPs-ZnO NPs-PANI solar cell device energy conversion efficiency is 1.35%.

ZnO exhibits a wurtzite hexagonal structure and a direct optical bandgap and has been studied for use in many attractive applications such as gas sensors [1], transport electrodes [2], piezoelectric devices [3], varistors [4] and surface acoustic wave devices [5]. Its direct optical bandgap of 3.4 eV at RT is wide enough to transmit most of the useful solar radiation in ZnO/CuInSe2 based solar cells [6]. Furthermore, doped ZnO such as AZO (Al-doped ZnO) is a good candidate to substitute for ITO (Sn-doped In2O3) and FTO (F-doped SnO2) in transparent conductive electrodes. Optically pumped UV emission of ZnO at RT has also been reported previously [7].
In our previous work [8], non-doped ZnO films were successfully grown on a polyethylene terephthalate film by a conventional spray pyrolysis at 150 degree C using a diethylzinc (DEZ)-based solution under an air atmosphere. The samples average optical transmittance had more than 80%, flat surfaces and a predominately a-axis orientation determined from optical transmittance, scanning electron microscopy (SEM) and x-ray diffraction (XRD) measurements, respectively. It is well known that the DEZ reacts with water and/or oxygen at low temperature, and ZnO can be generated [9, 10]. However, the DEZ reacts violently with water and easily ignites upon contact with air. Therefore, in its pure state, it should be handled using inert atmosphere techniques.
Non-doped ZnO films were successfully grown on glass substrates by conventional spray pyrolysis at low temperature using a DEZ-based solution under an air atmosphere. This is the first report of a ZnO film growth using a non-vacuum process at minimum temperature. The average optical transmittance of the film is above 80%. The surface is not smooth and homogeneous. The free carrier density is low, as shown by the high transmittance in the IR region. The growth using DEZ solution at low-temperature is attributed to atmospheric steam.
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Hematite (α-Fe2O3) has been receiving a lot of recent attentions as a material for photoanodes in photoelectrochemical (PEC) cells for water splitting owing to several advantages such as stability in water, abundance in earth crust, low material cost, proper band gap lying in the visible spectrum, valence edge below the water-oxidation level. While extensive experimental studies have been carried out, theoretical investigations, particularly at the first-principles level, are scarce. Furthermore, there are few studies on the point defects at the atomic level although the defect engineering has been found to be able to improve the material property.
In this presentation, the native point defects in Fe2O3 are theoretically investigated using ab initio methods based on the GGA+U formalism. In particular, we consider vacancy and interstitial defects of oxygen and iron as well as electron polaron as a FeII defect at the host FeIII site and determine relative stability by computing formation energies and the charge transition levels. The cell-size dependence of charged defects is carefully tested. We determine the oxygen deficiency at high-temperature equilibrium condition and find an excellent agreement with experiment. In the quenched condition, it is found that the Fermi level is pinned at ~0.5 eV below the conduction band minimum, which may limit the performance of Fe2O3 as photoanodes in solar water-splitting cells. Furthermore, it is found that intrinsic defects generally lead to n-type condition, where the oxygen vacancy is mostly neutral and the Fe interstitial responsible for electron carriers.

Nano- and mesostructured metal oxide electrode materials have proven to enhance the performance of hybrid solar cells compared to their flat counterparts.
Here, we present an alternative approach where, in a general organic bilayer solar cell structure, the electron acceptor rather than the electron extracting electrode has an intrinsic mesostructure and, in addition, offers an easy way to tune its absorption spectrum. Graphitic carbon nitride (g-C3N4), which is a very promising two-dimensional polymeric material for photocatalytic water splitting, is deposited as a mesoporous thin film on a flat ITO/TiO2 electrode. In contrast to previous methods, we use direct thermal thin-film formation from powder to deposit g-C3N4 and to control the mesostructure. We show how the properties of these films can be tuned and how the addition of barbituric acid (BA) can shift the absorption spectrum of g-C3N4 towards the red, shifting the lowest unoccupied molecular orbital (LUMO) providing efficient electron transfer from P3HT (poly(3-hexylthiophene)) to g-C3N4.
Inverted solar cells (ITO/TiO2/mp-g-C3N4/P3HT/MoO3/Au) have been fabricated surpassing previously reported values using sputtered carbon nitride by a factor of two.
Time-delayed collection field experiments are performed to evaluate the electric field dependence of charge generation and the implications of the mesostructure on charge transport in g-C3N4-based solar cells. Two-dimensional confocal photocurrent microscopy is employed to study local inhomogeneities in photocurrent generation in order to find mesostructures most useful for efficient photocurrent generation.
We demonstrate that graphitic carbon nitride may be a versatile and interesting candidate for a mesostructured polymeric electron acceptor with chemical tunability and we expect to see an increase in performance through optimization of both carbon nitride deposition and solar cell fabrication.

In dye-sensitized solar cells (DSSCs), the adsorption of two dyes with complementary absorption spectra has attracted considerable attention as a method to improve the light-harvesting features over a broad range of the solar spectrum. As a matter of fact, this approach has provided a significant enhancement of power conversion efficiencies (PCEs).1, 2
In the current work, two novel tetraphenylbenzoporphyrins (TPBPs) were synthesized and photophysically characterized as a novel class of sensitizers for TiO2-based DSSCs. Owing to their π-extended framework, they feature a strongly red-shifted absorption spectrum compared to standard porphyrins. As a matter of fact, their Soret and Q bands render those TPBPs promising candidates for cosensitization with N719. For example, the absorption spectrum of N719 shows two bands at 393 and 533 nm, which, indeed, flank the Soret bands of TPBPs at 460 nm. Additionally, the Q bands contribute in the far-red range, where N719 features no absorption.
In order to explore this unique feature in DSSCs, two cosensitization approaches were carried out, that is, the dye cocktail and the stepwise. While the earlier did not reveal improved solar cell performances, the latter featured an unforeseen adsorption behavior of the TPBPs. In particular, the free-base TPBP (H2TPBP) covered both the transparent and the light-scattering TiO2 layer, whereas the metallated TPBP (ZnTPBP) only attached to the transparent TiO2 layer. Thanks to this trend, N719 is selectively adsorbed onto the light-scattering layer. To shed light onto this unusual adsorption behavior electrodes with inverted layer architecture and with different particle sizes were probed. As a matter of fact, we were able to ascribe the unusual adsorption behavior to the particle size and not to the layer architecture.
Our most valuable finding is that DSSCs with the combination of ZnTPBP and N719 feature synergistic effects, resulting in an efficiency improvement of 39% when comparing devices with only TPBPs and N719. Importantly, the possibility to implement two dyes in different parts of the TiO2 network in a straightforward, two-step soaking manner constitutes a tremendous achievement in selective cosensitization.

1. N. C. Jeong, H.-J. Son, C. Prasittichai, C. Y. Lee, R. A. Jensen, O. K. Farha and J. T. Hupp, J. Am. Chem. Soc., 2012, 134, 19820-19827.
2. K. Lee, S. W. Park, M. J. Ko, K. Kim and N. G. Park, Nat. Mater., 2009, 8, 665-671.

The dye sensitized solar cell is gaining importance owing to its cost effective and non-toxic ingredients. However, to compete with conventional Si solar cells or other photovoltaic devices, further investigation is required to implement its use more competitively. In quest of gaining much improved efficiency and wide absorption range for a solar cell we have developed a TiO2 nanowire based novel hybrid material. Initially, a surface modified nanowire (SMoTiN) incorporating a surface trapping layer has been designed to overcome the recombination of electron hole pairs which is a major obstacle to achieve higher efficiency. In addition, by varying aspect ratio of nanowires, we aim to achieve materials with absorption capacity over a wide spectral range with coverage over the whole range of solar radiation. Further, we also investigate Novel SMoTiN-QD hybrid materials using TiO2 or graphene QDs to achieve improved efficiency.
These synthesized materials are studied by several microscopic and spectroscopic techniques to understand the chemistry and structural-microstructural characteristics responsible to achieve the desired properties. The surface chemical investigations by XPS are followed by gross chemical information by FTIR. The gross structural characterization was performed by X-ray diffraction analysis and detailed structural and microstructural investigation by use of transmission electron microscopy. In order to check the suitability of these materials for solar cell, photocatalytic activity was performed. A dye degradation approach was adopted using UV-Vis spectroscopy following observation of degradation of absorbed dye under UV radiation.

Semiconductor quantum dots (QDs) such as CdS, CdSe, and PbS have been widely employed as sensitizers of photochemical solar cells due to their specific advantages compared with dye molecules, such as easily tunable energy gap, broadband optical absorption, high extinction coefficients, larger intrinsic dipole moment facilitating rapid charge separation, and potentially higher stability and resistivity against oxygen and water. Among the semiconductor candidates, CdS is a promising material whose conduction band edge in bulk lies above that of TiO2, which is propitious to the injection of excited electrons from CdS into TiO2. However, the bulk band gap of CdS is 2.25 eV, which limits its absorption threshold below ca. 550 nm, leaving much of the visible and near-IR spectrum unutilized. Cosensitization with molecular dyes is a viable strategy to extend the spectral absorption to the near-infrared. Besides, the internal recombination in the QD absorber is always very serious, which could be possibly inhibited by introducing a fast hole scavenger like Ru dye.
Here, a highly efficient quantum dot/inorganic layer/dye molecule sandwich-structure was designed and applied in electrochemical solar cells with improved photovoltaic performance and high photostability. The key component TiO2/CdS/ZnS/N719 hybrid photoanode with ZnS insertion between the two types of sensitizers was demonstrated to not only efficiently extend the light absorption, but also dramatically suppress CdS corrosion and charge recombination from either TiO2 or CdS quantum dots (QDs) to electrolyte redox species. Notably, charge separation between two sensitizers, especially the hole transfer from CdS&’s valence band to N719&’s HOMO as CdS regeneration, was efficient thanks to the type-II alignment between CdS QD and N719 molecule according to Raman and PL measurements. Even in the presence of ZnS deposition between two sensitizers, the strong photoinduced charge transfer was not hindered. This was also confirmed by the excellent photovoltaic performance of the solar cell employing this TiO2/CdS/ZnS/N719 hybrid film, yielding a photocurrent density of 11.04 mA cm-2, an open-circuit voltage of 713 mV, a fill factor of 0.559, and an impressive overall energy conversion efficiency of 4.4%. It has been also found that the synergistic stabilizing effect of both the organic and inorganic passivation layers helped to achieve improved long-term photostability in the presence of a corrosive electrolyte. Cobalt complexes were then exploited to further enhance the cell stability. Such a strategy serves as a promising alternative to the conventional configuration of QD-sensitized solar cells and opens up a new way of fabricating highly efficient nanocrystalline hybrid solar cells by using combined inorganic-organic system.

Since Grätzel et al. reported the first efficient dye-sensitized solar cells (DSSCs) in 1991, which have attracted much attention due to their relatively high power conversion efficiency and potentially low cost production. Organic photo-sensitizers containing multi-acceptors in a chromophore have been synthesized and characterized for the application of dye-sensitized solar cell (DSSC). In this study, we have used the intramolecular push-pull system containing phenothiazine as the electron donor with different number of cyanoacetic acid moieties as electron acceptor/anchoring groups in a chromophore. The experimental results have revealed that when the induced electron acceptor increases, the larger amounts of dyes are adsorbed on the TiO2 surface in DSSC, resulting in the increase of short circuit photocurrent density. Our results suggest that the organic dyes with multi-electron acceptors moieties are promising for getting higher solar-to electricity conversion efficiencies in DSSC.

It is well known that the presence of traditional liquid electrolytes in dye-sensitized solar cells is related to problems such as precipitation of salts in the electrolyte at low temperature, evaporation of liquids of the electrolyte at high temperature, corrosion and lack of long-term stability of the cells. In order to overcome various problems associated with liquid electrolytes, quasi-solid-state polymer electrolytes which is well known PVDF-HFP can be used in dye-sensitized solar cells.
Moreover, Compared with the mechanistic complications of multiple electron transfer redox reactions such as iodide redox couple, we have studied alternative redox shuttles. Among these alternative mediators, cobalt bipyridyl complexes have provided the best performance to data. In a DSSCs, this implies a reduction in the driving force for sensitizer regeneration and a possible increase in photovoltage and power conversion efficiency, provided that the rate of charge recombination of the cobalt bipyridyl redox couple is comparable to or slower than that for iodide mediators. Interestingly, not all N719 dye perfomed poorly with cobalt redox mediators. Therefore, we used bulky derivatives such as Z907 of cobalt redox complexes which can help to avoid the fast backward electron transfer from working electrode to Cobalt.

Dye-sensitized solar cells (DSSCs) have attracted much attention of many research groups on account of their benefits, for instance, of low-cost fabrication process with decently high solar energy-to-conversion efficiencies compared to conventional p-n junction solar cells.
The organic dyes with double electron acceptors have recently been reported to be very favorable for the DSSCs, compared to those with single electron acceptor, due to the more efficient electron extraction paths from electron donor and the higher molar extinction coefficients.
However, the DSSCs using the organic dyes have yet been much less stable than the Ru-based dyes, due to the formation of unstable radicals under continuous light illumination and the dye desorption from the TiO2 nano-particle surfaces.
In this article, we have developed novel organic dyes which have triphenylamine moieties as an electron donor in their charge-transfer chromophore for application of DSSCs. We had synthesized a series of triphenylamine derivatives which have different wave length absorbing chromophore in the molecule with high molar extinction coefficient. The photovoltaic performance of DSSC composed of organic chromophores with broad wavelength absorption property were measured and evaluated by comparison with that of pristine ruthenium dye.

Dye-sensitized solar cells (DSSCs) have attracted considerable attention on account of their high solar energy-to-conversion efficiencies and low cost processes compared to conventional p-n junction solar cells. Despite potential advantages of their high molar extinction coefficiency, convenient, and customized molecular design for their photophysical and photochemical properties, the organic photo-sensitizer still remain rather limited application field mainly because of their lower conversion efficiencies (2~9%) and stabilities, compared to Ru-dyes. Recently, the performance of DSSCs with organic photo-sensitizer has remarkably been improved, suggesting that smartly designed organic dyes are highly competitive candidates for use as photosensitizing dyes in DSSCs.
In this work, the DSSCs were fabricated with double electron acceptor metal-free organic photo-sensitizer on the phenothiazine framework that was bridged with thiophene moieties. An organic dye with a single electron acceptor was also synthesized for comparison purposes. The photovoltaic performances of these dyes were measured and compared in order to characterize the effects of the multi-anchoring groups on the open-circuit voltage and the short-circuit current.

Nanostructured ZnO-based dye-sensitized solar cells(DSSCs) have attracted considerable attention in recent years due to the similarity of the energy bandgap and electron injection process of the ZnO to those of TiO2. Recent studies on ZnO-based DSSCs have primarily focused on the improvement of electron transport and the reduction of the recombination rate through ether a series of hopping events between trap states on neighboring particles or diffusive transport within the extended states slowed by trapping/detrapping events. Therefore, one method of achieving higher photovoltaic performance is to use ZnO nanorod. The decreased pathways of the electron travel avoid the recombination effects compared with those in nanocrystalline films. However Protons derived from the Ru-complexes make the dye-loading solution relatively acidic and dissolve ZnO, generating Zn2+/dye aggregates. Such aggregates are harmful to the cells because they lower electron injection efficiencies and fill nanorod of the ZnO photoanodes. So atomic layer deposition(ALD) of TiO2 were deposited onto ZnO nanorod to protect the columns.
The other one to increase the efficiency of DSSCs, we used a counter electrode which has ZnO nanorod. When using Pt on ZnO wider specific surface area is better caltalytic properties. In this paper, the ZnO nanostructures processing high electrochemical activity and large surface area were grown by the chemical deposition method. After ZnO nanorod applying to the working electrode and counter electrode each experiment was performed.

Dye sensitized solar cells are currently attracting widespread academic and commercial interest for the conversion of sunlight into electricity because of their low cost and high efficiency. Among of them, the performance of DSSCs based on organic dyes have been remarkably improved. Organic dyes have many advantages as photosensitizers, such as large molar extinction coefficient, control of absorption wavelength, facile design and synthesis, and lower cost than ruthenium complexes. Therefore, it is expected that organic dyes could compete with ruthenium complexes in the near future. So, We report that a combined experimental and computational study of several ruthenium(II) sensitizers and organic sensitizers.

All solid-state organic-inorganic hybrid perovskite sensitized solar cells have been intensively focused for next generation solar cells, promising high efficiency. Currently, Mesoporous oxide layers were used for scaffold or photoelectrode in the perovskite devices. Nanosphere-shaped mesoporous films, however, have limit in terms of infiltration of the hole transporting materials (HTM) into TiO2 mesoporous film. Therefore, we fabricate solid-state CH3NH3PbI3-sensitized solar cells based on TiO2 nanotube arrays which have open pore structure as a photoelectrode, and analyze their photovoltaic properties. Nb-doped TiO2 thin layers were deposited on fluorine-doped SnO2 glass substrate by rf-magnetron sputtering to prevent the electric field-assisted dissolution of NTAs. Ti metal thin films were deposited in the same manner, subsequently. Highly ordered TiO2 nanotube arrays (NTAs) were grown on a transparent conducting oxide (TCO) via electrochemical anodizing method. The diameter and thickness of the NTAs can be controllable by various applied bias and reaction time. CH3NH3PbI3 sensitizers were deposited by two-step solution process on the obtained NTAs photoelectrode. A solution of spiro-MeOTAD was deposited by spin-coating. We investigate morphology of fabricated perovskite device based on NTAs as photoelectrode, and photovoltaic performances were measured, depending on thickness of NTAs and pore condition.

We report on the development of the weavable dye sensitized solar cells (DSSC) by using carbon nanotube (CNT) yarns. For this purpose, fermat CNT yarns were spun from multi walled carbon nanotube (MWCNT) forests firstly. Then these yarns were heat set in a vacuumed furnace and coated with mesoporous TiO2 and this coating process repeated after eight individual yarns were plied together. Finally the electrode of the solar cell was obtained by heat setting these plied re-coated yarns in the same furnace. On the other hand archimedian yarns of MWCNTs coated with a thin layer of platinum prepared as a counter electrode to complete the architecture used in this DSSC. We improved the efficiency by using photonic-crystal filament between working and counter-electrodes. This weavable DSSCs enabled us to achive a power conversion efficiency over 4.6%

TiO2 photoelectrode layers were fabricated using one of dry deposition methods called nano-particle deposition system (NPDS) on transparent conductive oxide (TCO) glass for dye sensitized solar cells (DSSCs). Conventional paste type method requires numerous steps, which can be time consuming and cumbersome whereas dry deposition method, such as Aerosol Deposition Method (ADM) sprays raw TiO2 powders directly on TCO glass. In spite of its advantages of simple and quick process, dry deposition methods were not adequate to make moderate porous structure for DSSC. For this reason, the efficiency of DSSC using electrodes deposited by dry deposition method reported was below 5%. In this study, TiO2 layers were dry deposited by breaking agglomerates without particle fracture using NPDS in order to fabricate translucent photoelectrode with porous structure. Deposited TiO2 layer showed high level of surface area and light diffusion using its agglomerated structure with its size of 100nm and its transmittance of 20% at visible light spectrum. This indicated that enough light diffusion and absorption were taking place using TiO2 monolayer without any additional diffusion layer. Consequently, photovoltaic efficiency of 7.3% was measured using TiO2 monolayer deposited via NPDS and this efficiency value is close to the reported maximum efficiency of 8.75% using N719 dye and commercially available TiO2 powders used as monolayer of TiO2 photoelectrode. Therefore, dry deposited electrodes were shown to be highly promising materials to replace paste type method for DSSC fabrication.

Emerging technologies like dye-sensitized solar cells have known a steady development in the past decade as a promising way to reduce fabrication costs as well as to offer novel deployment schemes such as building integrated solutions. However, despite important progresses, DSSCs still exhibit lower efficiencies than their inorganic counterparts. New strategies have recently emerged to increase the solar cells efficiency by using carefully designed light harvesting structures. However, to avoid additional processing costs, important efforts remain to be deployed to propose simple and scalable fabrication solutions that result in robust and efficiency systems. In this project, we demonstrate highly efficient and tunable light trapping structures in the form of Distributed Bragg Reflectors prepared using simple Sol-Gel chemistry and spin coating processing. Periodic dielectric stacks are fabricated with dense TiO2 layers as the high index material (2.08 in the visible range), and macroporous silica layers as the low index counterpart (1.24 with 50% of porosity). The porous layers are prepared thanks to a porogen latex that is calcinated during a single annealing step of the full stack, highlighting the simplicity of our approach. A defect-free semi-transparent 9-layer stack is obtained and shows a high specular reflectivity up to 96% at normal incidence in its prescribed bandwidth. We also demonstrate the flexibility of our process by making highly reflective DBRs with a tunable reflection range from UV to IR (from 400nm to 1300nm). More over, thanks to the closed porosity, our approach enables to realize robust DBRs over time and in harsh conditions compared to open-porous reflectors made with colloids, highlighting their potential for long lasting solutions. Finally, we show, as a proof of concept that a wavelength optimized 8-layer DBR integrated in a a-Si:H thin-film solar cell increases the efficiency of the cell by 14.6%. Given their stability over time and inherent attribute to reflect light in a given bandwidth while remaining transparent in the visible, we propose that this scheme could be ideally exploited to realize semi-transparent DSSCs to be deployed in building integrated solutions.

Transient absorption and time-resolved photoluminescence measurements of high-performance mesoporous TiO2 photoanodes sensitized with CuInSexS2-x quantum dots reveal the importance of hole scavenging in the characterization of photoinduced electron transfer. The apparent characteristic time of this process strongly depends on the local environment of the quantum dot/TiO2 junction due to accumulation of long-lived positive charges in the quantum dots. The presence of long-lived photoexcited holes introduces artifacts due to fast positive-trion Auger decay (60 ps time constant), which can dominate electron dynamics and thus mask true electron transfer. We show that the presence of a redox electrolyte is critical to the accurate characterization of charge transfer, since it enables fast extraction of holes and helps maintain their charge neutrality. Although electron transfer is observed to be relatively slow (19 ns time constant), a high electron extraction efficiency (>95%) can be achieved because in well-passivated CuInSexS2-x quantum dots neutral excitons have significantly longer lifetimes of hundreds of nanoseconds.

In dye sensitized solar cells (DSSCs), the major roles of semiconductor photoanodes are dye adsorption, collection of photoexcited electrons from dye (charge injection) and efficient electron transport. An efficient DSSC demands quantitative charge injection with minimal recombination loss. It is therefore important to understand the energetic and kinetic processes involving these semiconductor materials, e.g. the conduction band energy, trap state distributions, recombination mechanism, etc. Accurate determination and characterization of the conduction band energy of porous nanostructured films, as well as trap state involvement in various electron transfer processes in DSSCs, still remain a challenge. Our lab has recently introduced a variable temperature spectroelectrochemical (VTSEC) method to determine the conduction band edge of nanostructured photoanode materials. In this presentation I will show results of VTSEC measurements of different photoanode materials in the presence of variety of solvents as well as electrolyte additives. Results of electrochemical measurements of the sub-bandgap trap state distribution will also be presented. The conduction band energy and trap-state measurements will be correlated with results of measurements of recombination to outersphere redox shuttles. These combined results provide a more detailed picture of the material controlled processes in a DSSC.

The current state-of-the-art dye loading process for modern dye-sensitized solar cells (DSCs), only slightly modified from the original procedure used in the seminal 1991 report, entails dip-coating nanoporous TiO2 photoelectrodes in a concentrated solution of dye for an average of 16 hours. This process constitutes up to 80% of the fabrication time, leads to significant amounts of dye waste, and necessitates the use of organic solvents. A promising gas-phase deposition technique, coined Functionalized Carboxylate Deposition (FCD), was used to rapidly deposit a self-assembled monolayer of targeted α-carbon modified carboxylic acid containing dye molecules on TiO2 photoelectrodes. The FCD process successfully reduced the dye loading time by 98% (i.e., 15 min compared to 16 hr for FCD and dip-coating, respectively). Moreover, the FCD-sensitized photoelectrodes produced DSCs with efficiencies over 1.5 times higher than dip-coating for all dyes tested. The performance of FCD dye sensitization in this study may indicate its applicability as a foundational technology for rapid and ultra-low cost fabrication of highly efficient DSCs.

Increasingly research is being dedicated to replacing rare earth elements in Transparent Conductive Materials (TCMs).In this work we present the application of innovative Transparent Conductive Oxide (TCO) films in the form of hybrid TCMs. The hybrid film we demonstrate exhibits excellent properties (sheet resistances (Rs) of about 10-100 Omega;/square and optical transparency of approximately 80% including losses due to substrate) with variable haze factors up to 60% for visible light.
These hybrid electrodes incorporate ZnO nanoparticles into a matrix of Fluorine doped Tin Oxide (FTO) and Ag nanowires in Aluminium doped ZnO, standard ZnO and FTO in order to enhance the properties of the two materials. The haze factor of electrodes has recently been shown to play a large role in the efficiency of a solar cell. Therefore have incorporated these hybrid electrodes in TiO2 Dye Sensitized Solar Cells (DSSCs) and we explore the impact of the electrode properties on the cell efficiency and on the electrical behavior of the devices.
This contribution represents a novel use of materials to modify a standard TCO thin film and enhance the properties of the resultant TCM. Combining materials and processes leads to the emergence of materials with distinctive properties that enhance the efficiency of DSSCs.

The role of deep interfacial traps on photovoltaic properties of dye sensitized solar cells is explained and simulated by a new charge transport model. The model incorporates electron capture/emission and oxidation/reduction processes mediated by the deep level interfacial trap states. The charge transport in DSSC was modeled through the formulation of electron conservation law in the nanoporous TiO2 network for both steady state and time dependent conditions. The steady state photovoltaic characteristic parameters like quantum efficiency, open circuit voltage, short circuit current, fill factor, efficiency, charge transport activation energy etc. and transient Photovoltaic characteristic parameters like charge transport lifetime were simulated. The validity of the model was investigated through series of experimental measurements which shows good consistency among simulated and experimental results.

Organic photovoltaics (OPV) have been modeled recently with different computational methods. However, as researchers are turning toward nanoscale morphologies and materials to increase OPV solar cell efficiencies, modeling interactions at these scales becomes computationally intensive. Recent work utilizes Finite Element Methods (FEM) to simulate absorption efficiencies of solar cell architectures with micron and nanoscale features. Using FEM-based COMSOL Multiphysics software, we pursued the modeling and analysis of nanomaterial functionalized OPV using the RF (Electromagnetic) Module within COMSOL. Modeling through FEM provides a cost-effective and waste-free characterization of potential device nano-architectures that lends experimentalists insight toward designing higher efficiency OPV solar cells. The effect of incorporating carbon nanomaterials is evaluated, such as carbon nanotubes (CNTs) and graphene as functional components into now standard bulk-heterojunction solar cells materials such as block copolymer poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM). Vertical pillar geometry is utilized for optimization of light interaction as well as increasing charge transport properties in the devices of interest. Optimization of film thickness and materials spacing in potential device architectures is investigated. Overall the results from this study are in good agreement with recent experiments and related simulations performed by independent researchers. This research contributes to advancing knowledge on the fundamental physical phenomena related to solar cell architectures and the efficient design of novel solar cells such as dye sensitized solar cells with nano-architectures, and represents a step toward their future use to bring innovative solutions to the global challenge of energy production.

Most of the time for the fabrication of dye sensitized solar cell(DSSC) by dip coating is consumed in dye loading. Functionalized Carboxylate Deposition(FCD) of dye aims at reducing time for dye loading and reduce cost for the fabrication of DSSCs. We deposited dye on a compact layer of TiO2 nanoparticles and studied the change in microscopic work function and surface potential of TiO2 after sensitizing with dye. Comparision of the result for compact TiO2 sensitized with dye by FCD with compact TiO2 sensitized with dye by dip coating reveals greater amount of dye absorbed on TiO2 nanoparticles for FCD process.

Dye sensitized solar cells have faced the challenge of limited lifetime owing to leakage of electrolyte arising from its chemical severity causing gradual degradation of the sealant materials, high vapour pressure. Gelation of electrolytes to enhance the long term stability of cells has been proposed by researchers. In the present study nanoparticles with widely differing isoelectric points have been used to prepare nanoparticle loaded electrolytes which undergo colloidal gelation. Owing to the different zeta potentials, cationic adsorption was observed on silica, alumina nanoparticles while anionic adsorption was seen for magnesia nanoparticles. These differences in adsorption behaviour resulted in significant differences in electrochemical characteristics of the cells as examined through cyclic voltammetry and impedance spectroscopy. Silica and alumina nanopowders loaded electrolytes performed similar to liquid electrolyte. Also, higher thermal stability was observed for such nanoparticle loaded electrolytes as compared to liquid electrolyte. Interestingly MgO loaded electrolytes achieved high open circuit potential even without use of tert-Butyl Pyridine (TBP). MgO nanopowder to electrolyte prevents recombination through adsorption of anions (triiodide/iodide) from electrolyte as supported by dark current measuements. Also, higher electron life time at the titania/electrolyte interface is observed in magnesia loaded electrolyte-based cells.

In recent years, Dye Sensitized Solar Cell (DSSC) has emerged as a promising alternative for Silicon based solar cells due to its low cost of manufacturing. Besides the photoanode, electrolyte the counter electrode characteristics also play a major role in determining cell performance. In the present study platinum nanoparticulate sol was synthesized and deposited over fluorine coated tin oxide (FTO) glass substrate by dip coating. The platinum nanoparticulate suspensions were synthesized by using a fixed concentration of chloroplatinic acid (H2PtCl6) and polyvinylpyrrolidone (PVP) as capping agent. The effect of time of deposition on cell performance has been studied. Field emission scanning electron microscope (FE-SEM) of electrodes showed uniformity of Pt deposition over wide area. Cyclic Voltametry was used to determine the catalytic activity and to estimate the electrochemical surface area (ESA) of the counter electrodes. The transmittance of dip coated Pt counter electrodes was higher (80 %) than the sputter coated (23 %) and it was close to the FTO coated glass (82 %). The dip coated counterelectode in the short term gives performance comparable to sputter coated electrodes at much lower platinum loading. Stability of counter electrode was checked by its static aging by dipping the EPD-Pt counter electrodes in electrolyte. I/V characteristics of cells prepared with the statically aged counter electrodes was measured over a gap of 20 days. The static aging analysis showed that the dip coated Pt counter electrodes were stable. Separate experiments on dynamic aging (illuminated, short circuit conditions) showed much lower stability for dip coated electrodes. Further work is being carried out to examine conditions which enhance stability of dip coated platinum electrodes under dynamic conditions.

Electrochromism refers to reversible changes in optical properties under an applied electric field. Because of their low power consumption and high coloration efficiency, electrochromic devices have many potential applications such as smart windows. As an energy saving devices, however, self-powered electrochromic devices with no power consumption are more desirable. In this sense, photoelectrochromic devices consisting of dye-sensitized solar cells (DSSCs) and electrochromic (EC) layers have an ideal configuration to realize an energy saving smart widow system. Nanostructured materials can provide a large surface area and a preferred path way for a charge transfer, so that they are utilized as electrodes in DSSCs or EC devices. Among the several synthetic routes to fabricate the nanostructured materials, an electrochemical anodization method is a simple way to form metal oxides with nanotube or nanoporous structures on metal substrates. In our study, we fabricated the photoelectrochromic devices consisting of TiO2 nanotube and WO3 nanoporous layers formed by anodization. For front-side illumination of DSSCs, free-standing TiO2 nanotube membranes detached from Ti substrate and fixed on to transparent conducting electrodes. The coloration efficiency of the device was also investigated as a function of the thickness of nanotube layer and the amounts of dye adsorbed.

A new method for the fabrication of highly transparent platinum counter electrode (CE) has been developed for efficient window-integrated dye-sensitized solar cells (DSSCs). The simplicity and low cost of this method provides a basis for an up-scalable fabrication process. The platinum consumption hence the cost of the counter electrode was reduced over 86%, compared to conventional Pt counter electrode prepared by sputter deposition methods. The prepared counter electrode film based on Pt nanoparticles (NPs) led to over 88% transparency and over 80% transparency when integrated with ITO/glass substrate. Scanning electron microscope (SEM) images show uniformly distributed Pt nanoparticles with the size ranging from few to 70 nanometers. These Pt nanoparticles act as a catalyst for the reduction of redox species in electrolyte while the spacing between them allow for the part of visible light to pass through the device, resulting in highly transparent DSSCs for window application. Surface morphology analysis confirmed that platinum forms a rough structure on the surface of ITO, suggesting that the fabricated counter electrode based on Pt nanoparticles has a high active surface area and high density of catalytic sites. DSSCs based on this new counter electrode showed 6.17% power conversion efficiency, comparable to the 6.46% efficiency of the corresponding reference opaque DSSC with sputtered Pt counter electrode.

Dye Sensitized Solar Cells (DSSCs) and Organic Photovoltaics (OPVs) are seen as possible sources of cost effective renewable energy. Recently, major advances have been made in electrolytic and solid state DSSCs through the use of perovskite nanocrystals as a sensitizing agent where power conversion efficiencies of over 12% have been realized [1-3]. Additionally, the uses of carbon nanotubes (CNTs) as a flexible, transparent, lightweight and robust electrode material have been demonstrated in both DSSC as well as OPV devices. The application of CNTs as a charge collector with perovskite sensitized DSSCs will be discussed. Performance characteristics of CNTs within perovskite based hybrid OPVs will be investigated and the role of CNTs as an efficient charge collector will be extended to the inverted geometry.
[1] Burschka, J.et al. (2013). Nature, 499(7458), 316-9
[2] Heo, J. H. et al. (2013). Nature Photonics, 7(6), 486-491.
[3] Im, J.-H et al. (2011). Nanoscale, 3(10), 4088-93.

Titanium dioxide is an indirect band gap semiconductor that has attracted significant interest as a photocatalyst for solar energy applications. This attribute has been utilized for the manufacture of self-cleaning glass, decomposition of organic molecules and microorganisms, and for hydrogen generation through water hydrolysis.¹ Special interest has been shown in the preparation of TiO₂ dye-sensitized solar cells (DSSC) as a potential low-cost alternative to traditional silicon-based photovoltaics. Preparing films and powders of titanium dioxide on transparent conduction oxides (TCO) is necessary for the construction of standard DSSC designs². Furthermore, <001>-orientation of anatase-phase TiO₂³ as well as the introduction of rutile-phase TiO₂⁴ have each been shown to increase the efficiency of the semiconductor&’s photocatalytic effect. In this work we present a low temperature hydrothermal synthesis method for the growth of titanium dioxide thin films on a TCO substrate that exhibit both strong preferred <001> orientation and a mixture of anatase and rutile phases.
Synthesis of <001> oriented titanium dioxide films used the method described by Ichimura, et al.⁵ Fluorine-doped tin oxide on glass was mounted vertically in a Teflon-lined autoclave with an aqueous solution containing varying concentrations of TiF₄ and NaF. The autoclave was then subjected to a ramp-and-soak regimen with 4 periods for each segment. The influence of reactant concentration, ramp, hold, and cool times, and reaction temperature were investigated. Films resulting from this method exhibit a primary anatase phase with over 80% <001> orientation as well as a minority rutile phase as determined by grazing-angle X-ray diffraction (XRD). Scanning electron microscopy (SEM) images show the polycrystalline films are continuous and composed of ~ 100 nm diameter crystals with well-formed square facets. SEM images show that film thicknesses are ~ 1 micron with this preparation. A summary of SEM, XRD, and photocatalytic study results will be presented.
(1) Chen, X.; Mao, S. S. Chem. Rev.2007, 107, 2891-2959.
(2) Gratzel, M. Inorg. Chem.2005, 44, 6841-6851.
(3) Han, X.; Kuang, Q.; Jin, M.; Xie, Z.; Zheng, L. J. Am. Chem. Soc.2009, 131, 3152minus;3153.
(4) Bakardjieva, S.; Subrt, J.; Stengl, V.; Dianez, M. J.; Sayagues, M. J. Applied Catalysis B, 2005, 58, 193-202.
(5) Ichimura, A. S.; Mack, B. M.; Usmani, S. M. Chem. Mater.2012, 24, 2324-2329.

Colloidal quantum dot solar cells (QDSCs) are of great interest due to their cost-effectiveness and solution-based processability. In particular, lead chalcogenide quantum dots (QDs) have large Bohr radii that result in a strong quantum confinement effect and a facile tunability of band gap energy, which provides better carrier transport characteristics and wide spectral responses for photovoltaic applications. With these great advantages, the performance of lead chalcogenide QDSCs has shown rapid improvements over the last few years. While such rapid developments have been achieved through engineering the optoelectronic properties of QD film and the device architectures, relatively little attention has been paid to the understanding and control of the interface properties in QDSCs. In the case of an interface between the QD film and the metal electrode, previous studies have showed that the strong Fermi-level pinning occurs at the interface which leads to low open-circuit voltages (Voc) and hence to low power conversion efficiencies (PCEs) in the QDSCs. Here, we demonstrate that the incorporation of an ultrathin oxide interfacial layer between the PbS QD film and the metal cathode significantly improves device performance and air stability in Schottky barrier QDSCs. Instead of introducing additional oxide material at the interface, we controllably produced an oxide layer through mild-oxidation treatments including air annealing at 60 oC, UV/Ozone treatment, and air exposure at room temperature: these treatments convert the top-most PbS QDs to an ultrathin wide-band-gap oxide layer. Through these processes, we fabricated a QD film/thin oxide/metal electrode structure, that prevents the formation of an unstable interface between the QD film and the metal electrode, leading to a decrease of the interface states. The oxide interfacial layer thus enables the generation of a Schottky junction with a significantly larger degree of band bending, which is desirable for the high performance of QDSCs. As a result, we demonstrate a much improved PCE of 3.39% (under AM1.5G illumination), which is 670% higher than that of a device without a thin oxide layer. Moreover, the device exhibits high stability of VOC under continuous light illumination, providing further understanding of the effects of oxide interfacial layers. Besides this, the oxide interfacial layer significantly enhances the air stability of Schottky barrier QDSCs to more than 500 hours, with a retention rate >55%, by acting as robust protection for the PbS QD film/metal interface from air.

Among solution-processed nanocrystals containing environmentally benign and earth-abundant elements, bismuth sulfide (Bi2S3) is a very promising n-type semiconductor for solar energy conversion due to the high absorption cross-section and the band gap in the near infrared. Till now, only a few types of Bi2S3-based solar cells have been reported. In the all-inorganic approach, the photovoltaic device is essentially based on a bulk nano-heterojunction between bismuth sulfide and lead selenide, a choice that could pose health hazard if deployed on large scale.[1] In the hybrid approach, the solar cell is based on a bulk-heterojunction between an organic semiconductor poly(3-hexylthiophene) and bismuth sulfide.[2,3] Here, we investigate hybrid device architectures in which we use spiro-OMeTAD as hole transport material, and bismuth sulfide as n-semiconductor.
Bi2S3 colloidal nanocrystals, with dimensions ranging between 3 and 30 nm, are obtained by mean of a synthetic route employing oleic acid as the capping ligand and toluene as the final solvent. Thin films are fabricated through dip-coating techniques followed by a ligand exchange procedure to replace the electrically insulating oleic acid with a shorter-chain capping agent (ethanedithiol). The effectiveness of the ligand exchange procedure are confirmed by FTIR spectroscopy. Carrier trapping mechanisms and trap density are assessed by time-resolved differential transmission spectroscopy.
Two types of cell architectures are investigated. The first type of cells is made by sandwiching the Bi2S3 layer between the n-type (TiO2) and p-type transport layers, i.e., with the structure FTO/TiO2/mesoscopic-TiO2/Bi2S3/SpiroOMeTAD-/Ag. Fluorine-doped tin oxide (FTO) and silver are used as carrier collecting electrodes. A second type of cell is fabricated without the use of TiO2, namely according to the structure FTO/Bi2S3/SpiroOMeTAD-/Ag forming a n-p heterojunction between Bi2S3 and SpiroOMeTAD. A maximum power conversion efficiencies around 1% is found. A comparative analysis of the photovoltaic response of the two cell architectures is reported.
[1] A. K. Rath et al., Nature Photonics, 2012, 6, 529.
[2] H-C. Liao et al., Cryst Eng Comm, 2012, 14, 3645.
[3] L. Martinez et al., Physical Chemistry Chemical Physics, 2013, 15, 5482.

Thin-film solar cells promise to reduce the cost of sunlight-to-electricity conversion compared to conventional monocrystalline silicon. A variety of approaches have been developed with different device architectures and materials systems. The most efficient thin-film absorber materials can fulfil the multiple roles of light-absorption, charge separation, and transport of both holes and electrons. Materials which can be processed with solution-based techniques at low-temperature, such as printing, should ultimately lead to the least expensive solar cell technology. However, those materials processable with the lowest cost methods are usually composed of complex architectures of distributed heterojunctions, which inherently introduce losses at the high density of internal material interfaces. Recently organometal halide perovskite absorbers have emerged as an exciting new material family for photovoltaics, with power conversion efficiencies having already exceeded 15%. Here, I will present our recent results on improving and understanding meso-superstructured and thin-film planar heterojunction perovskite solar cells, with both device based and spectroscopic investigations. A key focus of our work has been to eliminate the high temperature processing of all the layers in the solar cell. I will demonstrate how we have achieved this in a number of different device architectures, and also give insights as to why the organolead halide perovskite solar cells are already so efficient.

A porosity and a pore size are highly important for solid-state solar cells fabricated with submicron-thick mesoporous electrodes. A mixed paste of TiO2 and ZnO was prepared and the sacrificial material was removed from the film after etching in an acidic solution to make a highly porous structure. It was found that the modified films could supply larger space to load an organometallic sensitizer sustaining the original film thickness and enhancing the conversion efficiency of methyl ammonium lead iodide from lead iodide. As a result, the improved current density derived from the increased light absorption was observed in solar cell devices, which suggests great potential of this novel method for the modification of mesoporous electrodes.

A virtue of atomic layer deposition (ALD) for photovoltaic technologies is the control on film growth at (sub-)monolayer level. For example, in the field of dye-sensitized solar cells (DSCs), ALD has already been adopted to address the challenges related to the complex TCO/electron acceptor/dye/electrolyte interface, by developing ultra-thin passivation and recombination blocking layers.
In this work, we investigate other benefits of ALD not yet applied to DSCs. Low temperature (25 - 150 °C) processing by plasma-assisted ALD (PA-ALD) has been for the first time successfully applied in flexible, back-side illuminated DSCs to deliver the synthesis of a Pt counter-electrode (CE) on a conductive polymer [1]. This configuration requires a high catalytic activity to reduce the electrolyte redox couple, but also a high transparency of the Pt layer to improve light harvesting. The PA-ALD process is therefore designed to promote the synthesis of Pt nano-particles (NPs) serving as highly transparent CE.
The Pt NPs were deposited on ITO/PEN substrates by means of a newly developed Pt ALD consisting of three steps (MeCpPtMe₃ precursor dosing, O₂ plasma exposure and H₂ plasma exposure) [2]. In-situ spectroscopic ellipsometry was applied to screen the experimental conditions under which the nucleation process of Pt occurs on ITO/PEN. RBS and TEM were adopted to assess the Pt NPs density and size. Symmetric cells were used to evaluate the catalytic activity of the Pt NPs towards the liquid electrolyte using electrochemical impedance spectroscopy.
Being a trade-off between transparency and catalytic activity of the CE, the best cell performances of back-side-illuminated DSCs (~3.7% efficiency) are achieved for a number of ALD cycle between 50 and 100 at temperatures in the range of 100 - 150 °C. Under these conditions, a Pt loading in the range of 80-390 ng/cm² with a particle core size of 1.5-3 nm has been measured. The catalytic activity is controlled by the Pt loading and the Pt particle surface area. A larger number of ALD cycles leads to higher Pt loading, particle coalescence and therefore, a decrease in transmittance and short-circuit current. The deposition at 25 °C is also viable, although 300 ALD cycles are necessary because of the delayed Pt nucleation process. The best cell produced with ALD Pt CE outperforms cells fabricated with electrodeposited and sputtered Pt CEs, with a relative improvement in efficiency of 19% and 29%, respectively. For these conditions, ALD Pt showed a higher transmittance (99.5 - 93%) when compared to the ED and SP references (88%), whereas a similar catalytic activity was obtained (with a charge transfer resistance at the electrolyte/CE interface of 10 Omega;cm²). In addition, these parameters were used to fabricate a large area CE for a sub-module of 17.6 cm², with an efficiency of 3.1%.
[1] Garcia-Alonso et al., Adv. Energy Mat. 2013, DOI:10.1002/aenm.20130831
[2] Mackus et al., Chem. Mater. 2013, 25, 1769

Recently Organo-Metal Halide Perovskites have attracted considerable interest in view of their potential application in hybrid organic / inorganic solar cells. Efficiencies above 15% have been demonstrated , which are mainly attributed to large electron-hole diffusion lengths.[1] Yet, the energetics of these systems, i.e. the energy band diagram and related energy level offsets at perovskite interfaces with organic hole transport materials, have not been addressed.
We present direct measurements of valence and conduction band energies and associated ionization energy and electron affinity of CH3NH3PbI3, CH3NH3PbI3-xClx and CH3NH3PbBr3, using photoemission (PES) and inverse photoemission spectroscopy (IPES). Furthermore, we use PES and IPES to investigate the electronic structure of perovskite interfaces with the hole transport material Spiro-OMeTAD incrementally deposited in vacuum. We find subtle differences between these interface energetics, and provide a description for the hole transfer mechanism. We then extend our investigation to further hole transport materials and determine the barrier for charge carrier extraction from the perovskite absorber layer.
[1] S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T. Leitjens, L. M. Herz, A. Petrozza, H. J. Snaith, Science 2013, 243, 341-344

Dye-sensitized solar cells (DSSC) have attracted much attention recently due to its potential as a cost-effective alternative for silicon-based solar cells. Three major components are involved to make a highly efficient DSSC: a working electrode containing dye-sensitized nanostructured TiO2 layers, a counter electrode containing platinum-coated TCO substrate, and a composite electrolyte solution to fill between the two electrodes. In this lecture, I will present our recent developments on novel electrodes for highly efficient DSSC. For the counter electrodes, novel platinum nanostructures were fabricated according to a cyclic electro-deposition (CED) method for the Pt films with uniform nanograss structure having great electro-catalytic performance and intrinsic light-scattering, perfectly suitable for use as counter electrodes for DSSC. we have developed a strategy of nanofabrication to prepare monolayers of platinum nanoparticles self-assembled on a rough surface of TCO in two steps: first, preparation of a well dispersed Pt NP solution from polyol reduction at a pH well controlled on adding NaOH; second, dipping the SAM-functionalized TCO substrate, prepared beforehand using ‘piranha&’ and MPTMS surface treatments in turn, into the Pt NP solution near 295 K to complete the fabrication of the SAM-Pt CE for DSSC. The uniform SAM-Pt film with a narrow distribution of size can be produced on TCO without a stabilizer, so that the traditional thermal treatment at high temperatures is no longer necessary. The uniform nature of the SAM-Pt film has the advantages of minimizing the amounts of Pt loading and optimizing the catalytic function on the TCO surface. The DSSC devices prepared according to this SAM-Pt approach attained notable photovoltaic performance (PCE = 9.2 %) comparable to those fabricated with a conventional TD method (PCE = 9.1 %) or a CED method (PCE = 9.3 %) under the same experimental conditions. For the working electrodes, we constructed photoanodes, containing titania nanostructures of varied types - spherical nanoparticles (NP), one-dimensional nanorods (NR), and octahedron-like nanocrystals of varied size (HD1-HD3), in either a bi-layer (BL) or a multi-layer (ML) film configuration. We constructed the ML devices based on the NP-BL system with additional HDP layers inserted between the two components, NP and SL, to enhance JSC so as to attain the best performance, with efficiency of power conversion (PCE) = 10.1 %, at film thickness (L) 26 um. Because of the robust structural feature of the HD films, the devices fabricated with a simple BL film configuration, HD1/SL = 6/3, exhibited the best performance, PCE = 10.2 % at L = 29 um, which is a promising advance for Z907-based solar cells with a superior and enduring stability of performance for commercialization.

Co-assembly of block copolymer and inorganic nanoparticles provide a simple, direct synthesis route to fabricate well-ordered nanostructured materials that have been demonstrated to harness multiple properties, for example, charge carrier transport and light management in dye sensitized solar cells. We describe a new type of hybrid solar cell device using block copolymer nanostructures with the organic-inorganic hybrid perovskite absorbers. A clear understanding and control of the hybrid perovskite materials is the key to enhance the power conversion efficiency of the block copolymer perovskite-based solar cells. To the end, we utilize in-situ X-ray scattering to study the crystallization kinetics and film morphologies of the methylammonium lead iodide-chloride perovskite materials.

Metal-Organic Frameworks (MOFs) are highly porous supramolecular crystals assembled from a versatile array of functional molecular linkers, paddlewheels, and multidentate ligands, organized around metal ion hubs. By controlling the composition and chemistry of these building blocks, it is possible to tailor the molecular porosity, functionality, optical properties, and even electronic character of the MOF structures. We have recently learned to exploit the complex structural and functional character of MOFs in photovoltaic systems, where the atomically-organized interfaces between molecular building blocks can coordinate critical elements of photon absorption, energy transfer, and charge separation. Here, we specifically explore the application of MOFs as active elements in dye-sensitized solar cells (DSSCs). Guided by periodic Density Functional Theory calculations, we have synthesized and characterized a family of MOFs, designed as platforms to explore and optimize nanoscale ordering and energy-transfer processes, variable through modifications to MOF chemistry and structure. We describe the synthesis and integration of optically-active MOFs into DSSCs, and we investigate how MOF structure and chemistry influence the function of these materials as optical absorbers and charge transfer mediators in photovoltaic test cells. Exploiting these chemically and structurally versatile supramolecular structures promises creative new insights into the development of next generation photovoltaic development.
Sandia National Laboratories is a multi program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Graphene forest, or vertically-oriented graphene sheet network, was reported to have strong electrocatalytic reduction effects, such as I-/I3- redox pair reduction for dye-sensitized solar cells (DSSC) and hydrogen reduction reaction for water splitting, in substitution of the expensive platinum catalyst.
In this study, graphene forest (GF) was grown by microwave plasma chemical vapor deposition. The material nanostructure, chemical composition, electron transfer dynamics, and electrochemical properties of GF were examined. In particular, the electrocatalytic properties of GF were investigated with the aim to replace Pt for solar energy applications. It was found that the undoped GF, has strong catalytic function owning to its high density of sharp edges and other defects. Several unique properties of GF, such as superior electronic conductivity, facilitated ion transport in its straightforward porous structure, and large surface area with high density of active catalytic sites, make it a strong candidate for the cost-effective catalytic electrode for solar energy applications. The performance of DSSC with GF as counter electrode was compared with that based on Pt electrode. The results indicate very promising catalytic function of GF.

Solid-state, inorganic-absorber versions of sensitized solar cells provide a more stable alternative to liquid-electrolyte designs. Our work focuses on solid-state quantum dot-sensitized solar cells (ss-QDSSCs), fabricated by sensitizing nanostructured TiO₂ with quantum dots (QDs). QDs show favorable absorption properties due to their size-dependent band gap, as well as high molar extinction coefficients. However, QDSSC efficiencies remain low, especially in solid-state devices, due to high rates of recombination and low surface coverage of the TiO₂ by the QDs. The record efficiency for liquid QDSSCs is 6.7%,¹ and only 1.5% for ss-QDSSCs.² In this work, we examine two strategies for improving charge collection in ss-QDSSCs: reducing recombination by barrier layers, and increasing QD surface coverage.
We first demonstrate that ultra-thin barrier layers of Al₂O₃ deposited by atomic layer deposition (ALD) can improve the performance of CdS ss-QDSSCs fabricated with spiro-OMeTAD as the hole transport material.³ CdS QDs were grown by successive ion layer absorption and reaction (SILAR). We explored placement of the ALD Al₂O₃ barrier layers either before or after the QDs, resulting in TiO₂/Al₂O₃/QD and TiO₂/QD/Al₂O₃ configurations. The efficacy of the barrier layers were tracked through current-voltage (J-V) measurements and transient photovoltage (TPV) measurements of electron lifetimes. In both barrier layer configurations, the Al₂O₃ layers were found to increase V_OC, suppress dark current, and increase electron lifetimes - all indicators of decreased recombination. The higher electron lifetimes observed in the TiO₂/Al₂O₃/QD configuration suggest that passivation of the TiO₂ surface is of particular importance in these devices. For thin Al₂O₃ layers, gains in open-circuit voltage (V_OC) and concomitant increases in efficiency were observed. At greater Al₂O₃ thicknesses, losses in short-circuit current (J_SC) caused overall efficiency losses. Devices with 1 Al₂O₃ ALD cycle deposited before the QDs performed best, with an efficiency of 0.35%.
To achieve higher efficiencies, we also studied PbS QDs, which have a lower bandgap better matched to the solar spectrum. PbS QDs were grown on nanostructured TiO₂ substrates by SILAR as well as by ALD. With the ultimate goal of increasing QD surface coverage, we compared the impact of these two synthetic routes on the light absorption and electrical properties of the device. The results suggest a higher reverse-bias photocurrent in the case of SILAR-grown QDs. We hypothesize this is due to a higher density of defects in SILAR-grown QDs, as compared to the ALD-grown QDs. Investigation into methods to increase QD surface coverage by chemical treatments of the TiO₂ surface, such as ALD-grown seeding layers, prior to QD growth will also be presented.
1. J. Wang, et al., J. Am. Chem. Soc., 2013, 135.
2. H. Lee, et al., Adv. Func. Mater., 2009, 19, 2735.
3. K. Roelofs, et al., J. Phys. Chem. C, 2013, 117 (11), 5584.

Phthalocyanines (Pcs) [1-3] are among the few molecules that reveal an intense NIR absorption and therefore, constitute promising dyes as DSC photosensitizers.[4] On the other hand, Subphthalocyanines (SubPcs) are the lowest homologues of phthalocyanines. Their pi-electron aromatic core along with their non-planar cone-shaped structure make them attractive compounds with singular chemical and physical properties. Subphthalocyanines [5] are currently emerging as excellent chromophoric systems for studying electron and excitation transfer processes with applications in photovoltaic devices.
In the current paper, a series of peripherally carboxylic acid substituted Pcs and SubPc derivatives have been prepared and used as active components in photovoltaic devices.
1. de la Torre, G.; Claessens, C. G.; Torres, T. Chem. Commun. 2007, 2000.
2. (a) Martínez-Díaz, M. V.; de la Torre, G.; Torres, T. Chem. Commun. 2010, 46, 7090. (b) Ragoussi, M.-E.; Ince, M.; Torres, T. Eur. J. Org. Chem. 2013, 29, 6475.
3. Bottari, G.; de la Torre, G.; Guldi, D. M.; Torres, T. Chem. Rev. 2010, 110, 6768.
4. Ragoussi, M.E.; Cid, J.J.; Yum, J.H.; de la Torre, G.; Di Censo, D.; Graetzel, M.; Nazeeruddin, M.K.; Torres, T. Angew. Chem. Int. Ed. Eng. 2012, 51, 4375.
5. Claessens, C. G.; González-Rodríguez, D.; Rodríguez-Morgade, M. S.; Medina, A.; Torres, T. Chem. Rev., in press.

Metal oxide-based photoanodes are critical components of dye sensitized solar cells (DSSCs)[1], which act as electron transporter for electrochemical system. Nanoparticulate anatase TiO2, is the most common semiconducting metal oxide applied as photoanode, but considerable efforts are being devoted to identify alternative materials. In this frame ZnO[2,3] and SnO2[4]are promising candidates due to higher electron mobility and the possibility to exploit light harvesters working in the near infrared region of the solar spectrum, respectively.
Herein we present strategies for engineering ZnO and SnO2 nanostructures aimed at optimizing light management and charge transport in DSSCs. In particular, we will discuss the application of ZnO hierarchical aggregates[5] featuring photoconversion efficiencies as high as 4.5% and 7.5% thanks to high light scattering properties and the fabrication of hybrid ZnO@SnO2 photoanodes in which a ZnO capping layer acts as a blocking layer toward exciton recombination at the dye-photoanode interface.
The highly beneficial role exerted by a compact ZnO layer in inhibiting the electron back recombination at the TCO glass-electrolyte interface will be as well presented and discussed.
References:
1.B. O&’Regan, M. Gratzel, Nature,1991, 353, 737
2.Q. Zhang, T. P. Chou, B. Russo, S. A. Jenekhe, G. Cao, Angew. Chem, 2008, 120, 2436
3.E.Guillén, L. M. Peter, J. A. Anta J. Phys. Chem. C, 2011, 115, 2262.
4.A. Hossain, J.R. Jenning, Z.Y. Koh,Q. Wang, ACS Nano, 2011, 5, 3172.
5.N. Memarian, I. Concina, A. Braga, S.M. Rozati, A. Vomiero, G. Sberveglieri, Angew.Chem. 2011, 51, 12321

Dye Sensitized Solar Cells (DSSC) offer advantages over traditional semiconductor solar cells including lower costs, relaxed material purity requirements, and higher diffuse light performance. Key to DSSC performance is the nanocrystalline metal oxide layer serving as the base for attached photosensitive dye and acting as the charge transfer path for photoelectrons from the dye. The precise structure of the metal oxide layer is of vital importance to the overall performance of the cell.
Transport of photoelectrons through the nanocrystalline metal oxides is heavily reliant on the inter-connectivity of nanoparticles and the interface of metal oxides with the conducting substrate (MS interface). To enhance the conductivity of the MS interface and to allow a large surface area for attachment of dye, a graduated density profile of the nanocrystalline layer can be utilized. One method of varying the density of a deposited metal oxide layer is through modifying the layer&’s pore volume by adjusting the concentration and molecular weight of pore-forming additives [1]. Layer formation would entail depositing a dense initial layer onto a substrate followed by successive depositions of decreasing density.
In this study, inkjet printing of multi-layer dispersed titanium dioxide (TiO2) nanoparticles was explored for the application of DSSCs. A low-cost commercial inkjet printer was modified for this purpose. Three different suspensions were formulated each using pH adjusted DI water as the main solvent and each with variations in the pore-forming surfactant content. One suspension was formulated without a surfactant and the others used different molecular weights of polyethylene glycol (PEG), 600g/mol and 20000g/mol, to promote distinctly different pore volumes. Successive printing passes deposited the suspensions onto a substrate to form a final TiO2 with a graded density profile. The performance of DSSCs produced with graded TiO2 layers were compared with those with uniform layers. Also offered is an assessment on the viability of a multi-ink printing system for sequential depositions.
[1] Ren, D., Zou, Y., Zhan, C., & Huang, N. “Behaviors of different dispersers on morphologies of porous TiO2 films”, Front. Mater. Sci. China, 4(4), 394-397. (2010).

Reverse illuminated flexible DSCs can be manufactured onto a titanium substrate using printing technologies which allow the production costs to be reduced. A key component of a DSC is the counter electrode catalyst which is typically platinum for iodide based electrolytes. To deposit robust platinum catalysts onto flexible transparent substrates, such as tin doped indium oxide - polyethylene terephthalate (ITO-PET), requires expensive vacuum based sputtering.
It has been demonstrated that a reduced graphene oxide paste can replace platinum, catalysing a number of different redox couples¹. However it is opaque and requires high (350°C) temperature processing. Optically transparent graphene nanoplatelets (GNP) layers have been used for both iodide² and cobalt ³ based redox couples but their lack of adhesion with the substrate has meant that they do not have the lifetime required for commercial applications.
Functionalised GNPs were used as the active material in a high surface area ink with viscosity of 20mPas. The ink can be deposited onto an ITO-PET substrate by a number of inline printing methods, including flexographic printing at 0.4m/s. By tailoring the GNP functionalisation, GNP loading and binder content the ink was optimised. The optimised ink had a transmission at 550nm(T550) = 85% and an Rct of 6Omega;/cm². Although the Rct is higher than required for laboratory cells with Jsc of 20mA/cm ² under 1sun test conditions is it sufficient for the lower (Jsc =7mA/cm ² under 1 sun) Jsc of industrially produced reverse devices, especially when utilised for indoor applications where Jsc = 50µA/cm ² under 1000lux . This was demonstrated by reverse illuminated DSC efficiencies with flexible cathodes which were equivalent to cells with sputtered platinum catalysts when under 0.5sun illumination or less.
The printed catalysts are stable and can be manufactured into cells many months after printing, enabling optimisation of the print run and minimised set up costs. A modification of the ink, suitable for catalysing a Co(Bpy)₃^2+/3+ electrolyte, has an Rct of 2Omega;/cm² and T550= 85%. This demonstrates potential for use in high efficiency cobalt mediated DSCs. Printed catalysts are a versatile low cost replacement to sputtered platinum in reverse illuminated DSCs for low illumination applications.
References
1. Roy-Mayhew, J. D., Boschloo, G., Hagfeldt, A. & Aksay, I. Functionalized graphene sheets as a versatile replacement for platinum in dye-sensitized solar cells. ACS applied materials & interfaces 4, 2794-800 (2012).
2. Kavan, L., Yum, J. H. & Gra, M. Optically transparent cathode for dye-sensitised solar cells based on graphene nanoplatelets. ACS Nano 5, 165-172 (2011).
3. Kavan, L., Yum, J.-H. & Grätzel, M. Graphene nanoplatelets outperforming platinum as the electrocatalyst in co-bipyridine-mediated dye-sensitized solar cells. Nano letters 11, 5501-6 (2011).

Since their recent invention, perovskite-based solid-state solar cells have been attracting tremendous attention, leading to their rapid development. These new solar cells are based on organometal trihalide (CH3NH3PbX3; X=Cl, Br, I or combination) pervoskite absorbers, and they are able to deliver over 15% efficiency. Most importantly, the hybrid perovskites used in these solar cells can be solution-processed, making them potentially low cost. While progress is being made in understanding these new solar cells, there are several unresolved issues. One important issue is the micro- and nano-structuring of the dense TiO2 layer surface that is in contact with the perovskite. Here we have used solution-processing approaches to obtain different levels of surface micro- and nano-structuring, and built different perovskite solar cells on these surfaces. Results from the characterization of the TiO2 surfaces, the perovskites, and the entire solar cells are presented. Also, result from studies on the effects of surface micro- and nano-structuring on the photovoltaic mechanisms and performance of these solar cells are presented and discussed.

Dye-sensitized solar cells (DSCs) have been considered to be a promising photovoltaic technology to address the increasing energy and environmental challenges since the pioneering work of O&’Regan and Grätzel. To date, power conversion efficiencies (PCE) greater than 12% have been achieved. While encouraging, owing to the complexity of the multiple kinetic processes charge transport in DSCs has not been thoroughly and unambiguously understood. This talk will focus on our recent progress in the fundamental understanding of dye regeneration — one of the crucial, efficiency-determining processes, as relevant to DSCs. The predicted effect of sensitizer regeneration on the j-V characteristics of DSCs will be discussed and recent experimental affirmation will be highlighted. In addition, with cobalt polypyridyl complexes-based electrolyte as an example, the effects of electrolyte ionic strength on sensitizer regeneration will be discussed. We anticipate that these results strengthen our understanding on charge separation process of DSCs in order for more efficient and more stable solar energy conversion.

In the hybrid perovskites (CH3-NH3)PbI3-xClx, an organic molecule (methylammonia CH3-NH3) occupies the A-site of the common ABO3-type perovskite structure, Pb is at the B-site, and I and Cl anions sit at the oxygen positions. In the last 2 or 3 years these materials have been protagonist of an enthusiastic rise towards the fabrication of record-high efficient solar cell prototypes [1-3] and represent nowadays the latest and most promising frontier in photovoltaics developments. Solar cells based on (CH3-NH3)PbI3-xClx as photon absorber demonstrated up to 15% Power Conversion Efficiency and near 100% internal Quantum Efficiency. Furthermore, they can be grown by solution-based high-throughput approaches at low temperature, which make them suitable for large-scale mass productions. Even more impressively, it was shown in experiments that these hybrid perovskites are multifunctional, i.e. they are not only very efficient optical absorber, but even excellent n- and p-type transport material, thus prefiguring the possibility to overcome the efficiency loss due to charge injection from the optically active region to the charge-separated sides of the photovoltaic junction.
Our ab-initio calculations explore the fundamental properties of these systems and unveil the most relevant sources of this excellent performance. The role of organic and inorganic sides is revealed, together with the basic structural, electronic, optical, and transport properties. Large absorption coefficients (0.03-0.04 nmminus;1 for 500 nm wavelength ) and the small electron and hole effective masses are calculated, coherently with the expectation. An explanation of these results in terms of the basic characteristics of the perovskites is furnished. In particular, the presence of a direct band gap between highly dispersed Pb(6s)-I(5p) valence bands and Pb(6p) conduction bands is highlighted. Our understanding is not only relevant for these specific materials, but even as starting point for a material design activity headed to envision optimal candidates for the best device efficiency. Indeed, we have sketched simple design rules to search for more hybrid perovskites with enhanced capabilities or specific requirements, such as the Pb replacement with non-toxic cations. The flexibility of the perovskite structure, which counts literally thousands of different materials, represents an ideal template for testing the widest range of atomic and molecular substitutions.
[1] M. M. Lee, et. al., Science 338, 643 (2012).
[2] J. M. Ball, et. al., Energy Environ. Sci., 6, 1739 (2013).
[3] J. Burschka, et. al., Nature 499, 316 (2013).

Recently, organometal halide perovskite materials based solar cell with the characteristics of rather low raw materials cost, great potential of simple and scalable production, and extreme high performance, have been highlighted as one of the most competitive technology for next generation thin film photovoltaic (PV). Despite the successes on both mesoporous and planar structure, the latter one is considered with more flexibility on the device fabrication and efficient characterization on the underlying materials property and device physics. So far, solution processed perovskite thin films, toward planar structure, suffer from pinhole formation and incomplete surface coverage, which leads to low-resistance shunting paths and lost light absorption in the solar cell, and consequently low performance. Here, we develop a novel solution approach for perovskite materials with excellent controllability on the film quality with competitive performance. This method is based on the in-situ reaction between the organic and inorganic species, which is conceptually different from either the conventional solution process or vacuum deposition. It utilizes the kinetic reactivity of both species and thermodynamic stability of perovskite during the in-situ growth process, and provides the excellent films with 100 % surface coverage, and remarkable grain size up to micrometers. Primary experiment shows that the photovoltaic devices based on the resulting films have achieved power conversion efficiency of over 10% by adopting the architecture of FTO/condensed-TiO2/perovskite/Spiro-OMeTAD/Ag. This novel solution process can be further extended to other system to advance the development of optoelectronic devices.

A major deficit in suitable dyes is stiffling progress in the dye-sensitised solar cell (DSC) industry. Materials discovery strategies have afforded numerous new dyes; yet, corresponding solution-based DSC device performance has little improved upon 11% efficiency, achieved using the N719 dye over two decades ago. Research on these dyes has nevertheless revealed relationships between the molecular structure of dyes and their associated DSC efficiency. Here, such structure-property relationships have been codified in the form of molecular dye design rules, which have been judiciously sequenced in an algorithm to enable large-scale data mining of dye structures with optimal DSC performance. For the first time, we have a DSC-specific dye-discovery strategy that predicts new classes of dyes from surveying an representative set of chemical space. A lead material from these predictions is experimentally validated, showing DSC efficiency that is comparable to many well-known organic dyes. This demonstrates the power of this approach.

Recent achievements of perovskite solar cell based on 3-dimensional CH3NH3PbI3 are presented. CH3NH3PbI3 can be in-situ crystalized in nanostructured oxide film using one-step or two-step spin-coating procedure. One-step solution deposition in the mesoporous TiO2 thin film led to the average power conversion efficiency (PCE) of around 9%, whereas two-step coating resulted in higher PCE of more than 14%. Scanning electron microscopy confirmed that the mesoporous TiO2 film was fully covered with the perovskite layer by two-step coating procedure, leading to higher performance than one-step method with uncovered TiO2 surface. Comparative study on anatase and rutile TiO2 nanoparticle films showed that rutile TiO2 film covered with CH3NH3PbI3 demonstrated better photovoltaic performance due to higher photocurrent and fill factor. It was found that 1-dimensional nanorod ZnO film could be an effective n-type layer for the perovskite solar cell, where vertically grown ZnO nanorod film covered with CH3NH3PbI3 exhibited a PCE of 11%. Electron injection from the perovskite to TiO2 was evident from our initial study on the perovskite solar cell with discontinuous dot morphology of CH3NH3PbI3 deposited on TiO2 surface. Except for the electron accepting n-type oxides, a scaffold layer of ZrO2 with insulating property was investigated and confirmed to exhibit also photovoltaic performance, which indicates that CH3NH3PbI3-based perovskite solar cell can work even in the absence of electron accepting n-type oxide layer. Although opto-electronic property of CH3NH3PbI3 has not been well known, it is obvious from impedance study that CH3NH3PbI3 perovskite has ability of charge accumulation due to high density of state, which is associated with its superb photovoltaic property.

In this presentation, I will discuss low-cost and efficient dye-sensitized solar cells (DSSCs) through the use of nanostructured TiO2 (e.g., hierarchically structured nanotubes, nanoflowers, etc.) as photoanode, and earth abundant, environmentally friendly quaternary semiconductor copper zinc tin sulfide (CZTS) as counter electrode. The route to enhanced performance by incorporating upconversion nanocrystals to increase light harvesting will also be presented.

Organic-inorganic hybrid perovskite CH3NH3PbX3(X = Cl, Br, I) recently attracts much attention as a high-performance light-harvesting sensitizer for sensitized solar cells. Since the first report of CH3NH3PbI3-based solar cell in 2009 [1], the solar-energy conversion efficiency is being rapidly improved and reaches around 15 % to date [2-4]. For the further development of the perovskite solar cells, a thorough understanding of the fundamental optical properties and dynamical behaviors of photocarriers of CH3NH3PbI3 is necessary.
In this study, we revealed the near-band-edge optical responses of CH3NH3PbI3 on mesoporous TiO2 electrodes by means of diffuse reflectance (DR), photocurrent, photoluminescence (PL), and transient absorption (TA) spectroscopy. The samples were fabricated by spin-coating of CH3NH3PbI3 on meso-porous TiO2 films on silica glass substrates. We confirmed the solar cell using the fabricated CH3NH3PbI3 works well and the solar-energy conversion efficiency reaches 10.2 %.
At room temperature, in the Tauc plot of DR spectrum, the extrapolated line zero-crosses at 1.59 eV. On the other hand, PL spectrum shows a broad peak at 1.60 eV, indicating that the band-gap energy exceeds 1.60 eV. The PL excitation spectrum also shows a broad band at 1.64 eV. The TA spectrum shows a negative peak (photo-bleaching) at 1.61 eV, indicating direct band-gap energy. These complicated near-band-edge optical responses suggest the formation of the band tail states below the band edge of solution-processed CH3NH3PbI3. We also measured the temperature dependence of the DR and PL spectra. Temperature dependence of the PL peak energy and DR onset energy shows good agreement. The steepness of the absorption edge in DR spectrum linearly increases with the temperature. Such temperature-dependent steepness of the absorption edge is not observed in ideal direct-gap semiconductors. Based on these results, we concluded that the Urbach tail, which is usually attributed to structural disorder, determines the room-temperature band-edge optical absorption and that the band-gap energy is ~1.61 eV at room temperature.
In addition, we will present the recombination and charge-separation dynamics of band-edge photocarriers of CH3NH3PbI3 on meso-porous TiO2 studied by time-resolved PL and TA measurements.
This work was supported by The Sumitomo Electric Industries Group CSR foundation and JST-CREST.
References:
[1]A. Kojima, et al., J. Am. Chem. Soc., 131, 6050-6051 (2009).
[2] M. M. Lee, et al., Science, 338, 643-647 (2012).
[3] J. Burschka, et al., Nature, 499, 316-319 (2013).
[4] M. Liu, et al., Nature 501, 395-398 (2013).

Recent progress in hybrid organic-inorganic perovskite materials has demonstrated their potential as cheap, high-efficiency alternatives to silicon for use as solar cell absorber materials. In the few years since their introduction into solar cell devices, power conversion efficiencies up to 15% have been demonstrated.¹ For solar cell applications, mixed methyl ammonium organometallic halide perovskites, CH₃NH₃XY₃ (X = Pb or Sn and Y = I, Br, and/or Cl), are predominantly used.² To date, the properties of these materials on a fundamental level remain poorly understood. We have used a variety of electron and x-ray based spectroscopies with different information depth to study the chemical and electronic structures of these materials. Both lab- and synchrotron-based x-ray photoelectron spectroscopy have been used to investigate the core and valence levels of CH₃NH₃XY₃ perovskites probing the chemical and electronic surface properties. Resonant and non-resonant soft x-ray emission spectroscopy, and x-ray absorption spectroscopy have provided information about occupied and unoccupied valence states in the near-surface bulk of these materials. Further, we have used these techniques to investigate the chemical mechanism for degradation of the perovskites in humid environments. In this presentation we will compare our results obtained for CH₃NH₃PbI₃ to those for mixed halide perovskite materials, namely CH₃NH₃PbI_3-xCl_x and CH₃NH₃PbI_3-xBr_x.
References:
¹ J. Burschka et al., Nature 499, 316 (2013)
² A. Kojima et al., J. Am. Chem. Soc. 131, 6050 (2009)

Dye-sensitized solar cells (DSSCs) are low-cost photovoltaic devices and they are potentially promising alternatives to conventional solid-state cells, such as Si, CdTe and CuIn1-xGaxSe2. However, in the conventional DSSCs, the liquid iodide-containing electrolyte is found to be highly corrosive and volatile. This has resulted in reducing cell performance and long-term stability. In this regard, significant research efforts have been focused on replacing these fluid electrolytes by p-type semiconductors and solid-state hole-transport materials (HTMs). To date, they are mostly organic molecules such as sprio-OMeTAD (2,2&’,7,7&’-tetranis(N,N-di-p-methoxyphenyl-amine)9,9&’-spirobifluorene); and p-type conducting polymers such as polypyrole, polydiacetylene, poly(3-octylthiophene), poly(3,4-ethylenedioxythiophene) (PEDOT), 2,2&’-bis(3,4-ethylenedioxythiophene) (bis-EDOT). Despite many attempts, the development of inorganic HTMs has been slow. Examples include are CuI, CuSCN and NiO. However, these compounds are either chemically unstable or resulted in very low efficiency. Recently, our team has demonstrate the successful of P-type semiconductor CsSnI3-xFx as a hole conductor with very respectable performance.
Here, we report new a HTM based on air stable and solution processable p-type semiconductor Cs2SnI6. For the all solid state DSSCs, the mesoporous TiO2 film was first deposited on FTO glass by electrospray. After sintering and TiCl4 post treatment, we used commercially available Ru(4,4prime;-dicarboxy-2,2prime;-bipyridine)(4,4prime;-dinonyl-2,2prime;-bipyridine)(NCS)2 (Z907) as a sensitizing dye. The dye coated TiO2 film was spin coated with the solution of Cs2SnI6 dissolved in dimethylformamide (DMF).
A 100 nm gold electrodes was thermally evaporated onto the TiO2/ Dye/ Cs2SnI6 structure. The resulting cell has a Voc = 0.639 V, Jsc = 11.7 mA/cm2, and FF = 52.5%, and an efficiency = 3.92%.

After the first report of organo-metal-halide provskite-based photovoltaic cell by our group in 2009, there has been fast advancement in efficiency of the cell up to 15% by means of improvement of solid-state cell fabrication. The high efficiency is largely supported by high open-circuit voltage, 1.0-1.1V, in addition to strong light harvesting power of the perovskite band-gap photo-conductor. High voltage is a unique advantage of the cell structure in which thermal energy loss in charge transfers across interfacial hetero-junction barriers are minimized to the level down to 0.2 eV. Manipulation of the hybrid material and crystal layer preparation, however, needs highly controlled chemical processes against ambient conditions and is causing variation of cell performance. Process engineering is therefore the key to enhancing the perovskite cell performance. In this report we will show optimal conditions for the procedure of high efficiency perovskite cell that lead to 11% or more efficiency by quick processes under exposure to ambient air.
In the strategy to realize high open-circuit voltage (Voc), a well-designed organic inorganic hybrid structure contributes to minimal energy loss (in eV) at the charge-separating and current-rectifying interface. We found that Voc can reach 1.2V for a hybrid structure absorbing up to 700 nm. For example, perylene works as efficient organic hole conductor, simultaneously functioning as a light absorber, in junction with photovoltaic pigment films comprising organic dye-TiO2 interfacial complexes. Metal-ligand charge transfer (MLCT) complex serves as a strong light absorber on the surface of metal oxide. Cyclopentadiene forms a MLCT pigment on TiO2. A thin solid-state cell with hetero-junction of MLCT pigment and perylene generates Voc exceeding 1.2V. Here, crystallinity of the hole conductor, such as perylene, proved to strictly change the cell performance. The effect of crystalline and amorphous states of organic and inorganic conductors on the carrier conduction property (fill factor) of the photovoltaic cell will be reported.

Open-circuit voltage (V_OC) of Sb₂S₃-sensitized solid-state solar cell has been reported to be as low as 0.5 V, which is much lower than the theoretical V_OC of more than 1 V. Improving Voc in Sb₂S₃-sensitized solar cell has been therefore a challenging issue to be addressed. Charge recombination was reported to be one of major factors for lowering V_OC. To suppress the charge recombination, insertion of blocking layer (BL) at each interface of TiO₂/Sb₂S₃ and Sb₂S₃/HTM (hole transporting material) has been designed. Thin insulating oxide layer (BL1) was introduced in between TiO₂ and Sb₂S₃, and thin insulating chalcogenide layer (BL2) was introduced in between Sb₂S₃ and hole transporting material (HTM). Sb₂S₃ was deposited on TiO₂ surface using chemical bath deposition method. Spiro-MeOTAD was used as HTM. Compared to device without blocking layer (TiO₂/Sb₂S₃/HTM), V_OC was improved by about 8% upon insertion of BL1 (TiO₂/BL1/Sb₂S₃/HTM), while V_OC was significantly improved by about 19% when incorporating BL2 (TiO₂/Sb₂S₃/BL2/HTM). Moreover, photocurrent was simultaneously increased in the presence of BL1, associated with increase in the amount of Sb₂S₃ deposited on TiO₂ surface due to the increased surface area as confirmed by transmission electron microscopy image. The device structure with double blocking layers (TiO₂/BL1/Sb₂S₃/BL2/HTM) showed higher V_OC than the BL1-incorporated structure, but slightly lower V_OC than the BL2-contained structure. Transient photovoltage and impedance spectroscopic studies confirmed that the improved V_OC was ascribed to suppression of interfacial charge recombination. By optimizing blocking layer thickness and material in Sb₂S₃-sensitized solid state solar cell, a power conversion efficiency of 4.47% was achieved under one sun illumination.

Solution processed semiconductor quantum dot solar cells offer a path towards both reduced fabrication cost and higher efficiency enabled by novel processes such as hot-electron extraction and carrier multiplication. Here we use a new class of low-cost, low-toxicity CuInSexS2-x quantum dots to demonstrate sensitized solar cells with certified efficiencies exceeding 5%. Among other material and device design improvements studied, use of a methanol-based polysulfide electrolyte results in a particularly dramatic enhancement in photocurrent and reduced series resistance. Despite the high vapor pressure of methanol, the solar cells are stable for months under ambient conditions, which is much longer than any previously reported quantum dot sensitized solar cell. This study demonstrates the large potential of CuInSexS2-x quantum dots as active materials for the realization of low-cost, robust, and efficient photovoltaics as well as a platform for investigating various advanced concepts derived from the unique physics of the nanoscale size regime.

Despite the significant performance improvement of semiconductor sensitized solar cells (SSSCs) in recent years, their efficiency still falls short of that of dye sensitized solar cells (DSSCs). One of the dominant mechanisms leading to poor efficiency is associated with unwanted recombination at the interfaces between the anode and the sensitizer semiconductor, and between the anode and the hole conductor or electrolyte.Of the exploited methods, heterostructured sensitizer layers have shown significant promise in mitigating undesired recombination channels. Of particular interest in the context are type-II heterostructured QDs, comprised of a core localizing one charge carrier and a shell localizing the other, in which charge separation is induced already within the sensitizer layer. This system, which is in many senses analogous to heterostructures constructed by successive layering,should significantly reduce recombination losses of carriers already injected to the electrode with those remaining in the QDs. Another advantage is the significant red-shift of the absorption edge of the heterostructured QDs relative to its two constituents due to “spatially indirect” energy gap leading to improved absorption characteristics. The “spatially indirect” energy gap is determined by the energy separation between the conduction band edge of one semiconductor and the valence band edge of the other semiconductor. If a single semiconductor sensitizer is used (like CdS or CdSe), the absorption range can be increased by utilizing larger QDs with smaller band gap, with potentially higher monochromatic incident photon-to-current conversion efficiency (IPCE) values. On the other hand, it was shown for the CdSe case that smaller-sized QDs have greater electron injection rates into TiO2 and also higher IPCE at the excitonic band. A tradeoff between the two (absorption range and IPCE value) should be made in order to get the highest efficiency. Because the low energy photons in type-II heterostructure are absorbed due to the spatially indirect transition, and not only due to direct absorption in either of the constituents, both high IPCE and a wider absorption range can be potentially reached for a particular redox electrolyte or hole conductor system.
Here, Type-II heterostructure CdTe/CdSe core/shell nanocrystals (quantum dots, QDs) are explored as sensitizers in a quantum dot-sensitized photoelectrochemical solar cell. Upon incorporation in a sensitized solar cell utilizing a porous TiO2 and a polysulfide electrolyte these QDs exhibited efficient charge separation and high internal quantum efficiency despite hole localization in the CdTe core. Monochromatic incident photon-to-current conversion efficiency (IPCE) measurement shows a spectrally broad photoresponse spanning the whole visible spectrum and reaching up to about 900 nm.