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 inapp