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: Ahmed_El-Shafei@ncsu.edu

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).