Hidden surface photovoltages revealed by pump probe KPFM.

In this work, we use pump-probe Kelvin probe force microscopy (pp-KPFM) in combination with non-contact atomic force microscopy (nc-AFM) under ultrahigh vacuum, to investigate the nature of the light-induced surface potential dynamics in alumina-passivated crystalline silicon, and in an organic bulk heterojunction thin film based on the PTB7-PC71BM tandem. In both cases, we demonstrate that it is possible to identify and separate the contributions of two different kinds of photo-induced charge distributions that give rise to potential shifts with opposite polarities, each characterized by different dynamics. The data acquired on the passivated crystalline silicon are shown to be fully consistent with the band-bending at the silicon-oxide interface, and with electron trapping processes in acceptors states and in the passivation layer. The full sequence of events that follow the electron-hole generation can be observed on the pp-KPFM curves, i.e. the carriers spatial separation and hole accumulation in the space charge area, the electron trapping, the electron-hole recombination, and finally the electron trap-release. Two dimensional dynamical maps of the organic blend photo-response are obtained by recording the pump-probe KPFM curves in data cube mode, and by implementing a specific batch processing protocol. Sample areas displaying an extra positive SPV component characterized by decay time-constants of a few tens of microseconds are thus revealed, and are tentatively attributed to specific interfaces formed between a polymer-enriched skin layer and recessed acceptor aggregates. Decay time constant images of the negative SPV component confirm that the acceptor clusters act as electron-trapping centres. Whatever the photovoltaic technology, our results exemplify how some of the SPV components may remain completely hidden to conventional SPV imaging by KPFM, with possible consequences in terms of photo-response misinterpretation. This work furthermore highlights the need of implementing time-resolved techniques that can provide a quantitative measurement of the time-resolved potential.

Push-Pull Zinc Phthalocyanine Bearing Hexa-Tertiary Substituted Carbazolyl Donor Groups for Dye-Sensitized Solar Cells.

An asymmetrical, push-pull phthalocyanine bearing bulky tert-butylcarbazolyl moieties as electron donor and carboxylic acid as anchoring group was synthetized and tested as a photosensitizer in dye-sensitized solar cells (DSSC). The new photosensitizer was characterized by 1H and 13C NMR, UV-Vis and mass spectrometry. The bulky tert-butylcarbazolyl moieties avoid the aggregation of the phthalocyanine dye. DFT studies indicate that the HOMO is delocalized throughout the -electron system of the substituted phthalocyanine and the LUMO is located on the core of the molecule with a sizable electron density distribution on carboxyl groups. The new dye has been used as a photosensitizer in transparent and opaque dye-sensitized solar cells, which exhibit poor efficiencies related to a low Jsc.

Functional panchromatic BODIPY dyes with near-infrared absorption: design, synthesis, characterization and use in dye-sensitized solar cells.

We report two novel functional dyes based on a boron-dipyrromethene (BODIPY) core displaying a panchromatic absorption with an extension to the near-infrared (NIR) range. An innovative synthetic approach for preparing the 2,3,5,6-tetramethyl-BODIPY unit is disclosed, and a versatile way to further functionalize this unit has been developed. The optoelectronic properties of the two dyes were computed by density functional theory modelling (DFT) and characterized through UV-vis spectroscopy and cyclic voltammetry (CV) measurements. Finally, we report preliminary results obtained using these functional dyes as photosensitizers in dye-sensitized solar cells (DSSCs).

Electrodeposited ZnO nanowires as photoelectrodes in solid-state organic dye-sensitized solar cells.

A new approach for developing solid-state dye-sensitised solar cells (DSSCs) on glass/ITO and plastic substrates (PEN/ITO) is presented in this manuscript. A two step electrodeposition technique has been employed to realize the ZnO photoelectrodes. First a ZnO thin film is deposited on the ITO substrate and subsequently on this buffer layer 650 nm long ZnO nanowires are grown. The different nanostructured electrodes are crystallized and show a transparency close to 80% in the visible spectral range. The electrodes are then sensitized with a new purely organic dye, whose synthesis is presented here, which reveals a wide absorption spectrum and a high molar extinction coefficient. Finally, the sensitized electrodes were employed for the fabrication of liquid and solid-state DSSCs, using, respectively, a liquid iodine/iodide electrolyte and the spiro-OMeTAD hole transporter. These devices represent the first solid-state DSSCs fabricated using electrodeposited zinc oxide nanowires. Their power conversion efficiency is still limited, respectively, 0.18% and 0.03% under standard AM 1.5G sunlight (100 mW cm(-2)), nevertheless, these results prove the interest in this low-temperature deposition method for the realization of nanostructured electrodes on rigid and flexible substrates, and open up new perspectives for the development of solid state DSSCs on plastic substrates.

Lanthanide complexes based on ß-diketonates and a tetradentate chromophore highly luminescent as powders and in polymers.

A new type of octacoordinated ternary ß-diketonates complexes of terbium and europium has been prepared using the anionic tetradentate terpyridine-carboxylate ligand (L) as a sensitizer of lanthanide luminescence in combination with two ß-diketonates ligands 2-thenoyltrifluoroacetyl-acetonate (tta(-)) for Eu(3+) and trifluoroacetylacetonate (tfac(-)) for Tb(3+). The solid state structures of the two complexes [Tb(L)(tfac)2] (1) and [Eu(L)(tta)2] (2) have been determined by X-ray crystallography. Photophysical and (1)H NMR indicate a high stability of these complexes with respect to ligand dissociation in solution. The use of the anionic tetradentate ligand in combination with two ß-diketonates ligands leads to the extension of the absorption window toward the visible region (390 nm) and to high luminescence quantum yield for the europium complex in the solid state (Φ = 66(6)%). Furthermore, these complexes have been incorporated in polymer matrixes leading to highly luminescent flexible layers.

Push-pull photochromic dyes for semi-transparent solar cells with light-adjustable optical properties and high color-rendering index.

We report the design, synthesis and characterization of push-pull photochromic naphthopyran dyes, incorporating different carbazole moieties as the electron-donor group for use in dye-sensitized solar cells. Compared to a reference dye incorporating a diphenylamine-type donor moiety, the introduction of functionalized carbazoles allows for a hypsochromic shift of the absorption of the coloured isomers of the dyes in the visible region and a better tuning of their spectra to the photopic response of the human eye. Under illumination, the molecules exhibit a broad absorption with a maximum comprised between 546 nm and 571 nm in solution and they reveal relatively fast discoloration kinetics. By using these dyes to fabricate photochromic solar cells whose optical and photovoltaic properties vary with the light exposure, we have achieved a PCE of up to 3% in opaque cells. Using these molecules in semi-transparent solar cells with different electrolytes, a PCE of 2.3% was achieved. We also produced a semi-transparent mini-module with an average visible transmittance varying between 66% and 50% and a colour rendering index around 95 in both the uncoloured and coloured states.

Star-shape non-fullerene acceptor featuring an aza-triangulene core for organic solar cells.

We present the simple synthesis of a star-shape non-fullerene acceptor (NFA) for application in organic solar cells. This NFA possesses a D(A)3 structure in which the electron-donating core is an aza-triangulene unit and we report the first crystal structure for a star shape NFA based on this motive. We fully characterized this molecule's optoelectronic properties in solution and thin films, investigating its photovoltaic properties when blended with PTB7-Th as the electron donor component. We demonstrate that the aza-triangulene core leads to a strong absorption in the visible range with an absorption edge going from 700 nm in solution to above 850 nm in the solid state. The transport properties of the pristine molecule were investigated in field effect transistors (OFETs) and in blends with PTB7-Th following a Space-Charge-Limited Current (SCLC) protocol. We found that the mobility of electrons measured in films deposited from o-xylene and chlorobenzene are quite similar (up to 2.70 × 10-4 cm2 V-1 s-1) and that the values are not significantly modified by thermal annealing. The new NFA combined with PTB7-Th in the active layer of inverted solar cells leads to a power conversion efficiency of around 6.3% (active area 0.16 cm2) when processed from non-chlorinated solvents without thermal annealing. Thanks to impedance spectroscopy measurements performed on the solar cells, we show that the charge collection efficiency of the devices is limited by the transport properties rather than by recombination kinetics. Finally, we investigated the stability of this new NFA in various conditions and show that the star-shape molecule is more resistant against photolysis in the presence and absence of oxygen than ITIC.

Photochromic spiro-indoline naphthoxazines and naphthopyrans in dye-sensitized solar cells.

Photochromic dyes possess unique properties that can be exploited in different domains, including optics, biomedicine and optoelectronics. Herein, we explore the potential of photochromic spiro-indoline naphthoxazine (SINO) and naphthopyran (NIPS) for application in photovoltaics. We designed and synthesized four new photosensitizers with a donor-pi-acceptor structure embedding SINO and NIPS units as photochromic cores. Their optical, photochromic and acidochromic properties were thoroughly studied to establish structure-properties relationships. Then, after unravelling the possible forms adopted depending on the stimuli, their photovoltaic properties were evaluated in DSSCs. Although the photochromic behavior is not always preserved, we elucidate the interplay between photochromic, acidochromic and photovoltaic properties and we demonstrate that these dyes can act as photosensitizers in DSSCs. We report a maximum power conversion efficiency of 2.7% with a NIPS-based dye, a tenfold improvement in comparison to previous works on similar class of compounds. This work opens new perspectives of developments for SINO and NIPS in optical and photovoltaic devices, and it provides novel research directions to design photochromic materials with improved characteristics.

Photochromic naphthopyran dyes incorporating a benzene, thiophene or furan spacer: effect on photochromic, optoelectronic and photovoltaic properties in dye-sensitized solar cells.

We recently demonstrated that diaryl-naphthopyran photochromic dyes are efficient for sensitization of TiO2 mesoporous electrodes, thus allowing the fabrication of photo-chromo-voltaic cells that can self-adapt their absorption of light and their generation of electricity with the light intensity. Herein we report the synthesis, the characterisation of two novel photochromic dyes based on diaryl-naphthopyran core i.e NPI-ThPh and NPI-FuPh for use in Dye Sensitized Solar Cells (DSSCs). Compared to our reference dye NPI, the molecules only vary by the nature of the spacer, a thiophene or a furan, connecting the photochromic unit and the phenyl-cyano-acrylic acid moiety used as the anchoring function. We found that swapping a phenyl for a thiophene or a furan leads to an improvement of the absorption properties of the molecules both in solution and after grafting on TiO2 electrodes, however their photochromic process becomes not fully reversible. Despite better absorption in the visible range, the new dyes show poorer photochromic and photovoltaic properties in devices compared to NPI. Thanks to UV-Vis spectroscopy, DFT calculation, electrical characterization of the cells, and impedance spectroscopy, we unravel the factors limiting their performances. Our study contributes to better understand the connection between photochromic and photovoltaic properties, which is key to develop better performing molecules of this class.

Influence of Redox Couple on the Performance of ZnO Dye Solar Cells and Minimodules with Benzothiadiazole-Based Photosensitizers.

ZnO-based dye-sensitized solar cells exhibit lower efficiencies than TiO2-based systems despite advantageous charge transport dynamics and versatility in terms of synthesis methods, which can be primarily ascribed to compatibility issues of ZnO with the dyes and the redox couples originally optimized for TiO2. We evaluate the performance of solar cells based on ZnO nanomaterial prepared by microwave-assisted solvothermal synthesis, using three fully organic benzothiadiazole-based dyes YKP-88, YKP-137, and MG-207, and alternative electrolyte solutions with the I-/I3 -, Co(bpy)3 2+/3+, and Cu(dmp)2 1+/2+ redox couples. The best cell performance is achieved for the dye-redox couple combination YKP-88 and Co(bpy)3 2+/3+, reaching an average efficiency of 4.7% and 5.0% for the best cell, compared to 3.7% and 3.9% for the I-/I3 - couple with the same dye. Electrical impedance spectroscopy highlights the influence of dye and redox couple chemistry on the balance of recombination and regeneration kinetics. Combined with the effects of the interaction of the redox couple with the ZnO surface, these aspects are shown to determine the solar cell performance. Minimodules based on the best systems in both parallel and series configurations reach 1.5% efficiency for an area of 23.8 cm2.

Phosphorescent binuclear iridium complexes based on terpyridine-carboxylate: an experimental and theoretical study.

The phosphorescent binuclear iridium(III) complexes tetrakis(2-phenylpyridine)µ-(2,2':6',2''-terpyridine-6,6''-dicarboxylic acid)diiridium (Ir1) and tetrakis(2-(2,4-difluorophenyl) pyridine))µ-(2,2':6',2''-terpyridine-6,6''-dicarboxylic acid)diiridium (Ir2) were synthesized in a straightforward manner and characterized using X-ray diffraction, NMR, UV-vis absorption, and emission spectroscopy. The complexes have similar solution structures in which the two iridium centers are equivalent. This is further confirmed by the solid state structure of Ir2. The newly reported complexes display intense luminescence in dichloromethane solutions with maxima at 538 (Ir1) and 477 nm (Ir2) at 298 K (496 and 468 nm at 77 K, respectively) and emission quantum yields reaching ~18% for Ir1. The emission quantum yield for Ir1 is among the highest values reported for dinuclear iridium complexes. It shows only a 11% decrease with respect to the emission quantum yield reported for its mononuclear analogue, while the molar extinction coefficient is roughly doubled. This suggests that such architectures are of potential interest for the development of polymetallic assemblies showing improved optical properties. DFT and time-dependent-DFT calculations were performed on the ground and excited states of the complexes to provide insights into their structural, electronic, and photophysical properties.

Electroactive materials for organic electronics: preparation strategies, structural aspects and characterization techniques.

This critical review discusses specific chemical and physicochemical requirements which must be met for organic compounds to be considered as promising materials for applications in organic electronics. Although emphasis is put on molecules and macromolecules suitable for fabrication of field effect transistors (FETs), a large fraction of the discussed compounds can also be applied in other organic or hybrid (organic-inorganic) electronic devices such as photodiodes, light emitting diodes, photovoltaic cells, etc. It should be of interest to chemists, physicists, material scientists and electrical engineers working in the domain of organic electronics (423 references).

Imaging the carrier photogeneration in nanoscale phase segregated organic heterojunctions by Kelvin probe force microscopy.

In this work, we spatially resolve by Kelvin probe force microscopy (KPFM) under ultrahigh vacuum (UHV) the surface photovoltage in high-efficiency nanoscale phase segregated photovoltaic blends of poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester. The spatial resolution achieved represents a 10-fold improvement over previous KPFM reports on organic solar cells. By combining the damping contrast to the topographic data in noncontact atomic force microscopy under UHV, surface morphologies of the interpenetrated networks are clearly revealed. We show how the lateral resolution in KPFM can be significantly enhanced by optimizing the damping signal, allowing a direct visualization of the carrier generation at the donor-acceptor interfaces and their transport through the percolation pathways in the nanometer range. Henceforth, high-resolution KPFM has the potential to become a routine characterization tool for organic and hybrid photovoltaics.

Characterization of Photochromic Dye Solar Cells Using Small-Signal Perturbation Techniques.

Photochromic dye-sensitized solar cells (DSSCs) are novel semi-transparent photovoltaic devices that self-adjust their optical properties to the irradiation conditions, a feature that makes them especially suitable for building integrated photovoltaics. These novel solar cells have already achieved efficiencies above 4%, and there are multiple pathways to improve the performance. In this work, we conduct a full characterization of DSSCs with the photochromic dye NPI, combining electrical impedance spectroscopy (EIS) and intensity-modulated photocurrent spectroscopy (IMPS). We argue that the inherent properties of the photochromic dye, which result in a modification of the functioning of the solar cell by the optical excitation that also acts as a probe, pose unique challenges to the interpretation of the results using conventional models. Absorption of light in the visible range significantly increases when the NPI dye is in the activated state; however, the recombination rate also increases, thus limiting the efficiency. We identify and quantify the mechanism of enhanced recombination when the photochromic dye is activated using a combination of EIS and IMPS. From the comparison to a state-of-the-art reference dye (RK1), we were able to detect a new feature in the IMPS spectrum that is associated with the optical activation of the photochromic dye, providing a useful tool for assessing the electronic behavior of the device under different conditions of light excitation. This study provides guidelines to adequate characterization protocols of photochromic solar cells and essential insights on the interfacial electronic processes.

Non-Fullerene Acceptors with an Extended π-Conjugated Core: Third Components in Ternary Blends for High-Efficiency, Post-Treatment-Free Organic Solar Cells.

The synthesis of four non-fullerene acceptors (NFAs) with a "A-π-D-π-A" structure, in which the electron-donating core is extended, was achieved. The molecules differed by the nature of the solubilizing groups on the π-spacer and/or the presence of fluorine atoms on the peripheral electron-accepting units. The optoelectronic properties of the molecules were characterized in solution, in thin film, and in photovoltaic devices. The nature of the solubilizing groups had a minor influence on the optoelectronic properties but affected the organization in the solid state. On the other hand, the fluorine atoms influenced the optoelectronics properties and increased the photo-stability of the molecules in thin films. Compared to reference ITIC, the extended molecules showed a wider absorption across the visible range and higher lowest unoccupied molecular orbital energy levels. The photovoltaic performances of the four NFAs were assessed in binary blends using PM6 (PBDB-T-2F) as the donating polymer and in ternary blends with ITIC-4F. Solar cells (active area 0.27 cm2 ) showed power conversion efficiencies of up to 11.1 % when ternary blends were processed from non-halogenated solvents, without any thermal post-treatment or use of halogenated additives, making this process compatible with industrial requirements.

Implementation of data-cube pump-probe KPFM on organic solar cells.

An implementation of pump-probe Kelvin probe force microscopy (pp-KPFM) is reported that enables recording the time-resolved surface potential in single-point mode or over a 2D grid. The spectroscopic data are acquired in open z-loop configuration, which simplifies the pp-KPFM operation. The validity of the implementation is probed by measurements using electrical pumping. The dynamical photoresponse of a bulk heterojunction solar cell based on PTB7 and PC71BM is subsequently investigated by recording point-spectroscopy curves as a function of the optical power at the cathode and by mapping 2D time-resolved images of the surface photovoltage of the bare organic active layer.

Photochromic dye-sensitized solar cells with light-driven adjustable optical transmission and power conversion efficiency.

Semi-transparent photovoltaics only allows for the fabrication of solar cells with an optical transmission that is fixed during their manufacturing resulting in a trade-off between transparency and efficiency. For the integration of semi-transparent devices in building, ideally solar cells should generate electricity while offering the comfort for users to self-adjust their light transmission with the intensity of the daylight. Here we report a photochromic dye-sensitized solar cell (DSSC) based on donor-π-conjugated bridge-acceptor structures where the π-conjugated bridge is substituted for a diphenyl-naphthopyran photochromic unit. DSSCs show change in colour and self-adjustable light transmittance when irradiated with visible light and a power conversion efficiency up to 4.17%. The colouration-decolouration process is reversible and these DSSCs are stable over 50 days. We also report semi-transparent photo-chromo-voltaic mini-modules (23 cm2) exhibiting a maximum power output of 32.5 mW after colouration.

Remarkable tuning of the coordination and photophysical properties of lanthanide ions in a series of tetrazole-based complexes.

A series of seven new tetrazole-based ligands (L1, L3-L8) containing terpyridine or bipyridine chromophores suited to the formation of luminescent complexes of lanthanides have been synthesized. All ligands were prepared from the respective carbonitriles by thermal cycloaddition of sodium azide. The crystal structures of the homoleptic terpyridine-tetrazolate complexes [Ln(Li)(2)]NHEt(3) (Ln = Nd, Eu, Tb for i = 1, 2; Ln = Eu for i = 3, 4) and of the monoaquo bypyridine-tetrazolate complex [Eu(H(2)O)(L7)(2)]NHEt(3) were determined. The tetradentate bipyridine-tetrazolate ligand forms nonhelical complexes that can contain a water molecule coordinated to the metal. Conversely, the pentadentate terpyridine-tetrazolate ligands wrap around the metal, thereby preventing solvent coordination and forming chiral double-helical complexes similarly to the analogue terpyridine-carboxylate. Proton NMR spectroscopy studies show that the solid-state structures of these complexes are retained in solution and indicate the kinetic stability of the hydrophobic complexes of terpyridine-tetrazolates. UV spectroscopy results suggest that terpyridine-tetrazolate complexes have a similar stability to their carboxylate analogues, which is sufficient for their isolation in aerobic conditions. The replacement of the carboxylate group with tetrazolate extends the absorption window of the corresponding terpyridine- (approximately 20 nm) and bipyridine-based (25 nm) complexes towards the visible region (up to 440 nm). Moreover, the substitution of the terpyridine-tetrazolate system with different groups in the ligand series L3-L6 has a very important effect on both absorption spectra and luminescence efficiency of their lanthanide complexes. The tetrazole-based ligands L1 and L3-L8 sensitize efficiently the luminescent emission of lanthanide ions in the visible and near-IR regions with quantum yields ranging from 5 to 53% for Eu(III) complexes, 6 to 35% for Tb(III) complexes, and 0.1 to 0.3% for Nd(III) complexes, which is among the highest reported for a neodymium complex. The luminescence efficiency could be related to the energy of the ligand triplet states, which are strongly correlated to the ligand structures.

Activation Energy of Organic Cation Rotation in CH3NH3PbI3 and CD3NH3PbI3: Quasi-Elastic Neutron Scattering Measurements and First-Principles Analysis Including Nuclear Quantum Effects.

The motion of CH3NH3+ cations in the low-temperature phase of the promising photovoltaic material methylammonium lead triiodide (CH3NH3PbI3) is investigated experimentally as well as theoretically, with a particular focus on the activation energy. Inelastic and quasi-elastic neutron scattering measurements reveal an activation energy of ∼48 meV. Through a combination of experiments and first-principles calculations, we attribute this activation energy to the relative rotation of CH3 against an NH3 group that stays bound to the inorganic cage. The inclusion of nuclear quantum effects through path integral molecular dynamics gives an activation energy of ∼42 meV, in good agreement with the neutron scattering experiments. For deuterated samples (CD3NH3PbI3), both theory and experiment observe a higher activation energy for the rotation of CD3 against NH3, which results from the smaller nuclear quantum effects in CD3. The rotation of the NH3 group, which is bound to the inorganic cage via strong hydrogen bonding, is unlikely to occur at low temperatures due to its high energy barrier of ∼120 meV.

All-Polymeric Flexible Transparent Heaters.

All-polymeric flexible transparent heaters (THs) are demonstrated for the first time. Thin films of four poly(3,4-ethylenedioxythiophene) (PEDOT)-based materials embedding different dopants exhibit low sheet resistances, down to 57 Ω sq-1 associated with good transparencies (>87%) and a haze lower than 1%. These transparent thin films show excellent heating properties, with high heating rates (up to 1.6 °C s-1) and steady-state temperatures exceeding 100 °C when subjected to 12 V bias. Very high areal power densities were also measured, reaching almost 10 000 W m-2. The temperature increase is finely fitted to a thermal model. It is further demonstrated that these new THs can be efficiently integrated for applications in thermochromic displays and visor deicers.