Ionic Thermoelectric Generators in Vertical and Planar Topologies Based on Fluorinated Polymer Hybrid Materials with Ionic Liquids.

Ionic thermoelectrics (TEs), in which voltage generation is based on ion migration, are suitable for applications based on their low cost, high flexibility, high ionic conductivity, and wide range of Seebeck coefficients. This work reports on the development of ionic TE materials based on the poly(vinylidene fluoride-trifluoroethylene), Poly(VDF-co-TrFE), as host polymer blended with different contents of the ionic liquid, IL, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMIM][TFSI]. The morphology, physico-chemical, thermal, mechanical, and electrical properties of the samples are evaluated together with the TE response. It is demonstrated that the IL acts as a nucleating agent for polymer crystallization. The mechanical properties and ionic conductivity values are dependent on the IL content. A high room temperature ionic conductivity of 0.008 S cm-1 is obtained for the sample with 60 wt% of [EMIM][TFSI] IL. The TE properties depend on both IL content and device topology-vertical or planar-the largest generated voltage range being obtained for the planar topology and the sample with 10 wt% of IL content, characterized by a Seebeck coefficient of 1.2 mV K-1. Based on the obtained maximum power density of 4.9 µW m-2, these materials are suitable for a new generation of TE devices.

High-Performance Sustainable Electrochromic Devices Based on Carrageenan Solid Polymer Electrolytes with Ionic Liquid.

The development of sustainable functional materials with strong potential to be applied in different areas has been growing and gaining increasing interest to address the environmental impact of current materials and technologies. In this scope, this work reports on sustainable functional materials with electrochromic properties, based on their increasing interest for a variety of applications, including sensing technologies. The materials have been developed based on a natural derived polymer, carrageenan, in which different amounts of the ionic liquid (IL) 1-ethyl-3-methylimidazolium thiocyanate ([EMIM][SCN]) were blended. It is shown that the addition of different amounts of IL to the carrageenan matrix does not affect the properties of the samples in terms of morphology or physicochemical and thermal properties, the most significant difference being the increase of the ionic conductivity with increasing IL content, ranging from 2.3 × 10-11 S·cm-1 for pristine carrageenan to 4.6 × 10-4 S·cm-1 for the samples with 5 and 60 wt % IL content, respectively. A electrochromic device has been developed based on the different IL/carrageenan samples as electrolyte and poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) as electrodes. Spectroelectrochemistry testing demonstrates functional devices at low voltages between 0.3 and -0.9 V. Among the different samples, the one with 15 wt % IL content presents the best conditions for application, presenting an oxidation time of 6 s, a reduction time of 8 s, and a charge density of 1150 and 1050 µC·cm-2 for oxidation and reduction, respectively. The same sample also presents excellent optical density as a function of load density, presenting an optical switch (Δ%Tx) of 99%. Thus, it is demonstrated that it is possible to develop high efficiency and sustainable electrochromic devices based on natural polymers and ionic liquids.

Sustainable Collagen Blends with Different Ionic Liquids for Resistive Touch Sensing Applications.

Considering the sustainable development goals to reduce environmental impact, sustainable sensors based on natural polymers are a priority as the large im plementation of these materials is required considering the Internet of Things (IoT) paradigm. In this context, the present work reports on sustainable blends based on collagen and different ionic liquids (ILs), including ([Ch][DHP], [Ch][TSI], [Ch][Seri]) and ([Emim][TFSI]), processed with varying contents and types of ILs in order to tailor the electrical response. Varying IL types and contents leads to different interactions with the collagen polymer matrix and, therefore, to varying mechanical, thermal, and electrical properties. Collagen/[Ch][Seri] samples display the most pronounced decrease of the tensile strength (3.2 ± 0.4 MPa) and an increase of the elongation at break (50.6 ± 1.5%). The best ionic conductivity value of 0.023 mS cm-1 has been obtained for the sample with 40 wt % of the IL [Ch][Seri]. The functional response of the collagen-IL films has been demonstrated on a resistive touch sensor whose response depends on the ionic conductivity, being suitable for the next generation of sustainable touch sensing devices.

Pyridine Bridging Diphenylamine-Carbazole with Linking Topology as Rational Hole Transporter for Perovskite Solar Cells Fabrication.

Developing cost-effective and rational hole transporting materials is critical for fabricating high-performance perovskite solar cells (PSCs) and to promote their commercial endeavor. We have designed and developed pyridine (core) bridging diphenylamine-substituted carbazole (arm) small molecules, named as 2,6PyDANCBZ and 3,5PyDANCBZ. The linking topology of core and arm on their photophysical, thermal, semiconducting, and photovoltaic properties were probed systematically. We found that the 2,6PyDANCBZ shows higher mobility and conductivity along with uniform film-forming ability as compared to 3,5PyDANCBZ. The PSCs fabricated with 2,6PyDANCBZ supersede the performance delivered by Spiro-OMeTAD and importantly also gave improved long-term stability. Our findings put forward small molecules based on core-arm linking topology for cost-effective hole selective layers designing.

Elucidating the Doping Mechanism in Fluorene-Dithiophene-Based Hole Selective Layer Employing Ultrahydrophobic Ionic Liquid Dopant.

Perovskite solar cells have set a new milestone in terms of efficiencies in the thin film photovoltaics category. Long-term stability of perovskite solar cells is of paramount importance but remains a challenging task. The lack of perovskite solar cells stability in real-time operating conditions erodes and impedes commercialization. Further improvements are essential with a view to delivering longer-lasting photovoltaic (PV) performances. An ideal path in this direction will be to identify novel dopants for boosting the conductivity and hole mobility of hole transport materials (HTMs), and by so doing, the usage of hygroscopic and deliquescent additive materials can be avoided. The present work demonstrates the employment of ionic liquids into a dissymmetric fluorene-dithiophene, FDT (2',7'-bis(bis(4-methoxyphenyl)amino) spiro[cyclopenta[2,1-b:3,4-b']dithiophene-4,9'-fluorene]) based HTM to understand the doping mechanisms. N-Heterocyclic hydrophobic ionic liquid, 1-butyl-3-methylpyidinium bis(trifluoromethylsulfonyl)imide (BMPyTFSI) as p-type dopant for FDT was found to increase the conductivity of FDT, to higher geometrical capacitance, to facilitate homogeneous film formation, and to enhance device stability. Our findings open up a broad range of hole-transport materials to control the degradation of the underlying water-sensitive active layer by substituting a hygroscopic element.

Appraisement of Crystal Expansion in CH3 NH3 PbI3 on Doping: Improved Photovoltaic Properties.

Three-dimensional hybrid perovskite materials (CH3 NH3 PbI3 ) suffer from intrinsic instability owing to organic cation evaporation and ion migration. The inclusion of a large organic cation such as guanidinium has been probed to stabilize the structure. This work proposes the inclusion of imidazolium iodide (C3 N2 H5 I) as an organic cation inside the CH3 NH3 PbI3 matrix, as a reservoir to control the spontaneous loss of iodide. The introduction of imidazolium iodide in amounts below 20 % has an impact on the crystallization process but not on the optical properties. It also positively controls non-radiative recombination and improves the open-circuit voltage of the solar cells. The present study paves way for a deeper insight into the limit of multi-dimensional perovskite to further push the performance.

Understanding the Influence of Interface Morphology on the Performance of Perovskite Solar Cells.

In recent years, organo-halide perovskite solar cells have garnered a surge of interest due to their high performance and low-cost fabrication processing. Owing to the multilayer architecture of perovskite solar cells, interface not only has a pivotal role to play in performance, but also influences long-term stability. Here we have employed diverse morphologies of electron selective layer (ESL) to elucidate charge extraction behavior in perovskite solar cells. The TiO2 mesoporous structure (three-dimensional) having varied thickness, and nanocolumns (1-dimensional) with tunable length were employed. We found that a TiO2 electron selective layer with thickness of about c.a. 100 nm, irrespective of its microstructure, was optimal for efficient charge extraction. Furthermore, by employing impedance spectroscopy at different excitation wavelengths, we studied the nature of recombination and its dependence on the charge generation profile, and results showed that, irrespective of the wavelength region, the fresh devices do not possess any preferential recombination site, and recombination process is governed by the bulk of the perovskite layer. Moreover, depending on the type of ESL, a different recombination mechanism was observed that influences the final behavior of the devices.

Oxazolium Iodide Modified Perovskites for Solar Cell Fabrication.

Perovskites solar cells are gaining interest due to their attractive solar-to-electricity conversion efficiencies; however, they suffer from certain problems, such as suboptimal ion migration and stability issues. We report here on the inclusion of a phenyloxazolium salt (2-phenyl-3-methyloxazolium iodide) in perovskite solar cells based on methyl ammonium lead triiodide (MAPbI3 ). The fabricated solar cells not only displayed improved photovoltaic properties, but importantly the oxazolium cations can protect the perovskite layers from UV exposure as they down-convert electromagnetic irradiation; that is, the photons in the UV are absorbed and re-emitted at a different wavelength. The loading of 2-phenyl-3-methyloxazolium iodide in the perovskite precursor solution was optimized, the resulting perovskite films characterized, and the solar cells fabricated from them evaluated for their performance. Overall, this simple approach serves to optimize the performance parameters of perovskites films for solar cell applications.

Identifying the charge generation dynamics in Cs+-based triple cation mixed perovskite solar cells.

Triple cation based perovskite solar cells offer enhanced moisture tolerance and stability compared to mixed perovskites. Slight substitution of methyl ammonium or formamidinium cation by cesium (Cs+), was also reported to eliminate halide segregation due to its smaller size. To elucidate the device kinetics and understand the role of the Cs, we undertook different modes of scanning probe microscopy and electrochemical impedance spectroscopy (EIS) experiments. Kelvin probe force microscopy revealed that the incorporation of the Cs cation increases the contact potential difference (CPD), this CPD further increases when Spiro-OMeTAD is used as a hole transport material. The current at the nanoscale level shows improvement with Cs inclusion and further enhancement by the Spiro-OMeTAD deposition, studied under light illumination, which supports the high photocurrent density obtained from the cells. EIS demonstrates that in a triple cation environment, reduced carrier recombination at the TiO2/perovskite interface was also obtained which in turn allow us to achieve a higher Voc value.

Towards Extending Solar Cell Lifetimes: Addition of a Fluorous Cation to Triple Cation-Based Perovskite Films.

Organohalide perovskites have emerged as highly promising replacements for thin-film solar cells. However, their poor stability under ambient conditions remains problematic, hindering commercial exploitation. The addition of a fluorous-functionalized imidazolium cation during the preparation of a highly stable cesium-based mixed perovskite material Cs0.05 (MA0.15 FA0.85 )0.95 Pb(I0.85 Br0.15 )3 (MA=methylammonium; FA=formamidinium) has been shown to influence its stability. The resulting materials, which vary according to the amount of the fluorous-functionalized imidazolium cation present during fabrication, display a prolonged tolerance to atmospheric humidity (>100 days) along with power conversion efficiencies exceeding 16 %. This work provides a general route that can be implemented in a variety of perovskites and highlights a promising way to increase perovskite solar cell stability.

Cesium power: low Cs+ levels impart stability to perovskite solar cells.

Towards increasing the stability of perovskite solar cells, the addition of Cs+ is found to be a rational approach. Recently triple cation based perovskite solar cells were found to be more effective in terms of stability and efficiency. Heretofore they were unexplored, so we probed the Cs/MA/FA (cesium/methyl ammonium/formamidinium) cation based perovskites by X-ray photoelectron spectroscopy (XPS) and correlated their compositional features with their solar cell performances. The Cs+ content was found to be optimum at 5%, when incorporated in the (MA0.15FA0.85)Pb(I0.85Br0.15)3 lattice, because the corresponding device yielded the highest fill factor compared to the perovskite without Cs+ and with 10% Cs+. XPS studies distinctly reveal how Cs+ aids in maintaining the expected stoichiometric ratios of I : Pb2+, I : N and Br : Pb2+ in the perovskites, and how the valence band (VB) edge is dependent on the Cs+ proportion, which in turn governs the open circuit voltage. Even at a low content of 5%, Cs+ resides deep within the absorber layer, and ensures minimum distortion of the VB level (compared to 0% and 10% Cs+ perovskites) upon Ar+ sputtering, thus allowing the formation of a stable robust material that delivers excellent solar cell response. This study which brings out the role of Cs+ is anticipated to be of paramount significance to further engineer the composition and improve device performances.

Interface Play between Perovskite and Hole Selective Layer on the Performance and Stability of Perovskite Solar Cells.

Perovskite solar cells with variety of hole selective contacts such as 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD), poly(3-hexylthiophene-2,5-diyl), poly[bis(4-phenyl)(2,5,6-trimentlyphenyl)amine], 5,10,15-trihexyl-3,8,13-tris(4-methoxyphenyl)-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole (HMPDI), and 2',7'-bis(bis(4-methoxyphenyl)amino)spiro[cyclopenta[2,1-b:3,4-b']dithiophene-4,9'-fluorene] were employed to elucidate its role at the interface of perovskite and metallic cathode. Microscopy images revealed Spiro-OMeTAD and HMPDI produce smoother and intimate contact between perovskite/hole transporting materials (HTM) interfaces among others evaluated here. This morphological feature appears to be connected with three fundamental facts: (1) hole injection to the HTM is much more efficient as evidenced by photoluminescence measurements, (2) recombination losses are less important as evidenced by intensity-modulated photovoltage spectroscopy and impedance spectroscopy measurements, and (3) fabricated solar cells are much more robust against degradation by moisture. Devices with higher open-circuit photovoltages are characterized by higher values of the recombination resistance extracted from the impedance data. The variation in device hysteresis behavior can be ascribed mainly due to the molecular interaction and the core of HTM employed. In all cases, this fact is related with a larger value of the low-frequency capacitance, which indicates that the HTM can induce specific slow processes of ion accumulation at the interface. Notably, these processes tend to slowly relax in time, as hysteresis is substantially reduced for aged devices.

Specific cation interactions as the cause of slow dynamics and hysteresis in dye and perovskite solar cells: a small-perturbation study.

Hysteresis is one of the most remarkable features of perovskite solar cells; however, it is also present in other kinds of devices such as dye-sensitized solar cells. Hysteresis is due to underlying slow dynamic processes that interfere with the process of charge separation which depends critically on the selective contacts used. In this work we focus on the low-frequency (0.1-10 Hz) dynamics using impedance and intensity-modulated photocurrent spectroscopy and found that both perovskite solar cells (PSCs) and "viscous electrolyte containing" dye-sensitized solar cells (DSSCs) can be described on the same fundamental grounds. By comparing different electrolyte compositions in DSSCs and both methylammonium and formamidinium-based PSCs, we find a connection between the polar nature of the cations and the low-frequency component of these solar cells. There is evidence that in both cases ion transport and specific chemical interactions with the TiO2 surface give rise to the slow dynamics and the hysteresis. This is mainly inferred from the slope of the capacitance vs. applied voltage which shows accumulation behavior for the formulations with higher dipole moments only.

Influence of the mixed organic cation ratio in lead iodide based perovskite on the performance of solar cells.

Lead halide based perovskite solar cells are presently the flagship among the third generation solution-processed photovoltaic technologies. The organic cation part in the perovskite plays an important role in terms of crystal structure tuning from tetragonal to trigonal or pseudocubic or vice versa depending on the organic cations used, while it also displays different microstructure. In this paper, we demonstrate the influence of the organic cation part with respect to optical properties, hysteresis behavior, and stability. This study offers a clear understanding of the perovskite properties and how they can be modulated by compositional engineering. With a rational choice, light harvesting abilities and hysteresis behavior can be controlled in these systems. The substitution of formamidinium cation by methylammonium cation allows achieving low temperature annealing and inducing stability in perovskites together with enhanced photovoltaic properties. By the use of in-situ scanning force microscopy experiments the conversion of precursors to perovskite at a particular temperature can be visualized.

Extending the Lifetime of Perovskite Solar Cells using a Perfluorinated Dopant.

The principle limitation of perovskite solar cells is related to their instability and, hence, their limited lifetime. Herein, we employ an imidazolium iodide dopant, 1-methyl-3-(1H,1H,2H,2H-nonafluorohexyl)-imidazolium iodide, containing a perfluorous appendage, which leads to prolonged (unencapsulated, under Ar atmosphere) device activities exceeding 100 days without compromising the power conversion efficiency and other photovoltaic parameters. The extended lifetime of the device can be attributed, at least in part, to the hydrophobic nature of the imidazolium iodide salt. The functionalization of the perovskite material was found to have negligible influence on the perovskite crystal structure.

Unraveling Charge Carriers Generation, Diffusion, and Recombination in Formamidinium Lead Triiodide Perovskite Polycrystalline Thin Film.

We report on studies of the formamidinium lead triiodide (FAPbI3) perovskite film using time-resolved terahertz (THz) spectroscopy (TRTS) and flash photolysis to explore charge carriers generation, migration, and recombination. The TRTS results show that upon femtosecond excitation above the absorption edge, the initial high photoconductivity (∼75 cm(2) V(-1) s(-1)) remains constant at least up to 8 ns, which corresponds to a diffusion length of 25 µm. Pumping below the absorption edge results in a mobility of 40 cm(2) V(-1) s(-1) suggesting lower mobility of charge carriers located at the bottom of the conduction band or shallow sub-bandgap states. Furthermore, analysis of the THz kinetics reveals rising components of <1 and 20 ps, reflecting dissociation of excitons having different binding energies. Flash photolysis experiments indicate that trapped charge carriers persist for milliseconds.

Universal Features of Electron Dynamics in Solar Cells with TiO2 Contact: From Dye Solar Cells to Perovskite Solar Cells.

The electron dynamics of solar cells with mesoporous TiO2 contact is studied by electrochemical small-perturbation techniques. The study involved dye solar cells (DSC), solid-state perovskite solar cells (SSPSC), and devices where the perovskite acts as sensitizer in a liquid-junction device. Using a transport-recombination continuity equation we found that mid-frequency time constants are proper lifetimes that determine the current-voltage curve. This is not the case for the SSPSC, where a lifetime of ∼1 µs, 1 order of magnitude longer, is required to reproduce the current-voltage curve. This mismatch is attributed to the dielectric response on the mid-frequency component. Correcting for this effect, lifetimes lie on a common exponential trend with respect to open-circuit voltage. Electron transport times share a common trend line too. This universal behavior of lifetimes and transport times suggests that the main difference between the cells is the power to populate the mesoporous TiO2 contact with electrons.