Strong Light-Matter Coupling in Lead Halide Perovskite Quantum Dot Solids.

Strong coupling between lead halide perovskite materials and optical resonators enables both polaritonic control of the photophysical properties of these emerging semiconductors and the observation of fundamental physical phenomena. However, the difficulty in achieving optical-quality perovskite quantum dot (PQD) films showing well-defined excitonic transitions has prevented the study of strong light-matter coupling in these materials, central to the field of optoelectronics. Herein we demonstrate the formation at room temperature of multiple cavity exciton-polaritons in metallic resonators embedding highly transparent Cesium Lead Bromide quantum dot (CsPbBr3-QD) solids, revealed by a significant reconfiguration of the absorption and emission properties of the system. Our results indicate that the effects of biexciton interaction or large polaron formation, frequently invoked to explain the properties of PQDs, are seemingly absent or compensated by other more conspicuous effects in the CsPbBr3-QD optical cavity. We observe that strong coupling enables a significant reduction of the photoemission line width, as well as the ultrafast modulation of the optical absorption, controllable by means of the excitation fluence. We find that the interplay of the polariton states with the large dark state reservoir plays a decisive role in determining the dynamics of the emission and transient absorption properties of the hybridized light-quantum dot solid system. Our results should serve as the basis for future investigations of PQD solids as polaritonic materials.

Responsive Optical Materials Based on Ligand-Free Perovskite Quantum Dots Embedded in Mesoporous Scaffolds.

Herein we show that dispersing inorganic cesium lead bromide (CsPbBr3) perovskite quantum dots (QDs) in optical quality films, possessing an accessible and controlled pore size distribution, gives rise to fluorescent materials with a controlled and highly sensitive response to ambient changes. A scaffold-based synthesis approach is employed to obtain ligand-free QDs, whose pristine surface endows them with high sensitivity to the presence of different vapors in their vicinity. At the same time, the void network of the host offers a means to gradually expose the embedded QDs to such vapors. Under these conditions, the luminescent response of the QDs is mediated by the mesostructure of the matrix, which determines the rate at which vapor molecules will adsorb onto the pore walls and, eventually, condensate, filling the void space. With luminescence quantum yields as high as 60%, scaffold-supported ligand-free perovskite nanocrystals display intense photoemission signals over the whole process, as well as high photo- and chemical stability, which allows illuminating them for long periods of time and recovering the original response upon desorption of the condensed phase. The results herein presented open a new route to explore the application of perovskite QD-based materials in sensing.

Ultrastrong Exciton-Photon Coupling in Broadband Solar Absorbers.

The recent development of organic polaritonic solar cells, in which sunlight absorbers and photon modes of a resonator are hybridized as a result of their strong coupling, has revealed the potential this interaction offers to control and enhance the performance of these devices. In this approach, the photovoltaic cell is built in such a way that it also behaves as an optical cavity supporting spectrally well-defined resonances, which match the broad absorption bands of the dyes employed. Herein we focus on the experimental and theoretical analysis of the specific spectral and angular optical absorption characteristics of a broadband light harvester, namely a subphthalocyanine, when operating in the ultrastrong coupling regime. We discuss the implications of having a broad distribution of oscillator strengths and demonstrate that rational design of the layered structure is needed to optimize both the spectral and the angular response of the sunlight harvester dye.

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.

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.

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.

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.

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.

Hole-Transport Materials for Perovskite Solar Cells.

The pressure to move towards renewable energy has inspired researchers to look for ideas in photovoltaics that may lead to a major breakthrough. Recently the use of perovskites as a light harvester has lead to stunning progress. The power conversion efficiency of perovskite solar cells is now approaching parity (>22 %) with that of the established technology which took decades to reach this level of performance. The use of a hole transport material (HTM) remains indispensable in perovskite solar cells. Perovskites can conduct holes, but they are present at low levels, and for efficient charge extraction a HTM layer is a prerequisite. Herein we provide an overview of the diverse types of HTM available, from organic to inorganic, in the hope of encouraging further research and the optimization of these materials.