Surface engineering for injection control and hysteresis reduction in Perovskite Solar Cells

Treball Final de Màster Universitari en Física Aplicada. Codi: SIN019. Curs acadèmic 2015-2016Hybrid halide perovskites, such as CH3NH3PbI3 are among the most promising absorbers materials in photovoltaics due to its direct band gap, high absorption coefficient and low charge recombination.1,2 In addition, they have the benefits of being prepared by low temperature solution processes which are low cost.3,4 However, the interfaces between the perovskite and the selective contacts are still decisive for obtaining a better performance of the device. In addition, interfaces also play a critical role on the hysteresis present in the J-V curves that has been detected in Perovskite Solar Cells (PSCs), indicating that the current is dependent on the direction and speed of the scanning.5,6 Modifying the electron selective contact with a self-assembled monolayer (SAM) of fullerene derivatives has shown to enhance the photocurrent and to have a positive effect on the hysteresis, as reported for example by Wojciechowski et al.7 This fullerene derivative contains a functional group to anchor, by chemisorption, onto the selective contact surface, modifying the interface and improving the electron extraction.8 This work is focused on the use of some fullerene derivatives containing different functional groups to anchor onto the scaffold and the compact layer of TiO2. In addition, the effect of removing the compact layer has been studied in combination with the use of fullerenes

Impedance Spectroscopy Measurements in Perovskite Solar Cells: Device Stability and Noise Reduction

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Up-Converting Lanthanide-Doped YAG Nanospheres

The development of lanthanide-doped Y3Al5O12 (Ln:YAG) garnet nanostructures is a hot topic in the field of inorganic nanophosphors due to the current interest in developing small nanoparticles for solid-state lighting (SSL), displays, lasers and scintillation applications. In this study, we report the preparation of homogeneous Ln:YAG (Ln: Ho/Yb ions) nanospheres through a combined two-steps coprecipitationsolvothermal synthesis at low temperature. The crystal growth takes place in ethylene glycol, which is an inexpensive, non-toxic and easily available solvent. Monodisperse and crystalline spherical YAG particles of 80 nm in diameter were obtained. Furthermore, the protocol can be extended to other compositions (Tb/Yb, Tm/Yb. . .) to explore different luminescent properties, without affecting the morphology of the material, indicating the robustness and practical utility of the reported methodology. Thermal treatment of the nanogarnets at 1200◦C is necessary for making materials optically active upon both UV and NIR excitation. The spherical morphology of annealed samples is preserved, what helps their further dispersion in solvents, barbotines, inks or printing vehicles. The lanthanide-doped nanogarnets exhibited the characteristic blue, green and red emissions from lanthanide upconversion photoluminescence (UCPL) upon NIR excitation. The UCPL mechanism was studied and CIE chromate coordinates were obtained. These nanogarnets were further evaluated as functional ceramic phosphors by incorporating them into commercial glazes. The materials exhibited an exceptional chemical stability in a harsh medium such as a fused glaze. Consequently, the visible emissions of the nanoparticles were transferred to the whole glass matrix, thus providing a functional glaze that emits intense blue and green light upon NIR excitation. These luminescent nanogarnets have promising applications in smart enamels, but can also be useful for lighting displays (white LEDs. . .), smart paintings or plastics, and anti-counterfeiting systems

Transformation of PbI2, PbBr2 and PbCl2 salts into MAPbBr3 perovskite by halide exchange as an effective method for recombination reduction

Halide perovskite derivatives present unprecedented physical phenomena among those materials which are suitable for photovoltaics, such as a fast ion diffusion coefficient. In this paper it is reported how the benefits of this property can be used during the growth of halide perovskites in order to control the morphological and optoelectronic properties of the final thin film. Using a large enough halide reservoir, the nature of the halides present in the final perovskite layer can be exchanged and this depends on the initial salt used in the two-step deposition method. In particular, the preparation of a methylammonium lead bromide (MAPbBr3) thin film is reported, using a two-step method based on the transformation of lead(II) iodide (PbI2), lead(II) bromide (PbBr2) and lead(II) chloride (PbCl2) salts into MAPbBr3 perovskite after dipping in a methylammonium bromide (MABr) solution. The films prepared from different salts present different properties in terms of morphology and optoelectronic properties, thus providing significantly different performance when they are used for the preparation of photovoltaic devices. Interestingly, the use of PbI2 and PbCl2 salts reduce the charge recombination and increase the open circuit potential obtained, especially in the former case. However, the highest photocurrent is obtained when PbBr2 is used. For PbI2 and PbCl2 salts no traces of the former salt are observed in the MAPbBr3 layer obtained after 10 minutes of dipping time, however, the presence of PbBr2 has still been detected (using X-ray diffraction) when this salt has been employed

Outstanding nonlinear optical properties of methylammonium- and Cs-PbX3 (X = Br, I, and Br–I) perovskites: Polycrystalline thin films and nanoparticles

Metal Halide Perovskites (MHPs) have arisen as promising materials to construct cost-effective photovoltaic and light emission devices. The study of nonlinear optical properties of MHPs is necessary to get similar success in nonlinear photonic devices, which is practically absent in the literature. The determination of the third order nonlinear coefficients is typically done by the Z-scan technique, which is limited by the scattering of polycrystalline thin films. In this work, we have studied nonlinear optical properties of polycrystalline CH3NH3PbX3 (MAPbX3) thin films and colloidal CsPbX3 nanoparticles with three different bandgaps (X3 = I3, Br3, and Br1.5I1.5). Their bright generation of photoluminescence under infrared illumination demonstrates an excellent efficiency of multiphoton absorption. The nonlinear absorption coefficient ( ) was studied by analyzing the transmitted light through the samples, observing the expected Eg −3 dependence with values as high as = 1500 cm/GW. In addition, we proposed the use of a modified Z-scan technique with imaging processing to analyze the nonlinear refraction coefficient (n2) under the laser damage threshold. Our experimental data agree quite well with theoretical predictions, demonstrating the accuracy of the method and potential applications to other thin films. Moreover, n2 parameter reaches values of 3.5 cm2/GW, indicating the suitability of MHPs for nonlinear photonics

High Optical Performance of Cyan‐Emissive CsPbBr3 Perovskite Quantum Dots Embedded in Molecular Organogels

This is the pre-peer reviewed version of the following article: High Optical Performance of Cyan‐Emissive CsPbBr3 Perovskite Quantum Dots Embedded in Molecular Organogels, which has been published in final form at https://doi.org/10.1002/adom.202001786. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions."Perovskite quantum dots (PQDs) have fascinating optoelectronic properties, such as high photoluminescence quantum yield (PLQY) for a broad range of materials, and the possibility to obtain different bandgaps with the same material or halide combinations. Nevertheless, blue‐emissive materials generally present limited PLQY or color instability. Here, two molecular organogels, based on a derivative of an amino acid and succinic acid, are used to embed CsPbBr3 quantum dots, obtaining green and blue emission. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) measurements are performed to confirm that there are no significant changes in the average size of the QDs and the crystal structure. A high near‐unity PLQY is achieved for the blue emission. This contribution opens the door to a postsynthetic treatment for synthesizing blue‐emissive PQDs with high optical performance, which can be attractive for optoelectronic applications

Enhanced Open-Circuit Voltage of Wide-Bandgap Perovskite Photovoltaics by Using Alloyed (FA1–xCsx)Pb(I1–xBrx)3 Quantum Dots

We report a detailed study on APbX3 (A=Formamidinium (FA+), Cs+; X=I-, Br-) perovskite quantum dots (PQDs) with combined A- and X-site alloying that exhibit, both, a wide bandgap and high open circuit voltage (Voc) for the application of a potential top cell in tandem junction photovoltaic (PV) devices. The nanocrystal alloying affords control over the optical bandgap and is readily achieved by solution-phase cation and anion exchange between previously synthesized FAPbI3 and CsPbBr3 PQDs. Increasing only the Br- content of the PQDs widens the bandgap but results in shorter carrier lifetimes and associated Voc losses in devices. These deleterious effects can be mitigated by replacing Cs+ with FA+, resulting in wide bandgap PQD absorbers with improved charge-carrier mobility and PVs with higher Voc. Although further device optimization is required, these results demonstrate the potential of FA1–xCsx)Pb(I1–xBrx)3 PQDs for wide bandgap perovskite PVs with high Voc

Fullerene-Based Materials as Hole-Transporting/Electron Blocking Layers. Applications in Perovskite Solar Cells

Here we report for the first time an efficient fullerenebased compound, FU7, able to act as Hole-Transporting Material (HTM) and electron blocking contact. It has been applied on perovskite solar cells (PSCs), obtaining 0.81 times the efficiency of PSCs with the standard HTM, spiro-OMeTAD, with the additional advantage that this performance is reached without any additive introduced in the HTM layer. Moreover, as a proof of concept, we have described for the first time efficient PSCs where both selective contacts are fullerene derivatives, to obtain unprecedented “fullerene sandwich” PSCs

Study of lanthanide-dopedY3AL5O12 nanogarnets with energy conversion properties

Treball final de Grau en Enginyeria Química. Codi: QU0943. Curs academic 2014-201

Different approaches to improve perovskite-based photovoltaic and optoelectronic devices

Compendi d'articles; Doctorat internacionalDoctorat internacionalHalide perovskites have had a huge impact on different fields in the last decade due to their versatility and amazing optoelectronic properties. However, there are still some issues to improve towards their future utilization and commercialization. In this thesis, several topics associated with perovskite-based photovoltaic and optoelectronic devices are addressed. The different works contribute to enhance the optoelectronic properties of perovskite quantum dots, to replace the most employed hole transporting material in photovoltaic devices (spiro-OMeTAD) and, moreover, to study a methodology which can be incorporated in up-scaling procedures towards their future commercialization as solar cells.Programa de Doctorat en Cièncie