Vacuum processed perovskite solar cells

El objetivo de la tesis es el desarrollo de materiales y métodos de deposición de capas de perovskita utilizando procesos de alto vacío, principalmente mediante co-evaporación. Para la fabricación de los dispositivos, se han empleado y optimizado diferentes materiales transportadores de electrones y de huecos. La tesis está estructurada de la siguiente manera:Capítulo 1: se desarrolla un método de deposición novedoso para la preparación de capas delgadas de perovskitas mixtas iodo-bromo.Capítulo 2: se presenta el estudio de diferentes derivados del fullereno como materiales transportadores de electrones para conseguir células solares de perovskita de alta eficiencia.Capítulo 3: se optimiza la interfaz entre la capa transportadora de electrones y el electrodo metálico, con el objetivo de remplazar el bario cuya reactividad limita la estabilidad de los dispositivos.The aim of this thesis is the development of vacuum deposition methods for the fabrication of perovskite thin films, and their incorporation into devices using organic charge transport layers. The thesis will be structure as follows:Chapter 1: preparation of efficient single-junction perovskite solar cells with tunable bandgap using vacuum processed techniques.Chapter 2: study the effect of different fullerene derivatives as electron transport materials on the performance of perovskite solar cells. Additionally, the interface between the fullerene and the perovskite is optimized, in order to reduce the non-radiative recombination and maximize the photovoltage.Chapter 3: Interface engineering towards stable electron transport materials. Development of a new electrode buffer layer as an alternative to reactive metal cathodes in p-i-n perovskite solar cells

Perovskite solar cells prepared by flash evaporation

A simple vacuum deposition method for the preparation of high quality hybrid organic-inorganic methylammonium lead iodide perovskite thin films is reported. When sandwiched in between organic charge transporting layers, such films lead to solar cells with a power conversion efficiency of 12.2%

Efficient photovoltaic and electroluminescent perovskite devices

Planar diode structures employing hybrid organic-inorganic methylammonium lead iodide perovskites lead to multifunctional devices exhibiting both a high photovoltaic efficiency and good electroluminescence. The electroluminescence strongly improves at higher current density applied using a pulsed driving method

Quadruple-Cation Wide-Bandgap Perovskite Solar Cells with Enhanced Thermal Stability Enabled by Vacuum Deposition

Vacuum processing of multicomponent perovskites is not straightforward, because the number of precursors is in principle limited by the number of available thermal sources. Herein, we present a process which allows increasing the complexity of the formulation of vacuum-deposited lead halide perovskite films by multisource deposition and premixing both inorganic and organic components. We apply it to the preparation of wide-bandgap CsMAFA triple-cation perovskite solar cells, which are found to be efficient but not thermally stable. With the aim of stabilizing the perovskite phase, we add guanidinium (GA+) to the material formulation and obtained CsMAFAGA quadruple-cation perovskite films with enhanced thermal stability, as observed by X-ray diffraction and rationalized by microstructural analysis. The corresponding solar cells showed similar performance with improved thermal stability. This work paves the way toward the vacuum processing of complex perovskite formulations, with important implications not only for photovoltaics but also for other fields of application

Deposition Kinetics and Compositional Control of Vacuum Processed CH3NH3PbI3 Perovskite

Halide perovskites have generated considerable research interest due to their excellent optoelectronic properties in the past decade. To ensure the formation of high quality semiconductors, the deposition process for the perovskite film is a critical issue. Vacuum based processing is considered a promising method allowing, in principle, also for large areas. One of the benefits of vacuum processing is the control over the film composition through the use of quartz crystal microbalances (QCMs) that in-situ monitor the rates of the components. In metal halide perovskites, however, one frequently employed component or precursor, CH3NH3I, exhibits non-standard sublimation properties. Here, we study in detail the sublimation properties of CH3NH3I and demonstrate that by correcting for its complex adsorption properties and by modeling the film growth, accurate predictions about the stoichiometry of the final perovskite film can be obtained

Unravelling steady-state bulk recombination dynamics in thick efficient vacuum-deposited perovskite solar cells by transient methods

Accurately identifying and understanding the dominant charge carrier recombination mechanism in perovskite solar cells is of crucial importance for further improvements of this already promising photovoltaic technology. Both optical and electrical transient methods have previously been employed to strive for this warranted goal. However, electrical techniques can be strongly influenced by the capacitive response of the device which overlays with the steady‐state relevant bulk carrier recombination. To ascertain the identification of bulk charge carrier dynamics, it is beneficial to evaluate thicker films to minimize the impact of device capacitance. Herein, the electrical transient response in very efficient planar co‐evaporated n‐i‐p solar cells is studied by varying the active layer thickness from 500 nm to 820 nm and compared to a solution‐processed perovskite device with an active layer of 350 nm. In case of the n‐i‐p devices, the capacitance for the 500 nm solar cell leads to longer perceived decay times in the lower voltage regime, while in the higher voltage regime quite similar kinetics independent of active layer thickness are observed, allowing us to identify the transition from capacitance‐affected to the sought‐after bulk charge carrier dynamics. We show that increasing the perovskite thickness by more than 50 % does not affect the recombination dynamics significantly, confirming the high quality of the vacuum‐processed perovskite solar cells. Finally, it is demonstrated for the first time for perovskite solar cells that the recombination order in both thicker devices is ranging between 1.6 to 2, pointing towards trap‐assisted and free‐carrier recombination under operating conditions. We emphasize that the observed low recombination orders are in strong contrast to earlier literature as well as to the thinner solution‐processed device, which suffers from both shorter carrier lifetimes and a larger device capacitance

Narrowband Monolithic Perovskite-Perovskite Tandem Photodetectors

Narrowband photodetectors (PDs) are sought after for many applications requiring selective spectral response. The most common systems combine optical bandpass filters with broadband photodiodes. This work reports a method to obtain a narrowband response in a perovskite PD by the monolithic integration of a perovskite photoconductor and a perovskite photodiode. The spectral response of the tandem PD is determined by the bandgap energy difference of the two perovskites, and exhibits a full width at half maximum below 85 nm, an external quantum efficiency up to 68% and a high specific detectivity of ≈1012 Jones in reverse bias, enabling the device to detect weak light signals. The absorption profile of the narrowband PD can be tuned by changing the thickness and bandgap of the wide bandgap perovskite absorber

Reduced Recombination Losses in Evaporated Perovskite Solar Cells by Postfabrication Treatment

The photovoltaic perovskite research community has now developed a large set of tools and techniques to improve the power conversion efficiency (PCE). One such arcane trick is to allow the finished devices to dwell in time, and the PCE often improves. Herein, a mild postannealing procedure is implemented on coevaporated perovskite solar cells confirming a substantial PCE improvement, mainly attributed to an increased open-circuit voltage (V OC). From a V OC of around 1.11 V directly after preparation, the voltage improves to more than 1.18 V by temporal and thermal annealing. To clarify the origin of this annealing effect, an in-depth device experimental and simulation characterization is conducted. A simultaneous reduction of the dark saturation current, the ideality factor (n id), and the leakage current is revealed, signifying a substantial impact of the postannealing procedure on recombination losses. To investigate the carrier dynamics in more detail, a set of transient optoelectrical methods is first evaluated, ascertaining that the bulk carrier lifetime is increased with device annealing. Second, a drift-diffusion simulation is used, confirming that the beneficial effect of the annealing has its origin in effective bulk trap passivation that accordingly leads to a reduction of Shockley-Read-Hall recombination rates

Dual-source vacuum deposition of pure and mixed halide 2D perovskites: thin film characterization and processing guidelines

The dual-source vacuum deposition of 2D perovskite films of the type PEA2PbX4, (PEA = phenethylammonium and X = I−, Br−, or a combination of both) is presented. Low-temperature deposited 2D perovskite films showed high crystallinity with the expected trend of bandgap as a function of halide type and concentration. Importantly, we observed an unavoidable halide cross-contamination among different deposition runs, as well as a strong dependence of the material quality on the type of halide precursors used. These findings should be taken into account in the development of vacuum processing for low-dimensional mixed halide perovskites

Boosting inverted perovskite solar cell performance by using 9,9-bis(4-diphenylaminophenyl)fluorene functionalized with triphenylamine as a dopant-free hole transporting material

In this study, two newly developed small molecules based on 9,9-bis(4-diphenylaminophenyl)fluorene functionalized with triphenylamine moieties, namely TPA-2,7-FLTPA-TPA and TPA-3,6-FLTPA-TPA, are designed, synthesized and characterized. The electrochemical, optical and thermal properties of both materials are investigated using various techniques. Afterwards, these materials are employed as dopant-free hole transporting materials (HTMs) in planar inverted perovskite solar cell devices with the aim of determining the device performance and studying their stability in comparison with reference N-4,N-4,N-4,N-4-tetra([1,10-biphenyl]-4-yl)-[1,1:4,1-terphenyl]-4,4-diamine (TaTm)-based devices. Under 1 sun conditions, TPA-3,6-FLTPA-TPA-based devices achieve a power conversion efficiency (PCE) of 13.9% whereas TPA-2,7-FLTPA-TPA-based devices exhibit the highest PCE of 17.1% mainly due to an improvement in the fill factor (FF). Meanwhile, the devices prepared using TaTm as the reference HTM exhibit an overall efficiency of 15.9%. In addition to the higher efficiency, our newly developed HTM TPA-2,7-FLTPA-TPA-based devices demonstrate good stability which is comparable to those with TaTm under similar aging test conditions