Untersuchung von hybriden organisch-anorganischen Blei-Halogenid-Perowskiten mittels modulierter Oberflächenphotospannungsspektroskopie

Hybrid organic-inorganic metal halide perovskites (called here perovskites) have emerged as a new group of materials for highly efficient solar cells (SCs) based on earth abundant elements which can be processed from solutions at low temperature. CH3NH3Pb(I1-xBrx)3 perovskite films were studied in the thesis since it belongs to the materials which are of great interest for top SCs in tandem SCs with c-Si bottom SCs due to its tunable band gap. Electronic properties of CH3NH3Pb(I1-xBrx)3 perovskite films sensitively depend on crystallization and defect formation and are crucial for the performance and stability of SCs. The band gap (Eg), exponential tail states (Et) and the diffusion length (L) are decisive parameters for absorbers in SCs since they give principle limitations for photo-generation and Fermi-level splitting. In CH3NH3Pb(I1-xBrx)3 perovskites, Eg, Et and L are not necessarily constant and can depend, for example, on the preparation conditions and degradation. Therefore, Eg, Et and L of CH3NH3Pb(I1-xBrx)3 perovskites were studied with respect to stoichiometry, interfaces, degradation and temperature. Modulated surface photovoltage (SPV) spectroscopy was used as the main characterization technique. Modulated SPV spectroscopy allows for the contactless and very sensitive characterization of Eg, Et, direction of charge separation and L (after Goodman) without the requirement of the preparation of contacts after or during different stages of layer preparation, light soaking etc. Vegard’s law was applied to obtain the composition of CH3NH3Pb(I1-xBrx)3 films. The Eg of CH3NH3Pb(I1-xBrx)3 films varied between 1.59 eV to 2.30 eV depending on the stoichiometry whereas the bowing parameter was 0.36 eV. The influence of the substrate on Eg and Et of CH3NH3PbI3 films was investigated. It has been found, for example, that Eg and Et of CH3NH3PbBr3 sensitively depend on the substrate and on soaking in nitrogen atmosphere and that light soaking has strong influence on the direction of modulated charge separation. CH3NH3PbI3 deposited on double layers of TiO2-PCBM and SnO2-PCBM showed a constant band gap of 1.58 eV and a low scatter in Et. This was attributed to the modification of the TiO2 or SnO2/CH3NH3PbI3 interfaces by PCBM allowing for efficient charge separation and transfer and well reproducible conditions for the layer formation. A decrease of L with light soaking was observed and can be attributed to light induced degradation due to charging and discharging of trap states and formation of defects at the TiO2/ CH3NH3PbI3 interface. Furthermore, Eg and Et of CH3NH3PbI3 stabilized with PMMA increased with increasing temperature, i.e. thermal expansion gives the predominant contribution to the temperature dependence of Eg of CH3NH3PbI3 whereas dynamic disorder was limited by phonons. A jump-like increase of Eg in the region of phase transition was attributed to the phase transition from the tetragonal to the cubic phases.Hybride organisch-anorganische Metallhalidperovskite (hier kurz Perovskite genannt) sind eine neue Materialgruppe für hocheffiziente Solarzellen, welche auf in der Erdkruste reich verfügbaren Elementen basieren und welche bei niedrigen Temperaturen aus Lösungen hergestellt werden können. Perovskitschichten aus CH3NH3Pb(I1-xBrx)3 wurden in der Doktorarbeit untersucht, da diese für Anwendungen als Topzellen in Tandemsolarzellen mit c-Si-Bottomzellen aufgrund der einstellbaren Bandlücke von großem Interesse sind. Die elektronischen Eigenschaften von Perovskitschichten aus CH3NH3Pb(I1-xBrx)3 hängen empfindlich von der Kristallisation und der Bildung von Defekten ab und sind kritisch für die Leistung und Stabilität von Solarzellen. Die Bandlücke (Eg), die exponentiellen Bandausläufer (Et) und die Diffusionslänge (L) sind entscheidend für Absorber in Solarzellen, da sie die Photogeneration und die Aufspaltung der Fermi-Niveaus limitieren. Eg, Et und L sind in Perovskitschichten aus CH3NH3Pb(I1-xBrx)3 nicht zwingend konstant und hängen z.B von der Präparation und der Degradierung ab. Deshalb wurden Eg, Et und L mit Hinblick auf die Stöchiometrie, Grenzflächen, Degradierung und Temperatur untersucht. Als Hauptuntersuchungsmethode wurde die modulierte Oberflächenphotospannungs-spektroskopie (SPV) eingesetzt. Die SPV erlaubt eine kontaktlose und außerordentlich empfindliche Bestimmung von Eg, Et, Richtung der Ladungstrennung und L (nach Goodman) nach oder während verschiedener Schritte der Präparation oder Degradierung, z.B. unter Beleuchtung. Anhand des Vegard’schen Gesetzes wurde die Stöchiometrie von CH3NH3Pb(I1-xBrx)3 ermittelt. Eg von CH3NH3Pb(I1-xBrx)3 variierte zwischen 1.59 und 2.30 eV, wobei der Bowingparameter 0.36 eV betrug. Starke Einflüsse des Substrates und der Lagerung in Stickstoffatmosphäre auf Eg, Et und die Richtung der modulierten Ladungstrennung wurden u.a. in CH3NH3PbBr3 festgestellt. Eg betrug für auf TiO2/PCBM oder SnO2/PCBM abgeschiedenes CH3NH3PbI3 1.58 eV und Et zeigte nur geringfügige Variationen, PCBM hat also kaum Einfluss auf Eg und Et. und trägt sehr zur effizienten Ladungstrennung und zur gut reproduzierbaren Herstellung von Perovskitschichten bei. Die beobachtete Abnahme von L in CH3NH3PbI3 unter Beleuchtung kann der Umladung von Defekten sowie der Generation zusätzlicher Defekte besonders im Bereich der Grenzfläche zwischen TiO2 und CH3NH3PbI3 zugeordnet werden. Eg und Et stiegen mit steigender Temperatur für mit PMMA stabilisiertes CH3NH3PbI3 an, was auf den dominanten Einfluss der thermischen Ausdehnung und der Limitierung der dynamischen Unordnung durch Phononen zurückzuführen ist. Ein sprunghafter Anstieg von Eg bei etwas höheren Temperaturen wurde dem Übergang der tetragonalen Phase von CH3NH3PbI3 in die kubische Phase von CH3NH3PbI3 zugeordnet

Investigation of Structural and Electronic Properties of CH3NH3PbI3 Stabilized by Varying Concentrations of Poly(Methyl Methacrylate) (PMMA)

Studies have shown that perovskites have a high potential of outdoing silicon based solar cells in terms of solar energy conversion, but their rate of degradation is also high. This study reports on improvement on the stability of CH3NH3PbI3 by passivating it with polymethylmethacrylate (PMMA). Structural and electronic properties of CH3NH3PbI3 stabilized by polymethylmethacrylate (PMMA) were investigated by varying concentrations of PMMA in the polymer solutions. Stability tests were performed over a period of time using modulated surface photovoltage (SPV) spectroscopy, X-ray diffraction (XRD), and photoluminescence (PL) measurements. The XRD patterns confirm the tetragonal structure of the deposited CH3NH3PbI3 for every concentration of PMMA. Furthermore, CH3NH3PbI3 coated with 40 mg/mL of PMMA did not show any impurity phase even after storage in air for 43 days. The Tauc gap (ETauc) determined on the basis of the in-phase SPV spectra was found in the range from 1.585 to 1.62 eV for the samples stored during initial days, but shifted towards lower energies as the storage time increased. This can be proposed to be due to different chemical reactions between CH3NH3PbI3/PMMA interfaces and air. PL intensity increased with increasing concentration of PMMA except for the perovskite coated with 40 mg/mL of PMMA. PL quenching in the perovskite coated with 40 mg/mL of PMMA can be interpreted as fast electron transfer towards the substrate in the sample. This study shows that, with an optimum concentration of PMMA coating on CH3NH3PbI3, the lifetime and hence stability on electrical and structural behavior of CH3NH3PbI3 is improved

Correction: Awino, C., et al. Investigation of Structural and Electronic Properties of CH₃NH₃PbI₃ Stabilized by Varying Concentrations of Poly(Methyl Methacrylate) (PMMA). <em>Coatings</em> 2017, <em>7</em>, 115

In the original version of article [1], the lists of authors and affiliations, and the Acknowledgements and Author Contributions were incomplete [...

Thickness Dependence of Window Layer on CH3NH3PbI3-XClX Perovskite Solar Cell

CH3NH3PbI3-xClx has been studied experimentally and has shown promising results for photovoltaic application. To enhance its performance, this study investigated the effect of varying thickness of FTO, TiO2, and CH3NH3PbI3-xClx for a perovskite solar cell with the structure glass/FTO/TiO2/CH3NH3PbI3-xClx/Spiro-OMeTAD/Ag studied using SCAPS-1D simulator software. The output parameters obtained from the literature for the device were 26.11 mA/cm2, 1.25 V, 69.89%, and 22.72% for Jsc, Voc, FF, and η, respectively. The optimized solar cell had a thickness of 100 nm, 50 nm, and 300 nm for FTO, TiO2, and CH3NH3PbI3-xClx layers, respectively, and the device output were 25.79 mA/cm2, 1.45 V, 78.87%, and 29.56% for Jsc, Voc, FF, and η, respectively, showing a remarkable increase in FF by 8.98% and 6.84% for solar cell efficiency. These results show the potential of fabricating an improved CH3NH3PbI3-xClx perovskite solar cell

Temperature Dependence of the Band Gap of CH₃NH₃PbI₃ Stabilized with PMMA: A Modulated Surface Photovoltage Study

CH₃NH₃PbI₃ layers chemically stabilized with poly­(methyl methacrylate) (PMMA), relevant for photovoltaic applications, have been investigated by modulated surface photovoltage (SPV) spectroscopy at temperatures (<i>T</i>) between −182 and 60 °C. SPV is sensitive only to the PMMA/CH₃NH₃PbI₃ interface region where photogeneration and charge separation take place. The <i>T</i> dependencies of the Tauc gap (<i>E</i>_g‑Tauc, equivalent to absorption measurements) and of the gap determined from the maximum slope (<i>E</i>_g‑tp, almost at increased absorption) were analyzed on the basis of the in-phase SPV spectra. At 32 °C, the values of <i>E</i>_g‑Tauc and <i>E</i>_g‑tp were 1.540 and 1.560 eV, respectively. A jump of <i>E</i>_g‑Tauc and <i>E</i>_g‑tp by 10 meV at 40 °C was interpreted as the transition from the tetragonal to the cubic phase at <i>T</i> lower than values known from literature. <i>E</i>_g‑Tauc and <i>E</i>_g‑tp of the cubic phase decreased with increasing <i>T</i>. In contrast, <i>E</i>_g‑Tauc and <i>E</i>_g‑tp of the tetragonal phase decreased moderately with decreasing <i>T</i> to 1.528 and 1.546 eV at −182 °C, respectively. No signature has been observed in <i>E</i>_g‑Tauc and <i>E</i>_g‑tp for the transition from the tetragonal to the orthorhombic phase. Structural interactions at PMMA/CH₃NH₃PbI₃ interfaces seem important for phase transitions in CH₃NH₃PbI₃ layers