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The study carried out here is the ambient preparation of CH 3 NH 3 PbI 3 (methylammonium lead iodide, MAPI)-based perovskite solar cells (PSCs) without a hole transport layer (HTL). The TiO 2 electron transport layer (ETL) is deposited using three different methods, namely, chemical deposition, RF sputtering and e-beam evaporation, and the fabricated solar cell performances are compared. The common device structure is FTO/compact-TiO 2 /mesoporous-TiO 2 /MAPI/Au. The solar cell parameters are measured under standard AM 1.5 G light of intensity 1 sun. The e-beam-based PSCs attained a maximum power conversion efficiency of 3.54% among the 25 devices fabricated on a 1-in. 2 substrate. Notably, it is found that 85% or above of the initial efficiency is retained for 20 days of storage in the dark indicating slow degradation of the studied solar cells. All the solar cell performances are monitored for 130 days during the storage. Remarkably, the RF-sputtered compact TiO 2 layer-based PSCs have retained 99% of the initial average efficiency up to 112 days of storage.

Lead-free, silicon compatible materials showing large electromechanical responses comparable to, or better than conventional relaxor ferroelectrics, are desirable for various nanoelectromechanical devices and applications. Defect-engineered electrostriction has recently been gaining popularity to obtain enhanced electromechanical responses at sub 100 Hz frequencies. Here, we report record values of electrostrictive strain coefficients ( M 31 ) at frequencies as large as 5 kHz (1.04×10 −14 m 2 /V 2 at 1 kHz, and 3.87×10 −15 m 2 /V 2 at 5 kHz) using A-site and oxygen-deficient barium titanate thin-films, epitaxially integrated onto Si. The effect is robust and retained upon cycling upto 6 million times. Our perovskite films are non-ferroelectric, exhibit a different symmetry compared to stoichiometric BaTiO 3 and are characterized by twin boundaries and nano polar-like regions. We show that the dielectric relaxation arising from the defect-induced features correlates well with the observed giant electrostriction-like response. These films show large coefficient of thermal expansion (2.36 × 10 −5 /K), which along with the giant M 31 implies a considerable increase in the lattice anharmonicity induced by the defects. Our work provides a crucial step forward towards formulating guidelines to engineer large electromechanical responses even at higher frequencies in lead-free thin films. Lead-free piezoelectric materials with large electromechanical responses are important for nano electromechanical systems devices. Here the authors defect engineer Si integrated BaTiO3 to enhance the electromechanical response and report a large electrostrictive response at frequencies <10 kHz.

The open-circuit voltage (Voc) in a thin-film n-i-p-type perovskite solar cell (PSC) depends on the quasi-Fermi-level splitting (QFLS) in the perovskite layer. Recent photo/electroluminescence measurements show that the Voc in practical devices is lower than the QFLS in the bare perovskite film, suggesting that the device performance is limited by the interfaces. The losses at the perovskite/transport layer (TL) interface can be caused by (a) the suboptimal band alignment between perovskite and TL, or (b) the interface defects. Previously, we reported a polymer interlayer, poly-4-(5-(9,9-dihexyl-7-methyl-9H-fluoren-2-yl)thiophen-2-yl)-5,6-difluoro-7-(5-methylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (PF-DTDFBT) having efficient band matching with valence band maxima (VBM) of perovskite (MAPbI3), which enhanced the Voc of the n-i-p structured PSC. In this report, we established the reason for the higher Voc. Firstly, the x-ray photoelectron spectroscopy and carrier lifetime measurements show that the polymer does not passivate the surface of the perovskite, and hence interface defects do not limit the Voc. On the other hand, comparison of the highest occupied molecular orbital (HOMO) of the polymer with that of Spiro-OMeTAD, shows that the alignment with the VBM of MAPbI3 is better for polymer than for Spiro-OMeTAD. From these two observations, it is inferred that the enhancement in Voc is due to the efficient HOMO/VBM alignment of the polymer interlayer and the perovskite (MAPbI3). As perovskite film quality and defect density are heavily dependent on the fabrication process, the device fabrication and stability measurements were repeated with the widely accepted planar antisolvent-treated perovskite films (as compared with the methylamine vapor-treated films used previously). The planar, antisolvent recipe-based devices showed enhanced performance for both Spiro-OMeTAD (reference) and polymer-interlayer-based devices. The power conversion efficiency (PCE) of the polymer interlayer device was 16.51% with Voc of 1.11 V compared with PCE of 14.72% and Voc of 1.04 V for the reference Spiro-OMeTAD-based devices. The polymer being hydrophobic, enhanced the stability of the interlayer devices, whose PCE drops by only 22% after 240 h of exposure to 72 ± 3% relative humidity as compared with a 60% drop in efficiency of the reference Spiro-OMeTAD devices kept in the same ambient conditions and time frame.

Lead-free, silicon compatible materials showing large electromechanical responses comparable to, or better than conventional relaxor ferroelectrics, are desirable for various nanoelectromechanical devices and applications. Defect-engineered electrostriction has recently been gaining popularity to obtain enhanced electromechanical responses at sub 100 Hz frequencies. Here, we report record values of electrostrictive strain coefficients (M ) at frequencies as large as 5 kHz (1.04×10 m /V at 1 kHz, and 3.87×10 m /V at 5 kHz) using A-site and oxygen-deficient barium titanate thin-films, epitaxially integrated onto Si. The effect is robust and retained upon cycling upto 6 million times. Our perovskite films are non-ferroelectric, exhibit a different symmetry compared to stoichiometric BaTiO and are characterized by twin boundaries and nano polar-like regions. We show that the dielectric relaxation arising from the defect-induced features correlates well with the observed giant electrostriction-like response. These films show large coefficient of thermal expansion (2.36 × 10 /K), which along with the giant M implies a considerable increase in the lattice anharmonicity induced by the defects. Our work provides a crucial step forward towards formulating guidelines to engineer large electromechanical responses even at higher frequencies in lead-free thin films.

Solar cells based on crystalline silicon offer high efficiency but they are expensive due to the high temperatures required in their fabrication. The alternative approach using low-temperature processable organic-semiconductors is potentially cheaper, but the organic solar cells are not very efficient. In this thesis we explore if organic semiconductors can be integrated with silicon to form hybrid organic/silicon solar cells that are both efficient and low-cost. Specifically, we demonstrate that a) organic molecules can be used to reduce carrier recombination at the silicon (100) surface and b) a solution-processed organic/silicon heterojunction can replace the conventional silicon p-n junction to yield solar cells with high power conversion efficiencies (>10%). With decreasing wafer thicknesses and improving bulk lifetimes of silicon solar cells, losses due to carrier recombination at the silicon surface are becoming increasingly important. At a bare silicon surface, some of the silicon valencies remain unsatisfied. These \"dangling-bonds\" cause midgap states at the silicon surface where photogenerated carriers can recombine, resulting in lower performance. Typically, a layer of silicon oxide/nitride is deposited on the silicon, at high-temperatures (>350°C), to passivate the dangling-bonds and reduce surface recombination. Organic semiconductors can be deposited at much lower temperatures, but in general organic materials do not react with the silicon dangling-bonds and the surface remains unpassivated. In this work, we demonstrate that the organic molecule 9,10 phenanthrenequinone (PQ) reacts with and satisfies the silicon dangling bonds, leading to a relatively defect-free silicon surface with a very low surface recombination velocity (150 cm/s). Electrical measurements of the metal/insulator/silicon devices show that the Fermi-level at the PQ-passivated silicon surface can be modulated and an inversion layer can be induced in silicon. High electron mobility of 600 cm²/Vs is measured at the Si/PQ interface further proving the electronic quality of the PQ-passivated surfaces. To generate a photovoltage in a solar cell, the photogenerated carriers need to be spatially separated at two electrodes of opposite polarity. In solar cells this is typically accomplished using a p-n junction. While the p-n junction technology is well understood, the fabrication of p-n junctions on silicon is an expensive process because it requires ultra-clean furnaces, pure precursors and high temperatures. In this thesis we successfully replace the silicon p-n junction with an silicon/poly(3-hexylthiophene) heterojunction that can be manufactured at low temperatures (<150°C) with a simple spin-coating process. The key design rules to achieve a high quantum-efficiency and high open-circuit voltage are discussed and experimentally demonstrated. Finally we highlight the importance of reducing minority-carrier currents in these heterojunction devices, which gives a pathway for further improving the efficiency of heterojunction solar cells. Using the prescribed design rules and optimizing device structure, a silicon/organic heterojunction solar cell with an open-circuit voltage of 0.59 V and power conversion efficiency of 10.1% is demonstrated.

Mobility measurements in highly resistive methylammonium lead iodide (MAPI) are challenging due to high impedance, ion drift, and low mobility. We show that we can address the challenge using intensity-dependent photo-Hall measurements. The key is an improved photo-Hall setup, which enables reliable Hall measurements in the dark and under low-intensity illumination. By tuning the illumination over four orders of magnitude, we get the additional information to simultaneously extract hole mobility, electron mobility, and background doping. For the first time, we show that a MAPI single crystal, exhibiting n-type behaviour in the dark, can show p-type behaviour under light due to the difference in hole and electron mobility. The data partly explains the variability in mobility reported in the literature. We show that one can erroneously extract any mobility from 0 to 25 cm2/Vs if we restrict the experiment to a small range of illumination intensities. For our MAPI (310) crystal, the measured hole and electron mobility is 40 cm2/Vs and 25.5 cm2/Vs, respectively.

Mixed-cation Cesium-Methylammonium lead halide perovskite (CsxMA1-xPbI3-xBrx) thin-films have been used to demonstrate stable and efficient perovskite devices. However, a systematic study of the Cs incorporation on the properties of the perovskite films has not been reported. In this report, Impact of Cesium incorporation on the minority carrier recombination lifetime of Cesium-Methylammonium lead halide perovskite thin-films is studied. The lifetime for the as-deposited perovskite films decreases with increasing concentration of cesium. However, mixed cation perovskite film is more stable, showing higher lifetime (15-20 micro seconds) after 9 hours of ambient exposure than just after deposition (6-13 micro seconds). Methylamine Vapor Exposure (MVE) technique was used to improve the morphology of the as-deposited film. MVE treated films are more oriented along (110) direction and were even more stable in ambient, with Cs0.10MA0.90PbI2.90Br0.10 films showing lifetime of almost 50 micro seconds after 9 hours of ambient exposure, twice the lifetime of a comparable MAPbI3 film. These results throw light on why mixed-cation cesium-methylamine lead halide perovskite films are better for highly efficient and stable perovskite solar cells.

Short gas pulses introduced in a vacuum chamber have long been utilized to showcase the ultra-low mass resolutions achievable with nanomechanical resonators. The resonance frequency shifts are used as evidence of gas adsorption. However, there is very little clarity as to what exactly is adsorbing on to the resonators. We demonstrate that the physisorption of gases on cantilevers is predominantly the effect of moisture content that is present even in ultra-high purity gases. The experimental work is performed at low temperatures and in a high vacuum and is supported by theoretical calculations and simulation.

Biological neurons are electro-mechanical systems, where the generation and propagation of an action potential is coupled to generation and transmission of an acoustic wave. Neuristors, such as VO2, characterized by insulator-metal transition (IMT) and negative differential resistance, can be engineered as self-oscillators, which are good approximations of biological neurons in the domain of electrical signals. In this study, we show that these self-oscillators are coupled electro-opto-mechanical systems, with better energy conversion coefficients than the conventional electromechanical or electrooptical materials. This is due to the significant contrast in the material's resistance, optical refractive index and density across the induced temperature range in a Joule heating driven IMT. We carried out laser interferometry to measure the opto-mechanical response while simultaneously driving the devices electrically into self-oscillations of different kinds. We analyzed films of various thicknesses, engineered device geometry and performed analytical modelling to decouple the effects of refractive index change vis-a-vis mechanical strain in the interferometry signal. We show that the effective piezoelectric coefficient (d13*) for our neuristor devices is 660 pm/V, making them viable alternatives to Pb-based piezoelectrics for MEMS applications. Furthermore, we show that the effective electro-optic coefficient (r13*) is ~22 nm/V, which is much larger than that in thin-film and bulk Pockels materials.

Lead free, silicon compatible materials showing large electromechanical responses comparable to, or better than conventional relaxor ferroelectrics, are desirable for various nanoelectromechanical devices and applications. Defect-engineered electrostriction has recently been gaining popularity to obtain enhanced electromechanical responses at sub 100 Hz frequencies. Here, we report record values of electrostrictive strain coefficients (M31) at frequencies as large as 5 kHz (1.04 x 10-14 m2 per V2 at 1 kHz, and 3.87 x 10-15 m2 per V2 at 5 kHz) using A-site and oxygen-deficient barium titanate thin-films, epitaxially integrated onto Si. The effect is robust and retained even after cycling the devices >5000 times. Our perovskite films are non-ferroelectric, exhibit a different symmetry compared to stoichiometric BaTiO3 and are characterized by twin boundaries and nano polar-like regions. We show that the dielectric relaxation arising from the defect-induced features correlates very well with the observed giant electrostrictive response. These films show large coefficient of thermal expansion (2.36 x 10-5/K), which along with the giant M31 implies a considerable increase in the lattice anharmonicity induced by the defects. Our work provides a crucial step forward towards formulating guidelines to engineer large electromechanical responses even at higher frequencies in lead-free thin films.