Optical, Excitonic, and Electronic Properties of CH3NH3PbI3 Thin Films and Their Application in Photovoltaics

In the past two years, the highest power conversion efficiency of perovskite absorber (PA)–based photovoltaics has been 20.2%. The PA can be fabricated on flat substrates (for example, ZnO, TiO2, and PEDOT:PSS) using solution processes, which have a low-cost advantage in terms of industry production. In this report, the recent advances of PA-based photovoltaics will be mentioned. Then, the optoelectronic properties of PA, material fabrication, and photovoltaic performance will be discussed. On the other hand, we used scanning electron microscopy, two-dimensional X-ray diffractometer, and photoluminescence spectroscopy to investigate the fundamental properties of CH3NH3PbI3 thin films fabricated with and without toluene washing treatment, which provides an assessment of the development potential of PA-based photovoltaics

Novel Growth Methods of Organic-Inorganic Lead Tri-Halide Perovskite Material for Photovoltaic Applications

Whilst different methodologies have been used to grow perovskite materials, still, there are demands to develop commercial approach techniques that could alleviate the problem associated with the various phases formed within the perovskite material, which may cause of its crystallographic instability. Therefore, the research study was aimed to assist in moving towards this objective by developing novel methods for the deposition of perovskite thin films (CH3NH3PbI3 = MAPbI3) that are scalable and industry-acceptable. It was also aimed to focus, along with the scalable process, on an atomic scale of processing to deposit materials with ATM (Atom to Matter), where the defect density within the films are minimised. In view of that, different CVD deposition methods to deposit MAPbI3 thin films were proposed in this study, including Radio Frequency-Plasma Enhanced Chemical Vapour Deposition (RF-PECVD) and Atmospheric-Pressure Chemical Vapour Deposition (APCVD). The first proof-of-concept was demonstrated to obtain perovskite films with a tetragonal crystal structure by a PECVD process that was not reported till date. This achievement is encouraging as the PECVD process is fully scalable and already available technology in the industries. The growth was subsequently successfully achieved by using carbon, nitrogen and hydrogen radicals that were contained a gas such as methane (CH4) and ammonia (NH3). A prior deposition of PbI2 thin films by either spin-coating or thermal evaporating was implemented previously before they were exposed to the organic molecules in the plasma state that was formed by RF power via a PECVD technique. The effect of the variation of PECVD growth parameters (such as substrate temperature, RF power density and chamber pressure) on the properties of the films deposited was critically analysed. The conditions that yielded the best quality material achieved from this study were at substrate temperature of 100 ℃ and under low power density (22 mW/cm2) and high chamber pressure (1000 mtorr). An attempt was made to understand the properties of the resultant films, in terms of the physical, optical and electrical properties, which led to develop and provide appropriate explanations of the possible growth mechanisms. Considering the cost with the use of the scalable process that any an industrial technique would prefer, a modified non-vacuum CVD system was additionally demonstrated in this work to deposit MAPbI3 films at atmospheric pressure, hence the name APCVD. The growth of the MAPbI3 films was controlled based on the concentration of the precursor and the temperature. The lower concentration of the molecules passing over the substrate enhanced the crystallographic stability, where the in-situ cubic structure was achieved. With the use of this novel design of the reactor specifically for the deposition of MAPbI3 perovskite films; excellent stability of these films at room temperature without care of storage for more than two months in an open atmosphere provided an alternative approach to obtain the stable MAPbI3 perovskite film. In addition to the low temperature and lower concentration of the precursor used for the fabrication of a novel design of the reactor to grow films with the bottom-up approach (i.e. atom by atom or molecules), this study was successful in fabricating a proof-of-concept solar cell using this method for the first time. The obtained perovskite material has significantly contributed to understanding of the crystallographic stability issues reported in the photovoltaics which are incorporated with the perovskite materials over the last recent years

In-depth analysis of defects in TiO2 compact electron transport layers and impact on performance and hysteresis of planar perovskite devices at low light

Properties of the electron transport layer (ETL) are known to influence the performance of lead halide perovskite solar cells (PSCs). But so far very little emphasis has been given on the increased impact of this layer at low light. In this work we compare the effect of thickness and coverage of a TiO2 compact layer on the performance and hysteresis of methyl ammonium lead iodide planar devices tested under 200 lux vs. 1 sun illumination. Standard TiO2 layers are produced with incremental thickness and coverage using sequential spray pyrolysis of a Ti-acetylacetonate precursor (0–50 sprays, 1 spray ~ 1 nm TiO2). Thorough materials characterisation combining FEG-SEM, XPS, and cyclic voltammetry shows that a crystalline, nearly pin-hole free TiO2 layer is achieved by deposition of ≥15 sprays over small to large areas (0.2 mm2–1 cm2). Device performance is affected by two main parameters, namely the coverage yield and thickness of the TiO2 layer, especially under 200 lux illumination. A 25 vs. 50 sprays-TiO2 layer is found to provide the best compromise between coverage and thickness and avoid charge recombination at the TiO2/perovskite interface whilst minimizing resistive losses with 11.7% average PCE at 200 lux vs 7.8% under 1 sun. Finally, the analysis of I/V forward vs. reverse scans and open circuit voltage decay data suggests that hysteresis is greatly affected by the capacitive properties of the ETL at low light, whilst other phenomena such as ion migrations may dominate under 1 sun

Fabrication and characterization of bismuth-based environmental-friendly materials for solar cell application

Lead-based perovskites, APbX3, achieved over 23 % of efficiency within only 7 years after first their usage in solar cells. Although these lead-based perovskites have rapidly achieved the high efficiency, they have two intrinsic problems to be addressed to be commercialised; toxicity and instability. To address the problems of lead at the same time, new lead-free and air-stable materials such as inorganic materials including bismuth iodide and bismuth- or antimony-based perovskites are emerging. However, the efficiency of the solar cells employing these new materials is relatively low below 5%. This thesis explores these new materials especially bismuth-based perovskite and bismuth iodide inorganic material for their photoelectronic application with improved device performance. An inorganic material, BiI3 has suitable optical properties for photovoltaic application but due to the short carrier lifetime (180-240ps), it needs structure optimisation to achieve higher device performance. For this, in-situ processed BiSI interlayer was employed to enhance charge-carrier extraction before their recombination and it was monitored by transient absorption spectroscopy. Fabricated solar cell devices showed improved device performance with the BiSI interlayer. Regarding bismuth or antimony-based perovskites, large binding energy, high defects and large bandgap are main issues to be addressed. Bismuth/antimony mixed perovskites were fabricated and investigated using a combined experimental and computational approach in a collaboration with the university of Bath. In the study, it was found that Sb mixing reduces the binding energy and it leads to higher solar cell efficiency. By employing 2D bismuth perovskite, improved PLQY obtained and it might come from the reduced defect density. In addition, nanocrystals (NCs) of bismuth or antimony-based perovskites were fabricated. These perovskites have high binding energy to overcome for the high-performance solar cell application, but this high binding energy can be helpful for high PLQY. Also, the reduced size to nanometre scale brings less defects in NCs and excitons are more prone to recombine radiatively. Therefore, NCs of Bi/Sb perovskite were fabricated to improve their optical property for opto-electronic application. The fabricated NCs were investigated by a range of characterisation techniques.Open Acces

Spectroscopic and optoelectronic characterisation of solution-processed tin monosulfide solar cells

Tin mono-sulphide (SnS) has emerged as a promising photovoltaic material because of the earth-abundance, non-toxicity and its optical properties. SnS exhibits a high absorption coefficient (>104cm-1) with an ideal band gap of 1.3 eV (ca. 950 nm), which make it an excellent light absorber for photovoltaic applications. To date, the highest power conversion efficiency (PCE) of SnS-based solar cells is up to 4.4% although the theoretical PCE is 32%; and SnS layers were obtained by atomic layer deposition and thermal evaporation. There are very few attempts using fast and cost-efficient methods; therefore, solution-processed fabrication methods for SnS thin films as well as the conjugated organic semiconductors to prepare hybrid solar cells are particularly interesting. This thesis explores the possibility of SnS as a new and promising light-harvesting absorber material. It focuses on the fabrication of a straight-forward, solution-based route for the preparation of SnS films, and optimisation of the surface coverage and the morphology which are monitored by a range of characterisation techniques. In particular, an exhaustive transient absorption spectroscopy (TAS) study has elucidated the key parameters that influence the optoelectronic properties of SnS/organic hybrid systems. Furthermore, a systematic study of the effects of processing parameters on the device efficiency was carried out which reveals the optimal conditions for efficient SnS-based solar cell performance.Open Acces

Low Dose Analytical Electron Microscopy of Hybrid Perovskite Photovoltaic Devices

The rapid ascent of perovskite photovoltaics over the past decade has enabled this technology to now stand on the cusp of commercialisation. However, a successful entry into the market will only be feasible if the power conversion efficiencies of perovskite solar modules can at least approach those of laboratory-scale cells. Achieving this feat requires spatially homogeneous depositions of device layers over a large area and high-quality interconnections between adjacent cells in a module. Since perovskite photovoltaic devices are nanostructured, materials characterisation with a nanometre spatial resolution can provide valuable insights to optimise the processes involved in scalable film deposition and interconnection fabrication. This thesis presents nanoscale electron microscopy investigations of perovskite photovoltaic devices made using scalable deposition methods and the cell interconnections within. A characterisation workflow consisting of cross-sectional specimen preparation, data acquisition, and multivariate statistical data analysis is developed and validated. Preparation of electron-transparent specimens is performed using focused ion beam milling, which is shown to have minimum impact on the perovskite specimen. Nanoscale compositional mapping is performed using energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope, where the applied electron dose is minimised to suppress beam-induced specimen damage while still ensuring statistical significance in the data. Principal component analysis, a multivariate statistical analysis algorithm, is optimised and applied to improve the signal-to-noise ratio in the obtained datasets by an order of magnitude. This sequence allows acquisition of spatially resolved morphological and compositional data with minimum damage on the perovskite specimen, which are supported by complementary computational methods and other characterisation techniques. The optimised workflow is applied to study perovskite solar modules deposited by blade coating, where electron microscopy revealed how additives in the perovskite precursor solutions contribute towards a more homogeneous device stack and, ultimately, more efficient modules. Finally, the interconnections are studied as they are critical to ensure good electrical performance in solar modules. Compositional characterisation shows how laser pulses used in scribing the interconnection lines can decompose the perovskite layer next to those lines, and also how the decomposition is affected by the perovskite’s homogeneity. Furthermore, elemental mapping reveals diffusion of sodium from the glass substrate into the active layers through the interconnection lines, even before the devices are operated. Sodium diffusion results in passivated defect sites and stronger perovskite luminescence, but also carries an inherent risk of excessive diffusion throughout the device’s lifetime.Jardine Foundation; Cambridge Trus

Opto-electronic Characterisation of Thin Film Solar Cells with Confocal Microscopy

The purpose of this work is to develop novel characterisation methods using confocal microscopy to investigate perovskite solar cell materials. It begins with a preliminary study of light management in silicon thin film solar cells for the purpose of demonstrating the capability of confocal scanning microscopy as a solar cell imaging tool. This is followed by several supporting studies to identify the stability of CH3NH3PbI3 perovskite materials when exposed to the tightly-focussed, intense laser illumination of a confocal microscope. These studies include: estimation of the laser-induced heating of CH3NH3PbI3 films during confocal measurements in order to avoid the effects of thermal-induced degradation during data collection; and measurements of films in different ambient atmospheres, showing that the film must be fully encapsulated by epoxy or kept in N2 environment to ensure stability during the measurements. PMMA coated perovskite film are shown to be protected from moisture-induced degraded, but they also exhibit an enhancement of photoluminescence (PL) signal under light exposure. During the PL measurements on CH3NH3PbI3 perovskite films, the phenomenon of light- and oxygen-induced PL enhancement was observed, which led to the exploration of trap properties and recombination kinetics of perovskite films. A combination of experimental PL measurements and numerical modelling is used to investigate the dramatic enhancement in PL following prolonged light exposure with timescales ranging from minutes to hours. The time and spatial dependence of the PL enhancement is directly observed by combining localised illumination with PL imaging, which can be explained by a combination of trap de-activation and photogenerated carrier diffusion away from the light-exposed area. Further experiments demonstrate that trap de-activation is reversible once the illumination is turned off. The observed time and spatial dependence of laser induced PL enhancement in CH3NH3PbI3 films is modelled, taking into account trap de-activation and carrier diffusion. Following this topic, a complete physical model of recombination kinetics is implemented to extract recombination coefficients and trap parameters of CH3NH3PbI3 and Cs0.07Rb0.03(FA0.85MA0.15)0.9Pb(I0.85Br0.15)3 perovskite films by fitting to excitation-dependent steady-state and transient PL measurements simultaneously. Sensitivity analysis shows that fitting a single model to the two different PL measurements provides improved accuracy in multiple-parameter fitting. A comparison of the fitted parameters of the two perovskite films suggests that the improved performance of mixed cation perovskites may result from less active trap states rather than from a lower density of traps. This analysis technique provides a simple, non-contact method to rapidly characterise the key trap properties of perovskite films. The general recombination model is further used to interpret carrier lifetimes in perovskite films. The origins of bi-exponential transient PL decay observed for many perovskite films are investigated by varying the recombination parameters in the recombination model. This analysis demonstrates that the fast and slow decays in the transient PL curve are dominated by trap-assisted recombination and radiative recombination, respectively. Simulations of the steady-state carrier lifetime as a function of carrier density over a wide excitation range demonstrate that radiative and Auger recombination coefficients could be extracted from experimental measurements at high excitation levels, and trap energy levels can be estimated at low excitation levels. The transient carrier lifetime extracted from the simulated PL decay curve matches the excitation-dependent minority carrier lifetime simulated for steady-state illumination conditions when the excitation level is sufficiently high. Therefore, the transient carrier lifetime extracted from time-resolved PL measurements can be used to estimate radiative and Auger recombination coefficients as long as the carrier density is known during the decay. This thesis contributes novel characterisation methods based on confocal microscopy for improved understanding of material properties and recombination kinetics of perovskites for photovoltaic applications. Theoretical modelling is performed to support the experimental findings and gain new insights into the detailed chemical and electronic properties of these materials