IMPEDANCE SPECTROSCOPY FOR INTERFACE CHARACTERIZATION IN SEMICONDUCTOR DEVICES

Impedance spectroscopy (IS) is a powerful tool to characterize devices since it allows to easily decouple the contribution of different interfaces existing in the device by only accessing the external terminals. The collected data are interpreted by means of equivalent electrical circuit. In this thesis, an automated procedure is developed to automatically extract lumped circuit parameters from impedance measured data, adding physical constraints deriving from experimental capacitance. In this work, Graphene-Silicon solar cells are characterized using impedance spectra, allowing to assess a new front contact technology that ameliorates these cells performance compared to the conventional. Impedance spectroscopy is also employed to characterize perovskite solar cells. The equivalent circuit coming from these devices allows to gain knowledge on perovskite layer and recombination mechanisms. An important focus of this thesis concerns capacitance versus voltage curves in forward bias region. This analysis is made using both experimental data and numerical results obtained from TCAD environment. This study is made on Metal-Semiconductor structure, finding the analytical expression of the forward bias capacitance peak and considering the effects of interface defects on capacitance behavior. The observation of multiple peaks arising in the high forward bias region suggests that interface properties are not uniform in the entire structure. Capacitance is also investigated in SiC MOSFETs devices permitting the TCAD model calibration and SiC/SiO2 interface characterization

Low-frequency noise spectroscopy as an effective tool for electric transport analysis

2016 - 2017In this work, several experiments and analyses performed by means of noise spectroscopy, on a broad typology of materials and compounds, are presented. Structural, DC electrical transport and noise properties are exposed for each investigated sample, and theoretical models and possible explanations of the experimental results are given to unravel physical phenomena. In particular, two distinct types of iron-chalcogenide superconductors are investigated, in their pristine and aged state, suggesting the more likely mechanism which generates the resistance fluctuations and resorting to Weak Localization theory. In the case of the polymer/carbon nanotubes composites, the fluctuation-induced tunneling model is introduced to explain the measured temperature dependence of the electrical conductance and the I-V curve behaviors. Then, noise measurements prove the existence of a structural phase transition occurring around 160 K within the perovskite compound and highlight the correlation between electronic defect states distribution and device performance. The variety of investigated devices and materials validates the soundness of the noise spectroscopy as an effective tool for electric transport analysis. [edited by author]XXX cicl

Enabling selective absorption in perovskite solar cells for refractometric sensing of gases

Perovskite solar cells are currently considered a promising technology for solar energy harvesting. Their capability to deliver an electrical signal when illuminated can sense changes in environmental parameters. We have numerically analyzed the variation of the current delivered by a perovskite cell as a function of the index of refraction of air, that is in contact with the front surface of the cell. This calculation identifies which geometrical and material structures enhance this behavior. After replacing the top transparent electrode of a solar cell by an optimized subwavelength metallic grating, we find a large variation in the responsivity of the cell with respect to the change in the index of refraction of the surrounding medium. Such a refractometric sensor can be interrogated electronically, avoiding the cumbersome set-ups of spectral or angular interrogation methods. We present an adaptation of the performance parameters of refractometric sensors (sensitivity and figure of merit) to the case of opto-electronic interrogation methods. The values of sensitivity and Figure of Merit are promising for the development of refractometric perovskite-based sensors

Correlation between Electronic Defect States Distribution and Device Performance of Perovskite Solar Cells

In the present study, random current fluctuations measured at different temperatures and for different illumination levels are used to understand the charge carrier kinetics in methylammonium lead iodide CH3NH3PbI3 based perovskite solar cells. A model, combining trapping detrapping, recombination mechanisms and electron phonon scattering, is formulated evidencing how the presence of shallow and deeper band tail states influences the solar cell recombination losses. At low temperatures, the observed cascade capture process indicates that the trapping of the charge carriers by shallow defects is phonon assisted directly followed by their recombination. By increasing the temperature, a phase modification of the CH3NH3PbI3 absorber layer occurs and for temperatures above the phase transition at about 160 K the capture of the charge carrier takes place in two steps. The electron is first captured by a shallow defect and then it can be either emitted or thermalize down to a deeper band tail state and recombines subsequently. This result reveals that in perovskite solar cells the recombination kinetics is strongly influenced by the electron phonon interactions. A clear correlation between the morphological structure of the perovskite grains, the energy disorder of the defect states, and the device performance is demonstrate

evidence of bipolar resistive switching memory in perovskite solar cell

In hybrid inorganic-organic perovskite solar cells a very stable bipolar resistive switching behavior in the dark current-voltage characteristics at low-voltages has been observed. The possible use of the solar cell as an electrical memory with a moderate on-off contrast but very good stability over a prolonged time has been suggested. The reversible behavior and the long dynamics during the write/erase processes indicate that the physical mechanism behind the switching is related to polarization effects. A detailed analysis of the charge carrier trapping/detrapping, transport, and recombination mechanisms has been performed by taking the ion migration and the consequent charge carrier accumulation within the device into account. The charge transport during the write operation can be described by space-charge-limited conduction process. The formation and subsequent interruption of conducting pathways due to ion migration have been identified as the main cause of the resistive switching within the perovskite material. The strong interaction between the ion movement and the electron transport enables the operation of the perovskite solar cell also as a non-volatile memory

Device Modelling of Perovskite Solar Cells

This thesis is primarily concerned with the electrical characterization and modelling of perovskite solar cells. Perovskite cells are a new player in the photovoltaic arena with several intriguing properties. One of these is the presence of intrinsic mobile ions which make these semiconductors simultaneously ionic conductors at room temperature. The presence of mobile ions is significant in that it leads to a number of transient behaviours in optoelectronic measurements, including nominally simple current-voltage measurements where the phenomena are broadly labelled as aspects of ``I-V hysteresis''. The first two-thirds of this thesis describes our work on extended drift-diffusion models which incorporate the presence of mobile ions into the conventional equations of semiconductor physics. These allow us to uncover mechanistic explanations for a variety of transient behaviours which are broadly caused by coupling between electronic and ion dynamics. The first third (Chapter 2) deals with hysteresis in the form of rate-dependent I-V sweeps: a selection of unusual measurements of this type is presented including a temporary enhancement in open-circuit voltage following prolonged periods of negative bias, dramatically S-shaped current-voltage sweeps, decreased current extraction following positive biasing or ``inverted hysteresis'', and non-monotonic transient behaviours in the dark and the light. This initial study is supplemented with a more in-depth investigation of inverted hysteresis and its correlation with band-alignment. The second third (Chapter 3) delves deeper into electrical characterization with a first-principles study of electrical impedance spectroscopy. We focus on accounting for features in the measured capacitance spectrum (sufficient for a full account of the total impedance due to the Kramers-Kronig relations) of standard-structure (non-inverted) perovskite cells. Here our models make clear the necessity of distinguishing fundamental contributions to the measured capacitance due to charging, from those due to currents delayed by slow processes such as ion migration. With this distinction clearly established we provide a detailed account of all the major features observed in impedance measurements of these cells, including the exotic and previously puzzling appearance of giant photo-induced capacitance, loop features and negative capacitance. The final part of this thesis in Chapter 4 concerns the integration of perovskite cells into tandem arrangements with a partner such as the crystalline silicon cell. Of relevance to any thin-film solar cell, and to 4-terminal tandem cells in particular, is the specifications of its transparent conductor layers. We analyze transparent conductor requirements under different regimes of metallization (the addition of metallic bus-bars or fingers). Here a key parameter is the minimal achievable wire width, which dictates the necessary tradeoff between transparency and conductivity in the underlying transparent conductor. We identify \SI{30}{\micro \metre} as a critical width below which many emerging transparent conducting layers such as carbon nanotubes and graphene become competitive with state-of-the-art transparent conductive oxides such as ITO for a stand-alone perovskite cell. We also discuss a novel strategy for integrating perovskite and Si cells into a single monolithic structure without the need for a tunnel junction or recombination layer. This is identified as being possible due to the presence of interfacial sub-gap states which can facilitate high-conductivity ohmic contact between TiO$_2$ and p-type Si, and has significant advantages in terms of reducing optical losses and processing steps

A piperidinium salt stabilizes efficient metal-halide perovskite solar cells

Longevity has been a long-standing concern for hybrid perovskite photovoltaics. We demonstrate high-resilience positive-intrinsic-negative perovskite solar cells by incorporating a piperidinium-based ionic compound into the formamidinium-cesium lead-trihalide perovskite absorber. With the bandgap tuned to be well suited for perovskite-on-silicon tandem cells, this piperidinium additive enhances the open-circuit voltage and cell efficiency. This additive also retards compositional segregation into impurity phases and pinhole formation in the perovskite absorber layer during aggressive aging. Under full-spectrum simulated sunlight in ambient atmosphere, our unencapsulated and encapsulated cells retain 80 and 95% of their peak and post-burn-in efficiencies for 1010 and 1200 hours at 60° and 85°C, respectively. Our analysis reveals detailed degradation routes that contribute to the failure of aged cells

Integrating Diverse Materials for Carbon Perovskite Solar Cells: Examining the Performance and Stability

The emergence of perovskite solar cells (PSCs) in a "catfish effect" of other conventional photovoltaic technologies with the massive growth of power conversion efficiency (PCE) with a simple manufacturing process has given a new direction to the entire solar energy field. Usually, PSC components such as electron transport material (ETM), perovskite sensitizer, hole transport material (HTM), and electrode materials need to be appropriately aligned according to the electron transfer and recombination process in order to achieve the best out of the device. Despite the enormous amount of research, the stability, reactivity, and cost issues of noble metal (Au, Ag) electrode-based traditional PSC devices are becoming obstacles to marketization. Due to the low fabrication cost and enhanced ambient stability, carbon counter electrode-based PSC (CPSC) evolved as a suitable alternative in such scenarios. These CPSCs are still in a stage of development where different fabrication engineering, designs and materials are being investigated to attain a comparable state with the standard commercialized photovoltaics. To date, hardly any report is available on ambient CPSC with PCE over 15% and stability of ~1000h without encapsulation, which opens up the window for more research. The fundamental objective of this thesis work was to develop high-performance ambient CPSC with PCE > 15% under 1SUN AM 1.5 illumination, maintaining the stability of ~1000h. This was achieved using alternative ETM and HTM with strategic incorporation instead of traditional ones. Noticeably, the temperature is a crucial parameter to attain and, at the same time, retain the aimed PV performance and stability. Therefore, a physico-thermal investigation was performed to understand the effect of temperature on the fabricated CPSC devices. The understanding further helped to examine the possible futuristic application of CPSC as the semi-transparent device for energy savings build environment. To achieve the goals of this thesis, the 1st step involved finding out suitable combination of HTM and carbon counter electrodes highlighted in chapter 3. For the first time, the fully printable mesoporous CPSCs are demonstrated with concentration-dependent WO3 (5, 7.5, and 10% by volume) nanoparticles incorporated in carbon electrodes fabricated under ambient conditions. The highest PCE ~10.5% was obtained with the 7.5% WO3/carbon device; however, the 10% WO3/carbon device exhibited better ambient stability of ~600h. Besides, graphene/ poly(3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT: PSS) was introduced as an alternative HTM with novel light soaking and surface wettability strategies, and an enhanced PV performance with PCE >11% was achieved. In search of an alternative HTM/carbon combination with more superior performance, a novel and cost-effective synthesis process of graphitic CNP as a suitable counter electrode and its combination with NiO was visualized. The stability test of the high-temperature counter electrode strategy of CNP/NiO showed ~1000 h air stability with negligible efficiency loss having a maximum PCE of 13.2%, whereas the low-temperature strategy of CNP/NiO devices showed 14.2% PCE with ~650 h air stability. Thus CNP/NiO combination achieved performance very close to the aims of this thesis, which was enhanced to the required performance by introducing alternative ETM for the devices. Chapter 4 describes the strategic incorporation of morphology modulated BaSnO3 (BSO) and brookite TiO2 (BTO) nanostructures in place of conventional anatase TiO2 as ETM to successfully achieve PCE >13.5% and >15%, respectively, with stability >1000 h. The enhanced electron transport and reduced charge recombination by rod-based nanostructures of BSO and BTO displayed the best performance for the types to date in CPSC. Along with performance improvement, the understanding of CPSC’s temperature behaviour was considered in this thesis to understand the real-world feasibility of CPSC for the first time. The temperature coefficients (TC) of photovoltaic parameters for MAPbI3-based devices are demonstrated in chapter 5 with a detailed physico-chemical understanding. Besides CH3NH3PbI3, other perovskites such as CH3NH3PbI3-xClx and Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 were applied as an alternative sensitizer for the CPSCs and studied their temperature coefficients across a wide range of real-world temperatures to obtain behavioural differences between the halide perovskites. Finally, the suitability of semi-transparent CPSC for fenestration integration was evaluated for the first time via fabrication engineering and thickness control with the highest reported average visible transmittance/PCE combination to date, as discussed in chapter 6. Finally, in this thesis work, CPSC devices are explored, which highlights fascinating ambient fabrication processes and significant device performance with new series of HTMs, ETMs and designs for futuristic applications

Development and analyses of innovative thin films for photovoltaic applications

In solar cell current research, innovative solutions and materials are continuously requested for efficiency improvements. Si-based technology rules over 95% of the market, with silicon heterojunction (SHJ) solar cell reaching 26.7% record efficiency. Nonetheless, hydrogenated amorphous silicon (a-Si:H) layers employed in the structure still have challenges, resolvable with oxygen/nitrogen inclusion. In parallel, new technologies based on different materials still lack in the market due to stability issues or low efficiencies. However, a preliminary study of their properties creates a deeper knowledge exploitable in photovoltaic application. In this perspective, we investigated both innovative Si-based materials (nanocrystalline and amorphous silicon oxy-nitride and oxide thin films, nc-SiOxNy, a-SiOxNy and a-SiOx, respectively) and innovative materials (perovskite lanthanum-vanadium oxide LaVO3 thin films, indium gallium nitride InxGa1-xN and aluminium indium gallium nitride AlxInyGa1-x-yN layers) for solar cell concepts. Different deposition conditions have been employed to extract their influence on compositional, optical, and electrical properties. The study on nc-SiOxNy layers by conductive atomic force microscopy (c-AFM) and surface photovoltage (SPV) has allowed to clarify O, N, and B content, and annealing treatment role on microscopic transport properties. On a-SiOx and a-SiOxNy layers, by spectral ellipsometry, Fourier transform infrared spectroscopy, photoconductance decay and SPV, we can conclude that moderate insertions of O/N in a-Si:H lead to a decrease of optical parasitic absorption, preserving the passivation quality of the layers. The measurements by AFM and Kelvin probe force microscopy on LaVO3 have clearly shown that it is a poor charge-transport medium, thus not suitable for photovoltaic applications. The analysis on InGaN and AlGaInN by SPV measurements has shown how low In content, Si doping and no misfit dislocations in InGaN/GaN structure cause less recombination processes at the interface, whereas, the strain relaxation (tensile and compressive) with the formation of pinholes produces better interfaces in the AlGaInN/GaN samples