Chemical Modifications and Passivation Approaches in Metal Halide Perovskite Solar Cells

This dissertation describes our study on different physical properties of passivated and chemically modified hybrid metal halide perovskite materials and development of highly efficient charge transport layers for perovskite solar cells. We first developed an efficient electron transport layer via modification of titanium dioxide nanostructure followed by a unique chemical treatment in order to have clean interface with fast electron injection form the absorber layer in the perovskite solar cells. We then explored monovalent cation doping of lead halide perovskites using sodium, copper and silver with similar ionic radii to lead to enhance structural and optoelectronic properties leading to higher photovoltaic performance of the resulting perovskite solar cells. We also performed thorough experimental characterizations together with modeling to further understand the chemical distribution and local structure of perovskite films upon monovalent cation doping. Then, we demonstrate a novel passivation approach in alloyed perovskite films to inhibit the ion segregation and parasitic non-radiative losses, which are key barriers against the continuous bandgap tunability and potential for high-performance of metal halide perovskites in device applications, by decorating the surfaces and grain boundaries with potassium halides. This leads to luminescence quantum yields approaching unity while maintaining high charge mobilities along with the inhibition of transient photo-induced ion migration processes even in mixed halide perovskites that otherwise show bandgap instabilities. We demonstrate a wide range of bandgaps stabilized against photo-induced ion migration, leading to solar cell power conversion efficiencies of 21.6% for a 1.56 eV absorber and 18.3% for a 1.78 eV absorber ideally suited for tandem solar cells. We then systematically compare the optoelectronic properties and moisture stability of the two developed passivation routes for alloyed perovskites with rubidium and potassium where the latter passivation route showed higher stability and loading capacity leading to achieve substantially higher photoluminescence quantum yield. Finally, we explored the possibility of singlet exciton fission between low bandgap perovskites and tetracene as the triplet sensitizer finding no significant energy transfer between the two. We then used tetracene as an efficient dopant-free hole transport layer providing clean interfaces with perovskite layer leading to high photoluminescence yield (e.g. ~18%). To enhance the poor ohmic contact between tetracene and the metal electrode, we added capping layer of a second hole transport layer which is extrinsically doped leading to 21.5% power conversion efficiency for the subsequent solar cells and stabilised power output over 550 hours continuous illumination

Impact of Monovalent Metal Halides on the Structural and Photophysical Properties of Halide Perovskite

This chapter discusses the importance and impact of metal halide additives into perovskite to enhance its semiconductor quality and realize highly efficient and stable perovskite photovoltaic devices. Monovalent metal halides have been introduced as the most promising candidates due to their loading capacity and chemical compatibility with the perovskite materials, as well as ease of incorporation and their remarkable positive impact on the crystal growth, optoelectronic properties, and subsequently the performance of perovskite solar cells (PSCs). Among all the monovalent metal cations, Cs is the only one that could fit in the perovskite structure and forms photoactive perovskite. The other monovalent cations are located at the interstitials sites, grain boundaries, and crystalline surfaces. We also discuss the key roles of monovalent metal halide additives that include modulating morphology of perovskite films, modification of structural and optoelectronic properties, adjusting energy level alignment in PSCs, inhibiting non-radiative recombination in perovskites, eliminating hysteresis, and enhancing operational stability of PSCs

Mesoporous TiO₂ electrodes with different thickness for dye sensitized solar cell application

Mesoporous TiO2 films with different thicknesses were prepared for dye sensitized solar cell application using dip coating method. The crystal structure and morphology of the films were studied by scanning electron microscope and X-ray diffraction. The optical properties of the films were investigated through UV–Vis absorption. With increasing film thickness from 3.1 to 13.9 μm, the efficiency increases from 0.81 to 3.09 %

Morphology, structure and optical properties of low bandgap organic-inorganic halide perovskite based on CH₃NH₃SnxPb₁₋ₓI₃

Herein, we investigate morphology, structure and optical properties of low band gap organic-inorganic halide perovskite based on a mixture of lead and tin as the divalent cation in ABX3 structure. A significant change in morphology of CH3NH3SnxPb1−xI3 perovskite with x as well as an alteration in crystal structure from I4cm (β-phase) to pseudocubic P4mm (α-phase) space groups is observed when Sn is the dominant divalent cation (x ≥ 0.5). Photo thermal defection optical absorption spectroscopy (PDS) and photoluminescence (PL) of CH3NH3SnxPb1−xI3 show the non-linear change in the band edge of perovskite. The bandgap as low as 1.17 eV and the most red-shifted PL at 1035nm is achieved for perovskite with x=0.8. In addition, higher electronic disorder is measured for CH3NH3SnxPb1−xI3 compounds with higher x

Impacts of plasmonic nanoparticles incorporation and interface energy alignment for highly efficient carbon-based perovskite solar cells

This work utilizes a realistic electro-optical coupled simulation to study the (i) impact of mesoporous TiO2 removal; (ii) the embedding of Ag@SiO2 and SiO2@Ag@SiO2 plasmonic nanoparticles; (iii) utilization of solution-processed inorganic p-type copper(I) thiocyanate (CuSCN) layer at the perovskite/carbon interface; and (iv) the increase of the work function of carbon electrodes (via incorporation of suitable additives/binders to the carbon ink) on the performance of carbon-based PSCs. Removal of mesoporous TiO2 increased the power conversion efficiency (PCE) of the device from 14.83 to 16.50% due to the increase in exciton generation rate and charge carriers’ mobility in the vicinity of the perovskite-compact TiO2 interface. Subsequently, variable mass ratios of Ag@SiO2 and SiO2@Ag@SiO2 plasmonic nanoparticles are embedded in the vicinity of the perovskite-compact TiO2 interface. In the optimum cases, the PCE of the devices increased to 19.72% and 18.92%, respectively, due to light trapping, scattering, and strong plasmonic fields produced by the plasmonic nanoparticles. Furthermore, adding the CuSCN layer remarkably increased the PCE of the device with a 0.93% mass ratio of Ag@SiO2 nanoparticles from 19.72 to 26.58% by a significant improvement of Voc and FF, due to the proper interfacial energy band alignment and the reduction of the recombination current density. Similar results were obtained by increasing the carbon work function, and the cell PCE was enhanced up to 26% in the optimal scenario. Our results pave the way to achieve high efficiencies in remarkably stable printable carbon-based PSCs

Static Eccentricity Fault Detection in Brushless Doubly Fed Induction Machines based on MotorCurrent Signature Analysis

In this paper a new rotor eccentricity fault detection method is proposed for the first time for Brushless Doubly Fed Induction Machines (BDFIMs). Due to the fact that BDFIMs are attractive alternatives to doubly fed induction machines for wind power generation, paying attention to their fault diagnosis is essential. Existing fault detection methods for conventional induction machines can not be directly applied to the BDFIM due to its special rotor structure and stator winding configurations as well as the complex magnetic fields. In this paper a new fault detection technique based on stator current harmonic analysis is proposed to detect rotor eccentricity faults in the BDFIM. The validity of the proposed fault detection method is verified by analytical winding function method and finite element analysis on a prototype D180 BDFIM. Index Terms—Brushless doubly fed induction machines, Nested-loop rotor slot harmonics, Motor current signature analysis, Winding function method, Finite element analysis, Static eccentricity fault

Low temperature dye-sensitized solar cells based on conformal thin zinc oxide overlayer on mesoporous insulating template by atomic layer deposition

Low temperature processing of dye-sensitized solar cells (DSCs) is essential to enable commercialization with low cost plastic substrates and diminish the overall manufacturing cost.We report a low temperature processing route for photoanodes where thin ZnO nanoshell is deposited by atomic layer deposition at 150°C, on a mesoporous insulating template. We found that a 6 nm ZnO overlayer on a 3 μm mesoporous nanoparticle Al2O3 template shows a power conversion efficiency of 4.2 % with the standard organic sensitizer (coded Y123) and cobalt bipyridine redox mediator

The race between complicated multiple cation/anion compositions and stabilization of FAPbI3 for halide perovskite solar cells

Compositional modifications and passivating additives have been key enablers to achieve operationally stable halide perovskite devices with excellent optoelectronic properties. The thermal and structural instability of the most desirable single-cation metal halide perovskites motivates the use of multiple cation and mixed-halide compositions that indeed lead to superior optoelectronic and photovoltaic properties over the course of the development. The application of multiple cation/anion and additive-based alloyed perovskites could, however, be hindered by the formation of non-perovskite phases and bandgap increase. Recent studies have shown that exceptional performance can be achieved using simpler compositions, such as FAPbI3, with appropriate passivation methods. In this perspective, we will present the current status of multiple cation/anion perovskites and discuss the role of common monovalent cations such as FA, MA, Cs, Rb, and K on the stability, optoelectronic properties, and charge transport behavior of lead halide perovskites. We further present the common stabilization and passivation strategies for phase-pure FAPbI3. We highlight the key role of solid-state NMR in determining the atomic-level mechanism of action of the various dopants and passivation agents. We then summarize the current understanding of the benefits and drawbacks of the cation alloying approach relative to the passivated phase-pure FAPbI3. Finally, we discuss the perspective for future research directions to achieve stable perovskite solar cells that approach the theoretical limit

Back-Contact Perovskite Solar Cells

Interdigitated back-contact (IBC) architectures are the best performing technology in crystalline Si (c-Si) photovoltaics (PV). Although single junction perovskite solar cells have now surpassed 23% efficiency, most of the research has mainly focussed on planar and mesostructured architectures. The number of studies involving IBC devices is still limited and the proposed architectures are unfeasible for large scale manufacturing. Here we discuss the importance of IBC solar cells as a powerful tool for investigating the fundamental working mechanisms of perovskite materials. We show a detailed fabrication protocol for IBC perovskite devices that does not involve photolithography and metal evaporation. The interview is available at https://youtu.be/nvuNC29TvOY.The authors thank the Engineering and Physical Sciences Research Council (EPSRC). XMaS is a mid-range facility supported by the EPSRC. The authors also thank all the XMaS beamline team staff for their support. M.A.-J. thanks Cambridge Materials Limited and EPSRC (EP/M005143/1) for their funding and technical support. M.A. acknowledges support from the President of the UAE’s Distinguished Student Scholarship Program (DSS), granted by the UAE’s Ministry of Presidential Affairs

Reversible Removal of Intermixed Shallow States by Light Soaking in Multication Mixed Halide Perovskite Films.

The highest reported efficiencies of metal halide perovskite (MHP) solar cells are all based on mixed perovskites, such as (FA,MA,Cs)Pb(I1-x Br x )3. Despite demonstrated structural changes induced by light soaking, it is unclear how the charge carrier dynamics are affected across this entire material family. Here, various (FA,MA,Cs)Pb(I1-x Br x )3 perovskite films are light-soaked in nitrogen, and changes in optoelectronic properties are investigated through time-resolved microwave conductivity (TRMC) and optical and structural techniques. To fit the TRMC decay kinetics obtained for pristine (FA,MA,Cs)Pb(I1-x Br x )3 for various excitation densities, additional shallow states have to be included, which are not required for describing TRMC traces of single-cation MHPs. These shallow states can, independently of x, be removed by light soaking, which leads to a reduction in the imbalance between the diffusional motion of electrons and holes. We interpret the shallow states as a result of initially well-intermixed halide distributions, which upon light soaking segregate into domains with distinct band gaps.Z.A.-G. acknowledges funding from a Winton Studentship and ICON Studentship from the Lloyd’s Register Foundation. M.A.-J. thanks Cambridge Materials Limited and EPSRC (Grant Number EP/M005143/1) for their funding and technical support. S.D.S. acknowledges the Royal Society and Tata Group (UF150033) for funding