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

Impact of hybrid plasmonic nanoparticles on the charge carrier mobility of P3HT:PCBM polymer solar cells

The solution processable polymer solar cells have shown a great promise as a cost-effective photovoltaic technology. Here, the effect of carrier mobility changes has been comprehensively investigated on the performance of P3HT:PCBM polymer solar cells using electro-optical coupled simulation regimes, which may result from the embedding of SiO2@Ag@SiO2 plasmonic nanoparticles (NPs) in the active layer. Firstly, the active layer thickness, stemmed from the low mobility of the charge carriers, is optimized. The device with 80 nm thick active layer provided maximum power conversion efficiency (PCE) of 3.47%. Subsequently, the PCE has increased to 6.75% and 6.5%, respectively, along with the benefit of light scattering, near-fields and interparticle hotspots produced by embedded spherical and cubic nanoparticles. The PCE of the devices with incorporated plasmonic nanoparticles are remarkably enhanced up to 7.61% (for spherical NPs) and 7.35% (for cubic NPs) owing to the increase of the electron and hole mobilities to [Formula: see text] and [Formula: see text], respectively (in the optimum case). Furthermore, SiO2@Ag@SiO2 NPs have been successfully synthesized by introducing and utilizing a simple and eco-friendly approach based on electroless pre-treatment deposition and Stober methods. Our findings represent a new facile approach in the fabrication of novel plasmonic NPs for efficient polymer solar cells

Low-Frequency Carrier Kinetics in Triple Cation Perovskite Solar Cells Probed by Impedance and Modulus Spectroscopy

Organometallic halide perovskites-based solar cells have emerged as next-generation photovoltaic technology. However, many of its intriguing optoelectronic properties at low frequency are highly debated. Here, we investigate the low-frequency carrier kinetics of the state-of-the-art triple cation perovskite Cs0.06FA0.79MA0.15Pb(I0.85Br0.15)3 solar cells using bias-dependent impedance and modulus spectroscopy under dark and illumination conditions. We observe a strong dependence of dielectric permittivity on frequency in 1 Hz to 1 MHz region with a dielectric relaxation, which is observed to follow the Maxwell-Wagner type interfacial polarization possibly originating from the grain boundary/ionic defects. Furthermore, correlating the impedance and modulus spectra reveals the localized charge carrier relaxation in this triple cation device from which we obtain a phenomenological picture of the hopping process and developing an understanding of the charge carrier kinetics in these high-efficiency perovskite photovoltaics

Critical light instability in CB/DIO processed PBDTTT-EFT:PC<inf>71</inf>BM organic photovoltaic devices

Organic photovoltaic (OPV) devices often undergo ‘burn-in’ during the early stages of operation, this period describing the relatively rapid drop in power output before stabilising. For normal and inverted PBDTTT-EFT:PC71BM OPVs prepared according to current protocols, we identify a critical and severe light-induced burn-in phase that reduces power conversion efficiency by at least 60% after 24 hours simulated AM1.5 illumination. Such losses result primarily from a reduction in photocurrent, and for inverted devices we correlate this process in-situ with the simultaneous emergence of space-chare effects on the μs timescale. The effects of burn in are also found to reduce the lifetime of photogenerated charge carriers, as determine by in-situ transient photovoltage measurements. To identify the underlying mechanisms of this instability, a range of techniques are employed ex-situ to separate bulk- and electrode-specific degradation processes. We find that whilst the active layer nanostructure and kinetics of free charge generation remain unchanged, partial photobleaching (6% of film O.D.) of PBDTTT-EFT:PC71BM occurs alongside an increase in the ground state bleach decay time of PBDTTT-EFT. We hypothesise that this latter observation may reflect relaxation from excited states on PBDTTT-EFT that do not undergo dissociation into free charges. Owing to the poor lifetime of the reference PBDTTT-EFT:PC71BM OPVs, the fabrication protocol is modified to identify routes for stability enhancement in this initially promising solar cell blend.The authors would like to thank SABIC for partially funding this research. PEH, EC, RHF and NCG thank the EPSRC for funding through the Supergen Supersolar Consortium (EP/J017361/1). PEH also thanks CKIK for additional funding. KD thanks the Gates Cambridge Scholarship fund. MAJ thanks Nyak Technology Ltd for PhD scholarship funding. AJP thanks David Lidzey (University of Sheffield) for use of a sample chamber for X-ray scattering measurements and Adam Brown (University of Cambridge) for UPS measurements.This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.orgel.2015.12.02

Impact of A-Site Cation Modification on Charge Transport Properties of Lead Halide Perovskite for Photovoltaics Applications

Perovskite solar cells (PSCs) have reached a formidable power conversion efficiency of 25.7% over the years of development. One of the strategies that has been responsible for the development of stable and highly efficient PSCs is modifications of the monovalent A-site cations (methylammonium, MA; formamidinium, FA; cesium, Cs, etc.) in lead halide perovskites. Herein, the impact of modifying the monovalent cation (MA, FAMA, CsFAMA, potassium-passivated CsFAMA, rubidium-passivated CsFAMA) in lead halide perovskite on their optoelectronic, charge transport, and photovoltaic behavior is systematically studied. Reduced trap density and improved charge carrier mobility after introduction of FA and Cs in the MAPb(I0.85Br0.15)3 system are confirmed. Further passivation of the triple-cation perovskite with K and Rb enhances the optoelectronic characteristics, charge transport, and charge extraction efficiency in halide perovskite solar cells

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

A facile low temperature route to deposit a TiO<inf>2</inf> scattering layer for efficient dye-sensitized solar cells

Hydrolysis of TiCl4 at low temperature formed an efficient scattering layer in dye-sensitized solar cell architecture, which leads to an improvement in the light harvesting and a remarkable reduction of electronic disorder of mesoporous-TiO2.Nava Technology Limited, Iran Nanotechnology Initiative Council, Nyak Technology Limited, Engineering and Physical Sciences Research CouncilThis is the author accepted manuscript. The final version is available from Royal Society of Chemistry via http://dx.doi.org/10.1039/C6RA13273

Interface engineering of mesoscopic hybrid organic-inorganic perovskite solar cells

We report on the optimization of the interfacial properties of titania in mesoscopic CH3NH3PbI3 perovskite solar cells (PSCs). Modification of the mesoporous titania (mp-TiO2) film by TiCl4 treatment substantially reduced the surface traps, as is evident from the sharpness of the absorption edge with a significant reduction in Urbach energy (from 320 to 140 meV) determined from photothermal deflection spectroscopy, and led to an order of magnitude enhancement in the bulk electron mobility and corresponding decrease in the transport activation energy (from 170 to 90 meV) within a device. After optimization of the photoanode–perovskite interface using various sizes of TiO2 nanoparticles, the best photovoltaic efficiency of 16.3% was achieved with the mesoporous TiO2 composed of 36 nm sized nanoparticles. The improvement in device performance can be attributed to the enhanced charge collection efficiency that is driven by improved charge transport in the mesoporous TiO2 layer. Also, the decreased recombination at the TiO2–perovskite interface and better perovskite coverage play important roles

Understanding the Performance-Limiting Factors of Cs₂AgBiBr₆ Double-Perovskite Solar Cells

Double perovskites have recently emerged as possible alternatives to lead-based halide perovskites for photovoltaic applications. In particular, Understanding the Performance-Limiting Factors of Cs₂AgBiBr₆ Double-Perovskite Solar Cells has been the subject of several studies because of its environmental stability, low toxicity, and its promising optoelectronic features. Despite these encouraging features, the performances of solar cells based on this double perovskite are still low, suggesting severe limitations that need to be addressed. In this work we combine experimental and theoretical studies to show that the short electron diffusion length is one of the major causes for the limited performance of Cs₂AgBiBr₆ solar cells. Using EQE measurements on semitransparent Cs₂AgBiBr₆ solar cells we estimate the electron diffusion length to be only 30 nm and corroborated this value by terahertz spectroscopy. By using photothermal deflection spectroscopy and surface photovoltage measurements we correlate the limited electron diffusion length with a high density of electron traps. Our findings highlight important faults affecting this double perovskite, showing the challenges to overcome and hinting to a possible path to improve the efficiency of Cs₂AgBiBr₆ solar cells

Impact of Excess Lead Iodide on the Recombination Kinetics in Metal Halide Perovskites

Fundmental comprehension of light-induced processes in perovskites are still scarce. One active debate surrounds the influence of excess lead iodide (PbI2) on device performance, as well as optoelectronic properties, where both beneficial and detrimental traits have been reported. Here, we study its impact on charge carrier recombination kinetics by simultaneously acquiring the photoluminescence quantum yield and time-resolved photoluminescence as a function of excitation wavelength (450–780 nm). The presence of PbI2 in the perovskite film is identified via a unique spectroscopic signature in the PLQY spectrum. Probing the recombination in the presence and absence of this signature, we detect a radiative bimolecular recombination mechanism induced by PbI2. Spatially resolving the photoluminescence, we determine that this radiative process occurs in a small volume at the PbI2/perovskite interface, which is only active when charge carriers are generated in PbI2, and therefore provide deeper insight into how excess PbI2 may improve the properties of perovskite-based devices