Enhancing the stability of organolead halide perovskite films through polymer encapsulation

Perovskite solar cells based on organolead halides such as CH3NH3PbX3 (X = Cl, Br, and I) have rapidly established themselves as the frontrunners among emerging photovoltaic technologies. However, their commercial application has been hindered to date in part due to their susceptibility to degradation by UV radiation or heat in the presence of moisture. Herein we investigate the relationship between the physical properties of several polymer encapsulants (poly(methylmethacrylate) (PMMA), ethyl cellulose, polycarbonate and poly(4-methyl-1-pentene)) and their ability to function as barrier layers to improve the stability of CH3NH3PbI3−xClx films under prolonged thermal degradation at 60 °C, 80 °C and 100 °C. In all cases, polymer-coated CH3NH3PbI3−xClx films showed retarded thermal degradation compared to the uncoated films, as indicated by the quantitative decay of the perovskite band edge in the UV/Vis absorption spectrum and the appearance of PbI2 peaks in the powder X-ray diffraction pattern. However, the extent of this reduction was highly dependent on the physical properties of the polymer encapsulant. Notably, PMMA-coated CH3NH3PbI3−xClx films showed no visible signs of degradation to PbI2 after extended heating at 60 °C. However, concomitant studies by epifluorescence microscopy (FM) revealed deterioration of the CH3NH3PbI3−xClx film quality, even in the presence of a polymer-coating, at much shorter heating times (29 h), as evidenced by quenching of the film fluorescence, which was attributed to grain aggregation and the formation of associated non-radiative trap sites. Since grain aggregation occurs on a shorter timescale than chemical degradation to PbI2, this may be the limiting factor in determining the resistance of organolead halide perovskite films to thermal degradation

A simple method to evaluate the effectiveness of encapsulation materials for perovskite solar cells

Solution processed perovskite solar cells are an exciting development in the field of photovoltaics achieving power conversion efficiencies of over 20%. Nevertheless, stability issues are still limiting the successful entry of this technology into the PV market. Rapid degradation has been observed and reported as the result of different factors, such as light, humidity and temperature, simultaneously present during real operation. It is felt within the PV community that proper, effective encapsulation is one of the key contributors to increasing perovskite lifetimes. This work presents a simple and effective method based on RGB (red, green, blue) colour measurements to track perovskite degradation to lead iodide (PbI2) using time lapse photography and thus evaluate the effectiveness/reliability of different encapsulation methods and materials. This technique gives a clear indication of when the perovskite has fully degraded and the impact of different encapsulants on degradation rate. This is supported by other analytical techniques, such as UV-Vis spectroscopy and XRD

Raman mapping analysis for removal of surface secondary phases of CZTS films using chemical etching

Raman spectroscopy has been widely used as a non-destructive surface characterization method for the Cu 2ZnSnS4 (CZTS) thin films. Secondary phases, which often co-exist with CZTS, are detrimental to the device performance. In this work, removal of the secondary phases using sodium sulfide (Na 2S) aqueous solution etching in various time durations was investigated. Raman scattering mapping provides a direct visualization of phase distribution in CZTS-based materials on a relatively large scale (1 mm × 10 mm). Both as-grown and etched CZTS absorber layers were examined by Raman spectroscopy with a 532 nm excitation laser light in the range of 50–500 cm-1. A clear reduction of the secondary phases (mainly SnS) at the surface after etching was confirmed by Raman spectroscopy and scanning electron microscopy. Room temperature photoluminescence (PL) reveals a pronounced correlation between the amount of secondary phases and photoluminescence peak position. The PL spectra of the regions with more Sn-rich secondary phases show clearly a shift to high wavelength of the peak position, in comparison with regions with less Sn-rich secondary phases. These observed PL changes could be due to Sn-rich defects which may cause recombination processes

Near Infrared Radiation as a Rapid Heating Technique for TiO2Films on Glass Mounted Dye-Sensitized Solar Cells

Near infrared radiation (NIR) has been used to enable the sintering of TiO2 films on fluorine-doped tin oxide (FTO) glass in 12.5 s. The 9 µm thick TiO2 films were constructed into working electrodes for dye-sensitized solar cells (DSCs) achieving similar photovoltaic performance to TiO2 films prepared by heating for 30 min in a convection oven. The ability of the FTO glass to heat upon 12.5 s exposure of NIR radiation was measured using an IR camera and demonstrated a peak temperature of 680°C; glass without the 600 nm FTO layer reached 350°C under identical conditions. In a typical DSC heating step, a TiO2 based paste is heated until the polymeric binder is removed leaving a mesoporous film. The weight loss associated with this step, as measured using thermogravimetric analysis, has been used to assess the efficacy of the FTO glass to heat sufficiently. Heat induced interparticle connectivity in the TiO2 film has also been assessed using optoelectronic transient measurements that can identify electron lifetime through the TiO2 film. An NIR treated device produced in 12.5 seconds shows comparable binder removal, electron lifetime, and efficiency to a device manufactured over 30 minutes in a conventional oven

Solution processing of TiO2 compact layers for 3rd generation photovoltaics

In this study, we introduce a new method for the deposition of TiO2 compact layers which involves the deposition of a wet film of an inorganic titanium (IV) precursor followed by fast hydrolytic conversion to crystalline TiO2 under near infrared radiative (NIR) treatment. With this, we aim to provide a scalable alternative to methods conventionally employed in laboratories for the fabrication of 3rd generation photovoltaic devices, such as high temperature pyrolysis or spin coating of organic titanium (IV) precursors. Optimization of our solution process is presented in detail. Structural features and crystalline properties of solution processed compact layers are characterized by FEG-SEM imaging and x-ray diffraction analyses and compared to compact layers produced by conventional laboratory techniques. Minimization of electron recombination is evaluated in standard liquid I-/I3- dye-sensitized solar cells (DSC). The results show that a compact, homogenous, high coverage yield crystalline TiO2 anatase layer can be produced by sequential deposition of 2–3 solution processed titanium oxide layers, each in under 30 s. In standard liquid I-/I3- DSC the solution processed compact layers strongly increased the electron lifetime, τn, when compared to cells prepared on a bare FTO substrate

Recent developments in perovskite-based precursor inks for scalable architectures of perovskite solar cell technology

The progressive enhancements in solar-to-electrical conversion within the past decade have allowed organic–inorganic lead halide perovskite-based solar cell (PSC) technology to become a competitive candidate for creating affordable and sustainable electricity. This review highlights the developments in fabricating advanced precursor inks of organic–inorganic lead halide perovskite-based light harvesters for large-area perovskite solar cell technology. One of the key characteristics of this promising photovoltaic technology includes solution processing, which offers possibilities to scale up lab-sized solar cell devices into large-area perovskite solar modules comprising unique device architectures. These have been realized in recent years for their deployment in various applications such as building-integrated photovoltaics or internet of things (IoT) devices. In this regard, the presented overview highlights the recent trends that have emerged in the research and development of novel perovskite precursor ink formulations, and it also discusses their contribution toward demonstrating efficient, scalable, and durable PSC technology to create electricity and energize futuristic applications. Various reports were included aiming to showcase the robust photovoltaic performance of large-area perovskite solar modules in a variety of device configurations, hence providing a brief overview of the role of state-of-the-art scalable precursor ink development in transforming unstable lab-sized solar cells into robust, low-cost perovskite solar cell technology that can be scaled up to cover much larger areas

Fast and Balanced Charge Transport Enabled by Solution‐Processed Metal Oxide Layers for Efficient and Stable Inverted Perovskite Solar Cells

Metal oxide charge transport materials are preferable for realizing long-term stable and potentially low-cost perovskite solar cells (PSCs). However, due to some technical difficulties (e.g., intricate fabrication protocols, high-temperature heating process, incompatible solvents, etc.), it is still challenging to achieve efficient and reliable all-metal-oxide-based devices. Here, we developed efficient inverted PSCs (IPSCs) based on solution-processed nickel oxide (NiOx) and tin oxide (SnO2) nanoparticles, working as hole and electron transport materials respectively, enabling a fast and balanced charge transfer for photogenerated charge carriers. Through further understanding and optimizing the perovskite/metal oxide interfaces, we have realized an outstanding power conversion efficiency (PCE) of 23.5% (the bandgap of the perovskite is 1.62 eV), which is the highest efficiency among IPSCs based on all-metal-oxide charge transport materials. Thanks to these stable metal oxides and improved interface properties, ambient stability (retaining 95% of initial PCE after 1 month), thermal stability (retaining 80% of initial PCE after 2 weeks) and light stability (retaining 90% of initial PCE after 1000 hours aging) of resultant devices are enhanced significantly. In addition, owing to the low-temperature fabrication procedures of the entire device, we have obtained a PCE of over 21% for flexible IPSCs with enhanced operational stability

A Transparent Conductive Adhesive Laminate Electrode for High-Efficiency Organic-Inorganic Lead Halide Perovskite Solar Cells

A self-adhesive laminate solar-cell electrode is presented based on a metal grid embedded in a polymer film (x–y conduction) and set in contact with the active layer using a pressure-sensitive adhesive containing a very low quantity (1.8%) of organic conductor, which self-organizes to provide z conduction to the grid. This ITO-free material performs in an identical fashion to evaporated gold in high-efficiency perovskite solar cells

Research Update: Behind the high efficiency of hybrid perovskite solar cells

Perovskite solar cells (PSCs) marked tremendous progress in a short period of time and offer bright hopes for cheap solar electricity. Despite high power conversion efficiency >20%, its poor operational stability as well as involvement of toxic, volatile, and less-abundant materials hinders its practical deployment. The fact that degradation and toxicity are typically observed in the most successful perovskite involving organic cation and toxic lead, i.e., CH3NH3PbX3, requires a deep understanding of their role in photovoltaic performance in order to envisage if a non-toxic, stable yet highly efficient device is feasible. Towards this, we first provide an overview of the basic chemistry and physics of halide perovskites and its correlation with its extraordinary properties such as crystal structure, bandgap, ferroelectricity, and electronic transport. We then discuss device related aspects such as the various device designs in PSCs and role of interfaces in origin of PV parameters particularly open circuit voltage, various film processing methods and their effect on morphology and characteristics of perovskite films, and the origin and elimination of hysteresis and operational stability in these devices. We then identify future perspectives for stable and efficient PSCs for practical deployment