Search

We find significant differences between degradation and healing at the surface or in the bulk for each of the different APbBr3 single crystals (A = CH3NH3+, methylammonium (MA); HC(NH2)2+, formamidinium (FA); and cesium, Cs+). Using 1- and 2-photon microscopy and photobleaching we conclude that kinetics dominate the surface and thermodynamics the bulk stability. Fluorescence-lifetime imaging microscopy, as well as results from several other methods, relate the (damaged) state of the halide perovskite (HaP) after photobleaching to its modified optical and electronic properties. The A cation type strongly influences both the kinetics and the thermodynamics of recovery and degradation: FA heals best the bulk material with faster self-healing; Cs+ protects the surface best, being the least volatile of the A cations and possibly through O-passivation; MA passivates defects via methylamine from photo-dissociation, which binds to Pb2+. DFT simulations provide insight into the passivating role of MA, and also indicate the importance of the Br3- defect as well as predicts its stability. The occurrence and rate of self-healing are suggested to explain the low effective defect density in the HaPs and through this, their excellent performance. These results rationalize the use of mixed A-cation materials for optimizing both solar cell stability and overall performance of HaP-based devices, and provide a basis for designing new HaP variants. published

We present perovskite solar cells using ozone-treated aluminum doped zinc oxide (AZO:O-3) in the dual role: as a transparent electrode and as an electron transport layer (ETL). Aluminum doped zinc oxide (AZO) replaces the conventional FTO/TiO2(c)/TiO2(m) stack, considerably simplifying the fabrication process and reducing thermal budget. Photoelectron spectroscopy suggests that AZO is an effective ETL for perovskite (CH3NH3PbI3) thin films, with a large valence band offset and a small conduction band offset, but with a possible path for carrier recombination at the interface. We show that treating the surface of AZO with ozone gas (AZO:O-3) improves the charge carrier extraction at the interface. Detailed characterization of the AZO:O-3/CH3NH3PbI3 interface shows that ozone reduces the oxygen vacancies and de-dopes top 6-9 nm of the AZO surface. The gradient in doping induces an electric-field at the AZO surface which enhances the carrier extraction. AZO:O-3/CH3NH3PbI3/spiro-OMeTAD/Au devices show champion J(SC), V-OC and eta of 20.92 mA/cm(2), 1.03 V and 10.5% respectively. Meanwhile, average short-circuit current density (J(SC)) has improved from 16.32 mA/cm(2) (AZO) to 19.74 mA/cm(2) (AZO:O-3). This is attributed to the induced electric-field at AZO:O-3 surface leading to an enhanced extraction of photo-generated charge carriers. Devices hysteresis is also much lower than previously reported results.

Experimental data from a 17.5% efficient perovskite solar cell has been used to develop a self-consistent device model using Solar Cell Capacitance Simulator (SCAPS). Electronic properties of the individual layers, such as doping density of compact TiO2 (c-TiO2), doping and defect density of perovskite, etc. are extracted from experimental data. The only fitting parameters in the model are the defect densities at spiro-MeOTAD/perovskite interface (IL-1) and perovskite/TiO2 interface (IL-2). The simulated current-voltage (J-V) characteristics and Mott-Schottky plot results match extremely well with the experimentally measured plots, validating the device model. The model was further used to extract insights into the loss mechanism of the high-efficiency perovskite solar cells. Simulations revealed that defect density at IL-1 interface has strong impact on the open-circuit voltage (V-OC), while defect density in the perovskite and IL2 interface has a major impact on short-circuit current density (J(SC)). (C) 2018 Elsevier B.V. All rights reserved.

Spin coated perovslcite thin films are known to have an issue of pinholes & poor morphology control which lead to poor device-to-device repeatability, that is an impediment to scale-up. In this work, Methylamine vapor annealing process is demonstrated which consistently leads to high-quality perovskite thin-films with an average grain-size of 10-15 mu m. The improvement in film morphology enables improvement in effective carrier recombination lifetime, from 21 its in as-deposited films to 54 mu s in vapor-annealed films. The annealed films with large-grains are also more stable in ambient conditions. Devices made on annealed perovskite films are very consistent, with a standard deviation of only 0.7%. Methylamine vapor annealing process is a promising method of depositing large-grain CH3NH3PbI3 films with high recombination lifetime and the devices with improved performance. (C) 2017 Elsevier B.V. All rights reserved.

Aluminum doped zinc oxide (AZO) can function as effective electronic transport layer (ETL) cum transparent electrode for perovskite solar cells. Measured electronic band diagram shows that AZO/perovskite is a carrier-selective interface that blocks holes but allows transport of electrons. The performance of AZO/perovskite interface is significantly improved by exposing AZO surface to ozone. Ozone depletes carriers from top few nanometers of AZO, forming AZO/un-doped-AZO/perovskite interface with lower recombination losses, resulting in champion cell efficiency of 10.5% and open circuit voltage (Voc) up to 1.05V. AZO interface is also stable. Devices retained 90% of initial Voc even after 60 days of ambient storage.

Here we present a novel vapor-annealing technique to improve morphology of pure-iodide perovskite (CH3NH3PbI3) thin-films and get pin-hole-free coverage with > 10µm grain size. X-Ray diffraction (XRD) measurement confirms that the annealed films are phase pure perovskite. The larger grain size leads to a significant increase in the effective recombination lifetime of charge carriers, with the best films showing lifetime of >50 µs, Solar cells fabricated with the large grain perovskite thin film shows significant improvement in short-circuit current density (Jsc) and open-circuit voltage (V oc).