Ionic polarization-induced current-voltage hysteresis in ch3nh3pbx3 perovskite solar cells

CH3NH3PbX3 (MAPbX3) perovskites have attracted considerable attention as absorber materials for solar light harvesting, reaching solar to power conversion efficiencies above 20%. In spite of the rapid evolution of the efficiencies, the understanding of basic properties of these semiconductors is still ongoing. One phenomenon with so far unclear origin is the so-called hysteresis in the current–voltage characteristics of these solar cells. Here we investigate the origin of this phenomenon with a combined experimental and computational approach. Experimentally the activation energy for the hysteretic process is determined and compared with the computational results. First-principles simulations show that the timescale for MAþ rotation excludes a MA-related ferroelectric effect as possible origin for the observed hysteresis. On the other hand, the computationally determined activation energies for halide ion (vacancy) migration are in excellent agreement with the experimentally determined values, suggesting that the migration of this species causes the observed hysteretic behaviour of these solar cells

High Absorption Coefficient Cyclopentadithiophene Donor-Free Dyes for Liquid and Solid-State Dye-Sensitized Solar Cells

We report a series of "donor-free" dyes featuring moieties of oligo(4,4-dihexyl-4H-cyclopenta[1,2-b:5,4-b′]dithiophene) (CPDT) functionalized with cyanoacrylic end groups for mesoscopic titania solar cells based on I-/I3 - or Co(II)/Co(III) redox couple and spiro-OMeTAD hole transporter. These were compared with similar cells using an oligo(3-hexylthiophene) dye (5T), which we reported before. Extending the CPDT moiety of the dye molecules from one to three (denoted as CPDT-1, CPDT-2, and CPDT-3) widens the photoresponse overlap with the solar spectrum, increases the molar absorption coefficient up to 75-000 M-1 cm-1, and improves the short-circuit current (JSC), open-circuit voltage (VOC), and power conversion efficiency (PCE) for all types of DSSCs. Among these sensitizers, CPDT-3 shows the highest PCE of 6.7%, 7.3%, and 3.9% with I-/I3 -, Co(II)/Co(III) redox couple, and spiro-OMeTAD hole transporter, respectively, compared with 7.6%, 9.0%, and 4.0% for 5T. Benefiting from the high absorption of CPDT-3, we demonstrate 900 nm thick mesoporous TiO2 film with remarkable JSC of 10.9 mA cm-2 in solid-state DSCs

Hill climbing hysteresis of perovskite-based solar cells: a maximum power point tracking investigation

The surge of the power conversion efficiency of metal halide lead perovskite solar cells comes with concerns, such as the long-term ecotoxicity of lead compounds, their sensitivity toward moisture and oxygen, or the scarcity of some of their components. Most perovskite solar cells still suffer from serious stability problems when measured under real working conditions (maximum power point tracking at 60 degrees C). In the long run, stability will certainly decide on the fate of CH3NH3PbI3 and related lead perovskites for their use in photovoltaic modules. Herein, we show an effective and inexpensive strategy to perform ageing of perovskite solar cells under maximum power point tracking. For the first time, we analyze the issue of power extraction from solar cells exhibiting hysteresis. We show that a standard tracking algorithm such as perturb and observe fails to converge to the maximum power point of the solar cell if it exhibits j(V) hysteresis, and we present an effective strategy to stabilize the algorithm. We show that enforcing oscillations in forward bias can boost the mean power output of some perovskite solar cells by more than 10%, in contrast to a reference crystalline silicon solar cell. Copyright (c) 2017 John Wiley & Sons, Ltd

Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies greater than 20%

Perovskite solar cells (PSCs) with efficiencies greater than 20% have been realized only with expensive organic hole-transporting materials. We demonstrate PSCs that achieve stabilized efficiencies exceeding 20% with copper(I) thiocyanate (CuSCN) as the hole extraction layer. A fast solvent removal method enabled the creation of compact, highly conformal CuSCN layers that facilitate rapid carrier extraction and collection. The PSCs showed high thermal stability under long-term heating, although their operational stability was poor. This instability originated from potential-induced degradation of the CuSCN/Au contact. The addition of a conductive reduced graphene oxide spacer layer between CuSCN and gold allowed PSCs to retain >95% of their initial efficiency after aging at a maximum power point for 1000 hours under full solar intensity at 60 degrees C. Under both continuous full-sun illumination and thermal stress, CuSCN-based devices surpassed the stability of spiro-OMeTAD-based PSCs

The Role of Rubidium in Multiple-Cation-Based High-Efficiency Perovskite Solar Cells

Perovskite solar cells (PSCs) based on cesium (Cs)- and rubidium (Rb)-containing perovskite films show highly reproducible performance; however, a fundamental understanding of these systems is still emerging. Herein, this study has systematically investigated the role of Cs and Rb cations in complete devices by examining the transport and recombination processes using current-voltage characteristics and impedance spectroscopy in the dark. As the credibility of these measurements depends on the performance of devices, this study has chosen two different PSCs, (MAFACs) Pb(IIBr)(3) (MA = CH3NH3+, FA = CH(NH2)(2)(+)) and (MAFACsRb) Pb(IIBr)(3), yielding impressive performances of 19.5% and 21.1%, respectively. From detailed studies, this study surmises that the confluence of the low trap-assisted charge-carrier recombination, low resistance offered to holes at the perovskite/2,2', 7,7'-tetrakis(N, N-di-p-methoxyphenylamine)- 9,9-spirobifluorene interface with a low series resistance (R-s), and low capacitance leads to the realization of higher performance when an extra Rb cation is incorporated into the absorber films. This study provides a thorough understanding of the impact of inorganic cations on the properties and performance of highly efficient devices, and also highlights new strategies to fabricate efficient multiple-cation-based PSCs

Function Follows Form: Correlation between the Growth and Local Emission of Perovskite Structures and the Performance of Solar Cells

Understanding the relationship between the growth and local emission of hybrid perovskite structures and the performance of the devices based on them demands attention. This study investigates the local structural and emission features of CH3NH3PbI3, CH3NH3PbBr3, and CH(NH2) 2PbBr(3) perovskite films deposited under different yet optimized conditions using X-ray scattering and cathodoluminescence spectroscopy, respectively. X-ray scattering shows that a CH3NH3PbI3 film involving spin coating of CH3NH3I instead of dipping is composed of perovskite structures exhibiting a preferred orientation with [202] direction perpendicular to the surface plane. The device based on the CH3NH3PbI3 film composed of oriented crystals yields a relatively higher photovoltage. In the case of CH3NH3PbBr3, while the crystallinity decreases when the HBr solution is used in a single-step method, the photovoltage enhancement from 1.1 to 1.46 V seems largely stemming from the morphological improvements, i.e., a better connection between the crystallites due to a higher nucleation density. Furthermore, a high photovoltage of 1.47 V obtained from CH(NH2)(2)PbBr3 devices could be attributed to the formation of perovskite films displaying uniform cathodoluminescence emission. The comparative analysis of the local structural, morphological, and emission characteristics of the different perovskite films supports the higher photovoltage yielded by the relatively better performing devices

High photovoltage in perovskite solar cells: New physical insights from the ultrafast transient absorption spectroscopy

To understand the cause of the high open circuit photovoltage (VOC) achieved by todays' state of the art perovskite solar cells (PSCs), we examine formamidinium lead bromide CH(NH2)(2)PbBr3 films by ultrafast transient absorption spectroscopy (TAS). By using TiO2 and spiro-OMeTAD as charge extraction layers, the devices based on the CH(NH2)(2)PbBr3 films yield VOC as high as 1.5 V ascertaining their high quality. TAS establish that the presence of charge extraction layers has very little influences on the nature of a negative band at 535 nm corresponding to the bleaching of the absorption band edge and two positive bands in the CH(NH2)(2)PbBr3 films. Therefore, we contend that the V-OC in PSC is predominantly determined by the quasi Fermi level splitting within the perovskite layer. (C) 2017 Published by Elsevier B. V

Dopant-Free Donor (D)–p–D–p–D Conjugated Hole- Transport Materials for Efficient and Stable Perovskite Solar Cells

Three novel hole-transporting materials (HTMs) using the 4-methoxytriphenylamine (MeOTPA) core were designed and synthesized. The energy levels of the HTMs were tuned to match the perovskite energy levels by introducing symmetrical electron-donating groups linked with olefinic bonds as the bridge. The methylammonium lead triiodide (MAPbI(3)) perovskite solar cells based on the new HTM Z34 (see main text for structure) exhibited a remarkable overall power conversion efficiency (PCE) of 16.1% without any dopants or additives, which is comparable to 16.7% obtained by a p-doped 2,2,7,7-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9-spirobifluorene (spiro-OMeTAD)-based device fabricated under the same conditions. Importantly, the devices based on the three new HTMs show relatively improved stability compared to devices based on spiro-OMeTAD when aged under ambient air containing 30% relative humidity in the dark

Impact of a Mesoporous Titania-Perovskite Interface on the Performance of Hybrid Organic-Inorganic Perovskite Solar Cells.

We report on the optimization of the interfacial properties of titania in mesoscopic CH3NH3PbI3 solar cells. Modification of the mesoporous 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.Nava Technology Limited (Ph.D. scholarship), Indo−UK APEX project, Royal Society London (Newton Fellowship), Engineering and Physical Sciences Research Council, King Abdulaziz City for Science and Technology (KACST), CCEM-CH in the 9th call proposal 906: CONNECT PV, Swiss National Science FoundationThis is the author accepted manuscript. The final version is available from the American Chemical Society via http://dx.doi.org/10.1021/acs.jpclett.6b0161