Effect of Ultraviolet Radiation on Organic Photovoltaic Materials and Devices.

ACS applied materials & interfaces 11 (2019) 21543-21551

JB Patel, P Tiwana, N Seidler, GE Morse, OR Lozman, MB Johnston, LM Herz

Organic photovoltaics are a sustainable and cost-effective power-generation technology that may aid the move to zero-emission buildings, carbon neutral cities, and electric vehicles. While state-of-the-art organic photovoltaic devices can be encapsulated to withstand air and moisture, they are currently still susceptible to light-induced degradation, leading to a decline in the long-term efficiency of the devices. In this study, the role of ultraviolet (UV) radiation on a multilayer organic photovoltaic device is systematically uncovered using spectral filtering. By applying long-pass filters to remove different parts of the UV portion of the AM1.5G spectrum, two main photodegradation processes are shown to occur in the organic photovoltaic devices. A UV-activated process is found to cause a significant decrease in the photocurrent across the whole spectrum and is most likely linked to the deterioration of the charge extraction layers. In addition, a photodegradation process caused by UV-filtered sunlight is found to change the micromorphology of the bulk heterojunction material, leading to a reduction in photocurrent at high photon energies. These findings strongly suggest that the fabrication of inherently photostable organic photovoltaic devices will require the replacement of fullerene-based electron transporter materials with alternative organic semiconductors.

Controlling energy levels and Fermi level en route to fully tailored energetics in organic semiconductors.

Nature communications 10 (2019) 5538-

R Warren, A Privitera, P Kaienburg, AE Lauritzen, O Thimm, J Nelson, MK Riede

Simultaneous control over both the energy levels and Fermi level, a key breakthrough for inorganic electronics, has yet to be shown for organic semiconductors. Here, energy level tuning and molecular doping are combined to demonstrate controlled shifts in ionisation potential and Fermi level of an organic thin film. This is achieved by p-doping a blend of two host molecules, zinc phthalocyanine and its eight-times fluorinated derivative, with tunable energy levels based on mixing ratio. The doping efficiency is found to depend on host mixing ratio, which is explained using a statistical model that includes both shifts of the host's ionisation potentials and, importantly, the electron affinity of the dopant. Therefore, the energy level tuning effect has a crucial impact on the molecular doping process. The practice of comparing host and dopant energy levels must consider the long-range electrostatic shifts to consistently explain the doping mechanism in organic semiconductors.

Overcoming Zinc Oxide Interface Instability with a Methylammonium-Free Perovskite for High-Performance Solar Cells


K Schutt, PK Nayak, AJ Ramadan, B Wenger, Y-H Lin, HJ Snaith

Revealing the nature of photoluminescence emission in the metal-halide double perovskite Cs2AgBiBr6


SJ Zelewski, JM Urban, A Surrente, DK Maude, A Kuc, L Schade, RD Johnson, M Dollmann, PK Nayak, HJ Snaith, P Radaelli, R Kudrawiec, RJ Nicholas, P Plochocka, M Baranowski

Controlling competing photochemical reactions stabilizes perovskite solar cells

NATURE PHOTONICS 13 (2019) 532-+

SG Motti, D Meggiolaro, AJ Barker, E Mosconi, CAR Perini, JM Ball, M Gandini, M Kim, F De Angelis, A Petrozza

Enhancing the Charge Extraction and Stability of Perovskite Solar Cells Using Strontium Titanate (SrTiO<inf>3</inf>) Electron Transport Layer

ACS Applied Energy Materials (2019)

M Neophytou, M De Bastiani, N Gasparini, E Aydin, E Ugur, A Seitkhan, F Moruzzi, Y Choaie, AJ Ramadan, JR Troughton, R Hallani, A Savva, L Tsetseris, S Inal, D Baran, F Laquai, TD Anthopoulos, HJ Snaith, S De Wolf, I McCulloch

© 2019 American Chemical Society. Charge transport layers strongly influence the performance of perovskite solar cells (PSCs). To date, compact layers and mesoporous scaffolds of titanium dioxide have emerged as good electron transport layers (ETL), enabling record power conversion efficiencies (PCE). However, these ETLs require sintering above 400 °C, which excludes them from low-temperature applications such as flexible devices and silicon-heterojunction tandems. Furthermore, instability of TiO2 under prolonged exposure to sunlight appears to be a critical issue. Here, we present the promising characteristics of low-temperature processed strontium titanate (STO) as an ETL to realize PSCs with 19% PCE. STO is a wide bandgap transparent inorganic perovskite. Compared with other low-temperature processed interlayers, STO reduces the parasitic absorption in the ultraviolet and visible range, improves the electron transport, and greatly increases the stability of the devices, retaining ∼80% of their initial efficiency after 1000 h of constant white light illumination.

Imaging photoinduced surface potentials on hybrid perovskites by real-time Scanning Electron Microscopy.

Micron (Oxford, England : 1993) 121 (2019) 53-65

G Irde, SM Pietralunga, V Sala, M Zani, JM Ball, AJ Barker, A Petrozza, G Lanzani, A Tagliaferri

We introduce laser-assisted Time-Resolved SEM (TR-SEM), joining Scanning Electron Microscopy and laser light excitation, to probe the long-term temporal evolution of optically excited charge distributions at the surface of Metal Ammonium Lead Triiodide (MAPbI3) hybrid perovskite thin films. Laser-assisted TR-SEM relies on the optically induced local modification of Secondary Electron (SE) detection yield to provide mapping of photoexcited potentials and charge dynamics at surfaces, and qualifies as a complementary approach to near-field probe microscopies and nonlinear photoemission spectroscopies for photovoltage measurements. Real-time imaging of evolving field patterns are provided on timescales compatible with SEM scanning rates, so that temporal resolution in the millisecond range can be ultimately envisaged. MAPbI3 is an outstanding light-sensitive material candidate for applications in solar light harvesting and photovoltaics, also appealing as an active system for light generation. In this work, the real time temporal evolution of optically induced SE contrast patterns in MAPbI3 is experimentally recorded, both under illumination by a 405 nm blue laser and after light removal, showing the occurrence of modifications related to photoinduced positive charge fields at surface. The long term evolution of these surface fields are tentatively attributed to ion migration within the film, under the action of the illumination gradient and the hole collecting substrate. This optical excitation is fully reversible in MAPbI3 over timescales of hours and a complete recovery of the system occurs within days. Permanent irradiation damage of the material is avoided by operating the SEM at 5 keV of energy and 1-10 pA of primary current. Optical excitation is provided by intense above-bandgap illumination (up to 50 W/cm2). TR-SEM patterns show a strong dependence on the geometry of SE collection. Measurements are taken at different axial orientations of the sample with respect to the entrance of the in-column detection system of the SEM and compared with numerical modeling of the SE detection process. This enables to single out the information regarding the local potential distribution. Results are interpreted by combining data about the spectral distribution of emitted SEs with the configuration of the electric and magnetic fields in the specimen chamber. The present modeling sets a robust basis for the understanding of photoinduced SE electron contrast.

Photocatalytic water splitting by N-TiO2 on MgO (111) with exceptional quantum efficiencies at elevated temperatures.

Nature communications 10 (2019) 4421-

Y Li, Y-K Peng, L Hu, J Zheng, D Prabhakaran, S Wu, TJ Puchtler, M Li, K-Y Wong, RA Taylor, SCE Tsang

Photocatalytic water splitting is attracting enormous interest for the storage of solar energy but no practical method has yet been identified. In the past decades, various systems have been developed but most of them suffer from low activities, a narrow range of absorption and poor quantum efficiencies (Q.E.) due to fast recombination of charge carriers. Here we report a dramatic suppression of electron-hole pair recombination on the surface of N-doped TiO2 based nanocatalysts under enhanced concentrations of H+ and OH-, and local electric field polarization of a MgO (111) support during photolysis of water at elevated temperatures. Thus, a broad optical absorption is seen, producing O2 and H2 in a 1:2 molar ratio with a H2 evolution rate of over 11,000 μmol g-1 h-1 without any sacrificial reagents at 270 °C. An exceptional range of Q.E. from 81.8% at 437 nm to 3.2% at 1000 nm is also reported.

Revealing the stoichiometric tolerance of lead tri-halide perovskite thin-films

Chemistry of Materials American Chemical Society (ACS) (2019) acs.chemmater.9b02639

AJ Ramadan, M Ralaiarisoa, F Zu, LA Rochford, B WENGER, N Koch, H SNAITH

Revealing the stoichiometric tolerance of lead tri-halide perovskite thin-films - Raw Data


A RAMADAN, M Ralaiarisoa, F Zu, L Rochford, BL WENGER, N Koch, HJ SNAITH

Raw data associated with publication

Impurity Tracking Enables Enhanced Control and Reproducibility of Hybrid Perovskite Vapor Deposition.

ACS applied materials & interfaces 11 (2019) 28851-28857

J Borchert, I Levchuk, LC Snoek, MU Rothmann, R Haver, HJ Snaith, CJ Brabec, LM Herz, MB Johnston

Metal halide perovskite semiconductors have the potential to enable low-cost, flexible, and efficient solar cells for a wide range of applications. Physical vapor deposition by co-evaporation of precursors is a method that results in very smooth and pinhole-free perovskite thin films and allows excellent control over film thickness and composition. However, for a deposition method to become industrially scalable, reproducible process control and high device yields are essential. Unfortunately, to date, the control and reproducibility of evaporating organic precursors such as methylammonium iodide (MAI) have proved extremely challenging. We show that the established method of controlling the evaporation rate of MAI with quartz microbalances (QMBs) is critically sensitive to the concentration of the impurities MAH2PO3 and MAH2PO2 that are usually present in MAI after synthesis. Therefore, controlling the deposition rate of MAI with QMBs is unreliable since the concentration of such impurities typically varies from one batch of MAI to another and even during the course of a deposition. However once reliable control of MAI deposition is achieved, we find that the presence of precursor impurities during perovskite deposition does not degrade the solar cell performance. Our results indicate that as long as precursor deposition rates are well controlled, physical vapor deposition will allow high solar cell device yields even if the purity of precursors changes from one run to another.

Charge-Carrier Dynamics, Mobilities, and Diffusion Lengths of 2D-3D Hybrid Butylammonium-Cesium-Formamidinium Lead Halide Perovskites


LRV Buizza, TW Crothers, Z Wang, JB Patel, RL Milot, HJ Snaith, MB Johnston, LM Herz

Oxidative Passivation of Metal Halide Perovskites

JOULE 3 (2019) 2716-2731

JSW Godding, AJ Ramadan, Y-H Lin, K Schutt, HJ Snaith, B Wenger

Planar perovskite solar cells with long-term stability using ionic liquid additives.

Nature 571 (2019) 245-250

S Bai, P Da, C Li, Z Wang, Z Yuan, F Fu, M Kawecki, X Liu, N Sakai, JT-W Wang, S Huettner, S Buecheler, M Fahlman, F Gao, HJ Snaith

Solar cells based on metal halide perovskites are one of the most promising photovoltaic technologies1-4. Over the past few years, the long-term operational stability of such devices has been greatly improved by tuning the composition of the perovskites5-9, optimizing the interfaces within the device structures10-13, and using new encapsulation techniques14,15. However, further improvements are required in order to deliver a longer-lasting technology. Ion migration in the perovskite active layer-especially under illumination and heat-is arguably the most difficult aspect to mitigate16-18. Here we incorporate ionic liquids into the perovskite film and thence into positive-intrinsic-negative photovoltaic devices, increasing the device efficiency and markedly improving the long-term device stability. Specifically, we observe a degradation in performance of only around five per cent for the most stable encapsulated device under continuous simulated full-spectrum sunlight for more than 1,800 hours at 70 to 75 degrees Celsius, and estimate that the time required for the device to drop to eighty per cent of its peak performance is about 5,200 hours. Our demonstration of long-term operational, stable solar cells under intense conditions is a key step towards a reliable perovskite photovoltaic technology.

Giant Fine Structure Splitting of the Bright Exciton in a Bulk MAPbBr3 Single Crystal.

Nano letters (2019)

M Baranowski, K Galkowski, A Surrente, JM Urban, Ł Klopotowski, S Mackowski, DK Maude, R Ben Aich, K Boujdaria, M Chamarro, C Testelin, P Nayak, M Dollmann, HJ Snaith, RJ Nicholas, P Plochocka

Exciton fine structure splitting in semiconductors reflects the underlying symmetry of the crystal and quantum confinement. Since the latter factor strongly enhances the exchange interaction, most work has focused on nanostructures. Here, we report on the first observation of the bright exciton fine structure splitting in a bulk semiconductor crystal, where the impact of quantum confinement can be specifically excluded, giving access to the intrinsic properties of the material. Detailed investigation of the exciton photoluminescence and reflection spectra of a bulk methylammonium lead tribromide single crystal reveals a zero magnetic field splitting as large as ~200μeV. This result provides an important starting point for the discussion of the origin of the large bright exciton fine structure observed in perovskite nanocrystals.

Dual-Source Coevaporation of Low-Bandgap FA(1-x)Cs(x)Sn(1-y)Pb(y)I(3) Perovskites for Photovoltaics

ACS ENERGY LETTERS 4 (2019) 2748-2756

JM Ball, L Buizza, HC Sansom, MD Farrar, MT Klug, J Borchert, J Patel, LM Herz, MB Johnston, HJ Snaith

Charge-Carrier Cooling and Polarization Memory Loss in Formamidinium Tin Triiodide.

The journal of physical chemistry letters 10 (2019) 6038-6047

KJ Savill, MT Klug, RL Milot, HJ Snaith, LM Herz

Reports of slow charge-carrier cooling in hybrid metal halide perovskites have prompted hopes of achieving higher photovoltaic cell voltages through hot-carrier extraction. However, observations of long-lived hot charge carriers even at low photoexcitation densities and an orders-of-magnitude spread in reported cooling times have been challenging to explain. Here we present ultrafast time-resolved photoluminescence measurements on formamidinum tin triiodide, showing fast initial cooling over tens of picoseconds and demonstrating that a perceived secondary regime of slower cooling instead derives from electronic relaxation, state-filling, and recombination in the presence of energetic disorder. We identify limitations of some widely used approaches to determine charge-carrier temperature and make use of an improved model which accounts for the full photoluminescence line shape. Further, we do not find any persistent polarization anisotropy in FASnI3 within 270 fs after excitation, indicating that excited carriers rapidly lose both polarization memory and excess energy through interactions with the perovskite lattice.

Growth modes and quantum confinement in ultrathin vapour-deposited MAPbI3 films.

Nanoscale 11 (2019) 14276-14284

ES Parrott, JB Patel, A-A Haghighirad, HJ Snaith, MB Johnston, LM Herz

Vapour deposition of metal halide perovskite by co-evaporation of precursors has the potential to achieve large-area high-efficiency solar cells on an industrial scale, yet little is known about the growth of metal halide perovskites by this method at the current time. Here, we report the fabrication of MAPbI3 films with average thicknesses from 2-320 nm by co-evaporation. We analyze the film properties using X-ray diffraction, optical absorption and photoluminescence (PL) to provide insights into the nucleation and growth of MAPbI3 films on quartz substrates. We find that the perovskite initially forms crystallite islands of around 8 nm in height, which may be the cause of the persistent small grain sizes reported for evaporated metal halide perovskites that hinder device efficiency and stability. As more material is added, islands coalesce until full coverage of the substrate is reached at around 10 nm average thickness. We also find that quantum confinement induces substantial shifts to the PL wavelength when the average thickness is below 40 nm, offering dual-source vapour deposition as an alternative method of fabricating nanoscale structures for LEDs and other devices.

Interfacial charge-transfer doping of metal halide perovskites for high performance photovoltaics


NK Noel, SN Habisreutinger, A Pellaroque, F Pulvirenti, B Wenger, F Zhang, Y-H Lin, OG Reid, J Leisen, Y Zhang, S Barlow, SR Marder, A Kahn, HJ Snaith, CB Arnold, BP Rand

Deciphering photocarrier dynamics for tuneable high-performance perovskite-organic semiconductor heterojunction phototransistors.

Nature communications 10 (2019) 4475-

Y-H Lin, W Huang, P Pattanasattayavong, J Lim, R Li, N Sakai, J Panidi, MJ Hong, C Ma, N Wei, N Wehbe, Z Fei, M Heeney, JG Labram, TD Anthopoulos, HJ Snaith

Looking beyond energy harvesting, metal-halide perovskites offer great opportunities to revolutionise large-area photodetection technologies due to their high absorption coefficients, long diffusion lengths, low trap densities and simple processability. However, successful extraction of photocarriers from perovskites and their conversion to electrical signals remain challenging due to the interdependency of photogain and dark current density. Here we report hybrid hetero-phototransistors by integrating perovskites with organic semiconductor transistor channels to form either "straddling-gap" type-I or "staggered-gap" type-II heterojunctions. Our results show that gradual transforming from type-II to type-I heterojunctions leads to increasing and tuneable photoresponsivity with high photogain. Importantly, with a preferential edge-on molecular orientation, the type-I heterostructure results in efficient photocarrier cycling through the channel. Additionally, we propose the use of a photo-inverter circuitry to assess the phototransistors' functionality and amplification. Our study provides important insights into photocarrier dynamics and can help realise advanced device designs with "on-demand" optoelectronic properties.