Impact of Bi3+ Heterovalent Doping in Organic-Inorganic Metal Halide Perovskite Crystals.

Journal of the American Chemical Society 140 (2018) 574-577

PK Nayak, M Sendner, B Wenger, Z Wang, K Sharma, AJ Ramadan, R Lovrinčić, A Pucci, PK Madhu, HJ Snaith

Intrinsic organic-inorganic metal halide perovskites (OIHP) based semiconductors have shown wide applications in optoelectronic devices. There have been several attempts to incorporate heterovalent metal (e.g., Bi3+) ions in the perovskites in an attempt to induce electronic doping and increase the charge carrier density in the semiconductor. It has been reported that inclusion of Bi3+ decreases the band gap of the material considerably. However, contrary to the earlier conclusions, despite a clear change in the appearance of the crystal as observed by eye, here we show that the band gap of MAPbBr3 crystals does not change due the presence of Bi3+ in the growth solution. An increased density of states in the band gap and use of very thick samples for transmission measurements, erroneously give the impression of a band gap shift. These sub band gap states also act as nonradiative recombination centers in the crystals.

Consolidation of the optoelectronic properties of CH3NH3PbBr3 perovskite single crystals.

Nature communications 8 (2017) 590-

B Wenger, PK Nayak, X Wen, SV Kesava, NK Noel, HJ Snaith

Ultralow trap densities, exceptional optical and electronic properties have been reported for lead halide perovskites single crystals; however, ambiguities in basic properties, such as the band gap, and the electronic defect densities in the bulk and at the surface prevail. Here, we synthesize single crystals of methylammonium lead bromide (CH3NH3PbBr3), characterise the optical absorption and photoluminescence and show that the optical properties of single crystals are almost identical to those of polycrystalline thin films. We observe significantly longer lifetimes and show that carrier diffusion plays a substantial role in the photoluminescence decay. Contrary to many reports, we determine that the trap density in CH3NH3PbBr3 perovskite single crystals is 1015 cm-3, only one order of magnitude lower than in the thin films. Our enhanced understanding of optical properties and recombination processes elucidates ambiguities in earlier reports, and highlights the discrepancies in the estimation of trap densities from electronic and optical methods.Metal halide perovskites for optoelectronic devices have been extensively studied in two forms: single-crystals or polycrystalline thin films. Using spectroscopic approaches, Wenger et al. show that polycrystalline thin films possess similar optoelectronic properties to single crystals.

Controlling Nucleation and Growth of Metal Halide Perovskite Thin Films for High-Efficiency Perovskite Solar Cells.

Small (Weinheim an der Bergstrasse, Germany) 13 (2017)

N Sakai, Z Wang, VM Burlakov, J Lim, D McMeekin, S Pathak, HJ Snaith

Metal halide perovskite thin films can be crystallized via a broad range of solution-based routes. However, the quality of the final films is strongly dependent upon small changes in solution composition and processing parameters. Here, this study demonstrates that a fractional substitution of PbCl2 with PbI2 in the 3CH3 NH3 I:PbCl2 mixed-halide starting solution has a profound influence upon the ensuing thin-film crystallization. The presence of PbI2 in the precursor induces a uniform distribution of regular quadrilateral-shaped CH3 NH3 PbI3 perovskite crystals in as-cast films, which subsequently grow to form pinhole-free perovskite films with highly crystalline domains. With this new formulation of 3CH3 NH3 I:0.98PbCl2 :0.02PbI2 , this study achieves a 19.1% current-voltage measured power conversion efficiency and a 17.2% stabilized power output in regular planar heterojunction solar cells.

A generic interface to reduce the efficiency-stability-cost gap of perovskite solar cells.

Science (New York, N.Y.) 358 (2017) 1192-1197

Y Hou, X Du, S Scheiner, DP McMeekin, Z Wang, N Li, MS Killian, H Chen, M Richter, I Levchuk, N Schrenker, E Spiecker, T Stubhan, NA Luechinger, A Hirsch, P Schmuki, H-P Steinrück, RH Fink, M Halik, HJ Snaith, CJ Brabec

A major bottleneck delaying the further commercialization of thin-film solar cells based on hybrid organohalide lead perovskites is interface loss in state-of-the-art devices. We present a generic interface architecture that combines solution-processed, reliable, and cost-efficient hole-transporting materials without compromising efficiency, stability, or scalability of perovskite solar cells. Tantalum-doped tungsten oxide (Ta-WO x )/conjugated polymer multilayers offer a surprisingly small interface barrier and form quasi-ohmic contacts universally with various scalable conjugated polymers. In a simple device with regular planar architecture and a self-assembled monolayer, Ta-WO x -doped interface-based perovskite solar cells achieve maximum efficiencies of 21.2% and offer more than 1000 hours of light stability. By eliminating additional ionic dopants, these findings open up the entire class of organics as scalable hole-transporting materials for perovskite solar cells.

Cs2InAgCl6: A New Lead-Free Halide Double Perovskite with Direct Band Gap.

The journal of physical chemistry letters 8 (2017) 772-778

G Volonakis, AA Haghighirad, RL Milot, WH Sio, MR Filip, B Wenger, MB Johnston, LM Herz, HJ Snaith, F Giustino

A2BB'X6 halide double perovskites based on bismuth and silver have recently been proposed as potential environmentally friendly alternatives to lead-based hybrid halide perovskites. In particular, Cs2BiAgX6 (X = Cl, Br) have been synthesized and found to exhibit band gaps in the visible range. However, the band gaps of these compounds are indirect, which is not ideal for applications in thin film photovoltaics. Here, we propose a new class of halide double perovskites, where the B3+ and B+ cations are In3+ and Ag+, respectively. Our first-principles calculations indicate that the hypothetical compounds Cs2InAgX6 (X = Cl, Br, I) should exhibit direct band gaps between the visible (I) and the ultraviolet (Cl). Based on these predictions, we attempt to synthesize Cs2InAgCl6 and Cs2InAgBr6, and we succeed to form the hitherto unknown double perovskite Cs2InAgCl6. X-ray diffraction yields a double perovskite structure with space group Fm3̅m. The measured band gap is 3.3 eV, and the compound is found to be photosensitive and turns reversibly from white to orange under ultraviolet illumination. We also perform an empirical analysis of the stability of Cs2InAgX6 and their mixed halides based on Goldschmidt's rules, and we find that it should also be possible to form Cs2InAg(Cl1-xBrx)6 for x < 1. The synthesis of mixed halides will open the way to the development of lead-free double perovskites with direct and tunable band gaps.

Crystallization Kinetics and Morphology Control of Formamidinium-Cesium Mixed-Cation Lead Mixed-Halide Perovskite via Tunability of the Colloidal Precursor Solution.

Advanced materials (Deerfield Beach, Fla.) 29 (2017)

DP McMeekin, Z Wang, W Rehman, F Pulvirenti, JB Patel, NK Noel, MB Johnston, SR Marder, LM Herz, HJ Snaith

The meteoric rise of the field of perovskite solar cells has been fueled by the ease with which a wide range of high-quality materials can be fabricated via simple solution processing methods. However, to date, little effort has been devoted to understanding the precursor solutions, and the role of additives such as hydrohalic acids upon film crystallization and final optoelectronic quality. Here, a direct link between the colloids concentration present in the [HC(NH2 )2 ]0.83 Cs0.17 Pb(Br0.2 I0.8 )3 precursor solution and the nucleation and growth stages of the thin film formation is established. Using dynamic light scattering analysis, the dissolution of colloids over a time span triggered by the addition of hydrohalic acids is monitored. These colloids appear to provide nucleation sites for the perovskite crystallization, which critically impacts morphology, crystal quality, and optoelectronic properties. Via 2D X-ray diffraction, highly ordered and textured crystals for films prepared from solutions with lower colloidal concentrations are observed. This increase in material quality allows for a reduction in microstrain along with a twofold increase in charge-carrier mobilities leading to values exceeding 20 cm2 V-1 s-1 . Using a solution with an optimized colloidal concentration, devices that reach current-voltage measured power conversion efficiency of 18.8% and stabilized efficiency of 17.9% are fabricated.

Monolithic Wide Band Gap Perovskite/Perovskite Tandem Solar Cells with Organic Recombination Layers

JOURNAL OF PHYSICAL CHEMISTRY C 121 (2017) 27256-27262

R Sheng, MT Horantner, Z Wang, Y Jiang, W Zhang, A Agosti, S Huang, X Hao, A Ho-Baillie, M Green, HJ Snaith

Reproducible Planar Heterojunction Solar Cells Based on One-Step Solution-Processed Methylammonium Lead Halide Perovskites

CHEMISTRY OF MATERIALS 29 (2017) 462-473

S Bai, N Sakai, W Zhang, Z Wang, JT-W Wang, F Gao, HJ Snaith

V-Shaped Hole-Transporting TPD Dimers Containing Troger's Base Core

JOURNAL OF PHYSICAL CHEMISTRY C 121 (2017) 10267-10274

T Braukyla, N Sakai, M Daskeviciene, V Jankauskas, E Kamarauskas, R Komskis, T Malinauskas, S Jursenas, HJ Snaith, V Getautis

V-shaped hole transporting materials based on N,N,N′,N′-tetraarylbenzidine (TPD)-type moieties conjoined by Tröger’s base core were synthesized and investigated. These hole transporting materials were obtained by a three-step synthetic method, are fully amorphous, and demonstrate high glass transition temperatures and good thermal and morphological stability. Relatively high charge mobility (up to 0.036 cm2 V –1 s–1) was measured in these hole transporting materials, exceeding that of corresponding methyl and methoxy substituted TPD analogues without TB core by more than 2 orders of magnitude. Determined ionization potential and charge mobility values permit use of the synthesized compounds as hole transporting materials in fabrication of perovskite solar cells.

Efficient ambient-air-stable solar cells with 2D-3D heterostructured butylammonium-caesium-formamidinium lead halide perovskites

Nature Energy 2 (2017)

Z Wang, Q Lin, FP Chmiel, N Sakai, LM Herz, HJ Snaith

© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. Perovskite solar cells are remarkably efficient; however, they are prone to degradation in water, oxygen and ultraviolet light. Cation engineering in 3D perovskite absorbers has led to reduced degradation. Alternatively, 2D Ruddlesden-Popper layered perovskites exhibit improved stability, but have not delivered efficient solar cells so far. Here, we introduce n-butylammonium cations into a mixed-cation lead mixed-halide FA 0.83 Cs 0.17 Pb(I y Br 1-y ) 3 3D perovskite. We observe the formation of 2D perovskite platelets, interspersed between highly orientated 3D perovskite grains, which suppress non-radiative charge recombination. We investigate the relationship between thin-film composition, crystal alignment and device performance. Solar cells with an optimal butylammonium content exhibit average stabilized power conversion efficiency of 17.5 ± 1.3% with a 1.61-eV-bandgap perovskite and 15.8 ± 0.8% with a 1.72-eV-bandgap perovskite. The stability under simulated sunlight is also enhanced. Cells sustain 80% of their post burn-in' efficiency after 1,000 h in air, and close to 4,000 h when encapsulated.

Impact of the Halide Cage on the Electronic Properties of Fully Inorganic Cesium Lead Halide Perovskites

ACS ENERGY LETTERS 2 (2017) 1621-1627

Z Yang, A Surrente, K Galkowski, A Miyata, O Portugall, RJ Sutton, AA Haghighirad, HJ Snaith, DK Maude, P Plochocka, RJ Nicholas

Optoelectronic and spectroscopic characterization of vapour-transport grown Cu2ZnSnS4 single crystals


TM Ng, MT Weller, GP Kissling, LM Peter, P Dale, F Babbe, J de Wild, B Wenger, HJ Snaith, D Lane

Solution-Processed Cesium Hexabromopalladate(IV), Cs2PdBr6, for Optoelectronic Applications.

Journal of the American Chemical Society 139 (2017) 6030-6033

N Sakai, AA Haghighirad, MR Filip, PK Nayak, S Nayak, A Ramadan, Z Wang, F Giustino, HJ Snaith

Lead halide perovskites are materials with excellent optoelectronic and photovoltaic properties. However, some hurdles remain prior to commercialization of these materials, such as chemical stability, phase stability, sensitivity to moisture, and potential issues due to the toxicity of lead. Here, we report a new type of lead-free perovskite related compound, Cs2PdBr6. This compound is solution processable, exhibits long-lived photoluminescence, and an optical band gap of 1.6 eV. Density functional theory calculations indicate that this compound has dispersive electronic bands, with electron and hole effective masses of 0.53 and 0.85 me, respectively. In addition, Cs2PdBr6 is resistant to water, in contrast to lead-halide perovskites, indicating excellent prospects for long-term stability. These combined properties demonstrate that Cs2PdBr6 is a promising novel compound for optoelectronic applications.

Vapour-Deposited Cesium Lead Iodide Perovskites: Microsecond Charge Carrier Lifetimes and Enhanced Photovoltaic Performance.

ACS energy letters 2 (2017) 1901-1908

EM Hutter, RJ Sutton, S Chandrashekar, M Abdi-Jalebi, SD Stranks, HJ Snaith, TJ Savenije

Metal halide perovskites such as methylammonium lead iodide (MAPbI3) are highly promising materials for photovoltaics. However, the relationship between the organic nature of the cation and the optoelectronic quality remains debated. In this work, we investigate the optoelectronic properties of fully inorganic vapour-deposited and spin-coated black-phase CsPbI3 thin films. Using the time-resolved microwave conductivity technique, we measure charge carrier mobilities up to 25 cm2/(V s) and impressively long charge carrier lifetimes exceeding 10 μs for vapour-deposited CsPbI3, while the carrier lifetime reaches less than 0.2 μs in the spin-coated samples. Finally, we show that these improved lifetimes result in enhanced device performance with power conversion efficiencies close to 9%. Altogether, these results suggest that the charge carrier mobility and recombination lifetime are mainly dictated by the inorganic framework rather than the organic nature of the cation.

How to Avoid Artifacts in Surface Photovoltage Measurements: A Case Study with Halide Perovskites.

The journal of physical chemistry letters 8 (2017) 2941-2943

I Levine, G Hodes, HJ Snaith, PK Nayak

Dopant-Free Planar n-i-p Perovskite Solar Cells with Steady-State Efficiencies Exceeding 18%

ACS ENERGY LETTERS 2 (2017) 622-628

SN Habisreutinger, B Wenger, HJ Snaith, RJ Nicholas

A low viscosity, low boiling point, clean solvent system for the rapid crystallisation of highly specular perovskite films


NK Noel, SN Habisreutinger, B Wenger, MT Klug, MT Horantner, MB Johnston, RJ Nicholas, DT Moore, HJ Snaith

Near-Infrared and Short-Wavelength Infrared Photodiodes Based on Dye-Perovskite Composites


Q Lin, Z Wang, M Young, JB Patel, RL Milot, LM Maestro, RR Lunt, HJ Snaith, MB Johnston, LM Herz

Processing Solvent-Dependent Electronic and Structural Properties of Cesium Lead Triiodide Thin Films.

The journal of Physical Chemistry Letters (2017) 4172-4176

AJ Ramadan, LA Rochford, S Fearn, HJ Snaith

Cesium lead triiodide (CsPbI3) is an attractive material for photovoltaic applications due to its appropriate band gap, strong optical absorption, and high thermal stability. However, the perovskite phase suffers from moisture induced structural instability. Previous studies have utilized a range of solvent systems to establish the role of solvent choice in structural instabilities. Despite this, effects of different solvents on the electronic structure of this material have not been compared. We report substantial chemical and compositional differences in thin films of CsPbI3 prepared from a range of solvent systems. We confirm via X-ray diffraction thin films formed from DMF, DMSO, and a mixture of these solvent systems share the same crystal structure. However, secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and low energy ion scattering measurements reveal significant differences between films processed via different solvent systems. Our findings reveal the critical impact solvents have upon compositional stoichiometry and thin-film morphology.

Electron injection and scaffold effects in perovskite solar cells.

Journal of materials chemistry. C 5 (2017) 634-644

M Anaya, W Zhang, BC Hames, Y Li, F Fabregat-Santiago, ME Calvo, HJ Snaith, H Míguez, I Mora-Seró

In spite of the impressive efficiencies reported for perovskite solar cells (PSCs), key aspects of their working principles, such as electron injection at the contacts or the suitability of the utilization of a specific scaffold layer, are not yet fully understood. Increasingly complex scaffolds attained by the sequential deposition of TiO2 and SiO2 mesoporous layers onto transparent conducting substrates are used to perform a systematic characterization of both the injection process at the electron selective contact and the scaffold effect in PSCs. By forcing multiple electron injection processes at a controlled sequence of perovskite-TiO2 interfaces before extraction, interfacial injection effects are magnified and hence characterized in detail. An anomalous injection behavior is observed, the fingerprint of which is the presence of significant inductive loops in the impedance spectra with a magnitude that correlates with the number of interfaces in the scaffold. Analysis of the resistive and capacitive behavior of the impedance spectra indicates that the scaffolds could hinder ion migration, with positive consequences such as lowering the recombination rate and implications for the current-potential curve hysteresis. Our results suggest that an appropriate balance between these advantageous effects and the unavoidable charge transport resistive losses introduced by the scaffolds will help in the optimization of PSC performance.