Hybrid Perovskites Workshop 2017 – Abstracts

Invited Speakers

Ni Zhao (The Chinese University of Hong Kong)

Towards Highly Efficient and Stable Wide-Bandgap Perovskite Solar Cells

Yang Zhou, Jie Cao, Feng Wang, Ching-Ping Wong, and Ni Zhao

Mixed iodide-bromide organolead perovskites with a bandgap of 1.70-1.80 eV have great potential to boost the efficiency of current silicon solar cells by forming a perovskite-silicon tandem structure. Yet, the stability of the perovskites under various application conditions, and in particular combined light and heat stress, hasn’t been well studied. Here we used FA0.15Cs0.85Pb(I0.73Br0.27)3, with an optical bandgap of ~1.72eV, as a model system to investigate the thermal-photostability of wide bandgap mixed halide perovskites. We found that the concerted effect of heat and light can induce both phase segregation and decomposition in a pristine perovskite film. On the other hand, through a post-deposition film treatment with benzylamine (BA) molecules, the highly defective regions (e.g. film surface and grain boundaries) of the film can be well passivated, thus preventing the progression of decomposition or phase segregation in the film. Besides the stability improvement, the BA-modified perovskite solar cells also exhibited excellent photovoltaic performance, with the champion device reaching a power conversion efficiency of 18.1%, a stabilized power output efficiency of 17.1% and a Voc of 1.24V. By combining a stable perovskite layer with all-inorganic charge-transporting layers, we have achieved excellent long-term ambient stability of perovskite solar cells. The devices exhibit no degradation in efficiency after being exposed to ambient air (humidity level: 50~60%) for 4 months without encapsulation.

 

 

Ferdinand Grozema (Delft University of Technology)

Abstract: to be added

 

 

Graeme Blake (University of Groningen)

Multifunctionality of hybrid perovskites

Although hybrid perovskites have mainly attracted attention for their optoelectronic properties in recent years, they have a longer history in other fields such as magnetism and ferroelectricity. New functionalities are also being discovered, most recently in the field of thermoelectric materials for waste heat recovery and solid state cooling. Here an overview will be given of the hybrid perovskite materials that have been studied in Groningen in recent years, with a focus on properties and applications other than photovoltaics. An outlook will then be given on possible future applications that exploit the multifunctionality of these versatile materials.

Bruno Ehrler (AMOLF)

The Sobering Reality of Perovskite/Si Tandem Solar Cells under Realistic Operating Conditions

Perovskite/Si tandem solar cells have the potential to considerably out-perform conventional solar cells. Under standard test conditions, perovskite/Si tandem solar cells already outperform the Si single junction. Under realistic conditions, however, as we show, those tandem solar cells are hardly more efficient than the Si cell alone. We model the performance of realistic perovskite/Si tandem solar cells under real-world climate conditions, by incorporating parasitic cell resistances, non-radiative recombination, and optical losses into the detailed-balance limit. We show quantitatively that optimizing these parameters in the perovskite top cell, perovskite/Si tandem solar cells reach an efficiency advantage of up to 14% absolute, even while leaving the Si cell untouched. Despite the rapid efficiency increase of perovskite solar cells, our results emphasize the need for further material development, careful device design, and light management strategies, all necessary for highly efficient perovskite/Si tandem solar cells.

 

Aditya D. Mohite (Los Alamos National Laboratory)

The emergence of layered 2D perovskites for stable and high-efficiency optoelectronic devices

Hybrid (inorganic-organic) perovskites have demonstrated an extraordinary potential for clean sustainable energy technologies and low-cost optoelectronic devices such as solar cells; light emitting diodes, detectors, sensors, ionic conductors etc. In spite of the unprecedented progress in the past six years, one of the key challenges that exists in the field today is the large degree of processing dependent variability in the structural and physical properties. This has limited the access to the intrinsic properties of hybrid perovskites and led to to multiple interpretations of experimental data. In addition to this, the stability and reliability of devices has also been strongly affected and remains an open question, which might determine the fate of this remarkable material despite excellent properties.

In this talk, I will describe our recent work on Ruddlesden-Popper halide perovskites as a potential alternative to the bulk hybrid perovskites. I will describe the versatility of this novel system through our efforts on achieving photovoltaic devices, photodetectors and light emitting diodes with technologically relevant stability. At the heart of these high performance devices lies an unusual photo-physical behavior where counterintuitive to classical quantum-confined systems where there exists an internal mechanism for the dissociation of excitons to edges of the perovskite layers. These states provide a direct pathway for dissociating excitons into longer-lived free-carriers, which remain well protected from non-radiative processes.

Aditya Mohite is the PI of the Light-to-Energy team and directs an energy and optoelectronic devices lab working on understanding and controlling charge and energy transfer processes occurring at interfaces created with organic and inorganic materials for thin-film clean energy technologies. His research philosophy is applying creative and “out-of-the-box” approaches to solve fundamental scientific bottlenecks and demonstrate technologically relevant performance in devices that is on par or exceeds the current state-of-the-art devices.

He has published more than 90 peer reviewed papers in journals such as Science, Nature, Nature Materials, Nature Nanotechnology, Nano Letters, ACS Nano, Chemical Society Reviews, Applied Physics Letters and Advanced Materials amongst others. He has also delivered more than 60 invited talks.

 

Contributed talks

Hong-Hua Fang (University of Groningen)

Extreme sensitivity of optoelectronic properties of methylammonium-lead tribromide single crystals to environmental gases

Solmaz Torabi (University of Groningen)

A Diagnostic Tool for the Dominant Recombination Channel in Perovskite Solar Cells

Identifying the dominant recombination processes in perovskite solar cells is an essential step towards improved device performance. Especially when it comes to the non-radiative recombination losses at the interfaces between the perovskite and electron- and hole transport layers.
We introduce an experimental method to detect the dominant recombination channel in the device under operating condition. The device is illuminated by either red, blue or white light of various light intensity. The ideality factor of the device in dark and under illumination by different light sources is determined. The obtained ideality factors are key values for disclosing the type of dominant recombination process and the more defective interface, for the case of dominant interface recombination. Our method not only serves as a diagnostic tool of recombination loss channels but also sheds light on the meaning of the ideality factor in perovskite solar cells.

Dibyashree Koushik (Eindhoven University of Technology)

Atomic Layer Interface Architecture for High-Efficiency Hysteresis-Less and 60 Days Humidity-Stable Planar Perovskite Solar Cells

Dibyashree Koushik1, Yinghuan Kuang1, Wiljan J. H. Verhees2, Sjoerd Veenstra2, Marcel A. Verheijen1, Wilhelmus M.M. Kessels1,2, Ruud E.I. Schropp1, and Mariadriana Creatore1,2

1 Department of Applied Physics, Eindhoven University of Technology (TU/e), P.O. Box 513, 5600 MB Eindhoven, The Netherlands
2 ECN-Solliance, High Tech Campus 21, 5656AE Eindhoven, The Netherlands

Perovskite solar cells (PSCs) have emerged as a promising candidate toward the next-generation photovoltaic (PV) technology, but are hampered by issues such as intrinsic material and device instability, hysteresis loss and poor device reliability, making its successful commercialization challenging. In this work, we present an atomic layer deposition (ALD) assisted interface engineering approach, which consists of employing ALD Al2O3 directly on top of the perovskite film [1-2]. It is observed that 10 cycles of ALD Al2O3 substantially protects the underlying sensitive perovskite against humidity, and also provides protection from other cell components during their respective depositions on top of the perovskite, thus preventing premature device failure. It aids in suppressing the charge recombination at the perovskite/ hole transport layer (HTL) interface, leading to an improvement in the open circuit voltage and fill factor of the fabricated devices. The champion ALD tuned PSC yields a power conversion efficiency (PCE) of 18% (with respect to 15% of the pristine), a significant reduction in the hysteresis loss, and stability (beyond 60 days) as a function of the unencapsulated storage time in ambient air, under humidity conditions ranging from 40 to 70% at room temperature. PCE measurements after 70 days of aging study show that the devices incorporating 10 cycles of ALD Al2O3 retain about 60-70% of the initial PCE, while the reference devices experience a drastic fall to 12% of the initial PCE [1]. Based on these advances, the study of the ALD Al2O3 interface engineering is extended toward implementing PEDOT as HTL in n-i-p PSCs, achieving a PCE of over 11%. The ALD Al2O3/PEDOT combination on top of the perovskite film allows for the replacement of the costly Spiro-OMeTAD HTL counterpart, in view toward obtaining future low-cost PV technology [2].

[1] D. Koushik, W. J. H. Verhees, Y. Kuang, S. Veenstra, D. Zhang, M. A. Verheijen, M. Creatore, R. E. I. Schropp, Energy & Environmental Science (2017), 10, 91-100.[2] D. Koushik, W. J. H. Verhees, D. Zhang, Y. Kuang, S. Veenstra, M. Creatore, R. E. I. Schropp, Advanced Materials Interfaces (2017), 1700043.
María C. Gélvez-Rueda (Delft University of Technology)

Interconversion between Free Charges and Bound Excitons in 2D Hybrid Lead Halide Perovskites.

The opto-electronic properties of hybrid perovskites can be easily tailored by varying their components. Specifically, mixing the common short organic cation (methyl ammonium (MA)) with a larger one (e.g. butyl ammonium (BA)) results in 2-dimensional perovskites with varying thicknesses of inorganic layers separated by the large organic cation. These materials, known as Ruddlesden-Popper phases, have proven to results in highly efficient, solution-processed and stable LEDs (EQE = 8.8%) and photovoltaic solar cells (PCE = 12.5%). In both of these applications a detailed understanding of the dissociation and recombination of electron-hole pairs is of prime importance. In this work we give a clear experimental demonstration of the interconversion between bound excitons and free charges as a function of temperature by combining microwave conductivity techniques with photoluminescence measurements. We demonstrate that the exciton binding energy varies strongly (between 80 meV and 370 meV) with the thickness of the inorganic layers. Additionally, we show that the mobility of charges increases with the layer thickness, in agreement with calculated effective masses from electronic structure calculations.

Machteld Kamminga (University of Groningen)

The Role of Connectivity on Electronic Properties of Lead Iodide Perovskite-Derived Compounds.

We use a layered solution crystal growth method to synthesize high quality single crystals of two different benzylammonium lead iodide perovskite-like organic/inorganic hybrids. The well-known (C6H5CH2NH3)2PbI4 phase is obtained in the form of bright orange platelets, with a structure comprised of single -terminated sheets of corner-sharing PbI6-octahedra separated by bilayers of the organic cations. The presence of water during synthesis leads to formation of a novel minority phase that crystallizes in the form of nearly transparent, light yellow bar-shaped crystals. This phase adopts the monoclinic space group P21/n and incorporates water molecules, with structural formula (C6H5CH2NH3)4Pb5I14·2H2O. The crystal structure consists of ribbons of edge-sharing PbI6-octahedra separated by the organic cations. Density functional theory calculations including spin-orbit coupling show that these edge-sharing PbI6-octahedra cause the band gap to increase with respect to corner-sharing PbI6-octahedra in (C6H5CH2NH3)2PbI4. To gain systematic insight, we model the effect of the connectivity of PbI6-octahedra on the band gap in idealized lead iodide perovskite-derived compounds. We find that increasing the connectivity from corner-, via edge- to face-sharing causes a significant increase in the band gap. This provides a new mechanism to tailor the optical properties in organic/inorganic hybrid compounds.

Submitted for publication.

Shuxia Tao (TU Eindhoven)

The extraordinary semiconductor properties of metal halide perovskites, including high carrier motilities, low carrier recombination rates, and the tunable spectral absorption range are attributed to the unique electronic properties of these materials. While DFT provides reliable structures and stabilities of perovskites, it performs poorly in electronic structure prediction. Relativistic GW approximation is demonstrated to be able to capture accurate electronic structure but with an extremely high computational cost. Here we report an efficient and accurate band structure calculations of halide metal perovskites by using an approximate quasiparticle method, i.e. DFT-1/2 method. Using AMX3 (A=CH3NH3, CH2NHCH2, Cs; M=Pb, Sn, X=I, Br, Cl) as demonstrations, the influence of the crystal structure (cubic, tetragonal and orthorhombic), variation of ions (different A, M and X) and relativistic effects on the electronic structure are systematically studied and compared with experimental results. Our results show that DFT-1/2 method yields accurate band gaps with a precision of GW method with no more computational cost than standard DFT. This opens the possibilities of accurate electronic structure prediction of sophisticated halide perovskite structures and new materials design for lead free materials.

Poster Presentations

Benjamin Daiber (AMOLF)

We investigate and compare the optoelectronic properties of FAPbI3, CsPbI3, and MAPbI3 under hydrostatic pressure to try to unravel the origin of the electric field. We use hydrostatic pressure to change the bandgap and (a) the alignment of the dipole of the cation (b) the tilting of the PbI3 cage.
We study photoluminescence, time resolved photoluminescence and absorption under pressure to track the changes in optoelectronic behavior of different lead-iodide perovskite derivatives. By correlating the results of lifetime and shape of the photoluminescence spectrum with their changes under pressure, we hope to be able to determine the influence of the cation on the Rashba splitting in lead-iodide perovskites.

 

Hong-Hua Fang (University of Groningen)

In this study, we comparatively investigate the optical properties of the nanocrystals of FAPbI3 and polycrystalline thin film by means of steady-state and time-resolved photoluminescence. A promisingly high quantum yield above 70 % was obtained, and a large absorption cross section (5.2×10-13 cm-2) was measured in these NCs. At high excitation powers, biexciton recombination is observed, featuring a slow recombination lifetime of 0.4 ns. While in the polycrystalline thin film, the quantum efficiency is much lower at low excitation level, which is limited by nonradiative trap-assisted recombination. We clearly observe phase transitions in both NCs and polycrystalline thin film at low temperature. Interestingly, NCs show PL temperature anti-quenching, in contrast to the strong PL quenching effect in the polycrystalline thin film. This is explained in terms of the thermal activation of trapped carriers at the nanocrystal’s surface, as opposed to the exciton thermal dissociation and trap-mediated recombination which occurs in thin films at higher temperatures.

 

Moritz Futscher (AMOLF)

Perovskite/Si tandem solar cells have the potential to considerably out-perform conventional solar cells. Under standard test conditions, perovskite/Si tandem solar cells already outperform the Si single junction. Under realistic conditions, however, as we show, those tandem solar cells are hardly more efficient than the Si cell alone. We model the performance of realistic perovskite/Si tandem solar cells under real-world climate conditions, by incorporating parasitic cell resistances, non-radiative recombination, and optical losses into the detailed-balance limit. We show quantitatively that optimizing these parameters in the perovskite top cell, perovskite/Si tandem solar cells reach an efficiency advantage of up to 14% absolute, even while leaving the Si cell untouched. Despite the rapid efficiency increase of perovskite solar cells, our results emphasize the need for further material development, careful device design, and light management strategies, all necessary for highly efficient perovskite/Si tandem solar cells.

 

Bart Groeneveld (University of Groningen)

Hybrid inorganic-organic perovskite solar cells have attracted a lot of attention for their strong absorption in the visible region of the solar spectrum and their good charge transport properties. This enables researchers to fabricate solar cells with high An alternative way of making perovskite solar cells is the p-i-n structure, where the order of the electron- and hole transport layer is reversed. By using this structure the hysteresis in the solar cells can be reduced significantly. However, devices with the p-i-n structure typically have a lower power conversion efficiency (PCE) than the n-i-p counterparts. This is caused by use of PEDOT:PSS as the hole transport layer, which leads to a lower open-circuit voltage (VOC). Moreover, the organic PEDOT:PSS layer is susceptible to degradation due to moisture, leading to a lower device stability. Power conversion efficiencies, with the record value of 22%. In most cases, the structure of the perovskite solar cells is the so-called n-i-p structure. Here, the photo-active perovskite layer (i) is sandwiched between an electron transport layer (n) on the bottom and a hole transport layer (p) on the top. There are some downsides to this structure; the most important one is the hysteresis which is often present. The use of inorganic hole transport layers in perovskite solar cells has the potential to improve the stability of the devices and to increase the PCE due to the higher VOC. Nickel oxide is a promising inorganic hole transport layer which has been used in perovskite solar cells before. Most of these devices were made with the fullerene derivative PCBM (phenyl-C61-butyric acid methyl ester) as the electron transport layer. Recent work by our group demonstrates that the efficiency of PEDOT:PSS-based perovskite solar cells increases when PCBM is replaced by PTEG-1, a fulleropyrrolidine with triethylene glycol monoethyl ether side chains [1]. Inspired by these results, we have incorporated PTEG-1 as  the electron transport layer in perovskite solar cells with NiOx as the hole transport layer. As a result, we were able to improve the power conversion efficiency of these cells compared to PCBM-based devices, with a maximum PCE of 16.1%.

 

Thomas Jansen (University of Groningen)

Organic Cation Rotation and Immobilization in Pure and Mixed Methylammonium Lead-Halide Perovskites.

Three-dimensional lead-halide perovskites have attracted a lot of attention due to their ability to combine solution processing with outstanding optoelectronic properties. Despite their soft ionic nature these materials demonstrate a surprisingly low level of electronic disorder resulting in sharp band edges and narrow distributions of the electronic energies. Understanding how structural and dynamic disorder impacts the optoelectronic properties of these perovskites is important for many applications. Here we combine ultrafast two-dimensional vibrational spectroscopy and molecular dynamics simulations to study the dynamics of the organic methylammonium (MA) cation orientation in a range of pure and mixed trihalide perovskite materials. For pure MAPbX3 (X = I, Br, Cl) perovskite films, we observe that the cation dynamics accelerate with decreasing size of the halide atom. This acceleration is surprising given the expected strengthening of the hydrogen bonds between the MA and the smaller halide anions, but can be explained by the increase in the polarizability with the size of halide. Much slower dynamics, up to partial immobilization of the organic cation, are observed in the mixed MAPb(ClxBr1−x)3 and MAPb(BrxI1−x)3 alloys, which we associate with symmetry breaking within the perovskite unit cell. The observed dynamics are essential for understanding the effects of structural and dynamical disorder in perovskite-based optoelectronic systems.

Sudeep Maheshwari (Delft University of Technology)

Two dimensional derivatives of hybrid halide perovskites are currently receiving increasing attention not only for their applications in photo-voltaics but also for their characteristic high photoluminescence and color tunability. The hydrophobic long chain cations act as an asset for their higher stability. The low dimensionality of the inorganic layer in 2D perovskites leads to quantum confinement effects and thus high exciton binding energy and thus lesser separation of charges. Until now, the cations in the 2D structure of perovskites have acted as mere spacers to separate the inorganic sheets of lead iodide. In this study we have designed novel and ultra-efficient 2D perovskites by substituting the long organic cations with highly electron deficient and electron rich cations targeting charge separation between inorganic sheets and organic cations. We have performed DFT calculations with lead-iodide inorganic layers coupled with electron-deficient naphthalene diimide dibutylammonium (NDI-dbu), perylene diimide dibutylammonium (PDI-dbu)), and benzothienobenzothiophene dibutylammonium (BTBT-dbu) cations. Through this study we demonstrate the effects of electron deficient linker groups in between the inorganic layers on the electronic structure of the 2D perovskites. Our study shows that the electron deficient organic cations have low-lying electronic levels and thus lead to small band gap whereas the electron rich cations with a higher bandgap reverses the electron and hole localization. The pi-conjugated core of the organic molecules dominates the energy states of the valence or conduction band depending on the cation being a donor of electrons or an acceptor of electrons. With this study we can efficiently tune the charge mobility and device highly functional and efficient 2D perovskites.

Shuyan Shao (University of Groningen)

Efficient Perovskite Solar Cells over a Broad Temperature Window: the Role of the Charge Carrier Extraction

Shuyan Shao, Jian Liu, Hong-Hua Fang, Li Qiu, Gert H. ten Brink, Jan C. Hummelen, L. Jan Anton Koster and Maria Antonietta Loi

We systematically investigated the mechanism behind the temperature dependence of the device performance in hybrid perovskite solar cells (HPSCs). The power conversion efficiency (PCE) of the reference cell using [60]PCBM as electron extraction layer (EEL) drops significantly from 11.9% at 295 K to 7% at 180 K. By investigating the temperature, light intensity and morphology dependence of the device performance, we find the deteriorated charge carrier extraction as the dominant factor causing this degradation. Temperature dependent spectroscopy and charge transport studies demonstrate that the poor electron transport in the [60]PCBM EEL at low temperature leads to inefficient charge carrier extraction. We further demonstrate that the n-type doping of [60]PCBM EEL or the use of an EEL (fulleropyrrolidine with a triethylene glycol monoethyl ether side chain, PTEG-1) with higher electron transport capability is an effective strategy to achieve HPSCs working efficiently over a broad temperature range. The devices fabricated with these highly performing EELs have PCEs at 180 K of 16.7% and 18.2%, respectively. These results support the idea that the temperature dependence of the electron transport in the EELs limits the device performance in HPSCs, especially at lower temperatures and they also give directions towards further improvement of the PCE of HPSCs at realistic operating temperatures.

Gilles de Wijs (Radboud University)

Symmetry, dynamics, and defects in methylammonium lead halide perovskites

Wouter M.J. Franssen, Gilles A. de Wijs, Arno P.M. Kentgens

In this contribution, we show a detailed investigation of this methylammonium lead iodide material using multi-nuclear solid state NMR and NQR. 14N NMR was used to elucidate the space group of the material, and the quadrupole coupling constant (CQ) was shown to be directly related to the unit cell deformation in the tetragonal phase, with no necessary change in the methylammonium dynamics, even at high temperatures (~50 °C). Variable temperature 207Pb NMR shows a strong increase in transversal relaxation time T2 at lower temperatures in the tetragonal phase that are attributed to low frequency motions in the PbI6 octahedra.

The appearance and annealing of defects is observable with variable temperature 14N NMR and 127I NQR, which has sparked an interest to try and identify the nature of these defects, and to quantify the diffusion of ions that contributes to the annealing.

Apart from experiments in the bulk, we are performing investigations of the composition of thin film samples in order to determine the concentration and nature of any impurities. These experiments show lines in the 13C spectrum not associated with the main perovskite phase, and have intensities of around 20%.

Sampson Adjokatse (University of Groningen)

Organic-inorganic hybrid perovskite solar cells have for the past five years attracted an unprecedented research attention due to their ever increasing record efficiency which now exceeds 22%. Apart from the excellent opto-electronic properties of these perovskite absorbers, their high performances are also dependent on the interfacial layers which act as charge-selective layers. Among the top-performing perovskite solar cells, the record leading devices are based on high temperature-sintered mesoporous TiO2 as the electron-selective layer in the n-i-p configuration. This architecture is however limited in large-scale manufacturing up-scale and also, incompatible with flexible substrates. Recently, Sargent and co-workers have demonstrated an efficient planar perovskite solar cells based on contact passivation of low-temperature solution-processed TiO2 nanocrystals (NCs) [1]. The devices exhibited performances comparable to those of high temperature-sintered TiO2-based devices. Following the technique reported by Sargent and coworkers, we have demonstrated that the concentration of the TiO2 NCs have a profound effect on all the device parameters. Our solar cells with optimal TiO2 NCs concentration exhibits power conversion efficiencies above 19.8%.[1] Tan et al., Science 355, 722–726 (2017).