Interfacial Energy Level Alignment Research Articles

Room Temperature Fabrication of SnO2 Electrodes Enabling Barrier‐Free Electron Extraction for Efficient Flexible Perovskite Photovoltaics

AbstractRoom temperature‐processed electron transport layers (RT‐ETLs) demonstrate vast potential to be used in fabricating high‐performance flexible perovskite solar cells (PSCs) in an energy‐saving manner. However, the RT‐ETL normally suffers from inferior crystallinity, mismatched energy level, and high surface trap‐state density, which would result in under‐optimized interfacial electron extraction and undesirable interfacial charge recombination at ETL/perovskite interface, thus limiting the device performance. Herein, a novel strategy is demonstrated to prepare annealing‐free RT‐ETL based on precrystalline metal ion‐modified SnO2 nanocrystals, which perfectly optimizes the interfacial energy level alignment between ETL and perovskite layer, achieving nearly zero‐barrier charge transfer at the interface. As a result, the charge extraction has been remarkably accelerated and the interfacial charge recombination has been largely suppressed, leading to a ≈26% enhancement in device efficiency. The best‐performing flexible PSCs achieve efficiencies up to 19.3%, accompanied by outstanding mechanical strength under repeated bending cycle tests, which, to the best of the knowledge, is one of the highest reported values for the flexible perovskite photovoltaics fabricated with RT‐ETLs.

Complementary Triple-Ligand Engineering Approach to Methylamine Lead Bromide Nanocrystals for High-Performance Light-Emitting Diodes.

Conjugated and short-molecule capping ligands have been demonstrated as a valid strategy for achieving high-efficiency perovskite nanocrystal (NCs) light-emitting diodes (LEDs) owing to their advantage of allowing efficient carrier transport between NCs. However, monotonously utilizing conjugated ligands cannot achieve sufficient surface modification/passivation for perovskite NCs, leading to their poor photoluminescence quantum yield (PLQY) and dispersibility. This work designs a complementary ligand synthesis method to obtain high-quality methylamine lead bromide (MAPbBr3) NCs and then leverage them into efficient LEDs. The complementary ligand system combines a conjugated ligand 3-phenyl-2-propen-1-amine (PPA) and a long-chain ligand didodecyldimethylammonium bromide (DDAB) together with a well-known inductive inorganic ligand ZnBr2. With such complementary ligand engineering, we significantly improve the emissive features of MAPbBr3 NCs (PLQY: 99% ± 0.7%). Simultaneously, the complementary ligand strategy facilitated the adequate charge transportation in related NCs films and modified the interfacial energy-level alignment when the NCs assemble as an emitting layer into LEDs. Finally, based on this NCs synthesis method, high-efficiency green LEDs were achieved, exhibiting the maximum luminance of 1.59 × 104 cd m-2, a current efficiency of 23.7 cd A-1, and an external quantum efficiency of 7.8%. Our finding could provide a new avenue for further development of LEDs and their commercial application.

Illumination‐Driven Energy Level Realignment at Buried Interfaces between Organic Charge Transport Layers and a Lead Halide Perovskite

Tremendous progress in employing metal halide perovskites (MHPs) in a variety of applications, especially in photovoltaics, has been made in the past decade. To unlock the full potential of MHP materials in optoelectronic devices, an improved understanding of the electronic energy level alignment at perovskite‐based interfaces is required. This particularly pertains to such interfaces under device operation conditions, e.g. under illumination with visible light such as in a solar cell. Herein, it is revealed that the energy level alignment at the buried interface between a double cation lead halide perovskite film and charge‐selective organic transport layers changes upon white light illumination. This is found from photoemission experiments performed with the samples in dark and under illumination, and the interfacial energy level shift is reversible. The underlying mechanism is attributed to the accumulation of one charge carrier type within the perovskite film at the interface under illumination, as a result of the charge‐selective nature of the organic layer. The fact that the interfacial energy level alignment at MHP‐based junctions under illumination can differ from that in dark is to be taken into account to fully rationalize device characteristics.

Photoinduced Energy-Level Realignment at Interfaces between Organic Semiconductors and Metal-Halide Perovskites

In contrast to the common conception that the interfacial energy-level alignment is affixed once the interface is formed, we demonstrate that heterojunctions between organic semiconductors and metal-halide perovskites exhibit huge energy-level realignment during photoexcitation. Importantly, the photoinduced level shifts occur in the organic component, including the first molecular layer in direct contact with the perovskite. This is caused by charge-carrier accumulation within the organic semiconductor under illumination and the weak electronic coupling between the junction components.

Band Structure and Energy Level Alignment of Chiral Graphene Nanoribbons on Silver Surfaces.

Chiral graphene nanoribbons are extremely interesting structures due to their narrow band gaps and potential development of spin-polarized edge states. Here, we study their band structure on low work function silver surfaces. The use of a curved Ag single crystal provides, within the same sample, regions of disparate step structure and step density. Whereas the former leads to distinct azimuthal growth orientations of the graphene nanoribbons atop, the latter modulates the substrate’s work function and thereby the interface energy level alignment. In turn, we disclose the associated charge transfer from the substrate to the ribbon and assess its effect on the nanoribbon’s properties and the edge state magnetization.

Interfacial Passivation and Energy Level Alignment Regulation for Self‐Powered Perovskite Photodetectors with Enhanced Performance and Stability

AbstractInterfacial passivation and energy level alignment regulation at perovskite and charge transport layer interface are urgently desirable for constructing efficient and stable self‐powered perovskite photodetectors. Herein, a modified additive‐containing antisolvent strategy that not only facilitates high quality perovskite film formation but also modulates energy level alignment at interface is developed to improve the photoresponse and stability of perovskite photodetectors. Specifically, poly(3‐hexylthiophene) (P3HT) is introduced as functional additive into the antisolvent chlorobenzene. P3HT molecules penetrate along perovskite grain boundaries to fill the pinholes and passivate Pb2+ defects. Furthermore, the valence band maximum of the surface of the perovskite layer is modulated to form a favorable band energy alignment with the hole transport layer (HTL). The suppressed carrier recombination at the perovskite/HTL interface results in reduced noise current and improved photocurrent. Finally, the optimized device achieves a responsivity of 0.41 A W‐1, a detectivity of 0.61 × 1012 Jones at 700 nm, and a linear dynamic response range of 118 dB without external bias. In addition, the unpackaged device still maintains 78% of initial performance after 720 h under ambient conditions, indicating its superior stability compared with the original device.

Ti1–graphene single-atom material for improved energy level alignment in perovskite solar cells

Carbon-based perovskite solar cells (C-PSCs) are widely accepted as stable, cost-effective photovoltaics. However, C-PSCs have been suffering from relatively low power conversion efficiencies (PCEs) due to severe electrode-related energy loss. Herein, we report the application of a single-atom material (SAM) as the back electrode in C-PSCs. Our Ti1–rGO consists of single titanium (Ti) adatoms anchored on reduced graphene oxide (rGO) in a well-defined Ti1O4-OH configuration capable of tuning the electronic properties of rGO. The downshift of the Fermi level notably minimizes the series resistance of the carbon-based electrode. By combining with an advanced modular cell architecture, a steady-state PCE of up to 20.6% for C-PSCs is finally achieved. Furthermore, the devices without encapsulation retain 98% and 95% of their initial values for 1,300 h under 1 sun of illumination at 25°C and 60 °C, respectively. Carbon materials are promising for perovskite solar cells but suffer from poor interfacial energy level alignment. Now, Zhang et al. show that Ti atomically dispersed in reduced graphene reduces energy losses improving device performance.

D-π-D molecular layer electronically bridges the NiOx hole transport layer and the perovskite layer towards high performance photovoltaics

Nickel oxide (NiOx) has significant cost and stability advantages over poly[bis (4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) for inverted p-i-n perovskite solar cells (PSCs), but the poor NiOx/perovskite contact stemming from some reactive species at the interface led to suboptimal device performance. To solve this problem, we take a multiple donor molecule approach, using 3,3′-(4,8-bis(hexylthio)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(10-(6-bromohexyl)-10H-phenoxazine) (BDT-POZ) as an example, to modify the NiOx/perovskite interface. The primary goal was to reduce the under-coordinated Ni≥3+ cations via electron transfer from the donor molecules to NiOx, thus mitigating the detrimental reactions between perovskite and NiOx. Equally importantly, the hole extraction at the interface was greatly enhanced after the organic donor modification, since the hydrophobic species atop NiOx not only enabled pinhole-free crystallization of the perovskite but also properly tuned the interfacial energy level alignment. Consequently, the PSCs with NiOx/BDT-POZ HTL achieved a high power conversion efficiency (PCE) up to 20.16%, which compared excellently with that of the non-modified devices (17.83%). This work provides a new strategy to tackle the exacting issues that have so far impeded the development of NiOx based PSCs.

In Situ Interfacial Passivation of Sn-Based Perovskite Films with a Bi-functional Ionic Salt for Enhanced Photovoltaic Performance.

Environment-friendly Tin (Sn)-based perovskite solar cells (PSCs) have lately made significant development, showing tremendous promise in addressing the hazardous problems associated with Pb-based PSCs. However, even in N2 atmospheres, the thermodynamic stability of Sn-based perovskite films and long-term stability of Sn-based PSCs are demonstrated to be poor due to the presence of interfacial defect trap states. Here, we demonstrate the post-treatment of Sn-based perovskite films with ethylenediamine formate (EDAFa2) ion salt, serving as a bi-functional interface layer to in situ passivate the interfacial defect and improve the stability of Sn2+ by creating a thermodynamic chemical environment pathway. Moreover, the presence of EDAFa2 is shown to promote the interfacial energy level alignment, which is beneficial for the charge extraction at the interface. As a result, PSC devices with a bi-functional interface achieve a champion power conversion efficiency (PCE) as high as 9.40% and enhanced stability, retaining ∼95% of the original PCE stored in a N2 environment after ∼1960 h without encapsulation. This work highlights the significant role of an interfacial design in efficient and stable Sn-based PSCs.

Boosting performance and stability of inverted perovskite solar cells by modulating the cathode interface with phenyl phosphine-inlaid semiconducting polymer

Cathode interface modulation can improve the charge carrier management and inhibit the unwanted ion/molecular diffusion at the electrode/electron transport layer (ETL) interface, thus play a key role in the long-term operation of high-performance perovskite photovoltaics, but few studies have been focused on understanding the relationship among the molecular structure of cathode interlayer (CIL), the interfacial electronic properties as well as the passivation quality, and the ion/molecular diffusion within the cells. Herein, we report a semi-conducting phenyl phosphine inlaid polymer as a novel CIL between top Ag metal electrode and PCBM, which is essentially an interlayer, in an inverted perovskite solar cell. Even with a perovskite layer prepared in air, the phosphine-inlaid polymer improved power conversation efficiency (PCE) from 16.4% to 20.2%, with the device maintaining 85% of the original efficiency at T = 85 °C after 917 h operation and 80% after 1100 h. Light soaking stability testing showed that the device with PPDIBPP, 85% of the original efficiency could be retained after 560 h. We have proved that the main reason for the device stability enhancement was closely related to the introduction of phosphine in the interlayer, which, besides improving the interfacial energy level alignment and reducing the trap density, could anchor strongly to the Ag electrode as an effective diffusion barrier to Ag and I ions. To our knowledge, such a phenyl-phosphine based polymer is the first to be applied in perovskite solar cells with a simultaneous boost in device efficiency and thermal/lighting stability. • A judiciously designed phosphine-inlaid polymer that can coordinate with Ag electrode and interact with iodide. • The multi-functional polymer can reduce trap density and prevent ion diffusion. • PCEs up to 20.2% even with ambient fabrication, along with a T 80 lifetime > 1100 h under 85 °C aging conditions.

Obstructing interfacial reaction between NiOx and perovskite to enable efficient and stable inverted perovskite solar cells

In NiOx-based perovskite solar cells (PVSCs), the interfacial redox reaction between Ni3+ (on the surface of NiOx) and A-site cation salt (MAI in perovskite precursor solution) is invariably ignored. This adverse reaction will generate PbI2-rich hole extraction barriers at the NiOx-perovskite interface, which limits hole transmission and increases charge recombination, thus resulting in open-circuit voltage (Voc) loss. Furthermore, it will accelerate perovskite degradation by deprotonating the precursor amine and oxidizing iodide to interstitial iodine, which induces the severe instability of devices. Herein, a physical separation strategy by introducing a modifier layer to obstruct the detrimental reaction is explored. The results demonstrate that the trimethylolpropane tris(2-methyl-1-aziridinepropionate) (SaC-100) depositing onto NiOx can suppress the reaction between Ni3+ and MAI to endow the improvement of conductivity and reduction of interfacial defects, thus reducing Voc loss and enhancing device stability. Moreover, the interfacial energy level alignment and the morphology of perovskite are also optimized. As a result, the PVSCs device based on NiOx/SaC-100 presents the best power conversion efficiency (PCE) of 20.21% with a superior Voc value of 1.12 V. Furthermore, the device shows better light and thermal stability because of the hindering effect and defect passivation effect of SaC-100.

Interfacial Molecular Doping and Energy Level Alignment Regulation for Perovskite Solar Cells with Efficiency Exceeding 23%

Interfacial Molecular Doping and Energy Level Alignment Regulation for Perovskite Solar Cells with Efficiency Exceeding 23%

Robust Molecular Dipole‐Enabled Defect Passivation and Control of Energy‐Level Alignment for High‐Efficiency Perovskite Solar Cells

AbstractThe ability to passivate defects and modulate the interface energy‐level alignment (IEA) is key to boost the performance of perovskite solar cells (PSCs). Herein, we report a robust route that simultaneously allows defect passivation and reduced energy difference between perovskite and hole transport layer (HTL) via the judicious placement of polar chlorine‐terminated silane molecules at the interface. Density functional theory (DFT) points to effective passivation of the halide vacancies on perovskite surface by the silane chlorine atoms. An integrated experimental and DFT study demonstrates that the dipole layer formed by the silane molecules decreases the perovskite work function, imparting an Ohmic character to the perovskite/HTL contact. The corresponding PSCs manifest a nearly 20 % increase in power conversion efficiency over pristine devices and a markedly enhanced device stability. As such, the use of polar molecules to passivate defects and tailor the IEA in PSCs presents a promising platform to advance the performance of PSCs.

Robust Molecular Dipole-Enabled Defect Passivation and Control of Energy-Level Alignment for High-Efficiency Perovskite Solar Cells.

The ability to passivate defects and modulate the interface energy-level alignment (IEA) is key to boost the performance of perovskite solar cells (PSCs). Herein, we report a robust route that simultaneously allows defect passivation and reduced energy difference between perovskite and hole transport layer (HTL) via the judicious placement of polar chlorine-terminated silane molecules at the interface. Density functional theory (DFT) points to effective passivation of the halide vacancies on perovskite surface by the silane chlorine atoms. An integrated experimental and DFT study demonstrates that the dipole layer formed by the silane molecules decreases the perovskite work function, imparting an Ohmic character to the perovskite/HTL contact. The corresponding PSCs manifest a nearly 20 % increase in power conversion efficiency over pristine devices and a markedly enhanced device stability. As such, the use of polar molecules to passivate defects and tailor the IEA in PSCs presents a promising platform to advance the performance of PSCs.

Quasiparticle electronic structure of two-dimensional heterotriangulene-based covalent organic frameworks adsorbed on Au(111)

The modular nature and unique electronic properties of two-dimensional (2D) covalent organic frameworks (COFs) make them an attractive option for applications in catalysis, optoelectronics, and spintronics. The fabrications of such devices often involve interfaces formed between COFs and substrates. In this work, we employ the first-principles GW approach to accurately determine the quasiparticle electronic structure of three 2D carbonyl bridged heterotriangulene-based COFs featuring honeycomb–kagome lattice, with their properties ranging from a semi-metal to a wide-gap semiconductor. Moreover, we study the adsorption of these COFs on Au(111) surface and characterize the quasiparticle electronic structure at the heterogeneous COF/Au(111) interfaces. To reduce the computational cost, we apply the recently developed dielectric embedding GW approach and show that our results agree with existing experimental measurement on the interfacial energy level alignment. Our calculations illustrate how the many-body dielectric screening at the interface modulates the energies and shapes of the Dirac bands, the effective masses of semiconducting COFs, as well as the Fermi velocity of the semi-metallic COF.

Hot electron spectroscopy: A novel method to study molecular semiconductor

Molecular semiconductors, as the core component, determine the performance of different devices. It is essential to gain more knowledge of such materials. Hot electron spectroscopy, by applying a ballistic electron emission microscope and hot-electron transistor, has been demonstrated to be the most powerful method. This review concentrates on the achievements in several emerging fields, such as the interfacial energy level alignment and the charge transport characteristics in molecular film. As a versatile technique, hot electron spectroscopy has been found to be helpful in exploring the metal/molecule interface energy, the molecular levels, the electronic transport gap, and even the group structures. In comparison to the traditional techniques, such as ultraviolet photoemission spectroscopy, inverse photoemission spectroscopy, and scanning tunneling spectroscopy, this method offers in-situ characterizations, which makes the detection much more accurate and convenient. This review contributes to a deep comprehension of the intrinsic characteristics of molecular semiconductors and molecule-based devices. • Hot electron spectroscopy has recently been employed to determine the intrinsic lowest unoccupied molecular orbital. • Hot electron spectroscopy has been developed to determine the highest occupied molecular orbital. • Hot electron spectroscopy has applied to acquire more informations about molecular levels, such as the LUMO+1 level. • Hot electron spectroscopy provides a powerful approach to achieve a high-efficient spin injection.

Efficient p-n Heterojunction Perovskite Solar Cell without a Redundant Electron Transport Layer and Interface Engineering.

We report an electron transport layer-free perovskite solar cell (PSC) without interface engineering, whose key is a p-n heterojunction consisting of ITO/n-type FA0.9Cs0.1PbI3-xClx/p-type spiro-MeOTAD/Ag. The naturally matched energy levels between FA0.9Cs0.1PbI3-xClx and ITO make interface engineering unnecessary. The FA0.9Cs0.1PbI3-xClx film has a wide antisolvent processing window, favoring large-area production. The self-seeding growth method regulates the energy levels and work function of the FA0.9Cs0.1PbI3-xClx film via modulating the shape and distribution of residual PbI2. Interface band bending is confirmed by double-side photoluminescence (PL) measurement. Reduced PL is introduced for unambiguous charge transfer analysis without interference caused by different absorbance. Accordingly, enhanced ITO/perovskite interface energy level alignment and device performance (efficiency of 17.48% and Voc of 1.02 V) are demonstrated. Such simplified but efficient PSCs featuring a wide antisolvent processing window have great potential in practical applications.

Construction of an Iodine Diffusion Barrier Using Network Structure Silicone Resin for Stable Perovskite Solar Cells.

Long-term stability of organic-inorganic hybrid perovskite solar cells (PSCs) is inhibited by ion diffusion. Herein, we introduce a thermally stable and hydrophobic silicone resin layer with a network structure as an interfacial layer between the perovskite and the hole-transporting layer (HTL). Experimental and theoretical investigations confirm that the small Si-O-Si unit in the network forms both Si-I and Pb-O bonds with the perovskite surface, which physically and chemically inhibit the diffusion and self-release of iodine. Besides, the silicone resin layer suppresses the thermal crystallization of spiro-OMeTAD and optimizes the interfacial energy level alignment for hole extraction. The power conversion efficiency (PCE) of a perovskite solar cell with a silicone resin layer is improved to 21.11%. The device maintains more than 90.1% of its original PCE after 1200 h under 85 °C thermal stress, 99.6% after 2000 h under RH ∼55 ± 5%, and 83% of its original PCE after light soaking in air for 1037 h.

Surface Termination of Solution-Processed CH3NH3PbI3 Perovskite Film Examined using Electron Spectroscopies.

The interfaces of a perovskite solar cell significantly influence the charge processes in the cell, which contributes to the device performance with direct implication for surface potential, electronic structure, and chemical reactivity. The properties of the interface are strongly affected by the surface termination. In this work, the combination of ultraviolet photoelectron spectroscopy (UPS) and metastable-atom electron spectroscopy is demonstrated, to examine the surface termination of a solution-processed CH3NH3PbI3 perovskite film. The results show that the surface of the CH3NH3PbI3 perovskite film is terminated with a layer consisting of CH3NH3 and I. The interface energy level alignment for both occupied and unoccupied levels between CH3NH3PbI3 and C60 is also examined using UPS and low-energy inverse photoelectron spectroscopy. It turns out that an ideal energy level alignment is established for the electron collection and hole block at the perovskite and C60 interface.

Influence of the nature of the anchoring group on the interfacial energy level alignment in dye-sensitized solar cells: A theoretical perspective

We investigate at the theoretical (time-dependent) density functional theory level the influence of the nature of the anchoring group on the electronic and optical properties of oligothiophene dyes when adsorbed on ${\mathrm{Ti}\mathrm{O}}_{2}$ surface. The computed electronic structures point to a strong orbital hybridization between the dye and the substrate in the presence of the carboxylic acid and thiocarboxylic acid group, leading to a pronounced pinning effect of the lowest unoccupied molecular orbital (LUMO) level and faster electron injection. In contrast, phosphonic acid and catechol promote a weak electronic coupling between the two components and hence slower injection times. The simulated absorption spectra demonstrate that carboxylic and thiocarboxylic anchoring groups can induce a large redshift of the lowest optical transition of the dye upon adsorption due to a strong stabilization of the LUMO level triggered by the pinning effect while a small redshift prevails for phosphonic and catechol dyes. When pinning is active, the chain-size evolution of the lowest optical transition is also less sensitive to the conjugation length compared to the free dyes.