Unraveling the Electronic Properties of Lead Halide Perovskites with Surface Photovoltage in Photoemission Studies

Toward Understanding the Built-in Field in Perovskite Solar Cells through Layer-by-Layer Surface Photovoltage Measurements

The built-in voltage (VBI) is a key parameter for solar cell operation, yet in perovskite solar cells the distribution, magnitude, and origin of the VBI remains poorly understood. In this work, we systematically studied the VBI in pin-type perovskite solar cells based on different hole transport layers (TLs). To this end, we determine the surface photovoltage (SPV) of partial and complete device stacks layer-by-layer by measuring the work function (WF) under dark and light (equivalent AM1.5G) conditions with Kelvin probe (KP) and photoemission spectroscopy (UPS) measurements in 3 different laboratories. We demonstrate that the SPV increases upon the addition of each additional layer until it equals the open-circuit voltage (VOC) of the full device. This suggests that both the electron and hole transport layer (HTL/ETL) enlarge the SPV, by improving the separation of photogenerated carriers. Yet, the contribution of both transport layers to the total SPV of the device is small (in the range of ≈100 to 200 meV) and the largest contribution to the SPV originates from the top metal electrode (≈500 meV). The results suggest that the VBI of pin-type perovskite solar cells is largely a result of the work-function difference of the electrodes. With regard to films (or incomplete cell stacks), our simulations can reproduce the measured SPV, and measured quasi-Fermi level splitting (>VOC) in partial cell stacks without a significant internal field consistent with the experimental data. This work establishes layer-by-layer SPV measurements, which are easily accessible, as a key tool for understanding device performance and internal energetics, similar to layer-by-layer QFLS measurements.

A Universal Perovskite/C60 Interface Modification via Atomic Layer Deposited Aluminum Oxide for Perovskite Solar Cells and Perovskite-Silicon Tandems

The primary performance limitation in inverted perovskite-based solar cells is the interface between the fullerene-based electron transport layers and the perovskite. Atomic layer deposited thin aluminum oxide (AlOX) interlayers that reduce nonradiative recombination at the perovskite/C60 interface are developed, resulting in >60 millivolts improvement in open-circuit voltage and 1% absolute improvement in power conversion efficiency. Surface-sensitive characterizations indicate the presence of a thin, conformally deposited AlOx layer, functioning as a passivating contact. These interlayers work universally using different lead-halide-based absorbers with different compositions where the 1.55 electron volts bandgap single junction devices reach >23% power conversion efficiency. A reduction of metallic Pb0 is found and the compact layer prevents in- and egress of volatile species, synergistically improving the stability. AlOX-modified wide-bandgap perovskite absorbers as a top cell in a monolithic perovskite-silicon tandem enable a certified power conversion efficiency of 29.9% and open-circuit voltages above 1.92 volts for 1.17 square centimeters device area.

Electrochemical Doping of Halide Perovskites by Noble Metal Interstitial Cations

Metal halide perovskites are an attractive class of semiconductors, but it has proven difficult to control their electronic doping by conventional strategies due to screening and compensation by mobile ions or ionic defects. Noble-metal interstitials represent an under-studied class of extrinsic defects that plausibly influence many perovskite-based devices. In this work, doping of metal halide perovskites is studied by electrochemically formed Au⁺ interstitial ions, combining experimental data on devices with a computational analysis of Au⁺ interstitial defects based on density functional theory (DFT). Analysis suggests that Au⁺ cations can be easily formed and migrate through the perovskite bulk via the same sites as iodine interstitials (I_i ⁺ ). However, whereas I_i ⁺ compensates n-type doping by electron capture, the noble-metal interstitials act as quasi-stable n-dopants. Experimentally, voltage-dependent, dynamic doping by current density-time (J-t), electrochemical impedance, and photoluminescence measurements are characterized. These results provide deeper insight into the potential beneficial and detrimental impacts of metal electrode reactions on long-term performance of perovskite photovoltaic and light-emitting diodes, as well as offer an alternative doping explanation for the valence switching mechanism of halide-perovskite-based neuromorphic and memristive devices.

Surface Photovoltage Study of Metal Halide Perovskites Deposited Directly on Crystalline Silicon

Perovskite (PVK) films deposited directly on n-type crystalline Si substrates were investigated by two operating modes of the surface photovoltage (SPV) method: (i) the metal-insulator-semiconductor (MIS) mode and (ii) the Kelvin probe force microscopy (KPFM). By scanning from 900 to 600 nm in the MIS mode, we consecutively studied the relatively fast processes of carrier generation, transport, and recombination first in Si, then on both sides of the PVK/Si interface, and finally in the PVK layer and its surface. The PVK optical absorption edge was observed in the range of 1.61-1.65 eV in good agreement with the band gap of 1.63 eV found from photoluminescence spectra. Both SPV methods evidenced an upward energy band bending at the PVK/n-Si interface generating positive SPV. Drift-diffusion modeling allowed us to analyze the shape of the wavelength dependence of the SPV. It was also observed that the intense illumination in the KPFM measurements induces slow SPV transients which were explained by the creation and migration of negative ions and their trapping at the PVK surface. Finally, aging effects were studied by measuring again SPV spectra after one-year storage in air, and an increase in the concentration of shallow defect states at the PVK/n-Si interface was found.

Quantum Efficiency Enhancement of Lead-Halide Perovskite Nanocrystal LEDs by Organic Lithium Salt Treatment

Surface-defect passivation is key to achieving a high photoluminescence quantum yield in lead halide perovskite nanocrystals. However, in perovskite light-emitting diodes, these surface ligands also have to enable balanced charge injection into the nanocrystals to yield high efficiency and operational lifetime. In this respect, alkaline halides have been reported to passivate surface trap states and increase the overall stability of perovskite light emitters. On the one side, the incorporation of alkaline ions into the lead halide perovskite crystal structure is considered to counterbalance cation vacancies, whereas on the other side, the excess halides are believed to stabilize the colloids. Here, we report an organic lithium salt, viz. LiTFSI, as a halide-free surface passivation on perovskite nanocrystals. We show that treatment with LiTFSI has multiple beneficial effects on lead halide perovskite nanocrystals and LEDs derived from them. We obtain a higher photoluminescence quantum yield and a longer exciton lifetime and a radiation pattern that is more favorable for light outcoupling. The ligand-induced dipoles on the nanocrystal surface shift their energy levels toward a lower hole-injection barrier. Overall, these effects add up to a 4- to 7-fold boost of the external quantum efficiency in proof-of-concept LED structures, depending on the color of the used lead halide perovskite nanocrystal emitters.

Electronic properties of metal halide perovskites and their interfaces: the basics

We have witnessed tremendous progress of metal halide perovskite (MHP)-based optoelectronic devices, especially in the field of photovoltaics. Despite intensive research in the past few years, questions still remain regarding their fundamental optoelectronic properties, among which the electronic properties exhibit an interplay of numerous phenomena that deserve serious scrutiny. In this Focus article, we aim to provide a contemporary understanding of the unique electronic properties that has been resolved by the community. First introducing some of the basic concepts established in semiconductor physics, the intrinsic and extrinsic electronic properties of the MHPs are disentangled and explained. With this, the complex interplay of interface-, dopant-, and surface state-induced electronic states in determining the electrostatic landscape in the material can be comprehended, and the energy level alignment in device architectures more reliably assessed.

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.

Revisiting the Determination of the Valence Band Maximum and Defect Formation in Halide Perovskites for Solar Cells: Insights from Highly Sensitive Near-UV Photoemission Spectroscopy

Using advanced near-UV photoemission spectroscopy (PES) in constant final state mode (CFSYS) with a very high dynamic range, we investigate the triple-cation lead halide perovskite Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃ and gain detailed insights into the density of occupied states (DOS) in the valence band and band gap. A valence band model is established which includes the parabolic valence band edge and an exponentially decaying band tail in a single equation. This allows us to precisely determine two valence band maxima (VBM) at different k-vectors in the angle-integrated spectra, where the highest one, resulting from the VBM at the R-point in the Brillouin zone, is found between -1.50 to -1.37 eV relative to the Fermi energy E_F. We investigate quantitatively the formation of defect states in the band gap up to E_F upon decomposition of the perovskites during sample transfer, storage, and measurements: during near-UV-based PES, the density of defect states saturates at a value that is around 4 orders of magnitude below the density of states at the valence band edge. However, even short air exposure, or 3 h of X-ray illumination, increased their density by almost a factor of six and ∼40, respectively. Upon prolonged storage in vacuum, the formation of a distinct defect peak is observed. Thus, near-UV CFSYS with modeling as shown here is demonstrated as a powerful tool to characterize the valence band and quantify defect states in lead halide perovskites.

Assessment of interstitial potentials for rapid prediction of absolute band energies in crystals

Electronic band alignment is a demanding process for first-principles simulations, but an important factor in materials selection for applications including electrocatalysis and photoelectrochemistry. Here, we revisit a bulk alignment procedure, originally developed by Frensley and Kroemer, using modern computational tools. The electrostatic potential in the interstitial region, obtained from density functional theory, with four exchange correlation functionals, is used to predict the valence band offsets of 27 zinc blende semiconductors. The results are found to be in qualitative agreement with Frensley and Kroemer's original data. In addition to absolute electron energies, the possibility of extracting effective ionic charges is investigated and compared to Bader partial charges. With further developments, such a procedure may support rapid screening of the bulk ionization potential and electron affinity of crystals, as we illustrate with an extension to rock salt and perovskite structure types.

Recent Progress on Perovskite Surfaces and Interfaces in Optoelectronic Devices

Surfaces and heterojunction interfaces, where defects and energy levels dictate charge-carrier dynamics in optoelectronic devices, are critical for unlocking the full potential of perovskite semiconductors. In this progress report, chemical structures of perovskite surfaces are discussed and basic physical rules for the band alignment are summarized at various perovskite interfaces. Common perovskite surfaces are typically decorated by various compositional and structural defects such as residual surface reactants, discrete nanoclusters, reactions by products, vacancies, interstitials, antisites, etc. Some of these surface species induce deep-level defect states in the forbidden band forming very harmful charge-carrier traps and affect negatively the interface band alignments for achieving optimal device performance. Herein, an overview of research progresses on surface and interface engineering is provided to minimize deep-level defect states. The reviewed subjects include selection of interface and substrate buffer layers for growing better crystals, materials and processing methods for surface passivation, the surface catalyst for microstructure transformations, organic semiconductors for charge extraction or injection, heterojunctions with wide bandgap perovskites or nanocrystals for mitigating defects, and electrode interlayer for preventing interdiffusion and reactions. These surface and interface engineering strategies are shown to be critical in boosting device performance for both solar cells and light-emitting diodes.

Mechanism and Timescales of Reversible p-Doping of Methylammonium Lead Triiodide by Oxygen

Understanding and controlling the energy level alignment at interfaces with metal halide perovskites (MHPs) is essential for realizing the full potential of these materials for use in optoelectronic devices. To date, however, the basic electronic properties of MHPs are still under debate. Particularly, reported Fermi level positions in the energy gap vary from indicating strong n- to strong p-type character for nominally identical materials, raising serious questions about intrinsic and extrinsic defects as dopants. ​In this work, photoemission experiments demonstrate that thin films of the prototypical methylammonium lead triiodide (MAPbI3 ) behave like an intrinsic semiconductor in the absence of oxygen. Oxygen is then shown to be able to reversibly diffuse into and out of the MAPbI3 bulk, requiring rather long saturation timescales of ≈1 h (in: ambient air) and over 10 h (out: ultrahigh vacuum), for few 100 nm thick films. Oxygen in the bulk leads to pronounced p-doping, positioning the Fermi level universally ≈0.55 eV above the valence band maximum. The key doping mechanism is suggested to be molecular oxygen substitution of iodine vacancies, supported by density functional theory calculations. This insight rationalizes previous and future electronic property studies of MHPs and calls for meticulous oxygen exposure protocols.

Role of the Metal-Semiconductor Interface in Halide Perovskite Devices for Radiation Photon Counting

Halide perovskites are promising optoelectronic semiconductors. For applications in solid-state detectors that operate in low photon flux counting mode, blocking interfaces are essential to minimize the dark current noise. Here, we investigate the interface between methylammonium lead tri-iodide (MAPbI₃) single crystals and commonly used high and low work function metals to achieve photon counting capabilities in a solid-state detector. Using scanning photocurrent microscopy, we observe a large Schottky barrier at the MAPbI₃/Pb interface, which efficiently blocks dark current. Moreover, the shape of the photocurrent profile indicates that the MAPbI₃ single-crystal surface has a deep fermi level close to that of Au. Rationalized by first-principle calculations, we attribute this observation to the defects due to excess iodine on the surface underpinning emergence of deep band-edge states. The photocurrent decay profile yields a charge carrier diffusion length of 10-25 μm. Using this knowledge, we demonstrate a single-crystal MAPbI₃ detector that can count single γ-ray photons by producing sharp electrical pulses with a fast rise time of <2 μs. Our study indicates that the interface plays a crucial role in solid-state detectors operating in photon counting mode.

Influence of Surface Ligands on Energetics at FASnI3/C60 Interfaces and Their Impact on Photovoltaic Performance

Interfacial chemistry and energetics significantly impact the performance of photovoltaic devices. In the case of Pb-containing organic metal halide perovskites, photoelectron spectroscopy has been used to determine the energetic alignment of frontier electronic energy levels at various interfaces present in the photovoltaic device. For the Sn-containing analogues, which are less toxic, no such measurements have been made. Through a combination of ultraviolet, inverse, and X-ray photoelectron spectroscopy (UPS, IPES, and XPS, respectively) measurements taken at varying thickness increments during stepwise deposition of C₆₀ on FASnI₃, we present the first direct measurements of the frontier electronic energy levels across the FASnI₃/C₆₀ interface. The results show band bending in both materials and transport gap widening in FASnI₃ at the interface with C₆₀. The XPS results show that iodide diffuses into C₆₀ and results in n-doping of C₆₀. This iodide diffusion out of FASnI₃ impacts the valence and conduction band energies of FASnI₃ more than the core levels, with the core level shifts displaying a different trend than the valence and conduction bands. Surface treatment of FASnI₃ with carboxylic acids and bulky ammonium substituted surface ligands results in slight alterations in the interfacial energetics, and all surface ligands result in similar or improved PV performance relative to the untreated devices. The greatest PV stability results from treatment with a fluorinated carboxylic acid derivative; however, iodide diffusion is still observed to occur with this surface ligand.

Nonradiative Recombination in Perovskite Solar Cells: The Role of Interfaces

Perovskite solar cells combine high carrier mobilities with long carrier lifetimes and high radiative efficiencies. Despite this, full devices suffer from significant nonradiative recombination losses, limiting their V_OC to values well below the Shockley-Queisser limit. Here, recent advances in understanding nonradiative recombination in perovskite solar cells from picoseconds to steady state are presented, with an emphasis on the interfaces between the perovskite absorber and the charge transport layers. Quantification of the quasi-Fermi level splitting in perovskite films with and without attached transport layers allows to identify the origin of nonradiative recombination, and to explain the V_OC of operational devices. These measurements prove that in state-of-the-art solar cells, nonradiative recombination at the interfaces between the perovskite and the transport layers is more important than processes in the bulk or at grain boundaries. Optical pump-probe techniques give complementary access to the interfacial recombination pathways and provide quantitative information on transfer rates and recombination velocities. Promising optimization strategies are also highlighted, in particular in view of the role of energy level alignment and the importance of surface passivation. Recent record perovskite solar cells with low nonradiative losses are presented where interfacial recombination is effectively overcome-paving the way to the thermodynamic efficiency limit.