Determination of Energy Level Alignment within an Energy Cascade Organic Solar Cell

The interfacial band alignment among boron subnaphthalocyanine chloride (SubNc), boron subphthalocyanine chloride (SubPc), and α-sexithiophene (α-6T) is explored using ultraviolet, inverse, and X-ray photoemission spectroscopies (UPS, IPES, and XPS, respectively). With these tools, the ionization energy (IE) and electron affinity (EA) for each material are determined. Layer-by-layer deposition of SubPc and SubNc on α-6T as well as SubPc on SubNc, combined with UPS and IPES, allows for the direct determination of the energy level alignment at the interfaces of interest. A small dipole is found at the α-6T/SubNc/SubPc interface, expanding the donor-LUMO to acceptor-HOMO gap and explaining the large open circuit voltage obtained with these devices. However, there is a small electron barrier between SubNc and SubPc, which may limit the efficiency of electron extraction in the current device configuration. Excess chlorine may be responsible for the high IE and EA found for SubNc and could potentially be remedied with improved synthetic methods or further purification

Improved Absorber Phase Stability, Performance, and Lifetime in Inorganic Perovskite Solar Cells with Alkyltrimethoxysilane Strain-Release Layers at the Perovskite/TiO₂ Interface

All-inorganic β-CsPbI3 has superior chemical and thermal stability compared to its hybrid counterparts, but the stability of state-of-the-art β-CsPbI3 perovskite solar cells (PSCs) under normal operating conditions (i.e., under illumination in an inert atmosphere) remains inferior to their hybrid counterparts. Here, we found that the lattice distortion in CsPbI3 near the perovskite/electron transport layer (ETL) interface can induce polymorphic transformation in encapsulated CsPbI3 films aged under illumination. To suppress this lattice distortion, we introduced alkyltrimethoxysilane strain-release layers (SRLs) at the perovskite/ETL interface. We found the SRL with the longest alkyl chain is the most effective at reducing interfacial lattice distortion, leading to enhanced charge transfer at the perovskite/ETL interface and improved phase/device stability. Its incorporation in β-CsPbI3 solar cells resulted in a power-conversion efficiency of 20.1% and an operational lifetime with an extrapolated T80 of >3000 h for encapsulated devices tested under continuous illumination under maximum power point tracking conditions

Quantifying the Extent of Contact Doping at the Interface between High Work Function Electrical Contacts and Poly(3-hexylthiophene) (P3HT)

We demonstrate new approaches to the characterization of oxidized regioregular poly­(3-hexylthiophene-2,5-diyl) (P3HT) that results from electronic equilibration with device-relevant high work function electrical contacts using high-resolution X-ray (XPS) and ultraviolet (UPS) photoelectron spectroscopy (PES). Careful interpretation of photoemission signals from thiophene sulfur atoms in thin (ca. 20 nm or less) P3HT films provides the ability to uniquely elucidate the products of charge transfer between the polymer and the electrical contact, which is a result of Fermi-level equilibration between the two materials. By comparing high-resolution S 2p core-level spectra to electrochemically oxidized P3HT standards, the extent of the contact doping reaction is quantified, where one in every six thiophene units (ca. 20%) in the first monolayer is oxidized. Finally, angle-resolved XPS of both pure P3HT and its blends with phenyl-C₆₁-butyric acid methyl ester (PCBM) confirms that oxidized P3HT species exist near contacts with work functions greater than ca. 4 eV, providing a means to characterize the interface and “bulk” region of the organic semiconductor in a single film

Air-Exposure-Induced Gas-Molecule Incorporation into Spiro-MeOTAD Films

Combined photoemission and charge-transport property studies of the organic hole transport material 2,2′,7,7′-tetrakis­(<i>N</i>,<i>N</i>-di-<i>p</i>-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD) under air exposure and controlled environments of O₂, H₂O + N₂, and N₂ (1 atm and under dark conditions) reveal the incorporation of gas molecules causing a decrease in charge mobility. Ultraviolet photoelectron spectroscopy shows the Fermi level shifts toward the highest occupied molecular orbital of spiro-MeOTAD when exposed to air, O₂, and H₂O resembling p-type doping. However, no traces of oxidized spiro-MeOTAD⁺ are observed by X-ray photoelectron spectroscopy (XPS) and UV–visible spectroscopy. The charge-transport properties were investigated by fabricating organic field-effect transistors with the 10 nm active layer at the semiconductor–insulator interface exposed to different gases. The hole mobility decreases substantially upon exposure to air, O₂, and H₂O. In the case of N₂, XPS reveals the incorporation of N₂ molecules into the film, but the decrease in the hole mobility is much smaller

Effect of Doping Density on the Charge Rearrangement and Interface Dipole at the Molecule–Silicon Interface

The interface level alignment of alkyl and alkenyl monolayers, covalently bound to oxide-free Si substrates of various doping levels, is studied using X-ray photoelectron spectroscopy. Using shifts in the C 1s and Si 2p photoelectron peaks as a sensitive probe, we find that charge distribution around the covalent Si–C bond dipole changes according to the initial position of the Fermi level within the Si substrate. This shows that the interface dipole is not fixed but rather changes with the doping level. These results set limits to the applicability of simple models to describe level alignment at interfaces and show that the interface bond and dipole may change according to the electrostatic potential at the interface

High-Work-Function Molybdenum Oxide Hole Extraction Contacts in Hybrid Organic–Inorganic Perovskite Solar Cells

We investigate the effect of high work function contacts in halide perovskite absorber-based photovoltaic devices. Photoemission spectroscopy measurements reveal that band bending is induced in the absorber by the deposition of the high work function molybdenum trioxide (MoO₃). We find that direct contact between MoO₃ and the perovskite leads to a chemical reaction, which diminishes device functionality. Introducing an ultrathin spiro-MeOTAD buffer layer prevents the reaction, yet the altered evolution of the energy levels in the methylammonium lead iodide (MAPbI₃) layer at the interface still negatively impacts device performance

Valence and Conduction Band Densities of States of Metal Halide Perovskites: A Combined Experimental–Theoretical Study

We report valence and conduction band densities of states measured via ultraviolet and inverse photoemission spectroscopies on three metal halide perovskites, specifically methylammonium lead iodide and bromide and cesium lead bromide (MAPbI₃, MAPbBr₃, CsPbBr₃), grown at two different institutions on different substrates. These are compared with theoretical densities of states (DOS) calculated via density functional theory. The qualitative agreement achieved between experiment and theory leads to the identification of valence and conduction band spectral features, and allows a precise determination of the position of the band edges, ionization energy and electron affinity of the materials. The comparison reveals an unusually low DOS at the valence band maximum (VBM) of these compounds, which confirms and generalizes previous predictions of strong band dispersion and low DOS at the MAPbI₃ VBM. This low DOS calls for special attention when using electron spectroscopy to determine the frontier electronic states of lead halide perovskites

Evidence for near-Surface NiOOH Species in Solution-Processed NiO_<i>x</i> Selective Interlayer Materials: Impact on Energetics and the Performance of Polymer Bulk Heterojunction Photovoltaics

The characterization and implementation of solution-processed, wide bandgap nickel oxide (NiO_<i>x</i>) hole-selective interlayer materials used in bulk-heterojunction (BHJ) organic photovoltaics (OPVs) are discussed. The surface electrical properties and charge selectivity of these thin films are strongly dependent upon the surface chemistry, band edge energies, and midgap state concentrations, as dictated by the ambient conditions and film pretreatments. Surface states were correlated with standards for nickel oxide, hydroxide, and oxyhydroxide components, as determined using monochromatic X-ray photoelectron spectroscopy. Ultraviolet and inverse photoemission spectroscopy measurements show changes in the surface chemistries directly impact the valence band energies. O₂-plasma treatment of the as-deposited NiO_<i>x</i> films was found to introduce the dipolar surface species nickel oxyhydroxide (NiOOH), rather than the p-dopant Ni₂O₃, resulting in an increase of the electrical band gap energy for the near-surface region from 3.1 to 3.6 eV via a vacuum level shift. Electron blocking properties of the as-deposited and O₂-plasma treated NiO_<i>x</i> films are compared using both electron-only and BHJ devices. O₂-plasma-treated NiO_<i>x</i> interlayers produce electron-only devices with lower leakage current and increased turn on voltages. The differences in behavior of the different pretreated interlayers appears to arise from differences in local density of states that comprise the valence band of the NiO_<i>x</i> interlayers and changes to the band gap energy, which influence their hole-selectivity. The presence of NiOOH states in these NiO_<i>x</i> films and the resultant chemical reactions at the oxide/organic interfaces in OPVs is predicted to play a significant role in controlling OPV device efficiency and lifetime

Contorted Hexabenzocoronenes with Extended Heterocyclic Moieties Improve Visible-Light Absorption and Performance in Organic Solar Cells

The large band gaps of existing contorted hexabenzocoronene derivatives severely limit visible-light absorption, restricting the photocurrents generated by solar cells utilizing contorted hexabenzocoronene (cHBC). To decrease the band gap and improve the light-harvesting properties, we synthesized cHBC derivatives having extended heterocyclic moieties as peripheral substituents. Tetrabenzofuranyldibenzocoronene (cTBFDBC) and tetrabenzothienodibenzocoronene (cTBTDBC) both exhibit broader absorption of the solar spectrum compared to cHBC, with peak absorbances on the order of 10⁵ cm^–1 in the near-ultraviolet and in the visible. Planar-heterojunction organic solar cells comprising cTBFDBC or cTBTDBC as the donor and C₇₀ as the acceptor surpass those having cHBC in photocurrent generation and power-conversion efficiency. Interestingly, devices containing cTBFDBC/C₇₀ exhibit the highest photocurrents despite cTBTDBC having the smallest band gap of the three cHBC derivatives. X-ray reflectivity of the active layers indicates a rougher donor–acceptor interface when cTBFDBC is employed instead of the other two donors. Consistent with this observation, internal quantum efficiency spectra suggest improved charge transfer at the donor–acceptor interface when cTBFDBCas opposed to cTBTDBC or cHBCis used as the donor

Mixed-Halide Perovskites with Stabilized Bandgaps

One merit of organic–inorganic hybrid perovskites is their tunable bandgap by adjusting the halide stoichiometry, an aspect critical to their application in tandem solar cells, wavelength-tunable light emitting diodes (LEDs), and lasers. However, the phase separation of mixed-halide perovskites caused by light or applied bias results in undesirable recombination at iodide-rich domains, meaning open-circuit voltage (<i>V</i>_OC) pinning in solar cells and infrared emission in LEDs. Here, we report an approach to suppress halide redistribution by self-assembled long-chain organic ammonium capping layers at nanometer-sized grain surfaces. Using the stable mixed-halide perovskite films, we are able to fabricate efficient and wavelength-tunable perovskite LEDs from infrared to green with high external quantum efficiencies of up to 5%, as well as linearly tuned <i>V</i>_OC from 1.05 to 1.45 V in solar cells