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.

Combined photoemission and charge-transport property studies of the organic hole transport material 2,2',7,7'-tetrakis(N,N-di-p-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.

The inorganic lead halide perovskite CsPbBr3 promises similar solar cell efficiency to its hybrid organic-inorganic counterpart CH3NH3PbBr3 but shows greater stability. Here, we exploit this stability for the study of band alignment between perovskites and carrier selective interlayers. Using ultraviolet, X-ray, and inverse photoemission spectroscopies, we measure the ionization energy and electron affinities of CsPbBr3 and the hole transport polymer polytriarylamine (PTAA). We find that undoped PTAA introduces a barrier to hole extraction of 0.2–0.5 eV, due to band bending in the PTAA and/or a dipole at the interface. p-doping the PTAA eliminates this barrier, raising PTAA's highest occupied molecular orbital to 0.2 eV above the CsPbBr3 valence band maximum and improving hole transport. However, IPES reveals the presence of states below the PTAA lowest unoccupied molecular level. If present at the CsPbBr3/PTAA interface, these states may limit the polymer's efficacy at blocking electrons in solar cells with wide band gap materials like CsPbBr3 and CH3NH3PbBr3.

We have used ultraviolet and inverse photoemission spectroscopy to determine the transport gaps (E t ) of C 60 and diindenoperylene (DIP), and the photovoltaic gap (E PVG ) of five prototypical donor/ acceptor interfaces used in organic photovoltaic cells (OPVCs). The transport gap of C 60 (2.5 6 0.1) eV and DIP (2.55 6 0.1) eV at the interface is the same as in pristine films. We find nearly the same energy loss of ca 0.5 eV for all material pairs when comparing the open circuit voltage measured for corresponding OPVCs and E PVG . V C 2012 American Institute of Physics.The energy level alignment at the donor/acceptor (D/A) heterojunction of an organic photovoltaic cell (OPVC) is decisive for its performance. In particular, the energy offset between the lowest unoccupied molecular orbital level of the acceptor [LUMO(A)] and the highest occupied molecular orbital level of the donor [HOMO(D)] sets an upper limit for the open circuit voltage V oc . 1-6 This has been expressed as e Á V oc ¼ HOMO(D) À LUMO(A) À D, where e is the elementary charge and D a loss term, which has been suggested to be related to the exciton binding energy 2 or radiative and nonradiative temperature dependent losses. 1,3,5 The HOMO(D)/ LUMO(A) offset is denoted in various ways in the literature, such as charge transfer gap, intermolecular gap, or donor/ acceptor gap and is often estimated by optical spectroscopy, 5,6 or electrical characterization, e.g., cyclic voltammetry, 6 reverse saturation current analysis, 2,7 or temperature dependent measurements of the open circuit voltage. 4,5 To avoid ambiguity, we use the term photovoltaic gap (E PVG ) (cf Fig. 1). To minimize energy losses during the photon harvesting process, it is desirable to maximize E PVG within the constraint of keeping the LUMO-LUMO (DE L ) and HOMO-HOMO level offsets (DE H ) sufficiently large to drive charge separation across the D/A junction. To quantify D for unraveling its physical origin, it is mandatory to have reliable E PVG values for comparison with corresponding V oc values. Unfortunately, simple models for estimating the energy levels at organicorganic interfaces are often invalid (e.g., vacuum level alignment 8,9 ), and more involved models have been brought forward. 10,11 For the time being, experimental determination of interface energetics is indispensable to understand the processes inside an OPVC based on reliable values of E PVG , but only few pertinent studies have been conducted to date. [12][13][14][15] The present study focuses on E PVG values at prototypical organic D/A pairs formed between four organic semiconductors [sexithiophene (6T), fullerene (C 60 ), diindenoperylene (DIP, chemical structure shown in Fig. 1), and poly(3-hexylthiophene) (P3HT)] determined using the combination of ultraviolet and inverse photoemission spectroscopy (UPS and IPES). The experiment for DIP/C 60 demonstrates that E PVG can be reliably inferred from measuring the offset between the D/A HOMO levels, once the acceptor's transport gap (E t ) is known and no changes of E...

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.