Selina Olthof

Influence of charge balance and exciton distribution on efficiency and lifetime of phosphorescent organic light-emitting devices

We discuss the importance of appropriate charge carrier confinement and exciton management for the realization of highly efficient and stable organic light-emitting diodes (OLEDs). As an example, we choose red p-i-n-type OLEDs based on the iridium-based electrophosphorescent emitter Ir(MDQ)2(acac) doped in α-NPD as host material. We show how an appropriate choice of the hole blocking layer material allows external quantum efficiencies as high as 20% for this emitter. At the same time, the display-relevant brightness of 100 cd/m2 is reached at an operation voltage of only 2.4 V, which is close to the thermodynamic limit. As a result, a high total power efficiency of 37.5 lm/W at 100 cd/m2 is reached. In a further step, we study the influence of the blocker materials on device lifetime. We investigate the chemical reactions causing the degradation process by use of matrix assisted laser desorption time-of-flight mass spectrometry. It can be shown that discovered degradation reactions can be suppressed by an...

Origin of open circuit voltage in planar and bulk heterojunction organic thin-film photovoltaics depending on doped transport layers

The aim of this article is to investigate the origin of the open circuit voltage (Voc) in organic heterojunction solar cells. The studied devices consist of buckminsterfullerene C60 as acceptor material and an oligophenyl-derivative 4,4′-bis-(N,N-diphenylamino)quaterphenyl (4P-TPD) as donor material. These photoactive materials are sandwiched between indium tin oxide and p-doped hole transport layers. Using two different p-doped hole transport layers, the built-in voltage of the solar cells is independently changed from the metal contacts. The influence of the built-in voltage on the Voc is investigated in bulk and planar heterojunctions. In bulk heterojunctions, in which doped transport layers border directly on the photoactive blend layer, Voc cannot exceed the built-in voltage significantly. Though, in planar heterojunctions, Voc is identical with the splitting of quasi-Fermi levels at the donor-acceptor interface and is thus primarily determined by the difference of the lowest unoccupied molecular orb...

Impact of mesoscale order on open-circuit voltage in organic solar cells

Structural order in organic solar cells is paramount: it reduces energetic disorder, boosts charge and exciton mobilities, and assists exciton splitting. Owing to spatial localization of electronic states, microscopic descriptions of photovoltaic processes tend to overlook the influence of structural features at the mesoscale. Long-range electrostatic interactions nevertheless probe this ordering, making local properties depend on the mesoscopic order. Using a technique developed to address spatially aperiodic excitations in thin films and in bulk, we show how inclusion of mesoscale order resolves the controversy between experimental and theoretical results for the energy-level profile and alignment in a variety of photovoltaic systems, with direct experimental validation. Optimal use of long-range ordering also rationalizes the acceptor-donor-acceptor paradigm for molecular design of donor dyes. We predict open-circuit voltages of planar heterojunction solar cells in excellent agreement with experimental data, based only on crystal structures and interfacial orientation.

Photoelectron spectroscopy study of systematically varied doping concentrations in an organic semiconductor layer using a molecular p-dopant

We investigate the doping behavior of the strongly electron accepting molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane coevaporated with the host molecule N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine by photoemission spectroscopy and conductivity measurements. Using interface resolved measurements, we compare the alignment on different substrates and investigate the effects of varying doping concentrations on the Fermi level position. We find that at high doping concentrations the Fermi level gets pinned at the exponentially decaying tail of the highest occupied molecular orbital and compare these results with different dopants and host molecules. The measurement of the doping dependent space charge layer thickness yields information on the amount of free charge carriers and thereby the efficiency of the doping.

Mechanistic study on the solution-phase n-doping of 1,3-dimethyl-2-aryl-2,3-dihydro-1H-benzoimidazole derivatives.

The discovery of air-stable n-dopants for organic semiconductor materials has been hindered by the necessity of high-energy HOMOs and the air sensitivity of compounds that satisfy this requirement. One strategy for circumventing this problem is to utilize stable precursor molecules that form the active doping complex in situ during the doping process or in a postdeposition thermal- or photo-activation step. Some of us have reported on the use of 1H-benzimidazole (DMBI) and benzimidazolium (DMBI-I) salts as solution- and vacuum-processable n-type dopant precursors, respectively. It was initially suggested that DMBI dopants function as single-electron radical donors wherein the active doping species, the imidazoline radical, is generated in a postdeposition thermal annealing step. Herein we report the results of extensive mechanistic studies on DMBI-doped fullerenes, the results of which suggest a more complicated doping mechanism is operative. Specifically, a reaction between the dopant and host that begins with either hydride or hydrogen atom transfer and which ultimately leads to the formation of host radical anions is responsible for the doping effect. The results of this research will be useful for identifying applications of current organic n-doping technology and will drive the design of next-generation n-type dopants that are air stable and capable of doping low-electron-affinity host materials in organic devices.

Highly doped layers as efficient electron-hole recombination contacts for tandem organic solar cells

A key feature of stacked organic solar cells is an efficient recombination contact at the interface between the solar cells in the stack. Here, an electron current has to be converted into a hole current without loss of energy. Furthermore, the recombination contact has to be highly transparent. We present a new approach for small molecule organic solar cells using highly doped organic layers. Our approach adapts the use of tunnel diodes known from inorganic tandem solar cells. We compare a metal cluster based recombination contact reported in literature to the new approach using different organic tandem solar cell structures. For this purpose, current-voltage characteristics of adequate solar cells are measured. The experiments show that highly doped layers as recombination contacts in tandem organic solar cells are superior to the metal cluster based approach. The proposed concept allows an addition of the open circuit voltages of the subcells of a tandem solar cell, without absorption or reflection at ...

Highly efficient white organic light-emitting diodes based on fluorescent blue emitters

Beside inorganic LEDs and fluorescent lamps, organic light-emitting diodes (OLEDs) are evolving into a serious alternative to incandescent lamps. Up to now, it was assumed that all-phosphorescent OLEDs are required for reaching sufficiently high efficiencies. However, the stability of phosphorescent blue emitters is a major challenge. We present a novel approach to achieve highly efficient (up to 90 lm/W at 1000 cd/m2 using a macroextractor) white light emission from OLEDs. The here presented combination of a fluorescent blue and a phosphorescent red emitter simultaneously allows for a strong blue emission and efficient triplet transfer to the phosphor. The spectrum is extended in the green and yellow region by a full phosphorescent unit stacked on top of the triplet harvesting device. This superposition of four different emitters results in color coordinates close to illuminant A and a color rendering index of 80. Furthermore, color stability is given with respect to varying driving conditions and estima...

Impact of Film Stoichiometry on the Ionization Energy and Electronic Structure of CH3 NH3 PbI3 Perovskites.

The electronic structure of a large sample set of CH3 NH3 PbI3 -based perovskites is studied. Combined investigations by UV/X-ray photoelectron spectroscopy and X-ray diffraction reveal that interstitials present in the film lead to changes in the occupied density of states close to the valence band, which in turn influences the performance of solar cells. Changes in elemental composition tune the ionization energy of the perovskite film by almost 1 eV without introducing significant amounts of gap states.

Investigation of C60F36 as low-volatility p-dopant in organic optoelectronic devices

We demonstrate highly efficient small molecule organic light emitting diodes and organic solar cells based on the p-i-n-type structure using the fluorinated fullerene molecule C60F36 as p-dopant in the hole transport layer. We present synthesis, chemical analysis, and energy level investigation of the dopant as well as the conductivity of organic layers consisting of a matrix of N,N,N′,N′-tetrakis 4-methoxyphenyl-benzidine(MeO-TPD) or N,N′-[(Diphenyl-N,N′-bis)9, ?> 9,-dimethyl-fluoren-2-yl]-benzidine(BF-DPB) doped by the fullerene compound. State of the art organic p-i-n devices containing C60F36 show efficiencies comparable to devices with the commonly used p-dopant2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). The advantages of the fullerene based dopant are the low volatility and high thermal stability, which is beneficial for device operation under elevated temperature. These properties make C60F36 highly attractive for the usage as p-dopant in a broad spectrum of organic p-i-n device...

Suppressed decomposition of organometal halide perovskites by impermeable electron-extraction layers in inverted solar cells

The area of thin-film photovoltaics has been overwhelmed by organometal halide perovskites. Unfortunately, serious stability concerns arise with perovskite solar cells. For example, methyl-ammonium lead iodide is known to decompose in the presence of water and, more severely, even under inert conditions at elevated temperatures. Here, we demonstrate inverted perovskite solar cells, in which the decomposition of the perovskite is significantly mitigated even at elevated temperatures. Specifically, we introduce a bilayered electron-extraction interlayer consisting of aluminium-doped zinc oxide and tin oxide. We evidence tin oxide grown by atomic layer deposition does form an outstandingly dense gas permeation barrier that effectively hinders the ingress of moisture towards the perovskite and—more importantly—it prevents the egress of decomposition products of the perovskite. Thereby, the overall decomposition of the perovskite is significantly suppressed, leading to an outstanding device stability.