The research on the polymer-based solar cells (PSCs) has attracted an increasing amount of attention in recent years because PSCs pose potential advantages over mainstream inorganic-based solar cells, such as significantly reduced material/fabrication costs, flexible substrates, and light weight of finished solar cells. The research community has made great progress in the field of bulk heterojunction (BHJ) polymer solar cells since its inception in 1995. The power conversion efficiency (PCE), a key parameter to assess the performance of solar cells, has increased from 1% in the 1990s to over 8% just recently. These great advances are mainly fueled by the development of conjugated polymers used as the electron-donating materials in BHJ solar cells. In this Perspective, we first briefly review the progress on the design of conjugated polymers for polymer solar cells in the past 16 years. Since a conjugated polymer can be arbitrarily divided into three constituting components—the conjugated backbone, the si...

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Rational Design of High Performance Conjugated Polymers for Organic Solar Cells

Fabricating low-temperature solution-processed solar cells with good power-conversion efficiency and stability in ambient conditions has proved challenging. The use of ligands that protect colloidal quantum dots from degradation in air and tune their energy levels is now shown to be a viable approach for the realization of spin-coated solar cells with very high efficiency. Solution processing is a promising route for the realization of low-cost, large-area, flexible and lightweight photovoltaic devices with short energy payback time and high specific power. However, solar cells based on solution-processed organic, inorganic and hybrid materials reported thus far generally suffer from poor air stability, require an inert-atmosphere processing environment or necessitate high-temperature processing1, all of which increase manufacturing complexities and costs. Simultaneously fulfilling the goals of high efficiency, low-temperature fabrication conditions and good atmospheric stability remains a major technical challenge, which may be addressed, as we demonstrate here, with the development of room-temperature solution-processed ZnO/PbS quantum dot solar cells. By engineering the band alignment of the quantum dot layers through the use of different ligand treatments, a certified efficiency of 8.55% has been reached. Furthermore, the performance of unencapsulated devices remains unchanged for over 150 days of storage in air. This material system introduces a new approach towards the goal of high-performance air-stable solar cells compatible with simple solution processes and deposition on flexible substrates.

Improved performance and stability in quantum dot solar cells through band alignment engineering

During the last few years, transition metal oxides (TMO) such as molybdenum tri-oxide (MoO3), vanadium pent-oxide (V2O5) or tungsten tri-oxide (WO3) have been extensively studied because of their exceptional electronic properties for charge injection and extraction in organic electronic devices. These unique properties have led to the performance enhancement of several types of devices and to a variety of novel applications. TMOs have been used to realize efficient and long-term stable p-type doping of wide band gap organic materials, charge-generation junctions for stacked organic light emitting diodes (OLED), sputtering buffer layers for semi-transparent devices, and organic photovoltaic (OPV) cells with improved charge extraction, enhanced power conversion efficiency and substantially improved long term stability. Energetics in general play a key role in advancing device structure and performance in organic electronics; however, the literature provides a very inconsistent picture of the electronic structure of TMOs and the resulting interpretation of their role as functional constituents in organic electronics. With this review we intend to clarify some of the existing misconceptions. An overview of TMO-based device architectures ranging from transparent OLEDs to tandem OPV cells is also given. Various TMO film deposition methods are reviewed, addressing vacuum evaporation and recent approaches for solution-based processing. The specific properties of the resulting materials and their role as functional layers in organic devices are discussed.

Transition Metal Oxides for Organic Electronics: Energetics, Device Physics and Applications

Transition metal oxides are capable of a wide range of work functions. This quality allows them to be used in many applications that involve charge transfer with adsorbed molecules, for example as heterogeneous catalysts, as charge-injection layers in organic electronics, and as electrodes in fuel cells. Chemical and structural factors can alter transition-metal oxide work functions, often making their work functions difficult to control. Little is known about the effects of the cation oxidation state and point defects on the oxide work function. It is necessary to understand how such chemical and structural factors affect work functions in order to controllably tune transition metal oxides for desired applications. Here, a correlation between the oxide work function and cation oxidation state is demonstrated. This correlation is attributed to the change in cation electronegativity with oxidation state. A model is presented that relates the work function to the oxygen deficiency for d0 oxides in the limit of dilute oxygen vacancies. It is proposed that the rapid initial decrease in work function, observed for d0 oxides, is caused by an increase in the density of donor-like defect states. It is also shown that oxides tend to have decreased work functions near a metal/metal-oxide interface as a consequence of the relationship between defects and work function. These insights provide guidelines for tuning transition metal oxide work functions.

Transition Metal Oxide Work Functions: The Influence of Cation Oxidation State and Oxygen Vacancies

The energetics of an archetype charge generation layer (CGL) architecture comprising of 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA), tungsten oxide (WO3), and bathophenanthroline (BPhen) n-doped with cesium carbonate (Cs2CO3) are determined by ultraviolet and inverse photoemission spectroscopy. We show that the charge generation process occurs at the interface between the hole-transport material (TCTA) and WO3 and not, as commonly assumed, at the interface between WO3 and the n-doped electron-transport material (BPhen:Cs2CO3). However, the n-doped layer is also essential to the realization of an efficient CGL structure. The charge generation mechanism occurs via electron transfer from the TCTA highest occupied molecular orbital level to the transition metal-oxide conduction band.

Charge generation layers comprising transition metal-oxide/organic interfaces: Electronic structure and charge generation mechanism

The electronic structure and hole-injection properties of ambient contaminated molybdenum trioxide (MoO3) surfaces are studied by ultraviolet and inverse photoemission spectroscopy, and current-voltage measurements. Contamination reduces the work function (WF), electron affinity (EA) and ionization energy by about 1 eV with respect to the freshly evaporated film, to values of 5.7 eV, 5.5 eV, and 8.6 eV, respectively. However, the WF and EA remain sufficiently large that the hole-injection properties of MoO3 are not affected by contamination. The results are of particular importance in view of potential applications of transition metal oxides for low-cost manufacturing of devices in low-vacuum or nonvacuum environment.

Effect of contamination on the electronic structure and hole-injection properties of MoO3/organic semiconductor interfaces

The device comprises a transparent disc of a motor vehicle. The transparent disc has an organic light emitting diode (1) display arranged in or on the disc. The organic light emitting diode display is conducted in the form of a module, which is arranged in a recess in the disc. The disc is a laminated glass, where the recess is arranged between the two panels of the laminated glass. The recess is achieved by a section of panels of the laminated glass connecting the foils.

Transparent indicator device for displaying information, comprises transparent disc of motor vehicle, which has organic light emitting diode display arranged in or on disc, where display is conducted in form of module

Compared to established LCD and plasma technologies displays based on organic light emitting diodes (OLEDs)
promise more brilliant images, less energy consumption and lower production costs. Furthermore, the organic
layers that make up an OLED can be engineered to be transparent in the visible part of the spectrum. In
combination with transparent conductive oxides like Indium-Tin-Oxide (ITO) or Aluminum doped Zinc-Oxide
(AZO) as contacts OLEDs may be built entirely transparent. One major issue to be addressed in the fabrication
of these devices is the deposition of the top transparent contact without damaging the organic layers.
Transparent OLED pixels can be arranged to form entirely transparent OLED displays. For the active matrix
addressing of the individual OLED pixels, we use TFTs which are transparent themselves. Rather than silicon,
they are based on the wide-bandgap semiconductor Zinc-Tin-Oxide (ZTO) and transmit about 80 % of the visible
light (400-750 nm). The transistors typically have field-effect mobilities of 13 cm2/Vs (an order of magnitude
larger than a-Si TFTs) and an on-off ratio of 106. The OLED pixel which needs to be driven may be positioned
directly on top of the driver circuit. The pixels fabricated accordingly have an overall transmittance > 70 %
in the visible spectrum. The brightness of the OLED pixels could be varied from 0 to 700 cd/m2 via the gate
bias of the driving TFTs. These devices state the initial building blocks of future, large-area, high-resolution
transparent OLED displays. More complex transparent driving circuits, required to compensate eventual device
variations will be discussed.

See-through OLED displays

One of the unique selling propositions of OLEDs is their potential to realize highly transparent devices over the
visible spectrum. This is because organic semiconductors provide a large Stokes-Shift and low intrinsic absorption
losses. Hence, new areas of applications for displays and ambient lighting become accessible, for instance, the
integration of OLEDs into the windshield or the ceiling of automobiles. The main challenge in the realization of
fully transparent devices is the deposition of the top electrode. ITO is commonly used as transparent bottom
anode in a conventional OLED. To obtain uniform light emission over the entire viewing angle and a low series
resistance, a TCO such as ITO is desirable as top contact as well. However, sputter deposition of ITO on top of
organic layers causes damage induced by high energetic particles and UV radiation. We have found an efficient
process to protect the organic layers against the ITO rf magnetron deposition process of ITO for an inverted
OLED (IOLED). The inverted structure allows the integration of OLEDs in more powerful n-channel transistors
used in active matrix backplanes. Employing the green electrophosphorescent material Ir(ppy) 3 lead to IOLED
with a current efficiency of 50 cd/A and power efficiency of 24 lm/W at 100 cd/m 2 . The average transmittance
exceeds 80 % in the visible region. The on-set voltage for light emission is lower than 3 V. In addition, by vertical
stacking we achieved a very high current efficiency of more than 70 cd/A for transparent IOLED.

Michael Kröger

Papers

Charge generation layers comprising transition metal-oxide/organic interfaces: Electronic structure and charge generation mechanism

Effect of contamination on the electronic structure and hole-injection properties of MoO3/organic semiconductor interfaces

Transparent indicator device for displaying information, comprises transparent disc of motor vehicle, which has organic light emitting diode display arranged in or on disc, where display is conducted in form of module

See-through OLED displays

Highly efficient fully transparent inverted OLEDs