Solution-Processed p-Dopant as Interlayer in Polymer Solar Cells

Cross-Linking of Doped Organic Semiconductor Interlayers for Organic Solar Cells: Potential and Challenges

Solution-processable interlayers are important building blocks for the commercialization of organic electronic devices such as organic solar cells. Here, the potential of cross-linking to provide an insoluble, stable, and versatile charge transport layer based on soluble organic semiconductors is studied. For this purpose, a photoreactive tris-azide cross-linker is synthesized. The capability of the small molecular cross-linker is illustrated by applying it to a p-doped polymer used as a hole transport layer in organic solar cells. High cross-linking efficiency and excellent charge extraction properties of the cross-linked doped hole transport layer are demonstrated. However, at high doping levels in the interlayer, the solar cell efficiency is found to deteriorate. Based on charge extraction measurements and numerical device simulations, it is shown that this is due to diffusion of dopants into the active layer of the solar cell. Thus, in the development of future cross-linker materials, care must be taken to ensure that they immobilize not only the host but also the dopants.

Controllable coating and reshaping of gold nanorods with tetracyanoquinodimethane

Various nanoparticle surface layers allow unique functionalities. We developed a coating method with tetracyanoquinodimethane that forms solid layers through π stacking on gold nanorod surfaces. Its reaction mechanism was investigated with reaction time, aging time and surfactant concentration. Our method could be generalizable to different nanoparticle shapes and crystal facets.

Interlayer doping with p-type dopant for charge balance in indium phosphide (InP)-based quantum dot light-emitting diodes

A 2,3,4,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4-TCNQ) doping interlayer was developed to improve charge imbalance and the efficiency in indium phosphide (InP)-based quantum dot light-emitting diodes (QLEDs). The doping layer was coated between a hole injecting layer (HIL) and a hole transport layer (HTL) and successfully diffused with thermal annealing. This doping reduces the hole injection barrier and improves the charge balance of InP-based QLEDs, resulting in enhancement of an external quantum efficiency (EQE) of 3.78% (up from 1.6%) and a power efficiency of 6.41 lm/W (up from 2.77 lm/W). This work shows that F4-TCNQ interlayer doping into both HIL and HTL facilitates hole injection and can provide an efficient solution of improving charge balance in QLED for the device efficiency.

Re-evaluating how charge transfer modifies the conformation of adsorbed molecules

The archetypal electron acceptor molecule, TCNQ, is generally believed to become bent into an inverted bowl shape upon adsorption on the coinage metal surfaces on which it becomes negatively charged. New quantitative experimental structural measurements show that this is not the case for TCNQ on Ag(111). DFT calculations show that the inclusion of dispersion force corrections reduces not only the molecule-substrate layer spacing but also the degree of predicted molecular bonding. However, complete agreement between experimentally-determined and theoretically-predicted structural parameters is only achieved with the inclusion of Ag adatoms into the molecular layer, which is also the energetically favoured configuration. The results highlight the need for both experimental and theoretical quantitative structural methods to reliably understand similar metal-organic interfaces and highlight the need to re-evaluate some previously-investigated systems.

Highly stable doping of a polar polythiophene through co-processing with sulfonic acids and bistriflimide

Doping of organic semiconductors is currently an intensely studied field, since it is a powerful tool to optimize the performance of various organic electronic devices, ranging from organic solar cells, to thermoelectric modules, and bio-medical sensors. Despite recent advances, there is still a need for the development of highly conducting polymer:dopant systems with excellent long term stability and a high resistance to elevated temperatures. In this work we study the doping of the polar polythiophene derivative p(g42T-T) by various sulfonic acids and bistriflimide via different processing techniques. We demonstrate that simple co-processing of p(g42T-T) with an acid dopant yields conductivities of up to 120 S cm-1, which remain stable for more than six months under ambient conditions. Notably, a high conductivity is only achieved if the doping is carried out in air, which can be explained with a doping process that involves an acid mediated oxidation of the polymer through O2. P(g42T-T) doped with the non-toxic and inexpensive 1,3-propanedisulfonic acid was found to retain its electrical conductivity for at least 20 hours upon annealing at 120 °C, which allowed the bulk processing of the doped polymer into conducting, free-standing and flexible films and renders the di-acid a promising alternative to commonly used redox dopants.

High-Efficiency and High-Color-Rendering-Index Semitransparent Polymer Solar Cells Induced by Photonic Crystals and Surface Plasmon Resonance

Semitransparent polymer solar cells (ST-PSCs) show attractive potential in power-generating windows or building-integrated photovoltaics. However, the development of ST-PSCs is lagging behind opaque PSCs because of the contradiction between device efficiency and transmission. Herein, Ag/Au alloy nanoparticles and photonic crystals (PCs) were simultaneously introduced into ST-PSCs, acting compatibly as localized surface plasmon resonances and distributed Bragg reflectors to enhance light absorption and transmission. As a result, ST-PSCs based on a hybrid PTB7-Th:PC₇₁BM active layer contribute an efficiency as high as 7.13 ± 0.15% and an average visible transmission beyond 20%, which are superior to most of the reported results. Furthermore, PCs can partly compensate valley range of transmission by balancing reflection and transmission regions, yielding a high color rendering index of 95. We believe that the idea of two light management methods compatibly enhancing the performance of ST-PSCs can offer a promising path to develop photovoltaic applications.

Doping-induced carrier profiles in organic semiconductors determined from capacitive extraction-current transients

A method to determine the doping induced charge carrier profiles in lightly and moderately doped organic semiconductor thin films is presented. The theory of the method of Charge Extraction by a Linearly Increasing Voltage technique in the doping-induced capacitive regime (doping-CELIV) is extended to the case with non-uniform doping profiles and the analytical description is verified with drift-diffusion simulations. The method is demonstrated experimentally on evaporated organic small-molecule thin films with a controlled doping profile, and solution-processed thin films where the non-uniform doping profile is unintentional, probably induced during the deposition process, and a priori unknown. Furthermore, the method offers a possibility of directly probing charge-density distributions at interfaces between highly doped and lightly doped or undoped layers.

Polar Side Chains Enhance Processability, Electrical Conductivity, and Thermal Stability of a Molecularly p-Doped Polythiophene

Molecular doping of organic semiconductors is critical for optimizing a range of optoelectronic devices such as field-effect transistors, solar cells, and thermoelectric generators. However, many dopant:polymer pairs suffer from poor solubility in common organic solvents, which leads to a suboptimal solid-state nanostructure and hence low electrical conductivity. A further drawback is the poor thermal stability through sublimation of the dopant. The use of oligo ethylene glycol side chains is demonstrated to significantly improve the processability of the conjugated polymer p(g₄ 2T-T)-a polythiophene-in polar aprotic solvents, which facilitates coprocessing of dopant:polymer pairs from the same solution at room temperature. The use of common molecular dopants such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is explored. Doping of p(g₄ 2T-T) with F4TCNQ results in an electrical conductivity of up to 100 S cm^-1 . Moreover, the increased compatibility of the polar dopant F4TCNQ with the oligo ethylene glycol functionalized polythiophene results in a high degree of thermal stability at up to 150 °C.

Inverted organic photovoltaic cells

The advance in lifestyle, modern industrialization and future technological revolution are always at high expense of energy consumption. Unfortunately, there exist serious issues such as limited storage, high cost and toxic contamination in conventional fossil fuel energy sources. Instead, solar energy represents a renewable, economic and green alternative in the future energy market. Among the photovoltaic technologies, organic photovoltaics (OPVs) demonstrate a cheap, flexible, clean and easy-processing way to convert solar energy into electricity. However, OPVs with a conventional device structure are still far away from industrialization mainly because of their short lifetime and the energy-intensive deposition of top metal electrode. To address the stability and cost issue simultaneously, an inverted device structure has been introduced into OPVs, bridging laboratory research with practical application. In this review, recent progress in device structures, working mechanisms, functions and advances of each component layer as well their correlations with the efficiency and stability of inverted OPVs are reviewed and illustrated.