Chunki Kim

Optimization of energy levels by molecular design: evaluation of bis-diketopyrrolopyrrole molecular donor materials for bulk heterojunction solar cells

We report a series of solution-processable, small-molecule, donor materials based on an architecture consisting of two diketopyrrolopyrrole (DPP) cores with different aromatic π-bridges between the DPP units and different end-capping groups. In general, this architecture leads to desirable light absorption and electronic levels for donor materials. Out of the compounds investigated, a material with a hydrolyzed dithieno(3,2-b;2′,3′-d)silole (SDT) core and 2-benzofuran (BFu) end capping groups leads to the most favorable properties for solar cells, capable of generating photocurrent up to 800 nm while producing an open-circuit voltage of over 850 mV, indicating a small loss in electrical potential compared to other bulk heterojunction systems. Device properties can be greatly improved through the use of solvent additives such as 2-chloronaphthalene and initial attempts to optimize device fabrication have resulted in power conversion efficiencies upwards of 4%.

Solution-Processed Organic Solar Cells from Dye Molecules: An Investigation of Diketopyrrolopyrrole:Vinazene Heterojunctions

Although one of the most attractive aspects of organic solar cells is their low cost and ease of fabrication, the active materials incorporated into the vast majority of reported bulk heterojunction (BHJ) solar cells include a semiconducting polymer and a fullerene derivative, classes of materials which are both typically difficult and expensive to prepare. In this study, we demonstrate that effective BHJs can be fabricated from two easily synthesized dye molecules. Solar cells incorporating a diketopyrrolopyrrole (DPP)-based molecule as a donor and a dicyanoimidazole (Vinazene) acceptor function as an active layer in BHJ solar cells, producing relatively high open circuit voltages and power conversion efficiencies (PCEs) up to 1.1%. Atomic force microscope images of the films show that active layers are rough and apparently have large donor and acceptor domains on the surface, whereas photoluminescence of the blends is incompletely quenched, suggesting that higher PCEs might be obtained if the morphology could be improved to yield smaller domain sizes and a larger interfacial area between donor and acceptor phases.

Morphology control of solution processable small molecule bulk heterojunction solar cellsviasolvent additives

A new donor–acceptor type small molecule with dithienothiophene and diketopyrrolopyrrole units has been synthesized for application in organic solar cells. In order to optimize the nanoscale morphology of the small molecule solar cells, additives such as 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN) are used. The use of CN significantly suppresses molecular aggregation and allows suitable phase separation, enhancing power conversion efficiency from 0.5% to 2.2%.

DNA Interlayers Enhance Charge Injection in Organic Field‐Effect Transistors

By inserting DNA interlayers beneath the Au contact, the contact resistance of PC(70) BM field-effect transistorss is reduced by approximately 30 times at a gate bias of 20 V. The electron and hole mobilities of ambipolar diketopyrrolopyrrole transistors are increased by one order of magnitude with a reduction of the threshold voltage from 12 to 6.5 V.

Role of crystallinity of non-fullerene acceptors in bulk heterojunctions

Understanding the solid-state structure of the bulk heterojunction provides insight into how to improve the performance of nonfullerene acceptors in organic solar cells. We have characterized the self-assembly of three functionalized pentacene acceptors in single crystals, neat films and bulk heterojunctions formed by blending with a diketopyrrolopyrrole-based molecular donor. Atomic force microscopy, grazing incidence wide-angle X-ray scattering and optical spectroscopy indicate that the presence of the donor perturbs the packing and texture of acceptors with smaller substituents. The structural characterization explains the differences in performance among the three acceptors studied and suggests that, unlike fullerenes, disordered domains of molecular acceptors with planar molecular structure have inefficient electron transport in BHJ thin films.

High light intensity effects on nanoscale open-circuit voltage for three common donor materials in bulk heterojunction solar cells

White light photoconductive atomic force microscopy (pc-AFM) was employed to evaluate nanoscale open circuit (Voc) at high light intensities (up to 200 sun) of three donor:acceptor blends comprising two widely studied polymers and one small molecule donor material. By varying the work function of electron extraction contacts, the Voc observed in nanoscale measurements reveals a unified dependence on the electrode work functions regardless of the blend materials; in line with earlier macroscopic measurements at 1 sun. At high light intensities, an agreement between the nanoscale and bulk Voc is observed. Nonetheless, light intensity dependent Voc measurements suggest that under high light intensities, the Voc obtained by pc-AFM exhibits contact-limited behavior.