Youqin Zhu

An Azulene‐Containing Low Bandgap Small Molecule for Organic Photovoltaics with High Open‐Circuit Voltage

A simple azulene-containing squaraine dye (AzUSQ) showing bandgap of 1.38 eV and hole mobility up to 1.25×10(-4)  cm(2)  V(-1)  s(-1) was synthesized. With its low bandgap, an organic photovoltaic (OPV) device based on it has been made that exhibits an impressive open-circuit voltages (Voc ) of 0.80 V. Hence, azulene might be a promising structural unit to construct OPV materials with simultaneous low bandgap, high hole mobility and high Voc .

Marked effects of indolyl vs. indolinyl substituent on solid-state structure, carrier mobility and photovoltaic efficiency of asymmetrical squaraine dyes

Two solution-processed asymmetrical squaraines (ASQs) with cyclopenta[b]indolinyl (1a) and cyclopenta[b]indolyl (1b) as end cappers have been designed and synthesized. Although the internal molecular structure variations are minimal, the presence of the cyclopenta[b]indolinyl group endows 1a more planar molecular structure, which results in a much more compact solid-state structure (density is 1.317 g cm−3 for 1a but is 1.187 g cm−3 for 1b), dramatically affecting charge transport in the thin films. The hole mobility of 1a:PC71BM blended film is about 7 times higher than that of 1b:PC71BM. Consequently, the maximum power conversion efficiency (PCE) value of the organic photovoltaic cells (OPVs) based on 1a of up to 4.1%, approximately 80% higher than that of 1b, is one of the highest PCEs achieved for ASQ-based bulk-heterojunction (BHJ) OPVs.

N,N-Diarylamino end-capping as a new strategy for simultaneously enhancing open-circuit voltage, short-circuit current density and fill factor in small molecule organic solar cells

A series of new asymmetrical squaraine derivatives bearing N,N-diarylamino substituents as end-capping groups, namely ASQAr-1–6, were designed and synthesized. In comparison with the reference compound ASQB bearing a N,N-diisobutylamino end-capper, all the six target compounds exhibit improved thermal stability, red-shifted and broadened absorption bands as well as lower HOMO and LUMO energy levels. Despite the hole mobility of most of the compounds still being lower than that of ASQB, solution-processed bulk-heterojunction small molecule organic solar cells (BHJ-SMOSCs) using ASQAr-1–6 as electron donor materials all show drastically higher power conversion efficiency (PCE, 3.08–3.69%) than that of the ASQB-based reference device (PCE = 1.54%). The much enhanced photovoltaic performance of BHJ-SMOSCs based-on ASQAr-1–6 could be attributed to the simultaneously enhanced open-circuit voltage (Voc, 0.81–0.87 V vs. 0.75 V), short-circuit current density (Jsc, 8.07–9.06 mA cm−2 vs. 5.40 mA cm−2), and fill factor (FF, 0.45–0.47 vs. 0.38) relative to those of the reference ASQB-based device.

Improving the Charge Carrier Transport and Suppressing Recombination of Soluble Squaraine-Based Solar Cells via Parallel-Like Structure

Small molecule organic solar cells (SMOSCs) have attracted extensive attention in recent years. Squaraine (SQ) is a kind of small molecule material for potential use in high-efficiency devices, because of its high extinction coefficient and low-cost synthesis. However, the charge carrier mobility of SQ-based film is much lower than other effective materials, which leads to the pretty low fill factor (FF). In this study, we improve the performance of SQ derivative-based solar cells by incorporating PCDTBT into LQ-51/PC71BM host binary blend film. The incorporation of PCDTBT can not only increase the photon harvesting, but also provide an additional hole transport pathway. Through the charge carrier mobility and transient photovoltage measurement, we find that the hole mobility and charge carrier lifetime increase in the ternary system. Also, we carefully demonstrate that the charge carrier transport follows a parallel-like behavior.

Highly Efficient and Operational Stability Polymer Solar Cells Employing Nonhalogenated Solvents and Additives

The power conversion efficiencies (PCEs) of potential polymer solar cells have been shown to rapidly exceed 15%. However, these high-performance devices are based on halogenated solvents that pose a significant hazard to the atmospheric environment and human beings. The use of nonhalogenated solvents makes the device less efficient because of its solubility issues. In this work, we report high-efficiency devices utilizing PffBT4T-2OD and [6,6]-phenyl C71 butyric acid methyl ester system from nonhalogenated solvents such as o-xylene ( o-XY) and 1-methylnaphthalene (Me) hydrocarbon solvent. When Me was used as the additive, the PCE of prepared devices improved from 1.83 to 10.13%, which is rather higher than that of the devices processed with traditional solvents combined with chlorobenzene and 1,8-diiodooctane (8.18%). Both atomic force microscopy and transmission electron microscopy confirmed that after nonhalogen solvents are treated, a more finely phase-separated dense morphology of active layers than after halogen solvents. At the same time, grazing incident wide-angle X-ray scattering patterns show that the combination of nonhalogenated solvents o-XY and Me ingeniously formed an ordered crystal and π-π stacking. Also, the stability of devices prepared from nonhalogenated solvents was significantly better than that of halogenated solvents under continuous illumination in the air without encapsulation.