In Hwan Jung

Geometrically controlled organic small molecule acceptors for efficient fullerene-free organic photovoltaic devices

Organic small molecule (SM) acceptors containing a perylene diimide (PDI) moiety, F2B-T2PDI and T2PDI are synthesized, and the effects of their molecular geometry on the performance of fullerene-free organic photovoltaic (OPV) devices are investigated. The SM acceptors possess a PDI–core–PDI structure in which the PDI wing is connected to conjugated core units. By incorporation of a 2,5-difluorobenzene (F2B) moiety within the core unit, the planarity of the conjugated core is enhanced and the energy levels of the SM acceptor are down-shifted. In terms of molecular geometry, the F2B-containing SM acceptor, F2B-T2PDI, has a rigid core, which can symmetrically align the two PDI wings and enhance molecular packing. As a result, improved electron transport and bulk heterojunction morphology of the active layers are achieved. Furthermore, the incorporation of the F2B moiety effectively down-shifts the HOMO energy level, preventing back-transfer of holes from the acceptor to the cathode and enhancing the absorption of complementary wavelengths of the donor polymer, PTB7-Th. Leveraged by the beneficial geometric and energetic effects from the incorporation of F2B units, the power conversion efficiency of fullerene-free OPV devices using F2B-T2PDI reached 5%, whereas that using T2PDI was 3.63%.

Diphenyl-2-pyridylamine-Substituted Porphyrins as Hole-Transporting Materials for Perovskite Solar Cells

The susceptibility of porphyrin derivatives to light-harvesting and charge-transport operations have enabled these materials to be employed in solar cell applications. The potential of porphyrin derivatives as hole-transporting materials (HTMs) for perovskite solar cells (PSCs) has recently been demonstrated, but knowledge of the relationships between the porphyrin structure and device performance remains insufficient. In this work, a series of novel zinc porphyrin (PZn) derivatives has been developed and employed as HTMs for low-temperature processed PSCs. Key to the design strategy is the incorporation of an electron-deficient pyridine moiety to down-shift the HOMO levels of porphyrin HTMs. The porphyrin HTMs incorporating diphenyl-2-pyridylamine (DPPA) have HOMO levels that are in good agreement with the perovskite active layers, thus facilitating hole transfers from the perovskite to the HTMs. The DPPA-containing zinc porphyrin-based PSCs gave the best performance, with efficiency levels comparable to those of PSCs using spiro-OMeTAD, a current state-of-the-art HTM. In particular, PZn-DPPA-based PSCs show superior air stability, in both doped and undoped forms, to spiro-OMeTAD based devices.

Fullerene-Free Organic Solar Cells with an Efficiency of 10.2% and an Energy Loss of 0.59 eV Based on a Thieno[3,4-c]Pyrrole-4,6-dione-Containing Wide Band Gap Polymer Donor

Although the combination of wide band gap polymer donors and narrow band gap small-molecule acceptors achieved state-of-the-art performance as bulk heterojunction (BHJ) active layers for organic solar cells, there have been only several of the wide band gap polymers that actually realized high-efficiency devices over >10%. Herein, we developed high-efficiency, low-energy-loss fullerene-free organic solar cells using a weakly crystalline wide band gap polymer donor, PBDTTPD-HT, and a nonfullerene small-molecule acceptor, ITIC. The excessive intermolecular stacking of ITIC is efficiently suppressed by the miscibility with PBDTTPD-HT, which led to a well-balanced nanomorphology in the PBDTTPD-HT/ITIC BHJ active films. The favorable optical, electronic, and energetic properties of PBDTTPD-HT with respect to ITIC achieved panchromatic photon-to-current conversion with a remarkably low energy loss (0.59 eV).

High Thermoelectric Power Factor of a Diketopyrrolopyrrole-Based Low Bandgap Polymer via Finely Tuned Doping Engineering

We studied the thermoelectric properties of a diketopyrrolopyrrole-based semiconductor (PDPP3T) via a precisely tuned doping process using Iron (III) chloride. In particular, the doping states of PDPP3T film were linearly controlled depending on the dopant concentration. The outstanding Seebeck coefficient of PDPP3T assisted the excellent power factors (PFs) over 200 μW m−1K−2 at the broad range of doping concentration (3–8 mM) and the maximum PF reached up to 276 μW m−1K−2, which is much higher than that of poly(3-hexylthiophene), 56 μW m−1K−2. The high-mobility of PDPP3T was beneficial to enhance the electrical conductivity and the low level of total dopant volume was important to maintain high Seebeck coefficients. In addition, the low bandgap PDPP3T polymer effiectively shifted its absorption into near infra-red area and became more colorless after doping, which is great advantage to realize transparent electronic devices. Our results give importance guidance to develop thermoelectric semiconducting polymers and we suggest that the use of low bandgap and high-mobility polymers, and the accurate control of the doping levels are key factors for obtaining the high thermoelectric PF.

Dark current reduction strategies using edge-on aligned donor polymers for high detectivity and responsivity organic photodetectors

We synthesized the conjugated polymers PT2OBT, PVT2OBT, and PFBT2OBT for use in organic photodetecting devices. An octyloxy benzothiadiazole (OBT) moiety was used as a weak electron-accepting building block in the polymer system to cause the dominant absorption in the green-light region via weakening the intramolecular charge transfer (ICT) interactions between adjacent electron-donating moieties. In particular, indirect X-ray detection using a scintillator requires a low leakage current; thus, we focused on designing a molecular structure that can enhance the detectivity. The difluorobenzene-incorporated PFBT2OBT polymer showed a strong edge-on orientation both in the pristine film and in the blend film; however, PT2OBT and PVT2OBT have no preferred molecular orientation in the blend film. In the edge-on structure, the alkyl side chains of PFBT2OBT align on the surface of the electrode, forming an insulating layer, which decreases the tunneling leakage current, whereas in the latter cases, the interface between the semiconducting polymer backbone and PC70BM can come into contact with the electrode, forming a pathway for the leakage current. Consequently, the PFBT2OBT:PC70BM devices showed promising detectivities of over 1013 Jones over a wide range of reverse biases of up to −2 V, resulting from their low dark current density of less than 2.3 × 10−9 A cm−2.

Development of a julolidine-based interfacial modifier for efficient inverted polymer solar cells

To enhance the performance of inverted polymer solar cells (PSCs), we have designed and synthesized interfacial modifiers (IMs) which provide better surface properties for the ZnO layer. The synthesized IMs were composed of electron donating and accepting parts. Julolidine or diethyl aniline was used as a donor part, and cyano acrylic acid as an acceptor part. The julolidine-based B1 material exhibited a much stronger dipole moment of 9.86 D than diethyl aniline-based B2 (9.17 D), and the water contact angle of the ZnO surface treated with B1 (65.3°) was much larger than that with B2 (39.8°). These julolidine effects were much more powerful than diethyl aniline: the julolidine moiety significantly decreased the hydrophilicity of the ZnO surface and its work function (WF). The inverted devices with the configuration of ITO/ZnO/IMs (B1 or B2)/PTB7-Th:PC71BM/MoO3/Ag were fabricated and the photovoltaic properties were investigated. Both B1 and B2 worked well as IMs, but B1-treated devices exhibited a much higher power conversion efficiency of 8.35% than B2-treated devices (7.80%). B1 treatment on ZnO effectively reduced series resistance and leakage current in the devices due to the julolidine effects. We believe that julolidine is an excellent electron donating building block for IMs in inverted PSCs and our approach can be applied to other IM systems universally to improve photovoltaic performance.

Performance Improvement in Low-Temperature-Processed Perovskite Solar Cells by Molecular Engineering of Porphyrin-Based Hole Transport Materials

Porphyrin derivatives have recently emerged as hole transport layers (HTLs) because of their electron-rich characteristics. Although several successes with porphyrin-based HTLs have been recently reported, achieving excellent solar cell performance, the chances to improve this further by molecular engineering are still open. In this work, Zn porphyrin (PZn)-based HTLs were developed by conjugating fluorinated triphenylamine (FTPA) wings at the perimeter of the PZn core for low-temperature perovskite solar cells (L-PSCs). The fluorinated PZn-HTLs (PZn-2FTPA and PZn-3FTPA) exhibited superior HTL properties compared to the nonfluorinated one (PZn-TPA). Moreover, their deeper highest occupied molecular orbital energy levels were beneficial for boosting open-circuit voltages, and their enhanced face-on stacking improved the hole transport properties. The L-PSC using PZn-2FTPA achieved the highest performance of 18.85%. Thus far, this result is one of the highest reported power conversion efficiencies among the PSCs using porphyrin-based HTLs.