Myoung Hee Yun

Semi-crystalline photovoltaic polymers with efficiency exceeding 9% in a ∼300 nm thick conventional single-cell device

We report a series of semi-crystalline, low band gap (LBG) polymers and demonstrate the fabrication of highly efficient polymer solar cells (PSCs) in a thick single-cell architecture. The devices achieve a power conversion efficiency (PCE) of over 7% without any post-treatment (annealing, solvent additive, etc.) and outstanding long-term thermal stability for 200 h at 130 °C. These excellent characteristics are closely related to the molecular structures where intra- and/or intermolecular noncovalent hydrogen bonds and dipole–dipole interactions assure strong interchain interactions without losing solution processability. The semi-crystalline polymers form a well-distributed nano-fibrillar networked morphology with PC70BM with balanced hole and electron mobilities (a h/e mobility ratio of 1–2) and tight interchain packing (a π–π stacking distance of 3.57–3.59 A) in the blend films. Furthermore, the device optimization with a processing additive and methanol treatment improves efficiencies up to 9.39% in a ∼300 nm thick conventional single-cell device structure. The thick active layer in the PPDT2FBT:PC70BM device attenuates incident light almost completely without damage in the fill factor (0.71–0.73), showing a high short-circuit current density of 15.7–16.3 mA cm−2. Notably, PPDT2FBT showed negligible changes in the carrier mobility even at ∼1 μm film thickness.

Soft network composite materials with deterministic and bio-inspired designs

Hard and soft structural composites found in biology provide inspiration for the design of advanced synthetic materials. Many examples of bio-inspired hard materials can be found in the literature; far less attention has been devoted to soft systems. Here we introduce deterministic routes to low-modulus thin film materials with stress/strain responses that can be tailored precisely to match the non-linear properties of biological tissues, with application opportunities that range from soft biomedical devices to constructs for tissue engineering. The approach combines a low-modulus matrix with an open, stretchable network as a structural reinforcement that can yield classes of composites with a wide range of desired mechanical responses, including anisotropic, spatially heterogeneous, hierarchical and self-similar designs. Demonstrative application examples in thin, skin-mounted electrophysiological sensors with mechanics precisely matched to the human epidermis and in soft, hydrogel-based vehicles for triggered drug release suggest their broad potential uses in biomedical devices.

Synthesis of PCDTBT-based fluorinated polymers for high open-circuit voltage in organic photovoltaics: towards an understanding of relationships between polymer energy levels engineering and ideal morphology control.

The introduction of fluorine (F) atoms onto conjugated polymer backbone has verified to be an effective way to enhance the overall performance of polymer-based bulk-heterojunction (BHJ) solar cells, but the underlying working principles are not yet fully uncovered. As our attempt to further understand the impact of F, herein we have reported two novel fluorinated analogues of PCDTBT, namely, PCDTFBT (1F) and PCDT2FBT (2F), through inclusion of either one or two F atoms into the benzothiadiazole (BT) unit of the polymer backbone and the characterization of their physical properties, especially their performance in solar cells. Together with a profound effect of fluorination on the optical property, nature of charge transport, and molecular organization, F atoms are effective in lowering both the HOMO and LUMO levels of the polymers without a large change in the energy bandgaps. PCDTFBT-based BHJ solar cell shows a power conversion efficiency (PCE) of 3.96 % with high open-circuit voltage (VOC) of 0.95 V, mainly due to the deep HOMO level (-5.54 eV). To the best of our knowledge, the resulting VOC is comparable to the record VOC values in single junction devices. Furthermore, to our delight, the best PCDTFBT-based device, prepared using 2 % v/v diphenyl ether (DPE) additive, reaches the PCE of 4.29 %. On the other hand, doubly-fluorinated polymer PCDT2FBT shows the only moderate PCE of 2.07 % with a decrease in VOC (0.88 V), in spite of the further lowering of the HOMO level (-5.67 eV) with raising the number of F atoms. Thus, our results highlight that an improvement in efficiency by tuning the energy levels of the polymers by means of molecular design can be expected only if their truly optimized morphologies with fullerene in BHJ systems are materialized.

Toward the Realization of A Practical Diketopyrrolopyrrole‐Based Small Molecule for Improved Efficiency in Ternary BHJ Solar Cells

An easily accessible DPP-based small molecule (DMPA-DTDPP) has been synthesized by a simple and efficient route. The resulting molecule, when incorporated into a P3HT:PCBM-based BHJ solar cell, is found to significantly improve the efficiency. The utility of DMPA-DTDPP as an additive yields an increase in the short circuit current density (Jsc) because DMPA-DTDPP serves as an energy funnel for P3HT excitons at the P3HT:PCBM interfaces, resulting in an improved overall power conversion efficiency, compared to the P3HT:PCBM control device. Considering the trouble-free and cost effective synthesis of DMPA-DTDPP, it may prove very useful in high-performance solar cells.

Towards optimization of P3HT:bisPCBM composites for highly efficient polymer solar cells

The optimization of the polymer solar cells based on regioregular poly(3-hexylthiophene) (P3HT) and the bisadduct of phenyl C61-butyric acid methyl ester (bisPCBM) is studied thoroughly as a role of solvent-annealing effect as well as different concentration of bisPCBM. In the case of P3HT:bisPCBM of 1 : 0.8 w/w, more balanced electron and hole mobilities are observed, resulting in better performance of the solar cells. Under the best balance conditions such as P3HT:bisPCBM of 1 : 0.8 w/w, the solvent annealing is employed to further clarify the optimization of the devices. Such a treatment leads to the formation of crystalline P3HT domains in the blend films, which is determined by X-ray diffraction, UV-vis spectroscopy, and atomic force microscopy. From our experiment, one can conclude that the best power conversion efficiency of 3.75% is achieved in a layered structure of P3HT:bisPCBM of 1 : 0.8 w/w for a solvent-annealing time of 24 h.

Replacing 2,1,3-benzothiadiazole with 2,1,3-naphthothiadiazole in PCDTBT: towards a low bandgap polymer with deep HOMO energy level

With the rising interest in using the medium bandgap polymer, poly(2,7-carbazole-alt-4,7-dithienyl-2,1,3-benzothiadiazole) (PCDTBT) with deep HOMO energy level for polymer solar cells (PSCs), we have developed an analogous polymer with a lower bandgap, namely, poly(2,7-carbazole-alt-4,7-dithienyl-2,1,3-naphthothiadiazole) (PCDTNT) by replacing 2,1,3-benzothiadiazole (BT) with 2,1,3-naphthothiadiazole (NT) in PCDTBT. Its optical, electrochemical, and photovoltaic properties are fully characterized in comparison with PCDTBT. Clearly, the λmax position of PCDTNT is significantly red-shifted by ∼30 nm, corresponding to a lower optical bandgap (1.71 eV) from the absorption edge of the thin film than that of PCDTBT (1.88 eV). A bulk-heterojunction (BHJ) PSC that incorporated PCDTNT with the low-lying HOMO energy level as a p-type material delivers a higher VOC value of 0.81 V and a power conversion efficiency (PCE) value of 1.31%.

Enhanced performance of polymer bulk heterojunction solar cells employing multifunctional iridium complexes

We report on the enhanced performance of polymer bulk heterojunction solar cells composed of an iridium complex with pendant sodium cations (pqIrpicNa) as an energy donor, poly(3-hexylthiophene) (P3HT) as an energy acceptor, polyethylene oxide (PEO) as an ion channel, and PCBM as an electron acceptor. With the iridium complex and PEO as additives, we observe a 20% increase in the current density, from 8.57 mA cm−2 to 10.24 mA cm−2, and a photoconversion efficiency of up to 3.4%. The observed enhancement in current density comes primarily from an efficient triplet–singlet energy transfer from the iridium complex to P3HT. Transient photoluminescence studies reveal triplet–singlet energy transfer efficiency from pqIrpicNa to P3HT of over 99%. Because of this high energy transfer efficiency, an enhancement is observed in the incident photon-to-conversion efficiency spectrum between 350 and 550 nm, which overlaps with the absorption range of the iridium complex. We also observe enhanced nanophase segregation of the active layer with pqIrpicaNa and PEO by atomic force microscopy. We propose that the observed enhancement in the current density stems not only from the enhancement in the morphology with the iridium complex, but also from the enhanced mobility of the sodium cations toward the metal electrodes through the ion channel of PEO under sunlight, which results in an increased charge collection at the electrodes.

A hybrid solar cell fabricated using amorphous silicon and a fullerene derivative

Hybrid solar cells, based on organic and inorganic semiconductors, are a promising way to enhance the efficiency of solar cells because they make better use of the solar spectrum and are straightforward to fabricate. We report on a new hybrid solar cell comprised of hydrogenated amorphous silicon (a-Si:H), [6,6]-phenyl-C71-butyric acid methyl ester ([71]PCBM), and poly-3,4-ethylenedioxythiophene poly styrenesulfonate (PEDOT:PSS). The properties of these PEDOT:PSS/a-Si:H/[71]PCBM devices were studied as a function of the thickness of the a-Si:H layer. It was observed that the open circuit voltage and the short circuit current density of the device depended on the thickness of the a-Si:H layer. Under simulated one sun AM 1.5 global illumination (100 mW cm(-2)), a power conversion efficiency of 2.84% was achieved in a device comprised of a 274 nm-thick layer of a-Si:H; this is the best performance achieved to date for a hybrid solar cell made of amorphous Si and organic materials.

High-efficiency, hybrid Si/C60 heterojunction solar cells

Hybrid solar cells, based on combinations of organic and inorganic semiconductors, constitute a promising avenue to harness solar energy by taking advantage of the strengths of both organic and inorganic materials. In this work, we report the first high-efficiency hybrid solar cell of its type comprising p-type silicon with an organic n-type C60 layer. High efficiencies based on the Si/C60 heterojunction were realized by utilizing an ultra-thin, doped and highly conductive C60 layer. Fabrication parameters were thoroughly investigated and critical factors for the efficient operation of this type of device were found to include the C60 thickness, doping of the C60 layer (using tetrabutyl ammonium iodide, TBAI), age-induced surface passivation and the incorporation of anti-reflection coatings (ARCs). From current density–voltage (J–V) and capacitance–voltage (C–V) characteristics, we have characterized the influence of C60 doping and device aging on the depletion region width and electrical parameters. An optimal power conversion efficiency of 8.43% was realized after 4 days of aging and TBAI treatment, with the application of a quarter-wave Sb2O3 ARC.