Hyun-Sub Shim

Enhancement of near-infrared absorption with high fill factor in lead phthalocyanine-based organic solar cells

Enhancing the short circuit current (JSC) by extending the absorption to the near-infrared (NIR) spectrum, which has 50% of the total solar photon flux, is a remaining task in small molecular solar cells. Here, high efficiency NIR absorbing solar cells based on lead phthalocyanine (PbPc) are reported using copper iodide (CuI) as a templating layer to control the crystal structure of PbPc. Devices with CuI inserted between the ITO and PbPc layers exhibit a two times enhancement of the JSC compared to the case in the absence of the CuI layer. This is explained by the increase of crystallinity in the molecules grown on the CuI templating layer, which is investigated via an X-ray diffraction study. Moreover, fill factor is also enhanced to 0.63 from 0.57 due to low series resistance although the additional CuI layer is inserted between the ITO and the PbPc layer. As a result, the corrected power conversion efficiency of 2.5% was obtained, which is the highest one reported up to now among the PbPc based solar cells.

CuI interlayers in lead phthalocyanine thin films enhance near-infrared light absorption

The insertion of a CuI interlayer improved the preferential alignment of lead phthalocyanine (PbPc) molecules in the monoclinic phase, as demonstrated by x-ray diffraction analysis of ultra-thin (5 nm) and thin (30 nm) PbPc films. CuI triclinic phases were not observed. The improved preferential alignment and crystallinity in the monoclinic phase enhanced near-infrared photon absorption. The power conversion efficiency of a PbPc/C60 planar heterojunction organic photovoltaic device fabricated using the CuI templating layer structure could be improved from 1.3% to 2.5%.

An efficient interconnection unit composed of electron-transporting layer/metal/p-doped hole-transporting layer for tandem organic photovoltaics

We report an efficient interconnection unit (ICU) consisting of an electron transporting layer/metal/p-doped hole transporting layer (p-HTL) structure for tandem organic photovoltaic (TOPV) cells. The ICU satisfies all the requirements of optical transparency and low voltage loss and for functioning as an optical spacer. The variation of the short circuit current and open circuit voltage (VOC) of the TOPV cells with increasing thickness of the p-HTL in the ICU followed the theoretical predictions, proving that the ICU does not disturb the electrical characteristics of the TOPV cells up to a p-HTL thickness of 100 nm with minimal VOC loss (∼3%).

Efficient Vacuum-Deposited Ternary Organic Solar Cells with Broad Absorption, Energy Transfer, and Enhanced Hole Mobility.

The use of multiple donors in an active layer is an effective way to boost the efficiency of organic solar cells by broadening their absorption window. Here, we report an efficient vacuum-deposited ternary organic photovoltaic (OPV) using two donors, 2-((2-(5-(4-(diphenylamino)phenyl)thieno[3,2-b]thiophen-2-yl)thiazol-5-yl)methylene)malononitrile (DTTz) for visible absorption and 2-((7-(5-(dip-tolylamino)thiophen-2-yl)benzo[c]-[1,2,5]thiadiazol-4-yl)methylene)malononitrile (DTDCTB) for near-infrared absorption, codeposited with C70 in the ternary layer. The ternary device achieved a power conversion efficiency of 8.02%, which is 23% higher than that of binary OPVs. This enhancement is the result of incorporating two donors with complementary absorption covering wavelengths of 350 to 900 nm with higher hole mobility in the ternary layer than that of binary layers consisting of one donor and C70, combined with energy transfer from the donor with lower hole mobility (DTTz) to that with higher mobility (DTDCTB). This structure fulfills all the requirements for efficient ternary OPVs.

High efficiency and high photo-stability zinc-phthalocyanine based planar heterojunction solar cells with a double interfacial layer

The use of CuI and MoO3 as a double interfacial layer between indium tin oxide (ITO) and a zinc phthalocyanine (ZnPc) layer improves the power conversion efficiency (ηp) and the photo-stability at the same time in ZnPc based solar cells. Insertion of CuI without MoO3 increased ηp more than 2 times to 3.3%. However, the photo-stability is lowered even further due to diffusion of Cu. Insertion of the MoO3 layer between the ITO and CuI prevents the diffusion of Cu under UV illumination to achieve the improved photo-stability and ηp.

Multilayer Epitaxial Growth of Lead Phthalocyanine and C70 Using CuBr as a Templating Layer for Enhancing the Efficiency of Organic Photovoltaic Cells

The molecular orientation and crystallinity of donor and acceptor molecules are important for high-efficiency organic photovoltaic cells (OPVs) because they significantly influence both the absorption of light and charge-transport characteristics. We report that the templating effect extends to multilayers to increase the crystallinity and to modify the orientation of the crystals of lead phthalocyanine (PbPc) and C70 layers at the same time by adopting CuBr as a new templating layer on indium tin oxide (ITO). The formation of a monoclinic phase with a preferred orientation of (320) for PbPc and a fcc phase with a preferred orientation of (220) for C70 on the PbPc layer is revealed by X-ray diffraction (XRD) patterns. The multilayer epitaxy results in an increase of the exciton diffusion lengths from 5.6 to 8.8 nm for PbPc and from 6.9 to 13.8 nm for C70 to enhance the power conversion efficiency (PCE) of the planar heterojunction OPVs composed of PbPc and C70 from 1.4 to 2.3%. The quasi-epitaxy model is proposed to explain the multilayer epitaxy.

A high performance semitransparent organic photodetector with green color selectivity

We report a high performance green color selective semitransparent inverted organic photodetectors (OPDs) with a detectivity of 2.1 × 1012 cm Hz1/2/W at the wavelength of 530 nm which has the comparable performance to the reported metal electrode based OPDs due to low dark current density of 3.8 × 10−10 A/cm2 at −1 V. The transparent OPD showed high transparency of 26% and 60% in the blue and red regions, respectively. The relatively low transmittance in the blue region can be improved using appropriate selection of active materials which absorb only the green region.

The epitaxial growth of lead phthalocyanine on copper halogen compounds as the origin of templating effects

Templating effects on the growth of lead phthalocyanine (PbPc) and the performance of organic photovoltaic cells (OPVs) have been investigated by using three copper halogen compounds (CuCl, CuBr, CuI), possessing different lattice parameters, as templating layers. The crystallinity of the PbPc films was highest on CuI followed by CuBr and CuCl, resulting in the broadening of the Q-band absorption in the same order. The templating effects were described by heteroepitaxial growth of organic molecules on the templating layers. The dimensionless potential calculated using a lattice model for the overlayer–substrate systems showed good correlation between the degree of epitaxy and the crystallinity of the PbPc overlayers. Furthermore, the performance of the OPVs was consistent with both the prediction from the calculation results and the observation from the optical and structural analyses.

Correlation of the electronic structure of an interconnection unit with the device performance of tandem organic solar cells

We report the correlation of the electrical properties of the p-doped layer in an interconnection unit with the performance of tandem organic photovoltaic (TOPV) cells where the interconnection unit (ICU) is composed of an electron-transporting layer (ETL)/metal/p-doped hole-transporting layer (p-HTL) by systematically varying the doping concentration of the p-HTL in the ICU. The open circuit voltage is significantly increased as the doping concentration of the p-HTL increases due to the reduction of the difference between the Fermi level and the highest occupied molecular orbital level of the p-HTL. The fill factor is also enhanced with increases in the doping concentration of the p-HTL due to the enhancement of the conductivity in the p-HTL and efficient hole transport at the interface between Ag and the p-HTL through the tunneling process, rather than through the thermionic process.

Enhancement of the Fill Factor through an Increase of the Crystallinity in Fullerene-Based Small-Molecule Organic Photovoltaic Cells

We report that the crystallinity of C70 is improved significantly if CuI is used as a templating layer, leading to remarkable enhancement of hole mobilities from 8.32 × 10(-6) to 3.26 × 10(-5) cm(2)/(V s). As a result, the use of the templating layer in C70-based solar cells with low donor concentration resulted in significant improvement of the fill factor from 0.51 to 0.57 and the power conversion efficiency from 5.56% to 6.23% under simulated AM 1.5G, 1 sun irradiation. This result demonstrates that the CuI templating layer is effective at improving the crystallinity of the fullerene derivatives as well as the donor materials.