Gyeong Woo Kim

A highly efficient transition metal oxide layer for hole extraction and transport in inverted polymer bulk heterojunction solar cells

This paper demonstrates a comparative study of transition metal oxide materials, tungsten trioxide (WO3) and molybdenum trioxide (MoO3), as an effective hole extraction and transport layer in inverted polymer bulk heterojunction solar cells. Their device performances as an anode buffer layer are investigated based on thieno[3,4-b]thiophene/benzodithiophene (PTB7) and [6,6]-phenyl C70-butyric acid methyl ester(PC70BM) photoactive layers. The fabricated device with WO3 compared with MoO3 shows improved power conversion efficiency as high as 6.67% under a simulated AM1.5G illumination of 100 mW cm−2. The excellent performance of WO3 in inverted bulk heterojunction solar cells is attributed to an efficient hole extraction, excellent electron blocking capability, smooth morphology as well as better ohmic contact between the active layer and the metal electrode.

Diketopyrrolopyrrole-based copolymers bearing highly π-extended donating units and their thin-film transistors and photovoltaic cells

New diketopyrrolopyrrole (DPP)-based π-extended conjugated polymers containing relatively long conjugated donor monomers were successfully synthesized in order to investigate their physical properties and device performance in thin-film transistors and photovoltaic cells. The solubility of the polymers was improved via side-chain engineering, and relatively high molecular weights were achieved by modifying the synthetic procedures. The highest occupied molecular orbital and the lowest unoccupied molecular orbital and optical bandgaps of the polymers were easily tuned by incorporating different donor monomers with different donating abilities. All polymers were highly crystalline with a predominant edge-on orientation on the substrate. The polymers displayed different hole mobilities in TFTs depending on the donor monomer. The highest mobility was 4.17 cm2 V−1 s−1, which was shown by the TFT fabricated with a terselenophene-containing DPP polymer after thermal annealing. Furthermore, bulk heterojunction organic photovoltaic cells made from a blend film of the polymers and (6,6)-phenyl C71-butyric acid methyl ester demonstrated promising device performance with power conversion efficiencies in the range of 3.48–5.05%.

New interfacial materials for rapid hole-extraction in organic photovoltaic cells

Two new hole-extraction materials, TPDI (5,10,15-triphenyl-5H-diindolo[3,2-a:3′,2′-c]carbazole) and TBDI (5,10,15-tribenzyl-5H-diindolo[3,2-a:3′,2′-c]carbazole), were synthesized and explored in planar heterojunction organic solar cells (OSCs). The synthesized materials have good transparency in the visible spectrum, high hole mobility, and compatible highest occupied molecular orbital (HOMO) values with the donor material, which make them excellent hole-extraction layers (HELs) for OSCs. The SubPc (subphthalocyanine chloride)/C60 and SubNc (subnaphthalocyanine chloride)/C60 OSCs with a TBDI HEL show impressive 35.9% and 29.1% improvements in power conversion efficiencies compared to reference devices. Their HOMO values were evaluated as 5.2–5.3 eV and hole mobilities were measured as 5.9–6.1 × 10−3 cm2 V−1 s−1 at 0.3 MV cm−1 by the space-charge limited current method.

Effectiveness of a polyvinylpyrrolidone interlayer on a zinc oxide film for interfacial modification in inverted polymer solar cells

This paper investigate the effectiveness of non-conjugated polymer polyvinylpyrrolidone (PVP) at the interface of an n-type metal oxide buffer layer and the photoactive layer in inverted bulk heterojunction solar cells. A 15% enhancement in power conversion efficiency (PCE) is realized after the incorporation of a thin PVP layer between zinc oxide (ZnO) and polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC70BM) based photoactive layer in inverted polymer solar cells. The fabricated devices with the PVP layer show enhanced PCE as high as 7.30% under simulated AM 1.5 G (100 mW cm−2) illumination. The ZnO/PVP improves the electron extraction property of the ITO electrode, effectively blocks holes from the highest occupied molecular orbital of the donor, suppresses charge recombination at the interface of ZnO and the photoactive layer, and decreases the interfacial contact resistance.

Thermal Annealing Effect of Subphthalocyanine (SubPc) Donor Material in Organic Solar Cells

We report that the S-shaped kink of current density-voltage (J-V) characteristics, which reduce fill factor of organic photovoltaic cells, can be removed by substrate heating during donor layer deposition process. Subphthalocyanine (SubPc) donor film in combination with buckminsterfullerene (C60) acceptor film has been studied in a planar bilayer donor/acceptor heterojunction by J-V characterization under AM 1.5 simulated illuminations with various thicknesses of SubPc donor film. The substrate heating process enhances hole mobility of SubPc film, resulting 8.7% of efficiency enhancement with markedly improvement of S-shaped kink in J-V curves.

High Mobility Hole Extraction Material for Organic Solar Cell Application

We report a high mobility hole extraction material, N,N,N′,N′,-tetrakis(biphenyl-4-yl)benzidine (TBBD) for organic solar cell applications. Its highest occupied molecular orbital energy value at 5.3 eV is well matched with a donor layer and its high mobility of 1.74 × 10−2 cm2/Vs brings out an easy and rapid extraction of holes from the donor material. The TBBD incorporated device showed significant improvement of open-circuit voltage, resulting in power conversion efficiency improvement of 9% compared with reference device.

A New Exciton Blocking Material for Organic Solar Cell Applications

We report a new efficient exciton blocking material, tris(2,4,6-trimethyl-3-(pyridin-3-l)phenyl) (3TPYMB), for organic solar cell applications. The introduction of 3TPYMB as an exciton blocking layer instead of well known BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) results in the significant improvement of short circuit current (Jsc) and open circuit voltage (Voc) in the boron subphthalocyanine (SubPC)/C60 based organic solar cell devices. As the results, the power conversion efficiency is enhanced by 11.8%. Such improvement with this new exciton blocking layer is mainly attributed to the good electron transportation by deep penetration of Al cathode metal to 3TPYMB layer.

A Comparative Study of the VOC in CuPc and SubPc Organic Solar Cells

The open-circuit voltage (VOC) with respect to the reverse dark saturation current density in planar CuPc/C60 and non-planar boron subphthalocyanine chloride (SubPc)/C60 organic solar cell (OSCs) devices is compared in the present study. The fabricated OSCs devices were: ITO/CuPc/C60/BCP(90Å)/Al(1000Å) and ITO/SubPc/ C60/BCP(90Å)/Al(1000Å) organic solar cells. We observed that the dark saturation current and the VOC values vary in the CuPc/C60 solar cells, whereas almost no variation of the dark saturation current and VOC values in SubPC/C60 solar cells occur. All SubPC devices show almost similar value of dark current. These differences may be attributed to the surface morphologies of planar CuPc and non-planar SubPC donor layers.