Hanok Park

Synthesis and photovoltaic properties of narrow band gap copolymers of dithieno[3,2-b:2′,3′-d]thiophene and diketopyrrolopyrrole

Two new sets of donor–acceptor and donor–π–acceptor type copolymers with a narrow band-gap, PDTTDPP and PTDTTTDPP, based on dithieno[3,2-b:2′,3′-d]thiophene (DTT) and dithieno[3,2-b:2′,3′-d]thiophene bridged with thiophene (TDTTT) as a short π-conjugated spacer with diketopyrrolopyrrole (DPP), were successfully synthesized by Stille polymerization. Incorporation of linear long chain alkyl (decanyl) groups in the DTT core and branched alkyl (ethylhexyl) groups in the DPP unit improved the solubility and processability of the resulting co-polymers. UV-vis absorption spectroscopy and cyclic voltammetry results demonstrated that introduction of thiophene as a shorter conjugated spacer between the donor and acceptor units in the PTDTTTDPP copolymer facilitated tuning of the absorption capability between the donor and acceptor resulting in a red-shifted absorption as compared to the PDTTDPP copolymer. Thermogravimetric analysis of the copolymers showed high thermal decomposition temperatures of 375 °C for PDTTDPP and 370 °C for PTDTTTDPP. The hole mobility of the copolymers, measured using an organic thin film transistor device, was 1.0 × 10−6 cm2 for PDTTDPP and 4.5 × 10−3 cm2 for PTDTTTDPP. Photovoltaic properties of PDTTDPP and PTDTTTDPP with PC61BM were evaluated using different device fabrication conditions and photovoltaic devices based on the polymer : PC60BM (1 : 1 ratio (w/w)) bulk hetero-junction demonstrated a maximum PCE of 1.39% and 0.29% for PTDTTTDPP:PC61BM and PDTTDPP:PC61BM, respectively, in chlorobenzene.

Roles of interfacial modifiers in hybrid solar cells: inorganic/polymer bilayer vs inorganic/polymer:fullerene bulk heterojunction.

Hybrid solar cells (HSCs) incorporating both organic and inorganic materials typically have significant interfacial issues which can significantly limit the device efficiency by allowing charge recombination, macroscopic phase separation, and nonideal contact. All these issues can be mitigated by applying carefully designed interfacial modifiers (IMs). In an attempt to further understand the function of these IMs, we investigated two IMs in two different HSCs structures: an inverted bilayer HSC of ZnO:poly(3-hexylthiophene) (P3HT) and an inverted bulk heterojunction (BHJ) solar cell of ZnO/P3HT:[6,6]-phenyl C61-butyric acid methyl ester (PCBM). In the former device configuration, ZnO serves as the n-type semiconductor, while in the latter device configuration, it functions as an electron transport layer (ETL)/hole blocking layer (HBL). In the ZnO:P3HT bilayer device, after the interfacial modification, a power conversion efficiency (PCE) of 0.42% with improved Voc and FF and a significantly increased Jsc was obtained. In the ZnO/P3HT:PCBM based BHJ device, including IMs also improved the PCE to 4.69% with an increase in Voc and FF. Our work clearly demonstrates that IMs help to reduce both the charge recombination and leakage current by minimizing the number of defect sites and traps and to increase the compatibility of hydrophilic ZnO with the organic layers. Furthermore, the major role of IMs depends on the function of ZnO in different device configurations, either as n-type semiconductor in bilayer devices or as ETL/HBL in BHJ devices. We conclude by offering insights for designing ideal IMs in future efforts, in order to achieve high-efficiency in both ZnO:polymer bilayer structure and ZnO/polymer:PCBM BHJ devices.

Synthesis characterization and bulk-heterojunction photovoltaic applications of new naphtho[1,2-b:5,6-b′]dithiophene–quinoxaline containing narrow band gap D–A conjugated polymers

Alternating donor-acceptor (D–A) π-conjugated copolymers, poly[2,7-bis(3-hexadecylthiophene-2-yl)naphtho[1,2-b:5,6-b′]dithiophene-5,5′-diyl-alt-5,8-bis(4-hexadecylthiophen-2-yl)-2,3-bis(4-(octyloxy)phenyl)quinoxaline-5,5′-diyl] (PTNDTT-QX-I) and poly[2,7-bis(3-hexadecylthiophene-2-yl)naphtho[1,2-b:5,6-b′]dithiophene-5,5′-diyl-alt-5,8-bis(thiophen-2-yl)-2,3-bis(3-(octyloxy)phenyl)quinoxaline-5,5′-diyl] (PTNDTT-QX2-II), were designed and synthesized based on the same thiophene-bridged naphtho[1,2-b:5,6-b′]dithiophene donor moiety, differing only at the quinoxaline acceptor counterpart by either additional electron-donating alkyl chain substitution in the thienyl ring attached to the quinoxaline base (in PTNDTT-QX-I) or a change in the location of the outward alkoxy side chain substituent of the phenyl rings (to the meta-position) adjoining the quinoxaline base (in PTNDTT-QX-II). The effect of alkyl chain positioning on the thermal, optical, and electrochemical properties, as well as field effect transistors and solar cell performances of the copolymers, were investigated and the results were compared with a previously published copolymer, PTNDTT-QX, which features a similar quinoxaline unit but is alkoxy substituted at the position para to its peripheral phenyl rings. Both polymers exhibited excellent thermal stability, with thermal decomposition temperatures over 400 °C. They absorbed light in the 300–700 nm range and exhibited optical band gaps of about 1.70 and 1.73 eV for PTNDTT-QX-I and PTNDTT-QX-II, respectively. Precise control of the alkyl/alkoxy chain positioning has made it possible to tune the HOMO energy levels between −5.14 and −5.29 eV and the LUMO energy levels between −3.44 and −3.55 eV. Bulk heterojunction photovoltaic devices of the structure ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al were fabricated by using the polymers as the donors and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the acceptor. Power conversion efficiencies (PCEs) of 1.28% and 1.61% respectively were achieved for the photovoltaic devices based on PTNDTT-QX-I/PC71BM and PTNDTT-QX-II/PC71BM under AM 1.5 G simulated 1-sun solar illumination.

Naphtho[1,2-b:5,6-b′]dithiophene-based conjugated polymer as a new electron donor for bulk heterojunction organic solar cells

We report for the first time the synthesis and photovoltaic application of a naphtho[1,2-b:5,6-b′]dithiophene (NDT) containing donor–acceptor conjugated copolymer. The combination of a thiophene-bridge NDT building block with a quinoxaline acceptor unit results in a new copolymer that has an optical band gap of 1.77 eV, a HOMO level of −5.24 eV, and a hole mobility of 5.0 × 10−5 cm2 V−1 s−1. Preliminary characterization of the bulk-heterojunction solar cell device exhibits a PCE of around 1.5%.

Device performance of inverted polymer solar cells with AgSiO 2 nanoparticles in active layer

UNLABELLED Localized surface plasmon mediated polymer solar cells (PSCs) were fabricated using the Ag/SiO(2) nanoparticles (NPs). The inverted PSC structure without poly (3,4-ethylenedioxythiophene) polystyrene sulfonate ( PEDOT PSS) was prepared due to the efficient insertion of Ag/SiO(2) NPs in the vicinity of active layer, which led to an enhancement in photo-conversion efficiency (PCE). This enhancement mainly comes from the light scattering by the SiO(2) shell and the localized surface plasmon effect by the Ag core, but we also considered the structural issues such as the NP distribution, the swelling of the active layer and of the metal electrode.

Influence of the terminal donor on the performance of 4,8-dialkoxybenzo[1,2-b:4,5′]dithiophene based small molecules for efficient solution-processed organic solar cells

Two π-conjugated small molecules, BDT(TTBT)2 and BDT(PTBT)2, based on benzodithiophene (BDT) and benzothiadiazole (BT) substituted with different terminal groups such as hexyl-bithiophene and hexylphenyl-thiophene are investigated for use in solution-processed organic solar cells (OSCs). Investigations into the molecules reveal that variation of the terminal groups not only influences the optical and electronic properties, but also affects the crystallization and morphology of the small molecules. The BDT(TTBT)2 device showed a power conversion efficiency (PCE) of 1.73% as a consequence of deep HOMO (Voc = 0.81 V), improved charge delocalization, and strong light absorption (Jsc = 4.75 mA cm−2) when mild annealing was used as a result of improved texture in morphology, while the BDT(PTBT)2 device rather showed a moderate PCE of 1.22% with a Jsc of 2.88 mA cm−2, a Voc of 0.81 V, and a FF of 0.52.

Synthesis and Photovoltaic Properties of Moderate Band Gap Diketopyrrolopyrrole Based Small Molecules for Solution Processed Organic Solar Cells

Two new small molecules, based on diketopyrrolopyrrole core flanked by cyanothiophene units (CN-DPP-CN, CN-TH-DPP-TH-CN) were synthesized using CuCN and palladium catalyzed Suzuki coupling and explored in organic solar cells (OSCs). The HOMO/LUMO energy levels of CN-DPP-CN, CN-TH-DPP-TH-CN having moderate band gap of 1.83 eV and 1.44 eV were estimated to be −5.63/−3.84 eV, −5.20/−3.75 eV respectively. The device efficiency was found to be 0.013, 0.21% for CN-DPP-CN, CN-TH-DPP-TH-CN respectively as donors for BHJ solar cells. When CN-DPP-CN (0.05%) was added in P3HT:PC60BM device, its PCE was enhanced to 2.45% from 2.08% signifying its ability to be used as potential n-type additive.