Robert C. Coffin

Low Band Gap Donor‐Acceptor Conjugated Polymer Nanoparticles and their NIR‐mediated Thermal Ablation of Cancer Cells

Low band gap D-A conjugated PNs consisting of 2-ethylhexyl cyclopentadithiophene co-polymerized with 2,1,3-benzothiadiazole (for nano-PCPDTBT) or 2,1,3-benzoselenadiazole (for nano-PCPDTBSe) have been developed. The PNs are stable in aqueous media and showed no significant toxicity up to 1 mg · mL(-1) . Upon exposure to 808 nm light, the PNs generated temperatures above 50 °C. Photothermal ablation studies of the PNs with RKO and HCT116 colorectal cancer cells were performed. At concentrations above 100 µg · mL(-1) for nano-PCPDTBSe, cell viability was less than 20%, while at concentrations above 62 µg · mL(-1) for nano-PCPDTBT, cell viability was less than 10%. The results of this work demonstrate that low band gap D-A conjugated polymers 1) can be formed into nanoparticles that are stable in aqueous media; 2) are non-toxic until stimulated by IR light and 3) have a high photothermal efficiency.

High efficiency organic solar cells with spray coated active layers comprised of a low band gap conjugated polymer

Airbrush is a promising tool for large scale organic thin film deposition in photovoltaic devices fabrication. This paper reports a detailed study on solar cell performance using airbrush spray deposition for active layer composed with recently developed low band gap donor material poly[4,8-bis(1-pentylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-2,1,3-benzoxadiazole-4,7-diyl and [6,6]-phenyl-C61-butyric acid methyl ester. The effect of carrier solvent and substrate temperature on film morphology are studied; a formula in 1,2-dichlorobenzene sprayed at a substrate temperature of 80 °C is found to be the optimum condition that produces a peak power conversion efficiency of 5.8%.

Charge balance and photon collection in polymer based ternary bulk heterojunction photovoltaic devices containing cadmium selenide nanoparticles

Solar cells employing a ternary bulk heterojunction active layer comprised of poly(3-hexylthiophene) (P3HT), 6,6-phenyl C61-butyric acid methyl ester (PCBM) doped with composites constructed from a combination of 2.5 nm CdSe nanoparticles (NP), and methyl viologen (MV) have been examined. It was found that the devices containing the CdSe NP/MV composite exhibit significantly more photocurrent in a region surrounding the absorption peak of the particles (560-660 nm) when compared to pristine P3HT:PCBM devices. For a low ratio of CdSe to PCBM, the photocurrent collection was accompanied by space charge build up that limited the performance of the devices. When the ratio of CdSe to PCBM was raised, the space charge dissipated and performance recovered. JV curve shape analysis suggests that charge balance was achieved; however, electrode selectivity was reduced.

Exploring Spray-Coating Techniques for Organic Solar Cell Applications

We have investigated spray coating as a novel processing method for organic solar cell fabrication. In this work, spraying parameters and organic solvent influences have been correlated with cell performance. Using airbrush fabrication, bulk heterojunction photovoltaic devices based on a new low band gap donor material: poly[(4,8-bis(1-pentylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-2,1,3-benzoxadiazole-4,7-diyl] with the C60-derivative (6,6)-phenyl C61-butyric acid methyl ester (PCBM) as an acceptor, have achieved power conversion efficiencies over 3%. We show that airbrush fabrication can be carried out with simple solvents such as pristine 1,2-dichlorobenzene. Moreover, the influence of device active area has been studied and the 1 cm2 device by spray coating maintained an excellent power conversion efficiency of 3.02% on average.

Silver Nanoparticle-Doped Titanium Oxide Thin Films for Intermediate Layers in Organic Tandem Solar Cell

In this work we investigate the Ag nanoparticle doping of TiOx used as an intermediate layer between subcells of a tandem organic photovoltaic. We use a model polymer cell structure of P3HT:TiOx:PEDOT:P3HT to observe charge-trapping effects as a function of nanoparticle content in the TiOx, as determined by the shape of the dark and illuminated current voltage curves of the devices. There is a direct correlation between the amount of Ag nanoparticles in the TiOx, and interfacial charge buildup, and charge trapping being completely mitigated at around 0.2% mol. This suggests that such doping schemes might provide a simple approach to the creation and use of TiOx layers for tandem cells.

Variation of the Side Chain Branch Position Leads to Vastly Improved Molecular Weight and OPV Performance in 4,8-dialkoxybenzo[1,2-b:4,5-b′]dithiophene/2,1,3-benzothiadiazole Copolymers

Through manipulation of the solubilizing side chains, we were able to dramatically improve the molecular weight (𝑀𝑤) of 4,8-dialkoxybenzo[1,2-b:4,5-b′]dithiophene (BDT)/2,1,3-benzothiadiazole (BT) copolymers. When dodecyl side chains (P1) are employed at the 4- and 8-positions of the BDT unit, we obtain a chloroform-soluble copolymer fraction with 𝑀𝑤 of 6.3 kg/mol. Surprisingly, by moving to the commonly employed 2-ethylhexyl branch (P2), 𝑀𝑤 decreases to 3.4 kg/mol. This is despite numerous reports that this side chain increases solubility and 𝑀𝑤. By moving the ethyl branch in one position relative to the polymer backbone (1-ethylhexyl, P3), 𝑀𝑤 is dramatically increased to 68.8 kg/mol. As a result of this 𝑀𝑤 increase, the shape of the absorption profile is dramatically altered, with 𝜆max = 637 nm compared with 598 nm for P1 and 579 nm for P2. The hole mobility as determined by thin film transistor (TFT) measurements is improved from ∼1×10−6 cm2/Vs for P1 and P2 to 7×10−4 cm2/Vs for P3, while solar cell power conversion efficiency in increased to 2.91% for P3 relative to 0.31% and 0.19% for P1 and P2, respectively.

The Effect of Side-Chain Structure on Copolymer-Based Bulk Heterojunction Solar Cells

This paper describes the synthesis and photovoltaic studies of copolymer based on benzo[1,2-b:4,5-b′]dithiophene and 2,1,3-benzothiadiazole. Bulk-heterojunction solar cells were fabricated by using chlorobenzene, and 2% chloronapthalene as a solvent additive in chlorobenzene. The copolymers as the electron donor were blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the electron acceptor. The effect of side–chain on BDT was compared between 4,8-bis(1-pentylhexyl)benzo[1,2-b: 4,5-b′]dithiophene (C11BDT) and 4,8-bis(1-butylhexyl)benzo[1,2-b:4,5-b′]dithioph ene (C9BDT). The power conversion efficiency (PCE) was improved with the shorter side–chain. The highest PCE was found in PC9BDTBT with 2.29%.

Synthesis of Copolymer Thieno[3,4-b]Thiophene and Benzodithiophene for Application in Solar Cells

Synthesis of poly(2-hexylthieno[3,4-b]thiophene-co-4,8-didodecyloxybenzo[1,2-b:4, 5-b′]dithiophene) (PTB) is presented using a simple and low cost method. Anhydrous DMF and toluene was added into the mixture of 4,6-dibromo-2-hexylthieno[3,4-b] thiophene,2,6-Bis(trimethyltin)-4,8-didodecyloxybenzo[1,2-b:4,5-b′]dithiophene and Pd(PPh3)4. The polymerization was carried out at 120°C for 12 h. The PTB product was precipitated in methanol and dried in vacuum. The HOMO and LUMO energies of the polymer were calculated using Density Functional Theory (DFT) calculations to be compared with the experimental results.

Determining the effect of selenium substitution on low band-gap conjugated polymer-fullerene solar cells by optoelectronic characterization

The design of new, low band-gap polymers is a major part of organic photovoltaic research. Atomic substitution in a ring structure is one way to alter the electronic properties of a polymer (such as substituting the sulfur in a thiophene ring for selenium to make a selenaphene ring). We show that, starting with a benzodithiophene donor, that substituting benzoselenadiazole for benzothiodiazole leads to a polymer with a lower band-gap as characterized by the onset of optical absorption. Photovoltaic devices, optimized for 1,2-dichlorobenzene as the solvent, are comparable except that the polymer containing benzoselenidiazole collects over 1 mA/cm2 less short circuit current than the polymer containing benzothiodiazole, a counter intuitive result. Similar fill factors suggest that film morphology is not a contributor to this discrepancy. This result is further strengthened by optical time of flight measurements that show a better hole mobility for the benzoselenidiazole polymer (at room temperature and over a range of external fields). We then conducted further test in order to find the cause of this discrepancy.