Hammad Cheema

Conferring flame retardancy on cotton using novel halogen-free flame retardant bifunctional monomers: synthesis, characterizations and applications

Two novel halogen-free phosphorous-nitrogen flame retardant bifunctional monomers were synthesized and characterized using attenuated total reflectance/Fourier transform-infrared (ATR/FT-IR) and electrospray ionization mass spectrometry ((+)ESI-MS). The monomers were applied separately and graft polymerized on cotton in the presence of the thermal initiator K(2)S(2)O(8). The performance of each monomer was evaluated using thermal gravimetric analysis (TGA), grafting efficiency, and vertical flame test. It was shown that the performance of N,N-dimethyl di(acryloyloxyethyl)phosphoramide (DMDAEP) (monomer 2) as flame retardant outperformed that of ethyl di(acryloyloxyethyl)phosphorodiamidate (EDAEP) (monomer 1). The superior performance of DMDAEP was attributed to the presence of more nitrogen atoms compared to EDAEP. The increased nitrogen content in DMDAEP increased the synergistic effect of the P-N system. Cotton treated using padding methods showed more promising results than cotton treated by exhaust methods.

Influence of Number of Benzodioxan-Stilbazole-based Ancillary Ligands on Dye Packing, Photovoltage and Photocurrent in Dye-Sensitized Solar Cells

Two novel heteroleptic Ru(II) bipyridyl complexes, HD-2 and HD-2-mono, were molecularly engineered, synthesized and characterized for dye-sensitized solar cells (DSCs). The influences of mono versus bis electron-donor benzodioxan ancillary ligands on optical, dye packing, electrochemical and photovoltaic properties were examined and compared to the benchmark N719. HD-2 and HD-2-mono achieved solar-to-power conversion efficiencies (%η) of 9.64 and 9.50, respectively, compared to 9.32 for N719 under the same experimental device conditions. Optical results showed that HD-2 and HD-2-mono have much higher molar extinction coefficients, longer excited state lifetimes and narrower HOMO-LUMO gaps compared to N719. Although the molar extinction coefficient of HD-2-mono was 27% less than that of HD-2, it outperformed HD-2 in photovoltaic performance when anchored on TiO2, owing to better dye packing and loading of the former. Charge recombination at the dye/TiO2 interface by impedance spectroscopy analysis showed that the recombination resistance and the lifetime of injected electron in TiO2 conduction band is directly proportional to the open-circuit voltage (Voc) observed. Furthermore, compared to HD-2 and HD-2-mono, the greater Voc of N719 can be attributed to the greater negative free energy for dye regeneration. Both HD-2 and HD-mono have almost the same negative free energy, which explains why they achieved almost the same Voc. Decay dynamic analysis for solar devices fabricated from the named dyes, by time correlated single photon counting (TCSPC), elucidated that the lowest excited state decay lifetime for HD-2-mono, HD-2 and N719 are 3, 10 and 20 ps, respectively. The shorter the decay lifetime, the less kinetic redundancy, which leads to better photocurrent, and that explanation is consistent with the measured photocurrent and total solar-to-power conversion efficiency of the named dyes in the order of HD-2-mono > HD-2 > N719.

Influence of mono versus bis-electron-donor ancillary ligands in heteroleptic Ru(II) bipyridyl complexes on electron injection from the first excited singlet and triplet states in dye-sensitized solar cells

A novel heteroleptic Ru(II) bipyridyl complex (HD-1-mono) was molecularly designed with a mono-carbazole ancillary ligand, synthesized and characterized for DSCs. The aim was to systematically study the influence of mono (HD-1-mono) versus bis-carbazole ancillary ligand (NCSU-10) on molar absorptivity, light harvesting efficiency (LHE), ground and excited state oxidation potentials, incident-photon-to-current conversion efficiency (IPCE), electron injection from the first excited singlet and triplet states, short-circuit photocurrent density (Jsc), and total solar-to-electric conversion efficiency (η) for DSCs. This study showed that HD-1-mono exhibited slightly lower Voc but greater Jsc compared to NCSU-10. Though HD-1-mono showed lower extinction coefficient than NCSU-10, interestingly, it was found that the decrease in molar extinction coefficient of HD-1-mono is not directly related to the short-circuit photocurrent density (Jsc). For example, HD-1-mono showed a higher Jsc of 21.4 mA cm−2 without the presence of any additives. However, under optimized conditions, HD-1-mono showed a Jsc of 19.76 mA cm−2, Voc of 0.68 V, and (%η) of 9.33 compared to a Jsc of 19.58 mA cm−2, Voc of 0.71 and (%η) of 10.19 for NCSU-10, where N719 achieved a Jsc of 16.85 mA cm−2, Voc of 0.749 V and (%η) of 9.33 under the same experimental device conditions. Impedance results for HD-1-mono showed a shorter recombination time as compared to N719 and NCSU-10, which justify its lower Voc. Femtosecond transient absorption spectroscopy results elucidated that electron injection from the first triplet state is 63% more efficient for HD-1-mono than that of NCSU-10.

NIR Absorbing Metal-Free Organic, Porphyrin, and Phthalocyanine Dyes for Panchromatic DSCs

Dye-sensitized solar cells (DSCs) are a promising source of renewable energy. However, the power conversion efficiency (PCE) of devices has been limited largely by the difficulty of producing electricity using photons from the near-infrared (NIR) spectral region. Metal-free organic sensitizers frequently employ strong electron-donating or -withdrawing moieties to tune the optical band gap to allow the absorption of lower energy wavelengths in charge-transfer systems, whereas porphyrins and phthalocyanines use substituents to shift the Soret and Q bands toward lower energy absorption. Very few devices employing precious metal-free dyes have achieved panchromatic and NIR photon conversion for electricity generation at wavelengths >750 nm despite a tremendous number of sensitizers published over the last 25 years. This Minireview seeks to compile a summary of these sensitizers to encourage assimilation, analysis, and development of efficient future sensitizers with absorption extending into the NIR. Herein, we discuss common synthetic strategies, optical properties, and electronic properties of the most successful panchromatic organic sensitizers.

Harnessing Photovoltage: Effects of Film Thickness, TiO2 Nanoparticle Size, MgO and Surface Capping with DSCs

High photovoltage dye-sensitized solar cells (DSCs) offer an exceptional opportunity to power electrocatalysts for the production of hydrogen from water and the reduction of CO2 to usable fuels with a relatively cost-effective, low-toxicity solar cell. Competitive recombination pathways such as electron transfer from TiO2 films to the redox shuttle or oxidized dye must be minimized to achieve the maximum possible photovoltage (Voc) from DSC devices. A high Voc of 882 mV was achieved with the iodide/triiodide redox shuttle and a ruthenium NCS-ligated dye, HD-2-mono, by utilizing a combined approach of (1) modulating the TiO2 surface area through film thickness and nanoparticle size selection, (2) addition of a MgO insulating layer, and (3) capping available TiO2 film surface sites post film sensitization with an F-SAM (fluorinated self-assembled monolayer) treatment. The exceptional Voc of 882 mV observed is the highest achieved for the popular NCS containing ruthenium sensitizers with >5% PCE and compares favorably to the 769 mV value observed under common device preparation conditions.

Molecular Engineering of Near Infrared Absorbing Thienopyrazine Double Donor Double Acceptor Organic Dyes for Dye-Sensitized Solar Cells

The thienopyrazine (TPz) building block allows for NIR photon absorption in dye-sensitized solar cells (DSCs) when used as a π-bridge. We synthesized and characterized 7 organic sensitizers employing thienopyrazine (TPz) as a π-bridge in a double donor, double acceptor organic dye design. Donor groups are varied based on electron donating strength and sterics at the donor-π bridge bond with the acceptor groups varied as either carboxylic acids or benzoic acids on the π-bridge. This dye design was found to be remarkably tunable with solution absorption onsets ranging from 750 to near 1000 nm. Interestingly, the solution absorption measurements do not accurately approximate the dye absorption on TiO2 films with up to a 250 nm blue-shift of the dye absorption onset on TiO2. This shift in absorption and the effect on electron transfer properties is investigated via computational analysis, time-correlated single photon counting studies, and transient absorption spectroscopy. Structure-performance relationships were analyzed for the dyes in DSC devices with the highest performance observed at 17.6 mA/cm2 of photocurrent and 7.5% PCE for a cosensitized device with a panchromatic IPCE onset of 800 nm.

Sequential series multijunction dye-sensitized solar cells (SSM-DSCs): 4.7 volts from a single illuminated area

Sequential series multijunction dye-sensitized solar cells (SSM-DSCs), which are mechanically stacked single illuminated area DSC devices wired in series, are reported to have exceptionally high photovoltages (Voc) ranging from 1.9–4.7 V from 2–5 stacked subcells. The use of multiple photoactive films under one area within the SSM-DSC framework is made possible by fine-tuning the thickness of TiO2 in each device and by judicious dye selection to allow for excellent light distribution among the films, termed as “photon management”. The SSM-DSC approach allows for incorporation of materials designed to use the maximal potential energy of photons in each region of the solar spectrum. Importantly, SSM-DSCs were observed to maintain high Voc under low-light conditions, rendering these systems very attractive for indoor applications. Additionally, an SSM-DSC was found to have a solar-to-fuel conversion efficiency of 2% (2.7% including H2 production) for the reduction of CO2 to CO with IrO2 and Au2O3 electrocatalysts, without an external bias.

Effect of Donor Strength and Bulk on Thieno[3,4-b]-pyrazine-Based Panchromatic Dyes in Dye-Sensitized Solar Cells

Near-infrared-absorbing organic dyes are critically needed in dye-sensitized solar cells (DSCs). Thieno[3,4-b]pyrazine (TPz) based dyes can access the NIR spectral region and show power conversion efficiencies (PCEs) of up to 8.1 % with sunlight being converted at wavelengths up to 800 nm for 17.6 mA cm-2 of photocurrent in a co-sensitized DSC device. Precisely controlling dye excited-state energies is critical for good performances in NIR DSCs. Strategies to control TPz dye energetics with stronger donor groups and TPz substituent choice are evaluated here. Additionally, donor size influence versus dye loading on TPz dyes is analyzed with respect to the TiO2 surface protection designed to prevent recombination of electrons in TiO2 with the redox shuttle. Importantly, the dyes evaluated were demonstrated to work well with low Li+ concentration electrolytes, with iodine and cobalt redox shuttle systems, and efficiently as part of co-sensitized devices.

Ullazine Donor-π bridge-Acceptor Organic Dyes for Dye-Sensitized Solar Cells

A series of four ullazine-donor based donor-π bridge-acceptor (D-π-A) dyes have been synthesized and compared to a prior ullazine donor-acceptor (D-A) dye as well as a triphenylamine donor with an identical π-bridge and acceptor. The D-π-A ullazine series demonstrates an unusually uniform-in-intensity panchromatic UV/Vis absorption spectrum throughout the visible region. This is in part due to the introduction of strong high-energy bands through incorporation of the ullazine building block as shown by computational analysis. The dyes were characterized on TiO2 films and in DSC devices. Performances of 5.6 % power conversion efficiency were obtained with IPCE onsets reaching 800 nm.

Low-Recombination Thieno[3,4-b]thiophene-Based Photosensitizers for Dye-Sensitized Solar Cells with Panchromatic Photoresponses

We report four NIR photosensitizers employing a low-recombination donor and a thieno[3,4-b]thiophene (3,4-TT) π bridge for use in dye-sensitized solar cells. The inclusion of electron rich π spacers red-shifts the dye absorbance with solution absorption onsets reaching 700 nm. Dyes were found to have suitable energy levels for rapid electron transfers using cyclic voltammetry and UV/Vis-NIR absorption spectroscopy. Computationally optimized ground-state geometries show an increased torsional angle between π spacer and π bridge brought about by an added alkyl chain. This results in a widened optical band gap and increased oxidation potentials owing to a weakening of the electron-accepting ability of 3,4-TT for solution-state measurements. Interestingly in terms of device parameters, the alkylated π spacer had a nearly identical incident photon-to-current conversion efficiency (IPCE) curve onset when compared to a non-alkylated analogue, suggesting more similar dye geometries on the surface of TiO2 . Elevated short-circuit current density (JSC ) values and comparable open-circuit voltage (VOC ) values were observed in the alkylated-π-spacer-dye-based devices with power conversion efficiencies up to 6.8 % observed with IPCE onsets exceeding 800 nm.