@article{shamey_zubair_cheema_2019, title={Unique hue stimulus selection using Munsell color chips under different chroma levels and illumination conditions}, volume={36}, ISSN={["1520-8532"]}, url={https://doi.org/10.1364/JOSAA.36.000983}, DOI={10.1364/JOSAA.36.000983}, abstractNote={To determine the role of chroma and illumination conditions on assessments of unique hue stimuli (UHs: Y, B, R, and G) 25 color normal observers selected Munsell color chips under controlled but different light sources that included simulated D65 daylight, incandescent (A), CWF and TL84, from two rotatable trays-one containing low and the other high chroma samples. Three independent evaluations were obtained from each observer with a gap of at least 24 h between assessments. A total of 2400 UH assessments were thus obtained. The mean, as well as the range and variability, in UH selections was calculated according to gender, illumination condition, and chroma. Results show statistically significant differences for uB and uR stimuli selections from the low to the high chroma sets under all lighting conditions, and for uY under light source A and CWF while differences in uG selections in different lighting conditions were statistically insignificant. No significant difference was found between male and female UH selections. Hue shift predictions based on CAT02 transform were found to agree well with observer responses under different illumination conditions. It was also found that observers' UH choices varied more for low chroma samples under different lighting conditions compared to high chroma samples. The highest variability was obtained under light source CWF, while the simulated D65 source resulted in the lowest variability in selections.}, number={6}, journal={JOURNAL OF THE OPTICAL SOCIETY OF AMERICA A-OPTICS IMAGE SCIENCE AND VISION}, author={Shamey, Renzo and Zubair, Muhammad and Cheema, Hammad}, year={2019}, month={Jun}, pages={983–993} }
 @article{cheema_younts_gautam_gundogdu_el-shafei_2016, title={Design and synthesis of BODIPY sensitizers with long alkyl chains tethered to N-carbazole and their application for dye sensitized solar cells}, volume={184}, ISSN={["1879-3312"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84994504448&partnerID=MN8TOARS}, DOI={10.1016/j.matchemphys.2016.09.024}, abstractNote={In this study, three boron dipyrromethenes (BODIPY) dyes with extended conjugation and electron donating carbazole groups with different alkyl chain lengths tethered to N-carbazole were synthesized and characterized for dye-sensitized solar cells. The goal was to study the effect of different alkyl chain lengths on dye aggregation at TiO2 surface. The proposed molecular strategy resulted in BODIPY dyes which showed interesting electronic absorption and fluorescence properties. It was observed that intramolecular energy transfer decreases with the increase in alkyl chain length possibly due to induced changes in molecular geometry caused by long alkyl chains. Additionally, interface analysis by impedance spectroscopy in comparison to N719 sensitized TiO2 solar cell showed significant charge transport related losses (Nyquist plot) most likely due to impedance resulted from aggregated BODIPY dye on TiO2 surface. Femtosecond transient absorption studies showed the loss of excited electrons by recombination with oxidized ground state of the sensitizers.}, journal={MATERIALS CHEMISTRY AND PHYSICS}, author={Cheema, Hammad and Younts, Robert and Gautam, Bhoj and Gundogdu, Kenan and El-Shafei, Ahmed}, year={2016}, month={Dec}, pages={57–63} }
 @article{babu_elsherbiny_cheema_el-shafei_adhikari_2016, title={Highly efficient panchromatic dye-sensitized solar cells: Synergistic interaction of ruthenium sensitizer with novel co-sensitizers carrying different acceptor units}, volume={132}, ISSN={["1873-3743"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84969544853&partnerID=MN8TOARS}, DOI={10.1016/j.dyepig.2016.05.016}, abstractNote={Herein, we report the molecular design, synthesis and photovoltaic performance studies of three new organic co-sensitizers, N1-3 carrying indole and thiophene units linked to different acceptors/anchoring groups, as co-adsorbents for dye sensitized solar cells. We present the role of anchoring/acceptor units on co-sensitization properties N1-3. Their photo-physical and electrochemical results along with molecular geometry, obtained from Density Functional Theory are utilized to rationalize the influence of co-sensitizer structures on photovoltaic properties for DSSCs. We have shown that, the co-sensitization effect is profoundly dependent upon the anchoring/acceptor unit in the co-adsorbents. Among them, N3 containing 4-aminobenzoic acid shows promising co-sensitization results and exhibits an enhanced efficiency of 9.26%, when co-sensitized with a ruthenium dye, HD-14. Further, the study highlights the importance of molecular matching between the sensitizer and co-sensitizer in enhancing the efficiency. Furthermore, vertical electronic excitations are calculated using time dependent density functional theory studies.}, journal={DYES AND PIGMENTS}, author={Babu, Dickson D. and Elsherbiny, Dalia and Cheema, Hammad and El-Shafei, Ahmed and Adhikari, Airody Vasudeva}, year={2016}, month={Sep}, pages={316–328} }
 @article{cheema_younts_ogbose_gautam_gundogdu_el-shafei_2015, title={A femtosecond study of the anomaly in electron injection for dye-sensitized solar cells: the influence of isomerization employing Ru(II) sensitizers with anthracene and phenanthrene ancillary ligands}, volume={17}, ISSN={["1463-9084"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84949117754&partnerID=MN8TOARS}, DOI={10.1039/c4cp04741a}, abstractNote={In this study, an intriguing difference caused by structural isomerization based on anthracene and phenanthrene stilbazole type ancillary ligands in Ru(ii) sensitizers for dye sensitized solar cells (DSCs) has been investigated using femtosecond transient absorption spectroscopy. Both anthracene and phenanthrene based sensitizers HD-7 and HD-8, respectively, resulted in a similar extinction coefficient, photophysical and thermodynamic free energy of electron injection and dye regeneration as measured by UV-Vis, excited state lifetime and cyclic voltammetry measurements, respectively. However, TiO2 adsorbed HD-7 resulted in up to 45% less photocurrent density than HD-8 although photovoltage was similar owing to comparable thermodynamic characteristics. It was obvious from the measurement of incident photon to current conversion efficiency (IPCE) that excited electrons in HD-7 are prone to internal energy loss before injection into the TiO2 conduction band. Analysis of photo-induced spectral features measured by femtosecond transient absorption spectroscopy showed that excited electrons in HD-7 are prone to ISC (intersystem crossing) much more than HD-8 and those triplet electrons are not injected into TiO2 efficiently. Interestingly, from impedance measurements, HD-7 showed higher recombination resistance than HD-8 and N719, but a shorter lifetime for electrons injected into the TiO2 conduction band.}, number={4}, journal={PHYSICAL CHEMISTRY CHEMICAL PHYSICS}, author={Cheema, Hammad and Younts, Robert and Ogbose, Louis and Gautam, Bhoj and Gundogdu, Kenan and El-Shafei, Ahmed}, year={2015}, month={Jan}, pages={2750–2756} }
 @article{shamey_zubair_cheema_2015, title={Effect of field view size and lighting on unique-hue selection using Natural Color System object colors}, volume={113}, ISSN={["1878-5646"]}, DOI={10.1016/j.visres.2015.03.023}, abstractNote={The aim of this study was twofold, first to determine the effect of field view size and second of illumination conditions on the selection of unique hue samples (UHs: R, Y, G and B) from two rotatable trays, each containing forty highly chromatic Natural Color System (NCS) samples, on one tray corresponding to 1.4° and on the other to 5.7° field of view size. UH selections were made by 25 color-normal observers who repeated assessments three times with a gap of at least 24h between trials. Observers separately assessed UHs under four illumination conditions simulating illuminants D65, A, F2 and F11. An apparent hue shift (statistically significant for UR) was noted for UH selections at 5.7° field of view compared to those at 1.4°. Observers' overall variability was found to be higher for UH stimuli selections at the larger field of view. Intra-observer variability was found to be approximately 18.7% of inter-observer variability in selection of samples for both sample sizes. The highest intra-observer variability was under simulated illuminant D65, followed by A, F11, and F2.}, journal={VISION RESEARCH}, publisher={Elsevier BV}, author={Shamey, Renzo and Zubair, Muhammad and Cheema, Hammad}, year={2015}, month={Aug}, pages={22–32} }
 @article{babu_cheema_elsherbiny_el-shafei_adhikari_2015, title={Molecular Engineering and Theoretical Investigation of Novel Metal-Free Organic Chromophores for Dye-Sensitized Solar Cells}, volume={176}, ISSN={["1873-3859"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84938295841&partnerID=MN8TOARS}, DOI={10.1016/j.electacta.2015.07.079}, abstractNote={In this work we report design and synthesis of three new metal free D-D–A–π–A type dyes (E1-3) with different acceptor/anchoring groups, as effective sensitizers for nanocrystalline titanium dioxide based dye sensitized solar cells. All the three dyes carry electron donating methoxy group as an auxiliary and indole as a principal donor, cyanovinylene as an auxiliary acceptor and thiophene as a π-spacer. Whereas, cyanoacetic acid, rhodanine-3-acetic acid and 4-aminobenzoic acid perform as acceptor/anchoring moieties, respectively in the dyes E1-3. Though the dye containing 4-aminobenzoic acid unit (E3) exhibits comparatively lower λmax, it shows the highest power conversion efficiency arising from the higher electron life time and good light-harvesting capability. The DFT studies reveal a better charge separation between the HOMO and LUMO levels of E3, further substantiating the experimental results. Among the three dyes, E3 shows the best photovoltaic performance with short-circuit current density (Jsc) of 9.35 mA cm−2, open-circuit voltage (Voc) of 620 mV and fill factor (FF) of 0.71, corresponding to an overall conversion efficiency of 4.12% under standard global AM 1.5G.}, journal={ELECTROCHIMICA ACTA}, author={Babu, Dickson D. and Cheema, Hammad and Elsherbiny, Dalia and El-Shafei, Ahmed and Adhikari, Airody Vasudeva}, year={2015}, month={Sep}, pages={868–879} }
 @article{cheema_islam_han_el-shafei_2015, title={Monodentate pyrazole as a replacement of labile NCS for Ru (II) photosensitizers: Minimum electron injection free energy for dye-sensitized solar cells}, volume={120}, ISSN={["1873-3743"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84928481607&partnerID=MN8TOARS}, DOI={10.1016/j.dyepig.2015.04.005}, abstractNote={A mono-dentate ligand of 3-(trifluoromethyl) pyrazole, instead of the labile isothiocyanate ligand for Ru (II) photosensitizers, was developed. This molecular modulation resulted in a new structure motif (HD-11) with 50 nm red shift in the low energy metal-to-ligand charge transfer (MLCT) absorption peak. However, electrochemical measurements revealed that both ground and excited state oxidation potentials were stabilized, we attributed this to the presence of strong electron withdrawing group CF3 tethered to pyrazole, which resulted in more free energy for dye regeneration (−ΔGregenration), but less driving force for electron injection (less −ΔGinjection) into the conduction band of TiO2. Hence HD-11, resulted in lower Jsc of 12.89 mA cm−2 compared to 17.07 mA cm−2 of N719 under similar conditions. Additionally, Voc of HD-11 was lower than N719, owing to decreased eTiO2 life time of 0.674 ms compared to 8.8 ms of N719 for injected electrons as found by impedance measurements.}, journal={DYES AND PIGMENTS}, author={Cheema, Hammad and Islam, Ashraful and Han, Liyuan and El-Shafei, Ahmed}, year={2015}, month={Sep}, pages={93–98} }
 @article{cheema_ogbose_el-shafei_2015, title={Structure-property relationships: Steric effect in ancillary ligand and how it influences photocurrent and photovoltage in dye-sensitized solar cells}, volume={113}, ISSN={["1873-3743"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84906735583&partnerID=MN8TOARS}, DOI={10.1016/j.dyepig.2014.08.005}, abstractNote={Here we report structure–property relationships of naphthalene-based ancillary ligands for DSCs employing Ru (II) bipyridyl complexes containing methoxy at 4,7-positions (HL-41) and ethoxy at 2-position (HL-42) on the naphthalene moiety respectively. 2-ethoxy naphthalene stilbazole-based ligand was strategically chosen as an ancillary ligand to ascertain the influence of steric effect of alkoxy group ortho to the spacer group (CHCH) of stilbazole on the photovoltaic properties. It was found that under similar conditions photocurrent response was in the following order HL-41 > HL-42 > N719. Both HL-41 and HL-42 showed similar redox potential. The overall conversion efficiency for devices employing HL-41, HL-42 and N719 were 7.8%, 7.6% and 7.6%, respectively. DFT calculations showed that the torsion angles between the naphthyl moiety and CHCH spacer was 34.81° and 39° for HL-41 and HL-42, respectively. Thus, for HL-42 ethoxy ortho to CHCH produces more twisted naphthyl which precludes efficient charge transfer from the ancillary ligand to the metal.}, journal={DYES AND PIGMENTS}, author={Cheema, Hammad and Ogbose, Louis and El-Shafei, Ahmed}, year={2015}, month={Feb}, pages={151–159} }
 @article{el-sherbiny_cheema_el-essawy_abdel-megied_el-shafei_2015, title={Synthesis and characterization of novel carbazole-based terpyridyl photosensitizers for dye-sensitized solar cells (DSSCs)}, volume={115}, ISSN={["1873-3743"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84922373450&partnerID=MN8TOARS}, DOI={10.1016/j.dyepig.2014.12.009}, abstractNote={Two novel terpyridyl-based Ru(II) complexes, DH-1 and DH-2, were synthesized and characterized as photosensitizers for dye-sensitized solar cells. To enhance the thermal and photostability, a strong electron donor bidentate carbazole-based ancillary ligand was molecularly engineered and used to replace two monodentate labile isothiocyanates. Photovoltaic characterizations, I–V curve and IPCE, of DH-1 and DH-2 were carried out in presence of deoxycholic acid as an additive to determine the best device working conditions. DH-1 showed IPCE of 45% at 540 nm, short circuit photocurrent density (Jsc) of 11.69 mA cm−2, open circuit photovoltage (Voc) of 0.64 V, fill factor of 0.63, producing an overall conversion efficiency (η) of 4.68%, where DH-2 showed IPCE of 45% at 540 nm, Jsc of 10.90 mA cm−2, Voc of 0.59 V and FF of 0.59, which translated into η of 3.82%. DFT calculations rationalized the light harvesting efficiency and charge separation of why DH-1 > DH-2.}, journal={DYES AND PIGMENTS}, author={El-Sherbiny, Dalia and Cheema, Hammad and El-Essawy, Farag and Abdel-Megied, Ahmed and El-Shafei, Ahmed}, year={2015}, month={Apr}, pages={81–87} }
 @article{cheema_islam_han_el-shafei_2014, title={Influence of Number of Benzodioxan-Stilbazole-based Ancillary Ligands on Dye Packing, Photovoltage and Photocurrent in Dye-Sensitized Solar Cells}, volume={6}, ISSN={["1944-8252"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84905002608&partnerID=MN8TOARS}, DOI={10.1021/am502400b}, abstractNote={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.}, number={14}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Cheema, Hammad and Islam, Ashraful and Han, Liyuan and El-Shafei, Ahmed}, year={2014}, month={Jul}, pages={11617–11624} }
 @article{cheema_islam_han_gautam_younts_gundogdu_el-shafei_2014, title={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}, volume={2}, ISSN={["2050-7496"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84905571530&partnerID=MN8TOARS}, DOI={10.1039/c4ta01942c}, abstractNote={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.}, number={34}, journal={JOURNAL OF MATERIALS CHEMISTRY A}, author={Cheema, Hammad and Islam, Ashraful and Han, Liyuan and Gautam, Bhoj and Younts, Robert and Gundogdu, Kenan and El-Shafei, Ahmed}, year={2014}, pages={14228–14235} }
 @article{cheema_islam_younts_gautam_bedja_gupta_han_gundogdu_el-shafei_2014, title={More stable and more efficient alternatives of Z-907: carbazole-based amphiphilic Ru(II) sensitizers for dye-sensitized solar cells}, volume={16}, ISSN={["1463-9084"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84911879802&partnerID=MN8TOARS}, DOI={10.1039/c4cp04120h}, abstractNote={Here we report two novel amphiphilic Ru(II) heteroleptic bipyridyl complexes, HD-14 and HD-15, compared to previously reported NCSU-10. We have combined the strong electron donor characteristics of carbazole and the hydrophobic nature of different long alkyl chains, C7, C18 and C2 (NCSU-10), tethered to N-carbazole to study their influence on photocurrent, photovoltage and long term stability for dye-sensitized solar cells. Photon harvesting efficiency and electron donating characteristics of carbazole-based ancillary ligands were found to be unaffected by different alkyl chain lengths. However, a slight drop in the Voc of HD-14 and HD-15 was observed compared to that of NCSU-10. It was found by nanosecond flash photolysis transient absorption (TA) measurements that the faster the dye regeneration the higher the photocurrent density response, and the dye regeneration time was found to be 2.6, 3.6, and 3.7 μs for HD-14, HD-15, and N719 dyes, respectively. The difference in the amplitude of the transient absorption (TA) signal of the oxidized dye as measured by femtosecond TA studies is in excellent agreement with the photocurrent generated, which was in the following order HD-14 > HD-15 > N719. Under 1000 h light soaking conditions, HD-15 maintained up to 98% (only 2% loss) of the initial power conversion efficiency compared to 8% loss for HD-14 and 22% loss in the power conversion efficiency for NCSU-10. HD-15 was strikingly stable to light soaking conditions when employed in the presence of an ionic liquid electrolyte, which paves the way for widespread applications of dye-sensitized solar cells with long term stability. The total power conversion efficiency (η) was 9.27% for HD-14 and 9.17% for HD-15 compared to 8.92% for N719.}, number={48}, journal={PHYSICAL CHEMISTRY CHEMICAL PHYSICS}, author={Cheema, Hammad and Islam, Ashraful and Younts, Robert and Gautam, Bhoj and Bedja, Idriss and Gupta, Ravindra Kumar and Han, Liyuan and Gundogdu, Kenan and El-Shafei, Ahmed}, year={2014}, pages={27078–27087} }
 @article{cheema_el-shafei_hauser_2013, title={Conferring flame retardancy on cotton using novel halogen-free flame retardant bifunctional monomers: synthesis, characterizations and applications}, volume={92}, number={1}, journal={Carbohydrate Polymers}, author={Cheema, H. A. and El-Shafei, A. and Hauser, P. J.}, year={2013}, pages={885–893} }