Browsing by Author "Yohannes, Teketel (Professor)"
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Item Comparative Study on Solid-State Photoelectrochemical Solar Energy Conversion Based on P3ot and P3ot: Pcbm(Addis Ababa Universty, 2010-06) Derib, Kassa; Yohannes, Teketel (Professor)The photoelectrochemical properties of a solid-state photoelectrochemical cell (PEC) based on poly(3-octylthiophene), P3OT, and a blend of P3OT with phenyl-C61-butyric acid methyl ester, PCBM, an ion-conducting polymer electrolyte, amorphous poly(ethylene oxide), POMOE, complexed with iodine/triiodide redox couple has been constructed and studied. The current - voltage characteristics in the dark and under white light illumination, the dependence of the short - circuit (Isc) and the open - circuit voltage (Voc) of the two types of the devices have been compared. An open-circuit voltage of 344 mV and a shortcircuit current of 0.47 μA cm-2 were obtained for the device of P3OT as a photoactive electrode and an open-circuit voltage of 294 mV and a short-circuit current of 4.271 μA cm-2 were obtained for the device of P3OT:PCBM at light intensity of 100 mW/cm2. The transient photocurrent and photovoltage, the photocurrent action spectra of the blend P3OT:PCBM for front and back side illuminations and an open-circuit voltage and shortcircuit current dependence on light intensity have been studied. During illumination, a cathodic photocurrent was observed, indicating that the neutral poly(3-octylthiophene) behaves as a p-type semiconductor. IPCE% of 0.0682 % for front side illumination (ITO/PEDOT) and IPCE% of 0.0011 % for backside illumination (ITO/P3OT:PCBM) were obtained for the device ITO│P3OT:PCBM │POMOE:I3 -/I-│ PEDOT│ITO.Item Developing Polymer/Dye Based Radiochromic Dosimeters for Medical and Environmental Applications(Addis Ababa University, 2015-06-06) Tesfay, Hagos; Yohannes, Teketel (Professor); Megerssa, Negussie (PhD)Dye/polymer based dosimeters are developed by various researchers for medical, industrial and environmental applications. Most of these radiochromic dosimeters are focused on applications related to detection of high dose ionizing radiation. Development of dosimeters from different materials such as polymers, dyes, semiconductors, metal oxides and other materials is an active research area. The development of a radiochromic dosimeter from cheap materials such as polymers and dyes for low dose radiation dosimetry is also getting much attention. At present different research groups are working on developing radiochromic dosimeters using dyes and polymers. Some polymethine cyanine dyes were studied for their dosimetric characteristics. The results of the studies show that these dyes could be promising materials for developing radiation dosimeters for the measurement of low dose radiation in medicine and environment. A traditional turmeric dye called curcumin was also studied in film form for high dose dosimetry. However, heptamethine dyes and curcumin have not been profoundly investigated for low dose dosimetry. Page iii On the other hand, conjugated polymers and other conventional polymers are being investigated for developing polymer based radiochromic dosimeters. Some promising results were reported especially for low dose radiotherapy and high dose industrial applications. In this research work, dosimetric studies were carried out on the heptamethine cyanine dyes, curcumin dye and P3HT solutions and polyvinyl alcohol (PVA) films using UV-Vis spectrophotometry, spectrofluorometry, mass spectrometry (MS), optical densitometry and Fourier transform infra-red spectroscopy (FT-IR) methods. The two heptemethine cyanine dyes and curcumin showed promising dosimetric characteristics for low dose applications. P3HT in chloroform with the addition of DDT also showed promising results opening for a possibility of developing a low dose dosimeter using P3HT solutions in other non-toxic solvents in the presence of less hazardous chemicals such as chloral hydrate and trichloroacetic acid as radiosensitizers instead of DDT. The heptamethine/PVA/HCl film nanocomposite doped with TiO2 nanoparticles showed enhanced dosimetric response confirming the catalytic role TiO2 nanoparticles play in improving the dosimetric characteristics of these dyes in solid state for low dose applications. A mechanism for the radiation-induced decomposition of the heptamethine dye solutions in chloroform was proposed based on the MS, FTIR and UV-Vis spectroscopic data. The proposed mechanism involves a two step reaction. In the first step, radiolysis of chloroform occurs, which results in radicals and stable molecules. In the second step, decomposition of the heptamethine dye occurs via interaction of the dyes with these radicals (Cl. and .CHCl2) and stable molecule (HCl) to produce smaller chromophoric groups. In the proposed mechanism, the various Page iv chromophoric groups formed are described based on UV-Vis, MS and FTIR data showing extensive loss of conjugation due to breakdown of the -bond of the polymethine bridge on interaction with radiolysis products of chloroform. The heptamethine dosimetric dye solutions also show visual color changes with intensity of color being proportional to absorbed doses which is very useful for precise monitoring of doses delivered to patients in radiotherapy applications. The shelf life of the pre-irradiated dye solutions also show good stability up to 20 days. The dosimetric dye solutions (heptamethine dyes and curcumin) prepared from non-toxic solvents such as ethanol and acetone in the presence of radical generating species trichloroacetic acid and chloral hydrate showed promising results which will have great potential for real low dose applications in medicine and environment.Item The Effect of Chemical Additives on Photovoltaic Performance of P3HT:PCBM Bulk Heterojunction Solar Cells(Addis Ababa Universty, 2012-06) Daniel, Eyob; Yohannes, Teketel (Professor)Organic bulk heterojunction solar cell devices based on blend of poly(3- hexylthiophene) (P3HT) and [6, 6] phenyl C61 butyric acid methyl ester (PCBM) with different concentration of propylthiouracil (PTU) additive have been fabricated and characterized in an open air. The current density-voltage characteristics were measured in the dark and under white light illumination at illumination intensity of 80 mW/cm2. The incident monochromatic photon to current conversion efficiency (IPCE), and consistency of optical absorption and IPCE of the device without additive and with propylthiouracil have been compared. Optimizations were made by varying the concentration of propylthiouracil from 0.75% (w/w) to 1.5% (w/w). It was found that a system with 1% (w/w) of propylthiouracil showed the best photovoltaic performance with a short circuit current density, JSC of 5.69 mA/cm2, open circuit voltage, VOC of 0.648 V, fill factor, FF of 40%, and power conversion efficiency (PCE), of 1.8%, as compared to the system without propylthiouracil and with other concentrations of propylthiouracil. The best device showed IPCE of 62% at 480 nm. It was also observed clearly that IPCE spectra of each solar cell match the corresponding optical absorption. Our results in general show that addition of 1% (w/w) Propylthiouracil additive to P3HT:PCBM bulk heterojunction solar cell improves efficiency of the device from 1% to 1.8%.Item Effect of Low Boiling Point Solvent Additives on the Photovoltaic Performance of Tq1:Pc61bm Based Bulk Heterojunction Solar Cells(Addis Ababa University, 2014-06-06) Tukue, Hagos; Yohannes, Teketel (Professor)The need for clean, inexpensive and renewable energy has increasingly turned research attention towards polymer photovoltaic cells. However, the performance efficiency of these devices is still low in comparison with silicon-based devics. The recent introduction of solvent additives has resulted in a remarkable increase in power-conversion efficiency by controlling the interpenetrating network morphology of the organic bulk heterojunction solar cells. In this study the effect of low boiling point solvent additives such as iodomethane, iodoethane and di-iodomethane on the photovoltaic porformance of TQ1:PC61BM (1:2) based organic BHJ solar cells with the device architecture glass/ITO/PEDOT-PSS/TQ1:PC61BM/Al were studied. It was shown that the highest PCE of 4.53% was obtained by solar cell made from a blend solution containing 3% (v/v) diiodomethane. Similarlly, devices made from blend solutions containing 3% (v/v) of iodomethane and iodoethane exhibited PCE of 3.28 and 3.76%, respectively. The addition of these solvent additives has mainly increased the short-circuit current density (Jsc) of the solar cells compared to pristine TQ1:PC61BM based solar cell. As confirmed from UV-Vis absorption and photoluminescence spectra, addition of these solvent additives has enhanced photon absorption and photoluminescence quenching efficiency, respectively. Hence, this study have shown that despite the high boiling point solvent additives, low boiling point solvent additives can improve the phase separation of the active layer that in turn enhances charge dissociation and PCE.Item Effect of Low Boiling Point Solvent Additives on the Photovoltaic Performance of Tq1:Pc61bm Based Bulk Heterojunction Solar Cells(Addis Ababa University, 2014-06-06) Tukue, Hagos; Yohannes, Teketel (Professor)The need for clean, inexpensive and renewable energy has increasingly turned research attention towards polymer photovoltaic cells. However, the performance efficiency of these devices is still low in comparison with silicon-based devics. The recent introduction of solvent additives has resulted in a remarkable increase in power-conversion efficiency by controlling the interpenetrating network morphology of the organic bulk heterojunction solar cells. In this study the effect of low boiling point solvent additives such as iodomethane, iodoethane and di-iodomethane on the photovoltaic porformance of TQ1:PC61BM (1:2) based organic BHJ solar cells with the device architecture glass/ITO/PEDOT-PSS/TQ1:PC61BM/Al were studied. It was shown that the highest PCE of 4.53% was obtained by solar cell made from a blend solution containing 3% (v/v) diiodomethane. Similarlly, devices made from blend solutions containing 3% (v/v) of iodomethane and iodoethane exhibited PCE of 3.28 and 3.76%, respectively. The addition of these solvent additives has mainly increased the short-circuit current density (Jsc) of the solar cells compared to pristine TQ1:PC61BM based solar cell. As confirmed from UV-Vis absorption and photoluminescence spectra, addition of these solvent additives has enhanced photon absorption and photoluminescence quenching efficiency, respectively. Hence, this study have shown that despite the high boiling point solvent additives, low boiling point solvent additives can improve the phase separation of the active layer that in turn enhances charge dissociation and PCE.Item Effect of Polymer Structure and Composition on Photovoltaic Performance of Organic Conjugated Polymer Bulk Heterojunction Solar Cells(Addis Ababa University, 2012-06-05) Adam, Getachew; Yohannes, Teketel (Professor)In this thesis solar energy to electrical energy conversion using organic conjugated polymer based bulk heterojunction solar cells has been studied by aiming at the effect of polymer structure and composition on minimizing the limitations such as lower charge carrier mobility, narrow absorption spectrum, poor nanomorphology of these solar cells which improves the photovoltaic performance in such a way that: a) By random distribution of segments of linear octyloxy side chains, inducing ordering, and of branched 2-ethylhexyloxy side chain, inducing disorder on the backbone of anthracene containing poly(phenylene-ethynylene)-alt-poly (phenylene-vinylene) (PPE-PPV) a side chain based statistical copolymer, denoted AnE-PVstat, has proven to be very effective in terms of field independent higher intrinsic charge carrier mobility μ = 5.43 × 10-4 cm2/Vs demonstrated by CELIV that might be due to its highest --stacking distance of d= 0.393 nm and conformationally balanced morphology as compared to the well-defined congeners. The random combination of the underlying constitutional units seems to promote the side chain geared order and disorder enabling effective tuning of the nanoscale morphology of photoactive layer. Power conversion efficiency up to 3.77% in a bulk heterojunction photovoltaic system and a significant device performance in solid state photoelectrochemical solar cells made using AnEPVstat: PCBM as a photoactive active layer in an open air showing an openOrganic conjugated polymer bulk heterojunction solar cells iv circuit voltage of 320 mV is the present state-of-art value for PPV based materials. b) A 1:1 mixture of two thiophene based poly(p-phenylene-ethynylene)-alt-poly(pphenylene- vinylene)s denoted DO-PThE1-PPV2 (D1) and MEH-PThE1-PPV2 (D2), consisting of the same conjugated backbone but different types and volume fraction of alkoxy side chains on the phenylene-ethynylene unit, has lead to enhanced charge carrier mobility as compared to the individual polymers. The resulting ternary blend with PCBM showed better photovoltaic performance as compared to binary blends. This is due to improved active layer nanomorphology in the ternary system as revealed by AFM studies. c) By varying one moiety with thiophene, bithiophene or 3,4-ethylenedioxy thiophene at the X position in the thiophene containing poly(p-phenyleneethynylene)- alt-poly(p-phenylene-vinylene)s (PPE-PPV) copolymers with a general structural design (Ph-CC-X-CC-Ph-CH=CH-Ph-CH=CH-) bearing identical side chains at the phenylene rings were synthesized. The effect of this structural alteration on the properties such as photophysics, electrical, charge carrier mobility and morphology of the materials and its impact on their photovoltaic performance were studied. The copolymer with a single thiophene ring at the X positions showed the highest VOC of 930 mV and the copolymer with a bithiophene unit at X position showed the highest short-circuit current density and charge carrier mobility. Where as the copolymer with 3,4-ethylenedioxy thiophene showed the lowest photovoltaic performance. d) To change the structure of the polymer by doping, a 1:1 ratio of Poly(3- hexylthiophene) (P3HT) to [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was dissolved in chlorobenzene containg four different molar concentrations of Organic conjugated polymer bulk heterojunction solar cells v the dopant ferric chloride. The photophysical properties in solution as well as in their films were characterized using Uv-Vis absorption and bulk hetrojunction solar cell devices were fabricated from each solution were characterized. It was found that with increasing the dopant concentration, monochromatic photocurrent spectra and IPCE of the devices showed a red shift which is consistent with the optical absorptions and the instu spectroelectrochemical behavior of P3HT. By doping the short circuit current of the devices increased due to an increase in charge carrier mobiliy. It was also observed that the best photovoltaic performance was achieved at lower concentration of FeCl3 as compared to undoped devices used in this experiment. e) A ternary blend of two polymers poly(3-hexylthiophene) and poly[4,4-bis(2- ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b]dithiophene-2,6-diyl-alt-4,7-bis(2-thienyl )-2,1,3-benzothiadia-zole-5’,5’’-diyl] (PCPDTTBTT) with complementary absorption and an acceptor [6,6]-phenyl C61-butyric acid methyl ester (PCBM) were used to fabricate the bulk hetrojunction type solar cells inorder to increase (broadening) the absoprption in the visible spectrum. It was found that the ternary blend showed better absorption spectrum as evidenced from the optical absorption, photocurrent spectra and IPCE curves of the blend. Due to this increased absorption of light the ternary blend has shown improved short circuit current and open circuit voltage due to n-type behaviour of PCPDTTBTT as compared to the binary blends of each polymer with PCBM.Item Natural dye senistized photoelectrochemical cells Based on ZnO Nanoparticles(Addis Ababa Universty, 2012-06) Yirga, Getachew; Yohannes, Teketel (Professor)This thesis focused on the study of dye-sensitized solar cells (DSSCs) based on ZnO nanoparticle. Our work started by synthesizing ZnO nanopartcile using sol-gel method. ZnO nanoparticles were synthesized and the size of the nanoparticle (1.14 nm) was determined by using effective mass approximation model. Natural sensitizers extracted from Jacaranda mimosifolia, Bougainvillea spectabilis, Carissa ovata, Amarathus iresine, Beta vulgaris, and Hibiscus sabdariffa used for sensitizing ZnO nanoparticle. All natural sensitizers were extracted by water and ethanol, absorption spectra were measured and all natural sensitizers absorbs in the visible region. Devices were fabricated using ZnO nanoparticles and with all the above natural sensitizers, then the photoelectrochemical performance were measured. DSSCs based on ZnO nanoparticles and ethanol extract of Amarathus iresine sensitizers have shown relatively good conversion efficiency of 0.039. The incident photon to current conversion efficiency (IPCE), short circuit current density (Jsc) and Voc were measured for all the above sensitizers. Also comparative study were made using TiO2 nanoparticle based DSSCs sensitized by Bougainvillea Spectabilis, Carissa ovata, and Jacaranda mimosifolia with ZnO based DSSCs sensitized by the same sensitizers. In TiO2 nanopartcile based DSSCs the photoelectrochemical (PEC) performance we found relatively higher than the ZnO nanopartcles based DSSCs as expected. The data we got from TiO2 and ZnO based DSSCs was analyzed and finally some of the problem that limit the conversion efficiency were discussed.Item Screening of Natural Dyes for Use in Dye Sensitized Solar Cells(Addis Ababa Universty, 2012-06) Jara, Desalegn; Yohannes, Teketel (Professor)Five natural dyes, extracted from natural materials such as flowers, were used as sensitizers to fabricate dye-sensitized solar cells (DSSCs). Dye-sensitized solar cells (DSSCs) were fabricated using natural dyes extracted from flowers Amaranthus caudatus, Bougainvillea spectabilis, Delonix regia, Nerium oleander, Spathodea companulata and a mixture of the extracts. The PEC performance of the DSSCs based on these dyes showed that the fill factors of these DSSCs are mostly higher than 50%, the open circuit voltages (VOC) varied from 0.45 to 0.55 V, and the short circuit photocurrent densities (JSC) ranged from 0.013 to 1.82 mAcm−2. Specifically, a high open circuit voltage (VOC = 0.55 V) and short circuit photocurrent density (JSC = 1.82 mAcm−2) were obtained from the DSSC sensitized by the ethanol extract of flower of Amaranthus caudatus. The photon to electricity conversion efficiency of the dyesensitized solar cell (DSSC) sensitized with the ethanol extract of flower of Amaranthus caudatus reached 0.61%. The PEC performances of DSSCs using the dye mixed solutions were also investigated. However, the mixed extract does not show synergistic photosensitization compared to the individual extracts. Instead, the cell sensitized by the flower of Amaranthus caudatus extract extracted with ethanol alone showed the best sensitization. The devices showed that an incident monochromatic photon to current conversion efficiencies (IPCE) varied from 4.7% to 52% illuminated from front side. The results from the IPCE data are consistent with the results from the current density ‒ voltage (J − V) curves.Item Solid State Photoelectrochemical Solar Energy Conversion Based on a Mixture of Meh-Ppv and Mdmo-Ppv(Addis Ababa Universty, 2010-06) Wodaje, Anteneh; Yohannes, Teketel (Professor)Solid state photoelectrochemical solar energy conversion based on a mixture of MEHPPV and MDMO-PPV, coated on ITO glass as light harvesting unit, a solid polymer electrolyte, POMOE complexed with I3 - /I- redox couple and oxidized PEDOT as counter electrode have been fabricated and studied for its photoresponse behavior. The short circuit current (Isc), open circuit voltage (Voc), and fill factor (FF) of this mixed polymer based PEC is higher than that of MEH-PPV based device. A PEC with a structure ITO│ (MEH-PPV:MDMO-PPV)│POMOE│: I3 - /I-│PEDOT│ITO produce Voc of 289.7 mV, Isc of 0.21 LA/cm2 and FF of 0.32. The device showed an incident monochromatic photon to current conversion efficiency (IPCE) of 0.0034% illuminated from front side and 0.00013% illuminated from back side. The dependence of Isc and Voc on incident light intensity showed both Isc and Voc increases with increasing light intensity. The plot of logIsc versus logPi yielded straight line with the power factor α is equal to 0.793. Our results show that mixing of two donor type polymer, PEC show relatively improved performance when compared with individual MEH-PPV based device.Item Solid-State and Quasi-Solid State Photoelectrochemical Solar Energy Conversion Based on Polymer PDTSTTz(Addis Ababa University, 2014-06-06) Tibebu, Dawit; Yohannes, Teketel (Professor)The photoelectrochemical properties of a solid-state and a quasi-solid state photoelectrochemical cell (PEC) based on a polymer PDTSTTz and a blend of PDTSTTz:PCBM with an amorphous polymer electrolyte Poly[oxomethylene-olig(oxyethylene )] and ionic liquid complexed with redox couple iodide/triiodide, and oxidized PEDOT as counter electrode were studied. PEDOT:PSS were spin coated to the substrate and the photo-response of the fabricated device were studied and the followings results were obtained: open-circuit voltage of 311 mV and a short-circuit current of 45.4 μAcm-2 and fill factor (FF) 0.34 for the device PEDOT:PSS/PDTSTTz:PCBM/POMOE:I-3/I- at light intensity of 100 mWcm-2. For all devices the photocurrent action spectra of the front and back side were studied and compared. During illumination, a cathodic photocurrent was observed, indicating that the neutral polymer PDTSTTz behaves as a p-type semiconductor. IPCE% ranges from 0.019% to 0.6% from the front side illumination (ITO/PEDOT) were obtained from the different types of device, in addition the Uv-vis absorption spectra of the PDTSTTz film was compared with the photoresponse of the different device and the results were consistent.Item Stability and Photovoltaic Performance Studies of Bulk Heterojunction Polymer Solar Cells(Addis Ababa Universty, 2017-02) Tsegaye, Endale; Yohannes, Teketel (Professor)This dissertation reports a research done on the stability and photovoltaic performance studies of bulk heterojunction polymer solar cells. Different techniques were used, in order to study the stability and performance of polymer solar cells fabricated using different polymers and fullerenes. Two fullerene derivatives with different electron affinity were used to fabricate polymer solar cell based on P3HT as donor. Results obtained from UV-visible, FTIR and J-V measurement showed that the P3HT:PCBM film and solar cells fabricated from P3HT:PCBM as active layer have much better air stability than those made from P3HT:ICBA as active layer. Due to lower electron affinity of ICBA, the blend of P3HT:ICBA photobleached at a faster rate compared to PCBM. Effect of sol-gel synthesized TiOx layer on the stability and performance of P3HT:ICBA based solar cells were investigated by spectroscopic and electrical measurement methods. Results from J-V measurements indicates that P3HT:ICBA/TiOx solar cell has better performance and stability compared to the reference device. The higher value of shortcircuit current density and open-circuit voltage for P3HT:ICBA/TiOx solar cell indicate the role of TiOx layer as the optical spacer and hole blocking layer, respectively. The results from UV-visible absorbance, J-V and C-V measurement on light irradiated P3HT:ICBA/TiOx solar cell demonstrates that TiOx layer can be used to protect P3HT:ICBA active layer from oxygen and water attack under ambient air and to improve the working lifetime of P3HT:ICBA based device. Investigation on the effect of two fullerene derivatives with similar electron affinity on the photovoltaic performance and stability of P3HT based solar cells is presented. From UVvisible measurements it is found out that P3HT mixed with both PC61BM and PC71BM has good ambient air photochemical stability. However, device based on P3HT:PC61BM has better stability compared to P3HT:PC71BM based device. The impact of two different fullerenes with similar electron affinity on the photovoltaic performance and stability of PCDTBT based solar cells were studied. Results from UVvisible absorbance indicate that unlike that of pure film of PCDTBT, blend of PCDTBT with PC61BM, PC71BM and 1:1 mixture of PC61BM and PC71BM has relatively constant absorbance under light exposure. The results show the stabilizing power of acceptor fullerenes when mixed with PCDTBT. The results from J-V measurement indicates that the solar cells made from PCDTBT:PC61BM and PCDTBT:PC61+71BM have better stability under light exposure in ambient air compared to the device based on PCDTBT:PC71BM. When only PC71BM is used as acceptor the device has higher rate of decay of PCE. The effect of DIM on the PCE and stability of PSCs made of PCDTBT:PC71BM is investigated. PCE of the device based on PCDTBT:PC71BM processed with DIM is higher than the reference device. In terms of device stability, the PSCs processed with DIM showed poor stability at longer light exposure time. For the device without DIM especially as the light exposure time was increased, the device stability was better. The result from IS measurement shows that for pristine PCDTBT:PC71BM devices with DIM, the active layer resistance is lower compared to device without DIM. However, after irradiating the device for 5 hr the resistance of the device processed with DIM is higher and it is consistent with decreased PCE of aged device. The investigation presented on the application of NIL for performance improvement of organic solar cells shows that, NIL is really promising technique for improving efficiency of polymer solar cells. Nanopatterning of the PEDOT:PSS electrode can be used as a way to improve the cell efficiency. In our case relative increase of the 60%, from 0.8% to 1.3% of efficiency is obtained. The increased interfacial area and higher hole collection contributed for the improved photovoltaic performance of nanopatterned device. Keywords: polymer solar cells, degradation, photochemical stability, lifetime, charge carrier mobility, P3HT, PCDTBT, ICBA, PC61BM, PC71BM, TiOx, DIM, nanopatterningItem Stability Study and Photovoltaic Application of Quinoxaline Based Polymers and Effect of Low Boiling Point Solvent Additives on Photovoltaic Performance of Bulk Heterojunction Polymer Solar Cells(Addis Ababa Universty, 2016) Abdisa, Bedasa; Yohannes, Teketel (Professor)In this study the photochemical stability and photovoltaic performance of two polymer families are presented; one based on a thiophene-quinoxaline unit and the other one on a thiophene-pyridopyrazine unit. Copolymerization of these monomers together with thiophene-hexylthiophene was performed in order to make the polymers more black, i.e. to fill the gap between the high- and the low-energy peak in the absorption spectra. The study has focused on how an increasing fraction of thiophene-hexylthiophene affects the photo-oxidative stability of these polymers, as well as the solar cell performance. Accordingly, thiophene-pyridopyrazine devices displayed increased device efficiency. In addition, the stability is retrained upon inclusion of the extra monomer of 30% and 50% mole fraction of thiophene-hexylthiophene. In contrast, it was found that for the thiophene-quinoxaline based copolymer both, device efficiency and stability, decreased with inclusion of 30%, 50%, and also 80% thiophene-hexylthiophene.Furthermore, the effect of incorporation of low mole fraction of thiophene-hexylthiophene i.e 1% and 2.5% in to quinoxaline based polymer on photochemical and photovoltaic performance was also studied. In contrast to the incorporation of large fraction of thiophene-hexylthiophene in to quinoxaline based polymer, incorporation of low fraction of thiophene-hexylthiophene has improved both photochemical stability and photovoltaic performance of the copolymers. Additionally, the effect of low boiling point solvent additives such as iodomethane, iodoethane, and diiodomethane on photovoltaic performance of bulk heterojunction polymer solar cells was studied. According to our findings, the efficiency of TQ1:[60]PCBM based devices have increased from 3.41% (control device) to 4.28, to 4.41, and 4.66% when the devices are processed from 3% (v/v) IMe, IEt, and DIMe, respectively. Similarly, the PCE of PCDTBT:[70]PCBM based devices have also showed enhancement from 3.08% (control device) to 3.39, 3.80, and 4.37% when processed from 3% (v/v) IMe, IEt, and DIMe, respectively. All the solvent additives have improved current density and fill factor leading to enhanced power conversion efficiency compared to control device fabricated without any additive. This is due to formation of nanomorphology creating large D-A interface area for better charge carrier dissociation and interpenetrated networks for efficient charge carrier transport to electrodes as confirmed from the AFM images. As confirmed by comparing UV-Vis absorption spectra of pristine blend films and blend films soaked in the additives for 5 seconds, the solvent additives selectively dissolved the PCBM aggregates leading to enhancing their misciblity/interaction in to the polymer domains of the active layer. The enhanced misciblity and nanoscale formation of the blends were further confirmed by an increase of the quenching efficiency of the PL of the blends containing the solvent additives compared to the pristine blends. Therefore, the increase in interface area and formation of networks between the donors and acceptors up on addition of the low boiling point solvent additives are the main reasons for the improvement of current density and fill factor leading to enhanced PCEs.Item Study of Charge Transport in Conjugated Polymers and Tuning Photovoltaic Performance in Bulk heterojunction Solar Cells(Addis Ababa Universty, 2015-06) Kedir, Fedlu; Yohannes, Teketel (Professor)In this study, the effects of a moderate variation of macromolecular properties of poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene)(PPE-PPV) statistical copolymers (AnE-PVstat) on charge transport properties were studied using differential time of flight (TOF) method. The influence of the macromolecular properies on bulk-heterojunction photovoltaic parameters was also investigated. Appreciable effects were obtained in both charge transport and photovoltaic properties. Structural effects on drift mobility of holes and electrons also were investigated in series of the six anthracene-containing AnE-PV-ab, AnE-PV-ae, AnE-PV-bb, AnE-PVstat, AnE-PVstat-4 and AnE-PVstat-5 copolymers with different nature of side chains using TOF techniques. AnE-PV polymer with longer and linear octyloxy and dodecyloxy chains has shown negative effect on carriers mobility (µ) because of decreased interaction between adjacent hoping sites. AnE-PV polymers with the short and branched substituents of ethyl-hexyloxy exhibited higher mobility both for holes and electrons. Impact of changing position of anthracene unit in poly{1,4-(5-[(2-ethylhexyl) oxy]-2-methoxy)phenylene-ethynylene-1,4-(5-[(2-ethylhexyl)oxy]-2-methoxy)phenyl -lene-vinylene-9,10-anthracenylene-vinylene}(MEH-PPE1-PAnV2) and poly{(1,4-( 5-(2-ethylhexyl)-oxy)-2-methoxy)-phenylene-ethynylene-9,10-anthracenylene-vinyl-ene-1,4-[(5-(2-ethylhexyl)-oxy)-2-methyloxy]-phenylene-vinylene}(MEH-PAnE1-PPV2) copolymers backbone on the hole and electron mobility was investigated using TOF method. The polymer having anthracene unit at the center between double bond and triple bond (MEH-PAnE1-PPV2) has shown higher hole and electron mobility than the one having anthracene unit between two double bonds at edge of backbone (MEH-PPE1-PAnV2). Effect of shifting position of anthracene unit is more significant in electrons mobility between the two polymers. Moreover we studied hole mobility of phenylene-ethynylene (PE)/phenylene-vinylene (PV) hybrid conjugated polymer of general constitutional structure (-Ar-C≡C-Ar-CH=CH-)n using integration mode time-of-flight (I-TOF). The hole mobilities showed positive field dependent as expected for dispersive transport. We have also studied the transport of holes in thin films of AnE-PVs copolymers by means of admittance spectroscopy as a function of field and temperature. The hole mobility exhibited strong field and temperature dependence. The study in this thesis was also extended to investigate the effect of barrier height between cathode work function and LUMO of AnE-PVstat copolymer on the bulk hole mobility using admittance spectroscopy. Devices of AnE-PVstat with the same anode material indium-tin-oxide (ITO)/poly(3,4-ethylene dioxythiophene):poly styrene sulfonic acid (PEDOT:PSS) and different cathode materials of Al, Ag, Au and Ag/PEDOT:PSS were prepared. Different hole transport properties were obtained from devices having different cathode materials. This might be due to the differences in electron injection efficiency among these cathode materials. The influence of the low boiling point solvent additives such as iodomethane (IMe), iodoethane (IEt), iodobutane (IBu) and di-iodomethane (DiMe), and the addition of poly(3-hexylthiophene-2,5-diyl) (P3HT) on the performance of poly[2,6-(4,4‟-bis(2-ethylhexyl)dithieno[3,2-b:2‟,3‟-d]silole)-alt-5,5‟-(3,6-bis[4-(2-ethylhexyl) thienyl-2-yl]-s-tetrazine] (PDTSTTz) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) BHJ solar cells were studied. The results have shown that the power conversion efficiency (PCE) of PDTSTTz:PCBM blend cell increased with the increase in length of carbon chains of the additives. Addition of P3HT on the PDTSTTz/PCBM system also brought enhancement in Jsc as well as PCE in PDTSTTz/PCBM solar cells. Keywords: Charge transport, Time of flight, Admittance spectroscopy, Conjugated polymer, Macromolecular properties, side chain, interface barrier height, solvent additive, bulk heterojunction solar cellsItem Study of Charge Transport Properties of Conjugated Polymers and Photovoltaic Performance of Bulkheterojunction Solar Cells(Addis Ababa University, 2012-05-05) Esubalew, Debebe; Yohannes, Teketel (Professor)Alternative energy source of the world in the future is organic photovoltaic, which is sustainable and environmentally friendly. Among the different organic photovoltaics, the class of bulk heterojunction solar cells required to have 10% power conversion efficiency and 10 years of life time to be commercialized. However, there are many factors that limit their performance. One of these factors is the charge carrier mobility. Therefore, the mobilities of different polymers (APFO-Green 6, APFO-Green 5 and a novel Phenyl substituted Polythiophene compound called PPOPT) have been characterized in order to understand the effect of mobility in their performance. Transport of holes in a low band gap polyfluorene, APFO-Green6, was investigated by means of admittance spectroscopy in the modulation frequency range 1-5x105 Hz and found to be in the order of 10x-6 cm2V-1s-1. At room temperature hole mobility of APFO-Green6 is dependent on the applied electric filed, as commonly observed in disordered organic materials. The excess capacitance towards low frequencies provides evidence for charge relaxation in trap levels. A dispersion parameter of 0.4 was achieved from the trend of holes transit times with the electric field. CELIV technique was also applied, but the characteristic signal was not observed. However, it was used to determine the polymer iv dielectric constant and found to be 3, which is very common value to many organic materials. The bulk transport properties of positive carriers in thin films of a low band-gap conjugated polymer, called APFO-Green5, have also been investigated with the ac Admittance technique. The capacitance response at low frequency gave indication of a combination of trapping and double-injection effects, while in the intermediate-high frequency range was determined by the transit time of injected holes. Hole mobility in APFO-Green5 thin films exhibited a Frenkel-like dependence on the applied electric field, with a field-dependent coefficient of around 8 x 10−3 (Vcm−1) −1/2. A hole mobility close to 2 x 10−5 cm2 V−1 s−1 was achieved at the field of 3.5x105 Vcm−1, in excellent agreement with that already reported by using a different bulk investigation technique. The temperature effect was also studied and the charge transport parameters were extracted by analyzing the mobility data by the uncorrelated and the correlated Gaussian Disorder Models. DISCL transient technique was also applied and one order magnitude higher value to that of AS was achieved. Characterization of a novel polythiophene substituted with a 2’-pentyloxy-5’-(1’’’-oxooctyl) phenyl group (PPOPT) is also reported. Optical and electrochemical studies were done. The HOMO (-5.49 eV) and LUMO (-3.14 eV) levels have been determined. The bulk transport properties of thin films of PPOPT are investigated by admittance spectroscopy. The dramatic effect of the phenyl side chain on the mobility of positive carriers in films of PPOPT is described. The photophysics of PPOPT in both solution and thin film is also investigated and correlated to substituent-driven intrachain and interchain arrangements. v More over two other polymers were used to develop bulkheterojunction solar cells and characterized. One of the polymers that is used in a photovoltaic characterization is a novel family of fluorene–thiophene-benzothiadiazole containing, poly{[4’-(9,9-bis(2-ethylhexyl)fluoren-2-yl)-2’,1’,3’-benzothiadiazole-7,7’-diyl]-co-[2’-(9,9-bis(2-ethyl-hexyl)fluoren-2-yl)thien-7,5’-diyl]} (PFB-co-FT), random copolymers . The study includes selection of the best solvent, analyzing annealing effects, optimization of polymer: PCBM weight ratio, optimization of the active layer thickness and studying the effect of LiF buffer layer in order to get optimized performance. The effect of the incident light intensity was also investigated in order to get insight about the space charge effect on the device. The optimum performance is around 1% and the light intensity study indicates that little or no significant effect of space charge in the solar cell devices.The other bulk heterojunction solar cell characterized is Poly{[2,7-(9,9-bis-(2-ethylhexyl)-fluorene)]-alt-[5,5-(4,7-di-2’-thienyl-2,1,3-benzothiadiazole)]}:PCBM. The solar cells were also characterized under different incident light power intensities. Charge trapping effects take place at low fullerene content in the photoactive blend; an efficient polymer fullerene intermixing with formation of continuous phases is reached at a donor: acceptor ratio of 1:4. For an optimized active layer thickness of 100 nm a power conversion efficiency of 2.57% was obtained. Photocurrent measurements under reverse bias conditions show that a high percentage of the photogenerated excitons do not lead to the formation of free carriers; thus representing the major limiting factor for the devices efficiency.