Study of Charge Transport Properties of Conjugated Polymers and Photovoltaic Performance of Bulkheterojunction Solar Cells
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Date
2012-05-05
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Addis Ababa University
Abstract
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.
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Keywords
Hole Mobility, Transport Properties, Admittance Spectroscopy Conjugated Polymers, Polyfluorene, Phenyl Substitution, Organic Solar Cell, Bulk Heterojunction