Study of Charge Transport Properties of Conjugated Polymers and Photovoltaic performance of Bulkheterojunction Solar Cells
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Date
2012-05
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Addis Ababa University
Abstract
Alternative energy source of the world in the future is organic pholoYoltaic, which
is sustainabl e and environmentally friendl y. Among the different organic photovoltaics, the
class of bulk hetcrojunclion solar cells required to have 10% power conversion efficiency
and 10 years of li fe time to be commercialized.
However, there arc many factors that limit their performance. One of these fac tors
is the charge carrier mobility. Therefore, the mobil ities of diITerent polymers (A PFOGreen
6, APFO·Grcen 5 and a novel Phenyl substituted Poly thiophene compound called
PPOPT) have been characterized in order to understand the e fTect of mobili ty in their
performance.
Transport of holes in a low band gap polyfluorene, APFO-Green6, was in vesti gated
by means of admittance spectroscopy in the modulation frequency range 1-5x 1 05 Hz and
found to be in the order of 10x.ti cm2y -1s-l . At room tem perature hole mobility of APFOGreen6
is dependent on the applied electric fi led, as commonly observed in disordered
organic materials. The excess capacitance towards low frequencies provides evidence for
charge relaxation in trap levels. A di spersion parameter of 0.4 was achieved from the trend
of holes transit times with the electric field . CELIV tec hnique was also applied, but the
characteristic signal was not observed. However, it was used to determine the polymer
III
die lectric constant and found to be 3, which IS very common value to many organic
material s.
The bulk transport properties of positive carriers in thin films of a low band-gap
conjugated polymer, ca lled 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 intcnnediate-high
frequency range was determined by the transit time of injected holes. Hole mobility in
APFO-GreenS thin films exhibited a Frenkel-like dependence on the applied electric fi eld,
with a field-dependent coefficient of around 8 x 10-3 (Vcm - I) - 112. A hole mobility close to
2 x 10-5 cm2 V- I S- I was ac hieved at the field of3.5x lOs Vcm- I, in excellent agreement
with that already repo rted by using a different bulk invest igation 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. D1SCL transient technique was also applied and one order magnitude higher value
to that of AS was achieved.
Characteri zation of a novel poly thiophene substituted with a 2' -pentyloxy-S ' -( 1"'oxooclyl)
phenyl group (PPOPT) is also reported. Optical and electrochemical st udies
were done. The HOMO (-5.49 eV) and LUMO (-3.14 eV) levels have been detennined.
The bulk transport properties of thin films of PPOPT are in vestigated 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 subst ituent-driven intrachain and interchain
arrangements.
IV
More ovcr two olher polymers were used 10 develop bulkheterojunction solar cells and
characterized. One of the polymers that is used in a photovoltaie characterization is a
novel family of nuorene- thiophene-benzolhiadiazole containing. poly{[4'-(9,9-bis(2-
ethyl hex ylln uoren-2-yl)-2', I ' ,3 ' -benzothiadiazole-7, 7' -di yl ]-co-[2 ' -(9,9-bi s(2-ethylhexyll
fluoren-2-yllthien-7,5 ' -diyl]) (PFB-co-FT), random copolymers . The study
includes se lection of the best solvent, analyzing annealing effects, optimi7..ation of
polymer: PCBM weight rati o, optimization of the act ive layer th ickness and studying the
effect of LiF buffer layer in order 10 gel 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 I % and the light intensity study indicates
that little or no significant efTect of space charge in the solar ce ll devices. The other bulk
heterojunction solar ce ll characterized is Poly{f2,7-(9,9-bis-(2-ethylhexyl)-fluorene)]-ah[
5,5 -(4, 7-di-2' -thienyl-2, I ,3-bcnzothiadiazole)]) :PCBM. The solar cells were also
characterized under different inc ident light power intensities. Charge trapping effects take
place at low fullerene content in the photoactive blend; an effic ient polymer fu llerene
intermixing with formation of continuous phases is reached at a donor: acceptor ratio of
I :4. For an optim ized act ive layer thickness of 100 nm a power conversion efficiency of
2.57% was obtained. Photocurrent measurements under reverse bias condi tions show that a
high percentage of the photogenerated excilons do not lead to the fo rmation of free
carriers; thus representing the major limiting factor for the devices effi ciency
Description
Keywords
Hole mobi lity, Transport properties, Admittance spectroscopy Conjugated polymers, Polyfluorene, phenyl subst itution, organic solar cel l, Bulk heterojunction