Thiophene Based Low Band Gap Conjugated Polymer Solar Engineering Essay

Published: 2020-06-06 17:31:04
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As the universe energy demand continues lifting, and the cost for natural resources like oil and coal is increasing, new solutions for the inexpensive production of energy from renewable resources are needed. Solar cells are expected to be a major subscriber to make full the hereafter energy demands ; nevertheless, new attacks are required that make the transition of visible radiation to power less dearly-won [ 1 ] .
Silicon-based inorganic photovoltaics ( PV ) are the best utilised for the last few decennaries in this way. But, the drawbacks such as fabrication costs and cumbrous fiction procedure made research workers to look into easy processable nature and low-priced polymer stuffs. At present over 90 % of the solar cell faculties are silicon-based, which offer comparatively high power transition efficiencies, but the cost of these solar cells are much higher to make day-to-day life. The outgrowth of organic stuff based solar cell engineerings promise great possible towards low cost energy. The organic photovoltaic ( OPV ) offers low cost, low thermic budget, solution processing, flexible substrates and a really high velocity of processing.
Assorted sorts of polymers have been used to manufacture polymer hetero junction solar cell such as poly ( 2,6-bis- 3-alkylthiophene-2-yl ) dithieno- [ 3,2-b:2,3-d ] thiophene ) ( PBTDT ) , PCBM, P3HT, poly- ( 3,4-ethylene- dioxy- thiphene ) doped with poly ( styrenesulphonate ) ( PEDOT: PSS ) poly ( 3-,3- didode-cylquaterthiophene ) ( PQT-12 ) [ 2 ] . Until now most promising polymers in such a sort of solar cells are thiophene-based polymers.
In majority hetero junction ( BHJ ) solar cells, photo-generated excitons within the exciton diffusion length of the donor-acceptor interface are dissociated via an ultrafast negatron transportation from the giver to the acceptor. The mobility of bearers is an of import parametric quantity for solar cell public presentation because it determines how long the hopping procedure would be and so how much of the bearers can be extracted by the electrode [ 3 ] . A set spread is defined as the difference between the highest occupied molecular orbital ( HOMO ) and the lowest unoccupied molecular orbital ( LUMO ) energy degrees in the polymer. Low set spread polymers are slackly defined as a polymer with a set spread below 2 electron volt, i.e. absorbing visible radiation with wavelengths longer than 620 nanometers.
In this paper, an debut of basic operational rules every bit good as a brief overview of the fiction technique and different public presentation parametric quantities i.e. fill factor, power transition efficiency of P3HT: PCBM, PBTDT: PCBM blend solar cells are reviewed. The low set spread polymers described in the literature are homo polymer ( polythiophene ) based on thiophene.
BASIC OPERATING Principle
The procedure of transition of visible radiation into electric current in an organic photovoltaic cell can be described by three back-to-back stairss: ( I ) Absorption of a photon either by negatron giver and/or negatron acceptor taking to the formation of an aroused province, the edge electron-hole brace ( exciton ) , ( two ) Exciton diffusion at the interface between the negatron giver and the negatron acceptor, where the charge separation occurs. ( three ) Finally the charge conveyance within the polymer blend to the anode ( holes ) and cathode ( negatrons ) , which supply direct current ( DC ) for burden [ 4 ] .
The electric current delivered by a photovoltaic solar cell depends on figure of created charges that are collected at the electrodes. Two forces are responsible as the drive force for the charge bearers ; those are internal electric Fieldss and concentration gradients of the several charge bearer species. The difference between the LUMO energy degree of negatron giver and negatron acceptor is the headstream of charge bearers.
Ratio of end product negatrons to the incident photons is termed as external quantum efficiency ( EQE ) . EQE is equal to the generation of all the efficiencies in the energy transportation processes [ 5 ] .
These footings represents the photon soaking up efficiency, efficiency of the exciton diffusion procedure, the hole-electron separation, the bearer conveyance and the charge aggregation procedure, severally. is a map of the soaking up spectrum, the soaking up coefficient, the absorbing bed thickness, and of internal multiple contemplations. is determined by whether they diffuse into a part where charge separation occurs and on the charge separation chance at that place [ 6 ] . Excitons get separated in a clip graduated table of femto-second ( 50fs ) [ 7 ] . So is about 100 % . The overall power transition efficiency ( PCE ) can be expressed as:
Where, Pin = input solar power ; Voc = unfastened circuit electromotive force ; = short circuit current ; FF= Fill factor. Expression foris:
VOC of bulk hetero-junction cells is straight related to the energy difference between HOMO degree of the negatron givers and the LUMO degree of the negatron acceptors [ 8 ] . VOC is through empirical observation described by following expression [ 9 ] :
.
POLYTHIOPHENE BASED PV CELLS
OPV solar cell uses organic electronic stuffs for light soaking up and charge conveyance. The set spread of polythiophene is 2 electron volt [ 9 ] . These thiophene based low set spread polymer solar cells are lightweight options to conventional, silicon-based solar cells — have great potency for presenting cheap solar energy and take downing fabrication costs while keeping high power efficiency. In this subdivision some typical types of thiophene based OPV cells brief overview of the fiction technique and different public presentation parametric quantities are reviewed.
P3HT: PCBM Blend Solar Cell
Blend heterojunctions dwelling of a majority commixture of poly ( 3-hexylthiophene ) ( P3HT ) as giver and 6,6-phenyl C61-butyric acid methyl ester ( PCBM ) as acceptor are really promising constructions [ 8 ] .Poly- ( 3-hexylthiophene ) ( P3HT ) has been the largely used p-type stuff [ 10-12 ] in polymer photovoltaics along with a fullerene derivative, [ 6,6 ] -phenyl C61-butyric acid methyl ester ( PCBM ) as an negatron acceptor. Since hole is typically the high-mobility bearer in regioregular P3HT [ 11 ] , the enhanced negatron mobility was achieved by add-on of negatron acceptor. The set spread of P3HT is 1.9 electron volt [ 10 ] .
A majority heterojunction organic cell consists of negatron giver stuff P3HT and PCBM as acceptor has been fabricated. Regioregular P3HT and PCBM were used as active bed and P3HT and PCBM ( 1:1 weight ratio ) dissolved in chlorobenzene, this resulted in PCBM segregation taking to a thermally stable permeating web [ 13 ] .
Figure 1. Chemical Structure of P3HT.
Figure 2. Schematic of P3HT: PCBM blend solar cell.
PEDOT: Postscript: Baytron P HC V4 de ionized ( DI ) H2O and dimethyle sulphoxide ( DMSO ) were used as anodal buffer beds. DI H2O added to cut down viscousness and series opposition of solar cell and DMSO enhances conduction. Conventional diagram of this BHJ solar cell and its energy set construction is shown in fig.2.
Figure 3. UV-vis soaking up spectrum of P3HT: PCBM
Figure 4. Current density-voltage features of P3HT: PCBM solar cell for different concentrations.
In P3HT/PCBM blends used in bulk heterojunctions it has been reported that the hole and negatron mobility depends on the content of PCBM [ 14 ] . An addition in the PCBM content increases the negatron mobility while the hole mobility decreases merely somewhat [ 15 ] . Fig.3 shows UV-vis soaking up spectra of P3HT: PCBM at 1500C and soaking up extremum occurs at ~550nm. Current density-voltage ( J-V ) features consequences are in conformity with the soaking up spectra as shown in fig.3. The highest short-circuit current denseness ( Jsc ) could be obtained ( 7.25 mA/cm2 ) .
Further, it has been reported that the soaking up of P3HT depend on the molecular weight of the polymer [ 16 ] . That is, lmax additions with an addition in molecular weight and the soaking up spectrum becomes broader up to 600nm [ 16 ] . The Jsc was dramatically decreased by increasing P3HT: PCBM concentration, i.e. 5.96 mA/cm2 for 2wt % and 4.97 mA/cm2 for 3wt % . The FF of 1 wt % device was calculated to be 49 % , where as it was 44 % for 2 and 3 wt % devices. These consequences elucidate the public presentation of solar cell depend on concentration. Baek et Al. reported unfastened circuit electromotive force somewhat decreased by lessening in P3HT: PCBM concentration. The maximal theoretical current for P3HT with a set spread at 650nm ( 1.9 electron volt ) was found to be 14.3 mAcm2 ( AM1.5, 100 mWcm2 ) [ 17 ] . Power transition efficiencies of organic photovoltaic cells based on P3HT: PCBM majority heterojunctions up to 4.4 % have been reported [ 18 ] . Different procedure parametric quantities were optimized like dissolver, solvent drying conditions, electron giver to acceptor ratio, cathode constructions and buffer bed utilizing PEDOT: Postscript to accomplish higher PCE for P3HT: PCBM devices.
PBTDT: PCBM Blend Solar Cell
A survey has been performed on solar cell fabricated by PBTDT as giver and thiophene based polymer PCBM as acceptor. This cell has been prepared by fade outing PBTDT and PCBM in Chlorobenzene by heating and ultrasonicating at 800C for 30min. Then extremely conductive PEDOT: PSS spin casted into cleaned In Sn oxide ( ITO ) coated glass substrate and with chlorobenzene solution incorporating PBTDT and PCBM on substrate top. Consequently a ~100 nanometer aluminium movie was deposited on top of the active bed in vacuity.
PEDOT: Postscript is used as a hole conducting and transporting bed in electroactive devices and OPVs to better the public presentation [ 19 ] and has found widespread application.
Figure 5. Chemical Structure of PBTDT
Figure 6. UV-vis soaking up spectrum of PBTDT: PCBM cell.
This PBTDT: PCBM blend thin movie cell exhibits UV-vis soaking up spectrum is shown in figure.6. One extremum soaking up occurs at 335nm and another occurs at 550nm [ 20, 21 ] . PBTDT: PCBM have stronger soaking up capacity than other polymer used in photovoltaic application i.e. P3HT: PCBM [ 5 ] . Although stronger soaking up does non straight lead to stronger photocurrent in practical devices, but synthesized PBTDT polymer can be used in polymer PV devices efficaciously. Based on UV-vis measuring, the set spread of PBTDT is 1.8 electron volt. As PBTDT have low set spread therefore PBTDT-PCBM interface is suited for exciton ( a edge province of an negatron and an negatron hole in a semiconducting material ) dissipation.
The best public presentation was achieved at PBTDT: PCBM weight ratio 1:6 [ 14 ] . The device displays a Voc of 0.43V, a Joc of 3.21 mA/cm2, a FF of 0.30 and power transition efficiency ( PCE ) of 0.42 % . In polymer: PCBM blend system the negatron mobility increases strongly with addition of the PCBM content but is still ever lower than the hole mobility while the hole mobility somewhat decrease with addition of the PCBM concentration [ 22,23 ] . As hole mobility in pristine PBTDT is higher therefore a higher composing is necessary for better public presentation. The consequences here may show that the balance of electrical conveyance between holes and negatrons is more critical instead than single hole or negatron transportation rate in a blended majority heterojunction-based device. This explains why PBTDT has strong soaking up and high hole mobility while demoing hapless public presentation and indicate that an acceptor with higher negatron mobility in needed for better electrical conveyance balance between hole and negatron for better public presentation of PBTDT solar device.
Decision
Converting solar energy into electrical energy is going of import due to the crisis in conventional energy beginnings today. There are assorted natural resources available to bring forth energy. Converting solar energy into electrical energy is one of such development of the natural beginnings.
The spectrum of the Sun as received on Earth has been presented in the context of light reaping by low set spread polymer stuffs and an rating of the possible currents that can be obtained for a stuff with a given set spread has been considered.
In the 1980s and early 1990s, when the OPVs were on their babyhood as they showed low efficiencies and short life-times and were therefore non a existent rival of conventional Silicon solar cells. In recent old ages, this has changed dramatically and the public presentations of OPVs have improved significantly with efficiencies of around 5 % . Availability of cheap and varied natural stuffs accompanied by an easy fiction process and the ability to tune molecular belongingss has made organic photovoltaic an attractive proposition [ 24, 25 ] . These organic stuff based solar cells have first-class potency to absorb sunshine and generate negatrons. By experimenting with different stuffs composing in organic solar cells solar radiation spectra at broad scope can be absorbed. Besides the transition of photoelectrons into current depend on assorted factors, such as, fill factor, current transition efficiency etc. Different organic stuffs have been used to heighten and better these factors because they have the possible to countervail the capital cost in comparing to silicon based solar cell. Several organic stuffs have the great potency to be an first-class giver in polymer solar cells for high public presentation when an acceptor with high negatron mobility can be discovered.
As present researches are ongoing to happen out better negatron acceptor for organic solar cells thiophene could be proved as a promising and possible one hiking the overall efficiency along with fill factor and current transition efficiency of solar cells. Different organic stuffs of low cost with acceptable lastingness are tried by the research worker at different composing in the organic solar cell to happen out the best 1 in footings of cost, efficiency, effectivity and fabrication procedure. As thiophene has the set spread lower than 2.0 electron volt they are best suited to be used indoor applications and on cloud-covered cloudy yearss. Besides the soaking up extremum for thiophene based solar cell stuffs were found to be occurred at longer wavelength part of the solar spectra which provides the chance to bring forth comparatively greater sum of negatrons by absorbing more photons ensuing in higher current denseness with compared to conventional Si based solar cell. The range of traveling in the part of higher current denseness by absorbing solar spectra of larger wavelength is limited by the molecular weight of PCBM. Because a diminishing tendency in the current denseness was pointed out with the addition in the molecular weight of PCBM and an optimal bound is to be set out. So thiophene could be named as a possible organic stuff to be used in solar cell as public presentation heightening stuff and farther researches with thiophene may thrust the prevalence of organic solar cell in the close hereafter.

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