University of Groningen Fullerene based organic solar cells Popescu, Lacramioara Mihaela IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2008 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Popescu, L. M. (2008). Fullerene based organic solar cells. [s.n.]. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license. More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne- amendment. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 26-07-2022
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University of Groningen
Fullerene based organic solar cellsPopescu, Lacramioara Mihaela
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.
Document VersionPublisher's PDF, also known as Version of record
Publication date:2008
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):Popescu, L. M. (2008). Fullerene based organic solar cells. [s.n.].
CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.
Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.
Photovoltaics literally means light-electricity: photo comes from the Greek phos,meaning light, and volt from the Italian scientist Alessandro Volta; a pioneer inthe study of electricity. This technology has many advantages: it is modular,clean, easy to maintain, and can be installed almost anywhere to suit the needsof the user. The electricity produced can be used directly, stored locally or fedinto an existing electricity grid.
The discovery of the photovoltaic effect is ascribed to the French physicistEdmund Becquerel in 1839, who reported a photocurrent when a silver coatedplatinum electrode was illuminated in aqueous solution [1]. Forty years later,in 1873 and 1876, respectively, Smith and Adams were the first to report on ex-periments with selenium [2, 3]. By that time the first solid state photovoltaicdevices were constructed. In 1906 Pochettino [4], and in 1913 Volmer [5], dis-covered photoconductivity in anthracene. However, it was not the photovoltaicproperties of materials, like selenium which excited researchers, but the photo-conductivity. In the late 1950s and 1960s the potential use of organic materials asphotoreceptors in imaging systems was recognized [6]. The scientific interest aswell as the commercial potential led to increased research into photoconductiv-ity and related subjects. In the early 1960s it was discovered that many commondyes, as methylene blue, had semiconducting properties [7]. Later, these dyes
1
2 Chapter 1: Introduction
were among the first organic materials to exhibit the photovoltaic effect [8, 9].Most of the earlier understanding of the photovoltaic effect in organic photocellscomes from the study of devices fabricated from many biological molecules likecarotenes, chlorophylls and other porphyrins, and the structurally related ph-talocyanines.
The first fabricated inorganic solar cell was reported by Chapin, Fuller andPearson in 1954 at Bell Laboratories [10]. It was a crystalline silicon solar cell,which converted sunlight into electric current with an efficiency of 6%, six timeshigher than the best previous attempt. Over the years the efficiency of crys-talline silicon cells has reached more than 24% in a laboratory setting [11], whichis close to the theoretical upper limit of 31% [12, 13]. The technology usedto make most of the silicon solar cells fabricated so far, benefited greatly fromthe high standard of silicon technology developed originally for transistors andlately for integrated circuits.
Today, a range of photovoltaic (PV) technologies are commercially availableor are under development in the research laboratories. An overview of the cur-rent PV technologies and the efficiencies reached are presented in Table 1.1.
Wafer-based crystalline silicon has dominated the photovoltaic industrysince the dawn of the solar PV area [13, 14]. It is widely available, has a con-vincing track-record in reliability and its physical characteristics are well under-stood, in part thanks to its use in the half-century-old microelectronics industry.Purified silicon (polysilicon) is the basic ingredient of the silicon modules. It ismelted and solidified using a variety of techniques to produce ingots or ribbonswith different degrees of crystal perfection. The ingots are shaped into bricksand sliced into thin wafers. Wafers are processed into solar cells and intercon-nected in weather-proof packages designed to last for at least 25 years. For thepast few years the availability of polysilicon feedstock has been a critical issuefor the rapidly growing PV industry. The tight supply has caused very highpolysilicon spot market prices and has limited production expansion for part ofthe industry [15].
The highest solar cell efficiency has been demonstrated using III-V semicon-ductors [51]. GaAs, InP and their alloys have been used in single-junction andmulti-junction configurations. The attractive features of GaAs are its high ab-sorbtion coefficient (devices only few µm thick absorb most of the light) and itshigh temperature coefficient compared with silicon (GaAs solar cells performsbetter in situations where the cells operates at high temperature and in space).GaAs cells have been developed primarily for use in space applications wherethe priority is to reduce the cell weight. However, GaAs cells are used in ter-
1.1. Photovoltaics: A historical perspective 3
Table 1.1: Current photovoltaic technologies and their efficiencies.
Technology Photovoltaic device Conversion efficiencies and notes
restrial applications for power generation under concentrated light. The mainchallenges for GaAs solar cells relate to minimize front surface and junctionrecombination, minimize series resistance and substrates costs (yet depositingGaAs on GaAs substrates is prohibitively expensive).
Thin-film solar cell active layer materials are deposited directly on large areasubstrates, such as glass panels (square meter-sized and larger) or foils (severalhundred meters long). Thin-film PV has an inherent low-cost potential becauseits manufacture requires only a small amount of active (high cost) materials (filmthickness is typically 1 µm) and is suited for fully integrated processing and
4 Chapter 1: Introduction
high throughputs [14]. There are three major inorganic thin-film technologies:amorphous/microcrystalline silicon (TFSi), the polycrystalline semiconductorsCdTe, and Cu(In, Ga)(S, Se)2 (CIGS). The CIGS technology currently exhibits thehighest cell and module efficiencies of all inorganic thin film technologies [22].A main challenge specially facing CIGS thin-film technology is the reduction ofthe material costs: high cost materials (In, Ga) should be replaced, active layerthicknesses should be reduced, and tolerance impurity in the materials shouldbe increased. Another problematic issue is that the available resources of in-dium are very limited. The attractive features of CdTe are its chemical simplicityand stability. Because of its highly ionic nature, the surfaces and grain bound-aries tend to passivate and do not contain significant defects. Its ionic naturealso means that absorbed photons do not damage its stability. CdTe’s favorablethermo-physical properties and chemical robustness make the CdTe cells easyand cheap to manufacture, using a variety of deposition methods. The efficiencyof CdTe cells depends on how the CdTe layers are grown, the temperature atwhich the layers are deposited and the substrate on which they are deposited.The main concern with this technology is the toxicity of the materials involved,even though very small amounts are used in the modules.
Organic solar cells offer the prospect of low cost active layer material, low-cost substrates, low energy input and easy upscaling. Another advantage isthe possibility of printing active layers , thereby boosting production through-puts typically by a factor of 10 to 100 compared to other thin-film technologies.Within ”organic solar cells”, two technology branches exist. One is the hybridapproach in which organic solar cells retain an inorganic component (for exam-ple the Graetzel cell [27]). The other is the full organic approach (for examplebulk-heterojunction solar cells [26]). More details about organic solar cells willfollow in the next section. The main challenges for both approaches relate to theincrease of efficiency, stability improvement and the development of a technol-ogy adapted for these materials.
The novel PV technologies can be characterized as high-efficiency approaches.Within this category, a distinction is made between approaches that tailor theproperties of the active layer to better match the solar spectrum and approachesthat modify the incoming solar spectrum without fundamentally modifying theactive layer properties. By introducing quantum wells or quantum dots consist-ing of a low-band gap semiconductor within a host semiconductor with a widerband gap, the current might be increased while retaining (part of) the higheroutput voltage of the host semiconductor [32, 33]. Tailoring the incoming solarspectrum for maximum conversion efficiency in the active semiconductor layer
1.2. Organic solar cells 5
relies on up- and down-conversion layers and plasmonic effects [34–36]. Theapplication of such effects in PV is definitely still at a very early stage, but thefact that they can be ”bolted-on” to conventional solar technologies (crystallinesilicon, thin films) may reduce their time-to-market considerably. An improve-ment of at least 10% (relative) of the performance of existing solar cells technolo-gies thanks to up- and down-convertors or the exploitation of plasmonic effectsshould be demonstrated in the coming decade.
The concentrator technologies (CPV) are based on the idea of concentratingsunlight to generate photovoltaic electricity, which is as old as the science ofphotovoltaics itself. Concentrating the sunlight by optical devices like lenses ormirrors reduces the area of solar cells and/or of the modules that house them,and increases their efficiency. CPV’s reliance on beam irradiation and the ne-cessity to track the sun’s motion across the sky by moving the system, is partlycompensated by longer exposure of the cells to sunlight during the day. Themost important benefit of this technology is the possibility to reach system effi-ciencies beyond 30%, which can not be achieved by single-junction 1-sun (nonconcentrating) photovoltaic technology.
Although PV systems are commercially available and widely developed, fur-ther development of PV technology is crucial to enable PV to become a ma-jor source of electricity. Currently the research is primarily aimed at reduc-ing the generation cost of PV electricity which at present may vary from 4 to8 euro/watt peak [15]. Considering the present development rate the cost of PVelectricity is expected to be competitive with consumer electricity (grid parity)in Southern Europe by 2015. Specifically, this means reaching PV generationcosts of 0.15 euro/kWh, or a PV system price of 2.5 euro/watt peak.
1.2 Organic solar cells
An alternative approach for inorganic semiconductor photovoltaic devices is theuse of organic, molecular semiconductors, which can be processed over large ar-eas at relatively low temperatures, either by vacuum sublimation of molecularmaterials, or, preferably, by processing from solution of film-forming materialssuch as polymers. Compared with Si, high optical absorption coefficients arepossible with these materials, which allows production of thin solar cells withmany advantages and opportunities as a result. An additional feature of an or-ganic semiconductor is that they are less expensive compared with inorganicsemiconductors like Si. The challenges in developing organic semiconductorsfor use in photovoltaic applications are considerable; requiring new materials,
6 Chapter 1: Introduction
new methods of manufacture, new device architecture, new substrates, and en-capsulation materials.
Until the seventies, carbon-based molecules and polymers have been con-sidered to be insulating materials, and as such have been exploited as electricalinsulators in many applications. A step forward for the molecular electronic ma-terials was the discovery in 1977 by Heeger et al. that the conductivity of the con-jugated polymer polyacetylene can be increased by several orders of magnitudeupon doping with iodine [37]. This discovery was followed by the observationof electroluminescence in poly(p-phenylene vinylene)(PPV) in 1990 [38, 39], thatinitiated an exciting and rapidly expanding field of research into these materials.The novel electronic properties of both molecular and polymeric semiconduc-tors arise from their conjugated chemical structure. This means that the bondsbetween carbon atoms in the framework of the polymer are alternating singleor double (called conjugation). In conjugated materials three sp2 hybrid orbitalsform covalent bonds: one with each of the carbon atoms next to it, and the thirdwith a hydrogen atom or other group. The remaining electron occupies a pz
orbital. The mutual overlap of pz orbitals creates π bonds along the conjugatedbackbone, by that delocalizing π electrons along the entire conjugated path. Thehighest filled π orbital is named the highest occupied molecular orbital (HOMO)and the empty lowest π∗ orbital is named the lowest unoccupied molecular or-bital (LUMO). The energy gap between the HOMO and the LUMO ranges from1 to 4 eV which makes most semiconducting polymers ideally suited for appli-cations in optoelectronic devices operating in the visible light range.
An organic solar cell consists of a photoactive layer sandwiched betweentwo different electrodes. One of the electrodes must be (semi-)transparent sincethe cell will be exposed to light. The underlying principle of a light-harvestingorganic PV cell consists of six distinct processes: (a) absorption of a photon creat-ing an exciton (bound electron-hole pair); (b) exciton diffusion; (c) charge trans-fer at donor/acceptor interface; (d) dissociation and separation of the carrierpair; (e) charge carrier transport to the corresponding electrodes; (f) collectionof charges by the electrodes.
The photoactive layer is based on a single, a bi-layer, or a mixture of two(or more) semiconductors. PV devices based on dyes or polymers yield limitedpower conversion efficiencies, typically well below 0.1%, mainly because of thelow relative dielectric constant of organic materials. Due to this low dielectricconstant strongly bound excitons are formed upon light absorption. Thereforethe electric field in PV device, due to the work function difference between theelectrodes, is much too weak to dissociate the excitons. Tang et al. demonstrated
1.2. Organic solar cells 7
that bringing a donor and an acceptor together in one cell dramatically increasesthe power conversion efficiency to 1% [40]. This concept of the heterojunctionhas since been widely exploited in a number of donor-acceptor cells: dye sen-sitized solar cells [41–43], planar organic semiconductor cells [40, 44–46], andbulk heterojunction solar cells [47–50]. The idea behind a heterojunction is touse two semiconductors with different electron affinities and ionization poten-tials. This favors exciton dissociation at the interface: the electron is acceptedby the semiconductor with the larger electron affinity and the hole by the ma-terial with the lower ionization potential. In the planar heterojunction (bi-layerdevice) charge separation is much more efficient at the donor/acceptor (D/A)interface than at the electrode interface in the single layer device. In this devicethe excitons should be generated in the close proximity to the D/A interface,within the exciton diffusion length. Otherwise, the excitons decay instead of tocontribute to the photocurrent.
One of the most used acceptors in planar heterojunction solar cells is thefootball-shaped fullerene molecule C60. The sixty electrons from the pz orbitalsgive rise to a delocalized π system similar to that in conjugated polymers. Theelectron mobility of 0.08 cm2V−1s−1 [51], reported from field-effect measure-ments on thin films of evaporated C60, and mobilities of 0.5 cm2V−1s−1, mea-sured by time-of-flight experiments [52] for C60 single-crystals grown from thevapor phase, makes it ideal for use as an electron acceptor in organic PV cells.In 1992 Sariciftci et al. report the photoinduced electron transfer from a semi-conducting polymer onto C60 [53].
It is clear that exciton dissociation is most effective at the interface in theplanar heterojunction cells, thus the exciton should be formed within its diffu-sion length from the interface. Since typically diffusion lengths are only 5-7 nm[54, 55], this limits the effective light-harvesting layer. However, for most or-ganic semiconductors the film thickness should be more than 100 nm in order toabsorb most of the light. Therefore thick film layers increase light absorption butonly a small fraction of the excitons will reach the interface and dissociate. Thisproblem was solved by Yu et al. [49] who intimately mixed the donor and accep-tor. Therefore the interfacial area is greatly increased and the distance excitonshave to travel in order to reach the interface is reduced. This device structure iscalled bulk heterojunction, and has been extensively used since 1995. One of theinherent problems with bulk heterojunction is that of solid-state miscibility. Yuet al. showed that the solvent used plays an important role on the film quality,and implicitly on the performance of the bulk heterojunction solar cell. Furtherexperiments performed by Shaheen et al. strengthened the fact that the solid
8 Chapter 1: Introduction
state morphology has an important effect on the power conversion efficiencyof solar cells. In 2000 [56] they obtained the first truly promising results forbulk heterojunction solar cells when mixing MDMO-PPV and [60]PCBM (1:4by weight) and optimizing the nanoscale morphology of the film, yielding toa power conversion efficiency of 2.5%. Nowadays the power conversion effi-ciency of polymer/fullerene bulk heterojunction solar cells approaches 6% [26].
1.3 Characterization of organic solar cells
1.3.1 Determining the charge carrier mobility
The current density-voltage characteristics in the dark are the result of thebulk charge carrier transport properties and the electrical properties of theorganic-electrode interface. Generally, electrical characteristics are essentiallyinterface-dependent in the low voltage/low current regime, while they are bulk-dependent in the high voltage/high current regime (Figure 1.1). In the lat-ter case, when the applied voltage (Vbias) exceeds the built-in voltage (Vbi) thedark current density scales quadratically with de voltage indicative of a space-charge limited (SCL) transport. One of the frequently used tools to determinethe charge carrier mobilities is to examine the space charge limited conductionin the dark from current-voltage measurements. The SCL dark conduction oc-curs when the contacting electrodes are capable of injecting either electrons intothe conduction band or holes into the valence band of a semiconductor or aninsulator, and when the initial rate of such carrier injection is higher than therate of recombination or extraction, so that the injected carrier will form a spacecharge, limiting the current flow. Therefore the SCL current is bulk limited.
The mobility µ of carriers in molecularly doped polymers is empirically de-scribed by a stretched exponential dependence [57, 58]:
µ = µ0 exp
(γ
√V
L
)(1.1)
where µ0 is the zero-field electron mobility, γ the field activation factor, and V/L
is the applied electric field.For a trap free semiconductor, assuming that the injecting contact is Ohmic,
the SCL current density is given by the Mott-Gurney equation: [59]
JDark =98εµ
V 2
L3(1.2)
where ε is the dielectric constant of material, V is the internal voltage of the
1.3. Characterization of organic solar cells 9
Dar
k cu
rren
t den
sity
Voltage 0.0
J V/L(local leakage current)
J exp (qV/kT)Diffusion current
J V2/L3
Space-charge limitedcurrent
Figure 1.1: The J − V characteristic in the dark with the three different domains.
device, µ is the charge carrier mobility and L is the thickness of the active layer.In the foregoing discussion, only the dependance of the charge carrier mobilityon the electric field was taken into account. Thus the SCLC is given by: [60]
JDark =98εµ0 exp
(0.89γ
√V
L
)V 2
L3(1.3)
Here V is related to the applied voltage (Vbias) as:
V = Vbias − Vbi − VRS(1.4)
where the built-in voltage (Vbi) is the voltage at which the J − V characteristicbecome quadratic. The VRS
is the voltage drop across the series resistance of thesubstrate.
Recent developments shown that the application of Equation 1.1 to describethe electric field dependence of the charge carrier mobility in low mobility me-dia is not fully correct due to the fact that the density dependence of chargecarrier mobility has been neglected [61–63]. It should be noted, however, thatthe charge carrier density in solar cells is fairly modest [64] and the density de-pendence of charge carrier mobility is negligible.
One can choose the electrodes materials in such a way that the injection ofeither electrons or holes can be suppressed or enhanced, thereby enabling toselectively determine the charge carrier mobility of either electrons or holes.This can be achieved if the work function of one of the electrodes is close to the
10 Chapter 1: Introduction
Voltage
Cur
rent
den
sity
FF =
Jmax
x Vmax
VOC
JSC
0.0
0.0J
Dark
JLight
Maximum power point
Figure 1.2: Typical J − V curves of an organic bulk heterojunction solar cell in the dark(empty symbols) and illumination (full symbols) conditions. The short-circuit currentdensity (JSC ) and open-circuit voltage (VOC ) are shown. The maximum output power isgiven by the rectangle Jmax × Vmax .
energy level of the transport band under investigation, while a high barrier forinjection of the other carrier type into the material exists. An energy diagramfor such an electron-only and hole-only device will follow in Chapter 2.
1.3.2 Characterization under illumination
In order to determine the performance of a solar cell device, as well as its elec-trical behavior, current density-voltage (J − V ) measurements in the dark andunder illumination are performed. Figure 1.2 shows the typical J − V curveof a solar cell in the dark (empty symbols) and under illumination (full sym-bols). The photovoltage developed when the terminals are isolated is called theopen circuit voltage (VOC). The photocurrent drawn when the terminals areconnected together is the short-circuit current (JSC). The operating regime of asolar cell is the range of bias from 0 to VOC in which the cell delivers power.
The cell power density is given by:
P = J × V (1.5)
P reaches a maximum at the cell’s operating point or maximum power point. Themaximum of the obtained electrical power Pmax is located in the fourth quad-rant where the product of current density J and voltage V reaches its maximum
1.4. Motivation and outline of the thesis 11
value :Pmax = Jmax × Vmax (1.6)
The fill factor (FF ) measures the quality of the shape of the J − V curve and isdefined as:
FF =Jmax × Vmax
JSC × VOC, (1.7)
which is the ratio between the Pmax that can be drawn from the device and theproduct of JSC and VOC . The power conversion efficiency η of a solar cell isrelated to these three quantities by:
η =JSC × VOC × FF
Plight(1.8)
where Plight is the power of the incident light.These four quantities JSC , VOC , FF and η are the key performance charac-
teristic parameters of a solar cell. All these photovoltaic parameters should bedefined for particular illumination conditions. The Standard Test Conditions(STC) include a constant temperature of the PV cells (25 oC), the intensity ofradiation (1000 W/m2), and the spectral distribution of the light (air mass 1.5or AM 1.5 global, which is the spectrum of sunlight that has been filtered bypassing through 1.5 thickness of the earth atmosphere).
The photocurrent generated by a solar cell under illumination at short circuitis dependent on the incident light intensity. The relation between the photocur-rent density and the incident spectrum is defined as the cell’s external quantumefficiency (EQE). The EQE is the ratio between the photocurrent and incidentphoton flux at one particular wavelength. EQE depends upon the absorptioncoefficient of the solar cell material, the efficiency of charge separation and theefficiency of charge collection in the device. The EQE does not depend on theincident spectrum, and it is therefore a key quantity describing solar cell perfor-mance under different conditions.
1.4 Motivation and outline of the thesis
Photovoltaic technology converts the sun’s rays directly into electricity withoutmoving parts or emitting pollutants. If its costs can be lowered, photovoltaicelectricity could become a competitive source of energy. It would help to com-bat the global threat of climate change and improve the security of energy sup-ply. Although a lot of progress has been made with organic bulk heterojunctionsolar cells, which were briefly discussed in the previous sections, the power
12 Chapter 1: Introduction
conversion efficiency needs to be improved further if these cells are to be com-mercialized. Since 2000 the bulk heterojunction solar cells are prepared fromconjugated polymers and, with only few exceptions, always using the samefullerene derivative ([60]PCBM). So far [60]PCBM remains the best perform-ing soluble fullerene derivative used in polymer:fullerene blends. This thesisaddresses the possibility of using new fullerene derivatives in organic bulk het-erojunction photovoltaic devices since one of the challenges for organic solarcells relates to developing of new materials.
Chapter 2 presents an overview of the materials used in the active layer ofbulk heterojunction solar cells, and the protocols that we developed for repro-ducible device preparation and characterization.
In Chapter 3, experimental studies of the morphology, charge transport andsolar cell performance in blends of P3HT and a newly developed analogue of[60]PCBM ([60]ThCBM) are presented. [60]ThCBM was designed with the aimof improving miscibility with polythiophene donors, especially (regioregular)P3HT.
The influence of film organization on the performance of the P3HT:[60]ThCBMbulk heterojunction photovoltaic cells is discussed in Chapter 4. The photo-voltaic performance of the fast drying and subsequent annealing of the activelayer is compared with the one of the cells with a slow growth photoactive layer.The benefit of a well balanced and enhanced charge carrier transport for slowgrowing films makes it possible to fabricate thicker films maximizing the lightabsorption in these blends.
Chapter 5 addresses the electrical stability of P3HT:Methanofullerene bulkheterojunction solar cell devices which has been investigated under standardcondition of illumination and temperature. Electrical and optical properties ofthese devices have been monitored for a test period of 1000 hours of continuousillumination.
Finally, Chapter 6 describes the solar cells performance in blends of P3HT anda mixture of two n-type semiconductors with approximately the same electronaccepting ability (as measured by the reduction potential) such as C60 and C70
fullerene derivatives. The use of two or more different n-type semiconductorsconcomitantly in the same device is essentially unheard of in the field of organicelectronics, let alone solar cells. Furthermore, the effect that certain impuritiesin the n-type semiconductor composition (such as pure C60 or higher fullerenesthan C70) have on the electrical properties of the solar cell devices is quantified.
REFERENCES AND NOTES 13
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14 Chapter 1: Introduction
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