Electrospun Polymer-Fiber Solar Cell

A novel electrospun polymer-fiber solar cell was synthesized by electrospinning a 1 : 2.5 weight% ratio mixture of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) resulting in bulk heterojunctions. Electrospinning is introduced as a technique that may increase polymer solar cell efficiency, and a list of advantages of the technique applied to solar cells is discussed. The device achieved a power conversion efficiency of %. The absorption and photoluminescence of MEH-PPV nanofibers are compared to thin films of the same material. Electrospun nanofibers are discussed as a favorable structure for application in polymer solar cells.

iii Acknowledgement I would like to thank my mentor and advisor Dr. James T. McLeskey for providing me with such vital direction and support throughout this project. His guidance has led me towards new research opportunities and eventually brought me to my next career stage -a research position in California.
I would also like to express my gratitude towards Dr. Chunya Wu for showing me the basics and helping me get acquainted with the lab, Dr. Dimitry Pestov for giving me the most precise and well-observed suggestions for my research , Dr. Gary Tepper for allowing me to collaborate with his lab to perform research on his electrospinning experiment., Dr. Gary Atkinson for guiding me through several equipments at the Virginia Microelectronics center, Mr. Josh Starliper for being a tremendous support and a great friend for years to come, and Dr. Mikhail Reshchikov and Dr. Supryo Bandyopadthay for teaching me many valuable lessons in research which have proven to become critical inputs for my study. Single organic layer (e.g., tetracene) solar cells were the first reported in the late 1960s and early 1970s, and consisted of an organic layer sandwiched between a low-work-function metal layer (aluminum) and a high-work-function metal (gold) as shown in Figure 3 [15][16][17][18][19] ,... 11 Breakthrough advances in organic solar cells were made first by Harima et al in 1984, andTang in 1986 with the introduction of two-component organic photovoltaic cell [22,23]. A power conversion efficiency of 1% under AM2 illumination was achieved through the use of zinc phthalocyanine (ZnPc) and porphyrin derivative (TPyP) thin films [22], and also with copper phthalocyanine (CuPc) and a perylene tetracarboxylic (PV) derivative [23].   [47]. The electron-hole pair for most conjugated polymers is generally known to have a binding energy of 0.3 eV -0.4 eV [48,49]. With the energy level offset being greater than the exciton binding energy, the electron-hole pair is separated at a polymer (MEH-PPV)/electron-acceptor (C 60 ) interface where the holes will then travel through the polymer (µ~ 1.1×10 -7 cm 2 /Vs for MEH-PPV [27]) to the anode (ITO), and the electrons will travel through the energetically favorable C 60 by hopping (µ=2×10 -7 m 2 /Vs [50]) and toward the Au cathode. This process is shown in Figure 6

Introduction and Motivation
There is a strong demand for renewable energy sources in the world today. The IEA (International Energy Agency) has reported that in the year 2006, 87.1% of electricity in the world was generated from non-renewable resources (coal, nuclear, gas and oil) [1]. Each of these sources can impact the environment in ways such as acid rain, radioactive waste, CO 2 pollution, and imported greenhouse gasses.
In 1956, M. King Hubbert created a theoretical model predicting the peak of oil production to be around year 1965 to 1970 [2]. Many other models were developed predicting the time period of peak oil differing from the Hubbert Peak theory. However all of the models have shown that oil production will decline, and oil is limited in supply.
The limitation in supply can lead to an increase in price of gasoline with significant impact to the economy, and cause strain in the nation's electricity such as the 2008 spike in the price of gasoline. Considering that petroleum oil makes up 34.4% of the total primary energy supply along with all the negative impact of non-renewable resources, it is clear that alternatives to non-renewable resources must be investigated.
In response to the energy crisis we face around the world, solar energy is attracting considerable attention as one of next generation energy sources. It has been noted by the U.S. Department of Energy that covering 0.16 % of the earth's surface by 10 % efficient solar cells will provide ~20 TW (2x10 13 W) of electricity, more than enough to cover the energy demand of the whole planet [3]. With the sun emitting energy that is virtually unlimited in supply, solar cells have significant potential as a next generation energy source.
The modern solar cell was first demonstrated in 1954 at Bell Labs. It was 6 % efficient and made from crystalline silicon (c-Si) [4]. C-Si solar cells were further developed reaching an efficiency of 14-18 % [5], and more than 95 % of current solar cells in use are c-Si solar cells [6]. In 1974, the amorphous silicon (a-Si) solar cell was invented at RCA Laboratories as an alternative to c-Si device [7]. An a-Si solar cell was made by evaporation of silicon to produce amorphous thin films requiring less material that would lower the cost of solar cell production [8]. The a-Si solar cell has a relatively low 6-8 % efficiency due to high defect densities from evaporation processing.
However, the thin film approach used in a-Si solar cell allowed further development utilizing multilayer processing to produce multijunction solar cells, and has led to an efficiency higher than 40 % [9][10][11] for Ga x In 1-x As or Ga y In 1-y P solar cells by stacking different materials to absorb a larger portion of the available sunlight.
Regardless of the significant improvements made in efficiencies of inorganic photovoltaics, they continue to struggle in competing against non-renewable resources due to their high cost. The primary problem is in high energy processing of silicon, and the cost per kilowatt-hour for electricity from Si-based solar cells is as high as $0.25-0.65 /kWh which is roughly 5 times more than the price of electricity produced using fossil fuels [12]. Adding to the high cost of inorganic solar cells, the potential increase in demand for crystalline Si can lead to even higher costs for the devices. One estimate has calculated that for a family consuming 20 kWh/day using 15 % efficient solar cells, the amount of silicon needed would 10,000 times more than that in a computer [13].
Clearly, a low cost alternative to inorganic solar cells is needed.
Polymer solar cells are a low cost alternative to inorganic solar cells. In addition to the lower cost of polymer solar cells, the use of polymeric materials are known to have several advantages such as lighter weight, and less energy used for large scale production, thanks to solution based processing. Polymer solar cells have significant mechanical flexibility, and are capable of being directly fabricated onto most surfaces including plastics. However, the efficiency of polymer solar cells is still low compared to inorganic solar cells due to poor light harvesting, limited photocurrent generation, and poor charge transport. This dissertation outlines these challenges in polymer solar cells, and will discuss a unique approach of utilizing a technique called electrospinning to resolve these issues. The work is to produce the first ever electrospun polymer-fiber photovoltaic cells.

Objectives
The objective of this project is to synthesize and characterize polymer-fiber solar cells by electrospinning. It is expected that these new devices will yield increased absorption, improved charge transport and improved charge collection. Increase in these parameters can lead to improved efficiency of the device added to potential lower cost benefits of polymer solar cells. The intention of the dissertation outlined in this proposal is to demonstrate a functioning electrospun polymer-fiber solar cell and characterize its output. The specific goals of the work are fabrication of the first ever polymer-fiber solar cell, and an investigation of triaxial electrospinning as the next step to fabrication of quasi-one dimensional solar cells.

Dissertation Outline
Chapter 2 presents a background and literature review of organic solar cells.
Information on solar radiation and air mass are introduced. Basic characteristics of solar cells are explained. History in development of polymer solar cells is followed.
Properties of polymers in organic solar cells are discussed, and different device architectures are discussed with their advantages along with their disadvantages.
Chapter 3 starts with the history of the electrospinning experiment, and its operation. Electrospinning and its application to solar cells are discussed. Some advantages for solar cells made using electrospining are illustrated.
Chapter 4 introduces an unusual occurrence of bimodal fibers in electrospinning.
An experiment using a water soluble PTEBS polymer is discussed, and its applicability to solar cells is considered. Significance in humidity for electrospinning is illustrated through an experiment using PTEBS, and its applicability to solar cell synthesis is discussed.
In Chapter 5, three electrospun polymer-fiber solar cell device structures are presented. A bulk heterojunction type polymer-fiber solar cell is considered for its conventional metal-semiconductor-metal structure. Triaxial electrospining is considered for its structural advantages along with its simple solar cell fabrication process. The coplanar bimetallic interdigitated electrode substrate is then introduced for a simple solar cell fabrication process of spin-on device synthesis. In chapter 10, the device parameters of the electrospun polymer-fiber solar cell are discussed. An efficiency of the device is studied in consideration for the unique device structure which leads us to conclude that the device efficiency is 3.08 × 10 -7 % or better.
An equivalent circuit analysis is performed from the current-voltage characteristic equation to understand the relationship between different resistances and fill factor.
Chapter 11 concludes with the summary of this work, and future work to be considered for further research in synthesis of electropsun polymer-fiber solar cell.

Solar Radiation
Through series of nuclear fusion reaction of hydrogen and helium, the surface temperature of the sun at 5800 K is hot enough to ionize all elements at this temperature, and emits wide spectrum of radiation. This radiated energy is then either scattered or absorbed on a clear day, and roughly 76% of the incident energy reaches the earth surface (shown in Table 1). Considering the atmospheric loss of energy in solar radiation, the amount of radiation can be classified by the air mass coefficient (AM) defined as AM0 is the extraterrestrial radiation, AM1 is the vertical incidence of sunlight at the equator at sea level, and AM1.5 is the sunlight radiating through an air mass 1.5 greater than the vertical case [14]. A standard solar radiation has a spectrum around the visible wavelengths from 380 nm to 780 nm as shown in Figure below

Characteristics of Solar Cells
In characterizing a solar cell, the general interest is in the performance of the device, and its efficiency (η). The efficiency is measured by looking at the ratio between the power input of the incoming light, and the maximum power output of the device. The power output of a solar cell is measured, and characterized by observing the relationship between its current density and voltage known as the J-U curve. With a working solar cell, a J-U measurement will be a curve similar to a photodiode as shown below,

Open Circuit Voltage and Short Circuit Current
When there is no external load on a solar cell, there is a built-in electrical potential between the two terminals of the device under white light illumination, and this is called the open circuit voltage (U oc ). U oc is the measure of maximum voltage produced by the device, and it is measured when the load is connected to infinite resistance. The built-in electrical potential V oc will then cause a drift in the photogenerated charges of the device known as the short circuit current (J sc ). J sc is the measure of maximum current produced by the device, and it is measured when the load is connected to zero resistance. Using these values, the quality of a device can be characterized by looking at a number called the fill factor (FF).

Fill factor
The ratio between the maximum electrical power, and the theoretical maximum of electrical power estimated from the product of J sc and U oc gives the fill factor (FF) defined as, where J MP and U MP are current density and voltage of maximum electrical power output of the device. Fill factor is essentially a measure of quality of the device, and it can also be seen as the ratio between the dark shaded areas of the J-U curve to the lightly shaded area of the J-U curve ( Figure 2). With higher fill factor, the device is able to extract more electrical power from a constant current source with a maximum voltage.

Energy Conversion Efficiency
Solar cell efficiency is a measure of how effectively a device is able to convert the energy of the sun to electricity. Therefore, an efficiency of a solar cell is a ratio between the electrical power output of the cell, and the incident optical power. Solar cell efficiency η is then given by,

History
Single organic layer (e.g., tetracene) solar cells were the first reported in the late 1960s and early 1970s, and consisted of an organic layer sandwiched between a lowwork-function metal layer (aluminum) and a high-work-function metal (gold) as shown in For homojunctions, growth of a thin oxide layer on the low-work-function material was recognized as an electron acceptor in 1983, where it formed a metalinsulator-semiconductor (MIS) structure Schottky-type photodiodes demonstrating a photovoltaic effect [21]. Extremely low efficiencies of these devices (~10 -4 %) [16] led Glass Al Semiconductor (tetracene) Au to research in testing wide array of materials by different research groups. However, the efficiencies of homojunction organic solar cells remained below 1% [21].
Breakthrough advances in organic solar cells were made first by Harima et al in 1984, and Tang in 1986 with the introduction of two-component organic photovoltaic cell [22,23]. A power conversion efficiency of 1% under AM2 illumination was achieved through the use of zinc phthalocyanine (ZnPc) and porphyrin derivative (TPyP) thin films [22], and also with copper phthalocyanine (CuPc) and a perylene tetracarboxylic (PV) derivative [23]. Figure 4 shows the device layout of two-layer organic photovoltaic cell by Tang. Through research on these heterojunction polymer solar cells, the charge transfer at the interface between two materials were found to be energetically favorable with distinct electron donor and acceptor layers [24].

Properties of polymers in Organic photovoltaics
For organic solar cells, the most common polymers have included the poly(phenylenevinylenes) such as MEH-PPV and the polythiophenes such as: poly(3- Glass undecyl-2,2 '-bithiophene) or P3UBT and poly(3-hexylthiophene) or P3HT. These conjugated polymers have semiconducting characteristics resulting from their alternating single (σ-bonds) and double carbon-carbon bonds (σ-bond and π-bond combination).
With delocalized π-bonds over the entire molecule, the overlapping p z orbitals formulate two orbitals that are called a bonding (π) orbital and an antibonding (π*) orbital. These  Table 2 below.

Bilayer heterojunction devices
In 1992, Sariciftci et al showed that the MEH-PPV polymer had an ultrafast photoinduced electron transfer reaction to C 60 [43]. C 60 or Buckminsterfullerene, is a form of carbon, which stores up to 6 electrons, and is able to work as a strong electron acceptor [44]. Other common acceptors include TiO 2 [32] and CdSe [45].  electron-hole pair known as the Frenkel exciton [47]. The electron-hole pair for most conjugated polymers is generally known to have a binding energy of 0.3 eV -0.4 eV [48,49]. With the energy level offset being greater than the exciton binding energy, the electron-hole pair is separated at a polymer (MEH-PPV)/electron-acceptor (C 60 ) interface where the holes will then travel through the polymer (µ~ 1.1×10 -7 cm 2 /Vs for MEH-PPV [27]) to the anode (ITO), and the electrons will travel through the energetically favorable C 60 by hopping (µ=2×10 -7 m 2 /Vs [50]) and toward the Au cathode. This process is shown in Figure 6 as a circuit diagram. Since its discovery, C 60 has been the electron acceptor of choice for polymer solar cells, and one of the primary focuses in organic solar cell has been an engineering of interface between electron donor polymer, and the electron acceptor.

Bulk heterojunction devices
A bilayer device is limited by small interfacial area where the electron-hole pair separation occurs. In order to overcome this challenge, the bulk heterojunction device  The MEH-PPV:C 60 bulk heterojunction device showed slightly higher photosensitivity compared to the pure P3OT device. With the P3OT device, photosensitivity increased with light incident through the ITO glass. This was explained as carrier generation and transport dominating the device performance at the cathode interface.
For a mixture of polymer with C 60 , a discrepancy was found between calculated external quantum efficiency (EQE) and the results suggested from photoluminescence (PL) quenching [52]. This indicated a problem in charge transport through the active layer for bulk heterojunction devices, and the concentration of polymer and C 60 becomes important in establishing two percolation networks for the generated electron/hole pairs of the active layer.
The first step in processing light into electric current in organic solar cells is the absorption of a photon to produce an exciton. However, organic solar cells suffer from poor light harvesting since they tend to be only efficient in the blue region of the solar spectrum, and not in the red. Most organic semiconductors have relatively large bandgaps (> 2 eV) [53], and 1.4 eV has been reported to be the optimal value for better light harvesting [54]. Development of low bandgap polymer has been proven to be difficult [40] although recent efforts have made progress in this area [37]. Instead, other approaches to increase light harvesting have been demonstrated such as increasing absorption of fullerene component of the organic solar cell by replacing C 60 -PCBM with C 70 -PCBM [40].
Excitons formed through light absorption in organic solar cells have a lifetime of approximately 10 -9 s [55]. If an exciton fails to dissociate within its diffusion length of 4-20 nm [56], the energy is lost due to charge recombination. This loss in energy can be avoided through the use of bulk heterojunction structure where it has been shown that the charge transfer from conducting polymers to C 60 is 10 3 times faster than the decay of photoexcitations [57]. However, with C 60 dispersed throughout the bulk heterojunction  showed that below 20% PCBM in the MEH-PPV:PCBM mixture by weight (4:1) was below the percolation threshold, and an interpenetrating networks for PCBM form at 50% (1:1) and above [52].
In summary, the challenges to improvement of polymer solar cells include, charge transport through the active layer, light harvesting, and charge recombination. In a conjugated polymer solar cell, these challenges can be met by decreasing the length of charge transport, red shifting the absorption spectrum, and by providing an interpenetrating network of an electron acceptor. All of these maybe accomplished by transforming the conventional solar cells of two-dimensional structure to a quasi-one-dimensional structure. A technique for producing a quasi-one-dimensional structure is discussed in Chapter 3.

History
The fundamental concept of electrospinning otherwise known as "electrostatic spinning," dates back to as early as 1934, when Formhals published a series of patents for an experimental setup in production of polymer filaments using electrostatic force [58].
His setup consisted of an electrode placed inside a polymer solution, with the charged solution then jetting out of a metal spinnerette, and evaporating to form polymer filaments at the grounded collector. Through continued research and development of electrospinning, the technique has found its way into filtration, biomedical, protective clothing, electrical and optical, and many other applications [58]. With its ability to produce nanoscale fibers, due to a surge of interest in nanotechnology over the recent years, the number of publications in the area of electrospinning has continued to grow as can be seen from Figure 9 below. In addition to the ability to produce electrospun nanofibers, advances in the experimental setup have been made in producing with aligned nanofibers, and utilizing multi capillary techniques [59].

Operation
In electrospinning, a polymer solution is charged with an AC or DC voltage, and the polymer solution forms a jet of polymer fibers which will be collected at the nearest grounded object ( Figure 10). In some cases, a grounded object can be a conductive rotating drum. With control of the drum speed, it has been demonstrated that the velocity of the reel surface can be adjusted to closely match the speed of the drawing fiber to assist in alignment of the nanofibers [60]. A sample substrate can then be mounted on top of the rotating drum to form aligned polymer-fiber photovoltaics. Other methods of fiber alignment include the use of an auxiliary field, a thin wheel collector with a sharp edge, frame collector, and etc [58].
Not all materials can be electrospun. In 1987, Hayati et al showed the significance of liquid conductivity for an electrospinning solution. As opposed to insulating liquids, conducting liquids have produced unstable streams, and have been proven difficult to electrospin [61]. For this reason, the majority of research in electrospinning has been done with non-conducting polymeric solutions. Table 3 below shows a list of polymers and the corresponding solvents used in previous research done in electrospinning.   Also for nanofibers after removal of PCL, After the removal of PCL, the electrospun P3HT nanofiber was able to maintain not only its nanofiber structure, but also its electrical properties. The mobility measurement for a 20 wt% PCL nanofiber showed degradation of one order of magnitude, µ = 0.0012 cm 2 V -1 s -1 , while a 50 wt% PCL nanofiber showed two-order of magnitude degradation with µ = 0.00047 cm 2 V -1 s -1 [63]. Although there was some level of degradation in their electrical properties, mixing of a high molecular weight electrospinnable polymer has proven itself to be a good method for nanofiber structured electronic devices.
Coaxial electrospinning ( Figure 13) is another method that is able to produce nanofibers from conjugated polymer solutions of low viscoelasticity.  Using coaxial electrospinning with solvent assistance, good nanofiber morphology is maintained using the conjugated polymer P3HT. Whether with an electrospinnable polymer or with a pure solvent, coaxial electrospinning is a powerful technique for synthesizing conjugated polymer nanofibers.
There are other ways to utilize coaxial electrospinning such as making solution A an electrospinnable polymer solution B the non-electrospinnable conjugated polymer.
This method of inverted coaxial electrospinning has an advantage in that it does not require the removal of the electrospinnable polymer to access the electronic properties of conjugated polymer because it is exposed at the surface of the coaxial nanofiber. Zhao et al in 2007 were successful in using this inverted coaxial electrospinning method with PVP as the core, and the conjugated polymer MEH-PPV as shell in synthesizing a conjugated polymer nanofiber [64]. Figure 15 below are images of the MEH-PPV shell coaxial nanofiber. Good morphology was observed in MEH-PPV shell coaxial nanofiber. However, a significant blue shift was observed in the PVP/MEH-PPV nanofiber which can be damaging for its use in polymer electronics. Possible causes for the blue shift include nano-effect of the thin MEH-PPV fiber, and also the possibility of PVP diffusing into MEH-PPV shell to serve as a nanospacer to prevent π-π stacking [64].
As it is evident from coaxial electrospinning, the nozzle tip used for electrospinning is often a main factor in determining the morphological structure of the electrospun nanofiber. By incorporating a multi-channel nozzle in electrospinning, Zhao et al successfully prepared nanofibers of multi-channel multi-tubular structures [65].
Their multi-channel nanotubes were prepared from electrospinning Ti(OiPr) 4 with poly(vinyl pyrrolidone) for their outer shell, and paraffin oil for their inner fluid. Figure   16 below is an SEM image of the multi-channel tube nanofibers.  The TEM image clearly shows the triaxial structure of the nanofiber. Electrsopinning proves to be a highly customizable technique with multi-tubular to multi-layered nanofiber structures. Its ability to mold nanofibers into different structures presents it self with a possibility for applications such as diodes, drug delivery systems, and etc.

Application of Electrospinning to Solar Cells
Most solution-based electronics can be electrospun into nanofiber structures. However, electrospinning of PAn.HCSA nanofiber was made possible by adding a small fraction of PEO to the chloroform based solution. Figure 18 below is an SEM image of the PEO doped PAn.HCSA electrospun nanofiber. PAn.HCSA being a conductive solution, the change in its conductivity by the formation of the nanofiber structure was studied. Figure 19 compares the conductivity of a cast film to electrospun nanofibers with increasing concentration of PAn.HCSA in a blended solution.   Table 4 below.    Similar to the TiO 2 nanofibers of Chuangchote et al [68], the observed P3HT/PCBM nanofibers showed a slight redshift in their absorption spectrum as shown in Figure 23 below.

Advantages of Electrospun Nanofibers for Solar Cells
There are several advantages to conjugated polymer nanofibers formed through electrospinning. Studies of changes in the optical properties of conjugated polymer nanofibers were performed by Babel et al [71]. Blends of MEH-PPV/poly(3hexylthiophene) (P3HT), and MEH-PPV/poly (9,9-  Comparing the thin films to electrospun nanofibers, a red shift in the MEH-PPV absorption peaks from 520 nm to 550 nm were observed. As explained earlier, the red shift is considered to be an effect due to stretching of the polymer chains leading to extension of the π-conjunction length [72]. In studying the PL spectra, intensity decrease with increase in P3HT concentration was observed suggesting an efficient energy transfer from MEH-PPV to P3HT. Clear differences observed between the 20 wt % blend thin film and the 20 wt % nanofiber imply an enhanced interaction between MEH-PPV and P3HT from their confined nanostructures compared to bulk thin film.  fibers, no red shift in the P3HT absorption peak was observed in their experiment.
One unique feature of photovoltaic device fabrication from electrospinning is in its application of electric field. Application of an electric field is an essential part of the electrospinning technique, and a study was done by Padinger et al [73] showing the effects of electric current in preparation of a photovoltaic device. Figure 27 are current to voltage curves of P3HT/PCBM solar cells prepared in different fabrication process. shows mobility enhancement of charge carriers in the photoactive layer. Annealing of the polymer film has been known to help crystallize the polymer structure for better device performance [74]. With an electric field, there is an additional orientational effect that takes place, which is presumed to help in enhancement of mobility.
Considering this orientational effect with an electric field, electrospinning could be a simple way to implement mobility enhancement for bulk heterojunction photovoltaics.

Sodium poly[2-(3-thienyl)-ethoxy-4-butylsulfonate] (PTEBS) is a water-soluble
thiophene polymer semiconductor, which has been used as electron donor to prepare environmentally friendly water-soluble polymer thin film solar cells [32]. For an initial attempt in fabrication of an electrospun polymer-fiber solar cell, PTEBS was combined in an aqueous solution with PEO and then electrospun into fibers in order to study its electrospinning properties and the effect of fiber diameter on optoelectrical properties.
The result was the discovery of a new method for producing bimodal fibers via electrospinning [75].
While electrospun mats exhibiting varying and even bimodal size distributions have been produced [76][77][78], chemically and physically distinct fibers have not been previously electrospun from a single homogeneous solution. Gupta        These results are consistent with the TEM observations and confirm that the small and large diameter fibers are chemically as well as physically distinct.
A high speed CCD camera was used to observe the Taylor cone during the electrospinning process. We observed that only one Taylor cone was formed and only one jet emerged from the tip of the Taylor cone in both the unimodal and bimodal cases.
When NH 4 OH was replaced with NaOH, bimodal fibers were also obtained. Therefore, we believe the formation of bimodal fibers of chemically distinct polymers may be related to the presence of OH-radicals in the electrospinning solution. We also found that the conductivity of a pure PTEBS thin film was increased by about one order of magnitude when made from a solution containing NH 4 OH. The higher electrical conductivity indicates that more PTEBS molecules were ionized when using NH 4 OH.
The negatively charged PTEBS ions, unlike PEO, should move against the direction of the electric field. We believe that the different polarity and electrophoretic mobility of PTEBS and PEO leads to separation within the cone-jet region and the formation of chemically distinct fiber segments. The electric fields employed in the electrospinning process are comparable to the fields used in electrokinetic processes such as gel electrophoresis, but to our knowledge electrokinetic chemical separation has not been previously reported in electrospinning. Figure 31 shows the absorption spectra of the unimodal PTEBS/PEO nanofibers. The optical absorption of the larger bimodal PTEBS/PEO fibers was almost the same as that of a spin coated PTEBS/PEO composite thin film . We believe that this is due to the much larger (200 nm and 1 µm) diameters of the bimodal fibers, which are too large to produce any optical changes in the polymers.

Humidity and Electrospinning
Electrospinning from an aqueous PTEBS/PEO solution without NH 4   In preparation of an organic solar cell, a blue shift of the polymer absorption can decrease its performance. In contrast to the blue shift shown in composite nanofibers, coaxial electrospinning can be used to avoid a decrease in π-π stacking, and to cause a red shift in PTEBS nanofibers. To demonstrate this, coaxial PTEBS fibers were fabricated.
For this experiment, a 1.5% concentration of PTEBS in de-ionized water was used as a core solution, and a 2.5% concentration of 200000 g/mol PEO dissolved in Chloroform was used as the outer solution to perform coaxial electrospinning. The coaxial electrospinning was performed with the PTEBS solution infusion rate at 4.5 µl/min, and the PEO solution infusion rate at 40 µl/min. The distance from the coaxial needle tip to the grounded substrate was kept at 28 cm with a DC voltage of 16 kV, and the collected fibers were washed using an ethanol vapor in a 65°C oven for an hour to remove the PEO coatings of the collected coaxial nanofibers. Figure 33 is an image of PTEBS/PEO coaxial nanofibers taken from an optical microscope at 100× magnification.

Electrospun polymer-fiber solar cell structure
The objective of this project is to take advantage of the qualities of electrospun

Bulk heterojunction type polymer-fiber solar cells
Within a history of organic solar cells, its device structure has typically consisted of a two dimensional sandwich structure where an active layer was deposited onto a transparent conducting substrate, and a counter electrode was evaporated on an exposed surface of the active layer. With electrospinning, an active layer fibers may be collected onto a transparent conducting electrode, and a counter electrode evaporated on the exposed surface of electrospun nanofibers. This device structure is shown in Figure 35 where FTO is used for the transparent conductor, and gold is the counter electrode metal is evaporated onto the electrospun nanofiber surface. This is essentially the device structure fabricated by Sundarrajan et al [69] and described in Chapter 3. For an active layer film deposited onto a transparent conductor, one structural advantage to this design is in its ability to incorporate a hole blocking layer between a conductor and an active layer which has been shown to significantly increase the performance of organic solar cells. To add a hole blocking layer between a conductor and an electrospun polymer-fiber active layer, a hole-blocking layer must be prepared prior to the polymer fiber collection. However, coaxial electrospinning often requires washing of outer shell polymer to establish a connection between the conjugated polymer of the core, and this process can often interfere with the hole blocking layer film. In addition, there is a possible short circuit from an evaporated metal piercing through the polymer fiber openings. To avoid the short circuit of the two electrodes, thicker fiber matt of active layer nanofiber maybe considered, yet this will only produce a repeated problem encountered by Sundarrajan where a thicker fiber matt leading to poor performance of the device due to low charge carrier mobility of the active layer [69]. Due to these Glass FTO

MEH-PPV/PCBM nanofibers Au
Photon problems encountered in for sandwich structure of electrospun polymer-fiber solar cells, other structures are considered for the fabrication of electrospun polymer-fiber device.

Triaxial electrospinning of polymer-fiber solar cells
Triaxial electrospinning has been reported for organic LEDs [83], and the same structure offers several potential advantages for organic solar cells. Triaxial nanofiber organic solar cells would consist of a conductive core, semiconducting mid-layer, and a conductive outer shell as shown in Figure36. One of the challenges with the polymer-fiber solar cells is in establishing good electrical contact with the conjugated polymer to the two electrodes, and the triaxial fiber has the potential to eliminate the challenges experienced in establishing good electrical contact.

Conjugated polymer layer
Conductive layer In addition, the device will not require washing for conjugated polymer exposure, will be readily made with true reel-to-reel processing, and it will be significant step from a conventional two-dimensional structure to quasi-one-dimensional devices.
To test the stability of triaxial electrospinning, the core and shell component of the    Because of these challenges encountered in Triaxial electrospinning, other device structure for an electrospun polymer-fiber solar cell have been studied.

Co-planar bimetallic substrate for electrospun polymer-fiber device
Our first device was successfully prepared with a very simple structure where two metal-coated microscope glass slides were glued adjacent to each other onto another glass slide. Such a device structure allowed easy access to collected nanofibers for nanofiber processing, and mobility of the device will be dependent on electrode separation rather than the fiber matt thickness. A sample image is shown below. For these devices, the resulting current-voltage characteristics are shown in Figure 40 below. The two break-down voltages which are also sensitive to light exposure demonstrate that photodiode were characteristics obtained from an electrospun device prepared with co-planar bimetallic substrates. In addition, small open circuit voltage (U oc ) was recorded at 0.020 V. Based on this successful device fabrication using co-planar bimetallic substrate, it was felt that additional improvements could be made to the device structure to improve the output. We considered reducing the metal separation for more efficient charge collection, and the use of multiple two metal junctions to increase the charge collection area. Such device structure is further discussed in the next chapter for use in electrospun polymer-fiber solar cells.

Interdigitated Electrode Substrate
Polymer solar cells typically take the form of a sandwich structure with the active layer placed in between the anode and cathode electrodes. There are several challenges with this design. For example, at least one of the electrodes must be an optically transparent material such as Indium Tin Oxide (ITO) or Fluorinated Tin Oxide (FTO) [84]. These materials typically have lower conductivity than metal electrodes and are often deposited using high temperatures which can be harmful to the polymer. In addition, the device fabrication requires a two-layer coating (at a minimum) with fabrication of the active layer, and the counter electrode.
For use with electrospun fibers, one possible design would make use of co-planar interdigitated electrodes of dissimilar materials. This chapter describes the development of such a structure through photolithography onto a heavily oxidized Si wafer. This device structure offers some potential advantages in comparison to the conventional multilayered sandwich configuration. For example, because transparency is not required, co-planar interdigitated electrodes in organic solar cells allow the use of a wider variety of electrode materials. In addition, the interdigitated structure has an inherent reliability due to the incorporation of multiple junctions for charge collection.
Interdigitated electrodes of a single metal have been used in sensors [81], transistors [85], and even in photovoltaic devices [85]. There are limited reports of the use of verticallyoriented two-metal interdigitated electrodes in polymer solar cells [87] .

Experiment
The co-planar two-metal electrode substrates were fabricated on the oxidized surface of silicon wafers using photolithography. Two photomasks were prepared and the masks were used to pattern photoresist using UV light and standard photolithographic techniques.
Aluminum and nickel electrodes with a separation distance of between 1 and 3 µm were deposited at a thickness of approximately 100 nm. Figure 41 shows a schematic and microscope image of a completed interdigitated two-metal electrode substrate. The total device area was 0.11 cm 2 . The minimum electrode separation in these devices was limited by our photolithographic capabilities (about 1 µm) and we estimated, based on the short diffusion length of the donor material, that the separation between the two metal electrodes should be closer to 100-200 nm for efficient charge collection. Therefore, relatively poor device efficiency was expected due to incomplete charge collection. However, the primary goal of this work was to introduce and demonstrate the feasibility of new co-planar electrode geometry and no effort was made to optimize the device efficiency. Devices were tested in dark and under AM1.5 illumination of 80 mW/cm 2 intensity. The current density-voltage (J-U) curve was measured using a Keithley 236 source generator by varying the applied voltage from -2 to 2 V in 0.04 V steps across nickel and aluminum electrodes. In addition, the resistance of the silicon dioxide substrate film was tested by measuring the illuminated J-U characteristics of the electrodes prior to depositing the polymer film in order to make sure the current response was due to that of the donor:acceptor film and not the silicon substrate.

Results
The actual electrode separations of the co-planar interdigitated bi-metallic substrate used for the experiment were 1.21 µm on one side and 2.42 µm on the other.
The electrode fingers were roughly 10 µm wide. Figure 42 shows a 100× magnified image of the co-planar interdigitated bi-metallic electrode used for this experiment.   The device efficiency was calculated in two ways. It was first calculated using the total area of the co-planar interdigitated electrode device (approximately 0.11 cm 2 ).
By this method, the efficiency was determined to be 3.53x10 -4 %. However, the majority of this total area is occupied by the electrode pads and only the fractional area between the cathode and anode electrodes is expected to contribute to the photocurrent.
Neglecting the area of the electrode metal surfaces, the active area of the device was determined to be approximately 15 % of the total illuminated area of the device (approximately 0.017 cm 2 ), and from this, the estimated total device efficiency was found to be 0.0023 %. It is expected that this efficiency could be further increased by reducing the electrode separation distance and by optimizing the properties (e.g. weight fractions) of the organic solution. With respect to the electrospun polymer-fibers, co-planar bimetallic interdigitated electrode substrate offers a simple alternative for electrospun polymer-fiber solar cells.

Electrospun Polymer-fiber solar cell materials optimization
Co-planar bi-metallic interdigitated electrode substrates were used for testing all electrospun polymer-fiber solar cells. A variety of different materials, solvents, and coaxial shell extraction methods have been tested. The aim of this chapter is to further understand the characteristics of different materials, and compatibility of each experiment for electrospun polymer-fiber solar cells device synthesis.

P3HT and MEH-PPV
Earlier we have introduced P3HT and MEH-PPV as popular choices for a fabrication of organic photovoltaics due to their good solubility, processability, environmental stability, electroactivity, and other interesting properties [58]. Taking these properties into consideration, P3HT was tested for synthesis of electrospun polymer-fiber solar cells. At 44.8% humidity, the needle tip to substrate distance was set at 11 cm. A 45:55 weight ratio of P3HT:PCBM was dissolved in Chlorobenzene at 1% concentration, and was coaxially electrospun as core solution with an infusion rate of 4µl/min. The infusion rate of the outer 10 wt% PVP solution dissolved in 8.5 parts ethanol and 1.5 parts DI-water was set to 26 µl/min, and a rather unstable jet of coaxial nanofiber jet was found at 7.7 kV. The collected fibers were annealed in 60ºC oven for 30 minutes to promote crystallization of P3HT, and were further washed in ethanol for an immediate removal of PVP shell. The device was then transferred to 130ºC hot plate, and were heated for 2 minutes for improved metal to polymer contact. Figure 44 shows a 50× optical microscope image of the synthesized solar cell device. As can be seen from the figure, P3HT failed to maintain its nanofiber structure with PVP extraction. Additional experiments were performed using ethanol vapor for the PVP removal in hopes of avoiding destruction of the P3HT nanofibers.. Figure 45 shows the collected coaxial nanofibers of P3HT:PCBM with PVP shell. For this experiment, coaxial nanofiber jets were stable at 7.7-8.5 kV, and the fibers were washed in ethanol vapor at 80ºC.

Solvents
Organic photovoltaics are known to have better performance with good film morphology where a choice of solvent is a contributing factor [88]. In electrospinning, undissolved particles can lead to clogging, and particle accumulation that can cause an unstable jet. For electrospinning of a MEH-PPV:PCBM mixture, Chloroform and Chlorobenzene are are known solvents [89]. To determine an ideal solvent for electrospinning, two solutions of 0.5% by weight MEH-PPV dissolved in chloroform and chlorobenzene were prepared for comparison. Figure 46 is a normalized photoluminescence spectrum for comparison of MEH-PPV film using two different solvents.  For bulk heterojunction solar cell, more charge generation is expected for higher quantum yield, which will lead to better efficiency for a device. Due to the morphology and overall characteristics of MEH-PPV film, chlorobenzene is a better choice for fabrication of electrospun polymer-fiber solar cell.

MEH-PPV to PCBM ratio
For bulk heterojunction systems, PCBM has two roles for organic photovoltaics to properly function. One is in its role as an electron acceptor assisting in charge separation from the excitons generated from the donors, and the other with the transport of charge to the anode using its percolation network. To understand the ability of PCBM as a charge acceptor, photoluminescence of MEH-PPV:PCBM mixtures were studied as shown in Figure47. As for charge transport to the anodes, a higher PCBM fraction is necessary for the formation of percolation network. The highest performance of a bulk heterojunction film has been observed for a device prepared from 1:4 MEH-PPV:PCBM solution [57].
However for coaxial electrospinning, the electrospun fiber jet loses its stability after 1:3 MEH-PPV:PCBM ratio, and a more stable 1:2.5 MEH-PPV:PCBM solution has been used in synthesis of electrospun polymer-fiber solar cells.

PCBM and PCBB
PCBB is a PCBM derivative that is known to have slightly better solubility and has resulted in higher performing organic photovoltaic devices [91].  Similar to P3HT experiments, PCBB did not maintain its nanofiber structure after PVP extraction. We speculate that due to the improved solubility of PCBB, the polymer is not able to maintain its nanofiber structure. For the purpose of maintaining a nanofiber structure, PCBM is used for synthesis of electrospun polymer-fiber solar cells.

Electrospun Polymer-fiber solar cell
Through In our attempt to accurately characterize the performance of electrospun polymerfiber solar cell, the amount of fibers collected were kept to a minimum with a countable amount of fibers for total active area calculation, while also maintaining the device functionality. Figure 49 is an optical microscope image of the collected coaxial nanofibers at 100×. Before ethanol washing, the collected coaxial nanofibers show some beaded structures from unstable electrospinning jet. Figure 50 below is an optical micrscope image of the collected nanofibers after PVP extraction. The wavy structure observed in the 10× image shows a PVP film, and after ethanol washing, good nanofiber structure is maintained along with some conjugated polymer spots. However, under the dark field, we observe a good nanofiber structure throughout the whole suface of the co-planar bi-metallic interdigitated substrate as shown in  The dark field under an optical microscope filters out directly transmitted light, and is a technique known to be free of artifacts. From this dark field image, we conclude that a good nanofiber morphology is preserved after the ethanol washing of the MEH-PPV:PCBM nanofibers.
The electrospun polymer-fiber solar cell was further evaluated by measuring its J-U curve as shown in Figure 52. The J-U curve shows a photovoltaic response with a U oc recorded at 0.11 V, and a photovoltage I sc at 3 × 10 -7 mA where the total efficiency is then estimated to be 7.92 × 10 -10 % based on the total area of the co-planar bi-metallic interdigitated electrode.
However, as discussed in detail in Chapter 10, the total area can also be estimated from the number of gold-nanofiber-aluminum junctions given the thickness of electrospun nanofibers. The thickness of the nanofibers are measured to be around 1 µm from optical microscope, and thus the actual efficiency for the electrospun polymer-fiber solar cells may be estimated to as 3.08 × 10 -7 % or better considering the potential failure of the nanofibers to establish contact with the electrode junctions. Similar to the earlier device synthesized from MEH-PPV:PCBB film on co-planar bi-metallic interdigitated electrode, the electrospun polymer-fiber solar cells leaves room for improvement with a smaller electrode separation. With proper adjustments made for electrospun polymerfiber solar cells, this device may bring significant impact in the field of organic photovoltaics.

Absorption
The  As it was discussed earlier, nanofiber structure is poorly maintained for PCBB mixed MEH-PPV. However, the dark field still exhibits some nanofiber morphology and more importantly, the UV-Vis absorption measurement of the sample showed an absorption peak at 520 nm. This is a small red shift as compared to the MEH-PPV thin film. This experiment shows that the MEH-PPV red shift in nanofiber structures are observed even after a change in its solvent, and it agrees with the explanation that the red shift is mainly due to the stretching of the polymer chains from electrospinning [71]. The collected coaxial fibers on microscope glass slide were all soaked in Ethanol for 2 hours to remove the PVP shell followed by PL measurement of the MEH-PPV nanofibers.

Photoluminescence
A consistent increase in PL intensity has been observed excluding nanofibers of 0.7 % MEH-PPV solution. We notice a potential correlation to the low PL intensity of 0.7 % MEH-PPV solution with its absorption measurement resulting in the largest red shift of 76 nm. The cause of inconsistency in 0.7 % PL is not yet understood, and needs further investigation.

Discussion and Analysis
The efficiency of the synthesized Electrospun polymer-fiber solar cells has been estimated to be 7.92 × 10 -10 %, based on the total substrate area. The unique device structure with the Co-planar bimetallic interdigitated electrode substrate allows further evaluation of its efficiency from the total active nanofiber area. For this chapter, a detailed analysis of an actual efficiency extracted from the electrospun polymer-fiber solar cells is presented.

Active area of electrospun polymer-fiber solar cells
In preparation of the electrospun polymer-fiber device with the co-planar

Efficiency Validation
With the unique device structure of co-planar bimetallic interdigitated electrode substrate, the 3.08 × 10 -7 % efficiency of the electrospun polymer fiber solar cells has been shown to be better than the 8.7 × 10 -8 % efficiency of Solar cloth presented by Sundarrajan [69]. Some assumptions have been made in evaluation of the total active area of the electrospun polymer-fiber solar cells, but these assumptions actually yield an overestimate of electrospun polymer-fiber area.
The first assumption made is with regard to the alignment of the nanofibers. For every nanofiber counted, it is assumed to be aligned orthogonally to the electrode surface.
For every charge generated from MEH-PPV, charge collection is more likely for a shortest distance to the collector considering the effect of recombination. With this assumption, the active area for a single electrospun nanofiber is a product of 1 µm nanofiber diameter, and the average electrode separation of 1.97 µm. In addition, due to the cylindrical structure of the electrospun nanofibers, the actual point of nanofiber to electrode contact can be much smaller compared to the diameter of the electrospun nanofiber as indicated in Figure 56 below. Without alignment of electrospun nanofibers, the random orientation of the collected nanofibers will result in nanofiber stacking, and this will prevent the complete formulation of electrode-nanofiber-electrode junctions. However, this incomplete form of electrode-nanofiber-electrode junctions will appear to be complete from the optical microscope, but this potential lack of contact will lead to an over estimate in total number of complete electrode-nanofiber-electrode junctions. With all considerations for errors in the active area measurement, a more likely overestimate of our active area allows us to conclude that the efficiency of our electrospun polymer-fiber solar cell is 3.08 × 10 -7 % or better.

Equivalent Circuit
To further understand the performance of our device, an equivalent circuit analysis has been performed. In a typical solar cell, series resistance (R s ) of a device is the sum of limited conductivity of the material, contact resistance between semiconductor and the electrodes, and other contact resistance with the electrodes to the external circuit.
In addition, a solar cells will have shunt resistance (R sh ) which is lowered by the leakage current, and charge carrier recombination. An equivalent circuit for a solar cell is as shown in Figure 58.  factor of n=3.4 has been found to be unusually high for a typical solar cell. High ideality factor generally is an indication of multilevel charge recombination sites. In addition, electrospun nanofibers are known to reduce the depletion width resulting in an increased probability of tunneling along with thermionic emission at the electrode nanofiber junction [92], and thus the ideality factor of n=3.4 for the electrospun polymerfiber solar cell.

Influence of shunt resistance
To understand the influence of shunt resistance in our eletrospun polymer-fiber solar cell, series resistance R s = 0 Ω is chosen. Figure 60 are current-voltage characteristics of the elctrospun polymer-fiber solar cell with increasing shunt resistance.  Applying the current-voltage characteristic equation to the collected data, the shunt resistance of the electrospun polymer-fiber solar cell is found to be approximately 10 TΩ.

Influence of series resistance
To understand the influence of series resistance, we assume infinite shunt resistance. Figure 61 are current-voltage characteristics of the elctrospun polymer-fiber solar cell with increasing series resistance.

Characteristic of electrospun polymer-fiber devices
From the equivalent circuit analysis, n of 3.4, R sh of 10 TΩ assuming R s =0, and an R s of 1 MΩ or less assuming R sh =∞ have been found. The high ideality factor for the device has been attributed to the nanofibers structurally induced multi-level recombination sites. An interesting characteristic in the device is the overestimate of its 1 MΩ series resistance. Solar cells are generally known to have better performance under low series resistance, and this is evident from inverse proportionality of series resistance to the fill factor of solar cells. The electrospun polymer-fiber solar cell fill factor reached a limit in its improvement with the series resistance as high as 1 MΩ.
The series resistance of the device is 1 MΩ or less, and this is also an implication that an electrospun polymer-fiber solar cells are functional under high series resistance.
Similarly, high shunt resistance of the device implies stable device performance of the circuit. However, the efficiency of the device is relatively small compared with other organic solar cell devices, and further research is needed for better power extraction for an electrospun polymer-fiber solar cell.
Although there is some uncertainty in the exact value of the device parameters, our calculation shows that the device has a high ideality factor because of defects leading to recombination. It also has a high shunt resistance because of limited leakage current to the silicon dioxide substrate, and it has a relatively high series resistance due to long fiber length and poor contact with the electrodes.

Conclusion
In this work, MEH-PPV:PCBM based electrospun polymer-fiber solar cells have been studied. Organic solar cells synthesized with electrospinning offers improved carrier transport, charge transfer, shunt resistance, charge collection, mobility enhancement, and absorption tuning. Improvement in these parameters with proper power extraction will potentially improve device performance, and will add a new device structure to organic solar cells.
Electrospinnable polymer mixing, and multi-layer electrospinning have been and a series resistance of R s = 1 MΩ has been estimated. These parameters show stability with the device being functional under high series resistance, and an advantage of electrospinning leading to a burning of the shunts [73] is also verified with high shunt resistance of the device.

Future Studies
The electrospun polymer-fiber solar cell prepared in this work is the first device to be prepared from MEH-PPV. The unique device structure, and its device parameters needs further investigation through experiments with absorption, Photoluminescence, and mobility measurements. In addition, other optimization processes may be introduced for better performance, and stability of the device.

Mobility measurement
From generation of excitons, the holes travel to the cathode, and high hole mobility is required for efficient charge transfer in an organic solar cell. Increase in mobility of the generated charges are often reported in electrospinning experiments.
With successful synthesis of electrospun polymer-fiber solar cell, change in mobility from the structural change in organic solar cell needs to be investigated. Hall effect is often used for mobility measurements, but low mobility of polymers are often difficult for measurement [93]. Instead for low mobility materials, Time-of-flight (TOF) [94], field-effect transistor (FET) measurement [95], space-charge limited current (SCLC) measurement [34], and pulse radiolysis time-resolved microwave conductivity (PR-TRMC) [96] technique may be used.

Co-planar bimetallic interdigitated electrode
Our electrospun polymer-fiber solar cell was synthesized by nanofiber collection on the surface of Co-planar bimetallic interdigitated electrode. The electrode separations of 1.61 µm and 2.34 µm has been achieved for a functional device using MEH-PPV:PCBM nanofibers. However, this electrode separation much greater than the MEH-PPV exciton diffusion length of 20 ± 3 nm, and this will result in greater charge recombination, and decreased charge collection [27]. By reduction of co-planar bimetallic interdigitated electrode separation, an improved charge collection, increased short circuit current, and an overall improvement in device efficiency are expected. The co-planar bimetallic interdigitated electrode was prepared using photolithography, but a Scanning electron Microscope technique is expected to provide better resolution resulting in smaller electrode separation.

Control of fiber diameters
The electrospun polymer-fiber solar cell had a fiber diameter of ~1 µm. This fiber diameter is too large for efficient charge collection to consider for the MEH-PPV hole mobility. With a reduction in the polymer fiber diameter, there will be an improved charge collection. Control of the fiber diameter in electrospinning may be achieved through changing the solution concentration, and other parameters which may effect the resulting nanofiber diameter.

P3HT for electrospun polymer-fiber solar cells
P3HT is a conjugated polymer that has been a popular choice for fabrication of organic solar cells with its ability to produce high efficiency organic solar cells.
Although the P3HT polymer has shown good photoelectric activity in our experiments, electrospinning of P3HT has continued to fail in maintaining its nanofiber structure through the PVP polymer extraction process of coaxial electrospinning. However with proper optimization of the electrospinning experiment, P3HT nanofibers has been synthesized with it electronic properties maintained [63]. A mixture of PCBM in such polymer may produce a nanofiber of good solar cell performance collected over the coplanar bimetallic interdigitated electrode, and the device parameter maybe compared with those synthesized by Subramanian et al for the difference in their device structure [69].

Triaxial nanofiber solar cell
In the earlier chapter, an electrospun polymer-fiber solar cell of triaxial nanofiber structure has been discussed. Synthesis of such device proves to be difficult due to limited choices for the conductive solutions, a need for transparent outer electrode layer, and establishment of electrical contact between the core electrode layer, and the outer electrode layer. However, the triaxial nanofiber structure for a solar cell has many advantages such as its as-spun device preparation process, nanoscale active layer for low polymer diffusion length of polymers, potentially improved mobility, and red shift.
Further investigation in synthesis of conductive nanofibers may further the research in triaxial organic solar, and such device will introduce a new device structure to organic solar cells.