Electrospinning in Solar Energy Application

Electrospinning is a highly versatile process for the production of nanofibers. Many polymers can be electrospun and modified version of the process can be used for others. Many inorganic precursors can be electrospun with the help of carrier polymers which can be reduced to form inorganic fibers through annealing. This has spark numerous research in the construction of dye sensitized solar cell (DSSC) and other forms of solar energy generation.

Dye sensitized solar cell (DSSC) comprise of a transparent electrode, a photo-sensitized anode, a redox electrolyte and a cathode. Where electrosun fibers are used in the setup, it is normally used as the photo-sensitized anode. Although more commonly used for producing polymer nanofibers, electrospinning can also be used to manufacture inorganic nanofibers by electrospinning of their precursors followed by annealing. This makes it possible to produce TiO₂ nanofibers which are commonly used as the photo-sensitized anode in DSSC. It is thought that a network of inorganic nanorods allow higher rate of electron transfer [Adachi et al 2004] and better electrolyte dye penetration [Song et al 2005]. Various modifications have been made in terms of choice of anode [Zhang et al 2009], doping [Jin et al 2012] and structure [Hamadanian and Jabbari 2014]. Current energy efficiency from DSSC using electrospun component is limited to about 6.2% [Song et al 2005]. Application of electrospun fiber for DSSC is not limited to the photo-sensitized anode. Ma et al (2019) investigated the potential use of electrospun Fe-Co nanoparticles incorporated carbon nanofibers (Fe-Co/CNF) as counter electrodes in DSSC to replace more commonly used platinum. The Fe-Co/CNF was fabricated by electrospinning salts of Fe and Co dissolved in polyacrylonitrile (PAN) solution. The electrospun composite nanofibers were sintered to form Fe-Co nanoparticles in CNF. The Fe-Co/CNF were ground and coated on FTO conductive glasses. This was assembled with TiO₂ photo anode to form the DSSC cell and electrolyte was injected through a hole in the counter electrode. Comparison made with platinum and CNF counter electrodes. The addition of Fe-Co to CNF increases the photoelectric efficiency by 43% and comparable to that of Pt counter electrodes due to its good electron conduction capability.

Electrospinning has also been tested for the construction of polymer solar cells. The bulk heterojunction polymer solar cells produce electrical charges when photons are absorbed by the organic semiconductor. This creates mobile electron-hole bound pairs known as Frenkel excitons which can be separated at polymer/electron-acceptor interface. Nagata et al (2013) used coaxial electrospinning to produce heterojunctions polymer fibers. Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were mixed to form in bulk heterojunctions. Polyvinyl pyrrolidone (PVP) was the carrier shell polymer for electrospinning to form fibers. This was later washed off to retain MEH-PPV:PCBM fibers. To create the solar device, the fibers were deposited on a gold and aluminum coplanar bimetallic interdigitated electrode. Comparing the absorption peak of MEH-PPV thin film and electrospun MEH-PPV fibers, there is a broadening and significant red shift in the electrospun fibers. Such observation may be attributed to stretching of the polymer molecular chain which increases the π-conjugation length. The red shift and broadening of absorption may facilitate optical absorption and improve efficiency of the solar device. In their constructed device, a power conversion efficiency of 3.08 x 10^-7%, a short circuit current density of J_sc = 0.525 µ A/cm², an open circuit voltage of V_oc = 0.11 V, and a fill factor of ff = 0.43 was achieved.

Optical microscope image of the collected coaxial nanofibers on coplanar bimetallic interdigitated electrode substrate at 10x [Nagata et al 2013]

The ease of electrospinning in the process of constructing nano-structures has allowed researchers to revisit certain materials for specific applications. With ferroelectric materials, their spontaneous electrical polarization property helps in the separation of electrons and holes which negates the need of a junction to separate the charge carriers. This would simplify the construction of photovoltaic devices and may avoid efficiency losses at the junctions. However, one limitation of ferroelectric materials is their low electrical conductivity. By using electrospinning to generate a thin layer of ferroelectric fibers, the limitation of poor electrical conductivity may be compensated by reducing the dimensions of the material. Melo et al (2022) demonstrated the production of KBiFe₂O₅ (KBFO) nanofibers using electrospinning of ceramic precursors in the form of Fe, K and Bi nitrates and poly (vinylpyrrolidone) (PVP) as the carrier followed by calcination at high temperature. The resultant KBFO fiber which is a ferroelectric oxide with a perovskite like structure showed a low band gap of 1.72 eV.

In conventional silicon solar cell, one way to improve its performance is to expand the light spectrum that can be utilized by the solar cell and to reduce electrical losses. Samir et al (2018) seek to address this by using electrospun poly(vinyl alcohol) (PVA) embedded with ceria nanoparticles as a rear surface coating for silicon solar cells. Active tri-valent states of cerium ions inside ceria nanoparticles are conductive and are able to emit visible fluorescence emission, under violet or near UV-excitation photons. Using just 1 wt % ceria nanoparticles concentration inside PVA nanofibers on the solar cells rear side surface, there was an efficiency improvement of about 24% over PVA nanofibers coating without ceria nanoparticles and silicon without nanofibers coating. A maximum efficiency of 18.34% was recorded using 1 wt % ceria nanoparticles concentration but the efficiency reduces with higher concentrations, possibly due to light scattering and optical quenching.

One of the most desirable objectives of energy generation application is the construction of clothing that is able to generate electricity. Ideally, the device can be miniaturized into the form of yarn that can be woven to give energy producing fabric. Bohr et al (2022) moved a small step towards this realization through the production of a triaxial perovskite composite fiber using electrospinning. The three materials are copper(I) thiocyanate (CuSCN) layer in the core as hole transport material, an intermediate shell of the perovskite absorber layer (MAPbI₃) and zinc oxide as electron transport material in the outermost shell. All three materials were electrospun in their respective precursor form into fibers followed by heat treatment. To ensure that the electrospun fibers deposited on the collector are dry, the collector was heated to 130 °C and further heat drying was carried out after fiber collection to remove any residual solvents. However, more progress in this research is needed to demonstrate a working model.