Double-layer Solar Cells Research Articles

Greatly Enhanced Photovoltaic Performance of Crystalline Silicon Solar Cells via Metal Oxide.

Band-gap alignment engineering has now been extensively studied due to its high potential for application. Here we demonstrate a simple route to synthesize two metal oxide layers and align them together according to their bandgaps on the surface of crystalline silicon (c-Si) solar cells. The metal oxide layers not only extend absorption spectrum to generate extra carriers but also more efficiently separate electron–hole pairs. As a consequence, the photovoltaic performance of SnO2/CdO/Si double-layer solar cell (DLSC) is highly improved compared to the controlled Si solar cell, CdO/Si and SnO2/Si single-layer solar cells (SLSCs). Via alignment engineering, the SnO2/CdO/Si DLSC produces a short circuit photocurrent (Jsc) of 38.20 mA/cm2, an open circuit photovoltage (Voc) of 0.575 V and a fill factor (FF) of 68.7%, a conversion efficiency (η) of 15.09% under AM1.5 illumination.

Design of TiO2 dye-sensitized solar cell photoanode electrodes with different microstructures and arrangement modes of the layers

Different structures of TiO2 photoelectrodes were fabricated with various arrangement modes of the layers. TiO2 nanoparticles were synthesized by a combination of sol–gel and solvothermal methods and were employed as the active layer of solar cells. Hierarchical TiO2 spheres (HTS) and spherical TiO2 particles (STP) were also prepared by hydrothermal and sol–gel processes, respectively, and used as the scattering layer of solar cells. Field emission scanning electron microscope and X-ray diffraction analyses revealed that all synthesized morphologies showed anatase structure with particle size around 20, 200–400 nm and 1.5–2.0 µm for the nanoparticles, spherical particles and hierarchical spheres, respectively. Moreover, the nanoparticles, spherical particles and hierarchical spheres had surface areas of 131.1, 5.7 and 253.5 m2/g, respectively. The optical properties of these morphologies were studied through diffuse reflectance spectroscopy analysis. The indirect optical band gap energy of the nanoparticles, spherical particles and hierarchical spheres was calculated 3.10, 3.14 and 3.13 eV, respectively. Furthermore, the light scattering particles (i.e., HTS and STP) exhibited high diffuse reflectance because of their large particle size. The improvement of power conversion efficiency of monolayer cells was achieved by light harvesting mechanism aided by controlling the thickness of the film. The monolayer solar cell made of TiO2 nanoparticles with 20 μm thickness showed the highest efficiency of 7.21 %. Further enhancement of photovoltaic performance was obtained by light scattering mechanism aided by fabrication of double-layer solar cells with different arrangement modes of the layers. The enhancement was attributed to higher incident photon-to-current conversion yield due to greater fraction of light scattering as well as less recombination of photogenerated electrons. Hierarchical spheres had the highest amount of dye adsorption, whereas spherical particles showed the greatest light scattering property. In addition, incorporation of nanoparticles into hierarchical spheres and spherical particles would lead to better connection between the particles and retard charge recombination in the photoanode. It was found that the influence of electron transport rate on cell efficiency improvement is more dominant than that of light scattering. Therefore, the double-layer solar cell made of TiO2 nanoparticles as the active layer and mixtures of nanoparticles and hierarchical particles as the scattering layer showed the highest efficiency of 8.21 % among all fabricated solar cells. Different structures of TiO2 photoelectrodes were fabricated with various arrangement modes of the layers containing TiO2 nanoparticles, hierarchical TiO2 spheres and spherical TiO2 particles for dye-sensitized solar cell applications.

Double-layer dye-sensitized solar cells based on Zn-doped TiO2 transparent and light scattering layers: Improving electron injection and light scattering effect

A new strategy for enhancing the efficiency of TiO2 dye-sensitized solar cells (DSSCs) by design of a new double-layer film doped with Zn ions, with various morphologies and phase compositions, is reported. X-ray photoelectron spectroscopy (XPS) revealed that Zn2+ (in the range 0.25-0.1 at.%) was successfully incorporated into the TiO2 lattice without forming secondary phases. X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM) analyses showed that the synthesized nanoparticles had nanometer grains with different phase compositions and average crystallite sizes in the range of 8-48 nm, depending upon Zn atomic percentage. UV–vis absorption verified that Zn introduction enhanced the visible light absorption of TiO2 nanoparticles by shifting the absorption onset to visible light region. Furthermore, the band gap energy of nanoparticles was decreased with an increase in dopant concentration due to reduction of particle size (e.g., 3.17 eV for TiO2 and 2.83 eV for 0.5 at.% Zn-doped TiO2). It was found that, 0.5 at.% Zn-doped TiO2 DSSC in the form of a double-layer film composed of Zn-doped TiO2 nanoparticles, as the under-layer, and mixtures of Zn-doped TiO2 nano- and micro-particles, as the over-layer, (i.e., ZT5/NM5 solar cell) had the highest power conversion efficiency of 6.58%, short current density of 16.02 mA/cm2 and open circuit voltage of 680 mV. This can be related to achievement of a balance among the electron injection, light scattering effect and dye sensitization parameters. Optimization of light scattering effect of photoanode electrode led to improve the photovoltaic performance of ZT5/NM5 double-layer solar cell and was demonstrated by diffuse reflectance spectroscopy (DRS). The following well-incorporated study would present an intellectual development in the fabrication of low-cost DSSCs with high power conversion efficiency.

Double-Layer TiO2 Electrodes with Controlled Phase Composition and Morphology for Efficient Light Management in Dye-Sensitized Solar Cells

The light-scattering effect in the dye-sensitized solar cells (DSCs) was studied by controlling TiO2 phase composition and morphology by fabrication of double-layer cells with different arrangement modes. The starting material for preparation of TiO2 cells was synthesized by an aqueous sol–gel process. X-ray diffraction and field emission scanning electron microscopic analyses revealed that TiO2 nanoparticles had particle size ranging between 18 and 44 nm. The optical property and band gap energy of TiO2 nanoparticles were studied through UV–Vis absorption. The indirect optical band gap energy of anatase and rutile nanoparticles was found to be 3.47 and 3.41 eV, respectively. The double-layer DSC made of nanostructured TiO2 film with phase composition of 78 % anatase and 22 % rutile as the under-layer and mixtures of anatase-nanoparticles and anatase-microparticles as the over-layer (i.e., NM solar cell) was shown the highest power conversion efficiency (PCE) of 6.04 % and open circuit voltage of 795 mV. This was achieved due to the optimal balance between the light scattering effect and dye sensitization parameters. Optimum light scattering of photoanode led to improve the PCE of NM double-layer solar cell which was demonstrated by diffuse reflectance spectroscopy.

Interfacial charging originated from the conductivity decrease of C60 layer in IZO/pentacene/C60/Al organic double-layer solar cells

The origin of interfacial charging process in double-layer organic solar cells (OSCs) was studied by using the normal structure of Indium–Zinc–Oxide/pentacene/C60/Al and its inverted double-layer system. Optical electric-field-induced second-harmonic generation (EFISHG) measurement was employed and results suggested that interfacial charging in these two kinds of OSCs led to charge accumulation with opposite charge polarity, owing to the conductivity decrease of C60 layer. Applying the EFISHG measurements to the inverted OSCs also showed that the significant charge accumulation on donor–acceptor interface is responsible for the low I–V performance of the inverted OSCs. Thus, Maxwell–Wagner type interfacial charging, which is governed by the conductivity of C60, can cause the degradation of the I–V performance of OSCs. The protection of C60 layer from the conductivity decrease is a way to improve OSCs performance.

Improved efficiency of dye-sensitized solar cells by design of a proper double layer photoanode electrodes composed of Cr-doped TiO2 transparent and light scattering layers

A new strategy for enhancing the efficiency of TiO2 dye-sensitized solar cells (DSSCs) by design of a new double layer film doped with Cr ions, with various morphologies, is reported. X-ray diffraction and field emission scanning electron microscope (FE-SEM) analyses revealed that the synthesized nanoparticles had uniform and nanometer grains with different phase compositions and average crystallite size in the range of 10–12 nm depending upon Cr atomic percentage. UV–vis absorption showed that Cr introduction enhanced the visible light absorption of TiO2 nanoparticles by shifting the absorption onset to visible light region. Furthermore, the band gap energy of nanoparticles decreased with an increase in dopant concentration due to reduction of particle size. It was found that, 3 at.% Cr-doped TiO2 DSSC in the form of a double-layer film composed of TiO2 nanoparticles, as the under-layer, and mixtures of nano- and micro-particles with weight ratio of 80:20, as the over-layer, (i.e., CT3/NM3 solar cell) had the highest power conversion efficiency of 7.02 %, short current density of 17.32 mA/cm2 and open circuit voltage of 674 mV. This can be related to achievement of a balance among the electron injection, light scattering effect and dye sensitization parameters. Optimization of light scattering effect of photoanode electrode led to improve the photovoltaic performance of CT3/NM3 double-layer solar cell and was demonstrated by diffuse reflectance spectroscopy. The presented strategy would open up new insight into fabrication of low-cost TiO2 DSSCs with high power conversion efficiency.

Controlling electron transport rate and recombination process of TiO2 dye-sensitized solar cells by design of double-layer films with different arrangement modes

TiO2 dye-sensitized solar cells (DSSCs) in the form of double-layer films, containing an under-layer and an over-layer, with various crystal structures (i.e., anatase and rutile phases) and morphologies (i.e., nanoparticle and nanowire) were reported. It was found that the photovoltaic performance of TiO2 DSSCs depends on the morphology, crystal structure, light scattering effect, optical band gap energy and arrangement of the under- and over-layer films. The double-layer solar cell made of anatase-TiO2 nanoparticles as the under-layer and anatase-TiO2 nanowires as the over-layer (i.e., AW solar cell) showed the highest power conversion efficiency and fill factor of 6.34% and 62.6%, respectively. High electron lifetime, rapid transportation and less recombination of photogenerated electrons are the factors affect the efficiency improvement of AW film and was demonstrated by electrochemical impedance spectroscopy (EIS). X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM) analyses revealed that TiO2 nanoparticles had uniform and nanometer grains with particle size around 20nm, whereas TiO2 nanowires with length of several μm had diameter in the range 20–50nm. The optical properties and band gap energies of TiO2 nanoparticles and nanowires were studied through UV–vis absorption. The indirect optical band gap energy of TiO2 nanowires, anatase-TiO2 and rutile-TiO2 nanoparticles was calculated 3.61, 3.47 and 3.41eV, respectively. The design of double-layer solar cells by manipulation of morphology and crystal structure of TiO2 nanostructures will open a new concept for improvement of power conversion efficiency of dye sensitized solar cells.

Magnetic Studies of Photovoltaic Processes in Organic Solar Cells

In this paper, we use magnetic field effects of photocurrent (MFE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PC</sub> ) to study the photovoltaic processes in pristine conjugated polymer, bulk heterojunction, and double-layer solar cells, respectively, based on poly(3-alkylthiophene) (P3HT). The MFE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PC</sub> reveals that the photocurrent generation undergoes the dissociation in polaron pair states and the charge reaction in excitonic states in pristine conjugated polymers. As for the bulk-heterojunction solar cells consisting of electron donor P3HT and electron acceptor [6,6]-phenyl C61-butyric acid methyl ester (PCBM), the MFE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PC</sub> indicates that the dissociated electrons and holes inevitably form the intermolecular charge-transfer (CT) complexes at donor and acceptor interfaces. Essentially, the photocurrent generation relies on the further dissociation of intermolecular CT complexes. Moreover, we use double-layer solar cell to further study the intermolecular CT complexes with well-controlled donor-acceptor interfaces based on double-layer P3HT/TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> design. We find that the increase in free energies can significantly reduce the density of CT complexes upon thermal annealing.

Enhanced quantum yield in porphyrin/electron-donor double-layer solar cells

When the layer of 3-carboxymethyl-5-[(3-ethyl-2(3H)-benzothiazolylidine)ethylidene (MC(COOH)) is inserted into the Au/Zntpyp interface in Al/Zntpyp/Au sandwich-type solar cell (Zntpyp: 5,10,15,20-tetra(3-pyridyl)porphyrinatozinc), the photovoltaic properties are remarkably improved. For the Al/Zntpyp(thickness 10 nm)/MC(COOH)(20 nm)/Au cell, a short-circuit photocurrent ( J sc) of 0.93 μ Acm −2, open-circuit photovoltage ( V oc) of 0.71 V, fill factor (ff) of 0.41, and energy conversion yield (η) of 3.6% are obtained when illuminated at the Al/Zntpyp interface with 455 nm monochromatic light of 7.5 μ Wcm −2 intensity. A rapid electron-transfer from the donor MC(COOH) to photogenerated holes in Zntpyp suppresses the charge recombination of the photogenerated carriers. The energetically well-arranged valence band levels eventually enhance the η value about 9 times compared with the Al/Zntpyp/Au cell. Further the Al/HD(9 nm)/MC(COOH)(20 nm)/Au cell using a longer-lived sensitizer (HD) instead of Zntpyp gives a J sc value of 2.36 μ Acm −2, V oc value of 0.69 V, ff value of 0.34, and η value of 4.8% when illuminated with 445 nm monochromatic light of 11.7 μ Wcm −2 intensity at the Al/HD interface, where HD represents a heterodimer consisting of 5,10,15-tri(4-chlorophenyl)-20-(3-pyridyl)porphyrin(H 2pyp 3p(Cl)) and 5,10,15,20-tetra(2,5-dimethoxyphenyl)porphyrinatozinc(Zntpp(OMe) 2).