P. Ravirajan

Hybrid polymer–metal oxide thin films for photovoltaic applications

We review progress in the development of organic–inorganic hybrid photovoltaic materials consisting of a conjugated polymer as an electron donor and a nanocrystalline metal oxide as the electron acceptor. We distinguish two main approaches: (1) where a rigid porous metal oxide structure is filled with polymer and (2) where metal oxide nanoparticles and polymer are co-deposited from solution to form a blend film. In the case of porous structures, performance is limited by the infiltration of polymer into the pores of the metal oxide and control of the nanostructure dimensions. In the case of blends, control of the blend morphology and transport between nanoparticles are limitations. In both cases, further improvements are possible by modifying the metal oxide organic interface to optimise charge transfer, by improving both inter- and intra-particle transport within the metal oxide phase, for example by the use of single crystalline nanorods, and by optimising the choice of electrode materials. Though unlikely to achieve the highest photocurrents, the polymer–metal oxide composites provide a model system to study the effects of interface properties and film morphology on the performance of bulk heterojunction photovoltaic devices.

Hybrid polymer/metal oxide solar cells based on ZnO columnar structures

We focus on the preparation of hybrid polymer/zinc oxide (ZnO) solar cells, in which the metal oxide consists of ZnO columnar structures grown perpendicularly on a flat, dense “backing” layer, as a means to provide a direct and ordered path for photogenerated electrons to the collecting electrode. We used scanning electron microscopy, absorption spectroscopy and photovoltaic device measurements to study the morphology and device performance of the prepared structures. Different solution chemical routes were investigated for the synthesis of the inorganic device components, i.e. the ZnO columnar structures and the “backing” layers, which act as a seed-growth layer for the ZnO rods. The growth of the ZnO rods was dependent on the morphological and structural characteristics of the seed layer and moreover, the seed layer itself was also affected by the synthetic conditions for ZnO rod growth. Different polymers (high hole-mobility MEH-PPV based polymer and P3HT) were compared in these structures and power conversion efficiencies of 0.15 and 0.20% were achieved under 1 Sun illumination, respectively. Results are discussed in terms of the optoelectronic properties of the polymers.

Hybrid nanocrystalline TiO2 solar cells with a fluorene–thiophene copolymer as a sensitizer and hole conductor

We report the effects of layer thickness, interface morphology, top contact, and polymer–metal combination on the performance of photovoltaic devices consisting of a fluorene–bithiophene copolymer and nanocrystalline TiO2. Efficient photoinduced charge transfer is observed in this system, while charge recombination is relatively slow (∼100 μs–10 ms). External quantum efficiencies of 13% and monochromatic power conversion efficiencies of 1.4% at a wavelength of 440 nm are achieved in the best device reported here. The device produced an open-circuit voltage of 0.92 V, short-circuit current density of about 400 μA cm−2, and a fill factor of 0.44 under simulated air mass 1.5 illumination. We find that the short-circuit current density and the fill factor increase with decreasing polymer thickness. We propose that the performance of the indium tin oxide/TiO2/polymer/metal devices is limited by the energy step at the polymer/metal interface and we investigate this situation using an alternative fluorene-based ...

Efficient charge collection in hybrid polymer/TiO2 solar cells using poly(ethylenedioxythiophene)/polystyrene sulphonate as hole collector

We report a study of the optimization of power conversion efficiency in hybrid solar cells based on nanostructured titanium dioxide and a poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV) based conjugated polymer. Charge collection efficiency is enhanced by introducing a poly(ethylenedioxythiophene)/polystyrene sulphonate (PEDOT) layer (under the gold electrode) as the hole collector. Device performance is maximized for a device with a net active layer thickness of 100 nm. The optimized device has peak external quantum efficiencies ≈40% at the polymer’s maximum absorption wavelength and yield short circuit current density ⩾2mAcm−2 for air mass (AM) 1.5 conditions (100mWcm−2, 1 sun). The AM 1.5 open circuit voltage for this device is 0.64 V and the fill factor is 0.43, resulting in an overall power conversion efficiency of 0.58%.

Effect of morphology on electron drift mobility in porous TiO2

Porous titanium dioxide is an attractive material for solar cell application on account of its stability, electron transport properties, and the possibilities for controlling surface morphology as well as for its ease of fabrication and low cost. Nanostructured TiO2 has been intensively studied for applications to dye sensitised solar cells. The performance of the titanium dioxide based solar cells is influenced, among other factors, by the electron mobility of the porous titanium dioxide. Different fabrication processes for porous titanium films result in different film morphology, which in turn affects the electron transport. We have employed three different techniques namely, electrostatic spray assisted vapour deposition (ESAVD), D.C. reactive sputtering, and doctor blading of sol-gel dispersions to deposit thin TiO2 films onto indium tin oxide (ITO) coated glass substrates. All these films exhibited only the anatase phase as confirmed by X-ray diffraction analysis. Using the time-of-flight technique, the electron drift mobility in the porous TiO2 films was measured. The results show that in the low field region (< 55,000 V cm −1 ) the mobility, in all the films, were in the range of 10−7 to 10−6 cm2 Vs−1. The drift mobility in the films prepared by reactive sputtering was consistently higher than in the films prepared by the two other techniques. Sputter deposited films had lower porosity (∼ 10% and 36% for normal-, and oblique (60◦)-angle deposited films) compared to ∼ 50% for films deposited by the two other techniques. The relationship between the drift mobility and film morphology is discussed with the aid of scanning electron microscopy studies.

Influence of polymer ionization potential on the open-circuit voltage of hybrid polymer/TiO2 solar cells

We report studies of the dependence of the open-circuit voltage (VOC) of polymer/titanium dioxide hybrid devices on the ionization potential of the polymer (IP). Once corrected for differences in photocarrier generation by the polymers, the measured VOC values vary linearly with the polymer IP, with a slope of 0.8±0.1. This behavior agrees with recent studies of polymer/fullerene photovoltaic devices and is consistent with the hypothesis that VOC of an organic donor-acceptor solar cell is limited by the energy difference between the highest occupied molecular orbital of the donor (in this case, the polymer) and the lowest unoccupied electronic level of the acceptor (in this case, the conduction band edge of the TiO2).We report studies of the dependence of the open-circuit voltage (VOC) of polymer/titanium dioxide hybrid devices on the ionization potential of the polymer (IP). Once corrected for differences in photocarrier generation by the polymers, the measured VOC values vary linearly with the polymer IP, with a slope of 0.8±0.1. This behavior agrees with recent studies of polymer/fullerene photovoltaic devices and is consistent with the hypothesis that VOC of an organic donor-acceptor solar cell is limited by the energy difference between the highest occupied molecular orbital of the donor (in this case, the polymer) and the lowest unoccupied electronic level of the acceptor (in this case, the conduction band edge of the TiO2).

Solid state solar cell made from nanocrystalline TiO2 with a fluorene-thiophene copolymer as a hole conductor

We study the charge recombination kinetics and photovoltaic performance of composites of poly (9,9-dioctylfluorene-co-bithiophene) polymer with nanocrystalline TiO2. Transient optical spectroscopy confirms that photoexcitation of the polymer leads to electron transfer to the TiO2 and indicates that charge recombination is slow with a half-time of 100 μs to 10ms. Polymer penetration into thick porous TiO2 layers is improved by melt-processing and treatment of the TiO2 surface. We study the photovoltaic characteristics of devices with different layer thickness and interface morphology. Quantum efficiency (QE) of all devices is increased by reducing the TiO2 and polymer layer thickness. Inserting a thin porous TiO2 layer in to a thin bi-layer device increases the QE by a factor of five. The improved device shows peak QE and monochromatic power conversion efficiencies of over 11% and 1% at 440nm respectively. The device produced a short-circuit current density of 300μAcm-2, a fill factor of 0.24 and an open-circuit voltage of 0.8V under AM1.5 illumination. The fill factor is increased from 0.24 to 0.40 by introducing an additional dip-coating layer and overall power conversion efficiency is increased by 50%. However, the device produced degraded current-voltage characteristics. We investigate this using an alternative polymers and different top contact metals.

Role of Poly(Ethylenedioxythiophene)/Poly(Styrene Sulphonate) on the Performance of Nanocrystalline Titanium Dioxide/Poly(3-Hexylthiophene) Polymer Solar Cells

Hybrid nanocrystalline titanium dioxide (TiO2)/polymer solar cells draw intense interest due to the potential advantages of nanocrystalline TiO2. The poly(styrenesulfonate)-doped poly(ethylenedioxy thiophene) (PEDOT:PSS) layer spin-coated below the top electrode in these solar cells had shown enhanced performance in previous studies, which motivated to explore the dependence of the thickness of the PEDOT:PSS layer on its performance. This study focused on the characterization of solar cells fabricated with poly(3-hexylthiophene) (P3HT) polymer with a silver electrode and different PEDOT:PSS layer thicknesses, in the dark and under AM 1.5 stimulated illumination with the intensity varying from 10 to 100 mW/cm 2 . The variations in the photovoltaic parameters, particularly the open-circuit voltage, proved that the PEDOT:PSS layer significantly affects the photovoltaic parameters through the characteristic changes in the morphology as well as the electrical properties. Discussed herein is the possible influence wielded by the thickness of the PEDOT:PSS layer on different factors, such as the series and shunt resistances, the mode of recombination, the reduction of the energy barrier, and the diffusion of silver. The optimum power conversion efficiency was observed for the as-prepared devices with 50-nm-thick PEDOT layers. The optimum power conversion efficiency, however, shifted to that corresponding to the 80 nm thick PEDOT:PSS layer about 30 weeks after the fabrication. A sublinear variation of the short-circuit current density with the intensity was found in the aged cells with relatively lower PEDOT:PSS layer thicknesses, supporting the view of dominant recombination contributed from bimolecular recombination in the cells with lower PEDOT:PSS thicknesses. The significantly increased open-circuit voltage and the more stable current density in the aged devices are the main causes of the improved performance of the cells generally with above 60 nm thick PEDOT:PSS layers. These, along with the long-term stability found in the cells with reasonably thick PEDOT:PSS layers, may be a figure of merit, most probably attributable to the comparatively minimized diffusion of silver nanoparticles.

Efficient hybrid polymer/TiO2 solar cells using a multilayer structure

This study focuses on systems consisting of high hole-mobility MEHPPV based polymers or a fluorene-bithiophene co-polymer in contact with different nanocrystalline TiO2 films. We use photoluminescence quenching, time of flight mobility measurements and optical spectroscopy to characterize the exciton transport, charge transport and light harvesting properties, respectively, of the polymers, and correlate these material properties with photovoltaic device performance. We find that the polymer properties with greatest influence on device efficiency are the polymer exciton diffusion length and absorption range, followed by the hole mobility. We have also studied the photovoltaic performance of these TiO2/polymer devices as a function of active layer thickness. Device performances are significantly improved by introducing a PEDOT layer between the polymer and the top Au electrode and by reducing the thickness of the active layers. The optimized devices have peak external quantum efficiencies ≈ 40 % at the polymers maximum absorption wavelength and yield short circuit current densities ≥ 2 mA cm-2 for air mass (AM) 1.5 conditions (100 mW cm-2, 1 sun). The AM 1.5 open circuit voltage reaches 0.64 V and the fill factor 0.43, resulting in an overall power conversion efficiency of 0.58 %.