Nanoparticle sensitisation of solid-state nanocrystalline solar cell

Over the past fifteen years dye-sensitised nanocrystalline solar cells have been the subject of intense research and development efforts. These systems provide a technically and economically credible alternative to classical p-n junction solar cells, reaching over 10 % certified efficiency under standard solar illumination conditions (AM 1.5, 1000 W/m2). Recently, the liquid electrolyte, commonly used in these dye-sensitised solar cells, could successfully be replaced by a novel solid hole-conducting material (spiro-OMeTAD). The absence of volatile solvents and corrosive components like iodine presents a clear advantage of these solid-state devices over their photoelectrochemical counterparts. Yet, their maximum overall efficiency of about 3 % clearly lacks behind the performance features of classical dye-sensitized solar cells. The objective of this work is the replacement of the usually used sensitiser (transition metal complexes and organic dyes) by quantum-dots. It was motivated by the possibility to achieve panchromatic light absorption due to the size-dependent properties of the quantum-dots. Some studies have been published using quantum-dots of inorganic low-bandgap semiconductors as sensitisers for mesoporous wide bandgap semiconductor electrodes in conjunction with liquid electrolytes. These systems often suffer from corrosion and photo-corrosion mainly due to the aggressive nature of the electrolyte employed. Solid-state organic-inorganic heterojunctions might provide the desired environment for quantum-dot sensitisation, as the environment is much less aggressive. A large variety of inorganic quantum-dot materials like PbS, CdS, PbSe and CdSe were scrutinized for their use as sensitisers. All particles were synthesised in situ on the TiO2 surface using two different techniques: dip-coating and chemical bath deposition. Lead sulphide was the most investigated due to its superior photovoltaic characteristics. High Resolution Transmission Electron Microscopy (HRTEM) of PbS sensitised TiO2 electrodes fabricated via dip-coating clearly showed the distinct particles on the surface of the semiconductor. The electron transfer dynamics following optical excitation of quantum-dot sensitized TiO2/spiro-OMeTAD heterojunctions were thoroughly studied. Fluorescence spectroscopy was used to prove the possible electron injection from the PbS into the TiO2. Nanosecond laser spectroscopy was applied to monitor the interfacial recombination of the injected electron and the oxidised hole-conductor. Femtosecond laser spectroscopy allowed measuring the ultra-fast kinetics in the system, including electron trapping, exciton recombination and PbS regeneration. An overall efficiency of 0.5 % at 0.1 sun (AM 1.5) has been reached with such systems. Chemical bath deposition (CBD) of PbS leads to the formation of a layer rather than distinct particles on the TiO2. Solar cells fabricated via CBD exhibited open-circuit voltages which were generally 100 mV higher then those of comparable devices made via dip-coating. The PbS layer acts as blocking layer, hindering the contact between TiO2 and the hole-conductor. This technique allowed achieving devices highly linear with respect to the illumination power. Different strategies were pursued to impede interfacial recombination, among which the introduction of self-assembled monolayers at the interface between the organic and inorganic phases was proven to be the most efficient. Nanosecond laser spectroscopy showed a strong decrease of the interfacial recombination kinetic rates in the presence of hexadecylmalonic acid or decylphosphonic acid monolayers. Short-circuit currents could be raised by a factor two to four using such type of molecules. The overall efficiency of the device could be strongly increased, reaching 1 % at 0.1 Sun (AM 1.5). CBD was also used to deposit PbSe and CdSe layers at the surface of TiO2. Fluorescence spectroscopy was used to probe electron injection from CdSe quantum dots to TiO2. Generally it was observed that metal sulfides were more efficient in sensitizing TiO2/OMeTAD heterojunctions then the respective selenides.

Interface engineering in solid-state dye-sensitized solar cells

Jessica Krüger

Dye-sensitised nanocrystalline solar cells are currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. In particular dye-sensitised cells based on spiro-MeOTAD have gained attention as promising approach towards an organic solid-state solar cell. However, the efficiency in such dye-sensitised solid-state solar devices (SSD) was so far only ca. 10 % of the values reported at AM1.5 for the classical dye-sensitised solar cell with an electrolyte hole transporting medium (DSSC). The objective of the present work is to study the limitations that emerge from the exchange of the electrolyte by the solid-state system and that oppose photovoltaic photon-to-electron conversion as high as for the DSSC. Interfacial charge recombination is an important loss mechanism in dye-sensitised solar cells. This is particularly true for SSD, as the solid hole-transporting medium is less efficient in screening of internal fields which assist recombination. A variety of strategies were tested in the SSD to minimise interfacial charge recombination. The most promising approach was the blending of the hole-transporting medium with tert.-butylpyridine (tBP) and lithium ions. Optical and electrochemical techniques, such as nanosecond laser spectroscopy, impedance spectroscopy and photovoltaic characterisation measurements, were used to study the impact of the additives on the SSD. Both lithium ions as well as tBP were found to increase the open circuit potential of the SSD. At the same time tBP was found to considerably lower the current output. The interaction of the additives was studied and their concentration in the spiro-MeOTAD medium optimised. The doping of the spiro-MeOTAD film, which was intended to support the hole transport, was found to enhance interfacial recombination significantly. The morphological properties of the TiO2, in particular layer thickness, particle size and film porosity, play a more important role in the SSD than in the DSSC. Penetration of the hole conductor into the TiO2 pores and electron diffusion length are coupled to these properties. As a result the light harvesting cannot be controlled at will via the TiO2 film thickness and the active surface area for dye adsorption. An enhanced light harvesting for thin TiO2 layers offers advantages for the charge transport and the formation of the interpenetrating network. The dye uptake in presence of silver ions was found to increase the dye loading and to significantly improve the device performance of thin TiO2 layer devices. The mechanism of this simple dye modification technique was studied by a variety of spectroscopic techniques. From spectroscopic evidence it is inferred that the silver is binding to the sensitiser via the ambidentate thiocyanate, allowing the formation of ligand-bridged dye complexes. The beneficial influence of the silver ions on the photovoltaic performance was not limited to the application of the standard N3 dye nor to the spiro-MeOTAD. SSDs were furthermore studied by frequency resolved techniques. Intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS) were performed over a wide range of illumination intensities. The IMPS and IMVS responses provide information about charge transport and electron-hole recombination processes respectively. For the range of light intensities investigated, the dynamic photocurrent response appears to be limited by the transport of electrons in the nanocrystalline TiO2 film rather than by the transport of holes in the spiro-MeOTAD. The diffusion length of electrons in the TiO2 was found to be 4.4 μm. This value was almost independent of the light intensity as a consequence of the fact that the electron diffusion coefficient and the rate constant for electron-hole recombination both increase in the same way with light intensity but with opposite sign. The results of this work provide for a substantial improvement of the overall photovoltaic performance compared to earlier results for this type of SSD. However, this study reveals also that high conversion efficiencies as are measured for DSSC are not likely to be reachable with the spiro-MeOTAD system due to the significantly slower charge transport in the spiro-MeOTAD compared to the electrolyte redox mediator.

EPFL2003

Solid state nanocrystalline titanium oxide photovoltaic cells

The dye solar cell, is a novel photoelectrochemical solar cell presenting unique advantages, as the use of low cost materials and the potential simplicity of manufacturing. However, a liquid electrolyte is actually required for the transport of the photogenerated positive charges. A major breakthrough is the replacing of the liquid electrolyte by a solid-state hole conducting material – the aim of this work. First, the metal/TiO2 junction was investigated, with either silver or gold as rectifying contact. Sensitization was demonstrated, as action spectra were measured using different dyes. The evaporated metal could not enter the pores of the nanocrystalline TiO2 to contact all the dye molecules, hence only low photovoltaic performances were obtained. Furthermore, these metal/TiO2 junctions showed a high sensitivity to humidity and to forward bias polarization. The use of an electrically conducting polymer, poly(bithiophene), was studied in a metal/polymer junction and in TiO2/polymer devices with flat and nanocrystalline TiO2 electrodes. The photoelectric characterizations indicated that electrochemically reduced poly(bithiophene) was able to inject electrons into TiO2, i.e. sensitizing TiO2, and it acted as a hole conducting layer. Short-circuit current densities of 2-3 mA/cm2 were measured at an irradiation of 1.2 W/cm2. The open-circuit voltages were in the 250-350 mV range, depending on the light intensity and temperature. The use of a transparent hole conducting material, CuI, which is a wide bandgap p-type semiconductor, was demonstrated in a nanocrystalline junction made of highly porous sensitized TiO2 and solution-cast CuI. A short-circuit current of 1 mA/cm2 was achieved under one sun simulated light; the open-circuit voltage was 600 mV and the light-to-power conversion efficiency was 0.3 %. The peak quantum efficiencies of 8 % in the UV, and 3 % in the visible, showed that the nanocrystalline junction was working inefficiently, probably due to the poor electrical contact between the TiO2 nanocrystallites and the CuI crystals. Furthermore, the sensitizer was not tuned to inject holes into the valence band of CuI. As conclusion, a controlled-capillary pore based sensitized device is suggested. In this concept, all the internal surface of the TiO2 electrode coated with sensitizer is contacted with the transparent hole conducting layer, hence improving the photovoltaic properties of the device.

EPFL1996

Interface issues in organic semiconductor based devices

Mauro Castellani

The present doctoral thesis aims to contribute to the field of organic semiconductor physics and technology, both of which have become fast growing disciplines. Two technological applications are emerging from these research efforts: Organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs). OLEDs are already being fabricated on an industrial level today, while some fundamental issues still remain to be resolved for OFETs. During the last ten years, research interest has been continuously shifting from the bulk properties of organic semiconductors to their interfaces. Such issues are in fact addressed in this thesis work and include different interactions of thin organic layers with their environments. In the field of OLEDs, a multitude of issues have been investigated here. Most importantly and in contrast to most publications in this field, OLED's with complex device architectures that fully compete with standard devices performancewise have been fabricated and characterized. An oxygen plasma treatment of indium-tin-oxide anode has been shown to reduce the injection performance while enhancing the quantum efficiency. Two different mechanisms are proposed to explain this. Going a step further, the life-time behavior of such devices has been examined. Different aging mechanisms were identified: in short term and intermediate term aging, an oxygen plasma treatment of anode clearly reduces the operational voltage. Bulk degradation of the organic electroluminescent layer then becomes predominant for long term aging. Another important finding concerns the formation of exciplexes at organic/organic heterojunctions: In devices comprising a high hole injection barrier between the hole-transport and the electron-transport layer, the dominant formation of exciplexes is identified by a red-shifted shoulder in the electroluminescent spectrum and a low quantum yield. In contrast to that, the quantum efficiency is increased markedly by minimizing this hole barrier. In such devices, the formation of exciplexes is beneficial for device performance, since it facilitates hole transfer across the heterojunction. For the first time, the operational currents could be divided into different radiative and non-radiative recombination channels of exciplexes and excitons respectively. This quantification was further developed to simulate current-versus-voltage behavior and quantum efficiency. The work on pentacene based devices was initiated by fabricating ultrathin pentacene films on polished sapphire and by characterizing them with a temperature variable four probe setup. Oxygen plasma treatment of the substrate introduces oxygen radicals onto the surface and subsequent pentacene deposition leads to electron extraction from the pentacene molecules to defects at the oxide surface. Such an increase of charge carrier density within the semiconducting layer enhances its conductivity. Using a thick, aliphatic self-assembled monolayer (SAM) as an intermediate layer, both the activation energy and the film resistance increase by at least an order of magnitude compared to devices without SAM. Hence, charge transfer to the substrate is inhibited. The OFETs fabricated for this thesis work are all based on pentacene as a semiconductor and the gate interface was the focus of interest. Two different device types were fabricated: One device category comprises ultra-thin film OFETs (10nm). In such devices contact resistance is negligible. This gives rise to a dependence of device behavior on the gate interface. For this reason, the dielectric constant of the gate insulator was varied by changing the gate materials: the hole mobility is enhanced four-fold by using an aliphatic SAM at the gate interface instead of an oxygen plasma treated alumina (Al2O3). This effect has been interpreted in terms of self-trapping at the interface and its reduction by a low-k dielectric gate. In addition the on-off ratio was enhanced by a factor of 20, thanks to the SAM-inhibited pentacene doping due to a reduced semiconductor/substrate interaction. The second OFETdevice category was based on thicker (80nm) pentacene films. Their behavior is injection controlled. This finding originates in the application of a top contact configuration which implies that the carriers cross the granular material to reach the channel. Impedance spectroscopic measurements on different metal-insulator-semiconductor diodes unveiled a morphology dependent transveral mobility, which is linked to the degree of structural disorder in the film. In conclusion: Understanding of the various physical processes occurring at the different interfaces (anode and organic/organic interfaces in OLEDs, gate interface in OFETs) has considerably improved the performance of organic semiconductor based devices.

EPFL2006