Interface engineering in solid-state dye-sensitized solar cells

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.

Strategies to Optimizing Dye-Sensitized Solar Cells

Sophie Wenger

Dye-sensitized solar cells (DSCs) constitute a novel class of hybrid organic-inorganic solar cells. At the heart of the device is a mesoporous film of titanium dioxide (TiO2) nanoparticles, which are coated with a monolayer of dye sensitive to the visible region of the solar spectrum. The role of the dye is similar to the role of chlorophyll in plants; it harvests solar light and transfers the energy via electron transfer to a suitable material (here TiO2) to produce electricity — as opposed to chemical energy in plants. DSCs are fabricated of abundant and cheap materials using inexpensive processes (e.g. screen-printing) and are likely to be a significant contributor to the future commercial photovoltaic technology portfolio. The work conducted during this thesis aimed at optimizing the DSC using three different strategies: The use of versatile organic sensitizers for stable and efficient DSCs, the study of tandem device architectures in combination with other solar cells to harvest a larger fraction of the solar spectrum, and the development of a validated optoelectric model of the DSC. Organic donor-π-acceptor dyes are an interesting alternative to the standard metal-organic complexes used in DSCs. Efficient photovoltaic conversion and stable performance could be demonstrated with three classes of donor systems, namely diphenylamine, difluorenylaminophenyl, and π-extended tetrathiafulvalene. The highest conversion efficiencies were obtained with a difluorenylaminophenyl donor system (η = 8.3 % with a volatile electrolyte and η = 7.6 % with a solvent-free ionic liquid, which was a new record for organic dyes at the time of publication). Surprisingly, efficient regeneration of the oxidized dye by the I-/I3- redox mediator was found with the π-extended tetrathiafulvalene system, even though the thermodynamic driving force was as low as 150 mV. So far driving forces of 300-500 mV had been regarded as necessary for efficient regeneration of the dye cation. Also, important structure-property relationships pertaining to the recombination of electrons with the electrolyte and to the stability of the device could be identified (i.e. effect of linear vs. branched structure, linker length, and moieties used). The power conversion efficiency of solar cells can be extended beyond the limit for a single cell (∼ 30 %) by using multiple cells with different optical gaps in a tandem device. DSCs and chalcopyrite Cu(In,Ga)Se2 (CIGS) solar cells have complementary optical gaps and are thus suitable systems for integration in a tandem device. It was shown that a monolithic DSC/CIGS tandem device has the potential for increased efficiency over a mechanically stacked device due to increased light transmission to the bottom cell, and a monolithic DSC/CIGS device with an initial efficiency of η = 12.2 % was demonstrated. The degradation of the devices — induced by the corrosion of the CIGS cell in contact with the I-/I3- redox mediator — could be retarded with a protective thin conformal ZnO/TiO2 oxide layer coated on the CIGS cell by atomic layer deposition. Finally, an experimentally validated optical and electrical model of the DSC has been developed to assist the optimization process, which is predominantly conducted by empirical means in the DSC research community. The optical model allows to accurately calculate the internal quantum efficiency of devices, i.e. the ratio of extracted electrons to absorbed photons by the dye, a crucial and so far difficult to determine characteristic. Intrinsic parameters — like injection efficiency, electron diffusion length, or distribution of trap states in the TiO2 — can be extracted from experimental steady-state and time-dependent data with the electric model. The model allows to make a comprehensive and quantitative loss analysis of the different optical and electric loss channels in the DSC. The model has been implemented with a graphical user interface for straightforward usage. All three optimization strategies — organic dyes, tandem architecture, and device modeling — developed during this thesis make a valuable contribution to the development and commercialization of inexpensive and high efficiency DSCs. They enable a comprehensive view of the system and pave the way for a systematic analysis and reduction of losses, which has been the ultimate route to success for several established photovoltaic technologies.

EPFL2010

Electronic and structural modification of titanium dioxide/zinc oxide photoanode for dye-sensitized solar cells

Aravind Kumar Chandiran

Dye-sensitized solar cells (DSC) are a new class of molecular photovoltaics that mimics the natural photosynthesis, for the direct conversion of sunlight into electricity. A typical DSC is a sandwich of a dye sensitized nanoparticle TiO2 film and a catalyst coated counter electrode, with a redox electrolyte filling the space in between. The high internal surface area of the mesoporous TiO2 film ensures higher dye uptake for an efficient light harvesting. Following the light illumination, an electron from the highest occupied molecular orbitals (HOMO) of the dye is excited to its lowest unoccupied molecular orbitals (LUMO). Due to an effective electronic orbital coupling between dye and TiO2, the excited electrons are injected into TiO2. The oxidized dye is regenerated by the electron donor present in the electrolyte. The circuit is completed by the transfer of photogenerated electrons in TiO2 via an external load, to counter electrode regenerating the oxidized electrolyte species. In a conventional DSC, the transport rate of the photogenerated electrons in TiO2 competes with the parasitic electron recombination, as these two processes occur at a similar ms time domain. This thesis work is aimed at the fundamental understanding of these electron transfer processes at sensitized photoanodes that were subject to different bulk and surface electronic structural modifications. The implications were shown to be of profound interest for the fabrication of high efficiency and low cost dye-sensitized solar cells. In the first part of the thesis, the bulk electronic structure of the anatase TiO2 nanoparticles is modified by the incorporation of different concentrations of aliovalent cations (Nb5+, Ga3+, Y3+) in the crystal lattice. The cationic doping/substitution in the nanoparticles is achieved by hydrothermal process. The modified nanoparticles retained the parent anatase crystal structure of TiO2. The transparent dye-sensitized solar cells made with these modified films, using our standard heteroleptic metal complex sensitizers and iodide/triiodide redox electrolyte, displayed enhanced photovoltaic performances, at certain added concentrations. Charge extraction measurements in DSC, reveals a formation of deeper trap states below the conduction band of titanium dioxide when Nb ion is incorporated into the TiO2 lattice. On the other hand, Ga3+ and Y3+ did not significantly affect the trap states distribution compared to the reference device. The modification in the trap states and the oxygen stoichiometry due to these cation inclusions significantly influenced the electron recombination and transport rates. These properties are investigated in detail using transient decay techniques. Following the study on the electronic structure of the bulk TiO2, the influence of the surface modification of the TiO2 nanoparticles on the electron transfer dynamics is investigated. The kinetics of electron recombination from TiO2 to the single electron redox shuttles is fast, leading to a significant loss in the open-circuit potential (VOC) of devices. The emergence of cobalt complexes as a potential electrolyte motivated us to investigate the role of surface passivation of the titanium dioxide nanoparticles. Insulating sub-nanometer oxide tunneling layers deposited by atomic layer deposition (ALD) are known to block the electron recombination, thereby, leading to an increase in the VOC and the collection efficiency of the solar cell. A general perception in the DSC community is that any insulating oxide layer can block the recombination. However, in this work, it is unraveled that, just the insulating property of oxides is not sufficient. In addition, the properties such as the conduction band position and the oxidation state of the insulating oxide, the electronic structural modification induced to the underlying TiO2 mesoporous film, modification of surface charges (isoelectric point) and charge of the electrolyte species have to be considered. A complete photovoltaic study has been done by depositing different cycles (by ALD) of four insulating oxides (Ga2O3, ZrO2, Nb2O5 and Ta2O5) and their recombination characteristics, surface electronic properties, transport rate and injection dynamics are investigated with a standard organic dye and Co2+/Co3+ redox mediator. A comparison is made with the conventional iodide/triiodide electrolyte. The second part of the thesis is focused at the development of new type of photoanodes for effective electron transport to increase the charge collection efficiency. We present a way of utilizing thin and conformal overlayer of titanium dioxide or zinc oxide on an insulating mesoporous template as a photoanode for dye-sensitized solar cells. Different thicknesses of TiO2 ranging from 1 to 15 nm are deposited on the surface of the template by ALD. This systematic study helps unraveling the minimum critical thickness of the TiO2 overlayer required to transport the photogenerated electrons efficiently. A merely 6 nm thick TiO2 film on a 3 μm mesoporous insulating substrate is shown to transport 8 mA/cm2 of photocurrent density along with ≈900 mV of open-circuit potential, when using our standard donor-π-acceptor sensitizer and Co(bipyridine)3 redox mediator. The three dimensional conformal shell on the insulating core also exhibits one order of magnitude higher transport rate compared to the conventional TiO2 nanoparticles. Similar nanostructures were made by depositing ZnO on an insulating template and the electron transfer properties are compared with titania. We also developed ALD recipe for the deposition of crystalline TiO2 or ZnO at low temperatures (< 200 °C) on arbitrary mesoporous insulating templates, considering their prospects of making photoanodes on flexible plastic substrates. The experience gained about the deposition of conformal and pin-hole free TiO2 layer in the previous section is extended for the high efficiency solid-state mesoscopic solar cells based on CH3NH3PbI3 perovskite absorbers. We show that a 2 nm as-deposited ALD overlayer on a mesoporous TiO2 film, printed on a bare conducting glass, is sufficient to arrest the recombination process that lead to a power conversion efficiency of 11.5 %. A detailed investigation on the electron back reaction is presented using electrochemical impedance spectroscopy.

EPFL2013

Nanoparticle sensitisation of solid-state nanocrystalline solar cell

Robert Plass

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.

EPFL2004