Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

The photocurrent density-voltage (J(V)) curve is the fundamental characteristic to assess opto-electronic devices, in particular solar cells. However, it only yields information on the performance inte-grated over the entire active device area. Here, a method to deter-mine spatially resolved photocurrent images by voltage-dependent photoluminescence microscopy is derived from basic principles. The opportunities and limitations of the approach are studied by the investigation of III-V and perovskite solar cells. This approach allows the real-time assessment of the microscopically resolved local J(V) curve and the steady-state Jsc as well as transient effects. In addi-tion, the measurement contains information on local charge extrac-tion and interfacial recombination. This facilitates the identification of regions of non-ideal charge extraction and enables linking these to the processing conditions. The proposed technique highlights that, combined with potentiostatic measurements, luminescence microscopy can be a powerful tool for the assessment of perfor-mance losses and the improvement of solar cells.

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

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

Material Development for Perovskite/Silicon Tandem Photovoltaics

Peter Joseph Fiala

The developed world is built on the fact that energy is readily available and functionally infinite. The electricity from the wall, the gas at the station, and the heat in our homes are reliable and low-cost. But this comfort is so far only possible through the combustion of fossilized carbon sources. This is not a sustainable system. Not only are these sources finite, but the long-term effects of this much combustion are catastrophic for our planet and species. Transitioning from current energy production to sustainable sources will require the development and deployment of a diverse group of technologies. Chief among these is the generation of electricity from sunlight, or photovoltaics (PV).Most modern PV is based on crystalline silicon. Silicon PV alone could be scaled to provide the necessary energy production, but this could be realized more quickly and efficiently if more advanced technologies are used. The defining metric is the cost per generated electricity. In the case of silicon, the current cost breakdown of materials, manufacturing, installation, and system costs leaves little room for further price reduction; and light-to-electricity efficiencies are near their practical limit. Of the proposed alternatives, the pairing of silicon with a thin film of lead-halide perovskite in a multijunction or "tandem" architecture is one of the most promising. This splits the solar spectrum in two, and raises the theoretical maximum efficiency from 33.3% to 45.1% based on each material better using its segment than any one material could use the full spectrum. The central goal of this thesis was therefore to advance the capability of this type of solar cell while remaining within the constraints of industrially-plausible production methods.Towards this goal, we first isolated perovskite layers to study what limits their performance. This led to improvements in the bulk material quality, the interfaces with adjacent materials, and the layer morphology. We reduced bulk disorder via balancing the annealing temperature, time, and environment. The interfaces were improved by selecting passivating materials which were compatible with our deposition methods. We further observed a complex interplay between the materials used and the environment of the annealing, which illuminated critical dependencies of device performance and gave us a more complete understanding of our material. Finally the morphology was improved via high-temperature annealing. This was enabled by a surface-binding additive which reduced the detrimental effects of such an anneal while preserving the benefits, resulting in a perovskite with less internal resistance. From there, we moved outside of the perovskite layer and developed a light scattering ZnO layer which increased the optical performance and absorption of our cells. Returning to the main goal of the thesis, the lessons learned from work on isolated perovskites were applied in tandems on textured silicon. We additionally increased the perovskite layer thickness and made the bottom-cell more robust to processing damages. Together, the result was a stabilized efficiency of 27.3%. This is better than any silicon cell to date, but work remains to achieve 30% and beyond, extend material stability, scale up the deposition area, and transfer these capabilities to industry. Together, the work done as a part of this thesis represents a step forward in the journey towards carbon-neutral energy production.

EPFL2022