Nanoscale Organic / Inorganic Hybrid Photovoltaic Devices

There is an increasing trend of using various types of nanostructures as components of solar cells. This highlights the need to gain a deeper understanding of the nanoscale interface in such devices. The present thesis aims to study individual photoactive junctions as the elementary functional unit of bulk photovoltaic devices, and exploit the gained knowledge toward further improving the solar cells' efficiency. Along this direction, Schottky-contacts to cadmium sulphide nanowires were thoroughly studied using scanning photocurrent microscopy (SPCM). In conjunction with theoretical simulations of the measured photocurrent profiles, it was found that charge carriers can be very efficiently extracted out of a few micrometer long nanowire segments below the metal contact. Moreover, the carrier combination rate in this section could be determined as a function of an applied bias or backgate voltage. These findings provide valuable clues about how electric fields are distributed within semiconductor nanowire-based devices. Moreover, SPCM was employed to explore the local photoresponse along graphene/CdS heterojunctions, representing a first step toward implementing graphene as an active acceptor material into solar cells. The short circuit current of such devices could be enhanced by two orders of magnitudes through chemical tailoring of the graphene-CdS interface. In addition, evidence was obtained that surface plasmon excitation in the metal contacts can make significant contributions to the generated photocurrent. This effect was exploited in the fabrication of novel surface plasmon detectors, which support different plasmon modes depending on the polarization direction of the incoming light. Finally, it was attempted to realize photovoltaic devices comprising a pn-junction implemented into a π-conjugated organic polymer. While the electric-field assisted exciton dissociation in this system could be successfully modelled in the framework of the Braun-Onsager model, photoinduced charge separation at the interface between differently gated regions of the polymer could not be achieved due to insufficient electron conduction of the material.

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

Prediction of Photocurrent in Bifacial Photovoltaic Systems

Arttu Matias Tuomiranta

Motivated by the shift of the photovoltaic (PV) industry towards bifacial solar modules, desert markets, and ever narrower profit margins, this thesis study is aimed at improving the estimation accuracy of the output of bifacial PV systems that are potentially exposed to soiling. The scope is limited to the estimation of photocurrent, the performance parameter that is the most strongly affected by bifaciality and soiling. The thesis consists of three parts with the following topics: ground surface reflectance, module soiling, and bifacial effective irradiance. As for the first topic, the thesis aims at providing tools for surface reflectance estimation in any use case. The model evaluation results that are based on a global database of reflectance measurements can be used to choose a reflectance model that is appropriate for the surface type of interest. On a global average, the results show that data-driven estimation reduces the mean absolute error of reflectance estimates by 20-40% compared to the literature values. The proposed geographically generalised parametrisations of reflectance models can be used to produce temporally variable reflectance estimates if measuring campaigns for local model calibration cannot be implemented. If they can, the thesis provides detailed guidelines for their optimal timing. The choice of the reflectance model can result in deviations of up to 2.5% in the energy yield estimated for a bifacial PV system. In the second part of the thesis, a new physical model for forecasting soiling losses is proposed. The model incorporates new methods e.g., to simulate dust deposition on tilted surfaces, to physically capture the impact of relative humidity on dust accumulation, and to estimate the hemispherical transmittance of dust for diffuse light sources. The model can be used based on the public data products of numerical weather prediction models without the need for soiling measurements. The model is validated against its alternatives and measurements made at a field experiment in the United Arab Emirates. The impact of the soiling loss model on yield estimation is found to be lower than that of the reflectance model, plus minus 1% at maximum considering usual weekly or biweekly cleaning intervals. Finally, the third part of the thesis proposes a new model for simulating the photocurrent of each solar cell of a bifacial PV power plant using a computationally efficient method. It comprises a new algorithmic design that makes it possible to make all the detailed geometric calculations once and move to the temporal domain only to estimate irradiance. The most important novelty of the proposed model is that it estimates the photocurrent contributions as a distribution of the different light sources. The model is validated against the measurements made in France and Italy. The results of the validation show that the model's current estimates are more accurate than those of the widely used "unlimited sheds" approach. The most important reasons for the better performance are the new model's capability to estimate the spatial variability of effective irradiance on the array surface and that of ground-incident irradiance around the system. As per the impact assessment, the new model was found to overestimate yield and underestimate electricity cost by 1% on average. The "unlimited sheds" approach, in turn, resulted in more variable bias. In the case of large arrays, the approach was found to overestimate yield by 4%.

EPFL2020

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