Towards Quantitative Interpretation of Fourier-Transform Photocurrent Spectroscopy on Thin-Film Solar Cells

The method of detecting deep defects in photovoltaic materials by Fourier-Transform Photocurrent Spectroscopy has gone through continuous development during the last two decades. Still, giving quantitative predictions of photovoltaic device performance is a challenging task. As new materials appear, a prediction of potentially achievable open-circuit voltage with respect to bandgap is highly desirable. From thermodynamics, a prediction can be made based on the radiative limit, neglecting non-radiative recombination and carrier transport effects. Beyond this, more accurate analysis has to be done. First, the absolute defect density has to be calculated, taking into account optical effects, such as absorption enhancement, due to scattering. Secondly, the electrical effect of thickness variation has to be addressed. We analyzed a series of state-of-the-art hydrogenated amorphous silicon solar cells of different thicknesses at different states of light soaking degradation. Based on a combination of empirical results with optical, electrical and thermodynamic simulations, we provide a predictive model of the open-circuit voltage of a device with a given defect density and absorber thickness. We observed that, rather than the defect density or thickness alone, it is their product or the total number of defects, that matters. Alternatively, including defect absorption into the thermodynamic radiative limit gives close upper bounds to the open-circuit voltage with the advantage of a much easier evaluation.

High performance solid-state mesoscopic solar cells

Julian Burschka

Mesoscopic sensitized solar cells are one of the most promising third-generation photovoltaic technologies. Dye-sensitized solar cells (DSCs), imitating the photosynthesis of green plants, were the first photovoltaic devices to utilize a mesoscopic heterojunction for the conversion of solar irradiation into electrical power. Solid-state dye-sensitized solar cells (ssDSCs), that employ an organic hole-transporting material in place of a liquid redox electrolyte, have evolved as viable contenders to conventional liquid DSCs. A typical ssDSC is composed of a mesostructured wide-bandgap metal oxide semiconductor that is sensitized with a light-absorbing chromophore and infiltrated with a molecular organic hole-transporter, usually by solution-processing. In this device, photoexcitation of the sensitizer and subsequent ultrafast electron injection into the conduction band of the metal oxide semiconductor is followed by hole transfer from the oxidized sensitizer to the organic hole-transporter. Charge transport of both the electron and the hole through the two bicontinuous phases, followed by charge migration through the external circuit, completes the photovoltaic operation of the cell. Despite more than 15 years of development, ssDSCs have always been lagging behind their liquid counterparts in terms of both power conversion efficiency and long-term stability. My thesis presents three different approaches that are aimed at contributing to the development of high-performance solid-state mesoscopic solar cells. Firstly, I present a new class of Co(III) complexes as solution-processable p-type dopants for triarylamine-based hole-conductors such as the commonly employed 2,2’,7,7’-tetrakis-N,N-di-para-methoxyphenylamine-9,9’-spirobifluorene (spiro-MeOTAD). The proposed Co(III) complexes were characterized in detail using optical and electrochemical techniques. The application of the new p-dopants in ssDSC rendered it possible to directly relate the conductivity of the doped hole-transporter to the photovoltaic performance of the devices. The work shows that chemical p-doping is a powerful tool to control the charge transport properties of spiro-MeOTAD in ssDSCs, capable of replacing the commonly employed photo-doping, i.e. the light-assisted one-electron oxidation of spiro-MeOTAD by ambient oxygen. Combining this strategy with a state–of–the–art organic D–π–A sensitizer allowed us to achieve power conversion efficiencies of up to 7.2%—a new record for this kind of device architecture. Secondly, I investigated a series of new molecular hole-transporters based on the triarylamine-substituted 9,9’-spirobifluorene core. In total, we synthesized seven new materials and characterized them in detail using electrical and electrochemical techniques. Organic-field effect transistors were fabricated employing these materials in order to evaluate their hole mobilities. From the comparison of different substituents on the hole-conducting triarylamine moieties, a structure–property relationship was derived, highlighting the importance of processability and hole mobility of the hole-transporting material for an efficient application in a ssDSC configuration. We encountered the purity of the new materials as a key parameter for a proper functioning of the device and identified the use of functionalized metal-scavenging polystyrene beads as a valuable purification step. Thirdly, a new technique is introduced that allows to realize functional composites of a mesoporous metal oxide and the organic–inorganic hybrid perovskite CH3NH3PbI3. CH3NH3PbI3 has recently attracted great attention as a light-harvesting pigment in mesoscopic solar cells. We propose a two-step technique to deposit the perovskite absorber: PbI2 is first applied on a substrate by solution-processing and subsequently exposed to a solution of CH3NH3I. It was evidenced that the desired hybrid perovskite forms within seconds after contacting the two precursors. Scanning electron microscopy was employed to elucidate the changes in crystal morphology during the transformation reaction. The kinetics of the reaction was investigated in detail using X-ray diffraction and optical spectroscopy. Employing the two-step technique for the fabrication of mesoscopic perovskite-based solar cells enabled new record power conversion efficiencies of up to 15%. By following three different strategies, this thesis could contribute significantly to the development of high-performance solid-state mesoscopic solar cells. The power conversion efficiencies of 7.2% and 15%, achieved with a molecular dye and an organic–inorganic perovskite pigment, respectively, represent important milestones for the design of third-generation photovoltaic cells that hopefully one day can compete with the conventional silicon-based solar cell technology.

EPFL2013

L'oxyde de zinc par dépôt chimique en phase vapeur comme contact électrique transparent et diffuseur de lumière pour les cellules solaires

Sylvie Faÿ

Zinc oxide (ZnO) is a material that belongs to the family of Transparent Conductives Oxides (TCO). Its non-toxicity and the abundant availability in the Earth's crust of its components make it an ideal candidate as electrical transparent contact for thin-film amorphous and/or microcrystalline silicon solar cells. The Low-Pressure Chemical Vapour Deposition (LP-CVD) method allows one to obtain rough ZnO layers, which can effectively scatter the light that passes through them. This high scattering capacity increases the path of the light within the solar cell and therefore enhances its probability to be absorbed by the solar cell, and consequently the photogenerated current. This thesis work studies in detail the LP-CVD technology used for deposition of ZnO layers, which are employed as TCO layers for amorphous, microcrystalline and "micromorph" (microcrystalline/amorphous tandem) solar cells developped at the Institute of Microtechnology of the University of Neuchâtel. The chapter 3 is a detailed study of the growth of ZnO layers on a glass substrate. This study reveals a columnar microstructure of the ZnO layers, which are composed of large conical monocrystals. The tops of these monocrystals emerge at the surface of the ZnO layers as pyramids, which give to such layers their rough aspect and therefore their capacity of scattering the light. The two following chapters represent a study of the effect of variations in deposition parameters on the optical properties (transparency and scattering power), electrical properties (conductivity), and structural properties of ZnO layers. The deposition parameters varied thereby are the various gas flows, the substrate temperature, and the process pressure. In particular, a strong influence of the substrate temperature on the microstructure of the ZnO layers is observed. A variation of ten degrees of this deposition temperature leads to abrupt variations of the optical and electrical properties of the layers. From these various studies, a strong correlation is established, between the scattering power, the electrical properties, and the microstructure of the ZnO layers deposited by LP-CVD: The higher the pyramidal grains at the surface of the layers, the higher the scattering capacity of these layers. The larger the monocrystals within the bulk of the layers, the higher the mobility of the free electrons. This enhanced mobility leads to an increase of the conductivity of the layers. Finally, the chapter 6 of this thesis work describes the various situations where LP-CVD ZnO layers are used as transparent electrical contacts and light scattering layers in thin-film silicon solar cells. It is schown that, by incorporating ZnO layers developped in this work in the photovoltaic devices, an increase of 8% in the photogenerated current is obtained, compared to the performances of solar cells using standard commercially available TCO.

EPFL2003

Computational Modeling of Thin-Film Silicon Solar Cells and Modules

Thomas Andreas Lanz

Solar photovoltaic energy production relies on the direct conversion of sunlight into electricity. Silicon thin-film solar cells represent a reliable and environmentally friendly technology to make photovoltaics economically viable. As these solar cells do not contain any rare or toxic materials they are an ideal candidate for the large scale deployment of solar energy. At the same time photovoltaic energy production is ideal for off-grid, stand-alone installations. This thesis presents new methods and results on computational engineering of thin-film silicon solar cells and modules. Computational engineering employs numerical methods and simulation techniques to address engineering challenges. Predictive numerical simulations allow for device optimizations that otherwise require substantial experimental effort and they may lead to new insights into the device physics which is the basis for further improvements in terms of cell efficiency and lifetime. We discuss the development of two new simulation methods and present several case studies that illustrate their use in solar cell characterization and optimization. The first presented method addresses optical simulations, i.e. the interaction of the solar cell with sunlight. Our optical model is based on the net-radiation method that considers incoherent, ray-like propagation of light. We extend the net-radiation method with thin-film optics to allow for arbitrary sequences of thick and thin layers. Experimentally, advanced techniques such as optimized surface textures have been developed to increase the absorption in the solar cell and thereby to increase the produced current. We illustrate how our optical model takes into account these textures by means of azimuth integrated light scattering properties to formulate a computationally efficient model of light propagation within the solar cell. Our optical model thus integrates coherent and incoherent propagation of light as well as a non-iterative treatment of ligth scattering. It bridges the gap between wave and ray optics that alternative approaches are most often facing. We first apply the method to study a recently proposed rear electrode architecture based on lithium fluoride and aluminum in amorphous silicon solar cells. Our model-based optical analysis identifies the increased reflectivity of this back electrode as its main advantage. Secondly, we then explore the potential performance gains in microcrystalline silicon solar cells that can be reached by eliminating absorption in layers that do not contribute to the photogeneration of solar cell current. Finally, our optical model is also suited for other solar cell technologies and we illustrate its extension and application to light extraction calculations in light emitting diodes (LEDs). Our optical model has been incorporated into the software package setfos, commercialized by Fluxim Inc. It is thus available to the research community. We then turn to address large area solar modules. A new method based on the finite element method (FEM) is presented to calculate the electrical charge and heat transport in these devices and how it is influenced by defects as well as partial shading. We identify such defects with surface temperature measurements by means of lock-in thermography and demonstrate how our model accounts for the thermal signature of localized defects. Taking into account the large aspect ratio of thin-film solar modules we show how geometrical projections can be used to reduce the model complexity, leading to a numerically efficient approach based on the finite element method. We demonstrate this by presenting full current-voltage curves of solar modules under partial shading.

EPFL2013

High performance solid-state mesoscopic solar cells

Julian Burschka

EPFL2013

Computational Modeling of Thin-Film Silicon Solar Cells and Modules

Thomas Andreas Lanz

EPFL2013

L'oxyde de zinc par dépôt chimique en phase vapeur comme contact électrique transparent et diffuseur de lumière pour les cellules solaires

Sylvie Faÿ

EPFL2003