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Ge nanocrystals (Ge NCs) were grown in a multilayered superlattice structure using magnetron co-sputtering technology. Studies were taken to optimize the processing conditions including post-annealing temperature and duration. Structural properties of Ge NCs and multi-bilayers, such as crystallization process, precipitate crystallinity, size-control of nanocrystals and influence of interlayer diffusion, were particularly chosen to be investigated. The experimental results indicated that high quality and reproducible multilayered Ge NCs can be obtained with an appropriate thermal annealing condition. This investigation builds a technical foundation for fabrication of tandem solar cell applicable Ge NCs absorber films.

Nowadays the photovoltaics market is dominated by crystalline silicon solar cells, which are approaching their theoretical efficiency limit. Novel solutions have to be found for photovoltaics to further enhance its competitiveness with other energy sources. One of the most promising approaches lies in combining market-proven silicon solar cell technology, having a low optical band gap, with an efficient near-infrared transparent wide-band gap top cell to form a tandem cell. Organic-inorganic halide perovskite solar cells are promising candidates for top cells, showing high efficiencies with simple and cost-effective device fabrication. We first develop perovskite solar cells specifically for tandem applications. We develop and optimize a hybrid sequential deposition method combining thermal evaporation and solution processing to fabricate perovskite absorber materials with specifically tailored optoelectronic properties. These materials are systematically investigated for their optical, structural and electronic properties, and then applied in perovskite solar cells both in n-i-p and p-i-n configurations, for which we show several charge transport materials combinations. As a replacement for the standard metal opaque rear electrode of perovskite cells, we develop a TCO-based transparent electrode, for which sputtered IZO is presented as a good candidate thanks to its high carrier mobility and broadband transparency. Sputtering-induced damages are reduced by the introduction of a buffer layer: MoOx for n-i-p cells and SnO2 for p-i-n cells. The parasitic absorption losses in the MoOx are described in details. We then discuss the parasitic absorption losses in charge transport layers and TCOs, with experimental comparison of various materials. The optimisation of the perovskite absorber deposition method and the improvement of the charge transport layers and transparent electrode allows the fabrication 1cm2 area semitransparent perovskite solar cells with >16% efficiency. We then integrate the semitransparent perovskite cells in mechanically stacked 4-terminal tandem solar cells. The challenges in reducing the strong parasitic absorption and reflection losses are first discussed. We then demonstrate 4-terminal tandem measurements with >25% total efficiency with a small area top cell. With larger 1cm2 area top cells, we fabricate integrated 4-terminal tandem devices with both subcells having similar size and total efficiency >23%. The integration of perovskite solar cells in 2-terminal monolithically connected tandem solar cells with silicon heterojunction bottom cells is finally presented. First, we show the development of a TCO-based recombination layer and the important reflection losses and interference effects observed in all-flat devices. The origins of parasitic absorption losses in monolithic tandems are then explained and new architectures and materials are investigated supported by optical simulations. We then replace the polished wafers by fully textured silicon bottom cells for better light management. The development of a tandem device with the top cell conformally coated onto the textured bottom cell is explained, leading to >25% certified power conversion efficiency. The end of the thesis presents preliminary results on the up-scalability and light soaking stability of the developed textured tandems, as well as a proof-of-concept of a first perovskite/perovskite/silicon triple junction solar cell on textured wafers.

We show that two-terminal multijunction cells interconnected by tunnel junctions are fairly immune to individual local shunts, thanks to the shunt quenching. Interestingly, they may still suffer from global shunts. We revise the paradigm of a multijunction cell as a simple serial connection of component cells. This paradigm remains valid only for multijunction cells with laterally conductive interlayers. Instead, a new equivalent circuit is proposed and verified by measurement and simulations. As a main approach, selective illumination is applied and the voltage is measured at the end terminals. The global shunt is seen as a shift from logarithmic to linear intensity response. The presence of tunnel junction is important for an optimum configuration of tandem structures such as metal-halide perovskite with crystalline silicon solar cell. Fil: Ventosinos, Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Física del Litoral. Universidad Nacional del Litoral. Instituto de Física del Litoral; Argentina Fil: Klusacek, Jan. Institute Of Physics Of The Academy Of Sciences Of The Czech Republic; Fil: Finsterle, Tomas. Ceske Vysoke Uceni Technicke V Praze; Fil: Kunzel, Karel. Laboratoire de Physique Des Interfaces Et Des Couches Minces; Fil: Haug, Franz-Josef. Ecole Polytechnique Federale de Lausanne; Fil: Holovsky, Jakub. Institute Of Physics Of The Academy Of Sciences Of The Czech Republic; . Ceske Vysoke Uceni Technicke V Praze

Several aspects of the science and technology of thin film silicon for photovoltaic applications will be presented. The potential advantages of this technology over crystalline wafer technology will be discussed. A basic understanding of the material properties of thin film silicon layers enables to assess their potential and limitations when used in photovoltaic devices. A brief review of the production technology for thin films will be given with particular emphasis on amorphous and microcrystalline silicon. As for other photovoltaic technologies, the push for higher efficiency of thin film silicon devices is strong. An appealing feature of these materials is that they can be easily integrated in multi-junction tandem devices. For instance, stacking amorphous and microcrystalline silicon thin films in one tandem cell, the micromorph cell, increases the efficiency well above the characteristic values of single junction cells. The Institute of Microengineering (IMT) has been a pioneer in the research and development of thin film silicon photovoltaics over the last 20 years and several latest developments on are reviewed.

Monolithically integrated two-terminal perovskite/Si tandem solar cells promise to achieve high power conversion efficiency. However, there is a concern that the stability of the perovskite top cell will limit the long-term performance of tandem devices. To investigate the impact of perovskite cell degradation on the photocurrent generation and collection in the individual subcells, we employed light beam induced current mapping to spatially resolve the photocurrent under controlled humidity conditions. The evolution of the device behavior is consistent with the formation of an optically transparent hydrated perovskite phase that allows the bottom Si cell to continue to generate photocurrent at the probing wavelength (532 nm). Additional measurements were performed on perovskite thin films on glass substrates to verify the interpretation.

Multi-junction solar cells are known to have a considerably increased efficiency potential over their typical single junction counterparts. In order to produce low cost and lightweight multi-junction devices, the availability of suitable narrow (< 1.1 eV) bandgap bottom cells is paramount. A possible absorber for such a bottom cell is the Cu(In, Ga)Se-2 (CIGS) compound semiconductor, one of the most efficient thin film materials to date., In this contribution we report on the RbF post deposition treatment of narrow bandgap CIGS absorbers grown with a single bandgap grading approach. We discuss the necessary deposition conditions and the observed improvements on solar cells performance. A certified record efficiency of 18.0% for an absorber with 1.00 eV optoelectronic bandgap is presented and its suitability for perovskite/CIGS tandem devices is shown.

A novel molecular-ink deposition route based on thiourea and N,N-dimethylformamide (DMF) that results in a certified solar cell efficiency world record for non-vacuum deposited CuIn(S,Se)(2) (CIS) absorbers and non-vacuum deposited absorbers with a bandgap of 1.0 eV, is presented. It is found that by substituting the widely employed solvent dimethyl sulfoxide with DMF, the coordination chemistry of InCl3 could be altered, dramatically improving ink stability, enabling up to tenfold increased concentrations, omitting the necessity for elevated ink temperatures, and radically accelerating the deposition process. Furthermore, it is shown that by introducing compositionally graded precursor films, film porosity, compositional gradients, and the surface roughness of the absorbers are effectively reduced and device conversion efficiencies are increased up to 13.8% (13.1% certified, active area). The reduced roughness is also seen as crucial to realize monolithically interconnected CIS-perovskite tandem devices, where semitransparent MAPbI(3) devices are directly deposited on the CIS bottom cell. Confirming the feasibility of this approach, monolithic devices with near perfect voltage addition between subcells of up to 1.40 V are presented.

Due to advantages such as mechanical flexibility, light weight and the prospect to use low-cost roll-to-roll manufacturing processes, organic semiconductors have been widely investi-gated in many application areas as alternatives for their inorganic counterpart. In organic sem-iconductors, the rather weak Van der Waals interactions holding together the molecular build-ing blocks result in narrow absorption bands which endow organic electronics with important advantages for the development of smart functionalities. Transparent organic electronics (TOEs), for example, incorporate devices through which visible light is transmitted. Among other semiconducting devices, it is actually possible to construct sensors and photovoltaic de-vices that solely use ultraviolet (UV) and near infrared (NIR) light to produce electrical energy or signal. TOEs have been proposed for easy integration with other electronic devices. Among the different molecular materials, cyanine dyes stand out by sharp, intense absorption bands exhibiting the highest molar extinction coefficients. The absorption peak can be easily shifted into the NIR wavelength region by increasing the length of the conjugated polymethine chain. For example, NIR light absorbing heptamethine cyanine dyes (Cy7) are promising candidates as transparent and colorless photoactive film materials. In this thesis work, highly efficient TOE devices such as transparent solar cells and transparent photodetectors using NIR absorbing cyanine dyes as photosensitive materials have been successfully fabricated. To optimize these multilayer devices, various cyanine dyes were in-vestigated, device architecture and interfaces were engineered. Optical simulations of the stacked thin film structures allowed understanding and tuning device performance. Moreover, organic solar cells which are transparent in the visible range have been integrated into tandem and triple junction solar cells. Low bandgap materials that absorb NIR light were combined with cyanine cells which absorb visible light, thereby more sunlight could be harvested and power conversion efficiency was dramatically enhanced in such tandem solar cells. The photo-stability investigation of cyanine solar cells showed that cyanine dyes were photostable when illuminated in the absence of oxygen and water vapor. We found that the initial degradation of cyanine dye devices during operation was due to the photo-polymerization of the widely used electron acceptor material fullerene C60 and photo-chromism of the hole extraction interfacial layer molybdenum oxide (MoO3).

In monolithic perovskite/silicon tandem solar cells, it is important to know which sub-cells is limiting the overall current to adapt the perovskite absorber thickness and bandgap accordingly. The current matching situation is usually analyzed by integrating measured external quantum efficiencies. However, this method can lead to significant errors and misinterpretations if meta-stable perovskite solar cells are involved. In this work, spectrometric characterization is presented as an alternative approach avoiding these errors. Current-voltage curves are recorded under different spectral conditions. Spectral irradiance settings are varied in a systematic way from red-shifted spectra (the perovskite top solar cell limits the current) to blue-shifted spectra (the silicon bottom solar cell limits the current) around the Air Mass 1.5 global (AM1.5g) spectrum. This method not only allows for accurate determination of the current matching point, but also gives quantitative insight in the behavior of the single sub-cells and their influence on the tandem performance. As different current mismatching also influences other global cell parameters, an example is presented where the current loss due to the current mismatch is partly compensated by a strong fill factor increase when the silicon solar cell limits the current, resulting in a high-power output also at the AM1.5g condition.