Perovskite-based tandem solar cells have proven to be suitable candidates to increase the power conversion efficiency (PCE) of conventional single-junction photovoltaic devices, such as those based on silicon and Cu(In,Ga)Se 2 (CIGSe) absorbers, beyond the Shockley-Queisser single-junction PCE limit. Here, we present a highly efficient monolithic perovskite/CIGSe tandem solar cell with a solution processed perovskite top cell fabricated directly on an asgrown, rough CIGSe bottom cell. To prevent potential shunting due to the rough CIGSe surface, a thin NiO x layer is conformally deposited via atomic layer deposition (ALD) on the ITO front contact of the CIGSe bottom cell. The performance is further improved by an additional layer of the p-type polymer PTAA at the NiO/perovskite interface. This novel hole transport bilayer enables a 21.6% stabilized PCE of the monolithic perovskite/CIGSe tandem device at 0.778 cm 2 active area. We use TEM/EDX measurements to investigate the deposition uniformity and conformality of the NiO x and PTAA layers. By comparing the performance of single-junction subcells with absolute photoluminescence measurements, we determine the contribution of the individual subcells to the tandem V OC , revealing that further fine-tuning of the recombination layers between the two subcells might improve the tandem V OC further. Finally, based on the obtained results we give guidelines on how to further improve monolithic perovskite/CIGSe tandems towards predicted PCE estimates above 30%.

We report on the formation of wrinkle-patterned surface morphologies in cesium formamidinium based Cs x FA 1-x Pb(I 1-y Br y) 3 perovskite compositions with x = 0-0.3 and y = 0-0.3 under various spin-coating conditions. By varying the Cs and Br content, perovskite precursor solution concentration, and spin-coating procedure, the occurrence and characteristics of the wrinkleshaped morphology can be tailored systematically. Cs 0.17 FA 0.83 Pb(I 0.83 Br 0.17) 3 perovskite layers were analyzed regarding their surface roughness, microscopic structure, local and overall composition, and optoelectronic properties. Application of these films in p-in perovskite solar cells (PSCs) with ITO/NiO x /perovskite/C 60 /BCP/Cu architecture resulted in up to 15.3% and 17.0% power conversion efficiency for the flat and wrinkled morphology, respectively. Interestingly, we find slightly red-shifted photoluminescence (PL) peaks for wrinkled areas and we are able to directly correlate surface topography with PL peak mapping. This is attributed to differences in local grain size, while there is no indication for compositional de-mixing in the films. We show that perovskite composition, crystallization kinetics, and layer thickness strongly influence the formation of wrinkles which is proposed to be related to the release of compressive strain during perovskite crystallization. Our work helps to better understand film formation and to further improve efficiency of PSCs with widely used mixed perovskite compositions.

Agradecimientos:Agradezco a Cienciactiva y al Consejo Nacional de Ciencia y Tecnología (CONCYTEC) por financiar este trabajo en el marco de una beca completa de maestría (233-2015-1). Reconozco adicionalmente a la Oficina de Internacionalización de la PUCP, y al Servicio de Intercambio Alemán (DAAD) en conjunción con el Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT), por facilitar y financiar dos pasantías a Alemania, las cuales fueron vitales para la producción de este trabajo. A su vez, extiendo mi gratitud al Dr. Jan Amaru Palomino Töfflinger y Dr. Jorge Andrés Guerra Torres, del lado peruano, y al Dr. Lars Korte, Dr. Bernd Stannowski, Dr. Steve Albrecht y Prof. Dr. Bernd Rech, del lado alemán, por hacer posibles mis visitas y trabajos en el Helmholtz-Zentrum Berlin (HZB), así como a Lukas Kegelmann y al Dr. Steffen Braunger por su labor en los trabajos realizados. Finalmente reitero mi agradecimiento al Dr. Jorge Andrés Guerra Torres por su fuerte apoyo y guía en cada etapa de este trabajo. Resumen:Los índices de refracción complejos de películas delgadas de perovskitas de haluros mixtos de formamidinio-cesio de plomo (FA0.83Cs0.17Pb(I1 − xBrx)3), con composiciones variando de x = 0 a 0.4, y para topografías planas y de textura rugosa, son reportadas. Las películas se caracterizan por medio de una combinación de elipsometría espectral de ángulo variable y transmitancia espectral en el rango de longitudes de onda de 190 nm a 850 nm. Las constantes ópticas, espesores de las películas y las capas de microrugosidad, son determinadas con un método "punto a punto", minimizando una función de error global, sin hacer uso de modelos de dispersión, e incluyendo información topográfica proporcionada por un microscopio confocal láser. Para evaluar el potencial de ingeniería del ancho de banda del material, sus anchos de banda y energías de Urbach son determinadas con exactitud haciendo uso de un modelo de fluctuaciones de banda para semiconductores directos. Este considera las colas de Urbach y la región de absorción banda a banda fundamental en una sola ecuación. Con esta información, la composición que brindaría el ancho de banda óptimo de 1.75 eV para una celda solar tándem Siperovskita es determinada. Abstract:The complex refractive indices of formamidinium cesium lead mixed-halide (FA0.83Cs0.17Pb(I1 − xBrx)3) perovskite thin films of compositions ranging from x = 0 to 0.4, with both flat and wrinkle-textured surface topographies, are reported. Films are characterized using a combination of variable angle spectroscopic ellipsometry and spectral transmittance in the wavelength range of 190 nm to 850 nm. Optical constants, film thicknesses and roughness layers are obtained point-by-point by minimizing a global error function, without using optical dispersion models, and including topographical information supplied by a laser confocal microscope. To evaluate the bandgap engineering potential of the material, the optical bandgaps and Urbach energies are then accurately determined by applying a band flu...

Solar cells based on monovalent alkali or organic A-cation, divalent metal B-cation and monovalent halide anion (ABX3) perovskite semiconductors are emerging as a fast-growing research area with substantial technological potential. Discovered as an absorber in dye-sensitised solar cells, a range of processing strategies developed for printed organic photovoltaics have been used to deposit ABX3 solar cells from solutions. ABX3 semiconductors have comparable optoelectronic properties to GaAs and so far are the best solution-processed solar cell technology for small-area test devices. As device performances are comparable with other thin-film solar cell technologies, ABX3-based solar cells are reaching the phase of being evaluated for their potential in large-scale use for solar energy conversion. This chapter highlights the technological potential arising from the solution-processability of ABX3 materials. Recent insights into how ABX3 solution chemistry and lead–halide–solvent structural intermediates during film formation define the thin-film morphology of solution-processed ABX3 devices are discussed. Gaining control over film formation is a prerequisite to achieve further progress in scaling ABX3 devices to larger areas with solution-based processing methods. Apart from developing scalable process technology, rationalising material degradation pathways is of paramount importance, to make reliable predictions of device stability. Concerns regarding the potential ecotoxicity of lead-based materials has inspired the search for the next generation of ABX3-derived materials with similar favourable optoelectronic properties such as their solution-processability and defect tolerance.

The new di- and tetranuclear Fe(III) μ-oxido complexes [Fe 4 (μ-O) 4 (PTEBIA) 4 ](CF 3 SO 3 ) 4 (CH 3 CN) 2 ] ( 1a ), [Fe 2 (μ-O)Cl 2 (PTEBIA) 2 ](CF 3 SO 3 ) 2 ( 1b ), and [Fe 2 (μ-O)(HCOO) 2 (PTEBIA) 2 ](ClO 4 ) 2 (MeOH) ( 2 ) were prepared from the sulfur-containing ligand (2-((2,4-dimethylphenyl)thio)-N,N-bis ((1-methyl-benzimidazol-2-yl)methyl)ethanamine (PTEBIA). The tetrairon complex 1a features four μ-oxido bridges, while in dinuclear 1b , the sulfur moiety of the ligand occupies one of the six coordination sites of each Fe(III) ion with a long Fe-S distance of 2.814(6) Å. In 2 , two Fe(III) centers are bridged by one oxido and two formate units, the latter likely formed by methanol oxidation. Complexes 1a and 1b show broad sulfur-to-iron charge transfer bands around 400–430 nm at room temperature, consistent with mononuclear structures featuring Fe-S interactions. In contrast, acetonitrile solutions of 2 display a sulfur-to-iron charge transfer band only at low temperature (228 K) upon addition of H 2 O 2 /CH 3 COOH, with an absorption maximum at 410 nm. Homogeneous oxidative catalytic activity was observed for 1a and 1b using H 2 O 2 as oxidant, but with low product selectivity. High valent iron-oxo intermediates could not be detected by UV-vis spectroscopy or ESI mass spectrometry. Rather, evidence suggest preferential ligand oxidation, in line with the relatively low selectivity and catalytic activity observed in the reactions.