Revealing the beneficial effects of Ge doping on Cu2ZnSnSe4 thin film solar cells

Kesterite (CZTSe) is a promising thin film photovoltaic absorber material due to its composition of more earth abundant materials compared to mature thin film photovoltaic technologies. Up to now, power conversion efficiencies are still lower and its main problem is the low open circuit voltage (Voc). Recently, a novel sintering approach using a nanometric Ge layer showed a large increase in device performance and especially in Voc. In this work, in-depth solar cell characterization as well as Raman and Photoluminescence studies of devices employing different Ge doped CZTSe absorber layers is presented. The main focus is to reveal the beneficial effects of Ge doping and furthermore investigate the interaction of Ge and Na. For low Ge doping an increase in charge carrier concentration is observed, resulting in devices with Voc of 460 mV, which corresponds to Voc deficits (Eg/q–Voc) of 596 mV a value comparable to current record devices. For high Ge amounts admittance spectroscopy measurements identified the appearance of a deep defect which can explain the observed deterioration of solar cell performance. Additional Na provided during crystallization of high Ge doped devices can reduce the density of this deep defect and recover device performance. These results indicate that Na plays an important role in defect passivation and we propose a defect model based in the interaction of group IV elements and Na with Cu vacancies

Multiwavelength excitation Raman scattering analysis of bulk and 2 dimensional MoS2: vibrational properties of atomically thin MoS2 layers

In order to deepen the knowledge of the vibrational properties of two-dimensional (2D) MoS2 atomic layers, a complete and systematic Raman scattering analysis has been performed using both bulk single-crystal MoS2 samples and atomically thin MoS2 layers. Raman spectra have been measured under non-resonant and resonant conditions using seven different excitation wavelengths from near-infrared (NIR) to ultraviolet (UV). These measurements have allowed us to observe and identify 41 peaks, among which 22 have not been previously experimentally observed for this compound, and characterize the existence of different resonant excitation conditions for the different excitation wavelengths. This has also included the first analysis of resonant Raman spectra that are achieved using UV excitation conditions. In addition, the analysis of atomically thin MoS2 layers has corroborated the higher potential of UV resonant Raman scattering measurements for the non-destructive assessment of 2D MoS2 samples. Analysis of the relative integral intensity of the additional first- and second-order peaks measured under UV resonant excitation conditions is proposed for the non-destructive characterization of the thickness of the layers, complementing previous studies based on the changes of the peak frequencies

The importance of back contact modification in Cu2ZnSnSe4 solar cells: The role of a thin MoO2 layer

Cu2ZnSn(SxSe1−x)4 (CZTSSe) photovoltaic absorbers could be the earth-abundant and low toxicity replacement for the already commercialized CuIn1−xGaxSe2 (CIGS) thin film technology. In order to make this possible, specific research efforts applied to the bulk, front and back interfaces must be performed with the aim of improving CZTSSe performance. In this paper the importance of back contact modification to obtain high efficiency Cu2ZnSnSe4 (CZTSe) solar cells and to increase a paramount and limiting parameter such as VOC is highlighted. Several Mo configurations (monolayer, bi-layer and tri-layer) with different electrical and morphological properties are investigated in CZTSe solar cells. An optimum tri-layer configuration in order to minimize overselenization of the back contact during thermal annealing while keeping reasonable electrical features is defined. Additionally, a thin intermediate MoO2 layer that results in a very effective barrier against selenization and innovative way to efficiently assist in the CZTSe absorber sintering is introduced. The use of this layer enhances grain growth and subsequently the efficiency of solar cells increases via major VOC and FF improvement. An efficiency increase from 7.2% to 9.5% is obtained using a Mo tri-layer with a 20nm intermediate MoO2 layer

Kesterite Deposited by Spray Pyrolysis for Solar Cell Applications

En esta tesis se demuestra el uso de un sistema de spray pyrolysis utilizado para sintetizar kesterita de azufre puro (CZTS) un material que representa un reto tecnológico y científico en el campo de las celdas solares de películas delgadas. La síntesis de este material es llevada a cabo en un sistema de spray en atmosfera controlada en el marco de los parámetros del sistema y de la solución; evitando el uso de reactivos altamente peligrosos utilizando en su caso agua y alcoholes. Se demuestra la síntesis de materiales del tipo CZTSSe después de un proceso de selenización; las celdas solares resultantes muestran las posibilidades del material y del sistema.Solar cells generate electrical power by direct conversion of solar radiation into electricity using semiconductors. Once produced, the solar cells do not require the use of water; operate in silence and can be easily installed almost everywhere, as solar panels with low technological risk. In this thesis new photovoltaic materials and solar cells are investigated. From the beginning of the semiconductor era, silicon has been present; the semiconductor theory improved with the silicon technology, almost taking the idealized models to reality in within silicon. In the recent years plenty of new natural and artificially produced materials have seen the light; some of them are still waiting to be understood and explained by a new theory that has to be experimentally proved right. The clue for a better and faster progress is to work in a multidisciplinary frame; as this thesis shows, all research and knowledge has to have future protections and possibilities of been used in the benefit of our society. Today, the photovoltaic technology (PV) based on silicon solar cells dominates the market. The thin film PV such as GaAs, Cu(In,Ga)Se2 (GIGS) and CdTe have reached power conversion efficiencies above 20% which makes them industrially interesting despite the use of scare and/or very toxic elements. Researchers and investors are expectant for a new stable, eco-friendly, inexpensive, fast and easy to produce material that could be used in a big scale and long term for photovoltaic terrestrial applications. Some years ago it was believed that this ideal material was near to be confirmed when the reflectors were on a new chalcogenide material: Cu2ZnSn(SxSe1-x)4 (CZTSSe) called kesterite after its crystal structure. This semiconductor is very attractive due to its constituent elements, semiconductor properties. Its similarity with GIGS and compatibility with the already existing industrial processes made possible its rapid power conversion efficiency rise as absorber material in thin film solar cells. Through the years kesterite has been found to be a challenging material due to the energetically feasible mixture of stannite and kesterite structures, the high probability of defects, and the narrowness of the optimum compositional region (compared with that of chalcopyrites) that eases the formation of secondary phases limiting the efficiency of the solar cells. Recently, CZTSSe thin film solar cells with certified efficiency of 12.6% were produced by IBM; synthesized by spin-coating using a hydrazine-based pure solution approach on a soda-lime glass (SLG) Mo coated substrate. The use of hydrazine is the key for the record as well its mayor drawback however it demonstrates the robustness of the solution-based techniques. In this thesis the use of a cool-wall vertical pneumatic spray pyrolysis system (SP) is demonstrated as a synthesis technique of CZTS kesterite thin films from water and alcohol-based precursor solutions containing metal salts and thiourea. In the course of this thesis, the possibilities and limitations of this synthesis technique and the resulting films are explored in the frame of system- and solution-related parameters. The SP system used in this thesis is sophisticate and advantageous; is able of reproduce the open-air conditions used in typical spray systems but also it is capable of grow films by spraying in an oxygen-free atmospheres such as Ar or Ar-H2 or any other. It was completely new spray approach by the year of the publication of our first repot (2013) with a 0.5% efficient working solar cell. A remarkable efficiency value by the time of publication if considered the combo challenge: material + deposition technique. In this thesis, air, Ar and Ar-H2 were used as carrier gas and atmosphere, where the so sprayed films were studied in combination with other system parameters (solution flux, time of spraying, substrate temperature, etc.) and some solution related parameters (solvent, metal precursors concentration, solution stability, etc.). To obtain device grade films, the sprayed kesterite ought to be annealed; this annealing process is also subject of study in this thesis. One step annealing at high temperature (580°C) at room pressure in S-containing reactive atmosphere was optimized for the CZTS-based thin films with efficiencies of 1.4%. To synthesize CZTSSe films, different annealing approaches were tried; one step room pressure annealing (at 550°C) in Se- containing reactive atmosphere probed to be the optimum for sprayed kesterite from methanol-based precursor solutions for solar cells with the highest conversion efficiency of 1.9% obtained in this thesis. The results showed here open many new possibilities for the use of spray systems for the synthesis of PV quality materials for solar cells applications

Obtención de contactos de tipo CuxTe en celdas solares de CdTe mediante depósitos de Te por la técnica de CSVT y evaporación de Cu

Las celdas solares del tipo CdS/CdTe están formadas por películas delgadas de compuestos semiconductores de transición directa, su elemento activo (CdTe) tiene un alto coeficiente de absorción y casi todo su proceso de producción puede ser fabricado en el sistema CSVT-HW (Closed Sapaced Vapour Transport – Hot Wall). En este sistema se obtienen películas de buena calidad cristalina, la velocidad de depósito es del orden de los micrómetros por minuto, lo cual lo hace ideal para la fabricación de celdas solares de CdS/CdTe, pues las actividades industriales alrededor del mundo están enfocadas en la implementación de procesos de producción de alta calidad y bajo costo. Uno de los principales aspectos a mejorar en las celdas solares mencionadas, son sus contactos posteriores. Grupos como el de T. Potlog y X. Wu ha estado han estado enfocando gran parte de sus esfuerzos la obtención de una región p+ en el CdTe para la posterior evaporación de un contacto metálico. En nuestro grupo hemos dominando los paramentaros que caracterizan la región p+, obteniéndola por métodos físicos y químicos variando la estequiometria del CdTe; para la formación de un contacto posterior. En este trabajo exploramos la obtención de un compuesto que haga la función de contacto posterior para celdas solares del tipo CdS/CdTe. El material propuesto fue un calcogenuro de cobre (CuXTe), específicamente nos enfocamos en la obtención de Cu1.4Te. Logramos la formación de este calcogenuro mediante el depósito de teluro y la evaporación de cobre sobre vidrio, en función del tratamiento térmico y las proporciones de Cu/Te se obtienen diferentes calcogenuros de cobre. El siguiente paso consistió en la obtención de este compuesto sobre CdTe. El CdTe es un semiconductor tipo p por exceso de teluro, por lo cual si se logra incrementar la cantidad de teluro en los granos y/o en las fronteras de grano se obtiene un CdTe p+, esto se logró. El depósito de una película delgada de teluro disminuye la resistividad del CdTe. Sobre esta región rica en teluro se evaporó cobre, en este paso se investigo sobre la cantidad de cobre mínima necesaria para la formación del compuesto buscado. A las muestras se les proporcionaron distintos tratamientos térmicos y se evaluaron los resultados. El Cu1.4Te forma un contacto lineal, óhmico y cuya resistencia específica está influenciada por el alto valor de la resistividad del CdTe. Por lo tanto, los proceso tecnológicos que hemos optimizados en esta tesis permiten su aplicación en el procesamiento de contactos traseros en celdas solares de CdS/CdTe

Environmentally friendly and roll-processed flexible organic solar cells based on PM6:Y6.

Organic Solar Cells (OSCs) have reached the highest efficiencies using lab-scale on active areas far below 0.1 cm^2. This tends to widen the so-called “lab-to-fab gap”, which is one of the most important challenges to make OSCs competitive. The most commonly used fabrication technique is spin-coating, which has poor compatibility with large-scale techniques and substantial material waste. Moreover, other techniques such as blade or slot-die coating are much more suitable for roll-to-roll manufacturing processes, which is one of the advantages the technology has compared, for example, to silicon solar cells. However, only a few studies report solar cells using these fabrication techniques. Additionally, for the environmentally friendly OSC scale-up, inks based on non-hazardous solvent systems are needed. In this work, slot-die coating has been chosen to coat the PM6:Y6 active layer, using o-xylene, a green solvent, without additives. The optimal coating parameters are defined through fine-tuning of the coating parameters, such as the drying temperature and solution concentration. Moreover, ternary devices with PCBM, and fully printed devices are also fabricated. Power conversion efficiencies of 6.26% and 7.16% are achieved for binary PM6:Y6 and ternary PM6:Y6:PCBM devices, respectively