Quasi One Dimensional Antimony Selenide Thin Film Solar Cells for Next Generation Photovoltaics

Abstract

This PhD thesis explores the quasi one dimensional Sb2Se3 absorber material and its application in substrate thin film solar cells in a SLG/Mo/Sb2Se3/CdS/i-ZnO/ITO configuration. The body of the thesis is introduced by a careful review of the socio- economical context of the energy production in the world, pointing out the necessity of a solid renewable energy production and the evaluation, in this context, of the photovoltaic devices, the materials in which the solar cells are currently based, and the position as a promising absorber material of the Sb2Se3 system due to its abundancy and suitable physical properties. A theoretical introduction is presented devoted to the fundamentals needed to understand the basic physical properties of a solar cell based in a PN junction of semiconductors. The results start with the development of characterization procedures based in Raman spectroscopy and X-ray diffraction techniques to assess the basic physico-chemical properties of the Sb2Se3 material and generate fundamental knowledge to properly identify the phase in question and the possible detrimental secondary phases obtained during the synthesis process of the thin films. Thanks to these measurements, 27 of 30 Raman peaks theoretically predicted by group theory are identified by multiwavelength excitation sources ranging from ultraviolet to infrared, coupled together with results obtained from polarization and low temperature Raman measurements. These results are furthermore corroborated by comparison with expected the expected Raman shifts expected from density functional theory available in the literature. The structural measurements are at the same time coupled with the XRD characterization to ensure crystal quality of the samples and its difference between the as-grown thin films and a single crystalline sample. To finalize the structural characterization, a measurement protocol based in Raman spectroscopy is presented to properly analyze completed devices. Regarding the synthesis route of the Sb2Se3, it is explored by means of a two-step sequential process based in the reactive annealing under selenium atmosphere of a previously physically evaporated Sb metallic layer. The study is focused in the effects of the synthesis temperature, time and pressure of the reactive annealing step and how this affect the structural characteristics of the films by means of FE-SEM, XRF and the previously developed characterization methodologies based in Raman spectroscopy and XRD, a relation is found between the final optoelectronic properties with the compositional ratio of the Sb2Se3 system, reflected in an optimal 3[Sb]/2[Se] = 0.8-0.9 ratio with a maximum obtained device efficiency of 5.7 % in this range, where for ratios outside this range a decrease in performance is observed analyzed, a clear increase in shunt resistance is measured with increasing selenium content. These devices were further studied and analyzed by means of temperature dependent current voltage and external and internal quantum efficiency measurements to disclosure the limiting factors present mainly attributed to the device’s interfaces. Further exploration was carried out regarding the thermal stability of the Sb2Se3 based devices in the range of temperatures T = 50 – 350 ºC by means of post deposition annealing of absorber layers and complete devices. A comprehensive structural analysis by means of XRD characterization founded in the knowledge developed in the first stages of the thesis, permitted to identify a degradation path governed by selenium diffusion in the absorber layer. The degradation route starts at temperatures as low as 50 ºC with the shrinkage of the Sb2Se3 unit cell that clearly reflects in the degradation of the optoelectronic properties. Further increase in the temperature above 200 ºC leads to the formation of Sb oxides and Se secondary phases in self-standing absorbers, and the formation of CdS1-xSex solid solution in complete devices completely degrading photovoltaic performance.

Esta tesis doctoral explora el material absorbente cuasi unidimensional Sb2Se3 y su aplicación en células solares de película fina en una configuración substrato SLG/Mo/Sb2Se3/CdS/i- ZnO/ITO. Primeramente, se desarrollan mecanismos de caracterización basados en técnicas de espectroscopia Raman y difracción de rayos X para evaluar las propiedades físico- químicas básicas del material Sb2Se3 y generar conocimientos fundamentales para identificar adecuadamente la fase y las posibles fases secundarias obtenidas durante el proceso de síntesis de las películas delgadas. Se identifican 27 de los 30 picos Raman predichos teóricamente mediante mediciones de espectroscopia Raman de múltiples longitudes de onda y a baja temperatura, correlacionándose con el patrón de XRD observado y su diferencia entre las películas as-grown y monocristalinas. En un segundo paso se explora la ruta de síntesis del Sb2Se3 mediante un proceso secuencial de dos pasos basado en el recocido reactivo bajo atmósfera de selenio de una capa metálica de antimonio previamente evaporada físicamente. El estudio se centra en los efectos de la temperatura, el tiempo y la presión del proceso de recocido reactivo y cómo estos afectan a la relación de composición final del sistema Sb2Se3, reflejado en un rango óptimo de valores del ratio 3[Sb]/2[Se] = 0,8-0,9 y obteniendo así una eficiencia máxima del dispositivo del 5,7 %. Estos dispositivos se estudian y analizan más a fondo para revelar los factores limitantes presentes que se encuentran principalmente en las interfaces del dispositivo. Se realiza finalmente una indagación más profunda de los dispositivos ya optimizados en lo relativo a la estabilidad bajo temperaturas comprendidas entre T = 50-350 ºC. Este estudio pone de manifiesto una alta inestabilidad en el Sb2Se3 a temperaturas tan bajas como 50 ºC reflejada principalmente en la reducción del volumen de la celda unitaria del sistema Sb2Se3 y su correlación con la disminución de los parámetros optoelectrónicos obtenidos.