Synthesis and Characterization of UV-Selective Oxide-based Transparent Thin Film Solar Cells

Abstract

[eng] The work presented in this thesis book has been carried out during the period 2019-2022 within the Solar Energy Materials and Systems (SEMS) group at the Institut de Recerca en Energia de Catalunya (IREC) located in Sant Adrià del Besòs, Barcelona, Spain. The objective of this thesis was the synthesis and characterization of UV-selective transparent photovoltaic devices focusing on the use of inorganic wide-bandgap oxide-based materials and architectures, with the aim of obtaining high transparency and efficient TPV devices with a high integration potential and with features such as low-cost, stability and earth-abundance. For the consecution of results and development of this novel research field work jump-started from state-ofthe- art device architectures for UV-selective devices, which focused on ZnO absorbers. Subsequently, it was proposed to shift towards materials with a more appropriate bandgap that was best matched with the UV spectral onset in the AM1.5G spectra. The material studied was Zn(O,S) which involved the partial anionic substitution of oxygen by a chalcogen, sulfur in this case. After an initial phase of synthesis and characterization of different compositions lying in the whole range (from oxygen-rich to sulfur-rich) as well as a screening of different materials to work as carrier selective contacts, TPV devices were successfully fabricated for the first time in literature. Device characterization suggested some device limitations that have to be overcome in order to increase the performance of the TPV devices. The devices presented a record efficiency of PCE=0.5% and an average visible transmittance (AVT) of 69%, improving on the state-of-the-art pure oxide based approaches previously reported in scientific literature. Afterwards, a different approach was exploited in order to obtain transparent devices. This approach relied in the use of (lower) wide-bandgap materials, that lie in the visible range. To overcome opaqueness due to unwanted absorption in the visible range the deposition of the thin films were engineered to be ultrathin, having nanometric thickness below 30 nm. For this, a-Si:H nanometric films with a bandgap of 1.7 eV were embedded in pure oxide-based architectures. Similarly to the first approach, the work relied on the optimization of the deposition of ultrathin a-Si:H films and their optical characterization to assess their feasibility as candidates for TPV absorbers, relying on spectrophotometry and photothermal deflection spectroscopy. Different implementations of TPV devices were fabricated, yielding working devices that were able to overcome the bottleneck of PCE=0.5% found so far in state-of-the art pure oxide based approaches. It was shown that devices resulting from this approach presented the possibility of tuning the different optoelectronic parameters by small adjustments of the thickness of the ultrathin absorbers. Thicker films (30 nm) resulted in devices with lower transparency and colour rendering index (CRI) while increasing the PCE of the devices. Contrarily, using thinner films (8 nm) resulted in transparency values comparable to pure oxidebased devices while having a comparatively lower efficiency when compared to the devices made with absorbers of 30 nm. The best devices presented a PCE=2% with an AVT=35% for the devices using a 30 nm a- Si:H absorber embedded in oxide charge transport layers (CTL). Both approaches presented in the thesis were successful and the conclusions presented suggested that there is still room for device optimization by improving the contact selectivity of the charge transport layers that composed the devices.

[spa] El objetivo de esta tesis ha sido la síntesis y caracterización de dispositivos fotovoltaicos transparentes UVselectivos centrándose en el uso de materiales y arquitecturas inorgánicas basadas en óxidos de banda ancha, con el fin de obtener dispositivos TPV de alta transparencia y eficiencia con un alto potencial de integración y con características como bajo coste, estabilidad y abundancia. Para la consecución de los resultados se partió de las arquitecturas más avanzadas, que se centraban en absorbedores de ZnO. Posteriormente, se propuso cambiar hacia materiales con un bandgap que se ajustara mejor al inicio espectral UV. El material estudiado fue el Zn(O,S), que implicaba la sustitución aniónica parcial del oxígeno por un calcógeno, azufre en este caso. Tras una fase inicial de síntesis y caracterización de diferentes composiciones dentro de un amplio rango (desde ricas en oxígeno hasta ricas en azufre), así como un cribado de diferentes materiales para trabajar como contactos selectivos de portadores, se fabricaron con éxito dispositivos TPV por primera vez en la literatura. La caracterización de los dispositivos sugirió algunas limitaciones que deben superarse para aumentar el rendimiento de los dispositivos TPV. Los dispositivos presentaron una eficiencia récord de PCE=0.5% y una transmitancia visible ponderada del 69%, mejorando el estado del arte de las aproximaciones basadas en óxidos . Posteriormente, se adoptó un enfoque diferente para obtener dispositivos transparentes. Este enfoque se basaba en el uso de materiales (inferiores) de banda ancha, que se encuentran en el rango visible. Para superar la opacidad debida a la absorción no deseada en el rango visible, se diseñó el depósito de las películas finas para que fueran ultrafinas, con un grosor nanométrico inferior a 30 nm. Se demostró que los dispositivos resultantes de este enfoque presentaban la posibilidad de sintonizar los diferentes parámetros optoelectrónicos mediante pequeños ajustes del grosor de los absorbedores ultrafinos. Ambos enfoques presentados en la tesis tuvieron éxito y las conclusiones presentadas sugieren que todavía hay margen para la optimización del dispositivo mediante la mejora de la selectividad de contacto de las capas de transporte de carga que componen los dispositivos.