Characterization of Sb2s3 Based Extremely Thin Absorber Solar Cells Employing Tio2- nb Compact Layer

Abstract

An extremely thin absorber (ETA) solar cell of the type FTO/TiO2/Sb2S3/P3HT/Ag has been studied by theoretical method using software simulation as well as by experimental method with an inclusion of a porous layer of TiO2. The initial values for specific layer parameters in theoretical method were obtained from literature to mimic the best Sb2S3 ETA solar cell that has been reported. The results obtained from simulation showed that the performance of the solar cell can be enhanced by optimizing Sb2S3 thin film thickness, doping concentration (1.0×1017𝑐𝑚−3 for TiO2 and 3.0×1016𝑐𝑚−3 for Sb2S3), defect density of Sb2S3 absorber layer (1.0×1015𝑐𝑚−3) and also the electron affinity of the TiO2 layer (4.26 eV). The results of the final simulated Sb2S3 solar cell obtained using the attained optimum parameter values, were as follows: Voc of 750 mV, Jsc of 15.23 mA/cm2, FF of 73.55% and efficiency of 8.41%. These results show that Sb2S3 as an absorber layer can yield efficient solar cells. The TiO2:Nb electron transport layers were prepared by mixing 5 and 10 at% of Nb with TiO2 and deposited by spin coating. The doped thin films were found to have no structural change in reference to the undoped thin films as determined from the analysis of GIXRD spectra. SEM images show thin pin hole free layers of the cp-TiO2:Nb on FTO crystals that are agglomerates of particles. XPS survey scan spectra of the 50 nm thin films, as measured by Dektak profilometer, showed the presence of Niobium ions for the doped samples and their work function tends to increase with increase in dopant concentration which is attributed to change in the chemical composition. Analysis of the current density – voltage curves of the solar cells with the structure FTO/cp-TiO2:Nb/mp-TiO2/Sb2S3/P3HT/MoOx/Ag, showed increased short – circuit current, fill factor and efficiency of the solar cell from 1.3% to 1.7%. The enhancement in performance of this device is attributed to substitution of Ti ions with Nb ions in the TiO2 resulting in a change in the band alignment of the solar cells with Nb content caused by increased work function for Nb doped TiO2 thin films. This also results in increase in charge recombination resistance in the Sb2S3 layer as determined from the analysis of the impedance spectroscopy measurements. Capacitance – Voltage measurements showed a drop in the built-in voltage for solar cells with Niobium doped TiO2 compact layer as a result of decrease in barrier potential attributed to an interface that is poorly permeable to charge carriers between TiO2:Nb and Sb2S3 during chemical bath deposition of Sb2S3