Graphene Quantum dots (GQDs) are used as a surface-enhanced Raman substrate for detecting target molecules with large specific surface areas and more accessible edges to enhance the signal of target molecules. The electrochemical process is used to synthesize GQDs in the solution-based process from which the SERS signals were obtained from GQDs Raman spectra. In this work, GQDs were grown via the electrochemical process with citric acid and potassium chloride (KCl) electrolyte solution to obtain GQDs in a colloidal solution-based format. Then, GQDs were characterized by transmission electron microscope (TEM), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy, respectively. From the results, SERS signals had observed via GQDs spectra through the Raman spectra at D (1326 cm⁻¹) and G (1584 cm⁻¹), in which D intensity is defined as the presence of defects on GQDs and G is the sp² orbital of carbon signal. The increasing concentration of KCl in the electrolyte solution for 0.15M to 0.60M demonstrated the increment of Raman intensity at the D peak of GQDs up to 100 over the D peak of graphite. This result reveals the potential feasibility of GQDs as SERS applications compared to graphite signals.

In this work, a multi-electron transporting layer (ETL) for efficient perovskite solar cells is investigated. The multi-ETL consists of five conditions including SnO₂, SnO₂/SnO_x, TiO₂, TiO₂/SnO₂, and TiO₂/SnO₂/SnO_x. The best performance of PSC devices is found in the SnO₂/SnO_x double-layer and exhibits a power conversion efficiency equal to 18.39% higher than the device with a TiO₂ single-layer of 14.57%. This enhancement in efficiency can be attributed to a decrease in charge transport resistance (R_ct) and an increase in charge recombination resistance (R_rec). In addition, R_ct and R_rec can be used to explain the comparable power conversion efficiency (PCE) between a PSC with a SnO₂/SnO_x double-layer and a PSC with a triple-layer, which is due to the compensation effect of R_ct and R_rec parameters. Therefore, R_ct and R_rec are good parameters to explain the efficiency enhancement in PSC. Thus, the R_ct and R_rec from the electrochemical impedance spectroscopy (EIS) technique is an easy and alternative way to obtain information to understand and characterize the multi-ETL on PSC.

In this work, interface modification of SnO ₂ layer using p–n junction double layer is investigated for the efficiency enhancement of perovskite solar cell (PSC). For the double layer, a Sn additive layer was applied on a SnO ₂ layer by using the DC magnetron sputtering technique at various deposition times. The highest power conversion efficiency of 15.11% is obtained for PSC with a SnO _x additive layer at 5 s sputtering time, compared to 12.89% for the best PSC without the additive layer. The effect of the SnO _x additive layer on PSCs at optimum sputtering time is further explored via the photoconversion properties of both optical and electrical properties. From the results, it is found that the SnO _x additive layer is essential for efficiency enhancement by forming the p-n junction with a SnO ₂ electron transporting layer (ETL) and modifying the interface between the ETL and the perovskite layer. The p-n junction of the ETL is observed via the diode-like behavior of I – V characteristics. The interface modification can enhance the PSC efficiency by improving the quality of the perovskite layer due to the larger grain size and higher absorbance, and by improving the charge transfer. The faster photogenerated charge transfer is confirmed by lower PL intensity and the shorter charge transfer lifetime is confirmed by the fitted open-circuit voltage decay. In addition, the SnO _x additive layer can also eliminate the hysteresis effect of PSCs. This interface modification technique for PSC efficiency enhancement could be further explored for other ETLs.