In order to explain the observed trend in the photovoltaic performance with the variation in GCNS concentration, we divide our discussion into two parts: one for the increase and the other for the decrease in the efficiency trend. Comparative higher values of JSC in composite photo-anodes can be a consequence of the improved A-443654 transport mechanism  and . Compared to the pure TiO2, the 5 μL GCNS mixed TiO2 composite cell has shown 4.67% in efficiency accompanied by the increase in both JSC as well as VOC. An interesting and significant finding of our study is that the VOC had increased from 0.67 to 0.72 V by adding GCNS in various concentrations (5–20 μL).
Fig. 8. Energy level diagram of a GCNS–TiO2 composite photoanode based dye sensitized solar cell.Figure optionsDownload full-size imageDownload as PowerPoint slide
Now, we will discuss the variation of JSC with different concentrations of GCNS. From Table 1, it can be seen that the value of JSC increased with increase in GCNS concentration (up to 10 μL) and then it decreased with further addition of GCNS. These results suggest that addition of small amount of GCNS in TiO2 is enough to increase the injection as well as electron transfer rate in the cell, where the injection rate can be increased due to the ability of GCNS to improve the dye loading capacity in DSSC. Similar results have been reported in case of other carbon allotropes, such as graphene and CNT , , ,  and . We believe that addition of high concentrations (>10%) of GCNS acts as electron trapping sites centers resulting hindrance in the charge transfer in the dielectric medium (i.e., TiO2).