From the EIS results, it can be seen that the CdS QDSSC with Cu2S as CE has the lowest series resistance, R S. This is reasonable considering the highly conductive brass metal involved in comparison to the usual FTO layer used. R S is the resistance corresponding to the transport resistance of the conducting substrate. In this study, charge-transfer resistance at the QD-sensitized TiO2/electrolyte interface (R r) is not discussed as the value is not directly influenced by the choice of counter electrode materials. Under dark condition, the charge-transfer resistance at the CE/electrolyte interface, R CE is high

in all the cells. When the cells were tested under find more illumination, the R CE value reduced substantially for most of the cells due to more charge transfer taking place in the system. It is observed that the low R CE gives rise to higher open-circuit voltage of the cell as seen in the case of QDSSCs with carbon soot and platinum as their CEs. However, this is not the case for Cu2S as its photocurrent density PF477736 in vivo is few times lower than that of the cell with platinum as CE. The low R CE could be due to the excessive potential bias applied (0.45 V) to the cell as its open-circuit voltage is only 0.28 V. This high potential bias could have provided a more conductive state for the charge transfer. The overall low performance of the cell could be attributed to the low catalytic activity

at the Cu2S/electrolyte interface which implies a slow reduction rate for polysulfide S x 2- 3-mercaptopyruvate sulfurtransferase species. For the high-efficiency CdS QDSSCs having platinum, graphite or carbon soot as CEs, the good performance is due to low constant phase element (CPE) values. This translates to low true capacitance at the CE/electrolyte interface which could imply a better electrocatalytic activity. EIS results for the CdSe QDSSCs are shown in Figure 4 with the corresponding reference data under dark condition depicted in Figure 4a,b. The related series and charge-transfer resistances are tabulated in Table 4. Like in the case of the CdS QDSSC, low R S

is observed in the cell with Cu2S as the CE. In high-performing cells where platinum and Cu2S are the CEs, the observed low R CE values coupled with low CPE impedance values lead to high catalytic activity at the CE/electrolyte interface. On the other hand, cells with CE from carbon-based materials show high CPE values which result in slower charge transfer through the interface. However, as an exception, R CE for cell with carbon soot as the CE appears to be low due to the lower open-circuit voltage compared to the applied potential bias. The R CE could be even higher should the applied potential bias is equal to the open-circuit voltage. Contrary to general observation, the cell with RGO as the CE has a lower R CE in dark than the value obtained under illuminated condition.