P3HT:ZnS Based Photovoltaic Devices with Enhanced Performance Assisted by Oxidised Carbon Nanotubes

Abstract

Photovoltaic devices based of P3HT:ZnS bulk heterojunctions currently produce relatively low efficiencies, limiting the scope these devices have for further development. This work aims to produce a novel photoactive layer, by incorporating oxidised carbon nanotubes (CNTs) into a P3HT:ZnS bulk heterojunction, to investigate the potential CNTs in improving these devices. Furthermore, the active layers of these photovoltaic devices were created using a single-source precursor, thereby limiting potential barriers for scaling up this process to industrial scale. It was found that the CNTs that had been only mildly oxidised (LowOx-CNTs) were longer in length compared to the higher oxidised CNTs (HighOx-CNTs). This allows the HighOx-CNTs to disperse more evenly throughout the active layer, due to the increased oxygen functional groups on the surface of the CNTs, although this does reduce their comparative conductivity. Due to the reduced conductivity of the HighOx-CNTs, 5 and 10 wt% of CNTs performed at lower levels than that of the LowOx-CNTs, with the most notable difference at 5 wt% (0.0064 % and 0.0096 %, respectively). However, LowOx-CNT devices resulted in lower fill factors (FFs), likely due to increased recombination, which can be linked to the increase path length from the increased size of LowOx-CNTs. As the load of the CNTs increased, the FFs for both devices decreased and the Jsc values began to plateau at roughly 0.15 mA/cm2. As the Jsc plateaus, the beneficial conductive properties of LowOx-CNTs was overcome by the detrimental recombination mechanisms introduced by the longer path length. This caused the HighOx-CNTs to outperform the LowOx-CNTs after 15 wt% doping with a power conversion efficiency of 0.0122 %. This increases the efficiency of the undoped ZnS device by more than 7 times, which can be further increased with the incorporation of electron and hole transporting layers.