Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
Surface and interface engineering of lead sulphide colloidal quantum dot for better photovoltaic performance The performance of lead sulphide (PbS) colloidal quantum dot (CQD) solar cells is currently limited by low open circuit voltages (Voc) and fill factors (FF). In this study, we address both these issues by focusing on two aspects of materials chemistry- the surface passivation of CQDs and the interface material between them and the electrodes. First, we used cation-exchange method to produce PbS/CdS core/shell CQDs from PbS starting cores and then show that passivation by the CdS shell is more effective than traditional organic ligands. However, the energetic barrier imposed by the shell limits charge extraction and therefore, we develop a strategy that synergistically combines optimizing shell thickness and using appropriate surface ligands to maximise extraction while maintaining surface passivation. This avoids compromising short-circuit current while attaining a Voc of 0.7 V for PbS/CdS devices with a 1.3eV bandgap, an improvement of 0.3V over control devices of the same bandgap. In our second approach, we develop a three-step, solution-process compliant and low temperature procedure to fabricate a device with a FF of 0.72 which is air stable. A solution-phase ligand exchange procedure allows PbS CQDs to be capped with a shorter and yet protective iodine ligand and dispersed in butylamine at high concentrations, which enables direct deposition via spin coating to form the active layer. We utilize in combination colloidal zinc oxide and an organic semiconductor to act as electron and hole transport materials respectively. We show that the addition of the hole transport material gives an improvement in short-circuit current but with poor FF. However, upon storage of device in air, FF improves and surpasses that of control device without the hole transporter. We hypothesize that oxidation influences the hole transporter’s carrier properties and ultimately makes the device air stable.
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