Impedance analysis of PbS colloidal quantum dot solar cells with different ZnO nanowire lengths

Abstract

The photoconversion efficiency of colloidal quantum dot (QD) solar cells has been markedly improved by optimizing the surface passivation and device structure, and details of device physics are now under investigation. In this study, we investigated the resistance and capacitance components at the ZnO/PbS-QD interface and inside a PbS-QD layer by measuring the impedance spectrum while the interface area was controlled by changing the ZnO nanowire length. By evaluating the dependence of optical intensity and DC bias voltage on the ZnO nanowire length, only the capacitance was observed to be influenced by the interface area, and this indicates that photoinduced carriers are generated at the surface of PbS-QD. In addition, since the capacitance is proportional to the surface area of the QD, the interface area can be evaluated from the capacitance. Finally, photovoltaic performance was observed to increase with increasing ZnO nanowire length owing to the large interface area, and this result is in good agreement with the capacitance measurement.

Annealing-Temperature Dependent Carrier-Transportation in ZnO/PbS Quantum Dot Solar Cells Fabricated Using Liquid-Phase Ligand Exchange Methods

“…The external quantum efficiency (EQE) spectra were measured from 300 to 1500 nm using a constant photon flux density of 10 15 photon cm −2 (Bunko-keiki, CEP-2000). Complex impedance spectra of the solar cells were measured in the frequency range from 1 Hz to 8 MHz, and the amplitude of the alternating current voltage was fixed at 10 mV using a potentiostat and a frequency analyzer (Solartron, SI1260) [39]. The energy of the highest occupied quantum level of PbS (E HOQL ) was determined by the onset-photon energy observed in the air using photoelectron spectroscopy (Riken Keiki, AC-3).…”