Limiting factor of performance for solution-phase ligand-exchanged PbS quantum dot solar cell

Abstract

Despite great increases in photovoltaic performance due to the insertion of ligand-exchanged PbS quantum dot layer via 1,2-ethanedithiol (EDT), detailed carrier transport characteristics of solar cells with solution-phase ligand-exchanged PbS quantum dot (QD) still remains unknown. In this paper, relaxation time, recombination resistance, chemical capacitance, carrier lifetime, carrier diffusion length, and defect density corresponding to the depletion width were measured through electrochemical impedance spectroscopy using the Schottky diode model to understand limiting factors for photovoltaic performance. The activation energy, which is estimated from the temperature-dependence of resistance, was reduced from 0.06 eV to 0.02 eV by inserting the PbS-EDT layer, leading to the improved photovoltaic performance. We also evaluated QD solar cells with varying thicknesses of PbS QD layers to investigate the impedance analysis and photovoltaic measurements. The depletion width as well as the carrier diffusion length was larger than the physical thickness of the layer, and this indicates that an increased short-circuit current density can be realized at the thick PbS QD layer due to the resulting large absorbance. Furthermore, the reduced trap density of the QD layer improves the open circuit voltage and fill factor. As a result, a high photoconversion efficiency of 9.41% was achieved.