Development of low-temperature solution-processed colloidal quantum dot-based solar cells

Abstract

Solution-processed solar cells incorporating organic semiconductors and inorganic colloidal quantum dots (QDs) are potential alternatives to conventional solar cells fabricated via vacuum or high-temperature sintering processes for large-area, high-throughput, and low-cost manufacturing. In this work, we explored two types of solution-processed QD-based solar cells: all-inorganic solar cells and organic/inorganic hybrid solar cells. In the all-inorganic device, three QD deposition techniques (spin coating, dip coating, and spray coating) were first experimented in order to prepare high-quality QD thin film for the photovoltaic application. The device was based on the heterojunction formed between dip-coated PbS QD layers and CdS thin film that was deposited via a solution process at 80°'C. The resultant device, employing a 1,2-ethanedithiol ligand exchange scheme, exhibits comparable power conversion efficiency (3.5%) to that of high-temperature (260°C) sintered or sputtered ZnO/PbS (PbSe) QD devices. The initial device yield issue associated with the pinhole formation was addressed, and a procedure for the fabrication of reproducible devices was formulated. The demonstration of this device is a step towards low-cost solar cell manufacturing. Through a combination of thickness-dependent current density-voltage characteristics, optical modeling, and capacitance measurements, the combined diffusion length and depletion width in the PbS QD layer is found to be approximately 170 nm. In the organic/inorganic hybrid device, poly(3-hexylthiophene-2,5-diyl) (P3HT) nanofibers and CdS QDs were employed as electron donors and acceptors, respectively. Crystalline P3HT nanofibers, grown from amorphous P3HT solution, were blended with CdS QDs to form bulk heterojunctions. By adding a large quantity of poor solvent to the blended solution, we demonstrated preferential decoration of CdS QDs onto P3HT nanofibers and stronger interaction between these two materials. The resultant device also showed improved open-circuit voltage, short-circuit current density, and power conversion efficiency.