Band-Gap Tunable Colloidal Perovskite Nanocrystals Functionalized with Formamidinium Cation for Photodetector Application

Abstract

Colloidal quantum dots of lead halide perovskites (PQDs) have drawn much attention in the field of various optoelectronic applications such as solar cells, lasers and light emitting diodes (LEDs) due to their size- and composition-dependent optical bandgaps (Eg), high absorption coefficient, defect tolerance band structure and narrow band emission. Various PQDs such as CsPbI3, FAPbI3, and Cs1-xFAxPbI3 have been largely used as a photovoltaic absorber in room-temperature and solution-processed thin film solar cells because of the use of pre-crystallized PQDs while fabricating the PQD thin films, which lead to the absence of thermal an-nealing process normally used in conventional perovskite thin film approach. For analysing the charge transport properties, the PQDs could be employed on photodetectors. Because of low trap density, long carrier life time and diffusion length, PQDs are an ideal material to improve the performance of the photodetectors. PQDs are used a semiconducting material in the photodetector which absorbs the incident photon and gener-ate the electron-hole pair. Photocurrent is generated as result of extraction of the charge carries with the ap-plication of external or build-in electric field. In particular, transistor-type photodetector is used, as the pho-toresponsivity of the material increases without the loss of response speed. During the operation of the device, the high mobility of the material is obtained through the accumulation of charge carriers across the channel. Also, due to the reduced recombination rate of the photogenerated electron-hole pair, the carrier life time is increased.

In this work, we introduce a strategy to improve the inherent charge transporting of the PQDs thin film by selective removal of anionic oleates and cationic oleylammonium. First, to study on the optical properties of the PQDs, CsPbI3 and FAPbI3 are synThesesed through hot injection method. The A-site interplay between the CsPbI3 and FAPbI3 leads to the formation of Cs1-xFAxPbI3 through controlled cation exchange reaction. Also, since the amount of ligands bound to each PQDs varies after the synTheses, the removal and replacement of the surface passivating long chain ligands with a short chain ligand through ligand exchange mechanism im-proves the coupling between the quantum dots. Furthermore, to study the inherent charge transport mecha-nism, incorporating the PQDs in transistor-type photodetector lead to many significant observations.