Thin-Film Optoelectronics

Thin-film optoelectronics are an important class of devices for lighting, sensing and energy harvesting. We work with important classes of materials such as colloidal quantum dots and perovskites to advance the understanding and performance of devices.

Computational Design of Surface/Interface of Nano-structure Materials -- Heterostructure engineering at the surface/interface of nano-structure materials provides unique opportunities to control device properties. We reveal the possibility to optimize the key optoelectronic properties within a single-sheet solar cell made of a graphene sheet functionalized into 1D channels. Compared to vertical heterostructure architectures, the single-sheet solar cell shows potential for improved robustness against defects, enhancement of polaron dissociation, extra freedom for functionalization, and coverage of the entire solar spectrum. As another example in the 2D family, nanoplatelets (NPLs) are proposed as materials for novel optoelectronic devices, which present extraordinary robustness against off-stoichiometry as a result of surface homogeneity and sufficient cross-linking. Moreover, the crucial impacts of interfacial structure are illustrated by optimizing the electronic coupling and thus the external stimuli responsive behaviors in organic charge-transfer superstructures.

Colloidal Quantum Dots for Photovoltaics -- The efficiency of PbS based quantum dot (QD) photovoltaics (PV) has continued to rise to a record of 11.3%. Its good stability, scalable solution-based synthesis and low cost make it a promising candidate for emerging solar cell technologies. We have seen much improved understanding of the electronic, optical and transport properties of PbS QDPV and at the device level over the last few years. One major obstacle for further efficiency improvement is the low open circuit voltage (Voc) compared to the theoretical maximum. This is due to two major factors. (1) Stokes shift (2) fast carrier recombination rate due to trap states. We use ab initio calculations to investigate the effects of ligands and QD geometry on the Franck-Condon shift in QD, so as to understand their origin and suggest new design principles to synthesize more efficient quantum dot.