Emerging solution-processed materials as major resources of next generation solar cells include organic molecules, colloidal quantum dots (CQD), and perovskites. All materials have been actively studied due to light, inexpensive, and flexible/stretchable properties. Moreover, hybrid structures of those materials have been explored such as perovskite:CQD, CQD:organic, perovskite:organic to take advantages of both materials and thereby improve the device efficiency.
Among them, CQD:organic hybrid structure has been interested due to their complementary physical properties. In particular, CQD has an advantage to tune their bandgap across visible to infrared; however, it always suffers from the low extinction coefficient near the excitonic peak. On the other hands, organic molecules show strong and distinctive absorption features but their absorption window is narrow. Therefore, many previous works have studied to fabricate efficient CQD:organic hybrid structures for various optoelectronic applications such as solar cell, photodetector and light emitting diode (LED).
The major challenge of CQD:organic hybrids is to build efficient bulk-heterojunction structure via solution process due to the low exciton diffusion length of organics; However, the methods to prepare stable CQD:organic hybrid ink for solution-process is not proposed yet. Meanwhile, previous researches showed various alternative methods to fabricate hybrid structures for efficient solar cell such as stacked-structure or mixed-structure using post layer-by-layer (LBL) process. However, those strategies still have a limitation to control the morphology of hybrid structures. So far, the best CQD:organic hybrid solar cells showed the power conversion efficiency (PCE) of 13.7 %, lower than those CQD or organic-based solar cells.
Herein, a new method for fabricating CQD:organic hybrid ink, blending of CQD, polymer donor, and small-molecule acceptor as one solar ink, was successfully figured out to build efficient hybrid structures. I carefully exchanged CQD surface using benzoic acid (BA) ligand, and thereby controlled the CQD surface polarity, resulting that the CQD can fully dissolved in weakly polar solvents. This enables to blend with organic molecules and form a stable CQD:organic hybrid ink.
Based on the CQD:organic ink, efficient hybrid solar cells were fabricated using a single step spin-cast under ambient conditions. The hybrid device performed the broadband light absorption due to the three light absorbing materials and the ink-process enabled to reduce the surface roughness, resulting higher electrical properties. Under the AM 1.5 illumination, the power conversion efficiency (PCE) of 15.24% was achieved, which is the best solution-processed CQD:organic hybrid device under the ambient condition to date.