Jinseck Kim

Dopant-free polymeric hole transport materials for highly efficient and stable perovskite solar cells

We report a dopant-free polymeric hole transport material (HTM) that is based on benzo[1,2-b:4,5:b′]dithiophene and 2,1,3-benzothiadiazole, which results in highly efficient and stable perovskite solar cells (∼17.3% for over 1400 h at 75% humidity). The HTM comprises a random copolymer (RCP), which is characterized using UV-vis absorption spectroscopy, cyclic voltammetry, space-charge-limited current, and grazing-incidence wide-angle X-ray scattering. The RCP-based perovskite solar cell exhibits the highest efficiency (17.3%) in the absence of dopants [lithium bis(trifluoromethanesulfonyl)imide and tert-butylpyridine]. The observed efficiency is attributed to a deep HOMO energy level and high hole mobility. In addition, the long-term stability of the device is dramatically improved by avoiding deliquescent or hygroscopic dopants and by introducing a hydrophobic polymer layer. RCP devices maintain their initial efficiency for over 1400 h at 75% humidity, whereas devices made of HTMs with additives fail after 900 h.

Reducing Trap-Assisted Recombination in Small Organic Molecule-Based Photovoltaics by the Addition of a Conjugated Block Copolymer

The performance of organic photovoltaics (OPVs) based on the small-molecule organic semiconductor p-DTS(FBTTh2 )2 is greatly improved by the addition of a conjugated block copolymer composed of difluoroquinoxaline and thienopyrrolodione blocks (D130). The power conversion efficiency (PCE) of the p-DTS(FBTTh2 )2 -based OPV is improved from 5.08% to 6.75% by the addition of 5 wt% D130 to the photoactive layer, which is composed of p-DTS(FBTTh2 )2 and a fullerene derivative. Current-voltage and grazing incidence wide-angle X-ray scattering analyses revealed that the addition of D130 significantly reduces the trap density of the device and changes the packing orientation of p-DTS(FBTTh2 )2 from mostly edge-on to partially face-on. These changes greatly improve the charge carrier mobility of the OPV, indicating that D130 is highly compatible with p-DTS(FBTTh2 )2 . Furthermore, the addition of D130 improve the photostability of the OPV by reducing the burn-in loss under a light soaking intensity of 1 sun. The D130-based OPV maintained 34% of its initial PCE after a light soaking test for 858 h. In contrast, the PCE of the OPV without D130 reduced to 14% of its initial efficiency in the same time period.