Guan-Woo 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.

Green-Solvent-Processable, Dopant-Free Hole-Transporting Materials for Robust and Efficient Perovskite Solar Cells

In addition to having proper energy levels and high hole mobility (μh) without the use of dopants, hole-transporting materials (HTMs) used in n-i-p-type perovskite solar cells (PSCs) should be processed using green solvents to enable environmentally friendly device fabrication. Although many HTMs have been assessed, due to the limited solubility of HTMs in green solvents, no green-solvent-processable HTM has been reported to date. Here, we report on a green-solvent-processable HTM, an asymmetric D-A polymer (asy-PBTBDT) that exhibits superior solubility even in the green solvent, 2-methylanisole, which is a known food additive. The new HTM is well matched with perovskites in terms of energy levels and attains a high μh (1.13 × 10-3 cm2/(V s)) even without the use of dopants. Using the HTM, we produced robust PSCs with 18.3% efficiency (91% retention after 30 days without encapsulation under 50%-75% relative humidity) without dopants; with dopants (bis(trifluoromethanesulfonyl) imide and tert-butylpyridine, a 20.0% efficiency was achieved. Therefore, it is a first report for a green-solvent-processable hole-transporting polymer, exhibiting the highest efficiencies reported so far for n-i-p devices with and without the dopants.

Enhanced Efficiency and Stability of an Aqueous Lead-Nitrate-Based Organometallic Perovskite Solar Cell

We investigate the stability of an active organometallic perovskite layer prepared from a two-step solution procedure, including spin coating of aqueous lead nitrate (Pb(NO3)2) as a Pb2+ source and sequential dipping into a methylammonium iodide (CH3NH3I) solution. The conversion of CH3NH3PbI3 from a uniform Pb(NO3)2 layer generates PbI2-free and large-grain perovskite crystallites owing to an intermediate ion-exchange reaction step, resulting in improved humidity resistance and, thereby, improved long-term stability with 93% of the initial power conversion efficiency (PCE) after a period of 20 days. The conventional fast-converted PbI2-dimethylformamide solution system leaves small amounts of intrinsic PbI2 residue on the resulting perovskite and MAPbI3 crystallites with uncontrollable sizes. This accelerates the generation of PbI2 and the decomposition of the perovskite layer, resulting in poor stability with less than 60% of the initial PCE after a period of 20 days.

Thermally stable, planar hybrid perovskite solar cells with high efficiency

We report a highly effective interface engineering strategy for thermally stable perovskite solar cells (PSCs) by employing a zwitterion-modified SnO2 electron transport layer (ETL) and a dopant-free hole transport layer (HTL). A zwitterionic compound, 3-(1-pyridinio)-1-propanesulfonate, is used to modify the SnO2 ETL. The zwitterion, which forms interfacial dipoles, plays a few important roles: (1) it causes shifts in the work function of SnO2 resulting in more efficient charge extraction and an increase in the built-in potential. (2) It pulls electrons from perovskite layers to the ETL/perovskite interface, enhancing the electron transport ability. (3) Interfacial dipoles prevent back transfer of electrons from the ETL to the perovskite and suppress charge recombination. (4) Positively charged atoms in the zwitterion passivate Pb–I antisite defects, improving the stability of devices. With these desirable properties, the PSC with doped Spiro-OMeTAD obtained a power conversion efficiency of 21.43%. In addition, the PSC with the dopant-free HTL exhibited a record high efficiency of 20.5% among dopant-free polymeric HTLs using green solvents. The resulting PSCs without encapsulation showed excellent thermal stability. Accordingly, this work suggests that the use of a modified ETL and a dopant-free HTL is a promising strategy to overcome the thermal instability of planar-PSCs (P-PSCs).