Polyaromatic Anthracene Clenchers on Semiconducting Carbon Nanotubes for Growth and Bridging of Perovskite Crystal Grains in Perovskite Solar Cells
Authors: Jeon Il , Lin Hao-Sheng , Matsuo Yutaka , Maruyama Shigeo 
Published: May 2020

Perovskite solar cells (PSCs) have drawn much attention around the world as a promising energy source owing to high power conversion efficiency (PCE). This comes from the high absorption coefficient, long-range diffusion length, and high defect tolerance of the lead-halide perovskite photoactive materials, all of which can be further improved by increasing the perovskite crystal grain size and passivating the defective boundaries of the crystal grains. To date, polymers have been readily used as the crystal growth template that can homogeneously nucleate and retard the perovskite crystal growth, which as a result increases the crystal grain size. At the same time, they can passivate the grain boundaries by forming Lewis adducts, reducing the shallow trap sites and non-radiative recombination.Previously, we reported that semiconducting single-walled carbon nanotubes (s-SWNTs) can also function as effective crystal growth templates. In addition, they function as charge bridges between the grains owing to their compatible energy bandgap and high conductivity along their tube axis. The seminal point of this work is, in fact, the role of chiral-selective sodium deoxycholate (DOC) surfactants, which is used in producing the s-SWNTs. DOCs form Lewis coordinates with the perovskite precursors to function as the growth templates and the passivators, inducing a crystal size increase and a trap site reduction, respectively. Despite the enhancement of PCE from 18.1% to 19.7% upon addition of the DOC-wrapped s-SWNTs(aq), the PCE can potentially increase further. The fill factor (FF) was limited by the insulating nature of DOC and the limited solubility of the DOC-wrapped s-SWNTs in N,N-dimethylformamide (DMF), the perovskite precursor solvent. Moreover, the open-circuit voltage (V
OC) of the device was restricted by the limited passivation effect of the carboxylic group on DOC. Other functional groups, such as urea and amine, have been reported to exhibit a much greater passivation effect.

Polyaromatic anthracene, which contains a bent polyaromatic group on one end and a functional group on the other end, has been reported to clench SWNTs. While the polyaromatic end induces strong π-π interaction with the SWNTs, the functional end can easily be modified chemically. In our group, the modified polyaromatic anthracenes were used as charge-transporting layers in optoelectronics, harnessing their bandgaps and high mobility arising from the conjugated sp2bonds. Hence, we designed and synthesized 4,6-di(anthracen-9-yl)-1,3-phenylene bis(dimethylcarbamate) (DPB), which is essentially polyaromatic anthracene with two urea-analogues functional groups. Subsequently, we used them as new surfactants for s-SWNTs in the PSC application. DPB successfully attached to s-SWNTs and replaced DOC surfactants as confirmed by photoluminescence (PL) and UV-visible absorption spectroscopy (UV-Vis). DPB-attached s-SWNTs showed much higher solubility in DMF and dimethyl sulfoxide (DMSO) than the DOC-attached s-SWNTs. Accordingly, higher concentration of s-SWNTs could be added to the perovskite precursor solution. The DPB on SWNTs expectedly manifested a stronger interaction on Pb2+ than DOC owing to the strongly electron donating nature of the lone pair electrons on the carbamate group enhanced by its configurational orientation facing outwards. DPB-attached s-SWNT-added perovskite films exhibited excellent charge-selectivity and reduced charge trap density. The suitable molecular energy levels and higher mobility of DPB compared with DOC led to higher device performance. DPB-attached s-SWNTs-added PSCs exhibited a PCE of 20.7%, which is higher than those of DOC-attached s-SWNT-added PSCs (19.7%) and pristine PSCs (18.4%). All of the three photovoltaic parameters, namely, high short-circuit current density (J
SC), V
OC, and FF contributed to the high PCE of the DPB-attached s-SWNTs-added PSCs. The high J
SC came from the large perovskite crystal size of over 500 nm. The high V
OC came from strong passivation effect of the DPB-attached s-SWNTs. The high FF came from the improved carrier mobility and charge selectivity of DPBs on s-SWNTs.

Figure 1