Low-Temperature Alignment of Conjugated Polymers by Plasticizer-Aided Physical Rubbing.

Rubbing-induced alignment of conjugated polymers is systematically investigated in terms of intra- and inter-molecular interaction. Various polymer films with a broad range of polymer chain rigidity are rubbed, and the degree of polymer chain alignment is quantitatively characterized. The rubbing technique effectively aligns crystalline domains in conjugated polymer films when the temperature approaches the critical rubbing temperature ( T r c $T_{\mathrm{r}}^{\mathrm{c}}$ ), at which the rearrangement and the slip of polymer chains are possible. A polymer with significant intra-/inter-molecular interactions exhibits higher T r c $T_{\mathrm{r}}^{\mathrm{c}}$ , though quantitative analysis reveals an intermediately aligned state at temperature Tr ' lower than T r c $T_{\mathrm{r}}^{\mathrm{c}}$ . This state originates from polymer chain aggregation in an amorphous domain. The intermediately aligned state can be controlled by plasticizer, which enables low-temperature alignment of high-mobility polymer film by reducing Tr ' to near 100 °C, increases the crystallinity, and improves the alignment effect at this state comparable to that of the completely aligned state obtained at extremely high temperatures.

Surface Stabilization of a Formamidinium Perovskite Solar Cell Using Quaternary Ammonium Salt.

Dimensionality engineering is an effective approach to improve the stability and power conversion efficiency (PCE) of perovskite solar cells (PSCs). A two-dimensional (2D) perovskite assembled from bulky organic cations to cover the surface of three-dimensional (3D) perovskite can repel ambient moisture and suppress ion migration across the perovskite film. This work demonstrates how the thermal stability of the bulky organic cation of a 2D perovskite affects the crystallinity of the perovskite and the optoelectrical properties of perovskite solar cells. Structural analysis of (FAPbI3)0.95(MAPbBr3)0.05 (FA = formamidinium ion, MA = methylammonium ion) mixed with a series of bulky cations shows a clear correlation between the structure of the bulky cations and the formation of surface defects in the resultant perovskite films. An organic cation with primary ammonium structure is vulnerable to a deprotonation reaction under typical perovskite-film processing conditions. Decomposition of the bulky cations results in structural defects such as iodide vacancies and metallic lead clusters at the surface of the perovskite film; these defects lead to a nonradiative recombination loss of charge carriers and to severe ion migration during operation of the device. In contrast, a bulky organic cation with a quaternary ammonium structure exhibits superior thermal stability and results in substantially fewer structural defects at the surface of the perovskite film. As a result, the corresponding PSC exhibits the PCE of 21.6% in a reverse current-voltage scan and a stabilized PCE of 20.1% with an excellent lifetime exceeding 1000 h for the encapsulated device under continuous illumination.