Effect of multi-armed triphenylamine-based hole transporting materials for high performance perovskite solar cells

The performance of planar perovskite solar cells was enhanced by using hole transporting materials containing triphenylamine groups with a multi-armed structure.

The chemical shifts of NMR data were reported on the basis of tetramethylsilane (TMS, 1 H-NMR), CDCl 3 or CD 2 Cl 2 ( 13 C-NMR) and expressed as parts per million (ppm). 2 The splitting patterns of 1 H-NMR were shown as singlet (s), doublet (d), doublet of doublet (dd), triplet (t), quartet (q) and multiplet (m). To double-check the final products, matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was performed with trans-2- [3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) as a matrix. The ultra-violet/visible (UV/vis) absorption spectra of the hole transporting materials (HTMs) were obtained using a PerkinElmer Lambada 35 UV/vis spectrometer at room temperature. The photoluminescence (PL) spectra of HTMs were recorded using a Fluorolog3 spectrofluorometer system of HORIBA scientific. For the optical measurements, the solution samples were prepared by dissolving the HTMs in toluene and the films were prepared by spin coating on a pre-cleaned glass substrate. The differential scanning calorimetry (DSC) data of HTMs were recorded using Q20 system of TA instrument in the temperature range of 30 o C to 270 o C at the heating and cooling rate of 10 o C/min. Photoelectron spectroscopy in air (PESA) data were obtained using Riken Keiki AC-2 surface analyzer with a 10 nW of UV source. The samples for PESA measurements were spin-coated on the glass substrate as pristine HTMs or doped HTMs. Doped HTMs films were prepared with the same condition for the HTMs layer in solar cell devices. The crude products were purified by silica gel column chromatography using ethyl acetate:hexane=1:9 to obtain the title compound. (

Reorganization energy calculation
The reorganization energy( ) was calculated by the following equation: Here, is the energy of the cation with the optimized structure of the neutral molecule In the equation, ε 0 is the vacuum permittivity, ε r is the dielectric constant of HTMs, μ h is the hole mobility, V is the voltage drop across the device, and L is the thickness of the HTM layer; the thickness of the HTM layers were averaged of over 5 times measurements using an alpha-step. The thicknesses were 200 nm for Di-TPA and Tri-TPA, and 137 nm for Tetra-TPA.
The internal voltage in the device (V) was defined as V = V appl -V r -V bi , where V appl is the applied voltage to the device, V r is the voltage drop due to constant resistance and series resistance across the electrodes, V bi is the built-in voltage due to the difference in work functions of the two electrodes. 3,4 The dielectric constant (ε r ) of organic molecules are typically assumed to be 3 and the value was used in this analysis. The current density vs.
voltage (J-V) characteristics were obtained at a dark condition using a Keithley model 2400 source measuring unit. For comparsion, hole-only devices of the HTMs without the addtive were also fabricated using the same condition. The thicknesses were 90 nm for Di-TPA and 200 nm for Tri-TPA and Tetra-TPA. The calculated mobilities are 2.1 × 10 -5 cm 2 /Vs, 5.0 × 10 -5 cm 2 /Vs, and 6.9 × 10 -5 cm 2 /Vs for Di-TPA, Tri-TPA, and Tetra-TPA, respectively.

Conductive atomic force microscopy (c-AFM) measurements
The samples for c-AFM were prepared on a pre-cleaned ITO by spin-casting. The thickness of the HTMs films were carefully controlled to obtain same by adjusting the concentration of HTMs. And the ratios of the HTMs and additives (toluene and acetonitrile) were same as those of the HTM layers for the best solar cells. The average thicknesses were obtained by 7 times measurements of the AFM non-contact line profile mode with scratched HTM layer and the values were 90 nm for three HTMs. The c-AFM images for HTMs were obtained using XE-100 AFM of Park systems inc. with contact mode. All images were obtained using a single Pt-Ir coated cantilever (PPP-CONTSCPt, Nanosensors TM ) by the same measurement conditions. The external current amplifier (DLPCA-200, FEMTO ® ) was employed and the transimpedance gain was 10 9 V/A.