In Situ Thermal Cross-Linking of 9,9′-Spirobifluorene-Based Hole-Transporting Layer for Perovskite Solar Cells

A novel 9,9′-spirobifluorene derivative bearing thermally cross-linkable vinyl groups (V1382) was developed as a hole-transporting material for perovskite solar cells (PSCs). After thermal cross-linking, a smooth and solvent-resistant three-dimensional (3D) polymeric network is formed such that orthogonal solvents are no longer needed to process subsequent layers. Copolymerizing V1382 with 4,4′-thiobisbenzenethiol (dithiol) lowers the cross-linking temperature to 103 °C via the facile thiol–ene “click” reaction. The effectiveness of the cross-linked V1382/dithiol was demonstrated both as a hole-transporting material in p–i–n and as an interlayer between the perovskite and the hole-transporting layer in n–i–p PSC devices. Both devices exhibit better power conversion efficiencies and operational stability than devices using conventional PTAA or Spiro-OMeTAD hole-transporting materials.

The samples for the ionization potential I p measurement were prepared by dissolving materials in THF and were coated on Al plates pre-coated with ~0.5µm thick methylmethacrylate and methacrylic acid copolymer adhesive layer.The thickness of the HTM layer was 0.5-1 µm.The samples were illuminated with monochromatic light from the quartz monochromator with deuterium lamp.The power of the incident light beam was (2-5)×10 −8 W. The negative voltage of −300 V was supplied to the sample substrate.The counter-electrode with the 4.5 mm×15 mm slit for illumination was placed at 8 mm distance from the sample surface.The counter-electrode was connected to the input of the BK2-16 type electrometer, working in the open input regime, for the photocurrent measurement.The 10 −15 to 10 −12 A strong photocurrent was flowing in the circuit under illumination.The samples for mobility measurements were prepared from the neat material.The sample substrate was glass plates with conductive Al layer.The layer thickness was in the range of 3.5-5 µm.
The hole drift mobility was measured by xerographic time of flight (XTOF) technique.Positive corona charging created electric field inside the HTM layer.Charge carriers were generated at the layer surface by illumination with pulses of N 2 laser (pulse duration was 1 ns, wavelength 337 nm).
Scanning electron microscopy (SEM) was performed with a Hitachi S8010 ultra-high-resolution scanning electron microscope (Hitachi High-Tech Corporation).
For the time-resolved photoluminescene (TRPL) measurements, the samples were excited by a picosecond pulsed light with a wavelength of 688 nm (Advanced Laser Diode System).The excitation fluence was set at 100 nJ cm -2 .The PL signals were recorded using an avalanche photodiode (ID Quantique) and a time-correlated single photon counting board (PicoQuant).The PL lifetimes were obtained by fitting the PL decay curve with a double exponential function and calculating the average lifetime.The PL spectra were recorded using a N 2 cooled charge-coupled-device array equipped with a monochromator (Princeton Instruments).The samples were kept in an Ar-filled metallic box for the whole process to avoid oxygen contamination and degradation.X-ray photoelectron spectroscopy (XPS) was recorded with a JPS-9010 (JEOL Co., Ltd.) instrument.Perovskite film samples for XPS measurements were prepared in a N 2 -filled glove box and transferred to the XPS chamber through a N 2 -filled transfer vessel in order to avoid contamination.
Photocurrent-voltage (J-V) measurements for perovskite solar cells were measured in air with an OTENTO-SUNIII (BUNKOUKEIKI Co., Ltd.).The light intensity of the illumination source was adjusted by using standard silicon photodiodes (BS520).Each device was measured with a 20 mV voltage step and a 200 ms time step (i.e.scan rate of 0.1 V s -1 ) using a Keithley 2400 source meter.
The device active area was defined by an optimal mask (0.1 cm 2 ).Steady-state power output (SPO) measurements were performed by holding the device at the voltage of the maximum power point, as determined by the JV characteristic, and monitoring the current density over the course of 1000 s.

Figure S4 .
Figure S4.XPS spectra of (a) Pb 4f and (b) S 2p of the pristine perovskite and perovskite coated with cross-linked V1382/dithiol.

Figure S11 .J 2 Figure
Figure S11.Cross-sectional SEM images of perovskite films fabricated on cross-linked V1382/dithiol with difference concentrations of V1382.

Figure S14 .
Figure S14.The thermal stability test of the unencapsulated p-i-n PSCs using the cross-linked V1382/dithiol and PTAA as the HTMs at 85 C in air with a controlled humidity of 40%.Average data was taken from three devices for each HTM.

2 )Figure S15 .AuFigure S16 .
Figure S15.Complex impedance plots of p-i-n PSCs under an inert atmosphere at AM 1.5G illumination and 0 V bias.

Figure S24 .
Figure S24.(a) Steady-state PL and (b) time-resolved PL spectra of the pristine perovskite (Cs 0.05 FA 0.80 MA 0.15 PbI 2.75 Br 0.25 ) and perovskite/without or with HTM interlayer/Spiro-OMeTAD films excited at 688 nm with an excitation fluence of 100 nJ cm -2 .The perovskite is probed through the glass side.

Table S2 .
Photovoltaic Parameters of p-i-n PSCs Fabricated on Bare V1382 and on Cross-Linked V1382/dithiol with Difference Concentration of V1382 Derived from J-V Measurements HTMs (V1382:dithiol/1:2 molar ratio) were spin-coated on FTO substrates from PhCl solution. a

Table S3 .
Photovoltaic Parameters of n-i-p PSCs Fabricated without and with Cross-Linked V1382/dithiol Interlayer Derived from J-V Measurements