Unveiling the Effects of Hydrolysis‐Derived DMAI/DMAPbIx Intermediate Compound on the Performance of CsPbI3 Solar Cells

Abstract Introducing hydroiodic acid (HI) as a hydrolysis‐derived precursor of the intermediate compounds has become an increasingly important issue for fabricating high quality and stable CsPbI3 perovskite solar cells (PSCs). However, the materials composition of the intermediate compounds and their effects on the device performance remain unclear. Here, a series of high‐quality intermediate compounds are prepared and it is shown that they consist of DMAI/DMAPbIx. Further characterization of the products show that the main component of this system is still CsPbI3. Most of the dimethylammonium (DMA+) organic component is lost during annealing. Only an ultrasmall amount of DMA+ is doped into the CsPbI3 and its structure is stabilized. Meanwhile, excessive DMA+ forms Lewis acid–base adducts and interactions with Pb2+ on the CsPbI3 surface. This process passivates the CsPbI3 film and decreases the recombination rate. Finally, CsPbI3 film is fabricated with high crystalline, uniform morphology, and excellent stability. Its corresponding PSC exhibits stable property and improved power conversion efficiency (PCE) up to 17.3%.

Precursor solution preparation: The syn-PbI2 was prepared by using PbI2 (1.2 g) dissolved in 2.5 mL DMF at 80 °C under active stirring for 30 min in air atmosphere. Immediately after, different volumes of HI (2.5 mL, 5 mL, 7.5 mL, 10 mL, 12.5 mL) were added to the solution for 2 h and dry overnight using a vacuum oven. Finally, 0.7022 g syn-PbI2, 0.1468 g PbBr2 and 0.3638 g CsI were added to the solution of 2 mL DMF and DMSO (9:1) under active stirring for 1 h. The CsPbI3 precursor solutions were prepared. For comparison, we prepared the same ratio of solutions using PbI2 purchased directly.

Device Fabrication Section
Preparation of TiO2-blocking layer: FTO-coated glass was washed and O2-plasmatreated for 15 min before the deposition of TiO2. The clean substrates were immersed in a 40 mM TiCl4 aqueous solution for 1h at 70 °C and washed with distilled water and ethanol, followed by annealing at 200 °C for 30 min in air to form a compact TiO2 layer.
Growth of the CsPbI3 film: The CsPbI3 layer was fabricated via one-step spin-coating. The mixed solution was spin-coated on top of O2-plasma-treated TiO2 substrate at 1000 rpm for 10 s. The speed was then increased to 3500 rpm and maintained 40 s. Finally, the films were annealed at 180 °C for 15mim to form the films.
Assembly of the solar cells: An HTL film was prepared by spin-coating the HTL solution onto the CsPbI3 film at 5000 rpm for 30 s. A 70-nm-thick gold electrode was then thermally evaporated onto the HTL-coated film.

Characterization Section
SEM and AFM: The film surface morphology, elemental distribution, and crosssectional images were characterized by field emission scanning electron microscope (FESEM, Jeol SU-8020). Atomic force microscope (AFM, Bruker Dimension 5000 Scanning Probe Microscope) was used to measure surface roughness of the perovskite film in "tapping" mode.
Absorbance and PL: Absorbance spectra were collected using a Shimadzu UV-3600 double beam spectrometer using the slowest scanning rate with one-second integration and a 2 nm slit width. The PL spectra were measured using a PicoQuant FluoTime 300.
The source light was a xenon short arc lamp. PL (excitation at 532 nm) spectra were measured using a FLS980 spectrometer (Edinburgh Instruments Ltd) XRD, FTIR, NMR and XPS: XRD patterns of the samples were obtained using a Bruker D8 GADDS Diffractometer with the Cu Kα line. FTIR spectra were measured with a Bruker Vertex 70. NMR was performed by using JNM-ECZ400S/L1 with a frequency of 400 MHz and deuterated DMSO was used as solvent. The XPS measurements were performed in a VG ESCALAB MK2 system with monochromatized Al Kα radiation at a pressure of 5.0 × 10 -7 Pa. XPS was carried out by using a photoelectron spectrometer (ESCALAB250Xi, Thermo Fisher Scientifc).

J-V and EQE:
The active area of solar cell was restrained by a 9 mm 2 metal mask. The J-V characteristics of the PSCs were collected by using a digital source meter (Keithley Model 2400) under an illumination of an AM 1.5 solar simulator (100 mW cm -2 , SS-F5-3A, Enlitech), as calibrated by a NREL-traceable KG5-fltered silicon reference cell. This used reverse scan mode (from VOC to ISC) and forward scan mode (from ISC to VOC) with a scan rate of 30 mV/s. The external quantum efficiency (EQE) data were obtained by solar-cell spectral-response measurement system (QE-R3011, Enlitech). The monochromatic light intensity for EQE was calibrated using a reference silicon detector.
Long-term stability analysis: The steady photocurrent and PCE were measured at the maximum power point. The long-term stability was measured after storage in air (relative humidity of 30%-40% at 25 °C) over 7 days without any encapsulation. To test the device heat stability, we put the unencapsulated device in a nitrogen glovebox with 80 °C for 30 days.           Table S1. TRPL spectroscopy fitting parameters of the syn-CsPbI3 films (extracted from Figure S10). Figure S11. (a) XPS spectra of syn-CsPbI3 film before and after etching 300 s; (b) C 1s; (c) N 1s. Table S2. The measured elements value of the XPS spectra of syn-CsPbI3 film before and after etching 300 s (extracted from Figure S11a). Figure S12. Photovoltaic Performance of Syn-CsPbI3 PSCs. (a) J-V curves of 1:1 and 1:4 samples after longer annealing process; (b) J-V curves for 1:4 sample of optimized device and reference device with longer annealing process. Figure S13. Long-term stability of the best-performing device stored: (a) in N2 atmosphere at 80 ℃; (b) in air at relative humidity of 20%-30%.