To solve the toxic issue for new-generation photovoltaic applications, tin-based perovskite solar cells are a promising alternative to their lead counterparts, but they suffer from poor stability because of their tendency to exhibit tin oxidation. Herein we report a new sequential method of deposition based on solution processing using hexafluoro-2-propanol as a solvent to deposit eight bulky ammonium cations on top of the 3D layer to form a 3D/quasi-2D layer to protect the tin perovskite grains from penetration by moisture. The formation of the 2D layer was confirmed with grazing-incidence wide-angle X-ray scattering, scanning electron microscopy, conducive atomic force microscopy, photoluminescence, and transient absorption spectroscopy measurements. The anilinium (AN) device showed a remarkable performance with an efficiency of 10.6% and with great stability in ambient air without encapsulation. The AN device also showed a self-healing effect of performance when it was subjected to a severe environment under continuous light soaking in one-sun illumination and thermal stress between 20 and 50 °C for 10 cycles.

Alcohol-based bifunctional ammonium cations, 2-hydroxyethylammonium (HEA+), HO­(CH2)2NH3 +, were introduced into formamidinium (FA+) tin-based perovskites (HEA x FA1–x SnI3; x = 0–1) to absorb light in carbon-based mesoscopic solar cells. We found that HEA+ cations play a key role to control the crystal structures, the lattice structures altered from orthorhombic (x = 0) to rhombohedral (x = 0.2–0.4) with greater symmetry. When x was increased to 0.6–1.0, tin and iodide vacancies were formed to generate 3D-vacant perovskites (HEA x FA1–x Sn0.67I2.33, x ≥ 0.6) with a tetragonal structure. Tin-based perovskites in this series were fabricated into mesoporous solar cells using one-step drop-cast (DC), two-step solvent-extraction (SE), and SE + 3% ethylenediammonium diiodide (EDAI2) as an additive. After optimization of device performance with the SE + 3% EDAI2 approach, the HEA0.4FA0.6SnI3 (HEAI = 40%) device gave the best photovoltaic performance with J SC = 18.52 mA cm–2, V OC = 371 mV, FF = 0.562, and overall efficiency η = 3.9% after the device was stored for a period of 340 h.

This work is the first report on a hole-selective tin-based perovskite solar cell (PSC) using the concept of a self-assembled monolayer (SAM) to modify the ITO surface to fabricate a uniform tin perovskite layer via a two-step sequential deposition method. Herein, we developed a preheating procedure to diminish effectively the amounts of hydroxyl groups and oxygen vacancies on the ITO surface to produce a uniform SAM. The ITO substrate preheated at 400 °C gave the best device performance for an efficiency of power conversion (PCE) reaching 6.5%, and 80% of the initial PCE was maintained in a glovebox for ∼1900 h. Electrochemical impedance spectra and time-resolved spectra were used to understand the interfacial charge recombination and holeextraction kinetics in relation to the observed device performance. The present work thus provides a new direction for the development of SAM-based lead-free perovskite solar cells for their future scaled-up production.

CoO is a promising hole-extracting layer (HEL) for inverted planar perovskite solar cells with device configuration ITO/CoO/CHNHPbI/PCBM/Ag. The devices fabricated according to a simple solution procedure showed the best photovoltaic performance attaining power conversion efficiency (PCE) of 14.5% under AM 1.5 G 1 sun irradiation, which is significantly superior to those of materials fabricated with a traditional HEL such as PEDOT:PSS (12.2%), NiO (10.2%), and CuO (9.4%) under the same experimental conditions. We characterized the chemical compositions with XPS, crystal structures with XRD, and film morphology with SEM/AFM techniques. Photoluminescence (PL) spectra and the corresponding PL decays for perovskite deposited on varied HEL films were recorded to obtain the hole-extracting characteristics, for which the hole-extracting times show the order CoO (2.8 ns) < PEDOT:PSS (17.5 ns) < NiO (22.8 ns) < CuO (208.5 ns), consistent with the trend of their photovoltaic performances. The reproducibility and enduring stability of those devices were examined to show the outstanding long-term stability of the devices made of metal oxide HEL, for which the CoO device retained PCE ≈ 12% for over 1000 h.

Reduced graphene oxides (rGO) are synthesized via reduction of GO with reducing agents as a hole‐extraction layer for high‐performance inverted planar heterojunction perovskite solar cells. The best efficiencies of power conversion (PCE) of these rGO cells exceed 16%, much greater than those made of GO and poly(3,4‐ethenedioxythiophene):poly(styrenesulfonate) films. A flexible rGO device shows PCE 13.8% and maintains 70% of its initial performance over 150 bending cycles. It is found that the hole‐extraction period is much smaller for the GO/methylammonium lead‐iodide perovskite (PSK) film than for the other rGO/PSK films, which contradicts their device performances. Photoluminescence and transient photoelectric decays are measured and control experiments are performed to prove that the reduction of the oxygen‐containing groups in GO significantly decreases the ability of hole extraction from PSK to rGO and also retards the charge recombination at the rGO/PSK interface. When the hole injection from PSK to GO occurs rapidly, hole propagation from GO to the indium‐doped tin oxide (ITO) substrate becomes a bottleneck to overcome, which leads to a rapid charge recombination that decreases the performance of the GO device relative to the rGO device.