Organic-inorganic halide perovskite is believed to be a potential candidate for high efficiency solar cells because power conversion efficiency (PCE) was certified to be more than 22%. Nevertheless, mismatch of PCE due to current density (J)-voltage (V) hysteresis in perovskite solar cells is an obstacle to overcome. There has been much lively debate on the origin of J-V hysteresis; however, effective methodology to solve the hysteric problem has not been developed. Here we report a universal approach for hysteresis-free perovskite solar cells via defect engineering. A severe hysteresis observed from the normal mesoscopic structure employing TiO and spiro-MeOTAD is almost removed or does not exist upon doping the pure perovskites, CHNHPbI and HC(NH)PbI, and the mixed cation/anion perovskites, FAMAPbIBr and FAMACsPbIBr, with potassium iodide. Substantial reductions in low-frequency capacitance and bulk trap density are measured from the KI-doped perovskite, which is indicative of trap-hysteresis correlation. A series of experiments with alkali metal iodides of LiI, NaI, KI, RbI and CsI reveals that potassium ion is the right element for hysteresis-free perovskite. Theoretical studies suggest that the atomistic origin of the hysteresis of perovskite solar cells is not the migration of iodide vacancy but results from the formation of iodide Frenkel defect. Potassium ion is able to prevent the formation of Frenkel defect since K energetically prefers the interstitial site. A complete removal of hysteresis is more pronounced at mixed perovskite system as compared to pure perovskites, which is explained by lower formation energy of K interstitial (-0.65 V for CHNHPbI vs -1.17 V for mixed perovskite). The developed KI doping methodology is universally adapted for hysteresis-free perovskite regardless of perovskite composition and device structure.

device configuration usually adopting mesoporous TiO 2 scaffold for the purpose of increasing the contact area between electron transporting material (ETM) and perovskite materials. [1a,d,h] However, high-temperature sintering process (usually over 500 °C) required for preparing mesoporous TiO 2 films complicates device fabrication and increases energy consumption and thus device cost, which is incompatible with the fabrication of flexible PSCs. In order to overcome the above issues, low-temperature normal [1b,4] and inverted [5] planar PSCs were developed considering the long carrier diffusion length of commonly utilized perovskite compositions. [6] For low-temperature normal planar PSCs, developing low-temperature highquality ETMs are crucial to realize high PCE. Based on this consideration, several effective ETMs have been attempted and optimized in planar PSCs, such as TiO 2 , [1b,7] ZnO, [8] SnO 2 , [9] PCBM, [10] and so on. Among them, SnO 2 ETM as a promising alternative to TiO 2 possesses several appealing advantages, including wide optical bandgap (3.6-4.0 eV) beneficial for protecting UV-degradation, high bulk electron mobility (up to 240 cm 2 V −1 s −1 ), good band alignment with perovskites, low-temperature processability, and excellent chemical stability. [9] Consequently, SnO 2 ETM shows huge potentials in simultaneously achieving efficient and stable planar PSCs. To date, the PCEs over 21% have been reported for SnO 2 -planar PSCs based on optimization of SnO 2 film quality, [11] improvement of perovskite film quality, [1f ] and interface engineering. [12] In the past few years, various deposition methods have been developed to prepare high-quality SnO 2 film, such as solution process deposition, [1f,13] atomic layer deposition (ALD), [14] chemical bath deposition (CBD), [14,15] electrochemical deposition, [16] pulsed laser deposition (PLD), [17] etc. Particularly, solution process deposition from commercial SnO 2 nanoparticle colloidal dispersion solution attracts extensive attention because of high PCE and simple fabrication procedure. [1f,11,18] Most recently, a certified PCE up to 23.3% was reported based on SnO 2 -planar PSCs adopting commercial SnO 2 nanoparticle as ETM. [19] Although great progress has been made on SnO 2 -planar PSCs, the presently achieved PCE (over 23%) is still far from Chemical interaction at a heterojunction interface induced by an appropriate chemical linker is of crucial importance for high efficiency, hysteresis-less, and stable perovskite solar cells (PSCs). Effective interface engineering in PSCs is reported via a multifunctional chemical linker of 4-imidazoleacetic acid hydrochloride (ImAcHCl) that can provide a chemical bridge between SnO 2 and perovskite through an ester bond with SnO 2 via esterification reaction and an electrostatic interaction with perovskite via imidazolium cation in ImAcHCl and iodide anion in perovskite. In addition, the chloride anion in ImAcHCl plays a role in the improvement of crystallinity of perovskite film crystallinity. The...

Anomalous current-voltage (J-V) hysteresis in perovskite (PSK) solar cell is open to dispute, where hysteresis is argued to be due to electrode polarization, dipolar polarization, and/or native defects. However, a correlation between those factors and J-V hysteresis is hard to be directly evaluated because they usually coexist and are significantly varied depending on morphology and crystallinity of the PSK layer, selective contacts, and device architecture. In this study, without changing morphology and crystallinity of PSK layer in a planar heterojunction structure employing FACsPbI, a correlation between J-V hysteresis and trap density is directly evaluated by means of thermally induced PbI regulating trap density. Increase in thermal annealing time at a given temperature of 150 °C induces growth of PbI on the PSK grain surface, which results in significant reduction of nonradiative recombination. Hysteresis index is reduced from 0.384 to 0.146 as the annealing time is increased from 5 to 100 min due to decrease in the amplitude of trap-mediated recombination. Reduction of hysteresis by minimizing trap density via controlling thermal annealing time leads to the stabilized PCE of 18.84% from the normal planar structured FACsPbI PSK solar cell.

Fabrication of high-quality perovskite films with a large grain size and fewer defects is always crucial to achieve efficient and stable perovskite solar cells (PSCs).

The conventional precursor mixture based on highly pure and expensive PbI2 (purity >99.99%) is problematic in commercializing perovskite solar cells (PSCs) because of a large deviation in the batch-to-batch photovoltaic performance due to underlying nonstoichiometry and/or the nonperovskite phase. Here, we report on reproducibly efficient (>21%) PSCs based on the ambient-temperature-stable δ-phase FAPbI3 powder, synthesized by reacting PbI2 (either homemade or low-grade commercial product with purity <99%) with formamidinium iodide (FAI) at room temperature (∼93% yield). X-ray diffraction patterns confirm that annealing the spin-coated film leads to phase purity of cubic α FAPbI3. As compared to the conventional precursor mixture based on PbI2 (purity = 99.9985%), significantly reducing defects in the powder-based film is responsible for the stable and reproducible efficiency. In addition, the homogeneous light-intensity-dependent surface photocurrent contributes to the reproducibility of photovoltaic performance. The quasi-steady-state power conversion efficiency of 21.07% is certified for the device based on FAPbI3 powder.