Ngoc Duy Pham

Enhanced perovskite electronic properties via a modified lead(II) chloride Lewis acid–base adduct and their effect in high-efficiency perovskite solar cells

Methylammonium lead triiodide (MAPbI3) perovskite solar cells have gained significant attention with an impressive certified power conversion efficiency of 22.1%. Suppression of recombination at the interface and grain boundaries is critical to achieve high performance perovskite solar cells (PSCs). Here, we report a simple method to improve the performance of PSCs by incorporating a lead chloride (PbCl2) material into the MAPbI3 perovskite precursor through a Lewis acid–base adduct. The optimal concentration of PbCl2 that helps increase the grain size of MAPbI3 with introduction of the ideal amount secondary phases (lead iodide and methylammonium lead tri-chloride) is 2.5% (molar ratio, relative to lead iodide). Examination by steady-state photoluminescence and time-resolved photoluminescence has shown that devices based on MAPbI3-2.5% of PbCl2 facilitated longer charge carrier lifetime and electron–hole collection efficiency which is ascribed to reduced defects and concurrent improved material crystallinity. Electrochemical impedance spectra (EIS) of the corresponding PSCs have revealed that, compared to the pristine MAPbI3 perovskite film, the 2.5% PbCl2-additive increased the recombination resistance of the PSCs by 2.4-fold. Meanwhile, measurement of the surface potential of the perovskite films has indicated that the PbCl2-additive modifies the electronic properties of the film, shifting the fermi-level of the MAPbI3 film by 90 meV, leading to a more favourable energetic band matching for charge transfer. As a result, the performance of PSCs is enhanced from an average efficiency of 16.5% to an average efficiency of 18.1% with maximum efficiency reaching 19% due to the significantly improved fill-factor (from 0.69 to 0.76), while the hysteresis effect is also suppressed with the PbCl2-additive.

Hindered Formation of Photoinactive δ-FAPbI3 Phase and Hysteresis-Free Mixed-Cation Planar Heterojunction Perovskite Solar Cells with Enhanced Efficiency via Potassium Incorporation

Organic-inorganic hybrid lead halide perovskite solar cells have demonstrated competitive power conversion efficiency over 22%; nevertheless, critical issues such as unsatisfactory device stability, serious current-voltage hysteresis, and formation of photo nonactive perovskite phases are obstacles for commercialization of this photovoltaics technology. Herein we report a facial yet effective method to hinder formation of photoinactive δ-FAPbI3 and hysteresis behavior in planar heterojunction perovskite solar cells based on K x(MA0.17FA0.83)1- xPbI2.5Br0.5 (0≤ x ≤ 0.1) through incorporation of potassium ions (K+). X-ray diffraction patterns demonstrate formation of photoinactive δ-FAPbI3 was almost completely suppressed after K+ incorporation. Density functional theory calculation shows K+ prefers to enter the interstitial sites of perovskite lattice, leading to chemical environmental change in the crystal structure. Ultrafast transient absorption spectroscopy has revealed that K+ incorporation leads to enhanced carrier lifetime by 50%, which is also confirmed by reduced trap-assisted recombination of the perovskite solar cells containing K+ in photovoltage decay. Ultraviolet photoelectron spectroscopy illustrates that K+ incorporation results in a significant rise of conduction band minimum of the perovskite material by 130 meV, leading to a more favorable energy alignment with electron transporting material. At the optimal content of 3% K+ (molar ratio, relative to the total monovalent cations), nearly hysteresis-free, enhanced power conversion efficiencies from 15.72% to 17.23% were obtained in this solar cell.

Tuning the Amount of Oxygen Vacancies in Sputter-Deposited SnO x films for Enhancing the Performance of Perovskite Solar Cells

This work demonstrates the effect of oxygen vacancies in SnOx thin films on the performance of perovskite solar cells. Various SnOx films with different amounts of oxygen vacancies were deposited by sputtering at different substrate temperatures (25-300 °C). The transmittance of the films decreased from 82 to 66 % with increasing deposition temperature from 25 to 300 °C. Both X-ray photoelectron spectroscopy and electron-spin resonance spectroscopy confirmed that a higher density of oxygen vacancies was created within the SnOx film at a high substrate temperature, which caused narrowing of the SnOx bandgap from 4.1 (25 °C) to 3.74 eV (250 °C). Combined ultraviolet photoelectron spectroscopy and UV/Vis spectroscopy showed an excellent conduction band position alignment between the methylammonium lead iodide perovskite layer (3.90 eV) and the SnOx electron transport layer deposited at 250 °C (3.92 eV). As a result, a significant enhancement of the open-circuit voltage from 0.82 to 1.0 V was achieved, resulting in an increase of the power conversion efficiency of the perovskite solar cells from 11 to 14 %. This research demonstrated a facile approach for controlling the amount of oxygen vacancies in SnOx thin films to achieve a desirable energy alignment with the perovskite absorber layer for enhanced device performance.