Homologous Bromides Treatment for Improving the Open-Circuit Voltage of Perovskite Solar Cells.

The power conversion efficiency (PCE) of solution-processed organic-inorganic mixed halide perovskite solar cells has achieved rapid improvement. However, it is imperative to minimize the voltage deficit (Woc  = Eg /q - Voc ) for their PCE to approach the theoretical limit. Herein, the strategy of depositing homologous bromide salts on the perovskite surface to achieve a surface and bulk passivation for the fabrication of solar cells with high open-circuit voltage is reported. Distinct from the conclusions given by previous works, that homologous bromides such as FABr only react with PbI2 to form a large-bandgap perovskite layer on top of the original perovskite, this work shows that the bromide also penetrates the perovskite film and passivates the perovskite in the bulk. This is confirmed by the small-bandgap enlargement observed by absorbance and photoluminescence, and the bromide element ratio increasing in the bulk by time-of-flight secondary-ion mass spectrometry and depth-resolved X-ray photoelectron spectroscopy. Furthermore, a clear suppression of non-radiative recombination is confirmed by a variety of characterization methods. This work provides a simple and universal way to reduce the Woc of single-junction perovskite solar cells and it will also shed light on developing other high-performance optoelectronic devices, including perovskite-based tandems and light-emitting diodes.

Inorganic-Cation Pseudohalide 2D Cs2 Pb(SCN)2 Br2 Perovskite Single Crystal.

Most of the reported 2D Ruddlesden-Popper (RP) lead halide perovskites with the general formula of An +1 Bn X3 n +1 (n = 1, 2, …) comprise layered perovskites separated by A-site-substituted organic spacers. To date, only a small number of X-site-substituted RP perovskites have been reported. Herein, the first inorganic-cation pseudohalide 2D phase perovskite single crystal, Cs2 Pb(SCN)2 Br2 , is reported. It is synthesized by the antisolvent vapor-assisted crystallization (AVC) method at room temperature. It exhibits a standard single-layer (n = 1) Ruddlesden-Popper structure described in space group of Pmmn (#59) and has a small separation (d = 1.69 Å) between the perovskite layers. The SCN- anions are found to bend the 2D Pb(SCN)2 Br2 framework slightly into a kite-shaped octahedron, limiting the formation of a quasi-2D perovskite structure (n > 1). This 2D single crystal exhibits a reversible first-order phase transformation to 3D CsPbBr3 (Pm3m #221) at 450 K. It has a low exciton binding energy of 160 meV-one of the lowest for 2D perovskites (n = 1). A Cs2 Pb(SCN)2 Br2 -single-crystal photodetector is demonstrated with respectable responsivity of 8.46 mA W-1 and detectivity of ≈1.2 × 1010 Jones at a low bias voltage of 0.5 V.

Immediate and Temporal Enhancement of Power Conversion Efficiency in Surface-Passivated Perovskite Solar Cells.

This work reports strategies for improving the power conversion efficiency (PCE) by capitalizing on temporal changes through the storage effect and immediate improvements by interface passivation. It is demonstrated that both strategies can be combined as shown by PCE improvement in passivated perovskite solar cells (PSCs) upon ambient storage because of trap density reduction. By analyzing the dominant charge recombination process, we find that lead-related traps in perovskite bulk, rather than at the surface, are the recombination centers in both as-fabricated and ambient-stored passivated PSCs. This emphasizes the necessity to reduce intrinsic defects in the perovskite bulk. Furthermore, storage causes temporal changes in band alignment even in passivated PSCs, contributing to PCE improvement. Building on these findings, composition engineering was employed to produce further immediate PCE improvements because of defect reduction in the bulk, achieving a PCE of 22.2%. These results show that understanding the dominant recombination mechanisms within a PSC is important to inform strategies for producing immediate and temporal PCE enhancements either by interface passivation, storage, composition engineering, or a combination of them all to fabricate highly efficient PSCs.

Gas chromatography-mass spectrometry analyses of encapsulated stable perovskite solar cells.

Although perovskite solar cells have produced remarkable energy conversion efficiencies, they cannot become commercially viable without improvements in durability. We used gas chromatography-mass spectrometry (GC-MS) to reveal signature volatile products of the decomposition of organic hybrid perovskites under thermal stress. In addition, we were able to use GC-MS to confirm that a low-cost polymer/glass stack encapsulation is effective in suppressing such outgassing. Using such an encapsulation scheme, we produced multi-cation, multi-halide perovskite solar cells containing methylammonium that exceed the requirements of the International Electrotechnical Commission 61215:2016 standard by surviving more than 1800 hours of the Damp Heat test and 75 cycles of the Humidity Freeze test.

Acetic Acid Assisted Crystallization Strategy for High Efficiency and Long-Term Stable Perovskite Solar Cell.

Improving the quality of perovskite poly-crystalline film is essential for the performance of associated solar cells approaching their theoretical limit efficiency. Pinholes, unwanted defects, and nonperovskite phase can be easily generated during film formation, hampering device performance and stability. Here, a simple method is introduced to prepare perovskite film with excellent optoelectronic property by using acetic acid (Ac) as an antisolvent to control perovskite crystallization. Results from a variety of characterizations suggest that the small amount of Ac not only reduces the perovskite film roughness and residual PbI2 but also generates a passivation effect from the electron-rich carbonyl group (C=O) in Ac. The best devices produce a PCE of 22.0% for Cs0.05FA0.80MA0.15Pb(I0.85Br0.15)3 and 23.0% for Cs0.05FA0.90MA0.05Pb(I0.95Br0.05)3 on 0.159 cm2 with negligible hysteresis. This further improves device stability producing a cell that maintained 96% of its initial efficiency after 2400 h storage in ambient environment (with controlled relative humidity (RH) <30%) without any encapsulation.

The Impact of a Dynamic Two-Step Solution Process on Film Formation of Cs0.15 (MA0.7 FA0.3 )0.85 PbI3 Perovskite and Solar Cell Performance.

This paper provides deep understanding of the formation mechanism of perovskite film fabricated by sequential solution-based methods. It compares two sequential spin-coating methods for Cs0.15 (MA0.7 FA0.3 )0.85 PbI3 perovskite. First is the "static process," with a stoppage between the two spin-coating steps (1st PbI2 -CsI-dimethyl sulfoxide (DMSO)-dimethylformamide (DMF) and 2nd methylammonium iodide (MAI)-formamidinium iodide (FAI)-isopropyl alcohol). Second is the "dynamic process," where the 2nd precursor is dispensed while the substrate is still spinning from the 1st step. For the first time, such a dynamic process is used for Cs0.15 (MA0.7 FA0.3 )0.85 PbI3 perovskite. Characterizations reveal improved film formation with the dynamic process due to the "retainment" of DMSO-complex necessary for the intermediate phase which i) promotes intercalation between precursors and ii) slows down perovskite crystallization for full conversion. The comparison on as-deposited perovskite before annealing indicates a more ordered film using this dynamic process. This results in a thicker, more uniform film with higher degree of preferred crystal orientation and higher carrier lifetime after annealing. Therefore, dynamic-processed devices present better performance repeatability, achieving a higher average efficiency of 17.0% compared to static ones (15.0%). The new insights provided by this work are important for perovskite solar cells processed sequentially as the process has greater flexibility in resolving solvent incompatibility, allowing separate optimizations and allowing different deposition methods.