Taiyang Zhang

Controllable Sequential Deposition of Planar CH3NH3PbI3 Perovskite Films via Adjustable Volume Expansion

We demonstrate a facile morphology-controllable sequential deposition of planar CH3NH3PbI3 (MAPbI3) film by using a novel volume-expansion-adjustable PbI2·xMAI (x: 0.1-0.3) precursor film to replace pure PbI2. The use of additive MAI during the first step of deposition leads to the reduced crystallinity of PbI2 and the pre-expansion of PbI2 into PbI2·xMAI with adjustable morphology, which result in about 10-fold faster formation of planar MAPbI3 film (without PbI2 residue) and thus minimize the negative impact of the solvent isopropanol on perovskites during the MAI intercalation/conversion step. The best efficiency obtained for a planar perovskite solar cell based on PbI2·0.15MAI is 17.22% (under one sun illumination), which is consistent with the stabilized maximum power output at an efficiency of 16.9%.

Facile fabrication of large-grain CH3NH3PbI3-xBrx films for high-efficiency solar cells via CH3NH3Br-selective Ostwald ripening

Organometallic halide perovskite solar cells (PSCs) have shown great promise as a low-cost, high-efficiency photovoltaic technology. Structural and electro-optical properties of the perovskite absorber layer are most critical to device operation characteristics. Here we present a facile fabrication of high-efficiency PSCs based on compact, large-grain, pinhole-free CH3NH3PbI3−xBrx (MAPbI3−xBrx) thin films with high reproducibility. A simple methylammonium bromide (MABr) treatment via spin-coating with a proper MABr concentration converts MAPbI3 thin films with different initial film qualities (for example, grain size and pinholes) to high-quality MAPbI3−xBrx thin films following an Ostwald ripening process, which is strongly affected by MABr concentration and is ineffective when replacing MABr with methylammonium iodide. A higher MABr concentration enhances I–Br anion exchange reaction, yielding poorer device performance. This MABr-selective Ostwald ripening process improves cell efficiency but also enhances device stability and thus represents a simple, promising strategy for further improving PSC performance with higher reproducibility and reliability.

Hydrochloric acid accelerated formation of planar CH3NH3PbI3 perovskite with high humidity tolerance

We demonstrate high humidity tolerant one-step and sequential deposition methods to fabricate high quality planar CH3NH3PbI3 perovskite films with the assistance of HCl. The addition of stoichiometric HCl into PbI2 precursor solution from 33 wt% hydrochloric acid leads to the formation of a novel HCl·PbI2 precursor film, which can be easily thermally decomposed back into PbI2. This novel intermediate planar HCl·PbI2 precursor film can be completely converted into a compact planar MAPbI3 film within only 10 s at room temperature via sequential deposition. In another novel one step method the precursor solution of PbI2 + MAI + HCl obtained by adding stoichiometric HCl into regular PbI2 + MAI precursor solution was used to fabricate a very smooth planar MAPbI3 film by just spin coating. Both the one step and sequential deposition methods can be used to fabricate high quality planar perovskite films in a hood under ambient conditions with up to 60% humidity level for high efficiency planar perovskite solar cells.

Bication lead iodide 2D perovskite component to stabilize inorganic α-CsPbI3 perovskite phase for high-efficiency solar cells

A facile preparation of phase-stable cesium lead triiodide perovskite for high-performance solar cells. Among various all-inorganic halide perovskites exhibiting better stability than organic-inorganic halide perovskites, α-CsPbI3 with the most suitable band gap for tandem solar cell application faces an issue of phase instability under ambient conditions. We discovered that a small amount of two-dimensional (2D) EDAPbI4 perovskite containing the ethylenediamine (EDA) cation stabilizes the α-CsPbI3 to avoid the undesirable formation of the nonperovskite δ phase. Moreover, not only the 2D perovskite of EDAPbI4 facilitate the formation of α-CsPbI3 perovskite films exhibiting high phase stability at room temperature for months and at 100°C for >150 hours but also the α-CsPbI3 perovskite solar cells (PSCs) display highly reproducible efficiency of 11.8%, a record for all-inorganic lead halide PSCs. Therefore, using the bication EDA presents a novel and promising strategy to design all-inorganic lead halide PSCs with high performance and reliability.

Ion-Exchange-Induced 2D-3D Conversion of HMA1−x FAx PbI3 Cl Perovskite into a High-Quality MA1−x FAx PbI3 Perovskite

High-quality phase-pure MA1-x FAx PbI3 planar films (MA=methylammonium, FA=formamidinium) with extended absorption and enhanced thermal stability are difficult to deposit by regular simple solution chemistry approaches owing to crystallization competition between the easy-to-crystallize but unwanted δ-FAPbI3 /MAPbI3 and FAx MA1-x PbI3 requiring rigid crystallization conditions. Here A 2D-3D conversion to transform compact 2D mixed composition HMA1-x FAx PbI3 Cl perovskite precursor films into 3D MA1-x FAx PbI3 (x=0.1-0.9) perovskites is presented. The designed Cl/I and H/FA(MA) ion exchange reaction induced fast transformation of compact 2D perovskite film, helping to form the phase-pure and high quality MA1-x FAx PbI3 without δ-FAPbI3 and MAPbI3 impurity. In all, we successfully developed a facile one-step method to fabricate high quality phase-pure MA1-x FAx PbI3 (x=0.1-0.9) perovskite films by 2D-3D conversion of HMA1-x FAx PbI3 Cl perovskite. This 2D-3D conversion is a promising strategy for lead halide perovskite fabrication.

Mixed cation hybrid lead halide perovskites with enhanced performance and stability

Organic and inorganic hybrid perovskites have emerged as revolutionary optoelectronic semiconductors, which are promising for various applications especially in photovoltaics and light-emitting diodes. Perovskite solar cells have demonstrated unprecedented progress on PCE within a very short time in the history of photovoltaics. Perovskite solar cells have more than 20% PCE and advantages such as easy-to-process and tunable bandgaps have made them favorable for commercialization. Extending the absorption to a longer wavelength and addressing the challenge of stability in metal halide perovskites has become the most important research focus. However, composition engineering could be a potential solution by further tuning the band gap for the metal halide perovskites and, thus, enhance their stability. The mixed cation perovskite is one of the most practical and successful strategies for composition engineering. Here, recent progress on mixed cation metal halide perovskites is reviewed and this includes the development of binary, ternary and two-dimensional/three-dimensional mixed cation perovskites in chemical synthesis and device performance and stability progress. The prospects and challenges for the mixed cation hybrid perovskites are also considered.

A general non-CH3NH3X (X = I, Br) one-step deposition of CH3NH3PbX3 perovskite for high performance solar cells

We demonstrate a facile non-CH3NH3I one-step fabrication of high quality CH3NH3PbI3 perovskite via direct gas/solid deposition of spin coating HI + PbI2 precursor solution in low concentration CH3NH2 atmospheres. This HI + PbI2 precursor solution was prepared by direct addition of stoichiometric aqueous HI acid into PbI2 DMF solution without any other chemicals. This novel one-step approach can be extended to fabricate different lead halide perovskites such as CH3NH3PbI2Br and C2H5NH2PbI3 by spin coating HBr + PbI2 precursor solution in CH3NH2 atmosphere or HI + PbI2 in C2H5NH2 atmosphere, respectively. In all, our novel one-step approach is a promising candidate for fabricating high performance perovskites by one-step reaction of gaseous CH3NH2 with HX and PbX2 (X = I, Br).

Synergetic Effect of Chloride Doping and CH3NH3PbCl3 on CH3NH3PbI3−xClx Perovskite‐Based Solar Cells

The chloride-doped CH3 NH3 PbI3-x Clx perovskite has attracted great attention owing to clear performance enhancement by using a Cl additive and by the controversial arguments on Cl function and the mechanism behind it. Herein, a series of CH3 NH3 PbI3-x Clx perovskites with various Cl content was prepared through a gas/solid reaction between CH3 NH2 gas and HPbI3-x Clx (x=0-1). The small amount of Cl doping in CH3 NH3 PbI3-x Clx (x=0.05) could lead to band gap broadening and significantly increase the perovskite grain size, and the phase-pure CH3 NH3 PbI2.95 Cl0.05 perovskites exhibited up to 17.44 % efficiency. For Cl contents higher than 0.1 (x>0.1), CH3 NH3 PbCl3 formed and coexisted with CH3 NH3 PbI3-x Clx , and CH3 NH3 PbCl3 could help to improve the thermal stability of CH3 NH3 PbI3-x Clx . However, the excessive co-existing wide-band-gap CH3 NH3 PbCl3 perovskites would inhibit the electron transfer and lead to a deterioration of photovoltaic performance.

Recent Progress of Lead Halide Perovskite Sensitized Solar Cells

Low cost organic and inorganic hybrid perovskites such as CH3NH3PbI3 has demonstrated a rapid increase in efficiency and has attracted tremendous research effort within a few years. The perovskite solar cells has evolved from tradi- tional sensitized solar cell using liquid iodine electrolyte to solid state solar cell with either mesoporous or planar structure. The morphology controllable solution chemistry deposition of perovskite has progress into 15%~17% efficiency perovoskite solar cells reported by several groups. The perovskite sensitized solar cells has also progress from pure mesoporous structure into meso-planar hybrid one, which has the advantages of high efficiency and better stability. Here we review recent progress on the development of perovskite sensitized solar cells, different band gap perovskites and their deposition, different back- bone nanostructures and their charge transfer properties. Keywords lead halide perovskite; solar cell; sensitized; mesoporous

A simple fabrication of CH3NH3PbI3 perovskite for solar cells using low-purity PbI2 *

The CH3NH3PbI3 (MAPbI3) perovskite was usually prepared by high-purity PbI2 with high cost. The low cost and low-purity PbI2 was seldom reported for fabrication of MAPbI3 because it cannot even dissolve well in widely adopted solvent of DMF. We developed an easy method to adapt low-purity PbI2 for fabrication of high quality MAPbI3 just by the simple addition of some hydrochloric acid into the mixture of low-purity PbI2, MAI and DMF. This straightforward method can not only help dissolve the low quality PbI2 by reacting with some impurities in DMF, but also lead to a successful fabrication of high-quality perovskite solar cells with up to 14.80% efficiency comparable to the high quality PbI2 precursors.