Organic-inorganic hybrid halide perovskites (e.g., MAPbI3 ) have recently emerged as novel active materials for photovoltaic applications with power conversion efficiency over 22%. Conventional perovskite solar cells (PSCs); however, suffer the issue that lead is toxic to the environment and organisms for a long time and is hard to excrete from the body. Therefore, it is imperative to find environmentally-friendly metal ions to replace lead for the further development of PSCs. Previous work has demonstrated that Sn, Ge, Cu, Bi, and Sb ions could be used as alternative ions in perovskite configurations to form a new environmentally-friendly lead-free perovskite structure. Here, we review recent progress on lead-free PSCs in terms of the theoretical insight and experimental explorations of the crystal structure of lead-free perovskite, thin film deposition, and device performance. We also discuss the importance of obtaining further understanding of the fundamental properties of lead-free hybrid perovskites, especially those related to photophysics.

Lead-Free Organic-Inorganic Hybrid Perovskites for Photovoltaic Applications: Recent Advances and Perspectives.

Over the last several years, inorganic–organic hybrid perovskites have shown dramatic achievements in photovoltaic performance and device stability. Despite the significant progress in photovoltaic application, an in-depth understanding of the fundamentals of precursor solution chemistry is still lacking. In this review, the fundamental background knowledge of nucleation and crystal growth processes in solution including the LaMer model and Ostwald ripening process is described. This review article also highlights the recent progress in precursor-coordinating molecule interaction in solution along with the role of anti-solvent in the solvent engineering process to control nucleation and crystal growth. Moreover, chemical pathways from precursor solution to perovskite film formation are given. This represents identification of the intermediate phase induced by precursor-coordinating molecule interaction and responsible intermediate species for uniform and dense perovskite film formation. Further to the description of chemical phenomena in solution, the contemporary progress in chemical precursor composition is also provided to comprehend the current research approaches to further enhance photovoltaic performance and device stability. On the basis of the critical and comprehensive review, we provide some perspectives to further achieve high-performance perovskite solar cells with long-term device stability through precisely controlled nucleation and crystal growth in precursor solution.

Perovskite precursor solution chemistry: from fundamentals to photovoltaic applications.

It is extremely significant to study the trap state passivation and minimize the trap states of perovskite to achieve high-performance perovskite solar cells (PSCs). Here, we have first revealed and demonstrated that a novel p-type conductor Cu(thiourea)I [Cu(Tu)I] incorporated in perovskite layer can effectively passivate the trap states of perovskite via interacting with the under-coordinated metal cations and halide anions at the perovskite crystal surface. The trap state energy level of perovskite can be shallowed from 0.35–0.45 eV to 0.25–0.35 eV. In addition, the incorporated Cu(Tu)I can participate in constructing the p–i bulk heterojunctions with perovskite, leading to an increase of the depletion width from 126 to 265 nm, which is advantageous for accelerating hole transport and reducing charge carrier recombination. For these two synergistic effects, Cu(Tu)I can play a much better role than that of the traditional p-type conductor CuI, probably due to its identical valence band maximum with that...

A Breakthrough Efficiency of 19.9% Obtained in Inverted Perovskite Solar Cells by Using an Efficient Trap State Passivator Cu(thiourea)I

A Review on Additives for Halide Perovskite Solar Cells

Interfaces between perovskite solar cell (PSC) layer components play a pivotal role in obtaining high-performance premium cells and large-area modules. Graphene and related two-dimensional materials (GRMs) can be used to “on-demand” tune the interface properties of PSCs. We successfully used GRMs to realize large-area (active area 50.6 cm2) perovskite-based solar modules (PSMs), achieving a record high power conversion efficiency of 12.6%. We on-demand modulated the photoelectrode charge dynamic by doping the mesoporous TiO2 (mTiO2) layer with graphene flakes. Moreover, we exploited lithium-neutralized graphene oxide flakes as interlayer at the mTiO2/perovskite interface to improve charge injection. Notably, prolonged aging tests have shown the long-term stability for both small- and large-area devices using graphene-doped mTiO2. Furthermore, the possibility of producing and processing GRMs in the form of inks opens a promising route for further scale-up and stabilization of the PSM, the gateway for the c...

Graphene Interface Engineering for Perovskite Solar Modules: 12.6% Power Conversion Efficiency over 50 cm2 Active Area

Reduced Defects of MAPbI3 Thin Films Treated by FAI for High-Performance Planar Perovskite Solar Cells

Organic-inorganic halide perovskite solar cell has surged a great wave in photovoltaic field owing to its rapid advances, in which CH₃NH₃PbI₃ as the most conventional active layer has been attracting great attention. Generally, in a CH₃NH₃PbI₃ layer, the unreacted PbI₂ inevitably coexists with the perovskite crystals, especially in the two-step fabrication process. There seems a consensus that appropriate amount of unreacted PbI₂ is beneficial to the overall photovoltaic performance of the device, while as for the disadvantageous aspect of excessively remained PbI₂, it stays at regarding PbI₂ as an insulator. Further investigation and advisable control of the component in perovskite crystal call for a deeper understanding of the remnant PbI₂. In this work, PbI₂-enriched and PbI₂-controlled perovskite films as two extremes are prepared by modulating the crystallinity of the pre-deposited PbI₂ film, and the effects of excessively remained PbI₂ are elucidated on the basis of spectroscopic and optoelectronic studies. The initial charge separation, trap state density and trap state distribution properties are all found to be adverse for the PbI₂-enriched devices, i.e., to deteriorate the photovoltaic performances. Complementarily, the biphasic recombination process is reproduced and the charge carrier recombination dynamics was accelerated owing to the excessively remained PbI₂.

Adverse Effects of Excess Residual PbI2 on Photovoltaic Performance, Charge Separation, and Trap-State Properties in Mesoporous Structured Perovskite Solar Cells.

Charge recombination dynamics in intrinsic perovskite film and in meso-superstructured perovskite solar cells have been systematically studied, which are found to be mediated by the energetic distribution of intra-gap trap states as described by the trap-limited recombination theory. Besides, the passivation effect of the hole-transport material on trap states is discussed.

Trap-limited charge recombination in intrinsic perovskite film and meso-superstructured perovskite solar cells and the passivation effect of the hole-transport material on trap states

Two-dimensional (2D) perovskites, which have excellent stability compared with three-dimensional (3D) perovskites owing to the effective protection of the hydrophobic organic ligands, have become a research hotspot and have made great developmental progress in recent years. Herein, an n-butylammonium iodide (BAI) post-treatment process was developed to fabricate a 2D-3D hybrid perovskite with a thin layer of 2D perovskite covered on the surface of the 3D CH3NH3PbI3 perovskite. The growth process of 2D perovskite is formed through the chemical reaction between BAI and the residual PbI2, which improves stability and reduces the number of crystal defects of 3D perovskite by optimizing stoichiometry. Compared with the 3D counterpart, the 2D-3D hybrid perovskite shows outstanding light and air stability when exposed to external environments. Moreover, structure conversion from 3D to 2D-3D can induce the passivation of defects in the 3D films. The power conversion efficiency of the 2D-3D solar cell exceeds 18% and retains 80% of the initial value after more than 2000 h of storage without encapsulation.

https://iopscience.iop.org/article/10.1088/1361-6528/ab10f3/pdf

Highly efficient and stable 2D-3D perovskite solar cells fabricated by interfacial modification.

We report the in situ reduction and incorporation of gold nanoparticles into graphene oxide/polyethyleneimine composites with polyethyleneimine acting as the reducing and protecting agent for the gold nanoparticles. The resulting composites with porous structure are obtained through a simple freeze-drying method with the gold nanoparticles uniformly distributed on the graphene oxide sheets. The morphology and composition of the composite are characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction and Raman spectroscopy. The catalytic test indicates that the as-prepared porous gold nanoparticle-embedded composite catalyst could efficiently activate the reduction of 4-nitrophenol to 4-aminophenol. The recycling measurement reveals that the activity of the recovered catalyst decreases a little, mainly due to the restacking of the graphene sheets. Moreover, the porous composites used as packing material in chromatography column and injection syringe exhibit good catalytic performances in the rapid reduction of 4-nitrophenol, implying the potential applications of such porous heterogeneous catalysts in continuous catalytic reaction.

Hao-Yi Wang

Papers

Reduced Defects of MAPbI3 Thin Films Treated by FAI for High-Performance Planar Perovskite Solar Cells

Adverse Effects of Excess Residual PbI2 on Photovoltaic Performance, Charge Separation, and Trap-State Properties in Mesoporous Structured Perovskite Solar Cells.

Trap-limited charge recombination in intrinsic perovskite film and meso-superstructured perovskite solar cells and the passivation effect of the hole-transport material on trap states

Highly efficient and stable 2D-3D perovskite solar cells fabricated by interfacial modification.

Porous gold nanoparticle/graphene oxide composite as efficient catalysts for reduction of 4-nitrophenol