The formation of a dense and uniform thin layer on the substrates is crucial for the fabrication of high-performance perovskite solar cells (PSCs) containing formamidinium with multiple cations and mixed halide anions. The concentration of defect states, which reduce a cell’s performance by decreasing the open-circuit voltage and short-circuit current density, needs to be as low as possible. We show that the introduction of additional iodide ions into the organic cation solution, which are used to form the perovskite layers through an intramolecular exchanging process, decreases the concentration of deep-level defects. The defect-engineered thin perovskite layers enable the fabrication of PSCs with a certified power conversion efficiency of 22.1% in small cells and 19.7% in 1-square-centimeter cells.

Iodide management in formamidinium-lead-halide–based perovskite layers for efficient solar cells

The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within t...

Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells

A Review of Perovskites Solar Cell Stability

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

State of the Art and Prospects for Halide Perovskite Nanocrystals.

Additive Engineering for Efficient and Stable Perovskite Solar Cells

DOI: 10.1002/aenm.201702498 with chemical formula of A2Bn−1MnX3n+1 (n ≥ 2), wherein M is Pb2+ or Sn2+, B is CH3NH3 (MA+), HC(NH2)2 (FA+), or Cs+, A is the spacer cation including CH3(CH2)2NH3 (BA+), C6H5(CH2)2NH3 (PEA+), etc., have been developed and applied as the light-harvesting layer in PVSCs.[18–24] The usage of the spacer cation favors the formation of stable 2D perovskite, but the related device efficiency decreases rapidly. The partial replacement of MA+ with the spacer cations leads to the anisotropy in crystal structure and subsquently anisotropic charge carriers transportation. As a result, the movement of the charge carriers in 2D perovskite is blocked by the insulating spacer cation layer.[20] It is considered as one of the main reasons why 2D perovskite-based PVSCs show a much lower PCE than 3D-based PVSCs, for example, 4.73% for (PEA)2(MA)n−1PbnI3n+1 and 4.02% for (BA)2(MA)n−1PbnI3n+1 with n = 3. To improve the PCE of the 2D-based PVSCs, researchers tried to reduce the insulating effects of the organic layer by increasing the n value. Mora-Seró and coworkers[27] fabricated (C6H5NH3I)2(CH3NH3I)n−1(PbI2)n film on compact TiO2/mesoporous TiO2 substrate and obtained PCEs from 0.66% (n = 1) to 5.94% (n = 5). Sargent and co-workers[28] prepared (PEA)2(MA)n−1PbnI3n+1 perovskite on dense compact TiO2 substrate and achieved the highest PCE of 15.3% in the case of n = 60. But the PCE reduced to 9.22% with decreasing n to 6 due to the wide bandgap and poor carrier transport. Etgar and co-workers[29] fabricated (PEA)2(MA)n−1PbnBr3n+1 based Increasing the power conversion efficiency (PCE) of the two-dimensional (2D) perovskite-based solar cells (PVSCs) is really a challenge. Vertical orientation of the 2D perovskite film is an efficient strategy to elevate the PCE. In this work, vertically orientated highly crystalline 2D (PEA)2(MA)n–1PbnI3n+1 (PEA= phenylethylammonium, MA = methylammonium, n = 3, 4, 5) films are fabricated with the assistance of an ammonium thiocyanate (NH4SCN) additive by a one-step spin-coating method. Planar-structured PVSCs with the device structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/ (PEA)2(MA)n–1PbnI3n+1/[6,6]-phenyl-C61-butyric acid methyl ester /bahocuproine/ Ag are fabricated. The PCE of the PVSCs is boosted from the original 0.56% (without NH4SCN) to 11.01% with the optimized NH4SCN addition at n = 5, which is among the highest PCE values for the low-n (n < 10) 2D perovskitebased PVSCs. The improved performance is attributed to the vertically orientated highly crystalline 2D perovskite thin films as well as the balanced electron/ hole transportation. The humidity stability of this oriented 2D perovskite thin film is also confirmed by the almost unchanged X-ray diffraction patterns after 28 d exposed to the moisture in a humidity-controlled cabinet (Hr = 55 ± 5%). The unsealed device retains 78.5% of its original PCE after 160 h storage in air atmosphere with humidity of 55 ± 5%. The results provide an effective approach toward a highly efficient and stable PVSC for future commercialization. Perovskite Solar Cells

Orientation Regulation of Phenylethylammonium Cation Based 2D Perovskite Solar Cell with Efficiency Higher Than 11

Vertically oriented highly crystalline 2D layered (BA)2 (MA)n-1 Pbn I3n+1 (BA = CH3 (CH2 )3 NH3 , MA = CH3 NH3 , n = 3, 4) perovskite thin-films are fabricated with the aid of ammonium thiocyanate (NH4 SCN) additive through one-step spin-coating process. The humidity-stability of the film is certified by the almost unchanged X-ray diffraction patterns after exposed to humid atmosphere (Hr = 55 ± 5%) for 40 d. The photovoltaic devices with the structure of indium tin oxide(ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate)/(BA)2 (MA)n-1 Pbn I3n+1 (n = 3,4)/[6,6]-phenyl-C61 -butyric acid methyl ester/Bathocuproine/Ag are fabricated. The devices based on (BA)2 (MA)2 Pb3 I10 perovskite (n = 3) with the precursor composition of BAI:methylammonium iodide:PbI2 :NH4 SCN = 2:2:3:1 (by molar ratio) show an averaged power conversion efficiency (PCE) of 6.82%. In the case of (BA)2 (MA)3 Pb4 I13 (n = 4), a higher PCE of 8.79% is achieved. Both of the unsealed devices perform unique stability with almost unchanged PCE during the period of storage in purified N2 glove box. This work provides a simple and effective method to enhance the efficiency of the 2D perovskite solar cell.

Vertically Oriented 2D Layered Perovskite Solar Cells with Enhanced Efficiency and Good Stability

As an important category of perovskite materials, two-dimensional (2D) perovskites are attracting increasing research attention these days. Their possibility of combining high performance and stability for perovskite based optoelectronic devices has triggered a new wave of research. This review mainly focuses on the application of 2D perovskite materials for solar cells. We start with a brief introduction of 2D perovskite structures and their unique properties. The recent progress in 2D perovskite solar cells is summarized in three aspects according to the existing forms of the perovskite materials in the devices. In the end, a short outlook with our opinion is given to indicate the possible development trend for this kind of perovskite material.

/pdf/recent-progress-in-2d-quasi-2d-layered-metal-halide-3e2rceoq3a.pdf

Recent progress in 2D/quasi-2D layered metal halide perovskites for solar cells

High-Performance Thickness Insensitive Perovskite Solar Cells with Enhanced Moisture Stability

Due to the quantum confinement effects, the bandgap of two-dimensional (2D) perovskites increases when the thickness of the inorganic slabs decreases. This could result in insufficient light absorption that limits the overall performance of perovskite solar cells. Here we report a series of small bandgap 2D perovskites using mixed butylammonium (BA) and formamidinium (FA) cations, i.e. (BA)2(FA)n−1PbnI3n+1 (n = 1–5). In particular, 2D perovskite (BA)2(FA)2Pb3I10 shows a small bandgap of 1.51 eV, which is comparable to state-of-the-art three-dimensional perovskites. A strongly preferential out-of-plane crystallographic alignment of the inorganic perovskite component in the (BA)2(FA)2Pb3I10 film is achieved by the addition of thiourea in the precursor. This significantly improves charge transport and thus leads to highly efficient inverted solar cells with a planar structure of ITO/PEDOT:PSS/(BA)2(FA)2Pb3I10/PC61BM/BCP/Ag. With the best power conversion efficiency of 6.88%, we demonstrated the highest PCE reported for FA based low-n (n < 4) 2D perovskite solar cells. By virtue of the stable 2D perovskites, the unencapsulated device retains 80% efficiency after storing in air with a humidity of 25 ± 5% for 25 days, indicating excellent stability against moisture and oxygen.

Jielin Yan

Papers

Orientation Regulation of Phenylethylammonium Cation Based 2D Perovskite Solar Cell with Efficiency Higher Than 11

Vertically Oriented 2D Layered Perovskite Solar Cells with Enhanced Efficiency and Good Stability

Recent progress in 2D/quasi-2D layered metal halide perovskites for solar cells

High-Performance Thickness Insensitive Perovskite Solar Cells with Enhanced Moisture Stability

Highly oriented two-dimensional formamidinium lead iodide perovskites with a small bandgap of 1.51 eV