Solvent engineering for high-quality perovskite solar cell with an efficiency approaching 20%

Abstract: An organic–inorganic perovskite is comprised of an organic cation (CH3NH3+, FAI, or Cs), a metal cation (Pb2+ or Sn2+) and a halide (I−, Cl−, or Br−) molecule. Precursor salts containing these cations, molecules and halide ions dissolved in solvents are used to prepare perovskite films. Perovskite film formation takes place through the reaction of precursor elements, which is assisted by various processing conditions such as thermal annealing, moisture and solvent treatment. This review focuses on various perovskite formation and crystallization routes with respect to processing parameters including the precursor solvent, solvent mixture, temperature, time, formation of solvent led-intermediate complex species, doping and humidity. Adding water as the dopant to the precursor solvent and exposure to moisture from atmospheric humidity to improve perovskite film quality are also discussed. Processing conditions and crystallization processes are described in correlation with the perovskite film morphology, crystallinity, defects, charge transport and device performance. This article will aim to highlighting recent findings in the selection of solvents in the crystallization of perovskite films, solvent induced intermediate phases, and effects of water in assisting perovskite crystallization for improved film quality and device performance. The review will also present various structural and nanoscale characterization techniques that have been used to probe solvent based intermediate species transformation processes to the perovskite phase and understand the effects in correlation with device performance.

Abstract: Perovskite solar cells (PSCs) have emerged as a ‘rising star’ in recent years due to their high-power conversion efficiency (PCE), extremely low cost and facile fabrication techniques. To date, PSCs have achieved a certified PCE of 25.2% on rigid conductive substrates, and 19.5% on flexible substrates. The significant advancement of PSCs has been realized through various routes, including perovskite composition engineering, interface modification, surface passivation, fabrication process optimization, and exploitation of new charge transport materials. However, compared with rigid counterparts, the efficiency record of flexible perovskite solar cells (FPSCs) is advancing slowly, and therefore it is of great significance to scrutinize recent work and expedite the innovation in this field. In this article, we comprehensively review the recent progress of FPSCs. After a brief introduction, the major features of FPSCs are compared with other types of flexible solar cells in a broad context including silicon, CdTe, dye-sensitized, organic, quantum dot and hybrid solar cells. In particular, we highlight the major breakthroughs of FPSCs made in 2019/2020 for both laboratory and large-scale devices. The constituents of making a FPSC including flexible substrates, perovskite absorbers, charge transport materials, as well as device fabrication and encapsulation methods have been critically assessed. The existing challenges of making high performance and long-term stable FPSCs are discussed. Finally, we offer our perspectives on the future opportunities of FPSCs in the field of photovoltaics.

Abstract: Organic-inorganic hybrid perovskite solar cells have become dominant topics on photovoltaic field in the past decade The quality of the perovskite layer plays a crucial role in the photovoltaic performance of perovskite devices, and the solvent engineering is an attractive method for improving the quality of perovskite layer Here, we introduce pyridine additive in the precursor mixed solution to prepare high quality cation-anion-mixed perovskite of Cs005 (MA017FA083)095Pb(I0·83Br017)3 The morphology observation indicates that a large grain, dense and pinhole-free perovskite film is obtained by pyridine adding Photoluminescence and resistance measurements demonstrate that pyridine adding contribution to less defects and charge carrier recombination in the perovskite layer The perovskite solar cell with pyridine adding achieves a power conversion efficiency of 1903%, while the device without pyridine adding reaches an efficiency of 1694% at the same experiment conditions The simple and green procedure and significant improvement in the performance render this method promising

Abstract: The introduction of upconversion materials as spectral converter into photovoltaic devices to harvest and convert near infrared photons to visible photons that can thus be absorbed by perovskite layer is a promising strategy for enhancing the performance of perovskite solar cells. Hence, β-NaYF4:Yb3+/Er3+/Sc3+@NaYF4 rare-earth doped upconversion nanoparticles with core-shell structure are synthesized. The doped rare-earth ions Yb3+ and Er3+ convert near infrared light to red and green light, and the Sc3+ ions enhance the upconversion luminescence efficiency, which broaden the absorption range of solar irradiation and generate extra photocurrent in the solar cells. The surface defects of β-NaYF4:Yb3+/Er3+/Sc3+ is passivated by the even NaYF4 shell, which results in the obvious enhancement in upconversion emission intensity. Consequently, the perovskite solar cell based on the rare-earth doped upconversion nanoparticle mesoporous layer achieves a high power conversion efficiency of 20.19%, while the device based on TiO2 mesoporous layer gets an efficiency of 17.44%.

Abstract: We report highly bendable and efficient perovskite solar cells (PSCs) that use thermally oxidized layer of Ti metal plate as an electron transport layer (ETL). The power conversion efficiency (PCE) of flexible PSCs reaches 14.9% with a short-circuit current density (Jsc) of 17.9 mA/cm2, open-circuit voltage (Voc) of 1.09, and fill factor (ff) of 0.74. Moreover, the Ti metal-based PSCs exhibit a superior fatigue resistance over indium tin oxide/poly(ethylene terephthalate) substrate. Flexible PSCs maintain 100% of their initial PCE even after PSCs are bent 1000 times at a bending radius of 4 mm. This excellent performance of flexible PSCs is due to high crystalline quality and low oxygen vacancy concentration of TiO2 layer. The concentration of oxygen vacancies in the oxidized Ti metal surface controls the electric function of TiO2 as ETL of PSCs. A decrease in the oxygen vacancy concentration of the TiO2 layer is critical to improving the electron collection efficiency of the ETL. Our results suggest that...

Abstract: Two organolead halide perovskite nanocrystals, CH3NH3PbBr3 and CH3NH3PbI3, were found to efficiently sensitize TiO2 for visible-light conversion in photoelectrochemical cells. When self-assembled on mesoporous TiO2 films, the nanocrystalline perovskites exhibit strong band-gap absorptions as semiconductors. The CH3NH3PbI3-based photocell with spectral sensitivity of up to 800 nm yielded a solar energy conversion efficiency of 3.8%. The CH3NH3PbBr3-based cell showed a high photovoltage of 0.96 V with an external quantum conversion efficiency of 65%.

Abstract: Following pioneering work, solution-processable organic-inorganic hybrid perovskites-such as CH3NH3PbX3 (X = Cl, Br, I)-have attracted attention as light-harvesting materials for mesoscopic solar cells. So far, the perovskite pigment has been deposited in a single step onto mesoporous metal oxide films using a mixture of PbX2 and CH3NH3X in a common solvent. However, the uncontrolled precipitation of the perovskite produces large morphological variations, resulting in a wide spread of photovoltaic performance in the resulting devices, which hampers the prospects for practical applications. Here we describe a sequential deposition method for the formation of the perovskite pigment within the porous metal oxide film. PbI2 is first introduced from solution into a nanoporous titanium dioxide film and subsequently transformed into the perovskite by exposing it to a solution of CH3NH3I. We find that the conversion occurs within the nanoporous host as soon as the two components come into contact, permitting much better control over the perovskite morphology than is possible with the previously employed route. Using this technique for the fabrication of solid-state mesoscopic solar cells greatly increases the reproducibility of their performance and allows us to achieve a power conversion efficiency of approximately 15 per cent (measured under standard AM1.5G test conditions on solar zenith angle, solar light intensity and cell temperature). This two-step method should provide new opportunities for the fabrication of solution-processed photovoltaic cells with unprecedented power conversion efficiencies and high stability equal to or even greater than those of today's best thin-film photovoltaic devices.

Abstract: Organic-inorganic perovskites have shown promise as high-performance absorbers in solar cells, first as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, we report transient absorption and photoluminescence-quenching measurements to determine the electron-hole diffusion lengths, diffusion constants, and lifetimes in mixed halide (CH3NH3PbI(3-x)Cl(x)) and triiodide (CH3NH3PbI3) perovskite absorbers. We found that the diffusion lengths are greater than 1 micrometer in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. In contrast, the triiodide absorber has electron-hole diffusion lengths of ~100 nanometers. These results justify the high efficiency of planar heterojunction perovskite solar cells and identify a critical parameter to optimize for future perovskite absorber development.

Abstract: Organic-inorganic perovskites have shown promise as high-performance absorbers in solar cells, first as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, we report transient absorption and photoluminescence-quenching measurements to determine the electron-hole diffusion lengths, diffusion constants, and lifetimes in mixed halide (CH3NH3PbI(3-x)Cl(x)) and triiodide (CH3NH3PbI3) perovskite absorbers. We found that the diffusion lengths are greater than 1 micrometer in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. In contrast, the triiodide absorber has electron-hole diffusion lengths of ~100 nanometers. These results justify the high efficiency of planar heterojunction perovskite solar cells and identify a critical parameter to optimize for future perovskite absorber development.

Abstract: Many different photovoltaic technologies are being developed for large-scale solar energy conversion. The wafer-based first-generation photovoltaic devices have been followed by thin-film solid semiconductor absorber layers sandwiched between two charge-selective contacts and nanostructured (or mesostructured) solar cells that rely on a distributed heterojunction to generate charge and to transport positive and negative charges in spatially separated phases. Although many materials have been used in nanostructured devices, the goal of attaining high-efficiency thin-film solar cells in such a way has yet to be achieved. Organometal halide perovskites have recently emerged as a promising material for high-efficiency nanostructured devices. Here we show that nanostructuring is not necessary to achieve high efficiencies with this material: a simple planar heterojunction solar cell incorporating vapour-deposited perovskite as the absorbing layer can have solar-to-electrical power conversion efficiencies of over 15 per cent (as measured under simulated full sunlight). This demonstrates that perovskite absorbers can function at the highest efficiencies in simplified device architectures, without the need for complex nanostructures.