High-Efficiency Air-Stable Colloidal Quantum Dot Solar Cells Based on a Potassium-Doped ZnO Electron-Accepting Layer

High-efficiency colloidal quantum dot (CQD) solar cells (CQDSCs) with improved air stability were developed by employing potassium-modified ZnO as an electron-accepting layer (EAL). The effective potassium modification was achievable by a simple treatment with a KOH solution of pristine ZnO films prepared by a low-temperature solution process. The resulting K-doped ZnO (ZnO-K) exhibited EAL properties superior to those of a pristine ZnO-EAL. The Fermi energy level of ZnO was upshifted, which increased the internal electric field and amplified the depletion region (i.e., charge drift) of the devices. The surface defects of ZnO were effectively passivated by K modification, which considerably suppressed interfacial charge recombination. The CQDSC based on ZnO-K achieved improved power conversion efficiency (PCE) of approximate to 10.75% (V-OC of 0.67 V, J(SC) of 23.89 mA cm(2), and fill factor of 0.68), whereas the CQDSC based on pristine ZnO showed PCE of 9.97%. Moreover, the suppressed surface defects of ZnO-K substantially improved long-term stability under air. The device using ZnO-K exhibited superior long-term air storage stability (96% retention after 90 days) compared to that using pristine ZnO (88% retention after 90 days). The ZnO-K-based device also exhibited improved photostability under air. Under continuous light illumination for 600 min, the ZnO-K-based device retained 96% of its initial PCE, whereas the pristine ZnO-based device retained only 67%

Enhanced performance of lead sulfide quantum dot-sensitized solar cells by controlling the thickness of metal halide perovskite shells

The metal halide perovskite CH3NH3PbI3 (MAP) can be applied as the shell layer of lead sulfide quantum dots (PbS QDs) for improving solar power conversion efficiency. However, basic physics for this PbS core/MAP shell QD system is still unclear and needs to be clarified to further improve efficiency. Therefore, in this study, we investigate how MAP shell thickness affects device performance and dynamics of charge carriers for PbS QD-sensitized solar cells. Covering the PbS QDs with the MAP shell layers of an appropriate thickness around 0.34 nm greatly suppresses charge carrier recombination at surface defects along with improved carrier transport to neighboring oxide and polymer layers. Therefore, this MAP shell thickness provides the highest open-circuit voltage, short-circuit current density, and fill factor for solar cells. Overall power conversion efficiencies of these solar cells reached about 4.1%, which is about six-fold higher than that for solar cells without MAP (about 0.7%). Additionally, use of the MAP shell layers can prevent oxidation of PbS QDs and, therefore, makes a degradation of solar cell performance slower in air

Hot Gas‐Blowing Assisted Crystallinity Management of Bar‐Coated Perovskite Solar Cells and Modules

The bar‐coating technique for perovskite solar cells has been studied as a scalable process in relation to solar cell commercialization. In large‐area bar coating, solvents with the high boiling points like dimethylformamide or γ‐butyrolactone have difficulty in obtaining uniform and planar film at room temperature as they have slow evaporation rate. As an alternative, 2‐methoxyethanol is a volatile and polar solvent, which is useful on a bar coating if applied using an air‐blowing method with an air knife. Herein, a hot gas‐blowing method for the fabrication of a perovskite layer to achieve both proper solvent evaporation and high crystallinity is developed. With 75 °C of N2 blowing on the bar‐coated perovskite solution, highly crystalline perovskite films with large grains without voids are fabricated, showing excellent optical and electrical characteristics, such as long carrier lifetime, few carrier recombinations, and low trap density. Both small‐area solar cells and large‐area modules show good performance, 20.85% for the solar cell and 15.4% for the solar module. The results indicate that the newly proposed method can equally be applied to the fabrication of large‐area solar cells toward commercialization

Enhancing the Performance of Sensitized Solar Cells with PbS/CH₃NH₃PbI₃ Core/Shell Quantum Dots

We report on the fabrication of PbS/CH₃NH₃PbI₃ (=MAP) core/shell quantum dot (QD)-sensitized inorganic–organic heterojunction solar cells on top of mesoporous (mp) TiO₂ electrodes with hole transporting polymers (P3HT and PEDOT:PSS). The PbS/MAP core/shell QDs were in situ-deposited by a modified successive ionic layer adsorption and reaction (SILAR) process using PbI₂ and Na₂S solutions with repeated spin-coating and subsequent dipping into CH₃NH₃I (=MAI) solution in the final stage. The resulting device showed much higher efficiency as compared to PbS QD-sensitized solar cells without a MAP shell layer, reaching an overall efficiency of 3.2% under simulated solar illumination (AM1.5, 100 mW·cm^–2). From the measurement of the impedance spectroscopy and the time-resolved photoluminescence (PL) decay, the significantly enhanced performance is mainly attributed to both reduced charge recombination and better charge extraction by MAP shell layer. In addition, we demonstrate that the MAP shell effectively prevented the photocorrosion of PbS, resulting in highly improved stability in the cell efficiency with time. Therefore, our approach provides method for developing high performance QD-sensitized solar cells

Deep level trapped defect analysis in CH3NH3PbI3 perovskite solar cells by deep level transient spectroscopy

We report the presence of defects in CH3NH3PbI3, which is one of the main factors that deteriorates the performance of perovskite solar cells. Although the efficiency of the perovskite solar cells has been improved by curing defects using various methods, deeply trapped defects in the perovskite layer have not been systematically studied, and their function is still unclear. The comparison and anal. of defects in differently prepd. perovskite solar cells reveals that both solar cells have two kinds of deep level defects (E1 and E2)​. In the one-​pot soln. processed solar cell, the defect state E1 is dominant, while E2 is the major defect in the solar cell prepd. using the cuboid method. Since the energy level of E1 is higher than that of E2, the cuboid solar cell shows higher open-​circuit voltage and efficiency

Origins of High Performance and Degradation in the Mixed Perovskite Solar Cells

The origins of the high device performance and degradation in the air are the greatest issues for commercialization of perovskite solar cells. Here this study investigates the possible origins of the mixed perovskite cells by monitoring defect states and compositional changes of the perovskite layer over the time. The results of deep-level transient spectroscopy analysis reveal that a newly identified defect formed by Br atoms exists at deep levels of the mixed perovskite film, and its defect state shifts when the film is aged in the air. The change of the defect state is originated from loss of the methylammonium molecules of the perovskite layer, which results in decreased J(SC), deterioration of the power conversion efficiency and long-term stability of perovskite solar cells. The results provide a powerful strategy to diagnose and manage the efficiency and stability of perovskite solar cells

Microscopic Analysis of Inherent Void Passivation in Perovskite Solar Cells

The presence of voids in perovskite solar cells influences the efficiency because of accelerated charge recombination. The induced electric field near voids due to band bending attracts photogenerated electrons and holes toward the voids, leading to carrier recombination. However, if the surface of the voids is coated by materials with a band gap higher than that of the perovskite layer, the strong electric field induced near the voids in the opposite way prevents carriers from recombining. We identified voids in the perovskite layer by using an electron beam-induced current technique and found the influence of field-assisted passivation by organic materials on the efficiency of the solar cell

Selective growth of layered perovskites for stable and efficient photovoltaics

Perovskite solar cells (PSCs) are promising alternatives toward clean energy because of their high-power conversion efficiency (PCE) and low materials and processing cost. However, their poor stability under operation still limits their practical applications. Here we design an innovative approach to control the surface growth of a low dimensional perovskite layer on top of a bulk three-dimensional (3D) perovskite film. This results in a structured perovskite interface where a distinct layered low dimensional perovskite is engineered on top of the 3D film. Structural and optical properties of the stack are investigated and solar cells are realized. When embodying the low dimensional perovskite layer, the photovoltaic cells exhibit an enhanced PCE of 20.1% on average, when compared to pristine 3D perovskite. In addition, superior stability is observed: the devices retain 85% of the initial PCE stressed under one sun illumination for 800 hours at 50 °C in an ambient environment

Dimensionally Engineered Perovskite Heterostructure for Photovoltaic and Optoelectronic Applications

Although 2D|3D has shown potential for application in multifunctional devices, the principle of operation for multifunction devices (SOLAR Cell-LED: SOLED) has not yet been revealed. However, most studies have reported that the devices have only one auspicious characteristic. Here in this study the SOLED devices are monitored and investigated in a 2D|3D heterostructure with a multidimensional perovskite. It is fond that a 2D|3D heterostructure with a multidimensional perovskite interface induces carrier transmission from the interface, increasing the density of electrons and holes, and increasing their recombination. An interface-engineered perovskite 2D|3D-heterojunction structure is employed to realize the multifunctional photonic device in on-chip, exhibiting overall power conversion efficiencies of photovoltaics up to 21.02% under AM1.5, and external quantum efficiency of the light-emitting diode up to 5.13%. This novel phenomenon is attributed to carrier transfer resulting in a high carrier density and enhanced carrier recombination at the 2D|3D interface

Enhancing the Performance of Sensitized Solar Cells with PbS/CH3NH3PbI3 Core/Shell Quantum Dots

We report on the fabrication of PbS/CH3NH3PbI3 (=MAP) core/shell quantum dot (QD)sensitized inorganic organic heterojunction solar cells on top of mesoporous (mp) TiO2 electrodes with hole transporting polymers (P3HT and PEDOT:PSS). The PbS/MAP core/shell QDs were in situ-deposited by a modified successive ionic layer adsorption and reaction (SILAR) process using PbI2 and Na2S solutions with repeated spin-coating and subsequent dipping into CH3NH3I (=MAI) solution in the final stage. The resulting device showed much higher efficiency as compared to PbS QD-sensitized solar cells without a MAP shell layer, reaching an overall efficiency of 3.2% under simulated solar illumination (AM1.5, 100 mW.cm(-2)). From the measurement of the impedance spectroscopy and the time-resolved photoluminescence (PL) decay, the significantly enhanced performance is mainly attributed to both reduced charge recombination and better charge extraction by MAP shell layer. In addition, we demonstrate that the MAP shell effectively prevented the photocorrosion of PbS, resulting in highly improved stability in the cell efficiency with time. Therefore, our approach provides method for developing high performance QD-sensitized solar cellsclos