Preventing Hysteresis in Perovskite Solar Cells by Undoped Charge Blocking Layers

Preventing hysteresis in lead halide perovskite solar cells remains one of the key challenges hindering their integration into industrial applications. Herein, we numerically study a model solar cell system that is based on a mixed electron–ion conducting perovskite active layer and vary the configuration of undoped charge-blocking layers within the device. We find that the use of undoped blocking layers significantly reduces the potential drop across the perovskite active layer. This redistribution of voltage across the device suppresses the ion accumulation, or deficiency, which would otherwise develop at the two ends of the active layer. The fill factor is not compromised, provided that the blocking layers’ mobility value is not lower than 0.01 cm² V^–1 s^–1, which results in devices with power-conversion efficiencies surpassing 20% and minimal hysteresis. We believe that this method not only can suppress hysteresis effectively but could also contribute to the long-term stability of such cells that have been shown to be adversely affected by ion migration

Large-Scale Compositional and Electronic Inhomogeneities in CH₃NH₃PbI₃ Perovskites and Their Effect on Device Performance

Photovoltaic devices based on lead halide perovskite materials are under extensive investigation due to their remarkably high efficiencies. However, it has become evident that the photovoltaic performance of these devices shows wide distributions, even across individual perovskite samples, resulting in the practice of presenting performance histograms, rather than single values. We demonstrate that these variations in performance are related to the large-scale compositional and electronic inhomogeneities of perovskite films, which we characterize using X-ray and ultraviolet photoemission spectroscopy mapping. These inhomogeneities are observed for three different fabrication methods for the perovskite layers, and while they are unaltered by storage in nitrogen and dry air, long-term exposure to vacuum increases the homogeneity of the surface structure. We demonstrate that perovskite films with a nonuniform surface structure result in broadly varying photovoltaic performance despite seemingly similar bulk properties such as absorption and microstructure

Improved Performance of ZnO/Polymer Hybrid Photovoltaic Devices by Combining Metal Oxide Doping and Interfacial Modification

Photoinduced charge separation at hybrid organic–inorganic interfaces is poorly understood and challenging to control. We investigate charge separation at a model system of ZnO/poly­(3-hexylthiophene) (P3HT) and employ Sr doping of ZnO and phenyl-C61-butyric acid (PCBA) self-assembled modification to study and enhance the charge separation efficiency. We find that doping alone lowers the efficiency of charge separation due to the introduction of defect states at the oxide surface. However, with the combination of doping and molecular modification, charge separation efficiency is significantly enhanced due to the passivation of interfacial traps and improved modifier coverage. This demonstrates a complex noncumulative effect of doping and surface modification and shows that with the correct choice of metal oxide dopant and organic modifier, a poorly performing hybrid interface can be turned into an efficient one

Triptycene-Based Porous Metal-Assisted Salphen Organic Frameworks: Influence of the Metal Ions on Formation and Gas Sorption

Porous organic polymers (POPs) are chemically and thermally robust materials and have been often investigated for their gas sorption properties. From the related field of metal–organic frameworks (MOFs) it is known that open ligation sites at metal centers can enhance the performance of gas sorption significantly, especially the selectivity toward one gas of a binary mixture, such as CO₂/N₂ or CO₂/CH₄. POPs that contain metal centers are rarer. One possibility to introduce metals into POPs is by the synthesis of metal-assisted salphen organic frameworks (MaSOFs), where the framework development is associated with the formation of the metal–salphen pockets. Based on a hexakissalicylaldehyde, a variety of three-dimensional isostructural porous MaSOFs with different metal ions (Zn²⁺, Ni²⁺, Cu²⁺, Pd²⁺, and Pt²⁺) are introduced. All compounds show a very similar pore structure and comparable specific surface areas, which make these MaSOFs ideal candidates to study the influence of the nature of the incorporated metal center on gas sorption selectivity. Due to the environmental importance, the adsorption of CO₂ in comparison to N₂ and CH₄ was extensively studied. Depending on the metal ions, the heat of adsorption was different as well as the Henry and IAST selectivities. Cu–MaSOF₁₀₀ for instance shows a high <i>Q</i>_st of 31.2 kJ mol^–1 for CO₂ and an uptake of 14.9 wt % at 1 bar and 273 K. The IAST selectivity of CO₂/N₂ for an 80/20 mixture is with <i>S</i>_IAST = 52 very high for a metal containing POP and even comparable to some of the best performing MOFs. The MaSOFs are stable even in boiling water. This, as well as the simple synthesis, makes them potential good candidates for CO₂ removal of binary mixtures

Triptycene-Based Porous Metal-Assisted Salphen Organic Frameworks: Influence of the Metal Ions on Formation and Gas Sorption

Porous organic polymers (POPs) are chemically and thermally robust materials and have been often investigated for their gas sorption properties. From the related field of metal–organic frameworks (MOFs) it is known that open ligation sites at metal centers can enhance the performance of gas sorption significantly, especially the selectivity toward one gas of a binary mixture, such as CO₂/N₂ or CO₂/CH₄. POPs that contain metal centers are rarer. One possibility to introduce metals into POPs is by the synthesis of metal-assisted salphen organic frameworks (MaSOFs), where the framework development is associated with the formation of the metal–salphen pockets. Based on a hexakissalicylaldehyde, a variety of three-dimensional isostructural porous MaSOFs with different metal ions (Zn²⁺, Ni²⁺, Cu²⁺, Pd²⁺, and Pt²⁺) are introduced. All compounds show a very similar pore structure and comparable specific surface areas, which make these MaSOFs ideal candidates to study the influence of the nature of the incorporated metal center on gas sorption selectivity. Due to the environmental importance, the adsorption of CO₂ in comparison to N₂ and CH₄ was extensively studied. Depending on the metal ions, the heat of adsorption was different as well as the Henry and IAST selectivities. Cu–MaSOF₁₀₀ for instance shows a high <i>Q</i>_st of 31.2 kJ mol^–1 for CO₂ and an uptake of 14.9 wt % at 1 bar and 273 K. The IAST selectivity of CO₂/N₂ for an 80/20 mixture is with <i>S</i>_IAST = 52 very high for a metal containing POP and even comparable to some of the best performing MOFs. The MaSOFs are stable even in boiling water. This, as well as the simple synthesis, makes them potential good candidates for CO₂ removal of binary mixtures

Charge Dynamics in Solution-Processed Nanocrystalline CuInS₂ Solar Cells

We investigate charge dynamics in solar cells constructed using solution-processed layers of CuInS₂ (CIS) nanocrystals (NCs) as the electron donor and CdS as the electron acceptor. By using time-resolved spectroscopic techniques, we are able to observe photoinduced absorptions that we attribute to the mobile hole carriers in the NC film. In combination with transient photocurrent and photovoltage measurements, we monitor charge dynamics on time scales from 300 fs to 1 ms. Carrier dynamics are investigated for devices with CIS layers composed of either colloidally synthesized 1,3-benzenedithiol-capped nanocrystals or <i>in situ</i> sol–gel synthesized thin films as the active layer. We find that deep trapping of holes in the colloidal NC cells is responsible for decreases in the open-circuit voltage and fill factor as compared to those of the sol–gel synthesized CIS/CdS cell

Synthesis and Modeling of Uniform Complex Metal Oxides by Close-Proximity Atmospheric Pressure Chemical Vapor Deposition

A close-proximity atmospheric pressure chemical vapor deposition (AP-CVD) reactor is developed for synthesizing high quality multicomponent metal oxides for electronics. This combines the advantages of a mechanically controllable substrate-manifold spacing and vertical gas flows. As a result, our AP-CVD reactor can rapidly grow uniform crystalline films on a variety of substrate types at low temperatures without requiring plasma enhancements or low pressures. To demonstrate this, we take the zinc magnesium oxide (Zn_1–<i>x</i>Mg_<i>x</i>O) system as an example. By introducing the precursor gases vertically and uniformly to the substrate across the gas manifold, we show that films can be produced with only 3% variation in thickness over a 375 mm² deposition area. These thicknesses are significantly more uniform than for films from previous AP-CVD reactors. Our films are also compact, pinhole-free, and have a thickness that is linearly controllable by the number of oscillations of the substrate beneath the gas manifold. Using photoluminescence and X-ray diffraction measurements, we show that for Mg contents below 46 at. %, single phase Zn_1–<i>x</i>Mg_<i>x</i>O was produced. To further optimize the growth conditions, we developed a model relating the composition of a ternary oxide with the bubbling rates through the metal precursors. We fitted this model to the X-ray photoelectron spectroscopy measured compositions with an error of Δ<i>x</i> = 0.0005. This model showed that the incorporation of Mg into ZnO can be maximized by using the maximum bubbling rate through the Mg precursor for each bubbling rate ratio. When applied to poly­(3-hexylthiophene-2,5-diyl) hybrid solar cells, our films yielded an open-circuit voltage increase of over 100% by controlling the Mg content. Such films were deposited in short times (under 2 min over 4 cm²)

Ultrafast Charge- and Energy-Transfer Dynamics in Conjugated Polymer: Cadmium Selenide Nanocrystal Blends

Hybrid nanocrystal–polymer systems are promising candidates for photovoltaic applications, but the processes controlling charge generation are poorly understood. Here, we disentangle the energy- and charge-transfer processes occurring in a model system based on blends of cadmium selenide nanocrystals (CdSe-NC) with poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) using a combination of time-resolved absorption and luminescence measurements. The use of different capping ligands (<i>n</i>-butylamine, oleic acid) as well as thermal annealing allows tuning of the polymer–nanocrystal interaction. We demonstrate that energy transfer from MDMO-PPV to CdSe-NCs is the dominant exciton quenching mechanism in nonannealed blends and occurs on ultrafast time scales (<1 ps). Upon thermal annealing electron transfer becomes competitive with energy transfer, with a transfer rate of 800 fs independent of the choice of the ligand. Interestingly, we find hole transfer to be much less efficient than electron transfer and to extend over several nanoseconds. Our results emphasize the importance of tuning the organic–nanocrystal interaction to achieve efficient charge separation and highlight the unfavorable hole-transfer dynamics in these blends

Effect of Ozone on the Stability of Solution-Processed Anthradithiophene-Based Organic Field-Effect Transistors

We have investigated the degradation effects of ozone exposure on organic field-effect transistors based on 2,8-difluoro-5,11-bis­(triethylsilylethynyl)­anthradithiophene as the organic semiconducting channel layer, as well as on thin films of this widely used, high-mobility, small molecule semiconductor. Electrical <i>I</i>–<i>V</i> measurements showed a loss of transistor characteristic behavior. We present ¹H Nuclear Magnetic Resonance (NMR) spectroscopy results as well as X-ray Photoemission Spectroscopy (XPS) and Fourier Transform Infrared (FTIR) spectroscopy measurements showing the oxidation of the parent molecule, from which we suggest various possible reaction paths

Structure Influence on Charge Transport in Naphthalenediimide–Thiophene Copolymers

Reported here is a characterization of a series of NDI–thiophene copolymers with one, two, three, and four thiophene units synthesized using Stille polycondensation of dibromo-naphthalene diimide and the trimethylstannylthiophene monomers. The effect of extension of the thiophene donor group is studied in terms of structure-charge transport correlation. The influence of side chains located on the thiophene units of copolymers with two and four thiophene units per monomer is also investigated. Charge transport of both signs is studied experimentally in field-effect transistors. Microstructural data obtained by near-edge X-ray absorption fine structure (NEXAFS) and grazing incidence wide-angle X-ray scattering (GIWAXS) is supported by AFM topography scans. Ultraviolet photoelectron spectroscopy (UPS) and UV–vis spectroscopy data are employed in the measurement of energy levels, and changes with annealing temperature are also discussed. Most of the polymers reach excellent electron and hole mobility with one copolymer (NDI-T4) exhibiting an especially balanced ambipolar charge transport of 0.03 cm² V^–1 s^–1. An odd–even effect in hole mobility is observed with higher values for polymers with an even number of thiophene units. The reported findings indicate that the final charge transport properties are a result of the interplay of many factors, including crystallinity, planarity and linearity of chain, spacing between acceptor units and packing of solubilizing branched side chains