Boosting Photocurrent via Heating BiFeO3 Materials for Enhanced Self‐Powered UV Photodetectors

BiFeO₃ (BFO) is a potentially important Pb‐free ferroelectric with a narrow bandgap and is expected to become a novel photodetector. The photocurrent in BFO₃ strongly depends on the temperature but only a few studies have investigated in detail the relationships between photocurrent and temperature. Here, the temperature‐dependent photocurrent and the corresponding photosensing properties of a Ag/BFO/indiumtin oxide (ITO) photodetector based on an optimized planar‐structured electrode configuration are investigated. The photocurrent and responsivity of the BFO₃‐based photodetector can first be increased and then be decreased with increasing temperature. The largest photocurrent and responsivity can reach 51.5 µA and 6.56 × 10^?4 A W^?1 at 66.1 °C, which is enhanced 126.3% as compared with that at room temperature. This may be caused by the temperature‐modulated bandgap and barrier height in Ag/BFO/ITO device. This study clarifies the relationship between photosensing performance and the operating temperature of BFO‐based photodetector and will push forward the application of ferroelectric materials in photoelectric field.     

Solid‐State Photovoltaic Thin Films using TiO2, Organic Dyes,and Layer‐by‐Layer Polyelectrolyte Nanocomposites

We report photovoltaic devices consisting of patterned TiO₂, porphyrin dyes, and layer‐by‐layer (LBL) polyelectrolyte multilayer/oligoethylene glycol dicarboxylic acid (OEGDA) composite films. A composite polyelectrolyte LBL/OEGDA film was fabricated by formation of an alternating multilayer of linear polyethyleneimine (LPEI) and polyacrylic acid (PAA), followed by immersion of the LBL film into an OEGDA aqueous solution. The ionic conductivity attained in this LBL LPEI/PAA and OEGDA composite film was approximately 10^–5 S cm^–1 at room temperature and humidity. Investigations of dye‐sensitized photovoltaic devices constructed with the LBL (LPEI/PAA)/OEGDA composite films, TiO₂, and four types of porphyrin dyes resulted in optimization of the dye molecule and its orientation at the interface with the ionically conductive composite. The photocurrent value of photovoltaic devices constructed with the composite LBL/OEGDA film from illumination of a xenon white light source exhibited a nearly 1.5 times enhancement over the device without OEGDA. This enhancement of the photocurrent was due to the high room‐temperature ionic conductivity of the multilayer composite film. Further marked improvements of the photovoltaic performance were achieved by patterning the TiO₂ electrode using polymer stamping as a template for TiO₂ deposition. The device with patterned TiO₂ electrodes exhibited almost 10 times larger conversion efficiencies than a similar device without patterning.     

Ferroelectric,Photoelectric, and Photovoltaic Performance of Silver Niobate Ceramics

Ferroelectrics coupled with solar energy conversion are receiving intensive research interest. However, most ferroelectrics with a large remnant polarization can only harvest ultraviolet light in the solar spectrum. Herein, high‐quality silver niobate (AgNbO₃) ceramics prepared by spark plasma sintering is reported with a bandgap of ≈2.75 eV and a long tail absorption until 800 nm, leading to outstanding photoelectric properties featured by the visible‐light response over 550 nm. Instantaneous photoresponse measurement using a 355 nm nanosecond pulse laser shows a fast response speed in nanoseconds. Moreover, the ceramic exhibits an intriguing photovoltaic effect under either electric poling or mechanical polishing. Both approaches have switchable characteristics and produce a stable photovoltage as well as photocurrent, while temperature dependence behavior reveals distinctions between ferroelectric polarization and ferroelastic strains in determining the photovoltaic properties. Piezoelectric force microscopy characterization further confirms distinctions between the underlying mechanisms. The electric poling induced photovoltaic effect stems from the aligned polarization involving the ferroelectric component, whereas the mechanical polishing induced photovoltaic effect is associated with the flexoelectricity induced by strain gradients. These results not only show AgNbO₃ to be a promising material for photoelectric application but also deepen the understanding of the mechanism underlying ferroelectric photovoltaics.     

The Influence of Solubilizing Chain Stereochemistry on Small Molecule Photovoltaics

Three stereochemically pure isomers and two isomeric mixtures of a solution‐processable diketopyrrolopyrrole‐containing oligothiophene ( SMDPPEH ) have been used to study the effect of 2‐ethylhexyl solubilizing group stereochemistry on the film morphology and bulk heterojunction (BHJ) solar cell characteristics of small molecule organic photovoltaics. The different SMDPPEH stereoisomer compositions exhibit nearly identical optoelectronic properties in the molecularly dissolved state, as well as in amorphous films blended with PCBM. However, for films in which SMDPPEH crystallization is induced by thermal annealing, significant differences in molecular packing between the different stereoisomer formulations are observed. These differences are borne out in photovoltaic device characteristics for which unannealed devices show very similar behavior, while after annealing the RR‐ and SS‐SMDPPEH enantiomers show blue‐shifted peak EQE relative to the SMDPPEH isomer mixtures. Unannealed devices made from the most crystalline stereoisomer, meso RS‐SMDPPEH , are not completely amorphous, and show improved photocurrent generation as a result. Unlike the other compounds, after thermal annealing the RS‐SMDPPEH devices show reduced device performance. The results reveal that the chirality of commonly used 2‐ethylhexyl solubilizing chains can have a significant effect on the morphology, absorption, and optimum processing conditions of small molecule organic thin films used as photovoltaic device active layers.     

Impact of perovskite precursor solution temperature on charge carrier dynamics and photovoltaic performance of perovskite based solar cells

Even though perovskite based solar cells have been routinely fabricated with preheated perovskite solutions, currently the underlying mechanism of how the perovskite precursor solution temperature influences perovskite solar cells has not been studied yet. Therefore, we investigated the impacts of perovskite solution temperatures on charge carrier dynamics of perovskite film and perovskite solar cell performance that were quantitatively analyzed using steady-state photoluminescence (PL), time-resolved emission spectra (TRES), excitation-dependence of PL and current-voltage measurements. It is found that the perovskite solution temperature greatly influenced the morphologies, defect densities and states, and charge recombination dynamics of perovskite thin films. Particularly, steady-state and time-resoled PL measurements revealed that perovskite thin films prepared with the perovskite solution temperature around 70 °C produced lowest surface and bulk defect densities. In addition, it is found that the perovskite solution temperature around 70 °C led to exciton-like transitions while lower and higher solution temperatures led to defect-mediated recombination of perovskite thin films. Such recombination dynamics of perovskite films strongly influenced the light absorption and extraction efficiencies of photogenerated charge carriers which in turn influenced short-circuit current, fill factor, and open circuit voltage of perovskite solar cells. As a result, better photovoltaic performance of perovskite solar cells was observed when prepared with the precursor temperature around 70 °C.     

Growth Mechanism of Thermally Evaporated γ-CsPbI3 Film

Cesium lead triiodide (CsPbI₃) inorganic perovskite possesses excellent thermal stability and matched bandgap for silicon-based tandem photovoltaics. The solution method with high-temperature annealing process for CsPbI₃ film preparation creates challenges to scalable application and conformal growth on the textured silicon. Although additives can decrease the annealing temperature, it will introduce undesired organic components and increase material cost. Thermal co-evaporation for CsPbI₃ has intrinsic advantages to overcome these issues, but the vague growth mechanism impedes the photovoltaic device development. In this study, γ-CsPbI₃ films are directly obtained through co-evaporation at 50 °C without any additives or high-temperature post-annealing. Focusing on the molecular thermodynamic calculations, it is proposed that the unique kinetic energy of evaporated molecules and the in-situ substrate thermal energy synergistically provide the energy prerequisite for γ-CsPbI₃ formation. Furthermore, the γ phase stabilization is clarified by the crystal grain size effect with regard to the Gibbs free energy difference between the γ and δ phases, which is adjusted through substrate temperature and evaporation rate. The obtained p-i-n device realizes an efficiency of 12.75%, which is the highest value for the thermally evaporated γ-CsPbI₃ photovoltaics at low temperature without additives. This study deepens the understanding of thermal evaporation process, benefiting to high-performance CsPbI₃-textured silicon tandem photovoltaics.     

Enhanced Device Performance of Perovskite Photovoltaics by Magnetic Field‐Aligned Perovskites‐Magnetic Nanoparticles Composite Thin Film

Perovskite photovoltaics have drawn great attention in both academic and industrial sectors in the past decade. To date, impressive device performance has been achieved in state‐of‐the‐art device architectures through morphological manipulation and generic interface engineering. In this study, enhanced device performance of perovskite photovoltaics by magnetic field‐aligned CH₃NH₃PbI₃‐mixed Fe₃O₄ magnetic nanoparticles (CH₃NH₃PbI₃:Fe₃O₄) composite thin films is reported. It is found that magnetic field‐aligned CH₃NH₃PbI₃:Fe₃O₄ composite thin films possess superior film morphology, boosted and balanced charge carrier mobility, and suppressed trap density. Moreover, perovskite photovoltaics by magnetic field‐aligned CH₃NH₃PbI₃:Fe₃O₄ composite thin films exhibit suppressed charge carrier recombination and shorter charge carrier extraction time. As a result, perovskite solar cells by magnetic field‐aligned CH₃NH₃PbI₃:Fe₃O₄ composite thin films exhibit 20.23% power conversion efficiency with significantly reduced photocurrent hysteresis. Moreover, perovskite photodetectors by magnetic field‐aligned CH₃NH₃PbI₃:Fe₃O₄ composite thin films exhibit a photoresponsivity of 858 mA W^?1, a photodetectivity over 10¹³ Jones (1 Jones = 1 cm Hz^1/2 W^?1) and a linear dynamic range over 160 dB at room temperature. All these device performance parameters are significantly better than those by pristine CH₃NH₃PbI₃ thin film. Thus, these studies provide a facile way to boost device performance of perovskite photovoltaics.     

Ionic Additive Engineering Toward High‐Efficiency Perovskite Solar Cells with Reduced Grain Boundaries and Trap Density

Organic–inorganic hybrid perovskite solar cells have emerged as one of the promising photovoltaic candidates to generate renewable energy. However, the large amounts of grain boundaries and trap states that exist in the bulk or interfacial regions of perovskite films limit further enhancement of device efficiency. Herein, an additive engineering strategy is introduced employing trimethylammonium chloride in the methylammonium iodide precursor solution to prepare methylammonium lead iodide perovskite films with reduced grain boundaries and trap densities. This leads to an increased charge carrier diffusion coefficient and diffusion length, as evaluated by impedance and voltage decay measurements, intensity‐modulated photovoltage, and photocurrent spectroscopies. The proportion of nonradiative recombination processes is significantly reduced, consequently increasing device efficiency from 19.1% to 20.9% in these perovskite solar cells.     

Probing the Photovoltage and Photocurrent in Perovskite Solar Cells with Nanoscale Resolution

In this work, by coupling scanning Kelvin probe force microscopy (SKPM) and photoconductive atomic force microscopy (pcAFM), the variation of the surface potential, photogenerated voltage, and photocurrent networks of the perovskite solar cells (PSCs) with different film topography is studied. The nanoscale photovoltaic reaction of different perovskite capping layers with three different perovskite crystalline sizes is first studied by using the SKPM technique. The performance of the overall device is correlated with the local nanostructure of the perovskite film. Photocurrent maps under various applied voltages are also presented. The pcAFM measurements on three different morphology positions determine that the defect region on the capping layer can induce the charge recombination process in the complete PSCs and thus suppress the V_oc in the complete device. These results suggest that the performance of PSCs can still be improved through better control of morphology. Henceforth, SKPM coupled with pcAFM techniques has the potential to become a routine characterization tool for perovskite organic and hybrid photovoltaics.     

A Low Temperature Route toward Hierarchically Structured Titania Films for Thin Hybrid Solar Cells

The requirement of high‐temperature calcination for titanium dioxide in (solid‐state) dye‐sensitized solar cells (DSSCs) implies challenges with respect to reduced energy consumption and the potential for flexible photovoltaic devices. Moreover, the use of dye molecules increases production costs and leads to problems related with dye bleaching. Therefore, fabrication of dye‐free hybrid solar cells at low temperature is a promising alternative for current DSSC technology. In this work the authors fabricate hierarchically structured titania thin films by combining a polystyrene‐block‐polyethylene oxide template assisted sol–gel synthesis with nano‐imprint lithography at low temperatures. The achieved films are filled with poly(3‐hexylthiophene) to form the active layer of hybrid solar cells. The surface morphology is probed via scanning electron microscopy and atomic force microscopy, and the bulk film morphology is examined with grazing incidence X‐ray scattering. Good light absorption by the active layer is proven by UV–vis spectroscopy. An enhancement in light absorption is observed and ascribed to light scattering in mesoporous titania films with imprinted superstructures. Accordingly a better photovoltaic performance is found for nano‐imprinted solar cells at various angles of light incidence.     

Interesting Evidence for Template‐Induced Ferroelectric Behavior in Ultra‐Thin Titanium Dioxide Films Grown on (110) Neodymium Gallium Oxide Substrates

The first evidence for room‐temperature ferroelectric behavior in anatase‐phase titanium dioxide (a‐TiO₂) is reported. Behavior strongly indicative of ferroelectric characteristics is induced in ultra‐thin (20 nm to 80 nm) biaxially‐strained epitaxial films of a‐TiO₂ deposited by liquid injection chemical vapor deposition onto (110) neodymium gallium oxide (NGO) substrates. The films exhibit significant orthorhombic strain, as analyzed by X‐ray diffraction and high‐resolution transmission electron microscopy. The films on NGO show a switchable dielectric spontaneous polarization when probed by piezoresponse force microscopy (PFM), the ability to retain polarization information written into the film using the PFM tip for extended periods (several hours) and at elevated temperatures (up to 100 °C) without significant loss, and the disappearance of the polarization at a temperature between 180 and 200 °C, indicative of a Curie temperature within this range. This combination of effects constitutes strong experimental evidence for ferroelectric behavior, which has not hitherto been reported in a‐TiO₂ and opens up the possibility for a range of new applications. A model is presented for the effects of large in‐plane strains on the crystal structure of anatase which provides a possible explanation for the experimental observations.     

Specific optical and photoelectric properties of thin CuIn3Se5 films synthesized by laser deposition

Optical and photovoltaic properties of films produced by laser deposition of CuIn₃Se₅ and the effect of the temperature conditions of film synthesis on these properties are studied. The composition of the films deposited onto substrates held at room temperature coincides with that of the target, with the films obtained in the vitreous state. Heating of substrates during deposition leads to a larger fraction of the crystalline phase and lower fraction of Se in a film. Annealing of glassy CuIn₃Se₅ films in a vacuum at temperatures of up to 700 K causes their crystallization without changes in composition. The photosensitivity of the films is highest at annealing temperatures of 510–600 K.     

Dramatically enhanced performances and ideally controlled nano-morphology via co-solvent processing in low bandgap polymer solar cells

The device performance of photovoltaics with a polymer:fullerene bulk heterojunction (BHJ) structure, consisting of DT-PDPP2T-TT donor polymer and poly(3-hexylthiophene):[6,6]phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) acceptor compound, was investigated as a function of co-solvent composition. An enhancement of the photocurrent density and fill factor is observed in diodes made by spin-coating with chloroform mixed with ortho-dichlorobenzene, which allows a significantly higher device efficiency of 5.55% compared to diodes made from neat chloroform (efficiency = 3.61%). To clarify the role of the co-solvent, we investigated the nanoscale morphology with AFM, TEM and 2D-GIWAXS techniques and also the free-charge carrier mobility via space-charge limited current theory. We obtained the result that, under such supersaturated conditions, co-solvents induce increased polymer crystalline aggregation into a 3D phase structure and boost charge-carrier transport characteristics. This provides a rational basis for the development of ideally-controlled BHJ films that yield efficient DT-PDPP2T-TT:PCBM solar cells. Therefore, carefully selecting solvent mixtures provides an approach toward efficient low bandgap polymer solar cells.     

Tailoring Ordered Mesoporous Titania Films via Introducing Germanium Nanocrystals for Enhanced Electron Transfer Photoanodes for Photovoltaic Applications

Based on a diblock-copolymer templated sol–gel synthesis, germanium nanocrystals (GeNCs) are introduced to tailor mesoporous titania (TiO₂) films for obtaining more efficient anodes for photovoltaic applications. After thermal annealing in air, the hybrid films with different GeNC content are investigated and compared with films undergoing an argon atmosphere annealing. The surface and inner morphologies of the TiO₂/GeO_x nanocomposite films are probed via scanning electron microscopy and grazing-incidence small-angle X-ray scattering. The crystal phase, chemical composition, and optical properties of the nanocomposite films are examined with transmission electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible spectroscopy. Special focus is set on the air-annealed nanocomposite films since they hold greater promise for photovoltaics. Specifically, the charge–carrier dynamics of these air-annealed nanocomposite films are studied, and it is found that, compared with pristine TiO₂ photoanodes, the GeNC addition enhances the electron transfer, yielding an increase in the short-circuit photocurrent density of exemplary perovskite solar cells and thus, an enhanced device efficiency as well as a significantly reduced hysteresis.     

Template free titiania photoanodes modified with carbon black or multi-wall carbon nanotubes: Thermal treatment at low and high temperature for the fabrication of quasi-solid state dye sensitized solar cells

A simple procedure was developed to prepare modified titiania (TiO₂) photoanodes for dye sensitized solar cells at low and high temperature in order to improve overall cell efficiency. Modification of TiO₂ films achieved by the incorporation of either carbon black powder (CBP) or multi-wall carbon nanotubes (MWCNTs). A small quantity of titanium alkoxide was added in a dispersion of titiania (TiO₂) powder consisting of nanoparticles at room temperature, which after alkoxide׳s hydrolysis helps to the connection between titiania (TiO₂) particles and to the formation of mechanically stable relatively thick films on conductive glass substrates. The absence of surfactant allowed us to prepare films at relatively low temperature (~100 °C), while the effect of sintering at a higher temperature (500 °C) was also studied. The structural properties of the films were examined with porosimetry method and microscopy analysis. Better electrical results were obtained for the MWCNT (0.1 wt%) modified TiO₂ films, with 3.14% and 4.68% conversion efficiencies under 1 sun illumination after treatment at 100 °C and 500 °C, respectively. The enhancement in photocurrent for MWCNT-TiO₂ films compared to pure TiO₂ films is attributed to the improved interconnectivity between TiO₂ nanoparticles, which further improved the electron transport through the film. For carbon doped CBP-TiO₂ cells, lower efficiencies were observed compared to pure TiO₂.     

Mapping Morphological and Structural Properties of Lead Halide Perovskites by Scanning Nanofocus XRD

Scanning nanofocus X‐ray diffraction (nXRD) performed at a synchrotron is used to simultaneously probe the morphology and the structural properties of spin‐coated CH₃NH₃PbI₃ (MAPI) perovskite films for photovoltaic devices. MAPI films are spin‐coated on a Si/SiO₂/poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) substrate held at different temperatures during the deposition in order to tune the perovskite film coverage. The films are then investigated using nXRD and scanning electron microscopy (SEM). The advantages of nXRD over SEM and other techniques are discussed. A method to visualize, selectively isolate, and structurally characterize single perovskite grains buried within a complex, polycrystalline film is developed. The results of nXRD measurements are correlated with solar cell device measurements, and it is shown that spin‐coating the perovskite precursor solution at elevated temperatures leads to improved surface coverage and enhanced solar cell performance.     

Low‐Temperature Superionic Conductivity in Strained Yttria‐Stabilized Zirconia

Very high lateral ionic conductivities in epitaxial cubic yttria‐stabilized zirconia (YSZ) synthesized on single‐crystal SrTiO₃ and MgO substrates by reactive direct current magnetron sputtering are reported. Superionic conductivities (i.e., ionic conductivities of the order ～1 Ω^?1cm^?1) are observed at 500 °C for 58‐nm‐thick films on MgO. The results indicate a superposition of two parallel contributions – one due to bulk conductivity and one attributable to conduction along the film–substrate interface. Interfacial effects dominate the conductivity at low temperatures (<350 °C), showing more than three orders of magnitude enhancement compared to bulk YSZ. At higher temperatures, a more bulk‐like conductivity is observed. The films have a negligible grain‐boundary network, thus ruling out grain boundaries as a pathway for ionic conduction. The observed enhancement in lateral ionic conductivity is caused by a combination of misfit dislocation density and elastic strain in the interface. These very high ionic conductivities in the temperature range 150–500 °C are of great fundamental importance but may also be technologically relevant for low‐temperature applications.     

Photoconductivity of C60 as an Origin of Bias‐Dependent Photocurrent in Organic Photovoltaics

The bulk‐ionized photoconductivity of C₆₀ is reported as an origin of the bias‐dependent linear change of the photocurrent in copper phthalocyanine (CuPc)/C₆₀ planar heterojunction solar cells, based on the observation of the variation of the bias‐dependent photocurrent on excitation wavelengths and the thickness‐dependent photocurrent of the C₆₀ layer. A theoretical model, which is a combination of the Braun‐Onsager model for the dissociation of excitons at the donor/acceptor interface and the Onsager model for the bulk ionization of excitons in the C₆₀ layer, describes the bias‐dependent photocurrent in the devices very well. The bulk‐ionized photoconductivity of C₆₀ must generally contribute to the photocurrent in organic photovoltaics, since fullerene and fullerene derivatives are widely used in these devices.     

ZnO过渡层对BiFeO_3薄膜铁电性能的影响

采用溶胶–凝胶法,在Pt(111)/Ti/SiO2/Si(100)衬底上采用逐层退火工艺制备了BFO(BiFeO3)、ZnO/BFO和ZAO(掺铝氧化锌)/BFO薄膜,研究了ZnO、ZAO过渡层对BFO薄膜晶相以及铁电、漏电和介电性能的影响。结果表明:与BFO薄膜相比,ZnO/BFO薄膜的表面更加致密、平整,结晶性更好,双剩余极化强度(2Pr)有非常大的提高,漏电和介电性能也均有改善。ZAO/BFO薄膜的铁电性能比ZnO/BFO薄膜的铁电性能差,这与ZAO的导电性强于ZnO有关。     

Guard flow-enhanced organic vapor jet printing of photovoltaic donor materials in air

We demonstrate the ability to rapidly deposit (local deposition rates well exceeding 1000 Å/s) in air a small molecular electron donor material boron subphthalocyanine chloride (SubPc) for use in photovoltaic devices. The method employs a highly collimated jet of hot nitrogen gas carrying organic vapor toward a substrate, on which the organic molecules condense. A secondary jet of nitrogen coaxially surrounds the primary jet, shielding the organic vapor from the atmosphere. We study how the guard flow rate affects the printed film morphology and the resulting device properties. Notably, the crystallinity of the donor film can be increased while lowering the thin-film roughness, enhancing short circuit photocurrent nearly twofold over films printed without guard flow, allowing the photovoltaic power conversion efficiency of planar heterojunction SubPc/C₆₀ cells to exceed 2% with SubPc deposited at ambient conditions.