High Photovoltage Research Articles

Atomic Layer Deposited Transition Metal Oxide-Titania Alloys As Corrosion Resistant Schottky Contacts for Silicon Photoanodes

Metal-insulator-semiconductor (MIS) structures are promising candidates for integrated solar driven water splitting devices. Often, the metal serves a dual purpose, catalyzing water oxidation while also setting the built-in field for extracting photogenerated carriers from the semiconductor. Recently, incorporation of additional protection layers into the MIS junction has made it possible to use semiconductor materials, such as silicon, that would normally be unstable under the conditions required for water oxidation.1 Although the theoretical maximum photovoltage for silicon photovoltaics is 700-800 mV,2 there is a tradeoff between photocurrent and photovoltage in adventitiously-doped TiO2.3 For example, maximum photovoltages of ~400 mV have been reported for nSi / TiO2 / metal photoanodes protected by highly conductive TiO2 layers,4 with some devices achieving only 200-250 mV.5 Our goal is to develop a more ideal protection layer using transition metal oxide-titania alloys synthesized by ALD. An ideal protection layer should be corrosion resistant, highly conductive, and enable a high photovoltage. For n-type silicon Schottky photoanodes, this means that the protection layer must also possess a high work function. We previously demonstrated that TiO2-RuO2 alloys composed of 13-46% Ru were highly conductive and consistently achieved photovoltages >525 mV without any post-processing anneals.6 The built-in field was set by the TiO2-RuO2 alloy, which possessed a sufficiently high work function of 5.02 eV. Although TiO2-RuO2 alloys were ideal Schottky contacts to nSi, they lacked the long-term corrosion resistance required of a protection layer. RuO2 is known to be unstable at the conditions required for water oxidation. We also report on ALD alloys of TiO2 and IrO2, which exhibit high conductivity and photovoltage > 600 mV on nSi, but with improved stability. Additionally, TiO2-IrO2 alloys are capable of catalyzing oxygen evolution. Thus, TiO2-IrO2alloys have the potential to be an “all-in-one” catalyst, Schottky contact, and corrosion resistant protection layer.

One‐step Solution‐Processed Formamidinium Lead Tribromide Formation for Better Reproducible Planar Perovskite Solar Cells

AbstractLow‐cost solar cells based on solution‐processed organic–inorganic hybrid perovskites with high photovoltage are highly sought after, especially for their use in tandem cells or for driving electrochemical reactions. Towards this end, we herein present a single‐step method for the preparation of regular planar heterojunction solar cells based on formamidinium lead tribromide (FAPbBr3), which is fabricated through an antisolvent‐assisted crystallization process. This results in the formation of improved film quality of the perovskite layer in terms of uniformity, surface coverage, crystallinity, and light absorption. Devices fabricated using such films utilizing 2,2′,7,7′‐tetrakis‐(N,N‐p‐dimethoxyphenylamino)‐9,9′‐spirobifluorene (spiro‐OMeTAD) as the hole‐transport layer exhibits an open‐circuit voltage (Voc) of 1.32 V with a power conversion efficiency (PCE) close to 6 %. The device performance is much higher than when using devices based on the conventional one‐step process (i.e., without using an antisolvent), where a Voc of 1.04 V and a PCE of 1.1 % are obtained. Moreover, the devices show better reproducibility with very little hysteresis.

Interfacial engineering of metal-insulator-semiconductor junctions for efficient and stable photoelectrochemical water oxidation

Solar-assisted water splitting can potentially provide an efficient route for large-scale renewable energy conversion and storage. It is essential for such a system to provide a sufficiently high photocurrent and photovoltage to drive the water oxidation reaction. Here we demonstrate a photoanode that is capable of achieving a high photovoltage by engineering the interfacial energetics of metal–insulator–semiconductor junctions. We evaluate the importance of using two metals to decouple the functionalities for a Schottky contact and a highly efficient catalyst. We also illustrate the improvement of the photovoltage upon incidental oxidation of the metallic surface layer in KOH solution. Additionally, we analyse the role of the thin insulating layer to the pinning and depinning of Fermi level that is responsible to the resulting photovoltage. Finally, we report the advantage of using dual metal overlayers as a simple protection route for highly efficient metal–insulator–semiconductor photoanodes by showing over 200 h of operational stability.

Hematite-based photoelectrode for solar water splitting with very high photovoltage

Hematite photoelectrode is a promising candidate for photoelectrochemical (PEC) water splitting. However, the high density of surface defects and slow kinetics for oxygen evolution have restricted the PEC performance of hematite photoelectrodes, mainly their low photovoltage. This study provides the hematite surface modification combining two strategies, annealing at high temperatures (800°C) and coating with RuO2 and IrO2/RuO2 co-catalysts. IrO2/RuO2-coated hematite thin film exhibited a low turn-on potential of 0.48 VRHE and a final photovoltage of 1.20V, which is the highest photovoltage reported for hematite-based photoelectrodes. A photocurrent density of ca. 1.52mAcm−2 was obtained at 1.23 VRHE, corresponding to ca. 140% increase compared to bare hematite; the photocurrent obtained at 0.60 VRHE was 0.34mAcm−2, the highest value ever reported. The IrO2/RuO2-coated hematite photoelectrode showed a quite stable performance at 0.80 VRHE for over 72h. The water oxidation efficiency was therefore enhanced with values above 90% for potentials higher than 1.00 VRHE due to faster charge transfer and reduced electron-hole recombination at its surface.

Function Follows Form: Correlation between the Growth and Local Emission of Perovskite Structures and the Performance of Solar Cells

Understanding the relationship between the growth and local emission of hybrid perovskite structures and the performance of the devices based on them demands attention. This study investigates the local structural and emission features of CH3NH3PbI3, CH3NH3PbBr3, and CH(NH2)2PbBr3 perovskite films deposited under different yet optimized conditions using X‐ray scattering and cathodoluminescence spectroscopy, respectively. X‐ray scattering shows that a CH3NH3PbI3 film involving spin coating of CH3NH3I instead of dipping is composed of perovskite structures exhibiting a preferred orientation with [202] direction perpendicular to the surface plane. The device based on the CH3NH3PbI3 film composed of oriented crystals yields a relatively higher photovoltage. In the case of CH3NH3PbBr3, while the crystallinity decreases when the HBr solution is used in a single‐step method, the photovoltage enhancement from 1.1 to 1.46 V seems largely stemming from the morphological improvements, i.e., a better connection between the crystallites due to a higher nucleation density. Furthermore, a high photovoltage of 1.47 V obtained from CH(NH2)2PbBr3 devices could be attributed to the formation of perovskite films displaying uniform cathodoluminescence emission. The comparative analysis of the local structural, morphological, and emission characteristics of the different perovskite films supports the higher photovoltage yielded by the relatively better performing devices.

Dye-sensitized solar cells for efficient power generation under ambient lighting

Solar cells that operate efficiently under indoor lighting are of great practical interest as they can serve as electric power sources for portable electronics and devices for wireless sensor networks or the Internet of Things. Here, we demonstrate a dye-sensitized solar cell (DSC) that achieves very high power-conversion efficiencies (PCEs) under ambient light conditions. Our photosystem combines two judiciously designed sensitizers, coded D35 and XY1, with the copper complex Cu(II/I)(tmby) as a redox shuttle (tmby, 4,4′,6,6′-tetramethyl-2,2′-bipyridine), and features a high open-circuit photovoltage of 1.1 V. The DSC achieves an external quantum efficiency for photocurrent generation that exceeds 90% across the whole visible domain from 400 to 650 nm, and achieves power outputs of 15.6 and 88.5 μW cm–2 at 200 and 1,000 lux, respectively, under illumination from a model Osram 930 warm-white fluorescent light tube. This translates into a PCE of 28.9%. A dye-sensitized solar cell that has been designed for efficient operation under indoor lighting could offer a convenient means for powering the Internet of Things.

High photovoltage in perovskite solar cells: New physical insights from the ultrafast transient absorption spectroscopy

To understand the cause of the high open circuit photovoltage (VOC) achieved by todays’ state of the art perovskite solar cells (PSCs), we examine formamidinium lead bromide CH(NH2)2PbBr3 films by ultrafast transient absorption spectroscopy (TAS). By using TiO2 and spiro-OMeTAD as charge extraction layers, the devices based on the CH(NH2)2PbBr3 films yield VOC as high as 1.5V ascertaining their high quality. TAS establish that the presence of charge extraction layers has very little influences on the nature of a negative band at 535nm corresponding to the bleaching of the absorption band edge and two positive bands in the CH(NH2)2PbBr3 films. Therefore, we contend that the VOC in PSC is predominantly determined by the quasi Fermi level splitting within the perovskite layer.

Synthesis of a conjugated pyrrolopyridazinedione\u2013benzodithiophene (PPD\u2013BDT) copolymer and its application in organic and hybrid solar cells

Herein, we describe the synthesis and characterization of a conjugated donor–acceptor copolymer consisting of a pyrrolopyridazinedione (PPD) acceptor unit, and a benzodithiophene (BDT) donor unit. The polymerization was done via a Stille cross-coupling polycondensation. The resulting PPD–BDT copolymer revealed an optical bandgap of 1.8 eV and good processability from chlorobenzene solutions. In an organic solar cell in combination with PC70BM, the polymer led to a power conversion efficiency of 4.5%. Moreover, the performance of the copolymer was evaluated in polymer/nanocrystal hybrid solar cells using non-toxic CuInS2 nanocrystals as inorganic phase, which were prepared from precursors directly in the polymer matrix without using additional capping ligands. The PPD–BDT/CuInS2 hybrid solar cells showed comparably high photovoltages and a power conversion efficiency of 2.2%.Graphical abstract

Efficient Si/SiOx/ITO Heterojunction Photoanode with an Amorphous and Porous NiOOH Catalyst formed by NiCl2 activation for Water Oxidation

Solar-driven water splitting with silicon photoelectrodes exhibiting high solar-to-fuel conversion efficiency is a promising way for producing hydrogen fuel in the future. In this study, a heterojunction photoanode was fabricated by the deposition of a thin indium tin oxide (ITO) layer on n-type silicon/native SiOx. A NiCl2-containing precursor was sprayed on the top of the photoanode, affording a NiO-Cl-OH catalyst; this NiO-Cl-OH catalyst was then activated to form an amorphous and porous NiOOH (a-NiOOH) catalyst, which exhibited enhanced performance. The fabricated Si/SiOx/ITO/a-NiOOH photoanode exhibited a low photocurrent onset potential of ∼0.98V vs. RHE, a high saturation photocurrent density of 36.98mA/cm2, a photocurrent density of 27.4mA/cm2 at the standard oxidation potential of water, and a photovoltage as high as 545mV under a solar illumination of 100mW/cm2. The photocurrent marginally decreased after 30h. These results suggested that such heterojunctions can replace homogeneous p–n junctions formed from Si doping for high photovoltage generation. In addition, porous a-NiOOH can improve the electrocatalytic performance of Si-based photoanodes.

Origin of Photovoltage Enhancement via Interfacial Modification with Silver Nanoparticles Embedded in an a-SiC:H p-Type Layer in a-Si:H Solar Cells.

We used silver nanoparticles (Ag-NPs) embedded in the p-type semiconductor layer of hydrogenated amorphous silicon (a-Si:H) solar cells in the Schottky barrier contact design to modify the interface between aluminum-doped ZnO (ZnO:Al, AZO) and p-type hydrogenated amorphous silicon carbide (p-a-SiC:H) without plasmonic absorption. The high work function of the Ag-NPs provided a good channel for the transport of photogenerated holes. A p-type nanocrystalline SiC:H layer was used to compensate for the real surface defects and voids on the surface of Ag-NPs to reduce recombination at the AZO/p-type layer interface, which then enhanced the photovoltage of single-junction a-Si:H solar cells to values as high as 1.01 V. The Ag-NPs were around 10 nm in diameter and thermally stable in the p-type a-SiC:H film at the solar-cell process temperature. We will also show that a wide range of photovoltages between 1.01 and 2.89 V could be obtained with single-, double-, and triple-junction solar cells based on the single-junction a-Si:H solar cells with tunable high photovoltage. These solar cells are suitable photocathodes for solar water-splitting applications.

Directly Probing Charge Separation at Interface of TiO2 Phase Junction.

Phase junction is often recognized as an effective strategy to achieve efficient charge separation in photocatalysis and photochemistry. As a crucial factor to determine the photogenerated charges dynamics, there is an increasingly hot debate about the energy band alignment across the interface of phase junction. Herein, we reported the direct measurement of the surface potential profile over the interface of TiO2 phase junction. A built-in electric field up to 1 kV/cm from rutile to anatase nanoparticle was detected by Kelvin Probe Force Microscopy (KPFM). Home-built spatially resolved surface photovoltage spectroscopy (SRSPS) supplies a direct evidence for the vectorial charge transfer of photogenerated electrons from rutile to anatase. Moreover, the tunable anatase nanoparticle sizes in TiO2 phase junction leads to high surface photovoltage (SPV) by creating completely depleted space charge region (SCR) and enhancing the charge separation efficiency. The results provide a strong basis for understanding the impact of built-in electric field on the charge transfer across the interface of artificial photocatalysts.

Perovskite Solar Cells on the Way to Their Radiative Efficiency Limit – Insights Into a Success Story of High Open‐Circuit Voltage and Low Recombination

Inorganic‐organic lead‐halide perovskite solar cells have reached efficiencies above 22% within a few years of research. Achieved photovoltages of >1.2 V are outstanding for a material with a bandgap of 1.6 eV – in particular considering that it is solution processed. Such values demand for low non‐radiative recombination rates and come along with high luminescence yields when the solar cell is operated as a light emitting diode. This progress report summarizes the developments on material composition and device architecture, which allowed for such high photovoltages. It critically assesses the term “lifetime”, the theories and experiments behind it, and the different recombination mechanisms present. It attempts to condense reported explanations for the extraordinary optoelectronic properties of the material. Amongst those are an outstanding defect tolerance due to antibonding valence states and the capability of bandgap tuning, which might make the dream of low‐cost highly efficient solution‐processed thin film solar cells come true. Beyond that, the presence of photon recycling will open new opportunities for photonic device design.

Ethynylene-linked planar rigid organic dyes based on indeno[1,2-b]indole for efficient dye-sensitized solar cells

Three metal-free push–pull type organic dyes QX19–21 based on indeno[1,2-b]indole donor have been synthesized for dye-sensitized solar cells (DSSCs). In order to obtain good cell performance, an ethynylene linker was inserted into the dye skeleton to extend π-conjugated planar rigid structure and three hexyl groups were involved to prevent dye aggregation. The dyes QX19 and QX20 employing phenylene and thiophene as the π-bridges were designed as a D–π–A structure. As for the dye QX21, a benzothiadiazole was inserted as the secondary acceptor to construct as a D–A–π–A structure. All three dyes based cells exhibited high photovoltages and good DSSC performances. A high PCE of 7.99% was obtained by the dye QX21 under AM 1.5G irradiation. Especially, the co-sensitization of QX21 with QX20 achieved the further improved PCE up to 8.37%.

Dispersed Ni Nanoparticles Stabilize Silicon Photoanodes for Efficient and Inexpensive Sunlight-Assisted Water Oxidation

Ni electrodeposited on n-type Si from aqueous solutions in the form of isolated or coalescent nanoparticles (NPs) protects the underlying and partially exposed Si from photocorrosion-induced electrical passivation. Such photoanodes, fabricated without the need for additional protecting layers, a buried junction, and high-vacuum techniques, show a high photovoltage of ∼500 mV for the oxygen evolution reaction (OER), state-of-the-art photocurrents, and faradaic efficiencies > 90% under AM 1.5G illumination conditions at pH 14. Remarkably, these photoelectrodes are stable and can be operated at the light-limited catalytic current from 10 h to more than 40 h in 1 M NaOH. These findings demonstrate that robust and efficient Si-based photoanodes can be produced easily, which opens new opportunities for the implementation of low-cost Si-based monolithic photoelectrochemical cells for efficient solar fuel production.

Harnessing Photovoltage: Effects of Film Thickness, TiO2 Nanoparticle Size, MgO and Surface Capping with DSCs.

High photovoltage dye-sensitized solar cells (DSCs) offer an exceptional opportunity to power electrocatalysts for the production of hydrogen from water and the reduction of CO2 to usable fuels with a relatively cost-effective, low-toxicity solar cell. Competitive recombination pathways such as electron transfer from TiO2 films to the redox shuttle or oxidized dye must be minimized to achieve the maximum possible photovoltage (Voc) from DSC devices. A high Voc of 882 mV was achieved with the iodide/triiodide redox shuttle and a ruthenium NCS-ligated dye, HD-2-mono, by utilizing a combined approach of (1) modulating the TiO2 surface area through film thickness and nanoparticle size selection, (2) addition of a MgO insulating layer, and (3) capping available TiO2 film surface sites post film sensitization with an F-SAM (fluorinated self-assembled monolayer) treatment. The exceptional Voc of 882 mV observed is the highest achieved for the popular NCS containing ruthenium sensitizers with >5% PCE and compares favorably to the 769 mV value observed under common device preparation conditions.

Mathematical model and characteristic analysis of hybrid photovoltaic/piezoelectric actuation mechanism

Photovoltaic materials can turn light energy into electric energy directly, and thus have the advantages of high electrical output voltages and the ability to realize remote or non-contact control. When high-energy ultraviolet light illuminates polarized PbLaZrTi (PLZT) materials, high photovoltages will be generated along the spontaneous polarization direction due to the photovoltaic effect. In this paper, a novel hybrid photovoltaic/piezoelectric actuation mechanism is proposed. PLZT ceramics are used as a photovoltaic generator to drive a piezoelectric actuator. A mathematical model is established to define the time history of the actuation voltage between two electrodes of the piezoelectric actuator, which is experimentally validated by the test results of a piezoelectric actuator with different geometrical parameters under irradiation at different light intensities. Some important characteristics of this novel actuation mechanism are analyzed and it can be concluded that (1) it is experimentally validated that there is no hysteresis between voltage and deformation which exists in a PLZT actuator; (2) the saturated voltage and response speed can be improved by using a multi-patch PLZT generator to drive the piezoelectric actuator; and (3) the initial voltage of the piezoelectric actuator can be acquired by controlling the logical switch between the PLZT and the piezoelectric actuator while the initial voltages increase with the rise of light intensity.

Pinhole-Free Perovskite Films by Methylamine Iodide Solution-Assisted Repair for High-Efficiency Photovoltaics under Ambient Conditions.

In this study, we demonstrate a simple strategy for obtaining pinhole-free, homogeneous, well-crystallized perovskite films under ambient conditions. The preparation of perovskite film with high light-harvesting efficiency and long carrier lifetime is verified. By applying this film in TiO2-based perovskite solar cells (PVSCs), we achieved a high power-conversion efficiency (PCE) of 13.07%, which is doubled with respect to that of the PVSC not subjected to the same improvement procedure (6.54%). High open-circuit photovoltage, photocurrent density, and fill factor are the main contributions to the high PCE that results from low trap density and high recombination resistance of the resultant perovskite films. This work paves a new means for fabricating high-performance perovskite films and PVSC devices in an ambient atmosphere.

Copper Bipyridyl Redox Mediators for Dye-Sensitized Solar Cells with High Photovoltage.

Redox mediators play a major role determining the photocurrent and the photovoltage in dye-sensitized solar cells (DSCs). To maintain the photocurrent, the reduction of oxidized dye by the redox mediator should be significantly faster than the electron back transfer between TiO2 and the oxidized dye. The driving force for dye regeneration with the redox mediator should be sufficiently low to provide high photovoltages. With the introduction of our new copper complexes as promising redox mediators in DSCs both criteria are satisfied to enhance power conversion efficiencies. In this study, two copper bipyridyl complexes, Cu(II/I)(dmby)2TFSI2/1 (0.97 V vs SHE, dmby = 6,6'-dimethyl-2,2'-bipyridine) and Cu(II/I)(tmby)2TFSI2/1 (0.87 V vs SHE, tmby = 4,4',6,6'-tetramethyl-2,2'-bipyridine), are presented as new redox couples for DSCs. They are compared to previously reported Cu(II/I)(dmp)2TFSI2/1 (0.93 V vs SHE, dmp = bis(2,9-dimethyl-1,10-phenanthroline). Due to the small reorganization energy between Cu(I) and Cu(II) species, these copper complexes can sufficiently regenerate the oxidized dye molecules with close to unity yield at driving force potentials as low as 0.1 V. The high photovoltages of over 1.0 V were achieved by the series of copper complex based redox mediators without compromising photocurrent densities. Despite the small driving forces for dye regeneration, fast and efficient dye regeneration (2-3 μs) was observed for both complexes. As another advantage, the electron back transfer (recombination) rates were slower with Cu(II/I)(tmby)2TFSI2/1 as evidenced by longer lifetimes. The solar-to-electrical power conversion efficiencies for [Cu(tmby)2]2+/1+, [Cu(dmby)2]2+/1+, and [Cu(dmp)2]2+/1+ based electrolytes were 10.3%, 10.0%, and 10.3%, respectively, using the organic Y123 dye under 1000 W m-2 AM1.5G illumination. The high photovoltaic performance of Cu-based redox mediators underlines the significant potential of the new redox mediators and points to a new research and development direction for DSCs.

On the low-temperature photoluminescence and photovoltaic properties of fine-grained CdTe films

In the low-temperature (T = 4.2 K) photoluminescence spectrum of a fine-grained (the grain size is smaller than 1 μm) obliquely sputtered CdTe thin film, the dominant intrinsic emission band caused by potential barriers at grain boundaries and the edge doublet band representing the LO-phonon replica of the dominant band are observed. Doping of the film with an In impurity quenches the doublet band and subsequent heat treatment activates the intrinsic emission band. The full-width at half maximum and the short-wavelength shift of the red edge of the latter correlate with a maximum anomalously high photovoltage of 2 × 102–103 V generated by the film.

Intrinsic and Extrinsic Stability of Formamidinium Lead Bromide Perovskite Solar Cells Yielding High Photovoltage.

We report on both the intrinsic and the extrinsic stability of a formamidinium lead bromide [CH(NH2)2PbBr3 = FAPbBr3] perovskite solar cell that yields a high photovoltage. The fabrication of FAPbBr3 devices, displaying an outstanding photovoltage of 1.53 V and a power conversion efficiency of over 8%, was realized by modifying the mesoporous TiO2-FAPbBr3 interface using lithium treatment. Reasons for improved photovoltaic performance were revealed by a combination of techniques, including photothermal deflection absorption spectroscopy (PDS), transient-photovoltage and charge-extraction analysis, and time-integrated and time-resolved photoluminescence. With lithium-treated TiO2 films, PDS reveals that the TiO2-FAPbBr3 interface exhibits low energetic disorder, and the emission dynamics showed that electron injection from the conduction band of FAPbBr3 into that of mesoporous TiO2 is faster than for the untreated scaffold. Moreover, compared to the device with pristine TiO2, the charge carrier recombination rate within a device based on lithium-treated TiO2 film is 1 order of magnitude lower. Importantly, the operational stability of perovskites solar cells examined at a maximum power point revealed that the FAPbBr3 material is intrinsically (under nitrogen) as well as extrinsically (in ambient conditions) stable, as the unsealed devices retained over 95% of the initial efficiency under continuous full sun illumination for 150 h in nitrogen and dry air and 80% in 60% relative humidity (T = ∼60 °C). The demonstration of high photovoltage, a record for FAPbBr3, together with robust stability renders our work of practical significance.