Ultrathin Buffer Layers of SnO₂ by Atomic Layer Deposition: Perfect Blocking Function and Thermal Stability

This study pinpoints the advantages of ultrathin electron selective layers (ESL) of SnO₂ fabricated by atomic layer deposition (ALD). These layers recently caught attention in planar perovskite solar cells and appear as powerful alternatives to other oxides such as TiO₂. Here, we carry out a thorough characterization of the nature of these ultrathin ALD SnO₂ layers providing a novel physical insight for the design of various photoelectrodes in perovskite and dye-sensitized solar cells and in photoelectrochemical water splitting. We use a combination of cyclic voltammetry, electrochemical impedance spectroscopy, Hall measurements, X-ray photoelectron spectroscopy, atomic force microscopy, and electron microscopy to analyze the blocking behavior and energetics of as-deposited (low-temperature) and also calcined ALD SnO₂ layers. First, we find that the low-temperature ALD-grown SnO₂ layers are amorphous and perfectly pinhole-free for thicknesses down to 2 nm. This exceptional blocking behavior of thin ALD SnO₂ layers allows photoelectrode designs with even thinner electron selective layers, thus potentially minimizing resistance losses. The compact nature and blocking function of thin SnO₂ films are not perturbed by annealing at 450 °C, which is a significant benefit compared to other amorphous ALD oxides. Further on, we show that amorphous and crystalline ALD SnO₂ films substantially differ in their flatband (and conduction band) positionsa finding to be taken into account when considering band alignment engineering in solar devices using these high-quality blocking layers

The Transient Photocurrent and Photovoltage Behavior of a Hematite Photoanode under Working Conditions and the Influence of Surface Treatments

Hematite (α-Fe₂O₃) is widely recognized as a promising candidate for the production of solar fuels via water splitting, but its intrinsic optoelectronic properties have limited its performance to date. In particular, the large electrochemical overpotential required to drive the water oxidation is known as a major drawback. This overpotential (0.4 – 0.6 V anodic of the flat band potential) has been attributed to poor oxygen evolution reaction (OER) catalysis and to charge trapping in surface states but is still not fully understood. In the present study, we quantitatively investigate the photocurrent and photovoltage transient behavior of α-Fe₂O₃ photoanodes prepared by atmospheric pressure chemical vapor deposition, under light bias, in a standard electrolyte, and one containing a sacrificial agent. The accumulation of positive charges occurring in water at low bias potential is found to be maximum when the photocurrent onsets. The transient photocurrent behavior of a standard photoanode is compared to photoanodes modified by either a catalytic or surface passivating overlayer. Surface modification shows a reduction and a cathodic shift of the charge accumulation, following the observed change in photocurrent onset. By applying an electrochemical model, the values of the space charge width (5–10 nm) and of the hole diffusion length (0.5–1.5 nm) are extracted from photocurrent transients’ amplitudes with the sacrificial agent. Characterization of the photovoltage transients also suggests the presence of surface states causing Fermi level pinning at small applied potential. The transient photovoltage and the use of both overlayers on the same electrode enable differentiation of the two overlayers’ effects and a simplified model is proposed to explain the roles of each overlayer and their synergetic effects. This investigation demonstrates a new method to characterize water splitting photoelectrodesespecially the charge accumulation occurring at the semiconductor/electrolyte interface during operation. It finally confirms the requirements of nanostructuring and surface control with catalytic and trap passivation layers to improve iron oxide’s performance for water photolysis

Mesoscopic Oxide Double Layer as Electron Specific Contact for Highly Efficient and UV Stable Perovskite Photovoltaics

The solar to electric power conversion efficiency (PCE) of perovskite solar cells (PSCs) has recently reached 22.7%, exceeding that of competing thin film photovoltaics and the market leader polycrystalline silicon. Further augmentation of the PCE toward the Shockley–Queisser limit of 33.5% warrants suppression of radiationless carrier recombination by judicious engineering of the interface between the light harvesting perovskite and the charge carrier extraction layers. Here, we introduce a mesoscopic oxide double layer as electron selective contact consisting of a scaffold of TiO₂ nanoparticles covered by a thin film of SnO₂, either in amorphous (a-SnO₂), crystalline (c-SnO₂), or nanocrystalline (quantum dot) form (SnO₂-NC). We find that the band gap of a-SnO₂ is larger than that of the crystalline (tetragonal) polymorph leading to a corresponding lift in its conduction band edge energy which aligns it perfectly with the conduction band edge of both the triple cation perovskite and the TiO₂ scaffold. This enables very fast electron extraction from the light perovskite, suppressing the notorious hysteresis in the current–voltage (<i>J–V</i>) curves and retarding nonradiative charge carrier recombination. As a result, we gain a remarkable 170 mV in open circuit photovoltage (<i>V</i>_<i>oc</i>) by replacing the crystalline SnO₂ by an amorphous phase. Because of the quantum size effect, the band gap of our SnO₂-NC particles is larger than that of bulk SnO₂ causing their conduction band edge to shift also to a higher energy thereby increasing the <i>V</i>_<i>oc</i>. However, for SnO₂-NC there remains a barrier for electron injection into the TiO₂ scaffold decreasing the fill factor of the device and lowering the PCE. Introducing the a-SnO₂ coated mp-TiO₂ scaffold as electron extraction layer not only increases the <i>V</i>_<i>oc</i> and PEC of the solar cells but also render them resistant to UV light which forebodes well for outdoor deployment of these new PSC architectures

Optically Transparent FTO-Free Cathode for Dye-Sensitized Solar Cells

The woven fabric containing electrochemically platinized tungsten wire is an affordable flexible cathode for liquid-junction dye-sensitized solar cells with the I₃^–/I^– redox mediator and electrolyte solution consisting of ionic liquids and propionitrile. The fabric-based electrode outperforms the thermally platinized FTO in serial ohmic resistance and charge-transfer resistance for triiodide reduction, and it offers comparable or better optical transparency in the visible and particularly in the near-IR spectral region. The electrode exhibits good stability during electrochemical loading and storage at open circuit. The dye-sensitized solar cells with a C101-sensitized titania photoanode and either Pt–W/PEN or Pt–FTO cathodes show a comparable performance

Temperature Dependence of Transport Properties of Spiro-MeOTAD as a Hole Transport Material in Solid-State Dye-Sensitized Solar Cells

The internal transport and recombination parameters of solid-state dye-sensitized solar cells (ssDSCs) using the amorphous organic semiconductor 2,2′,7,7′-tetrakis(<i>N,N</i>-di-<i>p</i>-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD) as a hole transport material (HTM) are investigated using electrical impedance spectroscopy. Devices were fabricated using flat and nanostructured TiO₂ and compared to systems using nanostructured ZrO₂ to differentiate between the transport processes within the different components of the ssDSC. The effect of chemically p-doping the HTM on its transport was investigated, and its temperature dependence was examined and analyzed using the Arrhenius equation. Using this approach the activation energy of the hole hopping transport within the undoped spiro-MeOTAD film was determined to be 0.34 ± 0.02 and 0.40 ± 0.02 eV for the mesoporous TiO₂ and ZrO₂ systems, respectively

Co(III) Complexes as p‑Dopants in Solid-State Dye-Sensitized Solar Cells

Following our recent work on the use of Co­(III) complexes as p-type dopants for triarylamine-based organic hole-conductors, we herein report on two new Co­(III) complexes for doping applications. With the aim of ameliorating the dopant’s suitability for its use in solid-state dye-sensitized solar cells, we show how the properties of the dopant can be easily adjusted by a slight modification of the molecular structure. We prove the eligibility of the two new dopants by characterizing their optical and electrochemical properties and give evidence that both of them can be used to oxidize the molecular hole-transporter spiro-MeOTAD. Finally, we fabricate high-performance solid-state dye-sensitized solar cells using a state-of-the-art metal-free organic sensitizer in order to elucidate the influence of the type of dopant on device performance

Fine-tuning the Electronic Structure of Organic Dyes for Dye-Sensitized Solar Cells

A series of metal-free organic dyes exploiting different combinations of (hetero)cyclic linkers (benzene, thiophene, and thiazole) and bridges (4<i>H</i>-cyclopenta[2,1-<i>b</i>:3,4-<i>b′</i>]dithiophene (CPDT) and benzodithiophene (BDT)) as the central π-spacers were synthesized and characterized. Among them, the sensitizer containing the thiophene and CPDT showed the most broad incident photon-to-current conversion efficiency spectra, resulting in a solar energy conversion efficiency (η) of 6.6%

Highly Efficient Perovskite Solar Cells with Gradient Bilayer Electron Transport Materials

Electron transport layers (ETLs) with suitable energy level alignment for facilitating charge carrier transport as well as electron extraction are essential for planar heterojunction perovskite solar cells (PSCs) to achieve high open-circuit voltage (<i>V</i>_OC) and short-circuit current. Herein we systematically investigate band offset between ETL and perovskite absorber by tuning F doping level in SnO₂ nanocrystal. We demonstrate that gradual substitution of F^– into the SnO₂ ETL can effectively reduce the band offset and result in a substantial increase in device <i>V</i>_OC. Consequently, a power conversion efficiency of 20.2% with <i>V</i>_OC of 1.13 V can be achieved under AM 1.5 G illumination for planar heterojunction PSCs using F-doped SnO₂ bilayer ETL. Our finding provides a simple pathway to tailor ETL/perovskite band offset to increase built-in electric field of planar heterojunction PSCs for maximizing <i>V</i>_OC and charge collection simultaneously

Analysis of Electron Transfer Properties of ZnO and TiO₂ Photoanodes for Dye-Sensitized Solar Cells

Mesoporous TiO₂ nanoparticle films are used as photoanodes for high-efficiency dye-sensitized solar cells (DSCs). In spite of excellent photovoltaic power conversion efficiencies (PCEs) displayed by titanium dioxide nanoparticle structures, the transport rate of electrons is known to be low due to low electron mobility. So the alternate oxides, including ZnO, that possesses high electron mobility are being investigated as potential candidates for photoanodes. However, the PCE with ZnO is still lower than with TiO₂, and this is typically attributed to the low internal surface area. In this work, we attempt to make a one-to-one comparison of the photovoltaic performance and the electron transfer dynamics involved in DSCs, with ZnO and TiO₂ as photoanodes. Previously such comparative investigations were hampered due to the morphological differences (internal surface area, pore diameter, porosity) that exist between zinc oxide and titanium dioxide films. We circumvent this issue by depositing different thicknesses of these oxides, by atomic layer deposition (ALD), on an arbitrary mesoporous insulating template and subsequently using them as photoanodes. Our results reveal that at an optimal thickness ZnO exhibits photovoltaic performances similar to TiO₂, but the internal electron transfer properties differ. The higher photogenerated electron transport rate contributed to the performances of ZnO, but in the case of TiO₂, it is the low recombination rate, higher dye loading, and fast electron injection