Recently, metal halide perovskite solar cells (PSCs) of the general formular ABX3 where A is a monovalent cation, that is, methylammonium (MA) CH3NH3 +•, formamidinium CH2(NH2)2 +, Cs+, or Rb+, B ...stands for Pb(II) or Sn(II), and X for iodide or bromide have achieved solar to electric power conversion efficiencies (PCEs) above 22%, exceeding the efficiency of the present market leader polycrystalline silicon while using 1000 times less light harvesting material and simple solution processing for their fabrication. The top performing devices all employ formulations containing a mixture of up to four A cations and iodide as well as a small fraction of bromide as anion, whose emergence will be described in this Commentary. Apart from leading the current PV efficiency race, these new perovskite materials exhibit intense electroluminescence and an extraordinarily high stability under heat and light stress.
The rise of metal halide perovskites as light harvesters has stunned the photovoltaic community. As the efficiency race continues, questions on the control of the performance of perovskite solar ...cells and on its characterization are being addressed.
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) positionsa finding to be taken into account when considering band alignment engineering in solar devices using these high-quality blocking layers.
We introduce a simple solution-based strategy to decouple morphological and functional effects of annealing nanostructured, porous electrodes by encapsulation with a SiO2 confinement scaffold before ...high temperature treatment. We demonstrate the effectiveness of this approach using porous hematite (α-Fe2O3) photoanodes applied for the storage of solar energy via water splitting and show that the feature size and electrode functionality due to dopant activation can be independently controlled. This allows a significant increase in water oxidation photocurrent from 1.57 mA cm−2 (in the control case) to 2.34 mA cm−2 under standard illumination conditions in 1 M NaOH electrolytethe highest reported for a solution-processed hematite photoanode. This increase is attributed to the improved quantum efficiency, especially with longer wavelength photons, due to a smaller particle size, which is afforded by our encapsulation strategy.
Issues related to the use of meso-substituted porphyrins for dye-sensitized solar cells are discussed. Dye-sensitized solar cells are used to produce solar energy.
Recently organic–inorganic hybrid perovskites have attracted attention as light harvesting materials in mesoscopic cells. While a considerable number of deposition and formation methods have been ...reported for the perovskite crystalline material, most involve an annealing step. As such, the thermal behavior of this material and its individual components is of crucial interest. Here, we examine the thermal properties of the CH3NH3PbX3 (X = I or Cl) perovskite using thermogravimetric analysis. The role of the precursors is exposed, and the effect of the formation of excess organic species is investigated. The sublimation behavior of the organic component is intensively scrutinized. Furthermore, differential scanning calorimetry is employed to probe the crystal phase structure, revealing subtle differences depending on the deposition method.
The pressure to move towards renewable energy has inspired researchers to look for ideas in photovoltaics that may lead to a major breakthrough. Recently the use of perovskites as a light harvester ...has lead to stunning progress. The power conversion efficiency of perovskite solar cells is now approaching parity (>22 %) with that of the established technology which took decades to reach this level of performance. The use of a hole transport material (HTM) remains indispensable in perovskite solar cells. Perovskites can conduct holes, but they are present at low levels, and for efficient charge extraction a HTM layer is a prerequisite. Herein we provide an overview of the diverse types of HTM available, from organic to inorganic, in the hope of encouraging further research and the optimization of these materials.
Hole for whole: Semiconductor hole‐transport materials (HTMs) are an essential component for perovskite solar cells. The three classes of materials available, inorganic, polymeric, and small molecule HTMs are reviewed, particularly the optoelectrical properties of molecular HTMs, which seem to be the most effective materials.
Developing efficient systems for the conversion of carbon dioxide to valuable chemicals using solar power is critical for mitigating climate change and ascertaining the world’s future supply of clean ...fuels. Here, we introduce a mesoscopic cathode consisting of Cu nanowires decorated with Ag islands, by the reduction of Ag-covered Cu2O nanowires prepared via galvanic replacement reaction. This catalyst enables CO2 reduction to ethylene and other C2+ products with a faradaic efficiency of 76%. Operando Raman spectroscopy reveals intermediate formation of CO at Ag sites which undergo subsequent spillover and hydrogenation on the Cu nanowires. Our Cu–Ag bimetallic design enables a ∼95% efficient spillover of intermediates from Ag to Cu, delivering an improved activity toward the formation of ethylene and other C2+ products. We also demonstrate a solar to ethylene conversion efficiency of 4.2% for the photoelectrochemical CO2 reduction using water as electron and proton donor, and solar power together with perovskite photovoltaics to drive the uphill reaction.
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 (J–V) curves and retarding nonradiative charge carrier recombination. As a result, we gain a remarkable 170 mV in open circuit photovoltage (V oc ) 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 V oc . 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 V oc 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.
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 photoelectrodesespecially 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.
