Solar cells based on organometallic halide perovskite absorber layers are emerging as a high-performance photovoltaic technology. Using highly sensitive photothermal deflection and photocurrent ...spectroscopy, we measure the absorption spectrum of CH3NH3PbI3 perovskite thin films at room temperature. We find a high absorption coefficient with particularly sharp onset. Below the bandgap, the absorption is exponential over more than four decades with an Urbach energy as small as 15 meV, which suggests a well-ordered microstructure. No deep states are found down to the detection limit of ∼1 cm–1. These results confirm the excellent electronic properties of perovskite thin films, enabling the very high open-circuit voltages reported for perovskite solar cells. Following intentional moisture ingress, we find that the absorption at photon energies below 2.4 eV is strongly reduced, pointing to a compositional change of the material.
Photoelectrochemical cells employing organic semiconductors (OS) are promising for solar‐to‐fuel conversion via water splitting. However, despite encouraging advances with the half reactions, ...complete overall water splitting remains a challenge. Herein, a robust organic photocathode operating in near‐neutral pH electrolyte by careful selections of a semiconducting polymer bulk heterojunction (BHJ) blend and organic charge‐selective layer is realized. The optimized photocathode produces a photocurrent density of >4 mA cm−2 at 0 V vs the reversible hydrogen electrode (VRHE) for solar water reduction with noticeable operational stability (retaining ≈90% of the initial performance over 6 h) at pH 9. Combining the optimized BHJ photocathode with a benchmark BHJ photoanode leads to the demonstration of a large‐area (2.4 cm2) organic photoelectrochemical tandem cell for complete solar water splitting, with a predicted solar‐to‐hydrogen (STH) conversion efficiency of 0.8%. Under unassisted two‐electrode operation (1 Sun illumination) a stabilized photocurrent of 0.6 mA and an STH of 0.3% are observed together with near unity Faradaic efficiency of H2 and O2 production.
Overall water splitting is demonstrated for the first time in a photoelectrochemical tandem cell using both photoanode and photocathode that employ organic semiconductor bulk heterojunctions.
Photoelectrochemical water-splitting devices, which use solar energy to convert water into hydrogen and oxygen, have been investigated for decades. Multijunction designs are most efficient, as they ...can absorb enough solar energy and provide sufficient free energy for water cleavage. However, a balance exists between device complexity, cost and efficiency. Water splitters fabricated using triple-junction amorphous silicon or III-V semiconductors have demonstrated reasonable efficiencies, but at high cost and high device complexity. Simpler approaches using oxide-based semiconductors in a dual-absorber tandem approach have reported solar-to-hydrogen (STH) conversion efficiencies only up to 0.3% (ref. 4). Here, we present a device based on an oxide photoanode and a dye-sensitized solar cell, which performs unassisted water splitting with an efficiency of up to 3.1% STH. The design relies on carefully selected redox mediators for the dye-sensitized solar cell and surface passivation techniques and catalysts for the oxide-based photoanodes.
Graphene nanoplatelets (GNP) in the form of thin semitransparent film on F-doped SnO2 (FTO) exhibit high electrocatalytic activity for Co(L)2; where L is 6-(1H-pyrazol-1-yl)-2,2′-bipyridine. The ...exchange current densities for the Co2+/3+(L)2 redox reaction scaled linearly with the GNP film’s optical absorbance, and they were by 1–2 orders of magnitude larger than those for the I3 –/I– couple on the same electrode. The electrocatalytic activity of GNP films with optical transmission below 88% is outperforming the activity of platinized FTO for the Co2+/3+(L)2 redox reaction. Dye-sensitized solar cells with Y123 dye adsorbed on TiO2 photoanode achieved energy conversion efficiencies between 8 and 10% for both GNP and Pt-based cathodes. However, the cell with GNP cathode is superior to that with Pt-FTO cathode particularly in fill factors and in the efficiency at higher illumination intensities.
Good Co‐op: A novel cyclopentadithiophene‐bridged donor–acceptor dye applied as sensitizer in mesoscopic dye‐sensitized solar cells, in conjunction with the cobalt complexes CoII(bpy)3(B(CN)4)2 and ...CoIII(bpy)3(B(CN)4)3 as redox couple, yields high power conversion efficiencies (up to 9.6 %) under standard AM 1.5G solar irradiation. The results confirm that cobalt complexes are legitimate alternatives to the commonly used I−/I3− redox shuttle when used with judiciously engineered organic sensitizers.
The use of molecular modulators to reduce the defect density at the surface and grain boundaries of perovskite materials has been demonstrated to be an effective approach to enhance the photovoltaic ...performance and device stability of perovskite solar cells. Herein, we employ crown ethers to modulate perovskite films, affording passivation of undercoordinated surface defects. This interaction has been elucidated by solid-state nuclear magnetic resonance and density functional theory calculations. The crown ether hosts induce the formation of host–guest complexes on the surface of the perovskite films, which reduces the concentration of surface electronic defects and suppresses nonradiative recombination by 40%, while minimizing moisture permeation. As a result, we achieved substantially improved photovoltaic performance with power conversion efficiencies exceeding 23%, accompanied by enhanced stability under ambient and operational conditions. This work opens a new avenue to improve the performance and stability of perovskite-based optoelectronic devices through supramolecular chemistry.
The optimization of mediator/cathode in a dye sensitized solar cell (DSC) requires detailed knowledge about their electrochemical properties. Particularly, the kinetics of mediator's turnover on the ...cathode and the mass transport of mediator in the electrolyte solution are the crucial parameters which influence the overall efficiency of solar energy conversion in DSC. This review article aims at critical survey of the works carried out in the field. Attention is paid to graphene-based materials for the counter electrode, which may, eventually, meet all the benchmark parameters for a good DSC cathode, i.e. high optical transparency and electrical conductivity associated with high electrocatalytic activity for the redox mediator. So far, platinum and transparent conducting oxide (TCO) electrodes were used ubiquitously in DSC cathodes, but there is a motivation to avoid Pt and TCO to decrease the device cost without compromising of its performance.
The complex refractive index (dielectric function) of planar CH3NH3PbI3 thin films at room temperature is investigated by variable angle spectroscopic ellipsometry and spectrophotometry. Knowledge of ...the complex refractive index is essential for designing photonic devices based on CH3NH3PbI3 thin films such as solar cells, light-emitting diodes, or lasers. Because the directly measured quantities (reflectance, transmittance, and ellipsometric spectra) are inherently affected by multiple reflections, the complex refractive index has to be determined indirectly by fitting a model dielectric function to the experimental spectra. We model the dielectric function according to the Forouhi–Bloomer formulation with oscillators positioned at 1.597, 2.418, and 3.392 eV and achieve excellent agreement with the experimental spectra. Our results agree well with previously reported data of the absorption coefficient and are consistent with Kramers–Kronig transformations. The real part of the refractive index assumes a value of 2.611 at 633 nm, implying that CH3NH3PbI3-based solar cells are ideally suited for the top cell in monolithic silicon-based tandem solar cells.
The employment of 2D perovskites is a promising approach to tackling the stability and voltage issues inherent in perovskite solar cells. It remains unclear, however, whether other perovskites with ...different dimensionalities have the same effect on efficiency and stability. Here, we report the use of quasi-3D azetidinium lead iodide (AzPbI3) as a secondary layer on top of the primary 3D perovskite film that results in significant improvements in the photovoltaic parameters. Remarkably, the utilization of AzPbI3 leads to a new passivation mechanism due to the presence of surface dipoles resulting in a power conversion efficiency (PCE) of 22.4%. The open-circuit voltage obtained is as high as 1.18 V, which is among the highest reported to date for single junction perovskite solar cells, corresponding to a voltage deficit of 0.37 V for a band gap of 1.55 eV.
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