Impact of Semiconductor Band Tails and Band Filling on Photovoltaic Efficiency Limits

Since the seminal work of Shockley and Queisser, assessing the detailed balance between absorbed and emitted radiative fluxes from a photovoltaic absorber has been the standard method for evaluating solar cell efficiency limits. The principle of detailed balance is one dictated by reciprocity and steady state, so that photons can be absorbed and emitted with equal probability. This basic principle has also been extended to evaluate the effects of multiple junctions, hot carriers, nanostructured geometries, multiexciton generation, subunity radiative efficiency, and many other solar cell configurations and nonidealities to estimate limiting efficiencies via modifications to the detailed balance model

Impact of Semiconductor Band Tails and Band Filling on Photovoltaic Efficiency Limits

Since the seminal work of Shockley and Queisser, assessing the detailed balance between absorbed and emitted radiative fluxes from a photovoltaic absorber has been the standard method for evaluating solar cell efficiency limits. The principle of detailed balance is one dictated by reciprocity and steady state, so that photons can be absorbed and emitted with equal probability. This basic principle has also been extended to evaluate the effects of multiple junctions, hot carriers, nanostructured geometries, multiexciton generation, subunity radiative efficiency, and many other solar cell configurations and nonidealities to estimate limiting efficiencies via modifications to the detailed balance model

Heteroepitaxial growth of Pt and Au thin films on MgO single crystals by bias-assisted sputtering

The crystallographic orientation of a metal affects its surface energy and structure, and has profound implications for surface chemical reactions and interface engineering, which are important in areas ranging from optoelectronic device fabrication to catalysis. However, it can be very difficult and expensive to manufacture, orient, and cut single crystal metals along different crystallographic orientations, especially in the case of precious metals. One approach is to grow thin metal films epitaxially on dielectric substrates. In this work, we report on growth of Pt and Au films on MgO single crystal substrates of (100) and (110) surface orientation for use as epitaxial templates for thin film photovoltaic devices. We develop bias-assisted sputtering for deposition of oriented Pt and Au films with sub-nanometer roughness. We show that biasing the substrate decreases the substrate temperature necessary to achieve epitaxial orientation, with temperature reduction from 600 to 350 °C for Au, and from 750 to 550 °C for Pt, without use of transition metal seed layers. In addition, this temperature can be further reduced by reducing the growth rate. Biased deposition with varying substrate bias power and working pressure also enables control of the film morphology and surface roughness

Excitonic Effects in Emerging Photovoltaic Materials: A Case Study in Cu_2O

Excitonic effects account for a fundamental photoconversion and charge transport mechanism in Cu_2O; hence, the universally adopted “free carrier” model substantially underestimates the photovoltaic efficiency for such devices. The quasi-equilibrium branching ratio between excitons and free carriers in Cu_2O indicates that up to 28% of photogenerated carriers during photovoltaic operation are excitons. These large exciton densities were directly observed in photoluminescence and spectral response measurements. The results of a device physics simulation using a model that includes excitonic effects agree well with experimentally measured current–voltage characteristics of Cu_2O-based photovoltaics. In the case of Cu_2O, the free carrier model underestimates the efficiency of a Cu_2O solar cell by as much as 1.9 absolute percent at room temperature

Excitonic Effects in Emerging Photovoltaic Materials: A Case Study in Cu_2O

Excitonic effects account for a fundamental photoconversion and charge transport mechanism in Cu_2O; hence, the universally adopted “free carrier” model substantially underestimates the photovoltaic efficiency for such devices. The quasi-equilibrium branching ratio between excitons and free carriers in Cu_2O indicates that up to 28% of photogenerated carriers during photovoltaic operation are excitons. These large exciton densities were directly observed in photoluminescence and spectral response measurements. The results of a device physics simulation using a model that includes excitonic effects agree well with experimentally measured current–voltage characteristics of Cu_2O-based photovoltaics. In the case of Cu_2O, the free carrier model underestimates the efficiency of a Cu_2O solar cell by as much as 1.9 absolute percent at room temperature

Enhancing the activity of oxygen-evolution and chlorine-evolution electrocatalysts by atomic layer deposition of TiO₂

We report that TiO₂ coatings formed via atomic layer deposition (ALD) may tune the activity of IrO₂, RuO₂, and FTO for the oxygen-evolution and chlorine-evolution reactions (OER and CER). Electrocatalysts exposed to ∼3–30 ALD cycles of TiO₂ exhibited overpotentials at 10 mA cm⁻² of geometric current density that were several hundred millivolts lower than uncoated catalysts, with correspondingly higher specific activities. For example, the deposition of TiO₂ onto IrO₂ yielded a 9-fold increase in the OER-specific activity in 1.0 M H₂SO₄ (0.1 to 0.9 mA cm_(ECSA)⁻² at 350 mV overpotential). The oxidation state of titanium and the potential of zero charge were also a function of the number of ALD cycles, indicating a correlation between oxidation state, potential of zero charge, and activity of the tuned electrocatalysts

Interface engineering of the photoelectrochemical performance of Ni-oxide-coated n-Si photoanodes by atomic-layer deposition of ultrathin films of cobalt oxide

Introduction of an ultrathin (2 nm) film of cobalt oxide (CoO_x) onto n-Si photoanodes prior to sputter-deposition of a thick multifunctional NiO_x coating yields stable photoelectrodes with photocurrent-onset potentials of ~−240 mV relative to the equilibrium potential for O2(g) evolution and current densities of ~28 mA cm^(−2) at the equilibrium potential for water oxidation when in contact with 1.0 M KOH(aq) under 1 sun of simulated solar illumination. The photoelectrochemical performance of these electrodes was very close to the Shockley diode limit for moderately doped n-Si(100) photoelectrodes, and was comparable to that of typical protected Si photoanodes that contained np+ buried homojunctions

Stable solar-driven oxidation of water by semiconducting photoanodes protected by transparent catalytic nickel oxide films

Reactively sputtered nickel oxide (NiO_x) films provide transparent, antireflective, electrically conductive, chemically stable coatings that also are highly active electrocatalysts for the oxidation of water to O_2(g). These NiO_x coatings provide protective layers on a variety of technologically important semiconducting photoanodes, including textured crystalline Si passivated by amorphous silicon, crystalline n-type cadmium telluride, and hydrogenated amorphous silicon. Under anodic operation in 1.0 M aqueous potassium hydroxide (pH 14) in the presence of simulated sunlight, the NiO_x films stabilized all of these self-passivating, high-efficiency semiconducting photoelectrodes for >100 h of sustained, quantitative solar-driven oxidation of water to O_2(g)

Enhancing the activity of oxygen-evolution and chlorine-evolution electrocatalysts by atomic layer deposition of TiO₂

We report that TiO₂ coatings formed via atomic layer deposition (ALD) may tune the activity of IrO₂, RuO₂, and FTO for the oxygen-evolution and chlorine-evolution reactions (OER and CER). Electrocatalysts exposed to ∼3–30 ALD cycles of TiO₂ exhibited overpotentials at 10 mA cm⁻² of geometric current density that were several hundred millivolts lower than uncoated catalysts, with correspondingly higher specific activities. For example, the deposition of TiO₂ onto IrO₂ yielded a 9-fold increase in the OER-specific activity in 1.0 M H₂SO₄ (0.1 to 0.9 mA cm_(ECSA)⁻² at 350 mV overpotential). The oxidation state of titanium and the potential of zero charge were also a function of the number of ALD cycles, indicating a correlation between oxidation state, potential of zero charge, and activity of the tuned electrocatalysts