The Mechanism of Iodine Reduction by TiO₂ Electrons and the Kinetics of Recombination in Dye-Sensitized Solar Cells

Electron transfer from TiO₂ to iodine/iodide electrolytes proceeds via reduction of either I₃^– or uncomplexed I₂ (free iodine), but which route predominates has not previously been determined. By measurement of the electron lifetime while independently varying free iodine or I₃^– concentrations, we find the lifetime is correlated with free-iodine concentration and independent of I₃^– concentration. This trend supports the hypothesis that electron recombination to the electrolyte occurs predominantly by iodine reduction rather than reduction of triiodide

How Transparent Oxides Gain Some Color: Discovery of a CeNiO₃ Reduced Bandgap Phase As an Absorber for Photovoltaics

In this work, we describe the formation of a reduced bandgap CeNiO₃ phase, which, to our knowledge, has not been previously reported, and we show how it is utilized as an absorber layer in a photovoltaic cell. The CeNiO₃ phase is prepared by a combinatorial materials science approach, where a library containing a continuous compositional spread of Ce_<i>x</i>Ni_1–<i>x</i>O_<i>y</i> is formed by pulsed laser deposition (PLD); a method that has not been used in the past to form Ce–Ni–O materials. The library displays a reduced bandgap throughout, calculated to be 1.48–1.77 eV, compared to the starting materials, CeO₂ and NiO, which each have a bandgap of ∼3.3 eV. The materials library is further analyzed by X-ray diffraction to determine a new crystalline phase. By searching and comparing to the Materials Project database, the reduced bandgap CeNiO₃ phase is realized. The CeNiO₃ reduced bandgap phase is implemented as the absorber layer in a solar cell and photovoltages up to 550 mV are achieved. The solar cells are also measured by surface photovoltage spectroscopy, which shows that the source of the photovoltaic activity is the reduced bandgap CeNiO₃ phase, making it a viable material for solar energy

How Transparent Oxides Gain Some Color: Discovery of a CeNiO₃ Reduced Bandgap Phase As an Absorber for Photovoltaics

In this work, we describe the formation of a reduced bandgap CeNiO₃ phase, which, to our knowledge, has not been previously reported, and we show how it is utilized as an absorber layer in a photovoltaic cell. The CeNiO₃ phase is prepared by a combinatorial materials science approach, where a library containing a continuous compositional spread of Ce_<i>x</i>Ni_1–<i>x</i>O_<i>y</i> is formed by pulsed laser deposition (PLD); a method that has not been used in the past to form Ce–Ni–O materials. The library displays a reduced bandgap throughout, calculated to be 1.48–1.77 eV, compared to the starting materials, CeO₂ and NiO, which each have a bandgap of ∼3.3 eV. The materials library is further analyzed by X-ray diffraction to determine a new crystalline phase. By searching and comparing to the Materials Project database, the reduced bandgap CeNiO₃ phase is realized. The CeNiO₃ reduced bandgap phase is implemented as the absorber layer in a solar cell and photovoltages up to 550 mV are achieved. The solar cells are also measured by surface photovoltage spectroscopy, which shows that the source of the photovoltaic activity is the reduced bandgap CeNiO₃ phase, making it a viable material for solar energy

Process-Function Data Mining for the Discovery of Solid-State Iron-Oxide PV

Data mining tools have been known to be useful for analyzing large material data sets generated by high-throughput methods. Typically, the descriptors used for the analysis are structural descriptors, which can be difficult to obtain and to tune according to the results of the analysis. In this Research Article, we show the use of deposition process parameters as descriptors for analysis of a photovoltaics data set. To create a data set, solar cell libraries were fabricated using iron oxide as the absorber layer deposited using different deposition parameters, and the photovoltaic performance was measured. The data was then used to build models using genetic programing and stepwise regression. These models showed which deposition parameters should be used to get photovoltaic cells with higher performance. The iron oxide library fabricated based on the model predictions showed a higher performance than any of the previous libraries, which demonstrates that deposition process parameters can be used to model photovoltaic performance and lead to higher performing cells. This is a promising technique toward using data mining tools for discovery and fabrication of high performance photovoltaic materials

Quantum Efficiency and Bandgap Analysis for Combinatorial Photovoltaics: Sorting Activity of Cu–O Compounds in All-Oxide Device Libraries

All-oxide-based photovoltaics (PVs) encompass the potential for extremely low cost solar cells, provided they can obtain an order of magnitude improvement in their power conversion efficiencies. To achieve this goal, we perform a combinatorial materials study of metal oxide based light absorbers, charge transporters, junctions between them, and PV devices. Here we report the development of a combinatorial internal quantum efficiency (IQE) method. IQE measures the efficiency associated with the charge separation and collection processes, and thus is a proxy for PV activity of materials once placed into devices, discarding optical properties that cause uncontrolled light harvesting. The IQE is supported by high-throughput techniques for bandgap fitting, composition analysis, and thickness mapping, which are also crucial parameters for the combinatorial investigation cycle of photovoltaics. As a model system we use a library of 169 solar cells with a varying thickness of sprayed titanium dioxide (TiO₂) as the window layer, and covarying thickness and composition of binary compounds of copper oxides (Cu–O) as the light absorber, fabricated by Pulsed Laser Deposition (PLD). The analysis on the combinatorial devices shows the correlation between compositions and bandgap, and their effect on PV activity within several device configurations. The analysis suggests that the presence of Cu₄O₃ plays a significant role in the PV activity of binary Cu–O compounds

Open Circuit Potential Build-Up in Perovskite Solar Cells from Dark Conditions to 1 Sun

The high open-circuit potential (<i>V</i>_oc) achieved by perovskite solar cells (PSCs) is one of the keys to their success. The <i>V</i>_oc analysis is essential to understand their working mechanisms. A large number of CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> PSCs were fabricated on single large-area substrates and their <i>V</i>_oc dependencies on illumination intensity, <i>I</i>₀, were measured showing three distinctive regions. Similar results obtained in Al₂O₃ based PSCs relate the effect to the compact TiO₂ rather than the mesoporous oxide. We propose that two working mechanisms control the <i>V</i>_oc in PSCs. The rise of <i>V</i>_oc at low <i>I</i>₀ is determined by the employed semiconductor n-type contact (TiO₂ or MgO coated TiO₂). In contrast, at <i>I</i>₀ close to AM1.5G, the employed oxide does not affect the achieved voltage. Thus, a change of regime from an oxide-dominated <i>E</i>_Fn (as in the dye sensitized solar cells) to an <i>E</i>_Fn, directly determined by the CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> absorber is suggested

Electron-Hybridization-Induced Enhancement of Photoactivity in Indium-Doped Co₃O₄

We investigate the significance of indium (In) doping of Co₃O₄ in the operation of TiO₂|Co–In–O|RuO₂ all-oxide solar cells by employing combinatorial experiments and density functional theory (DFT) calculations. We observed an increase in the open-circuit voltage, <i>V</i>_oc, of more than 240 mV with an enhancement by a factor of 4 in the short-circuit current, <i>J</i>_sc, in the low-doping range. This constitutes a maximum power that is five times greater than that of pure Co₃O₄-based photovoltaic (PV) devices. Surprisingly, a concurrent marginal change in the band gap and a decrease in the optical absorption coefficient as a function of indium concentration was observed, contrary to what has been assumed previously. Using DFT in conjunction with joint density of states calculations, we show that with increasing amounts of In, there is a reduction in the low-energy photon absorption due to disallowed electronic transitions. Moreover, we show that emergence of In 5s states results in a free-electron-like band in the conduction band. We propose that this might reduce the rate of carrier recombination (reflected in higher open-circuit voltage) and enhance the electron diffusion lengths (reflected in higher short-circuit current), leading to improved PV activity. We expect that our results will advance the understanding and development of novel metal oxide semiconductors for low-cost PV applications

Utilizing Pulsed Laser Deposition Lateral Inhomogeneity as a Tool in Combinatorial Material Science

Pulsed laser deposition (PLD) is widely used in combinatorial material science, as it enables rapid fabrication of different composite materials. Nevertheless, this method was usually limited to small substrates, since PLD deposition on large substrate areas results in severe lateral inhomogeneity. A few technical solutions for this problem have been suggested, including the use of different designs of masks, which were meant to prevent inhomogeneity in the thickness, density, and oxidation state of a layer, while only the composition is allowed to be changed. In this study, a possible way to take advantage of the large scale deposition inhomogeneity is demonstrated, choosing an iron oxide PLD-deposited library with continuous compositional spread (CCS) as a model system. An Fe₂O₃–Nb₂O₅ library was fabricated using PLD, without any mask between the targets and the substrate. The library was measured using high-throughput scanners for electrical, structural, and optical properties. A decrease in electrical resistivity that is several orders of magnitude lower than pure α-Fe₂O₃ was achieved at ∼20% Nb–O (measured at 47 and 267 °C) but only at points that are distanced from the center of the PLD plasma plume. Using hierarchical clustering analysis, we show that the PLD inhomogeneity can be used as an additional degree of freedom, helping, in this case, to achieve iron oxide with much lower resistivity