Recently, a new field in photovoltaics (PV) has emerged, focusing on solar cells that are entirely based on metal oxide semiconductors. The all-oxide PV approach is very attractive due to the chemical stability, nontoxicity, and abundance of many metal oxides that potentially allow manufacturing under ambient conditions. Already today, metal oxides (MOs) are widely used as components in PV cells such as transparent conducting front electrodes or electron-transport layers, while only very few MOs have been used as light absorbers. In this Perspective, we review recent developments of all-oxide PV systems, which until today were mostly based on Cu2O as an absorber. Furthermore, ferroelectric BiFeO3-based PV systems are discussed, which have recently attracted considerable attention. The performance of all-oxide PV cells is discussed in terms of general PV principles, and directions for progress are proposed, pointing toward the development of novel metal oxide semiconductors using combinatorial methods.
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.
Nanoparticles possess exceptional optical, magnetic, electrical, and chemical properties. Several applications, ranging from surfaces for optical displays and electronic devices, to energy conversion, require large-area patterns of nanoparticles. Often, it is crucial to maintain a defined arrangement and spacing between nanoparticles to obtain a consistent and uniform surface response. In the majority of the established patterning methods, the pattern is written and formed, which is slow and not scalable. Some parallel techniques, forming all points of the pattern simultaneously, have therefore emerged. These methods can be used to quickly assemble nanoparticles and nanostructures on large-area substrates into well-ordered patterns. Here, we review these parallel methods, the materials that have been processed by them, and the types of particles that can be used with each method. We also emphasize the maximal substrate areas that each method can pattern and the distances between particles. Finally, we point out the advantages and disadvantages of each method, as well as the challenges that still need to be addressed to enable facile, on-demand largearea nanopatterning.
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