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Emerging Materials and Methods for 3rd generation solar cells

Final Report Summary - EMM3 (Emerging Materials and Methods for 3rd generation solar cells)

Increasing the efficiency of traditional solar cells relies on the use of the right bandgap material(s) and high quality of the device interfaces. Third generation solar cells rely on novel principles that can only be realized with materials that still have to be designed. Such principles require the fabrication of new materials, material combinations and structures which lead to novel photoconversion phenomena. This involves a multidisciplinary task, which combines physics, chemistry and engineering. Thus, this project combines two complementary aspects of third generation solar cells:

1.- New physical mechanisms for solar light conversion. The goal is to gain further understanding and mastering on the fundamental phenomena that rule photo-current conversion and limit the quantum efficiency in solar cells. In this project, optical spectroscopy and electronic transport techniques are combined for the study of physical phenomena ruling and limiting photocurrent conversion. The goal is not only to determine the presence of new phenomena, such as intermediate band generation (IBSC) or up/down conversion, but also the transport properties of such materials.

2.- New materials and material combinations. The aim is to obtain materials presenting physical phenomena required for 3rd generation solar cells with improved performance. We aim to analyze also the engineering aspects of these energy technologies and identify unique opportunities enabled by nanoscale design of materials in order to improve performance and reduce costs.

The main accomplishments of the project so far are:
- Development of new characterization methods to probe free carriers in InAs- and InN- like and confined volumes: Here, we propose a new experiment designed to modulate the electron concentration in the near-surface region of In-based semiconductors to achieve the measurement of the transport properties, which are proved to be difficult by conventional techniques. Such experiment involves the use of an electrochemical cell for the modulation of surface electrons and in-situ Raman spectroscopy for probing the interaction between free carriers and longitudinal optical phonons. In this project, it has been found that the electrons confined at the surface have a major role in the optical properties in In-rich III-N semiconductors.

• Engineering arsenide based nanowires with alternate wurtize–zincblende crystallographic superlattice structures: the fabrication of nanowires with new crystalline structures has been demonstrated by the host institution. The performance of wz-zb In(Ga)As superlattice is thought to give rise to intermediate band formation. By means of the intermediate band, it is possible to absorb below-band gap energy photons. Commonly, intermediate band formation is approached through stacked quantum dot arrays or impurity levels. The approach proposed here is promising for intermediate band generation in high quality material without the need of heterojunctions and/or impurities. Band gap engineering that combines both changes in the composition and structure of materials is a completely recent and innovative area in the field of nanotechnology. A different kind of periodic structure is that found in certain strained ferroelectric BiFeO3 thin films. The strain is responsible for the spontaneous periodic arrangement of ferroelectric domains, which in turn creates a saw-tooth shaped potential in the electronic structure. Such potential, along with the low mobility of carriers is able to simulate a large number of nanometer-sized solar cells connected in series. As a result, a large open circuit voltage (of the order of 20V) can be obtained under sunlight.

• New physical mechanisms for solar light conversion. In this regard, we have shown how the use of core-shell nanowires can allow increasing the solar cell efficiency in IBSCs. The idea behind is the use of the heterostructure to spatially separate the exciton, so that electrons in the IB will exhibit an increase in their lifetime by several orders of magnitude. As a consequence, the overall solar conversion efficiency is boosted.

The proposed studies impact different major research areas of contemporary research. For instance, the described and proposed studies are of interest to the physics, chemistry, biology and engineering scientific community. They can lead to a variety of discoveries, which may be not only interesting for solar cell applications but to other scientific fields. As an example, the study of electrolyte-gated Raman on InN material, has led to the discovery and first observation by the fellow of a pn rectification in InN. Another example is the study of light interaction in nanowire is of extreme relevance for the future design of more complex nanowire based solar cells.

Finally, the deep understanding of mechanisms that can lead to third generation solar cells is going to be essential for the progress of Europe in this century.