The sun offers an abundant, clean source free of energy capable of generating enough power to satisfy global energy needs. Currently, however, photovoltaics, the technology used to convert the sun's energy to useable power, remains relatively inefficient and expensive. Indeed, despite over fifty years of development, the large-scale implementation of photovoltaics remains in the balance.

The aim of our group at The University of Oklahoma is to develop innovative solar cell technology capable of considerably increasing the performance and therefore economic viability of this technology. Currently we have projects focused predominately on next (third) generation photovoltaics, which although considered longer-term, has the potential to generate a paradigm shift in the photovoltaics industry. Specific projects being investigated at OU include: quantum dot intermediate band solar cells, narrow gap semiconductors for hot carrier solar cells and carrier multiplication, and dilute nitrides for multi-junction solar cells.

A summary of these projects are given below:

Quantum-Dot Intermediate Band Solar Cells

Figure 1. (a) Schematic representation of an intermediate band solar cell. As well as the traditional absorption of photogenerated carriers across the semiconductor bandgap (hω1) extra absorption is possible via transitions between the valence band and intermediate band (hω2), and from the intermediate band to conduction band (hω3). (b) Atomic force image and (c) transmission electron microscope image of surface quantum dots grown at OU

Traditional solar cells are limited as to the amount of the sun's energy they can harness due to the fixed energy gap (or absorption edge) of the semiconductor technology used. A trade-off therefore exists in the amount of light absorbed in the visible and UV part of the solar spectrum, relative to the longer wavelength infrared region. As such, in current commercial solar cells - based mainly on silicon - much of the infrared spectrum is not absorbed and simply passes through the cell. This transmission constitutes a total loss of around 25%, the significance of which is evident when one considers that the maximum efficiency of single junction devices is around 20%.

Intermediate band solar cells (IBSCs) have been proposed as a means to improve the performance of single-junction devices through the insertion of a band within the energy gap of the host semiconductor, that absorbs lower energy photons, while retaining the operating voltage of the higher energy matrix material. A schematic illustration of the proposed operation of such a device is shown in Figure 1(a). At OU, we are interested in utilizing semiconductor quantum dots (QDs) to form intermediate bands. QDs are three-dimensionally confined semiconductor nanostructures that behave in an analogous fashion to atoms. These nanostructures have been shown to produce isolated bands within the higher energy host material, making them excellent candidates as an intermediate state in IBSCs.

This program at OU is funded through the Oklahoma Center for the Advancement of Science & Technology (OCAST) in collaboration with the group of Professor Santos and Amethyst Research Inc. of Ardmore, OK. Our particular focus is based upon the arsenide and antimonide QD systems. Examples of such dots are shown in Figures 1(b) and (c). These images show InSb quantum dots grown by MBE at OU, using atomic force microscopy (AFM) and transmission electron microscopy (TEM), respectively. These images were recorded on campus in the Samuel Roberts Noble Electron Microscopy Laboratory.

Hot Carrier Solar Cells, and Multi-exciton Generation

Although semiconductor solar cells efficiently absorb light above the energy gap of the material, high-energy photons (quanta of light) well in excess of the semiconductor band edge rapidly lose their excess energy via the generation of heat, which is energy that cannot be harnessed to produce usable current in a solar cell. As such, thermal losses (heat) constitute a significant loss mechanism in conventional solar cells.

Hot-carrier solar cells have been proposed as systems in which hot, high-energy carriers are removed prior to thermally mediated relaxation, via the use of energy selective contacts. A similar process is Multi-exciton generation (MEG), which describes a process whereby carrier relaxation occurs via the excitation of additional carriers across the bandgap, rather than through heat production. Both hot carrier solar cells and solar cells operating via MEG have the potential to significantly enhance the efficiency of third-generation solar cells.

Figure 2. Schematic of the multi-exciton generation process. Here, high energy charge carriers relax to the band edge via the excitation of additional carriers across the semiconductor energy gap

At OU we are investigating materials and nanostructures that may slow carrier cooling via the unique physical properties of the systems studied. Projects include investigations of arsenide and antimonide-based quantum dots and quantum wells, as well as, bulk indium nitride. In each case we are investigating the interesting physical properties of these systems, which have the potential to facilitate the implementation of hot carrier solar cells or MEG in the operation of real devices. This work involves strong collaboration with the group of Santos (MBE) at OU and colleagues at TU Vienna in Austria.

GaInNAs - dilute nitrides - for multi-junction solar cells

To date, the method closest to practical implementation for harnessing the full solar spectrum, and improving the efficiency of photovoltaic systems, is through the utilization of multi-junction (MJ) solar cells. In this technology the limitation of the finite absorption of the broad solar spectrum by single-gap semiconductors is circumvented (somewhat) by the stacking of multiple semiconductor systems. Indeed, the highest efficiency solar cells currently on the market are based on monolithically grown Ge/GaAs/GaInP triple junction solar cells. These systems are extensively used in space applications, on satellites and spacecraft. In MJs, each cell absorbs different regions of the solar spectrum increasing the efficiency over single junction cells to ~30%. Currently, however, such technology is expensive, making it prohibitive for domestic and/or utility scale power generation.

Figure 3. Schematic representation of the absorption of (a) current multi-junction technology and (b) the improved absorption for a MJ based in a fourth junction absorbing at 1eV.

The relative absorption of current commercial systems is shown in Figure 3(a). However, despite the improved efficiency of MJ solar cells, the semiconductors that can be implemented in tandem are limited by the monolithic growth process and performance. This limitation is the result of strict conditions for semiconductor growth, whereby the atomic spacing of the various materials must match to provide the maximum optical quality. If these conditions are not met, defects form, thus reducing solar cell efficiency. Therefore, to enhance the efficiency of MJ systems further the addition of a fourth junction has been proposed. This is shown schematically in Figure 3(b). Such a material, lattice matched to the GaAs system, and absorbing at 1eV, has the potential to increase the efficiency of MJ systems to 44%.

At OU we are working on dilute nitride semiconductors, which have been proposed as a suitable candidate material for the fourth junction in next generation MJ solar cells. Indeed, recently a MJ solar cell incorporating GaInNAsSb has shown near record efficiencies of 43.5%*. Despite this success, much work remains to understand the true nature of this material so the yield and lifetime can be improved. At OU we are investigating various structures, growth parameters, and passivation techniques to improve the stability and performance of these systems. This work is performed in collaboration with industrial partners at Amethyst Research Inc. of Ardmore, OK. In addition, we are collaborating with groups at SUNY Buffalo to study defect formation in GaInNAs material and its contribution to the device characteristics. Currently, we receive material from partners at Sharp Laboratories of Europe (Oxford, UK) and CRHEA-CNRS, in France.

Besides these core projects, we are also working with colleagues in the Department of Electrical and Computer Engineering at OU on colloidal quantum dot solar cells and type-II quantum dots. We are always interested in participating in novel collaborative projects, where our experimental techniques can support materials and device development in next generation photovoltaics.