Opportunities in nanometer sized Si wires for PV applications
Introduction
Due to its non-toxicity, abundance, stability and the availability of processes from the microelectronics, the photovoltaic industry has been dominated by crystalline Si technology for the last 50 years. Since then the major challenge has been to reduce the cost per watt-peak, either by cutting down the material or processing costs, or by increasing the device energy conversion efficiency. Standard single-junction c-Si solar cells are thermodynamically limited to the so-called Shockley-Queisser limit of 31%. In this ideal single-junction solar cell, the main source of loss is the thermalization of the excess energy of carriers generated deep in the bands. There exist in the literature a number of concepts aiming at tackling this type of losses. They are often referred to as “third generation concepts”. The most frequently discussed concepts are the following: multijunction solar cells, hot-carrier solar cells, intermediate band solar cells, up- and down-conversion and carrier multiplication devices [1], [2], [3]. Despite interesting advances in the research of all of these concepts, the only one so far that has been able experimentally to surpass the Shockley–Queisser limit is the multijunction solar cell, with a maximum reported efficiency of 41.6% under 364 suns concentration [4]. In multijunction solar cells, multiple junctions made of different materials are stacked on top of each other with increasing bandgap to filter the light as it impinges on the cell, absorbing each range of wavelength in the material with the optimal bandgap to reduce thermalization. The bandgaps of III–V materials can be tuned by changing stoichiometry, making them very interesting candidates for fabricating this type of device. The growth of III–V material is nonetheless expensive, and the resources are limited. Developing a similar device fully based on Si, i.e. an all-Si tandem solar cell, is therefore of primary interest and has been over the years a topic of very intense research. Crystalline Si is used as bottom material, and a Si-based material with a higher bandgap needs to be developed for the top junction. The concept of using quantum confinement to create a higher bandgap c-Si material for an all-Si tandem solar cell has first been proposed in a quantum-dot (QD) embodiment. Such devices have not yet been experimentally demonstrated. Nevertheless, some test structures where QDs are used as active regions are found in literature and the open circuit voltage was reported to increase, up to 556 mV, proportionally with reductions in QD size (down to 3 nm). Probably the main challenge with the present implementation is the charge transfer: charges have to tunnel from dot to dot through the dielectric matrix limiting the current flow. The necessity of controlling both the size of the quantum dots and the spacing in-between the quantum dots down to the nanometer proves to be very demanding. To achieve sufficient carrier mobility and hence a reasonable conductivity, formation of a true superlattice is required with overlap of the wavefunctions for adjacent quantum dots; which in turn requires either close spacing between QDs or low barrier height. As a result, good quantum confinement and good tunneling are often mutually exclusive requirements [5].
This review paper discusses the literature related to the fabrication of a device built with vertical Si quantum wires. This material can in principle provide the electronic and optical properties of 2D quantum-confined Si nanowires (Si NWs) along with better prospects for conductivity than quantum dots in the third dimension. Over the last years Si NWs have actually been extensively explored for several other types of devices: on the one hand, they provided very interesting properties for single-junction solar cells, such as very low reflectivity; on the other hand, small-diameter Si NWs with confinement of the excitons became an intensive field of investigation for opto-electronic-related Si devices in the 1990s (LEDs, lasers, etc.). Most of the literature dealing with Si NWs for PV applications refers to NWs with relatively large diameters (>20 nm) for single-junction cells [6]. Despite the nanowire architecture of the devices, the underlying material properties are still those of bulk Si. These approaches have their own merits but stop short of considering the possibilities that quantum size effects offer. Nonetheless some properties are also valid for nanometer-sized NWs and are therefore included in this review. On the opposite, most of the literature related to optoelectronic devices presents quantum-confined NWs. The nanostructures are characterized, investigated and their properties are analyzed but the PV application of such device is only rarely mentioned. This review therefore combines both the literature of large NWs for PV and small NWs for optoelectronics to provide a comprehensive overview of the opportunities for Si NWs for all-Si tandem cells.
The first challenge towards the fabrication of an all-Si tandem cell based on NWs is the activation of photovoltaic functionalities. To integrate quantum confined Si nanowires into a solar cell, a number of non-trivial questions must be answered regarding the device functionalities that this material needs to perform: (1) Bandgap engineering, (2) light absorption or exciton generation, (3) electron-hole splitting or rectifying junction (4) carrier collection and (5) electrical contacts. The first part of the review is therefore dedicated to the literature discussing such PV functionalities. The second part of this review is devoted to the synthesis methods that can potentially deliver Si NWs with nanometer-scale diameter.
Section snippets
Bandgap engineering
For a given number of junctions, there exists an optimal combination of bandgaps of the materials to reach the highest efficiency. In case of two junctions, the optimal combination of bandgaps is 1.1 eV (bottom cell) and 1.7–1.8 eV (top cell) respectively as shown in Fig. 1a, Fig. 1b. We therefore need to acquire a Si-based material with bandgap of ∼1.8 eV for the top cell. The quantum confinement of Si in two dimensions (NWs) enables the engineering of the bandgap. Hence the first parameter to
Fabrication
We have seen in the previous chapter that the requirements of NWs for all-Si tandem cell are rather demanding. The fabrication of the desired nanomaterial with tailored atomic structures and its assembly into functional devices is challenging for nanotechnologists. As mentioned above, various parameters such as length, shape, diameter, diameter distribution, orientation, strain and inter-spacing play an important role for the targeted PV application. Having a critical eye on the fabrication
Conclusion
Reviewing the literature on Si nanowires in view of the fabrication of the high-bandgap junction for an all-Si tandem solar cell, it appears that the requirements are demanding. First, in order to reach the desired bandgap of 1.7–1.8 eV, the diameter of the nanowire should not exceed 3 nm, should be preferentially [100]-oriented and should be as stress-free as possible. Furthermore, the high aspect ratio will increase the surface area and hence the surface recombination. Extreme care must be
Acknowledgements
The authors would like to acknowledge the financial support of the IWT (Agentschap voor Innovatie door Wetenschap en Technologie, Flemish Community, Belgium) through the project SiLaSol, and the PhD thesis of Rufi Kurstjens.
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