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5. Maximizing Cell Performance
5.1 Externally Biased pn Junction
Fig.5 shows a pn junction connected to an external battery with the internally generated electric field direction included. If (5a) is recalled, taking into account that the externally applied voltage, V, with the exception of any voltage drop in the neutral regions of the material, will appear as opposing the junction voltage, Equation 5a becomes ...
...where now nn and np are the total concentrations of electrons on the n-side of the junction and on the p-side of the junction, no longer in thermal equilibrium. Thermal equilibrium will exist only when the externally applied voltage is zero, meaning that np ni = 2 , is only true when V = 0. However, under conditions known as low injection levels, it will still be the case that the concentration of electrons on the n-side will remain close to the thermal equilibrium concentration. For this condition, (7) becomes ...
5.3 Minimizing Cell Resistance Losses
Any voltage drop in the regions between the junction and the contacts of a PV cell will result in ohmic power losses. In addition, surface effects at the cell edges may result in shunt resistance between the contacts. It’s thus desirable to keep any such losses to a minimum by keeping the series resistance of the cell at a minimum and the shunt resistance at a maximum. With the exception of the cell front contacts, the procedure is relatively straightforward.
Most cells are designed with the front layer relatively thin and highly doped, so the conductivity of the layer is relatively high. The back layer, however, is generally more lightly doped in order to increase the junction width and to allow for longer minority carrier diffusion length to increase photon absorption.
There must therefore be careful consideration of the thickness of this region in order to maximize the performance of these competing processes.
If the back contact material is allowed to diffuse into the cell, the impurity concentration can be increased at the back side of the cell. This is important for relatively thick cells, commonly fabricated by slicing single crystals into wafers. The contact material must produce either n-type or p-type material if it diffuses into the material, depending on whether the back of the cell is n-type or p-type.
In addition to reducing the ohmic resistance by increasing the impurity concentration, the region near the contact with increased impurity concentration produces an additional electric field that increases the carrier velocity, thus producing a further equivalent reduction in resistance. The electric field is produced in a manner similar to the electric field that is produced at the junction.
For example, if the back material is p-type, holes from the more heavily doped region near the contact diffuse toward the junction, leaving behind negative acceptor ions. Although there is no source of positive ions in the p-region, the holes that diffuse away from the contact create an accumulated positive charge that is distributed through the more weakly doped region. The electric field, of course, causes a hole drift current, which, in thermal equilibrium, balances the hole diffusion current. When the excess holes generated by the photoabsorption process reach the region of the electric field near the contact, however, they are swept more quickly toward the contact. This effect can be viewed as the equivalent of moving the contact closer to the junction, which, in turn, has the ultimate effect of increasing the gradient of excess carriers at the edge of the junction. This increase in gradient increases the diffusion current of holes away from the junction. Since this diffusion current strongly dominates the total current, the total current across the junction is thus increased by the heavily doped layer near the back contact.
At the front contact, another balancing act is needed. Ideally, the front contact should cover the entire front surface. The problem with this, however, is that if the front contact is not transparent to the incident photons, it will reflect them away. In most cases, the front contact is reflecting. Since the front/ top layer of the cell is generally very thin, even though it may be heavily doped, the resistance in the transverse direction will be relatively high because of the thin layer. This means that if the contact is placed at the edge of the cell to enable maximum photon absorption, the resistance along the surface to the contact will be relatively large.
The compromise, then, is to create a contact that covers the front surface with many tiny fingers. This network of tiny fingers, which, in turn, are connected to larger and larger fingers, is similar to the con figuration of the capillaries that feed veins in a circulatory system. The idea is to maintain more or less constant current density in the contact fingers, so that as more current is collected, the cross-sectional area of the contact must be increased.
Finally, shunt resistance is maximized by ensuring that no leakage occurs at the perimeter of the cell.
This can be done by nitrogen passivation or simply by coating the edge of the cell with insulating material to prevent contaminants from providing a current path across the junction at the edges.
6. Traditional PV Cells
6.1 Introduction
Traditional PV cells are based on the theoretical considerations of Sections 3 through 5. Cells currently commercially available are based on crystalline, multicrystalline, and amorphous (thin film) silicon (Si-C, Si, a-Si:H); copper indium gallium diselenide (CIGS) thin films; CdTe thin films; and III-V compounds such as gallium arsenide (GaAs). They all have pn junctions and all are subject to the optimization considerations previously discussed. This section will present a brief summary of the structures of each cell along with current (2011) performance information. For the interested reader, reference [8] considers all of the cells in this section in greater detail.
6.2 Crystalline Silicon Cells
Crystalline silicon cells can be either monocrystalline or polycrystalline. The monocrystalline cells are generally somewhat more efficient, but are also somewhat more energy intensive to produce. The cells are composed of approximately 200 µm thick slices of p-type single crystal ingots grown from a melt, with their circumferences squared up by slicing the round cross section into an approximately square cross section, similar to the way that lumber is processed from logs. The junction is formed by diffusing n-type impurities to a depth of approximately 1/a, where a is the absorption constant. The surfaces are then polished, textured and contacts are attached on front and back of the cell. The resulting cell cross section is essentially that of Fig.7. An adaptation of the structure of Fig.7 involves connecting the top layer of the cell through to the back of the cell so all contacts of the cell will be on the back. This increases cell efficiency by eliminating reflection of incident photons from front contacts. Typical cell conversion efficiencies for cells with front contacts can approach 17% and conversion efficiencies of back contact cells can exceed 20%.
By pouring molten silicon into a quartz crucible with a rectangular or square cross section under carefully controlled temperature conditions, it’s possible to form a bar of silicon that consists of crystalline domains, but is not monocrystalline. The advantage of this polycrystalline silicon ingot is that it does not have to be "squared up," thus saving a processing step and some energy as well. The disadvantage is that the crystal boundaries reduce mobilities and provide trapping centers, so the efficiency of the cell is somewhat reduced to the 14%-15% range.
The interesting point, however, is that since monocrystalline silicon cells have rounded corners, when they are assembled into a module to produce more power and more voltage, the module has voids at the corners of the cells, such that no electricity is produced at these locations. The polycrystalline cells, on the other hand, being rectangles, have less wasted module space and the overall module efficiency approximates the efficiency of a monocrystalline module, except for the back contact versions.
Fig.10 shows photos of front contact monocrystalline cells, front contact polycrystalline cells, and back contact monocrystalline cells.
Above: Fig. 10 (a) Crystalline, (b) multicrystalline, and (c) crystalline
back contact Si PV cells.
6.3 Amorphous Silicon Cells
Since a-Si:H is not in a crystalline form, it loses the advantages of high mobility and high diffusion constant. The non-crystalline lattice includes a large number of silicon atoms with outer shell electrons that are not covalently bonded to nearest neighbors. These "dangling" electrons create impurity levels in the bandgap and thus affect the lifetimes of the excited charge carriers. Fortunately, it’s possible to apply hydrogen to the material such that the hydrogen fills the dangling bonds, thus passivating these sites in the material and improving the electrical properties. The material is thus described as a-Si:H.
Despite the amorphous nature of this material, it has a very favorable direct bandgap, which enables the material to efficiently absorb photons over a short distance. A 2 µm thickness of the material will absorb most of the incident photons, thus the reason why a-Si:H is considered to be a thin-film PV material.
Fig.11 shows several different cell structures. Structures b and c are multi-junction structures and thus present important challenges to the cell designer. First of all, each layer tends to act as a current source, such that if current sources are connected in series, each layer must generate the same current as every other layer. Secondly, the layers appear as series diodes. This means that although the generating diode is supplying current, this current must flow across a reverse-biased junction between adjacent layers. This implies that significant current won’t flow until the voltage across the reverse-biased junction reaches the reverse breakdown potential. Fortunately, with heavy doping on each side of the junction, the reverse breakdown voltage can be reduced to zero and a tunnel junction is created that allows current to pass unimpeded.
Commercial a-Si:H PV modules are presently available with flexible structures and conversion efficiencies of 8%-10%. An advantage of the technology and module structure is that the module can be applied directly to an approved surface without the need for any additional mounting components.
Above: Fig. 11 Three configurations for a-Si:H cells. (a) Basic a-Si:H cell
structure, (b) stacked a-Si:H junctions, and (c) SiC-Si-SiGe triple junctions.
(From Messenger, R.A. and Ventre, J., Photovoltaic Systems Engineering, 3rd
ed., CRC Press, Boca Raton, FL, 2010.)
Above: Fig. 12 Typical CIGS thin-film structure.
6.4 Copper Indium Gallium Diselenide Cells
Another popular material for thin-film cells is copper indium gallium diselenide (CIGS). This material is also a direct bandgap that absorbs most photons with energies above the bandgap energy within a thickness of about 2 µm. The material is in commercial production at conversion efficiencies in the neighborhood of 12%.
While it’s possible to produce both n-type and p-type CIS, homojunctions in the material are neither stable nor efficient. A good junction can be made, however, by creating a heterojunction with n-type CdS and p-type CIS.
The ideal structure uses near-intrinsic material near the junction to create the widest possible depletion region for collection of generated EHPs. The carrier diffusion length can be as much as 2 µm, which is com parable with the overall film thickness. Fig.12 shows a basic ZnO/CdS/CIGS/Mo cell structure, which was in popular use in 2004. Again, CIGS technology is advancing rapidly as a result of the thin-film PV partnership program, so by the time this paragraph is read, the structure of Fig.12 may be only suit able for history books and general discussion of the challenges encountered in thin-film cell development.
Nearly a dozen processes have been used to achieve the basic cell structure of Fig.12. The processes include radio frequency (rf) sputtering, reactive sputtering, chemical vapor deposition, vacuum evaporation, spray deposition, and electrodeposition. Sometimes these processes are implemented sequentially and sometimes they are implemented concurrently. Recently a novel method of manufacturing CIGS modules using cylindrical tubes with spaces in between to allow photons to pass through the module and be reflected back to the cylindrical tubes from the surface upon which the module is attached has been developed. This structure presents minimal wind loading for the module and consequently for most wind zones it can be simply laid on a flat roof and attached to adjacent modules to form the PV array. This module is becoming popular for use with white membrane flat roofing material.
6.5 Cadmium Telluride Cells
In theory, CdTe cells have a maximum efficiency limit close to 25%. The material has a favorable direct bandgap and a large absorption constant, allowing for cells of a few µm thickness. By 2001, efficiencies approaching 17% were being achieved for laboratory cells, and module efficiencies had reached 11% for the best large area (8390 cm^2 ) module. Efforts were then focused on scaling up the fabrication process to mass produce the modules, with the result of achieving a production cost of less than $1.00/W in 2008.
This cost included an escrow account to be used for recycling the materials at the end of module life.
No fewer than nine companies have shown an interest in commercial applications of CdTe. As of 2001, depending on the fabrication methodology, efficiencies of close to 17% had been achieved for small area cells (˜1 cm^2 ), and 11% on a module with an area of 8390 cm^2.
After it was shown that no degradation was observable after 2 years, production-scale manufacturing began. CdTe modules are now being manufactured and marketed at the rate of more than 1 GW annually for utility scale projects and the magic $1.00/W production cost barrier has now been bro ken for these modules, with an announced 4th quarter 2008 production cost of $0.93/W [8]. Fig.13 shows the cross section of a typical CdTe cell. In this figure, antireflective coating (ARC), transparent conducting oxide (TCO) and ethylene vinyl acetate (EVA), which are used to bond the back contact to the glass.
6.6 Gallium Arsenide Cells
The 1.43 eV direct bandgap, along with a relatively high absorption constant, makes GaAs an attractive PV material. Historically high production costs, however, have limited the use of GaAs PV cells to extraterrestrial and other special purpose uses, such as in concentrating collectors. Recent advances in concentrating technology, however, enable the use of significantly less active material in a module, such that cost-effective, terrestrial devices may soon be commercially available.
Most modern GaAs cells, however, are prepared by the growth of a GaAs film on a suitable substrate.
Fig.14 shows one basic GaAs cell structure. The cell begins with the growth of an n-type GaAs layer on a substrate, typically Ge. Then a p-GaAs layer is grown to form the junction and collection region.
Above: Fig. 13 Basic structure of a CdTe PV cell. (From Ullal, H.S. et al.,
Proceedings of the 26th IEEE Photovoltaic Specialists Conference, Anaheim,
CA, pp. 301, 1997. © IEEE.)
Above: Fig. 14 Structure of a basic GaAs cell with GaAlAs window and passive
Ge substrate. Back contact Ge substrate n-GaAs p-GaAs p-GaAlAs Top contacts;
ARC
The top layer of p-type GaAlAs has a bandgap of approximately 1.8 eV. This structure reduces minority carrier surface recombination and transmits photons below the 1.8 eV level to the junction for more efficient absorption.
Cells fabricated with III-V elements are generally extraterrestrial quality. In other words, they are expensive, but they are high-performance units. Efficiencies in excess of 20% are common and efficiencies of cells fabricated on more expensive GaAs substrates have exceeded 34%.
An important feature of extraterrestrial quality cells is the need for them to be radiation resistant.
Cells are generally tested for their degradation resulting from exposure to healthy doses of 1 MeV or higher energy protons and electrons. Degradation is generally less than 20% for high exposure rates.
Extraterrestrial cells are sometimes exposed to temperature extremes, so the cells are also cycled between ~-170°C and +96°C for as many as 1600 cycles. The cells also need to pass a bending test, a contact integrity test, a humidity test, and a high temperature vacuum test, in which the cells are tested at a temperature above 140°C in vacuum for 168 h.
Fill factors in excess of 80% have been achieved for GaAs cells. Single cell open-circuit voltages are generally between 0.8 and 0.9 V.
7. Emerging Technologies
7.1 New Developments in Silicon Technology
While progress continues on conventional Si technology, new ideas are also being pursued for crystal line and amorphous Si cells. The goal of Si technology has been to maintain good transport properties, while improving photon absorption and reducing the material processing cost of the cells. It’s likely that several versions of thin Si cells will continue to attract the attention of the PV community, including recent research on thin Si on glass.
Another interesting opportunity for cost reduction in Si cell production is to double up on processing steps. For example, a technique has been developed for simultaneously diffusing boron and phosphorous in a single step, along with growing a passivating oxide layer [14].
As an alternative to the pn junction approach to Si cells, MIS-IL (metal insulator semiconductor inversion layer) cells have been fabricated with 18.5% efficiency. The cell incorporates a point-contacted back electrode to minimize the rear surface recombination, along with Cs beneath the MIS front grid and oxide window passivation of the front surface to define the cell boundaries. Further improvement in cell performance can be obtained by texturing the cell surfaces.
New developments in surface texturing may also simplify the process and result in additional improvement in Si device performance. Discovery of new substrates and methods of growing good quality Si on them is also an interesting possibility for performance improvement and cost reduction for Si cells.
Since new ideas will continue to emerge as interest in Si PV technology continues to grow, the interested reader is encouraged to attend PV conferences and to read the conference publications to stay up-to-date in the field.
7.2 CIS-Family-Based Absorbers
Much is yet to be learned about inhomogeneous absorbers and composite absorbers composed of combinations of these various materials. The possibility of multi-junction devices is also being explored.
Meanwhile, work is underway to reduce the material usage in the production of CIS modules in order to further reduce production costs. Examples of reduction of material use include halving the width of the Mo contact layer, reduction in the use of H2S and H2Se, and a reduction in ZnO, provided that a minimum thickness can be maintained.
7.3 Other III-V and II-VI Emerging Technologies
It appears that compound tandem cells will receive appreciable emphasis in the III-V family of cells over the next few years. For example, Ga0.84In0.16As0.68P0.32, lattice matched to GaAs, has a bandgap of 1.55 eV and may prove to be an ideal material for extraterrestrial use, since it also has good radiation resistance. Cells have been fabricated with Al0.51In0.49P and Ga0.51In0.49P window layers, with the best 1 cm^2 cell having an efficiency of just over 16%, but having a fill factor of 85.4%.
Cell efficiencies can be increased by concentrating sunlight on the cells. Although the homo-junction cell efficiency limit under concentration is just under 40%, quantum well (QW) cells have been proposed to increase the concentrated efficiency beyond the 40% level. In QW cells, intermediate energy levels are introduced between the host semiconductor's valence and conduction bands to permit absorption of lower energy photons. These levels must be chosen carefully so that they won’t act as recombination centers, however, or the gains of EHPs from lower energy incident photons will be lost to the recombination processes. Laboratory cells have shown higher VOC resulting from a decrease in dark current for these cells.
7.4 Other Technologies
7.4.1 Thermo-photovoltaic Cells
To this point, discussion has been limited to the conversion of visible and near infrared spectrum to EHPs. The reason is simply that the solar spectrum peaks out in the visible range. However, heat sources and incandescent light sources produce radiation in the longer infrared regions, and in some instances, it’s convenient to harness radiated heat from these processes by converting it to electricity. This means using semiconductors with smaller bandgaps, such as Ge. More exotic structures, such as InAsSbP, with a bandgap of 0.45-0.48 eV have also been fabricated.
7.4.2 Intermediate Band Solar Cells
In all cells described to this point, absorption of a photon has resulted in the generation of a single EHP. If an intermediate band material is sandwiched between two ordinary semiconductors, it appears that it may be possible for the material to absorb two photons of relatively low energy to produce a single EHP at the combined energies of the two lower energy photons. The first photon raises an electron from the valence band to the intermediate level, creating a hole in the valence band, and the second photon raises the electron from the intermediate level to the conduction band. The trick is to find such an intermediate band material that will "hold" the electron until another photon of the appropriate energy impinges upon the material. Such a material should have half its states filled with electrons and half empty in order to optimally accommodate this electron transfer process. It appears that III-V compounds may be the best candidates for implementation of this technology. Theoretical maximum efficiency of such a cell is 63.2%.
7.4.3 Super Tandem Cells
If a large number of cells are stacked with the largest bandgap on top and the bandgap of subsequent cells decreasing, the theoretical maximum efficiency is 86.8%. A 1 cm^2 four-junction cell has been fabricated with an efficiency of 35.4%. The maximum theoretical efficiency of this cell is 41.6% [22]. Perhaps 1 day one of the readers of this paragraph (or one of their great-great grandchildren) will fabricate a cell with the maximum theoretical efficiency.
7.4.4 Hot Carrier Cells
The primary loss mechanism in PV cells is the energy lost in the form of heat when an electron is excited to a state above the bottom of the conduction band of a PV cell by a photon with energy greater than the bandgap. The electron will normally drop to the lowest energy available state in the conduction band, with the energy lost in the process being converted to heat. Hence, if this loss mechanism can be overcome, the efficiency of a cell with a single junction should be capable of approaching that of a super tandem cell. One method of preventing the release of this heat energy by the electron is to heat the cell, so the electron will remain at the higher energy state. The process is called thermo-electronics and is currently being investigated.
7.4.5 Optical Up- and Down-Conversion
An alternative to varying the electrical bandgap of a material is to reshape the energies of the incident photon flux. Certain materials have been shown to be capable of absorbing two photons of two different energies and subsequently emitting a photon of the combined energy. Other materials have been shown to be capable of absorbing a single high-energy photon and emitting two lower-energy photons. These phenomena are similar to up-conversion and down-conversion in communications circuits at radio frequencies.
By the use of both types of materials, the spectrum incident on a PV cell can be effectively narrowed to a range that will result in more efficient absorption in the PV cell. An advantage of this process is that the optical up- and down-converters need not be a part of the PV cell. They simply need to be placed between the photon source and the PV cell. In tandem cells, the down-converter would be placed ahead of the top cell and the up-converter would be integrated into the cell structure just ahead of the bottom cell.
7.4.6 Organic PV Cells
Even more exotic than any of the previously mentioned cells is the organic cell. In the organic cell, electrons and holes are not immediately formed as the photon is absorbed. Instead, the incident photon creates an exciton, which is a bound EHP. In order to free the charges, the exciton binding energy must be overcome. This dissociation occurs at the interface between materials of high electron affinity and low ionization potential [22]. Photoluminescence is related to this process. Just to end this section with a little chemistry, the reader will certainly want to know that one material that is a candidate for organic PV happens to be poly{2,5-dimethoxy-1,4-phenylene-1,2-ethenylene-2-methoxy-5-(2-ethyl-hexyloxy) 1,4-phenylene-1,2-ethenylene}, which goes by the nickname M3EH-PPV. Whether M3EH-PPV will dominate the PV market 1 day remains to be seen. So far efficiencies of this very challenging technology have been in the 1% range.
8. PV Electronics and Systems
8.1 Introduction
Obviously the extent of research and development that has been covered in this section may not have been undertaken if a possible market for each technology had not been identified. Prior to 2000, most PV applications were stand-alone applications, such as off-grid cabins. Since that time, however, grid connected applications have mushroomed and far surpassed stand-alone applications. Grid-connected applications can be either noninteractive or interactive and can be direct grid connect or battery backup.
Noninteractive grid-connected systems simply use the grid as a backup source of power when the sun is not shining. Interactive systems are capable of selling energy back to the grid if the host demand is met and excess system output remains available.
Interactive systems require inverters to convert the dc power produced by the PV array into compatible ac power for use at the source as well as for return to the utility. In a utility interactive system, it’s necessary for the inverter to shut down if the grid shuts down. However, if a set of batteries are used, then it’s possible to energize selected loads when the grid is down as long as the selected loads can be isolated from the grid via switching within the inverter. IEEE Standard 1547 specifies performance parameters for waveform purity and disconnecting from the grid and UL 1741 specifies a testing protocol to ensure compliance with IEEE 1547.
If a system has batteries for backup power, then a battery backup inverter as well as a charge controller will normally be needed.
8.2 PV System Electronic Components
8.2.1 Inverters
Inverters can have output waveforms ranging from square, for the simplest units, to sine, for the best units. If a unit is to be connected to the grid in a sell mode, it must have no more than 5% total harmonic distortion, with individual harmonic maxima specified by IEEE 1547.
Straight grid-connected, utility interactive inverters are used in the simplest of utility interactive systems. So-called "string inverters" use series/parallel combination of modules that may produce up to 1000 V when open circuited, although installations in the United States must have maximum voltages less than 600 V unless they are on utility-owned property. String inverter power output ranges from the low kW range up to 1 MW, with larger units in the pipeline. String inverters generally have maximum power point tracking (MPPT) circuitry at their inputs, so they can operate the PV array at its maxi mum power point and thus deliver maximum power to the load. They also incorporate ground fault detection and interruption at their inputs to shut down the PV array if a current-carrying conductor should come in contact with a grounded object. Since they comply with UL1741, they shut down when ever any grid disturbance, such as undervoltage, overvoltage, or frequency error is present on the utility system, even if other inverters are connected to the same system.
Recently, a version of the straight grid-connected inverter, the microinverter, has become very popular.
The microinverter is used with either a single module or with a pair of modules. Generally the rated dc module power is between 175 and 240 W. The microinverter is mounted next to the PV module so that ac rather than dc is fed from a rooftop. Furthermore, the ac is connected directly to the utility connection so that if the utility loses power, the microinverter is disabled and no current or voltage is present at its output.
When battery backup is used to create an uninterruptible source, the battery backup inverter must have two separate ac ports. One port is connected to the utility and one is connected to the standby loads.
If the grid is operational and the sun is down, the inverter will pass utility power through to the standby loads. If the grid is down, then the inverter provides power to the standby loads via the standby port either directly from the PV array during daytime hours or from the batteries at night, or, for that matter, from a combination of batteries and PV array, depending upon the demand of the standby load. If the grid is down, the grid-connected port is automatically shut down, generally in less than 2 s. Present commercial battery backup inverters are rated at 8 kW or less, with the possibility of connecting series-parallel combinations to deliver up to 80 kW. A 100 kW unit has been developed but is still in the beta testing phase.
8.2.2 Charge Controllers
When a battery backup inverter is used in a so-called dc-coupled battery backup configuration, the input of the inverter is connected directly to the system batteries. The PV array is also connected to the batteries, but it’s connected through a charge controller. The charge controller is needed for two purposes. The primary purpose is to prevent the batteries from becoming overcharged. The system inverter is set to shut off if the batteries approach the maximum discharge limit. The secondary purpose of the charge controller is to maximize energy transfer from PV array to batteries, assuming that a MPPT charge controller is selected. There is no need for MPPT at the inverter input, since it’s always at the battery voltage.
9. Conclusions
Regardless of the technology or technologies that may result in low-cost, high-performance PV cells, it must be recognized that the lifecycle cost of a cell depends on the cell's having the longest possible, maintenance-free lifetime. Thus, along with the developments of new technologies for absorbers, development of reliable encapsulants and packaging for the modules will also merit continued research and development activity.
Every year engineers make improvements on products that have been in existence for many years.
Automobiles, airplanes, electronic equipment, building materials, and many more common items see improvement every year. Even the yo-yo, a popular children's toy during the 1940s and 1950s, came back with better-performing models. Hence, it should come as no surprise to the engineer to see significant improvements and scientific breakthroughs in the PV industry well into the next millennium. The years ahead promise exciting times for the engineers and scientists working on the development of new photovoltaic cell and system technologies, provided that the massive planning and execution phases can be successfully undertaken. This will certainly be the case as the world of nano-devices is explored.