engineering the solar age - Suppliers for Photovoltaics

Solar modules: a variety of technologies

Photovoltaics uses the internal photoelectric effect to generate electricity from sunlight. Incoming light which is absorbed by a semiconductor crystal releases electrons from their bonding state. An electric field is then required to conduct them away from the solid state.

Solar cells have an n-type layer which can emit electrons (emitter) and a p-type layer which can absorb electrons (base). At the boundary interface between the two layers (p-n junction) an electric field is formed, which separates the light-generated charge carriers. A voltage corresponding to the electric field is produced at the terminal contacts of the cell, meaning that current is able to flow as soon as the contacts are connected to each other.

A crystalline silicon solar cell consists of a mono- or multicrystalline silicon chip (wafer) approximately 180 mm thick. In order to increase its conductivity, the crystal structure is doped, i.e. foreign atoms are added to the material in a targeted manner. The p-type layer is created by doping the structure with boron; the n-type layer is created by doping it with phosphorus. Metallic, strip-like contacts on the front and rear of the cell draw off the light-generated charge carriers.

Monocrystalline cells have a perfectly regular crystal structure. Multicrystalline cells are made of silicon crystals with grain sizes measuring just a few millimeters. The grain boundaries represent crystal defects to which charge carriers can be bound (recombination). Their efficiency is therefore somewhat lower.

Monocrystalline silicon cells achieve an efficiency of 17 to 18 percent in commercial manufacturing; multicrystalline silicon cells achieve 16 to 17 percent. Special technologies have also been developed to increase efficiency. The following three are the best known of the commercially available technologies:

Buried contacts: Applying contacts to the upper cell surface in the form of metal fingers and busbars reduces the absorptive area. This can be avoided by cutting grooves into the cell using a laser, to create a path for the metal conductors.

Back contact cells: Another way of removing the metal conductors from the upper surface is to place them on the rear side of the cell. The solar cells which currently achieve the highest efficiencies were created using this method.

Heterojunction with Intrinsic Thin Layer (HIT): The HIT cell consists of a thin, single-crystal wafer which is covered on the front and rear with an ultra-thin layer of amorphous silicon. Using both crystalline and amorphous silicon layers increases the cell’s efficiency.

These technologies can increase the efficiency of a crystalline solar cell by over 20 percent.

Thin-film modules are less efficient than those made with crystalline cells, but this is offset by the following advantages:

• the energy payback time is relatively short due to the low amounts of material and energy consumed
• the manufacturing process can be automated for large-area modules
• a high degree of vertical integration can be achieved, with only a few production steps necessary
• flexible substrates open up new areas of use for the product and its integration into buildings

The most important advantage is that fully-developed production, which allows large production volumes and efficiency to match, brings such major cost benefits that crystalline technology can be equaled or even bettered. This means that a kWh of solar power can be generated at a lower cost. Three technologies have already made the transition to mass production and are enjoying commercial success:

Silicon based thin-film modules are generally made from amorphous silicon (a-Si). Since amorphous silicon can absorb light considerably better than crystalline silicon, a film thickness of about 1 µm is sufficient. However, this material generates fewer charge carriers so the efficiency is substantially lower, lying between four and eight percent. Thin-film modules made from amorphous silicon can be manufactured to very large dimensions (the maximum as at June 2011 is 5.7 m2).

In order to increase the efficiency of an a-Si module, microcrystalline silicon (µc-Si) is added to the amorphous silicon, thereby producing a tandem or micromorph solar cell. The µc-Si layer absorbs more light in the red and infrared parts of the spectrum, and the efficiency increases to ten percent.

Copper-indium-selenium is the active semiconductor material in a CIS module, and is often alloyed with gallium to produce a CIGS module. Sulfur can also be used as an alloying agent to increase efficiency. CIS and CIGS modules have the highest efficiencies of all thin-film modules; an efficiency of 20 percent has already been reached in laboratory conditions, and commercial modules have efficiencies of between seven and twelve percent. The crystal structure of these chalcopyrite compounds corresponds to that of silicon, with the lattice sites occupied by elements of different valences (monovalent copper, trivalent indium, and hexavalent selenium). The outstanding characteristic of these semiconductors is their high tolerance of crystal defects and impurities, as these can combine to form electrically-neutral complexes. Consequently, the demands placed on the raw materials and processes are lower than for other semiconductor materials, resulting in greater scope for savings. The most recent research has shown that, in principle, it is possible to replace indium, a particularly costly element, with zinc.

Cadmium telluride (CdTe) modules are characterized by a relatively high efficiency (eleven percent) combined with low production costs, and are already being manufactured in very high quantities at reasonable cost. Currently, this is the most economic thin-film technology. Use of the heavy-metal cadmium poses particularly high expectations on module recycling, however.

Organic solar cells use a process to generate electricity that works in a similar fashion to the way in which photosynthesis converts the radiant energy from sunlight into chemical energy. This mechanism can be exploited using a suitable combination of strongly absorbing chromophores (semiconducting organic molecules or polymers may be considered) as donors and strong electron acceptors (e.g. fullerenes) for the photovoltaic generation of charge carriers. A layer thickness of around just 0.1 µm is required for the incident light to be completely absorbed.

Organic solar cells produced in the laboratory can reach an efficiency of eight percent. It has been possible to increase their service life to some 5,000 hours, but this is still far too short. Improvements achieved thus far have concentrated above all on the packaging of the cells and less on extending the photoactive materials’ service life.

It will be necessary to synthesize new donor and acceptor materials if organic solar cells are to be further improved. These must firstly be capable of self-organization (as this is crucial for the resulting coatings to be highly ordered) and secondly offer as broad an absorption spectrum as possible, so as to be able to better utilize the sunlight. Hence there is still great potential for increasing photocurrent.

Multijunction or stacked cells are generally made of elements of the third and fifth group of the periodic table. Elements of the third main group are indium (In), gallium (Ga) and aluminum (Al), while arsenic (As), phosphor (P), nitrogen (N) and antimony (Sb) belong to the fifth main group. The compound semiconductors are separated onto monocrystalline germanium wafers, where the germanium is the solar cell with the lowest band gap.

As the production of stacked cells is very complex and expensive, generally only small cells (100 square millimeters) are produced for use in concentrator systems with up to 700-fold concentration.

In a module, solar cells are connected to form an electric unit. Under standard conditions, commercial solar modules achieve an output of between 60 and 360 W. At 25 to 50 V, open-circuit voltage is considerably below grid voltage.

When a string is created by series-connecting several modules, the open-circuit voltage rises. By connecting several strings in parallel, almost any voltage can be achieved.

The direct current supplied by this PV generator is converted into alternating current at grid voltage and frequency by one or several inverters. If required, this conversion can occur with a specified phase shift, in order to feed reactive power into the grid (e.g. in the event of grid failure) and lend it support. Thanks to state-of-the-art power electronics, converting direct current to alternating current now only incurs minimal losses.

The decentralized feed-in of solar power opens up the possibility of creating microgrids from several neighboring PV installations. Using intelligent control engineering, a variable, virtual, large-scale power station can be developed in conjunction with decentralized suppliers and consumers of electricity. As elements in this power plant, the PV installations would contribute to increasing the quantity of decentralized power consumed, thus decreasing the amount of electricity bought from the public grid. Moreover, PV plants could improve supply security through short-term island operation.

In future, inverters could take over grid management tasks and provide energy services. In addition to stabilizing voltage and frequency, these include controlling the power factor and the targeted production of harmonic components to improve grid quality.

For this reason, bidirectional network interfaces are required to ensure the necessary communications and to link the large number of decentralized suppliers and consumers together in “smart grids”. This is an important precondition for the power supply of the future, which will no longer be dominated by large fossil fuel or nuclear power plants but by a few centralized and many decentralized power generation units.

PV plants will then feed power into the public grid or the microgrid (individual households, for example) on demand. If supply considerably exceeds demand, the solar power will be stored locally. This turns the microgrids that communicate with each other into smart grids, which in turn play an important part within a controllable, virtual large-scale power plant.

As shown by the Australian annual solar mobile race, solar mobility is still in its experimental stage rather than a commercial sector. On many lakes, tourist boats are powered by special solar modules installed onto the roof of the passenger cabin. Unmanned zeppelins and solar planes whose wings are completely covered with solar cells have already been tested; a scientist is planning a trip around the world in a solar plane in a few years’ time.

The scalability of photovoltaics (from mini solar cells with just a few milliwatts output to large 360 W modules) has contributed to a wide variety of stand-alone systems around the world today. These range from minute applications (pocket calculators) to the electricity supply of remote settlements, or solar mobility.

Stand-alone systems play but a minor role due to the fact that they produce relatively little electricity. Since powerful inverters were developed in around 1990, and since the systematic subsidization of feeding solar power into the grid started in 2000, grid-connected photovoltaics has developed into the predominant method of solar power generation.