Photovoltaics in Germany and the world

With an installed capacity of about 43 GW_P, photovoltaics made an electricity contribution of about 38 TWh (38 billion kWh) in Germany in 2017. Dividing the electricity contribution by the installed capacity gives a full-load hourly figure of just under 900. The above PV electricity contribution corresponds to a 7.2% share of German electricity production. The rapid PV expansion since 2000 has been made possible by a feed-in tariff paid by electricity customers in Germany. From an initial level of around 0.50 €/kWh, this has fallen to around 0.12 €/kWh today. In total, around €130 billion has been paid by electricity customers in Germany to date.

Last but not least, the German feed-in tariff in particular has led to a drastic reduction in PV production costs within about ten years. In the meantime, these are below €0.05/kWh in tenders for solar parks in Germany, and below €0.025/kWh in sunnier countries such as Saudi Arabia, Dubai or Chile.

In the world, PV power plants with a capacity of around 400 GW_P had been installed by 2017. The annual PV addition is already 100 GW_P today - around 50 GW_P is accounted for by China. Currently, the most powerful solar farm, with a capacity of 850 MW_P, is located in China at the Longyangxia Dam. While the two leading Chinese PV producers, Jinko Solar and Trina Solar, together shipped modules with nearly 20 GW_P, German cell production has virtually ground to a halt after several bankruptcies.

Learning Curve Installation Costs

The cost reduction in photovoltaics is impressive. The "learning curve" for installation costs as a function of time shows that since the 1970s, a price halving has been achieved each time PV installations increased tenfold. Today, installation costs for rooftop systems are less than 1500 €/kW. About half of this is for the module, the other half for BOS (Balance-Of-System). BOS includes cabling, switches, mounting system, MPP tracker and inverter, and for stand-alone systems also charger and batteries.

The market shares of Si solar modules in installed photovoltaic systems in the world have been around 90% for 15 years, with a dip in 2009 when CdTe modules covered almost 15% of the market for a short time - but today contribute only about 5%. For the past 10 years, polycrystalline Si modules have dominated the market with about 70%. With further cost reduction in photovoltaics, the most efficient modules will capture the largest market share because they have the lowest BOS cost per watt. Thus, monocrystalline Si modules could reach a market share of nearly 50% by 2020.

Successful: Silicon PV

Standard Si modules today have a thickness of 150 to 180 μm. The Si input ("polysilicon") is just over 4 g per installed watt. With an installed PV capacity of 100 GW in 2017, this corresponds to a consumption of polysilicon of about 400,000 t. In fact, global polysilicon production was about 460,000 t, which includes 30,000 t of electronic-grade silicon for the semiconductor industry.

Traditional wire saws with wire diameters of about 100 μm and a slurry additive of SiC particles are currently increasingly being replaced by saws with diamond-tipped wire. A wafer thickness of 100 μm seems achievable in this way. Also, a higher sawing speed can be achieved and an even thinner wire can be used, which further reduces Si loss. The Si cut surface is extremely smooth with this technique. Special chemical solutions must be used so that it can be textured with regard to reduced reflection losses.

Most crystalline Si solar cells today are made from p-doped base material into which an n-doped thin cover layer is diffused as an emitter on the surface - that is, the side facing the light. Contact fingers on this layer pick up the light-generated electrons. An extremely thin, highly n-doped layer is sandwiched between the electrodes and the n-doped layer to reduce the recombination of the generated charge carriers at the Si surface. The Si surface is pyramid-textured by etching to reduce reflection losses. The backside of the cells is continuously contacted, e.g. with aluminum. A highly p-doped layer, e.g. of Al ions, is sandwiched between the p-Si and the Al electrode. It generates the so-called back-surface field (BFS), which reflects the electrons and reduces the recombination of electrons and holes. The best modules made from such cells today have an efficiency of up to 20% for polycrystalline Si. For monocrystalline Si it is slightly higher. With regard to a long life of the modules of up to 30 years, the cells are wrapped in glass on both sides. However, the modules age, which causes the power output to decrease by about 0.5% per year. The temperature also affects the power output: this drops by almost 1% if the temperature rises by 1 K.

Already in production are modules with so-called PERC cells (Passivated Emitter and Rear Cell). Here, the continuous rear contact is replaced by point contacts, which further reduces the recombination of the generated charge carriers. In TOP-Con cells (Tunnel Oxyde Passivated Contact), the metal contacts are separated from the cell interior by an extremely thin insulating layer (which can be tunneled by the charge carriers). In addition, n-doped Si is used as the base material. When front-side contacts are omitted, both p-type and n-type contacts are located on the back side of the cell. It is worth mentioning that cell efficiencies of over 26% have been achieved by various companies using different Si PV types - with the theoretical maximum being 29%.

Alternative thin-film PV

Thin-film modules are interesting because of the low use of active material and low energy payback times. Interesting niche markets include building facades and flexible applications.

Cadmium telluride modules accounted for just over 5% of the global photovoltaic market in 2017/18. The efficiency for today's modules is about 18%. The use of the rare element tellurium is about 40 g/kW_P, so with a production of 5 GW/a, about 200 t of tellurium are needed per year. This is roughly equivalent to the world's tellurium production. Although cadmium is a toxic heavy metal, the large-scale use of CdTe - a chemically very stable 2-6 compound - in giant PV plants is well advanced.

PV modules made of amorphous silicon are produced by plasma-enhanced vapor deposition. In this process, the Si amorphously precipitates on the substrate, absorbing hydrogen that saturates free valences of the Si. Module efficiency is 5%-7%, accounting for about 2% of installed power in the world.

CIGS or CIS thin-film modules are made of the elements copper, indium, gallium and selenium. They have an efficiency of about 17% and a share of installed power in the world of less than 2%. The rare tin is used in CIS cells at about 30 g/kW_P. Worldwide, about 1300 t of indium was supplied from deposits and through recycling in 2013. The main supplier was China.

There are great expectations regarding perovskite photovoltaics. Within a decade, cell efficiencies of up to 22% have been achieved. Liquid-processable thin-film technology is promising, but is not yet used in the PV power plant sector. Research is devoted to, among other things, long-term stability and replacement of toxic lead in the perovskite absorber CH₃NH₃PbI₃. Highly attractive could be stacks of crystalline Si with a thin perovskite top layer with efficiencies above 30%.

Sources

Dr. H. Wirth, Fraunhofer ISE, Freiburg, Feb. 2018, "Current Facts on Photovoltaics in Germany"
Press Release Bernreuter Research, Würzburg, Oct. 2017 "Photovoltaic additions run up to 100 GW in 2017"
IWR Online News - iwr.de, June 21, 2017, "Photovoltaics: analysts expect monocrystalline solar cells to catch up"
Dr. A. W. Bett, BWK70,No.3,P.38-41(2018) "The new possibilities of photovoltaics"