Printed Light Collectors

Costs sink to one Dollar
The USA company Nanosolar developed the technology. Currently, they are starting production in their new module factory in Berlin. The innovation could be an important step towards the competitive ability of photovoltaics. Nanosolar wants to produce the watt more cheaply than cost leader First Solar, according to Germany boss Erik Oldekop. First Solar produces its thin-film modules made of cadmium telluride for 0.87 Dollar per watt. On the other hand, the competition still lags behind, considerably over the one Dollar threshold.

The key to low costs is Nanosolar’s new printing technology. The company can use it as it manages to exactly arrange tiny nanoparticles next to each other in liquid. These nanoparticles have a maximum size of 20 nanometres (0.00002 millimetres). The technique makes it possible for these arrangements of nanoparticles to function as an efficient semiconductor. In comparison, conventional CIGS thin-film modules are made up of two micrometres (0.002 millimetres) thick semiconductor layers, meaning they are a hundred times thicker. Microscale CIGS cannot be solubilised as it would clump together and become unusable as a result. Therefore, the manufacturers have used the laborious high-vacuum procedure to date to deposit the CIGS layer onto glass or stainless steel. The systems for doing this are expensive and demand precise process controls in order to apply the CIGS homogeneously. In addition, depending on the process, only 30 to 80 percent of the CIGS is used. The rest ends up on the walls of the vacuum chambers or in the recycling process. Using printing, 100 percent of the semiconductor is used, states Oldekop. In addition, production is ten times as fast: 1,500 metres of aluminium foil can be coated each hour.

Thus, the giant roll already consists of two of the three most important components of a solar cell, absorber and back contact. The missing top electrode, which collects electrons, is subsequently produced by Nanosolar by atomising metal oxides. For this purpose, a metal oxide solid is fired upon with ions in a so-called sputter chamber, whereby the individual atoms dissolve away and settle on the CIGS. The specialist reduces the costs for this vacuum stage by only applying the metal oxide layer 50 nanometres thick, so twenty times thinner than is usual with CIGS modules.

Normally, the generated electrons would quickly disappear in such a thin layer as a result of the high resistance. For this reason, Nanosolar bores many little holes into the cell, which immediately channel the charge carriers onto the back contact. As the distance to the next hole is not far, there are virtually no losses, explains Oldekop. The CIGS cells, produced in San José, California, are then assembled into modules in Nanosolar's new factory in Luckenwalde. A panel can be produced here every ten seconds, 640 megawatt per year.