In terms of module compatibility, the distinction between transformerless and galvanically isolated inverters is probably the most important one. Thanks to their galvanic isolation, devices with a transformer allow grounding the PV array, a requirement for some module types. Thus, all modules of the PV system shift to positive potential when grounding the negative pole of the PV system, or to negative potential when grounding the positive pole.
This is not always possible with transformerless inverters – at least not with the devices currently available. Here, the generation potential is determined by the electronics, usually devided more or less symmetrically into positives and negatives. A certain percentage ofalternating current on the DC side is also determined by the electronics: particularly the more efficient topologies cause the potential of the PV array to oscillate at half of the grid amplitude. This potential oscillation may become a problem, however, if the PV modules have high parasitic capacities; in this case capacitive leakage currents may occur. Inverters with so-called "quiet rail" topology, such as the Sunny Tripower by SMA, avoid this oscillation and only have voltage ripples of a few volts, similar to transformer inverters (Fig. 1).

Until today, modules with cells made of mono- and polycrystalline silicon dominate the market with a share of more than 80 percent. They consist of approx. 0.2 mm thick silicon wafers that are either laminated between two panes of glass or between a film layer and a pane of glass. They are usually covered with a grid of contact fingers on the front; some are contacted with both poles through the back side.
Much less semiconductor material is required for silicon thin-film cells. Here, an amorphous silicon layer with a diameter of only a few micrometers is deposited in a high vacuum and divided into individual cells that are connected accordingly. On the front side, electric contact between the cells is established through a transparent conductive oxide (TCO) layer. The manufacturing process usually begins with the outer glass pane; after this the TCO, the amorphous silicon and the metallic contact layer on the back are applied. This so-called superstrate structure is also used in connection with cadmium telluride (CdTe), while copper-indium-selenide (CIS) modules usually use the reversely assembled substrate structure. Here you begin with the backside, onto which the backside contact, the semiconductor material and the TCO are vapor-deposited layer for layer (see Fig. 1). Most important difference: in the substrate structure the laminate film lies between the cover glass and the TCO, thereby preventing direct contact, in contrast to the superstrate structure.
Flexible laminates form their own, rather young market segment. Here the various functional layers are deposited onto a film, which makes it possible to produce flexible, thin and extremely light cells. These can then be attached directly to the surface of conventional building materials (e.g., metal roofs, awnings or aircraft wings).
Of all other module types, only few have reached the mass production stage. Other types of cells on the threshold of serial production include dye-sensitized solar cells, which are produced by applying a sort of organic or anorganic ink, as well as so-called solar concentrator cells, in which an optical system concentrates sunlight amplified at up to 1000 times onto a highly efficient stacking cell.

The question is of course: Which of the above problems apply for which module technology? And which inverter provides the matching solution? An initial orientation is provided by the following overview, as well as in the table in Fig. 3.
Crystalline silicon (also "c-Si): The thick, encapsulated cells are quite robust chemically and do not tend to corrode even in case of negative potentials. Grounding is usually not necessary. The considerable thickness of these modules usually also leads to their parasitic capacity being relatively small. Most crystalline modules can therefore easily be operated with all inverters. There are two exceptions to this rule, however:
• Some cell types, especially those with both poles on one side, tend to exhibit polarization effects when operating under positive potential (problem no. 2). Positive grounding of the PV array usually solves the problem – as mentioned earlier, most transformerless devices are not suitable for this.
• Some glass-film modules have a grounded metallic structure integrated into the backside film, so that their parasitic capacity can be surprisingly great (problem 3). In order to prevent capacitive leakage currents, only inverters should be used here that have no significant fluctuations of potential on the DC side (transformer devices or transformerless inverters with quiet rail topology).
Thin-film silicon (a-Si): Cells based on amorphous silicon have a tendency towards corrosion of the TCO, which leads to a permanent loss of output (problem no. 1). The solution is to negatively connect the generator to ground, which is why most transformerless inverters are not a viable option.
Cadmium telluride (CdTe): A similar relationship as for amorphous silicon is also suspected to exist for cadmium telluride-based thin-film modules. Negative grounding is also recommended here, unless the manufacturer explicitly recommends a different solution.
Copper indium selenide (CIS) or copper indium gallium selenide (CIGS): Due to their substrate structure, no TCO corrosion could be observed here so far; grounding is not necessary in most cases. However, it has to be considered, however, that there is a particularly large variety of manufacturing processes for CIGS modules. A manufacturer recommendation should be obtained in the individual case.
Flexible solar cells: Besides CIGS technology, flexible cells currently available base on amorphous silicon; however, it uses a substrate structure and has no glass contact. TCO corrosion has not been observed here, grounding is not required. However, their thin structure can lead to problems: parasitic capacities of flexible laminates can be particularly great when directly attached to a metal surface or in case of moisture and can therefore lead to especially large leakage currents when operated with certain transformerless inverters (problem no. 3). In order to avoid unwanted deactivation, it is recommended to use an inverter that has no notable fluctuations of potential on the DC side (transformer device or transformerless inverter with quiet rail topology).