Flexible modules
As silicon solar cells are getting thinner, they become flexible and can be integrated in flexible photovoltaic modules. Flexible panels are typically significantly lighter compared to their rigid counterparts and, consequently, they open up new applications. Most of the thin film solar cell technologies such as CIGS (copper indium gallium selenide), GaAs (gallium arsenide) and a-Si (amorphous silicon) are flexible, however, their application has been hindered by their relative low efficiency, particularly in the case of a-Si, and cost (particularly in the case of GaAs).
Figure 1: Photograph of a flexible silicon PV module taken at the SNEC Exhibition in 2018.
Glass- glass Modules
In these modules glass is used for both the front and the back. Glass-glass modules have lower yearly degradation rate when compared with their backsheet counterparts. This is reflected in the fact that most glass-glass modules are sold with a 30-year warranty instead of the conventional 25 years which is attractive from an application point of view. Glass on the rear provide more strength to the module in harsh environments in conditions of high humidity and high temperatures. Although these modules do not require an aluminium frame, they are 20% heavier than conventional modules which can be an issue if your roof has a limited loading capacity.
Split Modules and Half size cells
Instead of using typical full area square (or pseudo-square) cells, some recent modules are using cells which are cut in half. The main benefit of half cells is lower resistive losses in the module as each solar cell only yield half of the normal current. As resistive related losses scale with the square of the current and linear with the resistance, this reduces the resistive power losses in the strings by 75%. The solar cells are only cut in half at the end of the solar cell manufacturing process, hence, the rest of the production line does not need to be changed. Obviously, you need a new piece of equipment to cut the cells in two halves and your tabber-stringer needs to be able to handle half-cells which might require additional investment in your module manufacturing line. A picture of a PV module with half cells is shown in Figure 2.
Figure 2: A photograph of a PV module with half-cut cells taken at the SNEC exhibition in 2018.
Shingles
Photovoltaic shingles are made by directly interconnecting the cells by placing them onto each other like roof tiles. In the process, solar cells are cut into smaller strips along the busbars. The main advantages of shingles are their aesthetic appearance, lower currents resulting in potential lower resistive losses, and a higher packing density for photovoltaic modules increasing the PV module efficiency. However, interconnection of shingled solar cells is more complex compared to standard PV modules a. Shingles are for example of interest for building integrated photovoltaics as the appearance can be much more uniform compared to conventional modules.
Figure 2: Closeup photograph of a PV module with shingled solar cells.
Multi busbar solar cells
The metalisation pattern of a typical solar cell consist of relatively thin silver fingers which collect the current from the solar cell and transport it to relatively thick busbars. These busbars are subsequently connected to ribbons (typically coated copper wires) in the PV module process. In recent years, the number of busbars has been increased from 2 to typically 5. Some manufacturers even take it a step further and go for 9-12 busbars. Increasing the number of busbars reduces the current per ribbon and thus reduces the resistive losses (the power losses scales with the square of the current). In addition, the distance the current has to travel from collection to the busbar becomes shorter further reducing the resistance of the solar cell and consequently the resistive losses. This allows for thinner contact fingers and thus a reduction in the usage of silver paste, that is one of the most expensive consumables for silicon wafer solar cell production. In addition, the usage of a large number of busbars makes the solar cell also more resistive to cracking in case this would happen after module manufacturing as there is a larger chance that the solar cell parts are still all connected to a busbar.
Figure 3: Photograph of a solar cell with 12 busbars taken at the SNEC exhibition in 2018.
Bifacial modules
Bifacial modules are designed to illuminate a PV module from both the front and rear side. This requires a transparent backsheet or glass at the rear of modules. The light intensity at the rear of a PV module is typically significantly lower than that at the front, however, it can still be in the 10-40% range depending on the installation thus offering significant potential for increasing the PV module power. The front surface of a bifacial solar cell looks identical to its monofacial counterpart as can be seen in Figure 3. The rear of the solar cell has no full area contact as in the case of an aluminium back surface solar cell but it has a similar metalisation pattern as the front. As is shown in Figure 4 the colour of the rear of the solar cell can be different than that of the front. This is because bifacial solar cells are still optimised for an optimal frontside performance which can require a slightly thicker dielectric coating at the rear. Bifacial solar cells also require a more advanced placement of the junction box as this can potentially shade the rear side of some solar cells. One of the main challenges for bifacial PV modules is their power rating as the power generated from the rear side is very application dependent. A standard is currently under discussion on how to properly and consistently rate bifacial PV modules.
Figure 4: Photograph of the front (left) and rear (right) bifacial solar cell.