C.6 Test Results
C.6.1 H Cell versus Flower Cell
The flower cells were prototyped by University of Stuttgart and tested in the Sunsimulator. Standard H cells and flower cells are tested to provide data. The Sunsimulator used standard test conditions (STCs): 25̊C, an irradiance of 1000 W/m2, with an air mass 1.5 spectrum (AM1.5). During the simulation, the temperature was 25.2̊C.
The calibration cell of the H cell and flower cell was the same, providing the possibility to compare the cells with each other.
In PV technology, the performance of solar cells is illustrated in current–voltage (I–V) curves. The best performing H cell and best performing flower cell were selected for the I–V curves. From these curves it was apparent that both cells were performing well. The curve that compares the two cells is illustrated in Figure C.6.1.
Figure C.6.1 Measured current–voltage (I–V) curves of an H cell and flower cell
The flower cell had an average output of 0.6 V, 7.9 A, and a power output of 4.8 W. In reference to the demands and specifications, the data showed that the demand was fulfilled (Table C.6.1). This result gave a positive outlook for the development of new contact patterns.
Table C.6.1 Demands and specifications.
| Function | Requirement | Technical specification |
| The solar cell has to convert light into electricity. | The solar cell produces enough electrical output. | The new solar cell has an output of at least 0.5 V, 7 A, and 3.5 W at standard test conditions (STC). |
| The solar cell has to be able to transport the generated current. | The produced current is collected by metal contacts on the cell. | The cell has metal lines that are connected, leading the current to the contact points of the cell. |
| The solar cells have to be connectable in a product (e.g., a PV module). | The cells will be connected to each other by contact points. | The cell must have a minimum of four contact points, divided at the sides of the cell. |
| A cell generally has an output of 8 A. This output must be sufficiently distributed; otherwise, the cell can be overheated. | ||
| The solar cell can be screen printed. | The metal pastes can be applied on the cell. | A standard H cell fulfills these requirements. The metal contact-stripped H cell is the base cell for the new solar cell. Silver alloy paste and aluminium alloy paste can be applied on this cell. |
| The squeegee must be able to handle the metal paste (no metal paste must stay behind at the squeegee). | The direction of the squeegee is always in the direction of the small structures, like the fingers. | |
| The angle in the screen, between open structures and the wires, is in general 22.5̊. | ||
| The metal lines can be screen printed. | The screen fits in recent screen-printing machines. | |
| The height of the fingers and busbars can range between 10 and 20 μm. | ||
| The width of the fingers can range between 110 and 120 μm. The minimal width is 60 μm. | ||
| The spacing between fingers can be 1–5 mm. | ||
| The busbar width ranges between 1.6 and 2.0 mm. | ||
| An achievable minimal width is 110 μm. | ||
| The new solar cell has to function efficiently. | The efficiency will decrease by shadowing and resistances of the metallization. | The new solar cell has a minimal efficiency of 12%. |
| Efficiencies of standard cells (Solland Solar) range between 14.6% and 16.6%. The new solar cell must have an acceptable efficiency. | The absolute efficiency loss of the cell must not exceed 2%. | |
| The new solar cell's cost price must be acceptable. | The cost price must not be too high. The new solar cell adds aesthetical value and can be sold for a higher selling price. | The cost price may rise a maximum of 5%. |
C.6.2 Redesigned PV Modules
The PV modules were prototyped and flashed six times to provide data. The PV modules with flower cells have a lower output and efficiency than standard PV modules (e.g., 175 W). Several factors played a role in causing this result. The prototypes contained lower class cells, which had less output and efficiency. The soldered tabs were not completely running over the cells and led to higher resistances. This resulted in output losses. During production, the flower cells were soldered by hand and not by machine, causing less accuracy. Each hand-soldered cell took 0.5 W off the total output.
Finally, the glass top of a standard PV module is coated, while the prototyped modules did not have this coating. This led to a loss of 8–10 W. It was logical that the prototypes had lower outputs, but for a prototype of a “design” PV module this output of 150 W was acceptable.
C.6.3 Expected Costs
The flower cell has an increased value in aesthetics. This enhancement adds to the selling price of the solar cell and PV module. In comparison to a standard H cell, the flower cell of a 100,000-cell batch has a cost increase of only 0.5%.
The prototyped 50-celled PV module contains 18 flower cells. This module will have a cost increase of 0.95% compared to a standard 50-celled PV module. This result is acceptable, especially for a redesigned PV module.