SunLab B2B – instrument for measuring busbar to busbar resistance

To achieve high efficiency solar cells, series resistance plays a major role, and front side grid resistance is one of the components. The front side grid resistance on its turn is dependent on the print definition and print quality. A practical assessment of the grid resistance & print quality is to measure the resistance in between two busbars (B2B resistance).

SunLab B2B is made for measuring the resistance of all fingers between two busbars on an H-pattern solar cell. The B2B resistance is a direct measure for the print quality. At the same time the B2B resistance is the information needed to quantify the resistance of a finger and to obtain a direct value for modeling, etc.

Print quality

When optimizing the front side print definition, e.g. through varying on paste, or when testing new screens or improved line definition in the screens, it is important to have a good measure. The busbar to busbar measurement is a simple and straightforward method for this.

Day to day or run-to-run print quality in production can vary and is dependent for instance on screen wear, paste transfer though the screen, paste supply, local temperature, etc. It is therefore important to have a direct measure for the most critical part of the front side print: the fingers. Here too, the busbar to busbar resistance is a simple and straightforward measure.

So: the B2B resistance is the direct measure for the print quality. This data represents the run-to run or day-to-day print quality, and is indispensable when optimizing the screen print quality.

Grid resistance

In order to reduce power losses associated with front side metallization, efforts converge in producing fine conductors with high aspect ratio. Fine lines reduce shadow losses and allow finer pitch which reduces the resistive losses in the emitter. High aspect ratio, thus large cross section area, reduces resistive losses in the conductor. Practice involves balancing the resistive losses (finger and emitter resistance) and shadow losses by designing an optimum pattern. In case fingers can be optimized, the number of fingers on the front side may have to be increased. For this modelling the finger resistance is needed. For this the measurement using SunLab B2B is indispensable.

Covering all resistance losses with SunLab instruments

The SunLab B2B now enables together with the other SunLab instruments to cover mapping and/ or measurement of all relevant resistance losses on the front side of a solar cell as depicted in the following figure.

• Corescan for mapping of the contact resistance together with finger and busbar resistance, but also Voc, and Shunt resistance. The Corescan instrument has become more or less a standard in the cell manufacturing world for improving on cell efficiency through control of the contact resistance.
• Sherescan for mapping and measuring of the emitter sheet resistance. The Sherescan has found its place in cell manufacturing and R&D because of the extra interest the emitter formation and tuning has gained over the last years. Emitter sheet resistance control and optimization is the key to improve on cell efficiency
• B2B resistance: Recently front side printing of finer fingers with a high aspect ratio has gained also interest. Quite some work was done on improving on screen printing and stencil printing, and progress has been made. Automated microscopic vision systems have become available to assess the dimensions and topography of the printed fingers. Together with the SunLab B2B, now the line resistance can be validated with the geometrical dimensions.