Contact Resistivity and Sheet Resistance Measurements of Cells Extracted from Field-aged Modules

The electrical performance of a photovoltaic (PV) module is greatly hindered by the existence of parasitic resistance losses, such as high series resistance (R
_s
) and low shunt resistance (R
_sh
). Contact resistance at metal grid/semiconductor interface and emitter sheet resistance are two major contributors to cell R
_s
. Transmission Line Measurement (TLM) is a powerful method to estimate these resistance components. This paper presents the application of the TLM method to the cell strips extracted from field-aged PV modules at two different climates (Arizona and Florida) of the same design to investigate the influence of encapsulant material and microcracks on the contact resistivity and sheet resistance of the solar cell. The cells were initially extracted from the module laminate by dissolving the encapsulant polymer in trichloroethylene (TCE) solvent at 80
^o
C for around 3 hours. The recovered cell was then used to cut into rectangular strips of 1.5 cm wide. The effective contact resistivity and emitter sheet resistance between two fingers in each strip were measured and compared to quantify the contact degradation induced from longer field aging. The Arizona module suffered from higher resistance as compared to the Florida module due probably to longer field exposure and higher operating temperatures. This method serves as a good diagnostic tool to anticipate and understand the severity of the contact degradation of solar cells in the fielded modules.