Currently, only certification tests for photovoltaic modules are being made, so the exact cause of efficiency reductions due to aging are not fully analyzed. Before making a photovoltaic module in this study, we manufactured 3 line ribbon tabbed solar cells. After thermal shock and high-temperature and high-humidity testing, the changes in each solar cell component were analyzed and the cause of output reduction was identified. For the thermal shock testing, 500 cycles were performed for 15 min at each temperature, totaling 30 min per cycle, including ramp time to low temperature ($-40^{circ}C$) and high temperature ($85^{circ}C$). For the high-temperature and high-humidity tests, electrical characteristics were examined by exposing the cells to a temperature of $85^{circ}C$ and relative humidity (RH) of 85% for 1000 h. After thermal shock and high-temperature and high-humidity testing, the relative efficiency reduction rates were 11.3% and 5.9%, respectively, and relative fill-factor-reduction rates were 6.0% and 0.7%, respectively. The causes of output power reduction were found to be grid finger disconnect due to damage of the internal cell and cell cracking after the thermal shock test. This meant that concentrated current was not collected and therefore lost. The light collection ability was also degraded by surface damage. Through this experiment, it was identified that the major cause of output power reduction after thermal shock and hightemperature and high-humidity testing was increased series resistance (RS) caused by external damage and not by deterioration of the cell itself.