Effect of sodium addition on Cu_2SnS_3 thin-film solar cells fabricated on alkali-free glass substrates

摘要

Cu2SnS3 (CTS) p-type semiconductors are expected to be applied as a light absorption material for low-cost thin-film solar cells due to their advantageous physical properties. The influence of sodium addition to CTS thin films was investigated by comparing Na-free CTS and Na-doped CTS fabricated on alkali-free glass substrates. Grain growth for Na-free CTS, which has a Cu/Sn composition ratio of approximately 2.0, did not occur below 570 degrees C. In contrast, the addition of sodium to the CTS increased the grain sizes with an increase in the annealing temperature. Even with Na-free and Na-doped CTS, the grain sizes increased with a decrease in the Cu/Sn composition ratio. These results show that an excess of Sn combined with the presence of sodium accelerate the grain growth of CTS. Photovoltaic cells using the Na-doped CTS with a Cu/Sn ratio of 1.81 exhibited an open-circuit voltage of 242 mV, a short-circuit current density of 26.5 mA/cm(2), a fill factor of 0.523, and a conversion efficiency of 3.35%. The cells using CTS without sodium did not exhibit good photovoltaic characteristics due to the small grain sizes. (C) 2018 The Japan Society of Applied Physics