4.2.4. Preparation of hole transport materials (HTM) solution
To prepare the hole transport materials, 73.5 mg of spiro-OMeTAD (99.62%, Feiming Chemical Limited, Shenzhen, China), 17 µl of bis(trifluoromethane)sulfonamide lithium salt (Li-TFSI, 99.95%, Sigma Aldrich, St. Louis, MO, USA) solution (574.2 mg of Li-TFSI in 1 ml of acetonitrile), and 36.2 µl of 4-tert-butylpyridine (98%, Sigma Aldrich, St. Louis, MO, USA) were stirred at room temperature for 2 hours in 1 ml of chlorobenzene (99.8%, Sigma Aldrich, St. Louis, MO, USA).