Molecular structure effect of pyridine-based surface ligand on the performance of P3HT:TiO ₂ hybrid solar cell

Colloid TiO ₂ nanorods are used for solution-processable poly(3-hexyl thiophene): TiO ₂ hybrid solar cell. The nanorods were covered by insulating ligand of oleic acid (OA) after sol-gel synthesis. Three more conducting pyridine type ligands: pyridine, 2,6-lutidine (Lut) and 4-tert-butylpyridine (tBP) were investigated respectively to replace OA. The power conversion efficiency (PCE) of the solar cell was increased because the electronic mobility of pyridine-type ligand-modified TiO ₂ is higher than that of TiO ₂ -OA. The enhancement of PCE is in the descending order of Lut > pyridine > tBP because of the effective replacement of OA by Lut. The PCE of solar cell can be further enhanced by ligand exchange of pyridine type ligand with conjugating molecule of 2-cyano-3-(5-(7-(thiophen-2- yl)-benzothiadiazol-4-yl) thiophen-2-yl) acrylic acid (W4) on TiO ₂ nanorods because W4 has aligned bandgap with P3HT and TiO ₂ to facilitate charge separation and transport. The electronic mobility of two-stage ligand exchanged TiO ₂ is improved furthermore except Lut, because it adheres well and difficult to be replaced by W4. The amount of W4 on TiO ₂ -tBP is 3 times more than that of TiO ₂ -Lut (0.20 mol % vs. 0.06 mol %). Thus, the increased extent of PCE of solar cell is in the decreasing order of tBP > pyridine > Lut. The TiO ₂ -tBP-W4 device has the best performance with 1.4 and 2.6 times more than TiO ₂ -pyridine-W4 and TiO ₂ -Lut-W4 devices, respectively. The pKa of the pyridine derivatives plays the major role to determine the ease of ligand exchange on TiO ₂ which is the key factor mandating the PCE of P3HT:TiO ₂ hybrid solar cell. The results of this study provide new insights of the significance of acid-base reaction on the TiO ₂ surface for TiO ₂ -based solar cells. The obtained knowledge can be extended to other hybrid solar cell systems.