Molecular Structure Effect of Pyridine-Based Surface Ligand on the Performance of P3HT:TiO2 Hybrid Solar Cell

Colloid TiO2 nanorods are used for solution-processable poly(3-hexyl thiophene): TiO2 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 TiO2 is higher than that of TiO2-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 TiO2 nanorods because W4 has aligned bandgap with P3HT and TiO2 to facilitate charge separation and transport. The electronic mobility of two-stage ligand exchanged TiO2 is improved furthermore except Lut, because it adheres well and difficult to be replaced by W4. The amount of W4 on TiO2-tBP is 3 times more than that of TiO2-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 TiO2-tBP-W4 device has the best performance with 1.4 and 2.6 times more than TiO2-pyridine-W4 and TiO2-Lut-W4 devices, respectively. The pKa of the pyridine derivatives plays the major role to determine the ease of ligand exchange on TiO2 which is the key factor mandating the PCE of P3HT:TiO2 hybrid solar cell. The results of this study provide new insights of the significance of acid-base reaction on the TiO2 surface for TiO2-based solar cells. The obtained knowledge can be extended to other hybrid solar cell systems.