Physical Origin of the Impact of Different Nanocrystal Surface Modifications on the Performance of CdSe/P3HT Hybrid Solar Cells

Alkylamines were recently found to be suitable ligands for CdSe quantum dots with respect to applications in polymer/nanoparticle solar cells. However, the physical origin of the superior performance with respect to more widely used pyridine-capped quantum dots still remains unclear. Here, we report about the details of the surface modification procedure of CdSe quantum dots with alkylamines and subsequent application of the nanoparticles for the fabrication of hybrid CdSe/poly-3-hexylthiophene (P3HT) solar cells. As-synthesized nanocrystals were subjected to a pyridine treatment as an intermediate step to remove the high molecular weight species from the sample, and then the exchanges with octylamine and butylamine were carried out. Investigation based on nuclear magnetic resonance (NMR), X-ray spectroscopy (EDX), and thermal gravimetric analysis (TGA) demonstrated that pyridine ligand exchange as intermediate step is an effective procedure to reduce undesirable impurities which otherwise impede further surface modification and, therefore, the final performance of the CdSe/P3HT hybrid cells. Laboratory samples with butylamine- and octylamine-capped CdSe nanoparticles and P3HT were prepared and characterized by current-voltage (I-V) and external quantum efficiency (EQE) measurements. In order to find out the optimum parameters of the butylamine-capped CdSe/P3HT samples, we looked at the influence of the active layer thickness and annealing temperature on the solar cell performance. Power conversion efficiency (PCE) of 2.0% was reached for butylamine-stabilized CdSe quantum dots and P3HT. The superior performance with respect to pyridine-capped quantum dots was found to be mainly due to a higher photocurrent. Deeper analysis of the photocurrent improvement was performed by detailed comparison of the EQE spectra and investigations on the charge carrier generation and recombination processes by light-induced electron spin resonance (I-ESR). Therefrom, we have done a model based on different charge carrier trap states at the nanocrystal surface.