Silicon/organic semiconductor heterojunctions for solar cells

Using current-voltage measurements, photocurrent spectroscopy, and light-induced electron spin resonance (LESR), we have investigated hybrid solar cells consisting of composites of silicon nanocrystals (Si-ncs) and poly-3(hexylthiophene) (P3HT). For a more detailed understanding of the inorganic/organic interface, we have also studied the properties of crystalline silicon (c-Si)/P3HT heterostructures. Current-voltage characteristics of Si-ncs/P3HT blends show an open-circuit voltage of up to 0.76 V. Photocurrent spectroscopy measurements give a strong indication that both, the Si-ncs and P3HT, contribute to the photo-induced current. Further, we directly demonstrate a light-induced charge transfer between the Si-ncs and P3HT via LESR. Current-voltage measurements of c-Si/P3HT heterojunctions were performed under illumination with high power light emitting diodes (LEDs) of different wavelengths, which allow a selective excitation of charge carriers in P3HT, silicon, or both materials. We have observed a disadvantageous energy band alignment of the valence band of silicon and the highest occupied molecular orbital (HOMO) level of P3HT which favors the use of small particles with a larger band gap due to quantum confinement.