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.