Improved charge carrier transfer in squaraine-capped ZnO nanoparticles layer for electron transport of hybrid solar cells

Abstract

Blend of conjugated polymer poly(3-hexylthiophene) or P3HT and Zinc Oxide nanoparticles (ZnO-NPs) has been intensively used as active material for high performance hybrid solar cells. However, agglomeration of ZnO-NPs hinders efficient charge carrier or charge transfer both from P3HT to ZnO-NPs and its transport within ZnO-NPs which lead to low performance of solar cells. Capping of ZnO-NPs is currently applied to avoid this agglomeration effect. In this study, we used small molecule squaraine (SQ) to cap ZnO-NPs. SQ was chosen due to its lowest unoccupied molecular orbital (LUMO) level lower than conduction band of ZnO, therefore, SQ can assist charge transfer to ZnO-NPs. Moreover, SQ molecule also enhances light absorption due to its high light absorption in the near infrared region and large spectral overlap between the P3HT emission and SQ absorption. Here we report our current study on charge carrier transport in the blend of P3HT and SQ-capped ZnO-NPs by using photoluminescence spectroscopy with UV and blue excitation wavelengths. ZnO-NPs were synthesized by using sol-gel method. The capping process was employed by mixing small amount of SQ solution into the ZnO-NPs gel before its blend with P3HT. Small molecule SQ was successfully capped ZnO-NPs as observed from its TEM image. ZnO-NPs solution has absorption wavelength less than 380 nm, therefore, no emission spectrum is observed when it is excited with 473 nm excitation wavelength. However, a clear emission spectrum is observed from SQ-capped ZnO-NPs solution, which shows that charge transfer was occurred from SQ to ZnO-NPs. Similar behavior was also observed in the blend of P3HT and SQ-capped ZnO-NPs. Our study shows that small molecule squaraine can improve charge carrier transfer from ZnO-NPs and therefore the performance of solar cells might be increased.