Hybrid Bulk Heterojunction Solar Cells Based on P3HT and Porphyrin-Modified ZnO Nanorods

Abstract

This work presents hybrid bulk heterojunction solar cells based on dye-sensitized zinc oxide (ZnO) nanorods blended with poly(3-hexylthiophene) (P3HT). Tetra(4-carboxyphenyl)porphyrin (TCPP) molecules were grafted onto the surface of ZnO nanorods to enlarge the absorption spectrum of the blend. We demonstrate that additional bands in the external quantum efficiency (EQE) spectra corresponding to Soret and Q-band absorption can already been observed at very low dye concentration at the ZnO surface. Therefore, direct grafting of TCPP onto ZnO nanorods leads to very efficient electron injection process into the ZnO nanorods after the light absorption of the dye. However, the overall photocurrent of the devices decreases gradually with TCPP concentration at the ZnO nanorod surface. The recombination dynamics of the photogenerated charges at the P3HT:ZnO interface are investigated by transient absorption spectroscopy on micro-to millisecond time scales. We observe that the lifetime of the P3HT polarons is reduced by an order of magnitude by grafting TCPP of already low concentration at the ZnO surface. Furthermore, high-resolution transmission electronic microscopy analysis of the blend morphology reveals that aggregation of ZnO nanorods within the P3HT is strongly increased by TCPP grafting. Therefore, we conclude that TCPP grafting is beneficial for additional photocurrent generation in the P3HT:ZnO blend but introduces strong modification of the blend morphology and charge carrier dynamics at the P3HT/ZnO interface, which finally reduces the overall photocurrent generation.

P-type semiconductor surfactant modified zinc oxide nanorods for hybrid bulk heterojunction solar cells

“…In contrast to our studies using porphyrin grafting at ZnO NRs [21], for which the long-lived decay component was suppressed by the ligand and addressed to reduction of charge trapped at the hybrid interface, grafting of 1 is not affecting the intensity of the second long lived components. It is possible that rather direct interaction between the -conjugated systems with the ZnO surface is involved in the passivation of trap state at the hybrid interface than the carboxylic acid groups used for the grafting of the ligand to the ZnO surface.…”

“…Figure 1b shows the molecular structure of 1 as well as a simplified energy diagram of this hybrid solar cell taking into account the energy level of molecule 1 and ZnO [21,30]. It is worth mentioning that ZnO and P3HT as well as 1 form a type II heterojunctions.…”

“…We previously demonstrated that porphyrin dye surfactants on the surface of ZnO NRs contribute directly to the photocurrent generation in P3HT/ZnO blends and increase the spectral response of the solar cells. Unfortunately, we found that this dye, a tetra(4-caboxyphen)porphyrin, increases simultaneously the aggregation between nanorods due to strong dye-dye interaction leading finally to strong losses in all photovoltaic parameters of the devices [21]. Intensive studies were devoted to methods for improving the morphology of hybrid polymer blends by tailoring the compatibility of the nanoparticles with the polymer via ligand modification of the nanoparticles [13,[22][23][24][25][26][27].…”

“…These experiments reveal that in polymer/nanoparticle blends the surfactant controls not only the polymernanoparticle and the nanoparticle-nanoparticle interfaces, respectively, but also the nanoscale morphology of the blend that makes their choice critical for optimizing hybrid polymer blend solar cells. We could show recently that p-type bithiophene derivative monolayers grafted onto ZnO NRs form a coaxial p-n junction core-shell systems that can be used instead of the polymer making the nanoscale control of the bulk heterojunction simpler [21,28].…”

Surface Modification of Zinc Oxide Nanorods with Zn-Porphyrin via Metal-Ligand Coordination for Photovoltaic Applications

“…Therefore, metal oxide nanorods with a diameter less than 20 nm and a length in a range of 100-200 nm might provide an efficient, stable pathway for the transport of photogenerated charge carriers to electrodes as a channel when hybridized into the active layer of polymer-based solar cells. 8 Furthermore, it has been suggested that the dyes grafted at the nanorod surface might also improve the characteristics of light absorption and charge carrier separation when their energy levels are optimized with those of the metal oxides.…”