Nanoscale Morphology Control of Polymer/TiO₂ Nanocrystal Hybrids: Photophysics, Charge Generation, Charge Transport, and Photovoltaic Properties

We present a simple approach by using mixed solvent to control the morphology of poly(3-hexylthiophene) (P3HT)/TiO2 nanorod hybrid bulk heterojunction solar cells without any post-treatment. The effects of the controlled morphology on the optical and electrical properties are investigated. It has been a challenge to disperse polar inorganic nanocrystals at a relative high concentration into a relative nonpolar polymer. The use of mixed solvent which consists of pyridine (a poor solvent for P3HT), dichloromethane, and chloroform (a good solvent for P3HT) modifies the nanoscale morphology of P3HT/TiO2 nanorod hybrids, resulting in highly crystalline P3HT domains with well-dispersed TiO2 nanorods within polymer matrix. Study of photophysics reveals that charge carrier could form from emissive species upon photoexcitation and such a process is more efficient in highly ordered P3HT prepared by mixed solvent method. In the P3HT/TiO2 hybrid film, the formation of a bicontinuous phase-separated morphology largely improves charge separation, transport, and recombination in the hybrid devices, which are further supported by time-resolved photoluminescence spectroscopy, carrier extraction by linearly increasing voltage mobility measurement, and transient open-circuit voltage decay measurement, respectively. A result of threefold improvement of the device performance using mixed solvent has been demonstrated compared to that using a single solvent only. This simple process does not need any further thermal post-treatment and is therefore compatible with the room temperature process developed with commonly used plastic substrates for flexible solar cell applications. Our method for morphology control could also be applied to other donor−acceptor hybrid systems as a strategy for device optimization

Interplay of Three-Dimensional Morphologies and Photocarrier Dynamics of Polymer/TiO₂ Bulk Heterojunction Solar Cells

In this study, we investigated the interplay of three-dimensional morphologies and the photocarrier dynamics of polymer/inorganic nanocrystal hybrid photoactive layers consisting of TiO2 nanoparticles and nanorods. Electron tomography based on scanning transmission electron microscopy using high-angle annular dark-field imaging was performed to analyze the morphological organization of TiO2 nanocrystals in poly(3-hexylthiophene) (P3HT) in optimal solar cell devices. The Three-dimensional (3D) morphologies of these hybrid films were correlated with the photocarrier dynamics of charge separation, transport, and recombination, which were comprehensively probed by various transient techniques. Visualization of these 3D bulk heterojunction morphologies clearly reveals that elongated and anisotropic TiO2 nanorods in P3HT not only can significantly reduce the probability of the interparticle hopping transport of electrons by providing better connectivity with respect to the TiO2 nanoparticles, but also tend to form a large-scale donor–acceptor phase-separated morphology, which was found to enhance hole transport. The results support the establishment of a favorable morphology for polymer/inorganic hybrid solar cells due to the presence of the dimensionality of TiO2 nanocrystals as a result of more effective mobile carrier generation and more efficient and balanced transport of carriers

Interfacial Nanostructuring on the Performance of Polymer/TiO₂ Nanorod Bulk Heterojunction Solar Cells

This work presents polymer photovoltaic devices based on poly(3-hexylthiophene) (P3HT) and TiO2 nanorod hybrid bulk heterojunctions. Interface modification of a TiO2 nanorod surface is conducted to yield a very promising device performance of 2.20% with a short circuit current density (Jsc) of 4.33 mA/cm2, an open circuit voltage (Voc) of 0.78 V, and a fill factor (FF) of 0.65 under simulated A.M. 1.5 illumination (100 mW/cm2). The suppression of recombination at P3HT/TiO2 nanorod interfaces by the attachment of effective ligand molecules substantially improves device performance. The correlation between surface photovoltage and hybrid morphology is revealed by scanning Kelvin probe microscopy. The proposed method provides a new route for fabricating low-cost, environmentally friendly polymer/inorganic hybrid bulk heterojunction photovoltaic devices

Manipulation of Nanoscale Phase Separation and Optical Properties of P3HT/PMMA Polymer Blends for Photoluminescent Electron Beam Resist

A novel photoluminescence electron beam resist made from the blend of poly(3-hexylthiophene) (P3HT) and poly(methyl methacrylate) (PMMA) has been successfully developed in this study. In order to optimize the resolution of the electron beam resist, the variations of nanophase separated morphology produced by differing blending ratios were examined carefully. Concave P3HT-rich island-like domains were observed in the thin film of the resist. The size of concave island-like domains decreased from 350 to 100 nm when decreasing the blending ratio of P3HT/PMMA from 1:5 to 1:50 or lower, concurrently accompanied by significant changes in optical properties and morphological behaviors. The λmax of the film absorption is blue-shifted from 520 to 470 nm, and its λmax of photoluminescence (PL) is also shifted from 660 to 550 nm. The radiative lifetime is shorter while the luminescence efficiency is higher when the P3HT/PMMA ratio decreases. These results are attributed to the quantum confinement effect of single P3HT chain isolated in PMMA matrix, which effectively suppresses the energy transfer between the well-separated polymer chains of P3HT. The factors affecting the resolution of the P3HT/PMMA electron beam resists were systematically investigated, including blending ratios and molecular weight. The photoluminescence resist with the best resolution was fabricated by using a molecular weight of 13 500 Da of P3HT and a blending ratio of 1:1000. Furthermore, high-resolution patterns can be obtained on both flat silicon wafers and rough substrates made from 20 nm Au nanoparticles self-assembled on APTMS (3-aminopropyltrimethoxysilane)-coated silicon wafers. Our newly developed electron beam resist provides a simple and convenient approach for the fabrication of nanoscale photoluminescent periodic arrays, which can underpin many optoelectronic applications awaiting future exploration