Carrier Transport in Dye-Sensitized Solar Cells Using Single Crystalline TiO₂ Nanorods Grown by a Microwave-Assisted Hydrothermal Reaction

Single crystalline rutile nanorod was grown directly on top of fluorine-doped tin oxide (FTO) substrate via a microwave assisted hydrothermal reaction which dramatically increased a growth rate over a conventional hydrothermal method. In addition, the introduction of thin TiO2 seed layer to FTO substrates promotes heterogeneous nucleation and increases the density. Dye-sensitized solar cells (DSSCs) were fabricated using the rutile nanorods that were differently treated with TiCl4 solution and the carrier transport mechanism in the nanorod-based DSSCs was systematically examined. When the nanorods were treated with TiCl4, more dye was adsorbed on the TiO2 films and the energy conversion efficiency increased to 3.7% for a 2.5 μm thick TiO2 film. Stepped light induced-transient measurement of photocurrent and voltage measurements showed that the role of the nanorods in DSSCs is to increase an electron diffusion coefficient in TiO2 mesoporous films. In contrast to the diffusion coefficient, the lifetime of electron is not dependent on the presence of the nanorods. To explain the experimental observations, we propose a surface diffusion model for electrons that are injected into the rutile nanorods from dye molecules. This surface diffusion may originate from the high crystallinity of nanorods and the homogeneous contact between nanorod and coated nanoparticle layer

Controllable Sequential Deposition of Planar CH₃NH₃PbI₃ Perovskite Films via Adjustable Volume Expansion

We demonstrate a facile morphology-controllable sequential deposition of planar CH₃NH₃PbI₃ (MAPbI₃) film by using a novel volume-expansion-adjustable PbI₂·<i>x</i>MAI (<i>x</i>: 0.1–0.3) precursor film to replace pure PbI₂. The use of additive MAI during the first step of deposition leads to the reduced crystallinity of PbI₂ and the pre-expansion of PbI₂ into PbI₂·<i>x</i>MAI with adjustable morphology, which result in about 10-fold faster formation of planar MAPbI₃ film (without PbI₂ residue) and thus minimize the negative impact of the solvent isopropanol on perovskites during the MAI intercalation/conversion step. The best efficiency obtained for a planar perovskite solar cell based on PbI₂·0.15MAI is 17.22% (under one sun illumination), which is consistent with the stabilized maximum power output at an efficiency of 16.9%

Exceptional Morphology-Preserving Evolution of Formamidinium Lead Triiodide Perovskite Thin Films via Organic-Cation Displacement

Here we demonstrate a radically different chemical route for the creation of HC­(NH₂)₂PbI₃ (FAPbI₃) perovskite thin films. This approach entails a simple exposure of as-synthesized CH₃NH₃PbI₃ (MAPbI₃) perovskite thin films to HC­(NH)­NH₂ (formamidine or FA) gas at 150 °C, which leads to rapid displacement of the MA⁺ cations by FA⁺ cations in the perovskite structure. The resultant FAPbI₃ perovskite thin films preserve the microstructural morphology of the original MAPbI₃ thin films exceptionally well. Importantly, the myriad processing innovations that have led to the creation of high-quality MAPbI₃ perovskite thin films are directly adaptable to FAPbI₃ through this simple, rapid chemical-conversion route. Accordingly, we show that efficiencies of perovskite solar cells fabricated with FAPbI₃ thin films created using this route can reach ∼18%

Correlation between Photocatalytic Efficacy and Electronic Band Structure in Hydrothermally Grown TiO₂ Nanoparticles

The effects of electronic band structure, electron−hole recombination, and photocatalytic property of N- and/or Fe-doped TiO2 were systematically explored. Hydrothermal reaction was used to incorporate N and/or Fe into TiO2 nanoparticles. Structural analysis using Raman spectra, X-ray diffraction, and transmission electron microscope (TEM) indicates that hydrothermally grown TiO2 particles have anatase phase, and their average size is ∼10 nm. In addition, hydrothermal doping of N and/or Fe was found to significantly modify the electronic band structure. The photocatalytic performance of undoped and doped nanomaterials was examined under UV or visible light. N doping increased the photocatalytic efficacy of TiO2 under visible light by more than 2 times. In contrast, Fe-doped and N/Fe-codoped TiO2 show worse photocatalytic performance than pure TiO2 under both UV and visible light, in spite of their smaller band gaps. Fluorescence of terephthalic acid indicates that a change in the photocatalytic performance of doped TiO2 is closely related to the amount of photoinduced radical ions. X-ray photoelectron spectroscopy and low-temperature photoluminescence were employed to study the doping mechanism. While both N and Fe facilitate the absorption of the visible light, it is found that only Fe increases the electron−hole recombination rate, leading to the opposite effects of N and Fe doping on the photocatalytic performance of TiO2

Comparison of Recombination Dynamics in CH₃NH₃PbBr₃ and CH₃NH₃PbI₃ Perovskite Films: Influence of Exciton Binding Energy

Understanding carrier recombination in semiconductors is a critical component when developing practical applications. Here we measure and compare the monomolecular, bimolecular, and trimolecular (Auger) recombination rate constants of CH₃NH₃PbBr₃ and CH₃NH₃PbI₃. The monomolecular and bimolecular recombination rate constants for both samples are limited by trap-assisted recombination. The bimolecular recombination rate constant for CH₃NH₃PbBr₃ is ∼3.3 times larger than that for CH₃NH₃PbI₃ and both are in line with that found for radiative recombination in other direct-gap semiconductors. The Auger recombination rate constant is 4 times larger in lead-bromide-based perovskite compared with lead-iodide-based perovskite and does not follow the reduced Auger rate when the bandgap increases. The increased Auger recombination rate, which is enhanced by Coulomb interactions, can be ascribed to the larger exciton binding energy, ∼40 meV, in CH₃NH₃PbBr₃ compared with ∼13 meV in CH₃NH₃PbI₃

Multiple Step Growth of Single Crystalline Rutile Nanorods with the Assistance of Self-Assembled Monolayer for Dye Sensitized Solar Cells

A novel multiple step growth (MSG) process has been developed to synthesize rutile nanorods (NRs) on fluorine-doped tin oxide (FTO) glass with the assistance of a self-assembled monolayer (SAM) aiming to increase the internal surface area of the 1D materials for dye sensitized solar cell (DSSC) applications. The experimental result reveals that the SAM layer can be selectively decomposed at the tip of the nanorod, namely the rutile (001) surface, due to the anisotropic photocatalytic property of the rutile. The remaining SAM layer on the side-wall of the NRs remains intact and serves as water repellent which prevents the radial growth of the NRs during the next step hydrothermal synthesis; therefore, the spacing between the NRs and the porosity of the NR array can be retained after additional growth cycles. On the other hand, introduction of a middle layer formed via TiCl₄ solution treatment before the next growth cycle is found to be an effective way to control the diameters of the newly grown NRs. The performance of DSSC made from the rutile NRs grown using the MSG technique has been examined, and it is significantly affected by the internal surfaces of the NRs. Furthermore, the MSG combined with NR etching treatment by acid at low temperature (150 °C) leads to a significant enhancement in the solar cell performance. The gigantic wettability difference of the NRs before and after the SAM treatment as well as the MSG method could be adapted to prepare superhydrophobic and superhydrophilic nanostructured patterns for other applications

Toward Scalable Perovskite Solar Modules Using Blade Coating and Rapid Thermal Processing

Toward scalable manufacturing of perovskite solar panels, high-performance planar p–i–n perovskite solar cells (PVSCs) and modules have been demonstrated with blade coating and rapid thermal processing (RTP). The PVSCs made using RTP for less than 30 s have equivalent photovoltaic performance as devices fabricated from hot-plate annealing for 2 min. The resulting PVSCs show the best average power conversion efficiency (PCE) of over 18.47% from forward and reverse scans. Mini-modules with an active area of over 2.7 cm2 exhibit a champion average PCE of over 17.73% without apparent hysteresis. To the best of our knowledge, these efficiencies are the highest for PVSCs processed by the combination of blade coating and RTP. Furthermore, both the blade coating and RTP were performed in an ambient environment, paving the way for the large-scale production of PVSCs through high-speed roll-to-roll printing

Charge Transfer Dynamics between Carbon Nanotubes and Hybrid Organic Metal Halide Perovskite Films

In spite of the rapid rise of metal organic halide perovskites for next-generation solar cells, little quantitative information on the electronic structure of interfaces of these materials is available. The present study characterizes the electronic structure of interfaces between semiconducting single walled carbon nanotube (SWCNT) contacts and a prototypical methylammonium lead iodide (MAPbI₃) absorber layer. Using photoemission spectroscopy we provide quantitative values for the energy levels at the interface and observe the formation of an interfacial dipole between SWCNTs and perovskite. This process can be ascribed to electron donation from the MAPbI₃ to the adjacent SWCNT making the nanotube film <i>n</i>-type at the interface and inducing band bending throughout the SWCNT layer. We then use transient absorbance spectroscopy to correlate this electronic alignment with rapid and efficient photoexcited charge transfer. The results indicate that SWCNT transport and contact layers facilitate rapid charge extraction and suggest avenues for enhancing device performance

Ultrafast Imaging of Carrier Transport across Grain Boundaries in Hybrid Perovskite Thin Films

For optoelectronic devices based on polycrystalline semiconducting thin films, carrier transport across grain boundaries is an important process in defining efficiency. Here we employ transient absorption microscopy (TAM) to directly measure carrier transport within and across the boundaries in hybrid organic–inorganic perovskite thin films for solar cell applications with 50 nm spatial precision and 300 fs temporal resolution. By selectively imaging sub-bandgap states, our results show that lateral carrier transport is slowed down by these states at the grain boundaries. However, the long carrier lifetimes allow for efficient transport across the grain boundaries. The carrier diffusion constant is reduced by about a factor of 2 for micron-sized grain samples by the grain boundaries. For grain sizes on the order of ∼200 nm, carrier transport over multiple grains has been observed within a time window of 5 ns. These observations explain both the shortened photoluminescence lifetimes at the boundaries as well as the seemingly benign nature of the grain boundaries in carrier generation

Crystal Morphologies of Organolead Trihalide in Mesoscopic/Planar Perovskite Solar Cells

The crystal morphology of organolead trihalide perovskite (OTP) light absorbers can have profound influence on the perovskite solar cells (PSCs) performance. Here we have used a combination of conventional transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), in cross-section and plan-view, to characterize the morphologies of a solution-processed OTP (CH₃NH₃PbI₃ or MAPbI₃) within mesoporous TiO₂ scaffolds and within capping and planar layers. Studies of TEM specimens prepared with and without the use of focused ion beam (FIB) show that FIBing is a viable method for preparing TEM specimens. HRTEM studies, in conjunction with quantitative X-ray diffraction, show that MAPbI₃ perovskite within mesoporous TiO₂ scaffold has equiaxed grains of size 10–20 nm and relatively low crystallinity. In contrast, the grain size of MAPbI₃ perovskite in the capping and the planar layers can be larger than 100 nm in our PSCs, and the grains can be elongated and textured, with relatively high crystallinity. The observed differences in the performance of planar and mesoscopic-planar hybrid PSCs can be attributed in part to the striking differences in their perovskite-grain morphologies