Yu-Ching Huang

Tuning perovskite morphology by polymer additive for high efficiency solar cell.

Solution processable planar heterojunction perovskite solar cell is a very promising new technology for low cost renewable energy. One of the most common cell structures is FTO/TiO2/CH3NH3PbI3-xClx/spiro-OMeTAD/Au. The main issues of this type of solar cell are the poor coverage and morphology control of the perovskite CH3NH3PbI3-xClx film on TiO2. For the first time, we demonstrate that the problems can be easily resolved by using a polymer additive in perovskite precursor solution during the film formation process. A 25% increase in power conversion efficiency at a value of 13.2% is achieved by adding 1 wt % of poly(ethylene glycol) in the perovskite layer using a 150 °C processed TiO2 nanoparticle layer. The morphology of this new perovskite was carefully studied by SEM, XRD, and AFM. The results reveal that the additive controls the size and aggregation of perovskite crystals and helps the formation of smooth film over TiO2 completely. Thus, the Voc and Jsc are greatly increased for a high efficiency solar cell. The amount of additive is optimized at 1 wt % due to its insulating characteristics. This research provides a facile way to fabricate a high efficiency perovskite solar cell by the low temperature solution process (<150 °C), which has the advancement of conserving energy over the traditional high temperature sintering TiO2 compact layer device.

Nanoparticle-tuned self-organization of a bulk heterojunction hybrid solar cell with enhanced performance.

We demonstrate here that the nanostructure of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM) bulk heterojunction (BHJ) can be tuned by inorganic nanoparticles (INPs) for enhanced solar cell performance. The self-organized nanostructural evolution of P3HT/PCBM/INPs thin films was investigated by using simultaneous grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence wide-angle X-ray scattering (GIWAXS) technique. Including INPs into P3HT/PCBM leads to (1) diffusion of PCBM molecules into aggregated PCBM clusters and (2) formation of interpenetrating networks that contain INPs which interact with amorphous P3HT polymer chains that are intercalated with PCBM molecules. Both of the nanostructures provide efficient pathways for free electron transport. The distinctive INP-tuned nanostructures are thermally stable and exhibit significantly enhanced electron mobility, external quantum efficiency, and photovoltaic device performance. These gains over conventional P3HT/PCBM directly result from newly demonstrated nanostructure. This work provides an attractive strategy for manipulating the phase-separated BHJ layers and also increases insight into nanostructural evolution when INPs are incorporated into BHJs.

All‐Vacuum‐Deposited Stoichiometrically Balanced Inorganic Cesium Lead Halide Perovskite Solar Cells with Stabilized Efficiency Exceeding 11%

Vacuum-sublimed inorganic cesium lead halide perovskite thin films are prepared and integrated in all-vacuum-deposited solar cells. Special care is taken to determine the stoichiometric balance of the sublimation precursors, which has great influence on the device performance. The mixed halide devices exhibit exceptional stabilized power conversion efficiency (11.8%) and promising thermal and long-term stabilities.

Employing an amphiphilic interfacial modifier to enhance the performance of a poly(3-hexyl thiophene)/TiO2 hybrid solar cell

We have studied two amphiphilic interfacial modifiers: low cost Cu phthalocyanine dye containing ether side chains (Cu–ph–ether dye) and a carboxylic acid- and bromine-terminated 3-hexyl thiophene oligomer (oligo-3HT-(Br)COOH, Mw ∼ 5K) to enhance the interfacial interaction between poly(3-hexyl thiophene) (P3HT) and TiO2 nanorods. A large improvement in the performance of fabricated solar cells was observed using these relatively large molecular modifiers when compared to pyridine-modified TiO2 nanorods. UV-vis spectroscopy and X-ray photoelectron spectroscopy analyses reveal that the modifiers are adsorbed and chemically bonded to TiO2 through unshared electrons associated with the modifiers. Furthermore, the new modifiers increased the hydrophobicity of TiO2 with the order of oligo-3HT-(Br)COOH > Cu–ph–ether dye > pyridine. Synchrotron X-ray spectroscopy studies of the modified hybrid films indicate the crystallinity of P3HT is increased, following the same trend as the hydrophobicity, because the new modifiers function as plasticizers, increasing the flow characteristics of the film. Moreover, the same trend is also observed for the reduced recombination rate and increased lifetime of charge carriers in the device by transient photo-voltage measurement. Thus, the oligo-3HT-(Br)COOH outperforms the Cu–ph–ether dye and pyridine in enhancing the power conversion efficiency (PCE, η) of the solar cell. More than a two-fold improvement is shown compared to pyridine. The results are due to the large size, conductivity, and polar characteristics of the oligo-3HT-(Br)COOH unit, which facilitates both the crystallization of P3HT and the electron transport of the TiO2 nanorods. This study provides a useful route for increasing the efficiency of hybrid solar cellsvia the enhancement of interfacial interactions between organic donors and inorganic acceptor materials.

Using an Airbrush Pen for Layer-by-Layer Growth of Continuous Perovskite Thin Films for Hybrid Solar Cells

In this manuscript we describe hybrid heterojunction solar cells, having the device architecture glass/indium tin oxide/poly(3,4-ethylenedioxythiopene)/poly(styrenesulfonic acid)/perovskite/[6,6]-phenyl-C61-butyric acid methyl ester/C60/2,9-dimethyl- 4,7-diphenyl-1,10-phenanthroline/Al, fabricated using lead halide perovskite obtained through spray-coating at a low precursor concentration. To study the relationship between the morphology and device performance, we recorded scanning electron microscopy images of perovskite films prepared at various precursor ratios, spray volumes, substrate temperatures, and postspray annealing temperatures. Optimization of the spray conditions ensured uniform film growth and high surface area coverage at low substrate temperatures. Lead halide perovskite solar cells prepared under the optimal conditions displayed an average power conversion efficiency (PCE) of approximately 9.2%, with 85% of such devices having efficiencies of greater than 8.3%. The best-performing device exhibited a short-circuit current density of 17.3 mA cm(-2), a fill factor of 0.63, and an open-circuit voltage of 0.93 V, resulting in a PCE of 10.2%. Because spray-coating technology allows large-area deposition, we also fabricated devices having areas of 60 and 342 mm(2), achieving PCEs with these devices of 6.88 and 4.66%, respectively.

Bi-hierarchical nanostructures of donor–acceptor copolymer and fullerene for high efficient bulk heterojunction solar cells

Solvent additive processing has become the most effective method to tune the nanostructure of donor–acceptor (D–A) type copolymer/fullerene bulk heterojunctions (BHJs) solar cells for improving power conversion efficiencies. However, to date qualitative microscopic observations reveal discrepant results on the effects of solvent additives. Here, we present quantitative evolution of bi-hierarchical nanostructure of D–A copolymers and fullerenes by employing grazing-incidence small/wide angle X-ray scattering (GISAXS/GIWAXS) techniques and [2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]/[6,6]-phenyl-C71-butyric acid methyl ester (PCPDTBT/PCBM) BHJ as model materials. An accurate GISAXS model analysis is established herein for revealing the distinctive bi-hierarchical nanostructures from molecular level to a scale of hundreds of nanometers. The mechanisms of hierarchical formation and mutual influence between PCPDTBT and PCBM domains are proposed to correlate with photovoltaic properties. These results provide a comprehensive interpretation in respect to previous studies on the nanostructures of D–A copolymer/fullerene BHJs. It is helpful for optimum structural design and associated synthesis improvement for achieving high efficiency BHJ solar cells.

Quantitative nanoscale monitoring the effect of annealing process on the morphology and optical properties of poly(3-hexylthiophene)/[6,6]-phenyl C61-butyric acid methyl ester thin film used in photovoltaic devices

We have studied the nanoscale changes in morphology and optical properties during annealing for bulk-heterojunction poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) composite film. Thermal atomic force microscopy was used to monitor the morphology evolution of the film in situ quantitatively, which showed a migration and aggregation of PCBM with increasing temperature. Scanning near-field microscopy was used to investigate the quantitative changes in absorption behavior of the film in nanoscale with increasing annealing time at 140 °C, which indicated that the extent of absorption of the film was increased with increasing annealing time. However, a large PCBM aggregate (1 μm) was formed after the film annealed at 140 °C for 1 h. The aggregate interrupted the bicontinous morphology of the film and further affected the absorption behavior in nanoscale. Furthermore, the refractive index and extinction coefficient of the films increased after annealed 30 min at 140 °C, but d...

Regioregularity effects in the chain orientation and optical anisotropy of composite polymer/fullerene films for high-efficiency, large-area organic solar cells

In this paper, we demonstrate the strong influence of the regioregularity (RR) of poly(3-hexylthiophene) (P3HT) on the optical anisotropy of hybrid P3HT/fullerene films before and after thermal annealing. We determined the conversion efficiency and characterized the optical anisotropy of P3HT/6,6-phenyl-C61-butyric acid methyl ester (PCBM) blends featuring various values of RR. Unlike grazing-incidence X-ray diffraction analysis, optical anisotropic measurement provides a clear and convenient view of the polymer orientation and the device anisotropic absorption at the same time. By calculating the in-plane and out-of-plane optical constants (extinction coefficients and refractive indices), we determined that the optical anisotropy of P3HT/PCBM films was improved in both orientations upon increasing the RR. Upon increasing the thermal annealing temperature, the main chains of high-RR P3HT were converted from an amorphous structure to an alignment parallel to the substrate, resulting in higher optical anisotropy. The degree of anisotropy of the high-RR P3HT/PCBM blend was up to six times higher than that of the low-RR sample. This strong RR effect on optical anisotropy was also evident in the power conversion efficiency of large-area P3HT/PCBM-based organic solar cells.

Preparation of metal halide perovskite solar cells through a liquid droplet assisted method

Solution-processed organometal trihalide-based perovskites have attracted attention in the field of solar energy due to their high absorption and low temperature fabrication. We demonstrated the droplet-assisted two-step process of spin coating lead iodide (PbI2) followed by spray coating methylammonium iodide (CH3NH3I) to prepare a continuous lead iodide perovskite film. A simple airbrush gun was used to control the volume of CH3NH3I, in order to attain a uniform, stoichiometric and continuous perovskite film. An insufficient or excess volume of CH3NH3I gives poor crystallinity and morphology, which gradually reduces the device performance. A power conversion efficiency (PCE) of 11.66% was achieved for 100 nm of PbI2 followed by 300 μl of CH3NH3I and annealing at 100 °C for 120 min. To address the reproducibility of the device performance, 50 devices were fabricated for statistical analysis and 80% of the devices showed the average PCE of 10–11% with reproducible JSC, VOC and FF.

Band gap aligned conducting interface modifier enhances the performance of thermal stable polymer-TiO2 nanorod solar cell

In this paper, we show that the poly(3-hexyl-thiophene)/TiO2 nanorod hybrid material is more thermally stable than the poly(3-hexyl-thiophene)/[6,6]-phenyl C61-bntyric acid methyl ester (P3HT/PCBM) hybrid material. A metal free conducting interface modifier of oligo-3-hexyl thiophene carboxylic acid (oligo-3HT-COOH) has been synthesized that exhibits aligned band gap for the P3HT/TiO2 hybrid. The conducting modifier shows an increase in power conversion efficiency of 4.8 times over an insulating modifier of oleic acid and 2.2 folds improvement over small molecule modifier of pyridine. These increases are due to a reduced recombination rate (42 μs carrier life time) and fast electron injection time of 0.24 ps. This interface modifier makes thermally stable organic-inorganic hybrid materials useful for fabrication of all solution processable solar cells.