Doo Hyun Ko

Photonic Crystal Geometry for Organic Solar Cells

We report organic solar cells with a photonic crystal nanostructure embossed in the photoactive bulk heterojunction layer, a topography that exhibits a 3-fold enhancement of the absorption in specific regions of the solar spectrum in part through multiple excitation resonances. The photonic crystal geometry is fabricated using a materials-agnostic process called PRINT wherein highly ordered arrays of nanoscale features are readily made in a single processing step over wide areas (approximately 4 cm(2)) that is scalable. We show efficiency improvements of approximately 70% that result not only from greater absorption, but also from electrical enhancements. The methodology is generally applicable to organic solar cells and the experimental findings reported in our manuscript corroborate theoretical expectations.

The Patterning of Sub-500 nm Inorganic Oxide Structures†

Elastomeric perfluoropolyether molds are applied to pattern arrays of sub-500 nm inorganic oxide features. This versatile soft-lithography technique can be used to pattern a wide range of materials; in this work inorganic oxides including TiO2 , SnO2 , ZnO, ITO, and BaTiO3 are patterned on a variety of substrates with different aspect ratios. An example of TiO2 posts is shown in the figure.

Absorption and quasiguided mode analysis of organic solar cells with photonic crystal photoactive layers

We analyze optical absorption enhancements and quasiguided mode properties of organic solar cells with highly ordered nanostructured photoactive layers comprised of the bulk heterojunction blend, poly-3-hexylthiophene/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and a low index of refraction conducting material (LICM). This photonic crystal geometry is capable of enhancing spectral absorption by approximately 17% in part due to the excitation of quasiguided modes near the band edge of P3HT:PCBM. A nanostructure thickness between 200 nm and 300 nm is determined to be optimal, while the LICM must have an index of refraction approximately 0.3 lower than P3HT:PCBM to produce absorption enhancements. Quasiguided modes that differ in lifetime by an order of magnitude are also identified and yield absorption that is concentrated in the P3HT:PCBM flash layer.

Electrophotonic enhancement of bulk heterojunction organic solar cells through photonic crystal photoactive layer

We present one- (1D) and two-dimensional (2D) periodic nanostructured designs for organic photovoltaics where a photonic crystal is formed between blended poly-3-hexylthiophene/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and nanocrystalline zinc oxide. Absorption enhancements over the full absorption range of P3HT:PCBM of 20% (one polarization) and 14% are shown for the 1D and 2D structures, respectively. These improvements result in part from band edge excitation of quasiguided modes. The geometries are also shown to create excitons 26% (1D) and 11% (2D) closer to P3HT:PCBM exit interfaces indicating further photovoltaic improvement.

Large area nanofabrication of butterfly wing's three dimensional ultrastructures

The authors report a simple method for the artificial fabrication of the complex three-dimensional (3D) ultrastructures of butterfly wing scales. This method uses chemical vapor deposition, UV lithography, and chemical etching to create the ultrastructures over a large area surpassing previously used focused ion beam techniques that are limited to microscopic areas. Furthermore, this method shows flexibility to modify nanostructure types and can precisely control shapes and dimensions and periodicity. Fabricated 3D ultrastructures are also replicated using a nanoimprint method into soft polymer materials. Reflectivity measurements and simulations of the master and polymer replicas show selective UV reflection consistent with the length scales used in such butterfly-like nanostructures.

Nonideal parasitic resistance effects in bulk heterojunction organic solar cells

A common assumption in both experimental measurements and device modeling of bulk heterojunction (BHJ) organic solar cells is that parasitic resistances are ideal. In other words, series resistance (Rsr) is near zero while shunt resistance (Rsh) approaches infinity. Relaxation of this assumption affects device performance differently depending on the chosen BHJ material system. Specifically, the impact of nonideal Rsr is controlled by the electric field dependence of the probability of charge transfer (CT) state dissociation (PCT). This is demonstrated by evaluating the experimental current density versus voltage response within the framework of a drift/diffusion model for two BHJ systems that strongly differ in PCT. Second, light intensity measurements of devices with nonideal Rsr and Rsh are shown to convolute the scaling of short-circuit current and open-circuit voltage with light intensity, which is a common technique to study BHJ device physics. Finally, we show the connection between the drift/diffu...

Suppression of bimolecular recombination by UV-sensitive electron transport layers in organic solar cells

Incorporating UV-sensitive electron transport layers (ETLs) into organic bulk heterojunction (BHJ) photovoltaic devices dramatically impacts short-circuit current (Jsc) and fill factor characteristics. Resistivity changes induced by UV illumination in the ETL of inverted BHJ devices suppress bimolecular recombination producing up to a two orders of magnitude change in Jsc. Electro-optical modeling and light intensity experiments effectively demonstrate that bimolecular recombination, in the form of diode current losses, controls the extracted photocurrent and is directly dependent on the ETL resistivity.

Characterizing enhanced performance of nanopatterned bulk heterojunction organic photovoltaics

We present experimental and theoretical studies of a nanopatterned photonic crystal formed between the bulk heterojunction blend, poly-3-hexylthiophene:[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and nanocrystalline zinc oxide (nc-ZnO). The nanopattern is fabricated using the Pattern Replication in Non-wetting Templates (PRINT) technique. We summarize the fabrication method and show how it can be used to make a highly ordered hexagonal array of photovoltaic P3HT:PCBM posts. We also discuss theoretical studies of optical absorption for the nanopattern design that result in a 22% enhancement over a conventional planar cell. Spectroscopic ellipsometry is also used to determine the optical constants of solar cell materials that are used in the optical model. Finally, we calculate the local exciton creation profile within the photoactive nanopattern to relate the nanostructured geometry to electrical performance.

Electro‐optical model of photonic crystal bulk heterojunction organic solar cells

In this work, we present a two‐dimensional electro‐optical model for the operation of bulk heterojunction (BHJ) organic solar cells with photonic crystal (PC) geometries. The model is applied to the test case of a one‐dimensional (1‐D) periodic PC device and a conventional planar control cell with poly[2‐methoxy‐5‐(3’,7’‐dimethyloctyloxy)‐p‐phenylene vinylene]:[6,6]‐phenyl C61‐butyric acid methyl ester (MDMO‐PPV:PCBM) as the photoactive BHJ material. From the optical model we demonstrate absorption enhancements of 16% for the PC device compared to the planar control cell. In terms of the electrical model, the non‐planar geometry of the PC significantly alters the internal electric field in the photoactive region, which causes a 5% reduction in the exciton dissociation probability and a significant 8.4‐fold increase in the bimolecular recombination rate. Investigations are also discussed which are currently being conducted to resolve these predictions with experiment.