Yun Ju Lee

Improved performance of poly(3-hexylthiophene)/zinc oxide hybrid photovoltaics modified with interfacial nanocrystalline cadmium sulfide

To improve zinc oxide/poly(3-hexylthiophene) (ZnO/P3HT) hybrid solar cell performance, we introduce a nanocrystalline cadmium sulfide (CdS) film at the ZnO/P3HT heterojunction, creating a cascading energy band structure. Current-voltage characteristics under AM1.5 illumination show that, compared to unmodified ZnO/P3HT devices, CdS modification leads to an approximate doubling of the open-circuit voltage and a mild increase in fill factor, without sacrificing any short-circuit current. These characteristics double the power conversion efficiency for devices with an interfacial CdS layer. External quantum efficiency spectra reveal definite photocurrent contributions from the CdS layer, confirming the cascading band structure. The mechanisms behind open-circuit voltage increase are discussed.

Log-Pile TiO2 Photonic Crystal for Light Control at Near-UV and Visible Wavelengths

Three-dimensional photonic crystals (3DPCs) with an omnidirectional photonic bandgap (OPBG) are an important class of materials for the manipulation of photons. By creating a complete electromagnetic vacuum over a desired wavelength range, a 3DPC with an OPBG opens up new regimes of light–matter interaction, such as photon–atom bound states, spontaneous emission control, ultralow-loss waveguiding, and negative refraction, with significant impact in areas such as all-optical computing and circuitry, energy conversion, and subdiffraction optics. In particular, 3DPCs with OPBGs in the visible frequency regime can potentially enhance performance and efficiency of solid-state lighting, solar-energy conversion, and other applications by controlling energy transfer via modification of the photonic density of states (PDOS), as well as enable real-time subdiffraction imaging via control of photonic dispersion. Here, we demonstrate a 9-layer TiO2 log-pile 3DPC with a lattice constant of 250 nm and a rod size of 70 nm, fabricated using an optimized layer-by-layer processing approach involving TiO2 deposition, electron-beam patterning, and a reactive ion etch sequence. The 3DPC exhibits a stackingdirection bandgap characterized by a strong reflectance between 380and 500-nm wavelength. The realization of a 3DPC with uniform sub-100-nm features and a bandgap spanning the near-UV and blue–green regime is an important milestone because the potential complete control of the electromagnetic environment in the visible region offered by OPBG-3DPC structures is extremely attractive for a variety of high-impact applications. Fabrication of visible-frequency PCs has always posed an immense challenge because of the need to simultaneously satisfy the requirement of nanoscale dimensions and high refractive index (n) contrast in the PC structure. Since n of most non-absorbing materials in the visible region is less than 2.8, only diamondlike crystal structures such as the log pile enable 3DPCs with an OPBG. In fact, because a log pile can exhibit an OPBG with n as low at 2.0, the use of a log-pile 3DPC maximizes the bandwidth of the OPBG in the visible frequency region. TiO2 is often the material of choice for visible 3DPCs because it is transparent over the entire frequency range (extinction coefficient k 0) and exhibits a very high n 2.8 for a single-crystalline rutile phase. Many submicrometer lithographic techniques have been utilized for single-step, multiple-layer fabrication of 3DPCs, including holographic/ interference lithography, multiphoton polymerization, as well as direct laser writing. Researchers have successfully demonstrated templates with diamondlike crystal structures, some of which were subsequently infiltrated with high-n materials to create a negative replica. However, in all cases the fabricated device was not optimized for a visible OPBG due to either a lattice constant>500 nm or n<2.0. In contrast, a layer-by-layer lithographic technique based on a multilevel electron-beam direct-write method offers the capability to obtain small feature sizes and periodicity utilizing a high-nmaterial, which is required to fabricate PCs with OPBG in the visible regime. Two layer-by-layer processing methods (methods 1 and 2) were employed to fabricate the TiO2 log-pile 3DPCs. In method 1, we begin by depositing a planar TiO2 film by reactive sputtering. The TiO2 film exhibits n and k ranging from 2.25 and 0.002 at 800 nm to 2.55 and 0.01 at 400 nm (Fig. 1a), respectively, as expected. TiO2 is reactive ion etched through an electron-beam-patterned poly(methyl methacrylate) (PMMA) mask, backfilled and planarized with evaporated and spin-coated SiO2, and the process is repeated for additional layers (see Experimental section for more details). Using this process flow, shown schematically in Figure 1b, we fabricated a 4-layer log-pile 3DPC. Figure 2 shows a top-view scanning electronmicroscopy (SEM) image of a 4-layer structure with a 350-nm lattice constant. The image shows an average rod width of 140 nm and also reveals the underlying layers (out of focus), which are shifted by half of a lattice constant. Cross-section images taken along two perpendicular cut directions (1 and 2) reveal the four layers with accurate alignment between successive layers. We also note a distinct waviness in the nanorod appearance. This is due to our decision to over-etch the backfilled SiO2 film by 10% during the planarization step to ensure connectivity between different layers of TiO2 rods, which causes overlapping between layers and the wavy appearance. A relatively thick layer of PMMA ( 350 nm) was used to ensure sufficient thickness (>100 nm) of resist was left behind after the reactive ion etch step in order to have a clean lift-off after backfilling with SiO2. This is due to the fact that the etch selectivity of TiO2, as defined by the etch rate of TiO2 versus the etch rate of the PMMA mask, is only 0.5. However, the use of thicker PMMA results in poorer electron-beam dose control, which, in turn, leads to variation in rod width between layers (Fig. 2). We addressed this problem using a complementary pattern and evaporation of TiO2, as described in the next section

Open-Circuit Voltage Improvement in Hybrid ZnO–Polymer Photovoltaic Devices With Oxide Engineering

We present strategies to improve low open-circuit voltage (V_oc) for ZnO-poly(3-hexylthiophene) (P3HT) photovoltaic devices, which are typically ≤0.4 V, but vary among different reports. One factor affecting V_oc variability is the ZnO bandgap (E_g), which depends on detailed processing conditions. By decreasing the pyrolysis temperature of sol-gel ZnO films, we increased the ZnO E_g by 0.14 eV and V_oc of corresponding bilayer devices by 0.1 V. This is understood as increased donor-acceptor energy-level offset. Next, we demonstrate significant enhancement in V_oc by depositing conformal amorphous TiO_x films at the surface of planar ZnO films and ZnO nanorod arrays using a spin-coating method. The TiO_x coatings monotonically increased V_oc from 0.4 to 0.8 V for devices with increasing TiO_x thicknesses from 0 to ≥50 Å. Dark current-voltage measurement reveals that the TiO_x coating significantly decreases the reverse-bias current density, leading to an improvement in V_oc, in excellent agreement with predictions from the modified ideal diode equation. This is consistent with passivation of ZnO surface defects by TiO_x. In short, by varying the solution processing conditions, we modify the bulk and interfacial properties of the metal oxide acceptor, thus leading to systematic improvement in open-circuit voltage.

Charge collection in bulk heterojunction organic photovoltaic devices: An impedance spectroscopy study

Through thickness and applied bias variation, charge collection in poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction organic photovoltaic (OPV) devices was investigated with impedance spectroscopy. An equivalent circuit model incorporating chemical capacitance (Cμ), recombination resistance (R2), and transport resistance (R1) was used to analyze the results. Insufficient carrier extraction, exhibiting diffusion transport characteristics at high frequencies, was found in devices with a thick active layer. These devices also display a higher chemical capacitance, indicating greater carrier accumulation, and a lower recombination resistance, signaling increased bimolecular recombination. Increasing internal field with negative applied bias enhances carrier collection by reducing carrier accumulation and recombination. Moreover, we showed explicitly that charge collection can be quantified by (R2/R1)1/2, which is proportional to device fill factor. These results demonstrate that impedance spectroscopy is an effective tool for investigating charge collection in OPV devices.

Effects of Contact-Induced Doping on the Behaviors of Organic Photovoltaic Devices

Substrates can significantly affect the electronic properties of organic semiconductors. In this paper, we report the effects of contact-induced doping, arising from charge transfer between a high work function hole extraction layer (HEL) and the organic active layer, on organic photovoltaic device performance. Employing a high work function HEL is found to increase doping in the active layer and decrease photocurrent. Combined experimental and modeling investigations reveal that higher doping increases polaron-exciton quenching and carrier recombination within the field-free region. Consequently, there exists an optimal HEL work function that enables a large built-in field while keeping the active layer doping low. This value is found to be ~0.4 eV larger than the pinning level of the active layer material. These understandings establish a criterion for optimal design of the HEL when adapting a new active layer system and can shed light on optimizing performance in other organic electronic devices.

Sub-10 nm copper chromium oxide nanocrystals as a solution processed p-type hole transport layer for organic photovoltaics

We report the synthesis of CuCrO2 nanocrystals, a p-type transparent conducting oxide, and their application as an efficient hole transport layer (HTL) for organic photovoltaic (OPV) devices. A nanometer-sized mixture of Cu and Cr oxide/hydroxide is synthesized using microwave-assisted heating. With a 550 °C post-annealing treatment in N2, <10 nm CuCrO2 nanocrystals are successfully synthesized. XRD, XPS, EDAX, PESA, UV-vis spectrometry, and Kelvin probe technique are applied to confirm the delafossite phase, optical transmission, and p-type characteristics. Methanol is found to be a good solvent to disperse these nanocrystals for forming a smooth and transparent film. In comparison with the previously reported CuGaO2 HTL, the reduced film roughness enables the CuCrO2 HTL to produce highly efficient thin active layer OPV devices. UV-ozone treatment on the CuCrO2 HTL is found to increase the fill factor. Drift-diffusion modeling, energy level measurements, and XPS results reveal that the device improvement is not due to the reduced injection barrier, but due to an improved CuCrO2 conductivity arising from the formation of Cu2+ species.

Silicon-based near-visible logpile photonic crystal

A nanocavity structure is embedded inside a silicon logpile photonic crystal that demonstrates tunable absorption behavior at near visible wavelengths well beyond the absorption edge of silicon. This is due to silicon’s indirect bandgap resulting in a relatively slow increase in the absorption of silicon with decreasing wavelength. Our results open up the possibility of utilizing the wide, complete three dimensional photonic gap enabled by the large refractive index of silicon to create three dimensional photonic crystal based devices well into the visible regime.

Effect of metal/bulk-heterojunction interfacial properties on organic photovoltaic device performance

Interfacial properties between evaporated metal contacts and active layer in organic photovoltaic devices critically affect device performance. Through a controlled mechanical delamination method, the interfaces between annealed P3HT:PCBM BHJ layer and Al or Ag electrodes are revealed for direct chemical characterization. The difference in the interfacial, rather than bulk, properties account for the different OPV device performance.

In Situ Chemical Oxidation of Ultrasmall MoOx Nanoparticles in Suspensions

Nanoparticle suspensions represent a promising route toward low cost, large area solution deposition of functional thin films for applications in energy conversion, flexible electronics, and sensors. However, parameters such size, stoichiometry, and electronic properties must be controlled to achieve best results for the target application. In this report, we demonstrate that such control can be achieved via in situ chemical oxidation of MoOx nanoparticles in suspensions. Starting from a microwave-synthesized suspension of ultrasmall (d~2 nm) MoOx nanoparticles in n-butanol, we added H2O2 at room temperature to chemically oxidize the nanoparticles. We systematically varied H2O2 concentration and reaction time and found that they significantly affected oxidation state and work function of MoOx nanoparticle films. In particular, we achieved a continuous tuning of MoOx work function from 4.4 to 5.0 eV, corresponding to oxidation of as-synthesized MoOx nanoparticle (20% Mo6

Diffraction response of colloidal crystals: effect of numerical aperture

We present a quantitative experimental and theoretical study of the effect of numerical aperture (NA) on the Bragg diffraction from a dry polystyrene colloidal crystal. The diffraction peak parameters changed noticeably as the NA was increased from 0.017 to 0.5. The diffraction wavelength blueshifted 1.4% from 584 to 576 nm, and the normalized full width at half-maximum increased from 6.2% to 7.0%. These shifts occurred primarily for NA > 0.3 and agreed qualitatively with results predicted by a layered Korringa-Kohn-Rostoker method. Thus, by using focusing optics with NAs below 0.3, the diffraction response of low-photonic-strength mesostructures may be compared with normal-incidence experimental and theoretical data.