Danilo B. Romero

Effect of gold nanopillar arrays on the absorption spectrum of a bulk heterojunction organic solar cell

We report on the effect of arrays of Au nanopillars of controlled size and spacing on the spectral response of a P3HT: PCBM bulk heterojunction solar cell. Prototype nanopillar-patterned devices have nearly the same overall power conversion efficiency as those without nanopillars. The patterned devices do show higher external quantum efficiency and calculated absorption in the wavelength range from approximately 640 nm to 720 nm, where the active layer is not very absorbing. The peak enhancement was approximately 60% at 675 nm. We find evidence that the corresponding resonance involves both localized particle plasmon excitation and multiple reflections/diffraction within the cavity formed by the electrodes. We explore the role of the attenuation coefficient of the active layer on the optical absorption of such an organic photovoltaic device.

Interface effects on the external quantum efficiency of organic bulk heterojunction photodetectors

We investigate the effects of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and LiF interfacial layers on the efficiency of organic bulk heterojunction photodetectors fabricated with blends of regioregular poly(3-hexylthiophene-2,5-diyl) as electron donor and {6}-1-[3-(methoxycarbonyl)propyl]-{5}-1-phenyl-[5,6]-C61 as electron acceptor. At high reverse bias voltage, we find that the efficiency of these devices is limited only by the rate of creating excitons in the organic layer. Upon correction for the optical loss at the interfacial layers, the internal quantum efficiency can approach 100% in this highly disordered network of donor-acceptor molecular heterojunctions.

High External Quantum Efficiency from Organic Bulk Heterojunction Photodetectors

Abstract: From an organic bulk heterojunction photodetector fabricated from a blend of P3HT/PCBM-C60, we report an external quantum efficiency of 87 plusmn 2 % under an applied bias voltage of -10V, leading to an internal quantum efficiency of 97 %.

Organic photocells based on block copolymer / C60 blends

We investigate the dependence of the photovoltaic characteristics of organic photocells on the relative concentration of the donor-acceptor molecular complex. The devices were fabricated using a new [MEH-PPV] - co - [phenylene vinylene] blend with C60. We find that the morphology and device performance are strongly influenced by the molar fraction (x) of C60 in the electroactive layer of the device. The best device was obtained with x = 0.6 and manifested VOC = 0.85 V, JSC = 2.65 mA/cm2, FF = 0.42, and ηPext = 1%.

Surface compositions of atomic layer deposited Zn1−xMgxO thin films studied using Auger electron spectroscopy

In this paper, the authors present Auger electron spectroscopy (AES) studies of Zn1−xMgxO (ZMO) films grown via interrupted atomic-layer deposition (ALD) techniques. The ZMO films were fabricated by alternating ALD deposition of ZnO and MgO layers up to 1000 cycles. Zn1−xMgxO films with progressively decreasing Mg/Zn ratios (Mg/Zn = 1/1, 1/2, 1/3, 1/4, 1/5, 1/6, 1/9, and 2/8, 3/12, 4/16, and 5/20) were fabricated for this study. The AES results exhibit an abrupt drop of Mg composition on the ZMO surface when the Mg/Zn < 1/3. Additionally, the surface composition ratios of O to Mg, O to Zn, and Mg to Zn were estimated with known Auger sensitivity factors. The results indicate that Mg ions diffuse into the bulk, forming Zn1−xMgxO alloys.

Low-bandgap small molecules for near-infrared photovoltaic applications

We investigate the effects of interfacial layers on the photovoltaic properties of bi- layer organic heterojunction photovoltaic devices. The devices were fabricated using aluminum phthalocyanine chloride (AlPcCl) as electron donor and fullerene (C60) as electron acceptor. Two types of interfacial layers inserted between the transparent indium-tin-oxide anode and the AlPcCl layer were investigated: PEDOT:PSS and MoO3. We find that these interfacial layers have a strong influence on the device open-circuit voltage (VOC). The effects of temperature and illumination intensity on VOC were explored. C 2011 Society of Photo-Optical Instrumentation

Study of transient effects in photo-excited semiconducting polymer and bulk heterojunctions

Transient dynamics of photoexcited carriers in MEH-PPV, PCBM-C60 and their blend is investigated by THz-TDS using a 400 nm optical pump and terahertz probe. Free and bound carriers are revealed in the measurement of the complex conductivity.

F8T2 Copolymer/C 60 Heterojunction Photovoltaic Devices

We explore poly(9,9-dioctylfluorene-co-bithiophene) copolymer (F8T2) for photovoltaic applications. Devices fabricated with F8T2/C60blend show open-circuit voltage of 1V and power conversion efficiency of 1.15% for operation in the short-wavelength region of the solar spectrum.

Stoichiometric and Morphological Effects on the Photovoltaic Properties of Conjugated Polymer / C60 Distributed Heterojunction Devices

We report the effects of thin-film stoichiometry and morphology on the short-circuit current and open-circuit voltage of organic-based photocells. Performance of these devices depends strongly on the relative concentration in the polymer / C60 blend.