Christopher Lombardo

Scanning photocurrent microscopy of lateral organic bulk heterojunctions

Scanning confocal photocurrent microscopy has been used to characterize carrier collection efficiency in lateral bulk heterojunction devices. By analyzing the photocurrent mappings within these devices, the lateral extents of the space charge regions has been measured and reported. Modulation via white light bias or increased voltage bias is also shown to increase the size of the space charge regions.

Analysis of photocurrents in lateral-geometry organic bulk heterojunction devices

Lateral-geometry organic bulk heterojunction (BHJ) devices resemble field-effect transistors without a gate and are useful for probing electrical characteristics of BHJ blends. The lateral structure allows physical access to the BHJ during device operation and lends itself well to device modelling. Photocurrent-voltage data from P3HT:PCBM-based lateral devices were found to possess features of space-charge limited (SCL) extraction. Numerical simulations indeed suggest a physical picture of SCL operation, effectively dividing the device into a photogenerative zone and a photoconducting zone. Based on these simulations, an analytical model was created to describe SCL photocurrents that account for channel lengths and bimolecular recombination.

Nongeminate carrier recombination rates in organic solar cells

Ambipolar organic thin-film transistors and lateral resistor structures have been used to study the transport of charge carriers in bulk heterojunction (BHJ) organic photovoltaic devices. Active layers of a phase-separated blend of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester were chosen due to their wide use in BHJ solar cell devices. A new method for determining nongeminate recombination rates is reported. Field dependent measurements of the recombination rate show that recombination decreases with increasing electric field.

Mapping electric field distributions in biased organic bulk heterojunctions under illumination by nonlinear optical microscopy

How charge carriers are distributed in a bulk heterojunction (BHJ) under illumination is central to the understanding of organic photovoltics and photodetectors. Here, we apply nonlinear optical microscopy to quantitatively map the spatial distributions of electric fields in two lateral organic BHJs: poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (PCBM) and poly(4,4-dioctyldithieno(3,2-b:2′,3′-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl) and PCBM. For the former, we observe the development with time of a depletion region adjacent to the electron-collecting electrode. In the latter, the device is stable and characterized by a nearly linear potential drop. We discuss the origins of field distributions and space charge accumulation in organic BHJs.

Device physics and operation of lateral bulk heterojunction devices.

Measurements of lateral bulk heterojunction (BHJ) devices have recently been reported as a means to characterize charge transport and recombination properties within organic photovoltaic (OPV) materials. These structures allow for the direct measurement of the lateral extents of the space charge regions, potential and electric field profiles, current versus voltage characteristics, and other physical and chemical properties. This article describes numerical simulations that show three different transport regimes present within lateral BHJ devices and two different experimental methods, which verify those findings. These measurement techniques utilize typical confocal microscopy tools as well as steady-state current versus voltage measurements on high aspect ratio nanofabricated structures in order to probe the material properties between the electrodes. Experimental results show that the lateral extents of space charge regions within these devices are approximately 1-5 μm, which are related to the drift lengths of the charge carriers, and that the mechanism of bimolecular recombination is shown to be a bulk material property. The results within this article describe a series of methods to evaluate charge transport and recombination along the in-plane direction in BHJ films and provide complementary insights to those obtained from vertical-device-based measurements.

Lateral mobility measurements in organic bulk heterojunctions: comparison of field-effect and space charge mobilities

Measurement of charge transport and recombination parameters in bulk heterojunction (BHJ) materials is of great interest to better understand the science underlying organic solar cells. We discuss the use of space charge limited current measurements under solar illumination to measure the mobility of lateral structures in which charge transport is parallel to the plane of the substrate. We compare these mobilities with mobilities calculated from field-effect measurements of the same structures. Changes in mobility as a function of device length and electric field are consistent with numerical simulations of these lateral structures. Lateral organic photovoltaic structures are potentially very useful in evaluating a number of basic material properties of BHJs including three-dimensional transport.

Bimolecular recombination coefficient calculation by in situ potentiometry in a bulk heterojunction organic photovoltaic material

Polymer fullerene bulk heterojunction (BHJ) systems are an important class of active materials; however, charge transport and recombination mechanisms in these materials are not yet completely understood. We use lateral bulk heterojunction devices to perform in situ potentiometry on the BHJ system P3HT:PCBM. From current vs. voltage measurements performed at different light intensities, we provide evidence that the recombination mechanism in this material is bimolecular. The potential profile of the device channel is also constructed from these measurements, which is then used to determine the mobility of both charge carriers and calculate the bimolecular recombination coefficient.

Temperature-dependent charge transport in copper indium diselenide nanocrystal films

This manuscript reports the temperature dependence of majority carrier transport in p-type films of copper indium diselenide (CuInSe2) nanocrystals. Charge transport parameters, such as the carrier concentration and the electrical conductivity as well as the charge transport mechanisms have been characterized through measurements of electrical capacitance and electrical current as a function of applied voltage bias and temperature. At low temperatures, below 181 K, the temperature dependence of the conductivity is consistent with a variable range hopping mechanism for transport, while at higher temperatures, above 181 K, the transport mechanism shifts to nearest neighbor hopping. Charge transport measurements were also studied under AM1.5 illumination to show how energetic barriers for charge transport are reduced under solar cell-like operating conditions.

Analysis of bulk heterojunction material parameters using lateral device structures

Abstract. We review the key optoelectronic properties of lateral organic bulk heterojunction (BHJ) device structures with asymmetric contacts. These structures are used to develop a detailed model of charge transport and recombination properties within materials used for organic photovoltaics. They permit a variety of direct measurement techniques, such as nonlinear optical microscopy and in situ potentiometry, as well as photoconductive gain and carrier drift length studies from photocurrent measurements. We present a theoretical framework that describes the charge transport physics within these devices. The experimental results presented are in agreement with this framework and can be used to measure carrier concentrations, recombination coefficients, and carrier mobilities within BHJ materials. Lateral device structures offer a useful complement to measurements on vertical photovoltaic structures and provide a more complete and detailed picture of organic BHJ materials.

Characterization of charge transport via in situ potentiometry in bulk heterojunction organic photovoltaic materials

The most promising active material for organic photovoltaic (OPV) cells is the polymer/fullerene bulk heterojunction (BHJ) system, but charge transport and recombination mechanisms in these materials have yet to be completely understood. We report the use of lateral bulk heterojunction devices to perform novel material diagnostics on the BHJ system. Using electron beam lithography, we fabricate devices with up to 24 voltage probes embedded in the channel in order to perform in situ potentiometry. From current vs. voltage measurements performed at a variety of light intensities, we are able to describe the charge transport properties in three distinct regions of a polymer/fullerene BHJ device and determine the dominant recombination mechanism of the OPV material. We note that these are the first such measurements performed on OPV materials. Such measurements will be very useful for materials diagnostics.