Eric Danielson

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.

Photoconductivity and Photoconductive Gain in Organic Bulk Heterojunction Materials

A comprehensive study on photoconductive gain in organic bulk heterojunction (OBHJ) materials is reported. High photoconductive gains in OBHJs occur in operating regions where the device characteristics are dominated by photomodulated contact injection effects. The photomodulated injection current contributes to the total device current by augmenting the photocurrent resulting from photoabsorption in the device. An experimentally confirmed simulation of bulk photoconduction in lateral P3HT:PC71BM bulk heterojunction blend devices is presented, and it clarifies the origins and operating regimes in these devices. High gains occur at lower light intensities, where the total device current is dominated by photomodulated contact injection rather than bulk photoconduction effects. At higher light intensities, the total device current is dominated by bulk photoconduction effects, where the photocurrent results from light absorption in the bulk material. In agreement with previous research, gains also increase with higher applied biases. The inclusion of field-dependent mobilities plays a critical role in accurately modeling this relationship between applied voltage bias and gain.

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.

Unusual charge transport and reduced bimolecular recombination in PDTSiTzTz:PC71BM bulk heterojunction blend

Solar cells with bulk heterojunction active layers containing donor-acceptor copolymer PDTSiTzTz exhibit persistent high fill factors with thicknesses up to 400 nm. Transport and recombination in a blend of PDTSiTzTz and fullerene derivative PC71BM is studied using lateral organic photovoltaic structures. This material system is characterized by carrier-concentration-dependent charge carrier mobilities, a strongly reduced bimolecular recombination factor, and a negative Poole–Frenkel coefficient. The analysis provides an explanation for the relatively thickness-independent fill factor behaviour seen in solar cells using the copolymer PDTSiTzTz. Cumulative insights from this copolymer can be employed for future organic photovoltaic material development, study of existing high performance bulk heterojunciton blends, and improved solar cell design.

Use of lateral structures to monitor and evaluate degradation of key photovoltaic parameters in an organic bulk heterojunction material

Charge transport and recombination mechanisms within organic bulk heterojunction (BHJ) systems have been studied using lateral devices to perform in situ potentiometry. We have developed a simplified measurement technique using two types of lateral structures to elicit key charge transport parameters and study the time and process dependence of the carrier mobilities and their ratio. Small geometry lateral devices are used to evaluate the mobility of the slower carrier within the P3HT:PCBM material system. Larger structures with 5 in situ voltage probes are used to construct a simple potential profile of the device channel and accurately determine the carrier mobility ratio. These two measurements enable the calculation of carrier densities and the recombination coefficient. We monitor the change in these parameters as the P3HT:PCBM film degrades in the presence of oxygen and also examine the effect of the solvent additive 1,8-diiodooctane on this degradation mechanism. By exposing ethanol vapor to the BHJ film, we induce traps in the material and monitor the shift in dominant nongeminate recombination mechanism to a more unimolecular type. We are also able to measure the resulting decrease in carrier mobilities due to the presence of dipole-induced traps. Lateral devices are useful material diagnostic structures for studying degradation in 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.

Inkjet printing of carrier transport layers for inverted organic solar cells

Inverted organic solar cells, which utilize a transparent cathode and a high work function metal anode, have been the subject of extensive research. Their advantages over conventional organic solar cells include increased resistance to environmental degradation and compatibility with large area fabrication techniques. Carrier transport layers are essential for achieving high power conversion efficiencies in inverted organic solar cells and therefore need to be compatible with these large area fabrication techniques. Inkjet printing is one such technique that can be integrated into the low cost mass production of these cells via roll to roll fabrication. N-type metal oxides such as ZnO or zinc tin oxide (ZTO) have been previously used as electron transport layers for inverted cells, but only as spin coated films. We have developed inkjet printable ZTO solutions for use as electron transport layers in inverted organic solar cells, and achieve power conversion efficiencies of over 3% in inverted P3HT:PC71BM solar cells. We also discuss the effect of printing parameters on the electrical performance of these layers in inverted organic solar cells.

Effect of film nanostructure on in-plane charge transport in organic bulk heterojunction materials

Bulk heterojunction (BHJ) organic solar cells are a promising alternative energy technology, but a thorough understanding of charge transport behavior in BHJ materials is necessary in order to design devices with high power conversion efficiencies. Parameters such as carrier mobilities, carrier concentrations, and the recombination coefficient have traditionally been successfully measured using vertical structures similar to organic photovoltaic (OPV) cells. We have developed a lateral BHJ device which complements these vertical techniques by allowing spatially resolved measurement along the transport direction of charge carriers. This is essential for evaluating the effect of nanoscale structure and morphology on these important charge transport parameters. Nanomorphology in organic BHJ films has been controlled using a variety of methods, but the effect of these procedures has been infrequently correlated with the charge transport parameter of the BHJ material. Electron beam lithography has been used to create lateral device structures with many voltage probes at a sub-micron resolution throughout the device channel. By performing in-situ potentiometry, we can calculate both carrier mobilities and determine the effect of solvent choice and annealing procedure on the charge transport in BHJ system. Spin coated P3HT:PCBM films prepared from solutions in chloroform and o-xylene are characterized using this technique.