Session P42: Focus Session: Organic Electronics and Photonics -- Organic Photovoltaic Devices

Increasing Transport Efficiencies of Polymer Based Solar Cells by Electrophoresis

Organic polymer photovoltaic (PV) cells are an active area of Applied Physics research because of four unique characteristics: (1) relatively inexpensive costs, (2) transparent properties, (3) flexibility, and (4) ease of mass production. We are studying the effects of incorporating single-walled carbon nanotubes (SWCNs) into a mixture of poly-(3-hexylthiophene) (P3HT), to test the affects on transport characteristics. The experiment will be segregated into parallel trials, with fixed volume ratios of P3HT:SWCNs to test the effects of (1) random orientation of SWCNs or the control, and (2) an aligned orientation of SWCNs. An electrophoresis-based technique, similar to gel electrophoresis, used to separate DNA fragments of variable masses, is used for partial alignment of the SWCN. Fixed geometry metalized substrates in a four striped copper patternare used for the transport studies and the P3HT:SWCN film's resistivity is monitored in-situ. The oriented films show enhanced conductivity, indicating this plays a major role in the increased efficiencies found in P3HT:SWCN based polymer solar cells.   

Transparent Carbon Nanotube layers as cathodes in OLEDs

Organic Light Emitting diodes (OLEDs) have attracted high interest in recent years due to their potential use in future lighting and display applications. Reported work on OLEDs traditionally utilizes low work function materials as cathodes that are expensive to fabricate because of the high vacuum processing. Transparent carbon nanotube (CNT) sheets have excellent mechanical and electrical properties. We have already shown earlier that multi-wall (MWCNT) as well as single CNT (SWCNT) sheets can be used as effective anodes in bright OLEDs [1,2]. The true advantage of using the CNT sheets lies in flexible devices and new architectures with CNT sheet as layers in tandem devices [3] with parallel connection. In this work, we are investigating the possibility of using SWCNT as cathodes in OLEDs. SWCNT sheets have been reported to show lower work function compared to MWCNT. Our work attempts to demonstrate transparent OLED devices with CNT anodes and cathodes. In the process, OLEDs with CNT cathodes have been fabricated in normal and inverted configurations using inorganic oxides (MoO3,ZnO) as invertion layers.[1] C.D. Williams et al., Appl.Phys.Lett. 93, 1, 2008.[2] A. Kaskela et al. Nano Lett., 10,11, 4349 ,2010.[3] A. Papadimitratos et al. 8th ICEL,2010.   

Nanoscale electro-optical measurements of photovoltaic materials using scanning probe microscopy

The efficiency of photovoltaic devices based on inorganic thin-films or organic polymer blends is often determined by the nanoscale structure and properties of internal and contact interfaces. Measurements of local photo-conductivity, along with other scanning probe based measurements, can link the structural properties to the performance providing the desired feedback for the device optimization. However, the nature of the tip-to-sample contact can be quite different from contact interfaces in devices strongly affecting the injection and collection of charge carriers and complicating the data analysis. Here, we present the characterization of photoconductive channels in a model bulk heterojunction organic solar cell based on a p-type polymer and n-type small molecule. We directly compare the properties of the tip-to-sample interface to the nanocontact interface. We explore the nanoscale photocurrent response on two complementary device architectures using conductive tips suitable for the appropriate charge (i.e., electrons vs. holes) collection. In addition to the measurements at the top surface, we examine the response from the bulk of the film using novel sectioning technique. Our results provide significant insight into the origin of nanoscale variations in photoresponse and nanoscale morphology of such materials.   

Organic and hybrid organic-inorganic photovoltaic cells

The performance and limitations of the world's best organic and dye sensitized solar cells will be presented along with plans to increase the energy conversion efficiency to 15{\%}. Topics of more detailed discussion could include the formation of polymer-fullerene co-crystals and their implications for recombination, the use of energy transfer to improve light harvesting~in~dye sensitized solar cells, solution deposited transparent electrodes or the use of plasmonics to improve light absorption.   

Studying recombination in the bulk heterojunction solar cells using lateral solar cell geometries

Lateral structures are shown to be a very powerful tool to understand transport and recombination phenomena in bulk heterojunction materials and solar cells. Active layers of phase separated P3HT:PCBM were chosen due to their wide use in research devices and potential for commercialization. Studies of current-voltage curves for varying carrier transit lengths have resulted in information about the movement of charge carriers as well as carrier recombination. By examining typical solar cell parameters (open circuit voltage, short circuit current, fill factor, and power conversion efficiency) combined with photocurrent measurements as a function of electrode spacing, carrier density, applied electric field, and temperature under illumination conditions (0.1 -- 100 suns), we have determined how these parameters depend on the carrier concentration, electric field, and temperature. This work provides a clear picture of when bimolecular recombination dominates and also if the recombination is Langevin or non-Langevin.   

Scaling behavior and transport in bulk heterojunction materials

A lateral device geometry has been used to study charge transport in P3HT:C$_{71}$-PCBM bulk heterojunction devices. Analysis of current-voltage curves have previously been used to study charge transport in these materials. We perform ambipolar field effect transistor measurements on these structures to extract carrier mobilities. We are also able to describe the charge transport and recombination properties of these materials. Assymetric electrodes (Al, Au) separated by 100 nm-20$\mu $m enable us to gain considerable insight into transport physics. Photocurrent measruements as a function of channel length, electric field, and illumination intensity (0.1-100 suns) are used to measure the ambipolar mobility-lifetime product and study how this correlates with measured field-effect mobilities at various electric fields. Lateral structures are shown to be a powerful tool to understand transport and the role of carrier mobility on photovoltaic performance.   

Charge Carrier Lifetime in Poly(3-hexylthiophe)/ZnO Nanowire Array Based Photovoltaic Devices

Nanostructured electron donor and acceptor materials have shown potential for greatly improving the efficiency of organic photovoltaic (OPV) devices. Inorganic hybrid OPVs utilizing nanowires, nanorods and nanoparticles have been shown to greatly increase the current through P3HT based devices but have yet to achieve the efficiencies of their corresponding bulk-heterojunction OPVs. Determining the carrier properties and interface structure of these hybrid devices could greatly aid in determining limitation of the device structure on the overall efficiency. Here we report the use of terahertz, micro-Raman and micro-photoluminescence spectroscopy in determining carrier lifetime and optical properties of P3HT/ZnO based OPV devices. We will also discuss the effects of surface functionalization on the available phonon modes, carrier lifetimes and absorption properties.   

Spectral aspects of cavity tuned absorption in organic photovoltaics

In order to increase the power conversion efficiency of organic photovoltaic devices it is necessary to extend absorption to longer wavelengths and to concentrate and capture light in a thin bulk heterojunction (BHJ) layer. In this work, optical transfer matrix formalism is used to model absorption in organic photovoltaic devices as a function of BHJ thickness and incident wavelength in the optical cavity formed by the BHJ layer sandwiched between the aluminum cathode and indium tin oxide (ITO) anode. We have found that absorption can be finely tuned by adjusting the thicknesses of the BHJ and ITO layers within a relatively narrow range. We have also observed distinct spectral effects due to frequency pulling resulting in enhanced long- wavelength absorption. Because the absorption shifts arise purely from optical interference effects, tuning of the absorption spectrum can be achieved by careful cavity design without affecting the open circuit voltage. We have experimentally verified aspects of our modeling and suggest methods to improve device design. Additionally, we consider the effects of BHJ material gradients versus depth on absorption in these devices.   

Probing the Thickness Limits of Organic Solar Cells using Monte Carlo Simulation

Organic solar cell performance has increased dramatically in recent years, but in order to achieve higher efficiency devices, it is imperative to understand the remaining fundamental challenges. One major shortcoming is that thin film devices cannot absorb all of the targeted incoming light due to the limited optical density of the materials used. To overcome this, thicker devices that can maintain the high quantum efficiency and high fill factor, present in thin state-of-the-art devices, must be developed. We have taken advantage of recent advancements in dynamic Monte Carlo (DMC) simulation methods to study the current-voltage (J-V) behavior of organic solar cells with different thicknesses. This method allows all detailed physical mechanisms of the device to be simulated and as a result, the effects of device morphology and a range of material properties can be captured. Studying device behavior as a function of thickness highlights the importance of the competition between light absorption and charge recombination. The effects of carrier mobility and active layer morphology are also considered. Understanding this tradeoff between absorption and recombination will help direct future experimental efforts to design optimal materials and devices.   

Dehydration assisted nanoimprint of PEDOT:PSS nanogratings to improve organic photovoltaics

We demonstrate the fabrication of oly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)nanograting by a dehydration assisted nanoimprint lithographic technique. Dehydration of PEDOT:PSS increases its mechanical strength for high fidelity and fine precision nanoimprinting process, resuling in formation of high quality nanogratings of 60 nmin height, 70 nm in width, and70 nm in spacing. PEDOT:PSS nanograting ar used as hole injection and electron barrier layer in blended poly(3-hexylthiophene-2,5-diyl) (P3HT)[6,6]-penyl-C61-butyric-acid-methyl-ester PCBM) rganic bulk heterojuncton photovoltaic devices (OPV), showing enhancement of photocurrent and increased efficiency in comparison to non-patterned plane PEDOT:PSS film. Improved performance is discussed in terms of increased interface for charge collection and better distribution of internal electric field.   

High-performance inverted polymer solar cells with ITO coated with a thin layer of oxide for electron collection

Solar cells using organic or polymeric materials as the active material have been attracting strong attention due to the low fabrication cost and high mechanical flexibility. The photovoltaic efficiency has been improved to more than 8{\%} under AM1.5 G illumination. However, polymer solar cells are usually not very stable, which severely impedes the practical application. The stability is strongly affected by the electrodes. Both PEDOT:PSS used as the buffer layer on ITO for the hole electron and active metals like Ca for the electron collection are blamed to lower the stability of polymer solar cells. Polymer solar cells with an inverted structure can have much better stability than normal devices because they do not use PEDOT:PSS and active metals. One big challenge in building the inverted polymer solar cells is to lower the work function of ITO for effective electron collection. Here, we report a new method to effectively lower the work function of ITO by depositing a thin layer of oxide and demonstrate high-performance polymer solar cells. The photovoltaic efficiency of the inverted polymer photovoltaic cells is even higher than the normal devices. The mechanism for the oxide effect on the work function of ITO will be presented as well.