Recombination in polymer-fullerene bulk heterojunction solar cells

Recombination of photogenerated charge carriers in polymer bulk heterojunction (BHJ) solar cells reduces the short circuit current (Jsc) and the fill factor (FF). Identifying the mechanism of recombination is, therefore, fundamentally important for increasing the power conversion efficiency. Light intensity and temperature dependent current-voltage measurements on polymer BHJ cells made from a variety of different semiconducting polymers and fullerenes show that the recombination kinetics are voltage dependent and evolve from first order recombination at short circuit to bimolecular recombination at open circuit as a result of increasing the voltage-dependent charge carrier density in the cell. The "missing 0.3V" inferred from comparison of the band gaps of the bulk heterojunction materials and the measured open circuit voltage at room temperature results from the temperature dependence of the quasi-Fermi-levels in the polymer and fullerene domains - a conclusion based upon the fundamental statistics of Fermions.Comment: Accepted for publication in Physical Review B. http://prb.aps.org/accepted/B/6b07cO3aHe71bd1b149e1425e58bf2868cda2384d?ajax=1&height=500&width=50

Well-defined donor-acceptor rod-coil diblock copolymers based on P3HT containing C-60: the morphology and role as a surfactant in bulk-heterojunction solar cells

The synthesis of well-defined rod-coil block copolymers consisting of P3HT donor and C-60 acceptor chromophores (P3HT-b-P(S(x)A(y))-C-60) in a molecular architecture is reported for use in bulk-heterojunction (BHJ) solar cells. In thin films of the resulting block copolymer, reproducible self-assembly into well-defined "nanofibrils'' is observed. This is the first example of a block copolymer containing a C-60 derivative that shows exclusively a nanofibrilar structure. We have investigated the potential utility of the block copolymer as a "surfactant'' for controlling the interface morphology of the P3HT: PCBM donor-acceptor phase domains within the composite. We find a substantial improvement in device performance when 5% of block copolymer are introduced to the P3HT: PCBM blend system, resulting in ca. 35% improved efficiency relative to the P3HT: PCBM solar cell fabricated without the "surfactant''.close13212

Functional Interfaces in Polymer-Based Bulk Heterojunction Solar Cells: Establishment of a Cluster for Interdisciplinary Research and Training

Remarkable scientific progress has been demonstrated toward the creation of a low cost (“printable”) solar cell technology by the interdisciplinary group at UC Santa Barbara. Multi-layer architectures were implemented with clean interfaces were demonstrated; the various interfaces are sharp; there is no evidence of inter-layer mixing. This is indeed remarkable since each of these layers was processed from solution. The use of “Processing Additives” such as the alkanedithiols was demonstrated to increase the power conversion efficiency of BHJ solar cells by a factor of two. Equally important, the mechanism by which these Processing Additives function has been identified

Ambipolar organic field-effect transistors fabricated using a composite of semiconducting polymer and soluble fullerene

Organic field-effect transistors (FETs) with equivalent hole and electron mobilities have been demonstrated. The devices were fabricated using a phase separated mixture of regioregular poly(3-hexylthiophene) and [6,6]-phenyl C-61-butyric acid methyl ester as the active layer and using aluminum (Al) for the source and drain electrodes. Measurements of the source-drain current versus gate voltage gave an electron mobility of mu(e)=2.0x10(-3) cm(2)/V s and hole mobility of mu(h)=1.7x10(-3)cm(2)/V s. The ambipolar FET properties arise from the use of Al electrodes for the source and drain; the contacts between the Al electrodes and the active layer are improved by thermal annealing at elevated temperatures (150 degrees C), thereby enabling balanced injection for both holes and electrons in a single device.open413

Effect of Molecular Crowding on the Response of an Electrochemical DNA Sensor

E-DNA sensors, the electrochemical equivalent of molecular beacons, appear to be a promising means of detecting oligonucleotides. E-DNA sensors are comprised of a redox-modified (here, methylene blue or ferrocene) DNA stemloop covalently attached to an interrogating electrode. Because E-DNA signaling arises due to binding-induced changes in the conformation of the stem-loop probe, it is likely sensitive to the nature of the molecular packing on the electrode surface. Here we detail the effects of probe density, target length, and other aspects of molecular crowding on the signaling properties, specificity, and response time of a model E-DNA sensor.Wefind that the highest signal suppression is obtained at the highest probe densities investigated, and that greater suppression is observed with longer and bulkier targets. In contrast, sensor equilibration time slows monotonically with increasing probe density, and the specificity of hybridization is not significantly affected. In addition to providing insight into the optimization of electrochemical DNA sensors, these results suggest that E-DNA signaling arises due to hybridization-linked changes in the rate, and thus efficiency, with which the redox moiety collides with the electrode and transfers electrons

Photovoltaic effects on the organic ambipolar field-effect transistors

An organic multifunctional device, which can function as an ambipolar field-effect transistor (FET) and a photovoltaic (PV) cell, has been demonstrated using a phase separated mixture of poly(3-hexylthiophene) and [6,6]-phenyl C-61-butyric acid methyl ester. The gold (Au) electrode used for hole injection in the FET mode (source) acts as the anode in PV cell mode, and the aluminum (Al) electrode for electron injection in the FET mode (drain) acts as the cathode in PV cell mode. The device exhibits clear PV phenomena under illumination at zero gate bias with a power conversion efficiency of 0.6% as well as the properties of an ambipolar FET when the gate bias is applied.open292

Improved electron injection in polymer light-emitting diodes using anionic conjugated polyelectrolyte

We report improved performance in polymer light-emitting diodes incorporating conjugated polyelectrolytes as an electron injection layer (EIL). When we introduce water soluble conjugated polymers, poly[9,9'-bis(4-sulfonatobutyl)fluorene-co-alt-1,4-phenylene] (anionic PFP), between the aluminum (Al) cathode and emissive layer, the devices show an increased electroluminescence efficiency with a lowered turn-on voltage. We believe the mobile Na(+) ions in the EIL layer directly influences the device efficiency by forming a low work function layer at the interface between the EIL and Al cathode, thereby facilitating the electron injection into the emissive layer.open141

Multilayer bipolar field-effect transistors

Field-effect transistors comprising a layer of regioregular poly(3-hexylthiophene) (rr-P3HT) separated from a parallel layer of the soluble fullerene,[6,6]-phenyl C-61-butyric acid methyl ester (PCBM) by a layer of titanium suboxide (TiOx), are fabricated by solution processing. Because the TiOx is an electron transporting material and a hole blocking material, this multilayer architecture operates either in the p-channel mode with holes in the rr-P3HT layer or in the n-channel mode with electrons in the PCBM layer.open201

Carrier generation and transport in bulk heterojunction films processed with 1,8-octanedithiol as a processing additive

Improved performance of polymer-based solar cells based on poly[2,6-(4,4-bis-(2-ethylhexyl)- 4H -cyclopenta[2,1-b;3,4- b???] dithiophene)-alt-4,7-(2,1,3-benzo-thiadiazole)] PCPDTBT has been obtained by using 1,8-octanedithiol (ODT) as a processing additive in the polymer solution used to spin cast the bulk heterojunction films. Although ultrafast spectroscopy studies indicate that the carrier losses are reduced in the films processed with ODT [similar to the reduced carrier losses after thermal annealing of bulk heterojunction (BHJ) materials made from P3HT:PCBM], the magnitude of the reduction is not sufficient to explain the observed factor of 2 increase in the power conversion efficiency. From the analysis of carrier transport in field effect transistors, we find increased electron mobility in the PCPDTBT: PC70 BM composites when fabricated with ODT, which is indicative of enhanced connectivity of PC70 BM networks. The improved electron mobility appears to be the primary origin of the improved power conversion efficiency in BHJ films.open453