Patrick Denk

Effect of LiF/metal electrodes on the performance of plastic solar cells

The insertion of thin interlayers of LiF under the negative metal electrode (Al and Au) of bulk heterojunction solar cells significantly enhances the fill factor and stabilizes high open circuit voltages. Compared to devices without the LiF interfacial layer, the white light efficiencies increase by over 20% up to ηeff∼3.3%. Substitution of the LiF by another insulating interlayer SiOx results in lower overall efficiencies. In the case of a LiF/Au electrode, substantial efficiency enhancement is observed compared to a pristine Au electrode and white light efficiencies up to ηeff∼2.3% are reported.

Highly efficient inverted organic photovoltaics using solution based titanium oxide as electron selective contact

The challenge to reversing the layer sequence of organic photovoltaics (OPVs) is to prepare a selective contact bottom cathode and to achieve a suitable morphology for carrier collection in the inverted structure. The authors report the creation of an efficient electron selective bottom contact based on a solution-processed titanium oxide interfacial layer on the top of indium tin oxide. The use of o-xylene as a solvent creates an efficient carrier collection network with little vertical phase segregation, providing sufficient performance for both regular and inverted solar cells. The authors demonstrate inverted layer sequence OPVs with AM 1.5 calibrated power conversion efficiencies of over 3%.

Influence of the bridging atom on the performance of a low-bandgap bulk heterojunction solar cell.

Bulk heterojunction solar cells have attracted considerable attention over the past several years due to their potential for low-cost photovoltaic technology. The possibility of manufacturing modules via a standard printing/coating method in a roll-to-roll process in combination with the use of low-cost materials will lead to a watt-peak price of less than 1 US

Long-lived photoinduced charge separation for solar cell applications in phthalocyanine-fulleropyrrolidine dyad thin films{

within the next few years. [1] Despite the low-cost potential, the power conversion efficiency of bulk heterojunction devices is low compared to inorganic solar cells. Efficiencies in the range of 5‐6% have been certified at NREL and AIST usually on devices with small active areas. [2] The current understanding of bulk heterojunction solar cells suggests that the maximum efficiency is in the range of 10‐12%. [3] Several reasons for the power conversion efficiency limitation have been identified. [1] Some of the prerequisites for achieving highest efficiencies are donor and acceptor materials with optimized energy levels [highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)], efficient charge transport in the donor‐acceptor blend, efficient charge generation and limited recombination losses. Power conversion efficiency is strongly dependent on charge transport and charge generation, which are dominated by the phase behavior of the donor and acceptor molecules. The resulting, and often unfavorable, nanomorphology of this two-component blend limits the power conversion efficiency of bulk heterojunction solar cells. Precise control of the nanomorphology is very difficult and has been achieved only for a few systems. [4‐6] The relation between the chemical structure of donor and acceptor materials and the nanomorphology that they form when they are blended is currently not well understood, and as will be shown in this paper, minor changes in the chemical structure can cause major changes in the performance of the materials in organic solar cells. In this work we demonstrate the effect of replacing a carbon atom with a silicon atom on the main chain of the conjugated polymer. The approach has been used previously, and promising materials for field-effect transistors and organic solar cells have been demonstrated. [7‐9] We find that making this simple substitution in poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4b 0 ]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) yields a polysilole, e.g., poly[(4,4 0 -bis(2-ethylhexyl)dithieno[3,2b:2 0 ,3 0 -d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5 0 -diyl] (Si-PCPDTBT), with a higher crystallinity, improved charge transport properties, reduced bimolecular recombination, and a reduced formation of charge transfer complexes when blended with a fullerene derivative. This silole-based polymer is found to form a highly functional nanomorphology when blended with [6,6]-phenyl C71-butyric acid methyl ester (C70-PCBM), and solar cells prepared using this blend gave efficiencies of 5.2%, certified by the National Renewable Energy Laboratory. [1] The presented polymer is the first low-bandgap semiconducting polymer to have a certified efficiency of over 5%. The chemical structure of the subject polymer is shown in Figure 1. The material was synthesized following the procedure described previously. [10] The synthesis and properties of the carbon-bridged polymer have been described before. [11,12] Figure 2a shows the absorbance and photoluminescence (PL) spectra of a thin solid film of the pristine Si-bridged polymer and

Design of efficient organic tandem cells: On the interplay between molecular absorption and layer sequence

The photophysical properties of a new dyad molecule composed of a covalently linked Zn-phthalocyanine (antenna/donor) and a C60 derivative (acceptor) have been investigated. We report experimental evidence of long-lived charge separation in the solid state with a lifetime several orders of magnitude higher than in solution. Such a long lifetime, unusual for phthalocyanine–fullerene dyads, is the basis for possible photovoltaic applications. A first demonstration of a working solar cell using phthalocyanine–fullerene dyads as the active material is presented. Though the power conversion efficiency under simulated solar illumination of 80 mW cm−2 is found to be moderate (0.02%), it is an encouraging result for application of C60 dyad molecules to photovoltaics.

Realization, characterization, and optical modeling of inverted bulk-heterojunction organic solar cells

We have carried out detailed optical simulations of tandem solar cells based on the following organic semiconductors: poly(3-hexylthiophene) (P3HT), poly[2,6-(4,4-bis-(2-ethylhexyl)-4H- cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), 1-(3- methoxycarbonyl) propyl-1-phenyl[6,6] C61 (PC60BM), and 1-(3-methoxycarbonyl) propyl-1- phenyl[6,6] C71 (PC70BM). We demonstrate that out of the many possible combinations of the component materials, one specific combination emerges as the best to reduce the spectral overlap of the two bulk heterojunction blends and thereby to ensure an optimized short-circuit current density (Jsc). Furthermore, the calculations allow us to predict the maximum Jsc achievable in tandem cells based on P3HT and PCPDTBT. Finally, we show that the efficient tandem cell realized and described recently by Kim et al. [Science 317, 222 (2007)] ensures balanced absorption in the top and bottoms cells.

Non-Langevin bimolecular recombination in a silole-based polymer:PCBM solar cell measured by time-resolved charge extraction and resistance-dependent time-of-flight techniques

Inverted bulk-heterojunction organic solar cells (OSCs) using solution-processed layers possess significant advantages compared to the usual noninverted devices. To investigate the full potential of this type of OSC, we have carried out some optical modeling by rigorous coupled wave analysis. The influence of the thickness of several different layers in the device has been quantified, as well as the maximum possible number of photons absorbed in the poly(3-hexyltiophene):[6,6]-phenyl-C61-butyric acid methyl ester active layer for both conventional and inverted structures. It appears that the thickness of the hole injecting layer placed in front of the metallic mirror can influence the electromagnetic field distribution in the OSC, but no additional beneficial optical spacer effect is observed. The thickness of the electron injecting layer deposited on the semitransparent electrode also has a negligible influence on the photons absorbed in the active layer for the inverted structure.

Spectroscopic properties of PEDOTEHIITN, a novel soluble low band-gap conjugated polymer

The silole-based non-Langevin conjugated polymer KP115 has been used to demonstrate that circuit resistance is a crucial parameter in time-of-flight measurements of organic photovoltaic cells, providing a resistance-dependent bimolecular recombination coefficient. The origin of this behaviour is the biphasic decay dynamics present in KP115:PCBM devices (observed using a novel time-dependent charge extraction technique), which time-of-flight cannot accurately characterise.

A Fulleropyrrolidine-phthalocyanine dyad for photovoltaic applications

Polymers with narrow band gap are expected to posses appreciably high RT conductivities, luminescence in the NIR and improved solar energy harvesting properties. Here we report the spectroscopic properties of a soluble and environmentally stable copolymer (PEDOTEHIITN) with a band-gap of ca. 1.1 eV. The combination of this outstandingly narrow band gap, processability and stability are promising for future electronic and optoelectronic applications.

Barix multilayer barrier technology for organic solar cells

We report on photophysical properties of a novel dyad molecule having as antenna/donor a Zn-phthalocyanine derivative and as acceptor a C 60 derivative covalently attached. We found evidences for long living photoinduced electron transfer in solid state Photovoltate action of thin film devices of the dyad is demonstrated.