Christoph Waldauf

Recombination and loss analysis in polythiophene based bulk heterojunction photodetectors

The monochromatic external quantum efficiency of a bulk heterojunction photodetector based on a blend of poly-3(hexylthiophene) with a methanofullerene is reported to be as high as 76% at the peak maximum at 25 °C. Analysis of the temperature dependence, the illumination intensity dependence together with absorption measurements in reflection geometry, allow calculation of the internal quantum efficiency of the device close to 100% at the peak maximum. Recombination of photoinduced carriers is negligible or even absent in these photodetectors when operated in the photovoltaic mode. Optical losses in these bulk heterojunction devices are analyzed.

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%.

Simulation of light intensity dependent current characteristics of polymer solar cells

An extended replacement circuit describing the current–voltage characteristics of bulk heterojunction polymer solar cells at different light bias levels is introduced and discussed. A one diode-model is expanded by an extraction model for photogenerated carriers taking into account the effective reduction of the mean distance which the charge carriers cover when sweeping the electrical bias through the fourth quadrant of the solar cell. The model properly describes the current–voltage behavior of bulk heterojunction solar cells over more than three orders in light intensity with one set of parameters.

Physics of organic bulk heterojunction devices for photovoltaic applications

We present investigations of organic photovoltaic devices consisting of bulk heterojunction layers made from several material combinations. All of the investigated systems reveal close similarities to the behavior of classical pn-junction devices. The consequences of the pn-junction-like behavior on the device parameters and performance are presented. Furthermore, device characteristics and parameters of the pristine materials are correlated, resulting in a model that permits an identification of high potential materials, a performance prediction, and a device optimization. The resulting model is able to predict an open circuit voltage and a fill factor and their evolution with the light intensity or thickness of the active layer. It simplifies the identification of the internal morphology and therefore the choice of appropriate solvents. Necessary parameters concerning the choice of electrode materials are also provided.

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

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.

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

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.

Impedance Spectroscopy on Polymer-Fullerene Solar Cells

Impedance spectroscopy is used for studying the electrical transport properties of bulk heterojunction solar cells. A replacement circuit is needed to translate the frequency response of the circuit to the individual interfaces and layers of the solar cell. As a material combination and device architecture, composites of P3HT and PCBM, sandwiched between a transparent ITO front electrode and an aluminum back electrode, as well as a polymer buffer layer were investigated. By varying the film thickness we identified an equivalent circuit capable to fit our experimental data. We found a dielectric constant for the P3HT and for the P3HT:PCBM bulk.

Performance of bulk-heterojunction organic photodetectors

The performance of organic photodetectors has already reached a high level. In the near future organic photodetectors can be found in applications like imaging and sensing. The core of organic photodetectors is the bulk-heterojunction. Fullerenes and conjugated polymers forming islands in nanometer size are causing the outstanding quantum efficiencies. The conjugated polymers have a high absorption coefficient and the fullerenes are efficiently transferred electrons from the polymer. This leads to a high sensitivity to light. Organic photodetectors with low dark currents, external quantum efficiencies of 80% in the visible, linearity over several decades and fast transient behavior are demonstrated.

Conduction mechanisms of P3HT: PCBM solar cell

In order to get a deeper understanding of the conduction mechanisms limiting the electrical characteristics of ITO/PEDOT:PSS/P3HT:PCBM/Al solar cells, dark current-voltage measurements at different temperatures were analyzed using a compact electrical equivalent circuit previously used in p/n junctions. Between 0.2 V and 0.6 V, the current-voltage characteristic is modeled by an exponential term which can be described by Multi-Tunneling Capture Emission process. For larger voltage, the model takes into account Space-Charge Limited process and series resistance. In addition, the model is useful to calculate the built in potential of the solar cell using only dark current-voltage-temperature measurements.

Toward highly efficient photogeneration and loss-free charge transport in polymer-fullerene bulk heterojunction solar cells

Different material combinations of two conjugated polymers, each blended with the methanofullerene acceptor phenyl-C61 butyric acid methyl ester (PCBM) have been evaluated focusing on their potential for application as absorber material in polymer-fullerene bulk-heterojunction solar cells. Devices based on these solution processable composite materials have been studied by means of temperature dependent profiling of the photocurrent. In combination with measurements of the incident photon conversion efficiency, this technique probes the charge carrier recombination losses within the absorber material. Samples based on material composites with a low mobility-lifetime (μτ) product of the charge carriers (OC1C10-PPV: PCBM) exhibit a thermally activated photocurrent throughout the temperature range from 100 K to 350 K. The latter issue is attributed to the presence of shallow traps inside the bulk of the absorber limiting the photocurrent by recombination and scattering of the charge carriers with defects. Accordingly, the active layer thickness must be kept low at the expense of optical absorption. In contrast, the photocurrent in devices based on absorber materials with a high μτ product, P3HT: PCBM, saturates at a certain temperature and becomes constant, reflecting that all photogenerated charge carriers are efficiently extracted within their lifetime prior to recombination. Thus, solar cells with absorber materials demonstrating a high μτ product, have the potential to be designed with relatively thick absorber films above 100 nm. A large active layer thickness is a prerequisite for industrial deposition techniques, e.g., screen-printing, and improves the mechanical stability of large area flexible solar cells. As consequence of a high μt product the increase of the active layer thickness to L=350 nm in P3HT: PCBM photovoltaic devices results in a higher density of photogenerated charge carriers due to improved light absorption. Consequently, a strongly increased short-circuit current density of up to 15.2 mA/cm2 was obtained for devices with absorber thickness of 350 nm which rising the power conversion efficiency up to 3.1 %.