Pavel Schilinsky

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%.

Interface modification for highly efficient organic photovoltaics

We present highly efficient inverted polymer:fullerene bulk-heterojunction solar cells by incorporation of a nanoscale organic interfacial layer between the indium tin oxide (ITO) and the metal oxide electron-conducting layer. We demonstrate that stacking of solution-processed organic and metal oxide interfacial layers gives highly charged selective low ohmic cathodes. The incorporation of a polyoxyethylene tridecyl ether interfacial layer between ITO and solution-processed titanium oxide (TiOx) raised the power conversion efficiency of inverted organic photovoltaics to 3.6%, an improvement of around 15% in their performance over comparable devices without the organic interfacial layer.

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.

The impact of water vapor transmission rate on the lifetime of flexible polymer solar cells

In this paper we perform accelerated lifetime testing on high efficiency flexible poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) solar cells encapsulated with food package quality barrier films with a water vapor transmission rate of 0.2 g/(m2 day) at 65 °C/85% relative humidity. We show that lifetimes exceeding 1250 h, even at high temperature/high humidity conditions, may be reached, proving that organic solar cells are significantly less sensitive against the environmental effects of water and oxygen than previously expected.

On the effect of poly(3-hexylthiophene) regioregularity on inkjet printed organic solar cells

We investigate the impact of P3HT regioregularity on the performance of bulk heterojunction solar cells with inkjet printed P3HT:PCBM layers. Three polythiophenes with different regioregularities ranging from 93% up to 98% are inkjet printed from two different formulations at room temperature. It is found that the high RR-P3HT (98%) is not suitable for inkjet printing at room temperature. The fast formation of aggregates shortens the shelf life of the ink and thus, results in low reliability of the printing process, in the formation of inhomogeneous and very rough films with surface roughnesses up to 70 nm, and in a strongly reduced device performance. This phenomenon is not observed for solar cells processed via the doctor blading technique. Nevertheless, inkjet printing of 96% RR-P3HT:PCBM oDCB/mesitylene solutions at room temperature resulted in solar cells with 3.5% efficiency, while doctor blading of 98% RR-P3HT:PCBM oDCB/mesitylene solutions resulted in efficiencies as high as 4.4%.

Formation and impact of hot spots on the performance of organic photovoltaic cells

The failure mechanisms of organic solar cells under reverse bias conditions were investigated. Localized inhomogenities, so-called “hot spots,” leading to increased leakage currents under reverse bias, were identified as the dominant origin for failure. The intensity of hot spots does increase with the duration under reverse bias voltage. Cells with a higher leakage current density (i.e., >100 μA/cm−2 at −1 V) have a significant higher probability for dominant failure, while devices with low leakage current densities show less degradation under reverse bias stressing.

Solution-Processed Organic Photovoltaics

The technology of organic solar cells has matured to an extent that commercialization of first products has already started. However, with the first products pushing into the market, the research community realizes that a qualified product requires more than only high efficiency and good stability. Cost is of course as important as efficiency and lifetime, but to achieve high productivity, multiple technologic challenges have still to be solved. To reduce production costs, printing of functional layers from solution has evolved to a promising manufacturing technology for flexible organic electronics. Current processing of organic photovoltaic devices is mainly based on traditional methods like spin coating or doctor blading. However, these techniques have several disadvantages such as the incompatibility with a roll-to-roll setup and the processing of only small areas at laboratory scale. Enormous benefits in the manufacturing of organic photovoltaics are achieved by using low-cost roll-to-roll capable technologies including screen printing, spray coating, inkjet printing, gravure/flexographic printing and curtain/slot die coating. This review will shed some light on the role and importance of production technologies for organic photovoltaics and give an update on the most recent achievements in the field.

Barix multilayer barrier technology for organic solar cells

The effect of multilayer barrier materials on the lifetime of organic photovoltaic cells has been investigated. For thin film encapsulated cells a protective layer was used to prevent damage during barrier layer deposition. No post deposition effects developed after dry box storage. In accelerated temperature and humidity lifetime testing the degradation of the encapsulated cells can be related to the loss of effective cell area. An extrapolation of the lifetime at room conditions has been quantitatively determined by comparing the cell degradation with the loss of Ca in a Ca-oxidation test. The results indicate a barrier permeation rate of 10-4 gr/[m2* day] for these samples, corresponding to a lifetime of greater than 5000 hours. Routes to improvement of the OPV cell lifetime are discussed.