Advanced Functional Materials and Devices (AFMD) Group

Abstract:

In this paper we present detailed optical simulations of organic bulk-heteroj unction solar cells built with inverted layer sequence as compared to the commonly used setup which is based on indium tin oxide (ITO) covered glass or plastic substrates and where the metal electrode is evaporated on top of the active absorber blend. The inverted setup may have production related advantages over the conventional setup, as the metal electrode is first evaporated onto the substrate and afterwards only wet chemical processes are needed. Additionally ITO can be replaced with a suited module concept. The effects of light trapping with an optical spacer, namely a transparent conductive layer between the absorber and the metallic electrode are investigated for the inverted setup. The results show that the insertion of an optical spacer does not increase the maximal obtainable short circuit current density and is only beneficial if a decrease of film thickness of the active absorber results in a higher internal quantum efficiency, open circuit voltage or fill factor. In the experimental section we show that the inversion of the layer sequence can be realised without any loss in device efficiency as compared to devices with the conventional layer sequence.

Abstract:

We investigated organic bulk-heterojunction solar cells based on an absorber blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) by electrical impedance spectroscopy (EIS). A strong neck in the modulus plot of the EIS-spectra indicates that the absorber is divided into two regions of different conductivities. A similar behaviour was observed for pure P3HT-diodes. Hence, it can be concluded that the PCBM:P3HT absorber is p-doped by impurities of P3HT, so that a Schottky-like contact with aluminium is formed. It is known from literature, that annealing of PCBM:P3HT solar cells leads to drastic improvement of the photovoltaic performance. We compared the current-voltage characteristics and impedance spectra before and after consecutive annealing steps. After the annealing an expansion of the depletion region was observed, indicating that volatile dopants were evaporated out of the absorber. This contributes to an improved photovoltaic performance as the separation of the generated charges in the depletion region is more efficient than in the non-depleted region. Also an improved rectification behaviour might be caused by a lower doping level.

Abstract:

The working principle of organic bulk-heterojunction solar cells is a widely discussed topic. For thin film solar cells it is commonly supposed that the built-in potential Vbi is the driving force for charge separation and determines the open-circuit voltage Voc. In former works, V bi was estimated by measuring Voc in the saturation regime. To check the validity of this model, the direct measurement of the built-in potential is desirable. We have investigated the origin of the open-circuit voltage of organic bulk-heterojunction solar cells by means of electroabsorption spectroscopy. This technique allows measurement of the built-in potential directly and therefore permits an independent measurement of Vbi and Voc. In our experiments on indium tin oxide/poly(3,4-ethylendioxythiophene) : poly(styrene-sulfonate)/poly(2 -methoxy-5-(3′,7′-dimethyloktyloxy)-p-phenylene-vinylene) : 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61/metal bulk-heterojunction solar cells no significant correlation between the open-circuit voltage and the built-in potential was observed. For certain electrode materials, i.e. gold and copper, Voc exceeded V bi which is revealing of semipermeable membranes. © 2005 Elsevier B.V. All rights reserved.

Abstract:

Along with efficiency and lifetime, costs are one of the most important aspects for the commercialization of organic solar cells. Thinking of large scale production of organic solar cells by an efficient reel-to-reel process, the materials are expected to determine the costs of the final product. Our approach is to develop functional substrates for organic solar cells which have the potential for cost effective production. The functionality is obtained by combining periodically microstructured substrates with lamellar electrode structures. Such structured substrates were fabricated by cost effective replication from masterstructures that were generated by large area interference lithography. Two cell architectures were investigated - holographic microprisms and interdigital buried nanoelectrodes. A structure period of 20μm in combination with a 2μm wide metal grid was chosen for the microprism cells based on the results of electrical calculations. Current-voltage curves with reasonable fill factors were measured for these devices. A significant light trapping effect was predicted from optical simulations. Interdigital buried nanoelectrodes are embedded in the photoactive layer of the solar cell. Separated interdigital metal electrodes with a sufficiently high parallel resistance were manufactured despite a small electrode distance below 400 nm. Experimental results on first photovoltaic devices will be presented. We observe an insufficient rectification of the photovoltaic device which we attribute to partial electron injection into the gold anode.

Abstract:

We report on a concept for organic solar cells where the layer sequence is inverted compared to the conventional setup. In such a configuration a conducting polymer layer is used as the transparent anode which is able to transport the photocurrent laterally to a metal grid. For the anode a low sheet resistance and a work function matching approximately the chemical potential of the holes of the illuminated photoactive layer is required. We showed that the poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) fits to these requirements. In our setup an aluminium cathode was used. It turned out that for the inverted setup the interface between aluminium and the photoactive layer has to be protected against oxidation. Our investigations show that with a thin layer (20 nm) of electron-beam deposited titanium between aluminium and the photoactive layer the electrical contact is drastically improved. A solar cell efficiency of (1.4 ± 0.3)% was reached in this case. © 2005 Elsevier B.V. All rights reserved.