The production process of organic solar cells (OSCs) is investigated and the effects of parameter variations
on experimental results are analysed with the Principal Component Analysis (PCA). This statistical method
is applied to an exemplar data set, in which the materials' concentration in the absorber solution and the
spincoating speed of the absorber solution were varied intentionally. In addition to the remaining production
parameters, the time intervals between the steps were included in the analysis. A large part of the variance
in the experimental results can be explained with the evaporation conditions, the spincoating speed and the
concentrations in the absorber solution. The PCA also confirms that the OSC is a complex and interdependent
system, where one has to analyse the influence of several parameters at the same time in order to understand
their effects on the OSC properties. The PCA results will be used to focus further experiments on the identified
One key problem in optimizing organic solar cells is to maximize the absorption of incident light and to keep the charge carrier transport paths as short as possible in order to minimize transport losses. The large versatility of organic semiconductors and compositions requires specific optimization of each system. We investigate two model systems, the MDMO-PPV:PCBM blend and the P3HT:PCBM blend. Due to the small thickness of the functional layers in the order of several ten nanometers, coherent optics has to be considered and therefore interference effects play a dominant role. The influence of the thickness of the photoactive layer on the light absorption is investigated and compared with experimental data. The potential of an optical spacer which is introduced between the aluminium electrode and the photoactive layer to enhance the light harvesting is evaluated by optical modelling. Optical modelling becomes more complex for novel solar cell architectures based on nanostructured substrates. Exemplary optical simulations are presented for a nanoelectrode solar cell architecture.
In this paper we present detailed optical simulations of organic bulk-heterojunction 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.
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 pdoped 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.
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 20 <i>μm </i>in combination with a 2 <i>μm </i>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 <i>nm</i>. 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.