© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1338 COMMUNICATION wileyonlinelibrary.com www.MaterialsViews.com www.advenergymat.de Hui Jin, Almantas Pivrikas, Kwan H. Lee, Muhsen Aljada, Mike Hambsch, Paul L. Burn,* and Paul Meredith* Factors Influencing the Efficiency of Current Collection in Large Area, Monolithic Organic Solar Cells Dr. H. Jin, Dr. A. Pivrikas, Dr. K. H. Lee, Dr. M. Aljada, M. Hambsch, Prof. P. L. Burn, Prof. P. Meredith Centre for Organic Photonics & Electronics University of Queensland Brisbane, QLD 4072, Australia E-mail: p.burn2@uq.edu.au; meredith@physics.uq.edu.au DOI: 10.1002/aenm.201200254 Doped metal oxides have played a pivotal role in modern elec- tronics and optoelectronics–particularly as transparent, con- ducting electrodes (TCEs). In this regard, indium tin oxide (ITO) is commonly employed owing to its high optical transparency in the visible and acceptable sheet resistance (10–15 ohm sq -1 ). Organic solar cells (OSCs) in particular have relied upon ITO as the TCE of choice. This reliance has unfortunately hindered the scaling of OSCs to sub-module and module levels for multiple reasons: firstly cost, but it also appears that the sheet resist- ance of ITO prohibits effective current collection over pathways (the shortest linear distance to an electrode) of more than a few cms. [1–4] There have been previous attempts to understand these performance-limiting phenomena in small-scale ( < 1cm 2 ) devices. For example, Pandey et al. [5] simulated the power losses by using a one-dimensional current model, which they attributed to series resistance dissipation. Manor et al. [6] using a diode-model approach based upon standard Shockely theory reported the underlying physical mechanisms that reduce cur- rent and fill factor, including the voltage dependence of photo- current and the influence of dark current. However, to date, no systematic understanding of power losses has been presented for large devices at the sub-module scale where cell architec- ture, electrode geometry and current collection pathways are quite different to laboratory-scale cells. Although efficien- cies of > 8.3% or even > 10% have recently been achieved in small cells, these competitive performance figures are yet to be translated to large active areas–a fact in part due to this lack of knowledge. [7,8] For many doped metal oxides (including ITO) an inherent trade-off exists between doping (to achieve low resist- ance) and transparency. [9,10] This of course leads to an obvious problem for solar cells: the number of photons transmitted into the active junction through the TCE must be optimised, and yet in a device dominated by series resistance the maximum photocurrent needs to be extracted. Although several groups have studied these effects in combination with electrode geom- etry for serially connected strip architectures, there has been relatively little focus on monolithic devices with collection path- ways greater than a few cms. [11–13] Krebs et al. reported ITO-free monolithic solar cells with an area > 100 cm 2 by using silver grids as the transparent front electrode, and a large geometric fill factor of 74% was achieved. [14,15] From a manufacturing perspective, the monolithic architecture is undoubtedly prefer- rable since it can lead to good utilisation of the substrate and simplified processing. Therefore, irrespective of whether ITO becomes the dominant commercial TCE in organic solar cells, we must develop a more in depth understanding of how photo- current and photovoltage extraction can be optimised over large active areas at the sub-module and module levels. In this work, the device physics of 5 cm × 5 cm mono- lithic organic solar cells based on a blend of the model donor polymer poly(3- n-hexylthiophene) (P3HT) and the electron acceptor [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 60 BM) is investigated with particular emphasis on collection efficiency as a function of electrode geometry. By partially illuminating the cells at different positions via a narrow illumination beam, pho- tovoltaic parameters have been mapped. We show clearly that the short circuit current distribution is determined as expected by the effective series resistance (dominated by the ITO sheet resistance) while the open-circuit voltage is almost independent of the illumination position. Certain contact geometries are very susceptible to these effects. We confirm these findings using extraction current modeling and also use aperture illumination to determine a critical device area, which when exceeded causes the ITO sheet resistance to become debilitating. A series of large area monolithic (i.e., not serially connected strips) organic solar cells were prepared on ITO-coated glass sub- strates with the structure of ITO/PEDOT:PSS/P3HT:PC 60 BM/ Al. Three electrode geometries were implemented to achieve centrally symmetric and diagonally symmetric collection archi- tectures with device active areas of 5 cm × 5 cm. Schematic illustrations of the devices with the three electrode geometries ( G O , G M and G L ) are shown in Figure 1a –c, respectively. The top light grey parts represent metal aluminum (Al) cathodes, and the side Al bars are connected with the bottom ITO as anode contacts. The round and square symbols represent the contact positions for positive and negative probes, respectively. As such, all the round symbol positions were at equal positive potential and all the square symbol positions at equal negative potential. The contact area of the probe head was approximately 1.0 mm 2 . The collection circuit around the monolithic device does compromise the geometric fill factor, but in practise it would be integrated into the module architecture as is the case with inorganic solar cells. Due to the relatively high sheet resistance of ITO, it has been reported that power loss from the ITO becomes the only area- scaling parameter. [1] Since the resistance of Al is negligible, the Adv. Energy Mater. 2012, 2, 1338–1342