Current Collecting Grids for ITO‐Free Solar Cells

Abstract

Indium-tin-oxide (ITO) free polymer solar cells prepared by ink jet printing a composite front electrode comprising silver grid lines and a semitransparent PEDOT:PSS conductor are demonstrated. The effect of grid line density is explored for a large series of devices and a careful modeling study enabling the identification of the most rational grid structure is presented. Both optical and light beam induced current (LBIC) mapping of the devices are used to support the power loss model and to follow the evolution of the performance over time. Current generation is found to be evenly distributed over the active area initially progressing to a larger graduation in areas with different performance. Over time coating defects also become much more apparent in the LBIC images. Results and DiscussionA series of large area (2 cm × 2 cm) ITO-free organic solar cell devices were prepared on glass substrates (Figure 1). The devices contained current collecting grids/high conductivity PEDOT:PSS/P3HT:PCBM/LiF:Al. A schematic illustration of the devices is shown in Figure 1a. Current collecting grids are represented as parallel lines with different spacing (pitch size) ( Figure 1b). The devices with pitch sizes of 20, 10, 6.7, 5, 3.3, 2.5, 2, and 1 mm were prepared. The pitch size is defined as the distance between the centers of two neighboring grid lines. The number of the grid lines in the devices was changed as 1, 2, 3, 4, 6, 8, 10, and 20, respectively. The width of the grid lines was constant for each batch of devices. Thus shadowing losses increase with the number of grid lines. The effect of different pitch for the grid fingers was calculated at Fraunhofer ISE using a one dimensional numerical model developed by Glatthaar et al. [37,38] Each infinitesimal cell element with width dx delivers the current j(V(x))dx. The function j(V) is given by the current-voltage (JV)-curve of a small area device. Starting from x = 0 the current sums up to I(x) (Equation 1). This current leads to a voltage drop in the current collecting electrode corresponding to Ohm´s law (Equation 2), with ρ being the sheet resistance of the electrode(s). As the current output from the infinitesimal cell elements depends on the voltage V(x) at position x, also the current density j(V(x)) depends on the position x. Therefore the two differential Equations 1 and 2 are coupled:with the boundary conditions V (x = 0) = V 0 and I(x = 0) = 0.The area loss, which in a one dimensional model is the length l loss due to coverage with grid fingers or dead area due to series circuitry is accounted for by integrating the current I from 0 to the length of the active area l active but normalizing the current to the length including the lost "area" l active + l loss . The JV curve of the grid cell or module respectively therefore is given by:The model was used to consecutively calculate the ohmic and area losses in the PEDOT layer and metal fingers. The procedure was to first calculate the loss due to the distributed PEDOT resistance and only the shadow loss induced...

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