10.07.2015   SonSolar

Micro prisms — Grid dimensioning

The following criteria can be formulated for a suitable hole contact realized by the metal — grid supported organic p-conductor. The distance between the grooves of the microstruc­ture where the microgrid is located has to be sufficiently small to minimize the series re­sistance. In contrast, the ratio between the lattice distance of the grid and the grid width of the conducting lines should be as large as possible in order to minimize the shadowing effect. The metal grid lines have to be sufficiently conductive to minimize the series re­sistance. The optimum lattice distance for a given type of structure is calculated on the basis of the following assumptions: A width of 10^m for the gold grid conducting lines was taken which could be realised in experiments (figure 5). The contribution of the alu­minum top electrode and the gold grid to the series resistance is neglected. A conser­vative value of 105Qa was taken for the PEDOT CPP105d-sheet resistance. The current voltage characteristics of a infinitesimal small elementary cell with the following charac­teristics: Voc = 600mV, Isc = 15mA/cm2 and FF=0.55 was taken as input parameters.

As a consequence, the series resistance of the PEDOT-layer is excludet. The following non-linear differential equation (1) was solved under consideration of the listed boundary conditions (2) and (3) [8].

2 = Poj(U(x))

(1)

(2)

Boundary conditions:

w

w

U(x = 2 ) = Ub (3)

where po is the sheet resistance of the p-conducting layer and j(U) represents the current

voltage characteristics of an elementary cell. distance from the grid line boundary and the Ub is the voltage at x = w.

The device efficiency is calculated as a function of the grid distance shown in fig­ure 4. An optimum grid distance of 130^m was calculated which leads to an efficiency of 4.9%. In the case of a prism-structure with a groove angle of 90° a structure period of 92^m is calculated which is close to the experimental choosen pitch of 100^m. The strong dependence of the grid distance on the device efficiency of the cells strength­ens the necessity of simulations for proper dimensioning. A sheet resistance along the grid lines of 3,17 Qo was calculated for a grid of 100nm thick and 10 ^m wide gold conducting lines with a lattice distance of 130 ^m. This is a better value than the sheet resistance of commercially available ITO (po = 13 Qo).

The geometry parameters x and w are the distance between two grid lines respectively.

Figure 4: Calculation of the optimum grid distance

i( 2 ) = 0

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