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Photoconductivity: 2D Mapping of Fixed Charges

Fig. 1: Schematic drawing of the BiasMDP measurement setup. The effective carrier lifetime is determined as function of an applied bias voltage.

Highly efficient solar cells require excellent surface passivation. The best passivation for silicon surfaces is currently achieved with dielectric passivation layers, such as Al2O3, SiO2 or SiNx:H. Dielectric layers provide chemical and field-effect passivation. The chemical passivation results from a low density of open bonds at the silicondielectric interface. The field-effect passivation is caused by a high density of intrinsic fixed charges (Qf), which are formed in the dielectric layer. These fixed charges change the carrier statistics at the silicon surface and the asymmetry of electrons and holes very effectively reduces the surface recombination rate.

Fig. 2: Measured carrier lifetime as a function of the applied bias voltage. The position of the minimum lifetime is used to calculate fixed charge and interface trap densities.

For understanding and improving the passivation performance an accurate characterization of fixed charges and interface defects is desired. The common characterization methods are capacitance-voltage (C(V)) and corona charge techniques. However, with increasing importance of dielectric passivation layers in solar cell manufacturing, material homogeneity and process signatures become the focus of attention. A 2D mapping method with high spatial resolution and high throughput is required. However, 2D mapping is difficult to realize with state-of-the-art characterization methods.

Fig. 3: 2D map of fixed charge densities on a 150 mm wafer with inhomogeneous fixed charge distribution.

NaMLab presented a novel methodology for 2D mapping, termed BiasMDP. This method is based on a spatially resolved microwave detected photoconductivity (MDP) measurement commercialized by Freiberg Instruments GmbH. In BiasMDP, the carrier lifetime is measured while a bias voltage is applied to a back electrode (Fig. 1). The resulting carrier lifetime curve shows a distinct minimum when the external electric field compensates the field-effect passivation (Fig. 2). The fixed charge and interface defect densities of the dielectric passivation layer are determined by analyzing the curve shape. As the measurement is done with high spatial resolution (< 1 mm), this method allows a 2D mapping of the surface. Fig. 3 shows the measured fixed charge distribution on a test wafer with intentionally introduced inhomogeneity (shading with quarter wafer during deposition).

The potential of BiasMDP is already demonstrated. Currently, this method is further developed together with Freiberg Instruments GmbH and partners from solar industry. The development is focused on measurement sequences and data evaluation algorithms. Additionally, BiasMDP is applied to industrial mono- and multicrystalline solar wafers and solar cells to demonstrate the potential of this method for process monitoring.

 

Contact: Dr. Ingo Dirnstorfer

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