The ohmic properties and current-voltage characteristics of the screen-printed silicon solar cells with porous silicon surface

The ohmic properties and current-voltage characteristics of the silicon solar cells with fire-through screen-printed Ag metal contacts on the etched porous silicon surface have been investigated. The porous silicon layer of approximately 85 nm thickness was electrochemically etched on the n(+)-Si surface prior to the deposition of the front Ag grid contacts. Then, Ag contacts are formed by suitably applying a fire-through step at a temperature typically between 700-825 degrees C for 2 min in air ambient followed by annealing step at a lower temperature of 450 degrees C for 5 min in nitrogen ambient. The specific contact resistance (rho(c)) is one of the important parameters in studying the interfacial properties of the contact metalization system. The value of rho(c) of the contact structure is determined using the extrapolation technique based on the three-point probe (TPP) method. It shows that the best value of rho(c) is about 8.54 x 10(-4) Omega cm(2) is obtained for the contact structure which is an order less than the typical value of the rho(c) of 3 m Omega cm(2) reported for the sintered Ag metal contacts. The sintering of the Ag contacts on the etched porous silicon at temperature above 800 degrees C in air ambient has resulted in considerable reduction in the value of rho(c) of the contact structure. This improvement in rho(c) is attributed to the improvement in the barrier properties of the contact structure upon sintering at high temperature. The value of rho(c) follows a linear relationship with the surface doping level, N-s > 10(19) atoms/cm(3) and improves with an increase in the sintering temperature. The ohmic losses of the fire through Ag contacts are assessed using the power loss algorithm and have shown that relative ohmic losses of the contact structure have improved with higher firing temperatures. These results suggest that fire-through Ag metal contacts on the porous silicon have good ohmic quality and can be expected to provide low ohmic losses. (C) 2009 Elsevier Ltd. All rights reserved.