Date of Award

Spring 2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering - (Ph.D.)

Department

Committee for the Interdisciplinary Program in Materials Science and Engineering

First Advisor

N. M. Ravindra

Second Advisor

Bhushan L. Sopori

Third Advisor

Trevor Tyson

Fourth Advisor

Anthony Fiory

Fifth Advisor

Tao Zhou

Sixth Advisor

Przemyslaw Rupnowski

Abstract

Commercial multicrystalline silicon (me-Si) solar cells use screen-printing process for depositing both the Ag paste based gridded front and Al based back (whole area) metal contacts.. This thesis relates to experimental and theoretical studies of contact formation mechanisms in silicon solar cells. Temperature distribution studies during optical processing by. attached thermocouples to cells indicates that the maximum temperature reached under the front silver metal is less. than 800°C; this is lower than the eutectic point of Ag-Si (≈835°C). An analysis of the interaction of Ag particles and Si via the constituents of glass is given. This mechanism leaches metallic ions (solvent metals such as Pb, Bi or Zn), which cover the Ag particles and form a material of surface composition with low-melting-point. The low-temperature melt facilitates agglomeration of Ag and formation of a shallow alloy between Si, Ag, and the solvent metal. Ag-glass-Si interactions lead to the formation of Ag-rich and Si-rich areas at the metal-semiconductor .interface. The non-uniformity of the Ag-si interaction leads to degradation of various electrical parameters (i.e., fill factor and open circuit voltage (Voc)).

A hypothesis invoking ion .exchange phenomena for front contact formation is presented. Ag-Si, Ag-glass, glass-Si and Ag-glass-Si reactions are discussed. SIMS study on etched cells shows that a significant consumption of phosphorous occurs during Si-Ag interaction. Scanning Kelvin Probe Microscopy profiles have been studied to measure the surface potential of the metal semiconductor region. Current Voltage characteristics of the fired cells are presented. An improved technique to cross-section large lengths of wafers/solar. Cells for statistically meaningful analyses of the metal semiconductor interface is presented. Results and applications of study of the temperature distribution across. the cell during firing, by contact thermocouples are presented. Thermal modeling predicts a temperature gradient of more than 10°C across the cell width due to combined effect of shadowing and thermal mass of the metal grid. However, experimentally, no systematic effect of the temperature gradient is seen on the front contact formation mechanism. A study on the back Al -contact formation revealed that Si diffusion led to several defects (e,g. bumps, holes, shunts) in the cells.

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