III-V solar cells and doping processes

A GaAs/GaSb tandem solar cell having high conversion efficiency, comprising:

• a GaAs upper cell having a transparency for radiation to which GaSb is responsive and metallization on at least the upper surface thereof;
• a GaSb lower cell positioned beneath the GaAs upper cell to receive radiation through the GaAs upper cell with metallization on upper and lower surfaces thereof, said upper surface metallization including spaced grid lines;
• said GaAs cell having under its upper surface metallization a diffusion of a conductivity-type modifier that provides a graded doping with the conductivity-type modifier surface concentration being effective to preserve emitter minority carrier lifetime and to maintain a low series resistance and a high light generated current;
• said graded doping providing a junction depth that is sufficient to provide high light generated current;
• and
• said GaSb cell having at said upper surface a first light, relatively shallow, relatively uniform depth diffusion of a conductivity-type modifier to preserve emitter carrier lifetime between grid lines and a second, heavy diffusion of conductivity-type modifier that extends to a greater depth only under the grid lines to maintain a low series resistance.

A GaSb solar cell having:

• metallization on upper and lower surfaces thereof with said upper surface metallization including spaced parallel grid lines;
• a texture etch on said upper surface between said grid lines to provide lower reflection of incident solar energy and higher currents;
• and
• underlying zinc diffusion which has a shallow depth between the grid lines to provide a shallow junction cell having a favorable minority carrier lifetime and a greater diffusion depth under the grid lines to provide a low ohmic contact resistance.

A method for doping a III-V solar cell wafer to provide a non-uniform doping profile comprising:

• diffusing a dopant into a wafer surface;
• depositing a controlled amount of the dopant on the wafer surface;
• capping said dopant with an insulating material that is effective to reduce loss of dopant by evaporation at the water surface;
• heating the wafer to drive said dopant material into the wafer in a second diffusion step according to a profile which forms an emitter junction sufficiently distant from the wafer surface to provide high light current and a dopant concentration t the wafer surface that is effective to preserve emitter carrier lifetime;
• and
• removing the layer of insulating material and excess dopant, if any, before completion of the solar cell.

A method for producing a front side grid electrode on a III-V wafer, comprising the steps of:

• providing a III-V wafer of a first conductivity type;
• forming an insulating layer on the front side of said wafer having an opening to provide an exposed portion that defines a cell area;
• diffusing a conductivity modifier at a fist light homogeneous concentration to form an emitter having a substantially uniform thickness throughout the cell area;
• depositing a coating of protective material on said emitter cell area;
• forming openings in said protective coating material which expose positions in said cell area corresponding to grid line positions;
• diffusion a conductivity modifier at a second higher concentration to form a heavy diffusion in said cell area through only said exposed grid line positions;
• applying a further layer of photoresist on the front side before applying backside metallization and thereafter forming new openings in the photoresist to expose the grid lines which define a metallization pattern for said front side;
• depositing a metal on said front side photoresist and in said new openings formed therein;
• and
• thereafter removing said photoresist and any metal deposited thereon.

A method of making a diffused junction GaSb wafer having an emitter with a texture etched surface, comprising the steps of:

• masking grid line locations on the wafer upper surface;
• forming a texture etch on said upper surface between said grid line locations to reduce reflection of incident solar energy comprising parallel ridges and grooves that run substantially perpendicular to said grid line locations with the groove depth being larger than the solar energy wavelength to which GaSb is photoresponsive and smaller than the minority carrier diffusion length;
• forming a light, homogeneous diffusion of a conductivity modifier on said texture etched surface and grid line locations to provide a shallow junction emitter and preserve minority carrier lifetime;
• forming a heavy diffusion of a conductivity modifier only at the grid line locations to provide a low ohmic resistance contact between the emitter and the grid lines;
• and
• applying a patterned layer of metal along said grid line locations.

A tandem solar cell concentrator module comprising:

• (a) a concentrator lens for focusing incident solar radiation;
• (b) a tandem solar cell substantially at the focus of the lens, the cell including:
• (i) an upper AlGaAs/GaAs primary cell having:
• (I) an AlGaAs window covering a photoactive region of a generally n-type GaAs wafer, and
• (II) a zinc-diffused, p-type emitter region having a concentration of zinc of between about 10.sup.18 -10.sup.19 atoms/cm.sup.3 ;
• and
• (ii) a lower GaSb booster cell positioned to receive solar radiation that passes through the primary cell, the booster cell including grid lines on the upper surface for collecting current produced in the booster cell by incident radiation, a heavy concentration of zinc under the rid lines to provide low ohmic contact resistance, and a light concentration of zinc between the grid lines, the light concentration being effective to preserve the minority carrier lifetime in the booster cell to increase the short circuit current;
• wherein the booster cell includes texture etching to increase the conversion efficiency and to reduce reflection losses, the etching patterning the booster cell in the regions between the grid lines, and includes a backside metallization comprising layers of indium tin oxide, titanium tungsten nitride, and silver overlying one another, the indium tin oxide adjacent the GaSb and the silver at the exposed surface;
• (c) a heat spreader attached to both the primary cell and booster cell for holding the two in proper spaced relationship and for conveying excess heat away from either to a heat sink;
• and
• (d) current collection means connected to the primary cell and booster cell for collecting current generated in both when solar radiation is focused on the module.