Method and system for forming absorber layer on metal coated glass for photovoltaic devices

An apparatus for forming a solar cell includes a housing defining a vacuum chamber, a rotatable substrate support, at least one inner heater and at least one outer heater. The substrate support is inside the vacuum chamber configured to hold a substrate. The at least one inner heater is between a center of the vacuum chamber and the substrate support, and is configured to heat a back surface of a substrate on the substrate support. The at least one outer heater is between an outer surface of the vacuum chamber and the substrate support, and is configured to heat a front surface of a substrate on the substrate support.

FIELD

The disclosure relates to photovoltaic devices generally, and more particularly relates to a system and method for producing photovoltaic devices.

BACKGROUND

Photovoltaic devices (also referred to as solar cells) absorb sun light and convert light energy into electricity. Photovoltaic devices and manufacturing methods therefor are continually evolving to provide higher conversion efficiency with thinner designs.

Thin film solar cells are based on one or more layers of thin films of photovoltaic materials deposited on a substrate such as glass. The film thickness of the photovoltaic materials ranges from several nanometers to tens of micrometers. Such photovoltaic materials function as light absorbers. A photovoltaic device can further comprise other thin films such as a buffer layer, a back contact layer, and a front contact layer.

Copper indium gallium diselenide (CIGS) is a commonly used absorber layer in thin film solar cells. CIGS thin film solar cells have achieved excellent conversion efficiency (>20%) in laboratory environments. Most conventional CIGS deposition is done by one of two techniques: co-evaporation or selenization. Co-evaporation involves simultaneously evaporating copper, indium, gallium and selenium. The different melting points of the four elements makes controlling the formation of a stoichiometric compound on a large substrate very difficult. Additionally, film adhesion is very poor when using co-evaporation. Selenization involves a two-step process. First, a copper, gallium, and indium precursor is sputtered on to a substrate. Second, selenization occurs by a reaction of the precursor with H2Se/H2S at 500° C. or above.

DETAILED DESCRIPTION

Glass such as soda lime glass can be used as a substrate in thin film solar cells. In general, a back contact layer, a light absorber layer of photovoltaic material, a buffer layer, and a front contact layer can be deposited over the substrate, respectively. Examples of suitable materials for the back contact layer deposited over the glass include, but are not limited to copper, nickel, molybdenum (Mo) or any other metal or conductive material. Example of suitable materials for the light absorber layer include but are not limited to cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and amorphous silicon (α-Si). Depending on the type of the photovoltaic material for the absorb layer, the buffer layer can be either an n-type or a p-type semiconductor materials including but are not limited to CdS and ZnS. The front contact layer is a transparent conductive material such as indium tin oxide (ITO).

The inventors have determined that non-uniformity of temperature can cause a substrate—particularly a metal coated glass substrate—to deform and even crack when a subsequent layer is deposited over the substrate under heating conditions. The factors causing such a temperature variation include but are not limited to material difference between the substrate and the back contact layer; and material difference between the substrate and substrate holder contacting the substrate. The inventors have determined that a metal layer coated on glass as the back contact layer can reflect heat while glass absorbs heat inside the vacuum chamber, and can cause uneven temperature distribution cross the substrate. The substrate holder and other structural materials inside the vacuum chamber are generally metals. Different coefficient of thermal expansion (CTE) can worsen the problem to cause deformation, warping, cracking or other damage to the substrate.

In some embodiments, this disclosure provides an apparatus and a method for forming a solar cell, in which a substrate such as glass is heated without cracking or deformation, and an absorbed layer can be formed over the substrate. For example, in accordance with some embodiments in this disclosure, a good absorber layer can be deposited over a metal coated glass substrate when the substrate is heated under a temperature close to its glass transition temperature. The apparatus and the method are suitable for forming a solar cell or other photovoltaic devices using a substrate such as glass of different sizes including large glass panel.

Unless expressly indicated otherwise, references to a “front side” of a substrate made in this disclosure will be understood to encompass the side on which a light absorber layer will be deposited. References to a “back side” of the substrate made below will be understood to encompass the other side opposite to the side where the light absorber layer will be deposited. References to a “substrate” will be understood to encompass a substrate with or without a back contact layer, for example, a metal coated glass substrate. When the substrate is a metal coated glass, the “back side” is the glass layer while the “front side” is the metal layer deposited over the glass layer as the back contact layer.

FIG. 1is a plan view of an apparatus100for forming a solar cell. In accordance with some embodiments, apparatus100comprises a housing defining a vacuum chamber105, a rotatable substrate support120, at least one inner heater117and at least outer heater118.

As shown, apparatus100includes a housing105defining a vacuum chamber. In some embodiments, housing105may be shaped as a polygon. For example, as shown inFIG. 1, housing105may be octagonally shaped. In some embodiments, housing105has one or more removable doors built on one or more sides of the vacuum chamber. Housing defining a vacuum chamber105may be composed of stainless steel or other metals and alloys. For example, housing105can define a single vacuum chamber having a height of approximately 2.4 m (2.3 m to 2.5 m) with a length and width of approximately 9.8 m (9.7 m to 9.9 m). References to a “vacuum chamber”105or a “housing”105defining a vacuum chamber in this disclosure will be understood to encompass the same meaning.

Apparatus100includes a rotatable substrate support120. In some embodiments, substrate support120is configured to hold a substrate130on each of a plurality of surfaces122facing an interior surface of the vacuum chamber. In some embodiments, examples of suitable materials for substrate130include but are not limited to glass and metal coated glass. In some embodiments, rotatable substrate support120is shaped as a polygon. For example, as shown inFIG. 1, a plurality of substrates130are held on a plurality of surfaces122in a substantially octagonal shaped rotatable substrate support120. Any other suitable shape, such as rectangle or square, can be used for rotatable substrate support120.

Rotatable substrate support120comprises a plurality of metal posts121, and a plurality of metal frames124connected to the plurality of metal posts121in some embodiments. Each of the plurality of metal frames124may also comprise at least one fixture123in the edge. Metal posts121and metal frames124are configured to hold a respective substrate130.

FIGS. 3A and 3Billustrate examples of a portion of the substrate support120comprising a plurality of metal posts121and a plurality of metal frames124, which is configured to hold a respective substrate130, in accordance with some embodiments. Substrate130can be in any shape. In some embodiments, metal frames124are configured to hold substrate130in rectangular shape. As shown inFIG. 3A, metal frame124can be in one piece holding one respective substrate130in the middle. As shown inFIG. 3B, substrate can be held with multiple pieces of metal frames124. Examples of suitable materials for metal frames124and metal posts121can be any metal. In some embodiments, metal frames124and metal posts121are made of a material selected from the group consisting of stainless steel, titanium and molybdenum.

Metal posts121and metal frames124can be configured in different combinations to hold substrate130. In some embodiments, each of the plurality of metal frames124is configured to support a respective substrate130along its entire length, and each substrate edge is held along its entire length through the at least one fixture on the plurality of metal frames124.FIGS. 4A and 4Billustrate a plan view and a cross-sectional view of a substrate held by metal frame124along its entire length.

In other embodiments, each respective substrate130is attached with a respective one of metal frames124contacting along a portion of its length, and each substrate edge is retained by a respective fixture123at selected points on the respective metal frame124. FIGS.5A-5C illustrate a plan view and two cross-sectional views of an example of substrate130attached with metal frames124contacting along a portion of its length through at least one fixture on the same side of metal frames124.FIGS. 5B and 5Cillustrate a cross section along the section line5B-5B and5C-5C shown inFIG. 5A, respectively. In this example, all the fixtures123, are on one side of metal frame124.FIGS. 6A-6Cillustrate a plan view and two cross-sectional views of another example of a substrate130attached with metal frames124contacting along a portion of its length through at least one fixture on both sides of the metal frames.FIGS. 6B and 6Cillustrate a cross section along the section line6B-6B and6C-6C shown inFIG. 6A, respectively. In this example, some of fixtures123are on one side of metal frame124, while some other fixtures123are on the other side of metal frame124. One piece of metal frame124is shown inFIGS. 4A-4B,5A-5C, and6A-6C for purpose of illustration. Rotatable substrate support120may comprise a plurality of metal frames124.

Referring back toFIG. 1, in some embodiments, substrate support120is rotatable about an axis in the vacuum chamber.FIG. 1illustrates a clockwise direction of rotation for rotatable substrate support120. In some embodiments, substrate support120is configured to rotate in a counter-clockwise direction. In some embodiments, rotatable substrate support120is operatively coupled to a drive shaft, a motor or other mechanism that actuates rotation from a surface of the vacuum chamber. Substrate support120can be rotated at a speed, for example, between approximately 5 and 100 RPM (e.g., 3 and 105 RPM). In some embodiments, a speed of rotation is selected to minimize excessive deposition of absorption components over substrate130. In some embodiments, substrate support120rotates at a speed of approximately 80 RPM (e.g. 75-85 RPM).

In some embodiments, apparatus100also comprises a rotatable drum110, which is disposed within vacuum chamber105and coupled to a top or bottom surface of vacuum chamber105. Rotatable drum110may also comprise supporting beams111connected with rotatable substrate support120. Rotatable drum110can be operatively coupled to substrate support120through support beams111, and is configured to rotate substrate support120inside vacuum chamber105. In some embodiments, rotatable drum110has a shape that is substantially the same as the shape of substrate support120. In other embodiments, rotatable drum110can have any suitable shape.

In some embodiments, apparatus100also comprises at least one inner heater117and at least one outer heater118. In some embodiments, the at least one inner heater117is between a center of vacuum chamber105and substrate holder120, and is configured to face a back surface of substrate130and heat a back surface of substrate130on substrate support120. In some embodiments, the at least one inner substrate can the whole substrate130from the back surface. In some embodiments, the at last one inner heater117is configured to heat a plurality of substrates130held on rotatable substrate support120when substrate support120is rotated. The at least one inner heater117can have any suitable shape. In some embodiments, the rotatable substrate support120is polygonally-shaped, and the at least one inner heater117is configured to have a circular shape to avoid any collision between substrate support120and the at least inner heater117. In other embodiments the at least one inner heater117has a shape that is substantially the same as the shape of substrate support120. In some embodiments, the rotatable substrate support120is polygonally-shaped, and the at least one inner heater117has a shape substantially the same as the shape of substrate support120. For example, as shown inFIG. 1, the at least one inner heater117has a substantially octagonal arrangement.

In some embodiments, the at least one inner heater117is disposed to maintain a substantially uniform distance about the perimeter of substrate support120. In some embodiments, the at least one inner heater117is disposed between an interior surface of rotatable substrate support120and rotatable drum110. A power source of the at least one inner heater117can extend through a surface of rotatable drum110. In some embodiments, substrate support120is rotatable around the at least one inner heater117. In some embodiments, the at least one inner heater117can be coupled to a top or bottom surface of vacuum chamber105. The at least one inner heater117can be rotatable. In other embodiments, the at least one inner heater117is not rotatable. The at least one inner heater117can include, but is not limited to, infrared heaters, halogen bulb heaters, resistive heaters, or any suitable heater for heating a substrate130during a deposition process. In some embodiments, the at least one inner heater117can heat a substrate to a temperature between approximately 295° C. and 655° C. (e.g. 300° C. and 650° C.).

The at least one outer heater118is located between an outer surface (or shell) of vacuum chamber105(housing) and substrate support120, and is configured to heat a front surface of substrate130on substrate support120, in accordance with some embodiments. In some embodiments, the at least one outer heater118is attached on the interior surface of vacuum chamber105. The at least one outer heater118can be configured to heat substrate130from a front surface of substrate130during rotation of substrate support120.

The at least one outer heater118can include, but is not limited to, infrared heaters, halogen bulb heaters, resistive heaters, or any suitable heater for heating a substrate130during a deposition process. In some embodiments, the at least one outer heater118can heat a substrate130to a temperature between approximately 295° C. and 655° C. (e.g. 300° C. and 650° C.).

In some embodiments, the at least one outer heater118is configured to heat substrate130simultaneously while the substrate is heated by the at least one inner heater117. The heat emitted from the at least one outer heater118mitigates non-uniformity of temperature within sample130. For example, referring toFIGS. 7A-7B, when substrate130is a piece of glass with the front side coated with metal as the back contact layer of a solar cell, the metal layer may reflect heat from the at least one inner heater117to the glass layer.FIG. 7Ais a top view illustrating an example with at least one inner heater117and at least one outer heater118which are configured to heat a substrate130from both its back and front surface.FIG. 7Bis a magnified cross-section view of a portion of substrate130inFIG. 7A. As shown inFIG. 7B, the front surface of glass substrate is coated with a metal layer124, which will be the back contact layer of a solar cell. The back surface122of substrate130is glass. Metal layer124reflects the heat from the back surface122which will absorb such extra heat. The at least one outer heater118can be configured to generate additional heat to maintain uniform temperature distribution across substrate130. This example is used for illustration purpose only. The at least one outer heater118can be used in other ways.

In some embodiments, the at least inner heater117or the at least one outer heater118is configured to heat substrate130according to a predetermined heating profile. For example, a stepwise heating profile with various heating rates can be used in some embodiments. In some embodiments, both the at least one inner heater117and the at least one outer heater118is configured to operate according to a predetermined heating profile. In some embodiments, the heating profile comprises heating the substrate from room temperature to final high temperature step by step instead of straight heating. Each step of heating can be 100° C. interval and stay at that temperature for 5 minutes. Thus the temperature difference between substrate center and edge can be minimized. In some embodiments, the at least one inner heater117is configured to be operated according to a program which provides automatic adjustment according to actual temperature of a sample130during a process.

Apparatus100also comprises a cooling element115in some embodiments. Cooling element115is configured to cool down the temperature inside vacuum chamber105, for example, between two operations, or before a new plurality of substrates130is loaded onto rotatable substrate support120. For example, temperature of inner chamber wall can reach a temperature up to 700° C. after a deposition process. Cooling element115is a coil system having a coolant such as water in some embodiments. Apparatus100can be also configured to introduce conductive cooling gas such as nitrogen inside vacuum chamber105, in accordance with some embodiments of this disclosure. The method of cooling using gas inside vacuum chamber105and the method using coil cooling with a coolant can be used separately or simultaneously.

In some embodiments, apparatus100further comprises one or more additional heaters119coupled to the plurality of metal frames124or metal posts121in rotatable substrate support120. The additional heaters119are configured to rotate with the substrate support120and heat a respective substrate130during rotation. Examples of an additional heater119include, but are not limited to, infrared heaters, halogen bulb heaters, resistive heaters, or any suitable heater for heating a substrate130during a deposition process. Additional heaters119can be lamp heaters configured to emit infra-red radiation in some embodiments.

FIG. 8is a schematic view of a portion of an example of rotatable substrate support120in apparatus100.FIG. 8shows that one or more additional heaters119are coupled to metal frames124or metal posts121in rotatable substrate support120, in accordance with some embodiments. An additional heater119can be provided on each edge of a respective metal frame124or a metal post121. In some embodiments, additional heaters are only on a top or bottom edge of a metal frame124or a metal post121. In some embodiments, additional heaters119are only on side edges of a metal frame124or a metal post121. In some embodiments, additional heaters119are on both top or bottom edges and side edges of a metal frame124or a metal post121. In some embodiments, different combinations of additional heaters119are configured to provide different heating zones. The heat emitted from additional heaters119can be used to offset heat absorbed by metal posts121or metal frames124. Additional heaters119can also be operated according to a predetermined heating profile or a computer program providing automatic response to actual temperature on substrate130inside vacuum chamber105. Additional heaters119can be coordinated with the at least one inner heater117and the at least one outer heater118during a process.

Referring back toFIG. 1, in some embodiments, apparatus100further comprises at least one sputtering source135, at least one evaporation source140and at least one isolation pump152. Each of the at least one sputtering source135is configured to deposit a respective first ingredient. In some embodiments, the respective first ingredient is one ingredient of an absorber layer over a front surface of substrate130. The at least one evaporation source140is disposed in vacuum chamber105and configured to deposit a second ingredient. In some embodiments, the second ingredient is one ingredient of the absorber layer over the front surface of substrate130. Each of the least one isolation pump152is configured to prevent materials from the at least one evaporation source140from contaminating the at least one sputtering source135. A respective isolation pump152is disposed between each of the at least one sputtering source135and an adjacent one of the at least one evaporation source140in some embodiments.

The at least one sputtering source135can be disposed on the housing defining vacuum chamber105. Sputtering source135can be, for example, a magnetron, an ion beam source, a RF generator, or any suitable sputtering source configured to deposit a respective first ingredient for an absorber layer over the front surface of substrates130. Each sputtering source135includes at least one sputtering target137. Sputtering source135can utilize a sputtering gas. In some embodiments, sputtering is performed with an argon gas. Other possible sputtering gases include krypton, xenon, neon, and similarly inert gases.

As shown inFIG. 1, apparatus100can include at least two sputtering sources135. Each sputtering source135, having at least one sputtering target137, is configured to deposit a portion of a respective first ingredient for an absorber layer over a front surface of substrate130. Each respective first ingredient for the absorber layer can have a different chemical composition. In some embodiments, the at least two sputtering sources135are disposed adjacent to each other. In some other embodiments, the at least two sputtering sources135are disposed in two locations spaced apart from each other. For example, inFIG. 1, a first and second sputtering source135are disposed on opposing sides of vacuum chamber105and substantially equidistant from evaporation source140about the perimeter of vacuum chamber105.

In some embodiments, a first sputtering source135is configured to deposit atoms of a first type (e.g. copper (Cu)) in the first ingredient for absorber layer over at least a portion of a surface of substrate130. A second sputtering source135is configured to deposit atoms of a second type (e.g. indium (In)) in the first ingredient for absorber layer over at least a portion of a surface of substrate130. In some embodiments, the first sputtering source135is configured to deposit atoms of a first type (e.g. copper (Cu)) and a third type (e.g. gallium (Ga)) in the first ingredient for absorber layer over at least a portion of substrate130. In some embodiments, a first sputtering source135includes one or more copper-gallium sputtering targets137and a second sputtering source135includes one or more indium sputtering targets137. For example, a first sputtering source135can include two copper-gallium sputtering targets and a second sputtering source135can include two indium sputtering targets. In some embodiments, a copper-gallium sputtering target137includes a material of approximately 70 to 80% (e.g. 69.5 to 80.5%) copper and approximately 20 to 30% (e.g. 19.5 to 30.5%) gallium. In some embodiments, apparatus100has a first copper-gallium sputtering target137at a first copper: gallium concentration and a second copper-gallium sputtering target137at a second copper: gallium concentration for grade composition sputtering. For example, a first copper-gallium sputtering target can include a material of 65% copper and 35% gallium to control monolayer deposition to a first gradient gallium concentration and a second copper-gallium sputtering target can include a material of 85% copper and 15% gallium to control monolayer deposition to a second gradient gallium concentration. The plurality of sputtering targets137can be any suitable size. For example, the plurality of sputtering targets137can be approximately 15 cm wide (e.g. 14-16 cm) and approximately 1.9 m tall (e.g. 1-8-2.0 m).

In some embodiments, a sputtering source135that is configured to deposit a plurality of absorber layer atoms of indium over at least a portion of each substrate130can be doped with sodium (Na). For example, an indium sputtering target137of a sputtering source135can be doped with sodium (Na) elements. Doping an indium sputtering target137with sodium may avoid or minimize an alkali-silicate layer in the solar cell. This improvement may result in lower manufacturing costs for the solar cell as sodium is directly introduced to the absorber layer. In some embodiments, a sputtering source135is a sodium-doped copper source having between approximately two and ten percent sodium (e.g. 1.95 to 10.1 percent sodium). In some embodiments, an indium sputtering source135can be doped with other alkali elements such as, for example, potassium. In other embodiments, apparatus100can include multiple copper-gallium sputtering sources135and multiple sodium doped indium sputtering sources135. For example, apparatus100can have a 65:35 copper-gallium sputtering source135and an 85:15 copper-gallium sputtering source135for grade composition sputtering.

In some embodiments, an evaporation source140is configured to deposit a second ingredient of the absorber layer over at least a portion of each substrate130. In some embodiments, the second ingredient of the absorber layer comprises selenium, and can include any suitable evaporation source material. In some embodiments, evaporation source140is configured to produce a vapor of such an evaporation source material. The vapor can condense upon substrate130. For example, evaporation source140can be an evaporation boat, crucible, filament coil, electron beam evaporation source, or any suitable evaporation source140. In some embodiments, evaporation source140is disposed in a first sub-chamber of vacuum chamber105. In some embodiments, the vapor of source material can be ionized, for example using an ionization discharger, prior to condensation over substrate130to increase reactivity.

An isolation pump152is configured to prevent materials from the at least one evaporation source140from contaminating the at least one sputtering source135. In some embodiments, a respective isolation pump152is disposed between each of the at least one sputtering source135and an adjacent one of the at least one evaporation source140. In the embodiment ofFIG. 1, isolation pumps152are vacuum pumps. In some embodiments (not shown), one or more of the isolation pumps152is configured to maintain the pressure in an evaporator source140sub-chamber (not shown) lower than the pressure in vacuum chamber105. Isolation pumps152can be configured to evacuate absorber layer particles of the second ingredient (emitted by evaporation source240) from vacuum chamber105, prevent diffusion of these particles into the sputtering targets237, and prevent these particles from contaminating the two sputtering sources235.

In embodiments including a plurality of sputtering sources135and/or a plurality of evaporation sources140, apparatus100can include a plurality of isolation sources to isolate each of the evaporation sources from each of the sputtering sources135. For example, in embodiments having first and second sputtering sources135disposed on opposing sides of a vacuum chamber and an evaporation source140disposed there between on a perimeter of the vacuum chamber105, apparatus100can include a first isolation pump152disposed between the first sputtering source135and evaporation source140and a second isolation pump152disposed between the second sputtering source135and evaporation source140. In the illustrated embodiment, apparatus100includes an isolation pump152disposed between evaporation source140and one of the two sputtering sources135.

As shown inFIG. 1, apparatus100can include an isolation baffle170disposed about the evaporation source140. Isolation baffle170can be configured to direct a vapor of an evaporation source material to a particular portion of substrate130. Isolation baffle170can be configured to direct a vapor of an evaporation source material away from a sputtering source135. Apparatus100can include an isolation baffle170in addition to one or more isolation sources to minimize evaporation source material122contamination of one or more sputtering sources135. Isolation baffle170can be composed of a material such as, for example, stainless steel or other similar metals and metal alloys. In some embodiments, isolation baffle170is disposable. In other embodiments, isolation baffle170is cleanable.

In some embodiments, apparatus100can include a loading/unloading substrate chamber182, buffer chamber155, post-treatment chamber180and unload lock184. In various embodiments, post-treatment chamber180can be configured for post treatment of the solar cell such as, for example, cooling the solar cell.

In some embodiments, apparatus100can include one or more in-situ monitoring devices (not shown) to monitor process parameters such as temperature, chamber pressure, film thickness, or any suitable process parameter.

Apparatus100ofFIG. 1can also be used to form solar cells of different absorber films, for example, a copper-zinc-tin-sulfur-selenium (CZTSS) absorber film. In some embodiments, a number of CZTSS absorber layer are formed in apparatus100by further providing tin, copper, zinc, or copper/zinc targets, as targets137. Evaporation source140may use sulfur, selenium or both sulfur and selenium as source material. Additionally, another evaporation source140may be used to separately provide selenium and sulfur source material.

FIG. 2is a plan view of an apparatus200for forming a solar cell in accordance with some embodiments. Apparatus200comprises at least two (e.g., three) sputtering sources135, at least one evaporation source140, at least one inner heater and at least one outer heater. InFIG. 2, like items are indicated by like reference numerals, and for brevity, descriptions of the structure, provided above with reference toFIG. 1, are not repeated. The at least two (e.g., three) sputtering sources135are configured to deposit at least three different compositions as a respective first ingredient for an absorber layer over a front surface of substrate130. For example, a first sputtering source135is configured to deposit copper and gallium from a first target137comprising copper and/or gallium. A second sputtering source135is configured to deposit indium from a first target137comprising indium. In some embodiments, copper and gallium can be deposited in two different first sputtering sources135.

FIG. 9is a flow chart diagram illustrating an example of method900for fabricating solar cells comprising heating a substrate simultaneously from both its front surface and its back surface, in accordance with some embodiments.

In step910of this exemplary method900, a substrate130is first provided and secured on a rotatable substrate support120inside a vacuum chamber105in an apparatus for forming solar cells. For example, the apparatus100or200described above can be used. Substrate130has a front surface and a back surface. Examples of substrate130and configurations for holding substrate130are described inFIG. 1. Rotatable substrate support120is in a polygon shape and comprises a plurality of metal posts121and metal frames124in some embodiments. Metal posts121and metal frames are configured to hold substrate130, as described inFIGS. 3A-3B,4A-4B,5A-5C and6A-6C. The front surface of substrate130is disposed facing an interior surface of vacuum chamber105in some embodiments, as shown inFIGS. 7A-7B.

In step920, substrate support120is rotated. In some embodiments, substrate support120is continuously rotated at a certain speed as described inFIG. 1. In some embodiments, substrate support120is intermittently rotated.

Step930comprises heating substrate130simultaneously using at least one inner heater117and at least one outer heater118as described inFIG. 1. The inner heaters117faces a back surface of the substrate130and heats substrate130from the back surface of substrate130. The at least one outer heater is configured to heat a front surface of the substrate during rotation in some embodiments. In some embodiments, substrate130is simultaneously heated using the at least one inner heater117and the at least one outer heater118, according to a predetermined temperature profile. In some embodiments, the at least one inner heater117or at least one outer heater118, or both, heat substrate according to a predetermined temperature profile as described inFIG. 1. In Some embodiments, the predetermined temperature profile is used for minimizing temperature difference between the substrates and a plurality of metal posts or metal frames in the rotatable substrate support. In some embodiments, the at least one inner heater117or at least one outer heater118or both are operated based on a program providing automatic response based on the actual temperature of substrate130inside vacuum chamber105.

Step936is used in some embodiments, and is omitted from other embodiments. In some embodiments, the method900further comprises heating substrate130using one or more additional heaters119coupled to the plurality of metal frames124or metal posts121in rotatable substrate support120. As described inFIG. 1, examples of additional heaters119include, but are not limited to, lamp heaters emitting infrared radiation, for example, IR lamps. The one or more additional heaters119rotate with substrate support120in some embodiments.

In step940, an absorber layer is formed over the front surface of substrate130as described above. In some embodiments, the step of forming an absorber layer over the front surface of substrate130(in step940) includes depositing a respective first ingredient for an absorber layer over the front surface of substrate130from at least one sputtering source135; and depositing a second ingredient of the absorber layer over the front surface of substrate130from an evaporation source140disposed in the vacuum chamber105. In depositing the second ingredient, at least one isolation pump152as described inFIG. 1, is used to prevent materials from the at least one evaporation source140from contaminating the at least one sputtering source135.

FIG. 10is a flow chart diagram illustrating an example of a method for forming an absorber layer over the front surface of a substrate. In some embodiments, substrate130comprises a glass layer in the back surface and a metal layer deposited over the glass layer on the front surface. The step of depositing a respective first ingredient for an absorber layer (in step940ofFIG. 9) comprises at least three steps in the example ofFIG. 10. These include: depositing copper and gallium from a first sputtering source135(step942); and depositing indium from a second sputtering source135(step944). The step of depositing the second ingredient of the absorber layer comprises depositing selenium from an evaporation source140in step948.

Referring back toFIG. 9, in some embodiments, the method also comprises step950of cooling substrate130and vacuum chamber105with an inert conductive gas after forming an absorber layer, or any other layers of solar cells.

This disclosure provides an apparatus and a method for forming a solar cell. In accordance with some embodiments, the apparatus for forming a solar cell comprises a housing defining a vacuum chamber; a rotatable substrate support, at least one inner heater and at least outer heater. The rotatable substrate support inside the vacuum chamber is configured to hold a substrate. The at least one inner heater is between a center of the vacuum chamber and the substrate support, and is configured to heat back surface of a substrate on the substrate support. The at least one outer heater is located between the housing and the substrate support, and is configured to heat a front surface of a substrate on the substrate support. In some embodiments, the at least one outer heater is attached on the interior surface of the vacuum chamber. In some embodiments, the apparatus also comprises a rotatable drum configured to rotate the substrate support inside the vacuum chamber. In some embodiments, the apparatus of this disclosure further comprises at least one sputtering source, at least one evaporation source and at least one isolation pump. In some embodiments, the apparatus further comprises one or more additional heaters coupled to the plurality of metal frames or metal posts in the rotatable substrate support. The additional heaters are configured to rotate with the substrate support and heat a respective substrate during rotation.

In accordance with some embodiments, this disclosure provides an apparatus for forming a solar cell, comprising a vacuum chamber, a rotatable substrate support, a rotatable drum, at least one sputtering source, at least one evaporation source, at least one inner heater and at least one outer heater. The rotatable substrate support inside the vacuum chamber is of a polygonal shape configured to hold at least one substrate. The rotatable drum is disposed within the vacuum chamber. The at least one sputtering source is configured to deposit a respective first ingredient, for example, a respective first ingredient for an absorber layer over a front surface of the substrate. The at least one evaporation source is disposed in the vacuum chamber and configured to deposit a second ingredient, for example, a second ingredient for the absorber layer over the front surface of the substrate. Each inner heater faces a back surface of a respective substrate, and is configured to heat that substrate. The at least one outer heater is disposed between an outer surface (shell) of the vacuum chamber and the rotatable substrate support, is configured to heat a front surface of the at least one substrate during rotation. In some embodiments, the apparatus further comprises one or more additional heaters coupled to a plurality of metal frames or metal posts in the rotatable substrate support. The one or more additional heaters are configured to rotate with the substrate support and heat a respective substrate during rotation. In some embodiments, the apparatus also comprises at least one isolation pump. The at least one isolation pump is disposed between each of the at least one sputtering source and an adjacent one of the at least one evaporation source, and is configured to prevent materials from the evaporation source from contaminating the sputtering source.

This disclosure also provides a method for forming a solar cell. A substrate is first provided and secured on a rotatable substrate support inside a vacuum chamber. The front surface of a substrate is disposed facing an interior surface of the vacuum chamber. The method further comprises rotating the substrate support; heating the substrate simultaneously using at least one inner heater and at least one outer heater; and forming an absorber layer over the front surface of the substrate. The inner heater faces a back surface of the substrate and heats the substrate from the back surface of the substrate. The at least one outer heater is configured to heat a front surface of the substrate during rotation. The forming an absorber layer over the front surface of the substrate includes depositing a respective first ingredient for an absorber layer over the front surface of the substrate from at least one sputtering source; and depositing a second ingredient of the absorber layer over the front surface of the substrate from an evaporation source disposed in the vacuum chamber. In some embodiments, the substrate comprises a glass layer coated with a metal layer on the front surface. The depositing a respective first ingredient for an absorber layer comprises depositing copper, gallium and indium from at least two different sputtering sources, respectively. The depositing the second ingredient of the absorber layer comprises depositing selenium from the evaporation source.

In some embodiments, the method further comprises heating the substrate using one or more additional heaters coupled to the plurality of metal frames or metal posts in the rotatable substrate support. The one or more additional heaters rotate with the substrate support. In some embodiments, the method also comprises cooling the substrate and the vacuum chamber with an inert conductive gas after forming an absorber layer.