Abstract:
One embodiment of the present invention provides a solar cell. The solar cell includes a Si base layer, a passivation layer situated above the Si base layer, a layer of heavily doped amorphous Si (a-Si) situated above the passivation layer, a first transparent-conducting-oxide (TCO) layer situated above the heavily doped a-Si layer, a back-side electrode situated below the Si base layer, and a front-side electrode situated above the first TCO layer. The first TCO layer comprises at least one of: GaInO, GaInSnO, ZnInO, and ZnInSnO.

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/353,119, entitled “Transparent Conducting Oxide for Photovoltaic Devices,” by inventors Jianming Fu, Zheng Xu, Jiunn Benjamin Heng, and Chentao Yu, filed 9 Jun. 2010. 
    
    
     BACKGROUND 
     1. Field 
     This disclosure is generally related to solar cells. More specifically, this disclosure is related to a solar cell that includes a high work function transparent conducting oxide (TCO) layer. 
     2. Related Art 
     The negative environmental impact caused by the use of fossil fuels and their rising cost have resulted in a dire need for cleaner, cheaper alternative energy sources. Among different forms of alternative energy sources, solar power has been favored for its cleanness and wide availability. 
     A solar cell converts light into electricity using the photoelectric effect. There are several basic solar cell structures, including a single p-n junction, p-i-n/n-i-p, and multi-junction. A typical single p-n junction structure includes a p-type doped layer and an n-type doped layer. Solar cells with a single p-n junction can be homojunction solar cells or heterojunction solar cells. If both the p-doped and n-doped layers are made of similar materials (materials with equal band gaps), the solar cell is called a homojunction solar cell. In contrast, a heterojunction solar cell includes at least two layers of materials of different bandgaps. A p-i-n/n-i-p structure includes a p-type doped layer, an n-type doped layer, and an intrinsic (undoped) semiconductor layer (the i-layer) sandwiched between the p-layer and the n-layer. A multi junction structure includes multiple single junction structures of different bandgaps stacked on top of one another. 
     In a solar cell, light is absorbed near the p-n junction generating carriers. The carriers diffuse into the p-n junction and are separated by the built-in electric field, thus producing an electrical current across the device and external circuitry. An important metric in determining a solar cell&#39;s quality is its energy-conversion efficiency, which is defined as the ratio between power converted (from absorbed light to electrical energy) and power collected when the solar cell is connected to an electrical circuit. 
     For homojunction solar cells, minority-carrier recombination at the cell surface due to the existence of dangling bonds can significantly reduce the solar cell efficiency; thus, a good surface passivation process is needed. In addition, the relatively thick, heavily doped emitter layer, which is formed by dopant diffusion, can drastically reduce the absorption of short wavelength light. Comparatively, heterojunction solar cells, such as Si heterojunction (SHJ) solar cells, are advantageous.  FIG. 1  presents a diagram illustrating an exemplary SHJ solar cell (prior art). SHJ solar cell  100  includes front finger electrode  102 , a heavily doped amorphous-silicon (a-Si) emitter layer  104 , an intrinsic a-Si layer  106 , a crystalline-Si substrate  108 , and an Al back-side electrode  110 . Arrows in  FIG. 1  indicate incident sunlight. Because there is an inherent bandgap offset between a-Si layer  106  and crystalline-Si (c-Si) layer  108 , a-Si layer  106  can be used to reduce the surface recombination velocity by creating a barrier for minority carriers. The a-Si layer  106  also passivates the surface of crystalline-Si layer  108  by repairing the existing Si dangling bonds. Moreover, the thickness of heavily doped a-Si emitter layer  104  can be much thinner compared to that of a homojunction solar cell. Thus, SHJ solar cells can provide a higher efficiency with higher open-circuit voltage (V oc ) and larger short-circuit current (J sc ). 
     When fabricating solar cells, a layer of transparent conducting oxide (TCO) is often deposited on the a-Si emitter layer to form an ohmic-contact. However, due to the large band gap and high work function of the heavily doped p +  amorphous Si emitter layer, it is hard to form low-resistance ohmic contact between a conventional TCO material, such as indium tin oxide (ITO), and the heavily doped a-Si emitter. 
     SUMMARY 
     One embodiment of the present invention provides a solar cell. The solar cell includes a Si base layer, a passivation layer situated on a first side of the Si base layer, a layer of heavily doped p-type amorphous semiconductor situated on the passivation layer, a first transparent-conducting-oxide (TCO) layer situated on the heavily doped amorphous semiconductor layer, and a first electrode situated on the first TCO layer. The first TCO layer comprises at least one of: GaInO, GaInSnO, ZnInO, and ZnInSnO. 
     In a variation on the embodiment, the first side of the Si base layer is facing the incident sunlight. 
     In a variation on the embodiment, the solar cell includes a second electrode situated on a second side of the Si base layer, and the second side is opposite to the first side. 
     In a further variation, the second side of the Si base layer is facing the incident sunlight, and the second electrode includes a second TCO layer and a metal grid comprising Cu and/or Ni. 
     In a variation on the embodiment, the Si base layer includes a crystalline-Si (c-Si) substrate. 
     In a variation on the embodiment, the Si base layer includes an epitaxially formed crystalline-Si (c-Si) thin film. 
     In a variation on the embodiment, the passivation layer includes at least one of: undoped a-Si and SiO x . 
     In a variation on the embodiment, the heavily doped p-type amorphous semiconductor layer has a doping concentration between 1×10 17 /cm 3  and 5×10 20 /cm 3 . 
     In a variation on this embodiment, the first TCO layer has a work function between 4.9 eV and 6.1 eV. 
     In a variation on the embodiment, the solar cell further comprises a third TCO layer situated on the first TCO layer, and the third TCO layer has a lower resistivity than the first TCO layer. 
     In a further variation, the third TCO layer includes at least one of: indium tin oxide (ITO), tin-oxide (SnOx), aluminum doped zinc-oxide (ZnO:Al), and Ga doped zinc-oxide (ZnO:Ga). 
     In a variation on the embodiment, the first electrode comprises at least one of: Ag, Cu, and Ni. 
     In a variation on the embodiment, the p-type amorphous semiconductor comprises amorphous Si or amorphous Si containing carbon. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  presents a diagram illustrating an exemplary Si heterojunction (SHJ) solar cell (prior art). 
         FIG. 2  presents a diagram illustrating the band diagrams at the interface between high/medium/low work function TCO material and p-type amorphous Si. 
         FIG. 3  presents a diagram illustrating the process of fabricating a solar cell in accordance with an embodiment of the present invention. 
         FIG. 4  presents a diagram illustrating an exemplary solar cell in accordance with an embodiment of the present invention. 
         FIG. 5  presents a diagram illustrating an exemplary solar cell in accordance with an embodiment of the present invention 
     
    
    
     In the figures, like reference numerals refer to the same figure elements. 
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     Overview 
     Embodiments of the present invention provide an SHJ solar cell that includes a layer of novel TCO material with high work function. The relatively high work function, up to 6.1 eV, of the TCO material ensures lower contact resistance and higher V oc . 
     TCO film has been widely used in solar cells to form ohmic contact with the emitter layer. An SHJ solar cell can be formed by depositing a-Si layers on a c-Si substrate. Note that the a-Si layers include a layer of doped a-Si in order to form a junction with the c-Si substrate or to ensure good electrical contact with a subsequently formed electrode. A TCO layer is often deposited on the doped a-Si layer to form an ohmic contact. However, due to the large band gap and high work function of the p-type doped a-Si layer, it is difficult to find a TCO material with work function that is in alignment with the p-type a-Si in order to minimize the band bending at the TCO and p-type a-Si interface, and to reduce contact resistance and maximize open circuit voltage. For example, the work function of ITO is between 4.5 eV and 4.8 eV. This will cause band bending at TCO and p-type a-Si interface, and make it hard to achieve a low-resistance ohmic contact and high V oc .  FIG. 2  presents a diagram illustrating the band diagrams at the interface between high/medium/low work function TCO material and p-type amorphous Si. From the band diagram, one can see that, for TCO material with low or medium work function, potential barriers at the interface make it harder for charges (holes) to migrate from the p-type a-Si material to the TCO, thus resulting in higher contact resistance. Hence, it is desirable to use a TCO material that has a relatively high work function. 
       FIG. 3  presents a diagram illustrating the process of fabricating a solar cell in accordance with an embodiment of the present invention. 
     In operation  3 A, a substrate  300  is prepared. In one embodiment, substrate  300  is a c-Si substrate, which is textured and cleaned. C-Si substrate  300  can be either p-type doped or n-type doped. In one embodiment, c-Si substrate  300  is lightly doped with an n-type dopant, and the doping concentration of c-Si substrate  300  can be between 1×10 16 /cm 3  and 1×10 17 /cm 3 . Note that other than using c-Si substrate (which is more expensive) as a base layer, it is also possible to deposit a thin c-Si epitaxial film on a relatively cheaper metallurgical-grade Si (MG-Si) substrate to act as a base layer, thus lowering the manufacturing cost. The thickness of the c-Si epitaxial film can be between 5 μm and 100 μm. The surface of c-Si substrate  300  can be textured to maximize light absorption inside the solar cell, thus further enhancing efficiency. The surface texturing can be performed using various etching techniques including dry plasma etching and wet etching. The etchants used in the dry plasma etching include, but are not limited to: SF 6 , F 2 , and NF 3 . The wet etching etchant can be an alkaline solution. The shapes of the surface texture can be pyramids or inverted pyramids, which are randomly or regularly distributed on the surface of c-Si substrate  300 . 
     In operation  3 B, a passivation layer  304  is deposited on top of c-Si substrate  300 . Passivation layer  304  can significantly reduce the density of surface carrier recombination, thus increasing the solar cell efficiency. Passivation layer  304  can be formed using different materials such as intrinsic a-Si or silicon-oxide (SiO x ). In one embodiment, a layer of intrinsic a-Si is deposited on c-Si substrate  300  to form passivation layer  304 . Techniques used for forming passivation layer  304  include, but are not limited to: PECVD, sputtering, and electron beam (e-beam) evaporation. The thickness of passivation layer  304  can be between 3 nm and 10 nm. 
     In operation  3 C, a heavily doped p-type doped amorphous semiconductor layer is deposited on passivation layer  304  to form an emitter layer  306 . The p-type amorphous semiconductor can be a-Si or amorphous SiC (a-SiC). In one embodiment, emitter layer  306  includes a-Si. The doping concentration of emitter layer  306  can be between 1×10 17 /cm 3  and 5×10 20 /cm 3 . The thickness of emitter layer  306  can be between 3 nm and 10 nm. Techniques used for depositing emitter layer  306  include PECVD. Because the thickness of emitter layer  306  can be much smaller compared with that of the emitter layer in a homojunction solar cell, the absorption of short wavelength light is significantly reduced, thus leading to higher solar cell efficiency. 
     In operation  3 D, a layer of high work function TCO material is deposited on top of emitter layer  306  to form TCO layer  308 . Compared with conventional TCO material, such as ITO, used in solar cells, TCO layer  308  includes TCO material with a relatively higher work function. In one embodiment, the work function of TCO layer  308  is between 4.9 eV and 6.1 eV. Examples of high work function TCO include, but are not limited to: GaInO (GIO), GaInSnO (GITO), ZnInO (ZIO), ZnInSnO (ZITO), their combinations, as well as their combination with ITO. Techniques used for forming TCO layer  308  include, but are not limited to: PECVD, sputtering, and e-beam evaporation. Note that in addition to providing low-resistance ohmic contact, the higher work function of TCO layer  308  can also result in a higher V oc . 
     In operation  3 E, metal front electrodes  310  are formed on top of TCO layer  308 . Front metal electrodes  310  can be formed using various metal deposition techniques at a low temperature of less than 300° C. In one embodiment, front electrodes  310  are formed by screen-printing Ag paste. In another embodiment, front electrodes  310  are formed by electroplating Cu and/or Ni. 
     In operation  3 F, a back electrode  302  is formed on the opposite side to the front side. In one embodiment, the back electrode stack can include a passivation layer, an n-typed heavily doped semiconductor layer, a TCO or a metal layer with relatively low work function (such as between 4.0 eV and 5.0 eV), and a metal grid. 
     After the formation of front electrodes  310  and back electrode  302 , various techniques such as laser scribing can be used for cell isolation to enable series interconnection of solar cells. 
     Although adopting high work function TCO material can result in lower contact resistance between TCO layer  308  and emitter layer  306 , high work function TCO material tends to have a larger resistivity than that of the ITO. For example, an ITO material that has 5% tin oxide has a low resistivity of 200 μΩ·cm, which is much smaller than that of the high work function TCO materials. Hence, to reduce the overall resistance, TCO layer  308  may be a bi-layer structure that includes a high work function TCO sub-layer and an ITO sub-layer. 
       FIG. 4  presents a diagram illustrating an exemplary solar cell in accordance with an embodiment of the present invention. Solar cell  400  includes a base layer  402 , a passivation layer  404 , an emitter layer  406 , a TCO layer  408 , a back-side electrode  410 , and a front-side metal grid  412 . 
     Base layer  402  can be a c-Si substrate or an epitaxially formed c-Si thin film. Passivation layer  404  can be an oxide layer or a layer of intrinsic a-Si. Emitter layer  406  can be either p-type doped or n-type doped. In one embodiment, emitter layer  406  is p-type doped a-Si. TCO layer  408  includes two sub-layers  408 - 1  and  408 - 2 . Sub-layer  408 - 1  is on top of emitter layer  406 . To ensure a good ohmic contact with a low contact resistance, in one embodiment, sub-layer  408 - 1  is formed using high work function TCO material, including, but not limited to: GaInO (GIO), GaInSnO (GITO), ZnInO (ZIO), ZnInSnO (ZITO), and their combinations. Sub-layer  408 - 2  includes TCO materials having low resistivity, such as ITO, tin-oxide (SnO x ), aluminum doped zinc-oxide (ZnO:Al), or Ga doped zinc-oxide (ZnO:Ga). Back-side electrode can include a passivation layer, an n-typed heavily doped semiconductor layer, a TCO or a metal layer with relatively low work function (such as that between 4.0 eV and 5.0 eV), and a metal grid. Front-side metal grid  412  can include screen-printed Ag grid or electroplated Cu and/or Ni grid. 
     In addition to be deposited on the front side (the side facing the sun) of the solar cell, the high work function TCO layer can also be used on the side opposite to the incidence of sunlight. In one embodiment, the passivation layer and the heavily doped p-type semiconductor layer are deposited on the back side of the c-Si base layer, facing away from incident light. The high work function TCO layer is then deposited on the back side as well. The electrode on the front side of the solar cell includes a TCO layer with lower work function, such as ITO. The solar cell performance can still benefit from the low ohmic contact resistance between the high-work function TCO and the heavily doped p-type semiconductor layer. 
       FIG. 5  presents a diagram illustrating an exemplary solar cell in accordance with an embodiment of the present invention. Solar cell  500  includes a base layer  502 , passivation layers  504  and  506 , an emitter layer  508 , a BSF layer  510 , TCO layers  512  and  514 , a back-side electrode  516 , and a front-side electrode  518 . 
     Base layer  502  can be lightly doped c-Si. In one embodiment, base layer  502  is p-type doped. Passivation layers  504  and  506  can include an intrinsic a-Si or oxide layer or a combination thereof. Emitter layer  508  can be heavily doped n-type amorphous semiconductor, and BSF layer  510  can be heavily doped p-type amorphous semiconductor, such as a-Si or a-SiC. Front-side TCO layer  512  interfaces with n-type doped emitter layer  508 , and includes low work function TCO material, such as ITO. Back-side TCO layer  514  interfaces with p-type doped BSF layer  510 , and includes high work function TCO material, such as GIO, GITO, ZIO, ZITO, and their combinations. Back-side electrode  516  and front-side electrode  518  are similar to the ones shown in  FIG. 4 . 
     Note that it is also possible to place the heavily doped p-type emitter on the back side of the solar cell with a lightly doped n-type base layer, and to include a front surface field (FSF) layer. As long as the TCO material interfacing with heavily doped p-type material has a relatively high work function, the overall performance of the solar cell can benefit from the reduced ohmic contact resistance between the TCO and the heavily doped p-type material. 
     The foregoing descriptions of various embodiments have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention.