High-Efficiency Concepts of c-Si Wafer Based Solar Cells

There are 3 examples of highly efficient c-Si solar cells to be discussed.  Monocrystalline wafers are used to minimise bulk recombination.  The first example is the PERL (Passivated Emitter Rear Locally diffused) concept.  The top surface of PERL solar cells is textured with inverted pyramid structures, covered by a double layer anti-reflection coating (ARC) consisting of magnesium fluoride and zinc sulfide, and a passivation of silicon oxide covers the emitter.  Very thin metal finger contacts are processed with photolithography techniques, and where they contact the emitter, the region underneath is heavily doped with phosphorus (see diagram above).  At the rear surface, point contacts heavily doped with boron are used together with thermal oxide passivation layers for the non-contacted majority region.  An efficiency of 25% was achieved on a solar cell with an area of 4cm².  V_oc above 700mV has been obtained.

The 2nd example is the interdigitated back contact (IBC) solar cell.  There is no shading on the front surface of this type of solar cell because all contacts are located at the rear surface (see diagrams above).  N-type c-Si wafers are used here because they do not suffer from light induced degradation and are less sensitive to impurities, leading to a higher quality silicon with cheaper processing.  However, the doping concentration is less uniformly distributed, so electrical properties is uneven across the same wafer, leading to lower energy yields.  Fingers can be made larger because they do not cause shading, and the rear passivation should have a low refractive index to enable reflection of light above 900nm (a backside mirror), increasing the absorption path length.  A front surface field is created by a thin and higher doped n+ layer that keeps higher hole minority densities in the n-doped bulk.  ARC and texturing are also applied on the front surface.  An efficiency of 24.2% was achieved on a wafer size of 155cm².

The 3rd example is a c-Si wafer based heterojunction made of a n-type float zone c-Si and a hydrogenated amorphous silicon (a-Si:H), also known as a HIT cell (heterojunction with intrinsic thin film).  Homojunctions refers to junctions created by doping the same semiconductor material differently, resulting in the same band gap in the p- and n-doped material. Heterojunctions refers to junctions created with 2 different semiconductor materials.

At the front surface (see diagram above), there is a thin 5nm layer of intrinsic a-Si (shown in red), and a thin layer of p-doped a-Si (blue colour).  Holes will drift to the p-layer based on this heterojunction.  Similarly at the rear surface (see diagram below), there is a layer of intrinsic a-Si (red colour), and a layer of n-doped a-Si (yellow colour).  A-Si is a very good passivation layer, but it has poor conductivity.  Hence, a layer of transparent conductive oxide (TCO) material is applied on top of the a-Si layers, such as ITO (indium tin oxide).  Having similar front and back surfaces, the HIT cell can operate in a bifacial configuration, where light can be collected from both the front and back.  A-Si can also be cheaply and easily deposited by plasma-enhanced chemical vapour deposition at low temperatures.  An efficiency of 24.7% on a wafer size of 102cm² is achieved, with V_oc of 750mV.

Reference:
4.4 High-Efficiency Concepts of c-Si Wafer Based Solar Cells, Delft University of Technology, https://www.youtube.com/watch?v=BHl3tX6uk08