Current efforts in the bulk silicon solar cell technology to increase efficiencies and lower processing costs are leaning towards thinner wafers, low-temperature continuous processing, and superior passivation and metallization schemes.  Interdigitated-back-contact (IBC) crystalline silicon (c-Si) solar cells eliminate the shading loss resulting into higher short-circuit currents (JSC), decrease grid resistance due to wider coverage, and reduce stringing costs; SunPower Corp. has already demonstrated large-area efficiencies as high as 23.4% using this approach.  A second approach employed in Sanyo’s HIT structure utilizing silicon heterojunctions (SHJ) demonstrates high open-circuit voltages (VOC) (>730 mV) and efficiencies (>22.3%).  Both these high performance approaches are combined to develop high-efficiency low-cost IBC-SHJ solar cells.

Rear-junction, IBC c-Si solar cells potentially have a number of advantages over conventional front-junction solar cells.  The back surface of IBC solar cells can independently be optimized for low series resistance and high fill factors (FF), while the front surface is separately optimized for passivation and anti-reflection (AR) properties to achieve maximum optical coupling and high short circuit currents (JSC).  Back contact solar cells are far easier to incorporate into a module with a higher packing factor since all the contacts are on one side of the wafer, and the interconnection strips do not have to pass from the front of one cell to the back of the next.  Therefore, the advantages of all-back-contact solar cells include performance (no shading loss and avoiding the trade-off between series resistance and reflectance), manufacturability (for example, ease of series connection for module and allowing for higher packing factor) and aesthetics (PV modules with more uniform appearance is desirable for architectural applications).

While the advantages of all-back-contact c-Si solar cells are well known, their implementation is hindered by several design and processing constraints.  Further improvements in cell efficiency will require new contacts and junctions that have lower recombination than that achieved by diffused devices.  Moreover, high temperature processing induces thermal stress and bending of wafers.  These limitations can be circumvented using low temperature depositions and fabrication of heterojunctions mainly in three ways.  First, since thin wafers are attractive for back contact cells as they require minority carrier diffusion lengths greater than twice the device thickness, low temperature junction and contact formation processes are crucial for such solar cells. Second, heterojunction are able to achieve the low surface recombination velocities required to achieve high VOC’s.  Finally, the central challenge in back contact solar cells, patterning in the rear, is easier for deposited heterojunctions since it is easier to mask and etch deposition compared to diffusions.  In addition, rear junction devices with deposited amorphous silicon (a-Si) layers also mitigate possible disadvantages of the front junction technology that require front transparent conductive oxides (TCO) and a-Si layers causing absorption losses.