Band-gap profiling in amorphous silicon–germanium solar cells

Abstract

Profiled buffer layers at the interfaces of amorphous silicon–germanium (a-SiGe:H) solar cells are routinely used to avoid band-gap discontinuities and high-defect densities at the p/i and i/n interfaces. It is shown that such profiled a-SiGe:H buffer layers can be replaced by a constant band-gap a-Si:H buffer, an inverse profiled a-SiGe:H buffer, or even a 3-nm-thin (δ) buffer at some distance away from the interface without losses in the open-circuit voltage VOC and fill factor while maintaining the same short current density jSC. In view of these results, common model assumptions for a-SiGe:H solar cells have to be revised.

High Efficiency Inorganic/Inorganic Amorphous Silicon/Heterojunction Silicon Tandem Solar Cells

“…It is well-known that hydrogenated amorphous silicon germanium (a-SiGe:H) and hydrogenated nanocrystalline silicon (nc-Si:H) possess a higher absorption coefficient in the long wavelength region than hydrogenated amorphous silicon (a-Si:H) 12–17 . However, the narrow optical band gaps of an a-SiGe:H and nc-Si:H could cause band gap discontinuity at the p/i and i/n interfaces, leading to a detrimental open-circuit voltage (V oc ) and fill factor (FF) due to the high defect density at these interfaces 18,19 . Thus, we propose the possibility of sustaining high V oc , FF, and J sc simultaneously by using the band gap profiling of the highly absorbing a-SiGe:H alloy, along with buffer layers at the p/i and i/n interfaces, as shown in Fig.…”

Amorphous SiGe:H Thin Film Solar Cells with Light Absorbing Layers of Graded Bandgap Profile

“…In addition, a single junction cell structure with improved efficiency can be utilized as a component cell of multi-junction solar cells. Earlier works exploiting the concept of Ge composition grading or bandgap grading have been carried out to avoid band gap discontinuity and high defect density at the p/i and i/n interfaces 10,11 and to study the transport and recombination of carriers. 12,13 In the deposition of a-SiGe:H using PECVD, SiH 4 (100%) and GeH 4 diluted with H 2 (1.5% GeH 4 ) were used as source gases.…”

High efficiency triple junction thin film silicon solar cells with optimized electrical structure

“…Various photo‐electrical structure designs were applied in fabricating the a‐SiGe:H single‐junction solar cell to optimize the photo‐electrical performance. We applied a linear band grading along the growth direction to improve the fill factor ( FF ) while maintaining a high long‐wavelength response ; used a‐Si:H buffer layers with specific band gaps (1.82 eV) and thicknesses at the p‐i and i‐n interfaces to compensate the band gap discontinuity and reduce the electric‐field screening effect ; inserted an n‐µc‐SiO x :H layer after the n‐a‐Si:H layer to reinforce the long‐wavelength response and V oc ; and employed a p‐µc‐Si:H/p‐a‐SiC:H double‐layer structure to prevent the V oc loss resulting from the formation of a TCO/P contact potential , to enhance the short‐wavelength response, and for the interconnection with the top a‐Si:H component cell. The final optimized structure was glass/ZnO:Al/p‐µc‐Si:H/p‐a‐SiC:H/a‐Si:H (buffer)/a‐SiGe:H (grading)/a‐Si:H (buffer)/n‐a‐Si:H/n‐µc‐SiO x :H/Al.…”