Study Of Amorphous Silicon-Germanium Thin Film Solar Cell

In present work we fabricated a-SiGe:H films by radio frequency plasma enhancedchemical vapor deposition(RF-PECVD) technique. Effects of different process parameterson the films’ deposition rate, optical bandgap and electrical properties were investigated.We found that within the scope of the study properties of a-SiGe:H film were improvedfirst with increase of deposition pressure and discharge power and then degraded. It wasnecessary to combine discharge power and pressure to control the plasma properties so asto control ion bombardment and the plasma reactions. Under certain pressure and powerconditions, deposition rate of the film increased with the germane concentration while theoptical bandgap decreased and the electrical properties degraded. The germaneconcentration couldn’t be too high when the optical bandgap met the goal. Theinvestigations also showed that higher substrate temperature promoted diffusion of activeradicals on the growing surface. As a result, deposition rate and optical bandgap dropped.The relaxation of the film structure also resulted in improvement of the electricalproperties. However, electrical performances declined when substrate temperature was toohigh as the defect density of states climbed, not fallen. The role of hydrogen dilution wassimilar to that of substrate temperature. The higher hydrogen dilution, the less the densityof dangling bonds and the better the material qualities. But the etching effect would bedominant if hydrogen dilution was too high. Under the optimization process conditions,a-SiGe:H film with1.5eV bandgap and6×104photosensitivity was obtained.Further, we studied influences of various window layers, interface layers, thicknessof intrinsic layer and the back contact transparent conductive oxide (TCO) thin films onthe single-junction a-SiGe:H cell. Adoption of p-a-SiC:H window layer enhanced theopen-circuit voltage and the short-circuit current. Carbon buffer layer and graded germanebuffer layer at p/i interface improved the cell’s performance while the inverse gradedgermane buffer layer at i/n interface had little effect on the performance. The thickness of the intrinsic layer is a delicate trade-off between the absorption and the collection. It musthave a certain thickness but the thickness can’t exceed the collection length of chargecarriers. The back contact TCO improved the short-circuit current density (Jsc) and also theconversion efficiency by a large margin owing to lower absorption loss of the metal backelectrode. Finally we achieved single-junction a-SiGe:H cell with8.6%conversionefficiency.