Influence of alloy composition on carrier transport and solar cell properties of hydrogenated microcrystalline silicon-germanium thin films

Abstract

The authors report on carrier transport properties and spectral sensitivities of hydrogenated microcrystalline silicon-germanium (μc-Si1−xGex:H) alloys fabricated by low-temperature (∼200°C) plasma-enhanced chemical vapor deposition over the wide compositional range. Hall-effect and conductivity measurements reveal a change from weak n-type to strong p-type conduction for x>0.75 and a monotonic decrease in photoconductivity upon Ge incorporation. In a p-i-n diode structure, the Ge incorporation into i layer reduces quantum efficiencies in the short wavelengths, indicating an increased photocarrier recombination at p∕i interface. Nevertheless, under reverse biased condition, a 0.9-μm-thick μc-Si0.6Ge0.4:H absorber yields a large photocurrent of >27mA∕cm2 (air mass 1.5 global) with spectral sensitivities extending into infrared wavelengths, offering a potential advantage over conventional microcrystalline silicon solar cells.

Transport properties of nanocrystalline silicon and silicon–germanium

“…The value in Mx-Si:H is much higher than what we can expect in nc-Si:H, which can be deduced from the graph in Figure2.16 [49], a plot of simulated and experimental values of lifetimes in various materials.…”

“…Figure 2.12 Transport properties of µc-(Si,Ge):H measured at various Ge compositions [49] 2. Matsui et al [51], studied the Ge fraction as a function of GeH 4 concentration in the gas phase, defined as […”

Thin film solar cells incorporating microcrystalline Si1–xGex as efficient infrared absorber: an application to double junction tandem solar cells

“…On the other hand, open-circuit voltage (V oc ) and fill factor (FF) decrease with Ge content due to the band gap narrowing and the increased carrier recombination. As reported earlier, Ge incorporation larger than x$0.2 leads to the appreciable creation of Ge dangling bond defects (acceptorlike states) [17][18][19] by which solar cell performance is degraded severely. In contrast, the solar cell with smaller Ge content (x ¼ 0.1) maintains relatively high V oc and FF while exhibiting markedly high J sc , resulting in a higher conversion efficiency than for mc-Si:H solar cell.…”

Effect of Ge atoms on crystal structure and optoelectronic properties of hydrogenated Si–Ge films

“…[10][11][12][13] It has been reported that the electronic conductivity in the dark and under illumination depends strongly on the structural composition of µc-Si 1−x Ge x :H films. 6,10,14,15 However, the crystallization of the µc-SiGe:H films was considered complex because Si and Ge atoms have different incorporation efficiency in the crystalline matrix during the film growth. 10,14 So far the effects of crystallinity on the optoelectronic properties of the µc-Si 1−x Ge x :H films have not been carefully studied.…”

“…1-7 µc-SiGe:H features a higher absorption coefficient than microcrystalline silicon (µc-Si:H) throughout the solar spectrum, [4][5][6][7][8][9] whereas the optoelectronic quality of the µc-Si 1−x Ge x :H films is usually not satisfactory due to the increased defect density with Ge incorporation. [10][11][12][13] It has been reported that the electronic conductivity in the dark and under illumination depends strongly on the structural composition of µc-Si 1−x Ge x :H films.…”