Effect of illumination-induced space charge on photocarrier transport in hydrogenated microcrystalline Si1−xGex p-i-n solar cells

Abstract

Photocarrier transport in hydrogenated microcrystalline Si1−xGex (μc-Si1−xGex:H) p-i-n solar cells (0<x<0.42) is studied using spectral response measurement under various bias light illuminations. The solar cell composed of μc-Si0.8Ge0.2:H i layer reveals an injection-level-independent carrier collection, demonstrating a 6.1% conversion efficiency with infrared sensitivities higher than double-thickness μc-Si:H solar cells due to an enhanced optical absorption. However, the illumination of the solar cells for x>0.35, particularly in the wavelength range of <650nm, induces a strong carrier recombination near the p-i interface and a weak collection enhancement in the bulk, indicative of field distortion by the negative space charge generated near the p-i interface. This finding is consistently explained by the increased acceptorlike states in undoped μc-Si1−xGex:H for large Ge contents.

Thin film solar cells based on microcrystalline silicon–germanium narrow-gap absorbers

“…The decrease in short wavelength response can be attributed to the increased carrier recombination near the p-i interface. In addition, we found the strong injection-leveldependent p-i interface recombination in the mc-Si 1Àx Ge x :H p-i-n solar cells when xX0.35 [6]. Spectral response measurements indicate that the built-in field in i-layer is highly distorted by the negative space charge generated near the p-i interface under intense p-side illumination.…”

“…However, a further increase in Ge content leads to the considerable degradation in all solar cell parameters. For x40.2, the decrease in V oc is much greater than the band gap narrowing [6], indicating that the solar cell performance is dominated by the carrier recombination due to the increased dangling bond defects in the i-layer. Table 1 Illuminated J-V parameters of the mc-Si 1Àx Ge x :H p-i-n solar cells with different Ge contents in the i-layer.…”

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.…”

Carrier collection characteristics of microcrystalline silicon–germanium p–i–n junction solar cells

“…2, the lc-Si 0.8 Ge 0.2 :H solar cell exhibits larger QEs than the lc-Si:H solar cell for k > 600 nm. As a result, we obtained an increase of short-circuit current density by $4 mA/cm 2 and a conversion efficiency of 6.1% using a 1-lm-thick lcSi 0.8 Ge 0.2 :H absorber [6]. The solar cells with larger Ge contents (x P 0.35) show a further enhancement of the infrared response (k > 900 nm), whereas the QE peak at shorter wavelength deceases drastically.…”

“…Nevertheless, we find that the photocarrier collection in the lc-Si 1Àx Ge x :H solar cells depends strongly upon illumination condition. When x P 0.35, the QE spectrum showed a drastic decrease in the short wavelengths with increasing the intensity of the AM1.5 bias light [6]. In addition to the AM1.5 white bias illumination, as shown in Fig.…”