Chuang-Chuang Tsai

Applications of µc-SiOx:H as integrated n-layer and back transparent conductive oxide for a-Si:H/µc-Si:H tandem cells

We have prepared n-type hydrogenated microcrystalline silicon oxide [?c-SiOx:H(n)] films with oxygen contents from 0 to 37.3 at. % by varying the CO2-to-SiH4 flow ratio in a plasma-enhanced chemical vapor deposition (PECVD) system. By using ?c-SiOx:H(n) as an effective replacement for integrated ?c-Si:H(n) and indium?tin oxide (ITO), ?c-Si:H single-junction and a-Si:H/?c-Si:H tandem cells exhibited significantly improved efficiencies of 6.35 and 10.53%, respectively. The improvement of the single-junction and tandem cells mainly arose from the enhancement of long-wavelength optical absorption in ?c-Si:H absorbers, which was confirmed by a quantum efficiency instrument showing a markedly enhanced spectral response at wavelengths from 600 to 1100 nm. Moreover, all the PECVD processes, except the metal contact, had an advantage of in situ deposition without breaking vacuum, thereby minimizing contamination of the interface. The simplified cell fabrication can enhance the fill factor, which will benefit industrial production.

Development of Hydrogenated Microcrystalline Silicon-Germanium Alloys for Improving Long-Wavelength Absorption in Si-Based Thin-Film Solar Cells

Hydrogenated microcrystalline silicon-germanium (μc-:H) alloys were developed for application in Si-based thin-film solar cells. The effects of the germane concentration and the hydrogen ratio on the μc-:H alloys and the corresponding single-junction thin-film solar cells were studied. The behaviors of Ge incorporation in a-:H and μc-:H were also compared. Similar to a-:H, the preferential Ge incorporation was observed in μc-:H. Moreover, a higher significantly promoted Ge incorporation for a-:H, while the Ge content was not affected by in μc-:H growth. Furthermore, to eliminate the crystallization effect, the 0.9 μm thick absorbers with a similar crystalline volume fraction were applied. With the increasing , the accompanied increase in Ge content of μc-:H narrowed the bandgap and markedly enhanced the long-wavelength absorption. However, the bias-dependent EQE measurement revealed that too much Ge incorporation in absorber deteriorated carrier collection and cell performance. With the optimization of and , the single-junction μc-:H cell achieved an efficiency of 5.48%, corresponding to the crystalline volume fraction of 50.5% and Ge content of 13.2 at.%. Compared to μc-Si:H cell, the external quantum efficiency at 800 nm had a relative increase by 33.1%.

Enhancement of Spectral Response in -:H Thin-Film Solar Cells with a-Si:H/c-Si:H P-Type Window Layers

The hydrogenated amorphous silicon (a-Si:H)/hydrogenated microcrystalline silicon (c-Si:H) double p-type window layer has been developed and applied for improving microcrystalline silicon-germanium p-i-n single-junction thin-film solar cells deposited on textured SnO2:F-coated glass substrates. The substrates of SnO2:F, SnO2:F/c-Si:H(p), and SnO2:F/a-Si:H(p) were exposed to H2 plasma to investigate the property change. Our results showed that capping a thin layer of a-Si:H(p) on SnO2:F can minimize the Sn reduction during the deposition process which had H2-containing plasma. Optical measurement has also revealed that a-Si:H(p) capped SnO2:F glass had a higher optical transmittance. When the 20 nm c-Si:H(p) layer was replaced by a 3 nm a-Si:H(p)/17 nm c-Si:H(p) double window layer in the cell, the conversion efficiency () and the short-circuit current density () were increased by 16.6% and 16.4%, respectively. Compared to the standard cell with the 20 nm c-Si:H(p) window layer, an improved conversion efficiency of 6.19% can be obtained for the cell having a-Si:H(p)/c-Si:H(p) window layer, with  = 490 mV,  = 19.50 mA/cm2, and FF = 64.83%.

Improved light management in a-Si:H/a-Si1?xGex:H tandem cells by employing multi-functional n-type microcrystalline silicon oxide

In this work, the development of plasma-enhanced chemical vapor deposition (PECVD) ?c-SiOx:H(n) and its application to a-Si:H/a-Si1?xGex:H tandem cells as the intermediate reflecting layer (IRL) and back reflector (BR) is presented. The n-type microcrystalline silicon oxide [?c-SiOx:H(n)] was used as multifunctional layers in silicon thin-film solar cells owing to its wide bandgap and low refractive index. In the development of ?c-SiOx:H(n), increasing RF power increased film oxygen content, which widened the bandgap while reducing dark conductivity. Applying the ?c-SiOx:H to a-Si:H/a-Si1?xGex:H tandem cells as IRL and BR significantly improved cell performance. The ?c-SiOx:H(n) IRL increases the current of the top cell, thus improving the light management in a-Si:H/a-Si1?xGex:H tandem cells. On the other hand, the ?c-SiOx:H(n) can be used as the BR replacing the n-type a-Si:H and ITO layers. The ?c-SiOx:H increased cell conversion efficiency by 12.9% as IRL, and by 9.7% as BR, achieving 10.03% efficiency.

Study of Transition Region of p-Type SiO:H as Window Layer in a-Si:H/a-SiGe:H Multijunction Solar Cells

We have studied the p-type hydrogenated silicon oxide (:H) films prepared in the amorphous-to-microcrystalline transition region as a window layer in a-Si:H/a-:H multijunction solar cells. By increasing the -to- flow ratio () from 10 to 167, the :H(p) films remained amorphous and exhibited an increased hydrogen content from 10.2% to 12.2%. Compared to the amorphous :H(p) film prepared at low , the :H(p) film deposited at of 167 exhibited a higher bandgap of 2.04 eV and a higher conductivity of 1.15 × 10−5 S/cm. With the employment of :H(p) films prepared by increasing from 10 to 167 in a-Si:H single-junction cell, the FF improved from 65% to 70% and the efficiency increased from 7.4% to 8.7%, owing to the enhanced optoelectrical properties of :H(p) and the improved p/i interface. However, the cell that employed :H(p) film with over 175 degraded the p/i interface and degraded the cell performance, which were arising from the onset of crystallization in the window layer. Compared to the cell using standard a-:H(p), the a-Si:H/a-:H tandem cells employing :H(p) deposited with of 167 showed an improved efficiency from 9.3% to 10.3%, with of 1.60 V, of 9.3 mA/cm2, and FF of 68.9%.

Wide-Range Enhancement of Spectral Response by Highly Conductive and Transparent μc-SiOx:H Doped Layers in μc-Si:H and a-Si:H/μc-Si:H Thin-Film Solar Cells

The enhancement of optical absorption of silicon thin-film solar cells by the p- and n-type μc-SiOx:H as doped and functional layers was presented. The effects of deposition conditions and oxygen content on optical, electrical, and structural properties of μc-SiOx:H films were also discussed. Regarding the doped μc-SiOx:H films, the wide optical band gap () of 2.33 eV while maintaining a high conductivity of 0.2 S/cm could be obtained with oxygen incorporation of 20 at.%. Compared to the conventional μc-Si:H(p) as window layer in μc-Si:H single-junction solar cells, the application of μc-SiOx:H(p) increased the and led to a significant enhancement in the short-wavelength spectral response. Meanwhile, the employment of μc-SiOx:H(n) instead of conventional ITO as back reflecting layer (BRL) enhanced the external quantum efficiency (EQE) of μc-Si:H single-junction cell in the long-wavelength region, leading to a relative efficiency gain of 10%. Compared to the reference cell, the optimized a-Si:H/μc-Si:H tandem cell by applying p- and n-type μc-SiOx:H films achieved a of 1.37 V, of 10.55 mA/cm2, FF of 73.67%, and efficiency of 10.51%, which was a relative enhancement of 16%.

Optimization of μc-Si1−xGex:H Single-Junction Solar Cells with Enhanced Spectral Response and Improved Film Quality

Effects of RF power on optical, electrical, and structural properties of μc-Si1−xGex:H films was reported. Raman and FTIR spectra from μc-Si1−xGex:H films reflected the variation in microstructure and bonding configuration. Unlike increasing the germane concentration for Ge incorporation, low RF power enhanced Ge incorporation efficiency in μc-Si1−xGex:H alloy. By decreasing RF power from 100 to 50 W at a fixed reactant gas ratio, the optical bandgap of μc-Si1−xGex:H was reduced owing to the increase in Ge content from 11.2 to 23.8 at.%, while Ge-related defects and amorphous phase were increased. Consequently, photo conductivity of 1.62 × 10−5 S/cm was obtained for the μc-Si1−xGex:H film deposited at 60 W. By applying 0.9 μm thick μc-Si1−xGex:H absorber with of 48% and [Ge] of 16.4 at.% in the single-junction cell, efficiency of 6.18% was obtained. The long-wavelength response of μc-Si1−xGex:H cell was significantly enhanced compared with the μc-Si:H cell. In the case of tandem cells, 0.24 μm a-Si:H/0.9 μm μc-Si1−xGex:H tandem cell exhibited a comparable spectral response as 0.24 μm a-Si:H/1.4 μm μc-Si:H tandem cell and achieved an efficiency of 9.44%.