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

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Abstract

We report on the carrier collection characteristics of hydrogenated microcrystalline silicon–germanium (μc-Si1−xGex:H) p–i–n junction solar cells fabricated by low-temperature (∼200 °C) plasma-enhanced chemical vapor deposition. Spectral response measurements reveal that the Ge incorporation into absorber i-layer reduces the quantum efficiencies at short wavelengths. Furthermore, the illumination of the solar cells for x  0.35, particularly in the wavelength range of <650 nm, induces a strong injection-level-dependent p–i interface recombination and a weak collection enhancement in the bulk. These results indicate that space charges near the p–i interface are largely negative, which gives rise to an electric field distortion in the i-layer. We attribute the negative space charges to the presence of the acceptor-like states that are responsible for the strong p-type conduction observed in undoped μc-Si1−xGex:H films for large Ge contents.

Introduction

Hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) are currently combined into a tandem solar cell structure because of the better photovoltaic performance than their single junction devices. However, the achievable efficiency of the a-Si:H/μc-Si:H tandem solar cells is still limited by the lack of infrared response in the μc-Si:H bottom cell owing to its weak infrared absorption. To extend the spectral sensitivities of solar cells into longer infrared wavelengths, we have proposed the application of hydrogenated microcrystalline silicon–germanium alloys (μc-Si1−xGex:H) as a bottom cell material in a triple-junction structure, i.e., a-Si:H/μc-Si:H/μc-Si1−xGex:H. The μc-Si1−xGex:H films can be grown at low-temperature (∼200 °C) by plasma-enhanced chemical vapor deposition (PECVD), exhibiting the higher absorption coefficients compared to μc-Si:H over the entire solar spectrum [1], [2], [3], [4]. Although this material is advantageous in terms of optical absorption, the photocarrier collection in μc-Si1−xGex:H p–i–n solar cells has been shown to degrade severely as the Ge content increases in the i-layer particularly for x > 0.2 [3], [4]. Recently, we found that the substantial Ge incorporation gives rise to an electrical change from weak n-type to strong p-type conduction with a monotonic decrease in photoconductivity [4]. In this work, we have studied the impact of these electrical changes due to Ge incorporation on the carrier collection characteristics of μc-Si1−xGex:H p–i–n junction solar cells.

Section snippets

Experimental procedures

The μc-Si1−xGex:H films were prepared by capacitively-coupled 100-MHz PECVD using a SiH4–GeH4–H2 gas mixture. Films were deposited on glass substrates at a temperature of 200 °C. For film characterization, Hall-effect measurements were performed at room temperature using the van der Pauw method. The electrical properties were also studied by the coplanar conductivity measurement under dark and AM1.5 (air mass 1.5, 100 mW/cm2) illumination. The dangling bond defect density of the μc-Si1−xGex:H

Film properties

Fig. 1 shows the film properties of the μc-Si1−xGex:H including (a) carrier concentration, (b) ESR defect density and (c) coplanar dark and photoconductivities as a function of Ge content. Hall-effect measurement reveals that films exhibit a weak n-type character with electron concentrations of 1012–3 × 1013 cm−3 when x < 0.75. With further increase in Ge content, a transition occurs from n- to p-type conduction, where the hole concentration increases by more than five orders of magnitude. In Fig. 1

Discussion

Hall-effect measurement on μc-Si1−xGex:H films revealed an electrical change from n- to p-type conduction for x > 0.75 where free hole concentration increased by more than five orders of magnitude. In the smaller Ge content regime than this threshold, we observed a decrease in electron concentration by an order of magnitude when increasing x from 0 to 0.5. Although the microscopic origin is still unclear, we consider that Ge incorporation induces an acceptor state generation in the band gap,

Conclusions

We have studied the carrier collection characteristics of μc-Si1−xGex:H p–i–n junction solar cells prepared by low-temperature PECVD in the composition range 0  x  0.42. Although the Ge incorporation provides an enhanced infrared absorption, it gives rise to a significant p–i interface recombination when x  0.35. Spectral response measurements indicate that the built-in field in the i-layer is strongly distorted by the negative space charge generated near the p–i interface under blue bias

Acknowledgement

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO).

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