Photodetectors based on amorphous and microcrystalline silicon

doi:10.1016/j.tsf.2006.11.126

Thin Solid Films

Volume 515, Issue 19, 16 July 2007, Pages 7522-7525

https://doi.org/10.1016/j.tsf.2006.11.126 Get rights and content

Abstract

p-i-n diodes based on amorphous and microcrystalline silicon are used for different applications like sensors or solar cells. However, their long-term stability and dynamic behavior are still under discussion. Therefore, the optical properties of both types of diodes were investigated by a comparative study. The long-term stability of non-encapsulated devices was tested by means of light soaking, damp heat testing and high-temperature treatment of up to 2000 h. The dynamic properties of the thin-film silicon photodetectors were studied by measuring and analysing the admittance under different bias conditions.

Introduction

Amorphous and microcrystalline silicon and its alloys are promising candidates for the realization of advanced sensing systems, solar cells/modules and thin-film transistors (TFT). They exhibit excellent optoelectronic properties and can be fabricated on large area and different kinds of substrates like glass, foils and crystalline silicon. Although solar modules based on a-Si:H/μc-Si:H tandem cell structure are close to the market [1], [2] and advanced sensing systems as color sensor arrays, position detectors and spectrometers were realized [3], [4], [5], questions regarding (i) the long-term stability of amorphous and microcrystalline silicon diodes and (ii) their different transient behavior still remain open.

Aspects as in-diffusion and adsorption of chemical species can affect the lifetime of the contacts and the optoelectronic properties of both the transparent conductive oxide (TCO) and the thin-film silicon device. In particular for μc-Si:H, where the microstructure can change from a more porous-like structure to a regime where amorphous growth prevails, various phenomena are reported [6], [7]. Therefore, within a comparative study of a-Si:H and μc-Si:H diodes the long-term stability of non-encapsulated thin-film diodes was studied by damp heat testing (temperature (T) = 85 °C, humidity = 85%), light soaking (AM 1.5 illumination condition, T = 50 °C) and high T-treatment (T = 150 °C) at dry atmosphere. The latter study was performed to further stress the multilayer system and find out an upper T limit of operation. The performance of the detectors was measured as a function of the exposure time.

Additionally, the dynamic property of the p-i-n photodetector was studied, which is of particular interest for a precise optimisation of the read out process. Therefore, the small signal frequency response was studied by the voltage- and frequency-dependent admittance measured by a LCR meter.

Section snippets

Experiment

The investigated a-Si:H and μc-Si:H p-i-n diodes were prepared in superstrate configuration on fluorine doped tin oxide (FTO, Asahi substrate) or sputtered aluminum doped ZnO (AZO) coated glass substrates. The absorber layer of a-Si:H and μc-Si:H p-i-n diodes is between 300 nm and 1 μm. The μc-Si:H diodes were prepared close to the transition region of amorphous growth where the maximum cell efficiency is achieved. For more details regarding the ZnO sputtering and PECVD processes see Refs. [8],

Long-term stability

The instability of ZnO concerning humidity and thin-film silicon during light soaking is well known. Therefore, we have investigated the long-term stability of a-Si:H and μc-Si:H solar cells on FTO and AZO substrates with a ZnO/Ag back contact during light soaking and damp heat testing by a comparative study. Fig. 1 shows a typical dark IV behavior of an a-Si:H (top) and a μc-Si:H (bottom) diode with an i-layer thickness of 400 nm and 1 μm, respectively, in the initial state and after 2070 h of

Conclusion

Detectors of a-Si:H and μc-Si:H show very similar, inherent, long-term stability behavior even after harsh environmental tests which recommends these detectors for different indoor and outdoor applications. Damp heat testing for 2000 h has a minor influence on the cell performance and high T treatment at 150 °C results only in a deterioration of the blue response of both diode types. Light soaking leads to the well-known Staebler–Wronski effect of a-Si:H diodes whereas the bulk properties of

Acknowledgements

The authors like to acknowledge J. Kirchhoff, W. Reetz and C. Zahren for their technical assistance and T. Repmann for the helpful discussions.

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Photodetectors based on amorphous and microcrystalline silicon

Abstract

Introduction

Section snippets

Experiment

Long-term stability

Conclusion

Acknowledgements

Sol. Energy Mater. Sol. Cells

Thin Solid Films

Thin Solid Films

J. Non-Cryst. Solids

Appl. Phys. Lett.