Low-temperature deposition of amorphous silicon solar cells
Introduction
Besides glass, several novel substrates for amorphous silicon-based solar cells are recently used, such as sheet steel [1] or polyimide foils [2]. Low-cost polymer substrates like polyethylene (PE) or polyethylene terephtalate (PET) seem promising for further cost reduction of a-Si:H solar modules. The low glass transition temperature of these inexpensive materials, however, limits the deposition temperature Ts to values below 100°C. At such low temperatures, a-Si:H films from standard PECVD suffer from inferior structural quality, which causes a remarkable deterioration of their electronic properties. We observe a strongly increased structural disorder of our low-temperature a-Si:H reflecting in high values of the Urbach energy EU and sub-band gap absorption coefficient α1.2 eV. Low diffusion length for both electrons and holes, and poor electrical conductivity of doped layers prevent the use of such films in photovoltaics. It is known that hydrogen dilution of the feedstock gas silane compensates for material deterioration caused by low substrate temperatures. A low defect density and increased photosensitivity due to hydrogen dilution [3], [4] leads to device-quality material usable as photosensitive layer in a-Si:H-based solar cells [5].
This study reports on the improvement of the electronic quality of a-Si:H films deposited at temperatures of Ts=100°C and below. We present a comprehensive analysis of the majority carrier mobility-lifetime product μτ, the hole diffusion length LD, the Urbach energy, and sub-band gap absorption coefficient at different deposition temperatures and hydrogen dilution ratios for intrinsic films. Furthermore, a surprisingly strong correlation between the deposition temperature, hydrogen dilution ratio, and conductivity of n-type a-Si:H reveals the importance of developing low-temperature doped layers. The optimized intrinsic and doped materials prove to be well suited for the application in solar cells. We design, deposit, and investigate different single- and multi-junction cell structures. The desired low-cost polymers do not offer sufficient transparency for being used as a so-called ‘superstrate’ in pin structures, illuminated through the polymer film itself. Therefore, we develop not only pin- but also nip structures.
Section snippets
Experimental details
We deposit our a-Si:H layers and solar cells in a three chamber PECVD machine (load lock, doping chamber and intrinsic chamber) at a frequency of f=13.56 MHz. The feedstock gases are silane (SiH4), diborane (B2H6) and phosphine (PH3) for doping, methane (CH4) for the high band gap p-doped window layer and hydrogen (H2) for dilution. We define the hydrogen dilution ratio as rH=([H2]+[SiH4])/[SiH4]. For the Ts=100°C films we keep the deposition pressure at P=500 μbar and the plasma power density at
Single layers
In order to determine the influence of the deposition temperature Ts on structural, optical, and electronic properties of a-Si:H, we deposit films under standard conditions at different deposition temperatures. Keeping the pressure at P=150 μbar, and the plasma power density at p=50 mW/cm2, we vary the deposition temperature between Ts=50°C and Ts=150°C. At Ts=75°C and 100°C, we additionally deposit samples at a hydrogen dilution ratio of rH=10, a total pressure of P=300 μbar, and a plasma power
Conclusions
We analyze the impact of the deposition temperature between Ts=50°C and 150°C on growth rate rd, optical band gap Eg, sub-band gap absorption coefficient α1.2 eV, Urbach energy EU, and majority carrier mobility-lifetime product μτ of a-Si:H films deposited by standard PECVD. The increase in EU and α1.2 eV, and the drop of μτ at lower deposition temperatures reveal the structural disorder and deterioration of the electronic quality of low temperature a-Si:H films. We attribute this behavior to a
Acknowledgements
We would like to thank C. Köhler and S. Trottenberg for technical support, T. Rinke for carefully reading the manuscript, and one referee for useful remarks on an earlier version of this paper.
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