Material and solar cell research in microcrystalline silicon

TLDR: In this paper, the authors describe the introduction of hydrogenated microcrystalline silicon (μc-Si:H) as novel absorber material for thin-film silicon solar cells.

About: This article is published in Solar Energy Materials and Solar Cells.The article was published on 2003-07-01 and is currently open access. It has received 425 citations till now. The article focuses on the topics: Micromorph & Monocrystalline silicon.

Figures

Fig. 15. Temperature behavior of a high-Voc mc-Si:H cell compared with that of a c-Si cell, according to Ref. [48].

Fig. 8. mt products for majority carriers (deduced from sphoto measurements) and for minority carriers (deduced from Lamb measurements) plotted as a function of the parameter b (indicative of Fermi-level position) for a large number of mc-Si:H samples. Lines show typical values for device-quality, undegraded a-Si:H layers; from Ref. [31].

Fig. 13. Comparison of an intrinsic mc-Si:H film and the corresponding p–i–n solar cell in function of air exposure after deposition. The film was characterized by the dark conductivity measurement and the cell by I2V measurements under AM 1.5. The dashed lines are drawn to guide the eye [45].

Fig. 5. ‘‘Effective’’ (or ‘‘apparent’’) optical absorption of a mc-Si:H layer deposited by VHF-GD as directly measured by CPM [25] in comparison with amorphous and crystalline silicon.

Fig. 7. Hydrogen content for a whole series of mc-Si:H layers deposited at various values of hydrogen dilution, i.e. at various values of silane concentration as determined by IR spectroscopy (bonded hydrogen) and ERDA (bonded and non-bonded hydrogen). Data taken from Ref. [33].

Fig. 12. Comparative behavior of a microcrystalline silicon (mc-Si:H) solar cell and an amorphous silicon (a-Si:H) solar cell under intensive light illumination, from a sodium lamp. Efficiencies were measured before/after light soaking under standard conditions [9].

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Material and solar cell research in microcrystalline silicon" ?

Shah et al. this paper described the introduction of hydrogenated microcrystalline silicon ( mc-Si: H ) as novel absorber material for thin-film silicon solar cells. 

Q2. What is the promising scheme to enhance light trapping in a microcrystalline cell?

An ‘‘intermediate mirror layer’’ between the amorphous top cell and the microcrystalline bottom cell is an interesting scheme that has been tried in this context [56,59,60]. 

Q3. What is the advantage of a higher fexc?

Because deposition rate rd is one of the main existing bottlenecks when one wants to reduce the production cost of mc-Si:H solar cells and modules, the increase of rd with higher fexc is a significant advantage for an industrial plant. 

Q4. What is the way to improve the performance of a micromorph tandem?

If the tandem is top-cell limited, it suffers quite substantially from light-induced degradation, but it will have a favorably low temperature coefficient. 

Q5. How does one keep the thickness of the microcrystalline bottom cell?

In order to keep the deposition/fabrication time of the microcrystalline bottom cell within reasonable limits, one has to keep its thickness also low (below 1.5 mm). 

Q6. What is the promising method to increase the current of micromorph tandems?

To increase the current of micromorph tandems the most promising method is, as of now, to enhance light trapping within the solar cell: 

Q7. What is the effect of oxygen on the collection of a mc-Si:?

The detrimental effect of oxygen on collection in the intrinsic (i)-layer of a mc-Si:H pin-type solar cell certainly depends on the way oxygen is incorporated into the silicon crystallites. 

Q8. What was the first experimental method used for mc-Si:H?

In these first experiments, the deposition rates were very low, with values well below 1 (A/s. Such mc-Si:H layers had, in the early days, a strong n-type character; they could also be easily doped, e.g. with phosphine, so as to push the Fermi level upwards towards the conduction band edge and to obtain layers with an even more pronounced n-type character, or with diborane, so as to push the Fermi level downwards towards the valence band edge and to obtain p-type layers. 

Q9. What is the main reason why micromorph silicon is so attractive?

Its basic stability w.r.t. light-induced degradation makes it especially attractive for photovoltaic and optoelectronic applications. 

Q10. What is the effect of mc-Si:H solar cells on the efficiency?

It appears that mc-Si:H solar cells with high Voc values show a less pronounced drop in efficiency as the temperature increases, than wafer-based crystalline silicon (c-Si) solar cells (Fig. 15, [48]). 

Q11. What is the mc-Si:H layer's diffusion length?

The diffusion length Ldiff of minority carriers in growth direction can be measured for mc-Si:H layers by the SPV (surface photovoltage) technique. 

Q12. What is the effect of light on mc-Si:H?

On the other hand, mc-Si:H layers do suffer from another potential source of instability, i.e. from an increasing contamination by oxygen, when exposed to air over a period of weeks or longer. 

Q13. Why do shunts occur in mc-Si:H cells?

Because certain types of mc-Si:H layers exhibit cracks, shunts can constitute here a more pronounced problem than in other thin-film solar cells.