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Institute Silicon Photovoltaics

Surface dipoles by small organic molecules:

Functionalisation and passivation of Si surfaces by organic buffer layers

Organic layers are used for functionalisation (buffer layer) and passivation of Si surfaces. Benzene derivatives can be easily grafted on Si surfaces by an electrochemical reaction from aqueous diazonium salt solutions as sketched in figure 1.

enlarged view

Fig. 1: Pathway of the electrochemical grafting of organic molecules from aqueous solutions of diazonium salts [1,2].


Fig. 2: Current (top), change in photo-voltage, ΔUPV, and integrated PL intensity, IPL, as a function of time during grafting of 4-NB from 4-nitrobenzene diazonium tetrafluoroborate (4-NBDT).

The intermediate Si-dangling bonds (obviously a complex with water and/or benzene molecules) quenches the band to band related photoluminescence, IPL, of Si (red line in fig. 2) when the reaction sets on as reflected by an increase in cathodic current (fig. 2, top). However, IPL increases again and cures out with time pointing to a well passivated organic/c-Si interface. At the same time, the photovoltage, ΔUPV, changes by about -110 mV due to surface dipole induced by 4-nitrobenzene (4-NB).


DFT calculation of dipole strength and orientation of some molecules are presented in fig. 3.

Fig. 3: DFT calculations of the dipole strength and orientation of some organic molecules used (values in parenthesis are taken from literature)


Fig. 4: Band bending, ΔUPV, and work function, measured by CPD and UPS, as a function of the effective surface dipole

The strong variation in dipole strength and orientation of these molecules (indicated by a red arrow in fig. 3) leads to a strong influence on the surface dipoles and consequently on the band bending and work function of Si as measured by PV techniques, UPS and contact potential difference which are plotted as a function of the effective (perpendicular) dipole moment in fig. 4. The thickness of such organic buffer layers is in the range of 1-2 monolayers as revealed from TEM images.


Conclusion

  • Organic / c-Si are well passivated interfaces
  • Controlled tuning of the band bending and work-function
  • Bio-compatible
  • The use of thin polymeric layers as an emitter in solar-cells is under investigation and testing

References

[1] Surface dipole formation and non-radiative recombination at p-Si(111) surfaces during electrochemical deposition of organic layers; Hartig, P.; Dittrich, Th.; Rappich, J.; J. Electroanal. Chem. (524-525) (2002) 120-126.

[2] Stable electrochemically passivated Si surfaces by ultra thin benzene-type layers; Rappich, J.; Hartig, P.; Nickel, N.H.; Sieber, I.; Schulze, S.; Dittrich, Th.; Microelectron. Eng 80 (2005) 62-65.

[3] Electronic Properties of Si Surfaces and Side Reactions during Electrochemical Grafting of Phenyl Layers; Rappich J., Merson A., Roodenko K., Dittrich Th., Gensch M., Hinrichs K., Shapira Y.; J. Phys. Chem. B 110(3) (2006) 1332.