Paper NS1+PV-TuA11
A New Look at Si Inversion-Layer Solar Cells, Meeting Ground for Inorganic, Molecular and Organic Electronics

Tuesday, November 10, 2009, 5:20 pm, Room A8

We suggest a near-ambient temperature, simple and potentially low-cost approach to fabricate semiconductor solar cells. To this end we use a monolayer of organic molecules that self-assembles onto n-Si, to passivate and buffer the Si surface. The direct binding of the molecules to the oxide-free Si surface also provides a strong interface dipole and effectively reduces the semiconductor's electron affinity by as much as 0.7 eV. A thin layer of a conducting polymer with a high work function, that is spin-coated on top of the molecular layer plays a threefold role:

1. the high work function induces inversion in the Si and leads to an n-p⁺ homojunction in the Si;

2. with n~1.5 refractive index the polymer acts as an anti-reflective coating to the Si.

3. because it protects the molecular monolayer, subsequent metal deposition and encapsulation is now possible.

Good and stable interface passivation along with strong inversion allow minority carriers, generated by absorbed sun light, to move laterally within the inverted Si top layer and to be collected by a minimal area metal grid, deposited on the conducting polymer. For the same reason (lateral conductance) photo-current losses, due to polymer sheet resistance, are minimized. Not only are cells fabricated without high temperature steps, the use of small organic molecules appears to convey here a unique advantage over inorganic passivation or buffer layers. The approach, which includes no or minimal (for metal grid) high vacuum steps, should be applicable to other inorganic absorbers, amorphous materials and thin films to improve photovoltaic solar energy conversion.