n‐Si–Organic Inversion Layer Interfaces: A Low Temperature Deposition Method for Forming a p–n Homojunction in n‐Si

However, a simple, low-cost method for forming an inversion layer in a Si-based solid state device is lacking. Identifying such a method can open doors in Si device processing, with implications beyond photovoltaic devices. For PEDOT:PSS–n-Si junctions, the low-temperature processing has the potential to prevent formation of the interface defects that plague Si-metal junctions, avoiding Fermi level pinning and allowing large barrier heights. Another advantage of the low temperature processing is the stability of molecular passivation layers, such as methyl termination, [ 9 ] to deposition conditions. Additionally, a thin oxide passivation layer has been found to slightly enhance solar cell performance over H-terminated devices, [ 10 ] suggesting some improvement in surface passivation is possible for certain device conditions. Indeed, relatively high open-circuit voltages have been reported, reaching 610 mV for substrates with N D = 6 × 10 15 cm −3 when the Si surface was passivated with SiO x . [ 10 ] While this is slightly less than the V oc reached in HIT inversion layer devices for similar N D , it is high enough to suggest that the Si has been pushed to inversion. Additionally, V bi values indicative of inversion have been reported in the past, but they were not identifi ed as such, and the implications were not discussed. [ 11 ] In a recent review article [ 3a ] we suggested that these devices operate in inversion, but we did not have experimental evidence of strong inversion. The key distinction between an inversion layer model and a charge-selective electrode model is the role of the top contact. In an inversion layer device, charge separation has already occurred within the (n-type) semiconductor, because the surface is inverted (p-type). Charge selectivity of the contact is not necessary for device operation, as the p–n junction created in the semiconductor has already done the job. Within this model, a charge-selective electrode can further improve device performance by reducing recombination, as even a device operating in strong inversion can have some carrier density of both types Recent interest in hybrid solar cells has led to reports of above 11% effi ciency in n-Si–polyethylene dioxythiophene– poly(styrene)sulfonate (PEDOT:PSS) solar cells. [ 1 ] PEDOT:PSS is a transparent, wide-bandgap, organic hole conductor with a relatively high work function, φ m = 5.0 eV. [ 2 ] Silicon-PEDOT:PSS hybrid devices have been described as either Schottky barrier solar cells, [ 3 ] where the highly conductive PEDOT:PSS serves as the metallic contact, or as p–n junction type devices, where PEDOT:PSS acts as the “p-type” layer, providing charge separation by its hole-selective nature. [ 4 ] Parsing the relative importance of these two device models in these junctions is crucial to narrowing future efforts at device improvement. In a typical Schottky barrier junction, charge transfer between a high (low) work function metal in contact with an n-(p-)type semiconductor can form, across the barrier, a builtin voltage, V bi = φ m – E F , where E F is the semiconductor Fermi energy. The barrier height, φ b , determined from current–voltage measurements, is related to V bi by φ b = V bi + | E c – E F |, for an n-type semiconductor–metal junction ( Figure 1 a). It follows from analysis of Figure 1 that, if V bi exceeds a critical value, V bi > | E F – E i |, where E i is the intrinsic Fermi level, the majority carrier at the surface of the semiconductor is of opposite type to that in the bulk, generating a p–n junction without the need for dopant diffusion. This process is known as inversion and the p-type layer, in our example, as an inversion layer, which is generated by the large work function difference between the metal and semiconductor. Strong inversion occurs when the concentration of carriers in the inversion layer exceeds the dopant concentration in the bulk, at V bi > 2| E F – E i | (Figure 1 a). The fundamental difference between a standard Schottky-type solar cell with a barrier height resulting in depletion at the surface, and an inversion layer solar cell is the existence of a p–n junction within the semiconductor itself, leading to transport that is dominated by what are the minority carriers in the bulk of the material, regardless of the type of contact. In the strong inversion regime, the low bulk majority carrier density at the semiconductor surface prevents recombination at defects at the interface with the inversion-inducing contact, and the high bulk minority carrier density improves screening of donor ion