Junction formation and current transport mechanisms in hybrid n-Si/PEDOT:PSS solar cells

Abstract

We investigated hybrid inorganic-organic solar cells combining monocrystalline n-type silicon (n-Si) and a highly conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (

Pedot: PSS). The build-in potential, photo- and dark saturation current at this hybrid interface are monitored for varying n-Si doping concentrations. We corroborate that a high build-in potential forms at the hybrid junction leading to strong inversion of the n-Si surface. By extracting work function and valence band edge of the polymer from ultraviolet photoelectron spectroscopy, a band diagram of the hybrid n-Si/

Pedot: PSS heterojunction is presented. The current-voltage characteristics were analyzed using Schottky and abrupt pn-junction models. The magnitude as well as the dependence of dark saturation current on n-Si doping concentration proves that the transport is governed by diffusion of minority charge carriers in the n-Si and not by thermionic emission of majorities over a Schottky barrier. This leads to a comprehensive explanation of the high observed open-circuit voltages of up to 634 mV connected to high conversion efficiency of almost 14%, even for simple planar device structures without antireflection coating or optimized contacts. The presented work clearly shows that

Pedot: PSS forms a hybrid heterojunction with n-Si behaving similar to a conventional pn-junction and not, like commonly assumed, a Schottky junction.

Figure 1. Schematic of the device structure of a fabricated n-Si/PEDOT:PSS solar cell.

The surface of the monocrystalline n-type silicon wafer is structured by resist to define an active area for the spin coated polymer. The device is contacted by a top evaporated Au grid and a back side scratched In/Ga eutectic.

Figure 2. Photovoltaic properties of n-Si/PEDOT:PSS solar cells.

J-V-characteristics under AM1.5 spectrum irradiation of the hybrid PV-devices with differently doped (N_D) silicon substrates.

Figure 3. C-V characteristics of n-Si/PEDOT:PSS junctions.

1/C²-V plots for differently doped silicon substrates. The built-in voltage ψ_bi is extracted from the V-axis intercept of the extrapolation of the linear part of the data while the silicon substrate doping concentration N_D is given by the slope of the linear fit.

Figure 4. Inversion at the n-Si/PEDOT:PSS interface.

Built-in potential ψ_bi (apapted from Fig. 3) for differently doped silicon substrates with the treshold values for inversion and strong inversion.

Figure 5. Ultraviolet photoelectron spectrum of a PEDOT:PSS film using 6.5 eV excitation energy.

(a) Secondary electron cut-off (SECO) fittet by a Boltzmann sigmoid function for extraction of the work function qϕ_P and (b) valence band states near the Fermi level E_F with a linear extrapolation to the valence band edge E_V,P.

Figure 6. Junction formation at hybrid n-Si/PEDOT:PSS interfaces.

Schematic of the band structure for a silicon bulk doping concentration of N_D = 1.6 × 10¹⁷ cm⁻³ using values extracted from capacitance (C-V), UV photoelectron spectroscopy (UPS) measurements and literature data.

Figure 7. Dependence of (a) measured and calculated V_oc and (b) fitted and calculated J₀ on the silicon substrate doping concentration N_D for n-Si/PEDOT:PSS solar cells.

Figure 8. J-V characteristic of the n-Si/PEDOT:PSS interfaces.

The dark J-V plots of n-Si/PEDOT:PSS solar cells are fitted by the two-diode model following Equation 6 (dashed lines). The arrow illustrates the increase of the dark saturation current density J₀ with decrasing doping concentration N_D.