A space-charge-free CdS, and a new solar cell without pn-junction enabled by high-field domains

A review of the electrical properties of a copper doped CdS as a model semiconductor describing three independent thermodynamic states depending on optical excitation and bias. These are the traditional ground state that, dependent of optical excitation renders it from an insulator to a highly conductive n-type semiconductor. With higher applied voltage, a high-field domain occurs that is locked-in, attached to the cathode. It presents the second thermodynamic stable state that is n-type with constant drift current limited by the carrier density at a blocking electrode nc. With further increased bias, the high-field domain becomes anode-adjacent. The CdS is in its third thermodynamic stable state. Here, the CdS has flipped into p-type conductivity with the current limited by the hole density at the blocking anode pa. In both states, the CdS is free of space charges within the domain that permits defect level spectroscopy without broadening influence of fluctuating electric fields. The minimum entropy production principle connects the work functions of blocking cathode and anode by the constant domain current, and permits the measurement of their values that depend on the optical excitation. A new solar cell is described that works without a pn-junction, with a thin, pure CdS layer replacing the junction. The CdS extracts the holes for tunneling into the base electrode. The CdS/CdTe to scale model is given as an example. It has substantial increased efficiency by preventing junction leakage and permits an increase of the open circuit voltage to approach the theoretical limit of the band gap of the emitter when extrapolated to 0 K. The solar cell is series resistance limited by the CdS and requires a thin planar deposition of CdS. Since CdS is photoelectrically passive, its Fermi-level at the base electrode is fixed. A Gallium doping of a thin layer pins the Fermi level within 0.05 V to the conduction band and increases Voc further. All properties of CdS with high-field domains are described quantitatively and the domains are made visible by the Franz–Keldysh effect.