Numerical Analysis on Polarization-Induced Doping III-Nitride n-i-p Solar Cells

We design and numerically evaluate a new type of III-nitride n-i-p solar cells whose p- and n-type regions with equal carrier concentration of 3 × 10¹⁸ cm^-3 are not generated by extrinsic impurity doping but by the so-called polarization-induced doping, which is induced by the graded In_xGa_1-xN layers of linearly increasing (from x = 0% to 30%) and decreasing (from x = 30% to 0%) indium composition to construct the conductive p- and n-type regions, respectively. Because of the identical and uniform polarization charges within each unit cell, a smooth spatial variation of the potential profile of the device is, hence, expected, which mitigates the energy band discontinuities at heterointerfaces and facilitates transportation and collection of photogenerated carriers with high efficiency. Most importantly, as the conductive n- and p-type regions are formed by electrostatic field ionization but not by the thermal activation, the concentration of field-induced carriers is independent of thermal freeze-out effects. Thus, the polarization-induced doping III-nitride n-i-p solar cells can provide stable power conversion efficiency, even when operated at low temperatures.