Influence of polarization on the efficiency of In_xGa _1-xN/GaN p-i-n solar cells

The net polarization charge and induced internal electric field can positively or negatively affect the performance of InGaN-based optoelectronic devices, depending on the formation of the polarization charge. While positive polarization charges at the InGaN/n-GaN interface are supposed to affect solar efficiency positively by increasing the electric field, negative polarization charges at the InGaN/n-GaN interface negatively affect efficiency by forming an energy barrier. However, the solar performance is not improved in a beneficial case when the polarization charge reaches 10²⁰ cm^-3. In the detrimental case, the efficiency of p-GaN/i-In_0.1Ga _0.9N/n-GaN solar cells is seriously degraded from 2.75 to 0.49% as the polarization charge increases from 0 to 2.5 × 10¹⁸ cm ^-3. The insertion of graded layers with a slowly increasing In content is proposed to relax the polarization, and shown to improve solar efficiency.