Interplay of Surface Recombination and Diode Geometry for the Performance of Axial p-i-n Nanowire Solar Cells

Nanowires (NWs) with axial p-i-n junctions have been widely explored as microscopic diodes for optoelectronic and solar energy applications, and their performance is strongly influenced by charge recombination at the surface. We delineate how the photovoltaic performance of these diodes is dictated not only by the surface but also by the complex and seemingly counterintuitive interplay of diode geometry, that is, radius (R) and intrinsic length (Li), with the surface recombination velocity (S). An analytical model to describe these relationships is developed and compared to finite-element simulations, which verify the accuracy and limitations of the model. The dependence of the dark saturation current (I0), internal quantum efficiency (IQE), short-circuit current (ISC), and open-circuit voltage (VOC) on both geometric and recombination parameters demonstrates that no single set of parameters produces optimal performance; instead, various trade-offs in performance are observed. For instance, longer Li might be expected to produce higher ISC, yet at high values of S the ISC declines because of decreases in IQE. Moreover, longer Li produces a concurrent decline in VOC regardless of S due to increases in I0. We also find that ISC and VOC trends are radius independent, yet I0 is directly proportional to R, causing NWs with smaller R to display higher turn-on voltages. The analysis regarding the interplay of these parameters, verified by experimental measurements with various p-i-n geometries and surface treatments, provides clear guidance for the rational design of performance metrics for photodiode and photovoltaic devices.