Comparison of the device physics principles of planar and radial p-n junction nanorod solar cells

TLDR

A device physics model for radial p‑n junction nanorod solar cells, with densely packed rods oriented parallel to incident light, has been developed. The authors aim to demonstrate that radial nanorod designs can substantially improve efficiency over planar p‑n junction cells when minority‑carrier diffusion lengths are too short for planar absorption and depletion‑region recombination is low. High‑aspect‑ratio nanorods provide sufficient thickness for strong optical absorption while keeping carrier collection distances short, enabling efficient collection in materials with low minority‑carrier diffusion lengths and requiring depletion‑region lifetimes longer than ~10 ns. Modeling shows large efficiency gains for radial cells under both conditions, but only modest gains if the diffusion‑length condition alone is met.

Abstract

A device physics model has been developed for radial p-n junction nanorod solar cells, in which densely packed nanorods, each having a p-n junction in the radial direction, are oriented with the rod axis parallel to the incident light direction. High-aspect-ratio (length/diameter) nanorods allow the use of a sufficient thickness of material to obtain good optical absorption while simultaneously providing short collection lengths for excited carriers in a direction normal to the light absorption. The short collection lengths facilitate the efficient collection of photogenerated carriers in materials with low minority-carrier diffusion lengths. The modeling indicates that the design of the radial p-n junction nanorod device should provide large improvements in efficiency relative to a conventional planar geometry p-n junction solar cell, provided that two conditions are satisfied: (1) In a planar solar cell made from the same absorber material, the diffusion length of minority carriers must be too low to allow for extraction of most of the light-generated carriers in the absorber thickness needed to obtain full light absorption. (2) The rate of carrier recombination in the depletion region must not be too large (for silicon this means that the carrier lifetimes in the depletion region must be longer than ∼10ns). If only condition (1) is satisfied, the modeling indicates that the radial cell design will offer only modest improvements in efficiency relative to a conventional planar cell design. Application to Si and GaAs nanorod solar cells is also discussed in detail.