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We study n-GaN/i-InGaN/p-GaN solar cells on Ga-face substrates. We find that polarization charges at the heterointerfaces pin the value of VOC, JSC, and the fill factor to nearly optimal levels when the InGaN layer thickness exceeds a value dmin, even as the p- and n-layer thicknesses and doping concentrations vary widely. We verify an analytical approximation for dmin. We report nearly undiminished performance when an i-GaN setback layer is added between a p-doped layer and the i-InGaN layer - an addition that may be necessary to obtain good quality heterointerfaces. Additionally, the 2-D electron gas formed at the n-GaN/i-InGaN interface facilitates thinner n-GaN window layers for improved external quantum efficiency.