Optoelectronic losses due to recombination at the transition region between the asymmetric heterocontacts in interdigitated back contact (IBC) silicon (Si) heterojunction (HJ) solar cells (SCs) tend to hinder a respectable performance-to-cost ratio for c-Si photovoltaics. In this study, we investigated new principles of field-effect passivation incorporating built-in fixed charges in dielectric passivation thin films (DPTFs) on the rear transition/gap region for photovoltaic performance optimization. Negative/positive polarity rear gap fixed charges of a density vertical bar Q(f)vertical bar up to 10(13) cm(-2) significantly enhance hole/electron transport across the transition gap, hence boosting the power conversion efficiency (PCE). Numerical simulations of the electric fields, energy band structures, electron and hole concentrations and current densities revealed how high efficiencies can be achieved through suppression of carrier recombination at the front- and rear-side interfaces, enhancement of carrier selectivity at the heterocontacts, and widening of the minority carrier transport channel in the bulk c-Si, especially in thinner IBC-HJ SCs. While the vertical bar Q(f)vertical bar requirement for high PCE is less stringent for high quality chemical passivation of the transition region, negative polarity fixed charges are generally advantageous if vertical bar Q(f)vertical bar > 5 x 10(11) cm(-2), otherwise if vertical bar Q(f)vertical bar < 5 x 10(11) cm(-2), positive polarity fixed charges would allow a high efficiency. Furthermore, poor quality chemical passivation of the front surface or electron transport layer (ETL) can be sufficiently compensated by a high density of negative polarity transition region charges to obtain a superior PCE, and allowing a high tolerance to the ETL fill rate and device pitch.