Electrical Retention of Optically Recovered Current in Back-Junction Silicon Heterojunction Solar Cells with Nanocrystalline Hole Contacts

Abstract

Back-junction SHJ cells benefit from superb crystalline-silicon passivation while being subjected to challenging demands on doped-silicon carrier transport, transparent-conductive-oxide (TCO) conductivity and rear optical design. The key question studied here is to what extent the increase in photocurrent due to optical improvements is maintained in power output after the rear hole-selective contact replacement from p-a-Si:H to p-nc-Si:H. The comparison between four particular cells is used: a 25.26 % p-a-Si:H contact cell, a 26.30 % p-nc-Si:H cell with 86.59 % fill factor, a 26.74 % front-optics cell with \(J_{\mathrm{SC}}=\SI{41.16}{\milli\ampere\per\centi\meter\squared}\), and a 26.81 % MgF\(_2\)/Ag rear-stack cell with \(V_{\mathrm{OC}}=\SI{751.4}{\milli\volt}\), \(J_{\mathrm{SC}}=\SI{41.45}{\milli\ampere\per\centi\meter\squared}\) and \(\mathrm{FF}=\SI{86.07}{\percent}\). The transition from 25.26 % to 26.81 % efficiency is associated with an 2.9 mV increase in open-circuit voltage, an 1.97 mA cm⁻² rise in current density and an 0.57 % fill-factor improvement. The p-nc-Si:H rear-contact decreases the series resistance from 381 mΩ cm² to 206 mΩ cm² and minimizes the rear HSC power loss from 0.41 mW cm⁻² to 0.13 mW cm⁻². The latter optical step increases the current density by 0.29 mA cm⁻² but produces only 0.07 percentage points gain in efficiency, which proves that rear-stack increase in the current density is offset in part by the fill-factor dependence on TCO conductivity. In terms of device physics, this means that there is a retained current scenario: contact loss reduction makes the fill-factor high, optical optimizations of the front side are responsible for the major part of the current-density increase and the rear reflector contribution at the last step is limited by the current retention.