Modulation-doped ZnO as high performance electron-selective layer for efficient silicon heterojunction solar cells

Abstract

It has been a long-time dream for photovoltaic (PV) researchers to achieve high performance silicon (Si) heterojunction solar cells (SCs) relying only on low-temperature and solution-based processes. A newly emerging PV technique of carrier-selective contacts, which comprise high performance hole- and electron-selective layers (HSL and ESL) for both polarities on Si substrate, is highly expected to meet this dream. Here, we report a precise control of the doping level in solution-processed ZnO thin films and their successful application as ESLs for organic/Si heterojunction SCs. We show that addition of Li in ZnO (Li-ZnO) allows for the accurate tuning of its work function from 4.15 to 3.85 eV. In case of Si/ZnO/Al heterocontact, presence of the Li-ZnO layer with optimized Li concentration results in a remarkable enhancement in extraction of electrons, showing by a big reduction of contact resistivity (ρc) from 100 to 18 mΩ cm2. When used in combination with an interfacial passivation layer of intrinsic amorphous silicon (a-Si:H), the bilayer ESL of a-Si:H/Li-ZnO helps to yield an open-circuit voltage (Voc) of 643 mV and an efficiency up to 15.1%, both remarkable for an organic/Si SC fabricated using solution-based processes. Progress in tuning the Li-ZnO properties to yield improved interfaces between Si and rear-sided electrode may open up a new direction of exploring more functional materials for high performance ESLs via effectively solution-based method.