Bifacial silicon heterojunction solar cells with advanced Ag-free multi-wire metallization attached to ITO layers using new transparent conductive PAEK copolymers

Abstract

Previously, we reported the synthesis and application of transparent conductive polymer (TCP) films, poly(arylene ether ketone) copolymers (co-PAEKs), for forming direct contact between wires and transparent conductive oxide (TCO) layers in silicon solar cells. The polymers have the lowest peak strain temperature (Td), which determines the contact formation temperature, of 205 °C. To utilize such TCP films in silicon heterojunction (SHJ) solar cells with amorphous silicon layers, Td should be lowered. To solve the problem in question, a number of co-PAEKs with a decreased reduced viscosity (ηred) due to a decreased molecular weight of the polymer have been synthesized in this study. It has been shown that lowering ηred from 0.56 to 0.4 dl/g markedly improves the main properties of the co-PAEKs. In particular, (i) Td decreased from 205 to 189 °C, (ii) the peel strength, determined by pulling off the wires from the polymer surface, increased from 1.69 ± 0.26 to 3.55 ± 0.84 N/mm, and (iii) the resistivity of the wire/TCP/ITO (In2O3:Sn) contact, ρC, dropped from 1.20 to 0.67 mΩ cm2. At the same time, the optical properties of the copolymers remained unchanged. We have fabricated bifacial rear junction SHJ solar cells based on a ITO/(n)α-Si:H/(i)α-Si:H/(n)Cz-Si/(i)α-Si:H/(p)α-Si:H/ITO structure, with wire contact grids attached to the ITO layers using co-PAEK films. A solar cell produced using the co-PAEK film with the lowest reduced viscosity had an efficiency under front/rear illumination of 19.6%/18.4%. At 1-sun front illumination and 20/50% of 1-sun rear illumination, the equivalent efficiency is equal to 23.3%/28.8%.