Efficient inverted PM6:Y6 solar cells with tuned lateral phase separation and vertical composition distribution via bis(5-chlorothiophen-2-yl)alkane additives

Abstract

Finely controlling the morphology of bulk heterojunction (BHJ) films by additives provides a critical and versatile tool to boost performance of solution processed polymer solar cells (PSCs). In great need are efficient additives that can optimize the arrangement of energetically paired donor and acceptor materials to propel individual photoelectric conversion steps close to unity either in conventional or inverted PSCs. Alternative to the paradigm additives such as 1-chloronaphthlene (CN) etc. for highly efficient conventional PSCs with Y6-type acceptors, herein, bis(5-chlorothiophen-2-yl)alkanes (BCTA-m) with 3 ∼ 6 methylene units (m = 3, 4, 5 and 6) were incorporated as solvent additives to fabricate inverted PM6:Y6 PSCs. The lateral phase separation and vertical composition distribution between PM6 and Y6 were discovered tunable by varying the length (m) of alkanes. The BCTA-4 device offers the maximum power conversion efficiency (PCE) of 15.59%, which is superior to the ones processed from the other BCTA-m (m = 3, 5 and 6) additives and comparable to that of the CN reference (15.62%). The outperformed exciton generation and dissociation, charge transport and suppressed charge recombination are evidently observed within the BCTA-4 device. Furthermore, the lateral phase-separated domain sizes between the pure Y6 (2Rg-Y6) and the PM6:Y6 mixture (ξ) get an optimum ratio. The overwhelmingly vertical distribution of PM6 near the anode is also confirmed in the BCTA-4 device over the ones processed from the other three analogues. This work demonstrates the potential of BCTA-4 additive to realize high-performance inverted PSCs based on Y6-type acceptors.