Influence of Surface Energy on Organic Alloy Formation in Ternary Blend Solar Cells Based on Two Donor Polymers

Nemal S Gobalasingham; Sangtaik Noh; Jenna B Howard; Barry C Thompson

doi:10.1021/acsami.6b10144

Influence of Surface Energy on Organic Alloy Formation in Ternary Blend Solar Cells Based on Two Donor Polymers

ACS Appl Mater Interfaces. 2016 Oct 19;8(41):27931-27941. doi: 10.1021/acsami.6b10144. Epub 2016 Oct 5.

Authors

Nemal S Gobalasingham¹, Sangtaik Noh¹, Jenna B Howard¹, Barry C Thompson¹

Affiliation

¹ Department of Chemistry and Loker Hydrocarbon Research Institute, University of Southern California , Los Angeles, California 90089-1661, United States.

PMID: 27660888
DOI: 10.1021/acsami.6b10144

Abstract

The compositional dependence of the open-circuit voltage (V_oc) in ternary blend bulk heterojunction (BHJ) solar cells is correlated with the miscibility of polymers, which may be influenced by a number of attributes, including crystallinity, the random copolymer effect, or surface energy. Four ternary blend systems featuring poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) (P3HT₇₅-co-EHT₂₅), poly(3-hexylthiophene-co-(hexyl-3-carboxylate)), herein referred to as poly(3-hexylthiophene-co-3-hexylesterthiophene) (P3HT₅₀-co-3HET₅₀), poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%), and an analog of P3HTT-DPP-10% with 40% of 3-hexylthiophene exchanged for 2-(2-methoxyethoxy)ethylthiophen-2-yl (3MEO-T) (featuring an electronically decoupled oligoether side-chain), referred to as P3HTTDPP-MEO40%, are explored in this work. All four polymers are semicrystalline and rich in rr-P3HT content and perform well in binary devices with PC₆₁BM. Except for P3HTTDPP-MEO40%, all polymers exhibit similar surface energies (∼21-22 mN/m). P3HTTDPP-MEO40% exhibits an elevated surface energy of around 26 mN/m. As a result, despite the similar optoelectronic properties and binary solar cell performance of P3HTTDPP-MEO40% compared to P3HTT-DPP-10%, the former exhibits a pinned V_oc in two different sets of ternary blend devices. This is a stark contrast to previous rr-P3HT-based systems and demonstrates that surface energy, and its influence on miscibility, plays a critical role in the formation of organic alloys and can supersede the influence of crystallinity, the random copolymer effect, similar backbone structures, and HOMO/LUMO considerations. Therefore, we confirm surface energy compatibility as a figure-of-merit for predicting the compositional dependence of the V_oc in ternary blend solar cells and highlight the importance of polymer miscibility in organic alloy formation.

Keywords: open-circuit voltage; organic alloy; semi-random copolymer; solar cell; ternary blend.