Title: Molecular origin of photovoltaic performance in donor-block-acceptor all-conjugated block copolymers

Abstract

All-conjugated block copolymers may be an effective route to self-assembled photovoltaic devices, but we lack basic information on the relationship between molecular characteristics and photovoltaic performance. Here, we synthesize a library of poly(3-hexylthiophene) (P3HT) block poly((9,9-dialkylfluorene)-2,7-diyl-alt-[4,7-bis(alkylthiophen-5-yl)-2,1,3-benzothiadiazole]-2',2''-diyl) (PFTBT) donor-block-acceptor all-conjugated block copolymers and carry out a comprehensive study of processing conditions, crystallinity, domain sizes, and side-chain structure on photovoltaic device performance. We find that all block copolymers studied exhibit an out-of-plane crystal orientation after deposition, and on thermal annealing at high temperatures the crystal orientation flips to an in-plane orientation. By varying processing conditions on polymer photovoltaic devices, we show that the crystal orientation has only a modest effect (15-20%) on photovoltaic performance. The addition of side-chains to the PFTBT block is found to decrease photovoltaic power conversion efficiencies by at least an order of magnitude. Through grazing-incidence X-ray measurements we find that the addition of side-chains to the PFTBT acceptor block results in weak segregation and small (< 10 nm) block copolymer self-assembled donor and acceptor domains. This work is the most comprehensive to date on all-conjugated block copolymer systems and suggests that photovoltaic performance of block copolymers depends strongly on the miscibility of donor and acceptor blocks, which impactsmore » donor and acceptor domain sizes and purity. Lastly, strategies for improving the device performance of block copolymer photovoltaics should seek to increase segregation between donor and acceptor polymer domains.« less

Authors:

Smith, Kendall A. ^[1]; Lin, Yen -Hao ^[1]; Mok, Jorge W. ^[1]; Yager, Kevin G. ^[2]; Strzalka, Joseph ^[3]; Nie, Wanyi ^[4]; Mohite, Aditya D. ^[4]; Verduzco, Rafael ^[1]

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+ Show Author Affiliations

1. Rice Univ., Houston, TX (United States)
2. Brookhaven National Lab. (BNL), Upton, NY (United States)
3. Argonne National Lab. (ANL), Argonne, IL (United States)
4. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)

Publication Date:

2015-11-03

Research Org.:

Argonne National Laboratory (ANL), Argonne, IL (United States); Brookhaven National Laboratory (BNL), Upton, NY (United States)

Sponsoring Org.:

National Science Foundation (NSF); USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1245027

Alternate Identifier(s):

OSTI ID: 1246785

Report Number(s):

BNL-111803-2016-JA
Journal ID: ISSN 0024-9297; 125949

Grant/Contract Number:  

AC02-06CH11357; SC00112704

Resource Type:

Accepted Manuscript

Journal Name:

Macromolecules

Additional Journal Information:

Journal Volume: 48; Journal Issue: 22; Journal ID: ISSN 0024-9297

Publisher:

American Chemical Society

Country of Publication:

United States

Language:

English

Subject:

14 SOLAR ENERGY; 36 MATERIALS SCIENCE; P3HT; PF8TBT; PFTBT; all-conjugated; all-polymer; block copolymer; organic photovoltaics; organic solar cells; poly(3-hexylthiophene); self-assembly; 77 NANOSCIENCE AND NANOTECHNOLOGY