Graded Morphologies and the Performance of PffBT4T-2OD:PC71BM Devices Using Additive Choice.

Hugo Gaspar,Adélio Mendes,Andrew J Parnell,Luiz Pereira,Gabriel Bernardo,Stephen M King,Júlio C Viana,Gabriel E Pérez

doi:10.3390/nano11123367

Abstract

The impact of several solvent processing additives (1-chloronaphthalene, methylnaphthalene, hexadecane, 1-phenyloctane, and p-anisaldehyde), 3% v/v in o-dichlorobenzene, on the performance and morphology of poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldodecyl)-2,2′,5′,22033,5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T-2OD):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM)-based polymer solar cells was investigated. Some additives were shown to enhance the power conversion efficiency (PCE) by ~6%, while others decreased the PCE by ~17–25% and a subset of the additives tested completely eliminated any power conversion efficiency and the operation as a photovoltaic device. Grazing-Incidence Wide Angle X-ray Scattering (GIWAXS) revealed a clear stepwise variation in the crystallinity of the systems when changing the additive between the two extreme situations of maximum PCE (1-chloronaphthalene) and null PCE (hexadecane). Small-Angle Neutron Scattering (SANS) revealed that the morphology of devices with PCE ~0% was composed of large domains with correlation lengths of ~30 nm, i.e., much larger than the typical exciton diffusion length (~12 nm) in organic semiconductors. The graded variations in crystallinity and in nano-domain size observed between the two extreme situations (1-chloronaphthalene and hexadecane) were responsible for the observed graded variations in device performance.

Highlights

Organic photovoltaics (OPV) is an emerging solar cell technology that attracts great interest due to its promising performance and potential for low-cost manufacture, over large areas, on lightweight flexible plastic substrates, which can be fabricated using highthroughput, roll-to-roll (R2R) solution processing
We studied the impact of five different processing additives (1chloronaphthalene, 1-methylnaphthalene, hexadecane, 1-phenyloctane, and p-anisaldehyde), with concentrations of 3% v/v in o-dichlorobenzene, on the BHJ morphology and the efficiency of devices based on the donor:acceptor pair PffBT4T-2OD:PC71 BM
We tested the impact of several additives with different solubility pasolar cells based on the system PffBT4T-2OD:phenyl-C71-butyric acid methyl ester (PC71BM)

Summary

Introduction

Organic photovoltaics (OPV) is an emerging solar cell technology that attracts great interest due to its promising performance and potential for low-cost manufacture, over large areas, on lightweight flexible plastic substrates, which can be fabricated using highthroughput, roll-to-roll (R2R) solution processing. The small band gap donor polymer poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)alt-(3,3000 -di(2-octyldodecyl)2,20 ;50 ,200 ;500 ,2000 -quaterthiophen-5,5000 -diyl)] (PffBT4T-2OD), known as PCE11, exhibits relatively high hole mobility under space charge limited current conditions (SCLC) (over 1.5–3.0 × 10−2 cm2 ·V−1 ·s−1 [5]) due to its high crystallinity. These properties, together with its tendency to form relatively pure polymer domains when blended with fullerene acceptors, allow it to perform well in an OPV device, when used in relatively thick bulk-heterojunction (BHJ) layers (~300 nm). PffBT4T-2OD has, been the subject of numerous OPV studies either blended with PC71 BM [6,7,8,9,10,11,12,13] or with other fullerenes [5,14,15,16]

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