Article Cite This: ACS Appl. Energy Mater. 2018, 1, 1973−1980
www.acsaem.org
Robust Processing of Small-Molecule:Fullerene Organic Solar Cells via Use of Nucleating Agents Neil D. Treat,*,† Obadiah G. Reid,*,‡,§ Sarah Fearn,† Garry Rumbles,‡,§,⊥ Craig J. Hawker,∥ Michael L Chabinyc,∥ and Natalie Stingelin†,▽ †
Department of Materials and Center for Plastic Electronics, Imperial College London, London SW7 2AZ, United Kingdom National Renewable Energy Laboratory, Golden, Colorado 80401, United States § Renewable and Sustainable Energy Institute, University of Colorado Boulder, Boulder, Colorado 80309, United States ⊥ Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, Colorado 80309, United States ∥ Materials Department and Materials Research Laboratory, University of CaliforniaSanta Barbara, Santa Barbara, California 93117, United States ▽ School of Materials Science & Engineering and School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
Downloaded via UNIV OF WINNIPEG on January 29, 2019 at 06:26:20 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: The power conversion efficiency (PCE) of small-molecule bulk heterojunction solar cells is highly sensitive to the “ink” formulation used to produce the photoactive layer. Here we demonstrate that the addition of nucleating agents renders device fabrication notably less susceptible to the ink composition, promising a route toward more robust processing of efficient devices over large areas and enabling more facile materials screening. We selected as a model system blends of 7,7-[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b]dithiophene-2,6diyl]bis[6-fluoro-4-(5-hexyl-[2,2-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole](p-DTS(FBTTh2)2) as the donor and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the acceptor because this is one of the small-molecule OPV blends with a device performance that is most sensitive to ink formulation, especially when used with the processing aid diiodooctane (DIO). Addition of DIO is essential to obtain high device performances; however, a notable increase in device performance is only achieved over a very narrow DIO content regime. Use of nucleating agents drastically changes this situation and leads to well-performing devices even at extreme levels of DIO. We thus start to address here one of the great challenges in organic solar cell research: the fact that, too often, only a very limited composition range leads to high efficiency devices. This means that for every new donor or acceptor a multitude of formulations have to be tested, including in combination with processing aids, to ensure that promising materials are not overlooked. The use of nucleating agents, thus, promises to render materials discovery more straightforward as this dependency of device performance with composition can be reduced. KEYWORDS: organic solar cells, small-molecule:fullerene blends, processing aids, nucleating agents, processing reproducibility, device yield
■
INTRODUCTION
this objective, detailed insight into the mechanism of power conversion within an organic solar cell,4 and how it depends on microstructure and phase morphology of the donor:acceptor blend,5−10 is needed. Generally, it is established that upon illumination, photons are absorbed by either the donor or acceptor creating a bound excited state termed an exciton. Excitons that are sufficiently close to an interface between the donor and the acceptor (e.g., through initial location where the excitons are formed or exciton diffusion) will form free charges by complementary electron and hole transfer reactions, possibly proceeding through a charge transfer complex.4 As long as the
Solar cells fabricated with solution processable, carbon-rich materials are promising as future sources of scalable and renewable energy.1 Thereby, blends between electron-donating, organic small molecules and electron-accepting fullerenes are one area of interest because these systems have led to devices of power conversion efficiencies (PCE) > 10%,2,3 realized through precise control of the chemical structure of the donor leading to optimization, among other things, of carrier transport levels, extinction coefficients, and the crystallization behavior. Despite this rapid progress in recent years, there is still a need to develop strategies to improve not only device efficiencies but also reproducibility when processing and integrating such devices over larger areas, with the ultimate goal of facilitating large-area production at high yield. To realize © 2018 American Chemical Society
Received: January 22, 2018 Accepted: April 3, 2018 Published: April 3, 2018 1973
DOI: 10.1021/acsaem.8b00082 ACS Appl. Energy Mater. 2018, 1, 1973−1980
Article
ACS Applied Energy Materials
45 s on a room temperature PEDOT:PSS-coated glass/ITO substrate. After spin-coating, the devices were immediately transferred to a hot plate heated to 80 °C and annealed for 7 min. A bilayer electrode consisting of 5 nm calcium followed by 80 nm aluminum was evaporated through a shadow mask under high vacuum (>10−6 Torr). Each substrate contained six cells with an active area of 0.06 cm2. All devices were prepared and measured in the same manner to permit a fair comparison. The devices were prepared and tested in inert atmosphere. The current−voltage characteristics were measured using a xenon lamp (Newport) with a Keithley SMU measured in a nitrogen environment under 100 mW/cm2 illumination equipped with an AM 1.5G filter without shadow mask. External quantum efficiency (EQE) spectral measurements were made with a Xe source (Newport), a monochromator (McPherson EU-700-56), optical chopper, lock-in amplifier (Stanford Research Systems), and a National Institute of Standards and Technology traceable silicon photodiode for monochromatic power-density calibration. Device characteristics were averaged over 15 devices to limit errors because of the way the effective area was deduced. Short circuit values collected while illuminated with AM 1.5G light were verified with those values collected from EQE and found to be within 5%. Flash-Photolysis Time-Resolved Microwave Conductivity. Flash-photolysis time-resolved microwave conductivity (fp-TRMC) has been extensively described in other publications.38 Briefly, the technique consists of measuring the absorption of a continuous microwave probe (8.85 GHz) by charge carriers produced by a short (4 ns) laser flash. Thin film samples are fabricated on a quartz substrate that is mounted at the electric field maximum of a microwave resonance cavity, which enhances the signal-to-noise ratio at the expense of time resolution (∼10 ns response time for a cavity of moderate quality). In the present experimental configuration we use the third harmonic of a Nd:YAG laser (Continuum Powerlite) to pump an optical parametric oscillator (Continuum Panther), which delivers 4 ns fwhm pulses tunable from 410 to 670 nm. The beam is expanded to provide even illumination of the sample, and passed through a metal grid on the microwave cavity that transmits 70% of the light, while reflecting 99% of the microwave energy internally. The pulse energy is varied by a set of automated natural density filters, and pulse energy measurements are made at every filter combination concurrent with the experiment to account for any possible long-term drift in laser energy output. The microwave cavity is sealed with conductive elastomer O-rings at the joint, and epoxy mounted quartz windows at each end to exclude ambient air. In addition, ball valves at each end of the cavity permit active nitrogen purging of the sample. In this study, all samples were mounted in the microwave cavity inside of a nitrogen glovebox (
