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Robust processing of small-molecule:fullerene organic solar cells via use of nucleating agents Neil Treat, Obadiah Reid, Sarah Fearn, Garry Rumbles, Craig J. Hawker, Michael L. Chabinyc, and Natalie Stingelin ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00082 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018
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ACS Applied Energy Materials
Robust Processing of Small-Molecule:Fullerene Organic Solar Cells Via Use of Nucleating Agents Neil D. Treat,*,† Obadiah G. Reid,*,ǂ Sarah Fearn, † Garry Rumbles,ǂ Craig J. Hawker,ǁ Michael L Chabinycǁ and Natalie Stingelin†,§ †
Department of Materials and Center for Plastic Electronics, Imperial College London, London SW7 2AZ, United Kingdom School of Materials Science & Engineering and School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta Georgia 30332, USA ǂ Chemical and Materials Science Center, National Renewable Energy Laboratory, 15015 Denver West Parkway, Golden, Colorado 80401, USA ǁ Materials Department and Materials Research Laboratory, University of California Santa Barbara, Santa Barbara, California 93117, USA Keywords: Organic solar cells, small-molecule:fullerene blends, processing aids, nucleating agents, processing reproducibility, device yield §
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 towards 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)-4Hsilolo[3,2-b:4,5-b]dithiophene-2,6-diyl]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 wellperforming 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.
Introduction 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 largearea production at high yield. To realize 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 complimentary electron and hole transfer reactions, possibly proceeding through a charge-transfer complex.4 As long as the charge transfer rates are sufficiently high, the fraction of photons converted to free carriers is dictated by the efficiency with which excitons are created at these active interfaces and/or reach them; this strongly depends on the blend’s phase morphology, overall thin-film microstructure,11 and hence ink formulation. After exciton dissociation, the free charge carriers must be collected at their respective electrodes. This process relies on a bi-continuous pathway of the hole/electron transporting material that must exist to enable collection.12 For this, the vertical and lateral distribution of these molecular species – which often very sensitively depend on ink formulation and solidifying conditions– can have a profound effect on the charge carrier mobility. Increasing the processing window and manufacturability, and ultimately the commercial viability of OPV blends, is thus not an easy task. For instance, while the donor:acceptor blend microstructure and phase morphology to op-
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timize the photovoltaic performance can often be controlled by, e.g., the addition of high boiling-point solvent additives,13-15 one challenge to this approach is that even minute variations (e.g. ± 0.5 vol.%) in the solvent additive content can result in significant changes in the blend structure and, in turn, the PCE. This is especially pronounced for many small-molecule:fullerene blends. One illustrative example of such a system is 7,7′-[4,4-bis(2ethylhexyl)-4H-silolo[3,2-b:4,5-b′] dithiophene-2,6-diyl] bis[6fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl) benzo[c][1,2,5]thiadiazole], (p-DTS(FBTTh2)2) blended with [6,6]phenyl C71-butyric acid methyl ester (PC71BM), where the PCE can range from less than 1% up to 9% when the amount of solvent additive diiodooctane (DIO) is only slightly varied between 0.4 vol.% and 1.0 vol.%.16-18 Herein, we show that the addition of the nucleating agent di(3,4-dimethyl benzylidene) sorbitol (DMDBS) drastically reduces performance fluctuations observed in pDTS(FBTTh2)2:PC71BM devices of different DIO content. Nucleating agents are known to regulate the solidification process of (semi-)crystalline polymer solids, including organic semiconductors,19-23 and they are frequently exploited to control, among other things, crystallite dimensions and shape of bulk commodity polymers.24-28 Generally, most focus to date has been on two main nucleating agent groups: sorbitol derivatives and organic acid derivatives 29-37. We selected a sorbitol derivative because of its better compatibility with the donor:acceptor blend solution; this is important because the behavior of these agents is critically dependent on the phase diagram of the complex, multicomponent system.
Experimental Section Materials: PC71BM (Solenne BV), p-DTS(FBTTh2)2 (1Material), and DMDBS (Milliken & Company) were used as received. Solar Cell Fabrication and Characterization: Indium tin oxide (ITO) glass substrates (Thin Film Devices; 20Ω/sq) were cleaned via ultrasonication in acetone and isopropanol then dried with a stream of nitrogen. The substrates were coated with a layer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT: PSS, Clevios PVP Al 4083; Heraeus) that was filtered through a 0.45 µm polyvinylidene fluoride (PVDF) filter and spin coated at 4000 rpm for 40 seconds then annealed at 165°C for 10 minutes producing a ∼45 nm film. The active layers were spin-coated on top of the PEDOT:PSS layer from pDTS(FBTTh2)2:PC71BM (3:2) chlorobenzene (CB) solutions. The nucleated solutions contained DMDBS with respect to the total solids content. The active layer solutions were made by combining stock solutions of the p-DTS(FBTTh2)2, PC71BM, and DMDBS in CB with and without DIO, heated to 120 ˚C for 15 min, cooled to 90 ˚C and immediately spin coated at 1750 rpms for 45 seconds 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 6 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. 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: (fpTRMC) 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 410-670 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 joints, 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 (
