Subscriber access provided by CORNELL UNIVERSITY LIBRARY
Article
Suppressing Sub-Nanosecond Bimolecular Charge Recombination in a High Performance Organic Photovoltaic Material Kyra Noelle Schwarz, Paul B. Geraghty, David J Jones, Trevor A. Smith, and Kenneth Philip Ghiggino J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08354 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 16, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 13
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
Suppressing Sub-nanosecond Bimolecular Charge Recombination in a High Performance Organic Photovoltaic Material Kyra N. Schwarz1, Paul B. Geraghty2, David J. Jones2, Trevor A. Smith1 & Kenneth P. Ghiggino1* 1
School of Chemistry, University of Melbourne, Parkville, Victoria 3010, Victoria, Australia
2
School of Chemistry, Bio21 Institute, The University of Melbourne, 30 Flemington Road, Parkville, Victoria 3010, Australia
ABSTRACT: Nanoscale morphology and spin can have a significant impact on charge generation and short timescale recombination in organic photovoltaic materials. We reveal multiple efficient charge separation pathways and the suppression of triplet loss channels in a high performing nematic liquid crystalline electron donor, benzodithiophene terthiophene rhodanine (BTR). BTR:PC71BM bulk heterojunction photovoltaic devices have been shown to exhibit charge generation quantum yields of ~90% and power conversion efficiencies > 9.5%, even in thick devices. Solvent vapor annealing increases device efficiency, delivering performance almost twice as high compared to untreated blend films, despite reduced exciton quenching. Broadband femtosecond transient absorption spectroscopy reveals both efficient hole and electron transfer on different timescales in the bulk heterojunction blends. BTR triplet excitons are formed due to sub-nanosecond bimolecular recombination in untreated blend films, though their formation is significantly suppressed after solvent vapor annealing. This treatment results in more crystalline BTR domains with three dimensional percolation pathways that have an important impact on these terminal triplet loss channels formed through fast recombination of free charges. We propose that spin and nanoscale morphology have a significant and interconnected role in the prevention of loss channels that, with careful control, can lead to superior device performance in promising new photovoltaic materials.
INTRODUCTION Organic photovoltaics (OPVs) have potential to provide low-cost solar energy through facile manufacturing processes that result in short 1-3 energy payback times. These devices consist of a light absorbing electron donor (D) and electron acceptor (A) semiconductor blend, 4 sandwiched between electron and hole extracting contacts. The electron donor is generally a conjugated polymer or a non-polymeric molecular material, the latter receiving increasing recent attention for their ability to achieve high material purity in synthesis with positive repercussions for reproducible device fabrication. There has been enormous progress in solution processed molecular materials in the last 5-7 few years, with device power conversion efficiencies (PCEs) increasing from below 2 % in 2008, to over 10 %. A recent molecular 8 9 electron donor, benzodithiophene-terthiophene-rhodanine (BTR) has achieved a PCE of 9.6 % with the fullerene acceptor, PC71BM. In addition, even with an active layer that is > 300 nm thick, a high fill factor (FF) of ~75 % is maintained along with a PCE of > 9.5 %, a property promising for conversion to roll-to-roll printing. The morphological properties of bulk heterojunction (BHJ) D:A blends have long been realized as critical for producing optimized 10 devices, and the relationship between morphology and charge photogeneration remains an active topic of research. BTR has intriguing morphological properties including a high temperature nematic liquid crystalline phase. This liquid crystallinity has been demonstrated in 8 neat films as a phase change at elevated temperatures, and in room temperature blend films where surface features appear to be arranged 11 in a liquid crystalline manner. In the solid state, BTR has a coplanar structure to its conjugated backbone, facilitating crystal stacking, dominated by π-stacking 8 interactions. It is this rigid rod-like shape that can maintain a directional long-range order with long axes aligned parallel, as evident in a nematic liquid crystalline phase. There has been some interest in exploiting liquid crystalline material properties for enhancing OPV 12-14 performance and it has been suggested that these materials can have high crystallinity and high carrier mobility due to three8 dimensional charge transport. However, the role or correlation of this behavior with the device efficiency is still undetermined. In BTR:PC71BM devices, solvent vapor annealing (SVA) with a moderately good solvent, tetrahydrofuran (THF), enhances the performance from 5.2 % (as-cast) to 9.6 %, whereas thermal annealing significantly decreases device performance, through the pronounced 8 coarsening of both the donor and acceptor phases. SVA, which is conducted under ambient conditions, is a promising method of optimizing active layer morphology, and is compatible with the roll-to-roll printing process. This treatment has been shown by others to 15-17 increase the crystallinity of domains, and to enhance the performance of OPV devices. Another interesting property of the SVA 9 BTR:PC71BM blend is its significantly reduced Langevin recombination rate, recently demonstrated by Armin et. al. The bimolecular recombination rate reported here is ~150 times lower that predicted by Langevin theory. 18
Triplet excitons (spin one) have long been observed in organic photovoltaic materials. These species can be generated by intersystem crossing from photogenerated singlet excitons. Alternatively, charge recombination can lead to the generation of triplet excitons through the charge transfer (CT) state, which can either have singlet 1CT or triplet 3CT character. If energetically accessible, the 3CT state can then relax to the lowest lying triplet exciton state (T1). This is similar to the case in light emitting diodes (LEDs), where the recombination of electrically injected charges leads to singlet and triplet states, in a ratio of 1:3 as dictated by spin statistics. Figure 1 illustrates possible
ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 13
photophysical pathways following exciton formation in organic photovoltaics. The pathway where free charge carriers generate triplet excitons is indicated by the bolded black arrows, with other possible pathways also indicated in grey. In organic photovoltaic materials, triplet excitons are generally regarded as a terminal loss mechanism, where recombination to the ground state can occur as well as tripletcharge annihilation.
Figure 1. State diagram of possible photophysical pathways following exciton formation in organic photovoltaics. Black, bolded arrows show steps for triplet exciton formation caused by the bimolecular recombination of free charge carriers (polarons). Other pathways are shown in grey, where dotted arrows represent recombination to the ground state. Spin states are indicated by small arrows for each intermediate species, where free charge carriers have spin states that are uncorrelated. There are reports of significant donor triplet exciton generation occurring in polymer D:A heterojunctions as the result of geminate 19-22 23-25 recombination, and more recently, bimolecular charge recombination, though little has been reported with regard to molecular donor materials. There is currently some debate around the generation of these triplet loss channels, particularly with regard to the role of bulk heterojunction morphology, however, understanding the mechanisms of triplet state formation is crucial for the optimization of organic photovoltaic devices. Here we present a spectroscopic study of BTR:PC71BM blend films, providing a comprehensive picture of the different charge generation pathways relating to active layer morphology. We relate our findings to the significant change in photovoltaic efficiency observed upon blend SVA, revealing that its influence on donor-acceptor morphology significantly suppresses the formation of BTR triplet excitons formed by the recombination of free charges on the sub-nanosecond timescale. The results also show that charge photogeneration from the PC71BM acceptor component is an important factor in efficient performance of the device and suggest that the different timescales of electron and hole transfer may help to reduce the amount of bimolecular recombination. EXPERIMENTAL METHODS Synthesis BTR was synthesized according to literature procedure.8 Film Preparation Glass strip substrates with dimensions of 2.5 cm x 2.5 cm x 0.1 cm were cleaned by sonicating sequentially in NaOH (1M), distilled water, acetone, isopropanol and dichloromethane. The substrates were then kept in isopropanol until further use. Prior to casting thin films the substrates were dried with a strong flow of nitrogen and then subjected to UV/ozone treatment for 30 minutes. To prepare neat films, solutions of BTR and PC71BM (Nano-c, 99 %) were prepared by dissolving separated samples into HPLC grade chloroform to a concentration of 1 mg/100 µL. To prepare blend films, solutions of BTR and PC71BM were prepared by dissolving separated samples into HPLC grade chloroform to a concentration of 2 mg/100 µL. These solutions were then set to stir for three hours in the dark under ambient conditions. Blend solutions were then prepared in a 1:1 weight % ratio by combining equal volumes of PC71BM and BTR in another vial and set to stir in the dark for a further hour. Thin films were cast onto clean glass substrates via spin coating at 1000 rpm (2000 acceleration) and spun for 30 seconds. SVA was completed in THF with an exposure time of 20 seconds. All measurements were completed on thin films prepared the same day. Steady-state Spectroscopy Absorption spectra were recorded of neat BTR and blend films, using a Varian Cary 50 UV-Vis spectrophotometer. Fluorescence spectra were recorded on a Varian Eclipse spectrofluorimeter using an excitation wavelength of 630 nm, where all spectra were corrected.
ACS Paragon Plus Environment
Page 3 of 13
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
Transient Absorption Spectroscopy The high repetition rate picosecond transient absorption setup is described in detail elsewhere.26 Briefly, a mode-locked Ti:sapphire oscillator (Coherent, Mira Seed) seeded a Ti:sapphire regenerative amplifier system (Coherent, RegA 9050) to produce pulses of ~50 fs duration at a repetition rate of 92 kHz and a wavelength centered at 800 nm. A portion of the light was used to generate the 400 nm pump beam using a BBO (barium borate) crystal, and the 630 nm and 540 nm pump beams were generated with an optical parametric amplifier (OPA9450, Coherent). The pump beam was mechanically chopped at ~3.5 kHz, and arrival time of the pump pulses relative to the probe was manipulated using a variable optical delay line (Newport, UTS150PP with ESP 300 controller). The broadband probe was derived from the residual 800 nm beam focused onto a 3 mm sapphire substrate (Crystal Systems) for measurements in the visible region (450-800 nm), and a 5 mm undoped YAG substrate (Crystal Systems) for the infrared region (800-1400 nm). After passing through the sample, the probe beam was analyzed with a CMOS detector (Ultrafast Systems) at ~7077 spectra/s, and the excess 800 nm laser fundamental was blocked using a low and high pass filter for the visible and IR regions respectively. For the majority of measurements (unless otherwise noted) the pump beam was attenuated to a pulse power of 8 nJ/pulse with a spot area of 1.3 x 10-3 cm2, giving an excitation density of 6 µJ/cm2, to reduce artefacts associated with exciton-exciton27 and exciton-charge annihilation.28,29 The relative orientation of pump and probe polarization was 54.7° and all spectra were corrected for the chirp of the supercontinuum probe. Nitrogen was blown over films for all measurements, except those exposed to oxygen to demonstrate the quenching of triplet states. RESULTS AND DISCUSSION Steady-state absorption Figure 2 shows (a) the molecular structure of the electron donor BTR, and (b) the steady-state absorption spectra of neat BTR, PC71BM and blend films. The spectrum of the 1:1 BTR:PC71BM blend largely resembles the superposition of individual component spectra, with the exception of the red end of the BTR absorption around 650 nm. Typical of π-conjugated materials, a red shift and formation of a new 8 peak, at around 620 nm, results from the aggregation and crystallinity of the BTR donor material as compared to the material in solution. This aggregation is most apparent in the neat BTR films, where the addition of PC71BM in D:A blends is thought to interfere with BTR molecular packing. The aggregate peak is, however, more apparent in the SVA sample, where enhanced demixing leads to better molecular 8 alignment, and hence aggregation of BTR. A full morphological characterization has been reported by Sun et al. Recent grazing incidence x-ray diffraction (GIXD) measurements by Engmann et al., indicate that in blends the BTR phase has 11 pronounced crystallinity, while the PC71BM phase remains in an amorphous state, even following SVA. Upon SVA, the absorption spectrum shows an increase in absorption coefficient for the BTR portion of the spectrum, attributed to aggregation. There is no effect of the solvent vapor treatment on PC71BM, in contrast to a similar molecular material where PC71BM crystal size grew during SVA, and 15 PC71BM aggregation was thought to be significant in elevating the device performance.
ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 13
Figure 2. (a) Molecular structure of BTR. (b) Steady-state optical absorption spectra of chloroform cast thin films of neat BTR (black solid), neat PC71BM (grey solid), 1:1 BTR:PC71BM blend film (blue solid) and 1:1 BTR:PC71BM blend film following treatment with SVA (green dashed). Traces are unnormalized. Fluorescence quenching Steady-state photoluminescence spectra of neat BTR and 1:1 BTR:PC71BM blend films are shown in Figure 3. The BTR-containing thin films were excited at 630 nm and their emission, with a peak at 780 nm, was monitored. With the inclusion of PC71BM within the BTR film, 96% of that emission is quenched in the as-cast blend, indicating efficient exciton quenching at the interface between the two components. This implies that for the as-cast blend, most domains have diameters less than the exciton diffusion length, and excitons can undergo charge transfer at the interface. The reduced quenching for the SVA film indicates that a higher proportion of domains must be larger than the BTR exciton diffusion length and, even though some excitons (~10 %) remained unquenched, the SVA treatment still results in significantly improved device performance. 8
These observations are consistent with the work of Sun et al., and Engmann et al., where atomic force microscopy, TEM tomography and 8,11 GiWAXS studies show fine crystal domains for the as-cast active layer, with the blend becoming coarser following SVA. GiWAXS measurements of the blend films before and after SVA also revealed that this treatment improves the crystallinity and surface ordering of BTR. A random orientation of π-stacking evolves into the coexistence of both edge-on and face-on arrangements after SVA, beneficial for 8 3D charge transport. Hole mobility was also seen to increase by one order of magnitude following SVA. Detailed in-situ GiWAX studies of SVA BTR:PC71BM thin films indicate the vapor of the applied solvent can penetrate into a film, where BTR can undergo Ostwald 11 ripening to form fewer but larger crystalline regions.
ACS Paragon Plus Environment
Page 5 of 13
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
Figure 3. Photoluminescence of BTR film (black) and quenching of BTR emission with the introduction of PC71BM to form blends. Films were excited at 630 nm and quenching efficiencies are shown as 89 ± 1.8 % for the SVA blend (green triangles) and 96 ± 0.4 % for the blend as-cast (blue circles), where dotted lines show measurement error. Transient absorption spectroscopy exciting the BTR donor component Transient absorption (TA) measurements were first performed with an excitation wavelength of 630 nm, a region where PC71BM absorption is negligible, to predominately excite the BTR donor component. Therefore the transient signatures observed are considered to be solely from photophysical consequences of excitons generated within the BTR donor material. The TA spectra (a) and kinetics (b) of a neat BTR film excited at 630 nm in the absence of an acceptor are shown in the top panels of Figure 4. The negative-going ground state bleach (GSB) region at 470-670 nm and the broad, positive-going signal from 850-1400 nm (PIA 1) show mirrored kinetics (Figure 4(b)). This suggests that the BTR exciton decay is the primary photophysical process that occurs in the neat film, with a lifetime of ~110 ps. The TA spectra for as-cast and SVA BTR:PC71BM blends are shown in Figure 4 (c) and (e) respectively, while the associated kinetics at the indicated wavelengths are shown in (d) and (f). The steady-state absorption for each sample is shown for ease of comparison with the GSB. In neat BTR and blend films, enhanced vibronic transitions in the GSB relative to the ground state absorption spectrum of BTR show that excitons rapidly (< 1ps) collapse onto more ordered, crystalline regions of the BTR, transitions that are enhanced in the SVA sample where BTR is more crystalline. In the blend films, the BTR exciton signal is still visible in the 850-1400 nm region (PIA 1) at early times, and is more pronounced in the SVA sample. However, the BTR exciton signal decays faster in the blend compared to the neat film, indicating that some population of the exciton has been quenched. This is in agreement with the steady-state photoluminescence data, and morphological information which shows significant exciton quenching in both blend samples, though this is reduced in the SVA sample with its larger BTR domains. The ascast films show a decreased amount of exciton signal within the instrument response function (200 fs), and the reduced exciton lifetime suggests extremely fast charge transfer in the finely intermixed blend morphology where generated excitons are in close proximity to the donor-acceptor interface. The maintenance of the donor ground state bleach during this time also suggests that the BTR exciton has neither undergone energy transfer nor completely returned to the ground state. The most striking new feature of the BTR blend spectra is a new photo-induced (positive) signal stretching from 680-1000 nm (PIA 2). As this spectral signature is not present in either the donor or acceptor components, this suggests the formation of a new photoproduct as the result of BTR and PC71BM interaction. Chemical oxidation of BTR films with I2 vapor, and BTR in chloroform with FeCl3, gives rise to a new absorption signature in this same region, peaking at ~800 nm (shown in Supporting Information S1). This supports the assignment of the new transient signal to positive charge carriers, BTR hole polarons. Further evidence for electron transfer at the donor-acceptor interface is the contribution of a positive electroabsorption feature at the band edge, the result of the local electric field due to newly generated charge pairs. This signal is formed within the 200 fs limit of the experiment followed by an additional rise over ~10 ps. This indicates that some population of charges are generated on ultrafast timescales (90% Quantum Efficiency. Adv. Mater. 2013, 26, 1923–1928. Vandewal, K.; Himmelberger, S.; Salleo, A. Structural Factors That Affect the Performance of Organic Bulk Heterojunction Solar Cells. Macromolecules 2013, 46, 6379–6387. Gallaher, J. K.; Prasad, S. K. K.; Uddin, M. A.; Kim, T.; Kim, J. Y.; Woo, H. Y.; Hodgkiss, J. M. Spectroscopically Tracking Charge Separation in Polymer : Fullerene Blends with a Three-Phase Morphology. Energy Environ. Sci. 2015, 8, 2713–2724. Melianas, A.; Etzold, F.; Savenije, T. J.; Laquai, F, Inganas, O; Kemerink, M. Photo-Generated Carriers Lose Energy During Extraction From Polymer-Fullerene Solar Cells. Nat. Commun. 2015, 6, 8778.
Table of Contents artwork
ACS Paragon Plus Environment
