Removal of Residual Diiodooctane Improves Photostability of High

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Removal of Residual Diiodooctane Improves Photostability of High-Performance Organic Solar Cell Polymers Bertrand J. Tremolet de Villers, Kathryn A. O'Hara, David P. Ostrowski, Perry H. Biddle, Sean E. Shaheen, Michael L. Chabinyc, Dana C. Olson, and Nikos Kopidakis Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04346 • Publication Date (Web): 07 Jan 2016 Downloaded from http://pubs.acs.org on January 12, 2016

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Chemistry of Materials

Removal of Residual Diiodooctane Improves Photostability of High-Performance Organic Solar Cell Polymers Bertrand J. Tremolet de Villers,∗,† Kathryn A. O’Hara,‡ David P. Ostrowski,¶ Perry H. Biddle,§ Sean E. Shaheen,¶,k,⊥ Michael L. Chabinyc,‡ Dana C. Olson,† and Nikos Kopidakis∗,† †Chemical and Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado, 80401, USA ‡Materials Department, University of California, Santa Barbara, California, 93106, USA ¶Department of Electrical, Computer, and Energy Engineering, University of Colorado, Boulder, Colorado, 80309, USA §Goshen College, Goshen, Indiana, 46526, USA kDepartment of Physics, University of Colorado, Boulder, Colorado, 80309, USA ⊥Renewable and Sustainable Energy Institute (RASEI), University of Colorado, Boulder, Colorado, 80309, USA E-mail: [email protected]; [email protected] Abstract Solvent additives such as diiodooctane (DIO) are becoming ubiquitous in processing high performance organic photovoltaic (OPV) active layers. Here, we investigate the effects of DIO on the long-term stability of the active layer by studying the photodegradation under ambient white light illumination of the polymer PTB7-Th in pure polymer

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thin films and in blend films with PC71 BM. Using X-ray fluorescence we directly detect iodine in the active layer films, indicating the presence of residual DIO after casting from solution. Additionally, we show that this residual DIO dramatically decreases the photostability of the active layer. Structural changes in the films upon illumination are probed with grazing-incidence wide-angle X-ray scattering (GIWAXS). FTIR spectroscopy is used to monitor chemical changes in the polymer structure during irradiation in the presence of DIO. Furthermore, we demonstrate that film treatment with either high vacuum (10−8 Torr) for 60 minutes or with a high-temperature thermal anneal at 175 ◦C for 30 minutes removes residual DIO from the film and successfully delays photodegradation. Therefore, when processing polymer solar cells with DIO-containing solutions, it is imperative to remove any trace amounts of DIO from deposited films.

Introduction As organic photovoltaics (OPVs) continue to improve towards 12 % power conversion efficiency and beyond, it is imperative to demonstrate dramatic improvements in device stability. In the last decade, OPVs based on low-temperature and solution-processed carbon-containing materials are emerging as an attractive alternative solar technology. OPVs can be made into thin flexible forms and the opto-electronic properties of organic semiconducting materials can be tuned by appropriate choice of conjugated building blocks. While the efficiency of lab-scale OPVs recently surpassed 11 %, 1 the intrinsic stability of organic materials against chemical degradation from water and oxygen needs to be better understood and addressed. 2 Therefore, considering the three key success parameters for any solar energy technology– cost, efficiency, and lifetime–long-term stability is now the most pertinent obstacle for OPV success. 3 Until now, enhancement of the solar cell efficiency has motivated most of the synthesis of new OPV materials and the development of new device fabrication procedures. Consideration of a material system’s stability has only recently garnered more attention and standard 2


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stability testing protocols are now being implemented. 4,5 It has now been established that preventing degradation of the metal contacts is crucial to longer device lifetimes: inverting the cell architecture so that low work function metals, e.g. calcium, are replaced by less reactive contacts such as metal oxides represents an important advance. 6,7 In most inverted OPVs, changes in the organic semiconducting materials now represent the dominant degradation pathways. In an OPV device, the photoactive layer is often a two-component blend of a polymer (donor) and fullerene (acceptor) mixed together in a so-called bulk heterojunction (BHJ). 8 The photo-excited donor is usually the culprit during photo-induced degradation; 9 however, the fullerene may also be affected. 10 Furthermore, stability of the active layer is not only influenced by the choice of a particular donor or acceptor molecule, it also depends critically on the combination of the two components. 11 In an OPV, efficient light-to-electrical energy conversion depends on the solid-state microstructure of the active layer, including phase separation between the donor and acceptor components over length scales of tens to hundreds of nanometers. 12–14 Several methods have been utilized to control the initial morphology of the BHJ upon casting from solution including thermal annealing or solvent annealing in which the film is allowed to slowly dry in a saturated solvent atmosphere. In 2006, Bazan and co-workers discovered that adding a small amount of octanethiol (5 vol%) to a toluene solution of P3HT:PCBM led to an order of magnitude increase in the photoconductivity of films cast from the solution compared to films deposited from a pure solution with no “solvent additive”. 15 Use of a solvent additive that preferentially solvates one component, e.g. the fullerene, affects the kinetics of film drying during deposition and as such enables added control of the subsequent film morphology. 16,17 The use of solvent additives, such as diiodooctane (DIO), has become an almost universal processing strategy for increasing the power conversion efficiencies of organic photovoltaics (OPVs) toward 10 % and beyond, 18 with a number of materials systems requiring the additive to reach high performance. The development of “push–pull” low-bandgap copolymers like PTB7 combined with processing from DIO solution led to significant increases in the

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efficiency of OPVs. 19,20 Recently, a new high-performance OPV polymer, PTB7-Th, has been used in combination with PC71 BM to achieve solar cell PCEs near 10 %. 21,22 PTB7-Th is an analogue of PTB7 but with alkylthienyl side groups pendant off the benzodithiophene (BDT) backbone subunit. The molecular structures of PTB7, PTB7-Th, and PC71 BM are shown in Figure 1. Like most other high-performance OPVs based on low bandgap copolymers, this one requires casting the PTB7-Th:PC71 BM blend film from a solution containing a small amount of DIO. Details of our best device (9 % PCE) and fabrication procedure using DIO can be found in the Supporting Information (SI), Figure S1.

O O O

O

O

O S

F

O

S S S

S

F S

S S

n

O

S

n

S

PTB7

[70]PCBM PTB7-Th

Figure 1: Molecular structures of PTB7, PTB7-Th, and PC71 BM.

A decrease in an OPV’s performance over time can be influenced by deterioration of the blend morphology due to coarse donor-acceptor phase separation. 23 Evolution of the nanoscale donor-acceptor phase distribution away from an initial optimum structure often occurs with light exposure or elevated temperature. 24–27 One approach to combat instability of the BHJ morphology is to crosslink the polymer component with either photo or thermal activation to suppress phase separation that, for example, could lead to crystallization of the fullerene acceptor. 28,29 Mixtures of fullerenes can also stabilize the morphology by suppressing crystallization. 30,31 Another recent study highlights the benefits of increasing the crystallinity of the polymeric donor to reduce initial “burn in” loss of efficiency, 32 although high polymer 4


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crystallinity does not guarantee high OPV efficiency. 33 Despite improvements in thermal stability, these efforts have not yet led to high performance devices with long-term stability. Furthermore, in systems in which use of a solvent additive is requisite to ensure optimal initial blending, the additive may have unintended negative consequences for stability as we show in this paper. Although the addition of DIO to solvent provides a processing route to fabricating highefficiency OPVs, we have observed that PTB7-Th and PTB7-Th:PC71 BM films cast from DIO-containing solutions are substantially more prone to photodegradation when exposed to simulated sunlight in ambient conditions. These findings corroborate recent results by others investigating the drawbacks of what is assumed to be residual solvent additive in the BHJ film. 34–40 In general, it has been found that rinsing the BHJ film with a polar “non-solvent” such as an alcohol improves the device performance but the reasons for improvement remain unclear. 41–43 In systems where the film was processed with a solvent additive, it is likely the alcohol wash helps by removing residual additive. 35,37 Li and Brabec have recently shown that PTB7-Th can be processed in air without additional loss of stability as long as an methanol (or ethanol) is used to remove residual DIO from the film. 40 In this paper, we show unequivocally that processing from solutions with DIO leads to residual amounts of the DIO remaining in the film by directly detecting the X-ray fluorescence (XRF) originating from DIO. Furthermore, we find that if the residual DIO is not removed from the film, it accelerates photo-degradation of PTB7-Th and PTB7-Th:PC71 BM films. Nanostructural changes in the films after illumination were measured using grazingincidence wide-angle X-ray scattering (GIWAXS). We find that DIO can be removed by exposing the films to a high vacuum or a high temperature treatment. Alternatively, we show that a methanol rinse after film deposition washes out a substantial amount of the remaining residual DIO. DIO is unstable under light exposure and photolyzes into radical species that subsequently initiate photo-oxidation of the PTB7-Th. As discussed in more detail below, radical-induced degradation of the π-conjugated polymer likely commences by

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hydrogen abstraction on the solubilizing side-chains and proceeds to oxidize the conjugated backbone. 44

Results and discussion Photobleaching of PTB7-Th and PTB7-Th:PC71 BM films We made thin films of PTB7-Th and PTB7-Th:PC71 BM on quartz by blade-coating from solution and measured the UV-visible absorption spectrum of each film at various times during continuous illumination with a tungsten halogen bulb array in air. The Supporting Information (SI), Figure S2, describes our photobleaching apparatus in more detail. Figure 2 compares the bleaching of the absorption spectra for films of PTB7-Th (a) and PTB7-Th:PC71 BM (b) coated from a chlorobenzene (CB) solution containing no solvent additive. Panels (c) and (d) show changes in absorption after white light exposure of the neat polymer and blend films, respectively, deposited from a chlorobenzene solution with 3 vol% DIO added to it. Figure 2e shows the integrated absorption from 500 nm to 900 nm normalized to the initial (time=0) value. Regardless of processing with DIO or not, the film has decreasing absorbance in the visible region over time, particularly in the lower energy regions from 500 nm to 900 nm. In the case of pure PTB7-Th without DIO, the absorption spectra blue shift over time, indicating a decrease in the average chromophore length. The lowest energy band at ∼700 nm decreases faster than the band centered at 635 nm. Reduction of the average chromophore length could be a result of side-chain loss and subsequent twist of the backbone around that site, effectively reducing one low-energy chromophore into two or more higher-energy segments. This would have a more pronounced effect on the lowest energy polymer segments where π-electron density is most delocalized to begin with. Destruction of the entire chromophore, manifested in the loss of higher energy absorption, probably requires breaking conjugation throughout the polymer backbone, for example by photo-oxidative ring-opening of constituent benzene and thiophene rings. Figures 2b and 2e 6


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show that unlike in other polymer:fullerene systems such as P3HT:PCBM, blending fullerene with the PTB7-Th does not stabilize the photobleaching. 45 On the contrary, the polymer’s absorption decays faster in the presence of the fullerene, perhaps because the fullerene can enable indirect formation of an excited polymer triplet state that subsequently sensitizes reactive singlet oxygen. In fact, it is known that oxidative photobleaching of PTB7 is accelerated when blended with PC71 BM, so it is no surprise that PTB7-Th behaves similarly. 46,47 For films cast from a DIO-containing solution, Figures 2c and 2d, the photobleaching rates are significantly accelerated. We observe that 80-90% of the initial film absorption from 500 nm to 900 nm is lost within the first hour. The neat polymer film is entirely bleached. In the blend film, only the fullerene component continues to absorb light and the film color has changed from dark purple to yellowish-brown. Photodegradation is often enhanced under UV light emission and we note that our tungsten halogen light source has less UV emission than a corresponding 1-sun solar simulator; however, the absorption bleaching times we observe are already on the order of minutes for films with DIO. Changes in the nanostructure of the films upon illumination were measured using GIWAXS (Figure 3: line profiles and Figures S3-S4: 2D scattering images). Irradiation of the neat polymer and blend leads to several structural changes in the polymer, most noticeably in the alkyl stacking distance. Before illumination, the neat PTB7-Th film has an in-plane peak at q = 0.26 Å

−1

corresponding to an alkyl stacking distance of 24 Å. After exposure

to light, the alkyl stacking distance shifts to 23 Å. Similar variations in the alkyl stacking distance (∼ 1 Å) are commonly observed between as-cast and annealed polymer films and can be attributed to relatively small changes in the geometry of the sidechains. 48 When DIO is added to the neat polymer, the alkyl stacking distance is 23 Å prior to illumination and 24 Å after illumination, again relatively small shifts. Larger changes occur when PTB7-Th is blended with PC71 BM, the alkyl stacking distance shifts to 22 Å for blends with DIO and 21 Å without DIO. The distance increases by ∼ 1 Å for each blend after light exposure. More significant is the emergence of a new in-plane peak at q = 21 Å

−1

7


for samples exposed to


Absorbance /OD

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b) 1.0

PTB7-Th

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Norm. Integrated Abs.

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PTB7-Th:PC71BM Irradiation time /hrs 0 0.5 3 6 12

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800 400 600 Wavelength /nm PTB7-Th PTB7-Th:DIO PTB7-Th:PC71BM PTB7-Th:PC71BM:DIO

0.8 0.6 0.4 0.2 0.0 0

2

4 6 8 Irradiation time /hours

10

12

Figure 2: Absorption spectra versus irradiation time for thin films on quartz of (a) PTB7-Th and (b) PTB7-Th:PC71 BM processed from pure chlorobenzene (CB); and the corresponding films, (c) and (d), processed with a 3 vol% DIO/CB solvent mixture. (e): A plot of the integrated absorption intensity versus illumination time of each film.

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light, which corresponds to a d-spacing about 30 Å (see Figure S5 for zoomed-in line-cuts). This feature is present for PTB7-Th with DIO and the blend PTB7-Th:PC71 BM with and without DIO and is attributed to formation of products from photo-degradation rather than a simple shift in structure. PTB7-Th

Intensity (a.u.)

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


Dark Irradiated

PTB7-Th:DIO PTB7-Th:PC71BM PTB7-Th:PC71BM:DIO

0.5

1.0

qxy(Å-1)

1.5

2.0

Figure 3: In-plane GIWAXS line profiles of PTB7-Th and PTB7-Th:PC71 BM thin films. Samples were processed with or without DIO and either kept in the dark or exposed to ambient white light illumination for three hours. 2D scattering images can be found in the Supporting Information.

The process of polymer photo-degradation in going from dark to light appears to be exaggerated when DIO is present. There is a noticeable drop in intensity for the in-plane alkyl stacking peak at q = 0.26 Å

−1

and the out-of-plane π-π stacking at q ∼ 1.63 Å

−1

after

PTB7-Th films with and without DIO are illuminated. This is expected because light has been shown to cause degradation of PTB7-Th, 49 however the effect is more pronounced in the film cast with DIO. This greater degradation is evidenced by the change in the polymer alkyl peak full-width-half-maximum (FWHM) value, which was used to calculate the lower limit on the coherence length of crystallites in the in-plane direction of PTB7-Th (see Table S1). 50,51 Before illumination, the neat polymer film had an average coherence length of 134 Å, which increased to 212 Å when DIO was added. After illumination, the polymer coherence length decreased by 10 % to 121 Å, compared to a 35 % decrease to 138 Å with DIO present. Additionally, the coherence length of PBT7-Th in the blend without DIO decreased by ∼ 33 % after illumination, while the blend with DIO showed a 66 % decrease. 9



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The formation of photo-degradation byproducts can also be observed in the GIWAXS patterns. The dark PTB7-Th:DIO film has the characteristic alkyl stacking peak at 0.26 Å

−1

and π-π stacking at q ∼ 1.63 Å . However, after exposure to light, discrete spots and arcing −1

are present throughout the diffraction pattern. This same effect is observed in the DIO containing blend, however, it is unclear which peaks arise from the crystallization of PC71 BM versus the polymer photo-degradation byproduct, but both are likely present. Photographs of the PTB7-Th:PCBM:DIO film show the film is inhomogeneous with large aggregates present after irradiation, which are likely PC71 BM (Figure S6). Liu et al. noted that DIO was a good solvent for PCBM, but a poor solvent for PTB7, thus after the chlorobenzene in the film evaporates and the PTB7 solidifies, the DIO provides the PCBM with increased mobility to diffuse through the film and crystallize. 49

Detection and Removal of Residual DIO in Thin Films Given the evidence for accelerated degradation in the films processed with DIO as a solution additive, we were interested in directly detecting any residual DIO in a film after casting. Despite the widespread use of DIO as an additive, it is not always clear whether or not DIO remains in the film after the solvent dries. Hence, it is important to determine if the increased degradation in the sample processed with DIO is a result of residual DIO in the film, or instead is due to DIO-induced changes in microstructure. We used X-ray fluorescence (XRF) to identify iodine from the DIO. X-ray fluorescence (XRF) spectroscopy can be a useful elemental analysis technique. Upon absorption of a high-energy (X-ray) photon, an atom’s core electron is ejected and the atom is ionized. Core electrons in nearby but higherlying atomic orbitals subsequently relax to fill the void left by the ejected electron. During the relaxation process, a photon also of X-ray energy is emitted by process of fluorescence. Because emitted photons in XRF are characteristic of specific atomic orbital transitions, e.g. Kα = L2p → K1s , the emission spectrum can be used to reveal the chemical composition of a material sample. XRF is seldom used to probe conjugated polymers for OPV but has been 10


Intensity /cps

a)

PTB7-Th as-cast

6 4

b)

0.0% DIO/CB 1.5% DIO/CB 3.0% DIO/CB 6.0% DIO/CB Iodine

2 0 3600 4000 4400 4800 Energy /eV

c)

PTB7-Th 1hr vacuum

PTB7-Th:PC71BM as-cast

6 4 2 0 3600 4000 4400 4800 Energy /eV

d)

6

Intensity /cps

Intensity /cps

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Intensity /cps

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4 2 0 3600 4000 4400 4800 Energy /eV

PTB7-Th:PC71BM 1hr vacuum

6 4 2 0 3600 4000 4400 4800 Energy /eV

Figure 4: XRF spectra of (a) PTB7-Th and (b) PTB7-Th:PC71 BM thin films on quartz as a function of DIO/CB solution concentration. The films show characteristic iodine fluorescence with the relative intensity corresponding to the DIO solution concentration. (c) and (d): When the films are held at 10−8 Torr vacuum for 1 h, the residual DIO is removed from the film and the iodine XRF signal is no longer detected. The vertical lines indicate the relative intensities of atomic iodine Lα and Lβ emission energy lines.

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utilized in the past to detect trace metal impurities such as tin and palladium that are left over from the polymer synthesis and have adverse effects on solar cell performance. 52 Figure 4 shows XRF spectra of as-cast PTB7-Th and PTB7-Th:PC71 BM films deposited from solutions with different concentrations of DIO in CB solution. The vertical lines indicate the relative intensities of atomic iodine Lα and Lβ emission energy lines: Lα1 = 3.938, Lα2 = 3.926, Lβ1 = 4.221, Lβ2 = 4.508, Lγ1 = 4.800 keV. In Figures 4a and 4b, the films show characteristic iodine fluorescence with the relative intensity corresponding to the DIO solution concentration. A higher DIO concentration in solution translates to more residual DIO in the blade-coated film. Pure polymer PTB7-Th and blend PTB7-Th:PC71 BM films were also subjected to 10−8 Torr vacuum for approximately one hour, conditions that simulate the pressure used in thermal evaporation of top metal contacts when fabricating an OPV device. XRF spectra of these films, Figures 4c and 4d, show no measurable iodine fluorescence and suggest most of the residual DIO is removed from the films after high vacuum treatment, at least within the detection limit of iodine in the XRF measurement which is ∼ 1 ppm. 53 Figure 5 shows that the UV-visible absorption spectrum of the vacuum-treated blend film does not degrade like an untreated film with DIO but instead mimics that of a film processed without DIO. This adds further credence to the hypothesis that it is the presence of DIO, rather than DIO-induced structural changes, that accelerate the photo-oxidation of the film. We also subjected a DIO-containing film to a thermal treatment of 175 ◦C for 30 minutes. Because this temperature is above the boiling point of DIO and likely above the glass transition temperature, Tg , of PTB7-Th, we expected this heat treatment to drive off any residual DIO from the film. Indeed, UV-visible absorption spectra of this film show no signature of DIO-accelerated photobleaching (Figure S7). However, we note that thermal annealing generally does not improve OPVs utilizing low bandgap push-pull copolymers. 26,54 Another method that has been suggested to remove residual solvent additive is to wash the film with a simple alcohol such as methanol or isopropanol. 35,37 We spun 70 µL of methanol

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Absorbance /OD

a)

2.0 PTB7-Th:PC71BM:DIO Irradiation time/hrs 0 as cast 1

1.5 1.0 0.5 0.0 300

400

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700

800

700

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b)

Absorbance /OD

2.0

c)

PTB7-Th:PC71BM:DIO 1hr vacuum

1.5 1.0 0.5 0.0 300

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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


400


2.0 PTB7-Th:PC71BM as-cast

1.5 1.0 0.5 0.0 300

400


Figure 5: Absorption spectra of PTB7-Th:PC71 BM blend films taken before (black lines) and after (red lines) 1 h of light soaking. The film “as cast” from DIO/CB solvent (a) shows decreased absorption across its entire spectrum after irradiation. On the contrary, the film cast from DIO/CB solvent and subsequently placed in high vacuum for 1 h, (b), shows only degradation of the polymer’s lowest energy absorption and matches the degradation observed in a film cast without DIO in the solvent, (c).

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0.3 0.2

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0.0% DIO/CB 1.5% DIO/CB 3.0% DIO/CB

Absorbance /OD

a) Intensity /cps

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6.0% DIO/CB Iodine

0.1 0.0 3600 4000 4400 4800 Energy /eV

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Irradiation time /hrs: 0 1 12 29

0.8 0.6 0.4 0.2 0.0

PTB7-Th:PC71BM MeOH rinse


Figure 6: (a) XRF spectra of methanol-washed PTB7-Th:PC71 BM thin films on quartz as a function of DIO/CB solution concentration. Washing the films by spincasting methanol on top of them eliminates most of the iodine fluorescence. Although close to the limit of our resolution, it appears there is still a small amount of fluorescence, particularly at Lα1 = 3.938 keV, indicating not all of the DIO has been removed from the film. (b) UVvisible absorption spectra of methanol-rinsed films show the methanol wash is effective in preventing DIO-accelerated photobleaching of the blend film, at least withing the first hour of light exposure.

onto a PTB7-Th:PC71 BM blend film cast from a 97/3 CB/DIO solution. XRF measurement of the methanol-washed film shows almost no iodine fluorescence; although, it appears there may still be a small amount of fluorescence, particularly at Lα1 = 3.938 keV (Figure 6b). Whether or not all of the DIO is removed from the film, the UV-visible absorption spectra of the rinsed film (Figure 6d) shows the methanol wash is effective in preventing DIOaccelerated photobleaching of the blend film. To the best of our knowledge, all reports of OPVs using PTB7 and PTB7-Th processed with DIO used evaporated metal contacts and therefore films were exposed to high vacuum, fortuitously removing residual DIO. Hence, the adverse effects of DIO to stability were not experienced. However, processing OPV films with a high-vacuum step might be prohibitive as fabrication moves to larger scale methods such as roll-to-roll. Moving forward, our results suggest that DIO must be avoided during processing; alternatively, polymer systems resistant to DIO-induced degradation should be sought and used. We are currently undertaking further investigations to elucidate the importance of the copolymer subunits and side-groups

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on photostability.

Mechanism of DIO-accelerated Polymer Degradation Towards the goal of elucidating a mechanism by which these materials photo-degrade, we used FTIR to monitor specific chemical changes in the materials during irradiation. Figure 7 shows FTIR spectra of PTB7-Th and PTB7-Th:PC71 BM films taken as a function of exposure time to simulated sunlight in ambient. Spectra were collected from 4000 cm−1 to 650 cm−1 , but here we chose to focus on three particular regions of interest: region I from 3500 cm−1 to 2700 cm−1 corresponds to absorptions by hydroxyl (−OH) groups and aliphatic hydrogen (−CH2 and −CH3 ) groups; region II from 1800 cm−1 to 1630 cm−1 corresponds to carbonyl (−C−O) stretches, region III from 1575 cm−1 to 1350 cm−1 is where conjugated ring stretches take place. Table 1 summarizes the most important absorption peaks and the vibrational mode we assigned to each of them. The pure PTB7-Th FTIR spectrum, shown in Figure 7a, shows relatively few changes over time. In region I, there is a slight decrease in the methyl and methylene peaks from 2955 cm−1 to 2856 cm−1 . After 50 hrs of illumination, a weak intensity hydroxyl peak appears centered at 3250 cm−1 . This peak has been observed in the photodegradation of several other polymer systems and it has been attributed to a carboxylic −OH as opposed to an alcoholic −OH that has a broader absorption centered at 3400 cm−1 . 44,55 Rise of carbonyl absorption over time in region II is observed with the greatest increase at 1720 cm−1 that is indicative of additional ester or carboxylic acid formation. Carboxylic acid formation is consistent with the appearance of the aforementioned hydroxyl peak at 3250 cm−1 . We also observe some interesting changes in region III. Notably, there is a decrease at 1568 cm−1 and concomitant rises at 1525 cm−1 and 1428 cm−1 indicating changes are occurring within the thienothiophene backbone unit. Therefore, photo-oxidation of pure PTB7-Th likely involves reactions initiating on alkyl side chain sites as well as directly in backbone units. The addition of PC71 BM to the film complicates the FTIR spectra somewhat. Spectra 15



Absorbance /a.u.

a) PTB7-Th I

3400

II

3000

Irradiation time /hrs: 00 III 10 20 50

1800 1700 Wavenumber /cm-1

1500

1400

1500

1400

1500

1400

Absorbance /a.u.

b) PTB7-Th:PC71BM I

3400

II

3000

III


c)

Absorbance /a.u.

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PTB7-Th:PC71BM:DIO I

3400

II

3000

III


Figure 7: FTIR spectra vs irradiation time for (a) PTB7-Th, (b) PTB7-Th:PC71 BM without additive, and (c) PTB7-Th:PC71 BM with DIO.

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Table 1: FTIR band assignments for PTB7-Th and PTB7-Th:PC71 BM thin films ν, cm−1 3450 3240 2955 2926 2870 2856 1778 1732 1728 1652 1568 1525 1456 1429 1050

Assignment −OH −OH(COOH) −CH3 −CH2 −CH3 −CH2 −C−O −C−O −C−O −C−O −C−C, thienothiophene thiophene ring benzene ring −C−C, benzene −SO−

of the blend film without DIO, Figure 7b, show some reduction of the aliphatic hydrogen absorptions over time in region I. This is reflective of changes primarily in the fullerene, as has been observed by others. 56 Again, we observe a rise in the carbonyl absorption, particularly at 1718 cm−1 , and more pronounced changes in the thienothiophene band around 1525 cm−1 . At 1428 cm−1 , most of the intensity is now due to the fullerene. Contrary to films without DIO, Figure 7c shows more drastic temporal changes occur in the vibrational stretches within PTB7-Th:PC71 BM films cast from a 3 vol% DIO/CB. A large −OH absorption grows in over time signifying that hydroxylation is a prominent degradation product, including alcohols. The methyl and methylene stretches appear to decrease but we note that this may also be due to degradation of the DIO and not necessarily the PTB7Th or PC71 BM. As expected, the FTIR spectrum of DIO shows strong absorption in this region and makes comparison to films without DIO difficult. 57 Given the density of DIO (1.8 g mL−1 ) and the concentration of PTB7-Th used here (10 × 10−3 g mL−1 ), 3 vol% DIO in CB equates to approximately 15 DIO molecules per polymer repeat unit in solution. In the film, we used XRF to detect the presence of the residual DIO but we are unable to quantify its absolute concentration. Regardless, it is clear that DIO accentuates the photo17



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oxidative FTIR signatures we attribute to PTB7-Th degradation. The reduction of signal in the 1460 cm−1 to 1400 cm−1 energy range indicates that in the presence of the DIO, PC71 BM is also degrading more rapidly, as was observed in UV-Vis as well (Figures 2 and 5). Radical Initiation:

Scheme 1: (top) Photolysis of diiodooctane (DIO) results in formation of iodooctane radical. (bottom) PTB7-Th radical photo-oxidation can be initiated when the iodooctane radical abstracts a hydrogen atom from the α-carbon of the ethylhexylthienyl side-chain of the polymer’s benzodithiophene (BDT) subunit. For clarity, most of polymer has been abbreviated by (-BDT-TT-). Refer to Figure 1 for the full molecular structure of PTB7-Th. The photo-oxidation of the conjugated polymer P3HT is well studied. 55,58–60 Recently there have been several reports on the mechanisms of photo-oxidation of low bandgap conjugated polymers. 44,61–63 Although we are still unraveling the mechanistic details for our materials system, following the approach of others (referenced above), we propose the radical initiated mechanism for oxidation of PTB7-Th, Scheme 1. In the presence of a radical species, abstraction of the most acidic hydrogen, the one attached to the alpha carbon of the alkyl side chain pendant on the BDT backbone unit, likely occurs. Conjugation of the polymer backbone provides some stabilization for radical polymer species, however this product is still highly reactive with oxygen and can propagate more radical polymer formation, see propagation products in Scheme S1. In our case, the source for radicals is DIO, although in the absence of DIO degradation still occurs. 9 DIO is a liquid at room temperature and it can only be purchased with a copper stabilizer. The reason for this is that the carbon-iodine bond can easily dissociate in the presence of UV light. DIO likely dissociates into iodooctane 18


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and iodine radicals. I· is unstable and will likely terminate with another I· to form I2 . On the other hand, the iodooctane radical is somewhat stabilized by the hydrocarbon chain and, in the presence of PTB7-Th, is likely to initiate hydrogen abstraction from the polymer side-chain leading to the formation of chain oxidation products such as esters and carboxylic acids. If β-scission occurs, radicals will form on the thiophene ring and further reactions can propagate into the conjugated polymer units. It is also possible that sulfur within the backbone structure is oxidized directly resulting in formation of sulfoxide (−S(O)−) and sulfone (−S(O2 )−): indeed, in the FTIR spectra of the blend film with DIO, we observe the growth of a sulfoxide peak over time with a characteristic absorption at 1050 cm−1 (Figure S8). We identify the distal sulfur in the thienothiophene unit as the most susceptible to this reaction due to its lack of full aromaticity.

Conclusions We have shown that films of PTB7-Th and PTB7-Th:PC71 BM are not intrinsically stable to photo-oxidation. Their UV-visible absorption bleaches by more than 50 % of their initial values in just of few hours of ambient light exposure. Furthermore, when films are cast from solutions containing the OPV solvent additive DIO, residual DIO in the film greatly accelerates the photo-oxidative degradation by acting as a radical initiator. Therefore if used to make an OPV, DIO must be removed from the active layer film prior to deposition of additional layers, e.g. the top electrode. We have demonstrated that using high vacuum, such as that typically used in laboratory scale device fabrication, serves to remove residual DIO. However, this technique is not amenable to scale-up processing techniques such as roll-to-roll, where vacuum processing steps are not commonly utilized. A high temperature treatment of 175 ◦C for 30 minutes eliminates DIO from our films, but for most OPVs based on low-bandgap push-pull polymers like PTB7-Th, thermal annealing is detrimental to device performance. 26,54 Further studies must be done to investigate the

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effect of thermal annealing on the donor-acceptor morphology and the subsequent lifetimes of the solar cells. We also note that some OPV polymers may not be susceptible to DIO-induced degradation to the extent PTB7-type polymers are. A recent study highlights the remarkable photostability of another benzodithiophene-based (BDT) polymer, PBDTTPD, albeit in the absence of any solvent additive. 61 Polymers that are intrinsically more stable than PTB7 may be more amenable to processing with solvent additives. One reason for this may come from a recent investigation of the role of solvent additives on the photodegradation of P3HT:PCBM blend films by Tournebize et al. 36 In the study, they suggest DIO decomposition products decrease the photostability of P3HT:PCBM, but interestingly, they claim residual DIO is only a problem if the active layer is capped with another layer, e.g. PEDOT:PSS, that confines the solvent additive. With just the active layer on glass, photodegradation is not affected by DIO because the photodegradation by-products of DIO can migrate out of the film. On the other hand, here we find DIO contributes to the blend film degradation even without a top layer. Thus, given P3HT is a more photo-stable polymer than PTB7-Th, the relative intrinsic photostability of the polymer may directly correlate with its reaction rate to a solvent additive’s light-induced radical by-products.

Experimental Materials PTB7-Th was purchased from 1-Material (sold under the name PCE-10).

[70]-PCBM

was purchased from Nano-C. Chlorobenzene and diiodooctane were purchased from Sigma Aldrich. All materials were used as received, without further purification.

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Solutions and thin-films deposition Due to the relatively small volume ratios of diiodooctane (DIO) to chlorobenzene (CB) used in this study, we initially created a 6 vol% DIO/CB solution by adding 300 µL DIO to 5000 µL CB and stirring for at least 15 min. Subsequently, 3 vol% and 1.5 vol% DIO/CB solutions were made via serial dilution. PTB7-Th solutions were fabricated by adding 1000 µL of solvent to 10 mg of polymer. Solutions were stirred and heated at 60 ◦C for at least 12 h. PTB7-Th:PC71 BM solutions were created by blending 8.5 mg PC71 BM with 500 µL PTB7Th solution, resulting in an 8.5 to 5 (or 1.7 to 1) fullerene to polymer ratio. Blend solutions were also stirred and heated at 60 ◦C for at least 12 h before depositing films. For XRF and photobleaching studies, thin films of PTB7-Th and PTB7-Th:PC71 BM were deposited from 60 ◦C solutions using a Zehntner ZAA 2300/ZAU 2000 automatic film applicator coating system. The coating stage temperature was set to 50 ◦C and pre-cleaned quartz glass substrates were allowed to heat on the stage for approximately 15 min prior to film deposition. Three films were created in one pass of the blade using 20 µL of solution; with a gap height of 1200 µm and draw speed of 30 mm s−1 . PTB7-Th film thicknesses were 86 ± 12 nm, whereas PTB7-Th:PC71 BM films had thicknesses of 143 ± 27 nm as measured by a Dektak 8 stylus profiler.

Light-soaking and photobleaching measurements Films were exposed to continuous illumination in ambient by a halogen array of four Eiko MR16 50 W 4700 K bulbs. Samples were placed on a rotating platform that ensured each sample had the same average illumination intensity. The intensity of the bulb array was adjusted such that an NREL-certified reference photodiode, placed at sample height directly under one of the bulbs, produced a short-circuit current equal to that when illuminated by 1sun AM1.5G light from a calibrated Newport Oriel Sol 3A solar simulator. The light-soaking station is pictured in Figure S2. Absorption spectra were recorded using an Ocean Optics DH-2000-BAL deuterium21



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halogen light source, 600 µm-core solarization resistant UV-Visible optical fibers, and an OceanOptics HD4000 spectrometer. A 2.0 neutral density (ND) filter was used to attenuate the excitation light before the sample. The spectrometer integration time was 125ms per spectrum and the average of 5 spectra was recorded. In order to check the influence of film reflectance and scattering in our measurement, we also calculated the absorbance of select films by measuring the transmittance and reflectance using a Shimadzu 3600 UV-VIS-NIR spectrophotometer. Absorption spectra measured with both techniques were similar.

X-ray fluorescence (XRF) XRF spectra were recorded in air using a Fischerscope X-Ray XDV-SDD energy-dispersive X-ray spectrometer equipped with a rhodium target X-ray source. A source voltage of 50 kV was used with a 0.3 mm diameter aperture and nickel primary filter. A silicon drift detector with Peltier cooling was used for X-ray collection. Collection time was 120 s. A clean quartz glass slide was use to collect a background spectrum that was then subtracted from each sample spectrum.

Fourier transform infrared spectroscopy (FTIR) A Thermo Scientific Nicolet 6700 spectrometer with a liquid nitrogen cooled MCT/A detector was used to collect FTIR spectra. Scan resolution was approximately 1 cm−1 and the average of 128 scans was recorded. Thin-films of PTB7-Th and PTB7-Th:PC71 BM were deposited by drop-casting from solution onto Real CrystalTM IR sample cards with 19 mm-diameter KBr windows (International Crystal Labs).

Grazing incidence wide angleX-ray scattering (GIWAXS) GIWAXS measurements were conducted at the Stanford Synchrotron Radiation Lightsource at beamline 11-3. Data was collected at a sample to detector distance of 400 mm with an

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X-ray beam energy of 12.7 keV. Samples were kept under a helium atmosphere and tilted to an incidence angle of 0.10◦ during measurements. 2D scattering data collected was processed and analyzed using the WxDiff software developed at SSRL.

Acknowledgement BTV thanks Dr. Phil Parilla for help with XRF measurements and Drs. Zbyslaw Owczarczyk, Wade Braunecker and Logan Garner for helpful discussions of radical photo-oxidative mechanisms for polymer degradation. DCO, NK, and BTV were supported by the U.S. Department of Energy under Contract no. DE-AC36-08GO28308 with the National Renewable Energy Laboratory through the DOE Solar Energy Technology Program. PHB was supported by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS) under the Science Undergraduate Laboratory Internship (SULI). MLC and KAO acknowledge the Research Corporation for Science Advancement and NSF DMR 1436263 for support. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. DPO and SES were supported by NSF grants CHE-1125937 and DMR-1006930 and by the Research Corporation for Science Advancement Scialog Program.

Supporting Information Available Experimental details of solar cell fabrication, photo-soaking apparatus, detailed radical polymer degradation mechanisms, and GIWAXS 2D images and peak fitting results for PTB7 and PTB7-Th:PC71 BM are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Graphical TOC Entry After Light Exposure No DIO DIO PTB7-Th

PTB7-Th: PC71BM

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Removal of Residual Diiodooctane Improves Photostability of High

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