Promoting Morphology with a Favorable Density of States Using

Jun 6, 2017 - Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India. § Department of ...
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Promoting Morphology with a Favorable Density of States Using Diiodooctane to Improve Organic Photovoltaic Device Efficiency and Charge Carrier Lifetimes Logan E. Garner,† Abhijit Bera,‡ Bryon W. Larson,† David P. Ostrowski,§ Amlan J. Pal,*,‡ and Wade A. Braunecker*,† †

Chemistry & Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India § Department of Electrical, Computer and Energy Engineering, University of Colorado Boulder, Boulder, Colorado 80309, United States ‡

S Supporting Information *

ABSTRACT: Due to the inherent challenges in probing nanoscale properties within bulk heterojunction (BHJ) active layers of organic photovoltaic (OPV) devices, the relationship between morphology and nanoscale electronic structure is not well understood. Here, we employ scanning tunneling microscopy (STM) dI/dV imaging and localized density of states (DOS) spectra to investigate the influence of additives on morphology in a high-performance OPV system. In short, we are able to correlate the use of diiodooctane (DIO) additive with significant changes to the distribution of the localized DOS, most notably a broader distribution of PCE10 polymer HOMO levels and PC70BM fullerene LUMO levels, as well as significantly smaller domain sizes and significantly higher overall device efficiencies. We further correlate this data with a nearly 3-fold increase in charge carrier lifetimes in the active layer when DIO is employed, determined by time-resolved microwave conductivity (TRMC) measurements. The results are consistent with the growing body of literature evidence that DIO promotes the formation of a polymer/fullerene mixed phase and therefore highlight the unique information that this combination of techniques can provide when investigating OPV active layer morphology. device performance.6,7 However, simply improving structural order in the bulk does not completely account for the combinations of improved features such as more efficient generation of free charge carriers, increased carrier lifetime, and improved carrier mobility. Several groups have more thoroughly investigated the relationship between morphology and the bulk electronic features that strongly influence device performance, and a clearer picture of the full role of processing additives is emerging. One particularly well-studied and optimized highperformance donor polymer consists of alternating units of electron-donating benzodithiophene moieties with electronaccepting fluorothienothiophene units and is commonly

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ince the initial reports that processing additives such as alkane dithiols1 or diiodooctane (DIO)2 could afford a means to tune the morphology of bulk heterojunction (BHJ) organic photovoltaic (OPV) active layers and improve device performance, the use of such additives has become commonplace.3,4 Many reports have emerged that contribute to our overall understanding of how processing additives ultimately improve device performance. For example, additives with notably higher boiling points than the main processing solvent have been reported to facilitate favorable structural ordering and molecular packing in the bulk, resulting in increased charge carrier mobilities, more efficient charge carrier generation, and longer charge carrier lifetimes.5 Reports employing highly ordered systems based upon a number of alternating “push−pull” donor copolymers further support the idea that nanoscale ordering and/or crystallinity of molecular components induced by the presence of solvent additives plays a key role in improving the factors that govern optoelectronic © XXXX American Chemical Society

Received: April 11, 2017 Accepted: June 6, 2017 Published: June 6, 2017 1556

DOI: 10.1021/acsenergylett.7b00315 ACS Energy Lett. 2017, 2, 1556−1563

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http://pubs.acs.org/journal/aelccp

Letter

ACS Energy Letters

processing additives to as high as ∼9−10% PCE for optimized systems (hence, its namesake).21,22 Despite being composed of semiconductor materials, the contact and internal resistances of thick (∼100 nm) active layers required in OPV devices is known to skew STS/STM results; more accurate data can be obtained from thinner (10 nm) sample films in the latter technique. Thus, we first optimized blend ratios of PCE10 and PC70BM such that highquality, uniform films could be obtained for both thicknesses comprised of the same weight ratio of donor and acceptor components. This compromise led us to use a stock solution of PCE10:PC70BM prepared in 1:5 wt/wt ratio that was spun at 800 rpm to generate thick active layers for OPV devices and 6000 rpm to generate thin samples for STS/STM analysis (see the Supporting Information for more details). We then probed the surface morphology of the two samples (∼100 and ∼10 nm thick) with atomic force microscopy (AFM). While it is known that significant differences can exist between surface and bulk phase transitions,23 the AFM images in Figures S1 show that the two samples at least have very similar surface morphology. Devices with inverted architecture (ITO/ZnO/ PCE10:PC70BM/MoO3/Ag) were then prepared with and without DIO as a component of the active layer processing solvent (3 and 0% DIO, respectively). The active layer, MoO3 hole transport layer, and Ag hole collection contact thicknesses were 90−110, 10, and 100 nm, respectively. These devices were subjected to current−voltage (J−V) and external quantum efficiency (EQE) measurements for comparison of overall device performance between devices prepared with and without DIO. Figure 1A,B consists of J−V and EQE data corresponding to devices prepared with (red traces) and without (black traces) processing additive. Performance parameters, including PCE, short-circuit current density (JSC), open-circuit voltage (VOC), and fill factor (FF), are listed in the inset of Figure 1A (text color coded to traces). As anticipated and consistent with the aforementioned literature, significant improvements in device performance resulted from the use of DIO as a processing additive. The PCE of devices employing DIO more than doubled (6.57 vs 3.08% with no additive). These values are lower than optimized values reported in the literature but are still quite high given our blend ratio selected to optimize films for both OPV devices and STS/ STM analysis. Additionally, a nearly 2-fold increase in JSC was observed with DIO (11.8 vs 6.8 mA/cm2), as well as a notable increase in FF (0.70 vs 0.57). As seen in Figure 1B, devices prepared with DIO also exhibit a nearly 2-fold increase in EQE compared to no additive devices. The presence of DIO also appears to slightly alter the shape of the EQE spectrum; while this observation is somewhat unusual, we do note that it has been observed by several others that features of the EQE spectrum are sometimes sensitive to processing conditions and additives for this particular class of materials.24,25 While notable differences are observed for every other parameter, little change in VOC is observed between no additive and DIO-containing devices; this observation is typical for these materials8,13 and even expected as the VOC reflects the electronic structural features of the device components as a whole and therefore should not be significantly influenced by differences in local nanoscale electronic structure. The large increase in JSC and EQE values observed corresponding to DIO-containing devices are consistent with more efficient generation of free carriers, less recombination, and increased charge carrier mobilities, all

referred to as “PTB7”, having achieved between 7 and 9% photoconversion efficiency (PCE) depending on the interface materials and architecture employed in the device.8−10 Using Xray scattering techniques, Chen et al. concluded that DIO selectively dissolves PCBM aggregates, thereby allowing intercalation of the fullerene into PTB7 domains, resulting in both an optimized domain size and interface.11 Deibel et al. investigated morphology differences and performance in terms of dominant loss processes of PTB7-based solar cell devices prepared with and without DIO.12 They employed a combination of transient photovoltage, voltage-dependent charge extraction, and time-delayed collection field measurements in their study to provide strong evidence that both geminate and nongeminate recombination are dramatically decreased in devices processed with DIO that have smaller polymer and PCBM fullerene domains. Samual et al. proposed that the morphology obtained in active layers of PTB7:PCBM prepared with DIO additive results in donor/acceptor interfaces that effectively contain concentration gradients of the PTB7 and PCBM. These morphologies were observed to facilitate better charge extraction relative to morphologies obtained without the use of the processing additive.13 Studies such as these have inspired us to further consider the influence of processing additives on the nanoscale electronic environment and the subsequent effects on factors such as recombination and charge carrier lifetime, which play key roles in device performance. Indeed, research focused on understanding the role of active layer microstructure and morphology has been proposed as a critical next step in advancing the field.14 Unfortunately, few techniques exist that afford a means to probe localized electronic features within bulk materials. Scanning tunneling spectroscopy (STS) coupled with scanning tunneling microscopy (STM) is a particularly versatile and powerful technique for probing morphology in the nanoscale in terms of energy levels of the semiconductor materials.15−18 For example, the STS/STM technique affords the means to image and investigate localized electronic properties on the nanoscale and has been employed to map band edges of nanostructures.17 STS/STM has also been used to map the bands in a p−n junction and experimentally determine the width of the depletion region with nanometer resolution.19 The technique has further been used to investigate cross-sectional interfaces in a model BHJ OPV system.20 The powerful ability of STS/STM to image semiconductor materials at the nanometer scale allows transient conductance (dI/dV) images to differentiate semiconductor components based upon their electronic structure. The localized density of states (DOS), that is, polymer and fullerene HOMO and LUMO levels, can even be mapped at various positions throughout a given sample. We therefore contend that this technique is uniquely suited for correlating morphology and nanoscale electronic properties within OPV active layers when considering percolating networks or individual domains of the components in bulk heterojunctions. Here, we employ the high-performance organic polymer donor commonly known as “PCE10” with a fullerene acceptor (PC70BM) as a model BHJ OPV system. PCE10, sometimes called PTB7-Th, varies structurally from PTB7 only in its solubilizing side chains (alkyl thiophenyl vs alkoxy groups on the benzodithiophene unit, respectively). The performance of PCE10 is known to significantly improve with the use of DIO 1557

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Figure 2. STM dI/dV images corresponding to 10 nm thick active layer films prepared on Au(111) and imaged under inert atmosphere. Left panels: active layer films cast from chlorobenzene. Right panels: active layer films cast from chlorobenzene containing 3% DIO. Imaging conditions: a lock-in amplifier (16 mV rms, 971 Hz) was employed at 1.33 V. Blue/green and red regions correspond to PCE10-rich and PC70BM-rich domains, respectively. Figure 1. (A) Light current−voltage (J−V) traces and (B) external quantum efficiency (EQE) spectra corresponding to inverted devices with the structure ITO/ZnO/PCE10:PC70BM/MoO3/Ag and active layers cast from chlorobenzene with no additive (black traces) and chlorobenzene containing 3% DIO (red traces). Values of PCE, JSC, VOC, and FF are listed as insets of (A) and color coded with their corresponding J−V traces.

the films in order to ascertain the distributions of the local DOS. To obtain a single measurement, the tip was held at a given position and the sample bias was swept from 2.5 to −2.5 V. The tunneling current was measured simultaneously to obtain localized current−voltage profiles that correlate to the nanoscale electronic structure as a function of position. As the sample is biased in the negative and positive voltage range, the measured tunneling current corresponds to occupied and unoccupied states, respectively. In the case of pure materials, dI/dV curves afford accurate information regarding the relative positions of HOMO and LUMO energy levels. For example, the band gaps obtained from the dI/dV curves of neat PCE10 and PC70BM films (Figure S2) were determined to be 1.7 ± 0.1 and 2.15 ± 0.1 eV, in excellent agreement with experimental and computational band gap values previously reported for these materials.28,29 The tunneling spectroscopy characteristics of the pure materials then allow us to distinguish between donor and acceptor components in the blends. Figure 3 shows histograms of the distributions of the local DOS of PCE10 HOMO levels (which will influence hole transport) and PC70BM LUMO levels (responsible for electron transport) of both the neat films as well as blend films with and without DIO additive. Each histogram (represented with bars) was compiled from dI/dV data from at least 50 different positions for the neat films and over 100 positions for the blends. The line traces represent Gaussian fits of the histograms. While the histograms are not anticipated to reflect perfect Gaussian distributions, as both the donor polymer30 and the fullerene31 can be considered semicrystalline with amorphous and crystalline regions each having distinct HOMO and LUMO values, treatment of the data with a Gaussian fit has allowed us to better compare differences in the DOS among the different neat and blend samples. A summary of the peak values as well as the distributions of the local DOS can be found in the Supporting Information (Table S1). Examination of that data reveals significant differences between neat and blend films in the breadth of the distribution of both

of which have been reported as a result of the active layer morphologies afforded by use of processing additives.8,26,27 Bias-dependent localized DOS distributions were examined by STM measurements of conductance (dI/dV). As discussed, more accurate data can be obtained in the STS/STM technique with thinner film samples. Thus, we probed the morphologies of the 10 nm thick active layers cast at higher spin speeds from stock solutions of the same concentration as those used for OPV device preparation. Figure 2 consists of STM dI/dV images corresponding to active layers prepared with no additive (left) and with DIO (right) measured at a constant sample bias of 1.33 V. The bias chosen would largely populate the LUMO of the fullerene acceptor because it has a lower energy level than the LUMO of the polymer donor. The bias hence allowed us to “see” the acceptor domains as brighter regions. The morphology differences resulting from the use of DIO are significant and include an approximately 5- to 10-fold decrease in domain size of the donor (compare green regions of Figure 2 in the left and right panels). Following the observation of significant increases in PCE, JSC, FF, and EQE profile (Figure 1A,B) and pronounced differences in morphology as a result of employing DIO processing additive (Figure 2), we sought to investigate the correlation between the morphology and local electronic structure via STS. Neat films of PCE10 and PC70BM with ∼10 nm thicknesses were deposited onto Au(111) substrates (see the Supporting Information for preparation of neat films). Both of these neat films and the thin active layer blend films imaged by STM in Figure 2 were analyzed by STS at various positions throughout 1558

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Figure 3. Histograms illustrating the distribution of the local DOS corresponding to (A) the HOMO level of neat PCE10, (B) the LUMO of neat PC70BM, (C) the PCE10 HOMO levels in active layer blends prepared in the presence and absence of 3% DIO (red and black, respectively), and (D) the PC70BM LUMO levels in active layer blends prepared in the presence and absence of 3% DIO (red and black, respectively). Histograms represent dI/dV data collected from at least 50 different positions on each neat sample and 100 different positions for each blend.

Figure 4. Illustration highlighting how favorable differences in the local nanoscale electronic structure resulting from the presence of a threephase morphology could mitigate loss processes. Polymer molecular structure = PCE10; fullerene molecular structure = PC70BM.

significantly broader in the blend film (with no additive) than it is in the neat film, and it becomes broader yet when the DIO additive is introduced. The same trend is observed for the

the occupied (HOMO) states of the donor polymer as well as the unoccupied (LUMO) states of the fullerene. Qualitatively, the distribution of donor polymer occupied states becomes 1559

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figure of merit (ϕ∑μ) measured during the experiment is a product of the quantum efficiency of free carrier generation per photon absorbed (ϕ) and the sum of the mobilities of electrons and holes (∑μ). The TRMC figure of merit was recorded for the neat polymer, the blend with no additive, and the blend with DIO, over a range of light intensities spanning 3 orders of magnitude (Figure S3). First, the decline of ϕ∑μ at high light intensities observed in these experiments is typical of most samples and has been documented previously;48 the phenomenon can be attributed to quenching of excitons by the large number of free carriers generated during the laser pulse, a higher-order loss mechanism that ultimately limits ϕ. In the lower-intensity range of the experiment, the higher-order loss mechanisms are less prominent and ϕ∑μ increases. Second, the clear increase in ϕ∑μ for the blends compared to that for the pure polymer at all light intensities by approximately an order of magnitude can be attributed to the presence of a polymer/fullerene interface that efficiently dissociates excitons into free carriers.47,49 However, virtually no difference was observed in the overall magnitude of ϕ∑μ at any light intensity between samples processed with and without DIO. The result suggests that, despite the morphology changes observed with the STM imaging brought about by processing with DIO, the yield of free carrier generation measured by TRMC at these light intensities apparently remains unaffected by the additive, assuming that the local mobilities of the charge carriers measured with this technique are likewise unaffected. However, despite there being little difference in the overall magnitude of ϕ∑μ between blends processed with and without DIO, distinct differences in the decay dynamics of the transients in these two samples are observed at higher light intensities where higher-order loss mechanisms are more prominent. Figure 5 depicts the normalized TRMC transients

unoccupied states of the fullerene acceptor. If we attempt to quantify this broadening by comparing the half-widths of the Gaussian peaks, the distribution of the polymer donor occupied states in the blend film with no additive becomes 2.2 times wider than that in the neat film. In the blend film with DIO, that distribution becomes 2.7 times wider. Similarly, the distribution of fullerene acceptor unoccupied states in the blend film with no additive becomes 3.3 times wider than that in the neat film. In the blend film with DIO, that distribution becomes 4.0 times wider than that in the neat film. In Figure 4, we illustrate a three-phase morphology consisting of pure domains of the polymer donor and fullerene acceptor as well as a third mixed domain containing each component. A number of recent investigations have identified such an intermixed phase for different polymer/fullerene blends, including polymers of phenylenevinylene32 and P3HT33−36 as well as PTB737,38 with PCBM. Structural length scales, phase sizes, and molecular miscibility in these systems have been probed using various X-ray scattering and diffraction techniques,35−39 neutron scattering,33,34 and dynamic secondary ion mass spectrometery32,40 in conjunction with microscopy and spectroscopy techniques. Moreover, that such a three-phase morphology could in principle create an energy cascade that would facilitate the separation of both electrons and holes away from polymer/fullerene interfaces and into the respective pure domains is now proposed to improve yields of charge separation and collection, possibly even explaining what drives charge pair separation to begin with.37,38,40−42 The nanoscale electronic measurements that we illustrate in Figure 3 would be fully consistent with the presence of such intermixed polymer/fullerene domains in the blend films. Crystalline packing, intermolecular interactions, delocalized states, and/or charge stabilization by electronic polarization that all contribute to narrowing the band gap43−45 in pure domains would be partially disrupted in mixed regions, leading to a broader distribution of the DOS in blended films than would be observed in the respective neat films. Our electronic data suggest that the presence of DIO might enhance this interphase mixing, which would also be consistent with the aforementioned literature data that domains in PTB7:PCBM blends prepared with DIO may contain unique intermixed concentration gradients of the polymer and fullerene that are not observed in the absence of DIO.13 As enhanced mixing of polymer and fullerene domains in the presence of DIO might enhance both the free charge carrier generation and lifetime in these blends, we analyzed these metrics via time-resolved microwave conductivity (TRMC) measurements. This technique affords a unique means to probe charge carrier generation and decay without the requirement of collecting charges at electrical contacts. Recently, TRMC has been used to correlate photoinduced charge generation yield and dynamics with certain molecular properties, driving force, and local electronic coupling within organic BHJs.46 While TRMC principles and the instrumental setup employed are described in more detail elsewhere,47 briefly, active layer blend samples are prepared on quartz substrates. The sample is placed in a chamber configured to facilitate a standing microwave, and a 5 ns laser pulse is applied. Following laser excitation, charge carriers are generated that absorb microwave radiation. By measuring the change in microwave power within the sample cavity as a function of time following excitation, information concerning the amount, local mobility, and lifetime of free charge carriers can be obtained. More specifically, the TRMC

Figure 5. Normalized TRMC transients corresponding to blend samples processed without additive (black trace) and with 3% DIO (red trace) at absorbed photon flux (FAI0) values of 2 × 1015 photons/cm2/pulse. Dashed vertical lines correspond to transient signal half-lives (19 ns for no additive and 52 ns for DIO additive). Wavelength of laser excitation pulse = 640 nm (which excites the donor polymer, PCE10).

corresponding to PCE10:PC70BM active layer blends prepared with and without DIO additive (red and black traces, respectively). The normalized transient signals plotted as a function of time in Figure 5 were recorded at the relatively high absorbed photon flux of 2 × 1015 photons/cm2/pulse. The dashed vertical lines in Figure 5 (color coded to each transient) represent the time at which the signal has decayed to 50% of it 1560

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ACS Energy Letters maximum; using this metric, the half-life of charge carriers in the film processed with DIO was observed to be nearly 3-fold longer than that in the film processed with no additive (52 vs 19 ns for blends processed with and without DIO, respectively). While we note that differences in the decay dynamics become increasingly less pronounced at lower and lower flux (Figure S4), the results are generally consistent with literature observations12 that the application of DIO as an additive in structurally related PTB7:PCBM blends can dramatically decrease both geminate and nongeminate recombination. As discussed, a number of recent investigations have suggested that enhanced mixing of polymer and fullerene domains at their interface could promote an energy cascade that in turn would facilitate the separation of both electrons and holes away from that interface. Our evidence for a broader distribution of polymer donor occupied states and fullerene acceptor unoccupied states induced by DIO, as well as indications from TRMC that recombination and transient decay decrease with the application of DIO in our systems, is fully consistent with the energy level diagram illustrated in Figure 4. Our results thus provide further evidence, along with the growing body of literature work, that the energy cascade paradigm should be a targeted feature of OPV microstructure, either with the use of a processing additive or through some other technique. In this Letter, we have built on the work of many groups that have sought to answer how free charges are generated and persist in BHJ solar cells. To that end, significant evidence has been amassed in the literature to suggest that the presence of an intermixed polymer/fullerene phase is critical for improving the yields of charge separation, reducing charge recombination, and ultimately improving OPV device efficiency. Here, we have provided unique evidence by measuring the distribution of the localized DOS with STS/STM that the application of DIO as a processing additive may facilitate the formation of such a morphology. We fully agree with the emerging consensus44,45 that understanding the correlation between morphology and localized nanoscale electronic structures within active layers and at interfaces will be paramount to developing the next generation of high-performance OPV systems. We conclude that the combination of STM dI/dV imaging and measurement of DOS, in conjunction with a technique such as TRMC that can quantify transient decay dynamics, has significantly aided our understanding of the role of processing additives in affecting OPV active layer morphology and device performance.

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Wade A. Braunecker: 0000-0003-0773-9580 Present Address

L.E.G.: E-mail: [email protected]. Laxmi Therapeutic Devices, 75 Aero Camino Suite 204, Goleta, CA 93117. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is based on work supported in part by the Solar Energy Research Institute for India and the U.S. (SERIIUS) funded jointly by the U.S. Department of Energy subcontract DE AC36-08G028308 (Office of Science, Office of Basic Energy Sciences, and Energy Efficiency and Renewable Energy, Solar Energy Technology Program, with support from the Office of International Affairs) and the Government of India subcontract IUSSTF/JCERDC-SERIIUS/2012 dated Nov. 22, 2012. B.L. was 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 Technologies Program. D.O. acknowledges support from the Research Corporation for Science Advancement (RCSA) through the Scialog Collaborative Innovation Award #22355. The authors also thank the group of Sean Shaheen for graciously providing PCE10 for this study.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00315. Experimental details, AFM images, STM/STS analysis and dI/dV spectrum, TRMC methodology, and supplemental data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.J.P.). *E-mail: [email protected] (W.A.B.). ORCID

Amlan J. Pal: 0000-0002-7651-9779 1561

DOI: 10.1021/acsenergylett.7b00315 ACS Energy Lett. 2017, 2, 1556−1563

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DOI: 10.1021/acsenergylett.7b00315 ACS Energy Lett. 2017, 2, 1556−1563