Functional Scanning Probe Imaging of Nanostructured Solar Energy

Article pubs.acs.org/accounts

Functional Scanning Probe Imaging of Nanostructured Solar Energy Materials Rajiv Giridharagopal, Phillip A. Cox,† and David S. Ginger* Department of Chemistry, University of Washington, Seattle, Washington 98195, United States CONSPECTUS: From hybrid perovskites to semiconducting polymer/ fullerene blends for organic photovoltaics, many new materials being explored for energy harvesting and storage exhibit performance characteristics that depend sensitively on their nanoscale morphology. At the same time, rapid advances in the capability and accessibility of scanning probe microscopy methods over the past decade have made it possible to study processing/ structure/function relationships ranging from photocurrent collection to photocarrier lifetimes with resolutions on the scale of tens of nanometers or better. Importantly, such scanning probe methods offer the potential to combine measurements of local structure with local function, and they can be implemented to study materials in situ or devices in operando to better understand how materials evolve in time in response to an external stimulus or environmental perturbation. This Account highlights recent advances in the development and application of scanning probe microscopy methods that can help address such questions while filling key gaps between the capabilities of conventional electron microscopy and newer super-resolution optical methods. Focusing on semiconductor materials for solar energy applications, we highlight a range of electrical and optoelectronic scanning probe microscopy methods that exploit the local dynamics of an atomic force microscope tip to probe key properties of the solar cell material or device structure. We discuss how it is possible to extract relevant device properties using noncontact scanning probe methods as well as how these properties guide materials development. Specifically, we discuss intensity-modulated scanning Kelvin probe microscopy (IM-SKPM), timeresolved electrostatic force microscopy (trEFM), frequency-modulated electrostatic force microscopy (FM-EFM), and cantilever ringdown imaging. We explain these developments in the context of classic atomic force microscopy (AFM) methods that exploit the physics of cantilever motion and photocarrier generation to provide robust, nanoscale measurements of materials physics that are correlated with device operation. We predict that the multidimensional data sets made possible by these types of methods will become increasingly important as advances in data science expand capabilities and opportunities for image correlation and discovery.

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INTRODUCTION

goal that can improve the prospects for technological applications.14−16 For example, organometal halide perovskites provide a textbook example of how microscopy can reveal local structure/function relationships. In the Shockley−Queisser limit for photovoltaic efficiency, the only recombination of carriers is radiative. Thus, mapping local sites of nonradiative recombination can provide direct information about local efficiency losses. While stretched-exponential PL dynamics can be seen from bulk samples, scanning confocal microscopy reveals the origin of this decay as a heterogeneous spatial distribution of both PL intensity and lifetime, arising from a spatially varying density of nonradiative trap sites associated with interfaces and grain boundaries (Figure 2A,B).17 These results are also in good agreement with cathodoluminscence imaging via scanning electron microscopy (Figure 2C,D),18 which reveals similar heterogeneity in nonradiative recombina-

New materials are being studied for energy harvesting and storage applications as researchers look to enable solutionprocessable active layers,1−3 environmentally benign manufacturing from earth-abundant elements with facile processing,3−5 open transformative applications with semitransparent or flexible device architectures,6,7 and improve performance with active nanostructures.8 In the area of photovoltaics alone, materials including organic photovoltaics (OPVs),1,2,9 inorganics like Cu2ZnSnS4 (CZTS),10,11 and solution-processed hybrid inorganic−organic perovskites12,13 are active topics of research. These materials are often inherently nanostructured or microstructured (Figure 1). This structure may arise either by design, as in the case of nanostructured OPV bulk heterojunctions, or as a byproduct of a nonequilibrium deposition or growth process, as for many inorganic thin films. Regardless, the film’s local structure can influence the overall materials and device properties. Understanding such processing/structure/function relationships is an important © 2016 American Chemical Society

Received: May 26, 2016 Published: August 30, 2016 1769

DOI: 10.1021/acs.accounts.6b00255 Acc. Chem. Res. 2016, 49, 1769−1776

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Accounts of Chemical Research

properties can be altered even by the small current from a vacuum gauge.21 In this context, emerging scanning probe microscopy (SPM) methods offer the ability to combine nanoscale spatial resolution with noninvasive local measurements on inorganic, organic, and hybrid materials. More importantly, SPM can measure a range of local functional properties ranging from carrier lifetimes to surface potential, to conductivity, to piezoelectric response. Table 1 lists a number of SPM methods available to probe structure/function relationships as well as appropriate references for further detail. Recognizing that scanning tunneling microscopy and near-field methods have their place, we emphasize atomic force microscopy (AFM) here in part because of its modest requirements regarding sample preparation, widespread accessibility, and availability of robust commercial probes. In this Account, we focus on the prospects, challenges, and opportunities for advances in multifunctional AFM methods to obtain local electronic information. While we discuss these methods in the context of organic photovoltaics, we stress that they have broad applicability to other semiconductor families like chalcogenides and perovskites, as well as beyond photovoltaics. Furthermore, we note how the techniques listed extract device properties at the nanoscale that aid in the broader efforts to advance photovoltaics in general.

Figure 1. Representative morphologies of typical nanostructured materials used in thin film photovoltaic devices. (A) CH3NH3PbI3 perovskite solar cell, (B) Cu2ZnSn(S,Se)4, (C) polymer:fullerene organic photovoltaic, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5b]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno[3,4-b]thiophenediyl]]:[6,6]-phenyl C71-butyric acid methyl ester (PTB7:PC71BM) mixed with 1,8-diiodooctane, and (D) PbS quantum dots. Images (A)−(C) are AFM images, and (D) is a transmission electron microscopy image. Panel (B) reproduced with permission from ref 11. Copyright 2015 Royal Society of Chemistry. Panel (C) reproduced with permission from ref 55. Copyright 2013 American Chemical Society.

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POTENTIAL BEYOND THE SURFACE: MEASURING LOCAL CARRIER DYNAMICS Photovoltaic device efficiency is governed by a balance of charge carrier generation, recombination, and extraction at the contacts.22−24 These physical processes affect the defining characteristics of a solar device, such as the short-circuit photocurrent (JSC), open-circuit voltage (VOC), and fill factor (FF). Together, these metrics define the J−V curve (current vs voltage) of a device. Upon absorption of an incident photon, the resulting electron−hole pair (exciton) must dissociate into constituent carriers and then be extracted at respective contacts prior to recombining via either geminate or nongeminate processes. As a result, measuring local recombination is an important concern. Numerous methods can probe average recombination kinetics in macroscale samples, including transient photovoltage (TPV), transient absorption spectroscopy (TAS), and current extraction from linearly increasing voltage (CELIV).22,23,25 However, local probes of heterogeneous recombination dynamics can provide new insights that are inaccessible from macroscopic characterization. One method of measuring local recombination is intensitymodulated scanning Kelvin probe microscopy (IM-SKPM), which can measure carrier lifetimes with resolution below the optical diffraction limit (Figure 3A).26 IM-SKPM is a frequency-domain measurement that uses the slow response of a standard SKPM feedback loop to measure the timeaveraged contact potential difference (CPD) between the tip and sample in response to a modulated illumination source. As the experimental modulation frequency increases, the average CPD will also increase because recombination is too slow to decay completely during a single cyclethe rate at which average CPD evolves as a function of modulation frequency thus reflects the carrier lifetime in the film (Figure 3B). There are many possible sources of local heterogeneity in carrier recombination in thin film solar cells, and one of those is heterogeneity in the buried contact chemistry.25,27,28 Notably, we have shown that IM-SKPM can be used to probe variations

Figure 2. (A) Fluorescence image of a CH3NH4PbI3(Cl) perovskite film, and (B) corresponding PL lifetime decay traces for the blue circle and red square, showing significant local heterogeneity with far-field resolution. (C) SEM image of a perovskite film at 1.5 kV accelerating voltage and (D) corresponding cathodoluminescence image, showing that the local heterogeneity persists at structures well below the diffraction limit. Panels (A) and (B) reproduced with permission from ref 17. Copyright 2015 American Association for the Advancement of Science. Panels (C) and (D) reproduced with permission from ref 18. Copyright 2015 American Chemical Society.

tion while also separating surface from bulk effects by variations in the accelerating voltage. While such images are powerful, there exists a need for new functional methods that can map properties such as carrier lifetimes at resolutions far below the optical diffraction limit. Solid-state materials are generally not amenable to emerging super-resolution optical methods,19 and some soft materials are sensitive enough to electron beam damage20 that their 1770

DOI: 10.1021/acs.accounts.6b00255 Acc. Chem. Res. 2016, 49, 1769−1776

Article

Accounts of Chemical Research Table 1. Examples of Functional Scanning Probe Methods Used on Photovoltaics, with Typical Spatial and Temporal Resolution and Properties Measured resolution method (and representative references)

property

lateral

temporal

conductive and photoconductive AFM31−35 scanning Kelvin probe force microscopy11,65 intensity-modulated scanning Kelvin probe microscopy26−28 time-resolved electrostatic force microscopy36,41−44,63 frequency-modulated electrostatic force microscopy55 cantilever ringdown imaging56 band excitation57

current/voltage work function, contact potential difference, photovoltage carrier lifetime

5−10 nm 50 nm 50 nm

local EQE, charging rate of mobile carriers, charge carrier lifetime, photodegradation power dissipation, qualitative cantilever Q, charge trapping

10s of nm

>ms ms 1−10 μs, depending on modulation source 100 ns

10s of nm

N/A

power dissipation, quantitative cantilever Q, charge trapping power dissipation, cantilever Q, full resonance curves

10s of nm 50 nm

piezoresponse force microscopy61

piezoelectric and ferroelectric domains

50 nm

∼200 μs depending on frequency within 10s of Hz at the resonance frequency ∼5−10 J/cm2), trEFM could detect changes in electronic properties of films exposed to photon doses orders of magnitude lower, down as small as ∼10−20 mJ/cm2 (equivalent to less than a second of exposure to AM1.5G sunlight). Furthermore, because SKPM maps the tip−

Figure 6. (A) Schematic of the photooxidation grid administered with a 633 nm HeNe laser to a 1:1.5 PTB7:PC71BM film. (B) Topography, (C) resonance frequency shift, and (D) ΔQ/Q (amplitude) of a 1:1.5 PTB7:PC71BM film. Reproduced with permission from ref 55. Copyright 2013 American Chemical Society.

dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) and fullerene derivative PC71BM film that has been exposed to a series of different 633 nm light doses (Figure 6A) and then imaged with FM-EFM. By creating a grid in such a way, we capture a snapshot of how the cantilever parameters change over time in a single image. No structural changes occur in this range (Figure 6B). However, Figure 6C shows that while changes in the cantilever resonance frequency can be detected at higher doses, they are very small relative to the background noise. Lastly, in Figure 6D, we show that the oscillation amplitude of the cantilever changes significantly over the degraded areas due to a lower cantilever 1773

DOI: 10.1021/acs.accounts.6b00255 Acc. Chem. Res. 2016, 49, 1769−1776

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Accounts of Chemical Research

Figure 7. (A) Topography of a no additive 1:3 PTB7:PC71BM film cast from chlorobenzene. (B−E) Q images via cantilever ringdown of the same area shown in (A) photodegraded to photon doses of (B) 0, (C) 10, (D) 20, and (E) 50 J/cm2. (F) Masked area averages of cantilever Q as a function of photooxidation for fullerene aggregates (blue squares), areas in between aggregates (gray triangles), and the entire area in the dark with no tip bias (black circles). Reproduced with permission from ref 56. Copyright 2016 American Chemical Society.

trochemical strain microscopy,62 and band excitation57 are being developed to understand local structure/function relationships in fuel cell and battery materials. trEFM can also be used to probe ion motion63 and could be extended to study both ionic and electronic transport. Mixed ionic/ electronic conduction in organic semiconductors is strongly sensitive to film morphology,64 and offers another area where we anticipate functional scanning probes will be useful. In the future, we expect multidimensional scanning probe analyses that link composition with function will become increasingly important. As an example, we combined analysis using SKPM with energy-dispersive X-ray spectroscopy on CZTSSe to understand how local Se concentration affected work function.65 Kalinin and co-workers have used emerging data science tools, in part based on machine learning, to provide insight across large volumes of scanning probe data.66 We expect that complex analyses of local electronic transport and relaxation dynamics will be routinely correlated with local chemical composition obtained in situ, perhaps by combining electrical scanning probe methods with methods capable of mapping nanoscale film composition with infrared mechanical detection in AFM. These methods, including photoinduced force microscopy67 and photothermal induced resonance microscopy,68 obtain nanoscale chemical information across wide ranges of composition and, when combined with the methods discussed in this Account, hold the promise of revealing important correlations between local chemistry and local electronic properties.

quality factor, Q. The reduction in Q due to increased power dissipation in the cantilever is substantially higher upon photooxidation of the active layer materials, making dissipation well suited for degradation studies. In order to determine Q quantitatively, we measured the ringdown time of the cantilever.56 When the driving force is turned off, the cantilever amplitude relaxes with a time constant that is proportional to Q.60 Figure 7A−E shows the topography and cantilever Q images of a PTB7:PC71BM film in the initial stages of degradation. As the film is photooxidized, as seen in Figure 7C−E, the average Q drops, presumably due to the formation of trap states. At higher photon doses, Q increases, possibly because of polymer bleaching. Interestingly, Figure 7F shows that the most substantial changes in dissipation due to photooxidation occur in areas of high fullerene concentration, perhaps due to deep level traps that form in the fullerene.53 With further application of these methods to newer systems, trEFM, dissipation imaging, and other local scanning probe methods can provide an unprecedented look into the relationship between local film structure and device stability.

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OUTLOOK: SMALL FEATURES MEET BIG DATA Electronic and optoelectronic scanning probe microscopy methods now offer the capability to measure properties beyond surface potential and photocurrent such as carrier lifetime and energy dissipation related to carrier transport and trap formation. Understanding the distribution of these local properties, not merely the average values from a bulk measurement, is critical to improving the performance of nanostructured materials ranging from organic semiconductors to hybrid perovskites. The techniques outlined here tie into the larger effort to advance semiconductor photovoltaics by linking local information with device-level operation. Recombination lifetime, degradation, and photocurrent generation are all critical parameters underpinning the operation of these nanostructured systems. While we have focused on semiconductor photovoltaics, techniques including piezoresponse force microscopy,61 elec-

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

P.A.C.: Intel Corporation, Hillsboro, Oregon 97124.

Notes

The authors declare no competing financial interest. 1774

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Accounts of Chemical Research Biographies

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Rajiv Giridharagopal earned a B.S. with high honors from the University of Texas at Austin in 2004 and M.S. and Ph.D. degrees from Rice University in 2007 and 2010, all in electrical engineering. Following a postdoctoral position in David S. Ginger’s lab and a position at Intel Corp. in optical microscopy development, he returned to the University of Washington as a research scientist in 2015. Phillip A. Cox earned B.S. and Ph.D. degrees in chemistry from Pacific University in 2011 and University of Washington in 2016, respectively. He is currently employed at Intel Corp. in Oregon. David S. Ginger earned dual B.S. degrees in chemistry and physics from Indiana University in 1997 and a Ph.D. in physics from the University of Cambridge (U.K.) in 2001. After a postdoctoral fellowship at Northwestern University, he joined the University of Washington in 2003. He is currently Alvin L. and Verla R. Kwiram Endowed Professor of Chemistry and Associate Director of the University of Washington Clean Energy Institute

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ACKNOWLEDGMENTS

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REFERENCES

This Account includes work supported by the NSF (DMR1306079 and DMR-1337173), the Office of Naval Research (ONR N00014-14-1-0170), and the Department of Energy (DE-SC0013957), as well as the University of Washington Clean Energy Institute.

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Accounts of Chemical Research

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DOI: 10.1021/acs.accounts.6b00255 Acc. Chem. Res. 2016, 49, 1769−1776