Letter pubs.acs.org/NanoLett
Spatial Mapping of Efficiency of GaN/InGaN Nanowire Array Solar Cells Using Scanning Photocurrent Microscopy Sarah L. Howell,† Sonal Padalkar,† KunHo Yoon,† Qiming Li,‡ Daniel D. Koleske,‡ Jonathan J. Wierer,‡ George T. Wang,‡ and Lincoln J. Lauhon*,† †
Department of Materials Science and Engineering, Northwestern University, 2220 Campus Dr., Evanston, Illinois 60208, United States ‡ Sandia National Laboratories, Albuquerque, New Mexico 87185, United States S Supporting Information *
ABSTRACT: GaN−InGaN core−shell nanowire array devices are characterized by spectrally resolved scanning photocurrent microscopy (SPCM). The spatially resolved external quantum efficiency is correlated with structure and composition inferred from atomic force microscope (AFM) topography, scanning transmission electron microscope (STEM) imaging, Raman microspectroscopy, and scanning photocurrent microscopy (SPCM) maps of the effective absorption edge. The experimental analyses are coupled with finite difference time domain simulations to provide mechanistic understanding of spatial variations in carrier generation and collection, which is essential to the development of heterogeneous novel architecture solar cell devices. KEYWORDS: Nanowire, InGaN, SPCM, solar cell, photovoltaics
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scalable methods to fabricate a novel GaN−InGaN core−shell nanowire array solar cell with an power conversion efficiency of 0.3% and a short-circuit current density of 1 mA cm−2 under AM1.5G illumination.8 Despite the modest overall performance, the external quantum efficiency in the blue region of the spectrum reached 28%, which validates the principle of this approach while providing opportunity for improvement, particularly at longer wavelengths. Unlike conventional thin film photovoltaics that utilize in-plane junctions, vertical nanowire array devices possess a spatially varying structure by design. As such, a detailed understanding of the spatial variation in quantum efficiency is essential to improving device performance. Because variations in structure and composition occur on the submicrometer length scales, spatially and spectrally resolved measurements are needed to decompose the external quantum efficiency into contributions from absorption and collection efficiencies.19 Several scanned probe techniques, including Kelvin probe force microscopy and conductive atomic force microscopy (AFM), have been used to correlate morphology with the photoresponse and surface potential in next generation photovoltaics. 20,21 Scanning photocurrent microscopy (SPCM), also known as OBIC (optical beam induced current), has been used to spatially resolve the photocurrent of large-area devices based on organic bulk heterojunctions,22 quantum dot
nGaN alloys could provide a foundation for high-efficiency photovoltaics since the absorption coefficient is large,1 and the bandgap can in principle be tuned across the entire solar spectrum.2 However, the efficiencies of planar InGaN devices have been limited because InGaN grown on planar GaN is restricted to relatively low indium compositions and/or thin layers to minimize indium composition fluctuations and high defect concentrations that arise due to lattice mismatch.3,4 Nonplanar InGaN/GaN heterostructures provide a route to overcome these limitations5−9 and achieve a high radiative recombination efficiency, which has recently been shown to be essential to maximizing solar cell performance.10 Nanowire core−shell structures in particular have gained much attention for application in photovoltaics and light-emitting diodes (LEDs) in studies of both arrays 11−13 and individual wires.14−17 The advantages of the nanowire geometry for photovoltaics include: (1) a decoupling of the absorption axis from the carrier collection axis; (2) photonic effects that can enable enhanced light trapping due to nanoscale optical index changes that cause scattering and reduced reflection; and (3) elastic strain relief allowing higher indium mole fractions and lower defect concentrations. Dong et al. reported the first experimental realization and analysis of single nanowire coaxial GaN/InGaN solar cells.18 Such single-nanowire device studies provide important insights into fundamental processes of carrier absorption and collection in core−shell structures, but they do not address the potentially beneficial attributes of the vertical array geometry, including photonic crystal light trapping and the orthogonalization of absorption and charge separation. Recently, Wierer et al. used © XXXX American Chemical Society
Received: June 26, 2013 Revised: September 11, 2013
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Figure 1. (a) Schematic of the formation of InGaN/GaN MQW layer and p-InGaN canopy layer on hexagonal GaN nanowire array. (b) Crosssectional ADF-STEM images of nanowire array along the closest packed direction. (c) Spectrally resolved external quantum efficiency under global illumination for a representative device. 21 × 22 μm2 images of (d) AFM topography and (e) SPCM for 405 nm excitation.
thin films,23 and core−shell nanopillars.24,25 In this context, SPCM is a useful nondestructive technique for elucidating structure−performance relationships in devices to guide the optimization of both the materials components as well as the device design. Here we investigate the influence of variations in the local structure and composition on the performance of a GaN− InGaN core−shell nanowire array solar cell. The spatially and spectrally resolved photocurrent measured by SPCM is correlated with structure and composition inferred from (1) AFM topography; (2) scanning transmission electron microscopy (STEM); and (3) Raman microspectroscopy. The experimental results are compared with finite difference time domain (FDTD) simulations of absorption to understand how the spatial structure and compositional heterogeneities influence the device performance. We find that, while the radial nanowire p−n junctions perform well at shorter wavelengths, future efforts could focus on increasing the thickness and indium concentration within the photoactive regions to collect more of the solar spectrum while minimizing growth and defect generation at the low internal quantum efficiency nanowire apexes. We propose that device efficiencies can be increased by optimizing the pitch to enhance absorption on facets of high internal quantum efficiency (IQE) and by modifying fabrication processes to eliminate regions of lower IQE. The device structure is shown in Figure 1a,b. The fabrication and photovoltaic performance have been reported previously.8 Briefly, a self-assembled colloidal silica etch mask was used to fabricate a semiperiodic hexagonal array of GaN nanowires with an average pitch of 500 nm and heights of 900 nm. An
inductively coupled plasma etch was followed by a facetselective KOH-based wet etch to create nanowires with straight sidewalls free of plasma etch damage.26 Eight InGaN quantum wells (QWs) were grown as absorbing layers and then capped by a continuous p-type InGaN “canopy” layer that covers the tops and sides of all of the nanowires to form a p−n junction (Figure 1b). This structure is then fabricated into simple solar cell devices. The external quantum efficiency (EQE) of a typical device is high in the blue and drops rapidly upon moving into the green (Figure 1c). Energy conversion under AM1.5G illumination (Supporting Information S1b,c) is therefore determined by performance in the blue, and efficiency is greatly limited by the low EQE in the middle of the visible band. To identify the components of the heterostructure that perform well, and to characterize more quantitatively what factors limit the performance, we investigated both the spatial variation in the total EQE (Figures 1 and 3) as well as the spectral variation in the EQE at characteristic locations (Figures 2 and 4). We develop a qualitative understanding of these variations in Figures 1 and 2 before undertaking a more quantitative analysis in Figures 3 and 4. The AFM topograph in Figure 1d shows that the device consists of regions of hexagonally ordered arrays interrupted by defects that we refer to as “gaps” (dark regions of low height) and “plateaus” (bright regions of elevated height). The array and gap regions show the highest photocurrent response under convergent beam illumination by a scanning confocal microscope with 405 nm excitation (Figure 1e, normalized to the response of the array regions). The SPCM technique is described further in the Supporting Information S1. Because the EQE under global B
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absorbed, which is influenced by both InxGa1−xN composition and scattering? Second, how do the spatial and spectral dependence of the EQE depend on the efficiency with which the photogenerated carriers are collected (the IQE)? In the discussion below, we first consider the influence of the nanowire array geometry on the spatial dependence of the absorption (Figure 3). Quantitative 3-D simulations of absorption also shed light on the IQE of different regions of the nanowire heterostructure. Second, we consider the influence of spatial variations in composition (Figure 4). Lumerical FDTD simulations were performed for model structures shown in Figure 3. The FDTD model assumes that the p-InxGa1−xN capping layer (x ∼ 0.015)8,27 is perfectly nonabsorbing and the InGaN/GaN MQW absorber layer is modeled as a single material with a constant absorption coefficient of 2.5 × 104 cm−1. Additional details are provided in the Supporting Information S4. The 3-D distribution of absorption (Figure 3b,c,e,f) was calculated for illumination at three different positions within two structures: centered on a nanowire (position 1 in Figure 3a,g); centered in-between nanowires (position 2 in Figure 3a,g); and centered on a missing nanowire or gap (position 3 in Figure 3a,g). The bar graph in Figure 3g shows that the total absorption at each of these excitation points is nearly the same. Therefore, the large variations in EQE observed in Figure 2b can be attributed to variations in internal quantum efficiency depending on where the light is absorbed. Indeed, Figure 3g reveals that illumination of the gap region, which has the highest EQE, leads to a 7% increase in absorption in the “Sides” of neighboring nanowires, which support the high-quality quantum wells,27 with a corresponding decrease in absorption in the “Tops” of the nanowires. Similarly, illumination in the regions in between the nanowires (position 2) generates more photocurrent than illumination above a nanowire (position 1) in simulations and experiments (Figures 2a,b) again due to greater absorption in the Sides. Absorption in the Tops of the nanowires is associated with reduced internal quantum efficiency for at least two reasons. First, the defect density is higher in the absorber layer at the top regions,8,27 leading to increased minority carrier recombination. Second, the apex QWs are structurally imperfect, discontinuous, and nonuniform,27 and these defective QWs likely disrupt carrier extraction near the top of the wire. The Tops therefore act as sinks, depriving light from the Sides and Substrate while contributing little to the photocurrent. The Top sink effect is confirmed and enhanced when we simulate structures with a higher effective absorption coefficient, as is expected in the indium-rich Top (Supporting Information S3). We note that the simulated total absorption at 405 nm is 75%, which is three times larger than absorption of a simulated planar structure with the same absorber volume. The increase demonstrates the effective light trapping of these nanostructures but also suggests these structures could be improved. Two specific improvements are suggested by these analyses. First, if the whole device performed with the efficiency of the gaps, the device EQE would improve by ∼18%. In other words, the pitch and height of the nanowire array could be optimized to take better advantage of photonic light trapping within the vertically oriented quantum wells. Second, the distribution of absorption indicates that quantum wells on the substrate (Supporting Information S1a) are also contributing to energy conversion, so their contribution should be considered in the design optimization.
Figure 2. Some 11 × 4.3 μm2 images from boxed region in Figure 1d of (a) AFM surface topography, (b) SPCM at 405 nm, (c) SPCM at 500 nm, and (d) photocurrent threshold energy. To aid the eye, a grid identifies a nanowire array region, a green arrow identifies a gap region, and an orange arrow identifies a plateau region.
illumination is peaked around 405 nm, we can conclude that (1) the arrays make a significant contribution to power generation under AM1.5G illumination and (2) the defective gap regions make an even greater contribution (∼18%) per unit area. In contrast, the plateau regions are ∼60% less efficient than the array/gap regions per unit area. To further investigate the correlation between spatial and spectral variations in the photocurrent, Figure 2a and b show higher magnification views of the topography and photoresponse, respectively: example nanowire arrays, gaps, and plateaus are indicated by a grid, a green arrow, and an orange arrow, respectively. The gaps arise from voids in the colloidal silica particle mask that was used to etch the hexagonal nanowire array, whereas plateaus likely arise from regions in which two layers of masking particles were unintentionally deposited. While such defects could be eliminated with improved device processing, here they enable a more complete understanding of how radial core−shell nanowire heterostructure arrays perform. All regions show a marked decrease in photocurrent going from 405 nm (Figure 2b) to 500 nm (Figure 2c) excitation. However, the plateau regions actually perform better than the array regions at longer wavelengths. Qualitatively, we can capture the spatial variation in the spectral response by generating a map of the minimum photon energy required to generate a measurable photocurrent (Figure 2d). Red regions produce more photocurrent at longer wavelengths (the procedure used to generate this map is explained in the Supporting Information S3). This qualitative analysis immediately raises two important questions to be addressed by more quantitative analyses. First, how do the spatial and spectral dependence of the EQE depend on the number of photons C
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Figure 3. A 2-D cross-section of the simulated 3-D (a) array and (d) gap structures. (b, e) Top view of total absorption (3 × 3 μm2) at 405 nm from FDTD simulations corresponding illumination points 1 in a and 3 in d, respectively. (c and f) Side views of absorption in the vertical plane parallel to the white dotted lines in b and e, respectively. The arrows in f indicate regions described as Top, Sides, and Substrate. (g) Schematic depicting beam position for simulations and graph comparing the distribution of absorption for each beam position.
shift with literature data for planar InGaN (Table 1).28−30 The representative Raman spectrum from the plateau exhibits a much lower signal-to-noise ratio, which is expected for Raman spectra of InxGa1−xN for x > 0.2.28,29 Within the arrays, the Raman analysis assigns a higher indium mole fraction to the nanowire tips (Array 1, Figure 4b) than the regions between nanowires (Array 2, Figure 4b), in agreement with cathodoluminescence data from cross-sectional scanning transmission electron microscopy.27 The fact that the photocurrent threshold occurs at shorter wavelengths for the tips initially appears contradictory but is in fact consistent with our claim that collection of carriers within the high indium concentration nanowire tops is inefficient, leading to a lower photocurrent despite having a higher absorption coefficient. In contrast, both the Raman analyses and the photocurrent threshold indicate that the gap regions have a higher indium concentration than the nanowire array regions, which is consistent with absorption in the quantum well regions with higher internal quantum efficiency. The EQE and threshold data indicate that the indium mole fraction is highest for the
We now consider the effect of compositional variations on the spatially resolved EQE (Figure 4) and overall device performance. The EQE versus photon energy is plotted in Figure 4a for the characteristic array, gap, and plateau regions identified in Figure 2a. To facilitate analysis of the composition dependence, we consider both the photocurrent thresholds (plotted in Figure 2d and in Supporting Information S3) as well as confocal Raman microspectroscopy analysis of the E2(InxGa1−xN) peak position, which red-shifts with increasing x (Figure 4b).28,29 Raman spectra were acquired for multiple examples of each region in the backscattering configuration with a 100×/0.9NA objective under the following conditions: excitation wavelength 532 nm; 1/e2 spot diameter 720 nm (360 nm maximum resolution); 70 μW power; ∼2 min acquisition time; unanalyzed outgoing signal; and linearly polarized excitation. The E2(GaN) wavenumber30 is constant, while the E2(InGaN) wavenumber decreases with increasing indium concentration. Approximate indium mole fraction values for the array positions and the gap were estimated by fitting the E2(InGaN) peak to a Voigt function and comparing the peak D
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defects and impurities in the strained high-indium concentration top plane. (2) Despite a presumed higher absorption coefficient, the total absorption in the plateaus may be less than in the array due to the nonplanar structure of the array, which can have a larger effective absorption volume per unit area with the same absorber thickness. (3) The IQE of the plateaus may be lower due to additional absorption away from the junction in the thicker, p-type capping layer. Our analysis indicates the solar performance of future devices would be improved by modifying lithographic processes to eliminate “plateau” regions in favor of the more efficient nanowire arrays. Overall cell performance is also affected by electrical shorting paths resulting from incomplete coalescence of the p-InGaN canopy layer (∼0.1% of the total area), allowing p-contact metal layers to directly contact the n-type layers.8 However, these SPCM measurements do not probe regions under the contacts. We did observe one example of a region with zero EQE at all wavelengths; while such defects may also provide shorting paths, SPCM provides no further insights into the effect on open circuit voltage. With this cell design, there may also be an unrevealed trade-off between producing a low resistance capping layer with high hole injection efficiency and unwanted absorption in the capping layer, which occurs with a low internal quantum efficiency. A thinner p-type InGaN layer coupled to an alternative transparent conducting contact could represent an improved approach to improve overall cell performance. In summary, spectrally resolved SPCM measurements of external quantum efficiency were combined with electromagnetic simulations and Raman spectroscopy to analyze the performance of a novel nanowire solar cell and establish structure−property relationships. To take better advantage of nanowire array light trapping and orthogonal carrier collection, future efforts could focus on increasing the thickness of and indium concentration within the photoactive regions to collect more of the solar spectrum while minimizing growth and defect generation at the tops of the nanowires. The SPCM methods used here can be usefully applied to spatial characterization of other novel solar cell architectures, and the spatial resolution could be further enhanced by using two photon absorption.19
Figure 4. External quantum efficiency as a function of illumination energy for three representative locations (a). The inset is a cross section cartoon of a plateau region. (b) Raman spectra when the beam is above a nanowire tip (Array 1), between wires (Array 2), above a gap (Gap), and above a plateau (Plateau). The Voigt function fits (dotted black lines) of the E2 (InGaN) peaks shift to the left with increasing indium concentration, while the E2 (GaN) peak near 565 cm−1 remains stationary.
Table 1. Raman E2 (GaN) and E2 (InGaN) Shifts and the Estimates of Indium Concentration from the Raman Analysis Summarized for Each Photoactive Region of Interest E2 (GaN) (cm−1) E2 (InGaN) (cm−1) InxGa1−xN Raman estimate
Array (1)
Array (2)
Gap
Plateau
565.0 551.3 0.2
565.1 556.3 0.06
564.8 553.6 0.16
N/A N/A >0.2
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ASSOCIATED CONTENT
S Supporting Information *
plateau regions, and the Raman spectra of plateaus are consistent with this conclusion. As a general caveat, we note that since the nonplanar geometry decreases compressive strain, and strain influences the Raman E2(InGaN) peak shift, strain relaxation has the potential to cause an overestimate of the indium concentration when using planar E2(InGaN) peak shift reference data. Therefore, the indium mole fraction could be overestimated to a greater extent near the apex of the wires, but this does not impact our claims because the Raman-based indium concentration estimates are in general smaller than those measured by EDS8 and atom-probe tomography.27 While approaches to improving efficiency through modification of the array spacing were discussed above, inspection of Figure 1e highlights the need to modify processes to optimize indium concentrations and eliminate the plateau regions. Even though the plateau regions have a higher indium concentration, and thus a higher absorption coefficient, they produce less photocurrent at 405 nm than that of the surrounding regions (Figure 2b). There are at least three possible explanations for this finding. (1) The IQE of the plateaus may be lower due to increased recombination at
Supplementary figures illustrating the effective absorption edge calculation and additional FDTD simulation details. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Nanowire SPCM and Raman studies in the group of L.J.L. were supported by DOE BES grant DE-FG02-07ER46401. S.L.H. was supported in part by an NSF Graduate Research Fellowship under DGE-1324585. The nanowire solar cell growth, device fabrication, and STEM characterization were supported by Sandia’s Solid State Lighting Science Energy Frontier Research Center, funded by DOE BES. Sandia E
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National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
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