Influences of Surface Roughness of ZnO Electron Transport Layer on

Article pubs.acs.org/JPCC

Influences of Surface Roughness of ZnO Electron Transport Layer on the Photovoltaic Performance of Organic Inverted Solar Cells Zaifei Ma,† Zheng Tang,† Ergang Wang,‡ Mats R. Andersson,‡ Olle Inganas̈ ,† and Fengling Zhang*,† †

Biomolecular and Organic Electronics, Department of Physics, Chemistry and Biology, Linköping University, SE-581 83, Linköping, Sweden ‡ Department of Chemical and Biological Engineering/Polymer Technology, Chalmers University of Technology, SE-412 96, Göteborg, Sweden S Supporting Information *

ABSTRACT: Here, we demonstrate the correlation between the surface roughness of the ZnO interlayer used as an electron transporting interlayer (ETL) in organic inverted solar cells (ISCs) and the photovoltaic performance of the ISCs. Three different surfaces of the ZnO ETL are studied in ISCs with the polymer poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) mixed with [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as the active layer. The results obtained from these ISCs show that power conversion efficiency increases from 2.7% to 3.9% when the root-mean-square roughness of the ZnO layer decreases from 48 to 1.9 nm. Moreover, it is found that the short-circuit current density is higher in the ISC based on the smoother ZnO interlayer, with a larger donor/acceptor (D/A) interfacial area in the active layer that facilitates exciton dissociation. The reduced effective interfacial area between the photoactive layer and the ZnO interlayer with decreased ZnO surface roughness leads to an observed improvement in both fill factor and open-circuit voltage, which is ascribed to a reduced concentration of traps at the interface between the ZnO interlayer and the active layer. active layers in ISCs.18 There are many advantages of using an environmentally friendly ZnO interlayer prepared by a simple chemical bath method,19 such as low cost, roll-to-roll processing compatibility,20,21 and good air stability.17,22 Moreover, one of the most efficient ISCs reported up to now is based on ZnO.18 There have been many methods to construct highly efficient ISC based on ZnO. For instance, ultraviolet (UV) treatment of the ISCs was shown to improve performance of the ZnO-based ISCs via increasing the conductivity of the ZnO film.23,24 Postthermal treatment on the ISCs was also found to be effective in improving performance of the ISCs.25 Recently, Liang et al. investigated the influence of morphology and thickness of the ZnO interlayer in ISCs using poly(3-hexylthiophene-2,5-diyl) (P3HT)/[6,6]-phenyl C61 butyric acid methyl ester (PC61BM) as the photoactive layer, and they found that the photovoltaic performance of the ISCs was only sensitive to the morphology of the ZnO interlayer, but insensitive to the thickness of the ZnO layer.26 The surface energy of the substrate is also reported to affect the morphology of the film deposited atop.27,28 Thus, surface

1. INTRODUCTION Polymer solar cells (PSCs) have been attracting more and more attention during the past decade owing to their potential advantages in low-cost solar energy harvesting.1−4 State-of-theart PSCs are often constructed based on the bulk-heterojunction (BHJ) concept5 with the active layer sandwiched in between a low work function metal top cathode and a poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-coated indium tin oxide (ITO) anode. However, stability of such PSCs is severely limited: The acidic nature of PEDOT:PSS has been reported to erode the ITO substrate and cause degradation of the solar cells.6−8 In addition, the oxygensensitive nature of the top low work function metal is also found to reduce the lifetime of PSCs.9,10 Inverted organic solar cells (ISCs) based on modified ITO cathodes and top reflective metal anodes have thus been developed.11−13 The performance of the ISCs depends strictly on electrical properties and surface properties of the cathode interface modifiers. Thus, the choice of the electron-transporting interlayer is critical. Poly(ethylene oxide) (PEO),14 titanium oxide (TiOx),11 cesium carbonate (CsCO3),13 Al2O3,15 and ZnO16,17 are commonly used to convert ITO into the cathode for constructing ISCs. Among these buffer layer materials, ZnO is more often used owing to its low work function, which allows an Ohmic contact to be formed with © 2012 American Chemical Society

Received: August 27, 2012 Revised: October 24, 2012 Published: November 5, 2012 24462

dx.doi.org/10.1021/jp308480u | J. Phys. Chem. C 2012, 116, 24462−24468

The Journal of Physical Chemistry C

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interfacial area between electron donor and acceptor (D/A) in the active layers for exciton dissociation. Meanwhile, the opencircuit voltage (Voc) of the ISCs, which is mainly determined by the energy of the charge transfer (CT) state and external quantum efficiency of the electroluminescence (EQEEL)34−38 of the PSCs, depends also on the surface roughness of the ZnO interlayer.

energy modifications through UV-ozone treatment, plasma treatment, SAM layer, and the conjugated polymer interlayer are reported to improve the performance of various PSCs.18,28−31 Therefore, as an important parameter in determining surface energy, substrate surface roughness is also expected to determine the photovoltaic performance of PSCs, which has not been well-investigated.32 Taking advantage of the fact that the surface roughness of the ZnO layer can be easily tuned via controlling the concentration of the Zn(Ac)2 sol−gel solution, we present our investigation on the ISCs with a device geometry (Figure 1a): ITO/ZnO/

2. EXPERIMENTAL SECTION 2.1. ZnO Film Preparation and ISC Fabrication. Three different ZnO sol−gel solutions, in terms of concentration, were prepared by dissolving Zn(Ac)2·2H2O mixed with monoethanolamine (MEA) in a 1:1 molar ratio in isopropanol. Zn(Ac)2·2H2O (in 1.10, 3.29, and 6.59 g portions) was first dissolved in 50 mL of isopropanol, respectively. The Zn(Ac)2·2H2O concentrations of the prepared ZnO sol−gel solutions were, respectively, 0.1, 0.3, and 0.6 M (mol/L). MEA (in 316, 950, and 1900 μL aliquots) was then added into the three systems, respectively. After stirring for 1 h at 50 °C, the solutions were kept in the ambient overnight and were then filtered with a 0.45 μm filter to remove the large particles. ZnO films were then spin-coated on top of cleaned ITO substrates at 3000 rpm for 30 s. The substrates were heated on a hot plate that was preheated at 60 °C. After 30 min, the temperature was elevated from 60 to 220 °C with a ramping rate of 50 °C/min, and another 30 min is needed to heat the samples. After cooling down, the ITO substrates covered with ZnO films were then transferred into a glovebox filled with N2, where the active layers were spin-coated from TQ1:PC71BM in a 2:5 weight ratio o-dichlorobenzene (o-DCB) solution with a concentration of 25 mg/mL. After deposition of the active layer, the samples were removed from the glovebox and PEDOT:PSS (Baytron P VP Al 4083) mixed with 0.5% (by volume) surfactant (ZonylFS 300) was spin-cast on top at 1000 rpm for 60 s. Subsequently, the substrates were transferred into a vacuum chamber mounted in the glovebox, where 100 nm Ag was evaporated through a shadow mask at a pressure lower than 4 × 10−6 mbar. The active area of the prepared ISCs is about 4.5 mm2. 2.2. Characterization of ISCs. J−V Curves and EQE. Current density−voltage (J−V) curves were obtained by using a Keithley 2400 Source Meter under illumination of AM 1.5G with an intensity of 1000 W m−2 from a solar simulator (model SS-50A, Photo Emission Tech., Inc.). External quantum efficiency (EQE) spectra were collected by a Keithley 485 picoammeter under illumination of monochromatic light through the ITO glass side. AFM Measurement. ZnO films were prepared following the process described above. The active layers for AFM measurement were spin-coated from TQ1:PC71BM in o-DCB solution on top of ITO substrates covered with ZnO films. AFM were carried out with a Dimension 3100 system (Digital Instruments/Veeco) by using antimony (n) doped silicon cantilevers (SCM-PIT, Veeco) in tapping mode. Water Contact Angle Measurement. ZnO films were prepared on the cleaned ITO substrates following the process described above. The water is deionized water obtained from Ultrapure Water System. EL Spectra Measurement. The EL spectra of the ISCs was collected with an Andor spectrometer (Shamrock sr-303i-B, coupled to a Newton EMCCD detector) through connected ISCs to an external current/voltage source. EQEEL Measurements. The EQEELwere recorded from a home-built system consisting of a Hamamatsu silicon photo-

Figure 1. (a) Representation of the layer configuration of ISCs. (b) The chemical structure of the donor material TQ1. (c) Energy-level diagram for the ISC.

active layer/PEDOT:PSS/Ag using a blue polymer poly[2,3bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5diyl] (TQ1) with a number-average molecular weight (Mn) of 71 000 (Figure 1b) mixed with [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as the active layer33 based on different ZnO interlayers, in terms of surface roughness. As shown in the energy diagram of the ISC (Figure 1c), the low conduction band (CB) energy of ZnO allows an Ohmic contact to be formed between the cathode and the blend active layer, while the high electron mobility of ZnO makes the thickness of the ZnO layer in an ISC less critical.26 The high valence band (VB) energy of ZnO, on the other hand, could offer effective holeblocking to the cathode, which can reduce the undesired hole collection at the cathode, and improve both photocurrent and fill factor (FF) of the ISCs. ISCs with ZnO surface roughness varied by an order of magnitude are fabricated and characterized with atomic force microscopy (AFM), Fourier transform photocurrent spectroscopy (FTPS), and electroluminescence (EL). Our results indicate that the surface roughness of the ZnO can affect the PV performance parameters of the ISCs. The FF obtained from the ISCs depends on the surface roughness of the ZnO interlayer through controlling the ZnO/active layer interfacial area. The short-circuit photocurrent density (Jsc) of the ISCs shows the most significant change when the surface roughness of the ZnO layer alters from 1.9 to 48 nm through the reduced 24463



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Figure 2. 3D AFM images (5 μm × 5 μm) of ZnO films cast from (a) 0.1 M solution (RMS = 1.9 nm, named as S-Film), (b) 0.3 M solution (RMS = 17 nm, named as M-Film), and (c) 0.6 M solution (RMS = 48 nm, named as R-Film) and (d) corresponding line scanning profile figures obtained from the diagonal of the AFM images.

corresponding line scanning profile figures are given in Figure 2d. The smoothest film with a root-mean-square (RMS) roughness value of 1.9 nm is cast from Zn(Ac)2 sol−gel solution with a concentration of 0.1 M, which is named as SFilm. The roughest film (R-Film) with a RMS value of 48 nm is obtained from the sol−gel solution with a concentration of 0.6 M. The film cast from the 0.3 M sol−gel solution is named as M-Film (RMS = 17 nm), as it has an intermediate surface roughness. The line scanning profile figures suggest that the SFilm has the smallest effective surface area and thus the smallest effective interfacial area between the ZnO layer and the active layer on top, whereas the R-Film has the largest effective interfacial area with the active layer. J−V curves of the ISCs based on these different ZnO films are plotted in Figure 3, and their photovoltaic parameters are summarized in Table 1. It is noted that all the ISCs studied in this work are UV-illuminated and post-thermal-treated in order to obtain a better and more stable result.23,24 From the J−V curves, we found that the best performing ISC with a PCE of 3.9%, a Jsc of 7.0 mA/cm2, a Voc of 0.85 V, and an FF of 0.65 is

diode 1010B, a Keithley 2400 used to supply voltages and measure the injected current, and a Keithley 485 measuring the emitting light intensity. FTPS Measurement.35 FTPS was performed by using a Vertex 70 from Bruker optics with a QTH lamp, quartz beam splitter, and external detector option. The photocurrent produced upon illumination of the photovoltaic devices with light modulated by the FTIR is amplified by using a low noise current amplifier Stanford Research system (SR570). The output voltage from the current amplifier was returned into the external FTIR detector port.

3. RESULTS AND DISCUSSION 3.1. Characterization of the ZnO films and Photovoltaic Performance of the ISCs. To correlate the ZnO surface roughness to the PV performance of the ISCs, three different ZnO films were deposited from Zn(Ac)2 sol−gel solutions with different concentrations under the same spin speed and thermal conditions. AFM images obtained from these ZnO films are shown in Figure 2a−c, and their 24464



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3.3. Influence of ZnO Surface Roughness on Jsc of the ISCs. Moreover, the surface roughness of the ZnO film, which contributes to the surface energy,32 can influence the interfacial property of the ZnO layer/active layer and the morphology of the active layer on top;27,28,30 thereby, the PV performance of the ISC is expected to be dependent on the ZnO surface roughness. In this work, the surface energy of the ZnO layers is studied with water contact angle measurements. As shown in Figure 4, the water contact angle increases from 32° for the R-

Figure 3. J−V curves for the ISCs based on S-Film (black line), MFilm (red line), and R-Film (blue line).

based on the S-Film (RMS = 1.9 nm), whereas the R-Film (RMS = 48 nm) based ISC gives the lowest PCE (2.7%) with a decreased Jsc and FF, as compared with the other two devices. The PCE obtained from the M-Film based ISC is 3.3% with the lowest Voc of 0.79 V, although it has the highest Jsc of 7.3 mA/ cm2. (Standard deviation of the photovoltaic performance parameters is given in Figure S1 in the Supporting Information.) 3.2. Influence of ZnO Surface Roughness on FF of the ISCs. As shown in Figure 2, the effective interfacial area between the ZnO layer and the active layer depends on the ZnO surface roughness. Thus, the interface trap density can be expected to be different in the ISCs based on different ZnO layers: the larger ZnO/active layer interfacial area, the more interface traps. The interface traps, leading to an undesired trap-assisted recombination, could reduce the FF of the solar cell, since such trap-assisted recombination is one of the reasons that causes photocurrent to be dependent on the electric field,39 and can be significant when the field is reduced in the power generating quadrant. Thus, the observed higher FF in the ISC with a smoother ZnO film can be partially explained by the smaller effective ZnO/active layer interfacial area and fewer interface traps in the ISC, whereas the ISCs based M-Film and R-Film have larger effective interfacial areas and more interface traps and, therefore, relatively lower FFs. The traps in a solar cell can be visualized via EQEELmeasurements, since the trap-assisted recombination is generally nonradiative. In this work, EQEEL (Table 1) obtained from the ISCs based on the different ZnO interlayers agrees well with our predictions: the highest EQEEL is obtained from the ISC based on the smoothest ZnO interlayer with less interface traps, and the EQEELs are comparably lower in the ISCs based on the rougher ZnO interlayers. One should note that the series resistance (Rs) of the different ISCs is also found to be different, which could be another reason for the observed difference in FF. The lowest Rs (15 Ω cm2) is found in the SFilm based ISC, which also gives rise to the highest FF, whereas the M- and R-Film based ISCs are more limited by their relatively higher Rs (Table 1).

Figure 4. Measured water contact angle between a drop of deionized water and the ZnO films with different surface roughness.

Film to 61° for the S-Film. The M-Film has a similar hydrophobicity as the S-Film with a contact angle of 57°, indicating that the morphologies and D/A interfacial area in the active layers on top of the different ZnO layers should be different. The difference in Jsc is thus likely due to the different D/A interfacial areas in the active layers coated on top of the different ZnO interlayers.27,40 Topographic images of the active layers deposited on top of the different ZnO layers taken with tapping-mode AFM (Figure 5) show that the active layer on top of the S-Film has the finest structure, similar with the TQ1:PC71BM active layer coated on top of the ITO/ PEDOT:PSS substrate in conventional solar cells.33 However, the nanoridges in the active layers coated on the M-Film and RFilm make it difficult to deduce any useful information on the D/A interfacial area in the corresponding active layers of ISCs. The D/A interfacial area of the active layers is, therefore, probed by measuring the interfacial charge-transfer (CT) state absorption and emission with the highly sensitive Fourier transform photocurrent spectroscopy (FTPS)34,35 and electroluminescence (EL).41,42 Comparing to the EL from pure polymer (locates at roughly 1.7 eV27) (Figure 6a), the red shifted strong emission peaks obtained from the ISCs at roughly 1.3 eV in the normalized EL spectra are from the interfacial CT states. For a BHJ active layer in which donor and acceptor are homogeneously mixed, there should be only CT emission and no pure polymer emission in EL, since all the injected carriers recombine via CT states at the D/A interface.

Table 1. Photovoltaic Parameters for the ISCs Based on ZnO Layers with Different Surface Roughness ETL

Jsc (mA/cm2)

FF

Voc (V)

PCE (%)

Rs (Ω cm2)

EQEEL

S-Film M-Film R-Film

7.0 7.3 5.6

0.65 0.57 0.58

0.85 0.79 0.83

3.9 3.3 2.7

15 28 25

2.5 × 10−7 1 × 10−7 3 × 10−8

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Figure 6. (a) Normalized EL spectra and (b) normalized FTPS spectra for ISCs based on S-Film (black line), M-Film (red line), and R-Film (blue line). The neat TQ1 emission spectrum (cyan line) is given in the normalized EL spectra.

and this explains the lowest Jsc (5.6 mA/cm2) obtained from the ISC of R-Film. The difference in Jsc obtained from the ISCs based on different ZnO layers could also be due to the optical properties of the ZnO films, as shown in Figure S2 (Supporting Information). However, since the transmittance spectra of RFilm and M-Film are similar, and the Jsc obtained from the ISCs based on R- and M-Film are quite different, the effect of the optical properties of the three ZnO interlayers on the Jsc of the ISCs should be limited. In addition, false internal quantum efficiencies (fIQEs) of the ISCs based on different ZnO layers are estimated via eq 1

Figure 5. 3D AFM images (5 μm × 5 μm) for TQ1/PC71BM active layers on top of (a) S-film, (b) M-film, and (c) R-film.

When phase separation in the BHJ film is strong, polymer emission becomes detectable.41 It is thus clear that the D/A interfacial area in the active layer of the ISC based on R-Film is smaller, as it has the strongest polymer emission intensity in the EL spectra normalized at the peak position of CT emission.27 This is further confirmed in the normalized FTPS spectra (Figure 6b), in which the CT absorption is shifted to ∼1.46− 1.49 eV due to the Stokes effect. In FTPS, which is normalized in the region of polymer absorption, a stronger CT signal corresponds to a higher CT absorption, and thus more D/A interfacial area in the active layer. Both FTPS and EL spectra are roughly the same for the ISCs based on S-Film and M-Film, which indicates that the D/A interfacial area is similar in the active layers of the two ISCs, consistent with the observed similar Jsc (Figure 3 and Table 1). For the ISC based on the RFilm, the interface CT absorption and emission are weaker, indicating a smaller D/A interfacial area. The small D/A interfacial area results in inefficient exciton dissociation in the active layer and, thus, could seriously limit the Jsc of the ISC,

fIQE(λ )=

EQE(λ) 1 − R (λ )

(1)

which is a lower limit as the reflectance also includes effects due to power dissipation in the electrodes. The EQE(λ) (Figure S3a, Supporting Information) and R(λ) (Figure S3b, Supporting Information) are the external quantum efficiency and reflectance of the ISCs, respectively. As shown in Figure S3c (Supporting Information), the fIQE spectrum of the ISC based on S- or M-Film is significantly higher than that of the RFilm based ISC, proving that the observation of a difference in Jsc of the ISCs based on different ZnO layers is due to differences in exciton dissociation. Note that the Jsc of the S- or M-Film based ISC is only ∼30% higher than that of the R-Film based ISC, while the fIQE is ∼50% higher: This is due to the fact that the absorption in the active layer of the R-Film based ISC is higher, which could originate from the enhanced light scattering at the very rough ZnO surface. 24466



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3.4. Influence of ZnO Surface Roughness on Voc of ISCs. As observed from the EL and FTPS spectra obtained from the ISCs, the energy of CT states (ECT) of the ISCs based on S- and M-Film are similar, but ECT of the ISC based on RFilm is blue shifted compared with the ECT of ISCs based on Sand M-Film. The ECT determined by fitting the FTPS spectra obtained from the ISCs based on S/M-Film is ≈1.46 eV, and ECT of the R-Film based ISC is found to be 1.49 eV,36,37 as shown together with the low-energy FTPS spectrum used to deduce ECT in Figure 7. This unexpected shift in ECT may

FTPS spectra.36,37 Therefore, Vrad oc can be calculated when ECT is known. The calculated Vrad for ISC based on R-film is higher oc (Figure 7), due to the higher ECT, whereas Vrad oc for the ISC based on S- or M-Film is roughly 30 mV lower. This observation differs from the experimentally obtained Voc, which is higher in the ISC based on S-Film. This indicates that there is more nonradiative recombination pathways in the ISCs based on R-film, since nonradiative recombination lowers Voc, and the loss can be quantified as36−38 rad Voc = Voc +

kT ln(EQE EL) q

(3)

EQEEL, representative of the rate of nonradiative recombination in a solar cell, is always smaller than 1, making the second term on the right side of eq 3 negative, and leads to a reduction of Voc from Vrad oc . Using the EQEEL values listed in Table 1, we can calculate the theoretical Voc of the ISCs based on different ZnO layers. As given in Figure 7, the calculated Voc matches decently with the measured Voc for the different ISCs. This reveals the influence of ZnO surface roughness on the Voc of the ISCs and explains the observed difference in Voc of the ISCs: the ZnO surface roughness determines the effective ZnO/active layer interfacial area, and the amount of interface traps at the ZnO/ active layer interface. Because the trap-assisted recombination is nonradiative and reduces Voc of a solar cell, Voc's of the ISCs based on M-Film and R-Film with a relatively larger ZnO/ active layer interface are lower.

4. CONCLUSION In conclusion, the importance of the surface roughness of the ZnO layer (ETL) in determining the PV performance parameters of the ISCs was investigated by using TQ1/ PC71BM as the photoactive layer. Surface roughness was shown to determine the surface energy of the ZnO layer and thus determine the D/A interfacial area in the active layer deposited on top. Jsc of the ISCs could, therefore, be directly correlated to the RMS roughness of the ZnO substrate. The dependence of the D/A interfacial area in the active layer on the surface roughness of the ZnO layer was proven by comparing the interfacial CT absorption measured by FTPS and emission measured by EL with the absorption and emission from pure donor material. Hence, the lowest Jsc (5.6 mA/cm2) obtained from the ISC with the highest ZnO surface roughness (RMS = 48 nm) and the highest surface energy (32°) could be ascribed to the smallest D/A interfacial area in the active layer deposited on top. The surface roughness, on the other hand, also determines the effective interfacial area between the active layer and the ZnO layer and thus the density of trap sites at the interface that was observed via EQEEL measurement. Therefore, trap-assisted recombination at the ZnO surface was expected to be less in the ISC based on the smoothest ZnO layer (S-Film), and responsible for the higher FF. Furthermore, the different Voc obtained from the ISCs based on different ZnO layers was attributed to the different densities of the trap sites in different ZnO layers with different RMS values. As a result, the best ISC was the one based on the smoothest ZnO layer, with the largest D/A interfacial area, and lowest ZnO/active layer interfacial area. The correlation between surface roughness of the ZnO layer (ETL) and the PV performance parameters of ISCs deduced in this work is anticipated to help further the understanding of the working principle of ZnO-based ISCs.

Figure 7. Diagrams of the calculated Vrad oc and Voc for the ISCs of (a) SFilm, (b) M-Film, and (c) R-Film. The experimental Voc values obtained from the ISCs are given in parentheses.

originate from the larger ZnO/polymer interface formed as a consequence of the increased ZnO/active layer effective interfacial area between the ZnO layer and the active layer. The large ZnO/TQ1 interface allows more exciton dissociation and CT exciton formation to occur at the ZnO/TQ1 hybrid interface. The energy of the CT exciton formed between the polymer and ZnO should be different from that formed between TQ1 and PC71BM, thus it may lead to the existence of another CT state with a higher energy, and give rise to the observed blue shift in ECT. However, CT states at the interface between ZnO/TQ1 could not be detected even in a hybrid solar cell with TQ1/ZnO nanoparticles as the active layer due to the too weak current signal. A maximum Voc of an organic solar cell can be obtained when recombination occurs radiatively only. Under solar illumination, 36−38 the maximum obtainable Vrad oc can be derived from rad = Voc

⎞ Jsc h3c 2 ECT kT ⎛⎜ ⎟ + ln⎜ q q ⎝ fq2π (ECT − λ) ⎟⎠

(2)

Hereby, k is the Boltzmann constant, T is temperature, q is the elementary charge, and h is the Planck constant. The reorganization energy λ and factor f can be deduced by fitting 24467



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ASSOCIATED CONTENT

S Supporting Information *

Standard deviation figures for all PV parameters for the ISCs; transmittance spectra of the three ZnO films; and EQE, reflectance, and fIQE spectra for the ISCs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +46-13-281257. Fax: +46-13-13 7568. E-mail: fenzh@ ifm.liu.se. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Swedish Energy Agency (Energimyndigheten), the Swedish Research Council (VR), and VINNOVA for the financial support. Dr. KoenVandewal is gratefully acknowledged for valuable discussions.

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REFERENCES

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