Grain Boundary Engineering of Halide Perovskite CH3NH3PbI3 Solar

Feb 12, 2018 - In this study, we investigate the nanoscale effects of photochemically active additives of benzoquinone (BQ), hydroquinone (HQ), and te...
0 downloads 3 Views 1MB Size
Subscriber access provided by UNIV OF DURHAM

Article 3

3

3

Grain Boundary Engineering of Halide Perovskite CHNHPbI Solar Cells with Photochemically-Active Additives

Nastaran Faraji, Chuanjiang Qin, Toshinori Matsushima, Chihaya Adachi, and Jan Seidel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00804 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 13, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Grain Boundary Engineering of Halide Perovskite CH3NH3PbI3 Solar Cells with Photochemically-Active Additives

1

2,3

Nastaran Faraji , Chuanjiang Qin , Toshinori Matsushima

2–4

, Chihaya Adachi

2–4

, and Jan

1*

Seidel

1

School of Materials Science and Engineering, UNSW Australia, Sydney NSW 2052,

Australia 2

Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744

Motooka, Nishi, Fukuoka 819-0395, Japan 3

Japan Science and Technology Agency (JST), ERATO, Adachi Molecular Exciton

Engineering Project, 744 Motooka, Nishi, Fukuoka 819-0395, Japan 4

International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu

University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan *

correspondence: [email protected], phone +61 2 9385 4442

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 12

Abstract In this study, we investigate the nanoscale effects of photochemically-active additives of benzoquinone (BQ), hydroquinone (HQ), and tetracyanoquinodimethane (TCNQ) on grain boundaries in CH3NH3PbI3 solar cells. We employ scanning probe microscopy under light illumination, in particular Kelvin probe force microscopy, to study surface potential changes under laser light illumination. The recently found improvement in efficiency of BQ added solar cells can be clearly seen in vanishing contact potential differences at grain boundaries under illumination, rendering the material more uniform in solar cell operating conditions. These effects are observed for BQ, but not for HQ and TCNQ. Our findings shed light onto halide perovskite materials and functional additive design for improved solar cell performance.

Introduction Exploring new pathways for making inexpensive photovoltaic devices is a main goal in photovoltaic technology development

1, 2

. Organic-inorganic hybrid materials have been

shown to be promising candidates with facile processing, stable 3, tuneable band gap 4, extremely high optical absorption 5, and long electron-hole diffusion length 6. Although these materials have numerous properties which makes them ideal for applications in photovoltaics cells, detrimental effects such as water sensitivity and material degradation remain a challenge 7. Specifically, CH3NH3PbI3 perovskite has seen tremendous attention as light harvesting material

8, 9

. Chemical modifications such as replacing halide ions, e.g.

CH3NH3PbI3-xBrx have led to increasing photovoltaic efficiencies in this material

10

together

with attempts to attain a better morphology of the perovskite layer for improved efficiency. Similarly, mixed halides, e.g. CH3NH3PbI3−xClx, were investigated and an efficiency of 15.4% was achieved

11

. End of 2013 efficiencies of 16.2% using a mixed halide

CH3NH3PbI3−xBrx (10-15% Br) and a poly-triarylamine hole transport material reported. Later, a confirmed efficiency of 17.9%

13

12

were

was achieved by mixing the lower

bandgap CH(NH2)2PbI3 material with CH3NH3PbBr3 as the photovoltaic active layer. Zhou et. al. fabricated CH3NH3PbI3 perovskite on doped TiO2 with an yttrium and modified indium tin oxide cathode with polyethylenimine ethoxylated to reduce the contact barrier

14

. An

efficiency of 20.1% was independently confirmed in late 2014, as demonstrated by Seok and co-workers

[15]

. In this work, high-quality CH(NH2)2PbI3 films were fabricated by direct 2

ACS Paragon Plus Environment

Page 3 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

intramolecular exchange of dimethylsulfoxide (DMSO) molecules intercalated in PbI2 with formamidinium iodide. To date, efficiencies for a perovskite solar cells as high as 22.1% have been reported

16

. More recently, the influence of grain boundaries (GBs) in fabricated

solar cell devices has gained attention as a key factor for device performance

3, 17-20

and

21

chemical engineering of such boundaries has become a focus .

Besides excelling in efficiency, producing stable perovskite solar cells is also challenging. Qin et al. previously reported eliminating water inclusion which is a source of hole trap formation and degradation that can significantly improve the stability of perovskite solar cells

22

. Moreover, in

23

it was reported that introducing BQ as redox active-organic

molecule into a precursor solution of methylammonium iodide and PbI2 improves both morphology and stability. In this work, GBs of organic-inorganic halide perovskite films were characterized by Kelvin probe force microscopy (KPFM) to investigate their role in CH3NH3PbI3 solar cell devices with photochemically-active additives. Experimental KPFM has been used as a tool to measure the contact potential difference (CPD). These measurements were performed with a modified AIST-NT Smart SPM scanning probe microscope and a tunable laser source for measurements under light illumination. KPFM allows for high lateral resolution measurements of spatial variations of the electrical properties of the devices on the nanometre scale. KPFM is a surface potential detection method that determines the CPD during scanning by compensating the electrostatic forces between the probe and the sample, which has been used for photovoltaic SPM measurements 24

. KPFM measurements were performed using Pt-coated AFM cantilevers (Mikromasch

HQ:NSC35/Pt) as the probe. The wavelength of laser light used for the AFM beam deflection was 1300 nm, which is outside the absorption range of the investigated sample. Films of the perovskite CH3NH3PbI3 were prepared on a film of poly (3, 4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) coated on indium-tin-oxide (ITO) substrates by spin casting from a precursor solution of CH3NH3I (MAI), PbI2, and BQ (or HQ, TCNQ). During spin casting, intermolecular interactions between MAI and BQ (or HQ, TCNQ) compete with the reaction between MAI and PbI2 to form the three dimensional perovskite structures, thus reducing the speed of crystallization. After annealing at 90 °C in a nitrogen-filled glove box, CH3NH3PbI3 films containing BQ (or HQ, TCNQ) additives were 3

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

obtained. Experimental details are provided in our previous literature 23.

Results and discussion The topography of CH3NH3PbI3 films with and without additives was investigated by AFM measurements and is shown in Figure 1(a). The effect of additives on CH3NH3PbI3 thin films can be seen in changes in the grain sizes of the films. To rule out surface morphology effects on the device performance, thin films without additives were fabricated and then additives induced into the perovskite system. All films have similar rms surface roughnesses of a few nanometers, which were extracted from AFM images (BQ: 10 nm, HQ: 19 nm, TCNQ: 8 nm, without additive: 12 nm). Figure 1(b) shows X-ray diffraction (XRD) 2θ/θ scans of the investigated samples. The CH3NH3PbI3 thin films keep their tetragonal crystallinity for different additives.

Figure 1. Morphological and structural characteristic of perovskite films with different additives. a) AFM topography image of films without and with BQ, HQ and TCNQ additive. Scan area is 4×4 µm². b) XRD patterns for films without and with BQ, HQ and TCNQ additive.

Figure 2(b) indicates the external quantum efficiency measured over the broad range of 300 to 850 nm wavelength for the given device geometry schematically shown in Figure 2(a). The CH3NH3PbI3 thin film with BQ as an additive shows overall improved device efficiencies, as seen in Figure 2(b).

4

ACS Paragon Plus Environment

Page 4 of 12

Page 5 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Figure 2. External quantum efficiency (EQE) spectra of the CH3NH3PbI3 thin films containing 0.5% of additives. a) Schematic drawing of device. b) EQE of CH3NH3PbI3 thin films without any additive (black line), with BQ (red line), HQ (green line) and TCNQ (purple line).

To investigate correlations of device efficiency on the macroscale and potential nanoscale origins, we performed KPFM measurements on all samples. Figure 3 shows the topography

5

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. (a-d) AFM topography image of the CH3NH3PbI3 thin films without and with additives of BQ, HQ and TCNQ respectively. (e-h) Corresponding CPD images recorded for CH3NH3PbI3 thin films in dark and (i-l) under laser illumination. (m-p) Histogram distributions in dark and in light condition are shown for each sample.

and surface potential of halide thin films without and with BQ, HQ and TCNQ as additives. At grain boundaries surface potential changes in general are not expected unless the band structure bends upward or downward 25. In that case, changes in CPD images reflect the change of the work function in the measured area. Brighter areas in CPD images indicate 6

ACS Paragon Plus Environment

Page 6 of 12

Page 7 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

a higher work function. It needs to be mentioned that, since KPFM measures the contact potential difference between the probe and the sample surface, the same tip is used for all the measurements. CPD measurements under light illumination with 600 nm wavelength and intensity of 3.02 kW/m2 were carried out. Acquired KPFM data in dark condition is shown in Figure 3(e-h). Grain boundaries show a visibly lower contact potential difference than the grain interiors. Interestingly, under illumination (Figure 3(i-l)), this local difference or barrier at the GBs is reduced, the effect is most prominent in the BQ added samples, where CPD appears almost flat across GBs. This finding indicates that band bending at GBs is reduced under illumination, due to photovoltaic carrier generation and potential screening at the GBs. For this case the material thus looks more uniform in the surface potential landscape and we believe that it has a positive effect on the photovoltaic performance, as proven by the external quantum efficiency measurements discussed earlier. In addition, the overall average CPD change upon illumination is the largest in the BQ added samples at 300mV compared to 245…265mV for all other samples, which can be seen in the CPD histogram plots in Fig. 3(m-p).

Figure 4. Cross-section data of topography and CPD for CH3NH3PbI3 thin films a) without additives, b) with BQ, c) HQ and d) TCNQ as additives.

Cross-section profiles of Figure 3(a-l) were extracted for the same area of the thin films in dark and under laser illumination and are shown in Figure 4 to see the difference in CPD better and for simplicity of comparison. For each image the cross-section data for 7

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 12

topography and CPD are shown together, so the behaviour of grain boundaries can be seen clearly. In dark condition, nearly in all extracted profiles the contact potential drops at grain boundaries. The reduced CPD change at GBs is most visible for the BQ added sample. A schematic representation of associated qualitative changes in band structure is shown in Fig. 5. The schematic only shows a general concept, bending could be up or down. For the described effect the important thing is the amount of bending or lack thereof. a)

b) GB

GB

600 nm

c) GB

600 nm

∆ CPDGB EC

EC ∆ CPDGB

EF EV

Eg = 1.5 eV

EC

∆ CPDGB

EF

EF

EV

EV

HQ, TCNQ

BQ

Figure 5. Schematic band diagrams qualitatively illustrating the electronic structure around grain boundaries (GB) for a) sample in dark condition, b) for samples with added HQ and TCNQ under illumination, c) for samples with added BQ under illumination. ∆ CPDGB indicates the change of contact potential difference at the grain boundaries.

Conclusions We have demonstrated that the GBs in CH3NH3PbI3 films with additives of BQ, HQ, and TCNQ play a crucial role in halide perovskite solar cell performance. KPFM measurements showed that a potential barrier is formed along the GBs and higher CPD changes at GBs are found for HQ and TCNQ, while the opposite is true for BQ. These measurements confirm that photogenerated carriers are interacting differently with GBs in these samples leading to differences in overall device efficiencies. Further studies on the composition or different types of dopings at GBs are required to fully understand how the GB engineering can benefit halide solar cell device development.

Acknowledgements We acknowledge support by the Australian Research Council through Discovery Grants. JS further acknowledges travel support by I2CNER and UNSW strategic seed funding.

8

ACS Paragon Plus Environment

Page 9 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

References (1)

Edri, E.; Kirmayer, S.; Henning, A.; Mukhopadhyay, S.; Gartsman, K.; Rosenwaks, Y.; Hodes, G.; Cahen, D. Why Lead Methylammonium Tri-Iodide Perovskite-Based Solar Cells Require a Mesoporous Electron Transporting Scaffold (but Not Necessarily a Hole Conductor) Nano Lett. 2014, 14, 1000-1004.

(2)

Li, J.-J.; Ma, J.-Y.; Ge, Q.-Q.; Hu, J.-S.; Wang, D.; Wan, L.-J. Microscopic Investigation of Grain Boundaries in Organolead Halide Perovskite Solar Cells ACS

Appl. Mater. Interfaces 2015, 7, 28518-28523. (3)

Kim, Y. C.; Jeon, N. J.; Noh, J. H.; Yang, W. S.; Seo, J.; Yun, J. S.; Ho-Baillie, A.; Huang, S.; Green, M. A.; Seidel, J. et al. Beneficial Effects of PbI2 Incorporated in Organo-Lead Halide Perovskite Solar Cells Adv. Energy Mater. 2016, 6, 1502104.

(4)

Zhao, Y.; Zhu, K. Organic-Inorganic Hybrid Lead Halide Perovskites for Optoelectronic and Electronic Applications Chem. Soc. Rev. 2016, 45, 655-689.

(5)

Yin, W.-J.; Shi, T.;Yan, Y. Unique Properties of Halide Perovskites as Possible Origins of the Superior Solar Cell Performance Adv. Mater. 2014, 26, 4653-4658.

(6)

Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M., Grätzel, M., Mhaisalkar, S.,Sum, T. C. Long-Range Balanced Electron- and Hole-Transport Lengths in OrganicInorganic CH3NH3PbI3 Science 2013, 342, 344-347.

(7)

Hailegnaw, B.; Kirmayer, S.; Edri, E.; Hodes, G.; Cahen, D. Rain on Methylammonium Lead Iodide Based Perovskites: Possible Environmental Effects of Perovskite Solar Cells J. Phys. Chem. Lett. 2015, 6, 1543-1547.

(8)

Heo, J. H.; Im, S. H.; Noh, J. H.; Mandal, T. N.; Lim, C.-S.; Chang, J. A.; Lee, Y. H.; Kim, H.-J.; Sarkar, A.; Nazeeruddin, M. K. Efficient Inorganic-Organic Hybrid Heterojunction Solar Cells Containing Perovskite Compound and Polymeric Hole Conductors Nat. Photonics 2013, 7, 486-491.

(9)

Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells J. Am. Chem. Soc. 2009, 131, 60506051.

(10) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Chemical Management for Colorful, Efficient, and Stable Inorganic–Organic Hybrid Nanostructured Solar Cells

Nano Lett. 2013, 13, 1764-1769.

9

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(11) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition Nature 2013, 501, 395-398. (12) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent Engineering for High-Performance Inorganic–Organic Hybrid Perovskite Solar Cells

Nat. Mater. 2014, 13, 897-903. (13) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells

Nat. Photonics 2014, 8, 506-514. (14) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-B.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells Science 2014, 345, 542-546. (15) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. HighPerformance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange Science 2015, 348, 1234-1237. (16) Green, M. A. Corrigendum to ‘Solar Cell Efficiency Tables (Version 49)’(Prog. Photovolt: Res. Appl. 2017; 25: 3–13) Prog. Photovoltaics 2017, 25, 333-334. (17) Yun, J. S.; Ho-Baillie, A.; Huang, S.; Woo, S. H.; Heo, Y.; Seidel, J.; Huang, F.; Cheng, Y.-B.; Green, M. A. Benefit of Grain Boundaries in Organic–Inorganic Halide Planar Perovskite Solar Cells J. Phys. Chem. Lett. 2015, 6, 875-880. (18) Kim, J.; Yun, J. S.; Wen, X.; Soufiani, A. M.; Lau, C. F. J.; Wilkinson, B.; Seidel, J.; Green, M. A.; Huang, S.; Ho-Baillie, A. W. Y. Nucleation and Growth Control of HC(NH2)2PbI3 for Planar Perovskite Solar Cell J. Phys. Chem. C 2016, 120, 1126211267. (19) Yun, J. S.; Seidel, J.; Kim, J.; Soufiani, A. M.; Huang, S.; Lau, J.; Jeon, N. J.; Seok, S. I.; Green, M. A.; Ho-Baillie, A. Critical Role of Grain Boundaries for Ion Migration in Formamidinium and Methylammonium Lead Halide Perovskite Solar Cells Adv.

Energy Mater. 2016, 6, 1600330. (20) Zhang, M.; Yun, J. S.; Ma, Q.; Zheng, J.; Lau, C. F. J.; Deng, X.; Kim, J.; Kim, D.; Seidel, J.; Green, M. A. et al. High-Efficiency Rubidium-Incorporated Perovskite Solar Cells by Gas Quenching ACS Energy Lett. 2017, 2, 438-444. (21) Yun, J. S.; Kim, J.; Patterson, R.; Xia, H.; Kim, D.; Seidel, J.; Lim, S.; Young, T.; Chen, S.; Green, M. A.; Huang, S.; Ho-Baillie, A. Humidity Induced Degradation Via

10

ACS Paragon Plus Environment

Page 10 of 12

Page 11 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Grain Boundaries of HC(NH2)2PbI3 Planar Perovskite Solar Cells, Adv. Funct. Mater. 2018, in press. (22) Qin, C.; Matsushima, T.; Fujihara, T.; Potscavage, W. J.; Adachi, C. Degradation Mechanisms of Solution-Processed Planar Perovskite Solar Cells: Thermally Stimulated Current Measurement for Analysis of Carrier Traps Adv. Mater. 2016, 28, 466-471. (23) Qin, C.; Matsushima, T.; Fujihara, T.; Adachi, C. Multifunctional Benzoquinone Additive for Efficient and Stable Planar Perovskite Solar Cells Adv. Mater. 2017, 29, 1603808. (24) O’Dea, J. R.; Brown, L. M.; Hoepker, N.; Marohn, J. A.; Sadewasser, S. Scanning Probe Microscopy of Solar Cells: From Inorganic Thin Films to Organic Photovoltaics

MRS Bull. 2012, 37, 642-650. (25) Wang, P.; Zhao, J.; Wei, L.; Zhu, Q.; Xie, S.; Liu, J.; Meng, X.; Li, J. Photo-Induced Ferroelectric Switching in Perovskite Ch3nh3pbi3 Films Nanoscale 2017, 9, 38063817.

11

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TOC graphic

12

ACS Paragon Plus Environment

Page 12 of 12