Organic photovoltaics incorporating electron conducting exciton blocking layers

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Organic photovoltaics incorporating electron conducting exciton blocking layers Brian E. Lassiter, Guodan Wei, Siyi Wang, Jeramy D. Zimmerman, Viacheslav V. Diev et al. Citation: Appl. Phys. Lett. 98, 243307 (2011); doi: 10.1063/1.3598426 View online: http://dx.doi.org/10.1063/1.3598426 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v98/i24 Published by the AIP Publishing LLC.

Additional information on Appl. Phys. Lett. Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

Downloaded 03 Oct 2013 to 221.130.162.54. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

APPLIED PHYSICS LETTERS 98, 243307 共2011兲

Organic photovoltaics incorporating electron conducting exciton blocking layers Brian E. Lassiter,1 Guodan Wei,1 Siyi Wang,3 Jeramy D. Zimmerman,2 Viacheslav V. Diev,3 Mark E. Thompson,3 and Stephen R. Forrest1,2,4,a兲 1

Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109 USA 2 Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109 USA 3 Department of Chemistry, University of Southern California, Los Angeles, California 90089 USA 4 Department of Physics, University of Michigan, Ann Arbor, Michigan 48109 USA

共Received 18 April 2011; accepted 19 May 2011; published online 15 June 2011兲 We demonstrate that 3,4,9,10 perylenetetracarboxylic bisbenzimidazole 共PTCBI兲 and 1,4,5,8napthalene-tetracarboxylic-dianhydride 共NTCDA兲 can function as electron conducting and exciton blocking layers when interposed between the acceptor layer and cathode. A low-resistance contact is provided by PTCBI, while NTCDA acts as an exciton blocking layer and optical spacer. Both materials serve as efficient electron conductors, leading to a fill factor as high as 0.70. By using an NTCDA/PTCBI compound blocking layer structure in a functionalized-squaraine/C60-based device, we obtain a spectrally corrected power conversion efficiency of 5.1⫾ 0.1% under 1 sun, AM 1.5G simulated solar illumination, an improvement of ⬎25% compared to an analogous device using a conventional bathocuproine layer that has previously been shown to conduct electrons via damage-induced midgap states. © 2011 American Institute of Physics. 关doi:10.1063/1.3598426兴

a兲

Electronic mail: [email protected]

0003-6951/2011/98共24兲/243307/3/$30.00

electrons directly from acceptor to cathode.8 We show that 3,4,9,10 perylenetetracarboxylic bisbenzimidazole 共PTCBI兲 serves as an efficient electron conductor and forms a low energy barrier contact with the Ag cathode. This leads to an increased fill factor 共FF兲 from FF= 0.60 typical of analogous BCP-based devices, to FF= 0.70. Additionally, 1,4,5,8napthalene-tetracarboxylic-dianhydride 共NTCDA兲 is shown to function as a wide-gap electron conducting EBL. By using both NTCDA and PTCBI in a compound blocking layer structure as in Fig. 1共c兲, we obtain optimal optical spacing, leading to increased photocurrent. This results in a spectrally corrected power conversion efficiency of ␩ p = 5.1⫾ 0.1% under 1 sun, AM 1.5G simulated solar illumination, an improvement of ⬎25% compared to a conventional device with a BCP blocker. The HOMO and LUMO energies of C60 are 6.2 eV and 3.7 eV, respectively,9 while BCP has corresponding energies of 6.4 eV and 1.7 eV,10 as shown in Fig. 1. Although the low 1.7

6.2

4.0

6.2

6.2

4.3 Ag

4.0

PTCBI

C60

Ag

BCP 6.4

4.9

3.7 4.3

NTCDA

6.2

3.7 4.3

C60

3.7

c)

2.8

Ag

b)

Ru(acac)3

a)

C60

Significant progress has been made over the past 25 years in improving the efficiency of organic photovoltaic 共OPV兲 cells.1,2 An important milestone to increased efficiency was the introduction of a buffer layer interposed between the acceptor layer and cathode contact, forming a socalled double heterojunction solar cell.3 The ideal buffer serves multiple purposes: to protect the underlying acceptor material 共e.g., C60兲 from damage due to the evaporation of hot cathode metal atoms, to provide efficient electron transport to the cathode, to serve as an exciton blocking layer 共EBL兲 that prevents excitons generated in the acceptor from quenching at the cathode, and to act as a spacer that maximizes the optical field at the active donor-acceptor heterojunction. The most commonly used EBLs are wide energy gap 共and hence transparent兲 semiconductors, such as bathocuproine 共BCP兲, that transport carriers via cathode metaldeposition-induced damage that results in a high density of conducting trap states 关Fig. 1共a兲兴.3 However, as the layer is conductive only in the presence of traps, the thickness is limited by the depth of damage 共⬍10 nm兲, which may not be optimal for achieving a maximum optical field intensity in the active region of the device. One possible route to the use of thicker, wide energy gap EBLs is to dope the film to increase its conductivity.4–6 A second type of EBL was introduced based on tris-共acetylacetonato兲 ruthenium共III兲 关Ru共acac兲3兴 and related compounds that have a small highest occupied molecular orbital 共HOMO兲 energy. In this case, holes from the cathode are transported along the HOMO of Ru共acac兲3 and recombine with electrons at the acceptor/EBL interface, as shown in Fig. 1共b兲.7 In this work, we utilize a third type of EBL where the lowest occupied molecular orbital 共LUMO兲 is aligned with that of the acceptor, allowing for low-resistance transport of

8.0

FIG. 1. 共Color online兲 Energy level diagrams of EBLs that transport charge via 共a兲 damage-induced trap states, 共b兲 electron-hole recombination, and 共c兲 electron transport through the lowest unoccupied molecular orbital.

98, 243307-1

© 2011 American Institute of Physics

Downloaded 03 Oct 2013 to 221.130.162.54. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

Fill factor (unitless)

243307-2

Appl. Phys. Lett. 98, 243307 共2011兲

Lassiter et al.

0.6

0.4

BCP PTCBI NTCDA compound

0.2

0.0

0

10

20

30

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Buffer Thickness (nm)

FIG. 2. 共Color online兲 Fill factor under spectrally corrected 1 sun, AM 1.5G illumination for devices with BCP buffer layers 共squares兲, PTCBI 共circles兲, NTCDA 共triangles兲, and compound NTCDA/PTCBI 共stars兲 as functions of thickness. Lines are a guide to the eye. Inset: the molecular structure of 1-NPSQ.

LUMO energy of BCP suggests a large barrier to electron extraction at the cathode, transport in BCP occurs through damage-induced trap states created by the evaporation of hot metal atoms onto the BCP surface.11 Because the PTCBI and NTCDA LUMOs approximately align with that of C60,10 electron transport can occur between these materials in the absence of damage. Devices were grown on 150 nm thick layers of indium tin oxide 共ITO兲 precoated onto glass substrates. Prior to deposition, the ITO surface was cleaned in a surfactant and a series of solvents as previously,12 and then exposed to ultraviolet-ozone for 10 min before loading into a high vacuum chamber 共base pressure ⬍10−7 Torr兲 where MoO3 was thermally evaporated at ⬃0.1 nm/ s. Substrates were then transferred to a N2 glovebox where 2,4-bis关4-共Nphenyl-1-naphthylamino兲-2,6-dihydroxyphenyl兴 squaraine13 共1-NPSQ, see molecular structural formula in Fig. 2, inset兲 films were spin-coated from heated 6.5 mg/ml solutions in 1,2-dichlorobenzene, and thermally annealed on a hot plate at 110 ° C for 5 min to promote the growth of a nanocrystalline bulk heterojunction morphology.14 Substrates were once again transferred into the high vacuum chamber for deposition of purified15 organics at 0.1 nm/s, followed by a 100 nm thick Ag cathode deposited at 0.1 nm/s through a shadow mask with an array of 1 mm diameter openings. Current density versus voltage 共J-V兲 characteristics were measured in an ultrapure N2 ambient, in the dark and under simulated AM 1.5G solar illumination from a filtered 150 W Xe lamp. Lamp intensity was varied using neutral density filters. Optical intensities were referenced using an NREL-calibrated Si detector,16 and photocurrent measurements were corrected for spectral mismatch.17 Errors quoted correspond to the deviation from the average value of three or more devices on the same substrate. Devices were fabricated with the following structure: glass/150 nm ITO/8 nm MoO3 / 15 nm 1-NPSQ/40 nm C60 / buffer共s兲 / 100 nm Ag. The open-circuit voltage depends on the interfacial energy gap between the donor and acceptor,18,19 and is Voc = 0.90 to 0.96⫾ 0.01 V, independent of buffer layer composition. Figure 2 shows FF as a function of buffer layer thickness x for BCP, PTCBI, NTCDA, and compound buffers consisting of 共x − 5兲 nm NTCDA/5 nm PTCBI. Optimal performance for devices with BCP occurs at

FIG. 3. 共Color online兲 Spectrally corrected short-circuit current 共Jsc兲 under 1 sun, AM 1.5G illumination for devices with BCP buffer layers 共squares兲, PTCBI 共circles兲, NTCDA 共triangles兲, and compound NTCDA/PTCBI 共stars兲 as a function of thickness. Solid lines are a guide to the eye. The dashed line is Jsc modeled based on the optical intensity and exciton diffusion lengths in the device for the case of the NTCDA/PTCBI buffer.

a thickness of 5 nm, with FF= 0.60⫾ 0.01, beyond which there is a sharp drop in efficiency due to the limited depth of damage-induced transport states extending into the film from the surface.11 In contrast, devices with PTCBI exhibit FF = 0.70⫾ 0.01, with only a small reduction as x → 50 nm, confirming the low resistance transport in this material. The optimum thickness for PTCBI is 10 nm, where ␩ p decreases for thicker films due to a decrease in short-circuit photocurrent 共Jsc兲, since PTCBI absorption overlaps with that of the active acceptor and donor layers....

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