This document summarizes research on generating photocurrent in carbon nanotube photodiodes. It finds that applying a strong axial electric field can enable extremely efficient photocurrent generation. Photocurrent quantum efficiency was found to increase dramatically with higher applied fields, reaching over 60% at fields above 10 V/μm. Larger-diameter carbon nanotubes exhibited higher efficiencies at high fields due to their weaker exciton binding energies. The research provides evidence that impact ionization, the generation of multiple electron-hole pairs from single photon absorption, occurs at room temperature and is responsible for the high efficiencies observed at high fields.

1. Extremely efficient photocurrent generation in
carbon nanotube photodiodes
Mitchell Senger1, Daniel McCulley1, Andrea Bertoni2, Vasili Perebeinos3, and Ethan Minot1
1
2 3
McCulley, Senger, et al. Nano Letters. 20, 433 (2020).

2. Photocurrent Generation in 1D
Mitchell Senger - Oregon State University 2
𝜂 ≡
carriers extracted
photons absorbed
2009: Experimental
evidence of CM at
low temperature.
Gabor et al. Science. (2009).
𝜂 = ?Strong Coulomb interactions can
lead to novel relaxation pathways.
𝐸ph > 2𝐸g
𝜂 = 2 Previous Measurements of 𝜂 in CNTs
- Malapanis et al. Nano Lett. (2013).
𝜂 ≈ 5% at 𝑆11
- Kumamoto et al. Phys. Rev. Lett. (2014).
𝜂 = 50% at 𝑆22
- Aspitarte et al. Nano Lett. (2016).
𝜂 = 30% at 𝑆44
- Wang et al. ACS Nano. (2016).
𝜂 = 60% in bipolar junctions

3. Dissociation
Photocurrent
hole
electron
Strong Interactions also Tightly Bind Excitons
Dukovic et al. Nano Letters. (2005).
𝐸Coulomb =
𝑒2
4𝜋𝜖eff 𝜖0 𝐷
≈
0.4 eV ∙ nm
𝐷
Mitchell Senger - Oregon State University 3
Perebeinos et al. Nano Letters. (2007).
𝜖eff > 1
Field
𝐷 = 2 − 3 nm
𝐸b = 130 − 200 meV
Field
Can we efficiently separate excitons while still
allowing impact ionization?

4. Vg1 = -8 V Vg2 = +8 VVg1 = -12 V Vg2 = +12 VVg1 = -16 V Vg2 = +16 V
Vg1 = -20 V Vg2 = +20 V
1 𝜇m
Our suspended CNT devices
Mitchell Senger - Oregon State University 4
p n
-Vg +Vg
A
𝐹
Andrea Bertoni
Self-consistent field
calculations
p-type n-typeIntrinsic
region

5. p n
-Vg +Vg
A
𝐹
Mitchell Senger - Oregon State University 5
Changes in peak position, peak height, and peak width
Spectrally Resolved Photocurrent
at High Field
Normalized
photocurrent
(pA/μW)
Photon Energy (eV)
S22
0
1
2
3
4
0.80 0.810.790.780.77
3.2 V/μm
5.9 V/μm
8.2 V/μm
10.3 V/μm
12.3 V/μm
14.2 V/μm
Field
S33
S44
(𝑛, 𝑚) = (22,14)
Photon Energy (eV)
0
0.2
0.4
0.6
0.8
1.6 1.7 1.8 1.9
p n
Monochromatic light
-Vg +Vg
A
𝐹

6. Mitchell Senger - Oregon State University 6
𝑆22
𝐼pc 𝑑 ℏ𝜔 = 𝑒 𝜂22 Φ 𝐿i 𝑁𝐿
𝑆22
𝜎𝑐 𝑑 ℏ𝜔
Liu et al. PNAS. (2011) .
𝜂22 Increases Dramatically with Field
𝜂22 =
𝑆22
𝐼pc
𝑒 Φ 𝐿i
𝑑 ℏ𝜔
𝑁𝐿 𝑆22
𝜎𝑐 𝑑 ℏ𝜔
3.2 V/μm
5.9 V/μm
8.2 V/μm
10.3 V/μm
12.3 V/μm
14.2 V/μm
Field

7. Larger CNTs show Larger 𝜂22 at High
Field
Mitchell Senger - Oregon State University 7
ℏ𝜔22 ∝
1
𝐷
∝ 𝐸Coulomb

8. Calculated Decay Products of 𝑆22
Mitchell Senger - Oregon State University 8
Vasili Perebeinos
𝑆22 Decay Modeling
1/Le-h (nm-1)
Probabilitydensity(/nm-1)
Free Carriers Bound Carriers

9. Increased Low Field 𝜂 in Dielectric Fluids
Mitchell Senger - Oregon State University 9
𝜖eff > 1
Field
Oil

10. Mitchell Senger - Oregon State University 10
Conclusions
Ipc
e∙Φ∙Li
(nm)
- 𝜂 can be increased at lower fields by weakening
𝐸Coulomb with dielectrics.
- Efficient photocurrent generation in large-diameter
CNTs is enabled with large axial fields.
- Strong evidence for room temperature impact
ionization
→ McCulley et al. Nano Lett. 20, 433–440 (2020).

11. Vasili Perebeinos
Acknowledgements
Andrea Bertoni
Daniel McCulley Ethan Minot
Mitchell Senger - Oregon State University 11
Contact Info:
Mitchell Senger
sengermi@oregonstate.edu
Coauthors: Funding:

12. References
Mitchell Senger - Oregon State University 12
1. Gabor, N. M., Zhong, Z., Bosnick, K., Park, J. & McEuen, P. L. Extremely Efficient Multiple Electron-Hole Pair
Generation in Carbon Nanotube Photodiodes. Science. 325, 1367–1371 (2009).
2. Malapanis, A., Perebeinos, V., Sinha, D. P., Comfort, E. & Lee, J. U. Quantum Efficiency and Capture Cross
Section of First and Second Excitonic Transitions of Single-Walled Carbon Nanotubes Measured through
Photoconductivity. Nano Lett. 13, 3531–3538 (2013).
3. Kumamoto, Y. et al. Spontaneous exciton dissociation in carbon nanotubes. Phys. Rev. Lett. 112, 1–5 (2014).
4. Aspitarte, L., McCulley, D. R. & Minot, E. D. Photocurrent Quantum Yield in Suspended Carbon Nanotube p-n
Junctions. Nano Lett. 16, 5589–5593 (2016).
5. Wang, F. et al. High Conversion Efficiency Carbon Nanotube-Based Barrier-Free Bipolar-Diode Photodetector.
ACS Nano 10, 9595–9601 (2016).
6. Dukovic, G. et al. Structural Dependence of Excitonic Optical Transitions and Band-Gap Energies in Carbon
Nanotubes. Nano Lett. 5, 2314–2318 (2005).
7. Liu, K. et al. Systematic determination of absolute absorption cross-section of individual carbon nanotubes. Proc.
Natl. Acad. Sci. 111, 7564–7569 (2014).
8. McCulley, D. R., Senger, M. J., Bertoni, A., Perebeinos, V. & Minot, E. D. Extremely Efficient Photocurrent
Generation in Carbon Nanotube Photodiodes Enabled by a Strong Axial Electric Field. Nano Lett. 20, 433–440
(2020).