Skip to main page content
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
, 4, 6236

Breaking the Space Charge Limit in Organic Solar Cells by a Novel Plasmonic-Electrical Concept

Affiliations

Breaking the Space Charge Limit in Organic Solar Cells by a Novel Plasmonic-Electrical Concept

Wei E I Sha et al. Sci Rep.

Abstract

As a fundamental electrostatic limit, space charge limit (SCL) for photocurrent is a universal phenomenon and of paramount importance for organic semiconductors with unbalanced photocarriers mobility and high exciton generation. Here we proposed a new plasmonic-electrical concept to manipulate electrical properties of organic devices including photocarriers recombination, transport and collection. As a proof-of-concept, organic solar cells (OSCs) comprising metallic planar and grating electrodes are systematically investigated with normal and inverted device structures. Interestingly, although strong plasmonic resonances induce abnormally dense photocarriers around a grating anode, the grating-inverted OSC is exempt from space charge accumulation (limit) and degradation of electrical properties in contrast to the planar-inverted and planar-normal ones. The particular reason is that plasmonically induced photocarriers redistribution shortens the transport path of low-mobility holes, which are collected by the grating anode. The work demonstrated and explained the SCL breaking with the plasmonic-electrical effect. Most importantly, the plasmonic-electrical concept will open up a new way to manipulate both optical and electrical properties of semiconductor devices simultaneously.

Figures

Figure 1
Figure 1. A schematic pattern of inverted OSC devices.
(a) inverted OSCs with a planar metallic anode; (b) inverted OSCs with a grating metallic anode. The device structure of the planar-inverted OSC is: ITO (70 nm)/TiO2 (20 nm)/P3HT:PCBM (220 nm)/MoO3 (10 nm)/planar Ag or Au (100 nm); and the device structure of the grating-inverted OSC is: ITO (70 nm)/TiO2 (20 nm)/P3HT:PCBM grating (220 nm)/MoO3 (10 nm)/Ag or Au grating (100 nm). The short notation “SC” denotes the space charge. A long transport path of holes in the planar-inverted OSC induces the SCL characteristics. A short transport path of holes manipulated by the plasmonic-electrical effect in the grating-inverted OSC breaks the SCL. (c) 45°-tilt SEM image of the planar P3HT:PCBM film. (d) 45°-tilt SEM image of the P3HT:PCBM film with the grating structure.
Figure 2
Figure 2. SCL characteristics for Ag-inverted OSCs measured at room temperature (T = 300 K).
Left graphs are photocurrents versus effective applied voltage at different incident light intensities. (a) Ag-planar-inverted OSCs, and (c) Ag-grating-inverted OSCs. Light intensity is varied from 100, 63, 40, 25, to 10 mW/cm2 by using neutral density filters. The black solid lines in (a) represent the square-root dependence of photocurrent on effective applied voltage (SCL region). Right graphs are photocurrents versus incident light intensity at different effective applied voltages. (b) Ag-planar-inverted OSCs, and (d) Ag-grating-inverted OSCs.
Figure 3
Figure 3. Experimental temperature-dependent SCL characteristics for Ag-inverted OSCs.
Top graphs are the temperature dependence of photocurrent versus effective applied voltage for (a) Ag-planar-inverted device and (b) Ag-grating-inverted device. Bottom graphs for Ag-grating-inverted device: (c) incident light intensity dependence of photocurrent versus effective applied voltage. (d) effective applied voltage dependence of photocurrent versus incident light intensity. The black solid lines in (a) – (c) represent the square-root dependence of photocurrent on effective applied voltage (SCL region).
Figure 4
Figure 4. Abnormal exciton generation obtained from the theoretical model.
The graphs present the exciton generation of the Ag-grating-inverted devices over that of the Ag-planar-inverted ones, which is defined as log10(Gg(r)/Gp(r)). The active layer thickness for both types of devices is set to be the same. The exciton generation of the Ag-planar device is nonzero at the region corresponding to the nanopatterned anode of the Ag-grating device where zero exciton generation is achieved. (a) s polarization for the square grating; (b) p polarization for the square grating; (c) unpolarization for the square grating; (d) s polarization for the sinusoidal grating; (e) p polarization for the sinusoidal grating; (f) unpolarization for the sinusoidal grating.
Figure 5
Figure 5. Theoretical temperature dependence of photocurrent versus effective applied voltage.
(a) Ag-planar-inverted device at T = 300 K; (b) Ag-grating-inverted device at T = 300 K; (c) Ag-planar-inverted device at T = 270 K; (d) Ag-grating-inverted device at T = 270 K. The overlapped regions between fitted and simulated curves are the square-root regions of OSC devices, i.e. Jph ~ (V0-V)0.5. The incident light intensity is 100 mW/cm2.
Figure 6
Figure 6. Theoretical temperature dependence of photocurrent versus light intensity.
The orders α of the curves (Jph ~ Iα) are 0.70, 0.90, 0.72, and 0.84 respectively for Ag-planar-inverted device at T = 300 K, Ag-grating-inverted device at T = 300 K, Ag-planar-inverted device at T = 270 K, and Ag-grating-inverted device at T = 270 K. The effective applied voltage is set to V0-V = 4.0 V.
Figure 7
Figure 7. SCL characteristics for Ag-normal OSCs measured at room temperature (T = 300 K).
Left graphs are photocurrents versus effective applied voltage (V0-V) at different incident light intensities. (a) Ag-planar-normal device, and (c) Ag-grating-normal device. The black solid lines in (a) and (c) represent the square-root dependence of photocurrent on effective applied voltage (SCL region). Right graphs are photocurrents versus incident light intensity at different effective applied voltages. (b) Ag-planar-normal device, and (d) Ag-grating-normal device.
Figure 8
Figure 8. SCL characteristics for Au-inverted OSCs measured at room temperature (T = 300 K).
Left graphs are photocurrents versus effective applied voltage at different incident light intensities. (a) Au-planar-inverted OSCs, and (c) Au-grating-inverted OSCs. Light intensity is varied from 100, 63, 40, 25, to 10 mW/cm2 by using neutral density filters. The black solid lines in (a) represent the square-root dependence of photocurrent on effective applied voltage (SCL region). Right graphs are photocurrents versus incident light intensity at different effective applied voltages. (b) Au-planar-inverted OSCs, and (d) Au-grating-inverted OSCs.
Figure 9
Figure 9. SCL characteristics for Au-normal OSCs measured at room temperature (T = 300 K).
Left graphs are photocurrents versus effective applied voltage (V0-V) at different incident light intensities. (a) Au-planar-normal device, and (c) Au-grating-normal device. The black solid lines in (a) and (c) represent the square-root dependence of photocurrent on effective applied voltage (SCL region). Right graphs are photocurrents versus incident light intensity at different effective applied voltages. (b) Au-planar-normal device, and (d) Au-grating-normal device.

Similar articles

See all similar articles

Cited by 6 articles

See all "Cited by" articles

References

    1. Goodman A. M. & Rose A. Double extraction of uniformly generated electron-hole pairs from insulators with noninjecting contacts. J Appl Phys 42, 2823–2830 (1971).
    1. Juska G., Viliunas M., Arlauskas K. & Kocka J. Space-charge-limited photocurrent transients-the influence of bimolecular recombination. Phys Rev B 51, 16668–16676 (1995). - PubMed
    1. Koster L. J. A., Mihailetchi V. D., Xie H. & Blom P. W. M. Origin of the light intensity dependence of the short-circuit current of polymer/fullerene solar cells. Appl Phys Lett 87, 203502 (2005).
    1. Mihailetchi V. D., Wildeman J. & Blom P. W. M. Space-charge limited photocurrent. Phys Rev Lett 94, 126602 (2005). - PubMed
    1. Lenes M., Koster L. J. A., Mihailetchi V. D. & Blom P. W. M. Thickness dependence of the efficiency of polymer: fullerene bulk heterojunction solar cells. Appl Phys Lett 88, 243502 (2006).

Publication types

Feedback