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Energy Express

  • Editor: Bernard Kippelen
  • Vol. 18, Iss. S4 — Nov. 8, 2010
  • pp: A513–A521
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Realization of efficient semitransparent organic photovoltaic cells with metallic top electrodes: utilizing the tunable absorption asymmetry

Donggeon Han, Hoyeon Kim, Soohyun Lee, Myungsoo Seo, and Seunghyup Yoo  »View Author Affiliations


Optics Express, Vol. 18, Issue S4, pp. A513-A521 (2010)
http://dx.doi.org/10.1364/OE.18.00A513


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Abstract

Efficient semitransparent organic photovoltaic (OPV) cells are presented in an inverted geometry employing ZnS/ Ag/ WO3 (ZAW) as a top anode and ITO/ Cs2CO3 as a bottom cathode. Upon identification of the light absorption that differs depending on the illumination direction, the degree of the absorption asymmetry is tuned by varying the ZAW structure to maximize the efficiency for one direction or to balance it for both directions. Power conversion efficiency close to that of conventional opaque OPV cells is demonstrated in semitransparent cells for the ITO side illumination by taking advantage of the internal reflection occurring at the organic/ZAW interface. Cells with efficiencies that are reduced but balanced for both illumination directions are also demonstrated.

© 2010 OSA

1. Introduction

Organic photovoltaic (OPV) cells based on polymer:fullerene blends have been receiving growing attention as a potential solution as a low-cost renewable energy source. With continued efforts toward development of photoactive polymers and control over the nanoscale morphology of polymer:fullerene mixtures, power conversion efficiencies higher than 7% have recently been demonstrated [1

1. H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, and G. Li, “Polymer solar cells with enhanced open-circuit voltage and efficiency,” Nat. Photonics 3(11), 649–653 (2009). [CrossRef]

]. Such achievement as well as recent demonstrations of roll-to-roll based large-area OPV fabrication [2

2. A. J. Medford, M. R. Lilliedal, M. Jørgensen, D. Aarø, H. Pakalski, J. Fyenbo, and F. C. Krebs, “Grid-connected polymer solar panels: initial considerations of cost, lifetime, and practicality,” Opt. Express 18(S3), A272–A292 (2010). [CrossRef] [PubMed]

,3

3. F. C. Krebs, T. D. Nielsen, J. Fyenbo, M. Wadstrøm, and M. S. Pedersen, ““Manufacture, integration and demonstration of polymer solar cells in a lamp for the “Lighting Africa” initiative,” Energy Environ. Sci. 3(5), 512–525 (2010). [CrossRef]

] is regarded as a key step in realizing commercially viable OPV technologies. Along with the improved efficiencies and development of scalable low-cost processing technologies, useful characteristics that can differentiate OPV technologies from other PV technologies also have to be further developed. Semi-transparency, which can be utilized in applications such as energy-harvesting windows, is one of the characteristics for which OPV technologies are better positioned than existing PV technologies. A relatively narrow absorption band and thin-film fabrication of organic materials allows semitransparent OPV (ST-OPV) cells to be made simply and seamlessly [4

4. R. Koeppe, D. Hoeglinger, P. A. Troshin, R. N. Lyubovskaya, V. F. Razumov, and N. S. Sariciftci, “Organic solar cells with semitransparent metal back contacts for power window applications,” ChemSusChem 2(4), 309–313 (2009). [CrossRef] [PubMed]

10

10. T. Ameri, G. Dennler, C. Waldauf, H. Azimi, A. Seemann, K. Forberich, J. Hauch, M. Scharber, K. Hingerl, and C. J. Brabec, “Fabrication, optical modeling, and color characterization of semitransparent bulk-heterojunction organic solar cells in an inverted structure,” Adv. Funct. Mater. 20(10), 1592–1598 (2010). [CrossRef]

], as opposed to devices made of inorganic counterparts in which the numerous small cells need to be arranged with a given aperture ratio to define the average transmittance [11

11. P. Nath, and C. Vogeli, “Translucent photovoltaic sheet material and panels.” US patent, No.5,176,758 (1993).

,12

12. A. Ricaud, J. Schmitt, and J.-M. Siefert, “Semi-transparent solar module panel.” US Patent, No. D 353,129 (1994).

]. However, the power conversion efficiencies of most ST-OPV cells reported to date have been significantly lower than that of conventional cells with only a few exceptions [7

7. J. Huang, G. Li, and Y. Yang, “A semi-transparent plastic solar cell fabricated by a lamination process,” Adv. Mater. 20(3), 415–419 (2008). [CrossRef]

]. Although use of a relatively thick photoactive layer may provide a solution in some cases, as in the work by Huang et al. [7

7. J. Huang, G. Li, and Y. Yang, “A semi-transparent plastic solar cell fabricated by a lamination process,” Adv. Mater. 20(3), 415–419 (2008). [CrossRef]

], such method is not applicable unless active layers have a sufficient level of transport properties. In this work, we demonstrate a simple but highly effective strategy to enhance the power conversion efficiency of ST-OPV cells by utilizing the partial internal reflection from a multilayer top anode consisting of ZnS, Ag, and WO3.

2. Experiment

2.1 Overview of the device structure

2.2 Fabrication and characterization

A thin film of Cs2CO3 (Alfa Aesar, 99.994%) was thermally evaporated in vacuum (2 × 10−6 Torr) onto precleaned ITO-coated glass substrates. The substrates were treated with air plasma using a plasma cleaner (PDC-32G, Harrick Plasma) before being taken into the evaporation chamber (HS-1100, Digital Optics & Vacuum). After deposition of Cs2CO3, the samples were transferred to a nitrogen-filled glove box without exposure to ambient air. For the active layer, a mixture of poly(3-hexylthiophene)(P3HT) and [6

6. H. Schmidt, H. Flugge, T. Winkler, T. Bulow, T. Riedl, and W. Kowalsky, “Efficient semitransparent inverted organic solar cells with indium tin oxide top electrode,” Appl. Phys. Lett. 94(24), 243302 (2009). [CrossRef]

,6

6. H. Schmidt, H. Flugge, T. Winkler, T. Bulow, T. Riedl, and W. Kowalsky, “Efficient semitransparent inverted organic solar cells with indium tin oxide top electrode,” Appl. Phys. Lett. 94(24), 243302 (2009). [CrossRef]

]-phenyl C71 butyric acid methyl ester (PCBM70) dissolved in dichlorobenzene (20mg/ml, 1:0.7 by weight) was spun at 700 rpm for 60 s. The samples were then dried for 30 min at room temperature and subsequently annealed at 110 °C on a hotplate for 10 min. Finally, the samples were reloaded into the chamber for a successive deposition of WO3 (Alfa Aesar 99.99%), Ag (Alfa Aesar, 99.999%), and ZnS (Alfa Aesar, 99.99%). The active area of the fabricated devices was typically in the range of 0.07-0.13 cm2.

Current density-voltage (J-V) characteristics were recorded using a source-measure unit (Keithley 2400) in a 4-wire sensing mode. Simulated AM 1.5G illumination was done using a solar simulator (ABET technologies) with AM1.5G filters. Irradiance was measured each time using a Si photodiode the response of which was calibrated against the reference Si solar cell. External quantum efficiency (EQE) spectra were also measured using a monochromator coupled to a Xe arc lamp and a calibrated Si photodiode. During the photovoltaic testing, samples were kept in an N2-environment without exposure to an ambient atmosphere. Transmittance measurement was done in ambient air using a UV-VIS spectrometer (SV2100, K-MAC).

Optical analysis was done either with commercially available thin-film optic software (Essential MacleodTM) or with the custom MATLABTM code. Thick substrate effect was also taken into account in all the calculation done in this work [20

20. H. A. Macleod, Thin-Film Optics, 3rd Ed. pp.53–72 (Taylor & Francis, New York, 2001).

]. Optical constants of participating layers were taken either from the literature or from the software. When not available, they were measured using the spectroscopic ellipsometry[Woollam, M2000D (RCT)].

3. The optical properties of the proposed ST-OPVs and their optimization strategies

Organic photovoltaic cells in general may be considered as an assembly of thin films, and their optical properties are known to be well described within the framework of thin-film optics using the transfer-matrix or characteristic-matrix formalism [16

16. S. Han, S. Lim, H. Kim, H. Cho, and S. Yoo, “Versatile Multilayer Transparent Electrodes for ITO-Free and Flexible Organic Solar Cells,” IEEE J. Sel. Top. Quant. Electron. (available online. doi: ).

] (Also refer to the work by Koeppe et al. [4

4. R. Koeppe, D. Hoeglinger, P. A. Troshin, R. N. Lyubovskaya, V. F. Razumov, and N. S. Sariciftci, “Organic solar cells with semitransparent metal back contacts for power window applications,” ChemSusChem 2(4), 309–313 (2009). [CrossRef] [PubMed]

] and Ameri et al. [10

10. T. Ameri, G. Dennler, C. Waldauf, H. Azimi, A. Seemann, K. Forberich, J. Hauch, M. Scharber, K. Hingerl, and C. J. Brabec, “Fabrication, optical modeling, and color characterization of semitransparent bulk-heterojunction organic solar cells in an inverted structure,” Adv. Funct. Mater. 20(10), 1592–1598 (2010). [CrossRef]

] for similar approach previously applied in ST-OPVs). It is emphasized that the analysis of DMD-based OPVs should be done on the whole device structure rather than on the electrodes themselves for a correct and quantitative account of their optical properties, as was discussed in Refs. 16

16. S. Han, S. Lim, H. Kim, H. Cho, and S. Yoo, “Versatile Multilayer Transparent Electrodes for ITO-Free and Flexible Organic Solar Cells,” IEEE J. Sel. Top. Quant. Electron. (available online. doi: ).

and 18

18. H. Cho, C. Yun, and S. Yoo, “Multilayer transparent electrode for organic light-emitting diodes: tuning its optical characteristics,” Opt. Express 18(4), 3404–3414 (2010). [CrossRef] [PubMed]

. It is also noted that validity in optical analysis depends largely on the precise determination of the optical constants of participating layers. The optical constants of P3HT:PCBM70 films obtained by the ellipsometric measurement are presented in Fig. 2(a)
Fig. 2 (a) Refractive index (n) and extinction coefficient (k) of P3HT:PCBM70 blend films used in this work. The blend film was treated as if it were a single layer in ellipsometric analysis. (b) Comparison of the transmittance of P3HT:PCBM70 films on a glass calculated with the (n,k) values in (a) with respect to the experimental values.
. It can be easily seen in Fig. 2(b) that the transmittance calculated with the measured optical constants shows a reasonably good agreement with the experimental values.

In addition, A t of the proposed ST-OPV device can be varied by changing the thickness of ZnS layers (d ZnS) as presented in Fig. 3(c) showing A t calculated at the wavelength of 500 nm as a function of d ZnS. This is mainly because the net transmittance and reflectance (both external and internal) of the ZAW electrode is influenced sensitively by d ZnS [16

16. S. Han, S. Lim, H. Kim, H. Cho, and S. Yoo, “Versatile Multilayer Transparent Electrodes for ITO-Free and Flexible Organic Solar Cells,” IEEE J. Sel. Top. Quant. Electron. (available online. doi: ).

]. Similar trend has been reported by Tao et al. in ST-OPV cells with MoO3/Ag/MoO3 top anodes [5

5. C. Tao, G. Xie, C. Liu, X. Zhang, W. Dong, F. Meng, X. Kong, L. Shen, S. Ruan, and W. Chen, “Semitransparent inverted polymer solar cells with MoO3/Ag/MoO3 as transparent electrode,” Appl. Phys. Lett. 95(5), 053303 (2009). [CrossRef]

]. It is noteworthy that A t for ITO-side illumination [≡ A t (ITO)] and that for ZAW-side illumination [≡ A t (ZAW)] vary differently as d ZnS changes. As can be seen in Fig. 3(c), A t (ITO) has its minimum near d ZnS of 20 nm and peaks near d ZnS of 50 nm - 80 nm at λ of 500nm (≡λ 0) while A t (ZAW) exhibits almost opposite trend. Hence, not only the overall values of A t but also the degree of absorption asymmetry or dependence of A t on the illumination direction can be tuned by d ZnS. It is also noted that A t (ITO) (λ 0) is always larger than A t (ZAW) (λ 0) regardless of d ZnS. This is consistent with the report made by Pandey and Samuel [22

22. A. K. Pandey and I. D. W. Samuel, “Photophysics of solution-processed transparent solar cells under top and bottom illumination.” IEEE J. Sel. Top. Quant. Electron. (available online. doi: ).

], and is regarded to come from the fact that the light enters into the active region with little reflection when the light is incident on the ITO side. The effective optical path length can also be larger in this case due to the internal reflection at the active (buffer)/ metal interface.

Figure 4
Fig. 4 Calculated photocurrent density (J ph) for AM1.5G (1sun) illumination vs. the thickness of the ZnS layer (d ZnS) for each illumination direction. η QE of 80% assumed in Eq. (1).
presents the calculated photocurrent density (J ph) of the present devices as a function of d ZnS. The calculation was done for each illumination direction using the following relationship of J ph to absorption in active layers (A active):
Jph=eηQENAM1 .5G(λ)Aactive(λ;dZnS)dλ
(1)
in which e is the electronic charge, η QE is the quantum efficiency regarding exciton-to-carrier generation and N AM1.5G(λ) is the solar spectral photon flux density at λ under AM 1.5G (1Sun) condition. It can be easily seen that its trend over d ZnS is in fact very similar to that of A t (λ 0), although the calculation of J ph involves the integration over the whole spectrum, because P3HT:PCBM70 films have a major absorption near λ 0.

From the results shown above, one can now set the two distinctive optimization strategies according to the needs of a target application: (i) in applications where the orientation of an ST-OPV cell is fixed with respect to the position of a dominant illumination source, as in solar windows in building-integrated PV (BIPV) applications, d ZnS can be adjusted so that the asymmetry in A t is strengthened and that J ph is maximized for the illumination direction allowing for a higher A t ( = ITO-side in this work), at the expense of a reduced J ph in the opposite direction (≡ type-I optimization scheme). One can then let ST-OPV cells face the source in the illumination direction that allows for a higher A t and J ph; (ii) in applications where the orientation of an ST-OPV cell is not fixed with respect to a light source (≡ type-II application), d ZnS is chosen so that the asymmetry in A t is minimized and that J ph is balanced for both illumination directions (≡ type-II optimization scheme).

4. Results and discussions

Figure 5
Fig. 5 Experimental J-V characteristics of ST-OPV cells under study with d ZnS of (a) 50nm and (b) 20nm. That of a control inverted OPV cell with the same device structure except for the opaque anode [ = WO3(13 nm)/ Al (70 nm)] is also shown for comparison (dashed line).
presents the experimental current density (J) – voltage (V) characteristics of the ST-OPV cells with a d ZnS of 50 nm and 20 nm. Recall that they can be regarded as ST-OPV cells optimized under type I and type II schemes, respectively. (See Fig. 4). J-V of the best device among 13 different devices per each case is provided. The average PV parameters of the ST-OPV cells under study are also given in Table 1

Table 1. Average performance of semitransparent inverted OPV cells*

table-icon
View This Table
.

For the cell with d ZnS of 50 nm, the power conversion efficiency (η) for the ITO-side illumination was 3.7% (3.1% considering the mismatch factor with respect to the true AM1.5G; Refer to the following paragraph for further discussion), which corresponds to > 80-85% of that of our typical inverted cells with the opaque metal electrode. Note that this level of efficiency is among the highest that has been reported to date in ST-OPV cells based on P3HT:PCBM70 layers [7

7. J. Huang, G. Li, and Y. Yang, “A semi-transparent plastic solar cell fabricated by a lamination process,” Adv. Mater. 20(3), 415–419 (2008). [CrossRef]

]. On the other hand, η of the same cell measured for the ZAW-side illumination was only 1.4% in average, showing a severe asymmetry in J ph. For d ZnS of 20 nm, η was 2.8% and 1.8% for ITO-side and ZAW-side illumination, respectively, showing relatively good balance. Both of the results show a good agreement with the prediction made in Fig. 4, confirming the effectiveness of the proposed optimization strategies.

In order to check the spectrally resolved validity of the proposed analysis and to estimate J sc and η under the true AM1.5G illumination (≡ J sc(AM1.5G) and η (AM1.5G)), an additional batch of samples were prepared to measure their EQE spectra (Fig. 6
Fig. 6 Experimental and simulated EQE spectra of ST-OPV cells under study with d ZnS of (a) 50nm and (b) 20nm. That of a control inverted OPV cell with the same device structure except for the opaque anode [ = WO3(13 nm)/ Al (70 nm)] is also shown for comparison (grey hexagon). Note: EQE spectra were measured for a batch different from those used in Fig. 5. This batch of cells had a J sc of 9.9 (Ref), 8.6 (50nm; ITO), 4.1 (50nm; ZAW), 7.4 (20nm; ITO), 5.4 (20nm; ZAW) mA/cm2, respectively, under the illumination from the solar simulator.
). Comparison of J sc and η measured under illumination from the solar simulator with respect to those estimated from EQE data for the true AM1.5G spectrum indicates that the mismatch factors m of ST-OPVs and opaque reference cells for ITO side illumination are in the range of 0.74-0.85 while those of ST-OPVs for ZAW-side illumination are approximately 0.97. Recall that mismatch factor can differ and is dependent on the spectral response of OPV cells as well as the output spectrum of the solar simulator [23

23. S. E. Shaheen, C. J. Brabec, N. S. Sariciftci, F. Padinger, T. Fromherz, and J. C. Hummelen, “2.5% efficient organic plastic solar cells,” Appl. Phys. Lett. 78(6), 841–843 (2001). [CrossRef]

]. It appears that the spectral output of the simulator used has a tendency to overestimate the response for ITO-side illumination in comparison to that for ZAW-side illumination. Mismatch factors as well as J sc(AM1.5G) and η (AM1.5G) expected for the respective cases are also summarized in Table 1.

Figure 7(a)
Fig. 7 (a) Calculated internal reflectance (R ZAW (int)) at the organic/ ZAW interface for ITO-side illumination. (b) Distribution of the squared magnitude of electric field (≡ |E(z)|2) within ST-OPV cells when d ZnS is 20 nm or 50 nm in case of ITO-side (top figure) or ZAW-side (bottom figure) illumination. Values are normalized to the squared field strength of the incident light (≡ |E 0|2).
presents the internal reflectance R ZAW (int) at the organic/ ZAW interface vs. d ZnS calculated for the light that is incident from the medium of P3HT:PCBM70 to the ZAW-electrode at the wavelengths of 450 nm, 500 nm, and 550 nm, which represent the major absorption band of P3HT:PCBM70 films. One can note that R ZAW (int) becomes largest near a d ZnS of 50-80 nm and becomes lowest near a d ZnS of 20 nm at all those wavelengths in the ST-OPV cells under study, indicating that R ZAW (int) is one of the key factors in controlling the J ph in the present cells. Figure 7(b) shows the distribution of the squared magnitude of the optical electric field [≡ |E(z)|2] inside the present cells calculated at the wavelength (λ) of 500 nm along the symmetry axis (≡ z-axis) for both ITO-side and ZAW-side illumination. Numerical integration of |E(z)|2 over the P3HT:PCBM70 layer is directly proportional to the absorption within the active layer and thus to the available photocurrent at a given wavelength [16

16. S. Han, S. Lim, H. Kim, H. Cho, and S. Yoo, “Versatile Multilayer Transparent Electrodes for ITO-Free and Flexible Organic Solar Cells,” IEEE J. Sel. Top. Quant. Electron. (available online. doi: ).

]. In case of ZAW-side illumination, |E(z)|2 drops almost exponentially as in Beer-Lambert’s law, whether d ZnS is 20 nm or 50 nm, due to the lack of internal reflection from ITO electrodes. In case of ITO-side illumination, however, |E(z)|2 within the active layer varies differently depending on d ZnS: for d ZnS of 20 nm, a profile similar to the ZAW-side illumination is observed because R ZAW (int) is small; for d ZnS of 50 nm, |E(z)|2 is enhanced in the middle of the active layer, improving the absorption of photons by the active layer. This improvement is consistent with the enhanced R ZAW (int) at d ZnS near 50 nm because the reflected field can add up to yield the net improvement in |E(z)|2.

It would be of concern in some applications if the proposed optimization scheme involves a significant drop in T t of ST-OPV cells. As can be seen in Fig. 8
Fig. 8 Measured total transmittance (T t) of the ST-OPV cells. Inset: the photographs of the fabricated devices. Top parts of the finger electrodes correspond to the active regions. Bottom parts have additional 70 nm-thick Al layers for stable electrical contact.
, T t at a specific wavelength indeed shows some variation when d ZnS changes from 20 nm to 50 nm, and therefore, the apparent color looks different [see the inset of Fig. 8 for photographs of the actual samples]; however, T t averaged for the visible spectral range of 400-700 nm (≡ T avg) of the devices with a d ZnS of 20 nm and 50nm were 28.3% and 27.3%, respectively, showing only a small difference. It is also noted that these values are comparable to the typical average transmittance of tinting films used in applications such as automobiles [25].

5. Conclusions

In this study, we have presented semitransparent organic photovoltaic (ST-OPV) cells in inverted geometry in which a top anode is based on a multilayer transparent electrode consisting of ZnS, Ag, and WO3 (ZAW) layers. Upon identification of the asymmetric absorption that is characterized as a higher absorption in the ITO-side illumination, the ST-OPV devices were optimized by changing the thickness of the ZnS layer (d ZnS), for two distinctive objectives: (i) to let the asymmetry be strengthened so that the efficiency for the ITO-side illumination can be further enhanced to its maximum (≡ type-I optimization scheme); or, (ii) to let the asymmetry be lessened so that the efficiencies are balanced for both illumination directions (≡ type-II optimization scheme). The former method led to a P3HT:PCBM70-based ST-OPV cell with efficiency as large as 3.7% (3.1% estimated for the true AM1.5G, 1Sun condition) for the ITO-side illumination, which corresponds to ~80-85% of that of conventional opaque cells. Since the orientation of ST-OPV cells with respect to the major illumination source is fixed in many applications (e.g. building-integrated solar windows), the type I optimization method opens up the opportunity to obtain the most power out of a given ST-OPV cell with its ITO-side facing the major light source.

Acknowledgments

This work was supported in part by the Korea Energy Management Corporation (KEMCO) under the New and Renewable Energy R&D Grant (2008-N-PV08-02), by Korea Institute of Energy Technology Evaluation and Planning (KETEP) in Ministry of Knowledge Economy (MKE) under the New and Renewable Energy R&D Grant (N02090016), by EEWS program of KAIST, and by Korea Iron Steel corporation (KISCO). Authors are also grateful to W. S. Soun for spectroscopic ellipsometry measurement and to Dr. W. S. Shin and J.-U. Park for help in EQE measurement.

References and links

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2.

A. J. Medford, M. R. Lilliedal, M. Jørgensen, D. Aarø, H. Pakalski, J. Fyenbo, and F. C. Krebs, “Grid-connected polymer solar panels: initial considerations of cost, lifetime, and practicality,” Opt. Express 18(S3), A272–A292 (2010). [CrossRef] [PubMed]

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F. C. Krebs, T. D. Nielsen, J. Fyenbo, M. Wadstrøm, and M. S. Pedersen, ““Manufacture, integration and demonstration of polymer solar cells in a lamp for the “Lighting Africa” initiative,” Energy Environ. Sci. 3(5), 512–525 (2010). [CrossRef]

4.

R. Koeppe, D. Hoeglinger, P. A. Troshin, R. N. Lyubovskaya, V. F. Razumov, and N. S. Sariciftci, “Organic solar cells with semitransparent metal back contacts for power window applications,” ChemSusChem 2(4), 309–313 (2009). [CrossRef] [PubMed]

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6.

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A. K. Pandey and I. D. W. Samuel, “Photophysics of solution-processed transparent solar cells under top and bottom illumination.” IEEE J. Sel. Top. Quant. Electron. (available online. doi: ).

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S. E. Shaheen, C. J. Brabec, N. S. Sariciftci, F. Padinger, T. Fromherz, and J. C. Hummelen, “2.5% efficient organic plastic solar cells,” Appl. Phys. Lett. 78(6), 841–843 (2001). [CrossRef]

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OCIS Codes
(160.4890) Materials : Organic materials
(230.0250) Optical devices : Optoelectronics
(230.5170) Optical devices : Photodiodes
(250.2080) Optoelectronics : Polymer active devices
(310.4165) Thin films : Multilayer design
(310.6845) Thin films : Thin film devices and applications

ToC Category:
Photovoltaics

History
Original Manuscript: August 6, 2010
Revised Manuscript: September 4, 2010
Manuscript Accepted: September 6, 2010
Published: September 21, 2010

Citation
Donggeon Han, Hoyeon Kim, Soohyun Lee, Myungsoo Seo, and Seunghyup Yoo, "Realization of efficient semitransparent organic photovoltaic cells with metallic top electrodes: utilizing the tunable absorption asymmetry," Opt. Express 18, A513-A521 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-S4-A513


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References

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