Laser perforated ultrathin metal films for transparent electrode applications Martin Theuring, Volker Steenhoff, Stefan Geißendörfer, Martin Vehse,* Karsten von Maydell and Carsten Agert NEXT ENERGY • EWE Research Centre for Energy Technology at the University of Oldenburg, Carl-von-OssietzkyStr. 15, 26129 Oldenburg, Germany * [email protected]
Abstract: Transmittance and conductivity are the key requirements for transparent electrodes. Many optoelectronic applications require additional features such as mechanical flexibility and cost-efficient fabrication at low temperatures. Here we demonstrate a simple method to fabricate high performance transparent electrodes that is based on perforation of thin silver layers using picosecond laser pulses. Transparent electrodes have been characterized optically and electrically in order to determine the influence of specific surface coverage. Special attention was paid to maintaining sufficient conductivity in the metal-free areas. As a result, transmittance of a much higher bandwidth was achieved as compared to unpatterned metal films. Transparent electrodes have been fabricated on glass and plastic foil, as well as wafer-based silicon heterojunction solar cells, demonstrating their applicability for most relevant cases. ©2015 Optical Society of America OCIS codes: (040.5350) Photovoltaic; (140.3390) Laser materials processing; (160.3900) Metals; (230.4170) Multilayers; (310.6845) Thin film devices and applications; (310.7005) Transparent conductive coatings.
References and links 1.
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13. D. S. Ghosh, T. L. Chen, N. Formica, J. Hwang, I. Bruder, and V. Pruneri, “High figure-of-merit Ag/Al:ZnO nano-thick transparent electrodes for indium-free flexible photovoltaics,” Sol. Energy Mater. Sol. Cells 107, 338–343 (2012). 14. J. Zou, C.-Z. Li, C.-Y. Chang, H.-L. Yip, and A. K.-Y. Jen, “Interfacial engineering of ultrathin metal film transparent electrode for flexible organic photovoltaic cells,” Adv. Mater. 26(22), 3618–3623 (2014). 15. D. R. Sahu and J.-L. Huang, “The properties of ZnO/Cu/ZnO multilayer films before and after annealing in the different atmosphere,” Thin Solid Films 516(2-4), 208–211 (2007). 16. T. Schwab, S. Schubert, S. Hofmann, M. Fröbel, C. Fuchs, M. Thomschke, L. Müller-Meskamp, K. Leo, and M. C. Gather, “Highly efficient color stable inverted white top-emitting OLEDs with ultra-thin wetting layer top electrodes,” Adv. Opt. Mater. 1(10), 707–713 (2013). 17. J. R. Sambles, “The resistivity of thin metal films - some critical remarks,” Thin Solid Films 106(4), 321–331 (1983). 18. H. Han, N. D. Theodore, and T. L. Alford, “Improved conductivity and mechanism of carrier transport in zinc oxide with embedded silver layer,” J. Appl. Phys. 103(1), 013708 (2008). 19. S. Kim, H. W. Cho, K. Hong, J. H. Son, K. Kim, B. Koo, S. Kim, and J.-L. Lee, “Design of red, green, blue transparent electrodes for flexible optical devices,” Opt. Express 22(Suppl 5), A1257–A1269 (2014). 20. 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). 21. A. J. Morfa, E. M. Akinoglu, J. Subbiah, M. Giersig, and P. Mulvaney, “Transparent metal electrodes from ordered nanosphere arrays,” J. Appl. Phys. 114(5), 054502 (2013). 22. T. Gao, B. Wang, B. Ding, J.-K. Lee, and P. W. Leu, “Uniform and ordered copper nanomeshes by microsphere lithography for transparent electrodes,” Nano Lett. 14(4), 2105–2110 (2014). 23. N. P. Logeeswaran Vj, M. S. Kobayashi, W. Islam, P. Wu, N. X. Chaturvedi, S. Y. Fang, Wang, and R. S. Williams, “Ultrasmooth silver thin films deposited with a germanium nucleation layer,” Nano Lett. 9(1), 178– 182 (2009). 24. S. Liu, W. Liu, J. Yu, W. Zhang, L. Zhang, X. Wen, Y. Yin, and W. Xie, “Silver/germanium/silver: an effective transparent electrode for flexible organic light-emitting devices,” J. Mater. Chem. C 2(5), 835 (2014). 25. M. Theuring, S. Geißendörfer, M. Vehse, K. von Maydell, and C. Agert, “Thin metal layer as transparent electrode in n-i-p amorphous silicon solar cells,” EPJ Photovoltaics 5, 55205 (2014). 26. O. Madani Ghahfarokhi, K. von Maydell, and C. Agert, “Enhanced passivation at amorphous/crystalline silicon interface and suppressed Schottky barrier by deposition of microcrystalline silicon emitter layer in silicon heterojunction solar cells,” Appl. Phys. Lett. 104(11), 113901 (2014). 27. B. N. Chichkov, C. Momma, S. Nolte, F. Alvensleben, and A. Tünnermann, “Femtosecond, picosecond and nanosecond laser ablation of solids,” Appl. Phys., A Mater. Sci. Process. 63(2), 109–115 (1996). 28. K.-H. Leitz, B. Redlingshöfer, Y. Reg, A. Otto, and M. Schmidt, “Metal ablation with short and ultrashort laser pulses,” Phys. Procedia 12, 230–238 (2011). 29. A. V. Shah, H. Schade, M. Vanecek, J. Meier, E. Vallat-Sauvain, N. Wyrsch, U. Kroll, C. Droz, and J. Bailat, “Thin-film silicon solar cell technology,” Prog. Photovolt. Res. Appl. 12(23), 113–142 (2004). 30. P.-C. Hsu, S. Wang, H. Wu, V. K. Narasimhan, D. Kong, H. Ryoung Lee, and Y. Cui, “Performance enhancement of metal nanowire transparent conducting electrodes by mesoscale metal wires,” Nat. Commun. 4, 2522 (2013).
1. Introduction Transparent electrodes are essential components in many optoelectronic devices such as solar cells and displays. Two of the main criteria for these electrodes are a low sheet resistance and a high transmittance in a specific spectral range. However, for many applications more features are required: e. g. for solar cells (being mass-produced components) fabrication costs are of great importance in order to ensure price competitiveness with other energy technologies. Furthermore, low temperature fabrication and mechanical flexibility are desired because of a rising demand for electronic devices on plastic substrates. Indium tin oxide and other transparent conducting oxides [1] have been the state-of-the-art solution for both the display and photovoltaic industry. However, material costs, mechanical film stability issues and the high annealing or fabrication temperatures [2] required for maximum performance, hinder their application and have caused an increasing interest in alternative approaches [3]. A promising technology was found with transparent electrodes based on metal nanostructures. Metal nanowires can be chemically synthesized [4] and solution processed by e. g. spray-coating [5] or brush-painting [6] to form a conducting nanomesh. However, an uneven topography may cause electrical shunting of overlying thin films [7]. Further problems arise from wire-to-wire junction resistances, which require additional treatments of the nanomesh [8]. These drawbacks can be overcome by creating more defined metal
#232537 - $15.00 USD (C) 2015 OSA
Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A255
gratings. But this approach has not yet been transferred to large scale production because it depends on complex processing steps such as photolithography [9,10] or nanoimprinting [11]. Transparent electrodes based on ultrathin metal films (UTMFs) have also attracted considerable attention [12–16]. Despite the high reflection coefficient of metal surfaces, UTMFs exhibit optical transmittance due to destructive interference of waves reflected at their front surface with evanescent waves reflected at their back side. For this technology, established thin film deposition methods can be adopted without the need for elaborate structuring, allowing easy and cost efficient processing. However, for UTMFs the conductivity is limited by the clusterized growth of the material and electron scattering at the grain boundaries and material interfaces [17,18]. On the other hand, thicker layers cause higher reflectivities as the exponential decay of the evanescent waves in the metal impedes destructive interference. Additional antireflection layers are typically employed but these introduce strong wavelength dependence of the transmittance. Therefore, the application of UTMFs is preferred for electrodes requiring transmittance only in a confined spectral range [19,20]. In this paper, we propose a new type of transparent electrode based on UTMFs. The established technique is extended in such a manner that metal is removed in parts after its deposition using a pulsed laser. Compared to the known approach using nanosphere lithography [21,22], our method offers a higher degree of freedom in terms of the perforation pattern and requires fewer processing steps. In an example, we investigate a Ag layer perforated with holes in a hexagonal lattice (see Fig. 1(a)). The additional laser processing significantly enhances the transmittance of the metal film without overly degrading its conductivity. With our method, it is possible to overcome the spectral confinement of the known approach which makes it suitable for a much wider range of applications. 2. Experiment AF 32® eco thin glass from Schott AG and a polyethylene terephthalate (PET) plastic foil were used as substrates. All Ag films are of 15 nm thickness and were electron beam evaporated at a rate of 2 Å/s. The thickness was chosen to exceed the phase of island formation for very thin Ag films and thus to obtain a continuous layer. To improve metal adhesion on glass, an amorphous germanium layer was applied prior to the Ag evaporation using plasma enhanced chemical vapor deposition (precursor gases: GeH4 and H2) [23,24]. For the metal removal, a pulsed laser beam is guided line by line over the substrate using a scanning mirror (see sketch in Fig. 1(a)). A position offset for each second line was applied to create a hexagonal hole lattice. Perforation was carried out in atmospheric conditions using the laser processing tool microSTRUCTvario (3D Micromac AG) with a built-in diodepumped Nd:YVO4 solid-state picosecond laser. This type of laser is used in solar cell fabrication for monolithic interconnection and therefore suitable for high throughput application. While the laser power was varied, wavelength (1064 nm), pulse duration (8.5 ps), repetition rate (100 kHz) and scanning speed (6 cm·s−1) were constant for all experiments except for the pulse duration comparison. Aluminum doped zinc oxide (AZO) was deposited in a DC magnetron sputtering system (Al concentration 2 wt%) without previous substrate heating. The same DC magnetron sputtering system was used for the ITO deposition. A ceramic ITO target with 10 wt% SnO2 and a substrate heating temperature of 200°C were applied for this purpose. Silicon thin film solar cells were fabricated in the p-i-n configuration in a similar manner as previously described [25]. The active cell area of 9 mm2 was defined by evaporating Ag contacts through a mask. The fabrication of silicon heterojunction solar cells is based on an unstructured c-Si wafer and was previously described in great detail [26]. AZO and Ag layers for the front contact were deposited on a mask with 25 mm2 square openings, drawn with a marker pen. A lift-off was performed by dissolving the marker in acetone.
#232537 - $15.00 USD (C) 2015 OSA
Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A256
For the transmittance measurements, samples were positioned in front of an integrating sphere, which was attached to a spectrophotometer (Varian Cary 5000). Sheet resistances were measured with the four point measurement method (Jandel RM3-AR). We used currentvoltage measurements at 1000 W/m2 illumination (Wacom Super Solar simulator) to measure open circuit voltages and fill factors. Short circuit current densities were obtained by convoluting the external quantum efficiency with the sun spectrum (AM1.5G). The electrical simulations were performed with the software Sentaurus TCAD (by Synopsys Inc.), which solves Poisson's equation and the drift/diffusion of the charge carriers. The resistivity in the simulation was scaled to the value experimentally determined for the 100% coverage sample.
Fig. 1. (a) Sketch of the metal layer laser perforation. (b) Scanning electron micrograph of Ag layer exposed to ns laser pulse (i) and to ps laser pulse (ii), respectively (scale bar: 1 µm). (c) Optical microscope image of perforated Ag layers using laser powers of 0.31 W (i), 0.74 W (ii), 1.32 W (iii) and 2.08 W (iv) (scale bar: 50 µm). (d) Photographs of laser perforated Ag layers on glass (i) and plastic substrate (ii).
3. Results and discussions In preliminary experiments we compared nanosecond and picosecond laser pulse ablation of a thin Ag film on an AZO layer. Scanning electron micrographs (see Fig. 1(b)) reveal that a fundamental difference exists between these two regimes [27,28]. For pulse ablation in the nanosecond timescale (i), relatively long light-matter interaction allows a thermal energy transport into the metal, leading to material melting and particle formation. In the picosecond regime (ii), material is rapidly evaporated and heat conduction to the lattice of metal ions can be neglected. As a result, even in the rim of the hole, little melting damage or material accumulation occurs and a clean circular area is detached. Figure 1(c) shows microscope images of ps-laser pulse metal layer ablation on a glass substrate using four different laser powers. As the lateral beam profile can be approximated by a 2D Gaussian function and the metal is removed when a certain threshold power is exceeded, the hole size changes with the laser power. Almost circular spots ordered in a hexagonal lattice are obtained with the hole diameter depending on the laser power settings. Photographs of laser perforated Ag films on glass (i) and plastic foil (ii) are depicted in Fig. 1(d). The varying transmittance of the individual squares originates from different laser powers applied for the perforation. The transmittance measurements of Ag layers on glass and transparent foil for selected metal coverage are shown in Fig. 2(a) and 2(b), respectively (solid lines). The corresponding values for coverage, sheet resistance and laser power are summarized in the inset tables. The metal covered area can be calculated using the equation #232537 - $15.00 USD (C) 2015 OSA
Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A257
Coverage = 1 −
AHoles 2π rH 2 = 1− ATotal 3d P 2
(1)
with AHoles and ATotal defined as the hole covered area and the total area, respectively, rH for the hole radius and dP for the hole periodicity or lattice constant. Starting with the unperforated sample, a decrease of the metal coverage leads to an increase in the transmittance of the sample. Particularly for higher wavelengths, the improvement is significant, yielding a transmittance of almost 80% for the glass sample (21% metal coverage) and slightly lower values for the perforated Ag on foil. As the lateral dimensions of the holes are much larger than the optical wavelengths, the transmittance of the hole-perforated metal layers TCoverage should be predictable by a weighted mean from the uncovered (T0%) and the fully covered substrate (T100%):
TCoverage = Coverage ⋅ T100% + (1 − Coverage) ⋅ T0% .
(2)
The calculated values are plotted as dashed lines along with the measured curves in Fig. 2(a) and 2(b), respectively. For the infrared part of the spectrum, a good match is found with the experimental data. However, especially for the glass samples with lower metal coverage and for wavelengths below 600 nm, measurements deviate from the prediction. In this range, the localized surface plasmon resonance of Ag nanostructures on glass can be found [9]. As described above, small particles and damages to the metal film are present in the proximity of the hole edge, leading to an increased scattering and absorption for the corresponding plasmon resonance wavelengths. However, the drop in transmittance is below 5% and could possibly be further reduced by a surface cleaning procedure, an optimized laser processing (e.g. using shorter laser pulses) or by choosing a metal with different plasma frequency.
Fig. 2. Transmittance measurements (solid lines) of perforated Ag layers on glass (a) and on plastic foil (b). Dashed lines represent the transmission calculated with Eq. (2) using transmittance data of the 100% and 0% coverage samples. The inset table lists the laser power, the metal coverage and the measured sheet resistance RSQ of the corresponding samples. (c) Simulated and measured sheet resistance for perforated Ag layer. The inset figure shows the simulation domain. (d) Series resistance induced power loss for a solar cell of circular shape with rim contact for different cell radii as a function of the sheet resistance, calculated using Eq. (4) with JMPP = 40 mA/cm2 and VMPP = 0.6 V.
#232537 - $15.00 USD (C) 2015 OSA
Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A258
Concerning the electrical properties of the transparent electrode, the partially removed Ag naturally yields reduced sheet conductivity. In order to determine to what extent the metal layer can be removed without entirely degrading the conducting properties, we simulated the structure’s sheet resistance. A priori, we can calculate the minimum metal coverage for a circle packing of perfectly round holes by setting rH = 0.5·dP in Eq. (1). With this geometry, a metal coverage of 9.31% is calculated, which leaves no conducting paths between the holes. Hence, we start our simulation at a minimum coverage of 10% in order to generate relevant conductivity in the plane. The simulation results are plotted in Fig. 2(c) (inset shows the simulation domain). We observe a strong decrease of the sheet resistance for higher metal coverage due to an increased width of the tapered areas between the holes. The results are in good agreement with the measured sheet resistances of the devices shown in the Fig. 2(a) and 2(b). Deviation is only found for lower coverage, where both the glass and the foil sample, show higher values. We attribute this to small misalignments in the lattice and noncircularity of the holes leading to the occurrence of narrower or even disconnected conducting paths (see microscope image (iv) in Fig. 1(c)). Yet, in this configuration sheet resistances below 20 Ω are achievable with a metal coverage of less than 40%. A very important fact for transparent electrodes based on grating structures is that for many applications conductivity is also required in the openings of the grating. In the case presented here, the hole areas have diameters in the range of several tens of micrometers and in many devices (e.g. silicon thin film solar cells [29]), the conductivity of the active material is not sufficient to overcome this distance without substantial ohmic losses. Therefore, we estimate the influence of the conductivity in the hole region using the example of a solar cell. If we assume homogeneous current generation, perfectly conducting metal areas and no lateral conduction of the absorber, the power loss PLoss in the hole region due to the contact resistance and a corresponding loss ratio for the case of solar cell operation at the maximum power point (MPP) are calculated as follows rH
2
PLoss = ( J π r 2 ) ⋅ RS 0
Loss Ratio =
dr 2π r
PLoss RS J MPP rH 2 = PMPP 8VMPP
(3) (4)
with the current density J (JMPP at the MPP), voltage at the MPP, VMPP, sheet resistance, RS, and the hole radius, rH. For solar cells, the assumption of homogeneous current generation can be valid only as long as the voltage drop due to the series resistance is very small compared to VMPP, i.e. for low loss ratios. However, Eq. (4) allows a rough estimation of the limitations on hole size and sheet resistance. In an example, we study a worst case scenario for transparent electrodes in photovoltaic applications, which would, as it is apparent from Eq. (4), be a high current, low voltage solar cell design. Silicon heterojunction solar cells operate at the maximum power point with current densities and voltages of approximately JMPP ≈40 mA/cm2 and VMPP ≈0.6 V respectively. Figure 2(d) shows the loss ratio of such a solar cell for different hole radii as a function of the sheet resistance in the opening. For a radius up to 100 µm, a layer with a sheet resistance of 10 kΩ in the area of the hole suffices to keep the efficiency loss below 1%. However, our results show that, depending on the grating dimensions, additional conductivity in the metal-free areas is mandatory to minimize the efficiency loss. Highly transparent and/or cost effective materials, such as low-cost transparent conducting oxides or conducting polymers are established technologies in the thin film industry. In combination with a perforated metal film, layers from such materials would not only enable fully conductive surfaces, but could also provide further functionalities, such as encapsulation or anti-reflection properties.
#232537 - $15.00 USD (C) 2015 OSA
Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A259
Figure 3(a) shows a sketch of a perforated metal film, encapsulated in between two 35 nm thick AZO layers. This layout is a widely known example of transparent electrodes based on UTMFs [12]. The AZO layers are commonly applied for the purpose of anti-reflection but can also function as a physical and chemical barrier for the metal. The anti-reflection properties of the AZO layer are demonstrated by the transmittance measurements presented in Fig. 3(b). Compared to an uncovered metal layer (Fig. 2 (a)), the AZO significantly increases the transmittance for wavelengths around 450 nm. This result is in line with previous studies on UTMF transparent electrodes [13,14]. A decrease of metal coverage by laser perforation shifts the measurement curves towards the metal free sample (0% metal coverage, only AZO) and broadband transmittance is achieved for low coverage values. In the presented case of a perforated silver film, the AZO also provides conductivity in the hole area (sheet resistance of the AZO: RS ≈1550 Ω). With this configuration, conductivity is preserved throughout the entire structure. Although AZO and AZO/Ag/AZO layers in this paper are not fully optimized, the measurements of the laser-perforated metal films clearly indicate the prospects of our approach. Only a slight modification in the fabrication process of established UTMF transparent electrodes is necessary to significantly enhance the spectral bandwidth of such structures’ transmittance.
Fig. 3. Sketch (a) and transmittance measurements (b) of a perforated Ag layer encapsulated in an AZO layer with a total thickness of 70 nm.
Fig. 4. Sketch (a) and external quantum efficiency (b) of amorphous silicon solar cells, fabricated on transparent electrodes based on unperforated and perforated Ag layers and 70 nm encapsulation layers.
To provide a working example of the presented concept, we demonstrate its application in a silicon thin film solar cell. An optimized quarter-wave anti-reflection for an amorphous silicon layer can be realized with an AZO layer of approximately 70 nm [25]. To achieve
#232537 - $15.00 USD (C) 2015 OSA
Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A260
optimized light incoupling, a UTMF would be integrated not in the middle but on one side of the TCO (see sketch in Fig. 4(a)) [25]. Figure 4(b) shows the quantum efficiency measured for the described device with varying metal coverage. For wavelengths around 450 nm the sample with 100% coverage exhibits slightly higher EQE, which originates from the cavity resonance of the front contact. For all other wavelengths, the perforation is strongly beneficial for the efficiency with only a moderate influence on the electrical properties of the cell (fill factor decrease of 7.3%). An increase of the short circuit current density from 6.2 mA/cm2 (100% coverage) to 9.2 mA/cm2 (24% coverage) is measured, resulting in an overall power conversion efficiency (η) enhancement of more than 36% (η100% ≈3.6% compared to η24% ≈5%). A metal-free transparent electrode sample was not fabricated as the insufficient conductivity of the thin AZO layer as a front contact would not allow an efficient current collection [25]. Besides the laser structuring of UTMFs on transparent substrates, the perforation can also be performed directly on the device. We demonstrate this by furnishing a silicon heterojunction solar cell with a transparent electrode consisting of a 70 nm layer of AZO and 15 nm of Ag (see cell design in Fig. 5(a)). The measured EQE of the device with different coverages is shown in Fig. 5(b). For the unstructured Ag layer (100% curve), the efficiency is suppressed by the reflection of light at the Ag interface, especially of higher wavelengths. Decreasing the Ag coverage by laser-perforation yields a significant improvement of the quantum efficiency. With an 18% metal coverage, the photo current density is almost tripled from 11.6 mA/cm2 ( 1 0 0 % ) t o 3 0 . 7 m A / c m 2 ( η100% ≈4.7% compared to η18% ≈11.7%), illustrating the broadband transmittance of our transparent electrode solution. The electrical parameters of the cells are affected by the laser perforation only in a minor way. The fill factor is reduced from 69.3% (100% metal coverage) to 67.2% (18% metal coverage) and the open circuit voltage from 0.583 V (100%) to 0.567 V (18%) respectively. Both parameters are decreased only by about 3%, which demonstrates the applicability of the approach for this type of solar cell. For comparison, the EQE of an identical cell with a 70 nm ITO front contact is plotted in Fig. 5(b). The reference shows a slightly higher quantum efficiency with a short circuit current density of 34.2 mA/cm2. The optical performance of the perforated Ag transparent electrode could be further improved by positioning the UTMF adjacent to the silicon. However, Ag diffusion into the p-doped a-Si layer would strongly deteriorate the electrical properties of the cell, causing the need for a chemical barrier layer.
Fig. 5. Sketch (a) and external quantum efficiency measurements (b) from silicon heterojunction solar cells, furnished with AZO (70 nm) / perforated Ag layer and ITO (70 nm) transparent electrodes.
4. Conclusion
In conclusion, we have shown a new transparent electrode design based on laser structured UTMFs, which combines high optical transmittance with low sheet resistance and a simple
#232537 - $15.00 USD (C) 2015 OSA
Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A261
fabrication process. Using our method, it is possible to overcome the restrictions of established metal nanostructure transparent electrodes, such as confined spectral transmittance (UTMF) or complex fabrication (nanogratings). It is worth mentioning that material combination and applied perforation pattern, as discussed in this paper, are only one possible embodiment of the presented concept. Specifically, metals of lower cost (e.g. copper) have been demonstrated in both grid based [30] and UTMF transparent electrodes [15]. Neither is the laser processed metal structure limited to a hexagonal hole array. For instance, in the case of solar cell applications, a conceivable pattern would consist of microscopic tapered fingers, which features increasing sheet conductivity in the direction of the current flow. We are confident that the various design possibilities and optimization potentials of our approach will allow the development of a scalable and cost-efficient technology to fabricate versatile, high performing transparent electrodes. Acknowledgment
We would like to thank Omid Madani Ghahfarokhi, Antje Schweitzer and Ortwin Siepmann for discussions and sample fabrication, Richard Kessell, Stefan Strauf and Pang-Chieh Sui for proofreading and Thomas Faasch for the help with the photographs and the hand modeling. Also, the authors acknowledge financial support of the German Federal Ministry of Education and Research (BMBF) in the framework of the project SiSoFlex (FKZ 03SF0418B).
#232537 - $15.00 USD (C) 2015 OSA
Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A262
Abstract: Transmittance and conductivity are the key requirements for transparent electrodes. Many optoelectronic applications require additional features such as mechanical flexibility and cost-efficient fabrication at low temperatures. Here we demonstrate a simple method to fabricate high performance transparent electrodes that is based on perforation of thin silver layers using picosecond laser pulses. Transparent electrodes have been characterized optically and electrically in order to determine the influence of specific surface coverage. Special attention was paid to maintaining sufficient conductivity in the metal-free areas. As a result, transmittance of a much higher bandwidth was achieved as compared to unpatterned metal films. Transparent electrodes have been fabricated on glass and plastic foil, as well as wafer-based silicon heterojunction solar cells, demonstrating their applicability for most relevant cases. ©2015 Optical Society of America OCIS codes: (040.5350) Photovoltaic; (140.3390) Laser materials processing; (160.3900) Metals; (230.4170) Multilayers; (310.6845) Thin film devices and applications; (310.7005) Transparent conductive coatings.
References and links 1.
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Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A254
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1. Introduction Transparent electrodes are essential components in many optoelectronic devices such as solar cells and displays. Two of the main criteria for these electrodes are a low sheet resistance and a high transmittance in a specific spectral range. However, for many applications more features are required: e. g. for solar cells (being mass-produced components) fabrication costs are of great importance in order to ensure price competitiveness with other energy technologies. Furthermore, low temperature fabrication and mechanical flexibility are desired because of a rising demand for electronic devices on plastic substrates. Indium tin oxide and other transparent conducting oxides [1] have been the state-of-the-art solution for both the display and photovoltaic industry. However, material costs, mechanical film stability issues and the high annealing or fabrication temperatures [2] required for maximum performance, hinder their application and have caused an increasing interest in alternative approaches [3]. A promising technology was found with transparent electrodes based on metal nanostructures. Metal nanowires can be chemically synthesized [4] and solution processed by e. g. spray-coating [5] or brush-painting [6] to form a conducting nanomesh. However, an uneven topography may cause electrical shunting of overlying thin films [7]. Further problems arise from wire-to-wire junction resistances, which require additional treatments of the nanomesh [8]. These drawbacks can be overcome by creating more defined metal
#232537 - $15.00 USD (C) 2015 OSA
Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A255
gratings. But this approach has not yet been transferred to large scale production because it depends on complex processing steps such as photolithography [9,10] or nanoimprinting [11]. Transparent electrodes based on ultrathin metal films (UTMFs) have also attracted considerable attention [12–16]. Despite the high reflection coefficient of metal surfaces, UTMFs exhibit optical transmittance due to destructive interference of waves reflected at their front surface with evanescent waves reflected at their back side. For this technology, established thin film deposition methods can be adopted without the need for elaborate structuring, allowing easy and cost efficient processing. However, for UTMFs the conductivity is limited by the clusterized growth of the material and electron scattering at the grain boundaries and material interfaces [17,18]. On the other hand, thicker layers cause higher reflectivities as the exponential decay of the evanescent waves in the metal impedes destructive interference. Additional antireflection layers are typically employed but these introduce strong wavelength dependence of the transmittance. Therefore, the application of UTMFs is preferred for electrodes requiring transmittance only in a confined spectral range [19,20]. In this paper, we propose a new type of transparent electrode based on UTMFs. The established technique is extended in such a manner that metal is removed in parts after its deposition using a pulsed laser. Compared to the known approach using nanosphere lithography [21,22], our method offers a higher degree of freedom in terms of the perforation pattern and requires fewer processing steps. In an example, we investigate a Ag layer perforated with holes in a hexagonal lattice (see Fig. 1(a)). The additional laser processing significantly enhances the transmittance of the metal film without overly degrading its conductivity. With our method, it is possible to overcome the spectral confinement of the known approach which makes it suitable for a much wider range of applications. 2. Experiment AF 32® eco thin glass from Schott AG and a polyethylene terephthalate (PET) plastic foil were used as substrates. All Ag films are of 15 nm thickness and were electron beam evaporated at a rate of 2 Å/s. The thickness was chosen to exceed the phase of island formation for very thin Ag films and thus to obtain a continuous layer. To improve metal adhesion on glass, an amorphous germanium layer was applied prior to the Ag evaporation using plasma enhanced chemical vapor deposition (precursor gases: GeH4 and H2) [23,24]. For the metal removal, a pulsed laser beam is guided line by line over the substrate using a scanning mirror (see sketch in Fig. 1(a)). A position offset for each second line was applied to create a hexagonal hole lattice. Perforation was carried out in atmospheric conditions using the laser processing tool microSTRUCTvario (3D Micromac AG) with a built-in diodepumped Nd:YVO4 solid-state picosecond laser. This type of laser is used in solar cell fabrication for monolithic interconnection and therefore suitable for high throughput application. While the laser power was varied, wavelength (1064 nm), pulse duration (8.5 ps), repetition rate (100 kHz) and scanning speed (6 cm·s−1) were constant for all experiments except for the pulse duration comparison. Aluminum doped zinc oxide (AZO) was deposited in a DC magnetron sputtering system (Al concentration 2 wt%) without previous substrate heating. The same DC magnetron sputtering system was used for the ITO deposition. A ceramic ITO target with 10 wt% SnO2 and a substrate heating temperature of 200°C were applied for this purpose. Silicon thin film solar cells were fabricated in the p-i-n configuration in a similar manner as previously described [25]. The active cell area of 9 mm2 was defined by evaporating Ag contacts through a mask. The fabrication of silicon heterojunction solar cells is based on an unstructured c-Si wafer and was previously described in great detail [26]. AZO and Ag layers for the front contact were deposited on a mask with 25 mm2 square openings, drawn with a marker pen. A lift-off was performed by dissolving the marker in acetone.
#232537 - $15.00 USD (C) 2015 OSA
Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A256
For the transmittance measurements, samples were positioned in front of an integrating sphere, which was attached to a spectrophotometer (Varian Cary 5000). Sheet resistances were measured with the four point measurement method (Jandel RM3-AR). We used currentvoltage measurements at 1000 W/m2 illumination (Wacom Super Solar simulator) to measure open circuit voltages and fill factors. Short circuit current densities were obtained by convoluting the external quantum efficiency with the sun spectrum (AM1.5G). The electrical simulations were performed with the software Sentaurus TCAD (by Synopsys Inc.), which solves Poisson's equation and the drift/diffusion of the charge carriers. The resistivity in the simulation was scaled to the value experimentally determined for the 100% coverage sample.
Fig. 1. (a) Sketch of the metal layer laser perforation. (b) Scanning electron micrograph of Ag layer exposed to ns laser pulse (i) and to ps laser pulse (ii), respectively (scale bar: 1 µm). (c) Optical microscope image of perforated Ag layers using laser powers of 0.31 W (i), 0.74 W (ii), 1.32 W (iii) and 2.08 W (iv) (scale bar: 50 µm). (d) Photographs of laser perforated Ag layers on glass (i) and plastic substrate (ii).
3. Results and discussions In preliminary experiments we compared nanosecond and picosecond laser pulse ablation of a thin Ag film on an AZO layer. Scanning electron micrographs (see Fig. 1(b)) reveal that a fundamental difference exists between these two regimes [27,28]. For pulse ablation in the nanosecond timescale (i), relatively long light-matter interaction allows a thermal energy transport into the metal, leading to material melting and particle formation. In the picosecond regime (ii), material is rapidly evaporated and heat conduction to the lattice of metal ions can be neglected. As a result, even in the rim of the hole, little melting damage or material accumulation occurs and a clean circular area is detached. Figure 1(c) shows microscope images of ps-laser pulse metal layer ablation on a glass substrate using four different laser powers. As the lateral beam profile can be approximated by a 2D Gaussian function and the metal is removed when a certain threshold power is exceeded, the hole size changes with the laser power. Almost circular spots ordered in a hexagonal lattice are obtained with the hole diameter depending on the laser power settings. Photographs of laser perforated Ag films on glass (i) and plastic foil (ii) are depicted in Fig. 1(d). The varying transmittance of the individual squares originates from different laser powers applied for the perforation. The transmittance measurements of Ag layers on glass and transparent foil for selected metal coverage are shown in Fig. 2(a) and 2(b), respectively (solid lines). The corresponding values for coverage, sheet resistance and laser power are summarized in the inset tables. The metal covered area can be calculated using the equation #232537 - $15.00 USD (C) 2015 OSA
Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A257
Coverage = 1 −
AHoles 2π rH 2 = 1− ATotal 3d P 2
(1)
with AHoles and ATotal defined as the hole covered area and the total area, respectively, rH for the hole radius and dP for the hole periodicity or lattice constant. Starting with the unperforated sample, a decrease of the metal coverage leads to an increase in the transmittance of the sample. Particularly for higher wavelengths, the improvement is significant, yielding a transmittance of almost 80% for the glass sample (21% metal coverage) and slightly lower values for the perforated Ag on foil. As the lateral dimensions of the holes are much larger than the optical wavelengths, the transmittance of the hole-perforated metal layers TCoverage should be predictable by a weighted mean from the uncovered (T0%) and the fully covered substrate (T100%):
TCoverage = Coverage ⋅ T100% + (1 − Coverage) ⋅ T0% .
(2)
The calculated values are plotted as dashed lines along with the measured curves in Fig. 2(a) and 2(b), respectively. For the infrared part of the spectrum, a good match is found with the experimental data. However, especially for the glass samples with lower metal coverage and for wavelengths below 600 nm, measurements deviate from the prediction. In this range, the localized surface plasmon resonance of Ag nanostructures on glass can be found [9]. As described above, small particles and damages to the metal film are present in the proximity of the hole edge, leading to an increased scattering and absorption for the corresponding plasmon resonance wavelengths. However, the drop in transmittance is below 5% and could possibly be further reduced by a surface cleaning procedure, an optimized laser processing (e.g. using shorter laser pulses) or by choosing a metal with different plasma frequency.
Fig. 2. Transmittance measurements (solid lines) of perforated Ag layers on glass (a) and on plastic foil (b). Dashed lines represent the transmission calculated with Eq. (2) using transmittance data of the 100% and 0% coverage samples. The inset table lists the laser power, the metal coverage and the measured sheet resistance RSQ of the corresponding samples. (c) Simulated and measured sheet resistance for perforated Ag layer. The inset figure shows the simulation domain. (d) Series resistance induced power loss for a solar cell of circular shape with rim contact for different cell radii as a function of the sheet resistance, calculated using Eq. (4) with JMPP = 40 mA/cm2 and VMPP = 0.6 V.
#232537 - $15.00 USD (C) 2015 OSA
Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A258
Concerning the electrical properties of the transparent electrode, the partially removed Ag naturally yields reduced sheet conductivity. In order to determine to what extent the metal layer can be removed without entirely degrading the conducting properties, we simulated the structure’s sheet resistance. A priori, we can calculate the minimum metal coverage for a circle packing of perfectly round holes by setting rH = 0.5·dP in Eq. (1). With this geometry, a metal coverage of 9.31% is calculated, which leaves no conducting paths between the holes. Hence, we start our simulation at a minimum coverage of 10% in order to generate relevant conductivity in the plane. The simulation results are plotted in Fig. 2(c) (inset shows the simulation domain). We observe a strong decrease of the sheet resistance for higher metal coverage due to an increased width of the tapered areas between the holes. The results are in good agreement with the measured sheet resistances of the devices shown in the Fig. 2(a) and 2(b). Deviation is only found for lower coverage, where both the glass and the foil sample, show higher values. We attribute this to small misalignments in the lattice and noncircularity of the holes leading to the occurrence of narrower or even disconnected conducting paths (see microscope image (iv) in Fig. 1(c)). Yet, in this configuration sheet resistances below 20 Ω are achievable with a metal coverage of less than 40%. A very important fact for transparent electrodes based on grating structures is that for many applications conductivity is also required in the openings of the grating. In the case presented here, the hole areas have diameters in the range of several tens of micrometers and in many devices (e.g. silicon thin film solar cells [29]), the conductivity of the active material is not sufficient to overcome this distance without substantial ohmic losses. Therefore, we estimate the influence of the conductivity in the hole region using the example of a solar cell. If we assume homogeneous current generation, perfectly conducting metal areas and no lateral conduction of the absorber, the power loss PLoss in the hole region due to the contact resistance and a corresponding loss ratio for the case of solar cell operation at the maximum power point (MPP) are calculated as follows rH
2
PLoss = ( J π r 2 ) ⋅ RS 0
Loss Ratio =
dr 2π r
PLoss RS J MPP rH 2 = PMPP 8VMPP
(3) (4)
with the current density J (JMPP at the MPP), voltage at the MPP, VMPP, sheet resistance, RS, and the hole radius, rH. For solar cells, the assumption of homogeneous current generation can be valid only as long as the voltage drop due to the series resistance is very small compared to VMPP, i.e. for low loss ratios. However, Eq. (4) allows a rough estimation of the limitations on hole size and sheet resistance. In an example, we study a worst case scenario for transparent electrodes in photovoltaic applications, which would, as it is apparent from Eq. (4), be a high current, low voltage solar cell design. Silicon heterojunction solar cells operate at the maximum power point with current densities and voltages of approximately JMPP ≈40 mA/cm2 and VMPP ≈0.6 V respectively. Figure 2(d) shows the loss ratio of such a solar cell for different hole radii as a function of the sheet resistance in the opening. For a radius up to 100 µm, a layer with a sheet resistance of 10 kΩ in the area of the hole suffices to keep the efficiency loss below 1%. However, our results show that, depending on the grating dimensions, additional conductivity in the metal-free areas is mandatory to minimize the efficiency loss. Highly transparent and/or cost effective materials, such as low-cost transparent conducting oxides or conducting polymers are established technologies in the thin film industry. In combination with a perforated metal film, layers from such materials would not only enable fully conductive surfaces, but could also provide further functionalities, such as encapsulation or anti-reflection properties.
#232537 - $15.00 USD (C) 2015 OSA
Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A259
Figure 3(a) shows a sketch of a perforated metal film, encapsulated in between two 35 nm thick AZO layers. This layout is a widely known example of transparent electrodes based on UTMFs [12]. The AZO layers are commonly applied for the purpose of anti-reflection but can also function as a physical and chemical barrier for the metal. The anti-reflection properties of the AZO layer are demonstrated by the transmittance measurements presented in Fig. 3(b). Compared to an uncovered metal layer (Fig. 2 (a)), the AZO significantly increases the transmittance for wavelengths around 450 nm. This result is in line with previous studies on UTMF transparent electrodes [13,14]. A decrease of metal coverage by laser perforation shifts the measurement curves towards the metal free sample (0% metal coverage, only AZO) and broadband transmittance is achieved for low coverage values. In the presented case of a perforated silver film, the AZO also provides conductivity in the hole area (sheet resistance of the AZO: RS ≈1550 Ω). With this configuration, conductivity is preserved throughout the entire structure. Although AZO and AZO/Ag/AZO layers in this paper are not fully optimized, the measurements of the laser-perforated metal films clearly indicate the prospects of our approach. Only a slight modification in the fabrication process of established UTMF transparent electrodes is necessary to significantly enhance the spectral bandwidth of such structures’ transmittance.
Fig. 3. Sketch (a) and transmittance measurements (b) of a perforated Ag layer encapsulated in an AZO layer with a total thickness of 70 nm.
Fig. 4. Sketch (a) and external quantum efficiency (b) of amorphous silicon solar cells, fabricated on transparent electrodes based on unperforated and perforated Ag layers and 70 nm encapsulation layers.
To provide a working example of the presented concept, we demonstrate its application in a silicon thin film solar cell. An optimized quarter-wave anti-reflection for an amorphous silicon layer can be realized with an AZO layer of approximately 70 nm [25]. To achieve
#232537 - $15.00 USD (C) 2015 OSA
Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A260
optimized light incoupling, a UTMF would be integrated not in the middle but on one side of the TCO (see sketch in Fig. 4(a)) [25]. Figure 4(b) shows the quantum efficiency measured for the described device with varying metal coverage. For wavelengths around 450 nm the sample with 100% coverage exhibits slightly higher EQE, which originates from the cavity resonance of the front contact. For all other wavelengths, the perforation is strongly beneficial for the efficiency with only a moderate influence on the electrical properties of the cell (fill factor decrease of 7.3%). An increase of the short circuit current density from 6.2 mA/cm2 (100% coverage) to 9.2 mA/cm2 (24% coverage) is measured, resulting in an overall power conversion efficiency (η) enhancement of more than 36% (η100% ≈3.6% compared to η24% ≈5%). A metal-free transparent electrode sample was not fabricated as the insufficient conductivity of the thin AZO layer as a front contact would not allow an efficient current collection [25]. Besides the laser structuring of UTMFs on transparent substrates, the perforation can also be performed directly on the device. We demonstrate this by furnishing a silicon heterojunction solar cell with a transparent electrode consisting of a 70 nm layer of AZO and 15 nm of Ag (see cell design in Fig. 5(a)). The measured EQE of the device with different coverages is shown in Fig. 5(b). For the unstructured Ag layer (100% curve), the efficiency is suppressed by the reflection of light at the Ag interface, especially of higher wavelengths. Decreasing the Ag coverage by laser-perforation yields a significant improvement of the quantum efficiency. With an 18% metal coverage, the photo current density is almost tripled from 11.6 mA/cm2 ( 1 0 0 % ) t o 3 0 . 7 m A / c m 2 ( η100% ≈4.7% compared to η18% ≈11.7%), illustrating the broadband transmittance of our transparent electrode solution. The electrical parameters of the cells are affected by the laser perforation only in a minor way. The fill factor is reduced from 69.3% (100% metal coverage) to 67.2% (18% metal coverage) and the open circuit voltage from 0.583 V (100%) to 0.567 V (18%) respectively. Both parameters are decreased only by about 3%, which demonstrates the applicability of the approach for this type of solar cell. For comparison, the EQE of an identical cell with a 70 nm ITO front contact is plotted in Fig. 5(b). The reference shows a slightly higher quantum efficiency with a short circuit current density of 34.2 mA/cm2. The optical performance of the perforated Ag transparent electrode could be further improved by positioning the UTMF adjacent to the silicon. However, Ag diffusion into the p-doped a-Si layer would strongly deteriorate the electrical properties of the cell, causing the need for a chemical barrier layer.
Fig. 5. Sketch (a) and external quantum efficiency measurements (b) from silicon heterojunction solar cells, furnished with AZO (70 nm) / perforated Ag layer and ITO (70 nm) transparent electrodes.
4. Conclusion
In conclusion, we have shown a new transparent electrode design based on laser structured UTMFs, which combines high optical transmittance with low sheet resistance and a simple
#232537 - $15.00 USD (C) 2015 OSA
Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A261
fabrication process. Using our method, it is possible to overcome the restrictions of established metal nanostructure transparent electrodes, such as confined spectral transmittance (UTMF) or complex fabrication (nanogratings). It is worth mentioning that material combination and applied perforation pattern, as discussed in this paper, are only one possible embodiment of the presented concept. Specifically, metals of lower cost (e.g. copper) have been demonstrated in both grid based [30] and UTMF transparent electrodes [15]. Neither is the laser processed metal structure limited to a hexagonal hole array. For instance, in the case of solar cell applications, a conceivable pattern would consist of microscopic tapered fingers, which features increasing sheet conductivity in the direction of the current flow. We are confident that the various design possibilities and optimization potentials of our approach will allow the development of a scalable and cost-efficient technology to fabricate versatile, high performing transparent electrodes. Acknowledgment
We would like to thank Omid Madani Ghahfarokhi, Antje Schweitzer and Ortwin Siepmann for discussions and sample fabrication, Richard Kessell, Stefan Strauf and Pang-Chieh Sui for proofreading and Thomas Faasch for the help with the photographs and the hand modeling. Also, the authors acknowledge financial support of the German Federal Ministry of Education and Research (BMBF) in the framework of the project SiSoFlex (FKZ 03SF0418B).
#232537 - $15.00 USD (C) 2015 OSA
Received 16 Jan 2015; revised 19 Feb 2015; accepted 19 Feb 2015; published 25 Feb 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A254 | OPTICS EXPRESS A262
