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Ultrathin, high-efficiency, broad-band, omni-acceptance, organic solar cells enhanced by plasmonic cavity with subwavelength hole array

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Abstract

Three of central challenges in solar cells are high light coupling into solar cell, high light trapping and absorption in a sub-absorption-length-thick active layer, and replacement of the indium-tin-oxide (ITO) transparent electrode used in thin-film devices. Here, we report a proposal and the first experimental study and demonstration of a new ultra-thin high-efficiency organic solar cell (SC), termed “plasmonic cavity with subwavelength hole-array (PlaCSH) solar cell”, that offers a solution to all three issues with unprecedented performances. The ultrathin PlaCSH-SC is a thin plasmonic cavity that consists of a 30 nm thick front metal-mesh electrode with subwavelength hole-array (MESH) which replaces ITO, a thin (100 nm thick) back metal electrode, and in-between a polymer photovoltaic active layer (P3HT/PCBM) of 85 nm thick (1/3 average absorption-length). Experimentally, the PlaCSH-SCs have achieved (1) light coupling-efficiency/absorptance as high as 96% (average 90%), broad-band, and Omni acceptance (light coupling nearly independent of both light incident angle and polarization); (2) an external quantum efficiency of 69% for only 27% single-pass active layer absorptance; leading to (3) a 4.4% power conversion efficiency (PCE) at standard-solar-irradiation, which is 52% higher than the reference ITO-SC (identical structure and fabrication to PlaCSH-SC except MESH replaced by ITO), and also is among the highest PCE for the material system that was achievable previously only by using thick active materials and/or optimized polymer compositions and treatments. In harvesting scattered light, the Omni acceptance can increase PCE by additional 81% over ITO-SC, leading to a total 175% increase (i.e. 8% PCE). Furthermore, we found that (a) after formation of PlaCSH the light reflection and absorption by MESH are reduced by 2 to 6 fold from the values when it is alone; and (b) the sheet resistance of a 30 nm thick MESH is 2.2 ohm/sq or less–4.5 fold or more lower than the best reported value for a 100 nm thick ITO film, giving a lowest reflectance-sheet-resistance product. Finally, fabrication of PlaCSH has used nanoimprint on 4” wafer and is scalable to roll-to-roll manufacturing. The designs, fabrications, and findings are applicable to thin solar cells in other materials.

©2012 Optical Society of America

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Figures (9)

Fig. 1
Fig. 1 Plasmonic Cavity with Subwavelength Hole-array (PlaCSH) Solar Cell (SC). (a) Schematic. PlaCSH-SC consists of an Au metallic-mesh electrode with subwavelength hole-array (MESH), a Al backplane electrode, and in between a sandwich of P3HT/PCBM, TiOx, and PEDOT:PSS layers. (b) Energy band diagram. (c) Schematic of PlaCSH-SC fabrication: fabrication of MESH by nanoimprint on a fused silica substrate, spinning of PEDOT:PSS buffer layer and P3HT/PCBM active layer, and thermal deposition of TiOx buffer layer and Al electrode. (d) Tilt-view scanning electron micrograph (SEM) of Au MESH with 175 nm diameter and 200 nm pitch hole array. (e) Cross-sectional SEM of PlaCSH solar cell. (f) Optical image of 4-inch diameter nanoimprint mold used. (g) Optical image under Sun light of PlaCSH-SC (completely black) and reference ITO-SC (dark magenta).
Fig. 2
Fig. 2 Photocurrent of PlaCSH-SC and reference ITO-SC. Typical J-V characteristics measured under 100 mW/cm2 AM 1.5 global solar irradiation (a) and in the dark (b). PlaCSH-SC has an open-circuit voltage (Voc) of 0.62 V, a short-circuit current density (Jsc) of 10.4 mA/cm2, a fill factor (FF) of 67%, and a power conversion efficiency (ηeff) of 4.4%; while ITO-SCs have Voc = 0.62 V, Jsc = 7.4 mA/cm2, FF = 63%, and ηeff = 2.9%. PlaCSH-SCs have enhanced power conversion efficiency by 52%, and Jsc, and FF by 41% and 6% respectively.
Fig. 3
Fig. 3 EQE spectrum of PlaCSH-SC and ITO-SC and Absorption-length in P3HT/PCBM (85 nm thick on glass). Measured external quantum efficiency (EQE) spectrum of PlaCSH-SC and ITO-SC as well as the measured absorptance spectrum of 85 nm thick P3HT/PCBM film on glass (a), and EQE enhancement (EQE ratio of PlaCSH-SC to ITO-SC), and measured absorption-length in P3HT/PCBM (b). PlaCSH-SC achieved a maximum EQE of 69% at 575 nm wavelength where the 85 nm thick active layer’s single pass absorptance is only 27%. And PlaCSH-SC has an EQE enhancement factor always larger than one over the entire measured spectrum range, and can be as high as 2.2 fold (220%) at 650 nm.
Fig. 4
Fig. 4 Normal Incident Reflectance and Absorptance spectra of PlaCSH-SC, ITO-SC, and P3HT/PCBM (85 nm thick on glass). Measured normal incident optical reflectance spectrum (a) and measured absorptance spectrum (1-reflectance-transmittance). PlaCSH-SC has a normal incident reflectance as low as 5% and 10% average and absorptance as high as 96% and 90% average over a broad band (400 to 900 nm). ITO-SC has normal incident reflectance of minimum 20% and 56% average, and absorptance 80% maximum and 44% average. At 650 and 790 nm wavelength, the absorptance in PlaCSH is 2.9 and 18 fold higher than ITO-SC. The shape of absorptance spectrum in ITO-SC is dominated by that of P3HT/PCBM layer, but not in PlaCSH-SC.
Fig. 5
Fig. 5 Optical and electrical properties of metallic electrode with subwavelength hole-array (MESH). Measurements of sheet-resistance (a), reflectance (b), transmittance (c), and absorptance (d) of 30nm thick Au MESHs on glass (MESH-only) with 75nm, 125nm and 175 nm hole size and 200 nm period as well as the 100 nm thick annealed ITO film on glass (ITO-only). The measured sheet-resistance is 10 ohm/sq for of the ITO, but 2.2 ohm/sq or less for the MESH’s –making them at least 4.5 fold better. The smaller the hole diameter is, the smaller the sheet resistance of MESH, but higher light reflectance and absorptance. Compared with Fig. 4, after PlaCSH-SC formation, both the reflectance and absorptance of MESH drop significantly by 2 to 5 fold in reflectance and as large as 6.7 fold in absorptance at 500nm wavelength. In contrast, for the ITO, the reflectance, after ITO-SC formation, increases drastically by 2 to 5.8 fold.
Fig. 6
Fig. 6 Broad-band Omni acceptance (near angle and polarization independence) in PlaCSH-SC. Measured incident light angle and polarization dependence of photocurrent under a white light (a), reflectance at 500 nm and 750 nm wavelength (b), and reflectance spectra under a white light (c) for PlaCSH-SC and ITO-SC (The photocurrent is scaled to that of PlaCSH at normal light incident). They show that the angle and polarization dependence of photocurrent under white light in PlaCSH is significantly less than ITO-SC by a factor of 3 to 6 fold for s-wave and 4 to 7 fold for p-wave. The photocurrent changes observed are consistent with the measured reflectance changes. The property of PlaSCH-SC, that light coupling into solar cell is nearly independent of light polarization and incident angle over entire possible angle, is termed “Omni acceptance”. The achieved high, board-band, Omni light acceptance of PlaCSH is 2 to 3 fold better than that of Si subwavelength antireflection, yet PlaCSH has a thickness over 10 fold thinner.
Fig. 7
Fig. 7 Measured absorptance spectrum of MIM and Comparison with MAM. The measured absorptance spectrum of PlaCSH-SC (MESH/Absorbing active layer/Metal) and the structures same as PlaCSH-SC and ITO-SC except replacing the absorbing active layer (P3HT/PCBM) by PMMA of the same optical thickness (MESH/Insulating (lossless)/Metal). They show the absorbing layer changes the optical property of a plasmonic cavity significantly. Using of an absorbing layer changes a plasmonic cavity from narrow band to broadband, and increases the absorption substantially.
Fig. 8
Fig. 8 Cavity length effect on efficiency, photocurrent, and absorptance of PlaCSH-SC. Power conversion efficiency (a) and absorptance spectrum (b) for PlaCSH-SC with different P3HT/PCBM layer thickness of 50, 82, 100, and 130 nm, showing that the ~85 nm thickness gives the best performance.
Fig. 9
Fig. 9 Calculated upper-limit of additional enhancement in conversion efficiency of PlaCSH over ITO solar cells due to Omni acceptance for detecting s and p-wave and unpolarized scattered light as a function of material index. For s-polarized light and unpolarized light, the additional enhancement of PlaCSH-SC in power conversion efficiency over ITO-SC due to Omni acceptance (enhancement factor upper-limit) is 81% and 41%, respectively, for the PlaCSH-SC using P3HT/PCBM (index of 2.2); and 142% and 61% for silicon PlaCSH-SC (index of 3.5).

Tables (2)

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Table 1 Properties of PlaCSH Solar Cell and Reference ITO Solar Cell

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Table 2 Electrical and Optical Properties of MESH-only and ITO-only

Equations (1)

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Ratio(n,p)= A PLH A c = (1 R PLS )I(θ,p)dΩ (1 R c (n,θ,p))I(θ,p)dΩ
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