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Dual-microcavity resonance and plasmon-cavity polaritons enhanced the broadband absorption of polymer-based organic solar cells by employing micro-nano composite gratings
Organic Electronics 78 (2020) 105591
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Dual-microcavity resonance and plasmon-cavity polaritons enhanced the broadband absorption of polymer-based organic solar cells by employing micro-nano composite gratings Zhi-xiang Li a, Yu Jin a, *, Yu-wei Liu a, Xin Luo a, Ya-hui Chuai c, Zhi-jun Wu a, Chun-ping Xiang b, ** a
Fujian Key Laboratory of Light Propagation and Transformation, College of Information Science and Engineering, Huaqiao University, 668 Jimei Street, Xiamen, 361021, PR China b College of Information Engineering, Jimei University, 185 Yinjiang Road, Xiamen, 361021, PR China c School of Science, Changchun University of Science and Technology, Changchun, 130022, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Organic solar cells Micro-nano composite gratings Multi-absorption-enhancement mechanisms Dual-microcavity resonance Plasmon-cavity polaritons
A broadband-absorption enhancement in two kinds of polymer-based organic solar cells (OSCs) is demonstrated in this work by employing micro-nano composite gratings (MNCGs). The structure of MNCGs is nano-grating covering on the surface of micro-grating. By embedding the MNCGs at the interface of the active layer/Ag cathode, the multi-absorption-enhancement mechanisms, such as dual-microcavity resonance, plasmon-cavity polaritons and light-scattering effects, are excited and result in a broadband-absorption enhancement. The light absorption of active layer in the MNCG-based OSC is numerically calculated. As a result, comparing to the planar OSCs, the absorption intensity is increased by 50.41% and 20.43% in these two kinds of polymer-based OSCs, respectively. Meanwhile, the MNCGs can also be universally used in all kinds of polymer-based OSCs to increase the light absorption of the active layer.
1. Introduction Polymer-based bulk-heterojunction (BHJ) organic solar cells (OSCs) have received considerable attention due to their lower cost, simpler industrial processability and compatibility with flexible substrates over large areas [1–6]. In recent years, the power conversion efficiency (PCE) of polymer-based OSCs has steadily increased by improving energy harvest, enhancing excitons separation, optimizing device structure [7–13]. Up to now, a PCE record of 17.29% has been demonstrated in a 2-terminal monolithic solution processed tandem OSCs [14]. However, for either academic or commercial applications, obtaining higher PCE is still the major goal for OSCs. Functional nano or micro structures with controlled morphology can offer enhanced performance of the OSCs, such as metallic meander electrode [15–18], metallic nanoparticles [19–21] and nanopatterned metal films [22–24]. The micro or nano structures with different sizes and periods result in different performance-enhancement mechanisms. It has been demonstrated that the use of metallic nano-structure in OSCs
results in the enhancement of electromagnetic field and improvement of the optical absorption [25–27]. The use of two metallic meander elec trodes results in the plasmon-cavity polaritons (PCPs), which can confine electromagnetic field to a subwavelength scale [15]. Micro-structure can scatter the incident light and elongate the optical path in the active layer of OSCs [27]. However, the light absorption improvement in these OSCs is still limited due to the single performance-enhancement mechanism excited in these devices, which is mainly attributed to the simplex morphology and single period of these nano or micro structures. Therefore, for further increasing the light absorption in single polymer-based BHJ OSC, introducing more performance-enhancement mechanisms is the key point. One feasible way is to employ both micro and nano structures in the single OSC, simultaneously. In this work, such micro-nano composite gratings (MNCGs) are introduced in two kinds of polymer-based BHJ OSCs. In these two kinds of OSCs, the most commonly used materials of (poly [4,8-bis [(2-eth ylhexyl) oxy] benzo [1,2-b:4,5-bA] dithiophene-2,6-diyl] [3-fluoro-2-
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Jin), [email protected] (C.-p. Xiang). https://doi.org/10.1016/j.orgel.2019.105591 Received 15 August 2019; Received in revised form 5 November 2019; Accepted 9 December 2019 Available online 11 December 2019 1566-1199/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) The inverted schematic structure of polymer-based OSCs with MNCGs. The height and period of MGs and NGs are H1, P1, H2 and P2, respectively. The thickness of active layer is H3; (b) the AFM image of the surface morphology of the MNCG nanoimprint mold and the cross section image; (c) and (d) the refractive index and extinction coefficients of the PTB7:PC71BM, P3HT:PCBM and PEDOT:PSS, respectively.
[(2-ethylhexyl) carbonyl]thieno [3,4-b]-thiophenediyl] (PTB7):[6,6]phenyl-C71-butyric acid methyl ester (PC71BM) and poly (3-hexylth iophene) (P3HT):[6,6]-phenyl-C61 -butyric acid methyl ester (PCBM) are chosen as the active layer materials. The structure of the MNCGs is the nano-grating (NG) covering on the surface of micro-grating (MG). Rigorous coupled wave analysis (RCWA) and finite-difference timedomain (FDTD) numerical calculation methods are used to investigate the absorption-enhancement mechanisms induced by the MNCGs in polymer-based OSCs. By embedding the MNCG at the interface between the active layer and Ag cathode, multi-absorption-enhancement mech anisms, such as dual-microcavity resonance, PCPs and light-scattering effects, are excited. By tuning the period of NGs, the resonance wave length of the PCPs is tuned to the intrinsic absorption region of the active layer. As a result, a broadband-absorption enhancement is obtained.
2.2. Fabrication of MNCG molds Nanoimprint lithography is a cost-effective manufacturing method to produce polymer nanostructures [28,29]. However, the production of multi-morphological and multi-periodical polymer micro-structure with high precision period is still limited due to the high cost of the con ventional mold fabrication techniques, such as electron beam lithog raphy and ion beam lithography [30,31]. In this work, the low cost nanoimprint molds with high periodical precision micro-nano compos ite morphology were fabricated by the method of combinating the photolithography and two-beam laser interference. Firstly, a 200 nm film of SU-8 photoresist (2000.5, MicroChem) was spin-coated onto the cleaned glass substrate and exposed by lithography machine purchased from Institute of Optics and Electronics Chinese Academy of Science. The morphology of mask was choosed as periodical strip-type. The period was 5 μm. After the first exposure process, the glass substrate was immediately exposed by coherent two laser beams, which were split from a continuous laser with wavelength of 325 nm (KIMMON KOHA CO., LTD.). Then the sample was baked for 1 min at 95 � C and developed for 2 min. The morphology of MNCG mold was scanned by the atomic force microscope (AFM) as shown in Fig. 1 (b). It can be clearly seen that the NG with period of 350 nm covering on the MG with period of 5 μm. The height of the MG and NG is 80 nm and 30 nm, respectively. By tuning the pattern of mask in the process of photolithography and the incident angle of two laser beams in process of the interference sepa rately, the MNCG molds with adjustable morphology and period can be realized, which has been demonstrated in our previous work [32]. The realization of fabrication of micro-nano composite structure molds en sures the experimental availability of polymer-based OSCs with MNCGs, which made the following theoretical analysis meaningfully.
2. Materials and methods 2.1. Materials and structure of polymer-based OSCs The inverted schematic structure of polymer-based BHJ OSC with the MNCGs is shown in Fig. 1 (a). A 15-nm-thick Au film on the glass sub strate is used as the anode. A 60-nm-thick poly (3, 4-ethylenedioxythio phene):poly (styrenesulfonate) (PEDOT:PSS purchased from Xi’an Polymer Light Technology Corp.) film acts as the hole transport layer. The PTB7 or P3HT is selected as the donor. The PC71BM or PCBM is the acceptor. The BHJ of PTB7: PC71BM (1:1.5 wt) or P3HT: PCBM (1:1 wt) is selected as the active layer materials in two kinds of polymer-based OSCs, respectively. All of the donor and acceptor materials are pur chased from LumTec. Corp. A 100-nm-thick Ag film is used as the cathode. The anode Au was deposited on the glass substrate by vacuum thermal evaporation process and PEDOT:PSS and active layer were spincoating sequentially. Finally cathode Ag was evaporated on the active layer by vacuum thermal evaporation. The MNCGs are introduced at the interface between the active layer and Ag cathode, which can be realized by nanoimprint lithography technology.
2.3. Numerical simulation models and analysis methods RCWA and FDTD numerical calculation methods are used to inves tigate the absorption-enhancement mechanisms induced by the MNCGs in polymer-based OSCs. Firstly, a simple scheme is adopted. In this 2
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Fig. 2. The absorption spectra of MG-based OSCs as a function of wavelength and H3 under the TM polarization incident light: (a) the planar PTB7:PC71BM-based OSCs; (b) and (c) the PTB7:PC71BM-based OSCs with MGs, in which the P1 is 6 μm and H1 is 50 nm and 70 nm, respectively; (d) the P3HT:PCBM-based OSCs, in which the P1 is 6 μm and H1 is 50 nm.
simple scheme, NGs, MGs and MNCGs are employed in the polymerbased OSCs, respectively. The cross sections of the MGs and NGs are all rectangle with fill-factor of 50%. The extinction coefficients of PEDOT:PSS and PTB7:PC71BM (or P3HT:PCBM) are set to 0. Secondly, basing on the above models, the actual polymer-based OSC models with MGs, NGs and MNCGs are analyzed taking extinction coefficients of PEDOT:PSS and PTB7:PC71BM (or P3HT:PCBM) into account. The op tical constants of all materials are referenced from Refs. [33–37], and shown in Fig. 1 (c) and (d). The height and period of MGs and NGs and the thickness of active layer are set as H1, P1, H2, P2 and H3 (Fig. 1 (a)) respectively, which are applied in all theoretical analysis models. A transverse magnetic (TM) or transverse electric (TE) polarized plane wave source in wavelength range of 400 nm–780 nm is used in these two models. The glass substrate and the Ag cathode are set as semi-infinite region due to the 100% reflectance of 100-nm-thick Ag film. In this simulation, the grid size is set to 2 nm � 2 nm. The perfectly matched layer (PML) boundary condition is applied along the light propagation direction, and the periodic boundary condition (PBC) is applied in the lateral directions. 3. Result and discussion 3.1. Effect analysis induced by micro-grating Metal materials (such as Au, Ag and Al) are usually used as electrodes in the OSCs. Due to the high reflectivity of metal electrodes, the microcavity modes commonly form between the two metal electrodes in the OSCs. The optical resonance condition for normal incidence light in the microcavity can be described as [38]: X λ mλ ni di þ ðϕ1 þ ϕ2 Þ ¼ π 4 2 i
Fig. 3. The absorption spectra of NG-based OSCs as a function of wavelength and P2 under TM polarization incident light: (a) the PTB7:PC71BM-based OSCs, in which the H2 and H3 is 20 nm and 130 nm, respectively. (b) the P3HT: PCBM-based OSCs, in which the H2 and H3 is 10 nm and 125 nm, respectively.
(1)
Where ni and di are refractive index and thickness of the layers between two metal-electrode mirrors, respectively. ϕ1 and ϕ2 are the phase 3
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Fig. 4. Absorption spectra of NG-based OSCs as a function of wavelength and H3: (a), (b) and (c) the PTB7:PC71BM-based OSCs with NGs, in which P2 is 250 nm and H2 is 10 nm, 15 nm and 20 nm, respectively; (e) and (f) The P3HT:PCBM-based OSCs with NGs, in which P2 is 200 nm and H2 is 10 nm and 15 nm, respectively; (d) the magnetic field distribution of the point A in TM polarization.
change upon reflection from two mirrors. λ represents the wavelength of incident light and m is a positive integer. Firstly, the RCWA calculation method is used to analyze the micro cavity modes in the polymer-based OSCs with MGs. In this ideal model, the extinction coefficients of active-layer materials (PEDOT:PSS and PTB7:PC71BM or P3HT:PCBM) are set to 0. For comparison, an ideal planar polymer-based OSC model is also simulated. Fig. 2 (a) shows the absorption spectra of planar PTB7:PC71BM-based OSCs as a function of H3 (the thickness of PTB7:PC71BM). It can be seen that the microcavity modes exist in the absorption spectra and can be described by equation (1). The light incidents through the Au film and multi-reflects at two interfaces of Ag cathode and Au anode. The microcavity resonances form in the condition of a suitable thickness of H3 and exhibit absorption increment as shown in Fig. 2 (a). When H3 is 130 nm, the absorption of planar PTB7:PC71BM-based OSC is improved significantly below the wavelength of 540 nm. Meanwhile, due to the anomalous dispersion of refractive index in the material of PTB7:PC71BM from 650 to 720 nm as shown in Fig. 1 (c) [34], the microcavity modes perform a curve shape in the wavelength range from 650 nm to 720 nm. By employing the MG with P1 ¼ 6 μm (period) and H1 ¼ 50 nm (height) at the interface of PTB7:PC71BM/Ag cathode, the absorption spectra of PTB7: PC71BM-based OSC as a function of H3 is simulated and shown in
Fig. 2 (b). It can be seen that dual-microcavity modes generate in ab sorption spectra, which is mainly attributed to the thickness of active layer be a step function in the range of one grating period in BHJ OSCs. Moreover, the splitting distance of dual-microcavity modes increases with the increase of H1 as shown in Fig. 2 (c). When H3 is 130 nm, the absorption of PTB7:PC71BM-based OSC is improved significantly below the wavelength of 500 nm. The absorption spectra of P3HT:PCBM-based OSCs with MG (P1 ¼ 6 μm, H1 ¼ 50 nm) structures as a function of H3 are also simulated as shown Fig. 2 (d). It indicates that the absorption characteristics of P3HT:PCBM-based OSC exhibit the similar character istics to that of PTB7:PC71BM-based OSCs. 3.2. Effect analysis induced by nano-grating The absorption characteristics of polymer-based OSCs with NGs are also simulated by using RCWA method. Fig. 3(a) and (b) show the ab sorption spectra as a function of P2 (period of the NGs) in PTB7: PC71BM-based and P3HT:PCBM-based OSCs, respectively. It can be seen that the surface plasmon polaritons (SPPs) is excited and appears in the absorption spectra, when the momentum and energy of the incident light match to that of the SPPs along the dielectric/metal interface following the equation [39]: 4
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Fig. 5. The absorption spectra of MNCG-based OSCs as a function of wavelength and H3; (a) the PTB7:PC71BM-based OSCs, in which the P1 is 6 μm, H1 is 50 nm, P2 is 250 nm and H2 is 20 nm; (b) The P3HT:PCBM-based OSCs, in which the P1 is 6 μm, H1 is 50 nm, P2 is 200 nm and H2 is 10 nm. (c) and (d) The spatial magnetic field (Hy) intensity distribution of the MNCG-based OSCs in TM polarization at point D and E, respectively.
� � � 2πm�� kspp ¼ ��klight sin α � p2 �
dot circles). The anti-crossing coupling of these two modes results in an enhanced absorption band from 600 nm to 710 nm, when H3 is 130 nm. Meanwhile, the anti-crossing coupling between the two modes makes the feature of mode at point A, B and C (Fig. 4 (c)) resemble that of the SPPs. This new polariton modes name PCPs [15]. The spatial magnetic field (Hy) intensity distribution at the point A is simulated in TM po larization as shown in Fig. 4 (d). It indicates that the PCPs confine the magnetic field at the interface of PTB7:PC71BM/Ag cathode and enhance the absorption of PTB7:PC71BM layer. Meanwhile, the ab sorption of PTB7:PC71BM-based OSCs is also increased below the wavelength of 510 nm, when H3 is 130 nm. This is mainly attributed to the microcavity resonance. Fig. 4 (e) and (f) show the absorption spectra of the P3HT:PCBM-based OSCs, when the H2 is set to 10 nm and 15 nm, respectively. The period of NG is fixed at 200 nm. Comparing to PTB7: PCBM-based OSCs, the SPPs in the P3HT-PCBM-based OSCs only lo cates at around 620 nm (Fig. 4 (e)). The PCPs are also excited by anti-crossing coupling between SPPs and microcavity modes (Fig. 4 (f)).
(2)
Where kspp and klight are the in-plane wave vectors of the SPPs and the wave vector of light incident to the interface, respectively. p2 is the period of the NGs, α represents the incident angle and m is an integer. The SPPs enhanced absorption intensity appears as a function of both P2 and absorption wavelengths in these two kinds of polymer-based OSCs. In the P3HT:PCBM-based OSCs, the SPPs red shifts with the increase of P2, which can be described by equation (2). In PTB7:PC71BM-based OSCs, the SPPs perform a curve shape as the P2 increasing from 200 nm to 300 nm, which is attributed to the anomalous dispersion of refractive index of PTB7:PC71BM in the wavelength range from 650 nm to 720 nm as shown in Fig. 1 (c) [34]. In order to optimally enhance the absorption of active layer in these two kinds of polymer-based OSCs, the resonance wavelength of SPPs is needed to coincide with the intrinsic absorption region of active-layer materials. The intrinsic absorption region of PTB7:PC71BM and P3HT: PCBM is 400–750 nm and 400–650 nm, respectively. Therefore, the period of NGs (P2) is chosen as 250 nm and 200 nm for PTB7:PC71BMbased and P3HT:PCBM-based OSCs, respectively. The SPPs and the microcavity modes are analyzed as a function of H3 with different H2s. Fig. 4(a) - (c) show the absorption spectra of the PTB7:PC71BM-based OSCs (TE polarization), in which the H2 is set to 10 nm, 15 nm and 20 nm, respectively. Parameter P2 (the period of NG) is fixed at 250 nm. It can be see that both microcavity modes and SPPs (described as black dot lines) are excited and appear in the absorption spectra. When H2 is 10 nm, the SPPs locate at around 575 nm and 710 nm (Fig. 4 (a)). The SPP modes couple with the microcavity modes weakly. As the H2 in creases, the coupling between two modes become strongly. An anticrossing coupling between the SPPs and the microcavity modes gener ates in the absorption spectra as shown in Fig. 4 (c) (described as black
3.3. Effect analysis induced by MNCGs Finally, the MNCG with the optimal parameters with P1 ¼ 6 μm, H1 ¼ 50 nm, P2 ¼ 250 nm and H2 ¼ 20 nm is introduced in the PTB7: PC71BM-based OSC model. Fig. 5 (a) shows the absorption spectra as a function of H3. It can be seen that the PCPs are excited by the anticrossing coupling between dual-microcavity modes and SPPs. When H3 is 130 nm, the absorption of PTB7:PC71BM-based OSC is enhanced significantly from 550 nm to 600 nm and from 680 nm to 720 nm. In order to investigate the effects of MNCGs on absorption enhancement in PTB7:PC71BM-based OSCs, the field intensity distribution (Hy) is simulated in TM polarization at point D (Fig. 5 (a), wavelength ¼ 575 nm, H3 ¼ 130 nm) and shown in Fig. 5 (c). It can be seen that the magnetic field intensity exhibits maximum mainly at the interface of Ag 5
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based OSC, the absorption of PTB7:PC71BM layer in MG-based OSCs is increased by 50.95% at 700 nm (the intrinsic absorption peak of PTB7), which is attributed to the dual-microcavity resonance and light scattering effects of MGs. By employing the NG (H2 ¼ 20 nm, P2 ¼ 250 nm) in PTB7:PC71BM-based OSCs, the absorption of PTB7:PC71BM layer from 550 nm to 750 nm is increased obviously in TM polarization and is increased by 20.36% at the wavelength of 700 nm. This absorp tion increment is mainly due to magnetic field intensity enhancement induced by PCPs. In TE polarization, the absorption of PTB7:PC71BM layer is also increased by 12.72% at the wavelength of 700 nm, which mainly due to the microcavity resonances and light scattering effects of NGs. As shown in Fig. 6 (b), the absorption enhancement of P3HT:PCBM layer exhibits similar characteristics to that of PTB7:PC71BM layer. The absorption is increased by 15.22% and 3.20% at 525 nm (the intrinsic absorption peak of P3HT) in P3HT:PCBM-based OSCs with MG and NG, respectively. In order to make the theoretical simulation results can be used in the actual experiment, two types of MNCGs are introduced in the polymerbased OSCs, respectively. One type (type A) exhibits the morphology with NGs covering on the whole surface of the MGs as shown in Fig. 1(a). The other type (type B) exhibits the morphology with NGs covering only on the step top-surface of Ag MGs, which is closer to the actual nano imprint modes as shown in Fig. 1 (b). For the two types of PTB7: PC71BM-based and P3HT:PCBM-based OSCs, the increment of absorp tion spectra of the PTB7:PC71BM layer and the P3HT:PCBM layer are also shown in Fig. 6 (a) and (b), respectively. It indicates that the ab sorption of PTB7:PC71BM layer from 450 nm to 750 nm is increased obviously, comparing to the planar polymer-based OSCs. When the wavelength is 700 nm, the absorption is increased by 57.41% for type A and 40.43% for type B. The absorption increment is obviously higher than that in the PTB7:PC71BM-based OSCs only with MG or NG. It suggests that the dual-microcavity resonance, PCPs and light scattering effects all enhance the absorption of PTB7:PC71BM layer in BHJ OSCs. The multi-absorption-enhancement mechanisms (the dual-microcavity resonance, PCPs and light scattering effects) are also introduced in the P3HT:PCBM-based OSCs by employing the MNCGs. As shown in Fig. 6 (b), comparing to the planar polymer-based OSCs, the absorption of P3HT:PCBM layer is increased by 20.43% for type A and 17.47% for type B at the wavelength of 525 nm, respectively. It is also higher than that in the P3HT:PCBM-based OSCs only with MG or NG. Therefore, the MNCGs effectively enhance the broadband absorption of the active layer in these both kinds of polymer-based OSCs. For all above, it also suggests that the MNCGs can be universally used in polymer-based OSCs to increase the absorption of the active layer.
Fig. 6. The absorption spectrum of the planar OSCs (cyan); the increment of the absorption spectra in the MG-based (olive), TE-NG-based (yellow), TM-NGbased (orange) and MNCG-based (blue for type B and purple for type A) OSCs: (a) the PTB7:PC71BM-based OSCs; (b) the P3HT:PCBM-based OSCs. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
cathode/PTB7:PC71BM layer and decays exponentially into both ma terials, and in particular, extends to the PTB7:PC71BM layer. This sug gests that the absorption increment at wavelength of 575 nm is mainly attributed to the magnetic field intensity enhancement induced by PCPs. The MNCG with the optimal parameters (P1 ¼ 6 μm, H1 ¼ 50 nm, P2 ¼ 200 nm and H2 ¼ 10 nm) is also introduced in the P3HT:PCBM-based OSCs. The magnetic field intensity enhancement induced by PCPs also increases the absorption in intrinsic absorption region of P3HT:PCBM as shown in Fig. 5 (b) and (d). In order to illustrate the increment of intrinsic optical absorption in active layer, the FDTD numerical simulation method is used to analyze the absorption spectra in actual OSC device model with MNCGs (Fig. 1 (a)). In the actual OSC device model, the extinction coefficients of PTB7: PC71BM, P3HT:PCBM and PEDOT:PSS are taken into account. The ab sorption spectra of PTB7:PC71BM layer or P3HT:PCBM layer in planar polymer-based OSCs and the increment of absorption spectra of these layers in polymer-based OSCs with MG, NG and MNCG are shown in Fig. 6. The increment spectra are achieved from the equation: Absgrating
Absplanar
4. Conclusions In summary, the absorption of PTB7:PC71BM-based and P3HT: PCBM-based OSCs are broadband enhanced by employing the MNCGs at the interface between active layer and Ag cathode. Multi-absorptionenhancement mechanisms (dual-microcavity resonance, PCPs and light scattering effects) are excited by employing MNCGs in the single polymer-based OSC. As a result, the absorption is enhanced obviously in the intrinsic absorption region of PTB7:PC71BM layer (450 nm–750 nm) and P3HT:PCBM layer (400 nm–650 nm). Comparing to the planar polymer-based OSCs with the same active layer materials, the absorp tion of PTB7:PC71BM layer and P3HT:PCBM layer in MNCG-based OSCs is enhanced by 57.41% at 700 nm and 20.43% at 525 nm, respectively. Meanwhile, the low cost MNCG nanoimprint modes are obtained in this work, which insures the polymer-based OSCs with MNCG-based active layer can be universally realized by nanoimprint lithography.
(3)
Where Absgrating and Absplanar are the absorption intensity of gratingbased and planar polymer-based OSCs, respectively. The H3 of PTB7: PC71BM is set to 130 nm in considering both optical and electrical performance of polymer-based OSCs. As shown in Fig. 6 (a), by employing the MG (P1 ¼ 6 μm, H1 ¼ 50 nm) in PTB7:PC71BM-based OSCs, the absorption of the PTB7:PC71BM layer is enhanced obviously from 500 nm to 750 nm. Comparing with the planar PTB7:PC71BM-
Acknowledgements This work was supported by Fujian Natural Science Foundation of China (2019J01056), the Promotion Program for Young and Middle6
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aged Teacher in Science and Technology Research of Huaqiao University (ZQN-PY508), National Natural Science Foundation of China (61404053, 61505056), Research Project of Huaqiao University (13BS419), the Open Project Program of Fujian Key Laboratory of Light Propagation and Transformation (KF2019101), the science and tech nology research project of jilin education department (JJKH20181097KJ), the Subsidized Project for Postgraduates’ Innova tive Fund in Scientific Research of Huaqiao University (17013082006.)
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Organic Electronics journal homepage: http://www.elsevier.com/locate/orgel
Dual-microcavity resonance and plasmon-cavity polaritons enhanced the broadband absorption of polymer-based organic solar cells by employing micro-nano composite gratings Zhi-xiang Li a, Yu Jin a, *, Yu-wei Liu a, Xin Luo a, Ya-hui Chuai c, Zhi-jun Wu a, Chun-ping Xiang b, ** a
Fujian Key Laboratory of Light Propagation and Transformation, College of Information Science and Engineering, Huaqiao University, 668 Jimei Street, Xiamen, 361021, PR China b College of Information Engineering, Jimei University, 185 Yinjiang Road, Xiamen, 361021, PR China c School of Science, Changchun University of Science and Technology, Changchun, 130022, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Organic solar cells Micro-nano composite gratings Multi-absorption-enhancement mechanisms Dual-microcavity resonance Plasmon-cavity polaritons
A broadband-absorption enhancement in two kinds of polymer-based organic solar cells (OSCs) is demonstrated in this work by employing micro-nano composite gratings (MNCGs). The structure of MNCGs is nano-grating covering on the surface of micro-grating. By embedding the MNCGs at the interface of the active layer/Ag cathode, the multi-absorption-enhancement mechanisms, such as dual-microcavity resonance, plasmon-cavity polaritons and light-scattering effects, are excited and result in a broadband-absorption enhancement. The light absorption of active layer in the MNCG-based OSC is numerically calculated. As a result, comparing to the planar OSCs, the absorption intensity is increased by 50.41% and 20.43% in these two kinds of polymer-based OSCs, respectively. Meanwhile, the MNCGs can also be universally used in all kinds of polymer-based OSCs to increase the light absorption of the active layer.
1. Introduction Polymer-based bulk-heterojunction (BHJ) organic solar cells (OSCs) have received considerable attention due to their lower cost, simpler industrial processability and compatibility with flexible substrates over large areas [1–6]. In recent years, the power conversion efficiency (PCE) of polymer-based OSCs has steadily increased by improving energy harvest, enhancing excitons separation, optimizing device structure [7–13]. Up to now, a PCE record of 17.29% has been demonstrated in a 2-terminal monolithic solution processed tandem OSCs [14]. However, for either academic or commercial applications, obtaining higher PCE is still the major goal for OSCs. Functional nano or micro structures with controlled morphology can offer enhanced performance of the OSCs, such as metallic meander electrode [15–18], metallic nanoparticles [19–21] and nanopatterned metal films [22–24]. The micro or nano structures with different sizes and periods result in different performance-enhancement mechanisms. It has been demonstrated that the use of metallic nano-structure in OSCs
results in the enhancement of electromagnetic field and improvement of the optical absorption [25–27]. The use of two metallic meander elec trodes results in the plasmon-cavity polaritons (PCPs), which can confine electromagnetic field to a subwavelength scale [15]. Micro-structure can scatter the incident light and elongate the optical path in the active layer of OSCs [27]. However, the light absorption improvement in these OSCs is still limited due to the single performance-enhancement mechanism excited in these devices, which is mainly attributed to the simplex morphology and single period of these nano or micro structures. Therefore, for further increasing the light absorption in single polymer-based BHJ OSC, introducing more performance-enhancement mechanisms is the key point. One feasible way is to employ both micro and nano structures in the single OSC, simultaneously. In this work, such micro-nano composite gratings (MNCGs) are introduced in two kinds of polymer-based BHJ OSCs. In these two kinds of OSCs, the most commonly used materials of (poly [4,8-bis [(2-eth ylhexyl) oxy] benzo [1,2-b:4,5-bA] dithiophene-2,6-diyl] [3-fluoro-2-
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Jin), [email protected] (C.-p. Xiang). https://doi.org/10.1016/j.orgel.2019.105591 Received 15 August 2019; Received in revised form 5 November 2019; Accepted 9 December 2019 Available online 11 December 2019 1566-1199/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) The inverted schematic structure of polymer-based OSCs with MNCGs. The height and period of MGs and NGs are H1, P1, H2 and P2, respectively. The thickness of active layer is H3; (b) the AFM image of the surface morphology of the MNCG nanoimprint mold and the cross section image; (c) and (d) the refractive index and extinction coefficients of the PTB7:PC71BM, P3HT:PCBM and PEDOT:PSS, respectively.
[(2-ethylhexyl) carbonyl]thieno [3,4-b]-thiophenediyl] (PTB7):[6,6]phenyl-C71-butyric acid methyl ester (PC71BM) and poly (3-hexylth iophene) (P3HT):[6,6]-phenyl-C61 -butyric acid methyl ester (PCBM) are chosen as the active layer materials. The structure of the MNCGs is the nano-grating (NG) covering on the surface of micro-grating (MG). Rigorous coupled wave analysis (RCWA) and finite-difference timedomain (FDTD) numerical calculation methods are used to investigate the absorption-enhancement mechanisms induced by the MNCGs in polymer-based OSCs. By embedding the MNCG at the interface between the active layer and Ag cathode, multi-absorption-enhancement mech anisms, such as dual-microcavity resonance, PCPs and light-scattering effects, are excited. By tuning the period of NGs, the resonance wave length of the PCPs is tuned to the intrinsic absorption region of the active layer. As a result, a broadband-absorption enhancement is obtained.
2.2. Fabrication of MNCG molds Nanoimprint lithography is a cost-effective manufacturing method to produce polymer nanostructures [28,29]. However, the production of multi-morphological and multi-periodical polymer micro-structure with high precision period is still limited due to the high cost of the con ventional mold fabrication techniques, such as electron beam lithog raphy and ion beam lithography [30,31]. In this work, the low cost nanoimprint molds with high periodical precision micro-nano compos ite morphology were fabricated by the method of combinating the photolithography and two-beam laser interference. Firstly, a 200 nm film of SU-8 photoresist (2000.5, MicroChem) was spin-coated onto the cleaned glass substrate and exposed by lithography machine purchased from Institute of Optics and Electronics Chinese Academy of Science. The morphology of mask was choosed as periodical strip-type. The period was 5 μm. After the first exposure process, the glass substrate was immediately exposed by coherent two laser beams, which were split from a continuous laser with wavelength of 325 nm (KIMMON KOHA CO., LTD.). Then the sample was baked for 1 min at 95 � C and developed for 2 min. The morphology of MNCG mold was scanned by the atomic force microscope (AFM) as shown in Fig. 1 (b). It can be clearly seen that the NG with period of 350 nm covering on the MG with period of 5 μm. The height of the MG and NG is 80 nm and 30 nm, respectively. By tuning the pattern of mask in the process of photolithography and the incident angle of two laser beams in process of the interference sepa rately, the MNCG molds with adjustable morphology and period can be realized, which has been demonstrated in our previous work [32]. The realization of fabrication of micro-nano composite structure molds en sures the experimental availability of polymer-based OSCs with MNCGs, which made the following theoretical analysis meaningfully.
2. Materials and methods 2.1. Materials and structure of polymer-based OSCs The inverted schematic structure of polymer-based BHJ OSC with the MNCGs is shown in Fig. 1 (a). A 15-nm-thick Au film on the glass sub strate is used as the anode. A 60-nm-thick poly (3, 4-ethylenedioxythio phene):poly (styrenesulfonate) (PEDOT:PSS purchased from Xi’an Polymer Light Technology Corp.) film acts as the hole transport layer. The PTB7 or P3HT is selected as the donor. The PC71BM or PCBM is the acceptor. The BHJ of PTB7: PC71BM (1:1.5 wt) or P3HT: PCBM (1:1 wt) is selected as the active layer materials in two kinds of polymer-based OSCs, respectively. All of the donor and acceptor materials are pur chased from LumTec. Corp. A 100-nm-thick Ag film is used as the cathode. The anode Au was deposited on the glass substrate by vacuum thermal evaporation process and PEDOT:PSS and active layer were spincoating sequentially. Finally cathode Ag was evaporated on the active layer by vacuum thermal evaporation. The MNCGs are introduced at the interface between the active layer and Ag cathode, which can be realized by nanoimprint lithography technology.
2.3. Numerical simulation models and analysis methods RCWA and FDTD numerical calculation methods are used to inves tigate the absorption-enhancement mechanisms induced by the MNCGs in polymer-based OSCs. Firstly, a simple scheme is adopted. In this 2
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Fig. 2. The absorption spectra of MG-based OSCs as a function of wavelength and H3 under the TM polarization incident light: (a) the planar PTB7:PC71BM-based OSCs; (b) and (c) the PTB7:PC71BM-based OSCs with MGs, in which the P1 is 6 μm and H1 is 50 nm and 70 nm, respectively; (d) the P3HT:PCBM-based OSCs, in which the P1 is 6 μm and H1 is 50 nm.
simple scheme, NGs, MGs and MNCGs are employed in the polymerbased OSCs, respectively. The cross sections of the MGs and NGs are all rectangle with fill-factor of 50%. The extinction coefficients of PEDOT:PSS and PTB7:PC71BM (or P3HT:PCBM) are set to 0. Secondly, basing on the above models, the actual polymer-based OSC models with MGs, NGs and MNCGs are analyzed taking extinction coefficients of PEDOT:PSS and PTB7:PC71BM (or P3HT:PCBM) into account. The op tical constants of all materials are referenced from Refs. [33–37], and shown in Fig. 1 (c) and (d). The height and period of MGs and NGs and the thickness of active layer are set as H1, P1, H2, P2 and H3 (Fig. 1 (a)) respectively, which are applied in all theoretical analysis models. A transverse magnetic (TM) or transverse electric (TE) polarized plane wave source in wavelength range of 400 nm–780 nm is used in these two models. The glass substrate and the Ag cathode are set as semi-infinite region due to the 100% reflectance of 100-nm-thick Ag film. In this simulation, the grid size is set to 2 nm � 2 nm. The perfectly matched layer (PML) boundary condition is applied along the light propagation direction, and the periodic boundary condition (PBC) is applied in the lateral directions. 3. Result and discussion 3.1. Effect analysis induced by micro-grating Metal materials (such as Au, Ag and Al) are usually used as electrodes in the OSCs. Due to the high reflectivity of metal electrodes, the microcavity modes commonly form between the two metal electrodes in the OSCs. The optical resonance condition for normal incidence light in the microcavity can be described as [38]: X λ mλ ni di þ ðϕ1 þ ϕ2 Þ ¼ π 4 2 i
Fig. 3. The absorption spectra of NG-based OSCs as a function of wavelength and P2 under TM polarization incident light: (a) the PTB7:PC71BM-based OSCs, in which the H2 and H3 is 20 nm and 130 nm, respectively. (b) the P3HT: PCBM-based OSCs, in which the H2 and H3 is 10 nm and 125 nm, respectively.
(1)
Where ni and di are refractive index and thickness of the layers between two metal-electrode mirrors, respectively. ϕ1 and ϕ2 are the phase 3
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Fig. 4. Absorption spectra of NG-based OSCs as a function of wavelength and H3: (a), (b) and (c) the PTB7:PC71BM-based OSCs with NGs, in which P2 is 250 nm and H2 is 10 nm, 15 nm and 20 nm, respectively; (e) and (f) The P3HT:PCBM-based OSCs with NGs, in which P2 is 200 nm and H2 is 10 nm and 15 nm, respectively; (d) the magnetic field distribution of the point A in TM polarization.
change upon reflection from two mirrors. λ represents the wavelength of incident light and m is a positive integer. Firstly, the RCWA calculation method is used to analyze the micro cavity modes in the polymer-based OSCs with MGs. In this ideal model, the extinction coefficients of active-layer materials (PEDOT:PSS and PTB7:PC71BM or P3HT:PCBM) are set to 0. For comparison, an ideal planar polymer-based OSC model is also simulated. Fig. 2 (a) shows the absorption spectra of planar PTB7:PC71BM-based OSCs as a function of H3 (the thickness of PTB7:PC71BM). It can be seen that the microcavity modes exist in the absorption spectra and can be described by equation (1). The light incidents through the Au film and multi-reflects at two interfaces of Ag cathode and Au anode. The microcavity resonances form in the condition of a suitable thickness of H3 and exhibit absorption increment as shown in Fig. 2 (a). When H3 is 130 nm, the absorption of planar PTB7:PC71BM-based OSC is improved significantly below the wavelength of 540 nm. Meanwhile, due to the anomalous dispersion of refractive index in the material of PTB7:PC71BM from 650 to 720 nm as shown in Fig. 1 (c) [34], the microcavity modes perform a curve shape in the wavelength range from 650 nm to 720 nm. By employing the MG with P1 ¼ 6 μm (period) and H1 ¼ 50 nm (height) at the interface of PTB7:PC71BM/Ag cathode, the absorption spectra of PTB7: PC71BM-based OSC as a function of H3 is simulated and shown in
Fig. 2 (b). It can be seen that dual-microcavity modes generate in ab sorption spectra, which is mainly attributed to the thickness of active layer be a step function in the range of one grating period in BHJ OSCs. Moreover, the splitting distance of dual-microcavity modes increases with the increase of H1 as shown in Fig. 2 (c). When H3 is 130 nm, the absorption of PTB7:PC71BM-based OSC is improved significantly below the wavelength of 500 nm. The absorption spectra of P3HT:PCBM-based OSCs with MG (P1 ¼ 6 μm, H1 ¼ 50 nm) structures as a function of H3 are also simulated as shown Fig. 2 (d). It indicates that the absorption characteristics of P3HT:PCBM-based OSC exhibit the similar character istics to that of PTB7:PC71BM-based OSCs. 3.2. Effect analysis induced by nano-grating The absorption characteristics of polymer-based OSCs with NGs are also simulated by using RCWA method. Fig. 3(a) and (b) show the ab sorption spectra as a function of P2 (period of the NGs) in PTB7: PC71BM-based and P3HT:PCBM-based OSCs, respectively. It can be seen that the surface plasmon polaritons (SPPs) is excited and appears in the absorption spectra, when the momentum and energy of the incident light match to that of the SPPs along the dielectric/metal interface following the equation [39]: 4
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Fig. 5. The absorption spectra of MNCG-based OSCs as a function of wavelength and H3; (a) the PTB7:PC71BM-based OSCs, in which the P1 is 6 μm, H1 is 50 nm, P2 is 250 nm and H2 is 20 nm; (b) The P3HT:PCBM-based OSCs, in which the P1 is 6 μm, H1 is 50 nm, P2 is 200 nm and H2 is 10 nm. (c) and (d) The spatial magnetic field (Hy) intensity distribution of the MNCG-based OSCs in TM polarization at point D and E, respectively.
� � � 2πm�� kspp ¼ ��klight sin α � p2 �
dot circles). The anti-crossing coupling of these two modes results in an enhanced absorption band from 600 nm to 710 nm, when H3 is 130 nm. Meanwhile, the anti-crossing coupling between the two modes makes the feature of mode at point A, B and C (Fig. 4 (c)) resemble that of the SPPs. This new polariton modes name PCPs [15]. The spatial magnetic field (Hy) intensity distribution at the point A is simulated in TM po larization as shown in Fig. 4 (d). It indicates that the PCPs confine the magnetic field at the interface of PTB7:PC71BM/Ag cathode and enhance the absorption of PTB7:PC71BM layer. Meanwhile, the ab sorption of PTB7:PC71BM-based OSCs is also increased below the wavelength of 510 nm, when H3 is 130 nm. This is mainly attributed to the microcavity resonance. Fig. 4 (e) and (f) show the absorption spectra of the P3HT:PCBM-based OSCs, when the H2 is set to 10 nm and 15 nm, respectively. The period of NG is fixed at 200 nm. Comparing to PTB7: PCBM-based OSCs, the SPPs in the P3HT-PCBM-based OSCs only lo cates at around 620 nm (Fig. 4 (e)). The PCPs are also excited by anti-crossing coupling between SPPs and microcavity modes (Fig. 4 (f)).
(2)
Where kspp and klight are the in-plane wave vectors of the SPPs and the wave vector of light incident to the interface, respectively. p2 is the period of the NGs, α represents the incident angle and m is an integer. The SPPs enhanced absorption intensity appears as a function of both P2 and absorption wavelengths in these two kinds of polymer-based OSCs. In the P3HT:PCBM-based OSCs, the SPPs red shifts with the increase of P2, which can be described by equation (2). In PTB7:PC71BM-based OSCs, the SPPs perform a curve shape as the P2 increasing from 200 nm to 300 nm, which is attributed to the anomalous dispersion of refractive index of PTB7:PC71BM in the wavelength range from 650 nm to 720 nm as shown in Fig. 1 (c) [34]. In order to optimally enhance the absorption of active layer in these two kinds of polymer-based OSCs, the resonance wavelength of SPPs is needed to coincide with the intrinsic absorption region of active-layer materials. The intrinsic absorption region of PTB7:PC71BM and P3HT: PCBM is 400–750 nm and 400–650 nm, respectively. Therefore, the period of NGs (P2) is chosen as 250 nm and 200 nm for PTB7:PC71BMbased and P3HT:PCBM-based OSCs, respectively. The SPPs and the microcavity modes are analyzed as a function of H3 with different H2s. Fig. 4(a) - (c) show the absorption spectra of the PTB7:PC71BM-based OSCs (TE polarization), in which the H2 is set to 10 nm, 15 nm and 20 nm, respectively. Parameter P2 (the period of NG) is fixed at 250 nm. It can be see that both microcavity modes and SPPs (described as black dot lines) are excited and appear in the absorption spectra. When H2 is 10 nm, the SPPs locate at around 575 nm and 710 nm (Fig. 4 (a)). The SPP modes couple with the microcavity modes weakly. As the H2 in creases, the coupling between two modes become strongly. An anticrossing coupling between the SPPs and the microcavity modes gener ates in the absorption spectra as shown in Fig. 4 (c) (described as black
3.3. Effect analysis induced by MNCGs Finally, the MNCG with the optimal parameters with P1 ¼ 6 μm, H1 ¼ 50 nm, P2 ¼ 250 nm and H2 ¼ 20 nm is introduced in the PTB7: PC71BM-based OSC model. Fig. 5 (a) shows the absorption spectra as a function of H3. It can be seen that the PCPs are excited by the anticrossing coupling between dual-microcavity modes and SPPs. When H3 is 130 nm, the absorption of PTB7:PC71BM-based OSC is enhanced significantly from 550 nm to 600 nm and from 680 nm to 720 nm. In order to investigate the effects of MNCGs on absorption enhancement in PTB7:PC71BM-based OSCs, the field intensity distribution (Hy) is simulated in TM polarization at point D (Fig. 5 (a), wavelength ¼ 575 nm, H3 ¼ 130 nm) and shown in Fig. 5 (c). It can be seen that the magnetic field intensity exhibits maximum mainly at the interface of Ag 5
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based OSC, the absorption of PTB7:PC71BM layer in MG-based OSCs is increased by 50.95% at 700 nm (the intrinsic absorption peak of PTB7), which is attributed to the dual-microcavity resonance and light scattering effects of MGs. By employing the NG (H2 ¼ 20 nm, P2 ¼ 250 nm) in PTB7:PC71BM-based OSCs, the absorption of PTB7:PC71BM layer from 550 nm to 750 nm is increased obviously in TM polarization and is increased by 20.36% at the wavelength of 700 nm. This absorp tion increment is mainly due to magnetic field intensity enhancement induced by PCPs. In TE polarization, the absorption of PTB7:PC71BM layer is also increased by 12.72% at the wavelength of 700 nm, which mainly due to the microcavity resonances and light scattering effects of NGs. As shown in Fig. 6 (b), the absorption enhancement of P3HT:PCBM layer exhibits similar characteristics to that of PTB7:PC71BM layer. The absorption is increased by 15.22% and 3.20% at 525 nm (the intrinsic absorption peak of P3HT) in P3HT:PCBM-based OSCs with MG and NG, respectively. In order to make the theoretical simulation results can be used in the actual experiment, two types of MNCGs are introduced in the polymerbased OSCs, respectively. One type (type A) exhibits the morphology with NGs covering on the whole surface of the MGs as shown in Fig. 1(a). The other type (type B) exhibits the morphology with NGs covering only on the step top-surface of Ag MGs, which is closer to the actual nano imprint modes as shown in Fig. 1 (b). For the two types of PTB7: PC71BM-based and P3HT:PCBM-based OSCs, the increment of absorp tion spectra of the PTB7:PC71BM layer and the P3HT:PCBM layer are also shown in Fig. 6 (a) and (b), respectively. It indicates that the ab sorption of PTB7:PC71BM layer from 450 nm to 750 nm is increased obviously, comparing to the planar polymer-based OSCs. When the wavelength is 700 nm, the absorption is increased by 57.41% for type A and 40.43% for type B. The absorption increment is obviously higher than that in the PTB7:PC71BM-based OSCs only with MG or NG. It suggests that the dual-microcavity resonance, PCPs and light scattering effects all enhance the absorption of PTB7:PC71BM layer in BHJ OSCs. The multi-absorption-enhancement mechanisms (the dual-microcavity resonance, PCPs and light scattering effects) are also introduced in the P3HT:PCBM-based OSCs by employing the MNCGs. As shown in Fig. 6 (b), comparing to the planar polymer-based OSCs, the absorption of P3HT:PCBM layer is increased by 20.43% for type A and 17.47% for type B at the wavelength of 525 nm, respectively. It is also higher than that in the P3HT:PCBM-based OSCs only with MG or NG. Therefore, the MNCGs effectively enhance the broadband absorption of the active layer in these both kinds of polymer-based OSCs. For all above, it also suggests that the MNCGs can be universally used in polymer-based OSCs to increase the absorption of the active layer.
Fig. 6. The absorption spectrum of the planar OSCs (cyan); the increment of the absorption spectra in the MG-based (olive), TE-NG-based (yellow), TM-NGbased (orange) and MNCG-based (blue for type B and purple for type A) OSCs: (a) the PTB7:PC71BM-based OSCs; (b) the P3HT:PCBM-based OSCs. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
cathode/PTB7:PC71BM layer and decays exponentially into both ma terials, and in particular, extends to the PTB7:PC71BM layer. This sug gests that the absorption increment at wavelength of 575 nm is mainly attributed to the magnetic field intensity enhancement induced by PCPs. The MNCG with the optimal parameters (P1 ¼ 6 μm, H1 ¼ 50 nm, P2 ¼ 200 nm and H2 ¼ 10 nm) is also introduced in the P3HT:PCBM-based OSCs. The magnetic field intensity enhancement induced by PCPs also increases the absorption in intrinsic absorption region of P3HT:PCBM as shown in Fig. 5 (b) and (d). In order to illustrate the increment of intrinsic optical absorption in active layer, the FDTD numerical simulation method is used to analyze the absorption spectra in actual OSC device model with MNCGs (Fig. 1 (a)). In the actual OSC device model, the extinction coefficients of PTB7: PC71BM, P3HT:PCBM and PEDOT:PSS are taken into account. The ab sorption spectra of PTB7:PC71BM layer or P3HT:PCBM layer in planar polymer-based OSCs and the increment of absorption spectra of these layers in polymer-based OSCs with MG, NG and MNCG are shown in Fig. 6. The increment spectra are achieved from the equation: Absgrating
Absplanar
4. Conclusions In summary, the absorption of PTB7:PC71BM-based and P3HT: PCBM-based OSCs are broadband enhanced by employing the MNCGs at the interface between active layer and Ag cathode. Multi-absorptionenhancement mechanisms (dual-microcavity resonance, PCPs and light scattering effects) are excited by employing MNCGs in the single polymer-based OSC. As a result, the absorption is enhanced obviously in the intrinsic absorption region of PTB7:PC71BM layer (450 nm–750 nm) and P3HT:PCBM layer (400 nm–650 nm). Comparing to the planar polymer-based OSCs with the same active layer materials, the absorp tion of PTB7:PC71BM layer and P3HT:PCBM layer in MNCG-based OSCs is enhanced by 57.41% at 700 nm and 20.43% at 525 nm, respectively. Meanwhile, the low cost MNCG nanoimprint modes are obtained in this work, which insures the polymer-based OSCs with MNCG-based active layer can be universally realized by nanoimprint lithography.
(3)
Where Absgrating and Absplanar are the absorption intensity of gratingbased and planar polymer-based OSCs, respectively. The H3 of PTB7: PC71BM is set to 130 nm in considering both optical and electrical performance of polymer-based OSCs. As shown in Fig. 6 (a), by employing the MG (P1 ¼ 6 μm, H1 ¼ 50 nm) in PTB7:PC71BM-based OSCs, the absorption of the PTB7:PC71BM layer is enhanced obviously from 500 nm to 750 nm. Comparing with the planar PTB7:PC71BM-
Acknowledgements This work was supported by Fujian Natural Science Foundation of China (2019J01056), the Promotion Program for Young and Middle6
Z.-x. Li et al.
Organic Electronics 78 (2020) 105591
aged Teacher in Science and Technology Research of Huaqiao University (ZQN-PY508), National Natural Science Foundation of China (61404053, 61505056), Research Project of Huaqiao University (13BS419), the Open Project Program of Fujian Key Laboratory of Light Propagation and Transformation (KF2019101), the science and tech nology research project of jilin education department (JJKH20181097KJ), the Subsidized Project for Postgraduates’ Innova tive Fund in Scientific Research of Huaqiao University (17013082006.)
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