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Highly Efficient Top-Illuminated Flexible Polymer Solar Cells with a Nanopatterned 3D Microresonant Cavity Cheng Jin An, Changsoon Cho, Jong Kil Choi, Jong-Min Park, Ming Liang Jin, Jung-Yong Lee, and Hee-Tae Jung* Considerable attention has been devoted to polymer solar cells (PSCS) as potential next-generation solar energy-harvesting devices, owing to their low manufacturing cost and the possibility of fabricating PSCs in lightweight flexible plastic.[1] In spite of the intensive research efforts in this field, the performance of PSCs is still far from commercialization requirements,[2] primarily because the short exciton diffusion length of an organic semiconductor limits the active layer thickness for efficient light absorption.[3] Although polymer/ fullerene bulk heterojunction (BHJ) systems have been employed to increase the number of interfaces for facilitating charge separation, the low carrier mobilities and tortuous transport paths in these structures increases recombination losses in thicker devices.[4] Therefore, it is essential to develop efficient light-trapping structures that can maximize light absorption, while ensuring that the resulting carriers can be efficiently collected by electrodes with minimum recombination loss. Recently, optical microresonant cavities have received particular attention due to their low photon loss rate and ability to confine and store optical energy in small volumes.[5] The microresonant cavity system consists of a photoactive layer sandwiched in two metallic electrodes, in which an out-of-cell capping layer (CL) on a silver (Ag) electrode can improve light trapping inside the organic thin film.[6] This type of organic cell not only enhances the transmittance of the Ag electrode, enabling low reflectance of incident light over a broad range of wavelengths, but also improves the electric field intensity in the photoactive layers by an induced microresonant cavity arising from fine-tuning the thickness of the CL.[7] Similar to a BHJ system, the introduction of a nanostructure in the microresonant cavity system is a promising route to further improve the efficiency of microresonant cavity-based C. J. An, J. K. Choi, J.-M. Park, M. L. Jin, Prof. H.-T. Jung Department of Chemical and Biomolecular Engineering Korea Advanced Institute of Science and Technology Daejeon, 305–701, Republic of Korea E-mail: [email protected] C. Cho, Prof. J.-Y. Lee Graduate School of Energy Environment, Water and Sustainability (EEWS) Korea Advanced Institute of Science and Technology Daejeon, 305–701, Republic of Korea DOI: 10.1002/smll.201302653
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PSCs. Light trapping can be enhanced by a scattering effect generated from the nanostructure, leading to an increased path length of light inside the cell and enhanced absorption at longer wavelengths.[8] Also, the nanostructure can be deliberately tailored to improve carrier collection efficiency.[9] Unfortunately however, microresonant cavities with ultra-thin Ag transparent electrodes have thus far not been fabricated with three dimensional (3D) nanopatterned features, probably due to difficulties in controlling a thin, uniform thickness of each layer within the nanostructure. To derive the greatest benefits from nanostructure features in microresonant cavities, it is desirable for each layer to be uniformly and conformably coated with an underlying nanopatterned substrate. Indeed, conventional patterning methods, such as nanoimprinting on the top of the active layer, are not suitable, owing to the alternating thick and thin regions within the active layer.[10] Also, contamination during the embossing step can lead to Schottky barrier formation in the organic/metal interface.[11] In the studies described below, we designed and experimentally demonstrated highly efficient top-illuminated flexible polymer solar cells using nanopatterned polyethylene terephthalate (PET) substrates with a 3D nanopatterned microresonant cavity. To enable optimum carrier transport in the dielectric/metal/dielectric (DMD) transparent electrodebased cells through a nanopatterned micro resonant cavity, we precisely controlled the thickness of the photoactive and transporting layers. Then, the optical-field distribution inside the photoactive layer was tuned by the out-of-cell CL thickness and nanopatterns. The resulting 3D nanopatterned microresonant cavity shows that top-illuminated PSCs provide not only powerful light-trapping but also electrical enhancement, resulting in high performance solar cells with an excellent power conversion efficiency. Figure 1a shows the overall scheme for fabricating flexible top-illuminated PSCs with a nanopatterned 3D microresonant cavity, which involves the nanopatterning process, thermal evaporation of the bottom aluminum electrode on a nanopatterned PET substrate, spin-casting of the active materials, and final deposition of the anode. Figure 1b represents the device structure of the nanopatterned microresonant cavity cell. The polystyrene (PS) pattern was transferred from a polydimethylsiloxane (PDMS) mold by placing the PDMS mold onto the PS surface and heating it above the glass transition temperature (Tg), allowing capillary forces to drive the polymer into the void space of the pattern. The PET film under the PS mask was then etched by reactive ion etching
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with a line-shaped pattern 50 nm in height and 500 nm in width (Figure 2a). The shape and dimensions of the pattern were almost retained after evaporation on the flexible PET substrate (Figure 2b). After spin-coating, the line pattern was retained to a good degree by the bottom electrode, while the height of the line was reduced to 25 nm (Figure 2c). However, the device fabrication steps such as deposition of the anode did not change the dimensions of the periodic line pattern on the top electrode; the height of the line remained as 25 nm (Figure 2d), indicating that the line pattern was well retained. An effort was made to ensure a uniform coating of the photoactive layer in order to retain identical feature dimensions of the nanopatterned PET substrate by changing the solution concentration and spin-coating rate of the photoactive materials. In fact, Figure 1. Schematic diagram of the fabrication sequence of top-illuminated PSCs with a a photoactive layer thinner than 20∼30 nm 3-dimensional microresonant cavity (a), cross-sectional image of the PSC (b), chemical is not suitable in polymer solar cells, due structure of PCDTBT and PC70BM, employed as the photoactive layer (c). to low light absorption through ultrathin films. Thus, the thickness of the photousing a mixture of O2 (40 sccm) and CF4 (60 sccm) plasma active layer is a compromise in order to optimize the cell at a chamber pressure of 20 mTorr and a power of 80 W. The characteristics. In relatively thicker films ≈>30 nm, however, 65 nm thick active layer, consisting of a blend of the low-band the line height did not change significantly after spin-coating gap semiconducting polymer Poly [[9-(1-octylnonyl)-9H-car- of the photoactive layer; this is perhaps due to confinement bazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole- of the polymer in the nanostructure. Scanning electron 4,7-diyl-2,5-thiophenediyl, PCDTBT] as an electron donor microscope (SEM) images of the top and side of completed and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) devices further confirm the large area nanopatterned struc(Figure 1c) as an electron acceptor, was prepared on bottom ture of the top-illuminated flexible PSCs (Figure S2). The electrodes by spin-coating 3 wt% of PCDTBT and PC70BM increased electrode-BHJ interfacial areas on both sides of mixture (1:4 weight ratio) dissolved in dichlorobenzene, as the BHJ improve the carrier collection efficiency. To investigate the effect of nanopatterning of the microreported recently.[12] As a representative nanostructure, we used a highly periodic line pattern with a height of 50 nm and resonant cavities on the performance of PSCs, we measured a width of 500 nm; this allowed the thickness of the micro- the current density-voltage (J-V) characteristics of three difresonant cavity system to be controlled and retained more ferent devices (Figure 3): a flat cell without a microresonant easily than other structures, and metallic grating based light cavity (CL (MoO3) on the top electrode of Ag) (–䊉–), a trapping schemes have been investigated in traditional inor- non-patterned microresonant cavity cell (–䉱–), and a nanoganic photovoltaic cells, demonstrating effective light trap- patterned microresonant cavity cell (–䊏–). A summary of ping via both scattering effect and surface plasmon effect.[13] the results is shown in Table 1. The power conversion effiIn fact, we conducted FDTD simulation of line pattern with ciency (PCE) of the reference cell was improved from 3.56% other feature dimensions, for example, a height of 50 nm and to 3.89% by employing MoO3 as a CL on the Ag top eleca width of 1000 nm in order to test the effect of the patterned trode. Such an increase of the short current density (Jsc) is line width on the cell performance. The FDTD simulation attributed to two reasons:[14] Spectral transmittance of the results at 650 nm monochromatic wave show that the elec- top electrode was dramatically increased by sandwiching tric field intensity of nano-patterned cell with 50 nm height silver (Ag) electrodes between the hole transporting layer and 500 nm width shows higher intensity as compared to that MoO3 (10 nm) and the CL (MoO3) (40 nm) (Figure S3). with 1000 nm width (Figure S1, Supporting Information). Also, the CL effectively increases the transmittance of the Thus, we fixed the line pattern to a height of 50 nm and a transparent electrode, partially relying upon minimized interwidth of 500 nm to investigate the performance of the 3D ference effects. The electric field intensity within the photoactive layer could be enhanced by optimizing the thickness nanopatterned microresonant cavity. To investigate the surface morphology and dimensions of the CL so that incoming light can be coupled with outof flexible PCDTBT:PC70BM PSCs with nanopatterned 3D going light from reflection by the rear electrodes (Figure S4). microresonant cavities, atomic force microscope (AFM) More interestingly, the PCE was considerably increased: from images were collected for each process step (Figure 2). 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Figure 3. Current density-voltage (J–V) characteristics of devices with different structures measured under AM 1.5 illumination at 100 mW cm−2.
Figure 2. Surface morphology and surface profile of nanostructured top-illuminated PSCs at different stages during the fabrication process. (a) Nanopatterned PET substrate by conventional patterning method; (b) indirectly patterned rear electrode of aluminum by thermal evaporating 100 nm Al; (c) active layer of PCDTBT:PC70BM; (d) ultra-thin Ag top electrode sandwiched by MoO3.
device. The PCE improvement originates from the improvement of Jsc and the fill factor (FF); the higher FF is a consequence of the nanopattern whereby the interface area is increased.[15] Also, the decreased series resistance (Rs). from 31.39 Ω cm2 to 26.43 Ω cm2, and the increased parallel resistance, from 750.75 Ω cm2 to 824.72 Ω cm2, contributes to the enhanced FF. Figure 4a shows the incident photon-to-current efficiency (IPCE) spectrum for each device structure. The difference in IPCE indicates an increase in the number of carriers generated and/or an improvement in the charge collection efficiency at a particular wavelength. It should be noted that the short-circuit current measured under the irradiation of a solar simulator and integrated from IPCE is in the range of the rational error. This difference in Jsc between devices
is consistent with the IPCE spectrum. The formation of both the CL and nanopattern of the top-illuminated flexible PSCs caused a 17.3% increase in the photocurrent of the BHJ device. In order to further understand the relationship between the nanopattern of the rear electrode and the IPCE enhancement, we carried out diffuse reflectance measurements of the flexible top-illuminated PSCs with different microresonant cavity structures (Figure 4b). The intensity of reflectance is proportional to the number of unabsorbed photons, thus a lower reflectance for the nanopatterned cell with a CL indicates stronger absorption. Also, the reflectance peak of the nanopatterned cell device is broader than that of the flat device, indicating stronger absorption within active layers at longer wavelengths; this is probably due to the nanopatterned interface between the active layer and the aluminum cathode inducing scattering as well as reflection of light.[16] In addition, the reflectance of the devices with nanopatterns is significantly lower than that of the flat microresonant cavity devices. The lower reflectance (R) of the flexible PSCs as a consequence of enhanced diffuse reflectance indicate that a larger fraction of incident photons (ηabs = 1−R) are absorbed in the nanopatterned device with the CL, where ηabs represents the absorption of the entire device. Therefore, formation of nanopattern on both sides of the photoactive layers makes a significant contribution to the improvement of IPCE for λ > 450 nm. To further elucidate the enhancement of Jsc from 8.39 to 9.02 mA cm−2 caused by the nanopattern in the microresonant cavity device, we performed an optical simulation of the flat (–䉱–) and nanopatterned microresonant cavity device
Table 1. Summary of device performance of PSCs with different structures. Configuration Flat cell without CL
Voc [V]
Jsc [mA cm−2]
FF
PCE [%]
Rp [Ω·cm2]
Rs [Ω·cm2]
0.89
7.69
0.52
3.56
732.04
32.77
Flat cell with CL
0.91
8.39
0.51
3.89
750.75
31.39
Nano-patterned cell with CL
0.89
9.02
0.56
4.5
824.72
26.43
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Figure 4. (a) Measured EQE spectra and (b) diffuse reflectance spectra of the top-illuminated flat cell with CL and nanopatterned cell with CL.
(–䊏–) (Figure 5a). The finite elements method (FEM) was utilized to calculate the optical performance of each device, accounting for the wavelength dependence of the dielectric function of the devices. For the calculation of the nano-patterned device, the active layer thickness was assumed to be 65 nm in the convex region and 90 nm in the concave region. The thicknesses of other layers were assumed to be the same as the flat device, which had a structure of MoO3 (40 nm)/ Ag (13 nm)/MoO3 (10 nm)/PCDTBT:PC70BM (65 nm)/Al, following the actual device structure. As shown in Figure 5a, the light absorption spectra of the active layer obtained by the calculation have a similar trend as the experimental (IPCE) spectra results in Figure 4a. The active absorption of the patterned microresonant cavity device is lower than that of the flat microresonant cavity device in the short wavelength region (≈330–600 nm), while the absorption of the patterned microresonant cavity device is higher in the longer wavelength region (>600 nm). The difference in intensity between the experimental carrier collection by electrodes (Figure 4a) and the simulated light absorption by the active small 2014, 10, No. 7, 1278–1283
Figure 5. (a) Simulated active absorption and (b) internal quantum efficiency (IQE) of the top-illuminated flat cell with CL and nanopatterned cell with CL. For the patterned device, the active layer thickness was assumed to be 65 nm on the convex region and 90 nm on the concave region, and the thicknesses of the other layers were assumed to be same as the flat device; the flat device had a structure of MoO3 (40 nm)/ Ag (13 nm)/MoO3 (10 nm)/PCDTBT:PC70BM (65 nm)/Al, following the real device structure.
layer (Figure 5a) might be due to an electrical property such as charge transport or charge collection. As shown in Figure 5b, the internal quantum efficiency (IQE) for devices with and without the nanopattern was found by combining the calculated active absorption (Figure 5a) and the experimental IPCE (Figure 4a) according to the following equation: IQE(8) =
EQE(8) AbsAL (8)
where, AL denotes the active layer, AbsAL is the absorption in the active layer, and EQE is the number of charge carriers collected by the solar cell. IQE is the ratio of the number of collected electrons to the number of photons absorbed in the active layer. Hence, a high IQE indicates that the active layer of the solar cell is capable of making good use of the
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photons. For the whole wavelength range, the IQE is constantly higher in the patterned device than in the flat device. As shown in Figure S5, the enhancement of the IQE was also observed in the experimental results, where the total absorption of the entire device was estimated by 1−R(λ). Thus, the IQE enhancement can be explained by the improvement of charge collection and extraction with lower recombination.[17] Consequently, the enhancement in Jsc of the PSC with the nanopatterned 3D microresonant cavity originated from greater light absorption in the long-wavelength region and improvement in the electrical properties. To conclude, highly efficient flexible PSCs with a nanopatterned 3D microresonant cavity were successfully designed and fabricated. The cell efficiency was significantly improved (26.4%) by providing a 3D nanopattern on the microresonant cavity, which is due not only to greater absorption, but also to electrical enhancement: the CL enhanced the transmittance of the transparent electrodes, increasing electric field intensity in the photoactive layer, and the nanopattern on the rear electrodes of flexible PSCs with 3D cavity structures caused significant enhancement to the Jsc of the flexible cell. The IQE results show that the increased interfacial area of both electrodes effectively increased the Jsc and FF by enhancing the charge collection. As a result, highly efficient flexible top-illuminated PCDTBT:PC70BM PSCs were achieved, exhibiting excellent electrical characteristics with a high FF, Voc and PCE of up to 4.5%, offering great potential for the fabrication of highly stable and high performing plastic solar cells.
Characterization: AFM images were taken in scan mode on a Veeco multi-mode AFM with a nanoscope III controller. The J–V characteristics of the solar cells were tested in air using a Keithley 2400 source measurement unit, and a 150 W solar simulator with AM 1.5G filter from Newport Corporation at an intensity of 100 mW cm−2. All data on the device performance presented in the article are have been averaged over at least 10 devices from three repeated experiments. The series resistance and parallel resistance were calculated from the inverse of the slope in the J–V curve at 0.8 V and 0 V, respectively. Optical Simulation of Microresonant Effect: Transfer matrix formalism was used to calculate the interference of reflected and transmitted light waves at each interface in the stack. It is assumed that the materials are optically smooth and isotropic. The electric field is modeled considering normal incidence, so that both polarizations (s, p) behave the same. The complex refractive index (n = η + iκ) of the materials was measured by variable angle spectroscopic ellipsometry (VASE), and the imaginary part (κ) was corrected based on the transmission spectrum.[18] Optical Simulation of Nanostructure: A commercial FEM simulation tool in COMSOL multi-physics was used to simulate the nanopattern-based PSCs, and compare these PSCs with a flat structure. The 2D calculations were performed separately for s-polarized and p-polarized light then averaged. The simulation region had a width of 1000 nm and the left and right sides were assumed to have periodic boundary conditions (PBCs).
Supporting Information
Experimental Section Nanopatterning Process: PET substrates were sequentially cleaned by sonication in detergent, acetone and isopropyl alcohol, then dried under a N2 stream. Then a polystyrene solution (2 wt% in toluene) was spin coated on the PET substrate. The 120 nm height pattern was transferred from PDMS molds onto PS film. The PET substrate with the PS pattern film was etched using reactive ion etching under the following conditions: 40 sccm oxygen flow and 60 sccm tetrafluoromethane, at a power of 80 W for ≈30–40 s. Finally, the residual PS (∼70 nm) was removed in a cleaning process. Device Fabrication: the PCDTBT (Mw: 67000, PDI: 2.5) and PC70BM were supplied by 1-material. Flexible substrates with and without a nanopattern were cleaned sequentially by sonication in detergent, deionized (DI) water, acetone and isopropyl alcohol, and then dried under a N2 stream. The 100 nm bottom electrode of aluminum was deposited using thermal evaporation. The photoactive layers were obtained by spin-coating 3 wt% PCDTBT and PC70BM mixed solution (1:4 weight ratio) in dichlorobenzene onto the bottom electrodes. The photoactive layers were then slowly dried in a glass dish for 1 h. 10 nm of MoO3 was thermally deposited as a hole transporting layer under a base pressure of 2 × 10−6 Torr with a deposition rate of 0.2 䊐/s, then 13 nm of Ag was deposited as the top electrode at a rate of 2 䊐/s, and 40 nm of CL (MoO3) was deposited on the Ag electrode at a the rate of 0.1 䊐/s. The active area of 0.102 cm2 was determined by the overlap of the Al layer and the top transparent Ag electrode.
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Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the World Class University Program (R32–2008–000–10142–0, NRF) and the Global Frontier Research Center for Advanced Soft Electronics (no. 2011–0032062, MEST). Also this research was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology, Korea (MEST) (no.2012R1A2A1A01003537).
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Highly Efficient Top-Illuminated Flexible Polymer Solar Cells with a Nanopatterned 3D Microresonant Cavity Cheng Jin An, Changsoon Cho, Jong Kil Choi, Jong-Min Park, Ming Liang Jin, Jung-Yong Lee, and Hee-Tae Jung* Considerable attention has been devoted to polymer solar cells (PSCS) as potential next-generation solar energy-harvesting devices, owing to their low manufacturing cost and the possibility of fabricating PSCs in lightweight flexible plastic.[1] In spite of the intensive research efforts in this field, the performance of PSCs is still far from commercialization requirements,[2] primarily because the short exciton diffusion length of an organic semiconductor limits the active layer thickness for efficient light absorption.[3] Although polymer/ fullerene bulk heterojunction (BHJ) systems have been employed to increase the number of interfaces for facilitating charge separation, the low carrier mobilities and tortuous transport paths in these structures increases recombination losses in thicker devices.[4] Therefore, it is essential to develop efficient light-trapping structures that can maximize light absorption, while ensuring that the resulting carriers can be efficiently collected by electrodes with minimum recombination loss. Recently, optical microresonant cavities have received particular attention due to their low photon loss rate and ability to confine and store optical energy in small volumes.[5] The microresonant cavity system consists of a photoactive layer sandwiched in two metallic electrodes, in which an out-of-cell capping layer (CL) on a silver (Ag) electrode can improve light trapping inside the organic thin film.[6] This type of organic cell not only enhances the transmittance of the Ag electrode, enabling low reflectance of incident light over a broad range of wavelengths, but also improves the electric field intensity in the photoactive layers by an induced microresonant cavity arising from fine-tuning the thickness of the CL.[7] Similar to a BHJ system, the introduction of a nanostructure in the microresonant cavity system is a promising route to further improve the efficiency of microresonant cavity-based C. J. An, J. K. Choi, J.-M. Park, M. L. Jin, Prof. H.-T. Jung Department of Chemical and Biomolecular Engineering Korea Advanced Institute of Science and Technology Daejeon, 305–701, Republic of Korea E-mail: [email protected] C. Cho, Prof. J.-Y. Lee Graduate School of Energy Environment, Water and Sustainability (EEWS) Korea Advanced Institute of Science and Technology Daejeon, 305–701, Republic of Korea DOI: 10.1002/smll.201302653
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PSCs. Light trapping can be enhanced by a scattering effect generated from the nanostructure, leading to an increased path length of light inside the cell and enhanced absorption at longer wavelengths.[8] Also, the nanostructure can be deliberately tailored to improve carrier collection efficiency.[9] Unfortunately however, microresonant cavities with ultra-thin Ag transparent electrodes have thus far not been fabricated with three dimensional (3D) nanopatterned features, probably due to difficulties in controlling a thin, uniform thickness of each layer within the nanostructure. To derive the greatest benefits from nanostructure features in microresonant cavities, it is desirable for each layer to be uniformly and conformably coated with an underlying nanopatterned substrate. Indeed, conventional patterning methods, such as nanoimprinting on the top of the active layer, are not suitable, owing to the alternating thick and thin regions within the active layer.[10] Also, contamination during the embossing step can lead to Schottky barrier formation in the organic/metal interface.[11] In the studies described below, we designed and experimentally demonstrated highly efficient top-illuminated flexible polymer solar cells using nanopatterned polyethylene terephthalate (PET) substrates with a 3D nanopatterned microresonant cavity. To enable optimum carrier transport in the dielectric/metal/dielectric (DMD) transparent electrodebased cells through a nanopatterned micro resonant cavity, we precisely controlled the thickness of the photoactive and transporting layers. Then, the optical-field distribution inside the photoactive layer was tuned by the out-of-cell CL thickness and nanopatterns. The resulting 3D nanopatterned microresonant cavity shows that top-illuminated PSCs provide not only powerful light-trapping but also electrical enhancement, resulting in high performance solar cells with an excellent power conversion efficiency. Figure 1a shows the overall scheme for fabricating flexible top-illuminated PSCs with a nanopatterned 3D microresonant cavity, which involves the nanopatterning process, thermal evaporation of the bottom aluminum electrode on a nanopatterned PET substrate, spin-casting of the active materials, and final deposition of the anode. Figure 1b represents the device structure of the nanopatterned microresonant cavity cell. The polystyrene (PS) pattern was transferred from a polydimethylsiloxane (PDMS) mold by placing the PDMS mold onto the PS surface and heating it above the glass transition temperature (Tg), allowing capillary forces to drive the polymer into the void space of the pattern. The PET film under the PS mask was then etched by reactive ion etching
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with a line-shaped pattern 50 nm in height and 500 nm in width (Figure 2a). The shape and dimensions of the pattern were almost retained after evaporation on the flexible PET substrate (Figure 2b). After spin-coating, the line pattern was retained to a good degree by the bottom electrode, while the height of the line was reduced to 25 nm (Figure 2c). However, the device fabrication steps such as deposition of the anode did not change the dimensions of the periodic line pattern on the top electrode; the height of the line remained as 25 nm (Figure 2d), indicating that the line pattern was well retained. An effort was made to ensure a uniform coating of the photoactive layer in order to retain identical feature dimensions of the nanopatterned PET substrate by changing the solution concentration and spin-coating rate of the photoactive materials. In fact, Figure 1. Schematic diagram of the fabrication sequence of top-illuminated PSCs with a a photoactive layer thinner than 20∼30 nm 3-dimensional microresonant cavity (a), cross-sectional image of the PSC (b), chemical is not suitable in polymer solar cells, due structure of PCDTBT and PC70BM, employed as the photoactive layer (c). to low light absorption through ultrathin films. Thus, the thickness of the photousing a mixture of O2 (40 sccm) and CF4 (60 sccm) plasma active layer is a compromise in order to optimize the cell at a chamber pressure of 20 mTorr and a power of 80 W. The characteristics. In relatively thicker films ≈>30 nm, however, 65 nm thick active layer, consisting of a blend of the low-band the line height did not change significantly after spin-coating gap semiconducting polymer Poly [[9-(1-octylnonyl)-9H-car- of the photoactive layer; this is perhaps due to confinement bazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole- of the polymer in the nanostructure. Scanning electron 4,7-diyl-2,5-thiophenediyl, PCDTBT] as an electron donor microscope (SEM) images of the top and side of completed and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) devices further confirm the large area nanopatterned struc(Figure 1c) as an electron acceptor, was prepared on bottom ture of the top-illuminated flexible PSCs (Figure S2). The electrodes by spin-coating 3 wt% of PCDTBT and PC70BM increased electrode-BHJ interfacial areas on both sides of mixture (1:4 weight ratio) dissolved in dichlorobenzene, as the BHJ improve the carrier collection efficiency. To investigate the effect of nanopatterning of the microreported recently.[12] As a representative nanostructure, we used a highly periodic line pattern with a height of 50 nm and resonant cavities on the performance of PSCs, we measured a width of 500 nm; this allowed the thickness of the micro- the current density-voltage (J-V) characteristics of three difresonant cavity system to be controlled and retained more ferent devices (Figure 3): a flat cell without a microresonant easily than other structures, and metallic grating based light cavity (CL (MoO3) on the top electrode of Ag) (–䊉–), a trapping schemes have been investigated in traditional inor- non-patterned microresonant cavity cell (–䉱–), and a nanoganic photovoltaic cells, demonstrating effective light trap- patterned microresonant cavity cell (–䊏–). A summary of ping via both scattering effect and surface plasmon effect.[13] the results is shown in Table 1. The power conversion effiIn fact, we conducted FDTD simulation of line pattern with ciency (PCE) of the reference cell was improved from 3.56% other feature dimensions, for example, a height of 50 nm and to 3.89% by employing MoO3 as a CL on the Ag top eleca width of 1000 nm in order to test the effect of the patterned trode. Such an increase of the short current density (Jsc) is line width on the cell performance. The FDTD simulation attributed to two reasons:[14] Spectral transmittance of the results at 650 nm monochromatic wave show that the elec- top electrode was dramatically increased by sandwiching tric field intensity of nano-patterned cell with 50 nm height silver (Ag) electrodes between the hole transporting layer and 500 nm width shows higher intensity as compared to that MoO3 (10 nm) and the CL (MoO3) (40 nm) (Figure S3). with 1000 nm width (Figure S1, Supporting Information). Also, the CL effectively increases the transmittance of the Thus, we fixed the line pattern to a height of 50 nm and a transparent electrode, partially relying upon minimized interwidth of 500 nm to investigate the performance of the 3D ference effects. The electric field intensity within the photoactive layer could be enhanced by optimizing the thickness nanopatterned microresonant cavity. To investigate the surface morphology and dimensions of the CL so that incoming light can be coupled with outof flexible PCDTBT:PC70BM PSCs with nanopatterned 3D going light from reflection by the rear electrodes (Figure S4). microresonant cavities, atomic force microscope (AFM) More interestingly, the PCE was considerably increased: from images were collected for each process step (Figure 2). The 3.89% in the flat cell with a CL (MoO3) on the Ag top elecflexible nanopatterned PET substrates are highly periodic, trode to 4.5% in the nanopatterned microresonant cavity small 2014, 10, No. 7, 1278–1283
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Figure 3. Current density-voltage (J–V) characteristics of devices with different structures measured under AM 1.5 illumination at 100 mW cm−2.
Figure 2. Surface morphology and surface profile of nanostructured top-illuminated PSCs at different stages during the fabrication process. (a) Nanopatterned PET substrate by conventional patterning method; (b) indirectly patterned rear electrode of aluminum by thermal evaporating 100 nm Al; (c) active layer of PCDTBT:PC70BM; (d) ultra-thin Ag top electrode sandwiched by MoO3.
device. The PCE improvement originates from the improvement of Jsc and the fill factor (FF); the higher FF is a consequence of the nanopattern whereby the interface area is increased.[15] Also, the decreased series resistance (Rs). from 31.39 Ω cm2 to 26.43 Ω cm2, and the increased parallel resistance, from 750.75 Ω cm2 to 824.72 Ω cm2, contributes to the enhanced FF. Figure 4a shows the incident photon-to-current efficiency (IPCE) spectrum for each device structure. The difference in IPCE indicates an increase in the number of carriers generated and/or an improvement in the charge collection efficiency at a particular wavelength. It should be noted that the short-circuit current measured under the irradiation of a solar simulator and integrated from IPCE is in the range of the rational error. This difference in Jsc between devices
is consistent with the IPCE spectrum. The formation of both the CL and nanopattern of the top-illuminated flexible PSCs caused a 17.3% increase in the photocurrent of the BHJ device. In order to further understand the relationship between the nanopattern of the rear electrode and the IPCE enhancement, we carried out diffuse reflectance measurements of the flexible top-illuminated PSCs with different microresonant cavity structures (Figure 4b). The intensity of reflectance is proportional to the number of unabsorbed photons, thus a lower reflectance for the nanopatterned cell with a CL indicates stronger absorption. Also, the reflectance peak of the nanopatterned cell device is broader than that of the flat device, indicating stronger absorption within active layers at longer wavelengths; this is probably due to the nanopatterned interface between the active layer and the aluminum cathode inducing scattering as well as reflection of light.[16] In addition, the reflectance of the devices with nanopatterns is significantly lower than that of the flat microresonant cavity devices. The lower reflectance (R) of the flexible PSCs as a consequence of enhanced diffuse reflectance indicate that a larger fraction of incident photons (ηabs = 1−R) are absorbed in the nanopatterned device with the CL, where ηabs represents the absorption of the entire device. Therefore, formation of nanopattern on both sides of the photoactive layers makes a significant contribution to the improvement of IPCE for λ > 450 nm. To further elucidate the enhancement of Jsc from 8.39 to 9.02 mA cm−2 caused by the nanopattern in the microresonant cavity device, we performed an optical simulation of the flat (–䉱–) and nanopatterned microresonant cavity device
Table 1. Summary of device performance of PSCs with different structures. Configuration Flat cell without CL
Voc [V]
Jsc [mA cm−2]
FF
PCE [%]
Rp [Ω·cm2]
Rs [Ω·cm2]
0.89
7.69
0.52
3.56
732.04
32.77
Flat cell with CL
0.91
8.39
0.51
3.89
750.75
31.39
Nano-patterned cell with CL
0.89
9.02
0.56
4.5
824.72
26.43
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Flexible Polymer Solar Cells with a Nanopatterned 3D Microresonant Cavity
Figure 4. (a) Measured EQE spectra and (b) diffuse reflectance spectra of the top-illuminated flat cell with CL and nanopatterned cell with CL.
(–䊏–) (Figure 5a). The finite elements method (FEM) was utilized to calculate the optical performance of each device, accounting for the wavelength dependence of the dielectric function of the devices. For the calculation of the nano-patterned device, the active layer thickness was assumed to be 65 nm in the convex region and 90 nm in the concave region. The thicknesses of other layers were assumed to be the same as the flat device, which had a structure of MoO3 (40 nm)/ Ag (13 nm)/MoO3 (10 nm)/PCDTBT:PC70BM (65 nm)/Al, following the actual device structure. As shown in Figure 5a, the light absorption spectra of the active layer obtained by the calculation have a similar trend as the experimental (IPCE) spectra results in Figure 4a. The active absorption of the patterned microresonant cavity device is lower than that of the flat microresonant cavity device in the short wavelength region (≈330–600 nm), while the absorption of the patterned microresonant cavity device is higher in the longer wavelength region (>600 nm). The difference in intensity between the experimental carrier collection by electrodes (Figure 4a) and the simulated light absorption by the active small 2014, 10, No. 7, 1278–1283
Figure 5. (a) Simulated active absorption and (b) internal quantum efficiency (IQE) of the top-illuminated flat cell with CL and nanopatterned cell with CL. For the patterned device, the active layer thickness was assumed to be 65 nm on the convex region and 90 nm on the concave region, and the thicknesses of the other layers were assumed to be same as the flat device; the flat device had a structure of MoO3 (40 nm)/ Ag (13 nm)/MoO3 (10 nm)/PCDTBT:PC70BM (65 nm)/Al, following the real device structure.
layer (Figure 5a) might be due to an electrical property such as charge transport or charge collection. As shown in Figure 5b, the internal quantum efficiency (IQE) for devices with and without the nanopattern was found by combining the calculated active absorption (Figure 5a) and the experimental IPCE (Figure 4a) according to the following equation: IQE(8) =
EQE(8) AbsAL (8)
where, AL denotes the active layer, AbsAL is the absorption in the active layer, and EQE is the number of charge carriers collected by the solar cell. IQE is the ratio of the number of collected electrons to the number of photons absorbed in the active layer. Hence, a high IQE indicates that the active layer of the solar cell is capable of making good use of the
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photons. For the whole wavelength range, the IQE is constantly higher in the patterned device than in the flat device. As shown in Figure S5, the enhancement of the IQE was also observed in the experimental results, where the total absorption of the entire device was estimated by 1−R(λ). Thus, the IQE enhancement can be explained by the improvement of charge collection and extraction with lower recombination.[17] Consequently, the enhancement in Jsc of the PSC with the nanopatterned 3D microresonant cavity originated from greater light absorption in the long-wavelength region and improvement in the electrical properties. To conclude, highly efficient flexible PSCs with a nanopatterned 3D microresonant cavity were successfully designed and fabricated. The cell efficiency was significantly improved (26.4%) by providing a 3D nanopattern on the microresonant cavity, which is due not only to greater absorption, but also to electrical enhancement: the CL enhanced the transmittance of the transparent electrodes, increasing electric field intensity in the photoactive layer, and the nanopattern on the rear electrodes of flexible PSCs with 3D cavity structures caused significant enhancement to the Jsc of the flexible cell. The IQE results show that the increased interfacial area of both electrodes effectively increased the Jsc and FF by enhancing the charge collection. As a result, highly efficient flexible top-illuminated PCDTBT:PC70BM PSCs were achieved, exhibiting excellent electrical characteristics with a high FF, Voc and PCE of up to 4.5%, offering great potential for the fabrication of highly stable and high performing plastic solar cells.
Characterization: AFM images were taken in scan mode on a Veeco multi-mode AFM with a nanoscope III controller. The J–V characteristics of the solar cells were tested in air using a Keithley 2400 source measurement unit, and a 150 W solar simulator with AM 1.5G filter from Newport Corporation at an intensity of 100 mW cm−2. All data on the device performance presented in the article are have been averaged over at least 10 devices from three repeated experiments. The series resistance and parallel resistance were calculated from the inverse of the slope in the J–V curve at 0.8 V and 0 V, respectively. Optical Simulation of Microresonant Effect: Transfer matrix formalism was used to calculate the interference of reflected and transmitted light waves at each interface in the stack. It is assumed that the materials are optically smooth and isotropic. The electric field is modeled considering normal incidence, so that both polarizations (s, p) behave the same. The complex refractive index (n = η + iκ) of the materials was measured by variable angle spectroscopic ellipsometry (VASE), and the imaginary part (κ) was corrected based on the transmission spectrum.[18] Optical Simulation of Nanostructure: A commercial FEM simulation tool in COMSOL multi-physics was used to simulate the nanopattern-based PSCs, and compare these PSCs with a flat structure. The 2D calculations were performed separately for s-polarized and p-polarized light then averaged. The simulation region had a width of 1000 nm and the left and right sides were assumed to have periodic boundary conditions (PBCs).
Supporting Information
Experimental Section Nanopatterning Process: PET substrates were sequentially cleaned by sonication in detergent, acetone and isopropyl alcohol, then dried under a N2 stream. Then a polystyrene solution (2 wt% in toluene) was spin coated on the PET substrate. The 120 nm height pattern was transferred from PDMS molds onto PS film. The PET substrate with the PS pattern film was etched using reactive ion etching under the following conditions: 40 sccm oxygen flow and 60 sccm tetrafluoromethane, at a power of 80 W for ≈30–40 s. Finally, the residual PS (∼70 nm) was removed in a cleaning process. Device Fabrication: the PCDTBT (Mw: 67000, PDI: 2.5) and PC70BM were supplied by 1-material. Flexible substrates with and without a nanopattern were cleaned sequentially by sonication in detergent, deionized (DI) water, acetone and isopropyl alcohol, and then dried under a N2 stream. The 100 nm bottom electrode of aluminum was deposited using thermal evaporation. The photoactive layers were obtained by spin-coating 3 wt% PCDTBT and PC70BM mixed solution (1:4 weight ratio) in dichlorobenzene onto the bottom electrodes. The photoactive layers were then slowly dried in a glass dish for 1 h. 10 nm of MoO3 was thermally deposited as a hole transporting layer under a base pressure of 2 × 10−6 Torr with a deposition rate of 0.2 䊐/s, then 13 nm of Ag was deposited as the top electrode at a rate of 2 䊐/s, and 40 nm of CL (MoO3) was deposited on the Ag electrode at a the rate of 0.1 䊐/s. The active area of 0.102 cm2 was determined by the overlap of the Al layer and the top transparent Ag electrode.
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Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the World Class University Program (R32–2008–000–10142–0, NRF) and the Global Frontier Research Center for Advanced Soft Electronics (no. 2011–0032062, MEST). Also this research was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology, Korea (MEST) (no.2012R1A2A1A01003537).
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Received: August 14, 2013 Revised: October 1, 2013 Published online: November 28, 2013
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