1 Nature Communications 2013 Vol: 4():1446-. DOI: 10.1038/ncomms2411

A polymer tandem solar cell with 10.6% power conversion efficiency

An effective way to improve polymer solar cell efficiency is to use a tandem structure, as a broader part of the spectrum of solar radiation is used and the thermalization loss of photon energy is minimized. In the past, the lack of high-performance low-bandgap polymers was the major limiting factor for achieving high-performance tandem solar cell. Here we report the development of a high-performance low bandgap polymer (bandgap <1.4 eV), poly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:2′,3′-d]pyran)-alt-4,7-(5,6-difluoro-2,1,3-benzothia diazole)] with a bandgap of 1.38 eV, high mobility, deep highest occupied molecular orbital. As a result, a single-junction device shows high external quantum efficiency of >60% and spectral response that extends to 900 nm, with a power conversion efficiency of 7.9%. The polymer enables a solution processed tandem solar cell with certified 10.6% power conversion efficiency under standard reporting conditions (25 °C, 1,000 Wm−2, IEC 60904-3 global), which is the first certified polymer solar cell efficiency over 10%.

Mentions
Figures
Figure 1: Characterization of three DT-BT-based polymers.(a) Chemical structures of three different polymers (PCPDT-BT, PCPDT-DFBT and PDTP-DFBT) based on DT-BT backbone. (b) Normalized absorption spectra of the three polymers PCPDT-BT (black line), PCPDT-DFBT (red line) and PDTP-DFBT (blue line) spin-coated on glass substrate. (c) Electrochemical cyclic voltammogram of PCPDT-BT (black line) PCPDT-DFBT (red line) and PDTP-DFBT (blue line). The red arrows indicate the potential onset of the oxidation or reduction reactions in the electrochemical measurements. On the basis of equation (see Methods) it can be calculated the HOMO and the LUMO for PCPDT-BT are −5.18 and −3.56 eV, respectively; for PCPDT-DFBT, the HOMO and LUMO are −5.34 and −3.52 eV, respectively; for PDTP-DFBT, the HOMO and LUMO are −5.26 and −3.61 eV, respectively. a.u., arbitrary unit. Figure 2: J–V characteristics and EQE of single-cell devices for three different polymers blending with PC71BM.(a) J–V characteristics of single-cell devices PCPDT-BT:PC71BM (black line), PCPDT-DFBT:PC71BM (red line) and PDTP-DFBT:PC71BM (blue line) under simulated AM1.5G illumination from a calibrated solar simulator with irradiation intensity of 100 mW cm−2. (b) EQE of the corresponding devices, PCPDT-BT:PC71BM (black line), PCPDT-DFBT:PC71BM (red line) and PDTP-DFBT:PC71BM (blue line). Figure 3: The films’ morphology by energy-filtered TEM of polymer:fullerene blending films.(a–c) The energy loss image for PCPDT-BT:PC71BM spin-casted from CB solvent with DIO at energy loss of 0, 20 and 30 eV, respectively; (d–f) The energy loss image for PCPDT-DFBT:PC71BM spin-casted from CB solvent with DIO at energy loss of 0, 20 and 30 eV, respectively; (g–i) The energy loss image for PDTP-DFBT:PC71BM spin casted from DCB at energy loss of 0, 20 and 30 eV, respectively. The scale bars, 200 nm. Figure 4: Photo-CELIV of solar cells with different polymers blending with PC71BM.The delay times are 3 μs. And the tmax (the time when the extraction current reaches its maximum value) for PCPDT-BT:PC71BM, PCPDT-DFBT:PC71BM and PDTP-BT:PC71BM are 7.1, 2.8 and 1.1 μs, respectively. The device structure is ITO/ZnO/Polymer:PC71BM/MoO3/Ag. Figure 5: Absorption and device performance.(a) Absorption spectra of P3HT and PDTP-DFBT and solar spectrum. (b) Absorption spectra of P3HT:ICBA (black line), PDTP-DFBT:PC61BM (red line), PDTP-DFBT:PC71BM (blue line) blend. (c) J–V curve of P3HT:ICBA (black line), PDTP-DFBT:PC61BM (red line), PDTP-DFBT:PC71BM (blue line) under AM1.5G illumination from a calibrated solar simulator with an irradiation intensity of 100 mWcm2 (one Sun). (d) EQE of P3HT:ICBA (black line), PDTP-DFBT:PC61BM (red line), PDTP-DFBT:PC71BM (blue line)-based single cell devices. Figure 6: Tandem devices structure and performance.(a) Device structure of the tandem solar cell (Glass/ITO/ZnO/P3HT:ICBA/PEDOT:PSS/ZnO/PDTP-DFBT:PCBM/MoO3/Ag). (b) J–V curve of P3HT:ICBA/PDTP-DFBT:PC61BM combination (Tandem 1) and P3HT:ICBA/PDTP-DFBT:PC71BM combination (Tandem 2) under AM1.5G illumination from a calibrated solar simulator with an irradiation intensity of 100 mWcm2 (one Sun). (c) EQE of the tandem 1(black line) and 2 (red line) devices. A 700 and 550 nm light bias are used to get front and rear cell EQE, respectively. (d) The relationship of tandem cell FF and short circuit current (JSC) versus rear and front cell current ratio (JSC, rear/JSC,front). Figure 7: Original I–V characteristics of the Tandem 1 device.Device characteristics as measured by NREL. Figure 8: Tandem solar cell performance parameters under low light intensity.(a) J–V curve of the tandem devices under different light intensity from 1.2 to 100 mW cm−2 (0.012–1 Sun); (b–d) Variation of short circuit current (JSC), open circuit voltage (VOC), FF of the Tandem 1 device under different light intensity from 1.2 to 100 mW cm−2.
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References
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    • . . . Interestingly, by adding two F atoms onto the BT unit, the absorption spectrum of the new polymer PCPDT-DFBT shows a blue shift of around 30 nm (bandgap of around 1.51 eV), whereas the addition of F atoms did not affect the absorption spectrum of other reported polymer systems9, 29, 30, 31, 32, 33 . . .
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    • . . . Typically, for a double-junction cell, such a tandem structure consists of a front cell with a high-bandgap material, an interconnecting layer, and a rear cell with a low-bandgap (LBG) material11, 12, 13, 14, 15, 16, 17, 18, 19 . . .
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    • . . . Typically, for a double-junction cell, such a tandem structure consists of a front cell with a high-bandgap material, an interconnecting layer, and a rear cell with a low-bandgap (LBG) material11, 12, 13, 14, 15, 16, 17, 18, 19 . . .
    • . . . For polymer tandem solar cells, Hadipour et al.13 demonstrated a polymer tandem solar cell consisting of two subcells with two different materials with about 0.57% efficiency in 2006, which is higher than each of the subcell’s efficiencies . . .
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    • . . . To use solar radiation more effectively, a useful approach is to stack multiple photoactive layers with complementary absorption spectra in series to make a tandem PSC11, 12, 13, 14, 15, 16, 17, 18, 19. . . .
    • . . . Typically, for a double-junction cell, such a tandem structure consists of a front cell with a high-bandgap material, an interconnecting layer, and a rear cell with a low-bandgap (LBG) material11, 12, 13, 14, 15, 16, 17, 18, 19 . . .
    • . . . In 2007, Kim et al. used a new interconnecting layer structure to bridge two higher performance single junction polymer PV cells to realize a tandem structure with 6.5% PCE14 . . .
    • . . . It is clear that the JSC is proportional to illuminated light intensity, indicating no substantial space charge build-up in the tandem device in both the two subcells and in the interconnecting layer14 . . .
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    • . . . To use solar radiation more effectively, a useful approach is to stack multiple photoactive layers with complementary absorption spectra in series to make a tandem PSC11, 12, 13, 14, 15, 16, 17, 18, 19. . . .
    • . . . Typically, for a double-junction cell, such a tandem structure consists of a front cell with a high-bandgap material, an interconnecting layer, and a rear cell with a low-bandgap (LBG) material11, 12, 13, 14, 15, 16, 17, 18, 19 . . .
    • . . . However, the polymer tandem solar cells’ performance has been limited to around 7% efficiency in the last 4 years mainly due to the lack of high-performance low-bandgap polymers15, 16, 17, 18 with high VOC and high external quantum efficiency (EQE) at long wavelengths . . .
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    • . . . To use solar radiation more effectively, a useful approach is to stack multiple photoactive layers with complementary absorption spectra in series to make a tandem PSC11, 12, 13, 14, 15, 16, 17, 18, 19. . . .
    • . . . Typically, for a double-junction cell, such a tandem structure consists of a front cell with a high-bandgap material, an interconnecting layer, and a rear cell with a low-bandgap (LBG) material11, 12, 13, 14, 15, 16, 17, 18, 19 . . .
    • . . . However, the polymer tandem solar cells’ performance has been limited to around 7% efficiency in the last 4 years mainly due to the lack of high-performance low-bandgap polymers15, 16, 17, 18 with high VOC and high external quantum efficiency (EQE) at long wavelengths . . .
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    • . . . However, the polymer tandem solar cells’ performance has been limited to around 7% efficiency in the last 4 years mainly due to the lack of high-performance low-bandgap polymers15, 16, 17, 18 with high VOC and high external quantum efficiency (EQE) at long wavelengths . . .
    • . . . Recently, we designed a new low bandgap polymer PBDTT-DPP with improved quantum efficiency (EQE~;50%) at long wavelength, and successfully achieved an inverted tandem PSC17 with PCE of certified 8.6% (19, 21) . . .
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    • . . . Typically, for a double-junction cell, such a tandem structure consists of a front cell with a high-bandgap material, an interconnecting layer, and a rear cell with a low-bandgap (LBG) material11, 12, 13, 14, 15, 16, 17, 18, 19 . . .
    • . . . Recently, we designed a new low bandgap polymer PBDTT-DPP with improved quantum efficiency (EQE~;50%) at long wavelength, and successfully achieved an inverted tandem PSC17 with PCE of certified 8.6% (19, 21) . . .
    • . . . So far, with a low bandgap cell of ~1.4 eV, the PSC shows only 5–6% PCE19, 24, 25, 26, 27 . . .
    • . . . Particularly, most of the low bandgap polymers show low quantum efficiency (<50%)19, 21 and are not able to satisfy the requirement for a tandem cell. . . .
    • . . . It should be noted that the LUMO difference between polymer (~−3.6 eV) and PCBM (~−4.0 eV)19 is about 0.4 eV, there are sufficient driving force to dissociate the exciton at the bulk heterojunction interface3. . . .
    • . . . IC60BA has been shown to be a successful acceptor for the high bandgap polymer P3HT45, 46 used in both single junction and tandem PSCs19 . . .
    • . . . EQE results were first measured in University of California, Los Angeles (UCLA)48, and the tandem devices were then measured using the One-Sun Multi-Source Simulator (recently established at National Renewable Energy Laboratory, (NREL)) based on UCLA EQE data19 . . .
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    • . . . Inorganic multi-junction/tandem solar cells have gone a long way and the latest cell efficiency of 43.5% have been certified20, which shows the great potential of tandem solar cell. . . .
  21. Dou L. T. Systematic investigation of benzodithiophene- and diketopyrrolopyrrole-based low-bandgap polymers designed for single junction and tandem polymer solar cells J. Am. Chem. Soc. 134, 10071-10079 (2012) .
    • . . . Recently, we designed a new low bandgap polymer PBDTT-DPP with improved quantum efficiency (EQE~;50%) at long wavelength, and successfully achieved an inverted tandem PSC17 with PCE of certified 8.6% (19, 21) . . .
    • . . . Particularly, most of the low bandgap polymers show low quantum efficiency (<50%)19, 21 and are not able to satisfy the requirement for a tandem cell. . . .
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    • . . . In the PSC field, several polymers with ~1.9 eV such as poly-(3-hexylthiophene) (P3HT)2 and poly N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)22 have shown excellent performance, with short circuit current density (JSC) over 10 mA cm−2, and high EQE of ~70% from 400−600 nm . . .
    • . . . This type of morphology is expected to improve charge separation and transport10, 22 . . .
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    • . . . To achieve 10 mA cm−2JSC in a low bandgap polymer cell between 600 nm and longer wavelength, an EQE close to 90% are required for polymer that absorbs up to 800 nm (~1.55 eV), or 60% for one that absorbs to 900 nm (~1.38 eV)23 . . .
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    • . . . So far, with a low bandgap cell of ~1.4 eV, the PSC shows only 5–6% PCE19, 24, 25, 26, 27 . . .
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    • . . . So far, with a low bandgap cell of ~1.4 eV, the PSC shows only 5–6% PCE19, 24, 25, 26, 27 . . .
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    • . . . So far, with a low bandgap cell of ~1.4 eV, the PSC shows only 5–6% PCE19, 24, 25, 26, 27 . . .
    • . . . Starting from the reported LBG polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDT-BT)26, 27, 28, we introduced two strong electron-withdrawing fluoroine atoms on the benzothiadiazole (BT) unit to form the difluorobenzothiadiazole (DFBT) unit to lower the highest occupied molecular orbital (HOMO) level9, 29, 30, 31, 32, 33 . . .
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    • . . . So far, with a low bandgap cell of ~1.4 eV, the PSC shows only 5–6% PCE19, 24, 25, 26, 27 . . .
    • . . . Starting from the reported LBG polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDT-BT)26, 27, 28, we introduced two strong electron-withdrawing fluoroine atoms on the benzothiadiazole (BT) unit to form the difluorobenzothiadiazole (DFBT) unit to lower the highest occupied molecular orbital (HOMO) level9, 29, 30, 31, 32, 33 . . .
  28. Kirkpatrick J. A Systematic approach to the design optimization of light-absorbing indenofluorene polymers for organic photovoltaics Adv. Energy Mater. 2, 260-265 (2012) .
    • . . . Starting from the reported LBG polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDT-BT)26, 27, 28, we introduced two strong electron-withdrawing fluoroine atoms on the benzothiadiazole (BT) unit to form the difluorobenzothiadiazole (DFBT) unit to lower the highest occupied molecular orbital (HOMO) level9, 29, 30, 31, 32, 33 . . .
  29. Liang Y. Y. Highly efficient solar cell polymers developed via fine-tuning of structural and electronic properties J. Am. Chem. Soc. 131, 7792-7799 (2009) .
    • . . . Starting from the reported LBG polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDT-BT)26, 27, 28, we introduced two strong electron-withdrawing fluoroine atoms on the benzothiadiazole (BT) unit to form the difluorobenzothiadiazole (DFBT) unit to lower the highest occupied molecular orbital (HOMO) level9, 29, 30, 31, 32, 33 . . .
    • . . . Interestingly, by adding two F atoms onto the BT unit, the absorption spectrum of the new polymer PCPDT-DFBT shows a blue shift of around 30 nm (bandgap of around 1.51 eV), whereas the addition of F atoms did not affect the absorption spectrum of other reported polymer systems9, 29, 30, 31, 32, 33 . . .
  30. Price S. C. Fluorine substituted conjugated polymer of medium band gap yields 7% efficiency in polymer-fullerene solar cells J. Am. Chem. Soc. 133, 4625-4631 (2011) .
    • . . . Starting from the reported LBG polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDT-BT)26, 27, 28, we introduced two strong electron-withdrawing fluoroine atoms on the benzothiadiazole (BT) unit to form the difluorobenzothiadiazole (DFBT) unit to lower the highest occupied molecular orbital (HOMO) level9, 29, 30, 31, 32, 33 . . .
    • . . . Interestingly, by adding two F atoms onto the BT unit, the absorption spectrum of the new polymer PCPDT-DFBT shows a blue shift of around 30 nm (bandgap of around 1.51 eV), whereas the addition of F atoms did not affect the absorption spectrum of other reported polymer systems9, 29, 30, 31, 32, 33 . . .
  31. Zhou H. X. Development of fluorinated benzothiadiazole as a structural unit for a polymer solar cell of 7% efficiency Angew. Chem. Int. Ed. 50, 2995-2998 (2011) .
    • . . . Starting from the reported LBG polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDT-BT)26, 27, 28, we introduced two strong electron-withdrawing fluoroine atoms on the benzothiadiazole (BT) unit to form the difluorobenzothiadiazole (DFBT) unit to lower the highest occupied molecular orbital (HOMO) level9, 29, 30, 31, 32, 33 . . .
    • . . . Interestingly, by adding two F atoms onto the BT unit, the absorption spectrum of the new polymer PCPDT-DFBT shows a blue shift of around 30 nm (bandgap of around 1.51 eV), whereas the addition of F atoms did not affect the absorption spectrum of other reported polymer systems9, 29, 30, 31, 32, 33 . . .
  32. Li Z. Synthesis and applications of difluorobenzothiadiazole based conjugated polymers for organic photovoltaics J. Mater. Chem. 21, 3226-3233 (2011) .
    • . . . Starting from the reported LBG polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDT-BT)26, 27, 28, we introduced two strong electron-withdrawing fluoroine atoms on the benzothiadiazole (BT) unit to form the difluorobenzothiadiazole (DFBT) unit to lower the highest occupied molecular orbital (HOMO) level9, 29, 30, 31, 32, 33 . . .
    • . . . Interestingly, by adding two F atoms onto the BT unit, the absorption spectrum of the new polymer PCPDT-DFBT shows a blue shift of around 30 nm (bandgap of around 1.51 eV), whereas the addition of F atoms did not affect the absorption spectrum of other reported polymer systems9, 29, 30, 31, 32, 33 . . .
  33. Crouch D. J. Thiophene and selenophene copolymers incorporating fluorinated phenylene units in the main chain: synthesis, characterization, and application in organic field-effect transistors Chem. Mater. 17, 6567-6578 (2005) .
    • . . . Starting from the reported LBG polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDT-BT)26, 27, 28, we introduced two strong electron-withdrawing fluoroine atoms on the benzothiadiazole (BT) unit to form the difluorobenzothiadiazole (DFBT) unit to lower the highest occupied molecular orbital (HOMO) level9, 29, 30, 31, 32, 33 . . .
    • . . . Interestingly, by adding two F atoms onto the BT unit, the absorption spectrum of the new polymer PCPDT-DFBT shows a blue shift of around 30 nm (bandgap of around 1.51 eV), whereas the addition of F atoms did not affect the absorption spectrum of other reported polymer systems9, 29, 30, 31, 32, 33 . . .
  34. Li G. Efficient inverted polymer solar cells Appl. Phys. Lett. 88, 253503 (2006) .
    • . . . Single junction photovoltaic cells based on the three polymers (and PC71BM as acceptor) were fabricated in an inverted device structure34, 35, 36, 37, 38 . . .
    • . . . The inverted tandem structure was chosen because of its advantages of a simple, robust device fabrication process and better stability8, 34, 35, 36, 37. . . .
  35. White M. S. Inverted bulk-heterojunction organic photovoltaic device using a solution-derived ZnO under layer Appl. Phys. Lett. 89, 143517 (2006) .
    • . . . Single junction photovoltaic cells based on the three polymers (and PC71BM as acceptor) were fabricated in an inverted device structure34, 35, 36, 37, 38 . . .
    • . . . The inverted tandem structure was chosen because of its advantages of a simple, robust device fabrication process and better stability8, 34, 35, 36, 37. . . .
  36. Hau S. K. Air-stable inverted flexible polymer solar cells using zinc oxide nanoparticles as an electron selective layer Appl. Phys. Lett. 92, 253301 (2008) .
    • . . . Single junction photovoltaic cells based on the three polymers (and PC71BM as acceptor) were fabricated in an inverted device structure34, 35, 36, 37, 38 . . .
    • . . . The inverted tandem structure was chosen because of its advantages of a simple, robust device fabrication process and better stability8, 34, 35, 36, 37. . . .
  37. Sun Y. M. Inverted polymer solar cells integrated with a low-temperature-annealed sol-gel-derived ZnO film as an electron transport layer Adv. Mater. 23, 1679-1683 (2011) .
    • . . . Single junction photovoltaic cells based on the three polymers (and PC71BM as acceptor) were fabricated in an inverted device structure34, 35, 36, 37, 38 . . .
    • . . . The inverted tandem structure was chosen because of its advantages of a simple, robust device fabrication process and better stability8, 34, 35, 36, 37. . . .
  38. Park M. H. Doping of the metal oxide nanostructure and its influence in organic electronics Adv. Funct. Mater. 19, 1241-1246 (2009) .
    • . . . Single junction photovoltaic cells based on the three polymers (and PC71BM as acceptor) were fabricated in an inverted device structure34, 35, 36, 37, 38 . . .
  39. Drummy L. F. Molecular-scale and nanoscale morphology of p3ht:pcbm bulk heterojunctions: energy-filtered tem and low-dose HREM Chem. Mater. 23, 907-912 (2011) .
    • . . . Plasmon mapping based on energy-filtered transmission electron microscopy (EFTEM) was used to investigate the morphology of active layer39, 40, 41, 42 . . .
    • . . . It can be seen that the polymer and blend system has a peak around 22.5 and 24.2 eV, respectively, which are consistent with the previous reports39, 40, 41, 42, where the plasmon peak of [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) was found to be around 26 eV (39, 40, 41, 42) . . .
    • . . . For comparison, the zero loss images, formed with only elastically scattered electrons39, have also been collected . . .
  40. Pfannmoller M. Visualizing a homogeneous blend in bulk heterojunction polymer solar cells by analytical electron microscopy Nano Lett. 11, 3099-3107 (2011) .
    • . . . Plasmon mapping based on energy-filtered transmission electron microscopy (EFTEM) was used to investigate the morphology of active layer39, 40, 41, 42 . . .
  41. Vakhshouri K. Effect of miscibility and percolation on electron transport in amorphous poly(3-hexylthiophene)/phenyl-c61-butyric acid methyl ester blends Phys. Rev. Lett. 108, 026601 (2012) .
    • . . . Plasmon mapping based on energy-filtered transmission electron microscopy (EFTEM) was used to investigate the morphology of active layer39, 40, 41, 42 . . .
  42. Collins B. A. Miscibility, crystallinity, and phase development in P3HT/PCBM solar cells: toward an enlightened understanding of device morphology and stability J. Phys. Chem. Lett. 2, 3135-3145 (2011) .
    • . . . Plasmon mapping based on energy-filtered transmission electron microscopy (EFTEM) was used to investigate the morphology of active layer39, 40, 41, 42 . . .
  43. Juska G. Extraction current transients: new method of study of charge transport in microcrystalline silicon Phys. Rev. Lett. 84, 4996-4999 (2000) .
    • . . . To get reliable charge carrier mobility of the blending system, photo-induced charge carrier extraction in a linearly increasing voltages (Photo-CELIV) measurements have been conducted in bulk heterojunction solar cells based on the three systems43, 44 . . .
  44. Mozer A. J. Charge transport and recombination in bulk heterojunction solar cells studied by the photoinduced charge extraction in linearly increasing voltage technique Appl. Phys. Lett. 86, 112104 (2005) .
    • . . . To get reliable charge carrier mobility of the blending system, photo-induced charge carrier extraction in a linearly increasing voltages (Photo-CELIV) measurements have been conducted in bulk heterojunction solar cells based on the three systems43, 44 . . .
    • . . . Photo-induced charge carrier extraction in a linearly increasing voltages (Photo-CELIV) measurement: To further measure the mobility of the polymer:fullerene blend system, photo-induced charge carrier extraction in a linearly increasing voltages (Photo-CELIV) measurement was used to determine the charge carrier mobility in bulk heterojunction solar cells44 . . .
    • . . . From the active thicknesses of 100, 90 and 100 nm, respectively, the mobilities of PCPDT-BT:PC71BM, PCPDT-DFBT:PC71BM and PDTP-BT:PC71BM are calculated to be 1.2 × 10−5, 7.4 × 10−5 and 6.7 × 10−4 cm2 V−1 s−1, respectively, based on the following equation44. . . .
    • . . . In this measurement, the applied maximum voltage is 2 V, and Uoffset is chosen to be near the corresponding open-circuit voltage, the time t for voltage increase from Uoffset to maximum voltage is 30 μs, and j(0) is the capacitive displacement current44. . . .
  45. He Y. J. Indene−C60bisadduct: a new acceptor for high-performance polymer solar cells J. Am. Chem. Soc. 132, 1377-1382 (2010) .
    • . . . IC60BA has been shown to be a successful acceptor for the high bandgap polymer P3HT45, 46 used in both single junction and tandem PSCs19 . . .
  46. Laird D. W. Organic photovoltaic devices comprising fullerenes and derivatives thereof , (2008) .
    • . . . IC60BA has been shown to be a successful acceptor for the high bandgap polymer P3HT45, 46 used in both single junction and tandem PSCs19 . . .
  47. Wienk M. M. Efficient methano[70]fullerene/MDMO-PPV bulk heterojunction photovoltaic cells Angew. Chem. Int. Ed. 42, 3371-3375 (2003) .
    • . . . Two types of widely used fullerenes (PC61BM and PC71BM) with different absorption coefficients were examined47 here to blend with the low bandgap polymer . . .
    • . . . The successful application of PC61BM to achieve high-performance tandem cells also has the benefit of reducing the materials cost of PSCs47. . . .
  48. Shrotriya V. Accurate measurement and characterization of organic solar cells Adv. Funct. Mater. 16, 2016-2023 (2006) .
    • . . . Accurate tandem cell measurement is a quite complicated procedure and extra care was taken to get reliable data48, 49, 50 . . .
    • . . . EQE results were first measured in University of California, Los Angeles (UCLA)48, and the tandem devices were then measured using the One-Sun Multi-Source Simulator (recently established at National Renewable Energy Laboratory, (NREL)) based on UCLA EQE data19 . . .
  49. Boland P. Design of organic tandem solar cells using low- and high-bandgap polymer:fullerene composites Sol. Energ. Mat. Sol. C 94, 2170-2175 (2010) .
    • . . . Accurate tandem cell measurement is a quite complicated procedure and extra care was taken to get reliable data48, 49, 50 . . .
    • . . . In our system, when the front and rear subcell JSC matches, the tandem devices showed the optimized performance (Fig. 6d)49, 50. . . .
  50. Burdick J.; Glatfelter T. Spectral response and I-V measurement of tandem amorphous-Silicon alloy solar cells Solar Cells 18, 301-314 (1986) .
    • . . . Accurate tandem cell measurement is a quite complicated procedure and extra care was taken to get reliable data48, 49, 50 . . .
    • . . . In our system, when the front and rear subcell JSC matches, the tandem devices showed the optimized performance (Fig. 6d)49, 50. . . .
  51. Zhu Z. G. Panchromatic conjugated polymers containing alternating donor/acceptor units for photovoltaic applications Macromolecules 40, 1981-1986 (2007) .
    • . . . The synthesis procedure of CPDT, DTP and DFBT units can be found in Zhu et al.51 and Yoshimura and Ohya52 . . .
  52. Yoshimura K.; Ohya K. Polymer compound , (2011) .
    • . . . The synthesis procedure of CPDT, DTP and DFBT units can be found in Zhu et al.51 and Yoshimura and Ohya52 . . .
  53. You J. B. Metal oxide nanoparticles as an electron-transport layer in high-performance and stable inverted polymer solar cells Adv. Mater. 24, 5267-5272 (2012) .
    • . . . The synthesis process of ZnO nanoparticles can be found in You et al.,53 Sun and Sirringhaus54 and Beek et al.55 Then the active layer was spin-coated on the ZnO surface; PCPDT-BT:PCBM (1:2), PCPDT-DFBT (1:2) and PDTP-DFBT:PCBM (1:2) were dissolved in CB or DCB with a concentration ranging from 5 to 10 mg ml−1 and with stirring at about 80 °C for at least 2 h before spin coating the active layer (heating is helpful to improve the solubility); for the PCPDT-BT and PCPDT-DFBT-based device, to improve the active layer morphology, 3% by volume DIO was added . . .
    • . . . The P3HT:IC60BA (1:1 weight ratio) in DCB solutions with various solid content were spin-casted at 800 r.p.m. for 30 s on top of a ~30 nm layer of ZnO53, 54, 55 . . .
  54. Sun B. Q.; Sirringhaus H. Solution-processed zinc oxide field-effect transistors based on self-assembly of colloidal nanorods Nano Lett. 5, 2408-2413 (2005) .
    • . . . The synthesis process of ZnO nanoparticles can be found in You et al.,53 Sun and Sirringhaus54 and Beek et al.55 Then the active layer was spin-coated on the ZnO surface; PCPDT-BT:PCBM (1:2), PCPDT-DFBT (1:2) and PDTP-DFBT:PCBM (1:2) were dissolved in CB or DCB with a concentration ranging from 5 to 10 mg ml−1 and with stirring at about 80 °C for at least 2 h before spin coating the active layer (heating is helpful to improve the solubility); for the PCPDT-BT and PCPDT-DFBT-based device, to improve the active layer morphology, 3% by volume DIO was added . . .
    • . . . The P3HT:IC60BA (1:1 weight ratio) in DCB solutions with various solid content were spin-casted at 800 r.p.m. for 30 s on top of a ~30 nm layer of ZnO53, 54, 55 . . .
  55. Beek W. J. E.; Wienk M. M.; Janssen R. A. J. Efficient hybrid solar cells from zinc oxide nanoparticles and a conjugated polymer Adv. Mater. 16, 1009-1013 (2004) .
    • . . . The synthesis process of ZnO nanoparticles can be found in You et al.,53 Sun and Sirringhaus54 and Beek et al.55 Then the active layer was spin-coated on the ZnO surface; PCPDT-BT:PCBM (1:2), PCPDT-DFBT (1:2) and PDTP-DFBT:PCBM (1:2) were dissolved in CB or DCB with a concentration ranging from 5 to 10 mg ml−1 and with stirring at about 80 °C for at least 2 h before spin coating the active layer (heating is helpful to improve the solubility); for the PCPDT-BT and PCPDT-DFBT-based device, to improve the active layer morphology, 3% by volume DIO was added . . .
    • . . . The P3HT:IC60BA (1:1 weight ratio) in DCB solutions with various solid content were spin-casted at 800 r.p.m. for 30 s on top of a ~30 nm layer of ZnO53, 54, 55 . . .
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