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Energy Level Tuning of Poly(phenylene‐alt‐dithienobenzothiadiazole)s for Low Photon Energy Loss Solar Cells

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      Six poly(phenylene‐alt‐dithienobenzothiadiazole)‐based polymers have been synthesized for application in polymer–fullerene solar cells. Hydrogen, fluorine, or nitrile substitution on benzo­thiadiazole and alkoxy or ester substitution on the phenylene moiety are investigated to reduce the energy loss per converted photon. Power conversion efficiencies (PCEs) up to 6.6% have been obtained. The best performance is found for the polymer–fullerene combination with distinct phase separation and crystalline domains. This improves the maximum external quantum efficiency for charge formation and collection to 66%. The resulting higher photocurrent compensates for the relatively large energy loss per photon (E loss = 0.97 eV) in achieving a high PCE. By contrast, the poly­mer that provides a reduced energy loss (E loss = 0.49 eV) gives a lower photocurrent and a reduced PCE of 1.8% because the external quantum efficiency of 17% is limited by a suboptimal morphology and a reduced driving force for charge transfer.

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      Efficient tandem polymer solar cells fabricated by all-solution processing.

      Tandem solar cells, in which two solar cells with different absorption characteristics are linked to use a wider range of the solar spectrum, were fabricated with each layer processed from solution with the use of bulk heterojunction materials comprising semiconducting polymers and fullerene derivatives. A transparent titanium oxide (TiO(x)) layer separates and connects the front cell and the back cell. The TiO(x) layer serves as an electron transport and collecting layer for the first cell and as a stable foundation that enables the fabrication of the second cell to complete the tandem cell architecture. We use an inverted structure with the low band-gap polymer-fullerene composite as the charge-separating layer in the front cell and the high band-gap polymer composite as that in the back cell. Power-conversion efficiencies of more than 6% were achieved at illuminations of 200 milliwatts per square centimeter.
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        A polymer tandem solar cell with 10.6% power conversion efficiency

        Organic photovoltaics (OPV) is an emerging photovoltaics (PV) technology with promising properties such as low cost, flexibility, light weight, transparency and large-area manufacturing compatibility1 2 3 4 5 6. Polymer solar cells (PSCs) based on conjugated polymers as electron donor materials blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an electron acceptor material have been the leading candidates in OPV in the past several years and achieved ~8% power conversion efficiency (PCE) using a single bulk heterojunction device structure7 8 9 10. 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. Compared with a single-junction device using low-bandgap materials, the multijunction/tandem structure can reduce thermalization loss of photonic energy during the photon-to-electron conversion process, and maximize the open circuit voltage (V OC). The high bandgap material in the front cell, which is responsible for the absorption of high-energy photons provide higher V OC than the low bandgap material. Therefore, by adopting polymers with matched absorption spectra, a tandem solar cell can effectively utilize the photonic energy and optimize open-circuit voltage, which leads to high PCE. In serial connected tandem solar cell, it is very obvious that subcell current balancing is critical for achieving high efficiency18. 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. 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. 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. More recently, Janssen et al. 18 has studied the effect of current matching on the tandem device performance, which provides more insight to achieve high-performance tandem PSC. 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 V OC 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). These progresses have translated the tandem polymer PV from a concept to a real OPV technology breakthrough, and paved a solid ground for achieving higher efficiencies in the future. With the progress in inter-connecting layer development, the main technical obstacle to break 10% barrier with a tandem PSC is the low bandgap polymer. The high bandgap cell in an inorganic multi-junction solar cell typically has a band gap of ~1.9 eV. 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 (J SC) over 10 mA cm−2, and high EQE of ~70% from 400−600 nm. With 70% fill factor (FF) and over 1.5 V open circuit voltage in a tandem cell, a J SC of 10 mA cm−2 will lead to over 10% PCE. To achieve 10 mA cm−2 J SC 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. 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 ( 60% from 710 to 820 nm and the spectral response extends to 900 nm. This leads to a PCE of 7.9%. The polymer also enables over 10 mA cm−2 J SC in tandem solar cells. As a result, certified 10.6% PCE is achieved under standard reporting conditions (25 °C, 1,000 Wm−2, IEC 60904-3 global). Results Polymers design and characterization The chemical structure of PCPDT-BT, PCPDT-DFBT and PDTP-DFBT are shown in Fig. 1a. The synthesis procedures can be found in the Methods and Supplementary Fig. S1. All polymers showed molecular weight of around 20 kDa and poly dispersity index of around 2.5. The ultraviolet-visible absorption spectra of these polymers in solid state are shown in Fig. 1b. As shown in the figure 1b, the main absorption range of PCPDT-BT covers from 500 to 850 nm; the absorption onset is located at around 850 nm, indicating an optical bandgap of 1.48 eV. 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. To further lower the bandgap to match a P3HT-based wide bandgap cell in a tandem structure, a strong electron-donating oxygen atom is introduced into the CPDT unit to form the DTP unit. By co-polymerizing with the strong electron-withdrawing DFBT unit, the polymer PDTP-DFBT shows significantly lower bandgap. The absorption spectrum of PDTP-DFBT is ~80 nm red-shifted (Fig. 1b) compared with PCPDT-DFBT and the bandgap is calculated to be 1.38 eV. The HOMO and lowest unoccupied molecular orbital level (LUMO) energy levels of the three polymers were determined by cyclic voltammetry (CV), and the results are shown in Fig. 1c. The HOMO and LUMO levels of PCPDT-BT are located at −5.18 and −3.56 eV, respectively. After adding two F atoms, PCPDT-DFBT shows a much deeper HOMO level (−5.34 eV) whereas the LUMO level (−3.52 eV) is almost unchanged. The deeper HOMO level of PCPDT-DFBT is desired to enhance the V OC, the similar LUMO level satisfies the exciton dissociation requirement. After adding the O atom, PDTP-DFBT shows slightly higher HOMO level (−5.26 eV) and lower LUMO (−3.61 eV), agrees with the lower optical bandgap. All the measured CV results are in accordance with density functional theory (DFT) calculations (Supplementary Fig. S2). 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. Single junction solar cell devices 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 PV performances are shown in Fig. 2a. For PCPDT-BT, the device showed a V OC of 0.62 eV and a J SC of 10 mA cm−2, but the FF was only 50%. For PCPDT-DFBT, the V OC of the device was increased significantly to 0.85 V, as adding the F atoms lowered the HOMO level of the polymer (Fig. 1c). Both J SC and FF increased slightly compared with PCPDT-DFBT device. Upon the insertion of the electron-donating oxygen atom, the PDTP-DFBT-based devices showed significantly enhanced J SC of 17–18 mA cm−2, which is among the highest reported J SC in PSCs so far. At the same time it gave a relatively high V OC close to 0.7 V, indicating the energy level tuning was successful to balance photocurrent generation and photovoltage. The performance of devices based on these three polymers is summarized in Table 1. The EQE of these devices are shown in Fig. 2b. The EQE of PCPDT-BT and PCPDT-DFBT-based devices are around 40% peak (~35% average) and 50% peak (~45% average), respectively. The PDTP-DFBT-based devices showed much higher peak EQE over 60% (average ~55%), and the photoresponse extends to 900 nm. Plasmon mapping based on energy-filtered transmission electron microscopy (EFTEM) was used to investigate the morphology of active layer39 40 41 42. The electron energy loss spectra of PDTP-DFBT and PDTP-DFBT:PC71BM are shown in Supplementary Fig. S3. 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). Therefore, EFTEM enables us to use 20 and 30 eV energy loss to distinguish polymer and fullerene-rich domain, respectively, and provide more detailed morphology information than traditional TEM technique. The energy loss images of these three polymers blended with PC71BM at energy loss of 20 and 30 eV are shown in Fig. 3. The energy loss images at 20 and 30 eV for PCPDT-BT: PC71BM and PCPDT-DFBT: PC71BM active layers (using chlorobenzene (CB) with 3% (vol.) 1, 8-diiodooctane (DIO)) are shown in Fig. 3b, respectively. Nanoscale fibril features were clearly seen from on both polymer blend films, indicating nice phase separation of polymer and fullerene for PCPDT-BT and PCPDT-DFBT system. On the other hand, it can be found that the PC71BM-rich domain (the dark region in 20 eV energy loss image, and bright region in 30 eV energy loss image) are as large as ~200 nm, which could be not good enough for charge separation and transport. The PDTP-DFBT: PC71BM film shows finely phase separation with the feature size of about 20–30 nm, and it is achieved using pure dichlorobenzene (DCB) solvent without the assistance of solvent additive (Fig. 3h). This type of morphology is expected to improve charge separation and transport10 22. For comparison, the zero loss images, formed with only elastically scattered electrons39, have also been collected. Figure 3a,d and g are the zero loss images of PCPDT-BT:PC71BM, PCPDT-DFBT:PC71BM and PDTP-DFBT:PC71BM films, respectively. The zero loss images show the similar trend as plasmon energy loss mapping, with PBDT-DFBT:PC71BM owns a finer structure without large aggregation domain. Transport property is critically important for photovoltaic devices. Space-charge-limited current (SCLC) method was used to derive the hole mobilities of these three polymers. Hole-only devices were fabricated and the SCLC results are shown in Supplementary Fig. S3. On the basis of equation 2 (see Methods) the mobilities are estimated to be 5.1 × 10−4, 4.8 × 10−4 and 3.2 × 10−3 cm2 V−1s−1 for PCPDT-BT, PCPDT-DFBT and PDTP-DFBT, respectively. The carrier mobilities in the polymer, however, could be affected by morphology, field, recombination or carrier densities effect in polymer:fullerene bulk heterojunction active layers under operating conditions. 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. The maximum voltage is 2 V in our experiment, with active layer thickness ~100 nm, the maximum electric field is thus ~2 × 105 V cm−1. And the charge carriers were extracted after 3 μs fixed delay time. The Photo-CELIV results are shown in Fig. 4, where the t max (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 charge mobility can then be calculated to be 1.2 × 10−5, 7.4 × 10−5 and 6.7 × 10−4 cm2 V−1s−1 for PCPDT-BT:PC71BM, PCPDT-DFBT:PC71BM and PDTP-BT:PC71BM, respectively (equation 3, see Methods). The qualitative charge carrier mobility data in the blend systems thus show the same trend, but are lower than those in pure polymer cases, which is consistent with the larger disorder in bulk heterojunction film. The PDTP-BT:PC71BM owns a higher charge carrier mobility than the other two polymer systems, which could contribute to the higher PV performance. For more details of SCLC and Photo-CELIV, please see Methods. Tandem devices based on P3HT:ICBA and PDTP-DFBT:PCBM From the analysis of spectral coverage and the single junction solar cell result, PDTP-DFBT shows high potential for application in tandem solar cells as a rear cell the tandem structure is shown in Fig. 6a. In a tandem structure, a high bandgap polymer, P3HT, combined with Indene-C60Bisadduct (ICBA) fullerene are selected as front cell active materials, and the low bandgap polymer PDTP-DFBT with PC61BM or PC71BM are selected as the rear cell active materials. Sol-gel processed ZnO is used as the electron transport layer, and PEDOT:PSS and MoO3 are used as the hole transport layer for the front and rear cells, respectively. The inverted tandem structure was chosen because of its advantages of a simple, robust device fabrication process and better stability8 34 35 36 37. Figure 5a shows the ultraviolet-visible absorption spectra of P3HT and PDTP-DFBT in the solid state, and the solar radiation spectrum. It can be seen that these two polymers complementarily cover the solar spectrum from 350 to 900 nm with very little spectral overlap, which provides a favourable system combination for optimizing tandem cell performance through independent tuning of two subcells. IC60BA has been shown to be a successful acceptor for the high bandgap polymer P3HT45 46 used in both single junction and tandem PSCs19. Two types of widely used fullerenes (PC61BM and PC71BM) with different absorption coefficients were examined47 here to blend with the low bandgap polymer. The absorptions of polymers blended with acceptors are shown in Fig. 5b. When the low bandgap polymer PDTP-DFBT is blended with PC61BM, the absorption is enhanced slightly in the region of 300–400 nm due to the absorption of PC61BM, and the two subcells show little overlap. Whereas the PDTP-DFBT:PC71BM blend showed strong absorption in the entire visible region, and the overlap of two subcells is more significant. These different overlaps will affect the current match of the tandem solar cell, and thus the efficiency. Figure 5c shows the current versus voltage (J–V) characteristics of single-junction devices under AM1.5G illumination from a calibrated solar simulator with irradiation intensity of 100 mW cm−2. All the single junction cells (P3HT:IC60BA, PDTP-DFBT:PC61BM, and PDTP-DFBT:PC71BM) show excellent performance. Specifically, the P3HT:IC60BA cell shows 6.1% PCE, and the PDTP-DFBT-based single junction cells show 7.1% and 7.9% PCE when blended with PC61BM and PC71BM, respectively. The difference comes from the J SC, which is mainly due to the different absorption coefficients of PC61BM and PC71BM in the region of 300–600 nm. The detailed parameters of the single cell are summarized in Table 2. The EQE of P3HT:IC60BA, PDTP-DFBT:PC61BM, PDTP-DFBT:PC71BM are shown in the Fig. 5d. The wide bandgap polymer cell (P3HT:IC60BA) showed high quantum efficiency from 300–700 nm, with maximum EQE of 70% at about 520 nm. In both of the PDTP-DFBT-based single cells, the maximum EQE is 62% at ~800 nm, and EQE is over 60% from 710 to 820 nm. 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. The spectral mismatches associated with the re-measured EQEs and the simulator spectra were then recalculated. The spectral mismatches were found to be 1 cm2 (, both clearly show the importance of tandem structure in organic solar cells. Methods Materials P3HT was purchased from Rieke Metals. PC61BM and PC71BM were purchased from Nano-C. The synthesis procedure of CPDT, DTP and DFBT units can be found in Zhu et al. 51 and Yoshimura and Ohya52. The polymers were synthesized as follow (see Supplementary Fig. S1 for the chemical reaction equations): PCPDT-DFBT: CPDT (0.337 g, 0.463 mmol) and DFBT (0.148 g, 0.449 mmol) were dissolved into 10 ml toluene in a flask protected by argon. The solution was flushed by argon for 10 min, then 8 mg of Pd2(dba)3 and 16 mg of P(o-tol)3 was added into the flask. The solution was flushed by argon again for 20 min. The oil bath was heated to 110 °C gradually, and the reaction mixture was stirred for 10 h at 110 °C under argon atmosphere. Then, the mixture was cooled down to room temperature and the polymer was precipitated by addition of 100 ml methanol and the precipitated solid was collected and purified by Soxhlet extraction (acetone for 24 h and then hexane for 24 h). The solid was then dissolved in 30 ml toluene and purified by chromatography on silica gel. Then the solution was concentrated to 10 ml and precipitated in methanol. The title polymer was obtained as dark purple solid, yield ~50%. The polymer can be readily dissolved into CB, DCB and so on. PCPDT-BT and PDTP-DFBT were synthesized using the same procedure. Electrochemical CV The electrochemical CV was conducted with Pt disk, Pt plate and Ag/AgCl electrode as working electrode, counter electrode and reference electrode, respectively, in a 0.1 mol l−1 tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution1. The polymer films for electrochemical measurements were coated from a polymer chloroform solution of concentration ~5 mg ml−1. For calibration, the redox potential of ferrocene/ferrocenium (Fc/Fc+) was measured under the same conditions, and it is located at 0.42 V to the Ag/AgCl electrode. It is assumed that the redox potential of Fc/Fc+ has an absolute energy level of −4.80 eV to vacuum. The energy levels of the highest occupied (HOMO) and lowest unoccupied molecular orbital (LUMO) were then calculated according to the following equations: where E ox is the onset oxidation potential versus Ag/AgCl and E red is the onset reduction potential versus Ag/AgCl. Simulated results for the molecular orbital DFT calculations were executed to determine the electronic structures of the two polymers. Two repeating units of the polymers were used as the model compound in the simulation, and methyl groups were used instead of the long side chain. DFT calculations were done using HSE06/6-31G(d). The calculated HOMO/LUMO and bandgap for CPDT-BT, CPDT-DFBT and DTP-DFBT were −4.632/−2.714 and 1.918 eV, −4.827/−2.865 and 1.962 eV, −4.824/−2.914 and 1.910 eV, respectively. These results indicate that by adding two F atoms on the BT unit, CPDT-DFBT shows lower HOMO/LUMO levels and a larger bandgap than CPDT-BT due to the strong electron-withdrawing property of F atom; after further adding an O atom, DTP-DFBT shows lower HOMO/LUMO levels and a smaller bandgap than CPDT-BT. The simulation results are in accordance with experimental results. Morphology characterization EFTEM was used to characterize the morphology of the polymer:fullerene system. Thin films were analysed in a Titan Krios TEM equipped with an energy filter. By electron energy loss spectroscopy, plasmon energies of 20 and 30 eV were used to distinguish the polymer and PCBM, respectively. A slit width of 8 eV was used during the measurement. Mobilities measurement SCLC model measurement: Hole mobility was measured using the SCLC, using a diode configuration of ITO/PEDOT:PSS/polymer:/Au by taking current–voltage current in the range of 0–8 V and fitting the results to a space–charge-limited form, where the SCLC is described by where ε 0 is the permittivity of free space, ε r is the dielectric constant of the polymer, μ is the hole mobility, V is the voltage drop across the device and L is the polymer thickness. The dielectric constant ε r is assumed to be 3, which is a typical value for conjugated polymers. The thickness of the polymer films is measured by using a Dektek profilometer. The thickness of the polymer is ~100, 90 and 100 nm for PCPDT-BT, PCPDT-DFBT and PDTP-DFBT, respectively. The mobilities of these three polymers are 5.1 × 10−4, 4.8 × 10−4 and 3.2 × 10−3 cm2 V−1 s−1. 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. The device structure is ITO/ZnO/Polymer:PC71BM/MoO3/Ag. A 590-nm dye (Rhodamine Chloride 590) laser pumped by a nitrogen laser (LSI VSL-337ND-S) was used as the excitation source, the pulse energy and pulse width being about 3 μJ cm−2 and 4 ns, respectively. The current of the photodiode was first amplified by using a current amplifier (Femto DHPCA-100), then a preamplifier (SR SSR445A) and finally recorded using a digital oscilloscope (Tektronix DPO 4104). The t max values for PCPDT-BT:PC71BM, PCPDT-DFBT:PC71BM and PDTP-BT:PC71BM are measured to be 7.1, 2.8 and 1.1 μs, respectively. 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. where μ is the mobility, d is the thickness of the active layer, t max is the time when the extraction current is reached, A is the voltage rise speed A=dU/dt. In this measurement, the applied maximum voltage is 2 V, and U offset is chosen to be near the corresponding open-circuit voltage, the time t for voltage increase from U offset to maximum voltage is 30 μs, and j(0) is the capacitive displacement current44. Single-junction devices based on three different polymers The precleaned ITO substrates were first treated with ultraviolet-ozone for 15 min. ZnO nanoparticles were spin-coated on the ITO substrates. 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. After spin coating the active layer, the samples were transferred into the evaporation chamber for fabricating the MoO3/Ag electrode. Tandem devices fabrication The precleaned ITO substrates were first treated with ultraviolet-ozone for 15 min. 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. The P3HT:ICBA film is formed through a slow growth process and the thickness is between 180 and 240 nm2. PEDOT:PSS layer was applied on the first subcell active layer and annealed at 150 °C for 10 min. Then a thin layer of ZnO film was spin-casted, followed again by thermal annealing at 150 °C for 10 min. The second subcell active layer of PDTP-DFBT:PCBM (PC61BM or PC71BM) (1:2) from an DCB solution was then spin-casted at 1,500 r.p.m. on top of the ZnO layer, the thickness of PDTP-DFBT is about 80–120 nm. To control the thickness to tune the short circuit current of front and rear cell, we varied the concentration of P3HT (18–22 mg ml−1) and PDTP-DFBT (5–7 mg ml−1) in the solution. The optimized thickness of the front and rear cell are about 220 and 100 nm, respectively. The inverted tandem PSC devices fabrication were completed by thermal evaporation of MoO3/Ag as the anode under vacuum at a base pressure of 2 × 10–6 Torr. Tandem device characterization The device areas were measured in the NREL facility first using optical microscope, and the results range from 0.0998 to 0.104 cm2. EQEs were measured at UCLA and NREL, respectively. A 550- and 700-nm light bias was selected to excite the front and rear cells to measure the EQE of the rear and front cell, respectively. I–V measurements for the tandem cells were performed on the One-Sun Multi-Source Simulator of NREL first by using UCLA EQE data for spectral mismatch correction. The spectral mismatches associated with the re-measured EQEs and the simulator spectra were recalculated. In each case, the spectral mismatches were <0.2% different from those derived from the EQEs. Measurements in NREL were done at 1,000, 500 and 250 W m−2 with and without the mask (~0.04 cm2) on a representative device. The JSC with the mask was about 2% lower than without the mask. Some of this difference is likely due to the extended source of the One-Sun Multi-Source Simulator light (as opposed to a point-like source) and the thickness of the mask relative to the size or the opening. For the different light intensity response measurement, neutral density filters were used to tune the light intensity, and a Si-diode was used to calibrate the light intensity. In examining the NREL certified J-V data, we observed some fluctuation in the J-V curve. We would like to explain the phenomenon as the following: 1) We believe part of the reason for the noise in the data is that there are multiple light sources (we have 9 separated light sources to control the spectrum) each with different intensity versus time characteristics. 2) Smooth I-V curves do not mean they are more accurate then IV curves with fluctuations. For example a halogen light source will give very smooth curves but the spectrum is a terrible match to the reference solar spectrum. 3) More importantly than the noise in the I-V curve was the fact that the spectrum was adjusted so that each cell was within 1% of its target photocurrent under the reference spectrum. 4) Furthermore, from the expansion data in Figure 7 (NREL data, see Figure S7), we can see the fluctuations are at the 1% level or less within the requirements of a Class A simulator. Author contributions J.Y. carried out the fabrication, characterization of inverted single and tandem PSC devices, data collection and analysis. L.D. designed PCPDT-DFBT and performed materials synthesis and characterization of PCPDT-BT and PCPDT-DFBT. K.Y., T.K. and K.O. designed the chemical structure and performed materials synthesis of PDTP-DFBT. T.M. and K.E. performed the certification at NREL. C.C. modified the interlayer, C.C. and J.G. carried out TEM measurements. J.Y., L.D., G.L. and Y.Y. prepared the manuscript. All authors discussed the results and commented on the manuscript. Y.Y. and G.L. conceptualized, planned and supervised the project. Additional information How to cite this article: You, J. et al. A polymer tandem solar cell with 10.6% power conversion efficiency. Nat. Commun. 4:1446 doi: 10.1038/ncomms2411 (2013). Supplementary Material Supplementary Information Supplementary Figures S1-S7
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          An efficient triple-junction polymer solar cell having a power conversion efficiency exceeding 11%.

          Tandem solar cells have the potential to improve photon conversion efficiencies (PCEs) beyond the limits of single-junction devices. In this study, a triple-junction tandem design is demonstrated by employing three distinct organic donor materials having bandgap energies ranging from 1.4 to 1.9 eV. Through optical modeling, balanced photon absorption rates are achieved and, thereby, the photo-currents are matched among the three subcells. Accordingly, an efficient triple-junction tandem organic solar cell can exhibit a record-high PCE of 11.5%.

            Author and article information

            [ 1 ] Molecular Materials and Nanosystems Institute for Complex Molecular SystemsEindhoven University of Technology P.O. Box 513 5600 MB EindhovenThe Netherlands
            [ 2 ]Dutch Polymer Institute P.O. Box 902 5600 AX EindhovenThe Netherlands
            [ 3 ]Dutch Institute for Fundamental Energy Research De Zaale 20 5612 AJ EindhovenThe Netherlands
            Author notes
            Macromol Chem Phys
            Macromol Chem Phys
            Macromolecular Chemistry and Physics
            John Wiley and Sons Inc. (Hoboken )
            24 January 2017
            March 2017
            : 218
            : 5 ( doiID: 10.1002/macp.v218.5 )
            © 2017 The Authors. Published by WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim

            This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

            Figures: 7, Tables: 2, Pages: 11, Words: 7004
            Funded by: European Research Council under the European Union's Seventh Framework Programme
            Award ID: FP/2007‐2013
            Funded by: European Research Council
            Award ID: 339031
            Funded by: Ministry of Education, Culture and Science
            Award ID: 024.001.035
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