N-Doped Zwitterionic Fullerenes as Interlayers in Organic and

N‑Doped Zwitterionic Fullerenes as Interlayers in Organic and Perovskite Photovoltaic Devices Volodimyr V. Duzhko,*,†,‡ Brandon Dunham,§ Stephen J. Rosa,‡ Marcus D. Cole,‡ Abhijit Paul,‡ Zachariah A. Page,‡ Christos Dimitrakopoulos,§ and Todd Emrick*,‡ †

Laboratory for Electronic Materials and Devices, Institute for Applied Life Sciences, ‡Polymer Science and Engineering Department, and §Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts 01003, United States S Supporting Information *

ABSTRACT: The efficient operation of polymer- and perovskite-based photovoltaic devices depends on selective charge extraction layers that are placed between the active layer and electrodes. Herein, we demonstrate that integration of a tetra-n-butyl ammonium iodide-doped zwitterionic fulleropyrrolidine derivative, C60-SB, as a cathode modification interlayer significantly improves the photovoltaic device performance. Compared to the intrinsic (undoped) zwitterionic material, which is an efficient interlayer itself, the doped interlayers further improve average power conversion efficiencies from 8.37% to 9.68% in polymer-based devices and from 12.53% to 15.31% in perovskite-based devices. Ultraviolet photoelectron spectroscopy revealed that doping increases the interfacial dipole at the C60-SB/Ag interface, i.e., reduces the effective work function of the resultant composite cathode. This effect originates from the population of negative polaron states in C60-SB by extrinsic charges that prevent directional charge transfer from Ag to the integer charge-transfer states in C60-SB, pinning the Fermi level at higher energy. The reduced resistivity of the doped interlayer, as measured by impedance spectroscopy, enables efficient device operation with a broad range of interlayer thicknesses, thus simplifying the solution-based device fabrication process.

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Organic semiconductors bearing zwitterionic pendent groups have recently emerged as effective interlayers for modification of cathode work function in optoelectronic devices.8−10 Their orthogonal solubility to that of traditional active layers enables facile and scalable processing of multilayered structures using solution-based fabrication processes. The functionality of zwitterionic materials as cathode modification interlayers originates from the self-alignment of zwitterionic dipoles as a result of their interaction with the image charges at the metal surface.10 Although the favorable processability of zwitterionic materials, which is similar to that of other alcohol-soluble materials, 11 makes them attractive for applications as interlayers, especially as bottom layers in the inverted12 and tandem13 photovoltaic device architectures, the electronic functionality of tertiary amine-functionalized materials can lead to better device efficiency.8 The latter is attributed to selfdoping, i.e., lone-pair electron transfer from the tertiary amines to fullerene cage.14 Here, we report the application of tetra-nbutyl ammonium iodide (TBAI)-doped 2,3,4-tris(3-(propyl-

oping of semiconducting materials has enabled the creation of the fundamental units of electronics, for example, diodes, field-effect transistors, solar cells, and thermoelectrics.1 Adding a controlled amount of “impurity” atoms with valence different from that of the host semiconductor, such as phosphorus or boron to silicon, leads to nor p-type doping, respectively. Ionization of dopants increases the charge carrier density, which leads to modification of the electronic structure of interfaces, and improves electrical conductivity. In organic semiconductors, which are typically characterized by a Gaussian distribution of electronic states in the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), the population of deep electronic states with extrinsic charges may additionally improve charge carrier mobility by eliminating deep hopping sites.2 Although strategies for p-type/oxidative doping of organic semiconductors are evolving rapidly,3,4 development of efficient air-stable n-type/reducing dopants has proven to be challenging.5−7 Providing insight into fundamental doping mechanisms, developing effective p- and n-type doping strategies, and integrating doped organic semiconductors into practical device architectures are all crucial for advancing organic electronics and optoelectronics. © XXXX American Chemical Society

Received: February 23, 2017 Accepted: April 3, 2017

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DOI: 10.1021/acsenergylett.7b00147 ACS Energy Lett. 2017, 2, 957−963

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Figure 1. (a) Chemical structure of C60-SB and TBAI interlayer components; schematic architectures of photovoltaic devices with (b) PTB7Th:PC71BM and (c) MAPbI3 active layers.

Aesar, 99.9985% (metals basis)] and DMSO (anhydrous, Sigma-Aldrich) were dissolved in a 1:1 molar ratio at room temperature in DMF (anhydrous, Sigma-Aldrich) to form a 1.0 M PbI2(DMSO) complex. The solution was spun onto the PEDOT:PSS-coated substrates at 1500 rpm for 15 s, immediately followed by spin-coating of MAI (Dyesol) in IPA (anhydrous, Sigma-Aldrich) at 2500 rpm for 30 s. The resulting perovskite film was annealed at 100 °C for 1 h in the dark. After the methylammonium lead iodide (MAPbI3) perovskite was formed, 35 μL of a hot (90 °C) 20 mg/mL PC61BM solution in chlorobenzene was spin-coated onto the hot (∼100 °C) perovskite film at 2000 rpm for 30 s. Following procedures from a previous report,15 the films were covered with a Petri dish and allowed to solvent anneal in the dark for 24 h. The polymer and perovskite devices were completed by spin-coating of either C60-SB, TBAI, or C60-SB:TBAI cathode interlayers, followed by thermal evaporation of either a 130 nm thick Ag electrode for the polymer-based devices or a 100 nm thick Ag electrode for the perovskite-based devices at a base pressure of 3 × 10−6 Torr through a shadow mask. An overlap of Ag electrode and patterned ITO substrate defined a device area of 0.06 cm2. The C60-SB, TBAI, and mixed C60-SB:TBAI interlayers of different thicknesses were spin-coated from 2,2,2-trifluoroethanol (TFE) solutions with concentrations ranging from 0.25 to 12 mg/mL at 4000 rpm for 60 s. Film thicknesses greater than ∼30 nm were measured by surface profilometry (KLA Tencor, model Alpha-Step IQ) directly. The ultraviolet−visible−near infrared (UV−vis−NIR) absorbance of C60-SB films at 450 nm was used to determine the thickness of thinner films. The thickness of thin TBAI films was determined by extrapolating the film thickness versus solution concentration, using the surface profiler measurements of larger film thicknesses. Weight ratios (wt %) of 99:1, 95:5, 90:10, 80:20, 70:30, and 50:50 were used for mixed C60-SB:TBAI interlayer materials. The concentrations of all mixed solutions were kept at 2 mg/mL to produce consistent interlayer film thicknesses. UPS measurements were performed using an electron spectroscopy for chemical analysis instrument (Scienta Omicron Nanotechnology, model ESCA+S) operating at 4 × 10−10 mbar. Details of sample fabrication and experimental configuration are described in the Supporting Information. Room-temperature X-band electron paramagnetic resonance (EPR) spectra were collected on powders using the perpendicular mode of a dual-mode resonator cavity (Bruker Elexsys E-500 with ER-4116 cavity). The current density−

sulfobetaine)propoxy) fulleropyrrolidine (C60-SB) interlayers in both polymer-based (PTB7-Th:PC71BM) and perovskite-based (methylammonium lead iodide, MAPbI3) photovoltaic devices. We show that n-type doping due to anion-induced electron transfer leads to superior performance of C60-SB:TBAI interlayers relative to C60-SB itself, resulting in average power conversion efficiency (PCE) values of 9.68% for polymer-based devices and 15.31% for perovskite-based devices. We present a detailed ultraviolet photoelectron spectroscopy (UPS) study of the interfacial electronic structure of the zwitterionic semiconductor/metal interface and discuss the difference between the intrinsic and doped zwitterionic materials that resulted in improved device performance. The increased electrical conductivity of the doped materials enables efficient device operation with thick interlayers, hence improving device tolerance to thickness variations and simplifying device fabrication. The active layer materials selected for the polymer-based photovoltaic devices were poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate2-6-diyl)] (PTB7-Th) as the donor and [6,6]-phenyl-C71butyric acid methyl ester (PC71BM) as the acceptor. They were obtained from 1-Material and Nano-C, respectively. For perovskite-based devices, methylammonium iodide (MAI), lead iodide (PbI2), and PC61BM were purchased from Dyesol, Alfa Aesar, and American Dye Source, respectively. The synthesis of C60-SB is described elsewhere.8 Tetra-n-butyl ammonium iodide (TBAI, > 99%) was purchased from SigmaAldrich. The chemical structures of C60-SB and TBAI are shown in Figure 1a. A conventional device architecture was employed for polymer-based photovoltaic devices ITO/PEDOT:PSS/PTB7Th:PC71BM/interlayer/Ag (Figure 1b), while an inverted architecture was used for perovskite devices ITO/PEDOT:PSS/MAPbI3/PC61BM/interlayer/Ag (Figure 1c). Initially, a 35 nm thick PEDOT:PSS (Clevios P VP AI 4083, Heraeus) layer was spin-coated onto the ultraviolet (UV)− ozone-treated ITO-coated glass substrates (resistivity of 20 Ω/ sq, Thin Film Devices, Inc.) and annealed at 150 °C for 30 min. For polymer-based devices, a PTB7-Th:PC71BM solution [1:1.8 by weight, 25 mg/mL in chlorobenzene with 3 vol % of 1,8-diiodooctane (DIO)] was heated at 80 °C for at least 3 h prior to spin-coating inside a glovebox. The device active layers were spin-coated at 1350 rpm for 120 s and evacuated for over 2 h to remove DIO. For perovskite-based devices, PbI2 [Alfa 958

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Figure 2. Current density−voltage characteristics of device with (a) PTB7-Th:PC71BM and (b) MAPbI3 active layers and (c) their metrics for different C60-SB to TBAI weight ratios (weight %) in cathode interlayers.

SB:TBAI interlayers. Figure 2c summarizes the average device metrics. For the polymer-based devices, improvement of average PCE from 8.37 ± 0.27% for devices with C60-SB interlayers to 9.68 ± 0.14% for devices with 90:10 wt % C60SB:TBAI interlayers occurs mainly because of improvement in fill factore (FF), from 58.8% to 67.6%, respectively. While Voc continued to improve upon increasing the TBAI content, from 0.768 V for C60-SB to 0.786 V for 90:10 wt %, and to 0.798 V for 70:30 wt %, the Jsc and FF values decreased at TBAI content above 10%, resulting in a net decrease of PCE. Overall, the polymer-based devices proved relatively insensitive to TBAI content over a broad range, from 5 to 30 wt %. For comparison, devices with thin TBAI interlayers reached a maximum PCE value up to 7.17% (not shown). On the other hand, the improvement of perovskite-based device performance from average PCE of 10.11 ± 0.10% for Ag cathode to 12.48 ± 0.07% for C60-SB interlayer, and to 15.31 ± 0.13% for C60SB:TBAI (80:20 wt %) interlayer originates mainly from enhanced Voc from 0.865 to 0.973 V. Reference perovskitebased devices containing a Ca/Al cathode gave Voc = 0.912, Jsc = 20.16 mA/cm2, FF = 74.6%, and PCE = 13.72%. The origin of improved device performance that occurs upon adding TBAI to the C60-SB interlayer can be elucidated by comparing the electronic structure of C60-SB/Ag and C60SB:TBAI/Ag interfaces. Figure 3a (left panel) shows how the secondary electron cutoff range of UPS spectrum for bare Ag changes after deposition of C60-SB, TBAI, or C60-SB:TBAI films. A systematic increase of the binding energy onset with increasing TBAI content signified a decrease in the effective work function (Φ) values of C60-SB:TBAI/Ag. As shown in Figure 3b, Φ decreased from 4.45 eV for bare Ag to 4.0 eV after deposition of intrinsic C60-SB, i.e., 100:0 wt %. The reduction of metal work function by zwitterionic materials has been attributed to self-alignment of zwitterionic side chains due to interaction with an image charge in the metal.10 The

voltage (J−V) characteristics of photovoltaic devices were measured using a Keithley 2400 source-meter. Illumination was performed with a solar simulator (Newport 91160, 100 mW/ cm2) equipped with an AM1.5G filter and calibrated using a reference Si solar cell with a KG5 window (Newport certification). The photomask of 0.054 cm2 area (calibrated at NREL, ref 8) was overlapped with the Ag electrode to define the illuminated device area. The average device metrics are given for 12 polymer devices and 8 perovskite devices fabricated on 6 and 4 different substrates, respectively. Impedance spectroscopy was performed using an Agilent 4294A impedance analyzer on devices directly with an oscillation amplitude of 10 mV, applied bias, and illumination as described later. The fitting of Cole−Cole plots was done using ZView software package (Scribner Associates Inc.). All device characterization was performed inside the glovebox. The J−V characteristics for PTB7-Th:PC71BM devices with mixed C60-SB:TBAI cathode interlayers of different weight ratios are shown in Figure 2a. Insertion of a C60-SB interlayer significantly improves device PCE, as shown in going from 4.3% for devices with a bare Ag cathode to 8.05% for a C60-SBcoated Ag cathode (100:0 wt %), in agreement with our previous study.8 Adding TBAI to C60-SB further improved device performance, reaching a maximum PCE value of 9.98% for 90:10 wt % C60-SB:TBAI interlayer composition. Similarly, inserting a C60-SB interlayer into MAPbI3 devices improved device performance from 10.24% for Ag-only cathode to 12.57% for C60-SB/Ag cathode devices (Figure 2b). The mixed C60-SB:TBAI interlayers functioned more effectively than C60SB in the perovskite-based devices as well, producing PCE of up to 15.44% for 80:20 wt % interlayer composition. Moreover, comparison of measurements in forward and reverse direction (Figure S1) indicated that a hysteresis-free performance, typically attributed to passivated surface states of perovskite crystallites by PC61BM,16 was preserved in devices with C60959

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Figure 3. (a) UPS spectra of pristine Ag and C60-SB:TBAI-covered Ag for various weight ratios of C60-SB and TBAI; (b) effective work function of Ag/C60-SB:TBAI electrode for different doping levels of C60-SB by TBAI; (c) schematic electronic structure of Ag/C60-SB (left) and Ag/C60-SB:TBAI (right) interfaces; and (d) effective work function of Ag electrode covered by C60-SB, C60-SB:TBAI (50:50), or TBAI films of different thicknesses.

conjugated polyelectrolyte/metal interface.20 On the other hand, the work function of Ag is reduced to 3.0 eV by a 30 nm thick TBAI film (Figure 3d). Because TBAI lacks πconjugation, significantly larger Δ can be achieved because of interaction of ammonium/iodide functional moiety with the metal before electron transfer from the metal occurs. The larger Δ values for metal interfaces with aliphatic polymers and conjugated polymers with higher LUMOs underpin the efficient operation of ultrathin cathode interlayers.21,22 However, the effect in C60-SB:TBAI mixture on Φ reduction is not an additive of the two components, as seen from the Φ saturation at 3.65 eV for large TBAI content. Figure 4 compares the EPR spectra of C60-SB and C60-SB:TBAI with 50:50 wt %. A peak in the energy absorption in the latter case due to uncompensated spin provides evidence for charge transfer between TBAI, i.e., iodide anion, and C60-SB. This effect is similar to anionic doping5 and tertiary amine self-doping13 of fullerene derivatives. For the TBAI:C60-SB case, the extrinsic electrons from TBAI populate the negative polaron states in C60-SB (ENP), raising the Fermi level. The low binding energy portion of the UPS spectrum in Figure 3a shows that the HOMO level, and hence the LUMO level, of TBAI-doped C60SB (50:50 wt %) is positioned 0.2 eV lower than that of intrinsic C60-SB with respect to the common Fermi level (EF) in the C60-SB:TBAI/Ag heterostructure. This makes the ICT− states unavailable for electron transfer from Ag, leading to Fermi-level pinning at higher energies (Figure 3c, right). As a result, the doped C60-SB/Ag interface is characterized by a

macroscopic alignment of molecular dipoles, which creates an interfacial dipole (Δ), was confirmed by near-edge X-ray absorption fine structure spectroscopy and vibrationally resonant sum-frequency generation spectroscopy.17 As the TBAI content in C60-SB:TBAI mixture increases, Φ gradually decreases further and saturates at ∼3.65 eV at high TBAI content. The thickness dependence of work function modification for pristine C60-SB, TBAI, and C60-SB:TBAI that saturate at different energy levels (Figure 3d) provides insight into the origin of saturation. For the neutral organic semiconductor/metal interface that is characterized by a large interfacial dipole, the magnitude of Δ is dictated by the Fermi level pinning at the energy of integer charge transfer (ICT) states.18 The negative ICT states (ICT−) are located below the LUMO (Figure 3c, left), because of reduction of the electronic energy by the polaron effect, as quantified by the binding energy of negative polarons (EPB), and because of Coulomb interaction with an image charge in the metal (EICh), i.e., EICT− = ELUMO − (EPB + EICh). Because zwitterionic moieties produce large interfacial dipoles due to large individual molecular dipoles19 and high density,9 their macroscopic alignment should be counterbalanced by electron transfer from occupied states below the Fermi level in the metal to the unoccupied ICT− states in the zwitterionic semiconductor. The net result is an interfacial dipole of Δ = 0.45 eV and Φ = 4.0 eV for C60-SB/ Ag. Therefore, Fermi level pinning at the energy of ICT− states dictates the largest achievable interfacial dipole at the C60-SB/ Ag interface. This effect is similar to that described for the 960

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PC61BM in our perovskite-based devices, further facilitating electron extraction. Figure 5 shows the Cole−Cole plots of the imaginary versus real part of impedance measured for PTB7-Th:PC71BM devices with intrinsic C60-SB interlayers of different thicknesses and TBAI doping levels. The measurements were conducted under AM1.5G illumination and at an applied bias equal to Voc. Probing this regime of device operation allows not only access to the interlayer resistance but also comparison to the resistance of the active layer, effectively elucidating their individual contributions at the critical regime of operation near the Voc. Note that the experimental curves in Figure 5 consist of two elements: a small arc in the high-frequency range and a nearly complete semicircle in the low-frequency range. A systematic correlation between the resistivity that describes a large semicircle in Figure 5a and the interlayer thickness allows assigning this feature to the interlayer, while the smaller arc can be attributed to the active layer and anode. The experimental curves can be fitted by the equivalent circuit that includes active layer resistance (RAL) and capacitance (CAL), interlayer resistance (RIL) and capacitance (CIL), and contact resistance (RC), as shown in the inset of Figure 5b. As expected, the RIL decreases with decreasing interlayer thickness. Comparing RIL of the thinnest nondoped interlayer (9.7 nm) of 376.7 ± 2.6 Ω and RAL in the range from 48.46 to 85.70 Ω for different conditions (see Table S1), the significantly large magnitude of the former is responsible for the slightly distorted J−V curves and decreased FF in devices with nondoped interlayer. The resistance of doped C60-SB interlayer rapidly decreases with increasing TBAI content and becomes negligible above 20 wt % TBAI in comparison to RAL. Therefore, small doping levels improve FF of the devices, and higher levels have no effect on the device performance, unless percolation pathways of C60-SB are disrupted by aliphatic TBAI. Finally, Figure 5c compares the performance of polymer-based devices with C60-SB interlayers and TBAI-doped C60-SB interlayers of different thicknesses. Devices containing the intrinsic C60-SB interlayer demonstrate

Figure 4. Electron spin resonance spectra of C60-SB and C60SB:TBAI (50:50 wt %).

larger Δ = 0.8 eV as compared to 0.45 eV for intrinsic C60-SB/ Ag interface, and as such, Φ is reduced more effectively to 3.65 eV. Notably, the same Φ for self-doped C60-N and TBAI-doped C60-SB was observed (Figure S2), which correlated well with the identical LUMO/HOMO configurations of C60-N and C60SB, as indicated by their overlaid UV−vis−NIR absorption spectra.8 In polymer-based photovoltaic devices, a smaller cathode Φ induces a larger built-in electric field across the active layer, leading to more efficient extraction of photogenerated charge carriers at larger forward biases, which improves FF. In perovskite-based devices, a smaller cathode work function can increase the band bending at the perovskite/ PC61BM interface as well as improve charge extraction through the PC61BM layer. Moreover, doping can reduce the LUMO energy offset between the C60-SB interlayer and either active layer acceptor, i.e., PC71BM, in our polymer-based devices or

Figure 5. Cole−Cole plots of organic photovoltaic devices for (a) various C60-SB interlayer film thicknesses and (b) different doping levels; (c) dependence of PTB7-Th:PC71BM device metrics on the C60-SB and C60-SB:TBAI (90:10 wt %) interlayer film thickness. The inset in panel b shows the equivalent circuit of the devices. 961

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Keith Lehuta and Prof. Kevin R. Kittilstved assisted with EPR measurements.

satisfactory performance with PCE over 8% over a broad range of interlayer thicknesses. The reduced work function of composite C60-SB:TBAI/Ag electrode and better conductivity of doped C60-SB interlayers produce more efficient devices, with average PCE of 9.67%. The high PCE is maintained over a broad range of interlayer thicknesses, thus easing the tolerance of interlayer thickness and simplifying device fabrication. In summary, TBAI doping of zwitterionic fulleropyrolidine C60-SB was utilized to create solution-processable cathode interlayers for polymer- and perovskite-based photovoltaic devices that sustain high device PCEs. Population of polaron states in C60-SB by extrinsic charge carriers from TBAI makes negative ICT states unavailable for directional electron transfer from the metal electrode and leads to Fermi level pinning at higher energies for the doped C60-SB/Ag interface relative to the intrinsic C60-SB/Ag interface. This results in more efficient work function reduction of the cathode. The improved electrical conductivity of the doped interlayers sustains efficient operation of devices with a broad range of interlayer film thicknesses, thus simplifying the device fabrication process. The improvement of device performance with a composite interlayer is not an additive effect of two separate components, but results from the interaction between TBAI and C60-SB, i.e., from electron transfer. Solution processing and orthogonal solubility of doped zwitterionic interlayers to that of active layers as well as facile optimization of doping levels are advantageous for further development of flexible electronic and optoelectronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00147. Description of sample fabrication for UPS and EPR measurements, J−V curves of perovskite device in forward and reverse directions, UPS measurements of C60-N/Ag electrode, zoomed-in view of Figure 5a in the high-frequency range, fitting parameters of curves in Figure 5a,b, and polymer device metrics measured without the photomask (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-Mail: [email protected]. *E-Mail: [email protected]. ORCID

Volodimyr V. Duzhko: 0000-0001-9951-2599 Todd Emrick: 0000-0003-0460-1797 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Device fabrication and evaluation, UPS, and impedance spectroscopy were performed in the Laboratory for Electronic Materials and Devices of the Institute for Applied Life Sciences at the University of Massachusetts, Amherst. T.E. acknowledges the support for nanoscale materials synthesis from the National Science Foundation, Grant NSF-CHE-1506893 and use of facilities supported by NSF-MRSEC (DMR-0820506). Dr. 962

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ACS Energy Letters Origin of Electron Injection Enhancements with Fulleropyrrolidine Interlayers. Adv. Mater. Interfaces 2016, 3, 1500852. (20) Bao, Q.; Liu, X.; Wang, E.; Fang, J.; Gao, F.; Braun, S.; Fahlman, M. Regular Energetics at Conjugated Electrolyte/Electrode Modifier for Organic Electronics and Their Implications of Design Rules. Adv. Mater. Interfaces 2015, 2, 1500204. (21) He, Z.; Zhong, C.; Huang, X.; Wong, W.-Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y. Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit Current Density, and Fill Factor in Polymer Solar Cells. Adv. Mater. 2011, 23, 4636−4643. (22) Zhou, Y.; Fuentes-Hernandez, C.; Shim, J.; Meyer, J.; Giordano, A. J.; Li, H.; Winget, P.; Papadopoulos, T.; Cheun, H.; Kim, J.; et al. A Universal Method to Produce Low-Work Function Electrodes for Organic Electronics. Science 2012, 336, 327−332.

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DOI: 10.1021/acsenergylett.7b00147 ACS Energy Lett. 2017, 2, 957−963