Investigating Working Mechanism of Metallophthalocyanine

Sep 15, 2017 - Power conversion efficiency (PCE) values reached 8.1% for the devices with Ag as the cathode, while the PCE values could reach 8.2% for...
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Investigating Working Mechanism of Metallophthalocyanine Derivatives as a Cathode Interlayer in Polymer Solar Cells by Photoemission Spectroscopy Chengzhuo Yu, Jianxiong Han, Youchun Chen, Weilong Zhou, and Fenghong Li* State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry, Jilin University, Changchun 130012, P. R. China S Supporting Information *

ABSTRACT: Three metallophthalocyanine derivatives with different central metallocores (MPcXs, X = (OC8H17OPyCH3I)8), namely, ZnPcX, VOPcX, and TiOPcX, were synthesized and applied as a cathode interlayer (CIL) in the PTB7:PC71BM based polymer solar cells (PSCs). Power conversion efficiency (PCE) values reached 8.1% for the devices with Ag as the cathode, while the PCE values could reach 8.2% for the devices with Al as the cathode. Moreover the three MPcXs led to similar device performance in the PSCs with either Ag or Al as the cathode. It indicates that central metallocores in the MPcXs have a negligible effect on the device performance. Working mechanism of the MPcX as the CIL in the PSCs has been investigated by photoemission spectroscopy. Findings of ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS) demonstrated that the MPcX as the CIL not only decreased work function of the metal cathode and electron extraction barrier from PC71BM to cathode but also functioned as a buffer layer to protect the active layer from the damages during the thermal deposition of Al and Ag. cridone derivatives,31 8-hydroquinolatolithium,32 perylene diimides,33 pyridinium salt,34 porphyrin,35 rhodamine with inner salt,36 quinacridone tethered with sodium sulfonate,37 metallophthalocyanine (MPc) derivatives,38−41 and so on. Compared to polymers, small molecules have advantages of well-defined structures, high purity without batch-to-batch variation and easy modification. Recently we reported a water-soluble MPc derivative with pyridine salts, VOPc(OPyCH3I)8 as a CIL leading to a simultaneous enhancement in open circuit voltage (Voc), short circuit current (Jsc), and fill factor (FF) of the PSCs.38 However, spin-coated VOPc(OPyCH3I)8 on the active layer showed a rough morphology of surface where there are some visible islands. Formation mechanism of the islands may be selfaggregation of VOPc(OPyCH3I)8 itself due to its high polarity and adverse wettability of VOPc(OPyCH3I)8 aqueous solution on the hydrophobic active layer. Such a rough morphology is usually unfavorable for the device efficiency, especially for Jsc, FF, and stability. In order to improve film formation of the MPc derivatives on the active layer, alcohol-soluble VOPc(OPyC 2 H 5 Br) 8 , VOPc(OPyC 4 H 9 B r) 8 , an d VO Pc(OPyC6H13Br)8 have been synthesized by changing alkyl side

1. INTRODUCTION Polymer solar cells (PSCs) have attracted considerable attention due to their key advantages of synthetic variability, light weight, low cost, large-area roll to roll fabrication, and the lucrative possibility of direct integration into flexible devices.1−5 Currently, power conversion efficiency (PCE) of PSCs has rapidly increased to over 10% with good ambient stability for single cell devices.6−10 Many efforts have been made in terms of new material synthesis, device structure optimization, and controlling the morphology of the active layer to improve device performance.11−14 In addition, the PCE of PSCs can be significantly affected by charge carrier transport and collection at electrode/active layer interface. Holes are extracted from the highest occupied molecular orbit (HOMO) of the donor in the active layer to anode, while electrons are extracted from the lowest unoccupied molecular orbit (LUMO) of the acceptor in the active layer to cathode. Therefore, interfacial modification by incorporating an interlayer between the active layer and electrode to improve the PCE is an important research subject in the field of PSC.15−20 In particular, development of cathode interlayer (CIL) materials has attracted extensive research interest. Except polymers as a CIL,21,22 some alcohol/watersoluble small molecules were utilized successfully as CILs in the PSCs as well, which include fullerene derivatives,23−26 graphene quantum dots,27,28 surfactant-encapsulated polyoxometalate complex,29 isoindigo derivatives,30 dicyanomethylenated quina© 2017 American Chemical Society

Received: July 31, 2017 Revised: September 13, 2017 Published: September 15, 2017 21244

DOI: 10.1021/acs.jpcc.7b07561 J. Phys. Chem. C 2017, 121, 21244−21251

Article

The Journal of Physical Chemistry C

Figure 1. (a) Molecular structures of ZnPc(OC8H17OPyCH3I)8, VOPc(OC8H17OPyCH3I)8, and TiOPc(OC8H17OPyCH3I)8. (b) Molecular structures of PTB7 and PC71BM. (c) Conventional device configuration.

(UPS) measurements demonstrated that the MPcXs decreased not only the work function (WF) of metal cathodes but also barrier of electron extraction from PC71BM to cathode. X-ray photoemission spectroscopy (XPS) results demonstrated that inserting the MPcXs between the active layer and cathode can limit, even avoid the damages of the Al or Ag atoms to the PTB7:PC71BM during the thermal deposition of the metals. Therefore, the MPcXs as a CIL not only decreases the WF of the metal cathode and the electron extraction barrier from PC71BM but also functions as a buffer layer to protect the active layer from the damages during the thermal deposition of Al and Ag. It is expected that our findings can provide an insight for working mechanism of alcohol/water-soluble organic CIL materials in the PSCs.

chains attached to the pyridine functional groups in the MPc derivatives. PCE values of 7.9−8% were achieved in the PTB7:PC71BM based PSCs using the three MPc derivatives as a CIL.39 The longer alkyl chains afforded the MPc derivatives with sufficient solubility in alcohol and that were hardly soluble in water, which made them have better wettability as the CIL on the hydrophobic active layers. It provided a helpful suggestion for developing interfacial materials with good filmforming property. In order to further increase device performance of the PSCs, an alcohol-soluble MPc derivative ZnPc(OC8H17OPyCH3I)8 was synthesized by introducing eight alkoxy chains on the rigid skeleton as shown in Figure 1a. A PCE of over 8.5% has been achieved in the PTB7:PC71BM based PSCs with ZnPc(OC8H17OPyCH3I)8 as the CIL. The abundant flexible alkyl chains attached to the skeleton of ZnPc(OC8H17OPyCH3I)8 promoted uniform film formation on the underlying organic layer.41 As we know, the MPc derivatives are a big family including various central metallocores and outer organic functional groups. Our results have demonstrated that alkyl chains attached to the skeleton or the pyridine functional groups in the MPc derivatives can influence the film-forming property of the MPc derivatives on the active layer and then device performance. Therefore, it is necessary to figure out the effect of central metallocore in the MPc derivatives on the device performance and investigate working mechanism of the MPc derivatives as a CIL in the PSCs in order to determine the best CIL materials in the MPc derivatives. In this contribution, in order to explore the effect of central metallocores in the MPc derivatives on the device performance, we intentionally synthesized three MPc derivatives (MPcXs, X = (OC8H17OPyCH3I)8) with same molecular structures except central metallocores, namely, ZnPcX, VOPcX, and TiOPcX in Figure 1a. When the three MPcXs were applied as a CIL in the PTB7:PC71BM based PSCs, the PCE values reached 8.1% for the devices with Ag as cathode while the PCE values could reach 8.2% for the devices with Al as cathode. Moreover the three MPcXs led to similar device performance in the PSCs with either Ag or Al as cathode. It indicates that central metallocores in the MPcXs have a negligible effect on the device performance. Ultraviolet photoemission spectroscopy

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Fabrication for UPS and XPS Measurements. The synthesis procedure of ZnPcX, VOPcX, and TiOPcX can be referred to in ref 41. PTB7 and PC71BM were purchased from 1-Material and American Dye Sources Inc. Chlorobenzene (CB) and methanol were purchased from Sigma-Aldrich. All materials were used as delivered. All the substrates for UPS and XPS measurements were ultrasonically cleaned in acetone and isopropanol before spincoating organic materials. For UPS measurements, MPcX methanol solutions with various concentrations were spincoated on the substrates to obtain thin films with different thickness. PC71BM solutions with various concentrations were spin-coated on the substrates modified by MPcX to obtain thin films with different thickness. The thickness values of organic thin films were estimated by the attenuation of the substrate core-level signals in the XPS measurements. For XPS measurements, the PTB7 and PC71BM films (∼15 nm) were spin-coated onto a cleaned ITO substrates from 8 and 10 mg/ mL chlorobenzene solutions in the nitrogen-filled glovebox, respectively. Then, 2 nm Al or Ag was deposited by thermal evaporation under high vacuum (1 × 10−4 Pa) onto the films. After this step, the film samples were directly and quickly transferred into the load lock chamber of the ultrahigh vacuum system for measurements. 21245

DOI: 10.1021/acs.jpcc.7b07561 J. Phys. Chem. C 2017, 121, 21244−21251

Article

The Journal of Physical Chemistry C 2.2. Device Fabrication. PSCs were fabricated with a simple conventional structure of ITO/PEDOT:PSS/ PTB7:PC71BM/MPcX/Ag or Al. First, PEDOT:PSS (Baytron PVP Al 4083) was spin-coated onto a cleaned ITO and annealed in air at 110 °C for 30 min. Second, active layer was spin-coated from a 25 mg/mL blend solution of PTB7:PC71BM with 1:1.5 wt % in mixed solvent of chlorobenzene/1,8diiodoctane (97:3, vol %) on PEDOT:PSS and then dried in vacuum overnight. Third, ∼3 nm MPcX film was spin-coated on the active layer from methanol solution with the concentration of 1 mg/mL. Finally, 100 nm Ag or Al was deposited by thermally evaporation with a pressure of 1 × 10−4 Pa as a cathode. 2.3. Measurements and Characterization. Current density−voltage characteristics of the devices were measured under N2 atmosphere in the glovebox by using a Keithley 2400 under illumination. Solar cell performance was tested under 1 sun, AM 1.5G full spectrum solar simulator (Photo Emission Tech, model SS50AAA-GB) with an irradiation intensity of 100 mW cm−2 calibrated with a standard silicon photovoltaic traced to the National Institute of Metrology, China. XPS and UPS experiments were carried out using a XPS/UPS system equipped with VG Scienta R3000 analyzer in ultrahigh vacuum with a base pressure of 1 × 10−10 mbar. A monochromatized He Iα irradiation from discharged lamp supplies photons with 21.22 eV for UPS. WF(ϕ) was determined from the energetic position of the secondary electron cutoff (Ecutoff) of the UPS spectrum according to ϕ = 21.22 eV − Ecutoff. The vertical ionization potential (IP) was obtained from the frontier edge of the occupied density of states. A monochromatic Al (Kα) X-ray source provides photons with 1486.6 eV for XPS. All measurements were calibrated by referencing to Fermi level and Au 4f7/2 peak position of gold foil cleaned by argon ion sputtering.

for device 6, methanol/Al for device 7, ZnPcX/Al for device 8, VOPcX/Al for device 9, and TiOPcX/Al for device 10. Corresponding photovoltaic parameters derived from Figure 2 are presented in Table 1. Compared to the control devices, incorporating the MPcXs led to a large enhancement in PCE due to a simultaneous increase of Voc, Jsc, and FF of the PSCs. For the devices with Ag, the PCE values improved from 1.44% (device 1) to 8.10% for device 3, 8.06% for device 4, and 8.07% for device 5 due to a great enhancement of Voc from 0.299 V of device 1 to 0.75−0.755 V of devices 3−5, Jsc from 13.71 mA/ cm2 of device 1 to 16.28−16.33 mA/cm2 of devices 3−5, and FF from 35.1% of device 1 to 65.4−66.1% of devices 3−5. However, device 2 with methanol/Ag only exhibited slight increases in Voc, Jsc, and FF compared to device 1, leading to an average PCE of 1.73%. For the devices with Al, the PCE values improved from 5.22% (device 6) to 8.22% for device 8, 8.19% for device 9, and 8.21% for device 10 due to an enhancement of Voc from 0.675 V of device 6 to 0.75 V of devices 8−10, Jsc from 15.17 mA/cm2 of device 6 to 16.45−16.48 mA/cm2 of devices 8−10, and FF from 50.9% of device 6 to 66.3−66.5% of devices 8−10. However, device 7 with methanol/Al only exhibited slight increases in Voc, Jsc, and FF compared to device 6. It suggests that the enhanced PCEs of the devices with the CIL are mainly contributed by the CIL themselves rather than their solvent (methanol). In order to ensure the validity and repeatability of data, we have measured at least 30 pixels for all device configurations. Average PCE values of the PSCs are 1.30% for device 1, 1.73% for device 2, 8.03% for device 3, 7.97% for device 4, 8.01% for device 5, 5.09% for device 6, 5.82% for device 7, 8.15% for device 8, 8.11% for device 9, and 8.14% for device 10. Table 1 also presents series resistances (Rs) of devices obtained from the slops of J−V curves at Voc. Compared to the two control devices, a decrease of Rs is apparent when the MPcXs were utilized as a CIL. For the devices with Ag, Rs decreases from 12.9 Ω cm2 of device 1 to 5.7−6.0 Ω cm2 of devices 3−5, while for the devices with Al, Rs decreases from 11.1 Ω cm2 of device 6 to 4.7−5.3 Ω cm2 of devices 8−10. Obviously ZnPcX, VOPcX, and TiOPcX led to similar device performance in the PSCs with either Ag or Al as cathode. It indicates that central metallocores in the three MPcXs have a negligible effect on the device performance. Moreover introduction of the MPcXs makes noble metal Ag become a promising cathode in the PSCs with high PCE and stability. It is known that proper energy level alignments of metal/ organic and organic/organic interfaces are crucial for device performance of PSCs.42−46 In order to figure out the working mechanism of the MPc derivatives as a CIL in the PSCs, we first investigated energy level alignment of metal/MPcX interface by UPS. WF of Ag and Al modified by the three MPcXs was measured from the energetic position of the secondary electron cutoff (Ecutoff) of the UPS spectrum according to ϕ = 21.22 eV − Ecutoff. As shown in Figure S1 in Supporting Information, the final WF of Ag became 4.04 eV due to an interface dipole (Δ) of 0.57 eV when more than 3 nm ZnPcX was deposited on Ag. Similarly the final WF of Ag became 4.01 eV (or 4.03 eV) due to the Δ of 0.6 eV (or 0.58 eV) when more than 3 nm VOPcX (or TiOPcX) was deposited on Ag. When the three MPcXs were deposited on Al, the WFs of Al are 3.55 eV for ZnPcX, 3.56 eV for VOPcX, and 3.53 eV for TiOPcX as shown in Figure S2. Obviously the coverage of the MPcXs with different metallocentral cores leads to similar WF of Ag and Al. Therefore it is reasonable that there are the

3. RESULTS AND DISSCUSSION A set of PTB7:PC71BM based polymer solar cells were prepared using ∼3 nm ZnPcX, VOPcX, or TiOPcX as a CIL. For comparison, Ag-only device without (device 1) and with (device 2) methanol and Al-only device without (device 6) and with (device 7) methanol as control devices have been fabricated as well. Current density−voltage (J−V) characteristics of these devices under AM 1.5G illumination at 100 mW cm−2 are presented in Figure 2, where cathodes of devices are Ag for device 1, methanol/Ag for device 2, ZnPcX/Ag for device 3, VOPcX/Ag for device 4, TiOPcX/Ag for device 5, Al

Figure 2. Current density versus voltage (J−V) characteristics of PTB7:PC71BM based devices with various cathodes (device 1, Ag; device 2, methanol/Ag; device 3, ZnPcX/Ag; device 4, VOPcX/Ag; device 5, TiOPcX/Ag; device 6, Al; device 7, methanol/Al; device 8, ZnPcX/Al; device 9, VOPcX/Al; device 10, TiOPcX/Al). 21246

DOI: 10.1021/acs.jpcc.7b07561 J. Phys. Chem. C 2017, 121, 21244−21251

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The Journal of Physical Chemistry C

Table 1. Photovoltaic Parameters of PTB7:PC71BM Based PSCs with Various Cathodes under 100 mW cm−2 AM 1.5G Irradiation PCE (%) device

cathode

Voc (V)

Jsc (mA cm−2)

FF (%)

max

av

Rs (Ω cm2)

1 2 3 4 5 6 7 8 9 10

Ag methanol/Ag ZnPcX/Ag VOPcX/Ag TiOPcX/Ag Al methanol/Al ZnPcX/Al VOPcX/Al TiOPcX/Al

0.299 0.357 0.755 0.750 0.755 0.675 0.705 0.750 0.750 0.750

13.71 14.08 16.33 16.28 16.32 15.17 15.33 16.48 16.45 16.47

35.1 39.1 65.7 66.1 65.4 50.9 54.7 66.5 66.3 66.5

1.44 1.96 8.10 8.06 8.07 5.22 5.91 8.22 8.19 8.21

1.30 1.73 8.03 7.97 8.01 5.09 5.82 8.15 8.11 8.14

12.9 12.1 5.7 6.0 5.9 11.1 9.7 4.7 5.3 4.9

close Voc values in the PSCs with the three MPcXs as the CIL as shown in Table1. In addition, the ionization potentials (energy difference between HOMO level and vacuum level) of the three MPcXs from Figures S1 and S2 are about 5.2 eV, indicating that the MPcXs are not good hole blocking layers for polymer donors. In conventional PSCs based on polymer:PC71BM blend as the active layer, electrons are extracted from PC71BM to metal cathode. Accordingly we first studied energy level alignments of 20 nm PC71BM on Ag or Al in order to highlight functions of the MPcXs as a CIL in the PSCs. Figure 3a2 presents the energy

barrier from PC71BM to Al is 0.16 eV. It provides a logical explanation why Al-only PSC (device 6) has a better device performance than Ag-only PSC (device 1). UPS spectra and energy level alignments of PC71BM spincoated on Ag modified by MPcX are presented in Figure 4.

Figure 3. UPS spectra and energy level alignments of PC71BM on Ag (a1 and a2) or Al (b1 and b2).

Figure 4. UPS spectra and energy level alignments of PC71BM on Ag modified by ZnPcX (a1, a2), VOPcX (b1, b2) or TiOPcX (c1, c2).

level alignment of PC71BM on Ag derived from UPS spectra of bare Ag and 20 nm PC71BM on Ag in Figure 3a1. Figure 3b2 presents the energy level alignment of PC71BM on Al derived from UPS spectra of bare Al and 20 nm PC71BM on Al in Figure 3b1. As shown in both Figure 3a2 and Figure 3b2, HOMO level of PC71BM is 6.0 eV vs vacuum level. LUMO level of PC71BM can be estimated to be roughly equal to 4.2 eV vs vacuum level using HOMO and optical band gap (1.8 eV). At Ag/PC71BM interface, energy difference between HOMO level and Fermi level is 1.4 eV and then energy difference between LUMO level and Fermi level is 0.4 eV in Figure 3a2. Thereby electron extraction barrier from PC71BM to Ag is 0.4 eV. However, at Al/PC71BM interface, electron extraction

Figure 4a1 shows UPS spectra of 3 nm ZnPcX on Ag (black line), 1 nm PC71BM (red line) on Ag/ZnPcX, 5 nm PC71BM (blue line) on Ag/ZnPcX, and 10 nm PC71BM (green line) on Ag/ZnPcX. Figure 4b1 show UPS spectra of 3 nm VOPcX on Ag (black line), 1 nm PC71BM (red line) on Ag/VOPcX, 5 nm PC71BM (blue line) on Ag/VOPcX, and 10 nm PC71BM (green line) on Ag/VOPcX. Figure 4c1 show UPS spectra of 3 nm TiOPcX on Ag (black line), 1 nm PC71BM (red line) on Ag/TiOPcX, 5 nm PC71BM (blue line) on Ag/TiOPcX, and 10 nm PC71BM (green line) on Ag/TiOPcX. The UPS-derived energy level alignment diagrams for Ag/ZnPcX/PC71BM, Ag/ VOPcX/PC71BM, and Ag/TiOPcX/PC71BM interfaces are 21247

DOI: 10.1021/acs.jpcc.7b07561 J. Phys. Chem. C 2017, 121, 21244−21251

Article

The Journal of Physical Chemistry C

PC71BM, Al/VOPcX/PC71BM, and Al/TiOPcX/PC71BM interfaces are depicted in Figure 5a2, Figure 5b2, and Figure 5c2, respectively. The evolution of the WF and UPS-derived HOMO with increased coverage of PC71BM on Al/MPcX is presented in Figure S4. For the three Al/MPcX/PC71BM contacts, the interfacial dipoles at the Al/MPcX interfaces down-shift the WF from 3.78 eV to 3.5−3.6 eV and the interfacial dipoles at the MPcX/PC71BM interfaces up-shifts the WF from 3.5−3.6 to 4.3 eV, giving a total vacuum level upshift from Al to PC71BM of 0.5 eV. As a result, electron extraction barrier at Al/ZnPcX/PC71BM contact is 0.10 eV, electron extraction barrier at Al/VOPcX/PC71BM contact is 0.09 eV, and electron extraction barrier at Al/TiOPcX/PC71BM contact is 0.08 eV, which are 0.06−0.08 eV lower than electron extraction barrier of 0.16 eV at the pure Al/PC71BM cathode (Figure 3b2). The integer charge transfer (ICT) model has been applied to describe the energy level alignment of various organic−metal interfaces and organic donor−acceptor interfaces in the heterojunctions.44−46 Figure S5 shows the dependences of ΦORG/SUB variation on ΦSUB for PC71BM films spin-coated on substrates. For PC71BM films, when ΦSUB is smaller than the EICT− (∼4.33 eV), the Fermi level is pinned to EICT−. When ΦSUB is larger than EICT+ (∼5.31 eV), the Fermi level is pinned to EICT+. When ΦSUB is between 4.33 and 5.31 eV, the vacuum level alignment holds and the ΦORG/SUB is equal to the ΦSUB. For the PC71BM-on-Ag contact in Figure 3a, there is not an interfacial dipole at the interface and the WF is equal to the Ag due to vacuum level alignment. For the PC71BM-on-Al contact in Figure 3b, PC71BM-on-Ag modified by MPcX in Figure 4, and PC71BM-on-Al modified by MPcX in Figure 5, the Fermi level is pinned at the EICT− of PC71BM which will reduce voltage loss at these contacts.47,48 This can also explain why devices 1 and 2 have lower Voc and PCE than other devices. The above UPS measurements clearly demonstrate that the electron extraction barrier becomes ∼0.1 eV when the MPcX (ZnPcX, VOPcX, or TiOPcX) is inserted between cathode (Ag or Al) and PC71BM. Therefore a nearly ohmic contact forms between the modified cathode and PC71BM, which provides the reason why the MPcX simultaneously increases Voc, Jsc, and FF. In addition the fact that ZnPcX, VOPcX, and TiOPcX resulted in a similar electron extraction barrier indicates that the function of the three MPcXs as a CIL is independent of the central metallocores in the three MPcXs. It should be noted that Al-only device (device 6) presents much lower PCE than the devices with MPcX/Ag (devices 3− 5) in Table 1 even though there is an electron extraction barrier of 0.16 eV from PC71BM to Al in Figure 3b2, which is close to the electron extraction barriers (0.11−0.13 eV) from PC71BM to the Ag modified by the three MPcXs in Figure 4. Moreover bare Al (3.78 eV) has a lower WF than Ag modified the three MPcXs (4.01−4.03 eV). In addition, even though PC71BM-onAl contact also presents Fermi level alignment in Figure 3b, Alonly device (device 6) has a lower Voc than the devices with the MPcX as a CIL. It suggests that there could be other elements (for example, exciton-quenching and carrier-trapping at the metal-on-active layer in the real PSCs) also influencing the device performance of the PSCs besides WF of cathode and electron extraction barrier from PC71BM to cathode. Actual situation at active layer/cathode interface is much more complicated in the real PSCs. In order to further explore the working mechanism of the MPcX as a CIL in the PSCs with Al or Ag as a cathode, we

depicted in Figure 4a2, Figure 4b2, and Figure 4c2, respectively. In addition, the evolution of the WF and UPS-derived HOMO with increased coverage of PC71BM on Ag/MPcX is presented in Figure S3. For the three Ag/MPcX/PC71BM contacts, the interfacial dipoles at the Ag/MPcX interfaces down-shift the WF from 4.6 to 4.0 eV and the interfacial dipoles at the MPcX/ PC71BM interfaces up-shifts the WF from 4.0 to 4.3 eV, giving a total vacuum level down-shift from Ag to PC71BM of 0.3 eV. Hence, the effective work function for electron extraction from PC71BM layers to Ag beyond the MPcX interlayer is 4.3 eV, 0.3 eV lower than for the pure Ag/PC71BM cathode (Figure 3a2). As a result, electron extraction barrier at Ag/ZnPcX/PC71BM contact is 0.13 eV, electron extraction barrier at Ag/VOPcX/ PC71BM contact is 0.12 eV, and electron extraction barrier at Ag/TiOPcX/PC71BM contact is 0.11 eV, which are 0.3 eV lower than electron extraction barrier of 0.4 eV at the pure Ag/ PC71BM cathode (Figure 3a2). Therefore, the experimental results suggest that adding a thin MPcX film should improve electron extraction from PC71BM to Ag by down-shifting the effective work function. Similarly UPS spectra and energy level alignments of PC71BM spin-coated on Al modified by MPcX are presented in Figure 5. Figure 5a1 shows UPS spectra of 3 nm ZnPcX on

Figure 5. UPS spectra and energy level alignments of PC71BM on Al modified by ZnPcX (a1, a2), VOPcX (b1, b2) or TiOPcX (c1, c2).

Al (black line), 1 nm PC71BM (red line) on Al/ZnPcX, 5 nm PC71BM (blue line) on Al/ZnPcX, and 10 nm PC71BM (green line) on Al/ZnPcX. Figure 5b1 shows UPS spectra of 3 nm VOPcX on Al (black line), 1 nm PC71BM (red line) on Al/ VOPcX, 5 nm PC71BM (blue line) on Al/VOPcX, and 10 nm PC71BM (green line) on Al/VOPcX. Figure 5c1 shows UPS spectra of 3 nm TiOPcX on Al (black line), 1 nm PC71BM (red line) on Al/TiOPcX, 5 nm PC71BM (blue line) on Al/TiOPcX, and 10 nm PC71BM (green line) on Al/TiOPcX. The UPSderived energy level alignment diagrams for Al/ZnPcX/ 21248

DOI: 10.1021/acs.jpcc.7b07561 J. Phys. Chem. C 2017, 121, 21244−21251

Article

The Journal of Physical Chemistry C

ZnPcX completely protected the S atoms of PTB7 from the damage of Al. The F 1s and S 2p core level XPS spectra of PTB7/Ag and PTB7/ZnPcX/Ag are presented in Figure 6c and Figure 6d, respectively. When 2 nm Ag is thermally deposited on the PTB7 and the PTB7 covered by 3 nm ZnPcX, no changes can be observed in the F 1s and S 2p XPS spectra. It suggests that no chemical reaction happened between Ag and F atoms or between Ag and S atoms in the PTB7. In addition, in order to further check the influence of hot metal atoms Al and Ag on C atoms and O atoms in the PTB7 and PC71BM, we carried out the C 1s and O 1s XPS measurements as shown in Figure 7. Figure 7a shows the fitted

carried out XPS measurements for active layer/CIL/cathode interfaces. The above-mentioned three MPcXs (ZnPcX, VOPcX, and TiOPcX) as the CIL gave rise to similar device performance of the PSCs based on PTB7:PC71BM and similar energy level alignments at the interfaces. Hereby we select ZnPcX as a model of the MPcXs to measure F 1s and S 2p core level XPS spectra of PTB7/ZnPcX/Al and PTB7/ZnPcX/Ag. For comparison, F 1s and S 2p core level XPS spectra of PTB7, PTB7/Al, and PTB7/Ag are studied as well. Because only PTB7 molecule has F and S atoms among the used molecules, changes of the F 1s and S 2p core level XPS spectra can describe the chemical interactions at PTB7/Al, PTB7/ZnPcX/ Al, PTB7/Ag, and PTB7/ZnPcX/Ag interfaces in Figure 6.

Figure 7. C 1s core-level XPS spectra of PTB7, PTB7/Al, and PTB7/ Ag (a) and PC71BM, PC71BM/Al, and PC71BM/Ag (c). O 1s corelevel XPS spectra of PTB7, PTB7/Al, and PTB7/Ag (b) and PC71BM, PC71BM/Al, and PC71BM/Ag (d).

Figure 6. F 1s core-level XPS spectra of PTB7, PTB7/Al, and PTB7/ ZnPcX/Al (a) and PTB7, PTB7/Ag, and PTB7/ZnPcX/Ag (c). S 2p core-level XPS spectra of PTB7, PTB7/Al, and PTB7/ZnPcX/Al (b) and PTB7, PTB7/Ag, and PTB7/ZnPcX/Ag (d).

C 1s XPS spectra of PTB7, PTB7 covered by 2 nm Al, and PTB7 covered by 2 nm Ag. Figure 7c shows the fitted C 1s XPS spectra of PC71BM, PC71BM covered by 2 nm Al and PC71BM covered by 2 nm Ag. Apparently thermal deposition of Al and Ag atoms on the PTB7 and PC71BM resulted in the changes of C 1s XPS spectra. Namely, hot metal atoms Al and Ag made some C atoms in the PTB7 and PC71BM have the higher oxidation states at 285−292 eV. It is possible that high temperature harmed the C atoms at the surfaces of PTB7 and PC71BM during the thermal deposition of Al and Ag atoms, leading to such changes in Figure 7a and Figure 7c. Reversely the thermal deposition of Al and Ag on the PTB7 and PC71BM brought about more reduction states in the O 1s XPS spectra of PTB7/Al, PTB7/Ag, PC71BM/Al, and PC71BM/Ag as shown in Figure 7b and Figure 7d. It is acceptable that some O atoms of PTB7 and PC71BM were reduced by hot metal atoms Al and Ag during the thermal deposition. However, inserting the CIL between the active layer and Al (or Ag) can alleviate these damages from hot metal atoms. The XPS results demonstrate that thermal deposition of Al can lead to the reductions of some F, S, and O atoms and oxidation of some C atoms in the PTB7 and PC71BM. Though thermal deposition of Ag cannot lead to reductions of the F and S atoms in the PTB7, it gives rise to reductions of some O

Figure 6a shows the fitted F 1s XPS spectra of PTB7, PTB7/Al, PTB7/ZnPcX/Al. Obviously when about 2 nm Al is thermally deposited at the PTB7 surface, F 1s core level XPS spectrum of PTB7/Al presents two peaks which are the peak at 687.6 eV corresponding to original F atoms in PTB7 and a new peak at 685.9 eV assigned to a reduced state of F atoms in PTB7. Moreover height of the new peak is almost 2 times the peak at 687.6 eV. It means that F atoms are partially reduced by Al at PTB7/Al interface. However, inserting 3 nm ZnPcX between PTB7 and Al can buffer the reduction of F atoms because the height ratio of the new peak at 685.9 eV related to the peak at 687.6 eV becomes 1.2 as shown in the F 1s core level XPS spectrum of PTB7/ZnPcX/Al. Therefore ZnPcX to some extent protected the F atoms of PTB7 from the damage of Al. Figure 6b shows the fitted S 2p XPS spectra of PTB7, PTB7/Al, PTB7/ZnPcX/Al. Compared to bare PTB7, there are two new shoulder peaks at 162.5 and 163.7 eV assigned to a reduced state of S atoms in PTB7 besides two peaks at 164.2 and 165.4 eV corresponding to original S atoms in PTB7 in S 2p core level XPS spectrum of PTB7/Al. It suggests that a few S atoms are reduced by Al at PTB7/Al interface. However, the new shoulder peaks at 162.5 and 163.7 eV disappears when 3 nm ZnPcX is introduced between PTB7 and Al, indicating that 21249

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atoms and oxidation of some C atoms in the PTB7 and PC71BM. These chemical shifts can generate a lot of trap leading to exciton recombination in the PTB7:PC71BM. However, inserting the MPcX between the active layer and cathode (Al or Ag) can limit, even avoid these damages of the hot metal atoms to the PTB7:PC71BM. Therefore the MPcX as a CIL not only decreases the WF of the metal cathode and the electron extraction barrier from PC71BM to cathode but also functions as a buffer layer to protect the active layer from the damages during the thermal deposition of Al and Ag. Accordingly though there is a low electron extraction barrier of 0.16 eV from PC71BM to Al in Figure 3, the PCE (5.2%) of the Al-only PSC (device 6) without a CIL is much lower than the PSCs with the MPcX as a CIL (devices 3−5) because hot Al atoms damaged the PTB7 and PC71BM at PTB7:PC71BM/ Al interface.

4. CONCLUSION In summary, the three MPc derivatives with different central metallocores were synthesized and applied as a CIL in the PTB7:PC71BM based PSCs. The PCE values reached 8.1% for the devices with Ag as cathode, while the PCE values could reach 8.2% for the devices with Al as cathode. Moreover the three MPcXs led to similar device performance in the PSCs with either Ag or Al as cathode. It indicates that central metallocores in the MPcXs have a negligible effect on the device performance. UPS and XPS results suggest that the MPcX as a CIL not only decreases the WF of the metal cathode and the electron extraction barrier from PC71BM to cathode but also functions as a buffer layer to protect the active layer during the thermal deposition of Al and Ag. It is expected that our findings can provide an insight for working mechanism of alcohol/water-soluble organic CIL materials in the PSCs.

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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/acs.jpcc.7b07561. Full citation of refs 8, 20, 25, and 35, supplementary figures, and other related information (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 431 85168476. Fax: +86 431 85193421. E-mail: [email protected]. ORCID

Fenghong Li: 0000-0002-5707-7629 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the National Basic Research Program of China (Grant 2014CB643505), the Natural Science Foundation of China (Grant 51273077) and Graduate Innovation Fund of Jilin University (Grant 2017076).

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