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Lithium Doping of ZnO for High Efficiency and Stability Fullerene and Non-Fullerene Organic Solar Cells Anastasia Soultati, Azhar Fakharuddin, Ermioni Polydorou, Charalampos Drivas, Andreas Kaltzoglou, Muhammad Irfan Haider, Fotis Kournoutas, Mihalis Fakis, Leonidas C. Palilis, Stella Kennou, Dimitris Davazoglou, Polycarpos Falaras, Panagiotis Argitis, Spyros Gardelis, Apostolos Kordatos, Alexander I. Chroneos, Lukas Schmidt-Mende, and Maria Vasilopoulou ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01658 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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ACS Applied Energy Materials

Lithium Doping of ZnO for High Efficiency and Stability Fullerene and Non-Fullerene Organic Solar Cells Anastasia Soultati,† Azhar Fakharuddin,§,ǂ Ermioni Polydorou,†,⊥ Charalampos Drivas,╝ Andreas Kaltzoglou,† Muhammad Irfan Haider,§,‖ Fotis Kournoutas,⊥ Mihalis Fakis,⊥ Leonidas C. Palilis,⊥ Stella Kennou,╝ Dimitris Davazoglou,† Polycarpos Falaras,† Panagiotis Argitis,† Spyros Gardelis,⌡ Apostolos Kordatos,¶ Alexander Chroneos,¶ Lukas Schmidt-Mende,§ Maria Vasilopoulou†,*

†Institute

of Nanoscience and Nanotechnology (INN), National Center for Scientific

Research (NCSR) “Demokritos”, 15341 Agia Paraskevi, Attica, Greece §Department

‖Department

of Physics, University of Konstanz, 78457 Konstanz, Germany

of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan

ǂIMEC

Kapeldreef 75, Heverlee 3001, Belgium (present affiliation)

⊥Department

╝Department

⌡Solid

of Physics, University of Patras, 26504 Patras, Greece

of Chemical Engineering, University of Patras, 26504 Patras, Greece

State Physics Section, Physics Department, National and Kapodistrian University of Athens, Panepistimioupolis, 15784 Zografos, Athens, Greece

1 ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

¶Faculty

of Engineering, Environment and Computing, Coventry University, Priory Street, Coventry CV1 5FB, United Kingdom

ABSTRACT We report on the effect of lithium doping of zinc oxide used as electron transport layer in organic solar cells based on both fullerene and non-fullerene acceptors. The experimental and theoretical results indicate that lithium ions are intercalated within the ZnO lattice as interstitial dopants replace interstitial zinc defects that act as trap states and give rise to a higher electron conductivity without significantly altering the electronic properties (work function and valence band edge) of the doped oxide. The slightly improved energetic alignment of the doped oxide’s work function with the lowest unoccupied molecular orbital of both types of acceptors is considered to contribute to a reduction of the interfacial electron extraction barrier. The enhanced electron carrier extraction/collection efficiency, the suppressed bimolecular and interface trap-assisted recombination losses and the higher electron mobility of the photoactive blend synergistically contribute to the superior performance of PTB7-Th:PC71BM-based fullerene devices utilizing doped ZnO layers with an optimized lithium concentration of 5 wt.% Such devices increased their maximum PCE from 8.59% (average 8.05%) to 10.05% (average 9.53%) while, simultaneously, boosting their long-term stability. Moreover, non-fullerene solar cells based on the PTB7-Th:IT-4F blend exhibited PCEs up to 8.96% and maintained more than 80% of their initial efficiency after 1000 h upon using the lithium modified ZnO electron transport layer. KEYWORDS: Organic solar cells, zinc oxide, lithium doping, fullerene acceptor, nonfullerene acceptor.


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INTRODUCTION Organic solar cells (OSCs) have emerged as potential candidates for the realization of third generation photovoltaics as they exhibit significant advantages such as cost-effectiveness and large-scale manufacturing even on flexible substrates.1-3 For efficient and stable operation, the charge transport layers inserted between the electrodes and the photoactive layer play a vital role since they dictate charge transport and collection efficiency.4-11 The development of a large variety of charge transport layers with multiple functionalities has enabled the achievement of efficiencies over 11% and 13% in single-junction devices using fullerene and non-fullerene acceptors, respectively,12-18 while a record efficiency of 17.3% has been recently demonstrated in tandem organic solar cells.19 Solution-processed ZnO is widely applied as electron transport layer (ETL) since it shows adequate n-type conductivity, relatively low work function (WF), high transparency and offers a facile, cost-effective, device fabrication pathway.20-25 Several processes have been applied to further improve the surface properties of ZnO and to beneficially modify the ZnO/photoactive blend interface. Representative examples are its surface modification with alcohol/water-soluble conjugated polymers,26-28 conjugated polyelectrolytes

(CPEs),29,30

self-assembled

monolayers

(SAMs),31

inorganic

polyoxometalate molecular oxides,32,33 small molecules and ionic liquids (ILs) possessing high intrinsic dipole moments,34,35 and its post-deposition plasma surface treatment with appropriate gases.36-38 Additionally, doping of ZnO with metallic elements such as aluminum (Al), magnesium (Mg), strontium (Sr) and barium (Ba) has been reported to enhance its electron mobility and facilitate interfacial energy level alignment, thus improving the electron transport/collection in OSCs.39-41 Recently, Lin et. al. reported that the utilization of lithium (Li)-doped ZnO electron transport layers can improve the efficiency of fullerenebased OSCs up to 8.4% compared to 7.5% for the reference device.42 Furthermore, significant 3 ACS Paragon Plus Environment


efficiency enhancement has also been recently reported in perovskite and silicon-based photovoltaics upon using Li-doped electron selective contacts fabricated with solution-based processes.43,44 In this work, we further explore the possibility to apply Li-doped ZnO as a universal electron transport layer in both fullerene and non-fullerene based OSCs and investigate in depth the mechanisms responsible for the enhanced OSC performance. The fullerene OSCs based on poly[[2,6′-4-8-di(5-ethylhexylthienyl)benzo[1,2-b:3,3-b]dithiophene][3-fluoro-2[(2ethylhexyl) carbonyl]thieno[3,4-b]thiophenediyl:[6,6]-phenyl-C71-butyric acid methyl ester (PTB7-Th:PC71BM) as the photoactive blend upon utilization of ZnO with 5 wt % Li concentration (termed hereafter as ZnO:Li) as ETL exhibited high power conversion efficiency (PCE) up to 10.05%, representing a 17% enhancement compared to the reference device (8.59%). Importantly, Li-doped ZnO applied as ETLs in non-fullerene OSCs based on

PTB7-Th:3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-

5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6b’]dithiophene (IT-4F) blends also enabled significant efficiency enhancement up to 8.96% representing a 20% improvement compared to the reference value of 7.47%. Adding to the merits, Li-doped ZnO OSCs also exhibited improved long-term stability compared to pristine ZnO-based OSCs. The enhanced overall device performance of Li-modified OSCs has been investigated via both theoretical calculations and several material and device characterization techniques including X-ray (XPS) and ultraviolet photoelectron spectroscopy (UPS), steadystate and transient photoluminescence (PL), optical absorption spectroscopy, electrical conductivity, X-ray diffraction (XRD), atomic force microscopy (AFM) and contact angle measurements. It was found that Li ions intercalate as interstitial dopants thus increasing the oxide’s conductivity while also improving the energy level alignment at the modified cathode interface, reduce the electron extraction barrier and enhance the charge collection efficiency. The decreased charge carrier recombination combined with the improved nanomorphology 4 ACS Paragon Plus Environment

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and higher electron mobility of the photoactive blend when deposited on the Li-doped ZnO layer synergistically contribute to the enhanced device performance. Moreover, Li-doping of ZnO offers a significant boost to the device stability through the suppression of zinc interstitial Zni defect sites which have been previously identified to act as chemisorption centers for environmental oxygen which destructively react with photoactive organic semiconductors causing the poor stability of pristine ZnO-based OSCs.45

RESULTS AND DISCUSSION Pristine and Li-doped (with a 5 wt.% concentration) ZnO films with a thickness of ~50 nm were grown on FTO/glass substrates using a sol-gel method described in the Experimental Section. XPS was used to probe the surface elementary composition of these samples. Figure S1 (Supporting Information) presents the wide scans of both samples where the presence of Zn and O on their surface is evident. The presence of C is due to atmospheric contamination since the samples are exposed to the atmosphere before they are introduced into the UHV chamber for characterization. The Li 1s peak (expected at around 56 eV) is not detectable. This can be explained by taking into account the following reasons: (a) the Li concentration is low, (b) Li diffusion into the ZnO bulk due to its small atomic size reduces its concentration at the surface and (c) the relative sensitivity factor (RSF) for the Li 1s peak is very small. Figure S2 presents the XPS Zn 2p peak which appears as a well-defined doublet due to spin orbit splitting. The Zn 2p3/2 peak is at a binding energy (BE) of 1021.7±0.1 eV for both samples.46 No measurable change is observable between samples which implies no chemical interaction of Zn with Li in the ZnO:Li sample. Figure S3 shows the XPS O 1s peak. Its shape is asymmetric as a result of the contribution of two separate peaks; one with a BE of 530.4±0.1 eV attributed to the Zn-O bond and another at t 532±0.1 eV attributed to the –OH



groups from the atmosphere.47 No significant changes are observed at the O 1s peak upon Li doping. The Tauc plots ( (αhν)2 versus photon energy (hν)) for the estimation of the direct optical bandgap were derived from UV-Vis absorption/transmittance measurements on ZnO and ZnO:Li films deposited on FTO/glass substrates and shown in Figure 1a. Upon Li doping, the optical band gap is found to slightly increase (from 3.25 to 3.28 eV). This bandgap widening can be due to the Moss-Burstein shift indicating that some states close to the oxide’s conduction band are being populated which suggests n-type doping of ZnO upon Li intercalation.48 On the contrary, doping with acceptors (p-type) has shown to result in a bandgap reduction of ZnO.49 The functionality of Li intercalants as n-type dopants suggests that they probably occupy interstitial sites since Li+ substituting Zn2+ ions are considered to behave as acceptors.50 Such n-type doping effect can explain the increase in conductivity of the ZnO:Li films as indicated by current density versus voltage characteristics taken in FTO/ZnO or ZnO:Li/Al heterojunctions shown in Figure 1b. It is obvious that ZnO:Li/Al stacks provide better ohmic contact compared to the ZnO/Al ones which is expected to be beneficial for the operation of an electronic device. Note that the transmission of ZnO is not significantly altered upon Li doping (Figure S4) and remains high enough to allow most of the illuminated light to enter the photoactive layer. Figure 1c shows the room temperature PL emission of the pristine and Li-doped ZnO films normalized at the near-band-edge (NBE) peak. Both films exhibit similar visible emission which is attributed to mid-gap states mainly involving neutral (VO) and charged (VO2+) deeply-positioned oxygen vacancies.51,52 This indicates that Li ions intercalation within the ZnO lattice does not impact the oxide's concentration of oxygen vacancies which are considered responsible for the unintentional n-type conductivity of the oxide. However, compared to the pristine ZnO, the PL spectrum of the Li-doped oxide film shows an obvious 6 ACS Paragon Plus Environment

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hypsochromic shift in the NBE emission (Fig. 1c and Figure S5). In particular, the NBE band of ZnO appears at 3.13 eV (396 nm) whereas that of ZnO:Li at 3.23 eV (384 nm). However, the position of the NBE peak is less shifted relative to the estimated value of optical bandgap in the doped sample (smaller Stokes shift, ΔΕ, of 50 meV for ZnO:Li, versus 120 meV for ZnO). Zinc interstitials (Zni) are well-known localization centers for excitons in zinc oxide located few tens of meVs below the conduction band edge.45,53,54 A large density of defect states derived from Zni may act as deactivation centers thus causing a pronounced red shift in the NBE peak positioning of ZnO. We propose that the observed differences in ΔΕ for the pristine and Li-doped ZnO films investigated may be attributed to lower concentration of Zni sites in the Li-modified sample as a result of their replacement by Li+ ions as also indicated by conductivity measurements. The XRD patterns (Figure 1d) show relatively weak scattering of the ZnO lattice. The XRD patterns shows reflection of the (100), (002), (101), (102) and (110) crystallographic planes with a preferred crystallite orientation along the c-axis (Figure S6). The lattice parameters were refined for the hexagonal wurtzite structure while no traceable Li-containing phases (metallic lithium or lithium oxide) are observed. A small increase in the lattice parameters from a=b=3.222(11), c=5.186(13) Å for the pristine sample to a=b=3.225(4), c=5.194(4) Å is observed upon Li doping indicating longer inter-planar distances in the Li-doped sample. This lattice expansion may suggest the presence of Li in interstitial positions within the wurtzite lattice.55 As the ionic radius of Li+ (0.76 Å) is very close to that of Zn2+ (0.74 Å): (i) Li can easily penetrate into ZnO crystal lattice and (ii) replacement of Zn2+ with Li+ (substitutional doping) is not expected to increase the lattice parameters of the wurtzite structure. Interestingly, the full-width-at-half-maximum (FWHM) of the (002) peak decreases in the Li-doped sample (Figure S6) which implies better crystallinity in accordance with the enhanced electrical conductivity of the sample. Moreover, FTIR spectroscopy has been employed to study the influence of Li-doping on ZnO (Figure S7). The intense band at 7 ACS Paragon Plus Environment


around 430 cm-1 corresponds to the formation of Zn–O bond. The slight shift of this band towards lower wavenumbers in the Li-doped sample can be related to an increase in the bond length that takes place upon interstitial doping of ZnO with Li.44 Indeed, upon interstitial doping of ZnO with Li, bonds between Li and O are expected to be formed forcing the electron density on oxygen to be shifted (away from Zn) thereby making the O less electronegative and the relevant Zn-O bond becoming less ionic, thus increasing its length. The surface topography of both samples was also examined since it could affect the quality of the ZnO/photoactive blend interface. In Figures 1e and 1f, the 5×5 μm2 AFM topographic images of ZnO and ZnO:Li films, respectively, on FTO/glass substrates are shown. The pristine and Li-doped ZnO films exhibited a nanostructured-like surface topography consisting of columnar particles of different sizes. These particles grew bigger upon Lidoping of ZnO in accordance with a reduction in the density of Zni defects which are usually located at the particle boundaries acting as nucleation centers for the growth of new particles.56,57 Upon reduction of the amount of such defects, partial elimination of the particle nucleation centers takes place thus increasing the particle size of Li-doped ZnO. Note that, the root-mean-square (RMS) surface roughness decreases from 16.07 to 14.33 nm upon Li-doping which might enhance the interfacial contact at the Li-modified ZnO/photoactive blend interface and facilitate electron transport. Next, UPS measurements were carried out to gain more insight into the surface electronic properties of ZnO upon Li-doping. Figure 2a shows the UPS spectra of ZnO and ZnO:Li samples. In the middle, the full spectrum is presented whereas to the right and the left part, the region near the Fermi level and the high BE cut-off, respectively, are shown in detail. The valence band maximum (VBM) for the pristine ZnO is measured at 3.3 eV. There is no significant dependence of the VBM with Li-doping. For the pristine sample, the WF is


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calculated from the high BE cut-off region to be equal to 3.9±0.1 eV, whereas there is a +0.1 eV shift for the 5 wt. % Li-doped sample, in accordance with literature.58 To further explore the impact of Li in ZnO film we then performed density functional theory (DFT) calculations. Zinc oxide is a wide bandgap semiconductor that crystallizes in the hexagonal symmetry (wurtzite) that is the most stable at ambient conditions. The space group is P63mc and the calculated lattice parameters are a=b=3.25 Å and c=5.2 Å which are in good agreement with the experimental values (Figure S8). The structure has been initially optimized as a non-defective crystal. Thereafter, we introduced Li at all potential interstitial sites in the ZnO supercell. The lowest energy Li interstitial is in the hexagonal configuration along the ab plane (Figure 2b), while the Li distance from the closest Zn and O atoms are calculated to be 2.48 Å and 1.94 Å respectively. The electronic structure (partial density of states, PDOS) for the non-defective ZnO was first calculated (Figure S9 and Figure 2c) and was in good agreement with previous studies.59 For the electronic structure calculations, we used the HSE06 exchange–correlation hybrid functional with norm conserving pseudopotentials for every element. It is already reported that GGA/LDA calculations underestimate the ZnO band gap59-61 and thus they are not appropriate to reproduce the correct DOS. Specifically, the predicted band gap is approximately 0.9 eV far below the experimental value.60,61 The hybrid functionals have been considered to overcome this error and we confirmed the calculated band gap for ZnO to be 3.2 eV (Figure 2c), which is in agreement with the experimental results. The electronic structure for the Li-doped ZnO is presented in Figure 2d. In Figure S10, the detailed orbital contributions are shown. The valence band is governed by the O-p and Zn-d states whereas the Zn-s and Zn-p states correspond to the strongest contributions in the conduction band. The introduction of Li interstitial atoms has a minor influence on the DOS profile (in the form of Li-p states refer to Figure S10). Overall, the introduction of Li atoms corresponds to a doping process that does not drastically affect the electronic structure (in-gap states/deep levels in the band gap). This 9 ACS Paragon Plus Environment


indicates that ZnO can be a potential candidate for electronic applications with no anisotropic behavior on the electronic conduction mechanisms. Moreover, the crystal presents minimum volume divergence through the Li accommodation through interstitial sites and will preserve its good structural stability in operational conditions. Furthermore, the distribution of the Li states is expected to beneficially affect the materials performance via the promotion of redox processes. We then fabricated fullerene-based OSCs with the inverted architecture: FTO/ZnO (or ZnO:Li)/PTB7-Th:PC71BM/MoOx/Al (Figure 3a) to evaluate the effect of Li-doping of ZnO on

the device performance. The chemical structures of the organic semiconductors (PTB7-Th and PC71BM) used in the photoactive blend are also shown in Figure 3a. The energy level alignment (before contact) of the materials sequence used in the cathode side of the device is illustrated in Figure 3b. For the ZnO and ZnO:Li the VBM and WF positions were extracted from the UPS spectra; the WF of FTO and HOMO, LUMO levels of PTB7-Th and PC71BM were taken from the literature.62 It is evident that the application of ZnO:Li as ETL improves the energy alignment at the cathode interface through the excellent matching between its WF and PC71BM’s LUMO. In addition, the hole blocking barrier at the PTB7-Th/ZnO:Li interface is larger than that formed with the pristine ZnO which may also have a positive impact on the device performance. Figure 3c depicts the current density versus voltage (J–V) characteristics taken under simulated A.M. 1.5 illumination (100 mW cm-2) of the best performing PTB7-Th:PC71BMOSCs incorporating either pristine or doped with 5 wt. % Li concentration ZnO ETLs; the extracted device performance parameters are summarized in Table 1. Note that, the Li content was optimized to 5% by also testing devices using ZnO layers doped with Li of 2 and 10 wt. % concentration (Figure S11 and Table S1). The device using pristine ZnO exhibits an opencircuit voltage (Voc) of 0.76 V, a short-circuit current density (Jsc) of 16.62 mA cm-2 and a 10 ACS Paragon Plus Environment

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fill factor (FF) of 0.68 leading to a PCE of 8.59% which is comparable to other reported results.63,64 A significant enhancement in Voc (0.80 V) and Jsc (17.94 mA cm-2) and a small improvement in FF (0.70) was obtained upon 5 wt. % Li-doping of ZnO thus increasing the PCE to 10.05%. This value is among the highest reported for single junction PTB7-Th-based OSCs using fullerene acceptors (see also Table S2).65-67 The dark J-V curves of the same devices are also presented in Figure 3d where a pronounced decrease in the reverse saturation current and the below turn-on voltage leakage current is evident for the Li-doped incorporating device. This finding along with a decrease in the series resistance, Rs, (Table 1) indicates the improved quality of the cathode (FTO/ZnO:Li/PTB7-Th:PC71BM) contact which explains the enhancement in Jsc (and FF). The enhancement in Voc can be related to the improved interfacial energetic alignment, the reduced electron extraction barrier and the decrease in the reverse saturation current. Furthermore, we examined the consistency of the device performance by statistical analysis performed in a batch of 16 individual devices fabricated under identical conditions. Figure 4 a-d presents the statistical data for Jsc, Voc, FF and PCE, respectively; standard box plots are also included. It is easily seen that the average values of Jsc and Voc are significantly higher in the device incorporating the Li-doped ZnO compared to the reference device while the increase in FF is more modest. As a result, the average PCE of the Li-modified cells is 9.53% representing a considerable enhancement compared to the average PCE of the devices using the pristine ZnO (8.05%). In addition, the dispersion of the data is considerably lower with the Li-doped ZnO ETL, leading to more reproducible device performance. We next measured the external quantum efficiency (EQE) spectra of the fabricated OSCs (Figure 5a). The EQE spectra revealed an improvement in the entire visible range when using the ZnO:Li ETL which implies improved charge collection efficiency of the corresponding cell.

In addition, the more favorable photoactive blend nanomorphology, indicated by the UV-Vis absorption spectra (Figure S12) and AFM surface topographies (Figure S13) of PTB711 ACS Paragon Plus Environment


Th:PC71BM films deposited on pristine and Li-doped ZnO films, may also contribute to the enhanced electron transport/extraction. This alteration in thin film nanomorphology (i.e. smaller scale differences in the interpenetrating network of donor and acceptor phases when deposited on ZnO:Li with smaller surface roughness) can be probably explained by the different surface wetting characteristics of ZnO layers before and after Li-doping as suggested by contact angle measurements (Figure S14 and Table S3). Note that, the shortcircuit photocurrent densities estimated by EQE measurements, JscEQE (Table 1), are very close to that obtained from J-V characteristics (mismatch ˂1%) confirming the validity of measured Jsc and PCE values. The enhanced electron collection efficiency suggested by EQE spectra for OSCs using Li-doped ZnO layers was further supported by the J-V characteristics taken in electron-only devices with the structure FTO/ZnO (or ZnO:Li)/ PTB7Th:PC71BM/Al) shown in Figure 5b in a logarithmic plot. Dark current-voltage curves were measured, by applying the negative bias to the ZnO-modified FTO electrode. The higher electron current density obtained in the Li-doped ZnO- devices can be directly attributed to the enhanced electron extraction efficiency and the improved electron transport at the cathode interface due to the lower contact resistance and the enhanced ZnO:Li conductivity. The enhanced exciton dissociation and electron transfer was also confirmed by the reduction in steady-state photoluminescence of PTB7-Th when deposited on Li-doped ZnO (Figure S15). We also modeled space charge limited current conduction. The enhanced exciton dissociation and electron transfer was also confirmed by the reduction in steady-state photoluminescence of PTB7-Th when deposited on Li-doped ZnO (Figure S15). We also modeled space charge limited current conduction and determined the electron mobility of the PΤΒ7-Th:PC71BM blend; the measured current-voltage data were fitted to the Murgatroyd equation for field dependent mobility.68 For the PTB7-Th:PC71BM blend on the ZnO:Li-based device, we found μ0=2.8x10-4 cm2 (Vs)-1 and β=0.0025 (cm/V)1/2 (Table S4). These led to an electron mobility μ=8.6x10-4 cm2 (Vs)-1 at an electric field E=0.2 MV cm-1, which was approximately 12 ACS Paragon Plus Environment

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40% larger than the pristine ZnO-based device. The results obtained were in good agreement with those reported in similar, electron-only, PTB7-Th:PC71BM-based diodes.69 The trend observed in electron mobility enhancement with ZnO:Li can be readily correlated with the improved solar cell performance. Therefore, Li-doping of a ZnO electron transport layer effectively improves its transport ability and facilitates electron extraction at the modified cathode interface. We next measured the variation of Jsc and Voc versus illumination intensity to get a deeper understanding on the enhancement in the device performance upon Li-doping of ZnO (Figure 5c and 5d). The expression of power-law dependence of Jsc on the light intensity is: Jsc~(intensity)b,

where

the

exponential

factor

b

is

generally

related

to

monomolecular/bimolecular recombination under short-circuit conditions with a deviation from unity occurring primarily due to bimolecular recombination.70 The calculated values of b were 0.97 and 0.99 for pristine and Li-doped ZnO-based devices, respectively, indicative of weaker bimolecular recombination in the Li-modified device. In a similar manner, the slope of Voc versus the illumination intensity was lower (1.04 kT/q) in the Li-modified cell compared to the reference one (1.10 kT/q), where k is the Boltzmann constant, T is temperature in Kelvin and q is the elementary charge. The above indicates a lower probability/strength of trap-assisted or Shockley–Read–Hall recombination in the Limodified ZnO-based cell.71 The beneficial effect of Li-doping on ZnO was also indicated by an increase in the exciton lifetime of PTB7-Th when deposited on ZnO:Li, as compared to that deposited on ZnO, as observed in TRPL dynamics shown Figure 5e;46 the corresponding fitting parameters are included in Table S5. As the PTB7-Th film deposited on the two substrates is processed under identical conditions, the larger exciton lifetime of the organic film on the Li-modified substrate might indicate a reduction of the interface trap states (probably Zni according to the above discussion) that generally act as effective recombination centers. Note that a larger exciton lifetime is generally accompanied by a longer exciton 13 ACS Paragon Plus Environment


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diffusion length which is desirable since it will strongly facilitate excitons to reach the electron donor:acceptor interface and effectively dissociate into free carriers, as also suggested by the steady-state PL measurements. All these results verify that bimolecular and interfacial trap-assisted recombination processes are largely suppressed in the devices using the Li-doped ZnO layer. The enhanced electron transport/extraction and reduced recombination were further supported by the dependence of (net) photocurrent density Jph (Jph=Jlight-Jdark, where Jlight is the current density under 1.5 AM illumination) versus effective voltage Veff (Veff =Vo-Vappl, where Vo is the voltage at Jph equal to zero and Vappl is the applied voltage) characteristics (Figure 5f). Evidently, Jph is higher in the Li-modified cell suggesting that Li-doping of ZnO is beneficial for exciton dissociation into free carriers. Additionally, the maximum exciton generation rate, Gmax, is higher in the modified cell as Gmax~Jsat, where Jsat is the saturated (maximum) Jph obtained at large Veff when all the photogenerated excitons have dissociated into free charge carriers. Moreover, the exciton dissociation probability, P(E,T), which is determined by the ratio Jph/Jsat and is related to the charge separation efficiency, was also calculated (Figure S16) and found to be 94% and 99%, for the devices incorporating pristine and Li-doped ZnO, respectively. The increased P(E,T) of the Li-modified devices indicates that more excitons are dissociated into free carriers in those cells. The

long-term

stability

of

OSCs

represents

a

crucial

factor

for

the

industrialization/commercialization of this technology. Therefore, we next assessed the influence of Li-doping of ZnO on the device lifetime. In particular, the possible degradation of performance parameters (such as Jsc, Voc, FF and PCE) of non-encapsulated devices for a period of 2000 h was evaluated. Figure 6 presents the variation of the normalized values of those parameters over the aging period. Note that all measurements were performed at lab conditions (relative humidity 20-30%, temperature 25 oC) and that the devices were stored 14 ACS Paragon Plus Environment

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in the dark inside a nitrogen filled desiccator between measurements. In the reference cell a strong degradation of all device parameters is observed; after 1000 h the device loses more than 70% of its initial PCE while it hardly operates after 2000 h. On the contrary, in the Limodified cell Voc and FF preserve about 90% of their initial values after 2000 h. A higher degradation of Jsc to about 80% of the initial value leads to a PCE equal to 64% of the initial value representing, however, a notable enhancement compared to the reference cell. The high stability improvement of the Li-modified device can be attributed to the reduction of Zni defects of ZnO upon Li-doping. It has been established that oxygen chemisorption occurs readily on to interstitial zinc sites present on the oxide’s surface.45,72 Upon Li-doping, interstitial Li atoms replace Zni defects, thereby passivating them, prohibiting the adsorption of corrosive agents and boosting the stability of the Li-modified ZnO-based OSCs. Finally, the effect of Li-doping modification of ZnO on the performance of non-fullerene based devices was examined. The photoactive layer comprised of a PTB7-Th:IT-4F blend; the chemical structure of IT-4F acceptor and the energy level alignment at the cathode interface are illustrated in Figure 7a.73 A small decrease in the electron extraction barrier upon Li-doping is again evident. Figure 7b presents the J-V characteristics of OSCs using ZnO pristine or Li (5 wt. %)-doped ZnO ETLs and MoOx/Al as a hole selective contact.74 Table 2 summarizes the devices performance characteristics. The increased Jsc, Voc and FF upon Li modification resulted in an enhancement of the maximum PCE by 20% from 7.47 to 8.96% which lies among the highest reported values for non-fullerene OSCs based on a PTB7-Th:IT-4F blend.73,75 Figures 7c, 7d and 7e present the dark J-V characteristics, the EQE spectra and the variation of the net photocurrent versus the effective voltage of the same devices while Figure S17 shows the absorption spectra of the photoactive layers on top of the two different ZnO substrates. It is evident that the modified cells exhibited better characteristics relative to the reference ones which clearly indicates that applying Li-doping of ZnO is an effective and universal method to enhance OSC performance. Importantly, a 15 ACS Paragon Plus Environment


significant stability enhancement is again evident upon Li-doping of ZnO layer as in the case of fullerene devices (Figure 7f); the modified cell retains 81% of the initial PCE after 1000 h which represents a notable improvement compared to the reference cell. It is worth noting that the stability of non-fullerene devices was higher than that of their fullerene counterparts which can be mainly attributed to better resistance of non-fullerene acceptors to environmental corrosion agents compared to the fullerene ones.76

CONCLUSIONS In summary, we fabricated fullerene OSCs based on PTB7-Th:PC71BM blend with power conversion efficiency over 10% by employing Li-doped ZnO electron transport layers. Additionally, non-fullerene PTB7-Th:IT-4F-based OSC exhibited 20% performance enhancement reaching PCEs up to 8.96% upon Li-modification of ZnO. Our experimental and theoretical results suggest that Li atoms behave as interstitial dopants replacing Zni defects thus offering effective passivation of such trap states present in ZnO while also improving the oxide’s conductivity. The enhanced nanomorphology of the photoactive blend deposited on ZnO:Li also contributes to the enhanced exciton dissociation and charge extraction. Additionally, a small reduction in the electron extraction barrier upon doping results in increased charge collection, which is further assisted by the enhancement in electron mobility of the photoactive blend. The device parameters enhancement enabled by Li-doping of ZnO was also combined with a boost in long-term stability making them viable for technological applications. EXPERIMENTAL SECTION Preparation of ZnO and Li-doped ZnO. ZnO layers were prepared via sol-gel method as described in a previous publication.33 In brief, they were made by spin-coating 0.5 mol L-1 of 16 ACS Paragon Plus Environment

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zinc acetate dihydrate in ethanolamine and 2-methoxyethanol on a pre-cleaned FTO (Solaronix TCO10- 10)/glass substrate. For the lithium doped ZnO, lithium chloride (LiCl, Sigma Aldrich) was substituted in an appropriate weight ratio (5 wt. %). All the remaining processing such as spin coating parameters and annealing conditions was similar to a reference ZnO film deposition described in detail in our previous work.33 Theoretical methodology. We employ the plane wave DFT code CASTEP.77,78 The structure has been optimized with the exchange and correlation interactions modelled using the corrected density functional of Perdew, Burke and Ernzerhof (PBE)79 in the generalized gradient approximation (GGA), with ultrasoft pseudopotentials.80 The kinetic energy cut-off of the plane wave basis is 400 eV, in conjunction with a 2 x 2 x 2 Monkhorst-Pack (MP)81 kpoint grid and a 72-atomic site supercell. The calculations were under constant pressure conditions. For the partial DOS calculations, the kinetic energy cut-off of the plane wave basis is 800 eV and a denser mesh of 10x10x10 k-points was applied. Here, the exchange correlation of the Heyd, Scuseria, Ernzerhof HSE functional is applied separated into longrange and short-range using an error function. Additionally, it replaces a fraction of the shortrange GGA exchange by the respective fraction of a nonlocal Fock exchange potential. For the visualization of outputs, the OPTADOS subcode is employed.82,83 Device fabrication. OSCs were fabricated on ZnO or ZnO:Li deposited on pre-cleaned FTO/glass substrates. The photoactive blends consisted of PTB7-Th:PC71BM films spincoated from solutions having a concentration of 10 mg ml-1 in PTB7-Th (purchased from Ossila) and 15 mg ml-1 in PC71BM (purchased from Solenne). The solids were dissolved in 1,2-dichlorobenzene with the addition of 3% per volume of 1,8-diiodooctane (DIO). The films were spin-coated inside an argon filled glove box at 1000 rpm for 90 sec. After they were left to dry for 30 min they were transferred into a hot-wire deposition system for the deposition of MoOx. PTB7-Th/IT-4F (IT-4F was purchased from Ossila) blends (1:1.25 in 17 ACS Paragon Plus Environment


chlorobenzene, CB, with 0.5% DIO) with a solid concentration of 20 mg/mL were spincoated at 1200 rpm for 90 sec inside the glove box and then annealed at 150 oC for 10 min. The fabrication procedure was completed upon the deposition of the Al top electrode in a dedicated thermal evaporator. Characterization techniques. XPS and UPS measurements were carried out in order to characterize the surface chemical composition as well as their electronic structure of ZnO films on a FTO/glass substrate with nominal wt. % concentration of Li doping of 0% (ZnO) and 5% (ZnO:Li). All samples were measured as fabricated, without any treatment or cleaning before introducing into the ultra-high vacuum chamber. For the XPS measurements, an unmonochromatized Mg Ka line at 1253.6 eV (12 keV with 15 mA anode current) and an analyser (Leybold EA-11) pass energy of 100 eV, giving a full width at half maximum, FWHM, of 1.3 eV for the Au 4f 7/2 peak, were used. The analysed area was approximately a 2×5 mm2 rectangle positioned near the geometric centre of each sample. XPS analysis was carried out at 0 degrees take-off angle (normal to the sample surface). In all XPS spectra the BE of the predominant aliphatic contribution to the C 1s peak at 284.8 eV was used as a measured BE reference. For the UPS measurements the He I (21.2eV) excitation line was used and a voltage of 12.23V was applied to the specimen in order to separate the high binding energy cut-off from the analyser. Since ZnO is a photosensitive material, UPS spectrum was taken first followed by XPS measurements. The films of the undoped and Lidoped ZnO films (on silicon) were also studied with X-ray diffraction on a Siemens D-500 diffractometer with Cu radiation in Bragg-Brentano geometry. Structural refinements were performed with the Rietveld method using the Fullprof software.84 Photoluminescence measurements on ZnO and ZnO:Li were carried out using a Horiba Jobin-Yvon iHR320 Spectrometer with a He -Cd laser (325 nm) as excitation source. For monitoring steady-state PL spectra of PTB7-Th on ZnO and ZnO:Li substrates a blue laser diode module emitting at 450 nm was used as the excitation source and a Si photodiode was used as the detector. The 18 ACS Paragon Plus Environment

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recorded spectra were analyzed by using an Oriel 77200 monochromator. The PL decays were monitored under magic angle conditions by using the Time-Correlated Single Photon Counting (TCSPC) technique (Picoquant) with an Instrument’s Response Function of 100 ps. A pulse diode laser at 470 nm (Picoquant) was used for the excitation of the solid samples. The PL signal passed through a monochromator and detection of PL decays of PTB7-Th was performed at 790 nm by means of a Hamamatsu R3809U-5X series microchannel-plate photomultiplier. UV−Vis absorption and transmittance spectra were recorded with a Perkin Elmer Lambda 40 UV/Vis spectrometer. AFM topographies were monitored with an NTMDT atomic force microscope operating in tapping mode. For the OSC device measurements (J-V characteristics taken under light illumination and in dark) a Keithley 2400 sourcemeasure unit equipped with a Xe lamp with an AM 1.5G filter was used while EQE measurements were collected with a Autolab PGSTAT-30 potentiostat using a 300 W Xe lamp and an Oriel 1/8 monochromator.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Wide scan XPS spectra , Zn 2p and O 1s XPS peaks; theoretical calculated structure and DOS of ZnO; UV-Vis absorption spectra and AFM topographies of the photoactive blend; contact angle measurements and PL spectra of ZnO; J-V curves and exciton dissociation probability of OSCs.

AUTHOR INFORMATION 19 ACS Paragon Plus Environment


Corresponding Author *Email: [email protected]

ACKNOWLEDGMENTS ΙΚΥ Scholarship Programs, Strengthening Post-Doctoral Research Human Resources Development Program, Education and Lifelong Learning, co-financed by the European Social Fund – ESF and the Greek government is also acknowledged. A.F acknowledges Alexander von Humboldt Stiftung for the award of postdoctoral fellowship.

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(84) Roisnel, T.; Rodriguez-Carvajal, J. Fullprof, Version Sept. 2012, France.

Tables Table 1 Performance parameters of OSCs with the structure FTO/ZnO (or ZnO:Li)/PTB7Th:PC71BM/MoOx/Al. ETL

Jsc

Jsc(EQE)

Voc

(mA cm-2)

(mA cm-2)

(V)

ZnO

16.62

16.54

0.76

ZnO:Li

17.94

18.01

0.80

FF

PCEmax

PCEaverage

Rs

(%)

(%)

(Ω cm2)

0.68

8.59

8.05

1.4

0.70

10.05

9.53

1.1


Page 33 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2 Performance parameters of OSCs with the structure FTO/ZnO (or ZnO:Li 5 wt. %)/PTB7Th:IT-4F/MoOx/Al. ETL

Jsc

Jsc(EQE)

Voc

(mA cm-2)

(mA cm-2)

(V)

ZnO

15.01

15.09

0.79

ZnO:Li

16.12

16.14

0.83

FF

PCEmax

PCEaverage

Rs

(%)

(%)

(Ω cm2)

0.63

7.47

7.12

5.2

0.67

8.96

8.71

3.1

Figures



(b)

3

[ahv]

2

400

ZnO ZnO:Li

Normalized PL Intensity

-2

4

(c)

600

Current density (mA cm )

ZnO ZnO:Li

2

(eV cm-1)2

(a) 5

200 0

-200

1

-400

3.25 eV 0 2.0

(d)

1.4

ZnO ZnO:Li

1.2 1.0 0.8 0.6 0.4 0.2

3.28 eV 2.2

2.4

2.6

2.8

3.0

3.2

hv (eV)

3.4

3.6

3.8

-600 -2.0

4.0

-1.5

-1.0

-0.5

0.0

0.5

Voltage (V)

1.0

1.5

2.0

(f)

(e)

ZnO ZnO:Li

0.0

350

400

450

500

550

600

Wavelength (nm)

650

700

750

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 41

30

31

32

33

34

35

2theta (degrees)

36

37

38

Figure 1 (a) Tauc plots as derived from absorption measurements of pristine ZnO and ZnO with 5 wt. % Li doping (ZnO:Li) films deposited on FTO/glass substrates. (b) J-V curves of the samples with the architecture FTO/ZnO or ZnO:Li/Al contact. The related device structure is shown in the inset. (c) Photoluminescence intensity (normalized to Near-BandEdge emission peak) and (d) XRD patterns of pristine and Li-doped ZnO samples deposited on silicon substrates. (XRD data are presented without intensity normalization and correction of the zero-point shift). 2D 5x5μm2 AFM topographic images of (e) ZnO and (f) ZnO:Li films on FTO/glass substrates.


Page 35 of 41

ZnO ZnO:Li

(c)

UPS HeI

Normalized Intensity (a.u.)

(a)

(d) 3.9 eV

18

4.0 eV 17

3.3 eV 16 20 18 16 14 12 10

8

6

Binding energy (eV)

4

2

06

4

2

0

(b)

Figure 2 (a) UPS spectra of ZnO and ZnO:Li films on FTO/glass substrates. (b) Li relaxation in the ZnO supercell. The PDOS for ZnO (c) without doping and (d) with Li+ doping.

(a)

(d)

ZnO ZnO:Li

0 -4 -8 -12 -16 -20 -0.4

-0.2

0.0

10 -2

-2

4

(b)

Current densitydark (mA cm )

(c) 8 Current density (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2

0.4

0.6

0.8

1.0

3

10

2

10

1

10

0

10

-1

10

-2

10

-3

10

-4

ZnO ZnO:Li

-0.50 -0.25 0.00

Voltage (V)

0.25

0.50

0.75

Voltage (V)


1.00

1.25

1.50


Figure 3 (a) The inverted organic solar cell architecture and the chemical structures of the organic semiconductors used in this study. (b) Energy levels (considering vacuum level alignment before contact) of the materials sequence used in the cathode side of OSCs: WF of FTO, WF and VB of ZnO and ZnO:Li and HOMO, LUMO levels of PTB7-Th and PC71BM. (c) Current density-voltage (J-V) characteristics of PTB7-Th:PC71BM-based devices using ZnO, or ZnO:Li ETLs upon 1.5 AM illumination. (d) Dark J-V curves of the same devices.

19

(b) 0.84

18

0.82

Jsc (mA cm )

(a) -2

0.80

Voc (V)

17

0.78

16

0.76

15

0.74 14

(c)

ZnO

ZnO:Li

0.72

0.72

(d)

0.70

ZnO

ZnO:Li

11.0 10.5

PCE (%)

10.0

0.68

FF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 41

0.66

9.5 9.0 8.5 8.0 7.5

0.64

7.0 0.62

ZnO

6.5

ZnO:Li


ZnO

ZnO:Li

Page 37 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4 Statistical analysis of the performance parameters of PTB7-Th:PC71BM-based organic solar cells (16 devices for each type) using ZnO (or ZnO:Li) as ETLs: (a) Jsc, (b) Voc, (c) FF and (d) PCE.



(a) 100

(b)

80

EQE (%)

ZnO ZnO:Li

-2


ZnO ZnO:Li

90

70 60 50 40 30 20 10

10

2

10

1

0 300 350 400 450 500 550 600 650 700 750 800 850

Wavelength (nm)

(c)

ZnO ZnO:Li

10

0.5

(d) 0.85

T=300 K

0.80

1

1.5

Voltage (V) ZnO ZnO:Li

T=300 K

(slope 1.04KT/q)

Voc (V)

-2

Jsc (mA cm )

0.75 0.70

1

(slope 1.10KT/q)

0.65 0.60

b

Jsc ~ intensity (ZnO: b=0.97, ZnO:Li: b=0.99) 0.1

10

-2

Light intensity (mW cm )

(e)1

0.55

100

(f)

-2

100

20

ZnO ZnO:Li

ZnO ZnO:Li

15

excitation=470nm detection=790nm

-2

0.1

10

Light intensity (mW cm )

Jph (mA cm )

Normalized intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 41

10

0.01

1E-3 0

1

2

3

Time (ns)

4

5 0.01

5

0.1

Veff (V)

1

Figure 5 (a) EQE measurements of PTB7-Th:PC71BM-based devices using ZnO or ZnO:Li ETLs. (b) Dark J-V curves shown in a logarithmic plot of electron only devices with the structure glass/FTO/ZnO or ZnO:Li/PTB7-Th:PC71BM/Al. Variation of (c) Jsc (logarithmic plot) and (d) Voc (semi-logarithmic plot) with the intensity of 1.5 AM light illumination for the PTB7-Th:PC71BM-based devices shown in Fig. 3a. (e) Transient PL decays of PTB7-Th films deposited on ZnO or ZnO:Li on FTO/glass substrates. (f) Net photocurrent densityeffective voltage curves of PTB7-Th:PC71BM-based OSCs with ZnO and ZnO:Li ETLs.


Page 39 of 41

(a)

(b)

1.0

0.8

0.8

0.7

0.7

Normalized Voc

Normalized Jsc

1.0 0.9

0.9

0.6 0.5 0.4 0.3

0.1 0.0 0

0.6 0.5 0.4 0.3 0.2

0.2

ZnO ZnO:Li

0.1 0.0 0

200 400 600 800 1000 1200 1400 1600 1800 2000

ZnO ZnO:Li 200 400 600 800 1000 1200 1400 1600 1800 2000

Aging time (h)

Aging time (h) 1.0

(d) 1.0

0.9

0.9

0.8

0.8

Normalized PCE

(c) Normalized FF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

0.5 0.4 26%

0.3

0.1 0.0 0

200 400 600 800 1000 1200 1400 1600 1800 2000

64%

0.6

0.2

ZnO ZnO:Li

72%

0.7

15%

ZnO ZnO:Li

200 400 600 800 1000 1200 1400 1600 1800 2000

Aging time (h)

Aging time (h)

Figure 6 Stability measurements in ambient air: Variation of normalized (a) Jsc, (b) Voc, (c) FF and (d) PCE over a period of 2000 hours for PTB7-Th:PC71BM-based devices using ZnO or ZnO:Li ETLs.



4

(c)

ZnO ZnO:Li

0 -4 -8

-12 -16 -20 -0.4

90

0.0

0.2

0.4

Voltage (V)

0.6

0.8

Normalized PCE

70 60 50 40 30

0.7 0.6

60%

0.5 0.4

1

10

0

10

-1

10

-2

10

-3

10

-4

0.25

0.50

0.75

1.00

1.25

1.50

ZnO ZnO:Li 10

0.3 0.2

10

0.1

Wavelength (nm)

10

ZnO ZnO:Li

Voltage (V)

81%

0.8

20

0 300 350 400 450 500 550 600 650 700 750 800 850

2

(f)

0.9

80

10

-0.50 -0.25 0.00

1.0

(e)1.0

ZnO ZnO:Li

3

-2

(d) 100

-0.2

10

-2

-2


8

Current densitydark (mA cm )

(b)

Jph (mA cm )

(a)

EQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 41

0.0 0

ZnO ZnO:Li 100 200 300 400 500 600 700 800 900 1000

Aging time (h)

1

0.01

0.1

V

eff

1

(V)

Figure 7 (a) The chemical structure of IT-4F and the energy levels of ZnO, ZnO:Li andIT4F. (b) Current density-voltage (J-V) characteristics of PTB7-Th:IT-4F-based devices using ZnO, or ZnO:Li ETLs upon 1.5 AM illumination. (c) Dark J-V curves, (d) EQE spectra and (e) net photocurrent density-effective voltage characteristics of the same devices. (f) Variation of normalized PCE over a period of 1000 hours of the above cells.


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TOC


Lithium Doping of ZnO for High Efficiency and ... - ACS Publications

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