Yingzhi
Jin‡
a,
Jie
Xue‡
b,
Juan
Qiao
*b and
Fengling
Zhang
*a
aDepartment of Physics, Chemistry and Biology, Linköping University, Linköping SE-58183, Sweden. E-mail: fengling.zhang@liu.se
bKey Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China. E-mail: qjuan@mail.tsinghua.edu.cn
First published on 30th October 2019
Voltage losses in singlet material-based organic photovoltaic devices (OPVs) have been intensively studied, whereas, only a few investigations on triplet material-based OPVs (T-OPVs) are reported. To investigate the voltage loss in T-OPVs, two homoleptic iridium(III) complexes based on extended π-conjugated benzo[g]phthalazine ligands, Ir(Ftbpa)3 and Ir(FOtbpa)3, are synthesized as sole electron donors. T-OPVs are fabricated by mixing two donors with phenyl-C71-butyric acid methyl ester (PC71BM) as an electron acceptor. Insertion of oxygen-bridges as flexible inert δ-spacers in Ir(FOtbpa)3 has slightly elevated both the lowest unoccupied molecular orbital and the highest occupied molecular orbital levels compared to those of Ir(Ftbpa)3, which results in a lower charge transfer (CT) state energy (ECT) for Ir(FOtbpa)3-based devices. However, a higher Voc (0.88 V) is observed for Ir(FOtbpa)3-based devices than those of Ir(Ftbpa)3 (0.80 V). To understand the above result, the morphologies of the two blend films are studied, which excludes the influence of morphology. Furthermore, radiative and non-radiative recombination in two devices is quantitatively investigated, which suggests that a higher Voc can be attributed to reduced radiative and non-radiative recombination loss for the Ir(FOtbpa)3-based devices.
The open-circuit voltage (Voc) in OPVs is proportional to the energy of the charge transfer (CT) state (ECT) between the donor and acceptor.3 It has been found that the energetic difference between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor is roughly equal to ECT.4–6 Therefore, many reports are focused on increasing the Voc through increasing ECT by minimizing the energetic offset between donors and acceptors.7–9 Increasing ECT will however lead to a small driving force (defined as the energy difference between optical gap of the neat donor or acceptor and ECT) for exciton dissociating to free charges. Generally, fullerene based OPVs tend to show low PCEs with small driving forces (<0.3 eV), whereas, a reasonably high IQE (>85%) was obtained for P3TI:PC71BM blends with a small driving force of 0.1 eV.10 Recently, non-fullerene based OPVs have exhibited efficient exciton dissociation despite a negligible driving force.11–14 Furthermore, the voltage loss between ECT/q to Voc is due to radiative and non-radiative recombination. An empirical relation of , has been found for fullerene based OPVs, of which radiative recombination at donor/acceptor interfaces via the CT state causes ∼0.25 V loss and non-radiative recombination causes ∼0.35 V loss.3,15 Thus, reducing recombination losses is another important strategy to obtain a high Voc.16 It was reported that decreasing the donor/acceptor interfacial area is an effective way to reduce voltage losses.17 Therefore, high Voc can be achieved for organic materials with long exciton diffusion lengths, which will enable a reduced optimum interfacial area. Furthermore, reducing non-radiative recombination losses (<0.3 V) enabled high Voc for materials with high photoluminescence (PL) yields, which have also been reported.18,19
At present, the photo-induced charges mainly originate from singlet exciton dissociation in high performance OPVs. Triplet excitons, which have longer lifetimes or diffusion lengths than singlets, may provide a favorable approach to increase the photocurrent of OPVs due to the forbidden nature of recombination from the triplet state.20,21 In addition, the long diffusion lengths are beneficial to have large domains with decreased interfaces, which will further improve Voc.17 In general, the excitons generated by absorbing photons in organic materials are singlet due to the selection rule in the electronic dipole transition processes.22 The triplet excitons can be obtained by flipping the spin orientation of singlet excitons through the effective intersystem crossing (ISC) or by bimolecular singlet fission.23,24 Enlarging spin–orbit coupling (SOC) by chemically or physically introducing heavy atoms into the conjugated materials has been proposed to enhance ISC rate.25–27 So far, some research studies have been done on triplet material-based OPVs (T-OPVs)28–31 and the highest PCE for small-molecule Ir complexes is 3.81%.32 However, the voltage losses in T-OPVs were rarely investigated.33 In terms of recombination losses, the long exciton diffusion lengths and high emissive properties of triplet materials are beneficial for large Voc.
Here, we therefore investigate the voltage losses in T-OPVs via radiative and non-radiative recombination losses by employing highly sensitive external quantum efficiency and electroluminescence (EL) measurements. Two homoleptic iridium (Ir) complexes, tris(1-(2,4-bis(trifluoromethyl)phenyl)-4-(thiophen-2-yl)benzo[g]phthalazine) Ir(III) ((Ir(Ftbpa)3) and tris(1-(2,5-bis(trifluoromethyl)phenoxy)-4-(thiophen-2-yl)benzo[g]phthalazine) Ir(III) (Ir(FOtbpa)3), are designed as electron donors and phenyl-C71-butyric acid methyl ester (PC71BM) is used as the electron acceptor. OPVs based on Ir(Ftbpa)3 and Ir(FOtbpa)3 donors exhibit PCEs of 3.17% and 3.56%, which are decent performances regarding the studies on T-OPVs to date, and also showed great enhancement compared to poor photovoltaic performance of the 1-chloro-4-(thiophen-2-yl)benzo[g]phthalazine (Ftbpa) (0.001%) and 1-(2,5-bis(trifluoromethyl)phenoxy)-4-(thiophen-2-yl)benzo[g]phthalazine (FOtbpa) (0.007%) ligands as donors. More importantly, a higher Voc is achieved for Ir(FOtbpa)3-based devices despite a lower ECT, which is attributed to the reduced radiative and non-radiative recombination loss.
1-Chloro-4-(thiophen-2-yl)benzo[g]phthalazine, Ftbpa and Ir(Ftbpa)3 was synthesized according to the literature reports.34
(1) |
Current density–voltage (J–V) curves are measured by using a Keithley 2400 Source Meter under an illumination of AM 1.5 simulated by a solar simulator (LSH-7320 LED Solar Simulator, Newport). External quantum efficiency (EQE) spectra were obtained using a QE-R system (Enli Technology Co. Ltd, Taiwan). UV-vis absorption spectra were recorded using a PerkinElmer Lambda 900 spectrometer. Photoluminescence (PL) and EL spectra were recorded using an Andor spectrometer (Shamrock sr-303i-B, coupled to a Newton EMCCD silicon detector cooled to −60 °C). For the EL measurements, a Keithley 2400 Source Meter was utilized for applying an external electric field. EQEEL was measured using a homebuilt system using a calibrated large area Si photodiode 1010B from Oriel, a Keithley 2400 Source Meter to provide voltage and record injected current, and a Keithley 485 Picoammeter to measure the emitted light intensity. Fourier-transform photocurrent spectroscopy (FTPS)-EQE was carried out using a Vertex 70 from Bruker optics, equipped with a QTH lamp, quartz beamsplitter and external detector option. A low noise current amplifier (SR570) was used to amplify the photocurrent produced upon illumination of the devices with light modulated by the FTIR. The output voltage of the current amplifier was fed back into the external detector port of the FTIR, Atomic force microscopy (AFM) was performed using a Dimension 3100 system (Digital Instruments/Veeco) with antimony (n) doped silicon cantilevers (SCM-PIT, Veeco) in tapping mode. The active layer thickness was determined using a Veeco Dektak 6M Stylus profilometer.
The single crystals of Ir(FOtbpa)3 were readily grown from a chloroform/methanol mixture. As show in Fig. 1b, the single-crystal X-ray diffraction measurement verified that Ir(FOtbpa)3 possesses a facial configuration around the Ir center. The average C–O–C angles and the dihedral angles between the bis(trifluoromethyl)phenyl groups and the benzo[g]phthalazine cores are 117° and 86°. Consequently, the bis(trifluoromethyl)phenoxy groups could protect the benzo[g]phthalazine moieties and Ir center at one side.
The energy levels of Ir(Ftbpa)334 and Ir(FOtbpa)3 were estimated by cyclic voltammogram (CV) measurements (Fig. S1, ESI†). The LUMO/HOMO energy levels of Ir(Ftbpa)3, Ir(FOtbpa)3, and PC71BM are calculated to be −3.04/−5.20, −2.97/−5.13, and −3.75/−5.78 eV (Fig. 1c), respectively. It indicates that insertion of an oxygen-bridge has no obvious effect on the electrochemical LUMO–HOMO gap while both LUMO and HOMO levels are elevated slightly.
To give readers an intuitive understanding of the charge generation process in T-OPVs, the energetic states of the Ir(Ftbpa)3:PC71BM blend is presented in Fig. 1d where the singlet and triplet states of Ir(Ftbpa)3 were calculated in a previous report,34 and the energies of the CT states is obtained from the FTPS-EQE measurement. In the charge generation process of the singlet system, the CT states are formed directly from the S1 before being separated into free charges. While in the Ir(Ftbpa)3:PC71BM system, excitons go through a fast ISC from S1 to T1 (blue arrow in Fig. 1d). The energy offset between T1 and 3CT may be beneficial for triplet excitons to form 3CT and then dissociate into free charges (red arrow). However, this is also a possibility even in the triplet system, CT excitons might generate from S1 without going through T1 (green line).
The UV-vis absorption spectra of Ftbpa and FOtbpa ligands showed absorption bands below 450 nm (Fig. S2a, ESI†), which could be ascribed to the π–π* transition. Ir complexes, Ir(Ftbpa)3 and Ir(FOtbpa)3, exhibited significantly enhanced and broadened absorption compared to Ftbpa and FOtbpa ligands shown in Fig. 2a. The bands below 450 nm are attributed to the ligands’ absorption, while the absorption bands at 450–700 nm correspond to the mixed transitions of 1MLCT (metal-to-ligand charge transfer) and 3MLCT. The weak absorption band extending over 700 nm could be the excitation from the ground states to the lowest triplet state (S0 → T1). After blending with PC71BM, the blend films with a weight ratio of 1:1.5 showed similar absorption spectra due to the overlapped absorptions between Ir complexes and PC71BM. Compared with Ir(Ftbpa)3, Ir(FOtbpa)3 displayed similar NIR phosphorescence with an emission peak at 767 nm, but a lower PL quantum yield (ΦPL) of 10.8% and a shorter phosphorescent lifetime (τp) of 489 ns in degassed CH2Cl2 (Table S1 and Fig. S2b, ESI†), which are attributed to its slightly enlarged radiative transition rate constant (kr = 2.2 × 105 s−1) and significantly increased non-radiative transition rate constant (knr = 1.8 × 106 s−1). The significantly increased knr of Ir(FOtbpa)3 could be ascribed to the rotation of pendent bis(trifluoromethyl)phenoxy groups in the solution.
Fig. 2 (a) Absorption spectra of Ir(Ftbpa)3, Ir(FOtbpa)3 and corresponding blend films with PC71BM in a weight ratio of 1:1.5; (b) transient PL decay curves of Ir(Ftbpa)3 and Ir(FOtbpa)3 neat films. |
In neat films, the Ir(Ftbpa)3 complex showed slightly red-shifted emissions with peaks at 784 nm compared to that of Ir(FOtbpa)3 with peaks at 780 nm (Fig. S2c, ESI†), which should correspond to phosphorescence characteristics of the triplet excited states. Accordingly, the energies of T1 were estimated, by the highest energy vibronic band of the phosphorescence spectra, to be 1.58 eV and 1.59 eV for Ir(Ftbpa)3 and Ir(FOtbpa)3, respectively. The complete elimination of the ligand fluorescence emissions indicated the strong SOC and efficient ISC rate from S1 to T1. The ΦPL of Ir(FOtbpa)3 and Ir(Ftbpa)3 reduced to 2.4% and 2.6% (Table S1, ESI†), respectively, which could be ascribed to the ACQ with enlarged knr caused by the interactions of triplet excitons such as triplet–triplet annihilation. Also, the τp of Ir(FOtbpa)3 and Ir(Ftbpa)3 reduced to 49 ns and 19 ns, respectively (Fig. 2b). The knr of Ir(FOtbpa)3 and Ir(Ftbpa)3 were calculated to be 2.0 × 107 s−1 and 5.1× 107 s−1 in neat films, respectively, which are about 11 times and 43 times larger than their knr in degassed CH2Cl2. The values of kr were calculated to be 4.9 × 105 s−1 and 1.4 × 106 s−1 for Ir(FOtbpa)3 and Ir(Ftbpa)3 neat films, respectively. Since the only difference of Ir(FOtbpa)3 and Ir(Ftbpa)3 molecules is the pendent group, the much smaller enhancement of knr for Ir(FOtbpa)3 is ascribed to the usage of the bis(trifluoromethyl)phenoxy groups as δ-spacers, which hamper the interactions of triplet excitons in aggregated state and alleviate the reductions of ΦPL and τp. Thus, Ir(FOtbpa)3 displays longer τp in the pristine film, which is beneficial for the exciton diffusion.
To study the voltage losses in T-OPVs, the Ir complexes were evaluated using PC71BM as the electron acceptor with weight ratios of 2:1, 1:1.5 and 1:3. Photovoltaic parameters of the T-OPVs based on Ir(Ftbpa)3 and Ir(FOtbpa)3 are summarized in Table 1. For Ir(Ftbpa)3:PC71BM devices, a PCE of 3.17% with a short-circuit current density (Jsc) of 8.70 mA cm−2, Voc of 0.80 V, and fill factor (FF) of 0.46 is obtained at a weight ratio of 1:1.5. For Ir(FOtbpa)3:PC71BM devices, the best PCE increases to 3.56% with a Voc of 0.88 V, Jsc of 8.58 mA cm−2, and FF of 0.47 at the same weight ratio (1:1.5). On the other hand, the Ftbpa and FOtbpa ligands showed very poor performance with low PCEs of 0.001% and 0.007% in similar device structures (Table S2, ESI†), which confirms the significant contribution of Ir to the performance of corresponding T-OPVs. The typical J–V and EQE curves for Ir complex-based devices with a weight ratio of 1:1.5 are shown in Fig. 3a and b. The EQE curves of these Ir complex-based devices showed a spectral response from both donor and acceptor absorption regions (300 to 700 nm). The integrated Jsc values from the EQE curves are 8.26 and 8.11 mA cm−2 for Ir(Ftbpa)3:PC71BM and Ir(FOtbpa)3:PC71BM devices, respectively, which are consistent with the values from J–V measurement. The J–V characteristics of the hole-only and electron-only devices are shown in Fig. S3a and b (ESI†). The hole and electron mobilities are 6.6 × 10−7 and 1.76 × 10−4 cm2 V−1 s−1 for Ir(Ftbpa)3 blends (ratio 1:1.5) and 1.5 × 10−6 and 1.5 × 10−4 cm2 V−1 s−1 for Ir(FOtbpa)3 blends (ratio 1:1.5), as found through the SCLC measurements. The lower hole mobilities than the singlet materials resulted in unbalanced mobilities and the smaller FFs here.
Donor | Ratio | V oc (V) | J sc (mA cm−2) | FF | PCE (%) |
---|---|---|---|---|---|
Ir(Ftbpa)3 | 2:1 | 0.85 (0.85 ± 0.01) | 6.43 (6.47 ± 0.1) | 0.39 (0.38 ± 0.01) | 2.13 (2.07 ± 0.19) |
1:1.5 | 0.80 (0.80 ± 0.01) | 8.70 (8.72 ± 0.19) | 0.46 (0.43 ± 0.02) | 3.17 (3.01 ± 0.19) | |
1:3 | 0.78 (0.78 ± 0.01) | 8.62 (8.58 ± 0.07) | 0.42 (0.41 ± 0.01) | 2.97 (2.71 ± 0.05) | |
Ir(FOtbpa)3 | 2:1 | 0.93 (0.93 ± 0.01) | 5.07 (4.67 ± 0.23) | 0.32 (0.31 ± 0.01) | 1.51 (1.34 ± 0.09) |
1:1.5 | 0.88 (0.88 ± 0.01) | 8.58 (8.41 ± 0.51) | 0.47 (0.45 ± 0.02) | 3.56 (3.30 ± 0.26) | |
1:3 | 0.85 (0.85 ± 0.02) | 8.11 (8.14 ± 0.44) | 0.46 (0.41 ± 0.03) | 3.15 (2.80 ± 0.23) |
Comparing the devices based on these two Ir complexes with different weight ratios, we find that the Voc increases with increasing content of the Ir complexes. Similar phenomena have been reported and attributed to the changes in the interfacial area of the donor/acceptor.17,37 Atomic force microscopy (AFM) was used to investigate the morphologies of the blend films with different weight ratios. As shown in the images (Fig. S4, ESI†), there seem to be minor morphological differences between the different blend ratios for both Ir(Ftbpa)3:PC71BM and Ir(FOtbpa)3:PC71BM blend films. While AFM only examines the surface morphology, the phase separation of the whole active layer could be investigated by PL measurement. Steady state PL spectra of the pristine Ir(Ftbpa)3 and Ir(FOtbpa)3 films are compared with their corresponding blends with different weight ratios (Fig. S5, ESI†). The PL intensities from Ir(Ftbpa)3 and Ir(FOtbpa)3 triplet excitons are strongly quenched by PC71BM in all blends, indicating efficient excitons dissociation and charge transfer between the two Ir complex donors and PC71BM acceptor with highly mixed donors and acceptors. The CT state PL from 2:1, 1:1.5, and 1:3 Ir(Ftbpa)3:PC71BM blend films are presented in Fig. 4a. The interfacial CT state emission is observed at ∼950 nm, which is clearly red-shifted compared to Ir(Ftbpa)3 exciton emission at 784 nm. Furthermore, it shows a clear trend of suppressed CT PL from the films with a higher PC71BM content. Similar results have also been found in Ir(FOtbpa)3:PC71BM blends (Fig. 4b). Since the CT PL intensities are generally very low, EL measurement is a much more sensitive method to determine the ECT. Therefore, the EL emission from devices based on pristine Ir complexes and their blends are also recorded. As shown in Fig. 4c and d, these electrically generated CT state EL emissions are consistent with the CT state PL emissions generated by photoexcitation. The Ir(Ftbpa)3:PC71BM blend films showed red-shift EL emissions at around 950 nm compared to 780 nm for the pristine Ir(Ftbpa)3 devices (Fig. 4c). Similar red-shift EL emissions are observed in the Ir(FOtbpa)3:PC71BM blends (Fig. 4d) at around 973 nm. These indicate that the triplet energy of Ir(Ftbpa)3 and Ir(FOtbpa)3 are much higher than the ECT in the blends, which confirms the effective utilization of triplet excitons in the charge generation process.
More specifically, the ECT can be determined through fitting the FTPS-EQE spectra according to the model developed by Vandewal based on Marcus theory.
(2) |
Fig. 5 FTPS-EQE spectra of (a) Ir(Ftbpa)3:PC71BM and (b) Ir(FOtbpa)3:PC71BM. The dash curves are fits of the FTPS-EQE spectra using eqn (2); (c) EQEEL of the Ir(Ftbpa)3:PC71BM and (d) Ir(FOtbpa)3:PC71BM. |
As shown in Table 1, the Voc of the OPVs based on Ir(Ftbpa)3 are in the range of 0.85–0.78 V and the Voc of the OPVs based on Ir(FOtbpa)3 are in the range of 0.93–0.85 V. The contradiction between ECT and Voc for different blend ratios motivates us to further understand the voltage losses. Considering the detailed balance theory, the Voc of OPVs is then determined by eqn (3), where radiative (qΔVrad) and non-radiative (qΔVnon-rad) recombination losses can be experimentally determined by the fitting parameters and measured EQEEL.
(3) |
The qΔVrad and qΔVnon-rad for blends with different ratios were calculated (Table 2). The qΔVrad for both Ir(Ftbpa)3 and Ir(FOtbpa)3-based devices is independent with blend ratios. From the EQEEL measurements (Fig. 5c, d and Table 2), the EQEEL of the Ir(Ftbpa)3 and Ir(FOtbpa)3-based devices decreased with increasing content of PC71BM. These lead to low qΔVnon-rad for both Ir(Ftbpa)3 and Ir(FOtbpa)3-based devices resulting in a higher Voc with a low PC71BM content.
Donor | Ratio | qV oc (eV) | f 1 (eV2) | E CT (eV) | λ (eV) | qΔVrad (eV) | EQEEL (%) | qΔVnon-rad (eV) |
---|---|---|---|---|---|---|---|---|
Ir(Ftbpa)3 | 2:1 | 0.85 | 6 × 10−3 | 1.46 | 0.27 | 0.25 | 1 × 10−4 | 0.36 |
1:1.5 | 0.80 | 6 × 10−3 | 1.47 | 0.25 | 0.25 | 1 × 10−5 | 0.42 | |
1:3 | 0.78 | 9 × 10−3 | 1.48 | 0.27 | 0.26 | 5 × 10−6 | 0.44 | |
Ir(FOtbpa)3 | 2:1 | 0.93 | 9 × 10−4 | 1.41 | 0.19 | 0.21 | 2 × 10−3 | 0.27 |
1:1.5 | 0.88 | 6 × 10−4 | 1.38 | 0.12 | 0.19 | 7 × 10−4 | 0.31 | |
1:3 | 0.85 | 1 × 10−3 | 1.38 | 0.18 | 0.20 | 3 × 10−4 | 0.33 |
For the best device performances based on Ir(Ftbpa)3 and Ir(FOtbpa)3 blends (1:1.5), as shown in Table 1, the difference in the PCEs is mainly due to the difference in Vocs. When we compare the energy levels of these two donors, the HOMO level of Ir(Ftbpa)3 is lower than that of Ir(FOtbpa)3 (Fig. 1b), which indicates that the Ir(Ftbpa)3 blend may have a higher Voc. However, the Voc of Ir(Ftbpa)3-based devices is 0.08 V lower than that of the Ir(FOtbpa)3-based devices. The Ir(Ftbpa)3-based devices have a higher ECT of 1.47 eV compared with the value of 1.38 eV for the Ir(FOtbpa)3-based devices, which is consistent with the HOMO level difference. The qΔVrad for Ir(Ftbpa)3-based devices is 0.25 eV, which is higher than the value of 0.19 eV for the Ir(FOtbpa)3-based devices. The EQEEL of the device based on Ir(FOtbpa)3 is more than one order of magnitude higher than that of the Ir(Ftbpa)3. This leads to a calculated qΔVnon-rad of 0.31 eV for the Ir(FOtbpa)3-based devices, about 0.11 eV lower than that of the Ir(Ftbpa)3-based devices. Both radiative and non-radiative recombinations for the Ir(FOtbpa)3-based devices are lower than those of the Ir(Ftbpa)3-based devices, which results in a higher Voc for the Ir(FOtbpa)3-based devices. The calculated data fit well with Voc in these two blends.
Contradictory to the energy gap law (the non-radiative decay rate is exponentially increasing with decreasing energy difference between the excited and ground states), the Ir(FOtbpa)3-based device has a lower ECT, but a higher EQEEL. Considering the photophysical properties of the two Ir complexes, the larger kr (1.4 × 106 s−1) of Ir(Ftbpa)3 than that of Ir(FOtbpa)3 (kr = 4.9 × 105 s−1) in solid state may correlate with the larger radiative recombination loss in Ir(Ftbpa)3-based devices. The longer exciton lifetime (τ = 49 ns) and much smaller knr (2.0 × 107 s−1) compared with those of Ir(Ftbpa)3 (τ = 19 ns and knr = 5.1 × 107 s−1) in pristine films due to the flexible inert δ-spacer may decrease the non-radiative recombination loss in Ir(FOtbpa)3-based devices. In addition to the above reasons, some other charge carrier loss mechanisms may coexist in the Ir(Ftbpa)3-based devices.
The recombination mechanism was further studied by measuring the light intensity dependencies of Jsc and Voc (Fig. S6, ESI†). The Ir(Ftbpa)3 and Ir(FOtbpa)3-based devices (1:1.5) show figure-of-merit (α) values of 0.93 and 0.92, respectively, indicating that bimolecular recombination occurs in both systems at short circuit conditions. At open circuit conditions, a slope of 2 kBT/q for monomolecular (trap-assisted) recombination and a slope of 1 kBT/q for bimolecular recombination exist. In some cases, surface recombination would make the slope less than 1 kBT/q. The Ir(FOtbpa)3-based devices (1:1.5) show a slope of 1.03 kBT/q, while the Ir(Ftbpa)3-based devices (1:1.5) show a slope less than 1 kBT/q (0.95 kBT/q). Thus, the Ir(Ftbpa)3-based devices (1:1.5) is more dominated by surface recombination than the Ir(FOtbpa)3-based devices (1:1.5), which is consistent with the non-radiative recombination losses from EQEEL calculations.
Footnotes |
† Electronic supplementary information (ESI) available. CCDC 1916919. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9tc04914b |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2019 |