Article pubs.acs.org/JPCC
Energy Gap between Photoluminescence and Electroluminescence as Recombination Indicator in Organic Small-Molecule Photodiodes Gae Hwang Lee, Moon Gyu Han,* Dong-Seok Leem, Seon-Jeong Lim, Sungyoung Yun, Kwang-Hee Lee, Xavier Bulliard, Kyung-Bae Park, Tadao Yagi, Yeong Suk Choi, Yong Wan Jin,* and Sangyoon Lee Organic Materials Laboratory, Samsung Advanced Institute of Technology, Samsung Electronics, Co. Ltd., 130 Samsung-ro, Suwon-si, Gyeonggi-do 443-803, Republic of Korea S Supporting Information *
ABSTRACT: Charge transfer (CT) states at the donor−acceptor interfaces play an important role in organic optoelectronic devices, yielding photocarrier generation or recombination losses. In this study, we fabricate and characterize vacuum-deposited organic photodiodes (OPDs) composed of SubPc:C60 with various active-layer thicknesses and mixing ratios in terms of the charge separation efficiency (CSE) and charge collection efficiency (CCE). We demonstrate that the combined field-assisted quenching study using both photoluminescence (PL) and electroluminescence (EL) reveals detailed information on the device physics of bulk heterojunction photodiodes as related to the CT state. Our modified PL quenching efficiency approach allows us to reasonably evaluate the CSE. In addition, we find that the EL energy is closely related to the recombination loss factor, and the energy gap between PL and EL exhibits a strong linear relationship with the CCE. As a result, the energy gap is proved to be a meaningful indicator of the carrier transport properties, which allows prediction of the CCE in organic bulk heterojunctions.
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INTRODUCTION In recent years, researchers have devoted considerable attention to understanding the fundamental physics of charge carrier generation and transport as well as developing new photoactive materials, advanced device architectures, and optical enhancement structures, etc., in order to increase the efficiency of the organic photovoltaics (OPVs) and photodiodes (OPDs).1−4 In particular, an understanding of the physics of the charge transfer (CT) state at the donor−acceptor interface as an intermediate state from an exciton to free carriers is crucial for ensuring efficient charge separation and high photon-toelectron conversion efficiency.5−7 The CT state has been mainly studied by means of photoluminescence (PL)7−17 and electroluminescence (EL)13−18 observations through static and transient analyses. In particular, field-dependent PL and EL, which can provide insights into charge separation and recombination in optoelectronic devices, have been intensively investigated; however, the results of these studies are not always consistent among the multitude of donor−acceptor systems examined. The photoluminescence quenching efficiency (PLQE) does not correspond with the generated photocurrent value in many cases,15−17 and the PL and EL energies (luminescence maximum wavelength) are often found to be different.15−17 This discrepancy has led to the speculation that PL may not be generated from substantial CT states that generate free carriers. It also remains unclear whether PL or EL is generated in CT states directly related to the photocurrent. In this context, Inganäs et al.15 have recently carried out extensive fielddependent PL and EL analyses on pure polymers and © XXXX American Chemical Society
polymer:fullerene blends. They identified a red-shift of the EL in the blends only and found that the photocurrent did not correlate with the PLQE. They attributed these phenomena to the nature of PL; i.e., PL primarily probes disordered CT states, which are not dominant in the photocurrent generation process. However, previous results did not take into consideration the charge transport issue but only focused on charge separation. Since charge transport characteristics are involved in EL,18,19 a more detailed analysis considering charge transport is required to understand both the difference in PL and EL energies and the mismatch of the PLQE with the photocurrent. In this paper, we report on a novel methodology to estimate the charge collection efficiency (CCE) of OPDs through a newly introduced quenching efficiency methodology via fieldassisted PL analysis. The chosen system prepared by vacuum deposition is robust due to the high purity of materials used and the reproducible morphology. It is hence proved to be well adapted to fundamental studies, and the approach reveals hitherto undiscovered photophysical facts regarding organic bulk heterojunctions (BHJs).
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EXPERIMENTAL METHODS BHJ OPDs composed of organic small-molecule donors and C60 acceptor were investigated in this study. Boron subphthalocyanine chloride (SubPc)20 (Lumtec, >99% purity) Received: February 24, 2016 Revised: April 25, 2016
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Figure 1. (a) Simplified device structure and molecular structures of SubPc and C60. (b) External quantum efficiency spectra of bulk heterojunction (BHJ) organic photodiode (1:1 volume ratio of SubPc:C60, 50 nm thickness) observed at various reverse voltages. (c) Photoluminescence (PL) spectra of SubPc single layer and SubPc:C60 BHJ layer. (d) Transient PL spectra of the SubPc:C60 BHJ recorded at different applied voltages. (e) Electroluminescence (EL) spectra of the BHJ observed at different forward voltages. (f) EL peak wavelength position as a function of applied forward voltage.
measurements. Next, the absorptance (A) of the pure active layers was obtained as A = 1 − T − R − A′. The PL study was performed with a time-correlated single photon counting (TCSPC) setup (FluoTime 300, PicoQuant GmbH) by photoexcitation with a 510 nm picosecond laser (LDH-P-C510B, PicoQuant GmbH) operated at 40 MHz. Fielddependent PL spectra were measured on the OPD devices with the application of a reverse bias of 0−10 V with the use of a power supply. The EL spectra were measured from the devices at the least forward bias by using the same experimental setup as that for PL without photoexcitation. In order to characterize single layer properties of SubPc and C60, each layer was separately deposited by vacuum thermal evaporation. The highest occupied molecular orbital (HOMO) levels of deposited films were measured using an AC-2 photoelectron spectrophotometer (Hitachi High Tech). The lowest unoccupied molecular orbital (LUMO) levels were determined from the optical band gap calculated from the edge of the absorption spectra.
and C60 (Frontier Carbon Corp., >99.5% purity) were used as the standard donor and acceptor, respectively, without further purification. A 20 nm thick molybdenum oxide (MoOx) layer was deposited on an indium tin oxide (ITO)-coated glass substrate with a sheet resistance of 15 Ω per square. Next, an Al cathode with a thickness of 60 nm was evaporated through a shadow mask with an active area of 0.04 cm2 after deposition of the active materials by vacuum thermal evaporation under a pressure of 10−7 Torr. The external quantum efficiency (EQE) was measured with a spectral incident photon-to-electron conversion efficiency measurement system under monochromatic light generated by an optical filter from an ozone-free Xe lamp with a chopper frequency of 30 Hz. The measured incident light power was 0.2 mW/cm2 at a wavelength of 540 nm. The transmittance (T) and reflectance (R) of the OPD devices were measured by means of a UV−vis spectrophotometer (Shimadzu UV-240). The absorptance (A′) from the ITO and MoOx layers in the devices was obtained from refractive index (n) and extinction coefficient (k) values acquired via variable angle spectroscopic ellipsometry (VASE) B
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Figure 2. Photoluminescence (PL) (solid curve) and electroluminescence (EL) (open dotted curve) of the charge transfer state in devices with SubPc:C60 layer thickness of (a) 50, (b) 100, and (c) 200 nm. The PL spectra measured at an applied voltage increasing from Voc are depicted. The energy difference (ΔEPL−EL) between PL and EL is also indicated with arrows. The measured internal quantum efficiency (IQE), conventional photoluminescence quenching efficiency (PLQE), modified PLQE, estimated charge collection efficiency in devices with SubPc:C60 layer thickness of (d) 50, (e) 100, and (f) 200 nm. For the modified PLQE, α values of (a) 1.56, (b) 1.55, and (c) 1.44 were used. The SubPc:C60 layer of all devices has a 1:1 volume ratio. The critical electric field at which the collection efficiency reaches 99% is indicated by the dashed line.
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RESULTS AND DISCUSSION In order to investigate the CT states of OPDs, we chose a bulk heterojunction composed of SubPc as the p-type molecule and C60 as the n-type molecule as a model system. SubPc is a promising organic semiconductor material due to its high extinction coefficient in the visible and deep HOMO energy levels, permitting its use as an acceptor material while allowing the modification of the molecular structure.21,22 When it is combined with C60, the resulting BHJ has been known to exhibit good Voc and power conversion efficiency.20 Although the photoinduced CT state and the dynamics of SubPc:C60 BHJ have been recently reported,23 a detailed analysis of CT states of this system is still required to further understand the system. The molecular structures of SubPc and C60 together with the device structure are shown in Figure 1a. The EQE spectra presented in Figure 1b exhibit a selective response in the “green wavelength regime” with the maximum response at 590 nm, as reported earlier,24−26 and the EQE increases with increase in applied voltage, which indicates that the charge
separation efficiency (CSE) and CCE increase with the applied potential. The PL peak maximum obtained from the single donor layer (SubPc) is located at 620 nm (Figure 1c), which corresponds to a Stokes shift of approximately 30 nm. This PL originating from the exciton of the SubPc is totally quenched, but a new PL peak appears in the case of the SubPc:C60 blend, which is assumed to originate from the CT state of the blend. HOMO and LUMO energy levels of deposited thin films and the schematic diagram for forming of the charge transfer (CT) and emissive recombination procedure are available in Figure S1 of the Supporting Information. The PL lifetime, shown in Figure 1d, decreases with increase in applied voltage. This indicates that the charge separation from the CT states is enhanced with increasing voltage.14 This decrease in the PL lifetime observed in transient PL analysis exhibits a tendency similar to the PL intensity decrease observed in static PL.12 Figure 1e shows the EL spectrum as a function of the applied forward voltage, and Figure 1f depicts the peak wavelength position as a function of the voltage. The peak of the EL C
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Figure 3. Photoluminescence (PL) (solid line) and electroluminescence (EL) (open dotted line) of the charge transfer state in devices with SubPc:C60 volume ratios of (a) 1:5, (b) 2:1, and (c) 5:1. The PL spectra measured at an applied voltage increasing from Voc are shown. The energy difference (ΔEPL−EL) between PL and EL is also indicated by arrows. The measured internal quantum efficiency (IQE), conventional photoluminescence quenching efficiency (PLQE), modified PLQE, and estimated charge collection efficiency (CCE) in devices with SubPc:C60 volume ratios of (d) 1:5, (e) 2:1, and (f) 5:1. The SubPc:C60 layer of all devices has a thickness of 100 nm. For modified PLQE, α values of (a) 2.13, (b) 1.71, and (c) 1.56 were used. The critical electric field at which the collection efficiency reaches 99% is indicated by the dashed line.
spectrum due to radiative recombination after the formation of the CT state by injected charges from both electrodes remains in its position in the low-voltage regime. The peak is subsequently blue-shifted with further increase in the voltage mainly due to the recombination of the excitons of SubPc.18 In the context of preventing this overcharge injection that leads to increase in the EL energy, we examined the EL spectrum in the low-voltage regime for the following experiments. Based on the basic absorption, luminance, and quantum efficiency data, the behavior of the CT state was examined via field-assisted PL and EQE analyses of the devices with various active layer thicknesses and SubPc:C60 volume ratios. Figure 2a−c depicts the PL and EL spectra of the devices for different active-layer thicknesses. The PL spectra with the SubPc:C60 ratio of 1:1 for different layer thicknesses (50, 100, and 200 nm) exhibit the same peak position, which indicates that PL is mainly generated from the CT state at the donor− acceptor interface regardless of the layer thickness. In contrast,
EL is red-shifted with regards to PL, and the peak position difference between PL and EL exhibits a tendency to increase (from 0.02 to 0.061 eV) as the active-layer thickness increases. This indicates that the EL energy is determined not only by the CT states but also by other factors related to the collection efficiency. The internal quantum efficiency (IQE) and CSE were measured from the relationship ηIQE = ηCSE × ηCCE in order to quantify the collection efficiency. For obtaining the CCE, we assume that the exciton diffusion efficiency is close to 100% because the grain boundary dimension is considerably smaller than the exciton diffusion length in the case of well-mixed BHJs.26−28 The intensity variation in PL, which is indicative of charge separation of the CT exciton, has been utilized in many reports10−16 in order to estimate CSE. The field-dependent PLQE has been obtained using the relationship ηPLQE(V) = [IPL(Voc) − IPL(V)]/IPL(Voc)11,13,16 where IPL represents the steady-state PL intensity at an applied voltage (V). However, D
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Figure 4. (a) Collection efficiency at 0 V for active-layer thickness of 100 nm and (b) critical electric field at collection efficiency saturation for the devices shown in Figures 1 and 2 as a function of the energy difference (ΔEPL−EL). The solid lines indicate linear fits to the data.
this method is valid only when the CSE is close to 0% at Voc because the PLQE in the formula has to zero out at Voc. This limitation is one of the reasons why the PLQE of efficient photovoltaic devices does not correspond with photocurrent. To solve this issue, we suggest a modified PLQE (η′PLQE) that can effectively reflect the PL quenching and the corresponding charge separation even at Voc, as described below. ′ (α , V ) = ηPLQE
observed in both p-rich and n-rich blends. Interestingly, the PL and EL energies are identical in the n-rich blend (Figure 3a), and these are located at the shortest wavelength maximum (756 nm), which reflects CT states with the highest energy. Distinctively, EL shows a larger red-shift than PL, resulting in increased ΔEPL−EL with increase in the content of p-type molecules. Upon increasing the p-type molecule content, the CCE exhibits a tendency to decrease at low electric fields, thus requiring a larger electric field to reach saturation (Figure 3d− f). This result obviously indicates that the energy difference between PL and EL is closely associated with the CCE and in turn with the transport characteristics of the active layer, irrespective of the CT-state energy. It is also noticeable that the modified PLQE and IQE curves are almost perfectly coincidental for the SubPc:C60 1:5 blend (Figure 3d) due to high CCE even at low electric fields. Figure 4 provides an overview of the CCE and ΔEPL−EL for all devices investigated. The lower are the energy differences between PL and EL, the higher is the CCE (Figure 4a) and lower is the critical electric field (Figure 4b). In particular, the energy difference between PL and EL exhibits a close linear relationship with the critical electric field derived from the previous data corresponding to Figures 2 and 3. Parameters ΔEPL−EL and CCE at an applied bias of 0 V (short circuit voltage) exhibit a linear relationship when the blend layer is the same (100 nm), while the CCE at 0 V shows deviation from the linear relationship due to differences in the electric field when the thickness is varied. Therefore, this result provides direct estimation of the electric field range required to reach nearly 100% of charge collection after overcoming the Coulomb barrier of the exciton binding energy at the donor−acceptor interface. As a result, this linear relationship can provide researchers with a plausible rough approximation of the CCE via simply measuring ΔEPL−EL. The origin of the energy difference between PL and EL can be interpreted from the perspective of energy variation exhibited by the charge carriers during their transport through the organic active layer. Figure 5 depicts three possible energy distributions for charge extraction and injection: electron−hole pair in the CT state for PL and EL, charge carriers immediately after charge separation, and charge carriers after transport to the nearest electrode. In the charge extraction mechanism, excitons are produced by light absorption followed by the formation of CT states. The dissociated charges from the CT state at the donor−acceptor interface travel to each electrode, and their energy decreases during the transport via hopping
αIPL(Voc) − IPL(V ) αIPL(Voc)
Here, α represents a modification factor that can allow validity of the above expression even when the CSE is not close to 0% at Voc. Although no method is currently available to obtain α directly, we can deduce a reasonable value for α that satisfies the condition for which PLQE and IQE exactly match if we suppose that CCE is saturated at high bias limit. For example, the modified PLQE and IQE values perfectly match at electric fields over 0.28 × 106 V/cm when the α value is 1.55 (Figure 2e). In this manner, we could successfully deduce α values by repeating the same procedure for all the investigated devices. Figure 2d−f shows the IQE, conventional PLQE, modified PLQE, and CCEs of SubPc:C60 blends for different blend-layer thicknesses as a function of the electric field. IQE was obtained with the expression ηEQE = A × ηIQE, where the absorptance (A) of the active layer and EQE in the device were acquired at 510 nm (see the Experimental Methods section). The fielddependent EQE data were acquired at various electric fields, as shown in Figure 1b. It is noteworthy that IQE did not exhibit any wavelength dependency. With increase in the blend layer thickness, the CCE of the devices reduces, and larger electric fields are required for the CCE to reach saturation value. The low CCE is the consequence of additional loss factors in devices with thicker active layers. In addition, the critical electric field at which the collection efficiency reaches 99%, indicated by the dotted line in the figure, increases with increase in the thickness of the active layers. In summary, the red-shift energy (ΔEPL−EL) observed in Figure 2a−c shows a relationship with the CCE observed in Figure 2d−f. Furthermore, we traced the CCE and EL energy when changing the blend ratio from 5:1 to 1:5 with a fixed activelayer thickness of 100 nm. As shown in Figure 3a−c, the PL spectrum shifts with the blend ratios as a consequence of different CT states induced by the modification of the interface between p-rich and n-rich domains.14 Both the PL and EL maximum peaks in the 1:1 blend locate at the longest wavelength (780 nm, Figure 2b), and a peak blue-shift is E
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energy of the injected charge carriers can also decrease in the process of transport. This means that EL energy can decrease depending on the transport characteristics of the device. As shown in Figure 5a, in the case of the CCE being close to 100%, the PL and EL spectra should overlap (as observed in Figure 3a) because of the absence of energy loss during charge transport from the respective electrodes. In contrast, the CCE reduces and the energy of the CT states for EL decreases when the energy decrease in the charge carriers during transport is significant, as shown in Figure 5b. In this case, the energy of the CT states for EL becomes lower than that of PL, which concomitantly leads to the EL spectrum maximum being located at longer wavelengths. Such a trend can be observed in Figures 2 and 3 for increasing layer thickness or p-type content, for which an increase in the red-shift of EL spectrum is observed. This phenomenon is the reason for the observed relationship between CCE and ΔEPL−EL. The PL energy would change in the BHJ with different p:n ratios as was depicted in Figure S1. In such a case, EL energy will also follow the tendency for PL, while ΔEPL−EL is still dependent on the transport property of the system. It should be again noted that PL and EL energy observed in all the devices are dependent on the nature of BHJs. In particular, EL energy is the reflection of the loss of charge carrier energy during transport. The energy decrease in charge carriers during transport is observed in PL as well. Because charge carriers dissociated from the CT states can recombine in the CT states again during transport to the electrode,10 the PL spectrum contains not only the energy of the CT states formed by illumination but also the contribution from the CT states with lower energy. When a low field is applied to the device, PL contains a larger amount of the lower-energy component because the nongeminate recombination after dissociation is relatively higher.31 This low-energy
Figure 5. Schematics describing the generation of photoluminescence (PL) and electroluminescence (EL) in devices without free-carrier loss (case 1) and with free-carrier loss (case 2). The occupied density of the charge transfer state (left panel), electron state after charge dissociation (center panel), and electron state near Al electrode after transport (right panel) in cases of charge extraction process (red) and charge injection process (blue) are shown. The energy difference (ΔEPL−EL) between PL and EL appears when the energy of the charge transfer states for EL decreases due to energy loss in charge carriers.
before reaching quasi-equilibrium in the device.29,30 The injection process is the reverse of the extraction process. The
Figure 6. Normalized photoluminescence (PL) spectrum with respect to the applied voltage in devices with SubPc:C60 volume ratios of (a) 5:1 (100 nm thickness), (b) 2:1 (100 nm), (c) 1:1 (200 nm), and (d) 1:5 (100 nm). F
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Figure 7. Electroluminescence (EL) quantum efficiency with respect to the applied voltage in devices with SubPc:C60 volume ratios of (a) 2:1 and (b) 1:1 (100 nm thickness).
component in PL is expected to decrease with increase in the applied field. The field dependency of the PL spectrum is evidenced upon normalizing the PL spectrum, as clearly shown in Figure 6d. It should be noted that this behavior is different from blue-shifted PL at high electric fields due to the PL arising from excitons, reported in previous works,13,16 which is observed in p-rich BHJs (Figure 6a) due to the recombination of the excitons of SubPc. The charge carriers in organic materials undergo both trapassisted32 and bimolecular recombination.33 Trap-assisted recombination has been observed for a ZnPc:C60 system containing a trap concentration of ∼1016 cm−3,34,35 which system is similar to the small-molecule:C60 system examined in this study. In order to understand the nature of recombination, we measured the EL quantum efficiency variation as a function of the applied voltage although the quantum efficiency of EL is extremely low. From the forward-voltage dependency of the EL quantum efficiency presented in Figure 7, it can be definitely concluded that the nonradiative trap-assisted recombination coexists with radiative bimolecular recombination because the EL efficiency increases with voltage.19 The linearity observed in Figure 3 can be attributed to the field-dependent hopping characteristics of the charge transport process. A charge carrier can increase its local energy E by an increment ΔE = eFr through hopping against the electric field F over a distance r according to the model of hopping charges at elevated temperatures and under an electric field.36,37 We suppose that the increase in carrier energy under a critical electric field can lead to increase in the local energy, which is sufficient to compensate for the energy difference in PL and EL. The slope of the plot in Figure 4 is 0.678 nm−1, corresponding to a hopping distance r of 1.47 nm, which is similar to the values previously reported for polymer systems.38−40 Interestingly, the slope remains valid even when different p-type molecules are used in the bulk heterojunction (see Figure S2).
changes are presumed to be related with the transport properties of the devices. In the OPDs exhibiting a small gap between EL and PL energies, the IQE and modified PLQE against an electric field exhibit a close agreement for even low electric fields. We determined that ΔEPL−EL as a function of the critical electric field shows a strong linear relationship. Furthermore, ΔEPL−EL also exhibited an inverse relationship with the CCE at applied bias of 0 V, but it exhibited a deviation from linearity due to variation in the electric field when the thickness of the active layer was varied. Therefore, ΔEPL−EL can be considered as a good indicator of the device characteristics, i.e., the transport properties, since EL energy reflects the charge loss factors of the devices. This work aids in clarifying the CT state in bulk heterojunctions and also provides a plausible explanation regarding the discrepancy between EL and PL observed in earlier studies. In addition, we could deduce the hopping distance of carriers that affects the transport property from the slope of the relation between the critical electric field and ΔEPL−EL. This linearity was observed to be valid not only in BHJs composed of SubPc:C60 but also in BHJs with other ptype small molecules and C60. Consequently, we believe this finding can provide researchers with a quick estimation of the CCE via simply measuring ΔEPL−EL and enable researchers on organic electronics to further tune optoelectronic devices.
CONCLUSIONS We carried out an in-depth study of the CT state in organic photodiodes based on small-molecule:C60 as active layers through field-dependent PL and EL analyses. With our modified PLQE value, we evaluated the CSE even when this value is not close to 0% at Voc. In addition, we deduced the critical electric field in the devices at which the modified PLQE starts to match the IQE. This methodology to estimate the CSE is a surprising finding because the presence or absence of charge separation at Voc is still a matter of debate as regards OPVs. The EL energy decreased with increase in the thickness of the active layers or increase in p-molecule content, which
Corresponding Authors
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01876. Figure S1: schematics for the energy level and charge transfer state; Figure S2: critical electric field plot including different p-type materials (PDF)
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AUTHOR INFORMATION
*E-mail
[email protected]; Tel +82-31-8061-1395 (M.G.H.). *E-mail
[email protected]; Tel +82-31-8061-1400 (Y.W.J.). Notes
The authors declare no competing financial interest.
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