Exploring Deep and Shallow Trap States in a Non-Fullerene

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C: Energy Conversion and Storage; Energy and Charge Transport

Exploring Deep and Shallow Trap States in a Non-Fullerene Acceptor ITIC Based Organic Bulk Heterojunction Photovoltaic System Xixiang Zhu, Kai Wang, Jiaqi He, Lu Zhang, Haomiao Yu, Dawei He, and Bin Hu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b03515 • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 5, 2019

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The Journal of Physical Chemistry

Exploring Deep and Shallow Trap States in a Non-Fullerene Acceptor

ITIC

based

Organic

Bulk

Heterojunction

Photovoltaic System

Xixiang Zhu,1,‡ Kai Wang,1,‡,* Jiaqi He,1 Lu Zhang,1 Haomiao Yu,1 Dawei He,1 Bin Hu1,2*

1. Key Laboratory of Luminescence and Optical Information, Ministry of Education, School of Science, Beijing Jiaotong University, Beijing 100044, China 2. Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA

* corresponding authors: [email protected] (Kai Wang); [email protected] (Bin Hu)

‡ These

two authors contribute equally to this work.

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

Abstract Non-fullerene organic molecules that usually act as acceptors have exhibited prominent optoelectronic properties in organic bulk heterojunction (BHJ) solar cells. Although the highly efficient exciton dissociation can happen even with a relatively smaller driving force, the underlying energy loss including exciton dissociation and electron-hole recombination in the non-fullerene acceptor (NFA) system remains unclear. In principle, the energetic disorder characteristic is a common feature in organic semiconductors as well as their organic blends due to the molecular random distribution and the donor-acceptor interpenetration network. Owing to their completely different molecular structures in contrast to the bulky-ball shaped fullerenederivative counterpart such as PCBM, a comprehensive understanding for the trap associated energetic disorder characteristic and distribution are merits to further eliminate the open-circuit (𝑉oc) loss. In this work, a highly efficient planar organic BHJ solar cell comprising ITO(glass)/ZnO/PBDB-T:ITIC/MoO3/Al was fabricated. Both the polymeric donor PBDB-T and NFA ITIC form an interpenetrating network. With assist of the non-destructive impedance spectroscopic technique, surface and bulk trap states can be fully revealed. More importantly, both of them respectively contain deep and shallow traps with different energetic locations in an energy gap. The results help to explain the appearance of double and triple capacitance-voltage (C-V) bands. Further with pump-probe laser spectroscopic measurements, the trap states may locate far below the photo-absorption band-edge. This work serves an insightful view on the trap states in the NFA based organic BHJ system, and is valuable for minimizing the energy loss.


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1. Introduction Since the first demonstration of a highly efficient non-fullerene acceptor (NFA) based organic bulk heterojunction (BHJ) solar cell, there has been an enormous amount of efforts dedicated to its development.1-5 It has witnessed some great successes in material syntheses, device designs and interfacial optimizations, in order to ultimately reach applicable power conversion efficiencies (PCE).6-10 In the organic BHJ system, a donor (D) and acceptor (A) interface, sometimes called the D:A interface, is mainly responsible for the polaron pair dissociation. This, as a consequence, will increase charge densities for organic semiconductors of low charge carrier mobility.11-12 However, the system usually constitutes energetically disordered molecules arranged through the weakly bonded van der Waals force. Even though in a polymeric thin solid film, there usually exists a large number of trap or defect states in an energy gap. The states perform as non-radiative recombination centers which may cause the device degradation and the reduction of open circuit voltage (𝑉𝑜𝑐).13-15 By comparing with fullerene derivatives based acceptors with the bulky-ball structure such as PCBM, NFA can be more complicated due to their planar and twisted molecular configurations.16 It is, thus, of highly demanding to effectively characterize and deeply understand the origin of the trap states. Usually, the trap states can be classified as both deep and shallow traps depending on their energetic locations with respect to band-edges.14, 17-18 Both of them may coexist in the organic BHJ system. As a basic model suggests in Figure 1(a), they are assumed to appear in some localized states in the band-gap. The dotted line of Figure 1(a) indicates the location of the Fermi-energy (𝐸𝐹). Some localized states which are above and below 𝐸𝐹 refer to the shallow and deep trap states respectively. Their existences lead to the assumption that we should be able to detect relaxations for both trapping and de-trapping kinetics in some appropriate time and frequency domains. It is usually too difficult to reveal the complete mechanism with the conventional dcelectrical transport measurements. On the other hand, the ac-frequency based nondestructive impedance spectroscopy has been drawing some particular attentions in organic electronics since it offers fruitful information regarding electronically dynamic processes, such as charge dissociation, recombination and accumulation.19-21 With this technique and to make the BHJ system equivalent to an effectively electronic circuit, the traps, no matter they are deep or shallow, can be well recognized as a series resistor-



capacitor (R-C) circuit. The non-linear capacitive element represents a localized state with an occupancy certainty. In fact, the capacitance is not solely related to the conventional geometric capacitance (𝐶𝑔𝑒𝑜) in an organic solar cell; nevertheless, it belongs to the so-called chemical capacitance (𝐶𝜇), primarily due to an occupancy of trap states with respect to the electron and hole quasi-Fermi energies (𝐸𝐹𝑛 and 𝐸𝐹𝑝) under external stimuli such as light and electric field. The corresponding R is a kinetic factor due to the de-trapping mechanism. The resultant characteristic trap frequency ( 𝜔𝑡) is determined by 𝜔𝑡 = 1 (𝑅𝑡𝑟𝑎𝑝 ∙ 𝐶𝑡𝑟𝑎𝑝). Figure 1(b) shows the case before the metallic electrode of work-function 𝜙 is in the contact with the organic blend. As we can see from the interfacial contact at zero bias in Figure 1(c), the alignment of their electrochemical potentials leads to the interfacial band bending and the formation of depletion region of a width (𝑊𝑑) close to the interface. Here, we can imagine that both deep and shallow trap energy levels participate for the band bending at the metalorganic interface in Figure 1(c). An externally applied bias voltage throughout the interface can break the thermodynamic equilibrium and suppress the build-in potential (𝑉𝑏𝑖) (Figure 1(d)). In addition, electron-hole pairs can pile up at energy states close to 𝐸𝐹𝑛 and 𝐸𝐹𝑝 respectively under photo-excitations before they dissociate into free charge carriers. If the deep and shallow trap states co-exist, one may expect their different contributions for charge accumulations at different energy scales. In the present study, a NFA based organic blend such as PBDB-T:ITIC was used as the photo-sensitive layer in a BHJ solar cell comprising ITO(glass)/ZnO/PBDBT:ITIC/MoO3/Al. The schematic drawings of the molecular structures are given in SFigure 1(a) and (b) respectively. It is one of the prototypical systems in the class of highly efficient NFA based organic BHJ solar cells.6-7, 22-23 ZnO and MoO3 act as the electron transport layer and buffer layer respectively for the solar cell. In order to commendably reveal the complete feature for the deep and shallow trap states in the present PBDB-T:ITIC organic blend, the solar cell configuration was changed by the following ways: (i) the MoO3 buffer layer was removed in order to make the direct contact of Al and PBDB-T:ITIC; (ii) the thickness of the organic blend was intentionally increased leading to a complete presence of the depletion region. We will demonstrate below that both deep and shallow trap states can be fully revealed from the impedance spectra at steady states.20 Their co-existence can be utilized to explain the appearance of double and triple C-V bands at some ac-excitation frequencies. Further


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with assist of in-gap pump-probe laser spectroscopy, more details about the electronhole dynamic behavior at CT states can be found.

2. Results and discussion The UV-visible photo-absorption spectra of PBDB-T, ITIC and their organic blend are given in Figure 2(a). ITIC has a relatively stronger photo-absorption ability at longer wavelengths, which is one particular reason of using it. Figure 2(b) shows the photovoltaic J-V characteristic curves with and without illumination for the solar cell comprising ITO(glass)/ZnO/PBDB-T:ITIC(100 nm)/MoO3/Ag. The corresponding photovoltaic parameters have been summarized in Table 1. The solar cell produces an optimal PCE of 11.5%, with 𝑉𝑜𝑐 = 0.916 V, 𝐽𝑠𝑐 = 16.8 mA/cm2 and FF = 72.5%. The average PCE of the solar cells is approximately 10.8%. The corresponding external quantum efficiency (EQE) spectrum is given in S-Figure 1(c). After removing the buffer layer MoO3 while increasing the thickness of PBDB-T:ITIC to 250 nm, the results of the photovoltaic performance for the device is provided in S-Figure 2. For all electronic transport measurements, we adopted that the positive (i.e. forward) bias voltage corresponds to electrons flow from occupied electronic states of Al into unoccupied electronic states of ITO, and it is vice-versa for the negative (i.e. reverse) bias voltage. Figure 3(a) shows the steady state impedance spectra (dotted-line) for the device of

the

structure

ITO(glass)/ZnO/PBDB-T:ITIC(250

nm)/MoO3/Al

and

the

corresponding fitting curves (red solid-line). During the measurements, the light intensity can be tuned by several different sets of optical density filters. The acmodulated impedance spectroscopy was performed under an alternating electrical field of 50 mV (peak-to-peak value) at the ac-excitation frequency ranging from 20 Hz to 11 MHz. All the spectra comprise three distinct semi-circles at three different frequency ranges, (I) the low frequency region is from 20 Hz to 10 kHz, (II) the intermediate frequency region is from 10 kHz to 50 kHz, and (III) the high frequency region from 50 kHz to 11 MHz, respectively. Every spectrum can be modeled and interpreted as two R-CPE circuits and one R-C connected in series. As it is schematically drawn in the inset of Figure 3(a), the relatively small resistance 𝑅𝑠 that is nearly invariant under the illumination and bias voltage is the series resistance. The resistance 𝑅1 measured at the low frequency region is ascribed to the surface resistance. The CPE component



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stands for the constant phase element and it usually contains two major elements. One is called the pseudo-capacitance (CPE-T) and it is normally represented by Q. Another is the semi-circle depression element (CPE-P) and it is usually denoted by n. CPE can thus be expressed as 𝐶𝑃𝐸 = 𝑅

(

1―𝑛 𝑛

)

1

(𝑛)

∙ 𝑄 , in which 𝑅 still represents resistance. If n

approaches 1, CPE turns theoretically to a capacitor accordingly.24 Referring to STable 1, 𝐶𝑃𝐸1 behaves like a capacitor. More precisely, it acts as the so-called surface chemical capacitance (𝐶𝑠𝜇) since it is detected at the low frequency region in this case. The same thing can be applied for the intermediate frequency region. 𝑅2 and 𝐶𝑃𝐸2 should be the bulk related resistance and chemical capacitance (𝐶𝑏𝜇) respectively (referring to S-Table 1). At the high frequency region, the trapping and detrapping mechanisms cannot be well responded to the ac-field due to the low dielectric constant and dielectric loss in the organic semiconductor. As a consequence, both 𝑅3 and 𝐶3 originate from the electronic transport resistance (𝑅𝑡𝑟) and geometric capacitance ( 𝐶𝑔𝑒𝑜) respectively. Here, we would like to further address the critical role of 𝐶𝑠𝜇 and 𝐶𝑏𝜇 in the organic BHJ solar cell. Essentially, both of them are determined by the charge accumulations close to 𝐸𝑛𝐹 and 𝐸𝑝𝐹 under external stimuli in the organic blend. The variations of 𝐶𝑠𝜇 and 𝐶𝑏𝜇 can be used to estimate the distributions of both surface and bulk density of states (DOS) for the organic photo-active layer. More importantly, because of the inherently energetic disorder (𝜎) characteristic, the relatively flat bandedge and the energy broadening for the organic molecule, the trap-related DOS should be more adequate to describe the electron and hole occupations in this case. In electronic transport measurements, 𝐶𝑠(𝑏) is proportional to the charge carrier density 𝜇 (𝑛) and (𝑝) with respect to 𝐸𝑛𝐹 and 𝐸𝑝𝐹. Mathematically, it can be written as, 𝑑𝑛 𝐶𝑠(𝑏) = 𝐿𝑞2 𝑛 = 𝐿𝑞2𝑔(𝐸𝑛𝐹) 𝜇 𝑑𝐸𝐹

(1)

where, 𝑞 is the elementary charge, and 𝑔(𝐸𝑛𝐹) represents DOS at 𝐸𝑛𝐹. The total charge carrier density can be expressed as an integration of the DOS, 𝑛=

∫𝑔(𝐸 )𝑓(𝐸 ― 𝐸 )𝑑𝐸 𝑛 𝐹

𝑛 𝐹

(2)

where 𝑓(𝐸 ― 𝐸𝑛𝐹) represents the Fermi-Dirac distribution function. 𝐶𝑠(𝑏) can be 𝜇 further modified as,


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𝐶𝑠(𝑏) 𝜇

𝑞2 = 𝑘𝐵𝑇

∫𝑔(𝐸 )𝑓(𝐸 ― 𝐸 )[1 ― 𝑓(𝐸 ― 𝐸 )]𝑑𝐸 𝑛 𝐹

𝑛 𝐹

𝑛 𝐹

(3)

The change of 𝑔(𝐸𝑛𝐹) can be reflected by the change of 𝐶𝑠(𝑏) 𝜇 . 𝑔𝑒(ℎ)(𝐸𝑛(𝑝) 𝐹 )=

𝑁𝑒(ℎ)

𝑒 𝜎 2𝜋

[―

( ± 𝐸 ∓ 𝐸𝐻𝑂𝑀𝑂,𝐷 𝐿𝑈𝑀𝑂,𝐴)2 2𝜎2

]

(4)

Figure 3(b) and (c) show the plots of surface and bulk trap densities versus 𝑉𝑜𝑐 at the steady states respectively. Apparently, both feature a Gaussian distribution of equation 4 after the fitting. All the numerical values about the fitting parameters have been provided in Table 1 and 2 for the surface and bulk trap states respectively. 𝑁𝑒(ℎ) is the electron or hole density in an organic blend, 𝐸𝐻𝑂𝑀𝑂,𝐷 and 𝐸𝐿𝑈𝑀𝑂,𝐴 denote the donor HOMO and acceptor LUMO respectively, and 𝜎 quantifies the energetic disorder. In some semiconductors of high mobility, the occupancy of trap states above 𝐸𝐹 can be also approximated by the Boltzmann distribution.17 As we can see from the plots, both the surface and bulk trap-related DOS contain a shadow and deep trap states locating at a low and high energies. They also exhibit different magnitudes and 𝜎 (i.e. the full-width at half maximum of the distribution). In this case, the density of the surface shallow traps is smaller than the one of the surface deep traps. In contrast, the phenomenon is opposite for the bulk shallow and deep trap densities. Nevertheless, both cases possess the energetic disorder characteristic 𝜎. It is relative greater for the shallow trap states indicating the comparatively large energetic broadening effect close to the band-edge. Despite the impedance spectral analysis for the trap density in the organic solar cell, the C-V method can act as a diagnostic tool for revealing the impacts of trap states on the electron-hole recombination, dissociation and accumulation. In the following, CV spectra, which were modulated by different ac-excitation frequencies in the range from 100 Hz to 700 kHz, were captured for the same device (i.e. 250 nm thick PBDBT:ITIC). The corresponding experimental results are given in Figure 4(a) and (b). In fact, the presence of both deep and shallow trap states in PBDB-T:ITIC contribute significantly to the double and triple C-V bands at some particular frequencies. As it is displayed in Figure 4(a) for the spectra measured under ac-excitation frequency from 100 Hz to 10 kHz, the gradually increase of the frequency leads to the suppression of the C-V band at the reverse bias voltage (i.e. negative bias voltage). In contrast, there is



no clear variation for the band at the forward bias voltage (i.e. positive bias voltage). As it has been discussed previously in Figure 3, the low and intermediate frequency regions should correspond to the surface and bulk trap-related energy states, meanwhile the frequency can be utilized to modulate the trapping and de-trapping mechanisms. The decrease of C at higher frequencies close to 10 kHz elucidates the reduction of trap densities since the traps are more preferentially generated at the vicinity of organic blend and metallic electrode interface. Further increase of the ac-excitation frequency, such as the one given in Figure 4(b), will move from the surface to the bulk of PBDBT:ITIC film. Because of the presence of both deep and shallow trap states in the bulk form, the double and multiple C-V bands can be still detected in the spectra until approximately 50 kHz. Owing to the relatively lower mobility and dielectric constant of the organic semiconductors as well as their blends, the dielectric loss can happen at even higher frequencies such as above 50 kHz. As the spectra illustrated in Figure 4(b), all the C-V bands are eliminated for both forward and reverse bias directions. There exists a large and invariant capacitive value at the reverse bias part for the frequency exceeding 450 kHz. Figure 4(c) and (d) display the same measurements except the PBDB-T:ITIC film thickness is reduced to 100 nm. Apparently, the double and triple C-V bands can be still detected at the forward and reverse bias voltages. We have also attempted to replace the electron transport layer ZnO by the hole transport layer such as PEDOT:PSS. The frequency dependent C-V spectra are given in S-Figure 3(a) and (b) for organic solar cells with two different thicknesses of PBDB-T:ITIC blend such as 250 nm and 100 nm respectively. Apparently, the presence of the multiple C-V band is the inherent effect in the NFA ITIC based organic blends, which is irrespective of the organic blend thickness and the transport layer. By far, we have discussed about the electronic charge transport dynamic behavior and the trap states for the organic solar cell operated at steady states. It is also possible to detect the excitonic behavior at excited states and in-gap trap states related electronhole dynamic behavior using the pump-probe laser spectroscopic technique.25-26 During the measurement, a 100 nm thick PBDB-T:ITIC solid film was photo-excited by a 410 nm ultra-short laser pulse, and the relative reflectance were recorded under the photoexcitation by delayed probe laser beams of different photon energies from 1.771 eV to 1.938 eV. As we can see from Figure 2(a), PBDB-T has stronger photo-absorption than ITIC at 410 nm. As the schematic energy diagram illustrated in Figure 5(a), the abovegap pump by the laser pulse initially photo-generates firmly bound Frenkel excitons


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(FE) in PBDB-T. The excess photon energy is quickly consumed by some internal dissipations. As the excitons diffuse to the CT states, they are separated into less bound polaron pairs in less than 100 fs with the positive and negative polarons locate at the donor and acceptor moieties respectively. Part of the polaron pairs can be further dissociated into free electron and hole charge carriers at the interface, and part of them recombines non-geminately. Here, the energy loss mainly comes from the electron-hole recombination. In addition, the electron and hole can be captured by the traps within the energy gap leading to the trap-assist recombination. As it is depicted in Figure 5(a), the in-gap probe laser pulses of different probe energies excite the organic blend after the initial pumping. The results of the differential reflectance spectra are given in Figure 5(b) and the inset shows the polaron pair lifetime. Apparently, the differential reflectance and lifetime vary remarkably when the probe energy is tuned in-between 1.771 eV and 1.938 eV. Some traps locate far below the band-edge indicating the presence of deep trap states for PBDB-T:ITIC. Although NFA such as ITIC exhibits many desirable optoelectronic properties for future organic photovoltaic applications, we suggest that some efforts on the reduction of trap density in the material processing may further eliminate the energy loss, as it gives rise to the improvement of 𝑉𝑜𝑐.

3. Conclusions In summary, we have explored the deep and shallow traps in the NFA ITIC based organic BHJ system using the non-destructive and steady state based impedance spectroscopy. The traps exist in both the surface and bulk forms of the organic blend. We have found that they are different in the trap density and energetic disorder characteristic. The wide distributions of the trap states help to explain the appearance of the double and triple C-V bands at some ac-excitation frequencies. The phenomenon is the inherent feature for the NFA ITIC based organic blend. It does not depend on the photo-active layer thickness and the interfacial transport layer. The study provides not only the insightful view on the electronic property for the ITIC based photovoltaic system, but also guides for future designs of NFA molecules.

4. Experimental Section Device Fabrications Indium tin oxide (ITO) coated glass substrates were cleaned in an ultrasonic bath



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by standard chemical means. The substrates were dried by a pure-nitrogen gas and then oxidized by oxygen plasma for approximately 10 min in an enclosed chamber. The ntype ZnO layers were spin-coated onto the ITO(glass) substrates. After this, they were immediately transferred into a pure nitrogen-filled glove-box. PBDB-T and ITIC with the equal amount were dissolved in the organic solvent CB:DIO (99.5:0.5 in the volume ratio) producing 20 mg/ml concentration. The solution was spin-coated onto ITO(glass)/ZnO with the spin-speed of 800 or 2500 rpm for 60 s. Finally, all the samples were transferred into an integrated thermal evaporation system; a 10 nm thick MoO3 and 100 nm thick Al top electrode were thermally evaporated onto ITO(glass)/ZnO/ PBDB-T:ITIC forming the cross-bar structure. The effective electronic transport area is 0.038 cm2.

Electronic Transport and Optical Spectroscopic Measurements Photovoltaic dc-electrical current density-voltage (J-V) characteristic curves were measured by the two-wiring method using a source-meter unit (Keysight B2912A) and a solar simulator (AM 1.5 G). The ac-modulated impedance spectroscopy was performed by an L-C-R impedance analyzer (Keysight E4990A). Absorption spectra were obtained by a UV-visible spectrometer (Shimadzu UV-2600 PC). In the pumpprobe laser spectroscopic measurement, a 410 nm ultra-short laser pulse (pump fluence = 41.533 µJ/cm2, pump power = 100 µW) was used to inject excitons. The spatiotemporal dynamics of the excitons is monitored by measuring differential reflection of the time-delayed and spatially scanned in the range from 640 nm to 700 nm (probe power = 50 µW). The differential reflectance is defined as

(𝑅 ― 𝑅0) 𝑅0

, where 𝑅

and 𝑅0 are the reflectance of the probe light with and without injection respectively.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX Schematic drawings of the molecular structures, EQE spectrum measured in the ambient condition for the solar cell with the configuration ITO(glass)/ZnO/PBDBT:ITIC(100 nm)/MoO3/Al, photovoltaic performance for the device configuration


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ITO(glass)/ZnO/PBDB-T:ITIC(250 nm)/Al, fitting parameters for the impedance spectra of the solar cell with the configuration ITO(glass)/ZnO/PBDB-T:ITIC(250 nm)/Al, 3-D view of frequency dependent C-V spectra measured from 100 Hz to 10 kHz for the solar cell configuration ITO(glass)/ZnO/PBDB-T:ITIC/Al, photovoltaic performance for the device configuration ITO(glass)/ZnO/PBDB-T:ITIC(100 nm)/Al, steady state impedance spectra for the solar cell consisting of ITO(glass)/ZnO/PBDBT:ITIC(100 nm)/Al, fitting parameters for the impedance spectra of the solar cell with the configuration ITO(glass)/ZnO/PBDB-T:ITIC(100 nm)/Al

Acknowledgements We acknowledge financial supports from the National Natural Science Foundation of China (Grant No. 61604010, 61634001, U1601651), and the research funding from Beijing Jiaotong University Research Program (Grant No. S18JB00020).

Conflict of Interest The authors declare no conflict of interest.

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(8) Li, Z.; Jiang, K.; Yang, G.; Lai, J. Y. L.; Ma, T.; Zhao, J.; Ma, W.; Yan, H. Donor Polymer Design Enables Efficient Non-Fullerene Organic Solar Cells. Nat. Commun. 2016, 7, 13094. (9) Lu, H.; Zhang, J.; Chen, J.; Liu, Q.; Gong, X.; Feng, S.; Xu, X.; Ma, W.; Bo, Z. Ternary‐Blend Polymer Solar Cells Combining Fullerene and Nonfullerene Acceptors to Synergistically Boost the Photovoltaic Performance. Adv. Mater. 2016, 28, 95599566. (10) Fan, B.; Zhang, K.; Jiang, X. F.; Ying, L.; Huang, F.; Cao, Y. High‐Performance Nonfullerene Polymer Solar Cells based on Imide‐Functionalized Wide‐Bandgap Polymers. Adv. Mater. 2017, 29, 1606396. (11) Dou, L.; You, J.; Hong, Z.; Xu, Z.; Li, G.; Street, R. A.; Yang, Y. 25th Anniversary Article: A Decade of Organic/Polymeric Photovoltaic Research. Adv. Mater. 2013, 25, 6642-6671. (12) Heeger, A. J. 25th Anniversary Article: Bulk Heterojunction Solar Cells: Understanding the Mechanism of Operation. Adv. Mater. 2014, 26, 10-28. (13) Shenkun, X.; Yuxin, X.; Zhong, Z.; Xuning, Z.; Jianyu, Y.; Huiqiong, Z.; Yuan, Z. Effects of Nonradiative Losses at Charge Transfer States and Energetic Disorder on the Open‐Circuit Voltage in Nonfullerene Organic Solar Cells. Adv. Funct. Mater. 2018, 28, 1705659. (14) Graupner, W.; Leditzky, G.; Leising, G.; Scherf, U. Shallow and Deep Traps in Conjugated Polymers of High Intrachain Order. Phys. Rev. B 1996, 54, 7610-7613. (15) Blakesley, J. C.; Neher, D. Relationship between Energetic Disorder and OpenCircuit Voltage in Bulk Heterojunction Organic Solar Cells. Phys. Rev. B 2011, 84, 075210. (16) Yan, C. Q.; Barlow, S.; Wang, Z. H.; Yan, H.; Jen, A. K. Y.; Marder, S. R.; Zhan, X. W. Non-Fullerene Acceptors for Organic Solar Cells. Nat. Rev. Mater. 2018, 3, 119. (17) Bisquert, J. Beyond the Quasistatic Approximation: Impedance and Capacitance of An Exponential Distribution of Traps. Phys. Rev. B 2008, 77, 235203-1. (18) Ray, B.; Baradwaj, A. G.; Boudouris, B. W.; Alam, M. A. Defect Characterization in Organic Semiconductors by Forward Bias Capacitance-Voltage (FB-CV) Analysis. J. Phys. Chem. C 2014, 118, 17461-17466. (19) Fabregat-Santiago, F.; Garcia-Belmonte, G.; Mora-Sero, I.; Bisquert, J. Characterization of Nanostructured Hybrid and Organic Solar Cells by Impedance Spectroscopy. Phys. Chem. Chem. Phys. 2011, 13, 9083-9118. (20) Garcia-Belmonte, G.; Boix, P. P.; Bisquert, J.; Sessolo, M.; Bolink, H. J. Simultaneous Determination of Carrier Lifetime and Electron Density-Of-States in P3HT:PCBM Organic Solar Cells under Illumination by Impedance Spectroscopy. Sol. Energ. Mat. Sol. C 2010, 94, 366-375. (21) Zhu, X.; Wang, K.; Zhao, F.; Han, C.; Yang, Q.; Yu, H.; Zhang, F.; Hu, B. Revisiting the Impact of Interfacial Transport Layers on Organic Bulk Heterojunction Systems. ACS Appl. Energy Mater. 2018, 1, 3457-3468. (22) Yuan, J.; Qiu, L.; Zhang, Z.-G.; Li, Y.; Chen, Y.; Zou, Y. Tetrafluoroquinoxaline based Polymers for Non-Fullerene Polymer Solar Cells with Efficiency Over 9%. Nano Energy 2016, 30, 312-320. (23) Liu, T.; Guo, Y.; Yi, Y.; Huo, L.; Xue, X.; Sun, X.; Fu, H.; Xiong, W.; Meng, D.; Wang, Z. Ternary Organic Solar Cells based on Two Compatible Nonfullerene Acceptors with Power Conversion Efficiency> 10%. Adv. Mater. 2016, 28, 1000810015. (24) Changfeng, H.; Kai, W.; Xixiang, Z.; Haomiao, Y.; Xiaojuan, S.; Qin, Y.; Bin, H.


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Unraveling Surface and Bulk Trap States in Lead Halide Perovskite Solar Cells Using Impedance Spectroscopy. J. Phys. D Appl. Phys. 2018, 51, 095501. (25) Grancini, G.; Maiuri, M.; Fazzi, D.; Petrozza, A.; Egelhaaf, H. J.; Brida, D.; Cerullo, G.; Lanzani, G. Hot Exciton Dissociation in Polymer Solar Cells. Nat. Mater. 2013, 12, 29-33. (26) Dong, Y.; Cha, H.; Zhang, J.; Pastor, E.; Tuladhar, P. S.; McCulloch, I.; Durrant, J. R.; Bakulin, A. A. The Binding Energy and Dynamics of Charge-Transfer States in Organic Photovoltaics with Low Driving Force for Charge Separation. J. Chem. Phys. 2019, 150, 104704.



Table of Contents Graphic


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Page 15 of 19 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 1. (a) A schematic drawing for an electronic energy level of an organic semiconductor contains both shallow and deep trap states within the energy-gap. (b) Before the direct contact of an organic BHJ and a metal, there is an offset measured from their electrochemical potentials. (c) An energetic thermodynamic equilibrium is established when the organic BHJ is in the contact with the metal at 𝑉 = 0 V and dark condition. (d) With an application of forward bias voltage, the quasi-Fermi energy levels tend to split.


1.0 0.8 0.6

pump

0.4 0.2

5

300

probe 400

500

600

700

800

dark

0 -5 -10 -15

0.0

Page 16 of 19

10

(b)

PBDB-T ITIC PBDB-T:ITIC

2

(a)

J (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

Normalized Abs. (a.u.)


-20 -0.2

light

0.0

0.2

 (nm)

0.4

0.6

0.8

1.0

V (V)

Figure 2. (a) UV-visible photo-absorption spectra of PBDB-T, ITIC and PBDB-T:ITIC; (b) J-V characteristic curves for the solar cell of the configuration ITO(glass)/ZnO/PBDB-T:ITIC/MoO3/Al with and without illumination.

Table 1. Summary for the photovoltaic parameters of Figure 2. Device Structure

Voc (V)

ITO/ZnO/PBDB-T:ITIC/MoO3/Al

0.916

Jsc (mA/cm2) 16.8


FF (%) 72.5

Best PCE (%) 11.15

Ave. PCE (%) 10.8

CPE1

2

(II)

R2

(III) R

CPE2

C3

2

0.3 mW/cm - 0.15 V 2 3.0 mW/cm - 0.16 V 2 5.0 mW/cm - 0.17 V 2 10 mW/cm - 0.18 V 2 32 mW/cm - 0.20 V 2 50 mW/cm - 0.21 V 2 80 mW/cm - 0.22 V 2 100 mW/cm - 0.23 V Fitting

3

0

2

4

Z' (k)

6

-1

8

4

0 0.08

0.12

0.16

0.20

0.24

Voc (V)

(c)

1

0

-3

R1

17

(I)

8

-1

Rs

16

-3

3

12

12

17

(a)

t-DOS (10 cm eV )

(b)

Z'' (k

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


t-DOS (10 cm eV )

Page 17 of 19

8 4 0 0.12

0.15

0.18

0.21

0.24

Voc (V)

Figure 3. (a) Steady state impedance spectra measured in the ambient condition for the solar cell consisting of ITO(glass)/ZnO/PBDB-T:ITIC(250 nm)/Al, the solid red lines are the fitting curves and the inset displays the equivalent electronic circuit. I, II and III represent low (20 Hz–10 kHz), intermediate (10 kHz–50 kHz) and high (50 kHz–11 MHz) frequency regions respectively. (b) and (c) are trap-related DOS with Gaussian fittings (red and blue solid lines) for the low and intermediate frequency regions respectively.

Table 2. Fitting parameters for the low frequency region of the steady state impedance spectra in Figure 3(b). shallow trap deep trap

𝑁𝑡𝑟𝑎𝑝 (cm ―3eV ―1) 1.22 × 1016 4.00 × 1016

𝜎 (meV) 28 14

𝐸𝑐𝑒𝑛𝑡𝑒𝑟 (meV) 205 225

Table 3. Fitting parameters for the intermediate frequency region of the steady state impedance spectra in Figure 3(c). shallow trap deep trap

𝑁𝑡𝑟𝑎𝑝 (cm ―3eV ―1) 1.48 × 1017 7.50 × 1016

𝜎 (meV) 34 24


𝐸𝑐𝑒𝑛𝑡𝑒𝑟 (meV) 175 217


(a)

(b)

(c)

(d)

Figure 4. 3-D view of frequency dependent C-V spectra measured from (a) 100 Hz to 10 kHz, and (b) 10 kHz to 700 kHz for the solar cell configuration ITO(glass)/ZnO/PBDB-T: ITIC(250 nm)/Al; (c) and (d) are the spectra measured at the same condition for ITO(glass)/ZnO/PBDB-T: ITIC(100 nm)/Al from 100 Hz to 10 kHz, and from 20 Hz to 800 kHz respectively.


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Figure 5. (a) A schematic drawing of band diagram for the electronic transition at the charge transfer (CT) state, 𝐸𝐷𝐻𝑂𝑀𝑂 and 𝐸𝐷𝐿𝑈𝑀𝑂 denote the energies of the highest occupied molecular orbitals (~-5.33 eV) and the lowest unoccupied molecular orbitals (~-2.92 eV) of the donor PBDB-T, 𝐸𝐴𝐻𝑂𝑀𝑂 and 𝐸𝐴𝐿𝑈𝑀𝑂 denote the energies of the highest occupied molecular orbitals (~-5.45 eV) and the lowest unoccupied molecular orbitals (~-3.34 eV) of the acceptor ITIC. (b) Results of the pump-probe experiment for a 100 nm thick PBDB-T:ITIC organic blend, the pump photon energy is about 3.02 eV (~410 nm), the inset shows the extract lifetime (𝜏) at different probe energies.


Exploring Deep and Shallow Trap States in a Non-Fullerene

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