Highly Efficient Charge Collection in Bulk-Heterojunction Organic

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Letter

Highly efficient charge collection in bulk heterojunction organic solar cells by anomalous hole-transfer and improved interfacial contact Wen Feng, Chaoyu Song, Xiaofeng Hu, Shaobo Liu, Ruichen Yi, Xinju Yang, Hugen Yan, and Xiaoyuan Hou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08390 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

Highly efficient charge collection in bulk heterojunction organic solar cells by anomalous hole-transfer and improved interfacial contact Wen Feng, Chaoyu Song, Xiaofeng Hu, Shaobo Liu, Ruichen Yi*, Xinju Yang, Hugen Yan, and Xiaoyuan Hou

State Key Laboratory of Surface Physics, Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education) and Department of Physics, Fudan University, Shanghai 200433, CHINA

Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, CHINA Email: [email protected]

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ACS Applied Materials & Interfaces 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

The dilute donor-fullerene bulk-heterojunction (BHJ) has been proven as an efficient architecture of organic solar cells. However, the hole-extraction pathway and the origin of high open-circuit voltage (VOC) in this peculiar architecture remains elusive. Direct evidence is provided here that the photo-generated holes can be extracted via acceptor phase even under the operating conditions. Meanwhile VOC is found to be closely correlated with the surface composition at MoO3/BHJ interface. Extending these findings into device optimization, more than 37% enhancement is achieved in a prototype BHJ device. These results evoke renewed insights into the underlying physics in organic solar cells.

Keywords: organic solar cells, heterojunction, open-circuit voltage, MoO3, charge extraction


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The utilization of bulk heterojunction (BHJ) structure in organic photovoltaic (OPV) devices has made significant achievements1-2. One pivotal issue to further develop the OPV technique is to realize high quality charge collection, in other words, to enhance charge transport and to eliminate recombination pathways under the operating conditions of BHJ devices. Due to the energy offset at the donor-acceptor (D-A) interface (type-II heterojunction) in most cases, the charge-extraction is generally considered to be confined in donor phase for hole-carriers and in acceptor phase for electron-carriers3. Thus the existence of bi-continuous interpenetrating D-A network has been long regarded as a prerequisite for efficient charge collection3-6, and discrete donor/acceptor domains in BHJ mixtures are believed detrimental to the charge collection quality. However, the necessity of this design rule is questioned by the remarkable performance observed in a series of dilute donor-fullerene devices7-11. Efficient charge collection is even observed in devices using 2.5% donor concentration12, wherein the presence of sufficient hole-percolation-channels is highly suspicious. In addition, these devices usually have higher open-circuit voltage (VOC) compared with their counterparts using more balanced D-A ratio (e.g. 50%). In contrast to the high performance achieved, little has been done to clarify what triggers the efficient hole-extraction and high VOC in devices with such low donor concentration. The high VOC observed was previously assigned to the change of charge-transfer state energy (ECT)8 and the reduction of D-A interface area in the BHJ bulk13. To address the puzzle of the unexpected hole-extraction efficiency, the long-range hole tunneling between discrete donor sites7 and the formation of donor columns14 were proposed recently. Although these explanations



are hardly satisfactory to the results observed in the devices with ultralow donor concentration, the hole-extraction under the operating conditions was still believed to be accomplished by hole-transport through donor phase in both of these two works.

In this letter, two key factors lying behind the efficient charge collection in the dilute donor-fullerene devices are reported: i) hole-carriers can be extracted through acceptor (fullerene) phase even under the operating conditions of OPV devices, ii) the large interfacial contact area between MoO3 and C70 is crucial to achieve high VOC in fullerene-based BHJ devices with MoO3 buffer layer. Fill factor (FF) higher than 60% is observed in devices where donor molecules are isolated from the anode by a thick acceptor layer, suggesting that the general notion of confined hole-extraction via the donor phase of BHJ network is incomplete. On the other hand, from current density-voltage (J-V) and electroluminescence (EL) measurements, we find that the existence of abundant donor domains in contact with MoO3 aggravates the non-radiative recombination and gives rise to the voltage-loss in devices with high donor concentration. Extending these two findings into device structure optimization, we demonstrate a 37.3% improvement of power-conversion-efficiency (PCE) in a prototype BHJ system using moderate donor concentration.


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Figure 1. a) Energy level diagram of TAPC and C70, and illustration of the model system design. b) Molecular structures of TAPC and C70. c) J-V curves under illumination of ITO/MoO3/C70 (x nm)/TAPC (2 nm)/C70 (40 nm)/BPhen/Al and ITO/MoO3/C70 (60 nm)/BPhen/Al and ITO/MoO3/C70 (60 nm)/BPhen/Al devices (dash-dot).



The excellent hole-transport-ability has been long observed in the fullerenes7, 15-17, however, the deep-lying highest occupied molecular orbital (HOMO) levels of the fullerenes are likely to reject the hole-transfer from common donor materials. To investigate whether the hole-transfer from donor to fullerene is feasible under the operating conditions of OPV devices, we simplified the research objective to a series of devices with C70/donor/C70 stacks. As shown in Figure 1a, 5 nm-thick MoO3 anode buffer layer was deposited on pre-patterned indium-tin-oxide (ITO) glass, followed by a C70 layer with varying thickness (x nm), a 2 nm-thick donor layer, a second 40 nm-thick C70 layer, an 8 nm-thick bathophenanthroline (BPhen) exciton blocking layer and an 100 nm-thick Al cathode. Considering the surface roughness of the MoO3 layer (Rq ~ 2.6 nm, see Figure S3 in the Supporting Information), the minimum thickness of the first C70 layer is set to be 6 nm to eliminate the direct contacts between MoO3 and donor molecules. The illuminated J-V curves for devices using 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC, Figure 1b) as donor are shown in Figure 1c, together with the J-V curve of ITO/MoO3/C70 (60 nm)/BPhen/Al (dash-dotted). Comparing the TAPC/C70 device (x = 0 nm) to the C70-only device, it can be found that PCE is largely enhanced (from 0.40% to 2.47%) by introducing the TAPC/C70 heterojunction interface, wherein short-circuit current (JSC) increases by five-fold (from 0.94 mA/cm2 to 6.38 mA/cm2) and FF is almost doubled (from 33.1% to 63.4%). These results indicate that the charge-transfer process at the heterojunction interface is essential to the free-charge-carrier yielding in fullerene-based devices, and the hole-carriers can be efficiently collected through TAPC after the initial charge-separation. Surprisingly, in spite of


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the apparent hole-transport-barrier from TAPC to C70 (0.7 eV12), after adding the C70 layer (20 nm ≥ x ≥ 6 nm) between the MoO3 layer and the donor layer, the devices exhibit superior JSC, VOC and FF to the TAPC/C70 device (x = 0 nm), and PCE is improved by more than one-fold (for detailed device performance please see Table S1 in the Supporting Information). The J-V characteristics present decent FF with a maximum value of 71.6% at x = 10 nm, while VOC and JSC barely change with the varying thickness of C70 layer. Even for the device with x = 20 nm, the J-V curve still renders good charge-generation and charge-extraction (FF = 64.2%, JSC = 8.3 mA/cm2, PCE = 4.9%). These results strongly suggest that hole-carriers from light harvesting are able to transfer from TAPC to fullerene against the large barrier. Considering the limited D-A interface area and exciton diffusion length in these devices, the remarkable photovoltaic performances indicate that the hole-transfer is very efficient under the operating conditions. We further verified this mechanism by alternatively using another two common donor materials in devices with the same device structure (see Figure S1 and Table S1 in the Supporting Information). The similar results observed confirm the universality of the donor-to-acceptor hole-transfer pathway and provide a direct evidence to the existence of ambipolar charge-extraction via acceptor phase in fullerene-based BHJ devices, especially for those devices using ultralow donor concentration7, 12-13.



Figure 2. a) Schematic diagram, b) FF, JSC and PCE of ITO/MoO3/TAPC:C70 (x nm, 5%)/C70 (60 – x nm)/BPhen/Al devices as a function of the thickness of TAPC:C70 BHJ layer. c) Illustration of the hindered donor-to-fullerene hole-transfer under forward-bias in BHJ devices due to the insufficient local electric-field-strength.

Further increasing the thickness of C70 layer, JSC and VOC do not change obviously, indicating that the charge-generation is not affected18. However, FF drops to 47.5% at x = 25 nm and S-kink occurs at x = 30 nm (FF = 27.1%), implying that under forward-bias, the charge-extraction is progressively hindered by the increased distance between the MoO3 layer and the donor molecules18-20. Considering the strong band bending near the MoO3/fullerene interface21 and the reduced built-in-field-strength (due to increased device thickness), these results suggest that the hole-transfer mechanism still relies on the assistance of electric-field, though the threshold


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field-strength is much lower than previously predicted7. Therefore, it can be inferred that for BHJ devices wherein the hole-carriers can be extracted via acceptor phase, the charge collection quality under forward-bias (i.e. FF×VOC) will be deterred if the thickness of BHJ layer exceeds a certain

value,

despite

JSC

may

still

increase.

The

photovoltaic

parameters

of

ITO/MoO3/TAPC:C70 (x nm, 5%)/C70 (60 - x nm)/BPhen/Al devices are shown in Figure 2. Since TAPC is practically transparent to the solar irradiation, the change in light-absorption is minor in these devices and the influence from different charge-generation pathways can be excluded12. Meanwhile, due to the limited exciton diffusion length in C7022, the charge-generation-profile is gradually extended away from the MoO3 layer by increasing the thickness of the BHJ layer. As can be seen in Figure 2b, JSC linearly rises from 9.60 ± 0.14 mA/cm2 to 12.55 ± 0.22 mA/cm2 as x increases from 20 nm to 40 nm, whereas FF dramatically drops from about 65% to about 52% at x = 40 nm. Further increasing x from 40 nm to 60 nm, both FF and JSC change slightly. Comparing the device comprising halved BHJ thickness (x = 30 nm) to the full-BHJ device (x = 60 nm), the remarkably improved charge collection quality raises the device performance from 6.21 ± 0.08% (x = 60 nm) to 6.76 ± 0.10% (x = 30 nm) against the sacrifice of JSC. For devices with 25% TAPC concentration (Table 1), PCE is also improved from 4.77 ± 0.19% to 5.55 ± 0.21% by restricting the thickness of BHJ layer. In addition to the enhanced FF, VOC is marginally improved by 0.02 V (from 0.85 V to 0.87 V) in spite of the decreased JSC. Nevertheless, this value is still lower than the VOC of devices with 5% TAPC (0.94 V).



Figure 3. a) Normalized EL spectra and VOC of ITO/MoO3/TAPC:C70 (60 nm)/BPhen/Al devices using 5%, 12.5%, 20%, 25% TAPC concentration. b) VOC as a function of TAPC thickness in devices of ITO/MoO3/TAPC (x nm)/TAPC:C70 (60 nm, 5%)/BPhen/Al. The illuminated J-V curves of these devices are plotted in the inset. c) EL spectra of ITO/MoO3/TAPC (x nm)/TAPC:C70 (60 nm, 5%)/BPhen/Al devices. d) VOC as a function of TAPC concentration in ITO/MoO3/C70 (0/4 nm)/TAPC:C70 (30 nm)/C70 (30 nm)/BPhen/Al devices using 5%, 25% and 50% TAPC concentration. e) Illustration of the interfacial contact in BHJ devices using different donor concentration.


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To investigate the reduced VOC in devices with relative higher donor concentration, we measured the J-V and EL characteristics of TAPC:C70 BHJ devices with 5%, 12.5%, 20% and 25% TAPC concentration. The measured VOC values at one-sun intensity and normalized EL spectra are illustrated in Figure 3a. The EL emission is exclusively dominated by the radiative recombination of the charge-transfer (CT) states at TAPC/C70 interface. The peak position (~ 1.32 eV) of CT emission barely changes with the donor concentration, while a negative correlation between donor concentration and VOC is observed. These results suggest that the overall recombination rate is higher at open-circuit condition for device comprising more donor contents13.

For devices with different donor concentration, the surface composition at the MoO3/BHJ interface changes as well. To investigate the contact effect of the MoO3/BHJ interface without changing the bulk properties, we constructed devices with ultrathin TAPC layers inserted between the MoO3 layer and the BHJ layer, resulting in device structure of ITO/MoO3/TAPC (x nm)/TAPC:C70 (60 nm, 5%)/BPhen/Al. The illuminated J-V and EL spectra of these devices are illustrated in Figure 3b and 3c. As can be clearly seen, JSC is almost the same for these devices, suggesting that the charge generation and recombination dynamics are similar in these devices when the internal electric-field-strength is sufficient to drive most of the charge-carriers to the correct electrode. However, VOC progressively decreases from 0.94 V to 0.61 V as the nominal TAPC thickness increases from 0 to 2 nm, and the EL signal is wiped-off after the insertion of



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TAPC molecules. According to previous studies23-25, the roll-offs of VOC and EL indicate that the non-radiative recombination is largely enhanced under forward-bias, which is likely to stem from the reduced effective built-in potential21, 26.

Table 1. The photovoltaic parameters of ITO/MoO3/TAPC:C70 (60 nm)/BPhen/Al devices and ITO/MoO3/C70 (0/4 nm)/TAPC:C70 (30 nm)/C70 (30 nm)/BPhen/Al devices with different donor concentration. donor concentration

structure

C70 [nm]

JSC [mA/cm2]

VOC [V]

FF

PCE

ΔVOC [V]

BHJ a)

0

12.81 ± 0.12

0.94

52.05 ± 1.20%

6.21 ± 0.08%

-

0

11.12 ± 0.24

0.94

64.85 ± 1.33%

6.76 ± 0.10%

4

12.02 ± 0.17

0.94

61.62 ± 1.14%

6.96 ± 0.04%

0

10.47 ± 0.37

0.85

53.60 ± 0.54%

4.77 ± 0.19%

0

10.05 ± 0.30

0.87

63.32 ± 0.77%

5.55 ± 0.21%

4

10.79 ± 0.09

0.92

65.98 ± 0.30%

6.55 ± 0.08%

0

7.43 ± 0.25

0.86

58.60 ± 0.41%

3.75 ± 0.08%

4

8.08 ± 0.10

0.94

58.28 ± 0.10%

4.42 ± 0.05%

5% BHJ-C70 b)

BHJ 25%

0.00

BHJ-C70

50%

-

0.05

BHJ-C70

0.08

a) For BHJ devices, the active-layer comprises a 60 nm-thick TAPC:C70 blend. b) For BHJ-C70 devices, the active-layer comprises a 30 nm-thick TAPC:C70 blend and 30 nm-thick pristine C70. All values are averaged over 4 pixels from 2 different campaigns. The errors of VOC are less than 0.01 V. Since the surface roughness of the MoO3 layer (~2.6 nm) underneath is non-negligible regarding the nominal thickness of the TAPC layer (0.5 nm to 2 nm), the increasing TAPC thickness is similar to a progressive coverage of TAPC at the MoO3/BHJ interface. Thus, the continuous drop of VOC implies a positive correlation between the TAPC coverage and the non-radiative recombination. Conversely, for a relatively uniform donor distribution in the


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fullerene network7, the MoO3/C70 contact area is larger in devices with lower TAPC concentration (Figure 3e), which would lead to a higher VOC. Therefore, we comparatively investigated the VOC of TAPC:C70 BHJ devices with and without a C70 layer inserted at the MoO3/BHJ interface. The donor concentrations are 5%, 25% and 50% respectively, and the thickness of C70 layer is 4 nm (For higher C70 thickness, see Figure S6 and Table S4 in the Supporting Information). As shown in Figure 3d and Table 1, the original VOC values are 0.86 V (50% TAPC), 0.87 V (25% TAPC) and 0.94 V (5% TAPC) respectively. After the insertion of C70, the VOC values of BHJ devices using higher donor concentration (25% and 50%) converge closely to 0.94 V, and the PCE of both devices are strikingly enhanced. This result clearly indicates the interfacial contact between MoO3 and C70 are crucial to the high quality charge collection in fullerene-based devices. Comparing with the original value of 4.77 ± 0.19%, the PCE of device with 25% TAPC concentration has been improved by 37.3% after the structural and interfacial optimizations.

To summarize, inspired by the high performance realized in the dilute donor-fullerene devices, two important issues on charge collection in BHJ OPV devices have been revealed. One is that the hole-carriers can transfer from donor to acceptor under the operating conditions of solar cells. Despite the detailed process is still under study, the presented results experimentally manifest the possibility of hole-extraction via acceptor phase in BHJ blends with type-II heterojunction interface for the first time. We also find that the field-dependency of this hole-transfer



mechanism would lead to a trade-off between charge-generation and charge-extraction in BHJ devices, which should be especially noticed in device structure optimization. The second one is the important role of MoO3/fullerene contact in the fullerene-based BHJ devices. The heterogeneity of surface modification has been long recognized in inorganic semiconductors27, however, the analogous effects haven’t been valued adequately in the field of OPV yet in contrast to the massive utilizations of the multi-component BHJ structures and the buffer layers. A distinct example is provided here that the surface composition along with the heterogeneous contact effects can make great influences on the device performance. Deeper understandings on this issue will be undoubtedly beneficial to the development of new buffer layers and the design of device structure for both fullerene-based and non-fullerene28 OPV devices.


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Acknowledgement

This work was supported by National Natural Science Foundation of China (Grant No. 11574049) and the Natural Science Foundation of Shanghai (No. 17ZR1402600). H. Y. is grateful to the financial support from the National Young 1000 Talents Plan, the National Key Research and Development Program of China (Grant No. 2016YFA0203900 and 2017YFA0303504). Part of the experimental work have been carried out in Fudan Nanofabrication Laboratory. R. Yi thanks J. J. Qin, Prof. Y. Q. Zhan and Prof. Y. Yao for useful discussions. W. Feng and R. Yi stress their gratitude to Prof. X. Y. Hou for his instruction.

Supporting Information. Experiment Methods; Comprehensive information of devices using the C70/donor/C70 structure; AFM data of ITO, MoO3 on ITO, C70 on MoO3/ITO and TAPC on C70/MoO3/ITO; Representatives of illuminated J-V characteristic and photovoltaic parameters of ITO/MoO3/TAPC:C70 (x nm, 5%)/C70 (y nm)/BPhen/Al devices; Photovoltaic parameters of ITO/MoO3/TAPC (x nm)/TAPC:C70 (60 nm, 5%)/BPhen/Al devices; J-V curves under illumination and photovoltaic parameters of ITO/MoO3/C70 (x nm)/TAPC:C70 (30 nm, 25%)/C70 (30 nm)/BPhen/Al devices; J-V curves of ITO/C70 (10 or 20 nm)/TAPC (2 nm)/C70 (40 nm)/BPhen/Al devices; J-V curves of C70/TAPC/C70 devices with 5 nm-thick TAPC.



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Highly Efficient Charge Collection in Bulk-Heterojunction Organic

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