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Realizing efficiency improvement of polymer solar cells by using multi-functional cascade electron transport layers
Organic Electronics 76 (2020) 105482
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Organic Electronics journal homepage: http://www.elsevier.com/locate/orgel
Realizing efficiency improvement of polymer solar cells by using multi-functional cascade electron transport layers Yu Sun a, b, Mei Wang a, b, Chunyu Liu a, b, **, Zhiqi Li a, b, Da Fu a, b, Wenbin Guo a, b, * a
State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, People’s Republic of China b College of Materials Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: In2O3/ZnO/PCBM cascade layers Lattice matching Electron transfer Light absorption Acceptor
Contact property between metal oxide electron transport layer (ETL) and active layer is one of the key factors to the performance of polymer solar cells (PSCs). To achieve better lattice matching and fewer defects, indium trioxide (In2O3) and zinc oxide (ZnO) (In2O3/ZnO) were used as the inorganic composite ETL, combining with an organic material PCBM as interface layer to obtain homogeneous phase separation of the active layer. The resulting device demonstrates a high fill factor of 69.83% and power conversion efficiency of 9.036% for PTB7: PC71BM based PSCs. Moreover, the acceptor material of PCBM serving as interface layer can interact with donor material of active layer to promote the exciton dissociation. This study provides a new method to improve the performance of PSCs by using multi-functional cascade electron transport layers.
1. Introduction Polymer solar cells (PSCs) have a broad development prospect because of their flexibility, ease processing, low cost, good compatibility and so on [1–4]. However, the performance of PSCs is still limited by many factors, such as deficient light absorption, exciton dissociation, and high carrier recombination rate [5–8]. So far, many important strategies have been used to obtain high device performance, such as improving the device morphology, incorporating additives, and adopt ing different device structure. However, many problems are still needed to solve between active layer and metal oxide electron transport layer (ETL), such as zinc oxide (ZnO), indium oxide (In2O3), and titanium oxide (TiO2) [5,9–12]. The existence of charge transport barrier be tween the inorganic metal oxide and the active layer will lead to the severe interface charge loss. More importantly, the surface of inorganic material is not conducive to the homogeneous phase separation of the active layer [13–16]. Therefore, it is necessary to explore and utilize the organic interface layer [17–24]. The composite interface layers have been frequently used in solar cells [25–29], especially composite elec tron transport layers [30–33]. This structure also improves the electron selectivity of electrode and ensure good ohmic contact between
electrode and active layer [34–44]. However, the intrinsic in compatibility between inorganic charge transport layer and organic active layer still need to be further improved. In this work, an In2O3/ZnO/PCBM cascade layer was introduced into PSCs. The incorporation of underlying interface layer of In2O3 can be used as seed layer to improve the lattice mismatch of ZnO, which can reduce the recombination loss caused by defects and increase the available charge in PSCs. Meanwhile, the PCBM layer was further incorporated to form more matched energy level alignment with active layer, and the contact characteristics can be enhanced. The existence of PCBM interlayer is beneficial to exciton dissociation and electron transfer between electrode and active layer, which improves the per formance of PSCs. As a result, an improved PCE of 9.036% was realized, including an increased short-circuit current density (Jsc) of 17.030 mA/ cm2 and open-circuit voltage (Voc) of 0.760 V. 2. Experiment section ZnO precursor solution was prepared by dissolving 110 mg zinc ac etate (Zn(Ac)2) in 1 mL 2-methoxy ethanol containing 3% ethanolamine. To fabricate In2O3 layer, indium nitrate hydrate (In(NO3)3 xH2O) was
* Corresponding author. State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, People’s Republic of China. ** Corresponding author. State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, People’s Republic of China. E-mail addresses: [email protected] (C. Liu), [email protected] (W. Guo). https://doi.org/10.1016/j.orgel.2019.105482 Received 10 August 2019; Received in revised form 1 October 2019; Accepted 1 October 2019 Available online 7 October 2019 1566-1199/© 2019 Elsevier B.V. All rights reserved.
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Organic Electronics 76 (2020) 105482
PCBM (0.5, 1, 2 mg/mL) ETLs are investigated and displayed in Fig. 2a, and corresponding photovoltaic parameters are listed in Table 1. All values are typically average of 32 devices. The Device A with ZnO ETL presents an inferior PCE of 7.284%. The Device D shows the maximum PCE of 9.036% with the improved Voc from 0.725 to 0.76 V and FF from 60.88% to 69.83%. The Jsc demonstrates a little enhancement from 16.497 to 17.03 mA/cm2. The improvement of device performance is mainly attributed to the reduced defects and the enhanced electron transport capacity, thus resulting in increased Jsc and FF. However, it can be found the performance of Device E with PCBM of 2 mg/mL begins to deteriorate, which attributes to the decreased conductivity of In2O3/ ZnO/PCBM cascade layer. The J-V curves of the devices with ZnO, In2O3/ZnO, and In2O3/ZnO/PCBM (0.5, 1, 2 mg/mL) in dark were shown in Fig. 2b. The devices with PCBM display smaller leakage cur rent at negative voltages, which is attributed to the reduced series resistance (Rs) and increased shunt resistance (Rsh). Moreover, the larger current in the positive region was got, revealing the facilitated charge transport and decreased contact resistance. Furthermore, incident photon to-current conversion efficiency (IPCE) spectra of PSCs were measured to demonstrate the increased Jsc. From Fig. 2c, the Device A shows the maximum IPCE value of 64% at 665 nm, while the Device D exhibits the maximum IPCE of 74% at the same wavelength. The enhancement of IPCE can be seen from 350 to 700 nm compared with Device A, suggesting that there are more photons in this range are converted to effective charge carriers. It may be due to the suppressed excitons recombination and lowered interface charge transport barrier, which facilitates interface charge transfer. In order to guarantee the enough light absorption of active layer, the underlying ETL needs to have high transmittance, which is the key factor to the inverted PSCs. Therefore, the transmission spectra of ITO/ZnO, ITO/In2O3/ZnO without and with different PCBM layer were measured and displayed in Fig. 3a. It can be seen that the incorporation of PCBM layer has a little influence on transmission spectra. As the thickness of interface layer increases, the transmission decreases naturally. And the absorption spectra of ZnO/PTB7:PC71BM, In2O3/ZnO/PTB7:PC71BM, In2O3/ZnO/PCBM (0.5, 1 and 2 mg/mL)/PTB7:PC71BM were described in Fig. 3b. It can be noted that the In2O3/ZnO/PTB7:PC71BM shows similar absorption spectra, which is comparable to ZnO/PTB7:PC71BM layer. At the same time, we can observe that absorption of ITO/In2O3/ ZnO/PCBM/PTB7:PC71BM is higher than ITO/ZnO/PTB7:PC71BM. This
Fig. 1. The structure of the fabricated device.
firstly dispersed into 2-methoxy ethanol with a concentration of 20 mg/ mL. PCBM in chlorobenzene with different concentrations of 0.5, 1 and 2 mg/mL were pre-prepared. The PSCs with the structure of ITO/In2O3/ZnO/PCBM/PTB7: PC71BM/MoO3/Ag was fabricated, and In2O3/ZnO/PCBM cascade ETL improved the electron extraction ability of interface layer. The precleaned ITO substrates were treated with UV-zone for 15 min. The In2O3 precursor solution was spin-coated at 6000 rpm, followed by an annealing treatment at 200 � C for 1 h and extra UV-Ozone treatment for 10 min. Then ZnO layer was deposited onto the prepared In2O3 layer by spin-coating the Zn(Ac)2 solution at 3000 rpm for 30s and postannealing at 200 � C for 1 h. The thickness of In2O3 and ZnO are about 5 nm and 35 nm. PCBM layers with different thicknesses were prepared onto ZnO layer by varying the solution concentration, then annealing at 70 � C for 20 min. The thickness of PCBM layer with different concen trations is 5–10 nm. Active layer of PTB7:PC71BM (1:1.5 by weight) was subsequently fabricated. Finally, MoO3 (10 nm) and Ag (100 nm) were evaporated successively with a low vacuum (5 � 10 4 Pa). The device based on single ZnO layer and In2O3/ZnO composite layers were named as device A and B. The In2O3/ZnO based devices with spin-coating 0.5, 1, and 2 mg/mL PCBM were named as Device C to E.
Table 1 Photovoltaic parameters of devices, including Voc, Jsc, FF, PCE, Rs and Rsh.
3. Results and discussion The structure diagram of fabricated PSCs is shown in Fig. 1. To prove the effect of cascade ETL on the device performance, current-voltage (JV) characteristic of devices based on ZnO, In2O3/ZnO and In2O3/ZnO/
Device
Voc (V)
Jsc (mA/ cm2)
FF (%)
PCE (%)
Rs (ohm)
Rsh (ohm)
A B C D E
0.725 0.726 0.749 0.760 0.744
16.497 16.882 16.826 17.030 16.987
66.88 65.76 65.81 69.83 67.81
7.284 8.055 8.296 9.036 8.567
114.45 103.24 88.00 81.98 86.81
4112.02 6653.24 7739.00 10153.81 8347.13
Fig. 2. J-V characteristics of all devices measured (a) under illumination and (b) in dark, and (c) IPCE spectra of control and optimized devices. 2
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Organic Electronics 76 (2020) 105482
Fig. 3. (a) Transmission spectra of ITO/ZnO, ITO/In2O3/ZnO, ITO/In2O3/ZnO/PCBM (0.5, 1, 2 mg/mL) and (b) absorption spectra of ITO/ZnO/PTB7:PC71BM, ITO/ In2O3/ZnO/PTB7:PC71BM, and ITO/In2O3/ZnO/PCBM (0.5, 1, 2 mg/mL)/PTB7:PC71BM.
Fig. 4. (a) XRD patterns for the ITO/ZnO, ITO/In2O3/ZnO and (b) PL spectra of ITO/ZnO, ITO/In2O3/ZnO, and ITO/In2O3/ZnO/PCBM (1 mg/mL).
result indicates that the incorporation of PCBM can increase the light absorption of device and allow more electrons to reach the cathode. There is no doubt that the electrical properties of ZnO are greatly affected by the presence of defects. Fig. 4a shows the XRD diagram of ITO/ZnO and ITO/In2O3/ZnO. In Fig. 4a, it can be seen that the char acteristic peak of ZnO for In2O3/ZnO is higher than that of pure ZnO layer. When In2O3 is pre-spinning onto the ITO substrate, it can act as a seed layer, making a better lattice orientation. To verify the effect of In2O3 and PCBM on defects, the photoluminescence (PL) measurements were conducted and displayed in Fig. 4b. The PL spectra of devices were measured with a 325 nm exciting source and a broad band emission peak was observed at about 560 nm, which corresponds to the excitation peak of oxygen vacancy defects formed by the deep energy level [45,46].
Oxygen vacancy can play as potential electron wells, which can capture one or two electrons for each vacancy, thus increasing the electron recombination probability [47]. However, it is an exciting phenomenon that after the insertion of In2O3 and PCBM layers, the PL spectra are gradually decreased, indicating that the defect states are minimized, which contributes to the improved interface electron extraction ability. To conclude, the cascade layers possess stronger capacity to extract
Fig. 5. Water contact angles of (a) ITO/ZnO, (b) ITO/In2O3/ZnO, (c) ITO/ In2O3/ZnO/PCBM (1 mg/mL), AFM images of (d) ITO/ZnO, (e) ITO/In2O3/ ZnO, and (f) ITO/In2O3/ZnO/PCBM (1 mg/mL).
Fig. 6. Electrochemical impedance spectroscopy (EIS) of ITO/ZnO, ITO/In2O3/ ZnO, and ITO/In2O3/ZnO/PCBM (0.5, 1, 2 mg/mL). 3
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Organic Electronics 76 (2020) 105482
Fig. 7. (a) TRTPL spectra of ZnO/PTB7:PC71BM, In2O3/ZnO/PTB7:PC71BM and In2O3/ZnO/PCBM/PTB7:PC71BM composite films, (b) the J-V characteristics of electron-only device, as well as the values of electron mobilities of devices.
electrons. Next, the water contact angle (WCA) was measured in order to further understand the contact characteristics between ETL and active layer. The wet ability of underlying surface will affect the film formation of organic active layer. As shown in Fig. 5, the WCA of bare ITO/ZnO, ITO/In2O3/ZnO and ITO/In2O3/ZnO/PCBM (1 mg/mL) are 35� , 52� and 60� , respectively, which unravels that after the PCBM layer is inserted, the hydrophobicity is enhanced and the active layer solution can grow better films. Therefore, electron transfer between the active layer and ETL can occur well, reducing the interface charge leakage. Meanwhile, we suspect that the surface roughness of the electron transport layer may have changed. So the atomic force microscope (AFM) was measured and shown in Fig. 5d, e and f. It can be seen that the ZnO film shows ho mogeneous nanoparticles and lower roughness. While for the ITO/ In2O3/ZnO and ITO/In2O3/ZnO/PCBM (1 mg/mL) cascade layers, the roughness are gradually reduced. The performance of PSCs is closely related to the roughness of ETL in inverted solar cells and the lower roughness is beneficial for the film formation of active layer, which will lead to a reduced Rs and improved FF [48–52]. The electrochemical impedance spectroscopy (EIS) of devices are demonstrated in Fig. 6. The impedance spectra were determined in a frequency range from 20 Hz to 1 MHz. We can see the shapes of the impedance spectra are the ideal semicircle in PSCs. The diameter of the semicircle depends on the resistance in impedance spectroscopy. Seen from Fig. 6, the diameters of the semicircles decrease obviously from ZnO based device to In2O3/ZnO/PCBM based device. The introduction of PCBM interlayer provides an ideal interface for the deposition of active layer. So it reduces the barrier to electron extraction and thus results in further lessened resistance. To deeply demonstrate the much easier electron transfer process from active layer to ZnO, corresponding time-resolved transient pho toluminescence (TRTPL) spectra were measured. As seen from Fig. 7a, with the incorporation of the In2O3 and PCBM interfacial layers, the emission decay time is slightly decreased from 1.23 to 0.93 ns, sug gesting that electron transport from active layer to ZnO was improved, which also promotes the exciton dissociation, passivates the interface and decreases the bimolecular recombination. In order to further verify the effect of composite electron transport layer on the extraction and transport of electrons, electron-only devices with the structure of ITO/ ZnO/PTB7:PC71BM/BCP/Ag, ITO/In2O3/ZnO/PTB7:PC71BM/BCP/Ag and ITO/In2O3/ZnO/PCBM/PTB7:PC71BM/BCP/Ag were manufac tured, and the J-V characteristics are shown in Fig. 7b, as well as their electron mobility (the table in Fig. 7b). Compared with 4.37 � 10 4 cm2/(V⋅S) of the control device, the electron mobilities of devices with
In2O3/ZnO layer and with In2O3/ZnO/PCBM layers increased to 8.53 � 10 4 cm2/(V⋅S) and 1.01 � 10 3 cm2/(V⋅S), respectively. It is proved that the device with composite ETL is beneficial to promote electron transport and improve the performance of the device. 4. Conclusions In conclusion, it has demonstrated the improved efficiency of PSCs based on In2O3/ZnO/PCBM. The incorporation of In2O3 and PCBM can significantly increase the Voc and PCE. The results above show that the defect states of the optimized devices are significantly reduced. Due to the construction of In2O3/ZnO/PCBM structure, the interface barrier is reduced, which makes electron transfer more convenient. At the same time, the increase of Voc can be attributed to the decreased reverse saturation current, facilitated exciton dissociation, and the decreased charge loss. These methods are of great significance for reasonably decreasing the defects of PSCs. Acknowledgements The authors are grateful to National Natural Science Foundation of China (61875072), the Special Project of the Province-University Coconstructing Program of Jilin Province (SXGJXX2017-3), the Science and Technology Innovation Leading Talent and Team Project of Jilin Province (20170519010JH), the National Postdoctoral Program for Innovative Talents (BX20190135), and International Cooperation and Exchange Project of Jilin Province (20170414002GH, 20180414001GH) for the support to the work. References [1] C.J. Brabec, S. Gowrisanker, J.J.M. Halls, D. Laird, S. Jia, S.P. Williams, PolymerFullerene bulk-heterojunction solar cells, Adv. Mater. 22 (2010) 3839–3856. [2] C.W. Tang, Two-layer organic photovoltaic cell, Appl. Phys. Lett. 48 (1986) 183–185. [3] G. Li, V. Shrotriya, J.S. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Highefficiency solution processable polymer photo voltaic cells by self-organization of polymer blends, Nat. Mater. 4 (2005) 864–868. [4] X.C. Li, F.X. Xie, S.Q. Zhang, J.H. Hou, W.C. Choy, MoOx and V2Ox as hole and electron transport layers through functionalized intercalation in normal and inverted organic optoelectronic devices, Light Sci. Appl. 4 (2015) e273. [5] K.S. Lee, J.A. Lee, B.A. Mazor, S.R. Forst, Transforming the cost of solar to electrical energy conversion: integrating thin-film GaAs solar cells with non-tracking miniconcentrators, Light Sci. Appl. 4 (2015) e288. [6] C. Edwards, A. Arbabi, G. Popescu, L.L. Goddard, Optically monitoring and controlling nanoscale topography during semiconductor etching, Light Sci. Appl. 1 (2012) e30.
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Organic Electronics journal homepage: http://www.elsevier.com/locate/orgel
Realizing efficiency improvement of polymer solar cells by using multi-functional cascade electron transport layers Yu Sun a, b, Mei Wang a, b, Chunyu Liu a, b, **, Zhiqi Li a, b, Da Fu a, b, Wenbin Guo a, b, * a
State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, People’s Republic of China b College of Materials Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: In2O3/ZnO/PCBM cascade layers Lattice matching Electron transfer Light absorption Acceptor
Contact property between metal oxide electron transport layer (ETL) and active layer is one of the key factors to the performance of polymer solar cells (PSCs). To achieve better lattice matching and fewer defects, indium trioxide (In2O3) and zinc oxide (ZnO) (In2O3/ZnO) were used as the inorganic composite ETL, combining with an organic material PCBM as interface layer to obtain homogeneous phase separation of the active layer. The resulting device demonstrates a high fill factor of 69.83% and power conversion efficiency of 9.036% for PTB7: PC71BM based PSCs. Moreover, the acceptor material of PCBM serving as interface layer can interact with donor material of active layer to promote the exciton dissociation. This study provides a new method to improve the performance of PSCs by using multi-functional cascade electron transport layers.
1. Introduction Polymer solar cells (PSCs) have a broad development prospect because of their flexibility, ease processing, low cost, good compatibility and so on [1–4]. However, the performance of PSCs is still limited by many factors, such as deficient light absorption, exciton dissociation, and high carrier recombination rate [5–8]. So far, many important strategies have been used to obtain high device performance, such as improving the device morphology, incorporating additives, and adopt ing different device structure. However, many problems are still needed to solve between active layer and metal oxide electron transport layer (ETL), such as zinc oxide (ZnO), indium oxide (In2O3), and titanium oxide (TiO2) [5,9–12]. The existence of charge transport barrier be tween the inorganic metal oxide and the active layer will lead to the severe interface charge loss. More importantly, the surface of inorganic material is not conducive to the homogeneous phase separation of the active layer [13–16]. Therefore, it is necessary to explore and utilize the organic interface layer [17–24]. The composite interface layers have been frequently used in solar cells [25–29], especially composite elec tron transport layers [30–33]. This structure also improves the electron selectivity of electrode and ensure good ohmic contact between
electrode and active layer [34–44]. However, the intrinsic in compatibility between inorganic charge transport layer and organic active layer still need to be further improved. In this work, an In2O3/ZnO/PCBM cascade layer was introduced into PSCs. The incorporation of underlying interface layer of In2O3 can be used as seed layer to improve the lattice mismatch of ZnO, which can reduce the recombination loss caused by defects and increase the available charge in PSCs. Meanwhile, the PCBM layer was further incorporated to form more matched energy level alignment with active layer, and the contact characteristics can be enhanced. The existence of PCBM interlayer is beneficial to exciton dissociation and electron transfer between electrode and active layer, which improves the per formance of PSCs. As a result, an improved PCE of 9.036% was realized, including an increased short-circuit current density (Jsc) of 17.030 mA/ cm2 and open-circuit voltage (Voc) of 0.760 V. 2. Experiment section ZnO precursor solution was prepared by dissolving 110 mg zinc ac etate (Zn(Ac)2) in 1 mL 2-methoxy ethanol containing 3% ethanolamine. To fabricate In2O3 layer, indium nitrate hydrate (In(NO3)3 xH2O) was
* Corresponding author. State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, People’s Republic of China. ** Corresponding author. State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, People’s Republic of China. E-mail addresses: [email protected] (C. Liu), [email protected] (W. Guo). https://doi.org/10.1016/j.orgel.2019.105482 Received 10 August 2019; Received in revised form 1 October 2019; Accepted 1 October 2019 Available online 7 October 2019 1566-1199/© 2019 Elsevier B.V. All rights reserved.
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PCBM (0.5, 1, 2 mg/mL) ETLs are investigated and displayed in Fig. 2a, and corresponding photovoltaic parameters are listed in Table 1. All values are typically average of 32 devices. The Device A with ZnO ETL presents an inferior PCE of 7.284%. The Device D shows the maximum PCE of 9.036% with the improved Voc from 0.725 to 0.76 V and FF from 60.88% to 69.83%. The Jsc demonstrates a little enhancement from 16.497 to 17.03 mA/cm2. The improvement of device performance is mainly attributed to the reduced defects and the enhanced electron transport capacity, thus resulting in increased Jsc and FF. However, it can be found the performance of Device E with PCBM of 2 mg/mL begins to deteriorate, which attributes to the decreased conductivity of In2O3/ ZnO/PCBM cascade layer. The J-V curves of the devices with ZnO, In2O3/ZnO, and In2O3/ZnO/PCBM (0.5, 1, 2 mg/mL) in dark were shown in Fig. 2b. The devices with PCBM display smaller leakage cur rent at negative voltages, which is attributed to the reduced series resistance (Rs) and increased shunt resistance (Rsh). Moreover, the larger current in the positive region was got, revealing the facilitated charge transport and decreased contact resistance. Furthermore, incident photon to-current conversion efficiency (IPCE) spectra of PSCs were measured to demonstrate the increased Jsc. From Fig. 2c, the Device A shows the maximum IPCE value of 64% at 665 nm, while the Device D exhibits the maximum IPCE of 74% at the same wavelength. The enhancement of IPCE can be seen from 350 to 700 nm compared with Device A, suggesting that there are more photons in this range are converted to effective charge carriers. It may be due to the suppressed excitons recombination and lowered interface charge transport barrier, which facilitates interface charge transfer. In order to guarantee the enough light absorption of active layer, the underlying ETL needs to have high transmittance, which is the key factor to the inverted PSCs. Therefore, the transmission spectra of ITO/ZnO, ITO/In2O3/ZnO without and with different PCBM layer were measured and displayed in Fig. 3a. It can be seen that the incorporation of PCBM layer has a little influence on transmission spectra. As the thickness of interface layer increases, the transmission decreases naturally. And the absorption spectra of ZnO/PTB7:PC71BM, In2O3/ZnO/PTB7:PC71BM, In2O3/ZnO/PCBM (0.5, 1 and 2 mg/mL)/PTB7:PC71BM were described in Fig. 3b. It can be noted that the In2O3/ZnO/PTB7:PC71BM shows similar absorption spectra, which is comparable to ZnO/PTB7:PC71BM layer. At the same time, we can observe that absorption of ITO/In2O3/ ZnO/PCBM/PTB7:PC71BM is higher than ITO/ZnO/PTB7:PC71BM. This
Fig. 1. The structure of the fabricated device.
firstly dispersed into 2-methoxy ethanol with a concentration of 20 mg/ mL. PCBM in chlorobenzene with different concentrations of 0.5, 1 and 2 mg/mL were pre-prepared. The PSCs with the structure of ITO/In2O3/ZnO/PCBM/PTB7: PC71BM/MoO3/Ag was fabricated, and In2O3/ZnO/PCBM cascade ETL improved the electron extraction ability of interface layer. The precleaned ITO substrates were treated with UV-zone for 15 min. The In2O3 precursor solution was spin-coated at 6000 rpm, followed by an annealing treatment at 200 � C for 1 h and extra UV-Ozone treatment for 10 min. Then ZnO layer was deposited onto the prepared In2O3 layer by spin-coating the Zn(Ac)2 solution at 3000 rpm for 30s and postannealing at 200 � C for 1 h. The thickness of In2O3 and ZnO are about 5 nm and 35 nm. PCBM layers with different thicknesses were prepared onto ZnO layer by varying the solution concentration, then annealing at 70 � C for 20 min. The thickness of PCBM layer with different concen trations is 5–10 nm. Active layer of PTB7:PC71BM (1:1.5 by weight) was subsequently fabricated. Finally, MoO3 (10 nm) and Ag (100 nm) were evaporated successively with a low vacuum (5 � 10 4 Pa). The device based on single ZnO layer and In2O3/ZnO composite layers were named as device A and B. The In2O3/ZnO based devices with spin-coating 0.5, 1, and 2 mg/mL PCBM were named as Device C to E.
Table 1 Photovoltaic parameters of devices, including Voc, Jsc, FF, PCE, Rs and Rsh.
3. Results and discussion The structure diagram of fabricated PSCs is shown in Fig. 1. To prove the effect of cascade ETL on the device performance, current-voltage (JV) characteristic of devices based on ZnO, In2O3/ZnO and In2O3/ZnO/
Device
Voc (V)
Jsc (mA/ cm2)
FF (%)
PCE (%)
Rs (ohm)
Rsh (ohm)
A B C D E
0.725 0.726 0.749 0.760 0.744
16.497 16.882 16.826 17.030 16.987
66.88 65.76 65.81 69.83 67.81
7.284 8.055 8.296 9.036 8.567
114.45 103.24 88.00 81.98 86.81
4112.02 6653.24 7739.00 10153.81 8347.13
Fig. 2. J-V characteristics of all devices measured (a) under illumination and (b) in dark, and (c) IPCE spectra of control and optimized devices. 2
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Fig. 3. (a) Transmission spectra of ITO/ZnO, ITO/In2O3/ZnO, ITO/In2O3/ZnO/PCBM (0.5, 1, 2 mg/mL) and (b) absorption spectra of ITO/ZnO/PTB7:PC71BM, ITO/ In2O3/ZnO/PTB7:PC71BM, and ITO/In2O3/ZnO/PCBM (0.5, 1, 2 mg/mL)/PTB7:PC71BM.
Fig. 4. (a) XRD patterns for the ITO/ZnO, ITO/In2O3/ZnO and (b) PL spectra of ITO/ZnO, ITO/In2O3/ZnO, and ITO/In2O3/ZnO/PCBM (1 mg/mL).
result indicates that the incorporation of PCBM can increase the light absorption of device and allow more electrons to reach the cathode. There is no doubt that the electrical properties of ZnO are greatly affected by the presence of defects. Fig. 4a shows the XRD diagram of ITO/ZnO and ITO/In2O3/ZnO. In Fig. 4a, it can be seen that the char acteristic peak of ZnO for In2O3/ZnO is higher than that of pure ZnO layer. When In2O3 is pre-spinning onto the ITO substrate, it can act as a seed layer, making a better lattice orientation. To verify the effect of In2O3 and PCBM on defects, the photoluminescence (PL) measurements were conducted and displayed in Fig. 4b. The PL spectra of devices were measured with a 325 nm exciting source and a broad band emission peak was observed at about 560 nm, which corresponds to the excitation peak of oxygen vacancy defects formed by the deep energy level [45,46].
Oxygen vacancy can play as potential electron wells, which can capture one or two electrons for each vacancy, thus increasing the electron recombination probability [47]. However, it is an exciting phenomenon that after the insertion of In2O3 and PCBM layers, the PL spectra are gradually decreased, indicating that the defect states are minimized, which contributes to the improved interface electron extraction ability. To conclude, the cascade layers possess stronger capacity to extract
Fig. 5. Water contact angles of (a) ITO/ZnO, (b) ITO/In2O3/ZnO, (c) ITO/ In2O3/ZnO/PCBM (1 mg/mL), AFM images of (d) ITO/ZnO, (e) ITO/In2O3/ ZnO, and (f) ITO/In2O3/ZnO/PCBM (1 mg/mL).
Fig. 6. Electrochemical impedance spectroscopy (EIS) of ITO/ZnO, ITO/In2O3/ ZnO, and ITO/In2O3/ZnO/PCBM (0.5, 1, 2 mg/mL). 3
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Fig. 7. (a) TRTPL spectra of ZnO/PTB7:PC71BM, In2O3/ZnO/PTB7:PC71BM and In2O3/ZnO/PCBM/PTB7:PC71BM composite films, (b) the J-V characteristics of electron-only device, as well as the values of electron mobilities of devices.
electrons. Next, the water contact angle (WCA) was measured in order to further understand the contact characteristics between ETL and active layer. The wet ability of underlying surface will affect the film formation of organic active layer. As shown in Fig. 5, the WCA of bare ITO/ZnO, ITO/In2O3/ZnO and ITO/In2O3/ZnO/PCBM (1 mg/mL) are 35� , 52� and 60� , respectively, which unravels that after the PCBM layer is inserted, the hydrophobicity is enhanced and the active layer solution can grow better films. Therefore, electron transfer between the active layer and ETL can occur well, reducing the interface charge leakage. Meanwhile, we suspect that the surface roughness of the electron transport layer may have changed. So the atomic force microscope (AFM) was measured and shown in Fig. 5d, e and f. It can be seen that the ZnO film shows ho mogeneous nanoparticles and lower roughness. While for the ITO/ In2O3/ZnO and ITO/In2O3/ZnO/PCBM (1 mg/mL) cascade layers, the roughness are gradually reduced. The performance of PSCs is closely related to the roughness of ETL in inverted solar cells and the lower roughness is beneficial for the film formation of active layer, which will lead to a reduced Rs and improved FF [48–52]. The electrochemical impedance spectroscopy (EIS) of devices are demonstrated in Fig. 6. The impedance spectra were determined in a frequency range from 20 Hz to 1 MHz. We can see the shapes of the impedance spectra are the ideal semicircle in PSCs. The diameter of the semicircle depends on the resistance in impedance spectroscopy. Seen from Fig. 6, the diameters of the semicircles decrease obviously from ZnO based device to In2O3/ZnO/PCBM based device. The introduction of PCBM interlayer provides an ideal interface for the deposition of active layer. So it reduces the barrier to electron extraction and thus results in further lessened resistance. To deeply demonstrate the much easier electron transfer process from active layer to ZnO, corresponding time-resolved transient pho toluminescence (TRTPL) spectra were measured. As seen from Fig. 7a, with the incorporation of the In2O3 and PCBM interfacial layers, the emission decay time is slightly decreased from 1.23 to 0.93 ns, sug gesting that electron transport from active layer to ZnO was improved, which also promotes the exciton dissociation, passivates the interface and decreases the bimolecular recombination. In order to further verify the effect of composite electron transport layer on the extraction and transport of electrons, electron-only devices with the structure of ITO/ ZnO/PTB7:PC71BM/BCP/Ag, ITO/In2O3/ZnO/PTB7:PC71BM/BCP/Ag and ITO/In2O3/ZnO/PCBM/PTB7:PC71BM/BCP/Ag were manufac tured, and the J-V characteristics are shown in Fig. 7b, as well as their electron mobility (the table in Fig. 7b). Compared with 4.37 � 10 4 cm2/(V⋅S) of the control device, the electron mobilities of devices with
In2O3/ZnO layer and with In2O3/ZnO/PCBM layers increased to 8.53 � 10 4 cm2/(V⋅S) and 1.01 � 10 3 cm2/(V⋅S), respectively. It is proved that the device with composite ETL is beneficial to promote electron transport and improve the performance of the device. 4. Conclusions In conclusion, it has demonstrated the improved efficiency of PSCs based on In2O3/ZnO/PCBM. The incorporation of In2O3 and PCBM can significantly increase the Voc and PCE. The results above show that the defect states of the optimized devices are significantly reduced. Due to the construction of In2O3/ZnO/PCBM structure, the interface barrier is reduced, which makes electron transfer more convenient. At the same time, the increase of Voc can be attributed to the decreased reverse saturation current, facilitated exciton dissociation, and the decreased charge loss. These methods are of great significance for reasonably decreasing the defects of PSCs. Acknowledgements The authors are grateful to National Natural Science Foundation of China (61875072), the Special Project of the Province-University Coconstructing Program of Jilin Province (SXGJXX2017-3), the Science and Technology Innovation Leading Talent and Team Project of Jilin Province (20170519010JH), the National Postdoctoral Program for Innovative Talents (BX20190135), and International Cooperation and Exchange Project of Jilin Province (20170414002GH, 20180414001GH) for the support to the work. References [1] C.J. Brabec, S. Gowrisanker, J.J.M. Halls, D. Laird, S. Jia, S.P. Williams, PolymerFullerene bulk-heterojunction solar cells, Adv. Mater. 22 (2010) 3839–3856. [2] C.W. Tang, Two-layer organic photovoltaic cell, Appl. Phys. Lett. 48 (1986) 183–185. [3] G. Li, V. Shrotriya, J.S. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Highefficiency solution processable polymer photo voltaic cells by self-organization of polymer blends, Nat. Mater. 4 (2005) 864–868. [4] X.C. Li, F.X. Xie, S.Q. Zhang, J.H. Hou, W.C. Choy, MoOx and V2Ox as hole and electron transport layers through functionalized intercalation in normal and inverted organic optoelectronic devices, Light Sci. Appl. 4 (2015) e273. [5] K.S. Lee, J.A. Lee, B.A. Mazor, S.R. Forst, Transforming the cost of solar to electrical energy conversion: integrating thin-film GaAs solar cells with non-tracking miniconcentrators, Light Sci. Appl. 4 (2015) e288. [6] C. Edwards, A. Arbabi, G. Popescu, L.L. Goddard, Optically monitoring and controlling nanoscale topography during semiconductor etching, Light Sci. Appl. 1 (2012) e30.
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