Decreased Charge Transport Barrier and Recombination of Organic

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Decreased Charge Transport Barrier and Recombination of Organic Solar Cells by Constructing Interfacial Nanojunction with Annealing-Free ZnO and Al Layers Chunyu Liu, Dezhong Zhang, Zhiqi Li, Xinyuan Zhang, Wenbin Guo, Liu Zhang, Shengping Ruan, and Yongbing Long ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06235 • Publication Date (Web): 13 Jun 2017 Downloaded from http://pubs.acs.org on June 14, 2017

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ACS Applied Materials & Interfaces

Decreased Charge Transport Barrier and Recombination of Organic Solar Cells by Constructing Interfacial Nanojunction with Annealing-Free ZnO and Al Layers Chunyu Liu1, Dezhong Zhang1, Zhiqi Li1, Xinyuan Zhang1, Wenbin Guo1*, Liu Zhang2*, Shengping Ruan1, and Yongbing Long3 1

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering , Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China 2

College of Instrumentation & Electrical Engineering, Jilin University, 938 Ximinzhu Street, Changchun 130061, People’s Republic of China

3

School of Electronic Engineering, South China Agricultural University, Guangzhou, 510642, China

ABSTRACT To overcome drawbacks of electron transport layer, such as complex surface defects and unmatched energy levels, a smart semiconductor-metal interfacial nanojunciton was successfully employed in organic solar cells by evaporating an ultrathin Al interlayer onto annealing-free ZnO electron transport layer, resulting in a high fill factor of 73.68% and power conversion efficiency of 9.81%. The construction of ZnO-Al nanojunction could effectively fill the surface defects of ZnO and reduce its work function due to the electron transfer from Al to ZnO by Fermi level equilibrium. The filling of surface defects decreased the interfacial carrier recombination in mid-gap trap states. The reduced surface work function of ZnO-Al re-modulated the interfacial characteristics between ZnO and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM), decreasing or even eliminating the interfacial barrier against the electron transport, which is beneficial to improve the electron extraction capacity. The filled surface defects and reduced interfacial barrier were realistically observed by photoluminescence measurements of ZnO film and the performance of electron injection devices, respectively. This work provides a simple and effective method to simultaneously solve the problems of surface defects and unmatched energy level for the annealing-free ZnO or other metal oxide semiconductors, paving a way for the future popularization in photovoltaic devices. KEYWORDS: Interface Barrier, Carrier Recombination, Filled Surface Defects, Reduced Work Function,

ZnO-Al Nanojunction

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1. INTRODUCTION In organic solar cells (OSCs), even if the efficient charge separation only occurs at the donor/acceptor (D/A) interfaces, the quantum efficiency is still limited by the carrier extraction and collection efficiency. Thus, the charge transport layer acting as the bridge between the active layer and electrode, reducing the barrier against charge transport, has attracted much attention in the research process of OSCs, including electron transport layer (ETL) and hole transport layer (HTL).1-4 The suitable and outstanding transport materials play important roles on achieving high quantum efficiency and device performance.5-7 However, it is still problematic sometimes for some common transport materials, such as the mismatched energy level alignment with photovoltaic materials and poor contact of adjacent layers,8-10 leading to the lower carrier extraction efficiency from organic layer, which intensifies the free-carrier recombination at the D/A interfaces. At this moment, the subtle interface properties play a crucial role for the charge extraction, therefore these urgently needed advances of device performance encourage the study of interfacial layer between transport layer and organic layer to become a subject of intense research in recent years.11-13 It is generally known that ZnO with high transmittance and electron mobility has been regarded as a very attractive material for electron transport in OSCs.14,15 However, some drawbacks caused by intrinsic quality have to be overcome in the practical application, such as some structural defects (Zn/oxygen vacancies) which hinder local charge transfer at ZnO/organic acceptor,16-18 and high work function (WF) that is unsuitable to completely eliminate electron transport barrier at the interface with acceptor.19 The C60-SAM have been employed to passivate interfacial defects and fill surface trap states of N-type metal-oxides, thus decreasing the charge recombination at the interfaces and promoting the electron transfer.20,21 Meanwhile, some excellent functional materials such as polymer electrolyte22,23 and cross-linked fullerenes24,25 with the properties of reducing WF or building energy gradient intermediate have been applied to modify electron transport layer and further modulate the interface characteristics, facilitating the electron transport and extraction. These interlayers incorporated in the photovoltaic devices carefully modify the interfacial characteristics, undertaking the responsibility for the promotion of charge extraction and transfer basing on different mechanisms. But it is difficult to simultaneously solve various drawbacks of ETL using the single-material, meanwhile the synthesis of these functional materials may suffer from long period and cost a lot, setting up obstacles to the development of high performance devices. Perhaps, a simple approach can be adopted to achieve the same purposes, only inserting an ultrathin metal interlayer that just demands the facile experimental technique of thermal evaporation and low expense.26-28 In the present work, an ultrathin Al film was thermally evaporated onto annealing-free ZnO electron 2


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transport layer, ingeniously forming a semiconductor-metal nanojunction and demonstrating extraordinary capacity for facilitating electron extraction and transport, which contributes to the enhanced short-circuit current density (Jsc) and fill factor (FF). Similar metal-semiconductor contact nanojunctions have been reported to improve the photoelectrochemical performance or photocatalytic activity of semiconductor materials by inducing electron transfer and Fermi level (Ef) shift.29-32 When the ZnO comes into contact with Al interlayer, the electrons will be transferred to ZnO by Ef

equilibration, mainly due to higher Ef of Al. There is no doubt that the Ef of

ZnO will be driven upwards which means that the WF is reduced. Importantly, the transferred electrons from Al could effectively fill the ZnO surface defects, decreasing the interfacial carrier recombination. Furthermore, though ZnO demonstrates the lower lowest unoccupied molecular orbital (LUMO) than that of [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) acceptor, the interfacial energy barrier against electron transport still exists between them due to the lower WF of PC71BM, weakening the electron extraction capacity. The ZnO-Al nanojunction with lowered WF could reduce or even eliminate the interfacial barrier, significantly enhancing the electron extraction capacity of ZnO from PC71BM, which will be deeply investigated in our subsequent study. Strikingly, ZnO without annealing used here has no effect on the resistance and WF of ITO electrode. High annealing and sintering temperature would unfortunately increase the resistance of ITO, which is disadvantage for the achievement of high FF. Herein, the device with annealing-free ZnO-Al nanojunction obtains a boosted efficiency of 9.81% and a higher FF of 73.68% in the study of OSCs.

2. RESULTS AND DISCUSSION The synthesis of annealing-free ZnO was described in Supporting Information. In truth, the thermal annealing can decrease numerous defects of ZnO,33 while the higher sintering temperature will weaken the conductivity of ITO substrate. The square resistance of ITO suffering from some common annealing temperature (80, 150, 280, and 400 °C) are measured and shown in Figure S1 (Supporting Information). The resistance gradually increases from 16.3 to 66.8 Ω/sq with the annealing temperature rising from 20 to 400 °C, especially demonstrating large increase range, which loses the possibility to attain higher FF and performance for OSCs. Thus it is extremely necessary to employ the annealing-free ZnO.

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Figure 1. The device structure diagram of OSCs, insert is the Ef equilibration process between ZnO and Al.

Figure 2. The AFM phase images of (a) bare ZnO film and covered with (b) 0.5 nm, (c) 1.0 nm, (d) 1.5 nm, (e) 2.0 nm and (f) 2.5 nm Al interlayer.

The Figure 1 is the device structure diagram, an ultrathin Al interlayer was thermally evaporated onto ZnO electron transport layer, forming functional ZnO-Al nanojunction. The device with pristine ZnO ETL was named as control device, meanwhile the devices with ZnO-Al nanojuntion with different thickness of Al (0.5, 1.0, 1.5, 2.0, and 2.5 nm) were denoted as ZnO-Al0.5, ZnO-Al1.0, ZnO-Al1.5, ZnO-Al2.0, and ZnO-Al2.5 respectively. The device with annealed ZnO at 150 °C was also fabricated to prove the heat stability of ZnO film. The detailed fabrication processes of complete devices and devices characterization are provided in Supporting Information. The inset is the Ef equilibration process when ZnO contacts with Al, displaying that electrons were transferred from Al to ZnO due to the lower WF of Al. The surface morphologies of ZnO and ZnO-Al were displayed by AFM height and phase images in Figure S2 (Supporting Information) and Figure 2. Seen from the height images of 1 µm×1 µm in Figure S2, ZnO consists of homogeneous nanoparticles (NPs), but the evaporated Al can’t be 4


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clearly distinguished, which may be due to the sparse distribution or ultrathin thickness of Al interlayer. It is worth noting from the phase images with lager scan area that an additional phase starts to appear from the Figure 2b, and it demonstrates gradually dense distribution with the increased thickness of Al. Therefore, it is reasonable to deduce that the emerging phase should be Al, presenting the uniform distribution on ZnO surface.

Figure 3. (a) J-V characteristics curves and (b) EQE spectra of devices with bare ZnO and different ZnO-Al nanojunctions.

Table 1. Photovoltaic Parameters of Control Device and Optimized Devices with Different ZnO-Al Nanojunctions, Including Voc, Jsc, FF, and PCE. Device

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

control device

0.738±0.001

16.14±0.11

66.22±0.18

7.89±0.09

ZnO-Al0.5

0.739±0.001

16.56±0.12

69.67±0. 17

8.53±0.09

ZnO-Al1.0

0.741±0.002

17.39±0.10

72.12±0.20

9.29±0.10

ZnO-Al1.5

0.744±0.002

17.89±0.12

73.68±0.22

9.81±0.11

ZnO-Al2.0

0.743±0.002

17.50±0.13

71.17±0.16

9.25±0.11

ZnO-Al2.5

0.742±0.001

16.89±0.10

71.19±0.17

8.92±0.09

The photovoltaic parameters are the average values from thirty identical devices for each type.

Figure 3a plots the current density-voltage (J-V) characteristics of all annealing-free devices with bare ZnO and with different ZnO-Al nanojunctions. All the related photovoltaic parameters have been summarized in Table 1, including open-circuit voltage (Voc), Jsc, FF and power conversion efficiency (PCE). As clearly shown, the control device submitted an unsatisfactory PCE of 7.89%. While the champion performance of 9.81% for device with ZnO-Al1.5 nanojunction was realized, obtaining a Voc of 0.744 V, a Jsc of 17.89 mA/cm2, and a FF of 73.68%. The improved performance mainly attributes to the decreased interfacial recombination because of the filling of surface defects by transferred electrons from Al and improved electron extraction capacity of ZnO-Al due to the reduced WF, contributing to increased Jsc and FF. When the thickness of Al is less than 1.5 nm, the distribution of Al is relatively sparse, lacking of ability to fill surface defects or reduce WF. While the thickness is over than 1.5 nm, the light transmittance will be a problem deserving serious consideration that weakens the 5



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device performance. At the same time, the J-V curve of device with annealing ZnO at 150 °C is shown in Figure S3 (Supporting Information), which displays almost the same performance with annealing-free device, suggesting that the ZnO film possesses high quality and heat stability. The transmittance spectra of ITO/ZnO and ITO/ZnO-Al have been measured and shown in Figure S4 (Supporting Information). Therefore, the performance of these devices is lower than optimal device with ZnO-Al1.5 nanojunction. That being said, the device with ZnO-Al nanojunction of optimal Al thickness is the balance between facilitated electron transport and decreased light transmittance within a certain range in this study. The authentic working mechanism of ZnO-Al nanojunction will be given below in detail, which was carried out mainly around control device and devices with ZnO-Al0.5, ZnO-Al1.5 and ZnO-Al2.5. Meanwhile, the external quantum efficiency (EQE) was measured to further understand the photocurrent enhancement, displayed in Figure 3b. There are entire improvements ranging from 350 nm to 700 nm, achieving a maximum EQE value of 79.5% compared with a lower value of 69.2% for control device, which suggests the enhanced photons utilization. The mainly reasons for this enhancements can be ascribed to the facilitated electron extraction and transport capacity due to the structure of ZnO-Al nanojunciton, leading to the increase of Jsc.

Figure 4. (a) The PL spectra of ZnO and ZnO-Al films with an exciting light wavelength of 350 nm, (b) the TRTPL spectra of ITO/ZnO and ITO/ZnO-Al, and (c) the time response curves of devices with ZnO and ZnO-Al nanojunction.

Unquestionably, the presence of defects has a significant impact on the electronic properties of ZnO, which could be detailedly detected and analyzed by the photoluminescence (PL) spectra.34,35 The PL spectra of ZnO and ZnO-Al were measured with a 350 nm exciting source and displayed in Figure 4a. A broad band emission with the peaks at ca. 525 nm are observed, which can be indexed to the excitation peak of oxygen vacancies due to the formation of deep levels in the bandgap.36,37 Oxygen vacancies is potential wells that could trap one or two electrons for each vacancy, increasing the recombination probability of transported electrons.38 However, an exciting phenomenon that the insertion of thin Al interlayer remarkably quenches the PL of oxygen vacancies is captured, which can be attributed to transferred electrons from Al interlayer. Even if the thickness of Al 6


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interlayer is only 0.5 nm, the PL emission is still quenched by about seventy percent, cutting down the electrons loss from defect states trapping on the surface of ZnO. In addition, to explore the effect of Al interlayer on bandgap emission of ZnO in ultraviolet region, the PL spectra of ZnO and ZnO-Al were carried out under 230 nm light excitation and shown in Figure S5 (Supporting Information). There are no obvious changes for bandgap emissions (peaks locate at ca. 370 nm) of ZnO by the modification of Al interlayer, though the PL emissions of oxygen vacancies were significantly quenched. This may be mainly due to the fact that the defect emission is much weaker relative to the bandgap emission. Figure 4b is the time-resolved transient photoluminescence

(TRTPL) of ITO/ZnO and ITO/ZnO-Al by monitoring the emission peak at 525 nm, which could be used to probe the defects recombination of ZnO. It is clearly exhibited that the defects emission decay time can be gradually decreased by incorporating of Al interlayers, indicating electrons could more easily transfer instead of recombination by oxygen vacancies. In order to further estimate the impact of defects filling and easier electron transport, the time response characteristics of devices were measured by 532 nm light with intensity of 0.3 mW/cm2. Figure 4c is the time response curves, gradually faster response speeds are achieved for devices with ZnO-Al nanojunction compared with control device, suggesting that photocurrent can be more easily generated for ZnO-Al devices without suffering from the additional process of filling oxygen vacancies for the electrons transported from active layer. Thus, these behaviors could sufficiently verified that the defects of ZnO are filled by electrons from Al, benefiting to decreased carrier recombination and promoting electrons transport, which produce a positive influence on the photocurrent convergence.

Figure 5. (a)The reflectance spectra and (b) the IQE curves of control device and devices with ZnO-Al nanojunctions.

The internal quantum efficiency (IQE), an important property of OSCs, is the ratio of the collected electrons to the absorbed photons, reflecting the true ability of light to electricity.39 As mentioned above, the incorporation of ZnO-Al nanojunction decreases the interfacial carrier recombination, thus yielding higher IQE values and favorable photocurrent. Firstly, the reflection (R) spectra of devices without and with Al interlayer were 7



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measured and displayed in Figure 5a. Higher R spectra were displayed for devices with ZnO-Al nanojunction due to the insertion of Al thin layers. Then the IQE spectra were given in Figure 5b by the formula of IQE=EQE/(1-R),40,41 demonstrating the supposed changing tendency that the IQE is higher for the devices with ZnO-Al nanojunction compared with control device. This apparently verifies that more carriers were collected instead of being recombined by interfacial trap states.

Figure 6. (a) The WFs of bare ZnO and ZnO covered by different thickness of Al thin layers, (b) the energy levels diagrams of devices with ZnO or ZnO-Al nanojunction, (c) energy levels diagrams of ZnO and PC71BM without and with Al1.5 interlayer, (d) the structure and (e) I-V characteristics of electron injection devices without and with Al interlayer.

There have been already some reports that metals play the role on reducing the WF of material, such as Ca, Al, and Mg.42-44 Here, the WF of bare ZnO and ZnO-Al with different thickness were measured by Kelvin probe and the results were arranged into Figure 6a. With the increase of the Al thickness, the WF of ZnO-Al is gradually reduced. And it is justifiable to infer that when the Al layer reaches a certain thickness, the surface WF of ZnO-Al will close to the WF of Al (4.3 eV). Favorably, ZnO-Al with lower WF will prefer to accept electrons from the LUMO of PC71BM, which will be further introduced in following analysis for energy levels of devices. The energy levels diagrams of control device and devices with different ZnO-Al nanojunctions are provided in Figure 6b. It can be seen that the WF of bare ZnO is much higher than that of PC71BM, while the surface WFs of ZnO-Al nanojunctions progressively approach to PC71BM. The difference of WFs inevitably causes the energy band bending. Energy levels alignment of ZnO and PC71BM without and with 1.5 nm Al interlayer is given in Figure 6c. When pristine ZnO layer directly contacts with PC71BM, unified Ef will bend the conduction band of PC71BM upwards, forming a larger electron barrier (qϕ) in the bulk of PC71BM and limiting the electron transport to ZnO layer. However, after the modification with Al interlayer, the WF of ZnO is pulled close to that 8


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of PC71BM, the electron transport barrier is decreased, facilitating electron transport to ZnO. It can be seen that the surface WF of ZnO-Al2.5 is lower than PC71BM, completely eliminating the qϕ, which contributes to better electron transport property. In order to make a clear exhibition for the existence of interfacial barrier in PC71BM, the electron injection devices without and with different ZnO-Al nanojunctions were fabricated and dark current characteristics were measured. Figure 6d presents the device structure and energy levels of electron injection devices, and electrons are injected from the Ag electrode. Seen from the current-voltage (I-V) characteristics in Figure 6e, the curve for device with pristine ZnO demonstrates an obvious barrier, in other words, the electrons transfer must firstly jump the barrier in PC71BM. There is relatively large non-radiative recombination in OSCs,45 thus the existence of barrier in PC71BM will increase the carrier recombination and decrease the exporting carrier quantity, which is the serious loss for the device performance. While for the devices with Al layers over 1.5 nm, the interfacial barrier demonstrates the tendency of gradual decrease, resulting in the easier electrons transfer in the whole device. Moreover, it has to be mentioned that devices with Al layer less 1.5 nm similarly show interfacial barrier, but the interfacial barrier were obviously decreased and the performance is more or less improved compared with control device with pristine ZnO layer. At the same time, the decreased interfacial recombination by filling the surface defects is also an important factor for improved device performance.

Figure 7. (a) The Jph and (b) Pc characteristics versus Veff of devices with bare ZnO and ZnO-Al nanojunction.

The reduced interfacial barrier is conductive to enhance the electron extraction capacity from PC71BM acceptor, being a positive signal for vigorous charge generation and collection in devices, which can be estimated by analysis of photocurrent density (Jph) and charge carrier collection efficiency (Pc) versus effective voltage (Veff),14,46 displayed in Figure 7. Jph can be obtained from Jph=JL-JD, where JL and JD are the current density under illumination and in dark. Veff is given by Veff=Vo-Va, where Va is the applied bias voltage and Vo is the compensation voltage at Jph=0. From saturation region of Jph in the Figure 7a, the saturation photocurrent 9



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density (Jsat) is only limited by the maximum charge carrier generation rate (Gmax).47 Now assuming that all the photogenerated excitons dissociated and contributed to the current, the Gmax values of control device and ZnO-Al nanojunction device can be estimated from the equation of Jsat= qGmaxL, where q is the elementary charge and L is the thickness of the active layer (about 105 nm). Significantly increased Gmax from 9.95×1027 m-3 s-1 (Jsat=167.1 A m-2) to 1.02×1028 m-3 s-1 (Jsat=171.4 A m-2), 1.11×1028 m-3 s-1 (Jsat=185.9 A m-2) 1.06×1028 m-3 s-1 (Jsat=178.5 A m-2) for devices with ZnO-Al nanojunctions. It can be attributed the obvious increase of Gmax to the enhanced electron extraction and transport capacity of ZnO-Al compared with bare ZnO. However, the complete dissociation of photogenerated excitons is an ideal and irrealizable situation, there are some certain probability for excitons dissociation and charge carrier collection, which can be obtained from the curves the normalized Jph with Jsat. The Pc associates with maximum power conditions, which increases from 78.5 % for control device to 82.3 %, 84.6 % and 84.7 % due to the incorporation of Al interlayer. This is an imaginable result that the improved charge carrier extraction capacity will necessarily lead to the increased Pc of devices with ZnO-Al nanojunction electron extraction layer.

Figure 8. The electron mobility of devices without and with Al thin layer.

So far, the decrease of interfacial carrier recombination and electron transport barrier in devices with ZnO-Al nanojunctions have been adequately proved, which will directly lead to much smoother electron transport, contributing to stimulative electron transport and improved electron mobility. Thus, to further realistically verify the influence of ZnO-Al nanojunction on the electron extraction and transport, the electron-only devices with the structure of ITO/ZnO-Al/PTB7:PC71BM/BCP/Ag were fabricated, where the BCP is the hole-blocking layer.48,49 Figure 8 is the current density characteristics in dark, and the electron mobilities 10


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have been calculated and shown in the inset of Figure 8. The electron mobility of device with ZnO-Al nanojunction indeed demonstrates obvious increase to 5.98×10-3 cm2/Vs, higher than 8.98×10-4 cm2/Vs for control device, confirming the positive effect from ZnO-Al nanojunction on facilitated electron transport and enhanced device performance.

3. CONCLUSIONS In summary, the high efficiency OSCs were fabricated in a full annealing-free process with a smart ZnO-Al nanojunction by evaporating an ultrathin Al interlayer onto ZnO ETL. The resistance of ITO are unaffected by the preparation process of annealing-free ZnO film, ensuring higher performance of devices. The unappreciated oxygen vacancies of ZnO that can trap one or two electrons were filled by transferred electrons from Al due to the different WFs between them, contributing to decreased electrons loss from interfacial recombination. Meanwhile, the surface WF of ZnO-Al was reduced by Ef equilibrium, conducing to the enhanced electron extraction capacity of ZnO. The decreased carrier recombination and improved electron extraction will no doubt facilitate the electron transport and collection, which have been clearly proved in this work. Consequently, a high PCE of 9.81% were obtained, accounting for 24.3% enhancement compared with control device. This easily fabricated ZnO-Al nanojunction, a typical representation of metal oxide semiconductor-metal nanojunction, possesses the potential to fill defects as well as adjust WF of metal oxide and can be further applied into broader field.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The synthesis of annealing-free ZnO. The fabrication process and the characteristics of OSCs devices. The square resistance of ITO under room temperature and after annealing with different temperature. The AFM height images of bare ZnO film and covered with Al thin layers of different thickness. The J-V characteristic of device with annealed ZnO film at 150 °C. The transmittance spectra of ITO/ZnO, ITO/ZnO-Al nanojunctions. The bandgap emissions of bare ZnO, ZnO-Al0.5, ZnO-Al1.5 and ZnO-Al2.5 under 230 nm light excitation.

AUTHOR INFORMATION Corresponding Author *E-mail: W. B. Guo, [email protected]; L. Zhang, [email protected] 11



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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful to National Natural Science Foundation of China (61370046, 11574110), the Science and Technology Innovation Leading Talent and Team Project of Jilin Province (20170519010JH), Guangdong Natural Science Funds for Distinguished Young Scholar (Grant No.2014A030306005), Foundation for High-level Talents in Higher Education of Guangdong Province, China (Yue Cai-Jiao [2013]246, Jiang Cai-Jiao[2014]10) for the support to the work.

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Decreased Charge Transport Barrier and Recombination of Organic

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