Overcoming Defect-Induced Charge Recombination Loss in Organic

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Overcoming Defect-Induced Charge Recombination Loss in Organic Solar Cells by Förster Resonance Energy Transfer Chunyu Liu, Dezhong Zhang, Fangbin Lu, Mingrui Tan, Tiantian Luan, Haoyan Wu, Bichi Zhang, Liang Shen, and Wenbin Guo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00604 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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ACS Sustainable Chemistry & Engineering

Overcoming Defect-Induced Charge Recombination Loss in Organic Solar Cells by Förster Resonance Energy Transfer Chunyu Liu1, Dezhong Zhang1, Fangbin Lu1, Mingrui Tan2, Tiantian Luan1, Haoyan Wu1, Bichi Zhang1, Liang Shen1, and Wenbin Guo1 * 1

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

2

Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, People’s Republic of China

*Corresponding author: Wenbin Guo E-mail: [email protected]

ABSTRACT The strong charge recombination such as defect-induced recombination in some transition metal oxide interlayers is really existent, which has resulted in the severe charge loss and the deteriorated device performance. Herein, the improved performance of device is demonstrated with annealing-free ZnO:P3HT composite interlayer by minimizing the charge recombination loss. The P3HT incorporated in ZnO interlayer can reuse the wasted energy from the defect-induced charge radiative recombination by Förster resonance energy transfer, which will reduce the charge recombination loss in the device. Meanwhile, the highest occupied molecular orbital (HOMO) of P3HT than ZnO, can work as the hole traps in ZnO interlayer, which also contributes to the increased background electron density and improved electron conductivity. The reduced charge recombination loss and improved conductivity of ZnO interlayer are beneficial to acquire higher short-circuit current density of 17.78 mA/cm2 and fill factor of 74.47%, leading to an enhanced power conversion efficiency of 9.84%. Our study paves a new way to overcome the drawback of charge recombination loss by reusing the energy of charge radiative recombination, instead of suppressing the charge recombination in advance. KEYWORDS: defect-induced recombination, FRET, hole traps, ZnO:P3HT composite interlayer, improved

conductivity

INTRODUCTION In the last few decades, the transition metal oxides (TMOs) are emerging as ideal materials to be applied in the fields of photodetectors,1,2 light emitting diodes,3,4 field-effect transistors5,6 and solar cells7-9 because of their

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advantages of wide band-gaps, large carrier mobility and high light transmittance. Especially in organic solar cells (OSCs), TMOs used as the charge transfer bridge between active layer and electrodes effectively facilitate exciton dissociation and charge collection, wherein molybdenum tri-oxide (MoO3), vanadium pent-oxide (V2O5) and tungsten tri-oxide (WO3) have been widely treated as hole transport materials, and titanium dioxide (TiO2) and zinc oxide (ZnO) are extensively employed to extract and transport electrons. Notably, ZnO possesses a wide range of applications and plays a critical role on cathode interlayer due to its high electron mobility, low temperature and easy synthesis, as well as mechanical, thermal, and chemical stability.10,11 The efficient charge carrier are generated from the exciton dissociation in donor/acceptor interface,12 and then transferred to anode (hole) or cathode (electron) through interlayers.13 There is thereby urgent need but it is still a significant challenge to achieve loss-free charge transport blaming on the charge recombination in cathode interlayers, such as defect-induced recombination due to the existence of defect states.14-17 Similarly, ZnO has been suffering from this problem that the intragap energy level produced by defect states unpleasantly acts as the recombination center for the charge carrier, causing the severe charge loss and degrading the device performance.18-20 Thus, some endeavors so far have been made to overcome this shortcoming by sharing the lone electron pair,21 penetrating the oxygen,22 inserting self-assembled monolayers23 and providing hot electrons from excited metal nano-structures,24 which reduce the charge recombination loss in interlayer. On the basis of these worthwhile results we find that these methods all depress charge loss from the defect-induced recombination by modifying, passivating, modulating or controlling the defect states before the electrons are captured. However, we demonstrate here a device with annealing-free ZnO:P3HT composite interlayer to conquer this drawback of ZnO by reusing the wasted energy from the undesired charge recombination caused by the defect states via Förster resonance energy transfer (FRET).25-27 The light emission at the specific wavelength can be observed in ZnO film due to the defect-induced charge radiative recombination, which causes serious charge loss. Also, we notice that this light emission of ZnO is strongly overlapped by the absorption of P3HT, which implies that the energy can be transferred from the intragap states created by the defects to P3HT, instead of being quenched and wasted. Then generated electrons can be transferred by the appropriate energy level structure of P3HT and reused in the device.28-30 Thus, P3HT is incorporated into ZnO layer, accordingly saving the defect-induced recombination loss and evidently improving the available charge in the device. At the same time, P3HT with a higher highest occupied molecular orbital (HOMO) energy level than ZnO, can work as hole traps, which also contributes to the increased background electron density of ZnO, resulting in an enhanced conductivity.31,32 So to summarize, the introduction of P3HT


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into ZnO interlayer minimizes the charge recombination loss, facilitates the electron transfer, and makes the electron and hole transport more balanced, which expectedly realize the improved short-circuit current density (Jsc), fill factor (FF) and ideal device performance.

EXPERIMENTAL SECTION Device Fabrication: PTB7 and PC71BM were purchased from 1-material Chemscitech Inc. (St-Laurent, Quebec, Canada), and P3HT was purchased from Lumtec corp. (Taiwan, Mw: 10000-100000 by GPC). All materials were used as received. The pristine ZnO solution was prepared according to our previous report with 8 mg/mL.33 The P3HT was dissolved into dichlorobenzene with concentrations of 0.08, 0.2, 0.4 and 1 mg/mL, respectively. Whereafter, the P3HT solution and ZnO solution were mixed with a volume ratio of 1:3, obtaining the mixed solutions with the concentrations of 6 mg/mL for the ZnO, 0.02, 0.05, 0.10 and 0.25 mg/mL for P3HT, respectively. The prepared ZnO:P3HT composite films are indexed as ZnO:P3HT-1 to ZnO:P3HT-4 with gradually increased P3HT concentration. The PTB7 and PC71BM (10 mg and 15 mg) were dispersed into 1 mL chlorobenzene and stirred overnight. Firstly, the ITO substrates were cleaned with acetone, ethanol and deionized water in sequence. Then the ZnO:P3HT and PTB7:PC71BM solutions were successively spin-coated onto pre-cleaned ITO substrates without thermally post-annealing treatments. Finally, the devices were completed by evaporating MoO3 anode interlayer and Ag electrode, respectively. The devices with pristine ZnO interlayer and ZnO:P3HT composite interlayers are named as Device A to E. The effective area of device is 0.044 cm2. Film and Device Characterization: The light transmission and absorption spectra were measured on a UV 1700, Shimadzu. The contact angles were analyzed by a contact angle goniometer (JC2000D, Powereach Co., Shanghai, China). The current density-voltage (J-V) characteristics were measured using a Keithley 2601 source meter with an Oriel 300 W solar simulator intensity of ~100 mW cm-2. The measurement of the incident photon-to-current efficiency (IPCE) was carried out using a Crowntech QTest Station 1000 AD. The photoluminescence (PL) spectra were performed using a Shimadzu RF5301 fluorescence spectrophotometer. The X-ray photoelectron spectroscopy (XPS) was carried out using an ESCALAB 250 spectrometer. The impedance, Mott-Schottky curves and dark capacitance versus voltage (C-V) characteristics were measured by a Precision Impedance Analyzer 6500B Series of Wayne Kerr Electronics.



RESULTS AND DISCUSSION

Figure 1. (a) Device structure and (b) energy levels diagram of OSCs device with the ZnO:P3HT composite interlayer. (c) Transmission spectra of ZnO films without and with P3HT, and inset is the absorption spectrum of PTB7:PC71BM active layer.

Figures 1a and 1b are the device structure and energy levels diagram of devices with ZnO:P3HT composite interlayer. The transmission spectra of ZnO films are firstly measured and shown in Figure 1c, and the inset is the absorption spectrum of PTB7:PC71BM active layer. High transmittance is observed for ZnO and ZnO:P3HT films, as well as a negligible decrease for the doped films from 400 to 600 nm, which can ensure sufficient light absorption of active layer. This is within our understanding that the slightly decreased light transmission is attributed to the absorption of P3HT.

Figure 2. The contact angles of the water on the surface of the (a) pristine ZnO film, (b) ZnO:P3HT-2 and (c) ZnO:P3HT-4.

To further characterize the interface contact between the cathode interlayer and active layer after incorporating P3HT, the contact angle of the water on the surface of the pristine ZnO and ZnO:P3HT composite films are tested and displayed in Figure 2. The contact angle of the water on the pristine ZnO film is 52°, while the contact


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angles on ZnO:P3HT composite films are gradually enlarged to 56° and 63° with the increase of P3HT, manly attributing to the introduction of organic phase.34,35 Thus, it suggests that the composite films possess higher hydrophobicity feature, which contributes to the spin-coating of the polymer blend and the mechanical adhesion of active layer, resulting in the well electron transfer from active layer to ZnO interlayer and the decreased interfacial charge leakage loss.36,37

Figure 3. (a) J-V characteristics of devices with ZnO interlayer and ZnO:P3HT composite interlayer, (b) corresponding IPCE curves of all devices.

Table 1. All Photovoltaic Parameters of Devices with Pristine ZnO Interlayer and ZnO:P3HT Composite Interlayers, Including Voc, Jsc, FF, and PCE

a

Device

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

Device A

0.739±0.001

16.10±0.09

66.69±0.21

7.93±0.08

Device B

0.741±0.002

16.83±0.11

71.55±0.21

8.92±0.11

Device C

0.743±0.001

17.78±0.12

74.47±0.22

9.84±0.11

Device D

0.743±0.001

17.26±0.12

73.23±0.19

9.39±0.10

Device E

0.742±0.002

16.51±0.10

72.89±0.20

8.93±0.10

The photovoltaic parameters are the average values from 30 identical devices for each type.

The performance of devices with ZnO or ZnO:P3HT composite interlayers are compared by the J-V characteristics in Figure 3a. The related photovoltaic parameters, such as open-circuit voltage (Voc), Jsc, FF, and power conversion efficiency (PCE) have been summarized in Table 1. The performance of devices with ZnO:P3HT interlayers are significantly enhanced, achieving a PCE of 9.84% for the optimal device (Device C), much higher than that of the control device (Device A) with pristine ZnO interlayer, which just demonstrates an ordinary device performance of 7.93% for PTB7:PC71BM based device. The improved device performance primarily gives the credits to the increased Jsc and FF. The improved Jsc from 16.10 to 17.78 mA/cm2 can be attributed to the increased available electron density in ZnO, and the higher FF of 74.47% is the result of the more balanced electron and hole transport. It has been investigated and proved that the FF is related to the charge



loss from recombination and trapping.38,39 But devices based on the composite interlayer with excessive P3HT demonstrate gradually slipped performance mainly due to the decreased electron transport capacity of ZnO:P3HT interlayer. The P3HT is a p-type semiconductor and doesn’t participate to the electron conduction in the device, which would partly affect the electron transport capacity of the ZnO with excessive P3HT. In other words, the enhanced device performance is a trade-off between reduced charge loss in the device and affected electron transport capacity of cathode interlayer. The corresponding incident photon to current efficiency (IPCE) spectra of devices are shown in Figure 3b to estimate the improvement of Jsc, presenting that the IPCE for the optimal device (Device C) is 78% at 650 nm and much higher than control device of 69%. The improvement of IPCE in a wide spectrum range can be attributed to the improved electron transport capacity due to the incorporation of P3HT. When the excessive P3HT is doped, the damaged charge transport of ZnO:P3HT composite layer will inevitably degrade the photon utilization of device.

Figure 4. (a) Absorption spectrum of P3HT and PL spectra of ZnO and ZnO:P3HT with the exciting source of 310 nm, the charge recombination processes in (b) pristine ZnO film and (c) ZnO:P3HT composite film. (d) PL spectra of PTB7:PC71BM active layer and PTB7:PC71BM/ZnO with an exciting source of 680 nm. (e) TRPL spectra of pristine ZnO film and ZnO:P3HT-2 composite film with an exciting source of 310 nm. (f) PL spectra of pristine P3HT and ZnO:P3HT films.

In pristine ZnO interlayer, the electrons transported from active layer are unavoidably trapped or recombined, bringing about a negative impact on the device performance, especially on the Jsc. The PL spectra are widely used to explore the light emission from the charge radiative recombination or verify the existence of defect states. To clarify the working mechanism of the device more truthfully and intuitively, the PL spectra of ZnO films without and with P3HT are measured and shown in Figure 4a with a 310 nm exciting source, exhibiting a narrow


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emission peak at 350 nm ascribed to the band-to-band emission and a broad peak around 525 nm originating from the emission peak of defect-induced charge recombination.21,22 Due to the presence of defect states, the charge recombination in cathode interlayer will be happened in the working process of device, as seen from the Figure 4b. The distance from defect level corresponding to the emission peak of defect-induced charge recombination to the valence band is 2.36 eV, which can be calculated using Planck’s equation: E=1240/λ, where E is the corresponding energy and λ=525 nm is the value of light emission peak.40 It can be found that the absorption of P3HT in Figure 4a perfectly overlaps the emission peak of defect-induced recombination. After incorporating P3HT into ZnO, the emission peak of 525 nm is strongly quenched, attributing to the energy transfer from the defect states of ZnO to P3HT by FRET, which is clearly exhibited in the Figure 4c. The absorption spectrum of P3HT just covers the stronger emission spectrum of the defect-induced recombination, without considering the lower band-to-band emission, so the band-to-band emission is not quenched. To testify that the photogenerated charge transferred from active layer will be recombined in ZnO from the defect-induced charge recombination, the light emission of PTB7:PC71BM active layer and PTB7:PC71BM/ZnO have been measured with an exciting source of 680 nm, which is the absorption peak of PTB7, as shown in Figure 4d. A distinguishable light emission can be observed when inserting the ZnO film, indicating that the strong defect-induced charge recombination is indeed present in the ZnO film. Thus, it is responsibility for improved device performance by reusing the energy of charge recombination. The time-resolved photoluminscence (TRPL) measurements of ZnO and ZnO:P3HT-2 are performed to prove the energy transfer from ZnO to P3HT, as shown in Figure 4e. It can be noted that the PL quenching of ZnO:P3HT-2 composite film is faster than that of pristine ZnO interlayer, suggesting that the energy is transferred from ZnO to P3HT.25 At the same time, the PL emission of P3HT and ZnO:P3HT-2 are also measured with a 310 nm exciting source as shown in Figure 4f. A weak light emission around 650 nm for the ZnO:P3HT film is observed, which is consistent with the light emission of pristine P3HT when exciting by 525 nm light source. The obvious light emission at 620 nm is the second order diffraction peak of exciting source, which can’t be dispelled based on our present experimental conditions. It can be concluded that the light emission of ZnO:P3HT around 650 nm is excited by the energy of defect-induced charge recombination of ZnO. To sum up, the energy of defect-induced charge recombination in ZnO can be transferred to P3HT.



Figure 5. XPS high resolution survey scan of Zn 2p.

To further verify if the charge can be reused, the XPS measurements of Zn 2p are carried out for ZnO and ZnO:P3HT-2 films, which is displayed in Figure 5. The binding energies of 2p3/2 and Zn 2p1/2 for pristine ZnO film are 1021.6 and 1044.6 eV, respectively. There is an energy difference of 23 eV, which corresponds to the binding state of Zn2+.41 The ZnO:P3HT-2 film demonstrates the same energy difference, while the peaks of ZnO 2p shift to lower binding energy. It implies that the electrons are transferred to ZnO and the Zn2+ is not affected by the introduction of P3HT.42,43

Figure 6. (a) C-V and (b) Mott−Schottky curves of device with pristine ZnO interlayer and ZnO:P3HT composite interlayer.

Figure 7. Nyquist plots of devices with pristine ZnO and ZnO:P3HT composite interlayer.


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As mentioned above, the charge recombination loss is apparently reduced, benefiting for the increased charge carrier density, which can be calculated and proved by the C-V characteristic in Figure 6a.44,43 The subsequent study will be mainly carried out around the control device (Device A), the optimal device (Device C) and the device with excessive P3HT (Device E). The charge carrier density (n) can be obtained by the formula:

n=

1 Aed

∫

Vbi

0

C (V )dV (1)

where A is the device area (0.044 cm2), e is the elementary charge, d is the thickness of the active layer (about 105 nm), C is the chemical capacitance, and the Vbi attained from the Mott-Schottky curves in Figure 6b is the built-in voltage. For the Device A with the pristine ZnO interlayer, the value of n is 6.78×1016 cm-3. After adding P3HT, the values of n are increased to 1.06×1017 and 7.74×1016 cm-3, respectively. It is consistent with the results of XPS measurement in Figure 5. Though the ZnO:P3HT-4 demonstrate stronger capacity to reduce the charge recombination loss, the n is still affected by the electron transport capacity of the composite interlayer. It can be distinctly observed that the incorporation of P3HT noticeably enhances n in the device, which bounds to lower the resistant of device. To further confirm this, the Nyquist plots of impedance measurement are carried out in the frequency range from 20 Hz to 1 MHz and shown in Figure 7. The series resistance (Rs) can be extracted from the intercept on ReZ' axis at high frequencies (left part).32,46 The Rs of devices with ZnO:P3HT composite interlayers are 51 Ω for Device C, and 63 Ω for Device E, lower than 91 Ω for the Device A. It contributes to the decrease of the current loss across the contacting materials, which is in agreement with the improvement in Jsc and FF.

Figure 8. (a) J-V characteristics of electron-only devices with pristine ZnO interlayer, ZnO:P3HT-2 and ZnO:P3HT-4 composite interlayers, and (b) the J-V curve of hole-only device.

The incorporation of P3HT minimizes the charge recombination loss, which is beneficial for the enhanced electron mobility. To investigate the effect of ZnO:P3HT composite interlayer on the electron transport property



in device, the electron-only devices with the structure of ITO/ZnO or ZnO:P3HT/PTB7:PC71BM/BCP/Ag are fabricated and corresponding J-V characteristics are measured, as shown in Figure 8a. The charge carrier mobilities can be calculated by the charge transfer model of space-charge-limited-current (SCLC) and the Mott-Gurney law that includes field-dependent mobility,47,48 obtained by

9 V2 J = ε 0ε r µ 3 8 d

(2)

where ε0 and εr are the permittivity of free-space and relative dielectric constant of the active layer (about 3 for organic materials), and µ is the mobility. The electron mobility (µe) increases from 8.50×10-4 cm2 V-1 s-1 to 3.36×10-3 cm2 V-1 s-1 and 1.33×10-3 cm2 V-1 s-1 compared with the Device A, attributing to the reduced charge recombination loss and improved conductivity of ZnO. Meanwhile, the hole-only device with the structure of ITO/PEDOT:PSS/PTB7:PC71BM/MoO3/Ag is fabricated to further explore the charge carrier transport balance. The J-V curve is given in Figure 8b, and the calculated hole mobility (µh) is 5.10×10-3 cm2 V-1 s-1. Remarkably, the optimal device (Device C) achieves more balanced electron and hole transport than Device A, which contributes to the improved FF.49 The electron mobility of Device E is relatively lower, mainly attributing to the decreased electron transport capacity of ZnO layer due to the excessive P3HT.

Figure 9. Pc characteristics versus Veff of devices with pristine ZnO interlayer and ZnO:P3HT composite interlayers.

The dissociation of photogenerated exciton and collection of charge carrier by electrodes are critical steps in the working process of the device, but actually the complete exciton dissociation and exhaustive charge carrier collection are impossible with some certain probability due to the undesired exciton quenching and charge recombination. The exciton dissociation is related to the contact potential between the donor and the acceptor, work function difference of electrodes and so on. However, the charge carrier collection extremely associates with the charge carrier extraction, transport and recombination. At the short-circuit condition, the exciton dissociation efficiency was obtained from the curves of normalized photocurrent density (Jph) with the saturation


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photocurrent density (Jsat) (Jph/Jsat),47 as shown in Figure 9. The Jph can be obtained from Jph=JL−JD, where JL and JD are the current density under illumination and in dark, and the Jsat is the photocurrent in fully saturated part.51 For the Device A, the exciton dissociation efficiency is 93.2%, slightly lower than 97.1% and 94.6% for the devices with ZnO:P3HT composite interlayers, implying that the incorporation of P3HT is conducive to the facilitated exciton dissociation of donor-acceptor. It has been proved above that ZnO:P3HT composite interlayer can effectively reduce the charge recombination loss and improved the electron mobility, which inevitably improve the charge collection efficiency (Pc) and enhance device performance.52 Under the maximum power output conditions, the Pc increases from 70.3% for the Device A to 78.2% for the Device C and 75.5% for the Device E. An increased Pc suggests that the charge recombination loss is effectively reduced, leading to a higher FF.53,54

CONCLUSIONS To conclude, we have successfully showed a highly efficient OSCs with the ZnO:P3HT composite interlayer, providing an excellent example to effectively minimize the defect-induced charge recombination loss by FRET. The energy can be transferred to P3HT instead of being quenched due to the perfectly matched PL emission of the defect states of ZnO and the absorption of P3HT. The P3HT with proper energy level can reuse them to reduce the charge loss, which contributes to the improved charge carrier density and conductivity, thus facilitating the electron transport in cathode interlayer and collection by cathode. This work is of great importance to reduce the charge recombination loss from a new perspective, demonstrating bright application prospects to fabricate high-performance OSCs device and application in other optoelectronic devices.

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

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful to the Special Project of the Province-University Co-constructing Program of Jilin Province (SXGJXX2017-3), the Science and Technology Innovation Leading Talent and Team Project of Jilin



Province (20170519010JH), International Cooperation and Exchange Project of Jilin Province (20170414002GH, 20180414001GH), and Project of Graduate Innovation Fund of Jilin University (2017175, 2017170) for the support to the work.

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SYNOPSIS: The incorporation of P3HT can reuse the energy of defect-induced charge recombination in ZnO, achieving a high efficiency of 9.84% in OSCs.


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Overcoming Defect-Induced Charge Recombination Loss in Organic

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