Employing Pentance to Balance the Charge Transport in Inverted

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C: Energy Conversion and Storage; Energy and Charge Transport

Employing Pentance to Balance the Charge Transport in Inverted Organic Solar Cells Changhao Wang, Chang Li, Ge Wang, Chen Wang, Pengfei Ma, Lingchu Huang, Shanpeng Wen, Wenbin Guo, Liang Shen, and Shengping Ruan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05333 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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The Journal of Physical Chemistry

Employing Pentacene to Balance the Charge Transport in Inverted Organic Solar Cells Changhao Wang,†,‡ Chang Li,† Ge Wang, † Chen Wang,† Pengfei Ma,†,‡ Lingchu Huang,† Shanpeng Wen,†* Wenbin Guo, † Liang Shen, † Shengping Ruan†*

†

State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, P. R. China.

‡

College of Science, Jilin Institute of Chemical Technology, 45 Longtan Street, Jilin, 132022, P.R. China.

ACS Paragon Plus Environment

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ABSTRACT: The charge transport balance is regarded as a prerequisite for high-performance organic photovoltaics (OPVs) since it is able to suppress the space charge accumulation and reduce the probability of charge carrier recombination before they are extracted. However, electrons usually transport faster than holes in most fullerene-based bulk heterojunction (BHJ) active layers, which leads to the unbalanced charge transport and thereby degrades the device performance. Here, we report an easy-processed approach to overcome this unbalance. Upon doping high-hole-mobility Pentance (Pc) into the active layer, we found that the hole transport improves significantly and the original nano-morphology of the active layer undergoes subtle changes. Time-resolved photoluminescence spectrum reveal that the life time of exciton with in the blend films decreases in the presence of Pc, indicating that the Pc dopant may participate in the charge transfer process and facilitate the exciton dissociation. As a result, a light-doping level of 0.2 % significantly improves the power conversion efficiency (PCE) by 38 %, from 3.46 % to 4.79 % in P3HT/PC71BM-based solar cells, which demonstrates a feasible approach to increase the efficiency of OPVs.


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INTRODUCTION To solve the energy and environment crisis, solar cells have been widely developed in recent years.1-5 Among them, polymer:PCBM bulk heterojunction (BHJ) organic photovoltaics (OPVs) provide a very promising platform to achieve the low-cost, flexible, large-area and low-energy fabrication.6, 7 However, the power conversion efficiency (PCE) of OPVs is still considerably lower relative to their inorganic counterparts.8, 9 This is caused by a variety of limiting-factors in organics, for example, insufficient absorption overlap with the solar spectrum, tightly bound excitons with short diffusion lengths, slow charge carrier migration and large recombination losses.10-18 Accordingly, lots of strategies have been developed to enhance these fundamental properties, and the efficiency is promoted to over the 10% threshold.19-21 Whereas, despite of these improvements, the PCEs of state-of-the-art OPVs are still restrained by their relatively low and unbalanced charge transportation.13, 22 The doping method has been proven to be a viable strategy to modulate the charge transportation in optoelectronic devices. Many high-mobility Carbon-based dopants like carbon nanotubes (CNT) or quantum like g-C3N4 have been incorporated into the active layer, which dramatically enhances the ability of charge transport and manages to increase the device PCE.2326

On the other hand, it is well known that electrons travel faster than holes in BHJ of

polymer:fullerene blending, which leads to the unbalanced charge transport and thereby decreases the device performance.27,

28

Therefore, the specific doping of high-hole-mobility

materials, such as p-type conjugated polymers, has been developed to achieve the more efficient


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and equivalent charge transport.29 Unfortunately, different chemical and physical properties between these two donor polymers imposed the fabrication of such solar cells very tricky. The increased background carrier density induced by over-doping may also in turn aggravates bimolecular recombination and reduces the device fill factor (FF).30 Hence, finding out an appropriate dopant with high-hole-mobility, easy obtainable processing method and taking effect at low doping concentration is necessary to further increase the OPVs efficiency. In this aspect, Wei et al. proposes the strategy of combining polymers with p-type organic small molecules (SMs) by taking in advantage of the features of SMs, for example the high crystallinity and hole mobility (µh), to manipulate the phase separation and hole transport within active layers.31 Following these guideline, we reason that Pentance (Pc) would be a promising candidate as the SM dopant since it has high crystallinity and mobility. Reported results have shown that for Pc molecules the µh reaches as high as 3.0 and 35 cm2V-1 s-1 for single crystals and thin films, respectively. These are almost the highest values for an organic material.32-34 This fascinating feature originates from its large and planar conjugated structure since they can greatly promote the facial π-π stacking. Additionally, the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of Pc are located at -3.21 eV and 4.99 eV, respectively.35-37 This energy structure forms the energy cascade with the most commercially available P3HT/PC71BM and is favorable for improving the exciton dissociation and suppressing geminate recombination.38-41 All of these advantages suggest us to exploit the effects of Pc in OPVs as dopant.


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In this contribution, we reported in detail an easy-processed method of increasing OPVs performance by doping Pc into the active layer. It is demonstrated that very low doping level at parts per thousand (ppt) concentrations can synergistically enhances the charge dissociation, reduces the device series resistance and improves the hole transport properties of OPVs with original nano-morphology of the active layer undergoes subtle changes. These improved fundamental properties are confirmed by detecting the variation of morphology, the lifetime of excitons as well as the hole mobility. The fast excitons dissociation and charge transport contribute to a reduced recombination loss and promote the final collection of free carriers, which consequently enables a simultaneous improvement for both JSC and FF in the most commercially available P3HT/PC71BM systems. This study thus emphasizes the potential practical application of facile Pc doping in OPVs field.

2. EXPERIMENTAL SECTION 2.1 Preparation of Pc solution. Pc material is purchased from NICHEM fine technology co. Ltd., and used without further purification. Pc possess a rigid backbone, and insoluble in most organic solvent. In order to obtain a good dispersion of Pc solution, we use dichlorobenzene (DCB) as the solvent and adopt a heat dissolution method.42 To be specific, 2.5 mg Pc and 5 ml DCB are mixed in a round-bottom flask (25ml), then the mixture is stirred under nitrogen at 100 〬C and refluxed 12 h. The concentration of finally prepared Pc solution is 0.5 mg/ml. After that, various volume of precipitated solution is added into the active layer solution (P3HT:PC71BM)


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and corresponding weight ratios (wt) of Pc and P3HT:PC71BM blend are 0 wt%, 0.1 wt%, 0.2 wt% and 0.3 wt%. 2.2 Device fabrication and characterization. The control device is prepared on ITO-coated glass substrates with a sheet resistance of 15 Ω/sq by employing a conventional reverse device configuration with the structure of indium tin oxide (ITO)/Zinc oxide (ZnO)/P3HT:PC71BM/ molybdenum oxide (MoO3)/sliver (Ag), as shown in Figure 1. Here, we use ZnO NPs as the electron transport layer, and the preparation method follows a traditional way.43, 44 Briefly, 2.95 g zinc acetate dihydrate was reflux dissolved in 125 mL anhydrous methanol at 60 °C for 30 min. A solution of potassium hydroxide in anhydrous methanol (1.48g/65 mL) was added to the zinc acetate dihydrate solution dropwise under vigorous stirring. Then the reaction mixture was stirred at 60 °C for another 2 h 15 min. The precipitate was washed twice with anhydrous methanol. To obtain a cluster free ZnO NPs solution, small amount of ethanolamine (0.2 wt%) is added into ZnO methanol dispersion and ultrasonic treated for 30 min. This solution was only slightly translucent, almost transparent. The glass/ITO substrates are pre-cleaned with detergent, deionized water, acetone and isopropyl alcohol for 15 min sequentially by ultrasonic baths followed by UV ozone treatment for 10 min. After that, the prepared ZnO solution is spin-coated (1500 rpm) onto ITO substrates with a thickness around 35 nm, the substrates are annealed at 100 〬C for 10 min in a nitrogen-filled glove box to remove residual methanol and improve the crystal quality of the ZnO film. For the active layer, blend solution with and without varying concentration of Pc is spun onto ZnO layer at the speed of 1200 rpm for 30 s and are then


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annealed at 140 〬C for 10 min under nitrogen protection. A thickness of about 100 nm active layer is formed. Finally, hole transport layer MoO3 (4 nm) and Ag (100 nm) are deposited by the vacuum evaporation in sequence under a high vacuum level (6×10−4 Pa) with a shadow mask. The active device area is about 0.064 cm2 (measured with a Vernier caliper). For the device characterizations, current density-voltage (J-V) characteristics are measured using Keithley 2400 Source Meter in the dark and under Air Mass 1.5 Global (AM 1.5 G) solar illuminations with an Oriel 300 W solar simulator intensity of 100 mW/cm2. The external quantum efficiency (EQE) is measured with spectral measurement system (Crowntech QTest Station 10000AD). Atomic force microscope (AFM, Veeco Dimension 3100) is used to characterize the surface morphologies. The

UV-vis

absorption

spectra

are

taken

using

Shimadzu

3600

UV-visible-NIR

spectrophotometer. Impedance analyzer (Wayne Kerr Electronics 6520B) is used to analyze impedance spectrum.

Figure 1. (a) Chemical structure of Pc and the device structure, (b) the flat-band energy levels of the inverted polymer solar cells.


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3. RESULTS AND DISCUSSIONS Figure 2 shows the corresponding J-V curves of the control device and devices with different Pc doping levels. The measurement is performed under the AM 1.5 G illumination at 100 mW/cm2. Detailed photovoltaic parameters containing JSC, open circuit voltage (VOC), FF, and PCE are summarized in Table 1. From the Table 1 we can see that there is an obvious enhancement in the photocurrent upon trace amount of Pc doping. The best device performance is reached for the doping level at 0.2 wt%, with VOC of 0.62 V, JSC of 11.95 mA/cm2, FF of 64.30% and PCE of 4.79%. On the contrast, control device shows a relatively lower PCE of only 3.46% as a result of the JSC and FF losses. It can be seen that the VOC does not change, this illustrates that the Pc doping into active layer has negligible effects on the LUMO and HOMO level of active material as shown in Figure 1b.45,

46

Meanwhile, the improved JSC and FF

suggests that there are more free carriers generated inside the active layer and extracted out of the devices.47 The series resistance (RS) and shunt resistance (RSh) can be also calculated from formula ( = −(/ ) ) and formula ( = 1/(/ ) ). The Rs presents a trend of decreasing first and then increasing with the Pc doping. This can be understood that too much doping will destroy the arrangement of the active layer, thus reduce the performance of solar cells.48-50 It also can be seen from the Table 1 that when doping Pc (0.2 wt%) into the active layer, the Rsh increase and reach its maximum. The Rs decrease and RSh increase are both advantageous for the FF improvement.


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Figure 2. J-V curves of the control sample and polymer solar cells with different Pc doping levels.

Table 1. Detailed Performance Parameters of OPVs of the control device and polymer solar cells with different Pc doping levels. Doping ratios (wt%)

Voc (V)

JSC (mA cm-2)

FF (%)

PCE (%)

Rs(Ω cm2)

Rsh(Ω cm2)

0

0.62

9.71

57.51

3.46

144.82

2983.31

0.1

0.62

11.45

60.38

4.28

111.06

5115.08

0.2

0.62

11.95

64.30

4.79

81.47

11888.79

0.3

0.62

10.98

54.73

3.72

135.77

1638.06

To elucidate the improvement of JSC, the incident photon-to-electron conversion efficiency (IPCE) results are measured, as shown in Figure 3a. All devices demonstrate a similar photoelectric response ranging from 300 to 700 nm. It is consistent with the absorption spectrum of the P3HT:PC71BM film. With an appropriate Pc doping, IPCE exhibits significant increases in


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the main absorption range of P3HT/PC71BM (350 nm - 625nm). The integrated JSC calculated from IPCE spectra in combination with standard solar spectrum are also plotted in the corresponding figure (linked with right-Y). It turns out to be 8.86 mA/cm2, 10.16 mA/cm2, 10.56 mA/cm2, 9.64 mA/cm2 for doping ratio of 0 wt%, 0.1 wt%, 0.2 wt% and 0.3 wt%, respectively. The mismatches between calculated JSC and the measured JSC are within 10%, indicating the reliability of device performance and providing evidences for the Pc-induced JSC enhancement. Considering the photo-to-electron conversion to be a three-step operation: (i) the absorption of photons; (ii) the exciton dissociation and charge transport at the P3HT/PC71BM interface; and (iii) the collection of charge carriers at the electrodes.51 We perform the absorption test on different sample, as shown in Figure 3b. The absorption spectrum of P3HT:PC71BM composite films without and with Pc doping (0.2 wt%) shows some variation in the range of 300-475 nm. The enhanced absorption from 300 nm to 475 nm after doping Pc is consistent with the main absorption of Pc.42 Apart from this, we envision that after Pc doping, the positive changes in electrical properties could also potentially contribute to the improved device performance that will be discussed in detail later.


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Figure 3. (a) The IPCE of the control device and polymer solar cells with different Pc doping levels, (b) absorption curves of P3HT:PC71BM composite films without and with 0.2% Pc doping level.

It is well known that the nano-morphology plays an important role in the exciton dissociation.52, 53 Besides, the highly crystallized Pc dopants potentially impose influences on the phase separation in BHJ films.54 So the surface roughness and phase images of the BHJ films are investigated by AFM. As shown in Figure 4a and 4b, the BHJ films exhibit smooth surfaces with the root-mean-squared (RMS) surface roughness of 0.76 nm and 0.78 nm respectively. From Figure 4c and 4d, we observe that nearly no differences in the phase separation between BHJ films before and after Pc doping (0.2 wt%). It implies at least that incorporating Pc has a minimal influence on the BHJ films and guarantee the interfacial properties of the OPVs.


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Figure 4. The AFM height images and phase images of the BHJ films (a,c) without and (b,d) with 0.2% Pc doping.

To further reveal the improved electrical performance in the doping cell, time-resolved photoluminescence spectroscopy (TRPL) measurements are conducted. Figure 5 shows the PL decay profiles of pristine P3HT, P3HT:PC71BM films with (0.2 wt%) and without Pc doping. The laser excitation wavelength is 480 nm. Through a bi-exponential fitting, () = −

exp −

the intensity weighted average exciton lifetime τ%&' are 947.4 ps, 322.8 ps and 288.2 ps for pristine P3HT film, P3HT:PC71BM and P3HT:PC71BM:Pc (0.2 wt%), respectively.55-58 The fastest PL decay of doped films suggests the facilitated excitons dissociation caused by Pc.59-61


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Since the P3HT- PC71BM nano-interface undergoes only subtle change, the Pc dopant may participate in the charge transfer process by forming the energy cascade between P3HT and PC71BM. This is reasonable in view of the energy structure (Figure 1b), and energetically accelerates the charge transfer from P3HT to PC71BM.62 The improved charge transfer process is helpful to inhibit excitons geminate recombination and gives a better JSC in the solar cells.60, 63

Figure 5. Time-resolved PL decay spectra of the pristine P3HT film, the P3HT:PC71BM blends without and with 0.2 %Pc doping.

Apart from the exciton dissociation process, how the Pc doping affects the vertical charge transport within the active layer is further studied. The hole mobilities of the devices with and without Pc doping ware measured by using the SCLC method.64, 65 We use a diode structure of ITO/PEDOT:PSS/active layer (with and without doping)/Au to ensure there is only hole can be extracted. The devices are measured in dark with the applied voltage from 0 V to 5 V, and corresponding J-V curves (Figure 6) are fitted into SCLC regime in which the hole mobilities can be calculated by:


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9 = * *+ , . /0.8923 4 8

Where J is the current density, * is the vacuum permittivity, *+ is the relative dielectric

constant (assumed to be 3 in here), , is the hole mobility, L is the film thickness of the active

layer, 2 is the field activation factor, and the effective voltage V=Vappl - Vbi. Vappl is the applied voltage and Vbi is the built-in voltage. The calculated zero-field hole mobility of control and the optimal doping device are 1.12510-5 and 4.88510-5 cm2V-1s-1, respectively. It is apparent that upon Pc doping into the active layer, the hole mobility goes up. This is expected to reduce the space charge accumulation in P3HT:PC71BM device and cause the carrier sweep-out became more competitive than bimolecular recombination, also explained the improved device performance.

Figure 6. J-V curves of hole-only devices in dark with (0.2 wt%) and without doping.

In-depth understanding on the effect of Pc on charge generation and dissociation process are gained by determining the photocurrent density (Jph) as a function of the effective voltage (Veff),


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as shown in Figure 7. Jph can be calculated as Jph=JL-JD, where JL and JD represent the current under the illumination and dark conditions respectively. Veff is determined by compensation voltage (V0) minus externally applied voltages (Va). As shown in Figure 7a, Jph presented firstly a linear dependence on the voltage where charge transport/extraction was dominant. Then Jph gets saturated. In this regime, nearly all the photogenerated excitons separated into free carriers and carriers left the device efficiently, therefore Jph reflected charge generation directly. We also estimated charge generation rate (Gmax) by Jsat =qGmaxL, where L is the thickness of the active layers (153 nm for control device and 161 nm for Pc device). The values of Gmax for the control device and Pc doping device are 5.00 ×1027 m-3 s-1 (Jsat = 122.4 A m-2) and 5.61 × 1027m-3 s-1 (Jsat =144.6 A m-2), respectively. The slightly increased Gmax of Pc device can be understood by the enhanced light absorption after Pc doping. It also can be found that upon Pc doping, the Jph shows a higher value in both the linear and saturated region, indicating more efficient charge extraction and collection process. This conclusion is confirmed by estimating the exciton dissociation probabilities P(E,T) which could be obtained at the short-circuit condition where Va equals zero and charge collection efficiency (PC,M) which could be obtained under maximum power conditions where Va equals the maximum power point voltage, as shown in Figure 7b.43, 44 The doped device afforded increased P(E,T) from 92.3% to 95.1% and PC,M from 79.28% to 82.63%, which coincides with higher JSC and FF in Pc doped solar cells.66, 67


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Figure 7. (a) Jph–Veff curves and (b) P(E,T)–Veff curves of the device with (0.2 wt%) and without Pc doping.

To further probe the electron transport characteristics of the device, impedance spectroscopy (IS) is measured under the open circuit voltage condition from the frequency of 20 Hz to 1 MHz. As shown in Figure 8, the shapes of impedance spectra are all typically semicircle which can help us to investigate the resistance of different structures PSCs. Generally, the semicircle’s diameter represents the magnitude of series resistance.23 Notably, after doping Pc into the active layer, the resistance of the device decreases at first and then increases after the doping arrives at 0.2 wt%. The variation trend of the resistance is in line with the change of devices’ JSC and RS value, as shown in Table 1, and also partly resulted in the performance improvement in PSCs.


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Figure 8. The impedance spectra diagram of the devices with and without Pc doping. To understand about the recombination mechanisms in the device with and without Pc doping, intensity dependent studies also have been performed. Figure 9 depicts the light intensity dependence of the VOC to gain insight into the influence on trap-assisted recombination and second-order recombination in the devices. The light intensity (I) and VOC can be correlated by the following expression. 67 =

(1 − B)CDE 89:; => − ?@ A G < < BF

This formula contains the dependence of the VOC on I. Generally, the slope of VOC versus ln(I) is equal to kT/q for second-order recombination. Moreover, a stronger dependency of VOC on the I can be observed in a system suffered from obvious trap-assisted recombination, and thus the slope of S is equal to 2kT/q. Due to the sensitivity of VOC measurements at low light intensity, therefore, we just fitted the VOC versus light intensity relationship for both systems under high light intensities. As can be seen from the Figure 9, the device without and with the pentacene introduction exhibit a logarithmic dependence on light intensity with a slope of ~1.11 kT/q and ~1.10 kT/q respectively. It implies that doping Pc into the active layer didn’t introduce extra trap states that probably aggravate trap-assisted recombination.


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Figure 9. The measured Voc of (a) without and (b) 0.2 wt% doping devices as a function of light intensity (black), together with linear fits to the VOC data (red lines). To further study the bimolecular second-order recombination within the device with (0.2 wt%) and without Pc doping, we investigated the variation of Jsc as a function of light intensity (Figure 10). A power law dependence of Jsc versus light intensity, Jsc = β(I)M

in which α is the exponential factor and β is a constant. When the fitted α value is close to unity

(α = 1), this indicates negligible bimolecular recombination in a system. While the more α

deviates from 1 the larger recombination rate is. As exhibited in the figure 10, the fitted α of the doping device is ~1.037 while α of the control device equals 1.043.

From the above discussion, it can be concluded that the introduction of Pc does not introduce trap states or acts as recombination centers. Instead, it reduced bimolecular recombination rate. This should originate from the improved hole mobility in doping device and explain superior device performance after Pc doping.


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Figure 10. The measured Jsc of devices based on (a)with 0.2 wt% doping and (b) without doping plotted against illumination intensity on a logarithmic scale.

4. CONCLUSION In summary, a considerable PCE enhancement is achieved in the most commercialized P3HT:PC71BM solar cells by incorporating Pc into the active layer. Pc has a high hole mobility that is able to effectively improve the hole migration of the host mixture. The energy level of Pc matches well with P3HT:PC71BM to form an energy cascade and facilitate the charge transfer. Both these two factors lead to the increased charge collection by the electrodes, then improve the Jsc and FF. The highest PCE of 4.79% after Pc doping is among the best results based on P3HT:PC71BM system. We believe that these results offer a practical application of facile Pc doping in OPVs field.


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ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (Grant No. 11574110), the Project of Science and Technology Development Plan of Jilin Province (Grant No. 20180414020GH), the Project of Jilin Provincial Development and Reform Commission (2018C040-2), Opened Fund of the State Key Laboratory on Applied Optics, and the China Postdoctoral Science Foundation (Grant Nos. 2014T70288, 2013M541299).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Notes

The authors declare no competing financial interest.

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Employing Pentance to Balance the Charge Transport in Inverted

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