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Morphological and optical engineering for high-performance polymer solar cells Seo-Jin Ko, Jungwoo Heo, Byoung Hoon Lee, Su Ryong Ha, Sujoy Bandyopadhyay, Hong Joo Cho, Hyosung Choi, and Jin Young Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16490 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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ACS Applied Materials & Interfaces

Morphological and Optical Engineering for HighPerformance Polymer Solar Cells

Seo-Jin Ko,¶ Jungwoo Heo,§ Byoung Hoon Lee,† Su Ryong Ha,₤ Sujoy Bandyopadhyay,₤ Hong Joo Cho,₤ Hyosung Choi,₤,* and Jin Young Kim§,*

¶ Division

of Advanced Materials, Korea Research Institute of Chemical Technology

(KRICT), Daejeon 34114, Republic of Korea § Department

of Energy Engineering, Ulsan National Institute of Science and Technology

(UNIST), Ulsan 44919, Republic of Korea † Division

of Chemical Engineering and Materials Science, Ewha Womans University, Seoul

03760, Republic of Korea ₤ Department

of Chemistry and Institute of Nano Science & Technology, Hanyang

University, Seoul 133-791, Republic of Korea

*Corresponding

authors E-mail: [email protected] and [email protected]

Keywords: Morphology engineering; optical engineering; processing additive; polymer solar cells; bulk heterojunction

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Abstract We demonstrate morphological and optical engineering by using processing additives and optical spacer in for polymer solar cells. Among various processing additives, introduction of diphenyl ether into the active layer results in smoothest surface roughness with uniform and well-distributed donor:acceptor domains and the device with DPE shows the highest device efficiency of 10.22% due to enhanced charge collection efficiency and minimized recombination loss. Additional ZnO optical spacer on the active layer controls the distribution of the electric field in whole device and enhance the light absorption within the active layer, thereby improving device efficiency up to 10.81%.


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Polymer solar cells (PSCs) with bulk heterojunction (BHJ) structure have been regarded as next-generation solar cells because of their potentials for realization of low-cost and large-area device fabrication on flexible substrates via solution processing.1-5 Intensive efforts for developing new materials and device architectures have dramatically improved power conversion efficiencies (PCEs) of PSCs up to 12% over the last decade. These improvements have mostly resulted from development of new donor polymers. Although polymers with small bandgap are favorable for more light absorption, it is difficult for these polymers to have deep highest occupied molecular orbital (HOMO) level. Specifically, bandgap tuning process for designing small bandgap polymers simultaneously decreases lowest unoccupied molecular orbital (LUMO) level and increases HOMO level, leading to insufficient driving force for excition dissociation at the D:A interface and low VOC by small difference between HOMOdonor and LUMOacceptor. As a results of the needs for polymers that have both broad light absorption and deep HOMO level, high-performance donor polymers have been developed and singlejunction PSCs based on these polymers achieved PCEs of 9-11%.6-8 Morphology engineering is one of critical factors for determining device performance. In spite of excellent properties of donor polymers, unless morphology optimization is realized in BHJ films consisting of polymers and fullerene derivatives, there are poor device efficiencies due to dominant charge-carrier recombination loss by incomplete formation of pathways for exciton dissociation and charge transport. Among various methods including thermal annealing and solvent-vapor treatment, introduction of processing additives into BHJ film has been commonly used for morphology optimization in BHJ PSCs because of its simple applicability and high reproducibility. Small amount of additive (less than 3 vol.%) led to remarkable enhancement in device performance.9,10 When BHJ components of poly[4,8-bis(5-(2ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) and [6,6]-phenyl-C71



butyric acid methyl ester (PC71BM) dissolved in pure chlorobenzene (CB) were used for the active layer, this device exhibited PCE of 3.92%. In contrast, incorporation of 1,8-diiodooctane (DIO) additive into CB solution led to approximately 2-fold increase in PCE (7.40%).11,12 1Chloronaphthalene (CN) and 1,8-octanedithiol (ODT) were also incorporated in BHJ PSCs and yielded significant increase in device efficiency.13-16 These enhancements resulted from improvements in excition dissociation and charge transport by formation of D:A bicontinuous interpenetrating network. However, processing additives have not guarantee the improvement of device efficiency at all times. Qiao et al. reported that addition of CN and DIO additives in BHJ film of poly(diketopyrrolopyrrole-terthiophene) and PC61BM improved domain purity and led to fast charge collection, while ODT resulted in well-connected domains with smaller size and increased charge collection time.17 The device with CN and DIO exhibited 2-fold PCE enhancement, whereas there was PCE decrease for the device with ODT, compared to that of pristine device without additive. In the same group, they employed 2,3-dihydroxypyridine (DHP) additive in both BHJ films of PDPP3T: PC61BM and poly(3-hexylthiophene):indeneC60 bisadduct (ICBA). 18,19 The device with DHP showed remarkable improvement in device efficiency due to improved polymer crystallinity, bicontinuous interpenertrated phase separation and balanced charge transport. However, few processing additives have been used for morphology engineering. This implies that there are still considerable rooms for new processing additives that can effectively optimize morphology of BHJ film and thereby maximize the device performance. Here, we demonstrate that morphology engineering technique using optimum processing additive successfully optimizes the active layer morphology and improves device efficiency over 10%. In addition, a zinc oxide (ZnO) layer as both electron transport20,21 and optical spacer layer is introduced in between the active layer and top electrode to improve the short-circuit current density (JSC) with enhanced charge generation rate. We introduce various additives into


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BHJ system of poly[[2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b] dithiophene][3fluoro-2[(2-ethylhexyl)carbonyl]thieno-[3,4-b]thiophenediyl]]

(PTB7-Th)

and

PC71BM

(Figure 1a). The device with DPE achieves highest device efficiency of 10.22%. High PCE is attributed to maximized charge generation efficiency and minimized recombination loss by optimized morphology of the active layer and strong interaction between polymer backbones. To compare additive effect on device performance, we employed various additives that are DIO, ODT, CN, and DPE (Figure 1a). We also used PTB7-Th as electron donor and PC71BM as electron acceptor. PTB7-Th with a band gap of 1.7 eV has cascade energy level structure between PC71BM and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), enabling high-performance fullerene-based PSCs with PCE of over 10% (Figure 1b). We fabricated

conventional

PSCs

with

device

configuration

of

indium

tin

oxide

(ITO)/PEDOT:PSS/active layer/Al (Figure 1c). Figure 2a and 2b present current density-voltage (J-V) curves and external quantum efficiency (EQE) of PTB7-Th:PC71BM PSCs as a function of additives, respectively. The device prepared from pure chlorobenzene (CB) exhibited low PCE of 5.93%, which resulted from poor JSC and fill factor (FF). Introducing various additives into the active layer led to remarkable enhancement in device efficiency with the exception of ODT. The device with ODT only showed slight increase in FF. Among the devices with other additives, the device with DPE achieved the highest PCE of 10.22% with JSC of 18.04 mA cm-2, VOC of 0.80 V, and FF of 0.71. This high efficiency is mostly attributed to high JSC and FF, which values are ~13% and ~10% higher than those of the devices with CN and DIO, respectively. However, there were negligible differences of VOC (0.76-0.80 V) in all devices. JSC values calculated from EQE curves are in good agreement with those from J-V curves (Figure 2b). While shapes of EQE curves were similar, EQE values were different as a function of additives. As expected from JV characteristics, the device with DPE exhibited higher EQE of over 65% over the whole



wavelength region, compared to those of the devices with other additives. Accordingly, different EQE values may be attributed to charge-carrier recombination loss during exciton dissociation and charge transport, but not light absorption enhancement. This was confirmed by similar UV-vis absorption spectra of the BHJ films with different additive (Figure S1). To further understand effect of different additives on device perforamance, we performed the photocurrent (Jph) dependence on effective voltage (Veff) and light-intensity dependence of JSC (JSC vs. light intensity). As shown in Figure 2c, Jph as a function of Veff was plotted in log-log coordinates, where the Jph is the difference of the current density under illumination and in dark conditions (Jph = JL − JD) and Veff is the difference between the compensation voltage at Jph = 0 and applied voltage (Veff = V0 –V). The Jphs of all devices became saturated at high Veff of 1.9 ~2 V. The charge collection efficiency (Jph / Jsat) of devices with CN, DIO and DPE are 95.4, 95.7 and 96.3 %, respectively, while Jph / Jsat of the control device and device with ODT show only the 89.3 and 91.5 %, respectively. These increased Jph / Jsat ratios of device with CN, DIO and DPE indicates better exciton dissociation or charge collection efficiency than control device and device with ODT. Since beginning of bimolecular recombination is one of scales to know indirect evidence of decreasing FF value, the high Jph / Jsat value of 96.3% with the device using DPE is good agreement with the highest FF value of 0.71 among all devices. Recombination behavior was further investigated by analyzing the dependence of JSC on light intensity which follows a power law relationship JSC ~ Pα (where P is the light intensity) at the short-circuit condition.22,24 It is well known that when α = 1, non-geminate recombination represents a negligible loss mechanism. As shown in Figure 2d, the α values of optimized devices with CB, CB:ODT, CB:CN, CB:DIO, and CB:DPE were 0.91, 0.91, 0.92, 0.93, and 0.94, respectively. The deviations from α = 1 are ascribed to non-geminate (bimolecular and trap-assisted) recombinations which may limit the JSC.12,22 The high JSC of 18.04 mA cm-2 obtained from the device with CB:DPE is consistent with higher value of α (0.94) compared to


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other devices processed with CB, CB:ODT, CB:CN, and CB:DIO. We also measured atomic force microscopy (AFM) to study morphology change of BHJ films as a function of additives. As shown in Figure 3, the film prepared from CB had root-mean-square (RMS) roughness of 2.2 nm. All additives resulted in even BHJ films with smoother surface apart from ODT (ODT: 3.4e nm, CN: 1. nm, and DIO: 1.28 nm). It is noticeable that the film with DPE exhibited even surface with lowest RMS roughness of 0.9 nm. Uniform and smooth surface also supported high performance of the device with DPE. Furthermore, positive effect of DPE is in good agreement with previous literatures, where DPE led to desirable morphological changes and agglomerated phase separation in the form of fibril-like nanostructures in other BHJ films.23 To further investigate molecular ordering and packing characteristics of pristine polymer and BHJ films, we performed grazing incidence wide angle X-ray scattering (GIWAXS) measurement.24 The GIWAXS patterns of pristine polymer and BHJ films with different additives are shown in Figure S2. The detailed GIWAXS plots and parameters are listed in Figure S3 and Table S1, respectively. All pristine polymer films exhibited reflection peaks in in-plane direction. Corresponding inter-lamellar spacing were 19.7 Å, 20.1 Å, 18.5 Å, 19.0 Å, and 18.5 Å for CB, CB:ODT, CB:CN, CB:DIO, CB:DPE, respectively. Pronounced π-π stacking peaks in out-of-direction were also observed regardless of additives, indicating dominant face-on orientation of polymer backbones. Resultant face-on stacking distances were around 3.9 Å for all pristine polymer films with the except of CN (3.95 Å). The π-π stacking face-on orientation is beneficial to efficient charge transport in PSCs. However, the BHJ blend with any solvent did not show any evidence of differences in lamellar spacing or π-π stacking when blended. Among those processing additives, the DPE may be the most favorable to facilitate aggregation and thorough phase separation of the polymer mixture relative to the other processing conditions, supporting efficient charge transfer and consistent with the high JSC and FF observed with this solvent system.25



Transfer matrix formalism was carried out to estimate improved light manipulation by the insertion of ZnO optical spacer. The formalism is an effective technique to calculate the interaction between a thin film device and incident electric field. It calculates transfer matrix of each thin film layer using optical constants of the corresponding layer, and transmitted and reflected field at each interface between adjacent thin films are further calculated. This method is powerful especially for polymer solar cell, which field interferences is dominant inside the device owing to thin active layer; both incident light and reflected light by top metal electrode have significant influences for the device operation. For the calculation, optical constants of each material, n and k, were deduced from the spectra collected by UV-vis absorption spectroscopy. Refractive index, n, was further corrected using Kramers-Kronig relation. The optical constants are summarized in Figure S4a. Thickness-dependent JSCs were estimated for the devices w/ and w/o ZnO optical spacer and the results are shown in Figure S4b. Devices of the active layer thickness (tactive) less than 140 nm exhibited gradual decreases in JSC as increasing the thickness of ZnO (tZnO). By contrast, devices of tactive > 140 nm outperformed the device w/o optical spacer as a result of constructive interference of incident and reflected light in the proper position (in this case, active layer) of the device. Electric field distribution profiles of the devices with all considered cases in Figure S4b are summarized in Figure S5. The calculated field profiles reveal that JSC reduction for the devices with tactive < 140 nm is attributed to the localization of electric field (EI, the localized fields right under the metal) at the interface between active layer and ZnO, resulting in parasitic absorption by optical spacer. However, ZnO optical spacer effectively rearranged electric field inside the active layer and led to improved light absorption of the device when tactive > 140 nm. Based on the simulated results shown in Figure S4b and Figure S5, tZnO = 30 nm and tactive = 190 and 280 nm were chosen as optimal conditions.


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Spectrally resolved spatial distribution of electric field inside the optimal devices are shown in Figure 4a. For the device with tactive = 280 nm, it is shown that electric fields are strongly localized and form two peaks (EI and EII, EII is the localized fields below EI) inside the active layer at wavelengths from 400 to 800 nm, which are the effective wavelengths for PTB7Th:PC71BM blend system. While, in the case of the device with tactive = 190 nm, EII was not mostly localized within the active layer at wavelengths longer than 600 nm. Considering the fact that the intensity and distribution of residual electric fields are directly proportional to light absorption of the device, it is expected that the device with tactive = 190 nm would exhibit less light absorption in the ranges from 600 to 800 nm. With the help of ZnO optical spacer, however, EII of the device with tactive = 190 nm get included to the active layer even at wavelengths longer than 600 nm, which promotes enhanced light absorption in the wavelengths. For the device with tactive = 280 nm , however, the profile was not significantly distorted by ZnO optical spacer. Hence, it is expected that JSC of the device would be nearly identical. To investigate the role of ZnO is optical spacer and/or electron transport layer, we measured electrical impedance spectroscopy (EIS). EIS measurements were carried out 1 sun condition in the frequency range 0.1 to 1 MHz. The EIS spectra were plotted real and imaginary components to differentiate between the resistive and capacitive components, there single semicircle was observed in Figure S6. The EIS spectra of the solar cells were fitted using a equivalent circuit (Inset of Figure S6), where Rs and Rct are the series and charge transfer resistance, respectively. The introduction of ZnO slightly reduced Rs value from 34.5 to 30.5 ohm. In addition, Rct values were measured as 23.9 ohm and 15.4 ohm for the device without and with ZnO, respectively (Table S2). These similar Rs and Rct values supported similar FF of two devices. In conclusion, we demonstrate the morphology engineering using processing additive and optical engineering using ZnO optical spacer in PSCs based on BHJ film of PTB7-Th:PC71BM.



Among four processing additives (ODT, CN, DIO, and DPE), the device with DPE showed the highest charge collection efficiency and minimized bimolecular recombination during exciton dissociation and charge transport process. In addition, BHJ film with DPE had uniform and smooth surface morphology with well-distributed D:A network. As a result, the introduction of DPE into BHJ film led to the highest efficiency of 10.22% which mostly resulted from enhancements of JSC and FF. Furthermore, the ZnO additional layer coated on top of the active layer played role both as electron transport layer and optical spacer, thereby leading to increased device efficiency up to 10.81% through enhanced light absorption within the active layer and reduced series and charge transfer resistance. Therefore, the morphological and optical engineering for PSCs with BHJ structure by using processing additives and introduction of optical spacer are the effective methods for further improving the device efficiency of PSCs.

Acknowledgements This work was supported by National Research Foundation of Korea (NRF2018R1C1B6001015) and the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning(NRF-2015M1A2A2057506, 2016M1A2A2940914). We would like to thank Hanyang LINC+ Analytical Equipment Center (Seoul). Supporting Information Available: Further characterization of the materials and devices studied. This material is available free of charge via the Internet at http://pubs.acs.org/.


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Figure Legends Figure 1. (a) Chemical structures of PTB7-Th, PC71BM, and various processing additives as components of the active layer. (b) Energy band diagram and (c) device structure of PTB7Th:PC71BM-based PSCs.

Figure 2. Photovoltaic characteristics of ternary blend solar cells. (a) Current density-voltage (J-V) curves, (b) external quantum efficiency (EQE), (c) photocurrent versus effective voltage, and (d) JSC dependence on light intensity of optimized BHJ PSCs with different processing additives.

Figure 3. (a-e) AFM topography and (f-j) phase images of optimum BHJ films prepared from pure CB (a, f), ODT (b, g), CN (c, h), DIO (d, i), and DPE (e, j).

Figure 4. (a) Spectrally resolved spatial distribution of electric field inside the optimized devices, (b) J-V and (c) EQE curves of devices using thick (280nm) and thin (190nm) BHJ film without and with zinc oxide layer (ZnO).


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Figure 1



Figure 2


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Figure 3



Figure 4


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Table 1. Photovoltaic parameters of PTB7-Th:PC71BM PSCs with different additives. Solvent

JSC [mA cm-2]

VOC [V]

FF

PCE [%]

JSC (Cal.)a) [mA cm-2]

Pure CB

14.65 (14.40±0.59)

0.76 (0.76±0.01)

0.53 (0.52±0.02)

5.93 (5.54±0.35)

14.36

CB:ODT

14.69 (14.63±0.36)

0.76 (0.76±0.01)

0.58 (0.57±0.01)

6.47 (6.21±0.24)

14.48

CB:CN

16.60 (16.31±0.28)

0.80 (0.80±0.01)

0.63 (0.63+0.02)

8.27 (7.99±0.26)

16.25

CB:DIO

16.61 (16.42±0.34)

0.77 (0.76±0.01)

8.41 (8.18±0.21)

16.32

CB:DPE

18.04 (17.76±0.24)

0.80 (0.80±0.01)

10.22 (10.05±0.16)

17.72

a)

0.66 (0.65±0.02) 0.71 (0.69±0.02)

JSC (Cal.): Calculated JSC observed from EQE measurement.

Table 2. (a) J-V and (b) EQE curves of devices using thick (280nm) and thin (190nm) BHJ film without and with zinc oxide layer (ZnO).

Thickness of BHJ film [nm]

ZnO

JSC

VOC

[nm]

[mA/cm2]

[V]

280

0

19.76

0.78

280

30

19.65

190

0

190

30

a) Average

Best PCE [%]

JSC (Cal.)b)

(Ave. PCE)a)

[mA/cm2]

0.59

9.19 (9.1)

19.61

0.78

0.59

9.03 (8.9)

19.46

18.04

0.80

0.71

10.22 (10.1)

17.72

19.09

0.80

0.70

10.81 (10.6)

18.73

PCE obtained from 10 devices

b)

FF

JSC (Cal.): Calculated JSC observed from EQE measurement



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