Morphology-Dependent Hole Transfer under Negligible HOMO

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Morphology Dependent Hole Transfer Under Negligible HOMO Difference in Non-Fullerene Acceptor Based Ternary Polymer Solar Cells Taehyo Kim, Jungwoo Heo, Ji Young Lee, Yung Jin Yoon, Tack Ho Lee, Yun Seop Shin, In-Sik Kim, Hyojung Kim, Mun Seok Jeong, In-Wook Hwang, Bright Walker, Pil Sung Jo, Bogyu Lim, and Jin Young Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20884 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 2, 2019

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ACS Applied Materials & Interfaces

Morphology Dependent Hole Transfer Under Negligible HOMO Difference in Non-Fullerene Acceptor Based Ternary Polymer Solar Cells Taehyo Kim,a Jungwoo Heo b Ji Young Lee, c Yung Jin Yoon, a Tack Ho Lee, a Yun Seop Shin, a

In-Sik Kim, d Hyojung Kim, e Mun Seok Jeong, e In-Wook Hwang, f Bright Walker, g Pil

Sung Jo, h Bogyu Lim* ,c,i and Jin Young Kim,*,a,b aDepartment

of Energy Engineering, Ulsan National Institute of Science and Technology

(UNIST), Ulsan 44919, South Korea bDepartment

of Physics, Ulsan National Institute of Science and Technology (UNIST), Ulsan

44919, South Korea cFuture

Technology Research Center, Corporate R&D, LG Chem/LG Science Park, 30,

Magokjungang 10-ro, Gangseo-gu, Seoul, 07796, Republic of Korea dDepartment

of Physics and Photon Science, Gwangju Institute of Science and Technology,

Gwangju, 61005, Republic of Korea. eDepartment

of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of

Korea. fAdvanced

Photonics Research Institute, Gwangju Institute of Science and Technology,

Gwangju, 61005, Republic of Korea. gDepartment

of Chemistry, Kyung Hee University, Seoul, 02447, South Korea

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

hPlatform

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Technology Research Center, Corporate R&D, LG Chem, 188, Munji-ro, Yuseong-

gu, Daejeon, 34122, Republic of Korea iGreen

Fine Chemical Research Center, Korea Research Institute of Chemical Technology,

Ulsan, 44412, Republic of Korea

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

ABSTRACT

In the field of organic solar cell (OSCs), it has been generally accepted until recently that a difference in band energies of at least 0.3 eV between the highest occupied molecular orbital (HOMO) level of the donor and the HOMO of the acceptor is required to provide adequate driving force for efficient photo-induced hole transfer, due to the large binding energy of excitons in organic materials. In this work we investigate polymeric donor : nonfullerene acceptor junctions in binary and ternary blend polymer solar cells (PSCs) which exhibit efficient photoinduced hole transfer despite negligible HOMO offset and demonstrate that hole transfer in this system is morphology-dependent. The morphology of the organic blend


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was gradually tuned by controlling the amount of ITIC and PC70BM. High external quantum efficiency (EQE) was achieved at long wavelengths, despite a 1:9 ITIC to PC70BM ratio, which indicates efficient photo-induced hole transfer from ITIC to the donor despite an undesirable HOMO energy offset. Transient absorption spectra (TAS) further confirm that hole transfer from ITIC to the donor becomes more efficient upon optimizing the morphology of the ternary blend, compared to that of donor:ITIC binary blend.

KEYWORDS: ternary polymer solar cell, hole transfer, morphology, non-fullerene acceptor, organic solar cell, energy loss, conjugated polymer


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1. INTRODUCTION Light-weight, mechanical flexibility, large-area processibility and the potential for low-cost manufacturing have made polymer solar cells a promising candidate for next generation solar cells.1-5 The relatively low power conversion efficiency (PCE) of polymer solar cells, however, still prevents them from being a major competitor in the current photovoltaic market.6,7 Concerted efforts by chemists and physicists in the solar cell research community have led to new material development8-11 as well as elucidation and optimization of charge carrier dynamics.12-14 These efforts have paid off, with certified power conversion efficiencies reaching up to 17.3% recently15, which is greater than many researchers thought possible 15 years ago, yet still lower than most commercial polycrystalline silicon solar cells. The ability of conjugated polymers to effectively absorb solar irradiation into the near infrared region has developed considerably in recent years. For example Kim and coworkers reported PSC devices exhibiting high quantum efficiency at wavelengths up to 900 nm, resulting in more than 20 mA/cm2 of short-circuit current density (JSC).16 One may expect that there is a room for further improvement in the PCE of polymer solar cells; in particular the

VOC of polymer solar cells is generally thought to be limited by differences between the HOMO of donor (EHOMO,D) and lowest unoccupied molecular orbital (LUMO) of the acceptor (ELUMO,A). Until recently, a significant energy loss has been thought to be necessary to overcome the high binding energy of Frenkel type excitons in organic materials and separate electrons and holes into ELUMO,A and EHOMO,D, respectively. This energy loss causes the resultant open circuit


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voltage (VOC) of OPV devices to be relatively low compared to their band gaps. For the efficient charge transfer, effective HOMO and LUMO offsets between donor and acceptor (ΔHOMO, and ΔLUMO, simultaneously) energy bands have been empirically observed to be ~ 0.3 eV1718,

which limits the efficiency of device based on this design.

Interestingly, since the development of the efficient non-fullerene acceptor, 3,9bis(2methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d:2’3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene) (ITIC), a few papers have reported that ITIC-based bulkheterojunction (BHJ) PSCs occasionally do not necessarily required the aforementioned energy loss associated with charge dissociation. For instance, Zhang et al.19 reported that a ΔHOMO value of 0.2 eV is adequate in BHJ solar cells using a side-chain modified ITIC derivative, resulting in 9.3% PCE under optimal conditions. Bin et al.20 used ITIC with a trialkylsilyl substituted 2D-conjugated polymer possessing a deep HOMO level, and a reduced ΔHOMO in the blend of 0.11 eV. This system successfully yielded exceptionally high PCE values of up to 11.41%. Recently, Eastham et al.21 investigated several donor materials having different energy levels relative to ITIC and demonstrated a breakthrough result - that even in type I heterojunctions (where HOMO and LUMO energy levels of one material are positioned inside the bandgap of the other material), efficient hole transfer is possible and strongly depends on the blend morphology rather than the energy level alignment.


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Despite this remarkable result, questions remain about whether the effect is solely dependent on the morphology or whether the electronic band structure of the BHJ blend is necessarily changed concomitantly while changing the donor material. In order to reduce the complexity and number of variables, we designed an experiment by utilizing a single donor polymer in combination with ITIC, which forms a type I heterojunction, with an additional C70 fullerene derivative (PC70BM) which allows morphological optimization of the ternary BHJ blend. Strongly aggregated phases of the donor:ITIC binary blend were observed via atomic force microscopy and the agglomerates were reduced after adding a considerable amount of PC70BM in the mixture. Photovoltaic parameters and photoluminescence (PL) spectrum of donor:ITIC binary BHJ solar cells confirmed that exciton dissociation and charge transfer occur even in type I heterojunctions, consistent with the results of Eastham et al.21, although it was also noted that the ITIC agglomerates induced both radiative and non-radiative recombinations. The addition of PC70BM to the binary blend allowed the suppression of the recombination process by optimizing the blend morphology and resulted optimal PCEs of up to 9.73%. Furthermore, a significant improvement in the efficiency of hole transfer from ITIC to the donor phase was observed after the addition of PC70BM to the donor:ITIC binary blend via transient absorption spectroscopy. Considering energy band structure of PC70BM, it is hard to say that PC70BM plays a significant role in the hole transfer process; the improvement in hole transfer from ITIC to the donor is mainly dependent on the morphology of the blend. This experimental design allows the observation of morphology-dependent hole transfer in a single type I heterojunction system and confirms that hole transfer in polymer:ITIC systems is


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not strictly determined by the ΔHOMO value, but to the blend morphology as well. This result extends the choice of donor materials, which is currently limited to the material showing good morphological and energy compatibility with ITIC for high efficiency polymer solar cells.

2. EXPERIMENTAL SECTION General : Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) (Baytron AI 4083 and PH500) were purchased from H. C. Starck (Germany). PTFFTB was synthesized according to the literature.1 [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM) and 3,9-bis(2methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (ITIC) were purchased from Organic Semiconductor Materials (OSM) and 1-materials Index Co., Ltd. The optical properties of composite films were analyzed using a UV-vis spectrophotometer (Varian Carry 5000). Atomic force microscopy (AFM) images (2 μm × 2 μm) were obtained using a Veeco AFM microscope in tapping mode. 2D-GIXRD measurement: GIXRD measurements were carried out at the PLS-II 9A U-SAXS beam line of the Pohang Accelerator Laboratory, Korea. The X-ray beam, coming from the invacuum undulator (IVU), was monochromated (Ek = 11.06 keV, λ = 1.103 Å) using a Si(111) double crystal monochromator and focused horizontally and vertically at the sample position (450 (H) x 60 (V) μm2 in FWHM) using a KB type focusing mirror system. The GIXRD sample stage was equipped with a 7-axis motorized stage for the fine alignment of the thin sample and the incidence angle of X-ray was adjusted in the range of 0.12º~0.14º. GIXRD patterns were recorded with a 2D CCD detector (Rayonix SX165, USA) and X-ray irradiation time was 0.5 ~ 5 s


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dependent on the saturation level of the detector. The diffraction angle was calibrated using a reference sucrose crystal (Monoclinic, P21, a = 10.8631 Å, b = 8.7044 Å, c = 7.7624 Å, ß = 102.938o) and the sample-to-detector distance was about 232 mm. Femtosecond TA measurements: Femtosecond transient absorption spectra and decays were recorded using a commercial transient absorption spectrometer (HELIOS, Ultrafast Systems) combined with a 1 kHz repetition rate femtosecond Ti:sapphire regenerative amplifier system (Legend, Coherent). A pump pulse at 750 nm with a power density of 15 µJ/cm2 was produced using an optical parametric amplifier (TOPAS-OPA, Coherent) and a neutral density filter, while a broadband white-light continuum probe pulse was generated by focusing a small portion of the 800 nm amplifier output into a sapphire window. Transient absorption signals in respective optical delays of pump and probe pulses were collected using an optical fiber coupled to a multichannel spectrometer. The transient absorption decay time constants were obtained by first deconvoluting the measured signal from the pump time profile (characterized by a full width at half maximum of ~120 fs) and then fitting to a sum of exponential terms: -OD(t) = A1 exp(-t/1) + A2 exp(-t/2 + A3 exp(-t/), where -OD(t) is the transient photobleaching signal intensity, A the amplitude (noted in parentheses as the normalized percentage i.e., [Ai / (A1 + A2 + A3)] × 100), and  is the fitted decay time.

Fabrication and characterization of PSCs: PSC devices were fabricated with a configuration of ITO/PEDOT:PSS/PTFFTB:PC70BM:ITIC/Al. The devices were fabricated according to the following procedures: First, ITO-coated glass substrates were cleaned with detergent, then sequentially ultra-sonicated in acetone and isopropyl alcohol and dried in an oven overnight at 100 °C. PEDOT:PSS layers were spin-coated (after filtration through a 0.45 μm filter) at 5000


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rpm for 40 s, baked at 140 °C for 10 min in air and then moved into a nitrogen filled glove box. A mixed solution (12 mg/ml) of PTFFTB: PC70BM (or PTFFTB:ITIC or PTFFTB:(PC70BM +ITIC)) (blend ratio = 1:1.5 by weight) in chlorobenzene: diphenylether (DPE) (97:3 vol%) was spin-coated at 1300 rpm for 60 s on top of the PEDOT:PSS layers. Devices were then brought under vacuum (< 10–6 torr), and a 100 nm thick Al electrode was deposited on top of the active layer by thermal evaporation. The deposited Al electrode area defined the active area of the devices as 13 mm2. Measurements were carried out inside a glove box using a high quality optical fiber to guide the light from the solar simulator. J-V characteristics were measured under AM 1.5G illumination (100 mWcm-2) with a Keithley 2635A source measurement unit. EQE measurements were conducted in air using an EQE system (Model QEX7) by PV measurements Inc. (Boulder, Colorado).

3. RESULTS AND DISCUSSION As the VOC of polymer solar cells is primarily determined by the difference between EHOMO,D and ELUMO,A, a donor material with a deep HOMO value is required to achieve a high VOC. In this study, a thienopyrroledione and benzodithiophene containing random copolymer (PBTTFB, detailed molecular structure is shown in Figure 1a),22 which possesses a deep HOMO band energy of 5.65 eV and a wide bandgap of 1.89 eV, was used as donor material. PBTTFB absorbs light up to ~ 660 nm, as shown in Figure 1. Energy levels of each material were measured by collection of ultraviolet photoelectron spectra (Figure S1, and Table S1, measured


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relative to an Au reference with work function of 5.15 eV) and UV-vis absorption spectra of each material. The two acceptor materials exhibit complementary light absorption with PBTTFB covering a wide range of wavelengths from 300 nm to 800 nm; short wavelengths (300 to 450 nm) are mainly absorbed by PC70BM while long wavelengths (from 650 nm to 800 nm) are absorbed by ITIC. The ΔLUMO between PBTTFB and ITIC (PC70BM) is 0.25 eV (0.51 eV), which is close to the empirical value of 0.3 eV for efficient electron transfer from PBTTFB to acceptors. However, 0.26 eV of ΔLUMO between two acceptors doesn’t allow for efficient electron transfer from ITIC to PC70BM, a point which is confirmed by PL spectra (Figure S2) of the two acceptors blend and the photovoltaic parameters of a solar cell based on the blend (Figure S3). PL spectra of the ITIC:PC70BM blend reveal that exciton dissociation at the interface of the two materials hardly occurs, preserving the PL intensity of ITIC. Furthermore, the extremely low JSC of ITIC:PC70BM solar cells, in spite of a significant light absorption by ITIC, supports poor photo-induced charge transfer between the two acceptor materials. Hence, two specific electron transfer pathways are possible in the PBTTFB:ITIC:PC70BM ternary blend; (1) PBTTFB to ITIC and (2) PBTTFB to PC70BM. A ΔHOMO value of 0.52 eV between PBTTFB and PC70BM is sufficient for hole transfer from PC70BM to PBTTFB. Contrary to PC70BM, ITIC exhibits a relatively shallow HOMO compared to that of PBTTFB, thus forming a type I heterojunction. PBTTFB:ITIC binary and PBTTFB:ITIC:PC70BM ternary blend polymer solar cells were fabricated with the device configuration of ITO/PEDOT:PSS/active layer/Al, where


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PEDOT:PSS (poly (3, 4-ethylenedioxythiophene): poly (styrene-sulfonate)) was utilized as a hole transport layer. J-V characteristics of the fabricated ternary polymer solar cells (TPSCs) are shown in Figure 2a and photovoltaic parameters are summarized in Table 1. PBTTFB:ITIC solar cells exhibited a moderate PCE of 5.91%. Despite the moderate PCE, the JSC of 12.59 mA/cm2 implies that photo-induced hole transfer from ITIC to PBTTFB occurred readily despite being a type I heterojunction. The deep HOMO level of PBTTFB resulted an exceptional VOC of 1.04 V, however, the low fill factor (FF) limited the PCE to less than 6%. PL spectra of PBTTFB:ITIC binary blends (Figure S4) showed residual PL from ITIC even after blending with PBTTFB. That is, incomplete hole transfer from ITIC to PBTTFB causes radiative recombination from ITIC domains. Increasing the fraction of PC70BM in the active layer further increased the JSC and FF with a slight decrease in VOC, except for the device with a 7:3 acceptor ratio, which exhibited a particularly low JSC of 9.10 mA/cm2. PBTTFB:PC70BM binary solar cells exhibited 7.98% PCE with the highest FF of 0.74 among all the conditions, however, the JSC was relatively low to compared to devices using ITIC. The highest PCE was achieved in devices with a 1:9 acceptor ratio (Figure 2b). Compared to PBTTFB:PC70BM binary solar cell, the 1:9 ternary device maintained its high FF together with a significantly increased

JSC, arising from the extended light absorption range (up to 800 nm) afforded by ITIC (Figure 2c). Furthermore, the difference between EQE spectra in the wavelengths from 650 to 780 nm, between the devices with 1:9 and 10:0 acceptor ratio, was not dramatic even if the difference in the amount of ITIC in the active blend is huge, showing that only 10% ITIC component in the


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active layer was able to absorb sufficient amount of light in the wavelengths from 650 to 800 nm. The absorbance difference between 1:9 and 1:10 thin films are shown in Figure S6. Electric field distribution inside the device was calculated by the transfer matrix formalism23. Figure S5 reveals that localized electric field in the wavelengths from 650 to 800 nm is effectively attenuated upon increasing the contents of ITIC from 0 to 10%. (Compare Figure S5a and b). Based on the information of residual electric field and extinction coefficient of the active layer, the exciton generation rate was derived. (Figure S5c). Under the assumption that the area under the exciton generation rate curve is proportional to the JSC produced by each device, a 25.6% improvement in exciton generation rate (from 10.97 × 1021 cm-3 s-1 to 13.78 × 1021 cm-3 s-1 for PBTTFB:PC70BM binary and 1:9 ternary devices, respectively.) is highly consistent with the observation that the JSC of the 1:9 ternary device is improved by 24.9% compared to the PBTTFB: PC70BM binary device. From this result, it is apparent that the increase of JSC in the 1:9 ternary device is largely from the improved exciton generation rate due to extended light harvesting ability in the near IR range by ITIC. This result is noteworthy because only a small proportion (10%) of ITIC is enough to effectively extend absorption into the near IR under optimized conditions. We examined charge recombination characteristics by the incident light intensity dependent JSC and Voc.24-26 Figure 2d shows a log-log plot of JSC as a function of light intensity. The curve was fitted according to the power -law dependence of JSC on the light intensity, where α indicated the slope of log-log plot of JSC of versus light intensity. The light intensity


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dependence of JSC can be used to determine if the JSC is limited by bimolecular recombination. Except PBTTFB:ITIC binary blends, alpha value of PBTTFB:PC70BM binary blend and 1:9 ternary blend showed close to 1, showing low bimolecular recombination. The light intensity dependence on Voc is shown in Figure 2e using logarithmic scale on both axes. Slopes of two binary samples, PBTTFB:ITIC and PBTTFB:PC70BM, were 1.61 kT/q and 1.41 kT/q, respectively, where k is the Boltzmann constant, T is the temperature in K, and q is the elementary charge. The slopes close to 1.5 kT/q imply that Shockley-Read-Hall (SRH) recombination is dominant in the binary samples. While, the slope of the optimized ternary blend, PBTTFB:ITIC:PC70BM (1:9), exhibited 1.06 kT/q, which the optimized blend is relatively free from SRH recombination loss. To investigate the charge generation and extraction process, the photocurrent versus effective voltage was also measured for ternary blend PSCs27 (Figure 2f). Jph is defined as the difference between the current density and dark condition (Jph – JD). Veff is the voltage (V0) at

Jph =0 and applied voltage, V (Veff = V0 - V) The saturation of Jph suggests the effective charge sweep out and extraction of photogenerated charge carriers with negligible recombination. The PBTTFB:PC70BM binary blend and 1:9 ternary blend showed saturation of Jph at the lower

Veff = ~0.3V. This is good agreement with the improved JSC and FF observed in the ternary blend PSCs. In order to characterize the morphology of ternary blend films, AFM was carried out (See Figure 3). PBTTFB:ITIC binary blends showed large and inhomogeneous domains on the


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surface with a root-mean-square (RMS) roughness value of 4.64 nm. After increasing the amount of PC70BM in the blend, strong aggregation was observed; The blend with a 7:3 acceptor ratio showed a very high RMS roughness of 16.9 nm. Upon increasing the contents of PC70BM, however, the size of agglomerates become smaller with smoother RMS roughness. The aggregation observed in the ternary blends may arise from the strong intermolecular interaction between PC70BM and ITIC. These unintended agglomerates would cause inefficient charge transfer between active materials and, as a result, the performance of devices would decrease. The PBTTFB:PC70BM binary blend showed the smoothest surface roughness (1.17 nm) with homogeneous phase mixing, which is consistent with ideal mixing between PBTTFB and PC70BM and the least carrier recombination before charge collection at the electrodes, resulting in the device with high FF as shown in Figure 2. At a 1:9 acceptor ratio, smooth and homogeneous morphology, similar to the 0:10 binary blend was produced, which implies that 10% of ITIC in the blend preserves the ideal morphology formed with the binary PBTTFB and PC70BM mixture. The variation of photovoltaic parameters depending on the acceptor ratio, as shown in Figure 2 and Table 1 is well consistent with the morphological study. GIWAXS spectral linecuts and images further support our interpretations above. As shown in Figure 4, with 10% ITIC in the blend, molecular packing is nearly identical to PBTTFB:PC70BM binary blend with no peaks characteristic of ITIC. However, more than 10% of ITIC content in the blend causes diffraction peaks corresponding to ITIC aggregation to appear, including a sharp peak near q = 0.35 Å-1 in the x-y plane, which is not observed in PBTTFB:PC70BM binary blend. To find underlying carrier dynamics for the improved PCE of ACS Paragon Plus Environment

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ternary composite, we have performed femtosecond transient absorption (TA) spectroscopy for the binary (PBTTFB:ITIC) and ternary (PBTTFB:ITIC:PC70BM) composites (Fig. 5). When photoexcited at 750 nm i.e., exclusively photoexcited at ITIC, the binary composite shows a fast photobleaching (PB) time decay at 700 nm (characterized by time constants of 0.6 ps (34%), 2.8 ps (42%), and 45.4 ps (24%), yielding an average decay time of 12.3 ps) i.e., at the ITIC absorption regime (Fig. 5c). This fast time decay also coincides with the rise time at ~530 nm i.e., at the PBTTFB absorption regime. The ternary composite shows much faster PB time decay (characterized by time constants of 0.1 ps (84%) and 2.6 ps (16%), yielding an averaged decay time of 0.5 ps) at 700 nm (Fig. 5d). Because the PB signal-decay is a direct measurement of the ground state recovery from all the photogenerated excitations, the fast PB time decay at 700 nm may result from the hole transfer process from ITIC to PBTTFB after photoexcitation of ITIC, as demonstrated by Li et al. for the binary composite of J61 polymer and BTIC acceptor.28 The faster and thus more efficient hole transfer process of the ternary composite, principally resulting from PC70BM involvements to the energy level and the morphology of the system, is believed to lead to higher PCE of the ternary composite. 4. CONCLUSION In summary, we observed efficient hole transfer in polymer solar cells comprising a donor – acceptor pair (the wide band gap polymer, PBTTFB, and ITIC) with negligible ΔHOMO. Even without PC70BM addition to PBTTFB:ITIC, these type I heterojunction solar cells exhibited reasonable JSC and PCE, which demonstrates that a significant ΔHOMO is not strictly necessary


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to promote photo-induced hole transfer; a point which has been long-standing assumption in the field of OPVs. Despite reasonably good hole-transfer efficiency, significant radiative recombination from ITIC domains was observed in the PL spectra. Addition of PC70BM to the BHJ blend enabled control of the morphology of the active layer and suppressed radiative recombination energy losses. Optimization of solar cells with variable ITIC:PC70BM ratios led to significantly increased PCE values of up to of 9.73% by reducing ITIC agglomeration in the blend. Morphological study using AFM and GIWAXS revealed that strong intermolecular interactions exist between ITIC and PC70BM in this system and the amount of PC70BM strongly affect to the blend morphology to prevent ITIC agglomeration. Finally, TA decay profiles proved that fast hole transfer from ITIC to PBTTFB occurs and that the transfer rate is greatly accelerated upon adding PC70BM to the blend. Considering the energy level of PC70BM, the enhancement in hole transfer from ITIC to PBTTFB is primarily due to optimization of the blend morphology via inhibition of ITIC agglomeration. Our results suggest that relative position of EHOMO,D (in this case, slightly deeper EHOMO,D than EHOMO,A) does not necessarily determine the efficiency of hole transfer from ITIC to the donor, and that the morphology of BHJ blend has a greater impact on hole transfer rates in this system. This work also demonstrates that improvements in VOC, in conjunction with high JSCs, may provide a pathway to improve the efficiency of PSC beyond previous, empirically estimated efficiency limits in polymer solar cells, which until recently, were widely accepted as insurmountable limitations to PSCs.


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ASSOCIATED CONTENT Supporting Information. Experimental details, ultraviolet photoelectron spectroscopy, photoluminescence and simulated spatial distribution of squared electric field intensity with binary and ternary PSCs. AUTHOR INFORMATION Author Contribution T.Kim and J. Heo contributed equally to this work. J. Y. Kim and B. Lim conceived the project. T. Kim and J. Heo designed and performed the overall experiments. J. Y. Lee and P. S. Jo synthesized organic materials. performed device fabrication. Y. J. Yoon, T. H. Lee, Y. S. Shin and B. Walker measured and analysed UPS, GIWAXS, Abs. and PL, AFM, respectively. I.-S. Kim, I.-W. Hwang, H. Kim and M. Jeong analysed TAS. T. Kim and J. Heo wrote the manuscript. B. Lim and J. Y. Kim contributed and commented on the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20123010010140, No. 20173010012960), the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016M1A2A2940914), and the LG Chem Research Fund.


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REFERENCES (1)

Krebs, F. C., Fabrication and Processing of Polymer Solar Cells: A Review of Printing

and Coating Techniques. Sol. Energy Mater. Sol. Cells 2009, 93, 394-412. (2)

Liang, N.; Jiang, W.; Hou, J.; Wang, Z., New Developments in Non-Fullerene Small

Molecule Acceptors for Polymer Solar Cells. Mater. Chem. Front. 2017, 1, 1291-1303. (3)

Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L., Recent Advances in

Bulk Heterojunction Polymer Solar Cells. Chem. Rev. 2015, 115, 12666-12731. (4)

Song, S.; Kranthiraja, K.; Heo, J.; Kim, T.; Walker, B.; Jin, S.-H.; Kim, J. Y., Efficiency

Exceeding 11% in Tandem Polymer Solar Cells Employing High Open-Circuit Voltage Wide-Bandgap π-Conjugated Polymers. Adv. Energy Mater. 2017, 7, 1700782. (5)

Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J.,

Efficient Tandem Polymer Solar Cells Fabricated by All-Solution Processing. Science 2007,

317, 222-225. (6)

International Technology Roadmap for Photovoltaic Results 2016.

http://www.itrpv.net/Reports/Downloads/2017/ (accessed July 31, 2018) (7)

Fraunhofer Institute for Solar Energy Systems. Photovoltaics Report.

https://www.ise.fraunhofer.de/content/dam/ise/de/documents/publications/studies/Photovolt aics-Report.pdf (accessed July 31, 2018)


19


(8)

Page 20 of 30

Dou, L.; Liu, Y.; Hong, Z.; Li, G.; Yang, Y., Low-Bandgap Near-IR Conjugated

Polymers/Molecules for Organic Electronics. Chem. Rev. 2015, 115, 12633-12665. (9)

Zhang, M.; Guo, X.; Zhang, S.; Hou, J., Synergistic Effect of Fluorination on Molecular

Energy Level Modulation in Highly Efficient Photovoltaic Polymers. Adv. Mater. 2014, 26, 1118-1123. (10) Nguyen, T. L.; Choi, H.; Ko, S. J.; Uddin, M. A.; Walker, B.; Yum, S.; Jeong, J. E.; Yun, M. H.; Shin, T. J.; Hwang, S.; Kim, J. Y.; Woo, H. Y., Semi-Crystalline Photovoltaic Polymers with Efficiency Exceeding 9% in a ~300 nm Thick Conventional Single-Cell Device. Energy

Environ. Sci. 2014, 7, 3040-3051. (11) Lu, L.; Yu, L., Understanding Low Bandgap Polymer PTB7 and Optimizing Polymer Solar Cells Based on It. Adv. Mater. 2014, 26, 4413-4430. (12) Le Corre, V. M.; Chatri, A. R.; Doumon, N. Y.; Koster, L. J. A., Charge Carrier Extraction in Organic Solar Cells Governed by Steady-State Mobilities. Adv. Energy Mater. 2017, 7, 1701138. (13) Stuart, A. C.; Tumbleston, J. R.; Zhou, H.; Li, W.; Liu, S.; Ade, H.; You, W., Fluorine Substituents Reduce Charge Recombination and Drive Structure and Morphology Development in Polymer Solar Cells. J. Am. Chem. Soc. 2013, 135, 1806-1815.


20

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Mihailetchi, V. D.; Xie, H. X.; de Boer, B.; Koster, L. J. A.; Blom, P. W. M., Charge Transport and Photocurrent Generation in Poly(3-hexylthiophene): Methanofullerene BulkHeterojunction Solar Cells. Adv. Funct. Mater. 2006, 16, 699-708. (15) Meng, L.; Zhang, Y.; Wan, X.; Li, C.; Zhang, X. Wang, Y.; Ke, X.; Xiao, Z., Ding, L.; Xia, R., Organic and Solution-Processed Tandem Solar Cells with 17.3% Efficieney. Science 2018, 361, 1094-1098. (16) Choi, H.; Ko, S. J.; Kim, T.; Morin, P. O.; Walker, B.; Lee, B. H.; Leclerc, M.; Kim, J. Y.; Heeger, A. J., Small-Bandgap Polymer Solar Cells with Unprecedented Short-Circuit Current Density and High Fill Factor. Adv. Mater. 2015, 27, 3318-3324. (17) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J., Design Rules for Donors in Bulk-Heterojunction Solar Cells—Towards 10 % Energy-Conversion Efficiency. Adv. Mater. 2006, 18, 789-794. (18) Liu, J.; Chen, S. S.; Qian, D. P.; Gautam, B.; Yang, G. F.; Zhao, J. B.; Bergqvist, J.; Zhang, F. L.; Ma, W.; Ade, H.; Inganas, O.; Gundogdu, K.; Gao, F.; Yan, H., Fast Charge Separation in a Non-Fullerene Organic Solar Cell with a Small Driving Force. Nat. Energy 2016, 1, 16089. (19) Zhang, Z.; Feng, L.; Xu, S.; Liu, Y.; Peng, H.; Zhang, Z. G.; Li, Y.; Zou, Y., A New Electron Acceptor with meta-Alkoxyphenyl Side Chain for Fullerene-Free Polymer Solar Cells with 9.3% Efficiency. Adv. Sci. (Weinh) 2017, 4, 1700152.


21


Page 22 of 30

(20) Bin, H.; Gao, L.; Zhang, Z. G.; Yang, Y.; Zhang, Y.; Zhang, C.; Chen, S.; Xue, L.; Yang, C.; Xiao, M.; Li, Y., 11.4% Efficiency Non-Fullerene Polymer Solar Cells with Trialkylsilyl Substituted 2D-Conjugated Polymer as Donor. Nat. Commun. 2016, 7, 13651. (21) Eastham, N. D.; Logsdon, J. L.; Manley, E. F.; Aldrich, T. J.; Leonardi, M. J.; Wang, G.; Powers-Riggs, N. E.; Young, R. M.; Chen, L. X.; Wasielewski, M. R.; Melkonyan, F. S.; Chang, R. P. H.; Marks, T. J., Hole-Transfer Dependence on Blend Morphology and Energy Level Alignment in Polymer: ITIC Photovoltaic Materials. Adv. Mater. 2018, 30, 1704263. (22) Kim, T.; Lee, J. Y.; Heo, J.; Lim, B.; Kim, J. Y., Highly Efficient Polymer Solar Cells with a Thienopyrroledione and Benzodithiophene Containing Planar Random Copolymer.

Polym. Chem. 2018, 9, 1216-1222. (23) Burkhard, G. F.; Hoke, E. T.; McGehee, M. D., Accounting for Interference, Scattering, and Electrode Absorption to Make Accurate Internal Quantum Efficiency Measurements in Organic and Other Thin Solar Cells. Adv. Mater. 2010, 22, 3293-3297. (24) Lu, L.; Xu, T.; Chen, W.; Landry, E. S.; Yu, L., Ternary Blend Polymer Solar Cells with Enhanced Power Conversion Efficiency. Nat. Photonics 2014, 8, 716 (25) Kyaw, A. K. K.; Wang, D. H.; Gupta, V.; Leong, W. L.; Ke, L.; Bazan, G. C.; Heeger, A. J., Intensity Dependence of Current-Voltage Characteristics and Recombination in HighEfficiency Solution-Processed Small-Molecule Solar Cells. ACS Nano 2013, 7, 4569


22

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(26) Bauer, N.; Zhang, Q.; Zhao, J.; Ye, L.; Kim, J-H.; Constantinou, I.; Yan, L.; So, F.; Ade, H.; Yan, H.; You, W., Comparing Non-Fullerene Acceptors with Fullerene in Polymer Solar Cells: A Case Study with FTAZ and PyCNTAZ. J. Mater. Chem. A 2017, 5, 4886 (27) Mihailetchi, V. D.; Wildeman, J.; Blom, P. W. M., Space-Charge Limited Photocurrent. Phys. Rev. Lett. 2005, 94, 126602 (28) Li, Y.; Zhong, L.; Gautam, B.; Bin, H.-J.; Lin, J.-D.; Wu, F.-P.; Zhang, Z.; Jiang, Z.-Q.; Zhang, Z.-G.; Gundogdu, K.; Li, Y.; Liao, L.-S., A Near-Infrared Non-Fullerene Electron Acceptor for High Performance Polymer Solar Cells. Energy Environ. Sci. 2017, 10, 16101620.


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Figure 1. (a) Molecular structures, (b) UV-vis-NIR absorption spectra, and (c) schematic energy level diagram of PBTTFB, ITIC and PC70BM.


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Figure 2. (a) J-V characteristics of TPCSs with various blend ratios and (b) the characteristics at the optimum condition. (c) External quantum efficiency (solid) and integrated JSC (dotted). Light intensity dependent (d) JSC, (e) Voc, and (f) photocurrent versus effective voltage.


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Figure 3. AFM topographical images (2 μm by 2 μm) for ternary blend films prepared on ITO/PEDOT:PSS pre-coated glass substrates. (X:Y = ITIC:PC70BM).


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0.0

0

2.0 1000

10

)

100

0.5 0

qxy

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-1

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(Å

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(Å

(Å

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-1

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1.5

-1

)

0:10

1000 100 10

1.0 0.5 0.0

-1

0

qxy

)

1 (Å

1 (Å

-1

)

(c)

Figure 4. 2D-GIWAXS images for (a) ternary blend films on Si substrate with controlled ratio (X:Y = ITIC: PC70BM). (b) In-plane and (c) out-of-plane line-cuts of the data shown in (a).


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Figure 5. Femtosecond transient absorption (a, b) spectra and (c, d) decay profiles for films of binary (PBTTFB:ITIC) and ternary (PBTTFB:ITIC:PC70BM) composites. The spectra and decays were obtained using a pump wavelength of 750 nm and probe wavelength of 700 nm. Fitted lines for the decays are indicated.


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Table 1. Summary of TPSC photovoltaic parameters Acceptor ratio (ITIC:PC70BM) 10:0 7:3 5:5 3:7 0:10 1:9 (Opt.)

JSC

2

(mA/cm ) 12.52±0.32 (12.59) 8.81±0.30 (9.10) 12.56±0.85 (13.40) 13.95±0.41 (14.40) 10.62±0.16 (10.79) 13.55±0.18 (13.73)

VOC (V)

FF

η (%)

1.04±0.00 (1.04) 1.02±0.00 (1.02) 1.01±0.01 (1.02) 1.00±0.01 (1.01) 0.99±0.01 (0.99) 1.01±0.00 (1.01)

0.45±0.00 (0.45) 0.49±0.00 (0.49) 0.53±0.01 (0.54) 0.60±0.00 (0.60) 0.73±0.01 (0.74) 0.71±0.00 (0.71)

5.80±0.11 (5.91) 4.38±0.13 (4.51) 7.17±0.12 (7.29) 8.41±0.24 (8.66) 7.81±0.17 (7.98) 9.54±0.19 (9.73)


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TOC GRAPHICS


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Morphology-Dependent Hole Transfer under Negligible HOMO

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