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Optimized Phase Separation and Reduced Geminate Recombination in High Fill Factor SmallMolecule Organic Solar Cells Jun Yan, Quanbin Liang, Kai-Kai Liu, Jing-Sheng Miao, Hui Chen, Sha Liu, Zhicai He, Hong-Bin Wu, Jin-Liang Wang, and Yong Cao ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00556 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on December 1, 2016
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ACS Energy Letters
Optimized Phase Separation and Reduced Geminate Recombination in High Fill Factor Small Molecule Organic Solar Cells Jun Yan,† Quanbin Liang,† Kaikai Liu,‡ Jingsheng Miao,† Hui Chen,† Sha Liu,† Zhicai He,† Hongbin Wu,*,† Jinliang Wang,*,‡ and Yong Cao† †
Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of
Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, China. ‡
Beijing Key Laboratory of Photoelectronic/ Electrophotonic Conversion Materials, Beijing
Institute of Technology, Beijing, 100081, China. AUTHOR INFORMATION Corresponding Authors *E-mail:
[email protected].*E-mail:
[email protected] Author Contributions J. Yan and Q. Liang contributed equally to this work. Notes The authors declare no competing financial interest.
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ABSTRACT: In the recent few years, rapid increase in the power conversion efficiency above 10% in small-molecule-based organic solar cells (SM-OSCs) has been made possible. However, one of the key device parameters, fill factor, which is mainly limited by comprehensive courses, including charge generation, recombination, transport and extraction, still remains moderate. Here we demonstrate a record high FF of 78.35% in SM-OSCs obtained through dichloromethane solvent vapor annealing, which provides optimized phase-separation morphology for efficient charge generation and facilitates charge transport/extraction at the same time. A combined charge dynamic measurement and current-voltage characteristics reconstruction allow us to identify that geminate recombination loss that resulted from undesired film morphology is mainly responsible for low FF in the pristine devices. Even higher FF that comparable with that of crystal silicon solar cells in organic solar cells is very likely with the presence of charge mobility around 5×10-3 cm2V-1s-1 and proper film morphology. TOC GRAPHICS 0 -2
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Organic solar cells (OSCs) have attracted intense attentions as promising next-generation renewable energy source. 1-3 In the past few years, the power conversion efficiency (PCE) of polymer-based OSCs have been improved to over 11%.4 Small-molecules are another type of organic donor material for high performance OSCs, which are mono-dispersed and well-defined in chemical structure. Most importantly, the use of small-molecules for OSCs can offer unique features of ease of purification, superior batch-to-batch reproducible property, avoid end-group contaminants, and minimize energetic/structure disorder.5 Nowadays, the PCE of the state-of-theart SM-OSCs have exceeded 10%,6 proving the potential for future applications. Three device performance parameters, open circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF) determine the PCE (𝜂𝑃 ) of a solar cell via 𝜂𝑃 =
𝑉𝑂𝐶 ⋅ 𝐽𝑆𝐶 ⋅ 𝐹𝐹 , 𝑃𝐼𝑁
(1)
where 𝑃𝐼𝑁 is the incident light power. Therefore, in order to obtain high efficiency device, all of device performance parameters should be maximized at the same time. While a lot of studies have been reported to enhance Voc7,8 and JSC9,10of OSCs, relatively less attention had been focused on FF itself. Generally, the FF of a solar cell is determined by the competition between charge extraction and recombination,11 and has been identified to be closely related to the morphology of photoactive layer,12,13 the recombination process upon photo excitation,14-18 active layer thickness, 19,20 charge transport 21-24 and the balance between negative and positive charge carriers.25 Figure 1a shows a non-exhaustive survey of reports that focus on the FF of OSCs, as a function of charge mobility. As can be clearly seen, while the FF of the resultant devices shows a strong dependence on charge mobility, it also diverges even at the same charge mobility. Hence, in order to obtain high FF in OSCs, both improved charge carrier transport/extraction and suppressed recombination loss as obtained from an optimal phase-
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separated morphology in nanoscale need to be satisfied. Until now, the record high FF in polymer solar cells is ~80%,26 while the best FF in SM-OSCs is 76-77% (see Figure 1a)5,27,28. As more and more novel small molecules emerge as promising donor materials for high performance SMOSCs, especially some of them can deliver high charge mobility, higher FF should be obtainable if with the presence of more preferred film morphology and improved charge transport/collection properties. Recently, several simple and versatile processing techniques, including thermal annealing (TA),6,29,30 solvent vapor annealing (SVA)28,31 and solvent additives (SA)32,33have been proved to be very efficient in controlling the film morphology of the photoactive layer of SM-OSCs, hence become route technologies for enhanced device performance. In this work, we report the realization of record high FF (~78.35%) in highly efficient SM-OSCs (with a PCE of 9.00%) based on the blend of a newly developed small molecule BIT-4F reported by Wang et al.5and 6,6-phenyl C71-butyric acid methylester (PC71BM), by controlling the active layer morphology through dichloromethane (CH2Cl2)solvent vapor annealing treatment (referred to as "SVA annealed"). To look deep into the origin of this super high FF, we carried out various research methods, including atomic force microscope (AFM) and high-resolution transmission electron microscope (TEM) based photo-active layer morphology analysis, transient photovoltage (TPV) and photocurrent (TPC) based carrier dynamics study, and drift-diffusion based electrical model simulation. Table 1. The device performance with different SVA treatment times. SVA time(s) Voc (V) Jsc (mA cm-2) PCE (%) FF (FFMAX) (%) 0 0.95±0.00(0.95) 10.79±0.30(11.24) 6.24±0.17(6.56) 60.93±2.40(62.04) 10 0.89±0.00(0.89) 11.21±0.30(11.64) 7.23±0.23(7.59) 72.54±0.50(73.23) 20 0.87±0.01(0.88) 11.06±0.22(11.32) 7.31±0.18(7.47) 75.80±0.45(76.37) 30 0.89±0.01(0.89) 12.56±0.48(13.15) 8.61±0.32(9.00) 77.31±0.38(78.35) 40 0.87±0.01(0.88) 9.09±0.73(9.97) 5.62±0.49(6.28) 71.25±1.17(72.91) 60 0.90±0.01(0.91) 5.18±0.08(5.27) 3.29±0.06(3.36) 70.54±0.15(70.80) The average values calculated from >10 devices with standard deviation for the measurements.
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Al Cathode PFN Interlayer Active Layer PEDOT:PSS ITO Anode Glass Substrate
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Figure 1. Fill factor review, device/molecule structures and characterizations. a) The fill factor of organic solar cells published to date versus hole mobility from the science database. b) chemical structures of BIT-4F and PC71BM. c) devices structure.d)the typical J-V characteristics the devices with different treatment time, tested under AM 1.5G (1000 Wm-2) illuminationin air. (a) RMS = 0.450 nm
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Figure 2. Post-treatment-dependent morphology. a-f) Tapping mode AFM height images 5×5 μm2 and a'-f') high-resolution TEM images under different SVA treatment times: (a, a') 0s, (b, b') 10s, (c, c') 20s, (d, d') 30s, (e, e') 40s, (f, f') 60s, respectively.
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The chemical structure of the donor/acceptor material for this study and the OSCs device structure are presented in Figure 1b-c. Figure 1d shows the typical current density versus voltage characteristics (J-V) of a series of BIT-4F:PC71BM SM-OSCs with varied treatment time, tested under 1000 W m-2 air mass 1.5 global (AM 1.5G) illumination in air. The deduced device performance parameters of the devices were summarized in Table 1 for comparison. It is worthy to note that shorter time treatment (10s~30s) can also effectively increase the PCE from ~6.24% to over 9.00%, while longer treatment time (40s~60s) resulted in a much reduced PCE of ~3%, mainly due to an obvious decrease in JSC from ~12.6 mA cm-2(ca. 12.50 mA cm-2 in FigureS1a) to 5~6 mA cm-2. Nevertheless, all of the SVA annealed devices shows high FFs well above 70% regardless of treatment duration, which is a strong sign for improved charge transport and extraction properties. Moreover, the device with a 30s SVA treatment shows the highest FF (~78.35%) with high repeatability (see FigureS1b), which is the highest value reported in the literature to date for SM-OSCs. In order to understand the origin of the significantly improved FF, we firstly investigated the morphology of the photoactive layer through atomic force microscope (AFM) and highresolution transmission electron microscope (TEM). For the pristine films without treatment (referred to as "non-annealed"), the surface of the photoactive layer is very smooth and homogeneous, with a root-mean-square roughness (RMS) of ~0.450 nm and inconspicuous phase-separated feature. The average phase-separated domain size (PDS) is negligible (Figure 2a, a'), indicating that the electron donor material BIT-4F has good miscibility with PC71BM. With the increase in the SVA treatment time up to 60 sec., the RMS and the PDS of the film increases rapidly from 0.450 nm to 5.96 nm and from ~ 0 nm to 125±10 nm, respectively. The best FF device was obtained when a 30s SVA treatment was applied, which corresponds to a RMS and a PDS of 1.123 nm and 18±2 nm, respectively (see Figure 2d, d'). More importantly, obvious phase
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separation and bi-continuous interpenetrating network could be clearly observed, which is beneficial for exciton dissociation at the donor/acceptor interfaces and the subsequent charge generation and transport. On the other side, owing to the limited exciton diffusion length in SMOSCs (~typically 10-15 nm) and reduction of interfacial area between the donor/acceptor interface, phase separation with too large domain size (ca. 40 nm) can lead to insufficient exciton dissociation. Indeed, as shown in Figure S3a, the solid-state photoluminescence of neat BIT-4F film (with an emission peak at 837 nm) is nearly completely quenched by PC71BM when short SVA treatment time is employed (0-30s), which is an indicative of efficient exciton dissociation. However, under longer treatment time (40-60s), quenching efficiency decreases notably. Therefore, the decrease in JSC in devices with prolonged SVA treatment time can be attributed to the observed increasing average phase-separated domain size (see Figure 2e~f,e'~f') and a much less efficient charge generation therein. Besides the above mentioned change in morphology, the high FF in all of the SVA annealed devices suggested that the carrier dynamics properties of the devices should be improved as well. To shed light on these processes, we further apply transient photovoltage (TPV), photocurrent (TPC) and differential charging techniques to investigate the charge dynamics inside the operating devices. 34-37 The charge density as a function of VOC (tuned by changing the light intensity) for typical BIT-4F:PC71BM devices before and after SVA treatment, which was measured by using differential charging method,34 is shown in Figure 3a. The charge density at open circuit conditions in all devices show a clear exponential dependence on VOC,38 following 𝑛 = 𝑛0 𝑒 𝛾𝑉𝑂𝐶
(2)
Where 𝑛0 is the average charge density in the active layer in dark and 𝛾 is the slope of the ln(n)~ VOC curves. The values of 𝛾 of each cures are found to give similar results, but much less than the value expected for ideal semiconductor (which should corresponds to 𝑒⁄2𝑘𝐵 𝑇 and is ACS Paragon Plus Environment
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around 19.3 V-1 at room temperature).38 We note that the deviation can be attributed to the existence of an exponential distribution of tail states extending to the bandgap of the photoactive layer,36 which is a characteristics of charge transport in disordered medium. Surprisingly, at the same light intensity, the non-annealed device shows a highest charge density, which can be assigned to more efficient photocurrent generation and/or lower non-geminate recombination rate, as we show below. Indeed, less efficient photocurrent generation in the SVA annealed device can be assigned to the enhanced phase separation that lowering the extent of intermixing and the donor/acceptor interfacial area (see Figure 2), while the slower charge recombination in the nonannealed devices can be evidenced later in Figure 3c. 16 16 16
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Figure 3. Transient photovoltage and photocurrent analysis. a) The measured charge densities in the devices as a function of VOC for different bias light intensities. The dashed lines represent a best fit to an exponential dependence. b) The derived charge lifetime in the devices as function of charge density. The dashed lines represent a best fit to a power law dependence. The star symbols in Figure 3a and Figure 3b corresponds to one sun light intensity. c) The measured non-geminate recombination rate coefficient for the devices. The dashed lines represent Langevin recombination rate coefficient for the non-annealed (black) and SVA annealed devices (red), respectively. d) Calculated photo-generated current density as a function of voltage in the devices.
Besides the analysis on the charge density, we also apply transient photovoltage (TPV) technique to determine the small perturbation charge lifetime (τΔ𝑛 ) in the devices by fitting the ACS Paragon Plus Environment
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ACS Energy Letters
TPV transient with a mono-exponential decay course, with the results shown in Figure 3b. And the total charge lifetime (𝜏(𝑛)) can be determined through34-37 𝜏(𝑛) = 𝜏𝛥𝑛 (𝜆 + 1)
(3)
where (𝜆 + 1) represents the order of non-geminate recombination, and relate the nongeminate recombination rate 𝑅(𝑛) with 𝜏(𝑛) through 𝑅(𝑛) =
𝑛 ∝ 𝑛 𝜆+1 𝜏(𝑛)
(4)
Consistent with previous study,34,36,37 all of the devices shows a decreasing lifetime with increasing charge carrier density, following a power law dependence upon charge density according to τΔ𝑛 = τΔ𝑛0 ⋅ 𝑛−𝜆 , indicating that non-geminate recombination is the dominating loss pathway for charge carriers under open-circuit conditions. Furthermore, the magnitude of the slope for each curve in Figure 3b is found to be nearly the same, which can be explained by unchanged exponential distribution of tail states in the photoactive layers,37,39 even upon different SVA treatment. Nevertheless, as compared to that in the SVA annealed device, the charge lifetime in the non-annealed shows the slowest decay dynamic, in good agreement with highest charge density at the same light intensity (see Figure 3a). On the other side, when biased at 1 sun light intensity, all of the devices exhibits similar charge lifetime on order of 10-6 sec (as indicated by the stars in Figure 3b), which is similar with other observations in literature.36,40 The reason responsible for the observed nearly constant lifetime in all of the devices at the same light intensity is not clear at this stage, but was thought to be associated with the transition between bimolecular to monomolecular recombination.40 With the data in Figure 3a and Figure 3b, the non-geminate recombination rate coefficient 𝑘(𝑛) can be determined (shown in Figure 3c), which is given by
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𝑘(𝑛) =
1 1 = . 𝜏(𝑛)𝑛 𝜏∆𝑛 (𝜆 + 1)𝑛
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(5)
It can be clearly seen, 𝑘(𝑛) in the SVA annealed devices increases in all range of charge densities in this study, which is expected to occur as the coefficient for Langevin recombination (defined as 𝑘𝐿 = 𝜀
𝑞 0 𝜀𝑟
(𝜇𝑛 + 𝜇𝑝 ) ) increase with the charge mobility upon SVA annealing. It
should also be that with the increase in SVA treatment time, increased phase separation and reduced intermixing and the donor/acceptor interfacial area (see Figure 2), complicate the analysis of the influence of SVA treatment on charge dynamics. Nevertheless, consistent with the lower charge density and higher non-geminate recombination rate coefficient, both were obtained under open circuit condition, at which non-geminate recombination is the dominating loss pathway for charge carriers), the SVA annealed devices indeed show lower VOC (see Table 1). This is understandable since a reduced charge density in the photoactive layer corresponds to a reduced quasi Fermi level splitting, while a faster non-geminate recombination could result in an extra loss in VOC. From the above results of charge density and charge lifetime, together with the analysis model proposed by Durrant et al.,16,35 the non-geminate recombination flux (𝐽𝑅𝑒𝑐 ) for each device can be determined, which is given by 𝐽𝑅𝑒𝑐 (𝑛, 𝑉) = 𝑒𝑑𝑅(𝑛, 𝑉) = 𝑒𝑑𝑘(𝑛, 𝑉)𝑛2 = 𝑒𝑑𝑛(𝑉)⁄𝜏(𝑛, 𝑉) .
(6)
On the quantitative analysis of non-geminate recombination loss in the devices, we examine a more complete picture on the origin of high FF in the SVA annealed devices by considering the field-dependent photocurrent in the devices. We calculate the free charge generation current density 𝐽𝐺𝑒𝑛 (𝑉)1515,16 𝐽𝐺𝑒𝑛 (𝑉) = 𝐽𝐿𝑖𝑔ℎ𝑡 (𝑉)−𝐽𝑅𝑒𝑐 (𝑛, 𝑉),
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where 𝐽𝐿𝑖𝑔ℎ𝑡 (𝑉) is the measured current density as obtained under 1 sun AM 1.5G illumination. Figure 3d depicts the normalized 𝐽𝐺𝑒𝑛 (𝑉) as a function of applied voltage for all of devices. As compared to the SVA annealed devices, 𝐽𝐺𝑒𝑛 (𝑉)of the non-annealed device shows a much stronger field dependence, implying severe geminate recombination loss that could not be neglected in the pristine devices. As a result of this loss pathway for photocurrent, the nonannealed device only shows a moderate FF of 61%. In contrast, the severe geminate recombination loss is effectively suppressed upon the SVA treatment, leading to high FF over 70%. It is worthy to note that despite of their much faster non-geminate recombination rate (R) (defined as 𝑅(𝑛) = 𝑑𝑛⁄𝑑𝑡 = −𝑘(𝑛)𝑛2, see Figure S3b) in the SVA annealed devices, loss of photocurrent due to non-geminate recombination is compensated by a nearly constant photogenerated current density (Figure 3d). Therefore, considering non-geminate recombination loss alone can enable us to reconstruct the J-V characteristics of these devices.16,35,39 As expected, as shown in Figure S4, the reconstructed and the experimental data for the 30s SVA annealed devices coincide very well, confirming non-geminate loss is the only limiting loss mechanism in these devices (see Figure S4d). In contrast, the reconstructed curve of non-annealed device shows apparent deviation from the experimental data (see Figure S4a), indicating both severe geminate and non-geminate losses are responsible for the reduced device performance.35,39, 41 Furthermore, we can see a clear revolution in current generation through increasing SVA treatment time, which is consistent with Figure 3d and highlights the importance of treatment time. We further employ a classical mono-dimensional drift-diffusion model that includes fielddependent charge generation, geminate recombination, and field-independent non-geminate recombination dominated by bimolecular recombination of free charge carriers,42,43 and modified Langevin recombination coefficient, to quantify the charge generation in the devices. The ACS Paragon Plus Environment
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simulation is completed by solving a set of differential equations including the Poisson equation, the continuity equation and drift-diffusion equations simultaneously (see Supporting Information). (a) Exciton Dissociation Rate
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average: ~ 0.92
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Figure 4. Drift-diffusion model analysis. a) The calculated exciton dissociation rate (P) as a funciton of applied voltage. b-c) A two-dimensional contour plot showing the dependence of calculated fill factor (FF) (b) and power conversion efficiency (PCE) (c) on charge (electron/hole) mobilities, respectively. Each contour plot is displayed by isolines calculated from the drift-diffusion model and filled using colormap shown above the plot, respectively.
As shown in Figure S5, with the use of the data in Table S1 as the input parameters, all of the experimental J-V characteristics can be consistently described by the model. On the basis of the best fitting results, the exciton (or e/h pair) dissociation rate (P) vs. applied voltage characteristics is calculated and shown in Figure 4a. Impressively, the device with the SVA treatment for 30s shows an average P of 92% (which implies a low total geminate recombination loss of 8%), while the control device shows an average P of 62% (corresponding to a geminate recombination loss of 38%), consistent with the observations in Figure 3d. Thus, the SVA treatment facilitates exciton dissociation and reduces geminate recombination. Finally, by using the whole set of data in the column of 30s in Table S1 as input parameters with exclusion of charge mobility, we calculated the upper limit of the FF as a function of charge mobilities (Figure 4b). Ideally, an ultrahigh FF of >80%, which is comparable to that of silicon solar cells (75%~82%) can be reached in SM-OSCs if both of hole and electron mobility are within the range between 10-3 cm2 V-1 s-1and 1cm2 V-1 s-1. We also note that the calculation results agree with those by Bartesaghi et al.,11Bartelt et al.,44 and Proctor et al.,45 respectively. In case of higher mobilities exceeding 1cm2 V-1 s-1, a slight decrease in FF is expected, mainly due ACS Paragon Plus Environment
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to an increased non-geminate recombination as the mobilities increase. The calculated PCE values show similar dependence on charge mobilities (see Figure 4c), but the optimum mobility region towards high PCE is found to be narrower than that of the FF contour. This can be understood because a reduction in VOC (see Figure S6a) is expected as a result of faster charge extraction rate and concomitant reduction in the quasi-Femi level gaps.21 Differently, the calculated JSC shows a gradual increase at low mobilities (
