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Negligible Energy Loss During Charge Generation in Small-Molecule/Fullerene Bulk-Heterojunction Solar Cells leads to Open-Circuit Voltage over 1.10 V maxime babics, Tainan Duan, Ahmed H. Balawi, Ru-Ze Liang, Federico Cruciani, Ionela-Daniela Carja, Dale Gottlieb, Iain McCulloch, Koen Vandewal, Frédéric Laquai, and Pierre M. Beaujuge ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02020 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 30, 2019

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ACS Applied Energy Materials

Negligible Energy Loss During Charge Generation in Small-Molecule/Fullerene Bulk-Heterojunction Solar Cells leads to Open-Circuit Voltage over 1.10 V Maxime Babics,⸰,† Tainan Duan, §,†,* Ahmed H. Balawi,‡ Ru-Ze Liang,‡ Federico Cruciani,‡ Ionela-Daniela Carja,‡ Dale Gottlieb,‡ Iain McCulloch,‡,⸰ Koen Vandewal,⸰ Frédéric Laquai,‡ and Pierre M. Beaujuge*,‡ ⸰ Department of Chemistry and Centre for Plastic Electronics, Imperial College London, London, SW7 2AZ, United Kingdom §Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 401122, China. ‡Physical Science and Engineering Division, KAUST Solar Center (KSC), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia. ⸰Institute for Materials Research (IMO-IMOMEC), Hasselt University, Wetenschapspark 1, B3590 Diepenbeek, Belgium. 1

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Keywords: organic photovoltaics, small molecule donor, oligothiophene, fluorination, high opencircuit voltage, low energy loss

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ABSTRACT. Solution-processable small molecules (SM) that can serve as donors in bulkheterojunction (BHJ) solar cells are practical alternatives to their polymer counterparts. However, SM-fullerene blends commonly suffer severe voltage losses. In general, devices that reach open-circuit voltages (VOC) >1 V – yield low photocurrents in BHJ solar cells with fullerene acceptors (e.g. PC71BM) and modest power conversion efficiencies (PCEs). In this contribution, we report on the design, synthesis and BHJ device characteristics of a new SM donor – 2F-DRCN5T – yielding a VOC of up to 1.10 V with PC71BM as the fullerene acceptor, while maintaining PCEs >7% (over 8% achieved upon solvent-vapor annealing (SVA) treatment). The negligible energy loss during charge generation (ΔECT), the deep-lying HOMO of 2F-DRCN5T inferred from its large ionization potential (IP), the high charge transfer state energy (ECT) of the blend and a reduced non-radiative voltage loss account for the high VOC achieved in BHJ solar cells.

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Representing promising alternatives to their polymer counterparts for a wide range of optoelectronic applications, solution-processable small-molecule (SM) donors with precise molecular weight and size have two important benefits: (i) ease of purification and (ii) batch-tobatch reproducibility.1-2 In bulk-heterojunction (BHJ) solar cells with fullerene acceptors, such as PC71BM, a maximum open-circuit voltage of 1.10-1.15V is theoretically achievable,3 but SM donor-based BHJ solar cells with VOC values >1V have rarely been described.4-7 Often, in BHJ devices, the high VOC comes at the expense of the photocurrent and the resulting power conversion efficiencies (PCEs) fall in the range of 2-4%, thus limiting their value for practical applications. Therefore, achieving high-VOC devices that maintain high photocurrents and convincingly high PCEs remains challenging, and the design principles for efficient SM donor-fullerene BHJ solar cells with VOC values reaching 1.10V are deserving further examinations. With the recent development of non-fullerene acceptors (NFAs), VOC values >1V have more frequently been accessed.8-13 In particular, the higher-lying lowest unoccupied molecular orbital (LUMO) of NFAs have largely contributed to increasing the VOC in BHJ solar cells with polymer donors. In parallel, the voltage loss Vloss of the corresponding devices –defined as Vloss=Eg/qVOC– has been shown to reach values as low as 0.5V.14-15 Nevertheless, achieving efficient BHJ solar cells (PCEs>7%) with VOC values reaching 1.10V remains challenging also with NFAs.14, 16 In recent works, SM donors –including DRCN5T and analogous structures– were described as particularly efficient in BHJ solar cells when combined with the fullerene acceptor PC71BM, achieving PCEs >10% with a high VOC of ca. 0.92V.17 In this contribution, we report on a synthetic modification of DRCN5T, namely the new derivative 2F-DRCN5T, yielding a VOC of up to 1.10 V in BHJ solar cells with PC71BM as 5



fullerene acceptor. Importantly, the devices maintain PCEs >7%, and over 8% can be achieved upon solvent-vapor annealing (SVA). The fluorine substitution at the core unit of 2F-DRCN5T (see Chart 1) increases its ionization potential (IP), in turn suppressing its HOMO and raising the VOC in BHJ solar cells. Our investigations show that the high VOC is a result of a negligible loss during charge generation (ΔECT = Eopt - ECT), a high charge transfer (CT) state energy independent of the morphology and a reduced non-radiative voltage loss compared to the reference molecule.

Chart 1. Molecular Structures of the SM Donor 2F-DRCN5T and the Fullerene Acceptor PC71BM.

The experimental details for the synthesis of 2F-DRCN5T are provided in the Supporting Information (SI). First-level density functional theory (DFT) calculations (Figure S1) show that the SM donor is prone to adopting a fully planar conformation and that the -system is well delocalized along the SM backbone (Figure S2). Thermogravimetric analyses (TGA, Figure S3) 6


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confirms the thermal stability of the molecule within the temperature range usually used in BHJ solar cell device optimizations; here, noting an onset of decomposition (5% loss) at ca. 385ºC comparable to that of the reference system DRCN5T (389ºC).17 Differential scanning calorimetry (DSC, Figure S4) measurements indicate that the molecule undergoes melting upon heating to 238ºC (Tm) and crystallization upon cooling to 225ºC (TC), which shows the propensity of the SM donor to crystallize.

Figure 1. (a) Thin-film and solution UV-Vis absorption spectra of the SM donor 2F-DRCN5T; film cast from chloroform at room temperature (25 oC) and solution in chloroform. (b) PESAestimated IP and EA. EA values inferred by subtracting IP and Eg values. Figure 1a provides the superimposed thin-film and solution UV-Vis absorption spectra of the neat SM donor 2F-DRCN5T. In solution, the spectral absorption is broad and featureless, whereas, in thin films spin-coated from chloroform solutions, the absorption shifts to longer wavelengths with an absorption onset at ca. 725 nm. The large bathochromic shift between solution and thin film absorption arises from the propensity of the small molecule to aggregate.18 The inset of Figure 1a emphasizes the dramatic color change on going from solution to the solid state. After annealing the film at 120ºC for 10 minutes, clear vibronic features appear at ca. 600 7



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nm and 660 nm, which can be assigned to aggregation and increased order of the molecules upon annealing.19 The structural order of the neat SM donor 2F-DRCN5T before and after thermal treatment was examined by X-ray diffraction (Figure S5). As seen from Figure S5, both films show a pronounced diffraction peak (100) at 2θ= 4.9º corresponding to a d-spacing of 18 Å. In fact, the enhancement of the peak intensity after thermal annealing confirms the higher degree of order/crystallinity obtained upon thermal annealing. The IP of 2F-DRCN5T measured via photoelectron spectroscopy in air (PESA, see details in the SI) is 5.65 eV, which is significantly higher than those of commonly reported SM donors for which the IP values often fall in the range of 5.0-5.3eV.2 In comparison, the IP of the reference system DRCN5T obtained with the same technique is measured to be 5.50eV (Figure S6). As expected, the IP of the fluorine-substituted SM donor 2F-DRCN5T is higher than that of the reference system: by 0.15eV. From the onset of thin-film absorption (at 725 nm), the optical gap of 2F-DRCN5T can be inferred as 1.71 eV. Compared to the non-fluorinated counterpart DRCN5T, the substitution with fluorine increases the optical gap by ca. 0.15 eV. Looking at the schematic energy diagram provided in Figure 1b, a higher VOC can be expected in BHJ solar cells that would incorporate the fluorine-substituted SM analogue 2F-DRCN5T. Device Characterization Table 1. Summary of PV Device Performancea Parameters for 2F-DRCN5T:PC71BM Blend System Subjected to Various Post-processing Treatments.

Treatment

As-cast TAb

VOC

JSC

FF

[V] [mA/cm2] [%] 1.12 6.9 47.6 1.10 11.7 56.0

Avg.

Max.

[%] PCEa 3.66 7.22

[%] PCE 3.98 7.31

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SVAc aAdditional

1.08 11.9

63.6 8.13

8.30

device statistics are provided in Table S1 in the Supporting Information. bThermal

annealing cSolvent Vapor Annealing.

Figure 2. (a) Characteristic J-V curves and (b) EQE spectra of BHJ solar cells made with the 2FDRCN5T:PC71BM blend system subjected to various post-processing treatments: thermal annealing (TA) and solvent vapor annealing (SVA) with the solvent chloroform.

Solution-processed thin-film BHJ solar cells were fabricated using the direct device architecture ITO/PEDOT:PSS/SM:PC71BM/Phen-NaDPO/Al. (device area: 0.1 cm2) and were tested under simulated AM1.5G illumination (100mW/cm2); detailed experimental protocols and additional optimization results are provided in Table S1-4. Table 1 summarizes the device performance results obtained upon using different post-processing treatments and Figure 2a shows the J-V curves of the 2F-DRCN5T devices. The device performance of the reference DRCN5T:PC71BM blend is detailed Table S1. The reference blend DRCN5T:PC71BM have a VOC of 0.94 V, a shortcircuit current of 14.5 mA cm-2 and fill-factor (FF) of 58.5%. The average power conversion efficiency (PCE) is ca. 8.27%. These results are in agreement with the previous literature for annealed films.17 Turning to the 2F-DRCN5T:PC71BM blend, the “as-cast” devices have a high 9



VOC of 1.12 V, but a modest JSC of 6.9 mA cm-2 and a low FF of 47.6%. The average power PCE of these devices is ca. 3.7%. After thermal annealing (TA) at 120oC for 10 min, the JSC increases significantly to 11.7 mA cm-2 and the FF increases to 56%, yielding a PCE of ca. 7.2% on average. In comparison, after solvent vapor annealing (SVA) with DCM for 20 s, the Jsc reaches 11.9 mA cm-2, the FF increases further to 63.6% and, while the VOC can be maintained at 1.08 V, the BHJ solar cells exhibit average PCEs of 8.13% (Max. 8.30%). Here, we note that a high VOC of ca. 1.10 V is also attained when calcium is used as an electron-extracting layer, instead of Phen-NaDPO (see Table S4); therefore, the high voltage is not a consequence of interlayer optimization, but rather an intrinsic characteristic of the BHJ blend system 2FDRCN5T:PC71BM.20 Looking at the external quantum efficiency (EQE) data (Figure 2b), the “as-cast” device shows a rather modest EQE in the range of 30-40%, in agreement with the modest JSC measured. For optimized BHJ devices subjected to thermal annealing (TA) or solvent vapor annealing (SVA), the EQE spectra are virtually the same, showing significantly higher values across the whole spectrum, with an average of ca. 62% between 500 and 660 nm. Compared to the “as-cast” device, the EQE drop is seen only after 670 nm, which is consistent with the UV-Vis spectra of the blend films subjected to post-processing treatment and the more pronounced absorption seen for the first vibronic at 660 nm (see Figure S7). Integrated EQEs are in agreement (±0.3 mA/cm2) with the Jsc values reported in Table 1. To explain the performance differences between 2F-DRCN5T-based BHJ solar cells, we examined the evolution of JSC with different light intensities (Figure S8-9), data from which the extent of bimolecular recombination can be inferred.21 It has been shown that the relationship between JSC and light intensity follows the equation JSC∝Iα where a value of α equal to unity 10


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implies that bimolecular recombination in the devices is negligible at short-circuit conditions. α values 8%) without sacrificing the high ECT accounting for the negligible energy loss during charge generation; hence, the high VOC (1.08V). This work provides a practical example of material design to increase the open-circuit voltage of BHJ solar cells.

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ASSOCIATED CONTENT Experimental methods, characterization, and additional figures and tables are provided in the supporting information Author Contributions. † The authors contributed equally Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This publication is based upon work supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award No. CRG_R2_13_BEAU_KAUST_1. The authors acknowledge concurrent support under Baseline Research Funding from KAUST. Dr. Duan acknowledges financial support from National Natural Science Foundation of China (No. 21702202).

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