Research Article www.acsami.org
Impact of Thermal Annealing on Organic Photovoltaic Cells Using Regioisomeric Donor−Acceptor−Acceptor Molecules Tao Zhang,†,⊥ Han Han,‡,⊥ Yunlong Zou,† Ying-Chi Lee,‡ Hiroya Oshima,§ Ken-Tsung Wong,*,‡,∥ and Russell J. Holmes*,† †
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan § Department of Chemistry, Nagoya University, Nagoya 464-8602, Japan ∥ Institute of Atomic and Molecular Science, Academia Sinica, Taipei 10617, Taiwan ‡
S Supporting Information *
ABSTRACT: We report a promising set of donor−acceptor−acceptor (D−A−A) electron-donor materials based on coplanar thieno[3,2-b]/ [2,3-b]indole, benzo[c][1,2,5]thiadiazole, and dicyanovinylene, which are found to show broadband absorption with high extinction coefficients. The role of the regioisomeric electron-donating thienoindole moiety on the physical and structural properties is examined. Bulk heterojunction (BHJ) organic photovoltaic cells (OPVs) based on the thieno[2,3-b]indole-based electron donor NTU-2, using C70 as an electron acceptor, show a champion power conversion efficiency of 5.2% under AM 1.5G solar simulated illumination. This efficiency is limited by a low fill factor (FF), as has previously been the case in D−A−A systems. In order to identify the origin of the limited FF, further insight into donor layer chargetransport behavior is realized by examining planar heterojunction OPVs, with emphasis on the evolution of film morphology with thermal annealing. Compared to as-deposited OPVs that exhibit insufficient donor crystallinity, crystalline OPVs based on annealed thin films show an increase in the short-circuit current density, FF, and power conversion efficiency. These results suggest that that the crystallization of D−A−A molecules might not be realized spontaneously at room temperature and that further processing is needed to realize efficient charge transport in these materials. KEYWORDS: organic photovoltaic cells, push−pull molecules, thermal annealing, crystallinity, charge collection
1. INTRODUCTION Organic photovoltaic cells (OPVs) are promising for low-cost solar energy harvesting due to their mechanical flexibility and compatibility with high-throughput roll-to-roll manufacturing.1 Despite this, further increases in device efficiency are needed in order to realize widespread application.2 Photoconversion in an OPV can be divided into four component processes: light absorption, exciton diffusion, charge transfer and exciton dissociation, and charge collection.3 In order to maximize the efficiencies of both light absorption and charge transfer, significant effort has been directed at developing electron donor and acceptor materials with strong broadband light absorption and tunable molecular orbital energy levels.4−6 Recently, organic small molecules with a donor−acceptor− acceptor (D−A−A) molecular configuration have been demonstrated with easily tunable molecular orbital energy levels and optoelectronic properties.6 As both a narrow optical gap and a deep-lying highest occupied molecular orbital (HOMO) energy level can be achieved in D−A−A type molecules, these systems have the potential to simultaneously © XXXX American Chemical Society
show high short-circuit current density (JSC) and open-circuit voltage (VOC) when utilized as the electron donor in an OPV. To date, over 20 D−A−A electron donor molecules have been reported in bulk heterojunction (BHJ) and planar-mixed heterojunction (PMHJ) OPVs.5−13 Although these devices generally show high JSC and VOC, the fill factor (FF) rarely exceeds 0.5, which ultimately limits device efficiency.8−13 Therefore, in order for these promising molecules to realize their full efficiency potential, the origin of the low FF must be understood, as must strategies to improve device performance. Although there are many factors that can limit device FF, in D−A−A donors, the intrinsically large ground-state dipole moments could play a role.6 When film morphology is poorly optimized, a large ground-state dipole moment could contribute to energetic disorder.14,15 This could adversely affect charge-carrier recombination rates under forward bias and Received: April 15, 2017 Accepted: June 29, 2017
A
DOI: 10.1021/acsami.7b05304 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Molecular structures of NTU-1 and NTU-2. (b) Optical constants for 30 nm thick films of NTU-1, NTU-2, and DTDCPB on glass substrates and the extinction coefficient of C70 measured by spectroscopic ellipsometry.
Table 1. Physical Properties of NTU-1 and NTU-2
NTU-1 NTU-2
λmaxa (nm)
ε × 104 a (M−1·cm−1)
λmaxfilm b (nm)
Egopt c (eV)
Eoxonset d (V)
Eredonset e (V)
HOMOf (eV)
LUMOg (eV)
Egh (eV)
Tdi (°C)
569 602
4.49 5.59
552 560
1.69 1.60
0.74 0.58
−0.92 −0.96
−5.54 −5.38
−3.88 −3.84
1.66 1.54
298 314
ε is molar extinction coeffient measured in 10−5 M CH2Cl2 solution. bEstimated from maximum thin-film extinction coefficient. cEstimated from onset of thin-film extinction coefficient; dIn CH2Cl2 with 0.1 M TBAPF6 as supporting electrolyte, Fc/Fc+ = 0 eV. eIn THF with 0.1 M TBAP as supporting electrolyte, Fc/Fc+ = 0 eV. fHOMO = −4.8 − (Eox,onset − EFc,onset). gLUMO = −4.8 − (Ered,onset − EFc,onset). hDifference between HOMO and LUMO. iTd is the decomposition temperature corresponding to 5% weight loss from TGA analysis under N2 at heating rate of 10 °C·min−1. a
contribute to a low FF.16 Griffith et al.17 have previously reported that the D−A−A donor 2-{[7-(4-N,N-ditolylaminophenylen-1-yl)benzo[c][1,2,5]thiadiazol-4-yl]methylene}malononitrile (DTDCPB) tends to form nanocrystalline clusters when mixed with fullerene at room temperature. The formation of centrosymmetric DTDCPB dimers with cofacial π−π antiparallel alignment can act to reduce the overall dipole moment and lead to high conductivity. Accordingly, optimized vacuum-deposited OPVs based on the D−A pairing of DTDCPB and C70 have demonstrated a high FF of 0.67 and power conversion efficiencies of ηP = 8−10%.17,18 Despite these high efficiencies, additional modification of the donor to realize increased extinction coefficients and deeper HOMO levels could further increase JSC and VOC. Here we report two new D−A−A molecules: 2-{[7-(4-ethyl4H-thieno[3,2-b]indol-2-yl)benzo[c][1,2,5]thiadiazol-4-yl]methylene}malononitrile (NTU-1) and 2-{[7-(8-ethyl-8Hthieno[2,3-b]indol-2-yl)benzo[c][1,2,5]thiadiazol-4-yl]methylene}malononitrile (NTU-2). These molecules contain the same acceptor−acceptor moieties as the previously studied DTDCPB, but they include electron-donating moieties that are coplanar, so that they are likely to dimerize when forming crystals. The molecular structures of NTU-1 and NTU-2 are shown in Figure 1a. The electron-donating moieties thieno[3,2b]indole and thieno[2,3-b]indole have recently been used as building blocks for light sensitizers in efficient dye-sensitized solar cells. 19,20 They are more electron-donating than DTDCPB, leading to a red shift in the absorption spectrum.19 These electron-donating moieties and the electron-withdrawing dicyanovinylene are connected by an electron-withdrawing benzo[c][1,2,5]thiadiazole (BTD) block to form the D−A−A configuration.
In this work, we examine the regioisomeric effects of the thienoindole moieties on the physical and structural characteristics of NTU-1 and NTU-2. These materials are further characterized as electron donors in both bulk and planar heterojunction OPVs. Optimized BHJ OPVs based on the donor−acceptor pairings of NTU-1−C70 and NTU-2−C70 show promising efficiencies limited by a low FF. In order to more deeply examine the origin of the low FF in D−A−A systems, planar heterojunction devices are used to study donor layer charge transport and isolate the role played by morphology. In order to improve FF, we investigate the evolution of thin-film crystallinity with thermal annealing and construct OPVs based on annealed donor layers.
2. RESULTS AND DISCUSSION 2.1. Synthesis of NTU-1 and NTU-2. The syntheses of NTU-1 and NTU-2 are shown in Figure S1, and detailed synthetic procedures and characterizations of new compounds can be found in the Supporting Information. It is worth noting that the electron-donating moiety N-ethylthieno[3,2-b]indole of NTU-1 was achieved by a tandem Buchwald−Hartwig C−N coupling reaction. In contrast, the N-ethylthieno[2,3-b]indole used for the synthesis of NTU-2 was synthesized for the first time via an intramolecular cyclization to prevent formation of a possible isomeric side product.21 2.2. Physical Properties and Structural Analysis. Figure 1b shows the optical constants (refractive index n and extinction coefficient k) for 30 nm thick films of NTU-1 and NTU-2. Both D−A−A donors show broadband absorption with large extinction coefficients: the kmax values of NTU-1 (kmax = 0.85 at λmax = 550 nm) and NTU-2 (kmax = 1.05 at λmax = 560 nm) are significantly larger than that for DTDCPB (kmax B
DOI: 10.1021/acsami.7b05304 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces = 0.67 at λmax = 590 nm) . A similar trend is observed for measurements in dichloromethane solution (Figure S2). Density functional theory (DFT) calculations also suggest that NTU-2 should show a bathochromic shift in absorption position and increased absorption relative to NTU-1, consistent with observations. This is due to increased intramolecular charge transfer (ICT) character in NTU-2 relative to NTU-1, resulting from the stronger electron-donating ability and higher quinoidal character of N-ethylthieno[2,3-b]indole, as indicated by a lower oxidation potential found in cyclic voltammetry (CV) and the separate calculations of increased oscillator strength based on DFT (Table S1), as well as bond-length alternation (BLA) calculations (Table S2) based on the observed X-ray-determined structures. The photophysical behavior of both compounds in thin films and in dichloromethane solution is summarized in Table 1. The electrochemical properties of NTU-1 and NTU-2 were probed by CV. An irreversible oxidation peak with onset at 0.74 V (vs Fc/Fc+) for NTU-1 was observed (Figure S3). In contrast, a quasi-reversible oxidation (onset at 0.58 V) was observed in NTU-2, where the syn-positioned nitrogen (vs S atom) lone-pair electrons can be effectively delocalized with BTD ring, leading to increased conjugation and a lower oxidation potential (Figure S4a). For reduction, both NTU-1 and NTU-2 exhibit a quasi-reversible reduction, referring to the BTD-stabilized radical anion. The stronger electron-donating ability of thieno[3,2-b]indole leads to a slightly higher reduction potential of NTU-2 (−0.96 V) compared to NTU1 (−0.92 V). Based on the CV data, the calculated HOMO (LUMO) energy levels for NTU-1 and NTU-2 are −5.54 eV (−3.88 eV) and −5.38 eV (−3.84 eV), respectively. The fairly shallow lowest unoccupied molecular orbitals (LUMOs) suggest a favorable offset for exciton dissociation by charge transfer with common fullerene acceptors.22−24 Structural analysis of single crystals resulting from X-ray diffraction (XRD) is summarized in Figure S5 and Table S3. Neighboring molecules of NTU-1 pack in a brickwork manner, while NTU-2 molecules stack in a herringbone structure. Both molecules form centrosymmetrical antiparallel dimers that feature evident cancellation of the molecular dipoles. This dimerization may help to prevent energetic disorder and facilitate efficient charge transport (see additional discussion in section 5 of Supporting Information).25−28 2.3. Device Characterization. 2.3.1. Performance of Bulk Heterojunction Organic Photovoltaic Cells. In order to investigate the performance of NTU-1 and NTU-2 as donors in BHJ OPVs, composition and thickness optimization of the mixed active layer was carried out. Figure 2a−d shows the operating parameters for an NTU-x−C70 BHJ with a 55 nm thick active layer as a function of film composition. For both donors, the short-circuit current density (JSC) and FF peak at a donor concentration of 20 vol %, possibly indicating optimal morphology at this composition. In particular, NTU-1−C70 BHJ devices show a high VOC of 1.08 V at this composition. Both sets of devices show an improvement in VOC over devices based on DTDCPB−C70 (VOC = 0.94 V), consistent with the measured HOMO levels in Table 1.18 Device performance at this optimum composition (D:A = 1:4 v/v) is plotted as a function of active layer thickness in Figure 2e−h. Both sets of devices show a drastic decrease in FF with increasing activelayer thickness, indicating that the efficient charge collection observed in the DTDCPB−C70 mixtures likely does not exist in the NTU-x−C 70 mixtures.17,18 Peak power conversion
Figure 2. (a) Short-circuit current density (JSC), (b) open-circuit voltage (VOC), (c) fill factor (FF), and (d) power conversion efficiency (ηP) as a function of donor concentration for NTU-1−C70 (solid symbols) and NTU-2−C70 (open symbols) BHJ devices with a 55 nm thick active layer. (e−h) Device operating parameters as a function of active-layer thickness for a NTU-x−C70 BHJ with a donor−acceptor ratio of 1:4. (i) External quantum efficiency for devices with structures 10 nm MoOx/40 nm NTU-1−C70 (1:4) or 55 nm NTU-2−C70 (1:4)/ 10 nm BCP/100 nm Al.
efficiencies are realized at optimum active-layer thicknesses of 40 and 55 nm for devices based on NTU-1 and NTU-2, respectively. These thicknesses are considerably thinner than for the corresponding DTDCPB−C70 devices, which show peak efficiency for an active-layer thickness of ∼80 nm.17 For NTU-1 and NTU-2, the active-layer thickness is limited by charge collection. The limited thickness of these fullerene-rich mixed layers can lead to a limited current contribution from the donor materials. By simulating the absorption efficiencies of the optimized NTU-x−C70 BHJ devices, we find that 28% and 35% of the total current is contributed by the donors for NTU-1 and NTU-2 devices under AM 1.5G illumination, respectively. A representative external quantum efficiency (ηEQE) spectrum is shown in Figure 2i, with both devices showing a broad spectral response. The ηEQE for the device containing NTU-2 extends further into the near-IR region compared to NTU-1 devices, consistent with the extinction coefficients in Figure 1b. The JSC values for NTU-1 and NTU-2 devices obtained by integrating the ηEQE are 8.8 and 11.9 mA·cm−2, respectively, which are within 4% of the measured JSC in Figure 2. The optimized NTU-1−C70 and NTU-2−C70 BHJs show average power conversion efficiencies of (3.7 ± 0.1)% and (5.0 ± 0.2)%, respectively. These efficiencies are limited by a low FF (
