Isomer Effects of Fullerene Derivatives on Organic Photovoltaics and

Jul 18, 2019 - Solar energy conversion is one of the most important issues for creating ... properties may exert a negative effect on photovoltaic pro...
0 downloads 0 Views 3MB Size
Download PDF
Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

pubs.acs.org/accounts

Isomer Effects of Fullerene Derivatives on Organic Photovoltaics and Perovskite Solar Cells Published as part of the Accounts of Chemical Research special issue “Advanced Molecular Nanocarbons”. Tomokazu Umeyama*,† and Hiroshi Imahori*,†,‡ †

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan

Downloaded via BUFFALO STATE on July 19, 2019 at 00:34:14 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

CONSPECTUS: Solar energy conversion is one of the most important issues for creating and maintaining a future sustainable society. In this regard, photovoltaic technologies have attracted much attention because of their potential to solve energy and environmental issues. In particular, thin-film solar cells, such as organic photovoltaics (OPVs) and perovskite solar cells (PSCs), are highly promising owing to their flexibility, light weight, and low-cost production. One of the most important factors used to evaluate solar-cell performance is the power conversion efficiency (PCE), which is the ratio of the output electric power divided by the input light power. The PCEs of PSCs have become comparable to those of multicrystalline silicon solar cells in a laboratory level, but the PCEs of OPVs have yet to catch up with them and still need to be improved. The insufficient durability of PSCs and OPVs is also a challenge that needs to be addressed. Fullerene derivatives have been utilized as electron acceptors and electron-transport materials in OPVs and PSCs. However, the use of fullerene derivatives requires attention to their isomers if they are multiadducts or even monoadducts produced from fullerenes with low symmetry. Their nonuniform structures and electronic properties may exert a negative effect on photovoltaic properties. However, most researchers in the field of OPVs and PSCs have been unaware of the importance of the isomerism. Even the most prevalent, high-performance fullerene acceptor, [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM), has been used as an isomer mixture. In this Account, we summarize recent studies on the effects of isomer separation of fullerene derivatives on the device performances of OPVs and PSCs. Largely, fullerene derivatives containing various isomers are categorized into [60]fullerene bisadducts, [70]fullerene bisadducts, and [70]fullerene monoadducts. In all cases, the difference in isomerism was found to have a large impact on PCEs. The miscibility with polymer donors and film-forming property of fullerene derivatives were affected by the isomer separations, which exert the most potent influence on device performances. Although the disorders in energy levels among isomers are not definitely influencing on photovoltaic properties of isomer mixtures, the molecular packing structures of fullerene derivatives make a significant effect on their photovoltaic properties. Notably, isomerically pure fullerene derivatives oftenbut not alwaysexhibit higher PCEs than the isomer mixture. The search for the best isomers of fullerene derivatives and their optimal compositional ratios, which extensively depend on their roles and the combined materials, will be an indispensable step to achieving consistently higher device performances for OPVs and PSCs.

■

INTRODUCTION

(Figure 1a). The photoactive layer absorbs light and generates excitons that dissociate into respective charge carriers. The ETL and HTL rectify charge flow to prevent charge recombination, and the electrodes collect the charges (Figure 1b). Typically, an indium tin oxide (ITO) or fluorine-doped tin oxide (FTO), is used as the TE to allow sunlight to pass through it. PSCs have a semiconducting perovskite material as the photoactive layer, where an electron and a hole are loosely bound with an exciton binding energy comparable to the thermal energy, kT, at room temperature.1 Consequently, excitons rapidly dissociate to form free carriers within the perovskite layer. However, in the photoactive layers of OPVs,

Thin-film solar-cell technologies, such as organic photovoltaics (OPVs) and perovskite solar cells (PSCs), possess a huge potential to provide a route toward highly efficient, costeffective, large-area solar-energy-conversion systems to meet the world’s increasing energy demands. In contrast to currently market-dominant crystalline-silicon solar cells, these emerging thin-film solar cells employ materials that are solutionprocessable and easily scalable by roll-to-roll manufacturing. OPVs and PSCs also have advantages in terms of flexibility, light weight, and enhanced design, which will satisfy the demands for versatile and ubiquitous electronic devices. Both OPVs and PSCs commonly utilize an architecture composed of a photoactive layer in the center, an electron- or hole-transport layer (ETL or HTL) on both sides of the photoactive layer, and a transparent or back electrode (TE or BE) at both ends © XXXX American Chemical Society

Received: March 31, 2019

A

DOI: 10.1021/acs.accounts.9b00159 Acc. Chem. Res. XXXX, XXX, XXX−XXX

Article

Accounts of Chemical Research

Figure 1. (a) Typical device structure and (b) schematic energy level diagram of OPV and PSC. ETL: electron-transport layer. HTL: holetransport layer.

Figure 2. (a) Nomenclature of [60]fullerene bisadduct regioisomers. Structures of (b) [60]NCnBA and [60]NCBA, (c) [60]ICBA, and (d) trans-3 isomers of [60]ICBA.

suppressing hysteresis in the J−V curves and increasing device durability. The incorporation of fullerenes into OPVs and PSCs as electron acceptors or ETL materials has already been reviewed.3,7,8 In this Account, we focus on the isomer separation of fullerene derivatives for OPV and PSC device applications. To improve their solubility and modify their electronic properties, mono- or multiadducts of fullerenes are usually used as electron acceptors and ETL materials. Such functionalizations often yield isomers. The difference in isomerism can have a significant influence on their electronic properties, molecular arrangement, and miscibility with conjugated polymer donors, which are usually overlooked. Recent studies of the fullerene isomer separation effects on the OPV and PSC device performance have been overviewed. There are significant differences in PCEs with respect to each report in this Account. However, such values largely depend on solubilizing added groups of fullerene derivatives, combined polymer donors, and device fabrication techniques that are rather off-topic for this Account. Direct comparisons of PCEs from different reports are not scientifically meaningful in many cases. Therefore, we emphasize the comparisons of PCEs described in each paper to extract the intrinsic isomer effect on photovoltaic properties.

exists a larger exciton binding energy, which exceeds the ambient thermal energy.2 To generate dissociated charge carriers, the photoactive layers of OPVs typically consist of heterojunctions between electron-donor and electron-acceptor domains. At the donor−acceptor interface, a difference between the energy levels of the donor and acceptor supplies the requisite driving force to split the excitons. As a result, free electrons and holes are formed in the respective domains. To overcome the short exciton diffusion length in the photoactive layers of OPVs and maximize the interfacial area for exciton dissociation, a bulk heterojunction (BHJ) structure, where the donor and acceptor form an interpenetrating bicontinuous network, is generally employed. Fullerenes and their derivatives have been ubiquitous electron acceptors in BHJ OPVs because of their strong electron-deficient character and excellent semiconducting properties, which originate from their small reorganization energies of electron transfer (ET).3 The power conversion efficiency (PCE) of single BHJ OPVs based on a conjugated polymer donor and a fullerene acceptor has reached 11%.4 Although nonfullerene acceptors with excellent light-harvesting abilities have emerged recently as alternatives with compelling higher PCEs of up to 16% in single BHJ OPVs,5 a fullerene acceptor, [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM), and nonfullerene acceptors have been used in a tandem BHJ OPV that achieved a record PCE of 17.3%.6 This implies that fullerene acceptors seem to be old-fashioned materials, but still important components to attain enhanced performances of OPVs. The excellent electron-transport properties of fullerene derivatives and their well-matched energy levels with perovskites also make them attractive candidates for ETL materials in PSCs.7,8 PSCs with fullerenebased ETL materials can not only attain high PCEs of over 18%, but also reduce the density of trap states and passivate the grain boundaries of the perovskite photoactive layer,

■

ISOMER SEPARATION OF FULLERENE BISADDUCTS FOR OPVs

[60]Fullerene Bisadducts

The energy gap between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor sets an upper limit on the open-circuit voltage (VOC) of an OPV.2 Therefore, a strategy for developing high-performance fullerene acceptors is the use of fullerene multiadducts or endohedral fullerenes that are still difficult to obtain in large amount due to the limitation B

DOI: 10.1021/acs.accounts.9b00159 Acc. Chem. Res. XXXX, XXX, XXX−XXX

Article

Accounts of Chemical Research

Table 1. LUMO Energy Levels,a Electron Mobilities,b and Photovoltaic Performances of OPV Devices Based on P3HT: [60]Fullerene Bisadductsc acceptor

LUMO/eV

μe/10−5cm2 V−1 s−1

JSC/mA cm−2

VOC/V

FF

PCE/%

ref

trans-2-[60]NC6BA trans-3-[60]NC6BA mix-[60]NC6BA trans-2-[60]NC4BA mix-[60]NC4BA mix-[60]NC2BA trans-3-[60]NCBA mix-[60]NCBA trans-3(a)-[60]ICBA trans-3(b)-[60]ICBA trans-3(c)-[60]ICBA mix-[60]ICBA

−3.75 −3.68 −3.67 −3.73 −3.66 −3.69 −3.70 −3.76 −3.49 −3.49 −3.48 −3.51

6.9 5.5 −d 8.2 −d −d 26 8.4 47 −d −d 21

4.73 3.57 3.71 6.82 5.05 4.02 10.21 9.88 9.87 1.95 8.66 8.85

0.728 0.647 0.677 0.711 0.552 0.574 0.88 0.82 0.85 0.72 0.88 0.86

0.402 0.387 0.379 0.497 0.394 0.520 0.71 0.67 0.723 0.373 0.617 0.686

1.38 0.89 0.95 2.41 1.10 1.20 6.3 5.3 6.1 0.53 4.7 5.2

10 10 10 11 11 11 13 13 15 15 15 15

Deteremined by cyclic voltammetry. bMeasured with P3HT:[60]fullerene bisadducts blend films by SCLC method. cOPV device structures are ITO/PEDOT:PSS/P3HT:fullerene/Al for [60]NCnBA, and ITO/PEDOT:PSS/P3HT:fullerene/Ca/Al for [60]NCBA and [60]ICBA. dNot reported. a

of [60]NC6BA were higher than that of the regioisomer mixture (0.95%) (Table 1), whereas the trans-1 (0.12%), trans3 (0.89%), and cis-2+cis-3 (0.62%) resulted in lower PCEs.10,11 This result demonstrated that the relative positions of the solubilizing groups on the [60]fullerene cage are greatly influential on their photovoltaic properties. The fact that some of the single regioisomers show inferior photovoltaic performances to mix-[60]NC6BA revealed that the disorders of the energy levels in mix-[60]NC6BA do not have major adverse impact. Unexpectedly, the VOC of the device with trans-2[60]NC6BA (0.728 V) was higher than that with trans-3[60]NC6BA (0.647 V), despite the lower LUMO level of the former (−3.75 V) than the latter (−3.68 V) (Table 1). The inferior VOC of the trans-3-[60]NC6BA-based device may be attributed to other factors such as more frequent charge recombination in P3HT:trans-3-[60]NC6BA than P3HT:trans-2-[60]NC6BA. In spite of the similar lightharvesting properties of P3HT:trans-2-[60]NC6BA and P3HT:trans-3-[60]NC6BA, the external quantum efficiency (EQE) values of photocurrent generations for the trans-2[60]NC6BA-based device were higher than those for the trans3-[60]NC6BA-based one, which is consistent with the higher JSC of the former device (Table 1).10,11 This indicates the higher efficiency of exciton diffusion to the donor−acceptor interface, charge dissociation at the interface, and/or collection of the generated charges in the former device. The slightly higher electron mobility (μe) of P3HT:trans-2-[60]NC6BA (6.9 × 10−5 cm2 V−1 s−1) than that of P3HT:trans-3[60]NC6BA (5.5 × 10−5 cm2 V−1 s−1) estimated by the space-charge-limited current (SCLC) method (Table 1) may partially contribute to the higher charge collection efficiency. More importantly, the atomic force microscopy (AFM) measurement of the P3HT:trans-3-[60]NC6BA film surface revealed the formation of 10 μm sized domains of trans-3[60]NC6BA, whereas P3HT:trans-2-[60]NC6BA did not show such large aggregations (Figure 3a, b).10,11 Furthermore, the other superior isomers, i.e., trans-4 and e, showed rather smooth film surfaces, but the other inferior one, i.e., trans-1, displayed even intense aggregates with micrometer sizes in the blend films with P3HT (Figure 3c-e). The degree of aggregation tendency of the fullerene derivatives correlates well with the PCEs. The photoluminescence of P3HT was quenched more efficiently with trans-2-[60]NC6BA (85%)

of their synthesis and purification. As the degree of functionalization on the fullerene core is increased, due to the reduction of the π-conjugation size, the LUMO energy level of fullerene multiadducts is raised, resulting in improved VOC.9 However, considering the balance between the increase in VOC and plausible decrease in the electron mobility as a consequence of loose packing of fullerene cores by bulkier functionalization, the use of fullerene bisadducts is likely to be the best option. A conventional way to prepare fullerene bisadducts is addition reactions between fullerene and an excessive amount of reactants. However, even if the two added groups are identical and symmetric and addition reactions are limited to [6,6]-bonds, there are eight possible regioisomers of [60]fullerene bisadducts (Figure 2a). As a result of the different structural and electronic properties of each regioisomer, disorders in molecular packing and energy levels would occur in BHJ films when using a regioisomer mixture. This may deteriorate the other photovoltaic parameters, short circuit current density (JSC) and fill factor (FF), of BHJ OPV devices. Separation of [60]Fullerene Bisadduct Regioisomers by HPLC

A straightforward method to obtain regioisomerically pure [60]fullerene bisadducts is the separation of a regioisomer mixture prepared by the conventional synthetic method into respective isomers using high-performance liquid chromatography (HPLC). As the first systematic investigation of the regioisomer effects of [60]fullerene bisadducts on OPV performance, we separated the regioisomers of [60]NCnBA (n = 2, 4, 6; Figure 2b) using HPLC with a Buckyprep column.10,11 To reduce the plausible number of fullerene bisadduct isomers, a symmetrical dihydronaphthyl group was chosen as the substituent. We expected that symmetrical introduction of two alkoxycarbonyl chains into the dihydronaphthyl groups increases the solubility and facilitate isomer separation. Indeed, repeated separations of the [60]NC6BA regioisomer mixture (mix-[60]NC6BA) by HPLC afforded trans-1, trans-2, trans-3, trans-4, e, and a mixture of cis-2 and cis3 (denoted as cis-2+cis-3) isomers. The LUMO energy levels of [60]NC6BA isomers were 0.1−0.2 eV higher than those of the corresponding monoadduct. Combined with regioregular poly(3-hexylthiophene) (P3HT) as an electron donor, PCEs of the trans-2 (1.38%), trans-4 (1.44%), and e (1.41%) isomers C

DOI: 10.1021/acs.accounts.9b00159 Acc. Chem. Res. XXXX, XXX, XXX−XXX

Article

Accounts of Chemical Research

low anisotropy is a useful indicator for screening isomeric fullerene bisadduct isomers. Following our pioneering work, Li and co-workers carried out regioisomer separation of [60]NCBA with no substitution on the dihydronaphthyl groups (Figure 2b) by HPLC.13 Only the trans-2, trans-3, trans-4, and e isomers were isolable. PCEs of P3HT:[60]NCBA-based devices (5.3−6.3%) were higher than those of P3HT:[60]NCnBA-based ones ( 6.9 × 10−5 cm2 V−1 s−1 for P3HT:trans-2[60]NC6BA, Table 1). Despite the shorter alkyl chains, the AFM measurement of P3HT:trans-2-[60]NC4BA showed smooth film surface as is P3HT:trans-2-[60]NC6BA. However, even shorter alkyl chains of [60]NC2BA caused insufficient solubility for separating the respective bisadduct isomers.11 Devices based on trans-2 and a mixture of trans-4 and e of [60]NC4BA exhibited PCEs of approximately 2.4%, higher than that of the [60]NC4BA regioisomer mixture (mix[60]NC4BA; 1.10%) (Table 1).11 It is noteworthy that the superior photovoltaic isomers (i.e., trans-2, trans-4, and e) are common to [60]NC6BA and [60]NC4BA. Theoretical calculations by Sabirov demonstrated that the superior photovoltaic isomers commonly possess lower anisotropy of polarizability than the inferior isomers.12 Such a molecular structure−photovoltaic performance relationship signifies that D

DOI: 10.1021/acs.accounts.9b00159 Acc. Chem. Res. XXXX, XXX, XXX−XXX

Article

Accounts of Chemical Research

the predominant product, which is easily isolated by HPLC. In the BHJ OPVs with P3HT, cis-2-[60]BIEC showed a PCE of 2.8% (JSC = 6.60 mA cm−2, VOC = 0.80 V, FF = 0.53), outperforming the devices with mix-[60]BIEC (PCE = 1.8%, JSC = 4.70 mA cm−2, VOC = 0.80 V, FF = 0.47) and a BIE−C60 adduct with one unreacted indene unit (mono-[60]BIEC; Figure 4) (PCE = 1.6%, JSC = 5.15 mA cm−2, VOC = 0.61 V, FF = 0.51).16 The higher VOC values of P3HT:cis-2-[60]BIEC and P3HT:mix-[60]BIEC based devices than P3HT:mono[60]BIEC result from the higher LUMO levels of cis-2[60]BIEC and mix-[60]BIEC (−3.50 eV) than mono[60]BIEC (−3.66 eV). AFM measurements revealed that the surface roughness is suppressed in P3HT:cis-2-[60]BIEC than in P3HT:mix-[60]BIEC. This correlated well with the quenching efficiency of P3HT fluorescence in the blend films, P3HT:cis-2-[60]BIEC (90%) > P3HT:mix-[60]BIEC (84%), indicating the higher exciton diffusion efficiency in the former film. In addition, highly arranged fullerene cages of cis2-[60]BIEC, which is indicated by the X-ray crystal structure of cis-2-[60]BIEC, brought the higher μe of P3HT:cis-2[60]BIEC (3.5 × 10−5 cm2 V−1 s−1) than that of P3HT:mix[60]BIEC (2.5 × 10 −5 cm2 V−1 s −1 ). These results demonstrate that cis-2 isomers of fullerene bisadducts with suitable miscibility with polymer donor are highly fascinating as electron-acceptors in BHJ OPVs.

not significantly influencing on photovoltaic properties of regioisomer mixtures and higher LUMO energy levels do not necessarily result in improved VOC values. Because of the high crystallinity and low solubility, some single isomers show low miscibility with polymer donor, leading to poor PCEs. On the other hand, the regioisomer mixtures often exhibit good miscibility, resulting in decent PCEs. Controlled Synthesis of [60]Fullerene Bisadduct Regioisomers

As indicated in the comprehensive work of Li and co-workers on the [60]ICBA regioisomer separations, the conventional synthesis of [60]ICBA yields no cis-isomers owing to steric hindrance.14,15 Nevertheless, the lower steric hindrance of the cis-isomers is anticipated to give more closely packed structures in the BHJ film and superior photovoltaic properties. To obtain pure cis-isomers of indene-[60]fullerene bisadducts efficiently, we conducted tether-directed functionalization of [60]fullerene using ethylene-tethered indene dimer (1,2-bis(3indenyl)ethane, BIE).16 This method aims at increasing the regioselectivity of the addition reactions on the fullerene cage by using a linker between the two reactive groups.17,18 Although the Diels−Alder reaction between BIE and C60 yielded a regioisomer mixture of the bisadducts (mix[60]BIEC; Figure 4), cis-2-[60]BIEC (83%; Figure 4) was

Controlled Synthesis of [70]Fullerene Bisadduct Regioisomers

[70]Fullerene derivatives are superior to [60]fullerenes in attaining high PCEs of OPVs due to their better lightharvesting ability in the visible region, which can improve JSC.3 The higher solubility of [70]fullerene derivatives compared to [60]fullerene derivatives is also beneficial for solution processes during the formation of the photoactive layer. Along with their higher LUMO levels, regioisomer-controlled [70]fullerene bisadducts are predicted to be high-performance acceptors for BHJ OPVs. However, owing to the lower symmetry of [70]fullerene compared to [60]fullerene, a

Figure 4. Structures of [60]BIEC isomers.

Figure 5. (a) Structure of [70]fullerene. α-, β-, γ-, and κ-type [6,6]-bonds are represented by red, blue, green, and orange lines. Structures of (b) [70]BIEC regioisomers, (c) [70]PBC regioisomers, and (d) PCDTBT. The reacted bonds on the fullerene cage are represented by bold red lines to clarify the structure. Nomenclature; Greek letters such as α define the reacted [6,6]-bonds, and numbers represent the smallest number of bonds separating the addition sites. E

DOI: 10.1021/acs.accounts.9b00159 Acc. Chem. Res. XXXX, XXX, XXX−XXX

Article

Accounts of Chemical Research

Figure 6. Structures of (a) [70]NCMA regioisomers, (b) [70]PCBM regioisomers, (c) α-[70]PCBM enantiomers, (d) β-[70]PCBM diastereomers, (e) PTB2 with optically active/inactive side chains, and (f) PffBT4T-2OD.

these results proved the utility of [70]fullerene bisadduct cisisomers, encouraging us to prepare α-1-α isomer of [70]fullerene bisadduct acceptors with the closest proximity of the two added groups on the [70]fullerene cage.25 Propylenetethered α-1-α bismethano[70]fullerene with two ethyl groups (α-1-α-[70]PBC; Figure 5c) was synthesized and isolated. However, the PCE of a device based on α-1-α-[70]PBC with an amorphous conjugated polymer, PCDTBT (Figure 5d), was lower (PCE = 3.25%, JSC = 6.35 mA cm−2, VOC = 0.864 V, FF = 0.593) than that with cis-1-[70]PBC (PCE = 4.60%, JSC = 9.24 mA cm−2, VOC = 0.844 V, FF = 0.590), i.e., a mixture of α1-α and α-1-β isomers (Figure 5c), despite removing the inhomogeneity of the structure and electronic properties of the fullerene bisadducts. In spite of amorphous nature of PCDTBT, the decreased solubility and enhanced aggregation behavior of α-1-α-[70]PBC relative to cis-1-[70]PBC led to the formation of micrometer-sized aggregates in the PCDTBT:α1-α-[70]PBC blend film and deteriorated the OPV performance, whereas such large aggregates were absent in the PCDTBT:cis-1-[70]PBC film.25 These results suggest that the optimization of the substituent structure is indispensable for forming bicontinuous structures in the blend films and taking full advantage of regioisomerically pure α-1-α bisadducts of [70]fullerene as electron-accepting materials.

[70]fullerene cage has four different types of nonequivalent [6,6]-bonds, i.e., α-, β-, γ-, and κ-type bonds (Figure 5a).19 [70]Fullerene bisadducts have as many as 38 possible regioisomers, even if the two added groups are identical and symmetric, and addition reactions are limited to [6,6]-bonds. Nevertheless, the differences in reactivity (α > β > γ > κ) and steric hindrance reduce the number of bisadduct regioisomers obtained by the conventional method. The α bonds are the most reactive because they are positioned at the ends of the [70]fullerene cage and therefore undergo higher strain from the curvature of the cage.20−23 In this context, we applied the tether-directed bisfunctionalization method to [70]fullerene with BIE as the reactant to selectively obtain a close substituent pattern, i.e., the α-2-α isomer (Figure 5b).24 If the tether-directed effect was absent, steric hindrance between the indene units would inhibit the formation of such a close substitution pattern.21,23 An OPV based on α-2-α-[70]BIEC and P3HT showed a remarkable PCE of 4.2% (JSC = 7.95 mA cm−2, VOC = 0.79 V, FF = 0.67), which is higher than those with cis-2-[70]BIEC (mixture of α-2-α and α-3-β isomers; PCE = 2.2%, JSC = 6.66 mA cm−2, VOC = 0.72 V, FF = 0.46), mono-[70]BIEC (BIE− [70]fullerene monoadduct; PCE = 2.2%, JSC = 5.90 mA cm−2, VOC = 0.64 V, FF = 0.57) (Figure 5b), and cis-2-[60]BIEC (2.8%).16 The VOC values of the device with α-2-α-[70]BIEC and cis-2-[70]BIEC were higher than that with mono[70]BIEC owing to the elevated LUMO levels by the bisfunctionalization. As is the case with [60]fullerene bisadducts, structures of the blend films were found to cause a considerable impact on the photovoltaic properties; the surface roughness of P3HT:α-2-α-[70]BIEC was suppressed as compared with those of P3HT:cis-2-[70]BIEC and P3HT:mono-[70]BIEC. Consistently, the quenching efficiency of P3HT fluorescence was higher in P3HT:α-2-α-[70]BIEC (95%) than P3HT:cis-2-[70]BIEC (89%) and P3HT:mono-[70]BIEC (86%), that is, the excitons generated in the P3HT domains reach to the P3HT-fullerene interface efficiently in P3HT:α-2α-[70]BIEC with the relatively small domain size. Overall,

■

ISOMER SEPARATION OF [70]FULLERENE MONOADDUCTS FOR OPVs Despite the higher LUMO level and VOC of [70]fullerene bisadducts, [70]fullerene monoadducts are preferentially used in OPVs as electron acceptors. The reduced number of added groups may enhance molecular packing and electron transportation. However, [70]fullerene possesses four kinds of reactive [6,6]-bonds (Figure 5a), and therefore, as-prepared [70]fullerene monoadducts usually consist of regioisomers. As in fullerene bisadducts, the difference in isomerism may have a large influence on the miscibility with conjugated polymer donors and the molecular arrangement in the photoactive layer of OPVs. Nevertheless, [70]fullerene monoadducts including F

DOI: 10.1021/acs.accounts.9b00159 Acc. Chem. Res. XXXX, XXX, XXX−XXX

Article

Accounts of Chemical Research

[70]NCMA. The matching of the substituent position with a conjugated polymer strongly depends on the structure of the added group on C70. Subsequently, we investigated the regioisomer effects of [70]PCBM on the film structures and photovoltaic properties of composite films with the crystalline conjugated polymer P3HT instead of the amorphous PCDTBT.27 Remarkably, β[70]PCBM induced a face-on P3HT packing in the blend film, whereas an edge-on alignment of P3HT was observed in the composite films with α-[70]PCBM or mix-[70]PCBM. The difference in the positions of the appended group in [70]PCBM may influence the π−π interaction between P3HT and the rugby ball-shaped C70, resulting in the difference in the P3HT packing orientations. Higher hole mobility (μh) and PCE were achieved in the device with P3HT:β-[70]PCBM (1.3 × 10−5 cm2 V−1 s−1, 3.69%) than those with P3HT:α-[70]PCBM (0.49 × 10−5 cm2 V−1 s−1, 3.11%) and P3HT:mix-[70]PCBM (0.29 × 10−5 cm2 V−1 s−1, 3.21%) owing to the face-on stacking structure of P3HT. This result suggests that the use of regioisomerically pure [70]fullerene monoadducts can modulate the packing direction of crystalline polymers in the blend films. Given the unsymmetrical substituents on the cyclopropane, α-[70]PCBM is composed of two enantiomers, (R)- and (S)α-[70]PCBM, whereas β-[70]PCBM consists of two diastereomers in which the phenyl group is protruding toward the pole (β1-[70]PCBM) or the equator (β2-[70]PCBM) direction (Figure 6c, d). We recently separated α-[70]PCBM into enantiomers using HPLC with a Chiralpak column, and unambiguously characterized their structures using X-ray crystallography.28 The packing diagrams of (R)-α-, (S)-α-, and racemic α-[70]PCBM crystals similarly exhibited isotropically well-packed fullerene cage structures. The effects of enantiomer combination of the electron donor and acceptor on photovoltaic properties were investigated systematically using PTB2 polymer with optically active/inactive (2ethylhexyloxy)carbonyl side chains (Figure 6e) as an electron donor and α-/(R)-α-/(S)-α-[70]PCBM as an electron acceptor. In contrast to our expectation, all combinations resulted in comparable PCEs (3.2−3.5%), revealing insignificant effects of enantiomer separations and combinations on photovoltaic properties. The polymer main chain and optically active 2ethylhexyl side chain are separated by the ester group in PTB2 (Figure 6e). The direct connection between the conjugated body and the optically active branched chain may accentuate the enantiomer combination effect. On the other hand, we found a striking effect of β[70]PCBM diastereomer separation on the performances of OPVs using a high-performance conjugated polymer donor, PffBT4T-2OD (Figure 6f).29 Despite the strong tendency of PffBT4T-2OD to form a bicontinuous structure with fullerene derivatives, β1-[70]PCBM showed extraordinary cohesion in the blend film, forming micrometer-sized aggregates. This deteriorated the OPV performance of the PffBT4T-2OD:β1[70]PCBM-based device, resulting in an extremely poor PCE (0.43%). The strong intermolecular interaction of β1[70]PCBM was also confirmed by a densely packed structure of fullerene cages in a single crystal of β1-[70]PCBM, whereas the other isomers, i.e., α-[70]PCBM and β2-[70]PCBM, showed relatively loose packing. Considering that the intrinsic electron-transporting properties of α-[70]PCBM, β 1 [70]PCBM, and β2-[70]PCBM, which are estimated by timeresolved microwave conductivity measurements, are compara-

the most prevalent electron acceptor, [70]PCBM, have been used in OPVs without isomer separation. To precisely evaluate the regioisomer effect of [70]fullerene monoadducts on photovoltaic properties, we first designed new dihydronaphthyl-substituted [70]fullerenes with two butoxycarbonyl groups ([70]NCMA; Figure 6a).26 The symmetrical dihydronaphthyl group was chosen as the substituent to eliminate isomers derived from the different steric orientations. An as-prepared sample of [70]NCMA (mix-[70NCMA]) contained α and β isomers (Figure 6a) with a ratio of 3:2, which are successfully separated by HPLC with a Buckyprep column. The LUMO energy level of α-[70]NCMA (−3.63 eV) was slightly higher than that of β-[70]NCMA (−3.69 eV). Furthermore, their packing structure was strongly affected by the substitution position; single-crystal X-ray analysis revealed that α-[70]NCMA has an isotropically wellpacked structure, whereas β-[70]NCMA forms a regular nanoporous structure, which may inhibit the smooth electron transport. Although the surface morphologies of PCDTBT:α[70]NCMA, PCDTBT:β-[70]NCMA, and PCDTBT:mix[70]NCMA observed by AFM were similar, the OPV based on PCDTBT:α-[70]NCMA exhibited significantly higher OPV performance (PCE = 4.04%, JSC = 9.40 mA cm−2, VOC = 0.811 V, FF = 0.529) than those with β-[70]NCMA (PCE = 2.44%, JSC = 7.14 mA cm−2, VOC = 0.745 V, FF = 0.458) and mix[70]NCMA (PCE = 2.71%, JSC = 7.27 mA cm−2, VOC = 0.765 V, FF = 0.486).26 The higher LUMO level of α-[70]NCMA straightforwardly resulted in the higher VOC. The improved JSC of the α-[70]NCMA-based device arises from the relatively isotropic packing structure of fullerene cages in PCDTBT:α[70]NCMA, which is favorable for efficient charge transportation. This corroborates that the isolation of pure regioisomers of [70]fullerene monoadducts is very valuable for controlling the molecular packing structures in blend films with conjugated polymers, thereby improving the BHJ OPV performance. We have extended the HPLC technique to separate the regioisomers of a prevalent, high-performance [70]fullerene monoadduct acceptor, [70]PCBM. 26,27 A purchased [70]PCBM sample (mix-[70]PCBM) included α and β isomers (Figure 6b) with a ratio of 88:12. As is [70]NCMA, the LUMO level of α-isomer (−3.61 eV) was slightly higher than that of β-isomer (−3.66 eV). The packing diagrams of the single crystal of α-[70]PCBM visualized isotropically contacted fullerene cages as in α-[70]NCMA, whereas the single crystal of β-[70]PCBM was not formed on account of the presence of the diastereomers.26 Combined with PCDTBT as an electron donor, both α-[70]PCBM- (PCE = 6.20%, JSC = 11.3 mA cm−2, VOC = 0.804 V, FF = 0.681) and β-[70]PCBM-based (PCE = 6.46%, JSC = 11.2 mA cm−2, VOC = 0.847 V, FF = 0.686) devices considerably exceeded the mix-[70]PCBMbased one (PCE = 5.59%, JSC = 10.3 mA cm−2, VOC = 0.808 V, FF = 0.672). The LUMO energy levels of [70]PCBM showed no definite correlation with the VOC values. Although AFM measurements displayed no significant differences in the PCDTBT:[70]PCBM composite films, transient absorption spectroscopy revealed lower charge-trap density for PCDTBT:α-[70]PCBM and higher charge-dissociation effic i e n c y f o r P C D T B T : β - [ 7 0 ] P C B M co m p a r e d t o PCDTBT:mix-[70]PCBM, leading to enhanced device performance of the regioisomer-pure [70]PCBMs. Higher PCEs were obtained for both α- and β-isomers of [70]PCBM, whereas only the α-isomer showed higher PCE for G

DOI: 10.1021/acs.accounts.9b00159 Acc. Chem. Res. XXXX, XXX, XXX−XXX

Article

Accounts of Chemical Research

independent of isomer composition, the film surface morphology was influenced intensively; the α:β1:β2 = 17:1:2 film was very smooth, but the regioisomerically pure [70]PCBM films suffered from severe aggregation of the fullerene derivatives. This led to poor morphology of the ETL, which could decrease the contact interfaces between perovskite and ETL and between ETL and BE. As is the case of OPVs, the energy level disorders among isomers are not significantly influencing on the ETL properties, but the poor film-forming property of isomer-pure [70]PCBM considerably diminishes the device performance.

ble, the degree of aggregation tendency of the fullerene derivatives has a large impact on the photovoltaic properties. OPVs based on α-[70]PCBM (8.4%) and β2-[70]PCBM (8.4%) rivaled or slightly surpassed the device with mix[70]PCBM (8.2%) because of the absence of the poorperformance β1-[70]PCBM. Table 2 lists the PCE dependencies on the isomers of [70]PCBM with various conjugated polymer donors. As are Table 2. PCE Orders of OPV Devices with Various [70]PCBM Isomers donor a

PCDTBT P3HTb rac-/(R)-/(S)-PTB2b PffBT4T-2ODb

PCE order α (6.20%) ≃ β (6.46%) > mix (5.59%) β (3.69%) > α (3.11%) ≃ mix (3.21%) α ≃ (R)-α ≃ (S)-α (3.2%−3.5%) α (8.4%) ≃ β2 (8.4%) ≃ mix (8.2%) > β (7.9%) ≫ β1 (0.43%)

■

ref

SUMMARY AND OUTLOOK In this Account, we have surveyed the effects of isomer separation of fullerene derivatives on the device performance of OPVs and PSCs. Fullerene derivative acceptors had provided a significant breakthrough in the field of OPVs,3 but recently the development of new fullerenes has made little contributions to the progress in this field. [60]PCBM and [70]PCBM have remained as the most prevalent fullerene acceptors for more than 10 years.3 The emergence of highperformance nonfullerene acceptors with excellent lightharvesting abilities in the visible and NIR regions offers a continuous improvement in the PCEs of OPVs. However, further enhancement of OPVs requires the utilization of tandem configurations, where two or more subcells with complementary absorptions are stacked and connected in series or in parallel.32 The PCE of tandem OPVs is predicted to exceed 20%, and therefore there is still room for further improvement. Fullerene acceptors with excellent electron affinity and strong absorption at