F8bt Synthesis Essay

1. Introduction

Organic photovoltaics (OPVs) offer enormous potential as inexpensive coatings capable of generating electricity directly from sunlight [1,2]. These polymer blend materials can be printed at high speeds across large areas using roll-to-roll processing techniques, creating the tantalising vision of coating every roof and other suitable building surface with low-cost photovoltaics [3].

Conventionally, OPV devices are fabricated from mixtures of donor and acceptor materials dissolved in organic solvents, which are deposited to produce an interpenetrating network [4]. Current fabrication methodologies rely on the thermodynamics of demixing to produce phase segregated regions with the required optimum size of 20–50 nm [5]. However, two key aspects of current OPV fabrication are not well-suited to building large-area PV modules using high-speed printing techniques. First, using current fabrication approaches to control phase segregation across large areas is problematic [6]. Second, current OPV technology typically uses chlorinated solvents (e.g., chloroform), which are under continual regulatory pressure due to their hazardous and toxic nature [7,8]. As such, the increasingly harsh technical requirement for using these solvents means that their implementation in high-speed printing lines will be highly problematic, if not economically impractical [9].

Indeed, the need for an alternative, environmentally-friendly process for OPV device fabrication has only recently been recognized in the OPV literature. However, early attempts have focused on replacing halogenated solvents with aromatic hydrocarbons, which are not necessarily less hazardous [10,11,12,13]. The work of Søndergaard et al. on water-soluble [6,6]-phenyl-C61-buteric acid methylester (PCBM) and poly(3-hexylethiophene) (P3HT) analogues presented water-processed device structures producing an overall device efficiency of 0.7% [14]. The earliest reports of semiconducting polymer nanoparticles (NPs) dispersed in water showed that conductive coatings could be prepared by mixing colloidal (10–100 nm diameter) conducting polymer in a latex base [15,16]. In 2002, work by Landfester et al. showed that conjugated semiconducting polymers could be deposited from aqueous dispersions prepared by the miniemulsion process [17]. In 2003, Kietzke et al. reported the first OPV devices based on nanoparticles (50–250 nm diameter) of poly(9,9-dioctylfluorene-co-benzothiadiazole (F8BT) and poly(9,9-dioctylfluorene-co-N,N'-bis(4-butylphenyl)-N,N'-diphenyl-1,4-phenylenediamine) (PFB) semi-conducting polymers. However, their power conversion efficiency (PCE) was extremely low (<0.004%) and substantially less than the corresponding bulk heterojunction (BHJ) blend devices (~0.2%) [18]. Subsequently, Snaith reported even lower efficiencies using an electroplating technique to deposit the nanoparticles as the active layer in OPV devices [19]. More recently, roll-to-roll processing of water-dispersed nanoparticulate polymer solar cells with efficiencies of up to 0.55% for certain low band gap materials has been demonstrated with the relatively poor device performance ascribed to shunting and non-optimum morphology [20,21,22].

The relatively low efficiencies reported for OPVs derived from nanoparticulate films spin coated from water are perhaps unsurprising. To disperse these colloidal particles in water, a surfactant is added to lower the interfacial energy and to provide colloidal stability [23]. Upon spin coating, these surfactant molecules might be expected to form an insulating barrier between neighbouring particles that would tend to disrupt charge transport within the photovoltaic device [24,25]. In addition, semi-conducting polymer particles are usually highly sensitive to oxygen and especially water contaminants, which tend to rapidly degrade OPV performance [26].

2. Synthesis of NPs via the Miniemulsion Process

The nanoparticle fabrication method utilises ultrasound energy delivered via an ultrasound horn inserted into the reaction mixture to generate a miniemulsion (Figure 1). The ultrasound horn makes the formation of sub-micrometre droplets possible by applying high shear force. A liquid aqueous surfactant-containing phase (polar) is combined with an organic phase of polymer dissolved in chloroform (non-polar) to generate a macroemulsion, then ultrasonicated to form a miniemulsion. The polymer chloroform droplets constitute the dispersed phase with an aqueous continuous phase. This is a modification of the usual method for generating polymer nanoparticles where the dispersed phase was liquid monomer; this alternative method was first reported by Landfester and Kietzke [17,18]. Immediately after miniemulsification, the solvent is removed from the dispersed droplets via evaporation, leaving polymer nanoparticles. The final nanoparticle size can be varied by changing the initial concentration of surfactant in the aqueous phase.

Figure 1. Schematic of the NP synthesis process.

Figure 1. Schematic of the NP synthesis process.

3. Synthesis of NPs via Precipitation Methods

As an alternative to the miniemulsion approach, precipitation techniques offer a simple route to the production of semiconducting polymer nanoparticles via the injection of a solution of active material into a second solvent of poor solubility [27,28]. As such, the synthesis is quick, does not use surfactant, requires no heating (and therefore, no prefabrication annealing of the nanoparticles) in the nanoparticle synthesis phase and can readily be scaled up for the large-scale synthesis of material. In general, the dispersions have been shown to have lower stability and exhibit a compositional change upon standing due to preferential precipitation of particles of differing composition. However, the precipitation approach does offer the opportunity for inclusion of the nanoparticle synthesis as part of an active printing process, with particles being generated as and when required. Furthermore, Hirsch et al. have shown that by successive solvent displacement, it is possible to synthesise inverted core-shell particles where the structural arrangement is counter to the inherent surface energies of the materials [29].

4. The PFB:F8BT Nanoparticulate Organic Photovoltaic (NPOPV) Material System

Early measurements of the power conversion efficiency of PFB:F8BT nanoparticle devices under solar illumination reported devices with a Jsc = 1 × 10−5 A cm−2 and VOC = 1.38 V [23], which (assuming a best estimate unannealed fill factor (FF) of 0.28 from bulk blend devices [24]) corresponds to a PCE of 0.004%. The only other photovoltaic measurements of PFB:F8BT nanoparticle devices were external quantum efficiency (EQE) plots reported by both Kietzke [18,23] and Snaith [19].

In 2012, Stapleton et al. reported multilayered photovoltaic devices fabricated from PFB:F8BT nanoparticles, which demonstrated the highest power conversion efficiencies observed for these polyfluorene nanoparticle materials [30]. This increased performance was achieved through the control of the surface energies of the individual components in the polymer nanoparticle and the post-deposition processing of the polymer nanoparticle layers. Significantly, this work showed that the fabricated nanoparticulate organic photovoltaic (NPOPV) devices were more efficient than the standard blend devices (Figure 2).

Figure 2. Comparison of the electrical characteristics of nanoparticle and bulk heterojunction devices. (a) Variation of current density vs. voltage for a five-layer PFB:F8BT (poly(9,9-dioctylfluorene-co-N,N'-bis(4-butylphenyl)-N,N'-diphenyl-1,4-phenylenediamine) (PFB); poly(9,9-dioctylfluorene-co-benzothiadiazole (F8BT)) nanoparticulate (filled circles) and a bulk heterojunction (open circles) device; (b) Variation of external quantum efficiency (EQE) vs. wavelength for a five-layer PFB:F8BT nanoparticulate (filled circles) and a bulk heterojunction (open circles) device. Also shown (dashed line) is the EQE plot for the nanoparticulate film device reported by Kietzke et al. [18]. Reproduced with permission from [30], published by Elsevier B.V., 2012.

Figure 2. Comparison of the electrical characteristics of nanoparticle and bulk heterojunction devices. (a) Variation of current density vs. voltage for a five-layer PFB:F8BT (poly(9,9-dioctylfluorene-co-N,N'-bis(4-butylphenyl)-N,N'-diphenyl-1,4-phenylenediamine) (PFB); poly(9,9-dioctylfluorene-co-benzothiadiazole (F8BT)) nanoparticulate (filled circles) and a bulk heterojunction (open circles) device; (b) Variation of external quantum efficiency (EQE) vs. wavelength for a five-layer PFB:F8BT nanoparticulate (filled circles) and a bulk heterojunction (open circles) device. Also shown (dashed line) is the EQE plot for the nanoparticulate film device reported by Kietzke et al. [18]. Reproduced with permission from [30], published by Elsevier B.V., 2012.

Subsequently, Vaughan et al. compared the effect of Ca and Al cathodes (two of the most common electrode materials) in OPV devices based on polyfluorene blend aqueous polymer nanoparticle (NP) dispersions [31]. They showed that PFB:F8BT NPOPV devices with Al and Ca/Al cathodes exhibit qualitatively very similar behaviour, with a peak PCE of ~0.4% for Al (consistent with Stapleton’s work [30]) and ~0.8% for Ca/Al, and that there is a distinct optimized thickness for the NP devices (Figure 3). The optimal thickness is a consequence of the competing physical effects of the repair and filling of defects for thin films [32,33] and the development of stress cracking in thick films [34]. Indeed, Stapleton’s work showed that the optimal layer thickness in these devices corresponds to the critical cracking thickness (CCT) above which stress cracking occurs, resulting in low shunt resistance and a reduction in device performance [30].

Figure 3. Variation of power conversion efficiency (PCE) with the number of deposited layers for PFB:F8BT nanoparticulate organic photovoltaic (NPOPV) devices fabricated with an Al cathode (filled circles) and a Ca/Al cathode (open circles). Dotted and dashed lines have been added to guide the eye. An average error has been determined based upon the variance for a minimum of ten devices for each number of layers. Reproduced with permission from [31], published by American Institute of Physics, 2012.

Figure 3. Variation of power conversion efficiency (PCE) with the number of deposited layers for PFB:F8BT nanoparticulate organic photovoltaic (NPOPV) devices fabricated with an Al cathode (filled circles) and a Ca/Al cathode (open circles). Dotted and dashed lines have been added to guide the eye. An average error has been determined based upon the variance for a minimum of ten devices for each number of layers. Reproduced with permission from [31], published by American Institute of Physics, 2012.

Overall, the work by Vaughan et al. provided further confirmation that the intrinsic morphology of NPOPV PFB:F8BT devices enhances the exciton dissociation relative to the corresponding BHJ structure. Moreover, the use of a Ca/Al cathode results in the creation of interfacial gap states (Figure 4), which reduce the recombination of charges generated by the PFB in these devices and restores open circuit voltage to the level obtained for an optimized BHJ device, resulting in a PCE approaching 1%.

5. The P3HT:PCBM NPOPV Material System

The first NPOPV devices based on the more common P3HT:PCBM system were reported by Lee et al., who synthesised P3HT:PCBM NPs using the miniemulsion method and then demonstrated their PV effect using conducting atomic force microscopy (AFM) under illumination [35]. At almost the same time, Larsen-Olsen et al. reported fully-printed reel-to-reel (R2R) devices using an inverted geometry also based on P3HT:PCBM NPs that exhibited a best PCE of only 0.3% [22].

Figure 4. Energy level diagrams for PFB:F8BT nanoparticles in the presence of calcium. (a) Calcium diffuses through the nanoparticle surface; (b) Calcium dopes the PFB-rich shell, producing gap states. Electron transfer occurs from calcium producing filled gap states; (c) An exciton generated on PFB approaches the doped PFB material (PFB*), and a hole transfers to the filled gap state, producing a more energetic electron; (d) Electron transfer from an exciton generated on F8BT to either the higher energy PFB lowest unoccupied molecular orbital (LUMO) or the filled lower energy PFB* LUMO is hindered. Reproduced with permission from [31], published by American Institute of Physics, 2012.

Figure 4. Energy level diagrams for PFB:F8BT nanoparticles in the presence of calcium. (a) Calcium diffuses through the nanoparticle surface; (b) Calcium dopes the PFB-rich shell, producing gap states. Electron transfer occurs from calcium producing filled gap states; (c) An exciton generated on PFB approaches the doped PFB material (PFB*), and a hole transfers to the filled gap state, producing a more energetic electron; (d) Electron transfer from an exciton generated on F8BT to either the higher energy PFB lowest unoccupied molecular orbital (LUMO) or the filled lower energy PFB* LUMO is hindered. Reproduced with permission from [31], published by American Institute of Physics, 2012.

In a subsequent paper, Ulum et al. demonstrated NP-OPV devices fabricated from water-dispersed P3HT:PCBM nanoparticles that exhibited power conversion efficiencies (PCEs) of 1.30% and peak external quantum efficiencies (EQE) of 35% [36]. However, unlike the PFB:F8BT NPOPV system, the P3HT:PCBM NPOPV devices were less efficient than their bulk heterojunction counterparts. Scanning transmission X-ray microscopy (STXM) revealed that the active layer retains a highly structured NP morphology and comprises core-shell NPs consisting of a relatively pure PCBM core and a blended P3HT:PCBM shell (Figure 5). However, upon annealing, these NPOPV devices undergo extensive phase segregation and a corresponding decrease in device performance. Indeed, this work provided an explanation for the lower efficiency of the annealed P3HT:PCBM OPV devices, since thermal processing of the NP film results in an effectively “over-annealed” structure with gross phase segregation occurring, thus disrupting charge generation and transport.

Figure 5. (a) Transmission electron microscopy (TEM) image of the unannealed P3HT:PCBM NP film. Corresponding scanning transmission X-ray microscopy (STXM) maps of: (b) P3HT composition and (c) PCBM composition for the unannealed P3HT:PCBM NP film. (d) TEM image of the annealed P3HT:PCBM NP film. Corresponding STXM maps of: (e) P3HT composition and (f) PCBM composition for the annealed P3HT:PCBM NP film. The horizontal line in the composition images is produced by a small (<5%) beam intensity variation occurring in the P3HT mass plot. A 1-μm scale bar is shown. Reproduced with permission from [36], published by Elsevier B.V., 2013.

Figure 5. (a) Transmission electron microscopy (TEM) image of the unannealed P3HT:PCBM NP film. Corresponding scanning transmission X-ray microscopy (STXM) maps of: (b) P3HT composition and (c) PCBM composition for the unannealed P3HT:PCBM NP film. (d) TEM image of the annealed P3HT:PCBM NP film. Corresponding STXM maps of: (e) P3HT composition and (f) PCBM composition for the annealed P3HT:PCBM NP film. The horizontal line in the composition images is produced by a small (<5%) beam intensity variation occurring in the P3HT mass plot. A 1-μm scale bar is shown. Reproduced with permission from [36], published by Elsevier B.V., 2013.

Holmes et al. reported both the intra- and inter-particle morphology of P3HT:PCBM nanoparticles before and after thermal annealing treatment, as a function of P3HT molecular weight [37]. The morphological changes resulting from thermal annealing were shown to be highly dependent upon the molecular weight of the polymer (Figure 6

1. Introduction

The thrust towards energy conservation has fuelled intensive research into the development of alternative energy sources. Solar energy offers the advantages of being renewable and clean, thus making solar cells attractive as a prospective alternative energy source. Photovoltaic (PV) cells based on inorganic materials are currently the main commercially used devices because of their relatively high efficiencies (e.g., 15%–20% for silicon-based PVs); nevertheless, these devices are limited by the high fabrication cost and related environmental issues [1,2,3,4]. Consequently, organic photovoltaic cells (OPVs) which offer the advantages of relatively low fabrication cost, easy processing, and flexibility, have gained focus despite their relatively low efficiencies [5]. The development of OPVs has progressed rapidly with the synthesis of new organic materials, control of processing condition such as annealing and the use of additive [6], as well as the introduction of various device structures such as the tandem and inverted structure [7,8,9]. In addition, control of morphology of active layers [10] and the development of purification by removing residual catalysts in conjugated polymers [11] have also been considered as important issues to achieve consistent, high-performance OPVs. Currently, the highest power conversion efficiency (PCE) of 12% has been announced by Heliatek [12]. Despite the relatively low PCEs of OPVs compared to those of inorganic-based solar cells, the development of OPVs is nevertheless rapid based on the anticipation that the numerous advantages can outweigh the low PCE of OPVs.

OPVs comprise an active layer consisting of organic materials that is sandwiched between two electrodes with different work functions (e.g., indium tin oxide (ITO) and Al as anode and cathode, respectively), and interfacial (hole/electron transporting) layers can be added between both electrodes and the active layer. The active layers in OPVs are normally composed of two electron donor (D) and electron acceptor (A) materials for the generation of the Coulomb-bound electron-hole pair (exciton) by photoexcitation of the donor. The diffused excitons are separated into charges of electrons and holes on the D–A surface, followed by free charge transportation and collection at electrodes. The appropriate highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy level of the donors and acceptors, and low band-gap are known to be important for high OPV performance, as well as good film-forming properties, strong absorption ability, and high charge mobility. OPV cells have been fabricated in bi-layer and bulk-heterojuncton (BHJ) solar cells according to the configuration of the active layer. Bi-layer OPVs containing separate donor and acceptor layers were first reported by Tang in 1986 [13]; their performance is known to be limited by the small charge-generating interfacial area between the donor and acceptor layers [14,15]. The BHJ solar cells, developed by Yu and Heeger et al., can be fabricated by simple spin-coating of a blended solution of donor and acceptor, and have an interpenetrating network with a large D–A interfacial area [16]. BHJ solar cells have been extensively used in the fabrication of high efficiency OPVs, and various processing techniques have been developed to achieve good film morphology of the BHJ solar cells, such as thermal annealing and the use of small amounts of additives [17].

The material system comprising poly(3-hexylthiophene) (P3HT, D1) and [6,6]-phenyl-C61 butyric acid methylester (PC61BM) as respective electron donor and acceptor is archetypal of the active layer in OPVs (Figure 1). In recent decades, various polymeric and small-molecule electron donor and acceptor materials have been synthesized and developed to achieve high-efficiency OPV cells, with specific focus on the development of polymer donors with an extended conjugated system for solution-processable OPVs. At the present stage, high PCEs of up to 9.2% have been achieved by using the polymeric donor thieno[3,4-b]thiophene/benzodithiophene (PTB7) with an inverted device structure [18]. The development of donor materials for OPVs has mainly focused on the syntheses of low-band-gap conjugated materials composed of electron-rich and electron-deficient repeating units (e.g., D–A type) for efficient absorption of the solar spectrum. Based on this synthetic design rule, a number of low-band-gap conjugated polymers (optical energy band-gap, Eg <1.8 eV) have been synthesized and employed as donors in polymer photovoltaic cells. Most building blocks for electron-rich units are based on thiophene and/or phenylene in the fused form or with bridging atoms for increased planarity of the polymer backbone and consequently enhanced short circuit current (JSC) and PCE [19]. Examples of electron-rich units include cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) [20], dithieno[3,2-b:2′,3′-d]silole [21] and 5H-dithieno[3,2-b:2′,3′-d]pyran (DTP) [22]. Various electron-deficient units have been copolymerized, and examples of building blocks for electron-deficient units are presented below. The development of high efficiency small-molecule donors has been the focus in more recent studies, and a high PCE of 8.12% has been achieved using D–A type oligothiophenes with strong electron-withdrawing dye units at both ends [23]. To enhance the PCE, various polymeric and small-molecule donors have also been synthesized and developed.

Figure 1. (a) Representative device configuration of organic photovoltaic cells (OPVs) and (b) molecular structures of P3HT (D1), PC61BM, and PC71BM.

Figure 1. (a) Representative device configuration of organic photovoltaic cells (OPVs) and (b) molecular structures of P3HT (D1), PC61BM, and PC71BM.

On the other hand, fullerene derivatives such as PC61BM and PC71BM have been widely used as representative acceptor materials for obtaining high PCEs in OPVs because of their good electron mobility as n-type materials, adequate band-gaps, and good interaction with donor materials in OPVs. Recently, non-fullerene small-molecule acceptor materials based on strong electron-withdrawing units, which exhibited high electron mobility in organic field-effect transistor (OFET) applications, have also been reported and are discussed in other review papers [24,25,26,27]. Examples include rylene imide, metallophthalocyanins, vinazene, and diketopyrrolopyrrole (DPP) units. PCEs of 3.45% [28] and 4.03% [29] have respectively been achieved for OPV devices employing polymer acceptors and small-molecule acceptors. Despite their relatively low efficiencies, the polymer acceptors have some unique advantages such as high absorption coefficients in the visible spectral region and easily tunable energy levels, compared to fullerenes and non-fullerene small-molecule acceptors [30]. Furthermore, the concept of conjugated block copolymers (BCPs) has been recently introduced to combine a donor and acceptor block into a single macromolecular platform and emerged as a promising class of materials for OPVs [31,32,33,34]. A large scale macroscopic phase separation is impeded in the BCP due to the covalent connectivity of the two blocks and the self-assembly of BCPs into mesoscale (5−500 nm) well-ordered morphologies is ideal for the active layer of OPVs [35,36,37]. The performance of up to 3.1% was achieved at the present stage [38].

Herein, we focus on various polymer acceptors for all-polymer solar cells, which have been rarely reported compared to small-molecule acceptors. The polymer acceptors are categorized into four classes on the basis of their structures, i.e., rylene diimide-based polymers, fluorene- and benzothiadiazole (BT)-based polymers, cyano (CN)-substituted polymers, and other polymer acceptors containing various electron-withdrawing units.

2. Rylene Diimide-Based Polymer Acceptors

In addition to their good thermal, chemical, and photochemical stability, rylene diimide-based polymers also exhibit high electron affinity and good electron mobility derived from the electron accepting imide groups, thus making the polymers suitable for use in various electronic fields [24,39,40]. In this section, we summarize the rylene diimide-based polymers used as acceptors in OPVs. These include perylene diimide (PDI)-, naphthalene diimide (NDI)-, and dithienocoronene diimide (DTCDI)-based polymer acceptors.

2.1. PDI-Based Polymer Acceptors

The electron-withdrawing PDI cores can be substituted in the bay or imide position when copolymerized with various electron-rich units such as dithienothiophene (DTT) and DTP to form electron-accepting polymers [26]. PDI-based polymers substituted in the bay position may exhibit good solubility because of the long branched alkyl chain on the imide N-atom. Imide-substitution results in polymers containing the PDI unit in the backbone or in polymers with pendant PDIs. The photophysical properties and device performance parameters of PDI-based polymer acceptors (112) are summarized in Table 1.

Marder and co-workers first developed polymer acceptors having the bay-substituted PDI unit. Good solubility was achieved by introducing long and/or branched alkyl chains onto the imide N-atom. In 2007, they synthesized a new conjugated polymer (PPDI-DTT, 1, Figure 2) with alternating DTT and PDI units that exhibited high electron mobility of 1.3 × 10−2 cm2 V−1 s−1, excellent thermal stability (up to 410 °C), and high electron affinity, with a LUMO energy level of –3.9 eV. The weight average-molecular weight (Mw) of 1 was 15,000 with a narrow polydispersity index of 1.5 [41]. All-polymer solar cells were fabricated by using polymer acceptor 1 and a polymer donor of polythiophene derivative (D2, Figure 3). The BHJ device exhibited an average PCE of 1% with an open circuit voltage (VOC) of 0.63 V, a JSC of 4.2 mA/cm2, and a fill factor (FF) of 0.39 under white-light illumination (AM 1.5 solar simulator, 100 mW/cm2). Subsequently, they modified the polymer structures by adding more DTT moieties in the polymer backbones, resulting in the polymer acceptors 2 and 3 (Figure 2) in which the PDI cores were bay substituted with two and three DTT units, respectively [42,43]. The highest PCE was achieved with the polymer acceptor 2 having two DTT units in the polymer repeating unit when using D3 (Figure 3) as a donor, mainly because of the high JSC. The devices were optimized at a blend ratio of 3:1 (D:A, w/w) and exhibited a VOC of 0.69 V, a JSC of 5.02 mA/cm2, a FF of 0.43, and a PCE of 1.48% under simulated AM 1.5 illumination at 100 mW/cm2.

Recently, Zheng and co-workers introduced longer alkyl side chain into the polymer acceptor 1, resulting in the polymer 4 (Figure 2). They fabricated BHJ solar cells with two different donors based on conjugated side-chain isolated polythiophene derivatives (PT4TV (D4) and PT4TV-C (D5), Figure 3) [44]. Despite the structural similarity of the donors, D4 produced a better PCE of 0.99% than achieved with D5 (0.57%). The higher PCE of D4 was mainly attributed to the good FF (above 0.50) which was attributed to the high and balanced hole/electron mobility of the D4:4 blend with rapid transfer of the generated carriers. After adding 10% of chloronaphthalene as a solvent, the PCE of D4:4 was enhanced from 0.99% to 1.17%.

More recently, Cheng and co-workers fabricated devices with 1 and PBDTTT-C-T (D6) and showed the highest PCE of 3.45% using binary additives which is the best PCE achieved with all-polymer solar cells to date [28]. The nonvolatile additive enhanced miscibility of donor and acceptor suppressing aggregation of 1, and the other additive, 1,8-diiodooctane, increased aggregation and crystallization of D6 resulting in suitable phase separation and balanced charge transport.

Hasimoto and coworkers synthesized several PDI-based electron acceptors including variousco-monomer units of DTP (PDTP-PDI, 5), carbazole (PC-PDI, 6), vinylene, thiophene, fluorene, and dibenzosilole as replacements for the DTT unit in polymer 1 (Figure 2) [45,46]. Devices were fabricated with various donors of polythiophene derivative D7, DPP-based low band-gap polymer D8 (Figure 3), and D1 for comparison. The device performance varied in the range of 0.11%–1.15% based on the moieties juxtaposed to the perylene unit. For example, the BHJ solar cell fabricated with 5:D7 exhibited a PCE of 0.93% under AM 1.5 (100 mW/cm2) illumination, which was higher than achieved with the 5:D1 cells (0.17%). The decreased efficiency obtained with D1 was attributed to the lower JSC due to the rough surface and coarse phase separation morphology related to the poor miscibility of D1 and the PDI-based acceptors. Among the six acceptors, 6 produced the highest PCE of 1.15% with donor D7, using chlorobenzene solvent in the active layer. By changing the solvent to toluene/chloroform, the PCE achieved with D7:6 was improved to 2.23%.

Imide-substituted PDI-based polymers were initially developed by Janssen and co-workers for OPV in 2003 [47]. They synthesized two alternating polymers (7 and 8, Figure 2) consisting of oligo(p-phenylene vinylene) and PDI segments connected via saturated spacers of the flexible unconjugated alkyl or phenyl groups, thus forming a new class of donor-acceptor polymers. Devices with ITO/PEDOT:PSS/7 or 8/LiF/Al configuration exhibited high VOC values (1.20 V and 0.97 V, respectively), whereas the JSC values were extremely low because of fast geminate recombination.

Later, Sharma and co-workers synthesized the alternating phenylenevinylene and PDI copolymer 9 (Figure 2) via Heck coupling for use as an acceptor in BHJ solar cells [48]. Copolymer 9 exhibited broad absorption extending up to about 800 nm with a maximum peak at ca. 500 nm and an optical band gap of 1.66 eV. The solubility of 9 increased upon the introduction of tert-butyl and hexyloxy side groups with respective glass transition (Tg) and decomposition temperatures (Td) of 72 and 370 °C. A PCE of 1.67% was obtained by blending acceptor 9 and a poly(3-phenyl hydrazone thiophene) (PPHT, D9, Figure 3) donor. After annealing, the enhanced PCE (2.32%) was evidenced by an increase in the efficiency of separation of the exciton; this PCE is one of the highest reported values achieved with imide-substituted PDI-based polymer acceptors.

Table 1. Perylene diimide (PDI)-based polymer acceptors a.

Acceptor [Ref]Mn
Mw
Mobility, μe
[cm2V1s1]
HOMO/LUMO
(Eg [eV])
VOC
[V]
JSC
[mA/cm2]
FF
PCE
[%]
1[41]10,000
15,000
1.3 × 102 b−5.9/−3.9
(2.0)
0.634.20.391
(ITO/PEDOT:PSS/D2:1(1:1)/Al)
[28]3.37 × 105 c−5.9/−3.9
(2.0)
0.758.550.523.45
(ITO/PEDOT:PSS/D6:1(1:1)/Ca/Al)
2[42]20,000
43,000
−5.7/−3.8
(1.9)
0.695.020.431.48
(ITO/PEDOT:PSS/D3:2(3:1)/Ca/Al)
3[43]15,000
27,000
−5.4/−4.0
(1.4)
0.692.800.400.77
(ITO/PEDOT:PSS/D3:3(1:1)/Ca/Al)
4[44]−5.7/−3.8
(1.9)
0.673.240.511.17
(ITO/PEDOT:PSS/D4:4(2:1)/Ca/Al)
0.751.60

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