Oligothiophene-based photovoltaic materials for organic solar cells: rise, plateau, and revival

Oligothiophene derivatives are considered as one of the most promising donor materials in cost-effectiveness and large-scale production of organic solar cells (OSCs).Recent findings of over 15% power conversion efficiency for OSCs with oligothiophene-based materials have intensified research interest in these compounds.To ensure that this cost-effective donor family qualifies for the real production of commercialized OSCs in the future, further enhancement of the performance of oligothiophene donors is necessary. Oligothiophene derivatives have been positioned as one of the most promising donor materials in cost-effectiveness and large-scale production of organic solar cells (OSCs). Oligothiophene donors have been popularized in the era of fullerenes but have not been successful as nonfullerene acceptors (NFAs). The recent report of over 15% power conversion efficiency (PCE) for OSCs has attracted research interest in these materials. Herein, the development of oligothiophene-based photovoltaic materials and their applications in OSCs are chronologically presented. The evolution of oligothiophene donors and milestone materials are highlighted and results on oligothiophene-based NFAs are included. To offer guidance and spur further development of oligothiophene-based photovoltaic materials, we propose a perspective on future trends in the molecular design and applications of oligothiophene donors/acceptors. Oligothiophene derivatives have been positioned as one of the most promising donor materials in cost-effectiveness and large-scale production of organic solar cells (OSCs). Oligothiophene donors have been popularized in the era of fullerenes but have not been successful as nonfullerene acceptors (NFAs). The recent report of over 15% power conversion efficiency (PCE) for OSCs has attracted research interest in these materials. Herein, the development of oligothiophene-based photovoltaic materials and their applications in OSCs are chronologically presented. The evolution of oligothiophene donors and milestone materials are highlighted and results on oligothiophene-based NFAs are included. To offer guidance and spur further development of oligothiophene-based photovoltaic materials, we propose a perspective on future trends in the molecular design and applications of oligothiophene donors/acceptors. Through the efficient direct conversion of solar irradiation to electricity, solar cells have become one of the most important solutions to the demand for energy [1.Lewis N.S. Toward cost-effective solar energy use.Science. 2007; 315: 798-801Crossref PubMed Scopus (1866) Google Scholar]. 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Between the anode and cathode, a layer of organic electron-donating and -accepting materials was separately deposited to form a planar heterojunction (PHJ) to generate hole–electron pairs, thereby producing photocurrent (Figure IA). However, owing to the relatively short diffusion length of the exciton in organic semiconductors and extremely limited options of photovoltaic materials, PHJ OSCs exhibit inferior power conversion efficiencies (PCEs) [8.Mishra A. et al.Small molecule organic semiconductors on the move: promises for future solar energy technology.Angew. Chem. Int. Ed. 2012; 51: 2020-2067Crossref PubMed Scopus (1529) Google Scholar]. In 1995, Heeger and colleagues reported the groundbreaking discovery of the bulk heterojunction (BHJ) device architecture and fullerene-based electron acceptors, while Friend and colleagues reported all-polymer BHJ [9.Yu G. et al.Polymer photovoltaic cells: enhanced efficiencies via a network of internal donor-acceptor heterojunctions.Science. 1995; 270: 1789-1791Crossref Google Scholar,10.Halls J.J.M. et al.Efficient photodiodes from interpenetrating polymer networks.Nature. 1995; 376: 498-500Crossref Google Scholar]. Dissimilar to PHJ devices, the well-mixed donor–acceptor (D–A) in the BHJ device ensures sufficient D–A interfacial area that guarantees efficient photoinduced electron transfer from a donor molecule to acceptors, and ultrafast charge separation leaves holes/electrons to be synergistically transferred by the donor/acceptor phase, resulting in essentially enhanced PCEs (Figure IB) [11.Arunagiri L. et al.Selective hole and electron transport in efficient quaternary blend organic solar cells.Joule. 2020; 4: 1790-1805Abstract Full Text Full Text PDF Google Scholar]. The prototype of OSC devices was fabricated in a multilayer architecture [7.Tang C.W. et al.Two-layer organic photovoltaic cell.Appl. Phys. Lett. 1986; 48: 183-185Crossref Scopus (4473) Google Scholar]. Between the anode and cathode, a layer of organic electron-donating and -accepting materials was separately deposited to form a planar heterojunction (PHJ) to generate hole–electron pairs, thereby producing photocurrent (Figure IA). However, owing to the relatively short diffusion length of the exciton in organic semiconductors and extremely limited options of photovoltaic materials, PHJ OSCs exhibit inferior power conversion efficiencies (PCEs) [8.Mishra A. et al.Small molecule organic semiconductors on the move: promises for future solar energy technology.Angew. Chem. Int. Ed. 2012; 51: 2020-2067Crossref PubMed Scopus (1529) Google Scholar]. In 1995, Heeger and colleagues reported the groundbreaking discovery of the bulk heterojunction (BHJ) device architecture and fullerene-based electron acceptors, while Friend and colleagues reported all-polymer BHJ [9.Yu G. et al.Polymer photovoltaic cells: enhanced efficiencies via a network of internal donor-acceptor heterojunctions.Science. 1995; 270: 1789-1791Crossref Google Scholar,10.Halls J.J.M. et al.Efficient photodiodes from interpenetrating polymer networks.Nature. 1995; 376: 498-500Crossref Google Scholar]. Dissimilar to PHJ devices, the well-mixed donor–acceptor (D–A) in the BHJ device ensures sufficient D–A interfacial area that guarantees efficient photoinduced electron transfer from a donor molecule to acceptors, and ultrafast charge separation leaves holes/electrons to be synergistically transferred by the donor/acceptor phase, resulting in essentially enhanced PCEs (Figure IB) [11.Arunagiri L. et al.Selective hole and electron transport in efficient quaternary blend organic solar cells.Joule. 2020; 4: 1790-1805Abstract Full Text Full Text PDF Google Scholar]. Oligothiophenes, an old but thriving family of conjugated materials, are widely used as an active component in organic electronic devices, such as light-emitting diodes, field-effect transistors, and OSCs [39.Perepichka I.F. et al.Handbook of Thiophene-Based Materials: Applications in Organic Electronics and Photonics. John Wiley & Sons, 2019Google Scholar]. α-Octithiophene (8T, Figure 1B ) was first adopted as an electron donor in OSCs in 1995, and oligothiophenes have been intensively investigated as photoactive materials in OSCs since then [40.Noma N. et al.αThiophene octamer as a new class of photo-active material for photoelectrical conversion.Adv. Mater. 1995; 7: 647-648Crossref Scopus (174) Google Scholar,41.Ni W. et al.A-D-A small molecules for solution-processed organic photovoltaic cells.Chem. Commun. 2015; 51: 4936-4950Crossref PubMed Google Scholar]. This is majorly because of their intriguing optoelectronic properties, charge-transport properties, excellent intrinsic stability, and unique self-assembling properties on solid surfaces or in the bulk. Another key factor facilitating this development is that the well-established and enormously developed heterocyclic chemistry offers countless methods for modifying the thiophene ring and generating kaleidoscopic π-conjugated systems at a high yield and low cost [42.Weu A. et al.Energy transfer to a stable donor suppresses degradation in organic solar cells.Adv. Funct. Mater. 2019; 301907432Google Scholar, 43.Tanaka S. et al.Synthesis of well-defined head-to-tail-type oligothiophenes by regioselective deprotonation of 3-substituted thiophenes and nickel-catalyzed cross-coupling reaction.J. Am. Chem. Soc. 2011; 133: 16734-16737Crossref PubMed Scopus (79) Google Scholar, 44.Dang M.T. et al.P3HT:PCBM, best seller in polymer photovoltaic research.Adv. Mater. 2011; 23: 3597-3602Crossref PubMed Scopus (1053) Google Scholar]. Owing to the unremitting efforts of researchers, oligothiophene-based photovoltaic compounds have evolved from pure thiophene oligomers to delicately constructed solution-processable molecules (Figure 1B). Consequently, the PCE of oligothiophene-based OSCs has been boosted from below 1% to over 15% (Figure 1A). To ensure that this cost-effective donor family qualifies for the real production of commercialized OSCs in the future, further enhancement of the performance of oligothiophene donors is necessary. When designing a high-performance donor, delicate control of multiple factors, including energy levels, bandgap, solubility, crystallinity, and compatibility with acceptors, is mandatory [45.Wan X. et al.Acceptor–donor–acceptor type molecules for high performance organic photovoltaics – chemistry and mechanism.Chem. Soc. Rev. 2020; 49: 2828-2842Crossref PubMed Google Scholar]. Therefore, a comprehensive retrospect on oligothiophene-based photovoltaic materials (Box 2) is urgently needed. In this article, the development of oligothiophene-based organic photovoltaic materials in the past 27 years (1995–2022) has been reviewed chronologically. The efficiency enhancement and structural evolution of oligothiophenes are discussed in detail. Some recent advances are discussed, along with further optimization of oligothiophene prospects. This review will shed some light on the future development and application of oligothiophene-based OSCs for a wide range of readers.Box 2The scope and delimitations of oligothiophene-based photovoltaic materials for OSCsSince thiophene derivatives are indispensable building blocks for constructing organic photovoltaic molecules, it will take an overwhelming length to include all the ‘thiophene-containing’ materials for OSCs [46.Mishra A. et al.Functional oligothiophenes: molecular design for multidimensional nanoarchitectures and their applications.Chem. Rev. 2009; 109: 1141-1276Crossref PubMed Scopus (1216) Google Scholar]. Therefore, we narrowed the range of ‘oligothiophenes with simple add-ons’. Specifically, alkyl chains, terminal groups, fused thiophenes (thienothiophene, dithienothiophene, etc.), and certain single-ringed substituents/insertions (thiazole, selenophene, etc.) are allowed to appear in our targets, but molecules containing other symbolic chromophores (triarylamines [47.Shang H. et al.A solution-processable star-shaped molecule for high-performance organic solar cells.Adv. Mater. 2011; 23: 1554-1557Crossref PubMed Scopus (338) Google Scholar, 48.Lin Y. et al.A star-shaped oligothiophene end-capped with alkyl cyanoacetate groups for solution-processed organic solar cells.Chem. Commun. 2012; 48: 9655-9657Crossref PubMed Scopus (70) Google Scholar, 49.Lin Y. et al.One, two and three-branched triphenylamine–oligothiophene hybrids for solution-processed solar cells.J. Mater. Chem. A. 2013; 1: 5128-5135Crossref Scopus (20) Google Scholar], diketopyrrolopyrroles [50.Li W. et al.Diketopyrrolopyrrole polymers for organic solar cells.Acc. Chem. Res. 2016; 49: 78-85Crossref PubMed Scopus (389) Google Scholar], BDT [51.Lin Y. et al.Small-molecule solar cells with fill factors up to 0.75 via a layer-by-layer solution process.Adv. Energy Mater. 2013; 41300626Google Scholar], etc.) will not be discussed herein (Figure I).Owing to the electron-rich feature, oligothiophenes have been widely used to design electron donors, which is the primary content of this paper. A few papers were recently published on oligothiophene-based electron acceptors; these inspiring results are discussed. Oligothiophenes have been occasionally used as functional materials in OSCs (surface modifier of conductive glass [52.Timpel M. et al.Oligothiophene-based phosphonates for surface modification of ultraflat transparent conductive oxides.Adv. Mater. Interfaces. 2020; 71902114Crossref Scopus (1) Google Scholar], interlayer materials [53.Planells M. et al.A quarterthiophene-based dye as an efficient interface modifier for hybrid titanium dioxide/poly(3-hexylthiophene)(P3HT) solar cells.ACS Appl. Mater. Interfaces. 2014; 6: 17226-17235Crossref PubMed Scopus (22) Google Scholar], etc.); such relatively less common reports are excluded. Since thiophene derivatives are indispensable building blocks for constructing organic photovoltaic molecules, it will take an overwhelming length to include all the ‘thiophene-containing’ materials for OSCs [46.Mishra A. et al.Functional oligothiophenes: molecular design for multidimensional nanoarchitectures and their applications.Chem. Rev. 2009; 109: 1141-1276Crossref PubMed Scopus (1216) Google Scholar]. Therefore, we narrowed the range of ‘oligothiophenes with simple add-ons’. Specifically, alkyl chains, terminal groups, fused thiophenes (thienothiophene, dithienothiophene, etc.), and certain single-ringed substituents/insertions (thiazole, selenophene, etc.) are allowed to appear in our targets, but molecules containing other symbolic chromophores (triarylamines [47.Shang H. et al.A solution-processable star-shaped molecule for high-performance organic solar cells.Adv. Mater. 2011; 23: 1554-1557Crossref PubMed Scopus (338) Google Scholar, 48.Lin Y. et al.A star-shaped oligothiophene end-capped with alkyl cyanoacetate groups for solution-processed organic solar cells.Chem. Commun. 2012; 48: 9655-9657Crossref PubMed Scopus (70) Google Scholar, 49.Lin Y. et al.One, two and three-branched triphenylamine–oligothiophene hybrids for solution-processed solar cells.J. Mater. Chem. A. 2013; 1: 5128-5135Crossref Scopus (20) Google Scholar], diketopyrrolopyrroles [50.Li W. et al.Diketopyrrolopyrrole polymers for organic solar cells.Acc. Chem. Res. 2016; 49: 78-85Crossref PubMed Scopus (389) Google Scholar], BDT [51.Lin Y. et al.Small-molecule solar cells with fill factors up to 0.75 via a layer-by-layer solution process.Adv. Energy Mater. 2013; 41300626Google Scholar], etc.) will not be discussed herein (Figure I). Owing to the electron-rich feature, oligothiophenes have been widely used to design electron donors, which is the primary content of this paper. A few papers were recently published on oligothiophene-based electron acceptors; these inspiring results are discussed. Oligothiophenes have been occasionally used as functional materials in OSCs (surface modifier of conductive glass [52.Timpel M. et al.Oligothiophene-based phosphonates for surface modification of ultraflat transparent conductive oxides.Adv. Mater. Interfaces. 2020; 71902114Crossref Scopus (1) Google Scholar], interlayer materials [53.Planells M. et al.A quarterthiophene-based dye as an efficient interface modifier for hybrid titanium dioxide/poly(3-hexylthiophene)(P3HT) solar cells.ACS Appl. Mater. Interfaces. 2014; 6: 17226-17235Crossref PubMed Scopus (22) Google Scholar], etc.); such relatively less common reports are excluded. The debut of oligothiophenes occurred in 1995. A simple D–D′–D structured α-octithiophene 8T (1) was employed as an electron donor to construct a vacuum-processed (VP) planar heterojunction (PHJ) OSC with an electron-accepting perylene pigment PV [40.Noma N. et al.αThiophene octamer as a new class of photo-active material for photoelectrical conversion.Adv. Mater. 1995; 7: 647-648Crossref Scopus (174) Google Scholar]. Although only a modest PCE of ca. 0.59% was achieved, this study initiated the development of oligothiophene-based photovoltaic materials. However, owing to the considerably high PCEs of polymer donors [poly(3-hexylthiophene-2,5-diyl); P3HT] [54.Padinger F. et al.Effects of postproduction treatment on plastic solar cells.Adv. Funct. Mater. 2003; 13: 85-88Crossref Scopus (1904) Google Scholar], oligothiophenes did not attract sufficient attention until a new century. In 2006, Bäuerle and colleagues reported an oligothiophene derivative, DCV5T (2), structured with a pentathiophene backbone and two electron-withdrawing dicyanovinyl (DCV) terminal groups [55.Schulze K. et al.Efficient vacuum-deposited organic solar cells based on a new low-bandgap oligothiophene and fullerene C60.Adv. Mater. 2006; 18: 2872-2875Crossref Scopus (303) Google Scholar]. The DCV5T:C60-based VP PHJ OSCs exhibited a high open-circuit voltage (VOC) of 0.98 V with a short-circuit current density (JSC) of 10.6 mA cm−2 and a PCE of 3.4%, which was comparable with those of P3HT-based devices. Based on DCV5T, DCV6T (3) was developed afterward but because of its decreased VOC of 0.90 V, an inferior PCE of ca. 3.1% was observed [56.Wynands D. et al.Correlation between morphology and performance of low bandgap oligothiophene:C60 mixed heterojunctions in organic solar cells.J. Appl. Phys. 2010; 107014517Crossref Scopus (52) Google Scholar]. Judging from the molecular structure, the introduction of DCV substituents with low-lying lowest unoccupied molecular orbital (LUMO) energy levels considerably reduced the corresponding optical gap, compared with that of the unsubstituted oligothiophene. Moreover, the terminal groups of the molecules were prone to coupling with neighboring thiophene cores in the solid state via intermolecular CN∙∙∙H bonds [57.Casado J. et al.Spectroscopic and theoretical study of the molecular and electronic structures of a terthiophene-based quinodimethane.ChemPhysChem. 2004; 5: 529-539Crossref PubMed Scopus (42) Google Scholar]. This molecular arrangement might favor an attractive interaction between the transition dipoles of the intramolecular donor–acceptor (D–A) excitations, thereby resulting in a red-shift in the absorption of thin films, compared with that of the molecules in solution. The broadened absorption would indisputably benefit the light-harvesting and subsequently enhance the JSC of OSC devices. This A–D–A strategy greatly impacted subsequent research; the electron-withdrawing terminal groups gradually became an indispensable part of high-performance SM photovoltaic molecules. Attracted by the superiority of the BHJ architecture and the convenience of the solution-processing technique, researchers began to focus on highly soluble oligothiophene donors to extend the boundary of material development and break the limitations of the costly vacuum deposition technique. To alleviate the π–π stacking interaction and enhance the solubility of oligothiophenes, Roncali and colleagues synthesized ‘X-shaped’ oligothiophene 4 in 2006. The solution-processed (SP) BHJ OSCs based on 4:PC61BM exhibited a PCE of ca. 0.2%; the fill factor (FF) of 0.27 and low JSC of 1.33 mA cm−2 caused by the narrow absorption range of donor 4 drastically limited the device performance [58.Karpe S. et al.3D π-conjugated oligothiophenes based on sterically twisted bithiophene nodes.Adv. Funct. Mater. 2007; 17: 1163-1171Crossref Scopus (71) Google Scholar]. Similarly, a series of tetrakis oligothienylsilanes were developed to further alleviate the intermolecular stacking of oligothiophene. Compound 5 exhibited a modest PCE of ca. 0.29% because of the poor FF of 0.24. In other studies, the PCE was pushed to over 1.0% by modified molecules 6 and 7 with an extended π-conjugation and absorption range [59.Roquet S. et al.Three-dimensional tetra(oligothienyl)silanes as donor material for organic solar cells.J. Mater. Chem. 2006; 16: 3040-3045Crossref Scopus (104) Google Scholar,60.Kleymyuk E.A. et al.3D quater- and quinquethiophenesilanes as promising electron-donor materials for BHJ photovoltaic cells and photodetectors.Energy Environ. Sci. 2010; 3: 1941-1948Crossref Scopus (20) Google Scholar]. Certain dendritic oligothiophenes were designed and tentatively employed as donor materials. In 2009, Bäuerle and colleagues reported a branched oligothiophene salt, MePy+-21T (9), bearing a methylpyridinium terminal group. Considering that the VOC of 9:PC61BM-based BHJ OSCs was merely 0.6 V because of the high-lying highest occupied molecular orbital (HOMO) energy level of 9, the PCE was below 1% [61.Fischer M.K.R. et al.Core-functionalized dendritic oligothiophenes—novel donor-acceptor systems.J. Mater. Chem. 2009; 19: 4784-4795Crossref Scopus (26) Google Scholar]. Another approach to soluble oligothiophenes is alkylation. Dissimilar to the construction of a 3D or twisted structure, the attachment of flexible alkyl chains can enhance the solubility of molecules without sacrificing the planarity of the conjugated backbone or the intermolecular charge transfer from the electron-donating center to electron-withdrawing terminals. In 2008, Lanzi and colleagues synthesized an alkylated octithiophene, OCT (8); BHJ OSCs based on OCT films mixed with single-walled carbon nanotubes (as acceptors) were prepared and a PCE of 0.35% was realized [62.Lanzi M. et al.New photoactive oligo- and poly-alkylthiophenes.Polymer. 2008; 49: 4942-4948Crossref Scopus (34) Google Scholar]. Yu and colleagues synthesized an oligomer, MF, containing 16 regioregular conjugated thiophene rings and an electron-withdrawing thieno[3,4-b]thiophene center (10) [63.Liang Y. et al.Regioregular oligomer and polymer containing thieno[3,4-b]thiophene moiety for efficient organic solar cells.Macromolecules. 2009; 42: 1091-1098Crossref Scopus (64) Google Scholar]. Molecule 10 exhibited wide-range light-harvesting with an absorption onset of film approaching 800 nm and a fairly good PCE of 1.46%. This work revealed the pivotal role of regioregular π-linkers. In 2009, Chen and colleagues reported a series of oligothiophene donors, DCN3T (11), DCN5T (12), and DCN7T (13). The suitable energy levels and conjugation length of DCN7T (13) resulted in a well-balanced VOC of 0.88 V and JSC of 12.4 mA cm−2. A PCE of 3.7% was achieved using 13:PC61BM blends [64.Liu Y. et al.Synthesis and properties of acceptor–donor–acceptor molecules based on oligothiophenes with tunable and low band gap.Tetrahedron. 2009; 65: 5209-5215Crossref Scopus (65) Google Scholar,65.Yin B. et al.Solution-processed bulk heterojunction organic solar cells based on an oligothiophene derivative.Appl. Phys. Lett. 2010; 97023303Crossref PubMed Scopus (86) Google Scholar], which was among the best PCEs obtained for SP SM BHJ devices. DCN7T is historically recognized as one of the most important SM donor molecules. Its regioregular and axisymmetric A–D–A structure provides an excellent template for researchers and has guided them toward efficient oligothiophene donors. In this period, it was observed that solution processing was gradually replacing vacuum deposition in device fabrication. For material development, owing to the limited papers and designs for reference, most studies on oligothiophene donors were not inherited. They were based on the exploration of new structures, investigating their impact on the performance of OSC devices and using the