Digest PaperReview of carbazole based conjugated molecules for highly efficient organic solar cell application
Graphical abstract
Introduction
The solar cell is one of the key technologies to settle the vast amount of energy needed. Solar energy conversion devices that convert light energy into electrical energy are seen as inevitable future energy candidates due to the unlimited supply of light energy. Organic solar cells (OSCs) are more attractive features because of low cost, flexibility, lightweight and easy fabricability than inorganic solar cells.1, 2, 3, 4, 5, 6, 7, 8 The development of OSCs was started with a single component active layer sandwiched between two electrodes with different work functions reported in the 1980s.9 Tang et al., introduced the bilayer heterojunction in 1986. In the initial phase, bilayer device was developed using copper phthalocyanine as a donor (p-type) and perylene tetracarboxylic derivatives as an acceptor (n-type) followed by the thin film coating method with 1% efficiency.10 The performance of bilayer heterojunction devices was limited to a charge-generating interface between the donor and acceptor within a small area. Yu et al., overcame this difficulty using a bulk heterojunction organic solar cell (BHJ OSC) device where the blended active layer of the donor and acceptor are intermixed that resulted in an increase of interfacial area existing for the exciton dissociation.11 The device structures of BHJ OSCs are ITO/PEDOT:PSS/active layer/LiF/Al as shown in Figure 1. The benchmark active layer is blended with a donor like poly(3-hexylthiophene) and an acceptor like [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)12, 13, 14, 15 and this active layer fabricated with an additive showed power conversion efficiencies (PCEs) of upto ∼6%.16 The ideal p-type material is P3HT because it has more advantages. The presence of a hexyl side chain in P3HT provides high solubility in organic solvents, possessing high charge carrier mobility and good hole transporting properties.17, 18, 19 It is having the highest occupied molecular orbital (HOMO) energy level of −5.2 eV. The ideal n-type material is PC61BM synthesized from C60. C60 is limited such as buckminsterfullerene (C60, 70) which is considered as one of the most successful electron acceptor materials used in solution-processed organic solar cells so far. The molecule PC61BM effectively transports electrons from molecule to molecule as well.20, 21, 22, 23 In 1995, the first solubilizing C60 derivative was synthesized by Wudl et al., but, unfortunately, acceptor drawback of PC61BM comes from its insufficient absorption in the visible region due to its structural symmetry that forbids low energy transitions.2 The efficiency of BHJ OSC devices can be further improved by replacing acceptor PC61BM with its higher fullerene analogue of [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM). This has lower symmetry and allows more transitions.24, 25
In this review, we have given accounts of carbazole (Cz) based polymers, their synthesis, properties and applications. The Cz is a tricyclic ring system consisting of pyrrole ring fused to two benzene rings. The presence of the pyrrole ring system increases the electron rich nature of Cz and fully aromatic as well as hole transporting properties. The Cz itself has more thermal stability upto 240 °C, further it can be attached to any other moiety to enhance the thermal stability. The Cz is an inexpensive starting material, and the nitrogen atom can be easily functionalized with a variety of substituents.26, 27, 28
Section snippets
Solar cell working principle of BHJ OSCs
BHJ OSCs works on the general principle where light strike on a photoactive layer and photons are absorbed by small molecules or polymers based on the wavelength in its absorption region to generate excitons (electron-hole pair) followed by diffusion of excitons toward the donor/acceptor (D/A) interface. The electrons move to lowest unoccupied molecular orbital (LUMO) energy level of the donor and the holes will move toward the highest occupied molecular orbital (HOMO) energy level of a donor.
Perovskite type solar cells
They use perovskite structured compounds for the conversion of light energy into electrical energy. Here the perovskite structure is an active layer and it consists of hybrid organic–inorganic (lead or tin halide) based materials. The 2,2′, 7,7′-tetrakis(N,N-di-p-methoxyphenylamine) 9,9′-spirobifluorene (Spiro-OMeTAD) is most widely used as a hole-transporting material (HTM) in perovskite solar cells achieved for PCE of 19.3%.34
Lee et al., employ carbazole derivatives as hole-transporting
Effect of carbazole position attachments on polymer properties for OSCs
The Cz containing nitrogen atom can be easily functionalized with a variety of substituents like alkyl, alkyloxy, alkylated benzene, etc., to increase solubility and stability of the polymer moiety. Carbazole can be prepared by the substitutions in the 3,6; 2,7 and 1,8-positions shown in Figure 5.
The 1,8 carbazole position on polymers were reported with an attached alkyne unit and showed long wavelength absorption, strong fluorescence, and significant electrochemical activities.38, 39 A few
Carbazole donor–acceptor copolymers
Andersson et al., synthesized the carbazole–thiophene–quinoxaline–thiophene (D–π–A–π) polymers (P1, P2, P3) using various side chains exhibiting excellent stability and degradation temperature around 400 °C. The active layers of the solar cells were spin-coated by polymer:PC71BM with different weight ratios of 1:1, 1:2, 1:3 to 1:4 using o-dichlorobenzene (ODCB) as a solvent. Among the four blend ratios, the 1:3 shows good PCE, low HOMO and LUMO energy levels. The polymer P3 has the most side
Indolo[3,2-b]carbazole based polymers
Cz is one of the molecules to produce a tricyclic fused system with plenty of attractive features and their polymers are being used in polymer light-emitting diodes and polymer solar cells (PSCs). Indolo[3,2-b]carbazole have penta-fused heterocyclic compounds and more π-conjugation than Cz. The indolocarbazole units contain two binding sites for the alkyl substituent that have pronounced solution processability compared to the Cz moiety, such type of indolo[3,2-b]carbazole based polymer have
Carbazole based small molecule used in dye sensitized solar cells (DSSCs)
Working principle of the DSSCs is composed of nanocrystalline semiconductor oxide film electrode, dye sensitizers, electrolytes, a counter electrode and transparent conducting substrate. In general, nanocrystalline titania films is used as a photoanode, platinized counter electrode filled with (I−/I3−) in the electrolyte solution. The incident photon was absorbed by photosensitizers on the TiO2 surface. The absorbed electrons are excited from the ground state to the excited state and quickly
Carbazole attached fullerene derivatives
The advantages of Buckminster fullerene (C60) are high electron mobility, photostability, thermodynamic favorability to accept an electron from donors and reduce up to six electrons. Still now mostly fullerene (C60) derivatives are used as electron acceptor in OSC devices. C60 cannot be used for direct OPV application because of it’s poor solubility and lower LUMO. Fullerene can be functionalized with various methods and the aldehyde compound attached with C60 in the presence of amino acids.78,
Conclusion
In this review, the working principle of BHJ OSCs and a huge variety of Cz derivatives are discussed. Many research groups optimized carbazole as donor material for a well-known light energy into electrical energy converting device. The different positions of Cz derivatives and the effect of position, properties were discussed. Among this the 2,7-carbazole position was found to have more potential because of more planar conjugation. Many intrinsic advantages of carbazole derivatives led us to
Acknowledgments
This work was supported by VIT University for providing laboratory facilities. The financial support by DST-SERB-SB/FT/CS-185/2011.
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