A Mini Review on the Development of Conjugated Polymers: Steps towards the Commercialization of Organic Solar Cells

Abstract

This review article covers the synthesis and design of conjugated polymers for carefully adjusting energy levels and energy band gap (EBG) to achieve the desired photovoltaic performance. The formation of bonds and the delocalization of electrons over conjugated chains are both explained by the molecular orbital theory (MOT). The intrinsic characteristics that classify conjugated polymers as semiconducting materials come from the EBG of organic molecules. A quinoid mesomeric structure (D-A ↔ D⁺ = A^-) forms across the major backbones of the polymer as a result of alternating donor-acceptor segments contributing to the pull-push driving force between neighboring units, resulting in a smaller optical EBG. Furthermore, one of the most crucial factors in achieving excellent performance of the polymer is improving the morphology of the active layer. In order to improve exciton diffusion, dissociation, and charge transport, the nanoscale morphology ensures nanometer phase separation between donor and acceptor components in the active layer. It was demonstrated that because of the exciton's short lifetime, only small diffusion distances (10-20 nm) are needed for all photo-generated excitons to reach the interfacial region where they can separate into free charge carriers. There is a comprehensive explanation of the architecture of organic solar cells using single layer, bilayer, and bulk heterojunction (BHJ) devices. The short circuit current density (J_sc), open circuit voltage (V_oc), and fill factor (FF) all have a significant impact on the performance of organic solar cells (OSCs). Since the BHJ concept was first proposed, significant advancement and quick configuration development of these devices have been accomplished. Due to their ability to combine great optical and electronic properties with strong thermal and chemical stability, conjugated polymers are unique semiconducting materials that are used in a wide range of applications. According to the fundamental operating theories of OSCs, unlike inorganic semiconductors such as silicon solar cells, organic photovoltaic devices are unable to produce free carrier charges (holes and electrons). To overcome the Coulombic attraction and separate the excitons into free charges in the interfacial region, organic semiconductors require an additional thermodynamic driving force. From the molecular engineering of conjugated polymers, it was discovered that the most crucial obstacles to achieving the most desirable properties are the design and synthesis of conjugated polymers toward optimal p-type materials. Along with plastic solar cells (PSCs), these materials have extended to a number of different applications such as light-emitting diodes (LEDs) and field-effect transistors (FETs). Additionally, the topics of fluorene and carbazole as donor units in conjugated polymers are covered. The Stille, Suzuki, and Sonogashira coupling reactions widely used to synthesize alternating D-A copolymers are also presented. Moreover, conjugated polymers based on anthracene that can be used in solar cells are covered.

Keywords: Sonogashira coupling reaction; Suzuki coupling reaction; benzothiadiazole (BT); bulk heterojunction (BHJ) device; conjugated polymers; naphthothiadiazole (NT); organic solar cells.

Figure 1

Molecular orbital energy levels diagram demonstrating the formation of σ-bonding and σ^× -antibonding molecular orbitals.

Figure 2

The diagram of molecular orbitals showing the formation of π- and π*-bonding orbitals.

Figure 3

Energetic diagram profile showing the effect of increasing the number of conjugated π-bonds on the magnitude of the EBG of the conjugated polymer.

Figure 4

Molecular structures of some important heterocyclic polymers (second generation).

Figure 5

Essential parts of an OLED.

Figure 6

Schematic diagram of the device configuration of an OFET.

Figure 7

(a) Schematic of a single organic device. (b) In a p-type Schottky contact at the metal cathode, photogenerated excitons are dissociated in a thin depletion region W.

Figure 8

(a) Schematic of a bilayer organic structure. (b) Schematic of the phase separation of a bilayer device. The donor contacts the higher-work-function electrode and the acceptor contacts the lower-function electrode.

Figure 9

Schematic of a bulk heterojunction device. The donor component is blended with the acceptor component throughout the whole film. Consequently, photogenerated excitons can be separated into charge carriers at any place within a thin film.

Figure 10

Schematic device structure of the OPV (glass/ITO/PEDOT: PSS/ active layer (polymer-fullerene bulk)/Li/Al).

Figure 11

Illustration of the main process of photoinduced electron transfer from the donor (RO-PPV) to the acceptor (PCBM) as well as energy-level diagram of the working process for a D–A heterojunction solar cell.

Figure 12

(a) Schematic diagram demonstrating the operating mechanism of a donor–acceptor BHJ device. (b) Basic energy level diagram for the photovoltaic process.

Figure 13

Common current-density–voltage (J–V) curve under dark and illumination for a typical organic solar cell.

Scheme 1

The suggested catalytic cycle mechanism for Stille coupling reaction.

Figure 14

Schematic energy-band diagram of a simple BHJ solar cell consisting of polymer/fullerene derivatives.

Figure 15

The chemical structure of PT polymers, showing the effect of adding ethylene bonds between the thiophene units in the molecular chain.

Figure 16

Intermolecular interactions between EDOT molecules.

Figure 17

Incorporation of electron donor and acceptor on the thiophene molecules exhibits QD structures.

Figure 18

The molecular structure of incorporating a fused ring in the thiophene system.

Figure 19

The energy diagram demonstrates the effect of polymerising electron-rich donor segments and electron-deficient acceptor moieties on the size of the EBG polymer.

Scheme 2

The proposed catalytic cycle for the Suzuki cross-coupling reaction.

Scheme 3

The general mechanism suggested for the Sonogashira cross-coupling reaction.

Figure 20

Molecular structures of: (a) P Si-FDTBT; (b) PFDT2BT.

Figure 21

Molecular structures of: (a) PFDTBTDI-DMO and PFDTBTDI-8; (b) PDBSDTBTDI-DMO and PDBSDTBTDI-8.

Figure 22

Molecular structure of PCDTBT.

Figure 23

Molecular structures of: (a) PCDTBTDI-DMO and PCDTBTDI-8; (b) P2F-CDTBTDI-DMO and P2F-CDTBTDI-8 [109].

Figure 24

Molecular structure of PCDEBT.

Figure 25

Molecular structure of PTADTDFBT.

Figure 26

Molecular structures of PPADTBTDI-DMO and PPADTBTDI-8.

Figure 27

Molecular structure of BT and NT monomers as well as molecular structure of PCDTNT.

Figure 28

Molecular structure of PBDT-DTBT and its counterpart PBDT-DTNT.

Figure 29

Molecular structure of NT-based polymers.

Figure 30

Schematic illustration for obtaining AC power from solar panels.

Figure 31

Levels of power for portable electronic devices and harvesting of energy capabilities of PV cells for indoor uses.

Figure 32

Heliatek’s solar panels on wind turbines. Bottom: ≈185 m² of Heliatek’s solar panels applied on an industrial building in the seaport of Duisburg, Germany.

Figure 33

(A) Tailor-made smart bag by Kolon Industries, integrating an organic solar panel energy harvester (outlined with red rectangle) and NFC technology. (B) Star Tent by Kolon Industries, integrating LED lighting with OPV as the energy source. (C) Smart jacket integrating an OPV module, LED, and Bluetooth by Kolon Industries. (D) Backpack by Kolon Industries, integrating OPV as the power source for cooling fans. (E) Solar bag designed and manufactured by Armor using ASCA© photovoltaic films as the source of energy.

Figure 34

The capacity to produce the modules as big rolls of thin film using high-speed techniques is a main component of the low cost of developing photovoltaic devices.

Figure 35

Thin organic solar-cell panels are not restricted to be used in merely glass on building facades but may also be designed to be integrated into cement and metal components.

Figure 36

Ultra-thin layers and flexible organic photovoltaic panel for a phone-charging device.