π-Conjugated Polymers and Their Application in Organic and Hybrid Organic-Silicon Solar Cells

Abstract

The evolution and emergence of organic solar cells and hybrid organic-silicon heterojunction solar cells have been deemed as promising sustainable future technologies, owing to the use of π-conjugated polymers. In this regard, the scope of this review article presents a comprehensive summary of the applications of π-conjugated polymers as hole transporting layers (HTLs) or emitters in both organic solar cells and organic-silicon hybrid heterojunction solar cells. The different techniques used to synthesize these polymers are discussed in detail, including their electronic band structure and doping mechanisms. The general architecture and principle of operating heterojunction solar cells is addressed. In both discussed solar cell types, incorporation of π-conjugated polymers as HTLs have seen a dramatic increase in efficiencies attained by these devices, owing to the high transmittance in the visible to near-infrared region, reduced carrier recombination, high conductivity, and high hole mobilities possessed by the p-type polymeric materials. However, these cells suffer from long-term stability due to photo-oxidation and parasitic absorptions at the anode interface that results in total degradation of the polymeric p-type materials. Although great progress has been seen in the incorporation of conjugated polymers in the various solar cell types, there is still a long way to go for cells incorporating polymeric materials to realize commercialization and large-scale industrial production due to the shortcomings in the stability of the polymers. This review therefore discusses the progress in using polymeric materials as HTLs in organic solar cells and hybrid organic-silicon heterojunction solar cells with the intention to provide insight on the quest of producing highly efficient but less expensive solar cells.

Keywords: heterojunction solar cell; hole transporting layer; n- and p-type doping; organic photovoltaic cell; π-conjugated polymer.

Scheme 1

Chemical structures of the mostly studied and applied $\pi$-conjugated polymers. (a) Polyacetylene, (b) polyphenylene vinylene, (c) polyaniline, (d) polythiophene, (e) polypyrrole, and (f) poly(3,4-ethylenedioxythiophen).

Scheme 2

The domain boundary (neutral soliton) of two degenerate ground states of t-PA and charged solitons (positive and negative) [24]. (a) positive soliton, (b) neutral soliton, and (c) negative solution.

Figure 1

Doped and undoped (soliton, polaron, bipolaron) conjugated polymer orbital and band structures. Reproduced with permission from reference [43], copyright (2013), John Wiley and Sons.

Scheme 3

Chemical structures of charged positive/negative polaron and bipolaron in PPY. (a) Positive polaron, (b) positive bipolaron, (c) negative polaron, and (d) negative bipolaron.

Scheme 4

The n- and p-type doping procedure in conjugated polymers. Reproduced with permission from reference [41], copyright (2013), John Wiley and Sons.

Scheme 5

Grafting of polystyrene (PS) and poly(3-hexylthiophene) (P3HT) on polypropylene (PP). Reproduced with permission from reference [61], copyright (2018), Elsevier.

Scheme 6

General mechanism of oxidative polymerization through 5 steps [63].

Scheme 7

Synthesis of new EDOT-based monomer (top), and electrochemical oxidative polymerization of PEDOT-graft-HPG (bottom). Reproduced with permission from reference [68], copyright (2019), Elsevier.

Scheme 8

General Suzuki–Miyaura cross-coupling reaction.

Scheme 9

Synthesis of (i) PFDTBTDI-DMO, PFDTBTDI-8, PDBSDTBTDI-DMO, and (PDBSDTBTDI-8 via Suzuki polymerization. Reagents and conditions (i) Pd(OAc)₂/P-(o-tol)₃, NaHCO₃, anhydrous THF, 90 °C, 21–30 h [87].

Scheme 10

The catalytic cycle of the Suzuki–Miyauya cross-coupling reaction.

Scheme 11

General Stille cross-coupling reaction.

Scheme 12

Synthesis of P1 and P2 using Stille polycondensation reaction. Reproduced with permission from reference [106], copyright (2015), John Wiley and Sons.

Scheme 13

The catalytic cycle of the Stille cross-coupling reaction.

Scheme 14

General Kumada cross-coupling reaction.

Scheme 15

Synthesis of poly(3-alkylthiophenes) (P3AT) [112].

Scheme 16

The catalytic cycle of the Kumada cross coupling reaction.

Figure 2

General operating principle of a photovoltaic cell. The entire process is outlined as follows: (a) photon absorption generates excitons, (b) excitons diffuse to the heterojunction, (c) excitons dissociate into charge carriers, and (d) carriers are transported to the electrodes for collection. The diagrams also depict other loss mechanisms as follows: (1) non-absorbed photons, (2) exciton decays, (3) geminate recombination of the bound pair and (4) bimolecular recombination. Reproduced with permission from reference [116], copyright (2013), Elsevier.

Figure 3

A typical general solar cell structure (a) conventional and (b) inverted.

Figure 4

A solar cell device’s typical current−voltage curve. Reproduced with permission from reference [121], copyright (2013), Taylor & Francis Online.

Figure 5

Comparisons of the cross-sectional area SEM images of the different morphologies of Si/PEDOT:PSS films. (a) Silicon nanowires (SiNWs) prepared by chemical etching with different etching time [145], (b) Silicon nanoholes (SiNHs) with micro-deserted structures (Reproduced with permission from reference [142], copyright (2014), Royal Society of Chemistry), (c) Silicon nanocones (SiNCs) at different etching times (Reproduced with permission from reference [146], copyright (2015), Royal Society of Chemistry), (d) Silicon nanopillar arrays (SiNPs) at different etching times (Reproduced with permission from reference [147], copyright (2012), Royal Society of Chemistry), and (e) Textured-Si with and without phthalic acid ester (Reproduced with permission from reference [143], copyright (2017), Wiley & Sons).

Figure 6

Schematic illustration of the fabrication process of organic-silicon hybrid heterojunction solar cell. (a) Fabricated Silicon Nanowires (SiNWs), (b) deposited Ti and Ag on the rear side of the Si wafer as cathode material, (c) Spin-coated PEDOT:PSS on ITO/glass substrate, and (d) immersion of SiNWs into wet PEDOT:PSS film. Reproduced with permission from reference [141], copyright (2015), AIP Publishing.

Scheme 17

Structures of common conjugated polymer hole transporting layers in OSCs.