Category Towards High-Efficiency Organic Solar Cells: Polymers and Devices Development

Criteria for an efficient BHJ solar cell polymer

For a conjugated polymer to suit in organic photovoltaic bulk heterojunction solar cell, it should possess favorable physical and chemical properties in order to achieve reasonable device efficiency. Key words are: large absorption coefficient; low band gap; high charge mobility; favorable blend morphology; environmental stability; suitable HOMO/LUMO level and solubility.

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Conjugated Polymers for Organic Solar Cells

Qun Ye and Chunyan Chi

Department of Chemistry, National University of Singapore,

Singapore

1. Introduction

Energy shortage has become a worldwide issue in the 21st century (Lior, 2008). The urge to look for renewable energy to replace fossil fuel has driven substantial research effort into the energy sector (Hottel, 1989). The solar energy has enormous potential to take the place due to its vast energy stock and availability worldwide (Balzani et al., 2008). Conventional solar energy conversion device is based on silicon technology. However, wide use of silicon based solar cell technology is limited by its high power conversion cost (Wohrle & Meissner, 1991). To address this issue, solution-processing based organic solar cell has been developed to replace Si-solar cell (Tang, 1986)...

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Thiophene-based conjugated polymers

Polythiophenes (PTs) are considered as the most important conjugated polymers for a broad spectrum of optoelectronic applications such as light-emitting diodes, field-effect transistors, and OSCs due to their excellent optical and electrical properties as well as exceptional

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By alternating quaterthiophene and diketopyrrolopyrrole (DPP) units, Janssen (Wienk et al., 2008) developed PT P27 (P27-36 structure in Chart 3), with an Eg of 1.4eV (Table 3). The absorption spectrum of P27 in DCB showed stronger red-shift than its solution in CF. Fast evaporation of CF did not allow P27 to crystallize, which results in poor morphology when spin-coating P27:PCBM blend film from CF solution. Certain degree of crystallization could be achieved by heating the film at 130oC...

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Carbazole-based conjugated polymers

Structurally analogous to FL, carbazole has been also known as 9-azafluorene. The central fused pyrrole ring makes tricyclic carbazole fully aromatic and electron-rich with its donating nitrogen. The alkyl chains introduced into nitrogen of carbazole improve the solubility. Carbazole derivatives have been widely used as electron-donor materials due to their excellent thermal and photochemical stabilities as well as relatively high charge mobility and good solubility with 9-position alkylation. When two carbazoles fused together to form indolo[3,2-fe]carbazole (IC) unit, it exhibits even stronger electron-donating properties, higher hole mobility and better stability than carbazole (Boudreault, 2007b; Li,

2006) ...

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Fluorene-based conjugated polymers

Fluorene (FL) and its derivatives have been extensively investigated for their application in light-emitting diodes due to its rigid planar molecular structure, excellent hole-transporting properties, good solubility, and exceptional chemical stability.

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Chart 1. Flourene based narrow band gap polymers.

Along with their low-lying HOMO levels, polyfluorenes (PFs) are expected to achieve higher Voc and Jsc in their PSC device, which makes fluorene unit a promising electron – donating moiety in D-A narrow band gap polymers’ design. Besides, feasible dialkylation at 9-position and selective bromination at the 2,7-positions of fluorene allow versatile molecular manipulation to achieve good solubility and extended conjugation via typical Suzuki or Stille cross-coupling reactions...

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Towards High-Efficiency Organic Solar Cells: Polymers and Devices Development

Enwei Zhu1, Linyi Bian1, Jiefeng Hai1, Weihua Tang1,* and Fujun Zhang2*

1Nanjing University of Science and Technology, 2Beijing Jiaotong University People’s Republic of China

1. Introduction

The effective conversion of solar energy into electricity has attracted intense scientific interest in solving the rising energy crisis. Organic solar cells (OSCs), a kind of green energy source, show great potential application due to low production costs, mechanical flexibility devices by using simple techniques with low environmental impact and the versatility in organic materials design (Beal, 2010). In the past years, the key parameter, power conversion efficiencies (PCE), is up to 7% under the standard solar spectrum, AM1.5G (Liang et al., 2010)...

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