Organic non-fullerene acceptors for organic photovoltaics

Heterojunction organic photovoltaics have been the subject of intensive academic interest over the past two decades, and significant commercial efforts have been directed towards this area with the vision of developing the next generation of low-cost solar cells. Materials development has played a vital role in the dramatic improvement of organic solar cell performance in recent years, and this is driven primarily by the advancement of p-type semiconductors as donor materials. With the highest performing solar cells today dominated by acceptors based on members of the fullerene family, much less attention has been devoted to other classes of n-type acceptors. In this review, we will provide an overview of the progress in the synthesis, characterization and implementation of the various classes of non-fullerenebased n-type organic acceptors for photovoltaic applications

Structure and properties of nano-confined poly(3-hexylthiophene) in nano-array/polymer hybrid ordered-bulk heterojunction solar cells

The ordered-bulk heterojunction (BHJ) photovoltaic device comprising a semiconducting donor polymer incorporated into pristine/unmodified vertically aligned arrays of metal oxide acceptor nanotubes/nanorods is widely perceived as being structurally ideal for energy conversion but the power conversion efficiencies of such devices remain relatively low (in the order of η = 0.6%) when compared with bilayer or non-ordered bulk heterojunction systems. We explain the incongruity by investigating the morphology and microstructure of regio-regular poly(3-hexyl thiophene) (P3HT) infiltrated and confined within the cavities of TiO2 nanotube arrays. A series of TiO2 nanotube arrays with different nanotube diameters and inter-nanotube spacings are fabricated by the liquid-phase atomic layer deposition (LALD) technique, and P3HT is infiltrated into the array cavities via a vacuum-annealing technique. X-Ray diffraction studies reveal that the P3HT chains in both nano-confined and non-confined (i.e. planar film) environments are well-aligned and oriented edge-on with respect to the underlying substrate. Up to 2.5-fold improvement in the incident-photon-to-converted-electron efficiency (IPCE) is observed in ordered-BHJ structures over benchmark planar devices which we attribute to the increase in interfacial area resulting from the use of the nanostructures. However, the large effective surface area conferred by the nano-arrays (up to 9.5 times that of the planar system) suggests that much higher efficiencies could be harnessed. Our study shows that the morphology and orientation of the infiltrated polymer play a critical role in the charge transport of the device, and suggests that better understanding and control of polymer morphology under nano-confinement in the nano-array will be the key to fully reaping the promised benefit of ordered-BHJ devices

Pyrrolo[3,2-b]pyrrole-1,4-dione (IsoDPP) end capped with napthalimide or phthalimide: novel small molecular acceptors for organic solar cells

We introduce two novel solution-processable electron acceptors based on an isomeric core of the much explored diketopyrrolopyrrole (DPP) moiety, namely pyrrolo[3,2-]pyrrole-1,4-dione (IsoDPP). The newly designed and synthesized compounds, 6,6'-[(1,4-bis{4-decylphenyl}-2,5-dioxo-1,2,4,5-tetrahydropyrrolo[3,2-]pyrrole-3,6-diyl)bis(thiophene-5,2-diyl)]bis[2-(2-butyloctyl)-1-benzo[]isoquinoline-1,3(2)-dione] (NAI-IsoDPP-NAI) and 5,5'-[(1,4-bis{4-decylphenyl}-2,5-dioxo-1,2,4,5-tetrahydropyrrolo[3,2-]pyrrole-3,6-diyl)bis(thiophene-5,2-diyl)]bis[2-(2-butyloctyl)isoindoline-1,3-dione] (PI-IsoDPP-PI) have been synthesized via Suzuki couplings using IsoDPP as a central building block and napthalimide or phthalimide as end-capping groups. The materials both exhibit good solubility in a wide range of organic solvents including chloroform (CF), dichloromethane (DCM), and tetrahydrofuran (THF), and have a high thermal stability. The new materials absorb in the wavelength range of 300-600 nm and both compounds have similar electron affinities, with the electron affinities that are compatible with their use as acceptors in donor-acceptor bulk heterojunction (BHJ) organic solar cells. BHJ devices comprising the NAI-IsoDPP-NAI acceptor with poly(3--hexylthiophene) (P3HT) as the donor were found to have a better performance than the PI-IsoDPP-PI containing cells, with the best device having a V of 0.92 V, a J of 1.7 mAcm, a FF of 63%, and a PCE of 0.97%

ZnO layers for opto-electronic applications from solution-based and low-temperature processing of an organometallic precursor

As printed and flexible plastic electronic gadgets become increasingly viable today, there is a need to develop materials that suit the fabrication processes involved. Two desirable requirements are solution-processable active materials or precursors and low-temperature processability. In this article, we describe a straightforward method of depositing ZnO films by simple spin coating of an organometallic diethylzinc precursor solution and annealing the resulting film at low temperatures (≤200 °C) without involving any synthetic steps. By controlling the humidity in which annealing is conducted, we are able to adjust the intrinsic doping level and carrier concentration in diethylzinc-derived ZnO. Doped or conducting transport layers are greatly preferable to undoped layers as they enable low-resistance contacts and minimize the potential drops. This ability to controllably realize doped ZnO is a key feature of the fabrication process that we describe in this article. We employ field-effect measurements as a diagnostic tool to measure doping levels and mobilities in ZnO and demonstrate that doped ZnO with high charge carrier concentration is ideal for solar cell applications. Respectable power conversion efficiencies (up to 4.5%) are achieved in inverted solar cells that incorporate diethylzinc-derived ZnO films as the electron transport layer and organic blends as the active material. Extensions of this approach to grow ternary and quaternary films with organometallic precursor chemicals will enable solution based growth of a number of semiconductor films as well as a method to dope them