An alcohol-soluble hyperbranched conjugated polymer HBPFN with a dimethylamino moiety is synthesized and used as a cathode interlayer. A PCE of 7.7% is obtained for PBDTTT-C-T/PC71 BM based solar cells. No obvious interfacial dipole is found at the interface between the active layer and HBPFN however, an interfacial dipole with the cathode could be one of the reasons for the enhanced performance.
We present a strategy to fabricate polymer solar cells in inverted geometry by self-organization of alcohol soluble cathode interfacial materials in donor-acceptor bulk heterojunction blends. An amine-based fullerene [6,6]-phenyl-C61-butyric acid 2-((2-(dimethylamino)-ethyl)(methyl)amino)ethyl ester (PCBDAN) is used as an additive in poly(3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM) blend to give a power conversion efficiency of 3.7% based on devices ITO/P3HT:PCBM:PCBDAN/MoO3/Ag where the ITO alone is used as the cathode. A vertical phase separation in favor of the inverted device architecture is formed: PCBDAN is rich on buried ITO surface reducing its work function, while P3HT is rich on air interface with the hole-collecting electrode. The driving force of the vertical phase separation is ascribed to the surface energy and its components of the blend compositions and the substrates. Similar results are also found with another typical alcohol soluble cathode interfacial materials, poly[(9,9-bis(3'-(N, N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN), implying that self-organization may be a general phenomenon in ternary blends. This self-organization procedure could eliminate the fabrication of printing thin film of interlayers or printing on such thin interlayers and would have potential application for roll-to-roll processing of polymer solar cells.
Polymer solar cells (PSCs) have attracted great attention in recent years due to their advantages of low-cost fabrication, light weight, and the capability of being fabricated into fl exible devices. [ 1,2 ] At present, research into PSCs is focused on improvement of the power conversion effi ciency (PCE) and device stability for future applications. The key points for increasing the PCE and stability are the design and synthesis of high-effi ciency photovoltaic materials [ 3,4 ] and the construction of new device structures. With regard to device optimization, great effort has been devoted to the modifi cation of interfacial layers between the electrodes and active layer of PSCs; [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] in particular, solutionprocessable cathode buffer layer materials have become a hot research topic recently. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] Modifi ed interfacial layers can not only improve the photovoltaic performance but also increase the stability of the PSCs. [ 11c , 13a ] Some water-or alcohol-soluble polar polymers have been successfully used as the cathode buffer layer in PSCs, such as poly(ethylene oxide) (PEO), [ 10 ] conjugated polyfl uorene derivatives with the sidechains containing amino end groups (PFN, [ 12,13 ] PFN-Br, [ 14 ] and PSFNBr, [ 15 ] ) or phosphoryl endgroups (PF-EP), [ 16 ] or metal ion-intercalated crown ether (PFCn6:K + ), [ 17 ] and amine-containing polyethylenimine derivatives PEI [ 18,19 ] and PEIE. [ 18 ] Actually, the cathode buffer layer needs to form an ohmic contact between the active layer and the cathode, and to possess lower workfunction and higher electron mobility for effi cient electron collection and transportation to the cathode. From these points of view, fullerene derivatives could be ideal cathode buffer layer materials in considering their n-type semiconductor character, higher electron mobility, and the good energy level matching with the fullerene acceptors (such as [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)) used in the active layer of the PSCs. Therefore, some fullerene derivatives have been synthesized and applied as the cathode buffer layer in PSCs, including self-assembled, [20][21][22][23] cross-linkable, [ 24,25 ] or solvent-orthogonal [ 26,27 ] C 60 derivatives.Considering the success of the water-/alcohol-soluble polar polymers with sidechains attaching amine endgroups and the fullerene derivatives as the cathode buffer layers mentioned above, herein, we design and synthesize two new amine group-modifi ed PCBM derivatives, [6,6]-phenyl-C61-butyric acid 2-((2-(dimethylamino)ethyl)(methyl)amino)ethyl ester (PCBDAN) and [6,6]-phenyl-C61-butyric acid 2-((2-(trimethylammonium)ethyl)(dimethyl)ammonium)ethyl ester diiodonium (PCBDANI) (see Scheme 1 ), and used them as cathode buffer layer materials in PSCs. The selection of [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) derivatives was made in considering the structural and electronic energy level ma...
The electrochemical properties of poly (3,4-ethylenedioxythiophene) are studied using the bending beam method to detect volume changes during electrochemical transformations of the material. Thin films of poly (3,4-ethylenedioxythiophene) immersed in different supporting electrolytes first contract very rapidly and then expand on doping, while upon undoping they contract directly, or first expand and then contract, to their original positions. It is clearly observed that the oxidation or reduction of the polymer contains two steps, one due to a redox potential close to -0.5 V vs Ag/AgCl, and another potential around 0 V. We find that the volume changes cannot be understood as a simple consequence of ion transport but must be due to the structural change of the polymer between the different states. A hypothetical picture is that during the transition from the neutral to the polaron state, the polymer is slightly charged and thus contracted; on further doping to the bipolaron and to the metallic state, the coulomb repulsion between charged sites become stronger, and the polymer expands.
The synthesis and thin film properties of a conjugated polymer bearing graft chains that are compatible with a fullerene, chemically modified with a similar motif, are described. The graft copolymer, obtained by nitroxide-mediated radical polymerization of a vinyl triazole onto a postfunctionalized poly(3-hexylthiophene) (P3HT) backbone, is blended with a fullerene modified with a pendant triazole functionality (TAZC60). For a given ratio of polymer:TAZC60, graft copolymer (P3HT-g-PVTAZ:TAZC60) blends exhibit substantially reduced photoluminescence compared to P3HT:TAZC60 blends, while TEM analysis reveals the graft polymer undergoes extensive mixing with the fullerene to form bicontinuous 10 nm phase domains. Graft polymer blends annealed for 1 h at 140 degrees C retain their nanometer phase separation as evidenced by TEM, UV-vis, XRD, and photoluminescence analysis, and phase purity was enhanced. In contrast, P3HT:TAZC60 blends exhibit micron-sized phase-segregated morphologies before and after annealing. The chemical similarity of the triazole functionality attached to P3HT and the fullerene leads to the formation of films with uniform, stable, nanophase morphologies. This strategy may prove a useful strategy for controlling the extent of phase segregation in electron donor and acceptor blends of pi-conjugated polymers (piCPs) and fullerenes.
Poly(3‐hexylthiophene) was quantitatively brominated and subsequently used in the Suzuki cross‐coupling with a boronic ester of a nitroxide to form a macroinitiator bearing a TEMPO group on each thienyl ring. This macroinitiator initiated the nitroxide‐mediated radical polymerization of styrene and 4‐chloromethylstyrene (CMS), and subsequently reacted with C60 to yield soluble graft, rod‐coil polymers. Films of the polymers display a bi‐continuous phase structure as revealed by AFM. Similar polymers, in which only a fraction of the thienyl units boasted C60‐bearing side chains, displayed optical properties representative of extensive π‐delocalization. The potential application of this methodology for the synthesis of graft polymers for photovoltaic devices is discussed.
Nano‐ and microstructured architectures of π‐conjugated polymers (see figure) are obtained by solution‐casting blends of poly(methyl methacrylate) (PMMA) and a conjugated polymer bearing solubilizing tetrahydropyranyl groups followed by thermally induced, solid‐phase deprotection of the conjugated polymer and subsequent dissolution of PMMA.
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