Ternary blend all-polymer solar cells: enhanced performance and evidence of parallel-like bulk heterojunction mechanism
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olymers/Soft Matter Research Letter
Ternary blend all-polymer solar cells: enhanced performance and evidence of parallel-like bulk heterojunction mechanism Ye-Jin Hwang†, Brett A. E. Courtright†, and Samson A. Jenekhe, Department of Chemical Engineering and Department of Chemistry, University of Washington, Seattle, Washington 98195-1750, USA Address all correspondence to Samson A. Jenekhe at [email protected] (Received 23 March 2015; accepted 19 May 2015)
Abstract
All-polymer solar cells composed of binary blends of donor poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′ ]dithiophene-2,6diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene-)-2–6-diyl)] (PBDTTT-CT ), and acceptor polymers naphthalene diimide-selenophene copolymer (PNDIS-HD) and perylene diimide-selenophene copolymer (PPDIS) had power conversion efficiencies (PCEs) of 1.3 and 2.1%, respectively. Ternary blend solar cells composed of [PBDTTT-CT][PNDIS-HD]1−x[PPDIS]x at 75 wt% PPDIS had a PCE of 3.2%, which is about a 50%–140% enhancement compared with the binary blend devices. Equality of the ternary blend short-circuit current to the sum of those of the binary blend devices, among other results, provided evidence of a parallel-like bulk heterojunction mechanism in the ternary blend solar cells. These results provide the first example of enhanced performance in ternary blend all-polymer solar cells.
Binary polymer/polymer (all-polymer) blend solar cells,[1–5] in which electron-donating and electron-accepting semiconducting polymers are mixed to form the active layer, are currently of growing interest as alternatives to polymer/fullerene blend solar cells. Advantages of all-polymer solar cells include: (i) appreciable contribution to light absorption by the acceptor polymer, which can enhance the photocurrent of devices; (ii) the ready tunability of the energy levels of both bulk heterojunction (BHJ) components allows for energetically well-aligned blends to be rationally designed; and (iii) an all-polymer blend enjoys improved morphological stability over a blend containing a fullerene derivative, which tends to aggregate with time. Despite these potential advantages and recent progress, the power conversion efficiency (PCE) of all-polymer solar cells (5%–6%)[1–5] remains significantly below that of polymer/fullerene solar cells (9%–10%).[6–8] New approaches and innovations in materials, processing, and device engineering are thus needed to improve the performance of all-polymer solar cells. An approach that has recently led to improvement in some polymer/fullerene devices is to add a third component to a binary blend to create ternary blend solar cells.[9–14] Such a ternary blend solar cell operates by one of two mechanisms. In the first mechanism, referred to here as the “sensitizer” mechanism, a small amount (about 1%–20%)[9,11] of the third component when added to the binary blend improves the light absorption but transfers energy to the majority donor or acceptor component. To be operative, the energy levels of the sensitizer
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