Effects of interlayer properties on the performance of tandem organic solar cells with low and high band gap polymers
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Effects of interlayer properties on the performance of tandem organic solar cells with low and high band gap polymers Zenan Jiang1,a)
, Bobak Gholamkhass1, Peyman Servati1
1
Flexible Electronics and Energy Lab (FEEL), Department of Electrical and Computer Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada a) Address all correspondence to this author. e-mail: [email protected] Received: 4 March 2019; accepted: 22 April 2019
Tandem organic solar cells with two stacked cells were fabricated using semiconducting polymers and fullerene derivatives. A thin intermediate multilayer of calcium, silver, and molybdenum oxide connects the front and the back cells. Bulk heterojunction (BHJ) films of the low band gap (BG) polymer, poly[N-90-heptadecanyl-2,7carbazole-alt-5,5-(49,79-di-2-thienyl-29,19,39-benzothiadiazole)] (PCDTBT), and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) are used for the front cell. As for the back cell of the tandem structure, the same PCDTBT:PC71BM BHJ (T1) or the high BG polymer poly(3-hexylthiophene) (P3HT) blended with [6,6]-phenyl-C61butyric acid methyl ester (PC61BM) BHJ (T2) are used. The critical role of interlayer properties on the photovoltaic performance of devices are investigated. The observed open-circuit potential for the tandem cell approaches the sum of the potentials of the two respective subcells, demonstrating the potential for increasing the voltage of the solar cell using the tandem structure even with same or lower band gap polymer in the front.
Introduction Polymer solar cells (PSCs) based on conjugated polymer and fullerene composites offer special opportunities for capturing solar energy in view of the potential for fabrication over large area flexible substrates [1] by means of low cost, roll-to-roll solution processing [2, 3]. The power conversion efficiency (PCE) of PSCs has reached up to 17% based on “tandem cell architecture,” emphasizing the prospects for PSCs [4, 5, 6, 7, 8]. PSCs make use of a nanometer-sized bicontinuous network of the polymer and fullerene, commonly referred to as a bulk heterojunction (BHJ) [9]. The nanostructure facilities dissociation of the exciton, and the bicontinuity of the network provides pathways for the transportation of electrons and holes to their corresponding electrodes [10]. In a tandem cell, a multilayer structure equivalent to two or more photovoltaic cells is used in series or in parallel [11]. The multijunction cells in series mode offer potential to improve efficiency and the open-circuit potential (Voc) to values close to the sum of the Voc values of each individual cell. A recent fabrication of a tenfold-stacked tandem device with a high Voc
ª Materials Research Society 2019
(5.89 V) emphasizes the great potential for charging energy storage devices [12] using PSCs. Whereas for the devices in a parallel mode, the short-circuit current density (Jsc) equals the sum of the Jsc values of each subcell [13]. Conventional inorganic tandem cells focus on using two semiconductors with different band gaps
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