Carrier Transport and Recombination in Efficient “All‐Small‐Molecule” Solar Cells with the Nonfullerene Acceptor IDTBR

Abstract

Reaching device efficiencies that can rival those of polymer-fullerene BHJ solar cells (>10%) remains challenging with the "All-Small-Molecule" (All-SM) approach, in part because of (i) the morphological limitations that prevail in the absence of polymer and (ii) the difficulty to raise and balance out carrier mobilities across the active layer. In this report, we show that blends of the SM donor DR3TBDTT (DR3) and the nonfullerene SM acceptor O-IDTBR are conducive to "All-SM" BHJ solar cells with high open-circuit voltages (VOC) >1.1 V and PCEs as high as 6.4% (avg. 6.1%) when the active layers are subjected to a post-processing solvent vapor-annealing (SVA) step with dimethyl disulfide (DMDS)-a low-partial-pressure solvent found to markedly impact the structural development of the BHJ network. Combining electron energy loss spectroscopy (EELS) analyses and systematic carrier recombination examinations, This article is protected by copyright. All rights reserved. 3 we show that SVA treatments with DMDS play a determining role in improving charge transport and reducing non-geminate recombination for the DR3:O-IDTBR system. Correlating our experimental results and device simulations, we find that substantially higher BHJ solar cell efficiencies of >12% could be achieved if the IQE and carrier mobilities of the active layer could be increased to >85% and >10-4 cm 2 V-1 s-1 , respectively, while suppressing the recombination rate constant k to <10-12 cm 3 s-1. This publication is based upon work supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award No. CRG_R2_13_BEAU_KAUST_1. The authors acknowledge concurrent support under Baseline Research Funding from KAUST. The authors thank KAUST ACL for technical support in the mass spectrometry analyses. V. S. acknowledges financial support from the NDSEG fellowship. Use of

Absence of Charge Transfer State Enables Very LowVOCLosses in SWCNT:Fullerene Solar Cells

“…At this point, we can state that the SWCNT:PC 70 BM system exhibits a remarkably small ΔV OC,nonrad loss regarding the ultranarrow bandgap of the (6,5) SWCNTs. This suggests that for SWCNTs and some nonfullerene acceptor systems, new models have to be developed beyond the well-accepted CT state describing conventional polymer:fullerene OSCs.…”

“…Critically for solar cell optimization, the device performance strongly depends on the quality of the used SWCNT material. The heterojunction based on (6,5) SWCNTs and PC 70 BM yields an open-circuit voltage (V OC ) of 0.59 V, a short-circuit current density (J SC ) of 8.8 mA cm −2 , and a fill factor of 57% resulting in an overall high power conversion efficiency of 2.9% for an all-carbon allotrope active layer. Due to recent advances in SWCNT preparation by Graf et al, [34] material with excellent purity and low defect concentration allowed us to fabricate highly efficient solar cells in this work.…”

“…[15,28,29] Typically, high-performing OSCs exhibit E gap /q − V OC losses greater than 0.8 V. [30] To give an example, the highly investigated benchmark system poly [4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo [1,2-b;4,5-b'] dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno [3,4-b] thiophene-)-2-carboxylate-2-6-diyl)]: [6,6]-phenyl-C 71 -butyric acid methyl ester that achieves PCEs greater than 10% exhibits a fundamental bandgap of around 1.7 eV, while the solar cell V OC is around 0.81 V. [30] The resulting voltage loss of 0.89 V is far beyond the thermodynamic limit of 0.3 V, which is described by the Shockley-Queisser (SQ) theory for an idealized solar cell. [15,28,29] Typically, high-performing OSCs exhibit E gap /q − V OC losses greater than 0.8 V. [30] To give an example, the highly investigated benchmark system poly [4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo [1,2-b;4,5-b'] dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno [3,4-b] thiophene-)-2-carboxylate-2-6-diyl)]: [6,6]-phenyl-C 71 -butyric acid methyl ester that achieves PCEs greater than 10% exhibits a fundamental bandgap of around 1.7 eV, while the solar cell V OC is around 0.81 V. [30] The resulting voltage loss of 0.89 V is far beyond the thermodynamic limit of 0.3 V, which is described by the Shockley-Queisser (SQ) theory for an idealized solar cell.…”

“…In this work, we perform a detailed analysis of the V OC and respective V OC losses in the nearly single-chirality (6,5) SWCNT:PC 70 BM system. The employed (6,5) SWCNT batches exhibit a very high quality with long nanotubes (>1 µm) and low number of defects, [34] making them perfectly suitable to obtain detailed insights into this material system.…”

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cells, [1][2][3][4] but also other types based on small molecule:fullerene [5,6] blends as well as solar cells incorporating singlewalled carbon nanotubes (SWCNTs) [7][8][9][10][11][12][13][14] were investigated with increased efforts. All of these new concepts have been introduced to tackle several drawbacks and outstanding issues related to polymer:fullerene solar cells.

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