Enhanced Uniformity and Area Scaling in Carbon Nanotube–Fullerene Bulk‐Heterojunction Solar Cells Enabled by Solvent Additives

Abstract

blends have recently demonstrated power conversion effi ciencies (PCEs) as high as 3.1% (2.5% NREL certifi ed) for device areas of 0.06-1.2 mm 2 . [7][8][9][10][11][12][13] These devices show excellent ambient stability and nearinfrared (NIR) absorption, [ 11 ] in contrast to other widely researched organic and perovskite photovoltaic technologies. [ 14,15 ] Despite these advances, SWCNT-fullerene solar cells have to date failed to demonstrate high performance over device areas larger than ≈1 mm 2 . In contrast, other TFPV technologies are routinely demonstrated at areas of 6 mm 2 and above. [ 15,16 ] Importantly, SWCNT-fullerene solar cells must maintain performance over areas on par with other TFPV technologies in order to exploit their exceptional absorption and chemical stability in hybrid or tandem photovoltaics, where an SWCNT-fullerene subcell would add value by harvesting NIR light and slowing degradation. Recently, it has been shown that trap-assisted recombination within the active layer limits performance in SWCNT-fullerene solar cells. [ 17 ] This recombination is in part due to the spatial inhomogeneities resulting from fullerene or SWCNT aggregates within the BHJ active layer. Such aggregates decrease the interfacial area between the donor and acceptor domains, limiting effi cient exciton dissociation and charge extraction. These aggregates also provide conductive pathways between both electrodes that electrically short the cell and limit both shortcircuit current density ( J sc ) and open-circuit voltage ( V oc ). [ 18 ] As the solar cell areas are increased, these negative effects begin to dominate the overall device performance, oftentimes with catastrophic consequences.Here, we report improved BHJ morphology and the resulting large-area device performance by incorporating the solvent additive 1,8-diiodooctane (DIO) within the SWCNT-fullerene active layer. DIO has previously been used as an additive in polymer solar cells and is known to break apart PC 71 BM aggregates, [ 16,19 ] which can have tremendous impact on solar cell performance. [ 20 ] We systematically vary the DIO concentration in the SWCNT-fullerene blend, fi nding that an optimal concentration of 1 vol% DIO enables large-area device performance that is comparable to smaller area devices. Atomic force microscopy (AFM) reveals that the addition of DIO signifi cantly reduces the Single-walled carbon nanotube (SWCNT) fullerene solar cells have recently attracted attention due to their low-cost processing, high environmental stability, and near-infrared absorption. While SWCNT-fullerene bulk-heterojunction photovoltaics employing an inverted architecture and polychiral SWCNTs have achieved effi ciencies exceeding 3% over device areas of ≈1 mm 2 , large-area SWCNT solar cells have not yet been demonstrated. In particular, with increasing device area, spatial inhomogeneities in the SWCNT fi lm have limited overall device performance. Here, 1,8-diiodooctane (DIO) is utilized as a solvent additive to reduce fullerene domain size and to impro...

Developing near-infrared quantum-dot light-emitting diodes to mimic synaptic plasticity

“…The conduction band minimum (CBM) and valence band maximum (VBM) of Si QDs are located at −4.0 and −5.4 eV, respectively [42]. The small-step cascade alignment from the work function of ITO to the highest occupied molecular orbital (HOMO) of PEDOT, the HOMO of P3HT and the VBM of Si QDs ensures that holes are effectively injected into the Si-QD layer [43][44][45]. Similarly, the small-step cascade alignment from the work function of Ag to the CBM of ZnO [46], and the CBM of Si QDs facilitates the injection of electrons into the Si-QD layer.…”

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

“…Following the results published in the literature on single‐chirality SWCNT:fullerene devices, the photocurrent generating active layer was spin coated in two steps from a SWCNT dispersion and a PC 70 BM solution. Devices based on one spin coating step with one solution, containing SWCNTs as well as PC 70 BM, did not yield well‐working devices in our case (see Section S6 in the Supporting Information).…”

“…The use of nearly monochiral SWCNTs in a bilayer architecture with C 60 facilitated high fill factors (FFs) greater than 60% and a high peak external quantum efficiency (EQE) of 43% at 1050 nm. Employing a bulk heterojunction (BHJ) architecture in combination with multichirality SWCNTs achieved a broad absorption in the nIR with power conversion efficiencies (PCEs) surpassing 3% for all‐carbon allotrope absorbers . In a recent study performed by Shea et al on nearly single‐chirality (6,5) SWCNT:C 60 devices, the V OC was close to 0.5 V despite the very narrow bandgap of the SWCNTs.…”

Single‐Walled Carbon Nanotubes as an Additive in Organic Photovoltaics: Effects on Carrier Generation and Recombination Dynamics

“…[16] However, up to date, the certified maximum PCE is only 3.2%, reported for SWNT-fullerene bulk-heterojunction (BHJ) devices [17] Among many, insufficient semiconducting purity, [21] the narrow absorption bands, and the tendency of bundling are argued to be the main drawbacks of SWNTs that suppress their advantages in photovoltaic devices. [22][23][24] Vertical demixing can be a severe problem in OPV, leading to charge accumulation and recombination losses. The introduction of SWNTs as a high-mobility, high aspect ratio additive is expected to avoid charge accumulation and consequently Employing single-walled carbon nanotubes (SWNTs) as an additive in the active layer of organic photovoltaics (OPV) to improve charge extraction is gaining weight in the research community.…”

“…[ 16 ] However, up to date, the certified maximum PCE is only 3.2%, reported for SWNT‐fullerene bulk‐heterojunction (BHJ) devices [ 17 ] Among many, insufficient semiconducting purity, [ 21 ] the narrow absorption bands, and the tendency of bundling are argued to be the main drawbacks of SWNTs that suppress their advantages in photovoltaic devices. [ 22–24 ]…”