A Chemically Orthogonal Hole Transport Layer for Efficient Colloidal Quantum Dot Solar Cells

Abstract

passivation and device architecture have led to improvements in CQD solar cell performance, [19][20][21] and these recently enabled power conversion efficiencies (PCE) above 12% for lead sulfide (PbS) CQDs. [19] The conventional CQD solar cell architecture consists of a transparent cathode, electron transport layer (ETL), a lightabsorbing active layer, a hole transport layer (HTL), and a metal anode. To achieve high-performing devices, the optoelectronic properties of the ETL, HTL, and active layer, which determine the charge absorption and extraction capacity of the devices, require accurate control. A number of excellent studies have illuminated the role of the ETL [22][23][24][25][26][27][28] and the active layer; [29] while the HTL is relatively less examined. Organic p-type semiconductors [30,31] and metal oxides (e.g., MoO 3 , NiO x , etc.) [32] have been explored to replace the thiol-passivated CQD HTL. Non-thiol ligands (e.g., NaHS) [33,34] have also been reported to produce p-type CQD solids; however, to date, device performance has not yet surpassed that of thiolpassivated CQD-based HTLs.State-of-art CQD solar cells employ 1,2-ethanedithiol (EDT) in the process of fabricating the CQD HTL. [19] This EDT HTL has been used in most high-performing CQD solar cells; but EDT has long been suspected of negatively affecting the underlying CQD active layer. In particular, its high reactivity [35] is proposed to be implicated in interfering with efficient charge extraction at the back-junction. [36] Herein we seek experimental evidence of any role of the EDT HTL in performance; and we find, using a new spatial collection efficiency (SCE) technique, [37] that the EDT HTL does indeed cause a rapid drop in the collection efficiency at the interface between HTL and active layer. We then develop an orthogonal CQD HTL that employs malonic acid (MA) instead of EDT. As a result of the lower reactivity of carboxylic acids compared to thiols, [38] the MA HTL substantially preserves the original surface chemistry of the CQD active layer after its deposition, as evidenced by X-ray photoelectron spectroscopy (XPS) analyses.The orthogonality of the MA HTL enables full charge collection at the back interface in CQD solar cells. This advance Colloidal quantum dots (CQDs) are of interest in light of their solutionprocessing and bandgap tuning. Advances in the performance of CQD optoelectronic devices require fine control over the properties of each layer in the device materials stack. This is particularly challenging in the present best CQD solar cells, since these employ a p-type hole-transport layer (HTL) implemented using 1,2-ethanedithiol (EDT) ligand exchange on top of the CQD active layer. It is established that the high reactivity of EDT causes a severe chemical modification to the active layer that deteriorates charge extraction. By combining elemental mapping with the spatial charge collection efficiency in CQD solar cells, the key materials interface dominating the subpar performance of prior CQD PV devices is demonstrat...

Orthogonal colloidal quantum dot inks enable efficient multilayer optoelectronic devices

“…Solid-state EDT-exchanged CQD films are obtained by deposition of OA-capped CQDs, soaking the OA-capped film in EDT/ acetonitrile (ACN) solution, and washing away unbound ligands by spin-coating ACN. These procedures result in partial loss of ligands in the underlying light-absorbing layer 22,23,33 .…”

Stability of Quantum Dot Solar Cells: A Matter of (Life)Time

“…[78] A particularly promising route to replace EDT was recently suggested by Biondi et al, who used malonic acid instead of EDT, leading to an increase in the photovoltaic performance and device stability. [79] Another approach is based Figure 3. The relationship between PbS QD size, morphology and facet ratio: a) the shape of the dots depending on their size.…”

Optimizing Solid-State Ligand Exchange for Colloidal Quantum Dot Optoelectronics: How Much Is Enough?

“…We argue this happens because higher [EDT] likely harm the underlying PbX 2 -PbS CQD active layer, in accordance with a recent finding. 50 Higher [EDT] increase p-character of the HTL evident from a slight enhancement of the V OC , yet damage to the underlying absorber harms the FF, limiting the overall PCE. These subtle but statistically significant changes in device parameters between [EDT] = 1.0%, 10 -1 % and 10 -2 % are highlighted in the histograms shown in Figure S15.…”