Improving Charge Collection from Colloidal Quantum Dot Photovoltaics by Single-Walled Carbon Nanotube Incorporation

Abstract

Improving charge collection is one of the key issues for high-performance PbS colloidal quantum dot photovoltaics (CQDPVs) due to the considerable charge loss resulting from the low mobility and large defect densities of the 1,2-ethanedithiol-treated PbS quantum dot hole-transporting layer (HTL). To overcome these limitations, single-walled carbon nanotubes (SWNTs) and C60-encapsulated SWNTs (C60@SWNTs) are incorporated into the HTL in CQDPVs. SWNT-incorporated CQDPV demonstrates a significantly improved short-circuit current density (J SC), and C60@SWNT-incorporated CQDPV exhibits an even higher J SC than that of pristine SWNT. Both result in improved power-conversion efficiencies. Hole-selective, photoinduced charge extraction with linearly increasing voltage measurements demonstrates that SWNT or C60@SWNT incorporation improves hole-transporting behavior, rendering suppressed charge recombination and enhanced mobility of the HTL. The enhanced p-type characteristics and the improved hole diffusion lengths of SWNT- or C60@SWNT-incorporated HTL bring improvement of the entire hole-transporting length and enable lossless hole collection, which results in the J SC enhancement of the CQDPVs.

Hybrid Surface Passivation for Retrieving Charge Collection Efficiency of Colloidal Quantum Dot Photovoltaics

“…It has been demonstrated that the major heterojunction in the PbS CQDPV device (which leverages the built-in potential driving force for major carrier transport) is the ZnO ETL/CQD active layer interface. Thus, suppression of charge recombination in the CQD-based HTL resulted in negligible V OC enhancement, which is in line with previous literature 39,45 ■ CONCLUSIONS Systematic investigation of the effects of EDT + SCN hybrid passivation on CQDs was conducted. XPS analysis showed reduction of oxygen moieties as well as suppression of unbound thiolate, strongly suggesting that the SCNs incorporated in the hybrid treatment not only substituted for the bound OH species but also led to additional coordination of unbound thiolates of EDT to the CQDs.…”

“…The resulting values plotted in Figure 5c are qualitatively in good agreement with previous reports and suggest that the hybrid HTL exhibited enhanced μ h (average μ h from 1.52 × 10 −4 to 1.61 × 10 −4 cm 2 V −1 s −1 ), both ascertaining the reliability of this technique and mobility enhancement by hybrid passivation. 39,44 Another intriguing feature can be observed from the temporal evolution of extracted hole concentrations from each HTL, as plotted in Figure 5d.…”

“…37,38 The region (known as the depletion width) shows that the drift potential dominates the charge transport. 32,39 While the ETL/active layer heterojunction provides strong leverage for electron collection within the CQDPV, a relatively weak energy level bending at the active layer/HTL interface has raised interest regarding the hole collection capability of the HTL. 18,19 In this regard, the pronounced energy level bending provided by the hybrid-passivated HTL can augment the local drift field at the interface, resulting in more effective charge separation at the interface.…”

“…This implies that a stronger internal electric field can be applied across the heterojunction, facilitating separation/transfer of holes from the active layer to the HTL. , The region (known as the depletion width) shows that the drift potential dominates the charge transport. , While the ETL/active layer heterojunction provides strong leverage for electron collection within the CQDPV, a relatively weak energy level bending at the active layer/HTL interface has raised interest regarding the hole collection capability of the HTL. , In this regard, the pronounced energy level bending provided by the hybrid-passivated HTL can augment the local drift field at the interface, resulting in more effective charge separation at the interface. ,, Since the depletion width is associated with doping concentrations of the materials, the feature is possibly attributed to enhanced p-doping concentration of the hybrid-passivated HTL, as evidenced from the E F – E V reduction. Capacitance–voltage ( C – V ) measurements with Schottky junction devices further support this hypothesis. , From the Mott–Schottky plots (Figure S7a), p-doping concentrations ( N A ) of the CQD HTLs were estimated and showed an ∼31% enhancement of N A (1.08 (±0.10) × 10 17 to 1.42 (±0.10) × 10 17 cm –3 ) by hybrid passivation (Figure S7b), which was qualitatively consistent with depth-dependent UPS analysis. ,,− Note that the doping concentration may be a consequence of extra trap passivation. In other words, the seamless surface passivation by hybrid treatment led to an enhanced doping concentration, preventing charge scavenging from bound OH.…”

“…To gain insight into the hole transport behavior of the CQD HTLs, hole-selective photoinduced charge extraction by a linearly increasing voltage (photo-CELIV) technique was employed for the HTLs using Al 2 O 3 as an electron blocking layer. ,− Figure a,b exhibits the hole-selective photo-CELIV current transients of both HTLs with various delay times. First, the hole mobilities (μ h ) of the HTLs were estimated from the time of the peak of transients ( t max ) based on the equation suggested by Lorrmann et al …”