Size-Dependent Photovoltaic Performance of CdSe Supraquantum Dot/Polymer Hybrid Solar Cells: “Goldilocks Problem” Resolved by Tuning the Band Alignment Using Surface Ligands

Abstract

This study explored the size dependence of colloidal CdSe nanocrystals (NCs) on the photovoltaic properties of CdSe NC/poly(3hexylthiophene) (P3HT) hybrid bulk-heterojunction (BHJ) solar cell devices. The size-dependent photovoltaic performance was achieved by utilizing CdSe supraquantum dots (SQDs), which are three-dimensionally interconnected colloidal superstructures composed of hundreds of CdSe quantum dots (QDs). The average size of the SQDs can span tens of nanometers, which allow the formation of percolation networks in BHJ films. The open-circuit voltage of the devices was observed to be proportional to the size of the SQDs because of their ideal percolation networks. The photocurrents were determined by the competition between the charge separation and charge transport abilities controlled by the SQD sizes. Overall, the 46 nm-sized CdSe SQD-device demonstrated the highest power conversion efficiency (PCE) of 0.95%, which was 3.2 times higher than that of the control 4.3 nm-sized CdSe QD device. However, further increasing the SQD size resulted in a decrease in the PCE because of the inherent carrier recombination loss within the SQDs. To overcome this "Goldilocks problem," we tuned the energy level at the surface region of the CdSe SQDs via electron-donating 4-methylthiophenol (MTP) ligand exchange. The MTP-treated CdSe SQDs further improved the device performance by enhancing the charge separation and increasing the energy level offset at the CdSe SQD/P3HT interface.

Organic building blocks at inorganic nanomaterial interfaces

“…31 This motivated research to tune the band energies of PbS and CdSe for organic-inorganic solar cells with functional group engineering. 234,235 Additionally, Sargent and co-workers made the impressive innovation of creating a bulk PbS heterojunction solar cell by combining PbS NCs functionalized with different ligands in the same device. 236 By respectively using thioglycerol and methylammonium iodide 237 as ligands for PbS, the band offsets between these two resulting NCs were engineered to create a bulk heterojunction solar cell with a PCE of 10.7%.…”

Blue-Emitting ZnSeTe/ZnSe/ZnS Quantum Dots for Efficient Electroluminescent Application

“…The energy levels in optoelectronic devices can be changed by adjusting the QD band structure with surface ligands. 40,42 To further verify the enhanced hole injection, hole-only devices were fabricated based on the architecture of ITO/PEDOT:PSS/PVK/QD/CBP/MoO 3 /Al (Figure S5). The current density−voltage (J−V) curve for the device fabricated using pristine-CHCl 3 QDs exhibits a higher hole injection current density than that for the device fabricated using pristine QDs.…”

Ultrafast Cation Exchange in Supra-Quantum Dots through Nanoporous Internal Structure

“…Such versatility and the unique structure of SQD can be useful for future applications in photovoltaic and photocatalyst. As an example, the internal inorganic scaffold can act as a channel for a charge carrier, 21 while the counter charge carrier can be easily captured by the internally diffused electrolyte. The expanded internal surface area can boost the surface chemical reaction with a certain type of charge carrier, while the other charge carrier can be collected through the internal inorganic solid channel.…”

“…Because it is constructed by discrete stepwise growth of oriented attachment and subsequent ripening of the primary units, it is expected to have sophisticated internal structure. Connectivity of the internal scaffold in SQDs has been verified by studying their size-dependent photovoltaic performance . Charge carrier transporting within the photovoltaic devices composed by SQDs/P3HT hybrid bulk heterojunction is enhanced with increasing size of the CdSe SQDs by reducing the interparticle hopping events.…”

“…Connectivity of the internal scaffold in SQDs has been verified by studying their sizedependent photovoltaic performance. 21 Charge carrier transporting within the photovoltaic devices composed by SQDs/ P3HT hybrid bulk heterojunction is enhanced with increasing size of the CdSe SQDs by reducing the interparticle hopping events. However, the internal structure of SQDs has not been explored yet.…”