Improving Open Circuit Potential in Hybrid P3HT:CdSe Bulk Heterojunction Solar Cells via Colloidal tert-Butylthiol Ligand Exchange

Abstract

Organic ligands have the potential to contribute to the reduction potential, or lowest unoccupied molecular orbital (LUMO) energy, of semiconductor nanocrystals. Rationally introducing small, strongly binding, electron-donating ligands should enable improvement in the open circuit potential of hybrid organic/inorganic solar cells by raising the LUMO energy level of the nanocrystal acceptor phase and thereby increasing the energy offset from the polymer highest occupied molecular orbital (HOMO). Hybrid organic/inorganic solar cells fabricated from blends of tert-butylthiol-treated CdSe nanocrystals and poly(3-hexylthiophene) (P3HT) achieved power conversion efficiencies of 1.9%. Compared to devices made from pyridine-treated and nonligand exchanged CdSe, the thiol-treated CdSe nanocrystals are found to consistently exhibit the highest open circuit potentials with V(OC) = 0.80 V. Electrochemical determination of LUMO levels using cyclic voltammetry and spectroelectrochemistry suggest that the thiol-treated CdSe nanocrystals possess the highest lying LUMO of the three, which translates to the highest open circuit potential. Steady-state and time-resolved photoluminescence quenching experiments on P3HT:CdSe films provide insight into how the thiol-treated CdSe nanocrystals also achieve greater current densities in devices relative to pyridine-treated nanocrystals, which are thought to contain a higher density of surface traps.

In situ syntheses of semiconducting nanoparticles in conjugated polymer matrices and their application in photovoltaics.

“…As a consequence immense effort was put in the development of approaches towards ligand exchange with e.g. pyridine [33,42], other "smaller" amines, such as butylamine [43], or thiols such as t-butylthiol [44,45].…”

Surface Chemistry of Colloidal Semiconductor Nanocrystals: Organic, Inorganic, and Hybrid

“…Because of its steric bulk, tBT is not able to pack well on the NC surface and resulted in ∼5-6% organic content (as compared to >20% organic content for the purified CdSe NCs prior to ligand exchange). Greaney et al demonstrated that ligand exchange with tBT resulted in a significant improvement in the power conversion efficiency of hybrid poly(3-hexylthiophene) (P3HT) : CdSe solar cells [38]. The tBTexchanged CdSe NC acceptors allow for an increase in both the short-circuit current (J SC ) and open circuit potential (V OC ) of the devices relative to pyridine-exchanged CdSe NC acceptors and those with native ligands.…”

Improved electronic coupling in hybrid organic–inorganic nanocomposites employing thiol-functionalized P3HT and bismuth sulfide nanocrystals

“…25 The absolute position of the P3HT HOMO level (5.28 eV) with respect to vacuum was found to be close to that already reported using cyclic-voltammetry measurements. 26 The band diagram of the system based on a ternary blend of Bi 2 S 3 , P3HT-SH and P3HT is depicted in Fig. 6b, showing the possible energy gradient driving photogenerated holes into the more crystalline P3HT.…”