Enhancing the open-circuit voltage of molecular photovoltaics using oxidized Au nanocrystals

Retarding oxidation of copper nanoparticles without electrical isolation and the size dependence of work function

Copper nanoparticles (CuNPs) are attractive as a low-cost alternative to their gold and silver analogues for numerous applications, although their potential has hardly been explored due to their higher susceptibility to oxidation in air. Here we show the unexpected findings of an investigation into the correlation between the air-stability of CuNPs and the structure of the thiolate capping ligand; of the eight different ligands screened, those with the shortest alkyl chain, -(CH2)2-, and a hydrophilic carboxylic acid end group are found to be the most effective at retarding oxidation in air. We also show that CuNPs are not etched by thiol solutions as previously reported, and address the important fundamental question of how the work function of small supported metal particles scales with particle size. Together these findings set the stage for greater utility of CuNPs for emerging electronic applications.

Plasmonic effect of spray-deposited Au nanoparticles on the performance of inverted organic solar cells

Gold nanoparticles with varying sizes were prepared by the spray process under an electric field (DC voltages of 0 V and 1 kV applied to the nozzle) for studying their role in inverted organic solar cells (ITO/Au/ZnO/P3HT:PCBM/Ag). The application of electric field during the spray process resulted in a smaller size (35 nm as compared to 70 nm without the electric field) of the nanoparticles with more uniform distribution. This gave rise to a difference in the surface plasmon resonance (SPR) effect created by the gold nanoparticles (Au NPs), which then affected the solar cell performance. The photovoltaic performances of plasmonic inverted organic solar cells (ITO/Au/ZnO/P3HT:PCBM/Ag) using spray-deposited Au and ZnO layers (both at 1 kV) showed improved efficiency. Fast exciton quenching in the P3HT:PCBM layer was achieved by using a spray-deposited Au layer in between ITO and ZnO layers. The absorption spectra and internal power conversion efficiency (IPCE) curve showed that the Au nanoparticles provide significant plasmonic broadband light absorption enhancement which resulted in the enhancement of the JSC value. Maximum efficiency of 3.6% was achieved for the inverted organic solar cell (IOSC) with an exceptionally high short circuit current density of ∼15 mA cm(-2) which is due to the additional photon absorption and the corresponding increase observed in the IPCE spectrum. The spray technique can be easily applied for the direct formation of Au nanoparticles in the fabrication of IOSC with improved performance over a large area.

Au@Ag core-shell nanocubes for efficient plasmonic light scattering effect in low bandgap organic solar cells

In this report, we propose a metal-metal core-shell nanocube (NC) as an advanced plasmonic material for highly efficient organic solar cells (OSCs). We covered an Au core with a thin Ag shell as a scattering enhancer to build Au@Ag NCs, which showed stronger scattering efficiency than Au nanoparticles (AuNPs) throughout the visible range. Highly efficient plasmonic organic solar cells were fabricated by embedding Au@Ag NCs into an anodic buffer layer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and the power conversion efficiency was enhanced to 6.3% from 5.3% in poly[N-9-hepta-decanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] (PCDTBT):[6,6]-phenyl C71-butyric acid methyl ester (PC70BM) based OSCs and 9.2% from 7.9% in polythieno[3,4-b]thiophene/benzodithiophene (PTB7):PC70BM based OSCs. The Au@Ag NC plasmonic PCDTBT:PC70BM-based organic solar cells showed 2.2-fold higher external quantum efficiency enhancement compared to AuNPs devices at a wavelength of 450-700 nm due to the amplified plasmonic scattering effect. Finally, we proved the strongly enhanced plasmonic scattering efficiency of Au@Ag NCs embedded in organic solar cells via theoretical calculations and detailed optical measurements.

Plasmonic Forward Scattering Effect in Organic Solar Cells: A Powerful Optical Engineering Method

In this report, plasmonic effects in organic photovoltaic cells (OPVs) are systematically analyzed using size-controlled silver nanoparticles (AgNPs, diameter: 10 ~ 100 nm), which were incorporated into the anodic buffer layer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The optical properties of AgNPs tuned by size considerably influence the performance levels of devices. The power conversion efficiency (PCE) was increased from 6.4% to 7.6% in poly[N-9-hepta-decanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] (PCDTBT):[6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₀BM) based-OPVs and from 7.9% to 8.6% in polythieno[3,4-b]thiophene/benzodithiophene (PTB7):PC₇₀BM based-OPVs upon embedding the AgNPs. The external quantum efficiency (EQE) was significantly enhanced by the absorption enhancement due to the plasmonic scattering effect. Finally, we verified the origin of the size-dependent plasmonic forwarding scattering effect of the AgNPs by visualizing the scattering field with near-field optical microscopy (NSOM) and through analytic optical simulations.

Boron subphthalocyanines as organic electronic materials

Boron subphthalocyanines (BsubPcs) are an emerging class of high performing materials in organic electronics. Since the first use of chloroboron subphthalocyanine in an organic electronic device 6 years ago subphthalocyanines have shown potential as functional materials in organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). Here we review the material properties of chloroboron subphthalocyanine (Cl-BsubPc) and its use as an organic semiconductor. We then highlight our efforts toward derivatives of boron subpthalocyanine beyond Cl-BsubPc and discuss the impact of molecular design on the material properties and the performance of the BsubPc. Finally, we comment on the status of BsubPcs in the field of organic electronics and discuss how we believe future progress can be made.

Nanoscale geometric electric field enhancement in organic photovoltaics

Generic design rules for electrode-organic semiconductor contacts that transcend specific materials are urgently required to guide the development of new electrodes and provide a framework for engineering this important class of interface. Herein a novel nanostructured window electrode is utilized in conjunction with three-dimensional electrostatic modeling to elucidate the importance of geometric electric field enhancement effects at the electrode interfaces in organic photovoltaics. The results of this study show that nanoscale protrusions at the electrode surfaces in organic photovoltaics dramatically improve the efficiency of photogenerated charge carrier extraction to the external circuit and that the origin of this improvement is the local amplification of the electrostatic field in the vicinity of said protrusions. This wholly geometric approach to engineering electrodes at the nanoscale is materials generic and can be employed to enhance the efficiency of charge carrier injection or extraction in a wide range of organic electronic devices.

Effect of nanoscale SubPc interfacial layer on the performance of inverted polymer solar cells based on P3HT/PC71BM

The effect of a nanoscale boron subphthalocyanine chloride (SubPc) interfacial layer on the performance of inverted polymer solar cells based on poly (3-hexyl thiophene) (P3HT) and [6,6]-phenyl-C(71)-butyric acid methyl ester (PC(71)BM) was studied. When a 1 nm SubPc layer was introduced between the active layer (P3HT:PC(71)BM) and MoO(x) in the device with ITO/ZnO/P3HT:PC(71)BM/SubPc/MoO(x)/Al configuration, the power conversion efficiency (PCE) was increased from 3.42 (without SubPc) to 3.59%. This improvement is mainly attributed to the enhanced open-circuit voltage from 0.62 to 0.64 V. When the Flory-Huggins interaction parameters were estimated from the solubility parameters through the contact angle measurement, it revealed that the interaction between SubPc and PC(71)BM is more attractive than that between SubPc and P3HT at the interface of P3HT:PC(71)BM/SubPc, through which charges are well transported from the active layer to the anode. This is supported by a decrease of the contact resistance from 5.49 (SubPc 0 nm) to 0.94 MΩ cm (SubPc 1 nm). The photoelectron spectra provide another evidence for the enhanced PCE, exhibiting that the 1 nm thick SubPc layer extracts more photoelectrons from the active layer than other thicknesses.

Absorption enhancement in solution processed metal-semiconductor nanocomposites

We present a full wave 3D simulation study of optical absorption enhancement in solution processed metal-semiconductor nanocomposite ultrathin films, which consist of colloidal metallic nanoparticles (MNPs) and semiconductor matrices of polymer and colloidal quantum dots (CQD). We present an approach for modeling the optical properties of a CQD film, and study the effect of the optical properties of the semiconductor in the near field enhancement showing that CQD is a very promising platform to exploit the benefits of the near-field effects. We show that over a 100% enhancement can be achieved in the visible-near infrared region of the spectrum for CQD PbS films, with a maximum gain factor of 4 when MNPs are on resonance. We study in detail the effect of MNP capping for different ligand lengths and materials and propose solutions to optimize absorption enhancement.