Improving the efficiency of polymer solar cells by incorporating gold nanoparticles into all polymer layers

We demonstrate efficiency improvement in polymer solar cells (PSCs) by ∼22 through incorporating Au nanoparticles (NPs) into all polymer layers. Au NPs are found to have distinct mechanisms in improving device performance when incorporated in different polymer layers. Au NPs in poly-(3,4- ethylenedioxythiophene):poly(styrenesulfonate) mainly contribute to better hole collection while Au NPs in active layer contributes to the enhanced optical absorption and more balanced charge-transport. Our theoretical result shows that the absorption enhancement at the active layer is attributed to plasmon resonances with strong near-field distributions penetrated into absorption polymers. These findings can be applied to design high-efficiency metallic NPs-incorporated PSCs. © 2011 American Institute of Physics.published_or_final_versio

Optical and electrical properties of efficiency enhanced polymer solar cells with Au nanoparticles in a PEDOT-PSS layer

We unveil new device physics and provide details of device mechanisms by investigating polymer solar cells (PSCs) incorporating Au nanoparticles (NPs) into the hole collection poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) layer. Theoretical and experimental results show that the very strong near field around Au NPs due to Localized Surface Plasmonic Resonance (LSPR) mainly distributes laterally along the PEDOT:PSS layer rather than vertically into the adjacent active layer, leading to minimal enhancement of light absorption in the active layer. This finding can be extended to a typical class of solar cells incorporating metallic NPs in spacing layers adjacent to the active layer. With optical effects proven to be minor contributors to device performance improvements, we investigate the electrical properties of the PSCs and obtain insights into the detailed device mechanisms. Improvements in power conversion efficiency (PCE) of solar cells are found to originate from the enlarged active layer/PEDOT:PSS interfacial area and improved PEDOT:PSS conductivity. At high NP concentrations, reduced exciton quenching at donor/acceptor junctions is found to cause PCE deterioration. Our findings indicate that it is highly important to investigate both optical and electrical effects for understanding and optimizing PSC performances. © 2011 The Royal Society of Chemistry.postprin

Optical and electrical effects of gold nanoparticles in the active layer of polymer solar cells

The effects of Au nanoparticles (NPs) incorporated into the active layer of polymer solar cells (PSCs) with a newly synthesized donor polymer are investigated in detail. Our work shows that localized surface plasmon resonance (LSPR) introduced by the metallic NPs can experimentally and theoretically enhance the light absorption in the active layer of PSCs because the strong LSPR near field mainly distributes laterally along the active layer. The understanding can be applied to other metallic NP incorporated organic solar cells. Meanwhile, our results show that electrical properties can counter-diminish the optical enhancement from LSPR and thus reduce the overall performance improvement. It is important that both optical and electrical properties need to be studied and optimized simultaneously for achieving improved power conversion efficiency. The study contributes to better understanding the uses of Au NPs for enhancing PSC performances. © The Royal Society of Chemistry 2011.postprin

Experimental Studies of Plasmonic Nanoparticle Effects on Organic Solar Cells

The incorporation of plasmonic nanoparticles (NPs) into different layers of organic solar cells (OSCs) is studied in this chapter. First, we incorporate NPs into the hole collection layer of OSCs. The resulting improvements in Power Conversion Efficiency (PCE) are found to originate mainly from improvement in hole collection efficiency, while Localized Surface Plasmon Resonance (LSPR) effects are found to have negligible effect on active layer absorption. Next, we incorporate NPs into the active layer of OSCs. In this case, the absorption of the active layer improves, but we also showed that consideration of electrical properties including carrier mobility, exciton dissociation efficiency, and active layer morphology is required to account for the PCE trend. In both studies, we theoretically show that the very strong near field of NPs is found to distribute laterally along the layer in which the NPs are incorporated in, and hence leading to active layer absorption improvements only when NPs are incorporated into the active layer. Lastly, we incorporated NPs into both active layer and hole collection layer in which the accumulated effects of NPs in the different layers achieved 22 % improvement in PCE as compared to the optimized control OSCs using poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester (P3HT:PCBM) as the active layer

Introduction to Organic Solar Cells

Organic solar cells (OSCs) have attracted strong attention in recent years, due to the advantages of flexibility, thinness, and simple manufacturing process. In this chapter, we overview the basics of OSCs. The basics of organic semiconductors are first described. We then provide details of the four steps in the operation principles of OSCs, including exciton generation, exciton diffusion, exciton dissociation, and charge collection. The basic architecture of OSC and the methods of characterization of OSCs are also explained. This chapter provides the fundamentals of OSCs to facilitate understanding of more advanced topics

Efficient inverted polymer solar cells with directly patterned active layer and silver back grating

We have investigated the effects of a directly patterned active layer together with silver nanograting arrays as the anode of inverted polymer solar cells (PSCs). The patterned nanostructure not only greatly enhances the light absorption of the active layer through both light diffraction and coupling to surface plasmon polariton (SPP) modes but also obviously promotes the fill factor of the patterned device. The absorption spectrum shows improvement over a broad wavelength range, especially around the 400 nm region and the near-infrared region surrounding 700 nm, which can also be reconfirmed from Incident Photon to Electron Conversion Efficiency (IPCE) and zeroth-order reflection spectra. Most importantly, our physical study shows that the absorption peak of 400 nm is due to the resonant waveguide mode, and the absorption peak of 700 nm is attributed to the excited SPP mode induced by the metallic back grating. Besides, another splitting SPP mode, called plasmonic band edge, around 800 nm is clarified from our detailed model. Consequently, on one hand, our work offers the fundamental physical understanding of plasmonic band edge resonances in periodic grating nanostructures for enhancing the optical absorption of thin-film photovoltaics. On the other hand, the study contributes to improving the power conversion efficiency of inverted PSCs by about 19% through incorporating grating structures that can be a potential candidate for improving the performances of other PSCs. © 2012 American Chemical Society.link_to_subscribed_fulltex

Charge dynamics in solar cells with a blend of π-conjugated polymer-fullerene studied by transient photo-generated voltage

The biphasic feature of transient photo-generated voltage (TPV) is investigated in organic solar cells (OSCs) with a blend active layer of poly(3-hexylthiophene) (P3HT) and phenyl C61 butyric acid methyl ester (PCBM). The positive and negative components in biphasic TPV are explained through PCBM only and P3HT only devices. The negative and positive components are ascribed to the dipole formation at the buried interface of P3HT/indium tin oxide (ITO) and PCBM/ITO respectively. Based on these findings, two fundamental phenomena are revealed as follows: (1) interfacial modification on the buried interface inverts the negative component in biphasic TPV to a positive component, which prevents the leakage current channel in the conventional OSC structure; and (2) the solvent chosen transforms the positive component in biphasic TPV into a negative signal, which blocks the leakage current channel in the inverted OSC structure. Consequently, the study of TPV polarity provides the justification of the interaction at the buried interface. Besides, the decay of TPV is found to be bi-exponential, which can be used as a tool to estimate the degree of charge balance in OSCs. © 2012 the Owner Societies.link_to_subscribed_fulltex

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 similar to 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-carbazolealt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] (PCDTBT):[6,6]-phenyl C-71-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