Triplet Exciton Generation in Bulk-Heterojunction Solar Cells based on Endohedral Fullerenes

Organic bulk-heterojunctions (BHJ) and solar cells containing the trimetallic nitride endohedral fullerene 1-[3-(2-ethyl)hexoxy carbonyl]propyl-1-phenyl-Lu3N@C80 (Lu3N@C80-PCBEH) show an open circuit voltage (VOC) 0.3 V higher than similar devices with [6,6]-phenyl-C[61]-butyric acid methyl ester (PC61BM). To fully exploit the potential of this acceptor molecule with respect to the power conversion efficiency (PCE) of solar cells, the short circuit current (JSC) should be improved to become competitive with the state of the art solar cells. Here, we address factors influencing the JSC in blends containing the high voltage absorber Lu3N@C80-PCBEH in view of both photogeneration but also transport and extraction of charge carriers. We apply optical, charge carrier extraction, morphology, and spin-sensitive techniques. In blends containing Lu3N@C80-PCBEH, we found 2 times weaker photoluminescence quenching, remainders of interchain excitons, and, most remarkably, triplet excitons formed on the polymer chain, which were absent in the reference P3HT:PC61BM blends. We show that electron back transfer to the triplet state along with the lower exciton dissociation yield due to intramolecular charge transfer in Lu3N@C80-PCBEH are responsible for the reduced photocurrent

Optimization of an Electron Transport Layer to Enhance the Power Conversion Efficiency of Flexible Inverted Organic Solar Cells

The photovoltaic (PV) performance of flexible inverted organic solar cells (IOSCs) with an active layer consisting of a blend of poly(3-hexylthiophene) and [6, 6]-phenyl C61-butlyric acid methyl ester was investigated by varying the thicknesses of ZnO seed layers and introducing ZnO nanorods (NRs). A ZnO seed layer or ZnO NRs grown on the seed layer were used as an electron transport layer and pathway to optimize PV performance. ZnO seed layers were deposited using spin coating at 3,000 rpm for 30 s onto indium tin oxide (ITO)-coated polyethersulphone (PES) substrates. The ZnO NRs were grown using an aqueous solution method at a low temperature (90°C). The optimized device with ZnO NRs exhibited a threefold increase in PV performance compared with that of a device consisting of a ZnO seed layer without ZnO NRs. Flexible IOSCs fabricated using ZnO NRs with improved PV performance may pave the way for the development of PV devices with larger interface areas for effective exciton dissociation and continuous carrier transport paths

Comparative study: the effect of annealing conditions on the properties of P3HT:PCBM blends

This paper presents a detailed study on the role of various annealing treatments on organic poly(3-hexylthiophene) and [6]-phenyl-C61-butyric acid methyl ester blends under different experimental conditions. A combination of analytical tools is used to study the alteration of the phase separation, structure and photovoltaic properties of the P3HT:PCBM blend during the annealing process. Results showed that the thermal annealing yields PCBM ‘‘needle-like’’ crystals and that prolonged heat treatment leads to extensive phase separation, as demonstrated by the growth in the size and quantity of PCBM crystals. The substrate annealing method demonstrated an optimal morphology by eradicating and suppressing the formation of fullerene clusters across the film, resulting in longer P3HT fibrils with smaller diameter. Improved optical constants, PL quenching and a decrease in the P3HT optical bad-gap were demonstrated for the substrate annealed films due to the limited diffusion of the PCBM molecules. An effective strategy for determining an optimized morphology through substrate annealing treatment is therefore revealed for improved device efficiency.Web of Scienc

P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance

Each year we are bombarded with B.Sc. and Ph.D. applications from students that want to improve the world. They have learned that their future depends on changing the type of fuel we use and that solar energy is our future. The hope and energy of these young people will transform future energy technologies, but it will not happen quickly. Organic photovoltaic devices are easy to sketch, but the materials, processing steps, and ways of measuring the properties of the materials are very complicated. It is not trivial to make a systematic measurement that will change the way other research groups think or practice. In approaching this chapter, we thought about what a new researcher would need to know about organic photovoltaic devices and materials in order to have a good start in the subject. Then, we simplified that to focus on what a new researcher would need to know about poly-3-hexylthiophene:phenyl-C61-butyric acid methyl ester blends (P3HT: PCBM) to make research progress with these materials. This chapter is by no means authoritative or a compendium of all things on P3HT:PCBM. We have selected to explain how the sample fabrication techniques lead to control of morphology and structural features and how these morphological features have specific optical and electronic consequences for organic photovoltaic device applications

Influence of nanomorphology on the photovoltaic action of polymer–fullerene composites

Composites of conjugated poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) demonstrate an efficient photogeneration of mobile charge carriers. Thermal annealing of P3HT:PCBM based devices gives rise to a significant increase of the photovoltaic efficiency, as follows from measurements of the external quantum efficiency and the current-voltage characteristics. Upon annealing, the absorption spectrum of the P3HT:PCBM composite undergoes a strong modification, whereas in the pure components it remains unchanged. The absorption of the annealed blends becomes stronger and red shifted in the wavelength region ascribed to P3HT, while the absorption due to the PCBM contribution does not change. Atomic force microscope measurements on P3HT:PCBM disclose some variation in morphology due to the crystallization of PCBM. The concentration of the PCBM clusters and their size (up to 500 nm) were found to be correlated with the amount of PCBM in the blend. We have studied the performance of photovoltaic devices with different weight ratios of P3HT:PCBM, namely, 1:3, 1:2, 1:1.5, 1:1, 1:0.9, 1:0.8, and 1:0.7. The photocurrent and the power conversion efficiency showed a maximum between 1:1 and 1:0.9. We conclude the variation in the absorption spectrum and the red shift to result from molecular diffusion of PCBM out of the polymer matrix upon annealing. The growth of the PCBM clusters leads to formation of percolation paths and, therefore, improves the photocurrent. Above a certain concentration, the PCBM crystals provide mechanical stress on the metal electrode, therefore possibly damaging the interface. Optimization of the composite weight ratio reveals the important role played by morphology for the transport properties of bulk heterojunction P3HT:PCBM based solar cells.

Electrical and optical design and characterisation of regioregular poly(3-hexylthiophene-2,5diyl)/fullerene-based heterojunction polymer solar cells

Electrical and optical properties of poly(3-hexylthiophene-2,5diyl) (P3HT-2,5diyl) used as the main component in a bulk heterojunction polymer/fullerene solar cell were investigated. The HOMO level of the polymer was estimated at about 4.7–5.1 eV, from the observed space charge limited current (SCLC) studies in ITO/P3HT-2,5diyl/Au hole-only devices, which confirmed the formation of ohmic contacts between the polymer and the Au and ITO electrodes. The values calculated for hole mobility and density range from 1.4 × 10-6 cm2/(V s) and 5.3 × 10e14 cm-3 at 150 K to 8.5 × 10-5 cm2/(V s) and 1.1 × 10e15 cm-3 at 250 K, respectively. A HOMO–LUMO gap of 2.14 eV was estimated from an absorption spectrum of the polymer. Photoinduced charge transfer from polymer to PCBM was evidenced by strong photoluminiscence quenching, which was observed when the polymer was mixed with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Charge carrier transport properties of the polymer/fullerene solar cells were studied by analysing the dependence of J–V characteristics on temperature and illumination intensity. A linear decrease of the open-circuit voltage with increasing temperature, with a local maximum around 320 K, was observed. The short-circuit current density increased with temperature, having a maximum around 300 K and decreased thereafter. Efficiency and fill factor presented maxima around 3 mW/cm2 white light intensity, and this was attributed to the poor bulk transport properties of the active layer. Typical values recorded for the solar cell at 300 K under white light of 100 mW/cm2 intensity were: open-circuit voltage 0.48 V, and current density 1.28 mA/cm2, with an efficiency of 0.2% and fill factor of 30.6%.

Temperature Dependent Characteristics Of Poly(3 Hexylthiophene)-fullerene Based Heterojunction Organic Solar Cells

Electrical and optical properties of poly(3-hexylthiophene-2.5diyl) (P3HT) used as the main component in a polymer/fullerene solar cell were studied. From the study of space-charge limited current behavior of indium-tin-oxide (ITO)/P3HT/Au hole-only devices, the hole mobility and density were estimated to range from 1.4x10(-6) cm(2)/V s and 5.3x10(14) cm(-3) at 150 K to 8.5x10(-5) cm(2)/V s and 1.1x10(15) cm(-3) at 250 K, respectively. The highest occupied to lowest occupied molecular orbital energetic difference was estimated from absorption spectrometry to be about 2.14 eV. Strong quenching of photoluminescence when the polymer was mixed with [6,6]-phenyl-C(61) butyric acid methyl ester (PCBM), provided evidence of photoinduced charge transfer from P3HT to PCBM. Characterization of ITO/PEDOT:PSS/P3HT:PCBM/Al solar cells was done by analyzing the dependence of current density-voltage characteristics on temperature and illumination intensity. The main solar cell characteristics recorded at 300 K under 100 mW/cm(2) white-light intensity were: Open-circuit voltage 0.48 V, current density 1.28 mA/cm(2), with an efficiency of 0.2%, and fill factor of 30.6%. Open-circuit voltage decreased almost linearly with increasing temperature, while short circuit current density increased with temperature, saturating at around 320 K, and decreased thereafter. Power conversion efficiency and fill factor were maximum around 3 mW/cm(2) due to the poor bulk transport properties of the active layer