Correlation between Surface Photovoltage and Blend Morphology in Polyfluorene-Based Photodiodes

Chiesa, Marco; Bürgi, Lukas; Kim, Jinhyun; Shikler, Rafi; Friend, Richard H.; Sirringhaus, Henning

doi:10.1021/nl047929s

Cited by 161 publications

(173 citation statements)

References 27 publications

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“…[ 14 ] Similar surface-stratifi cation has been seen in other conjugated polymer-based blend systems and has been argued to improve/reduce the injection/ extraction of electrons and holes in organic light-emitting diodes (OLEDs) and OPVs depending on the location of the blend components. [15][16][17] On thermal annealing at 70 ° C, we fi nd that the negative d 5 -PC 60 BM concentration gradient increases slightly, suggesting that thermal annealing further encourages the migration of PCBM towards the fi lm surface. Such a migration process is again consistent with our previous observations.…”

mentioning

confidence: 94%

The Nanoscale Morphology of a PCDTBT:PCBM Photovoltaic Blend

Staniec

Parnell

Dunbar

et al. 2011

Advanced Energy Materials

106

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Organic photovoltaic devices (OPVs) based on conjugated polymers and fullerene blends offer an attractive method to produce renewable energy. It has been demonstrated that they can be manufactured on fl exible substrates and at low cost over large areas using the high volume technique of roll-to-roll printing. [1][2][3] Whilst much work has been devoted to the donoracceptor system P3HT:PCBM (poly(3-hexylthiophene):[6,6]-phenyl-C 61 -butyric acid methyl ester), attention is now turning to the use of donor materials which have a reduced energy gap (permitting more of the sun's spectral emission to be harvested) and an increased ionization potential (leading to an increased open-circuit voltage and thus greater power conversion effi ciency [ 4 , 5 ] ).One promising new donor polymer for OPV applications is PCDTBT (poly [N-9 ′ -heptadecanyl-2,7-carbazole-alt-5,5-(4 ′ ,7 ′ -di-2-thienyl-2 ′ ,1 ′ ,3 ′ -benzothiadiazole)] whose chemical structure is shown in Figure 1 . [ 4 ] When PCDTBT is blended with the fullerene acceptor PC 70 BM, OPV devices have been created having a power conversion effi ciency of ∼ 6%, [ 6 ] one of the highest OPV effi ciencies to date. A marked difference in the fabrication process of these devices compared to the benchmark P3HT:PCBM system is the requirement for thermal annealing. It has been established that thermal annealing at ∼ 150 ° C [ 7 ] (between the glass transition and melting temperatures of P3HT) is required to drive the crystallization of the two components to form nanoscale, phase-separated domains. [8][9][10] Without such a thermal anneal process, the power conversion effi ciency (PCE) of P3HT:PCBM is limited to around 1%-2% [ 11 ] -a value that is improved to between 4 and 5% in an annealed device. [ 7 , 11 ] Recently we have established that this thermal anneal also modifi es the vertical structure of P3HT:PCBM thin fi lms. [ 12 ] In as-cast P3HT:PCBM fi lm, the surface is relatively depleted in PCBM. However by annealing the fi lm, PCBM is driven to the fi lm surface, an effect that we believe improves the electronic functionality of the device. In PCDTBT:PCBM OPVs however, a high temperature annealing process has been reported to be detrimental to device effi ciency. [ 6 ] Rather a low-temperature anneal/ drying stage at 70 ° C appears suffi cient to fully optimize device effi ciency although the benefi t of this stage has not been explicitly quantifi ed. Devices incorporating a TiO x optical spacing layer are also subsequently annealed at 80 ° C. [ 6 ] In this paper, we use neutron refl ectivity (NR) and grazing incidence wide-angle X-ray scattering (GI-WAXS) to explore the nanoscale structures formed in freshly cast PCDTBT:PCBM thin fi lms and those that have been annealed at a relatively low temperature. We make two main fi ndings. Firstly, we fi nd that the surface of freshly cast PCDTBT:PCBM fi lms is relatively enriched in PCBM with a negative concentration gradient existing away from the fi lm surface. Secondly, thermal annealing at 70 ° C does not signifi cantly m...

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mentioning

confidence: 94%

The Nanoscale Morphology of a PCDTBT:PCBM Photovoltaic Blend

Staniec

Parnell

Dunbar

et al. 2011

Advanced Energy Materials

106

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“…For example, conductive AFM (C-AFM) measures locally the electrical properties of the polymer blend, 41 where as Kelvin probe AFM measures locally the surface potential of the sample. [42][43][44] Since these scanning probe techniques exploit the electron donating or accepting properties of the photovoltaic blend, they obtain better contrast. The lateral resolution of these measurements techniques may be better than 100 nm.…”

Section: Toolbox To Probe the Morphologymentioning

confidence: 99%

Nanoscale structure of solar cells based on pure conjugated polymer blends

Veenstra

Loos

Kroon

2007

Progress in Photovoltaics

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This paper gives an overview of the status of photovoltaic devices based on blends of semiconducting polymers. The polymer blends form the bulk heterojunction in these photovoltaic devices. The fundamental mechanisms governing the performance of these devices are discussed as well as the specificities of these all-polymer solar cells. The morphology of the polymer blend layer is expected to influence the device performance of these bulk heterojunctions. An overview is presented of factors that influence the morphology of the active layer of polymer blend photovoltaic devices together with a summary of tools available to study the structure of this layer. An advanced electron microscopy technique is applied to study the morphology of polymer blends of MDMO-PPV and PCNEPV with differing molecular weights. It is shown that the molecular weight of the polymer influences the typical domain size from $200 nm down to less than 5 nm in these bulk heterojunction PV devices. No strong relation is observed between the typical length scale of the phase separated domains and the measured external quantum efficiency, indicating that the phases are intermixed.

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“…5 However, while these techniques broadly control the domain size, crystallinity, or packing of the components of the BHJ, these characteristics of the blend morphology are not uniform throughout the film. Indeed, studies of the structure of polymer-polymer, 3,6,7 polymer-fullerene, [8][9][10] and polymernanocrystal 11 blends show hierarchies of phase separation and vertical variations in the blend morphology. How this local variation in blend morphology affects the local mobility of carriers, and in particular the PV performance, is largely unknown.…”

Section: Introductionmentioning

confidence: 99%

The effect of morphology upon mobility: Implications for bulk heterojunction solar cells with nonuniform blend morphology

Groves

Koster

Greenham

2009

Journal of Applied Physics

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Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. We use a Monte Carlo model to predict the effect of composition, domain size, and energetic disorder upon the mobility of carriers in an organic donor-acceptor blend. These simulations show that, for the changes in local morphology expected within the thickness of a typical bulk heterojunction photovoltaic device, changes in mobility of more than an order of magnitude are expected. The impact of nonuniform mobility upon space-charge-limited diode and photovoltaic ͑PV͒ device performance is examined using a drift-diffusion model. The current passing through a space-charge-limited diode is shown to depend upon the position of the layers with differing mobility. Accurate modeling of the current in such devices can only be achieved using a drift-diffusion model incorporating nonuniform mobility. Inserting a 20 nm thick layer in which the mobility is less by one order of magnitude than in the rest of the 70 nm thick PV device reduced the device efficiency by more than 20%. Therefore it seems vital to exert a high degree of control over the morphology throughout the entire blend PV device, otherwise potential PV performance may be lost.

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Correlation between Surface Photovoltage and Blend Morphology in Polyfluorene-Based Photodiodes

Cited by 161 publications

References 27 publications

The Nanoscale Morphology of a PCDTBT:PCBM Photovoltaic Blend

The Nanoscale Morphology of a PCDTBT:PCBM Photovoltaic Blend

Nanoscale structure of solar cells based on pure conjugated polymer blends

The effect of morphology upon mobility: Implications for bulk heterojunction solar cells with nonuniform blend morphology

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