Jonathan Halls

Polymer–Fullerene Bulk‐Heterojunction Solar Cells

Solution-processed bulk heterojunction organic photovoltaic (OPV) devices have gained serious attention during the last few years and are established as one of the leading next generation photovoltaic technologies for low cost power production. This article reviews the OPV development highlights of the last two decades, and summarizes the key milestones that have brought the technology to todays efficiency performance of over 7%. An outlook is presented on what will be required to drive this young photovoltaic technology towards the next major milestone, a 10% power conversion efficiency, considered by many to represent the efficiency at which OPV can be adopted in wide-spread applications. With first products already entering the market, sufficient lifetime for the intended application becomes more and more critical, and the status of OPV stability as well as the current understanding of degradation mechanisms will be reviewed in the second part of this article.

Exciton diffusion and dissociation in a poly(p‐phenylenevinylene)/C60 heterojunction photovoltaic cell

We report measurements of the photovoltaic response of two‐layer photocells formed with layers of the conjugated polymer poly(phenylenevinylene), PPV and fullerene, C60, formed between indium‐tin oxide and aluminum electrodes. Peak quantum efficiencies of up to ∼9% (electrons collected per incident photon) were measured under short‐circuit conditions. We model the photovoltaic response as arising from excitons photogenerated in the PPV layer which are able to diffuse to the interface with the C60 layer where they are ionized. We obtain a value for the exciton diffusion range of 7±1 nm, both from the spectral response and from the absolute efficiency. We demonstrate that the branching ratio for the creation of singlet excitons from absorbed photons is close to unity.

Dual electron donor/electron acceptor character of a conjugated polymer in efficient photovoltaic diodes

The authors report efficient photovoltaic diodes which use poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (F8TBT) both as electron acceptor, in blends with poly(3-hexylthiophene), and as hole acceptor, in blends with (6,6)-phenyl C61-butyric acid methyl ester. In both cases external quantum efficiencies of over 25% are achieved, with a power conversion efficiency of 1.8% under simulated sunlight for optimized F8TBT/poly(3-hexylthiophene) devices. The ambipolar nature of F8TBT is also demonstrated by the operation of light-emitting F8TBT transistors. The equivalent p- and n-type operation in this conjugated polymer represent an important extension of the range of useful n-type materials which may be developed.

The photovoltaic effect in a poly(p-phenylenevinylene)/perylene heterojunction

Abstract We have measured the photovoltaic properties of heteroj unctions consisting of a layer of the molecular pigment bis(phenethylimido)perylene, sublimed onto a film of the conjugated polymer poly(p-phenylenevinylene) [PPV], sandwiched between electrodes of indium tin oxide (ITO) and aluminium. Quantum yields (electrons collected per incident photon) of up to 6 % , fill factors of 0.6 and open-circuit voltages approaching 1 V were obtained, representing significant improvements over single-layer PPV devices. We interpret the photoresponse as arising from the efficient dissociation of electrons and holes at the PPV/perylene interface. Application of a simple model to the photocurrent action spectra yields an estimate of the exciton diffusion range of 90±10 A.

Electronic excitations in luminescent conjugated polymers

Abstract We report progress in the processing and application of poly(phenylene vinylene), PPV, as the emissive layer in electroluminescent diodes, LEDs. Photoluminescence efficiencies above 60% for solid films of PPV are now achieved and single-layer EL diodes achieve luminous efficiencies above 2 Lumen W−1 and peak brightnesses up to 90 000 cd m−2. We discuss measurements of photoconductivity, photovoltaic response, photoluminescence excitation spectra and stimulated emission in films of PPV. We consider that the photoexcited state in these films of PPV is the intrachain singlet exciton. We demonstrate that PPV of this type can show stimulated emission in sub-picosecond pump-probe experiments and can be used as the active lasing medium when incorporated in suitable microcavity structures.

Electronic processes of conjugated polymers in semiconductor device structures

Abstract We report progress in the processing and application of poly(phenylene vinylene), PPV, as the emissive layer in electroluminescent diodes, LEDs. Photoluminescence efficiencies above 60% for solid films of PPV are now achieved, and single-layer EL diodes achieve luminous efficiencies above 2 Lumens/W and peak brightnesses up to 90,000 cd/m 2 . We demonstrate that PPV of this type can show stimulated emission in sub-picosecond pump-probe experiments, and can be used as the active lasing medium when incorporated in suitable microcavity structures.

Exciton dissociation at a poly(p-phenylenevinylene)/C60 heterojunction

Abstract We report measurements of the photovoltaic effect in heterojunction structures consisting of a layer of C 60 thermally evaporated onto a film of the conjugated polymer poly( p -phenylenevinylene) (PPV) sandwiched between electrodes of indium-tin oxide and Al. Peak quantum efficiencies of up to about 9% (electrons collected per incident photon) were measured under short-circuit conditions, with corresponding open-circuit voltages approaching 0.9 V. Increased quantum yields were obtained under forward and reverse biases. We model the response as arising from the dissociation of photogenerated excitons at the PPV/C 60 interface, with electron transfer to the C 60 . The C 60 layer acts as a dielectric spacer, providing a constructive enhancement of the optical fields at the PPV/C 60 interface. We have applied this model to estimate an exciton diffusion range of 60 to 80 A.

Light-emitting and photoconductive diodes fabricated with conjugated polymers

Processible conjugated polymers can be used to fabricate a range of thin-film diodes which can be designed to show good characteristics both as electroluminescent diodes and also as photoconductive diodes. We consider the present understanding of the operation of light-emitting diodes which use conjugated polymers for both charge transport and emission. We highlight the improvement to the electroluminescence efficiency that can be produced by the use of two polymer layers selected so that the heterojunction between the two layers is able to confine charge and thus bring about electron-hole capture to generate excitons at this interface. Photon absorption produces excitons which are considered to be bound at room temperature. Charge penetration requires ionisation of these excitons, which can be achieved efficiently at heterojunctions between layers with different electronegativities, and we report how this can be achieved at the interfaces between interpenetrating networks of electron- and hole-accepting polymers.

Controlled synthesis of conjugated random copolymers in a droplet-based microreactor

We report the highly controlled synthesis of conjugated random copolymers in a droplet-based microfluidic reactor. Using two optically distinct polymers, poly(3-hexylthiophene) (P3HT) and poly(3-hexylselenophene) (P3HS), a series of highly regioregular random copolymers is generated with physical properties intermediate to those of the parent homopolymers. Analysis by 1H nuclear magnetic resonance spectroscopy reveals the co-polymerisation process to follow ideal Bernoullian behavior.

Modelling of organic field-effect transistors for technology and circuit design

As organic field-effect transistors (OFETs) are preparing to take a key role in the flexible and low cost electronics applications, there is a pressing need for predictive device models to support technology optimization and circuit design. This paper focuses on the specific OFET features that challenge current modelling approaches. The presented modelling techniques range from the fundamental semiconductor equations to compact device model representations as required for implementation in advanced TCAD and EDA commercial tools. The models are verified with measured characteristics of advanced OFET device structures.