First Year Report

Magnetic Field ModulatedPhotovoltage

Author:Edward Peter Booker

Supervisor:Prof. Neil Greenham

October 20, 2015

Abstract

Magnetic fields applied to polymer-fullerene bulkheterojunction [BHJ] photovoltaic devices have beenfound to modulate the photovoltage. The effect ofmagnetic field on photovoltage has been found to beindependent of applied light intensity leading to theconclusion that magnetic field effects modulate therecombination of charge transfer excitons to long-livedtriplet excitons. Heavy metal doped polymer-fullereneBHJ photovoltaic devices have been found to have theirmagnetic field effect on photovoltage reduced with thelevel of heavy metal doping.

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Acknowledgements

I would like to thank:

• Neil for his patience and supervision

• Akshay and Dan for their useful suggestions

• Mark for helping me get started

• Rob for making the CDT training happen and getting Python going

• Maxim, Robin, Tom and Stephen for help with proofreading

• Everyone around the coffee table for thought-provoking conversation

• Harriet for putting up with me

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Contents

1 Introduction 41.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.2 Aims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3 Organic Magnetic Field Effects . . . . . . . . . . . . . . . . . . . 51.4 CDT-PV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Theoretical Background 72.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2 Origin of Photovoltage . . . . . . . . . . . . . . . . . . . . . . . . 82.3 Organic Semiconductors . . . . . . . . . . . . . . . . . . . . . . . 102.4 Magnetic Field Effects . . . . . . . . . . . . . . . . . . . . . . . . 13

2.4.1 Hyperfine Interaction and Zero-Field Splitting . . . . . . 132.4.2 ∆g-mechanism . . . . . . . . . . . . . . . . . . . . . . . . 142.4.3 Bipolarons . . . . . . . . . . . . . . . . . . . . . . . . . . 142.4.4 Exciton Charge Model . . . . . . . . . . . . . . . . . . . . 152.4.5 Trion Model . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3 Experimental Methods 183.1 Device Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . 183.2 Current-Voltage Characterisation . . . . . . . . . . . . . . . . . . 193.3 Magneto-Photovoltage . . . . . . . . . . . . . . . . . . . . . . . . 203.4 Transient Photovoltage . . . . . . . . . . . . . . . . . . . . . . . . 21

4 Preliminary Results 224.1 Current-Voltage Characterisation . . . . . . . . . . . . . . . . . . 224.2 Magneto-Photovoltage . . . . . . . . . . . . . . . . . . . . . . . . 244.3 Transient Photovoltage . . . . . . . . . . . . . . . . . . . . . . . . 284.4 Varying Blend Ratio . . . . . . . . . . . . . . . . . . . . . . . . . 334.5 Iridium Doped PTB7 . . . . . . . . . . . . . . . . . . . . . . . . . 35

5 Discussion 38

6 Conclusion 41

A CDT-PV Training 44A.1 Liverpool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44A.2 Cambridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44A.3 Sheffield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45A.4 Southampton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46A.5 Bath . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46A.6 Oxford . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47A.7 Loughborough . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47A.8 Python . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

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1 Introduction

1.1 Overview

Photovoltaics [PV] is a field which could provide clean, sustainable electricalpower generation. The possibility for sustainable, localised power production isa great incentive for research in this field. Current market leaders in solar powerare crystalline silicon and other inorganic solar cells, which typically make useof rare or costly to process materials such as tellurium. Organic photovoltaics[OPV] are a possible alternative to these more traditional solar cells: they couldpotentially be continuously processable from solution, and also have low cost(both in embodied energy and financially) chemical feedstocks, being made fromorganic compounds.

Currently lifetimes and efficiencies [1] are not competitive with traditional es-tablished solar cells for applications in large scale power production, but nichessuch as building integrated solar power (as they may be produced in a varietyof colours and work under low light) and flexible solar power could give OPVa place in the market. In order to compete on larger power production scalesefficiencies and lifetimes will need to be improved. To this end it is necessary tounderstand the processes which govern recombination, degradation, and chargeseparation in OPV.

The following report will detail how recombination in OPV will be studied withmagnetic fields. In this section I will introduce the various aspects of the study;Section 2 will cover the theoretical background pertinent to this study; Section 3will cover the experimental methods used to study recombination and magneticfield effects on OPV performance; Section 4 will detail the preliminary resultsof this study; Section 5 will discuss some of the implication of these resultsand some areas where further work should be carried out; finally Section 6 willconclude the work.

The appendix covers training carried out as part of the Centre of DoctoralTraining in New and Sustainable Photovoltaics [CDT-PV].

1.2 Aims

This study aims to investigate whether or not the recombination from chargetransfer states to triplet exciton states can be probed with the change in photo-voltage when magnetic fields are applied in solution processed polymer-fullerenephotovoltaic devices. This will be investigated using several techniques: OPVdevices will be fabricated, current-voltage curves will be obtained to charac-terise the devices’ responses under varying magnetic fields, the devices’ voltageresponses to pulsed light will also be used to directly measure the effect ofmagnetic field on the response of the devices to light.

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1.3 Organic Magnetic Field Effects

Organic electronic devices exhibit magnetic field effects [MFEs] despite havingno ferromagnetic components. The effect magnetic fields can have on the inter-actions of free carriers and excitons in organic semiconductors was first studiedby Ern & Merrifield in the 1960s [2] and has recently excited renewed interest.

MFEs are seen in the electrical performance of optoelectronic devices, both or-ganic photovoltaic cells [3] and organic light emitting diodes [OLEDs] [4]. Thesemagnetic field effects include both magneto-resistance (a change in the currentat a given applied voltage with magnetic field) and magneto-electroluminescenceis also seen (a change in the light emitted by devices with applied magnetic field).

These effects have characteristic line-shapes specific to the device and architec-ture [5][6][7]. Both bipolar and homopolar devices (devices with either bothelectrons and holes able to be injected into the device, or just electrons or holesrespectively) exhibit magnetic field effects [8]. There are several different modelswhich attempt to explain the origin of MFEs in organic semiconductor devices.These will be explained further in the theoretical background.

The MFEs can be positive or negative (i.e. the device resistance can increase ordecrease with magnetic field), and the sign of the effect depends on the partic-ular system and temperature [9]. Magnetic field effects have also been seen inorganic semiconductor films, nanoparticle devices and hybrid organic-inorganicperovskite photovoltaic devices and films. Magneto-photoluminescence (wherethe yield of photons emittted after being pumped with a light source is affectedby magnetic field) has been observed in polymer films such as PPV (poly(p-phenylene vinylene)) derivatives, and this can be used to study bimolecularrecombination [10][11]. Triplet dissociation into free charges can be studiedwith applied magnetic fields, and this can then feed into suitability of poly-mers for singlet fission devices, one potential route to high-efficiency organic, ororganic-nanocrystal devices [12].

Colloidal nanoparticles and quantum dots have potential as a disruptive tech-nology both in photovoltaic applications and light emitting diodes [LEDs] andother electronic applications. The ability to sensitively tailor their bandgap totheir application [13] and the potential for multiple exciton generation (whichallows more than one electron/hole pair per incident photon) could allow modi-fications to existing solar technologies which could break the Shockley-Queisserlimit. Further the polymer matrix that the nanoparticles are suspended in mayexperience magnetic field effects, and magnetic nanoparticles may allow intrinsicmagnetic fields to exist within the device [14].

In recent years organic-inorganic hybrid perovskite semiconducting devices havemade rapid progess with unstabilised power conversion efficiencies exceeding20%. Perovskite device architectures have been designed to include organicelectron and hole transporting layers. In these devices MFEs have been seensuch as:

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• Magneto-resistance of up to 1 %

• Change in the electroluminescence of 0.1 % with applied field

• Magneto-photoluminscence of 0.1 % in perovskite pristine films [15]

1.4 Centre of Doctoral Training in New and SustainablePhotovoltaics

The funding for this training has come both from the University of Cambridgeand the Engineering and the Physical Sciences Research Council [EPSRC] CDT-PV. The CDT-PV is a joint program between seven UK universities: Bath, Cam-bridge, Liverpool, Oxford, Sheffield, Southampton, and Loughborough. Thetraining provided by the CDT-PV consisted of attendance at two-week trainingmodules at each of these institutions, totalling fourteen weeks.

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2 Theoretical Background

2.1 Introduction

Here the central theoretical background to the study will be discussed, moredetailed theoretical background will follow.

In a bulk heterojunction [BHJ] PV device charge is generated by photons excit-ing electrons from the highest occupied molecular orbital [HOMO] to the lowestunoccupied molecular orbital [LUMO] [16](see Section 2.3 for more details). Ina BHJ PV device, due to the low dielectric constant causing the exciton bindingenergy to be higher than the thermal energy at room temperature, photoexcitedelectrons and holes usually form bound electron-hole pairs [17], also known asexcitons.

The photo-excitation of the electron from the HOMO to the LUMO is a dipoletransition, and so must obey spin selection rules, so the photo-generated excitonsare typically formed as singlet excitons [18]. The rate of generation of excitonsis directly proportional to the flux of incident photons.

Excitons may diffuse to the heterojunction and there they separate into freecharges via an intermediate charge-transfer exciton [CT state] (Figure 1).

Figure 1: Schematic taken from [19] of photophysical processes in OPV. Singlet exci-tons are photoexcited (1), at the heterojunction they form the relaxed CT state excitons(2) and may dissociate into free charges [FC] (3) or recombine to long-lived triplet stateexcitons (4) which are long lived as their radiative decay (5) is spin-forbidden.

The CT states may undergo intersystem crossing (where their spin state pre-cesses from singlet to triplet) and the triplet CT states may recombine to formlong-lived triplet excitons [20].

Any free charges dissociated from CT states may also recombine to form singletor triplet CT states again. This gives three main factors which limit the numberof free charges:

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• Generation, which is directly proportional to the incident photon flux

• Recombination to triplet excitons from CT states, which is influenced byintersystem crossing between CT states

• Recombination to CT states from free charges

The intersystem crossing between CT states may be modulated by magneticfields (see Section 2.4), and so it is the recombination from these CT states thatwe hope to probe. We can do this as the photovoltage depends logarithmicallyon the free charge density [21](see Section 2.2 for more details):

V oc = Eg + kBT ln(np/(NcNv)) (1)

Here:

• n is the free electron density,

• p the free hole density,

• Nc,v the intrinsic charge density in the LUMO or HOMO respectively,

• T the temperature and

• Eg is the band gap.

So with the photovoltage we can measure the free charge density. By mea-suring the change in photovoltage with applied magnetic field (the ‘magneto-photovoltage’) we hope to measure the effect of magnetic fields on recombinationto long lived triplet excitons, which gives a change in the number of CT statesthat can dissociate to free charges.

2.2 The Origin of Photovoltage

In a photovoltaic device a potential difference is produced when light promotesvalence electrons in the device’s active layer to the conduction band. The energyof the incident light must be greater than the energy difference between Ec andEv, the energy of the conduction and valence band edges respectively. Thedensity of the holes created in the valence band by this promotion and theconduction band electrons generate an electrical potential.

The upper limit on the voltage that could be produced is this energy difference,the band gap of the semiconductor, Eg. This parameter is often used to estimatean upper bound on the efficiency of a single junction photovoltaic device due tothe Shockley-Queisser limit [22].

The true value for the photovoltage can be obtained from considering the mobileelectrons and holes, and the following analysis is valid for a conventional crys-talline, inorganic semiconductor and is based on material in Jenny Nelson’s The

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Figure 2: Illustration of the band configuration in a conventional semiconductor. Pho-tons incident on the active layer with energy greater than the band gap (Ec − Ev)promote electrons from the valence band to the conduction band.

Physics of Solar Cells [23] and R.A. Smith’s Semiconductors [21]. The electronenergy at the lower edge of the conduction band compared to the vacuum is:

Ec = −Ea − qV (2)

Here Ea is the electron affinity, the energy to remove the electron from theconduction band edge to infinity;

q is the electron charge;

V is the electrical potential across the device.

The electron density is calculated using Boltzmann statistics:

n = Nc exp

(− (Ec − EFn)

kBT

)(3)

The hole density, p, is calculated similarly:

p = Nv exp

(− (EFp − Ev)

kBT

)(4)

Taking the natural logarithm we get the quasi-Fermi energy for the electronsand holes, EFn,p. The quasi-Fermi energy is the electrochemical potential of anelectron or hole in the conduction or valence band.

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The difference between the electron and hole quasi-Fermi energies is the maxi-mum possible open-circuit voltage obtained from a photovoltaic device:

V oc = Eg + kBT ln(np/(NcNv)) (5)

The electron and hole densities (n, p) are sensitively dependent on the generationand recombination rate constants for free charges in the semiconductor. A slowerphoto-generation should give a lower density, and slower recombination shouldgive higher steady state density.

Although the above analysis is carried out with respect to an inorganic semicon-ductor, the outcome that the open-circuit photo-voltage with be logarithmicallydependant on the electron and hole densities will remain valid.

2.3 Organic Semiconductors

Organic compounds are made primarily of carbon. Carbon has four valenceelectrons, one s and three p. In diamond and alkanes, for example, these elec-tron orbitals can be thought of as coming together to form four sp3 hybridisedorbitals, which are tetrahedrally arranged in space. Electrons in these orbitalscan take part in covalent bonding through the formation of σ-bonds.

In some other compounds such as benzene and alkenes, one s and two p orbitalsform three sp2 hybrised orbitals. These three orbitals are in a plane, and thefourth, un-hybridised, p orbital is perpendicular to the plane of the other threeelectrons. In a compound such as benzene the perpendicular p orbitals overlapside-to-side with those of their neighbours to form π-bonds. These bonds aredelocalised over the benzene ring, with three bonding and three antibondingorbitals being formed. In an extended system with many benzene rings joinededge-to-edge (e.g. pentacene) there are 11 bonding and 11 antibonding orbitalsper molecule. In a poly-alkene (Figure 3) there are as many bonding and anti-bonding orbitals as there are π-electrons which leads to a band structure similarto inorganic semiconductors except the fermi energy lying between bands ofbonding and antibonding orbitals [16]. With these organic semiconductors manydevices can be made including: transistors, photovoltaic cells and light emittingdiodes (which are already a commercial reality [24]).

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Figure 3: Illustration of how a pseudo-band structure can occur in an organic molecule.In benzene 3 bonding orbitals are all doubly occupied by the six π electrons. In extendedπ-systems the number of bonding orbitals grows until the picture in a conjugated (i.e.with an extended π-system) molecule has many thousand bonding molecular orbitals,at which point the distinction with a valence band becomes blurred.

Although the HOMO and LUMO bands in organic electronics may be comparedto the valence and conduction bands in conventional semiconductors, they arenot identical. Part of the difference is due to the small dielectric constant ofconjugated polymers compared to inorganic semiconductors, which results ina strong interaction between electrons and holes. This allows Frenkel excitons(strongly bound electron-hole pairs) to quickly form after photoexcitation of theelectrons from the HOMO to the LUMO [25].

Another difference is that inorganic semiconductors have their electronic bandstructure extended over the entire crystal or active area. In organic semicon-ductors this is not the case as the band structure (Figure 3) extends only over asingle molecule. Excitons transfer energy via Forster resonant energy transfer.This is a mechanism whereby the exciton transfers its energy from one moleculeto another through dipole-dipole interactions. Free charges in organic semicon-ductors move via Dexter transfer, which occurs over short distances (0.6–2 nm[26]) and requires electron/hole wavefunction overlap between the donor andacceptor molecules.

In order to collect the photo-generated charges to do work it is necessary toseparate the excitons generated in the semiconductor. This is done at a junctionbetween two different semiconductors, where there is a sufficient energy gap toprovide the exciton binding energy. This necessity for an energy gap places arestriction on the efficiency of OPV systems as the exciton binding energy (0.7-1eV[27]) is lost.

At the junction the hole from the photo-exciton stays in the donor and the

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electron moves to the acceptor, at this point they may still be bound in acharge transfer state exciton [CT state], which can then either recombine to anexciton (geminate recombination) or separate to free charges [25]. If the freecharges later recombine with other free charges this is known as bi-molecular,or non-geminate, recombination.

Figure 4: Cartoon of heterojunction in OPV. Excitons are photogenerated and dif-fuse to heterojunction, where they dissociate into free charges via the charge transferexciton.

Since excitons are composite particles of two spin- 12 particles they can have oneof three spin-1 triplet states:

|1, 1〉 = |↑〉 |↑〉 (6)

|1, 0〉 = (|↓〉 |↑〉+ |↑〉 |↓〉) /√

2 (7)

|1,−1〉 = |↓〉 |↓〉 (8)

or one spin-0 singlet state:

|0, 0〉 = (|↑〉 |↓〉 − |↓〉 |↑〉) /√

2 (9)

These are the spin-states at zero-magnetic field when the magnetic dipole-dipoleinteractions between electrons and holes are neglected (See Section 2.4.1)

The photogenerated excitons will be predominantly in the spin-0 state since itis a forbidden dipole transition to generate the spin-1 state from the groundstate. The exciton and the CT state may change from singlet to triplet excitonsby intersystem crossing [ISC], where the electron’s and hole’s spins precess atdifferent rates in magnetic fields, allowing the total spin state to change.

In addition, since organic semiconductors are comprised of small molecules orpolymer chains they are more sensitive to lattice vibrations so free charges existas quasi-particles together with phonons, or polarons.

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2.4 Magnetic Field Effects

Large, low-field organic magneto-resistance [OMAR] has been observed in or-ganic bulk-heterojunction devices without ferromagnetic components (more than7 % at 100 mT [28]), with several spin-blockade mechanisms being proposed toexplain the findings. There are several pairs of particles whose reactions mayproduce free charges and whose rate constant depends on the overall spin of thepair. Some of these proposed pairs are:

• Bipolarons

• Exciton-charge

• Trions (composite particles of polarons and trapped excitons)

These pairs’ proposed reactions are given in Sections 2.4.3, 2.4.4, and 2.4.5.

The overall idea behind magneto-resistance is:

1. The spins of a pair of particles undergo ISC between spin states.

2. The pair of particles undergo a reaction at a rate which depends on theoverall spin of the pair.

3. In order to observe a magnetic field effect on resistance the spin-dependantreaction must influence polaron transport and ISC must be modulated byapplied magnetic field [29].

Some of the proposed causes of ISC are:

• Precession due to hyperfine [HF] fields & zero-field-splitting [ZFS]

• The ∆g-mechanism

2.4.1 Modulation of Hyper-Fine Interaction and Zero-Field Splittingby Magnetic Fields

Hyperfine fields are due to the magnetic fields from magnetic centres such ashydrogen nuclei. In conventional semiconductors the modulation of electronicspin-states by hyperfine fields doesn’t occur due to large spin-orbit coupling dueto heavy nuclei, but in many organic semiconductors the nuclei are typicallyvery light with negligible spin-orbit coupling [30]. The varying hyperfine fieldsthroughout the organic semiconductors cause the charge carriers to precess atdifferent rates and so intersystem crossing can occur.

Zero-field splitting [ZFS] is the effect of magnetic dipole-dipole interactionschanging the spin eigenvectors of electrons in triplet spin-states. This effectsthe ISC as the previously good quantum number of spin-up and spin-down mayno longer apply, so the spin-state of an exciton will precess between singlet and

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triplet spin-states. This zero-field splitting can be characterised by an effectivemagnetic field of around 100 mT [31][32].

The precession of the spin-state of the excitons and CT state excitons in anorganic semiconductor caused by HF interaction and ZFS can be interrupted ormodulated by external magnetic fields, which can change the ratio of singlet totriplet excitons in organic semiconductors.

2.4.2 ∆g-mechanism

In an applied magnetic field, B, electrons’ and holes’ spin vectors will precessat a rate:

ω = gµBB/h (10)

Here g is the Lande g-factor, and µB is the Bohr magneton.

The g-factor depends sensitively on the environment of the charge carrier - soelectrons and holes will have different g-factors. A polaron’s g-factor will bedifferent from a free electron or hole’s. This is especially the case for CT-stateexcitons where the electron and hole are on different types of molecule.

This gives dephasing of the electron and holes spin-states (in addition to ISCfrom hyperfine and ZFS) at a rate ∆ω = ∆gµBB/h (∆g is the difference betweenthe electron and hole g-factors). This can lead to magneto-resistance as tripletstates are much longer-lived than singlet excitons and as such will an increasein triplet state population will lead to a reduced charge mobility [33].

2.4.3 Bipolarons

As charge transport mechanisms in organic semiconductor are based on hoppingalong, and between molecules (Dexter transfer) [34] bottlenecks may form asthere may be small a number of hopping sites that allow current to flow betweentwo reservoirs of charge.

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Figure 5: Schematic of two-site bipolaron model based on [34]. An electron may hopfrom the left-hand reservoir to site A (1) and occupy site A with its spin up or down.The electron on site A may hop onto site B where another electron is already present(3) only if their final spin state is anti-parallel. This requires initially parallel electronsto undergo ISC to allow hopping to occur. Applied magnetic fields modulate this ISCand hence the hopping rate. The anti-parallel pair may then dissociate and an electronmay hop the the right-hand reservoir (4). In addition the electron on site A may hopdirectly to the right-hand reservoir (2) if the Coulombic repulsion between the anti-parallel electron pair (U) is too high.

The model illustrated in Figure 5 demonstrates how magneto-resistance arisesfor two sites. The double occupancy on site B is only allowed in a spin-antiparallel configuration due to the Pauli exclusion principle. The probabilityof initially spin-parallel/antiparallel polarons on site A and B forming a bipo-laron on site B is dependent on the applied magnetic field as this will modulatethe precession of the electron on site A’s spin, so it can be seen that magneto-resistance could occur. The alternative is that the electron on A can hop aroundB. The size of the Coulombic interaction between the polarons will determinewhether or not this is favourable to making the hop through B.

2.4.4 Exciton Charge Model

Scattering of free charges from triplet excitons will reduce the mobility of thesefree charges in organic semiconductors. Since the current is related to the chargecarrier mobility this gives higher resistance with higher scattering rates. Thisleads to magneto-resistance if a magnetic field modulates the ISC leading tohigher ratios of triplet excitons. [?]

2.4.5 Trion Model

The trion model incorporates both HF and ZFS effects as causes of ISC. Themodel is shown schematically in Figure 6.

Figure 6 shows how the decay rate constant of electron hole pairs is ultimatelymodulated by magnetic fields in the trion model. Triplet and quartet decaysare forbidden transitions. Singlet and doublet decays are dipole transitions and

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Figure 6: Schematic taken from [7]. Non-geminate recombination of trapped electrons(nt) and free holes (p) forms trapped excitons (1,3(nt-p) which can change from singletto triplet via ISC. The singlet may decay to the ground state (S0), whereas this decayis a forbidden dipole transition for the triplet state. The ISC is modulated by magneticfield so the decay rate constant to the ground state depends on the applied magneticfield). Similarly the trapped triplet exciton (T) combines with a hole to form a trion(2,4(T-h)) which can change from doublet (2) to quartet (4) via ISC which is modulatedby magnetic field. Similarly here the decay to the ground state (S0+p) is forbidden fromthe quartet state so magnetic fields modulate the decay rate constant of trions whichwill determine the free charge density.

so happen much faster than forbidden transitions. The ratios triplet to singletand quartet to doublet are modulated by magnetic field and so the overall rateof holes being liberated from trions is modulated by magnetic field, as is the

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generation rate of trions.

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3 Experimental Methods

Here the experimental techniques used in this investigation will be discussed:the methods used to make photovoltaic devices, current-voltage characterisationof these devices, how the photovoltage was measured, and how photovoltagetransients were taken.

3.1 Device Fabrication

Figure 7: Schematic of typical device structure for bulk-heterojunction photovoltaic cell.The cathode used was ITO, and the for anode aluminium, or a mixture of aluminiumand calcium, was used. The polymer-fullerene bulk-heterojunction is spin-cast in onestep and phase separates into donor and acceptor rich regions.

The devices in the architecture depicted in Figure 7 were fabricated as fol-lows: indium doped tin oxide coated substrates [ITOs] were cleaned by soni-cating them in acetone followed by isopropyl alcohol for ten minutes in eachsolvent. Then the substrates were plasma-etched in oxygen plasma for ten min-utes to ensure that any organic matter was cleaned from the substrates. Thecleaned substrates were then spin-coated with Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS) at 4000 rpm for 60 s. These coated sub-strates were then annealed under a nitrogen atmosphere for approximately 30minutes at 220 ◦C.

The coated ITOs were transferred to a nitrogen atmosphere glovebox where theywere spin-coated with the specific polymer blend under investigation. Some ofthe films were annealed after spin coating (see Table 1). Electrodes were madeby evaporating metal on top of the polymer film. Electrical contacts were addedto the devices and then the films were encapsulated with epoxy and glass whichhad been cleaned as the ITOs had.

Iridium doped PTB7 (poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’] dithiophene-2,6-diyl} {3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl})) deriva-tives were used to make devices (as by Qian et al. [35]). This allowed us toinvestigate the effect of increasing spin-orbit coupling on recombination andmagneto-resistance.

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Polymer-Fullerene PTB7-PCBM PIDTphanQ-ICBA PIDTphanQ-PCBM

PEDOT spin speed (rpm) 4000 4000 4000

Anneal Time (min), Temperature (◦C) 30,150 30,140 30,140

Polymer blend solvent DCB and DIO DCB DCB

Polymer film spin speed (rpm) 2000 800, 120 s then 2000 for 5 s 800, 120 s then 2000 for 5 s

Anneal Time (min), Temperature (◦C) No anneal 10,110 10,110

Metal contact Al,Ca Al Al

Table 1: Summary of cells made in these investigations. The acronyms DIO andDCB stand for diiodo-octane and dichlorobenzene respectively. DIO is a dopant usedto reduce the number of trap states in a BHJ device, and DCB is a common solvantused for producing organic solutions. PTB7 here gives the procedure as well for PTB7derivative devices.

Other organic semiconductors used were PIDTphanQ (poly(indacenodithiophene-co-phenanthro[9,10-b]- quinoxaline)), PCBM (Phenyl-C61-butyric acid methylester), and ICBA (indene-C60 bisadduct).

3.2 Current-Voltage Characterisation

Current-voltage characterisation [IV curves] measurements allow the perfor-mance of devices to be assessed. In particular, for photovoltaic cells the IVcurve allows the device power conversion efficiency, maximum power output,short-circuit current [Isc] and open-circuit photovoltage [Voc] to be determined.The Voc is the difference between the quasi-Fermi levels (Equation 5). Isc is thecurrent that flows through the device when no external bias is passed across thedevice.

IV curves were taken under illumination by a white light LED ring which hadbeen calibrated for intensity against the light from a solar simulator by compar-ing the current from a photodiode under illumination from both sources. Theintensity spectrum was not measured, and it will not match the solar spectrum.The setup used is shown in Figure 8.

The IV curves were taken with a Keithley 2400 source-meter by measuring thecurrent output of the cell at certain applied voltages. The IV curves measuredhad around 100 points with around a 100 ms wait between points. The appliedvoltage was swept from negative to positive bias. The experimental method hereand in subsequent sections was automated using the Labview software package.

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Figure 8: Schematic of experimental apparatus. The device voltage and current weresupplied and measured by a Keithley 2400 source-meter. The device was held betweenthe poles of an electromagnet. The white light was generated by an LED ring whichwas focused on the device. For the transient measurements pulsed light was suppliedby a blue LED which was placed on the axis of the LED ring and also focussed on thedevice. The setup was enclosed in a black box.

3.3 Magneto-Photovoltage

To measure the effect of magnetic field on photovoltage the devices were heldbetween the poles of a GMW 3470 electromagnet. The field was swept fromnegative to positive and back down again as illustrated in Figure 9 so that driftin the device due to heating or other effects could be averaged out.

Voc can be extracted from IV curves, but this requires stable devices to giveappropriate IV curves at all magnetic fields. A technique which takes less timeand is less sensitive to the manner in which IV curves are taken is to use thesource-meter as a meter instead and measure the photovoltage at different fieldstrengths (Figure 9). It may also be useful to measure the magneto-photocurrentin this way and to compare this to the magneto-photovoltage, but this has notbeen done yet.

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Figure 9: Illustration of the sweep used to measure IV curves, photovoltage and tran-sient photovoltage as a function of magnetic field. At each point on the line a mea-surement is made and then the magnetic field is changed again.

3.4 Transient Photovoltage

In order to complement the magneto-photovoltage experiments transient-photovoltage[TPV] experiments were carried out. A pulsed LED provided a perturbation ontop of a white light bias. The transient photovoltage produced was measuredwith an Agilent Technologies DSO6032A oscilloscope and this trace (e.g. Figure17) was analysed to attempt to probe the recombination time. As before themagnetic field applied to the devices was swept up and down to average outsystematic effects such as heating (Figure 9).

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4 Preliminary Results

4.1 Current-Voltage Characterisation

We took IV curves under white light provided by white LEDs. Figure 10 showsan IV curve for a PIDTphanQ-PCBM 1:1 blend device taken under 0.1 sunintensity white light.

Figure 10: Current-voltage characteristic of a PIDTphanQ-PCBM device with a blendratio of 1-1. The incident light had an intensity of 0.1 suns. The IV curve has theVoc and the Isc picked out to highlight how they are determined.

The functional form for the IV curve in Figure 10 is a standard IV curve [23].Curves such as Figure 10 allow us to extract the Voc and Isc.

We then varied the intensity of white light provided by the LEDs and the re-sultant variation in Isc is shown in Figure 11 and that for Voc can be seen inFigure 12.

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Figure 11: Variation of short circuit current with applied white light intensity for aPIDTphanQ-PCBM 1:1 blend ratio device. The light was provided by a white LEDring. The straight line is provided as a guide to the eye.

The variation in Isc seen in Figure 11 can be is linear over the range of the exper-iment. This confirms that the rate of exciton generation is directly proportionalto the arrival rate of incident photons (see 2.1).

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(a) (b)

Figure 12: Variation in photovoltage with applied light intensity for a PIDTphanQ-PCBM 1:1 blend ratio device. Graph (a) is plotted on a linear scale and (b) is loga-rithmic with a straight line as a guide to the eye. The light was provided by a whiteLED ring.

Figure 12 shows that the variation in photovoltage depends logarithmically onthe intensity of incident light with roughly a 0.06 V change per decade of incidentlight intensity (corresponding to the thermal voltage at 300 K). We see thatthe photovoltage is logarithmically dependant on the incident light intensity inagreement with the theory (Equation 1) since the total number of free chargesis proportional to the incident light intensity.

4.2 Magneto-Photovoltage

We then varied the magnetic field as illustrated in Figure 9 as we measured theIV curve under 0.1 sun illumination for a PIDTphanQ-PCBM 1:1 blend ratiodevice. The variation in Voc with magnetic field taken from these IV curves isshown in Figure 13.

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Figure 13: Magneto-photovoltage for PIDTphanQ-PCBM 1:1 blend ratio device under0.1 sun white light intensity. The dashed and solid lines correspond to averages of theupwards and downwards sweeps through the magnetic field respectively.

Figure 13 shows the variation in Voc as the magnetic field changed. The figureshows the averages of the up and down sweeps through the magnetic field sepa-rately to illustrate the repeatability of the experiment. The downwards tails atnegative fields are due to an initial period when the device was first inserted inthe setup. As can be seen the tails only effect the upwards sweeps, and thesewere the first measured. This may be as the magnet was not ramped down tothe initial magnetic field slowly enough.

The variation of Voc with magnetic field shows a local maximum at zero appliedfield with two minima either at ∼ ± 15 mT and then an increase at higher fieldstrengths. The functional form is similar to that seen for OMAR [36].

Similar measurements were made with varying light intensity from the LED ringfor the same PIDTphanQ-PCBM device and the results can be seen in Figure14.

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(a) (b)

Figure 14: Change in photovoltage with applied magnetic field measured inPIDTphanQ-PCBM 1:1 blend ratio device. The graph contains the data for variousdifferent white light intensities. (b) shows the same data as (a) but with the downwardstails at low field strength cut out. The dashed and solid lines correspond to averagesof the upwards and downwards sweeps through the magnetic field respectively.

The variation of Voc with magnetic field and illumination intensity (Figure 14)shows that the magnitude of the change with magnetic field and the functionalform is consistent over the range of intensities used, sweeping both up and downin field strength. The tails at low negative magnetic field are artifacts due tonot ramping down to the starting magnetic field slowly enough.

The same measurements could be used to extract the Isc from the IV curvesmeasured at different magnetic fields, and the results can be seen in Figure 15.

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Figure 15: Change in Isc with applied magnetic field for a PIDTphanQ-PCBM 1:1blend ratio device under varying illumination intensities from a white light LED ring.The dashed and solid lines correspond to averages of the upwards and downwardssweeps through the magnetic field respectively.

The variation of Isc in Figure 15 displays similar functional form to the changein photovoltage (Figure 14). The magnitude increases with white light intensityhowever.

Taking the value of Voc from IV curves required the measurement of around 100data points and for the device to give IV curves of sufficient quality over theentire measurement period to be able to extract the Voc. An alternative methodwas used to measure the change in photovoltage with applied magnetic field wasto simply measure the voltage from the device and to change the magnetic field.The results of this technique can be seen in Figure 16.

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Figure 16: Photovoltage measurement from a PIDTphanQ-PCBM 1:1 blend ratio de-vice under 0.1 sun illumination. The photovoltage was measured directly, and notextracted from IV curves.

The functional form shown in Figure 16 is similar to that measured with theprevious technique (Figure 13). This demonstrates that measuring the photo-voltage alone is a legitimate substitute for measuring IV curves at each appliedfield strength.

4.3 Transient Photovoltage

Transient photovoltage experiments were carried out for the same PIDTphanQ-PCBM 1:1 blend ratio device under 0.2 sun white light illumination to comple-ment the steady-state photovoltage measurements above. The pulsed light wasgenerated by a blue LED which was set up so that the photovoltage transientwas visible on the oscilloscope screen and the trace was judged to return to thesteady-state value between each pulse. An example of such a trace is shown inFigure 17.

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Figure 17: Decay of photovoltage transient used to analyse the recombination withmagnetic field applied to the PIDTphanQ-PCBM 1:1 blend ratio device. The devicewas held under constant 0.2 sun illumination and pulsed with a blue LED. The red lineis the transient photovotlage data, and the blue crosses are the fit to the data.

The Figure 17 shows a photovoltage transient and a decay (Equation 11) fit tothe measurement. In a similar method to above (see Figure 9) the magneticfield was varied between measurements.

Several methods were used to analyse the data. Decays of the form:

Vo + Vstrect

(11)

were fitted to the voltage transients (Figure 17). Here:

• trec is the decay rate constant

• Vo was the steady-state value of the photovoltage

• Vs was a fitting parameter related to the maximum voltage during theLED pulse

• t was time after the pulse was turned off

We see the variation of these trec values with magnetic field in Figure 18.

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Figure 18: Variation of recombination time taken from decay fits to photovoltage tran-sients (e.g. Figure 17).

The variation in fitted recombination times (Figure 18) shows a large variationin the fitted values of trec. This variation is not due to magnetic field, butrandom error possibly due to white light contamination.

Another method used to analyse the photovoltage transients was to plot the dataon a log-scale (Figure 19) and to compare the voltage transients for differentapplied magnetic field.

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Figure 19: Normalised voltage transient for PIDTphanQ-PCBM 1: blend ratio devicewith 0.2 sun white light bias plotted logarithmically. The vertical line indicates wherethe decay starts.

Figure 19 has two clear regimes (early times, <14 ms, and late times, >14 ms)and the gradient can be extracted from both regimes, although this was notdone at early times as the data at early times were very similar for differentapplied magnetic field strengths. The gradients of the late times were takenand used to compare trec at different magnetic fields as can be seen in Figure20. The response at later times (>15 ms) has already fallen low enough to beindistinguishable due to the resolution of the oscilloscope.

Figure 20 shows that at ∼ 14 ms the voltage decays have very similar formregardless of applied field and may not be distinguishable due to the low reso-lution, and as such more resolution will be needed to determine any magneticfield effects on the TPV.

In order to test the repeatability of the measurement we compared the log-plotsof two TPV traces with the same magnitude applied magnetic field, but differentpolarity (Figure 20).

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(a) (b)

Figure 20: Late time tails for photovoltage transients for PIDTphanQ-PCBM 1:1 blendratio devices with 0.2 sun white light bias such as in Figure 19 for different appliedmagnetic field strengths. The (a) shows two of the traces in figure (b) of the samemagnitude applied magnetic field, but different polarity.

The gradient at the tail end of the voltage transient can be seen to be similar formagnetic fields of ideally the same amplitude (Figure 20). This implies that themeasurement is repeatable. The gradients from the tail end of the exponentialdecay are plotted in Figure 21.

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Figure 21: Variation in recombination time taken from gradients in the tail end of thelog-plots of photovoltage transients similar to those in Figure 20

Figure 21 shows the variation in recombination time as extracted from the gra-dients of the tail ends of the voltage transients. There is either no trend, or thesignal-to-noise ratio is far too high due to the gradients being extracted fromthe low resolution tails. This suggests that the light pulses used were too strongto be in the regime of a small perturbation (the fractional change in voltage inFigure 14 was around 11%) and the next step in this part of the investigationwill be to attempt to resolve smaller light pulses, or apply a larger white lightbias. The artifacts at low negative magnetic field in Figure 14 also suggest thatthe ramp to the initial magnetic field will also need to be slower so that theagnet has time to stabilise.

4.4 Varying Blend Ratio

The effect of wavefunction delocalization on the recombination at the hetero-junction is discussed by Rao et al. [19] and the effect of polymer-fullerne blendratio on wavefunction delocalization is also discussed. To determine whether ornot the blend ratio also has an effect on recombination from CT states to tripletexcitons we investigated the magneto-photovoltage in different blend ratio de-vices.

We measured the magneto-photovoltage using Voc extracted from IV curvesunder 0.1 sun intensity white light to compare PIDTphanQ-ICBA devices withboth 1:1 and 1:3 blend ratios (Figure 22).

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Figure 22: Variation of open-circuit voltage with magnetic field for different blendratios of PIDTphanQ-ICBA devices under 0.1 sun white light illumination. The ratioa:b is the ratio of polymer to fullerene. The graph shows the same devices measuredon different days.

Figure 22 shows the same devices measured on different days, and the lackof repeatability between two days and the absence of measured magnetic fieldeffects suggest that another method should be used to measure the magneto-photovoltage.

We used the same method as used in Figure 16 to measure the variation in pho-tovoltage with magnetic field for the two different blend ratios of PIDTphanQ-ICBA devices under 0.1 sun illumination (Figure 23).

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Figure 23: Variation in photovoltage with magnetic field for two different blend ratiosof PIDTphanQ-ICBA devices under 0.1 sun illumination. There are only a smallnumber of the 1:3 blend ratio as the rest are off the scale. They are not shown as theywere considered to be noise.

Comparing the magneto-photovoltage from two different polymer-fullerene blendratios (Figure 23) we see that the current data for this comparison are not con-clusive as the data for the high ratio blend is not good enough to draw mean-ingful conclusions. We do see a simlar line-shape in the 1:1 blend to in thePIDTphanQ-PCBM device (Figure 16), which indicates that a similar effect iscausing the change in photovoltage with magnetic field for the two devices.

The intention was to compare different blend ratios for the same ingredients,but failure to produce good devices meant that this could not be carried outyet. Further work in this area will be to prepare more devices to investigate theeffect of wavefunction dissociation on recombination and magnetic field effects.

4.5 Iridium Doped PTB7

We investigated the magneto-photovoltage for devices with Iridium complexesdoped along the PTB7 backbone [35] under 0.1 sun white light illumination.The photovoltage was measured similarly as for Figures 16 & 23 and the resultsare shown in Figures 24 & 25.

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Figure 24: Photovoltage measured at 0.1 suns white light illumination for differentfractions of the PTB7 backbone replaced with Ir complex.

There may be a trend in the photovoltage with increasing iridium doping, butthis can not be seen cleary from the data in Figure 24 and more devices willneed to be made to investigate the effect of iridium doping more thoroughly.

Figure 25: Variation in magneto-photovoltage for PTB7-PCBM devices with a dif-ferent fraction of the PTB7 repeat units replaced with Ir complexes. The magneto-photovoltage was measured under 0.1 sun illumination.

Figure 25 shows the changing lineshape for the Ir doped devices. The deviceshave similar photovoltage response to the applied magnetic field. The magnitude

36

of the response decreases with increasing doping of iridium complex.

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5 Discussion

The IV curve for the PIDTphanQ-PC(70)BM (Figure 10) device looks muchlike that for a generic photovoltaic cell[23]. This not only means that the deviceworks, but also that parameters such as Voc and Isc can be obtained straight-forwardly from the IV curves.

The variation in Voc with magnetic field is independent of white light intensity,whereas a trend is visible in Isc (Figure 15). This indicates that in addition tothe amount of free charge produced being modulated by the applied magneticfield there is also magneto-resistance. This could be explained by the bipolaronmodel operating independently of the cause of the magneto-photovoltage. Inthe bipolaron model magneto-resistance occurs in free charges through a spinblockade mechanism, so the total amount of free charges (directly resultingfrom the incident light bias) will vary the strength of the magneto-resistance.Therefore the current will vary differently with magnetic field for different whitelight biases. Since there is magneto-resistance independent of the magneto-photovoltage it follows that the cause of the magneto-photovoltage is a modu-lation of the geminate recombination from CT states to triplet excitons, other-wise the magneto-photovoltage would have had a similar intensity dependenceto the magneto-photocurrent (Figure 26). The effect of white light intensity onthe magneto-photovoltage and magneto-photocurrent should be studied over abroader range of intensities. To this end a new LED ring is being constructedwhich should be able to reach 10 sun white light intensity. This could be studiedwith a variety of different polymer blends which have been reported in the lit-erature with either negative or positive magneto-resistance. This would enablea fuller picture of the effect of magnetic field on recombination from both CTstates to excitons and free charges to CT states.

The failure of the method employed to compare photovoltage and photocurrentmeasured from IV curves can be seen by comparing Figures 22 & 23. The devicestested were the same, but the magneto-photovoltage acquired had very differentlineshapes. The reason for this may be that although the device works and givesa photovoltage, to compare the Isc and Voc necessitated a sufficiently good devicethat had a well-defined IV curve (such as in Figure 10) over a long periodof time (∼2 hrs). To generate the magneto-photovoltage alone this necessitywas relaxed. However the devices still needed to function and have BHJs thatallowed for the magneto-photovoltage to be seen, which is demonstrated inFigure 23. The 1:3 device gives only noise, but the 1:1 device gives a well-defined trend in magneto-photovoltage. Investigation a larger range of blendratios of polymer to fullerene for a variety of different polymers and fullereneswould allow the impact of wavefunction dissociation on recombination [19] tobe studied through magnetic field effects or with transient photovoltage.

One of the reason why the spin-blockade effects can be so readily seen in or-ganic semiconductors is due to the low spin-orbit interaction between the chargecarriers and the organic molecules, as the organic molecules typically have low

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nuclear masses (e.g. around 12 atomic mass units). This being the case an inter-esting way to study magnetic field effects, and indeed any effect that magneticfields have on recombination is to dope the polymer chains with heavy-metal or-ganic complexes (i.e. Iridium complexes being doped along the PTB7 backbonein Figure 25). Figure 25 shows that the different ratios of heavy metal complexto the rest of the backbone produce the same general lineshape, but with modi-fied parameters such as width of the magnetic field effect and the level at whichthe magneto-photovoltage plateaus. We can see that as the level of doping ofIridium complexes into the polymer chain increases from 0 complexes per repeatunit (2Ir0) to 5 for every one hundred repeat units (1Ir5) the magnitude of themagneto-photovoltage decreases by around a factor of 5. This clearly confirmsthe idea that low spin-orbit coupling is responsible for organic magnetic fieldeffects. By producing a greater number of devices fabricated from the Iridiumdoped PTB7-PCBM blend the effect of introducing spin-orbit coupling on thephotovoltage - and therefore the recombination from CT state to triplet-exciton- may be explored.

Figure 26: Cartoon of processes in BHJ leading to magnetic field effects. Photogen-erated singlet excitons (S1) can dissociate at rate τ sing to CT state exciton, or viaintersystem crossing to triplet excitons. The CT states here incorporate both singletand triplet CT state excitons which also undergo intersystem crossing. The CT statesare formed by recombination from free charges (τ rec, free charges) and triplet excitons(τ trip), and can recombine to form triplet excitons (τ rec,triplet) or to singlet excitons.The CT states may also dissociate to free charges (τdis, free charges). The dashed ellipsedenotes the processes and states where magnetic fields could potentially effect photovolt-age. Modulating the ISC between triplet and singlet excitons and triplet and singlet CTstates will modulate the fraction of singlet state CTs which dissociate into free chargesand the number of long-lived triplet excitons. This modulation of free charge densitywill thus effect the photovoltage. Other magnetic field effects such as the bipolaronmodel may occur to the free charges.

Using Equation 1 we can obtain an estimate on the change in recombination rate

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constant after applying a magnetic field from the change in photovoltage. Thephotovoltage depends logarithmically on the free charge density, which will de-pend on the difference in the recombination rate constant to a power, althoughthe power will depend on the particular recombination mechanism. Looking tothe data for a PIDTphanQ-PCBM 1:1 blend ratio device under 0.2 sun intensitywhite light the percentage change in recombination time Equation 1 might leadus to expect would be of the order of 1 in 1000 or less, depending on the partic-ular system, whereas in the experiments so far this is more like 50%. It will beattempted to produce rigorous TPV traces and to determine the effect - if any -of magnetic field upon these traces by decreasing the pulsed light intensity, us-ing slower magnetic field ramps and producing more efficient devices to improvethe signal. This will be part of an effort to set up a piece of experimental ap-paratus capable of measuring electroabsorption; photovoltage and photocurrenttransients; and magnetic effects on photocurrent and photovoltage.

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6 Conclusion

In summary we made polymer-fullerene devices and took their current-voltagecharacteristics under varying intensity white light. We saw the variation in pho-tovoltage and photocurrent we expected from theory. Applying magnetic fieldsto the devices gave us a change in photovoltage and photocurrent with magneticfield with similar lineshapes to that reported for OMAR in the literature. Wesaw decreased geminate recombination in polymer:fullerene photovoltaic devicesthrough the qualitative difference between the trends in magneto-photovoltageand magneto-photocurrent at different light biases, however to confirm thistrend more devices will need to be produced and tested at a larger varietyof white light biases.

Through magneto-photovoltage measurements in PTB7-PCBM devices dopedwith varying levels of an Iridium complex along the polymer backbone we sawdiminished magneto-photovoltage. As the doping was increased spin-orbit cou-pling would also have increased, lowering the spin-coherence time. This lowerspin-coherence time explains the diminished magnetic field effects we saw.

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A Centre of Doctoral Training in New and Sus-tainable Photovoltaics First Year Training

As part of the funding arrangements for this study training in the seven univer-sities involved in the CDT-PV (Bath, Cambridge, Liverpool, Oxford, Sheffield,Southampton, and Loughborough) was provided. Each of the training mod-ules were two weeks long and required attendance at the university. Here thetraining provided by each of these modules will be discussed in chronologicalorder.

A.1 Liverpool – Fundamentals of New and SustainablePV – November 3rd – 14th 2014

This module was carried out at the Stephenson Institute of Renewable Energy.A lecture course on the fundamental physics of solar cells (based on Jenny Nel-son’s book [23]) and their characterisation was given. This was in conjunctionwith several practical classes to introduce the cohort to the characterisationtechniques studied in the lecture course.

The techniques carried out included: obtaining IV & EQE curves, optical mi-croscopy, using a hall-effect probe, UV-Vis absorption spectrophotometry, pro-filometry with an atomic force microscope.

This course was assessed by an examination, a brief experimental report, andalso by group presentations on the subject of terawatt implementation of pho-tovoltaics (i.e. the use of solar power for large scale power production).

A.2 Cambridge – Renewable Energy, Developing World,Entrepreneurship – December 1st-12th 2014

This module consisted of two parts: a course given by Professor Neil Greenhambased on his part III interdisciplinary course: Materials, Electronics and Renew-able Energy; and part of the ETech programme given by the Judge BusinessSchool.

Professor Greenham’s course provided a holistic background for renewable en-ergy in the UK and covered the practicalities of implementation of renewableenergy to provide a nation’s energy budget. The lectures covered back-of-the-envelope style calculations for energy use and production. In addition back-ground was given on heat engines, electrochemistry and also photosynthesis.In conjuction with these lectures, lectures from Professor David MacKay andMichael Herzog were given to provide further insight into reasons why researchinto renewable energy is a worthwhile endeavour.

The ETech course provided an introduction to concepts and techniquecs whichmay be required to take any ideas discovered during the course of a PhD or

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Academic research in general to market. Some of the ideas covered included:building a successful team; assessing the quality of an innovation; analysing thecurrent market; building business models; basic of accounting and finance; andhow to handle intellectual property in the context of academic research.

This course was assessed with an examination given by Professor Greenham inJanuary 2015, and through a “Dragon’s Den” style pitch to a panel of investorsat the Judge Business School.

A.3 Sheffield – Research Skills and PV in Action – Jan-uary 12th-23rd 2015

During the time at Sheffield several lectures were given on topics pertinent tothe laboratory techniques that were studied there, although it should be notedthat there was some redundancy with the work carried out in Liverpool. Thelectures covered a wide variety of topics such as:

• High-vacuum systems and residual gas analysis,

• Atomic force microscopy,

• Statistical analysis for experimental physicists,

• Optical microscopy and lens ray diagrams,

• UV-Vis-IR spectroscopy and

• Raman spectroscopy.

The practical work carried out included: spin coating polymers on glass slidesand using an AFM to measure the thicknesses obtained; residual gas analysisof a high-vacuum system as it was being pumped down from ambient pressure;optical microscopy and image analysis to determine the size and concentrationof polystyrene beads in a solution; UV-Vis-IR to see that food dyes of a certaincolour absorb their complementary colours, to determine the concentration ofan unknown solution using colorimetry, and measure vibronic transitions inpolymer films and the Stokes shift between emission and absorption for thesame films. Raman spectroscopy of various organic compounds such as alcoholsand paracetamol, both in solution and powder, were taken to analyse theircomposition.

This module was assessed with four pieces of written work: two experimentalreports on the procedures carried out and also two short research reviews onone of the techniques studied in the lectures.

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A.4 Southampton – Nanotechnology and Winning Tech-nology – February 2nd-13th 2015

An interesting lecture series was given by Giles Richardson in Southampton onthe subject of models of carrier motion in semiconductors and semiconductordevices. A hopping model was developed for charges in an inorganic semicon-ductor and followed that through to a drift-diffusion model on the scale of adevice, and so obtained equations for the current through a semiconductor het-erojunction on exposure to light above the semiconductor’s bandgap. Theseequations were then compared to the Shockley equivalent circuits for a PV cell.

In addition to these lectures the cohort were required to investigate severalapplications of nanophotonics to photovoltaics. The application the team I wason carried out studied the application of plasmonics to PV to improve absorptioneither through scattering or enhancement of EM fields near to nanoparticlesembedded in the cell, or due to surface plasmon polaritons on the back contactof the device.

Further, there were four demonstrations of practical skills useful for PV researchincluding taking IV and EQE curves, determining the lifetime of carriers in acell using RF magnetic induction to track how the photoconduction decays withtime. There was also a demonstration of techniques to measure reflectance froma sample. This module was assessed by a short examination and a group pre-sentation based on the use of plasmonic enhancement of photovoltaic systems.

A.5 Bath – Design for High Performance and BusinessSkills – March 2nd-13th 2015

Instruction was given on the implementation and analysis of impedence spec-troscopy in the context of photovoltaics research. This was followed up bypractical demonstration of impedence spectroscopy both on dummy systemsand real photovoltaic device. This technique’s use in analysing electrochemicalcells was demonstrated as well.

Professor Aron Walsh ran a series of lectures on materials modelling methods,focusing on the implementation of density functional theory and related mod-elling methods to determine electronic properties of materials. These lecturesshowed how someone might get started in this area, and also how they mightuse these modelling techniques to assess a system’s viability for photovoltaicapplications, from inorganic to hybrid and organic PV systems.

A lecture was given on the basics of organic photovoltaics, and was consolidatedwith a laboratory session to produce some organic semiconductor films andmeasure their properties. This included a demonstration of the application oftransmission electron microscopy to OPV.

The course coordinator (Professor Alison Walker) was asked to help assess thesuitability of Bath Abbey to the installation of PV on its roof. This assess-

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ment was carried out using the PVSyst software package and the results of thisassessment gave information about the potential power output, likely paybacktimes, and alternative renewable energy investments for Bath Abbey.

A.6 Oxford – Advanced Materials and High PerformancePV – April 13th-24th 2015

During the training in Oxford many varied lectures were given on topics fromdensity functional theory assessment of prospective structures to large scaleimplementation of roll-to-roll vacuum processes. Lectures were also given onorganic electronics, carbon nanotubes, perovskite solar cells and quantum-dotsolar cells. These lectures were followed up with an investigation into the varyinghalide compositions in perovskite PV cells. The measurements taken includedtime-resolved photoluminescence and kelvin-probe analysis of perovskite films,and JV curves of perovskite devices produced by the cohort with the aid of themembers of the research group in Oxford.

The course was assessed through a report written in groups about the resultsseen in the experiments, and also by a “peer-review” process, where the cohortcritiqued each-other’s work.

A.7 Loughborough – Real PV Performance and PV inBusiness – May 11th-22nd 2015

This module gave the cohort an opportunity to make up a PV module fromseveral cells bought from a commercial manufacturer. The cells were wired inseries and encapsulated between EPA sheets and glass. The modules JV curveswere measured and electroluminescence measurements were taken before andafter the module was subjected to accelerated aging at 85 % humidity and at85 ◦C for a week.

Lectures on concentrating photovoltaics systems, commercial implementation ofphotovoltaic power production and also explanation of some practical points ofa photovoltaic installation were given. These were complemented by a visit toa 30 MW solar farm near Loughborough.

The lectures were reinforced by a project to investigate the feasibility of in-stalling a photovoltaic system in both domestic and agricultural settings. Thismade use of the PVSyst software and required investigation of current policy onfeed-in-tariffs. The assessment for the course was in the form of a presentationdisplaying the results of the PVSyst investigation and experimental results.

A.8 Python – CDT commandos additional work

The course coordinator, Rob Treharne, organised some additional, optionaltasks for the cohort so that they may get acquainted with the Python program-

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ming language, and its implementation in a research context. Tasks includedextracting variables from a data set, reading and writing files, presentation ofdata and modelling.

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