Maximizing the open-circuit voltage of polymer: Fullerene solar cells

Bijleveld, JC Johan; Verstrijden, Ram Melanie; Wienk, MM Martijn; Janssen, Raj René

doi:10.1063/1.3480598

Cited by 44 publications

(45 citation statements)

References 22 publications

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“…The vast majority of OPVs reported have V OC s below 1 V, 3 and even the best performing tandem devices often contain subcells which produce nearly identical voltages. 5 However, despite their widespread use, the reported V OC for devices with fullerene acceptors are typically between 0.6-0.8 V, 3 with selected systems producing V OC up to 1.15 V. 6,7 In an attempt to circumvent these issues, a wide variety of non-fullerene acceptor systems have been developed. While substantial progress has been made in formulating numerous donors which yield high performance, the corresponding diversity of efficient acceptors is lacking.…”

Section: Introductionmentioning

confidence: 99%

Symmetry-Breaking Charge Transfer in a Zinc Chlorodipyrrin Acceptor for High Open Circuit Voltage Organic Photovoltaics

Bartynski

Gruber

Das³

et al. 2015

J. Am. Chem. Soc.

123

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Low open-circuit voltages significantly limit the power conversion efficiency of organic photovoltaic devices. Typical strategies to enhance the open-circuit voltage involve tuning the HOMO and LUMO positions of the donor (D) and acceptor (A), respectively, to increase the interfacial energy gap or to tailor the donor or acceptor structure at the D/A interface. Here, we present an alternative approach to improve the open-circuit voltage through the use of a zinc chlorodipyrrin, ZCl [bis(dodecachloro-5-mesityldipyrrinato)zinc], as an acceptor, which undergoes symmetry-breaking charge transfer (CT) at the donor/acceptor interface. DBP/ZCl cells exhibit open-circuit voltages of 1.33 V compared to 0.88 V for analogous tetraphenyldibenzoperyflanthrene (DBP)/C60-based devices. Charge transfer state energies measured by Fourier-transform photocurrent spectroscopy and electroluminescence show that C60 forms a CT state of 1.45 ± 0.05 eV in a DBP/C60-based organic photovoltaic device, while ZCl as acceptor gives a CT state energy of 1.70 ± 0.05 eV in the corresponding device structure. In the ZCl device this results in an energetic loss between E(CT) and qV(OC) of 0.37 eV, substantially less than the 0.6 eV typically observed for organic systems and equal to the recombination losses seen in high-efficiency Si and GaAs devices. The substantial increase in open-circuit voltage and reduction in recombination losses for devices utilizing ZCl demonstrate the great promise of symmetry-breaking charge transfer in organic photovoltaic devices.

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Section: Introductionmentioning

confidence: 99%

Symmetry-Breaking Charge Transfer in a Zinc Chlorodipyrrin Acceptor for High Open Circuit Voltage Organic Photovoltaics

Bartynski

Gruber

Das³

et al. 2015

J. Am. Chem. Soc.

123

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“…Although there have been reports of polymer:fullerene devices with V oc values above 1.0 V, [8][9][10] and as high as 1.15 V, [ 11 ] all of these Voltage-dependent, steady state and time-resolved photoluminescence measurements indicate that energy transfer occurs from PBDTTPD to ICBA and that back hole transfer from ICBA to PBDTTPD is ineffi cient. By analyzing the absorption and emission spectra from fullerene and charge transfer excitons, we estimate a driving free energy of -0.14 ± 0.06 eV is required for effi cient hole transfer.…”

mentioning

confidence: 96%

Recombination in Polymer:Fullerene Solar Cells with Open‐Circuit Voltages Approaching and Exceeding 1.0 V

Hoke

Vandewal

Bartelt

et al. 2012

Advanced Energy Materials

216

246

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respectively. While much synthetic research has focused on the development of lower bandgap materials with absorption spectra that better match the solar irradiation spectrum, [ 1 , 2 , 5 ] new materials will also be required which maximize the photovoltage obtained from absorbed photons. Many organic photovoltaic materials have optical bandgaps in the range of 1.7-2.0 eV, which make them suitable for the large bandgap junction of a tandem solar cell. However, the highest effi ciency allorganic tandem solar cells reported thus far only obtain open-circuit voltages ( V oc 's) of 0.9 V or less from the large bandgap junction. [ 4 , 6 ] Internal quantum effi ciencies (IQEs) approaching 100% have been demonstrated in single junction polymer:fullerene BHJ solar cells. [ 3 , 7 ] In order to generate and extract charges with high quantum effi ciency from bound photogenerated excitons, a heterojunction is employed where the ionization potential and electron affi nity of the fullerene are substantially higher than for the polymer, providing suffi cient free energy ( Δ G ) for charge transfer and exciton dissociation. Consequently, in the most effi cient BHJ devices, the V oc is 0.7-1.0 V less than the optical bandgap potential of the individual materials, E g / q .[ 1-3 , 5 , 7 ] The C 60 and C 70 fullerene derivatives currently employed in organic solar cells with efficiencies above 4% have an optical bandgap of 1.7 eV. Although there have been reports of polymer:fullerene devices with V oc values above 1.0 V, [8][9][10] and as high as 1.15 V, [ 11 ] all of these Voltage-dependent, steady state and time-resolved photoluminescence measurements indicate that energy transfer occurs from PBDTTPD to ICBA and that back hole transfer from ICBA to PBDTTPD is ineffi cient. By analyzing the absorption and emission spectra from fullerene and charge transfer excitons, we estimate a driving free energy of -0.14 ± 0.06 eV is required for effi cient hole transfer. These results suggest that the driving force for hole transfer may be too small for effi cient current generation in polymer:fullerene solar cells with V oc values above 1.0 V and that non-fullerene acceptor materials with large optical gaps ( > 1.7 eV) may be required to achieve both near unity internal quantum effi ciencies and values of V oc exceeding 1.0 V.

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“…Whilst fullerenes are undoubtedly effi cient electron transport materials, this class of acceptor material has inherent issues with long-term stability [ 8 , 9 ] and relatively low bandgaps which limit the maximum obtainable V OC in single heterojunction systems. [ 10 ] Overcoming these issues by use of alternative acceptor materials, whilst maintaining or improving overall effi ciencies, is an important challenge in the OPV fi eld.Initially, the V OC in OPVs was believed to be primarily limited by the work function difference between the two electrodes, however, it has since become clear that the difference in energy between the highest occupied molecular orbital (HOMO) of the donor (D) and the lowest unoccupied molecular orbital (LUMO) of the acceptor (A), i.e. the interface gap ( I G ), is the primary determinant of the maximum V OC obtained.…”

mentioning

confidence: 99%

Halogenated Boron Subphthalocyanines as Light Harvesting Electron Acceptors in Organic Photovoltaics

Sullivan

Duraud

Hancox

et al. 2011

Advanced Energy Materials

152

165

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The fi eld of organic photovoltaics (OPVs) is attracting enormous interest due to its promise of providing a low-cost, easily processable technology for energy generation. Whilst the shortcircuit current density ( J SC ) of OPV devices can be optimized through morphology control, [ 1 ] intermixing, [ 2 , 3 ] nanostructuring [ 4 ] or templating [ 5 ] amongst other techniques, the opencircuit voltage ( V OC ) is critically dependent on the choice of photoactive materials. The relative abundance of suitable commercially available materials has led to a wealth of reports on optimization of the donor layer; [ 6 , 7 ] however the same is not true for the acceptor layer, with most work focusing on the ubiquitous fullerenes. Whilst fullerenes are undoubtedly effi cient electron transport materials, this class of acceptor material has inherent issues with long-term stability [ 8 , 9 ] and relatively low bandgaps which limit the maximum obtainable V OC in single heterojunction systems. [ 10 ] Overcoming these issues by use of alternative acceptor materials, whilst maintaining or improving overall effi ciencies, is an important challenge in the OPV fi eld.Initially, the V OC in OPVs was believed to be primarily limited by the work function difference between the two electrodes, however, it has since become clear that the difference in energy between the highest occupied molecular orbital (HOMO) of the donor (D) and the lowest unoccupied molecular orbital (LUMO) of the acceptor (A), i.e. the interface gap ( I G ), is the primary determinant of the maximum V OC obtained. [ 6 , 11 ] Importantly, the frontier orbital energies of organic materials can be readily tuned through introduction of electron-donating or electron-withdrawing substituents. For example, halogenation of conjugated molecules has been shown to lower the HOMO and LUMO levels with respect to the vacuum level whilst having only a small effect on other electronic properties such as the bandgap, or the position of the Fermi level within the bandgap. [ 12 , 13 ] As an example of the extent of modifi cation possible by this method, complete fl uorination of copper phthalocyanine (CuPc) to its hexadecafl uoro derivative (F 16 CuPc) results in a shift of the HOMO level by over 1 eV. [ 14 ] The D/A heterojunction system of boron subphthalocyanine chloride (SubPc)/fullerene (C 60 ) is amongst the current fi eld-leaders for single small molecule heterojunction devices both in terms of V OC ( ∼ 1.1 V) and overall power conversion effi ciency ( PCE ∼ 3%). [ 15 , 16 ] However, consideration of the interfacial energetics of this system reveals that C 60 is far from optimal as the acceptor (vide infra). In this communication, we report the synthesis of several new selectively chlorinated and fl uorinated SubPcs, and demonstrate that effi cient OPV devices can be constructed using these materials as electron acceptors. Moreover, it is shown that peripheral halogenation can be used to maximize the interface gap, and hence the V OC obtained. As well as a remarkably high V ...

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Maximizing the open-circuit voltage of polymer: Fullerene solar cells

Cited by 44 publications

References 22 publications

Symmetry-Breaking Charge Transfer in a Zinc Chlorodipyrrin Acceptor for High Open Circuit Voltage Organic Photovoltaics

Symmetry-Breaking Charge Transfer in a Zinc Chlorodipyrrin Acceptor for High Open Circuit Voltage Organic Photovoltaics

Recombination in Polymer:Fullerene Solar Cells with Open‐Circuit Voltages Approaching and Exceeding 1.0 V

Halogenated Boron Subphthalocyanines as Light Harvesting Electron Acceptors in Organic Photovoltaics

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