On the Differences between Dark and Light Ideality Factor in Polymer:Fullerene Solar Cells

Kirchartz, Thomas; Deledalle, Florent; Tuladhar, Pabitra Shakya; Durrant, James R.; Nelson, Jenny

doi:10.1021/jz4012146

Cited by 253 publications

(286 citation statements)

References 38 publications

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“…Ideality factors are unitless and typically range from 1 to 2, though values outside this range are possible. 69,74 An ideality factor of 1 is consistent with more ideal band-to-band recombination processes, whereas an ideality factor of 2 is consistent with trap-assisted recombination through midgap states. The recombination processes that give rise to intermediate ideality factors between 1 and 2 are not readily evident and must be evaluated on a contextual basis, though these values are often closely tied to trap-assisted recombination.…”

Section: Comparing the Device Physics Of Matchedmentioning

confidence: 85%

Comparing Matched Polymer:Fullerene Solar Cells Made by Solution-Sequential Processing and Traditional Blend Casting: Nanoscale Structure and Device Performance

Hawks¹,

Aguirre²,

Schelhas³

et al. 2014

J. Phys. Chem. C

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Polymer:fullerene bulk heterojunction (BHJ) solar cell active layers can be created by traditional blend casting (BC), where the components are mixed together in solution before deposition, or by sequential processing (SqP), where the pure polymer and fullerene materials are cast sequentially from different solutions. Presently, however, the relative merits of SqP as compared to BC are not fully understood because there has yet to be an equivalent (composition-and thickness-matched layer) comparison between the two processing techniques. The main reason why matched SqP and BC devices have not been compared is because the composition of SqP active layers has not been accurately known. In this paper, we present a novel technique for accurately measuring the polymer:fullerene film composition in SqP active layers, which allows us to make the first comparisons between rigorously composition-and thickness-matched BHJ organic solar cells made by SqP and traditional BC. We discover that, in optimal photovoltaic devices, SqP active layers have a very similar composition as their optimized BC counterparts (≈44−50 mass % PCBM). We then present a thorough investigation of the morphological and device properties of thickness-and composition-matched P3HT:PCBM SqP and BC active layers in order to better understand the advantages and drawbacks of both processing approaches. For our matched devices, we find that small-area SqP cells perform better than BC cells due to both superior film quality and enhanced optical absorption from more crystalline P3HT. The enhanced film quality of SqP active layers also results in higher performance and significantly better reproducibility in larger-area devices, indicating that SqP is more amenable to scaling than the traditional BC approach. X-ray diffraction, UV−vis absorption, and energy-filtered transmission electron tomography collectively show that annealed SqP active layers have a finer-scale blend morphology and more crystalline polymer and fullerene domains when compared to equivalently processed BC active layers. Charge extraction by linearly increasing voltage (CELIV) measurements, combined with X-ray photoelectron spectroscopy, also show that the top (nonsubstrate) interface for SqP films is slightly richer in PCBM compared to matched BC active layers. Despite these clear differences in bulk and vertical morphology, transient photovoltage, transient photocurrent, and subgap external quantum efficiency measurements all indicate that the interfacial electronic processes occurring at P3HT:PCBM heterojunctions are essentially identical in matched-annealed SqP and BC active layers, suggesting that device physics are surprisingly robust with respect to the details of the BHJ morphology.

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Section: Comparing the Device Physics Of Matchedmentioning

confidence: 85%

Comparing Matched Polymer:Fullerene Solar Cells Made by Solution-Sequential Processing and Traditional Blend Casting: Nanoscale Structure and Device Performance

Hawks¹,

Aguirre²,

Schelhas³

et al. 2014

J. Phys. Chem. C

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“…within the allowed limits of a diode in which diffusion (n~1) or recombination (n~2) dominates transport. Whilst n-values exceeding 2 are often reported when they are determined directly from the gradient of ln (J)-V curves [9][10][11][12], this should not be the case when using this methodology since the main junction component is isolated from any influence by the shunt components and series resistance. However, the n value of the secondary exponential term is 5.9 (A 2 = 6.5 V À1 ) which may imply that the term describes a tunnelling current (this is further investigated in Section 4.2).…”

Section: Methodsmentioning

confidence: 99%

Identifying parasitic current pathways in CIGS solar cells by modelling dark JV response

Williams

Smit

Kniknie³

et al. 2014

2014 IEEE 40th Photovoltaic Specialist Conference (PVSC)

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Document VersionPublisher's PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication:• A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. ABSTRACTThe non-uniform presence of shunting defects is a significant cause of poor reproducibility across large-area solar cells, or from batch-to-batch for small area cells, but the most commonly used value for shunt parameterisation (the shunt resistance) fails to identify the cause for shunting. Here, the use of equivalent circuit models to describe dark current-voltage characteristics of ZnO:Al/i-ZnO/CdS/CIGS/Mo devices in order to understand shunting behaviour is evaluated. Simple models, with a single shunt pathway, were tested but failed to fit experimental data, whereas a more sophisticated model developed here, which includes three shunting pathways, yielded excellent agreement throughout the temperature range of 183-323 K. The temperature dependence of fitting parameters is consistent with known physical models. Activation energies and contact barriers are determined from the model, and extracted diode factors are unique across the voltage range. A case study is presented whereby the model is used to diagnose poor reproducibility for CIGS devices (efficiency~3-14% across a 100 cm 2 plate). It's shown that lower efficiencies correlated with greater prevalence of Ohmic and non-Ohmic shunt currents, which may form due to pinholes in absorber and buffer layers respectively, whereas the quality of the main junction was constant for all cells (diode factor~1.5-2). Electron microscopy confirmed the presence of ZnO:Al/i-ZnO/Mo and ZnO:Al/CIGS/Mo regions, supporting the mult...

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“…Note that the light ideality factor is not necessarily equivalent to the diode ideality factor. [16][17][18] At open-circuit conditions (steady-state…”

Section: Wileyonlinelibrarycommentioning

confidence: 99%

2D and Trap‐Assisted 2D Langevin Recombination in Polymer:Fullerene Blends

Nyman

Sandberg

Österbacka

2014

Advanced Energy Materials

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developed a theory to describe the rate at which anions and cations recombine in an ion gas; the rate depends only on the velocity at which the ions move in their mutual Coulomb fi eld since it is assumed that once the ions are close enough to each other recombination (ion bond formation) is instantaneous. Langevin theory does not describe the reverse process (anioncation formation); the time reversed process was described by Onsager and further extended by Braun. [ 2,3 ] When applying standard Langevin theory to BHJ blends two basic assumptions are made, namely the charge transport in the active material is isotropic and the rate of nongeminate polaron pair formation is much lower than the rate of polaron pair recombination. If these assumptions hold the recombination rate is simply given by the probability of an electron and a hole to meet in coordinate space, so that R = β L np , and the Langevin recombination coeffi cient β L is:where e = elementary charge, µ n(p) = the electron (hole) mobility, ε = relative permittivity, and ε 0 = the vacuum permittivity. In general either one of these assumptions-or both, may not be valid. For example, if the charge-carrier mobility is highly anisotropic an electron will not necessarily recombine with the hole that is closest; it will recombine with the one that is most easily accessible. This could occur for example in lamellar systems; it has been shown that the mobility in the plane of a polymer lamella can be up to 100 times the out of plane mobility. [ 4 ] Several BHJ blends have been reported to show a bimolecular recombination that is signifi cantly reduced as compared with Langevin, [5][6][7] in particular a reduction in the recombination rate of a factor of 10 2 -10 4 has been reported in the BHJ blend poly(3-hexylthiophene) (P3HT) and PCBM. [ 5 ] In these cases a recombination prefactor, defi ned as / L ξ β β = , is typically introduced. Several models have been proposed to clarify the underlying mechanism of reduced recombination, some of which can be directly related to restricted validity of the assumptions in Langevin theory listed above. Koster et al. [ 8 ] noted that the complex created when an electron and a hole meet in coordinate space (charge encounter complex) can be resplit into a free electron and hole with a certain probability. Murthy et al. [ 7 ] noted that for several BHJ systems exhibiting reduced

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On the Differences between Dark and Light Ideality Factor in Polymer:Fullerene Solar Cells

Cited by 253 publications

References 38 publications

Comparing Matched Polymer:Fullerene Solar Cells Made by Solution-Sequential Processing and Traditional Blend Casting: Nanoscale Structure and Device Performance

Comparing Matched Polymer:Fullerene Solar Cells Made by Solution-Sequential Processing and Traditional Blend Casting: Nanoscale Structure and Device Performance

Identifying parasitic current pathways in CIGS solar cells by modelling dark JV response

2D and Trap‐Assisted 2D Langevin Recombination in Polymer:Fullerene Blends

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