Efficiency limiting factors of organic bulk heterojunction solar cells identified by electrical impedance spectroscopy

Glatthaar, Markus; Riede, Moritz; Keegan, Nicholas; Sylvester-Hvid, Kristian O.; Zimmermann, Birger; Niggemann, Michael; Hinsch, Andreas; Gombert, Andreas

doi:10.1016/j.solmat.2006.10.020

Cited by 237 publications

(173 citation statements)

References 8 publications

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“…In an ideal diode and solar cell, R s = 0, R p = ∞, n = 1, and J ph = J ph (V ). Apart from non-zero R s and finite R p , some common deviations from an ideal diode observed for organic heterojunctions are the following: R p dependent on light intensity [28][29][30] as R p ∝ P −1 0 , R s sometimes decreases with increased light intensity causing a crossing of the light and dark curves at positive current densities, 31 n > 2, a "kink" in the J-V curve near open-circuit, [31][32][33] and a voltagedependent photocurrent. 28,29,32 The inverse and linear dependence of R p on P 0 suggests an increase in free charge carriers in the bulk produced either as a result of exciton annihilation transferring energy to trapped charges, 28,34 or from photoconductive gain, as these processes are dependent on exciton density which is directly proportional to P 0 .…”

Section: Photovoltaic Fundamentalsmentioning

confidence: 99%

“…The reduced R s which has been observed under light illumination likely comes from photoconductive gain in the organic film which decreases the resistance to carrier flow through the bulk film. The "kink" which has been reported near open-circuit remains a topic of continued research, but could be due to poor carrier extraction at the contacts 33 or to poor charge transfer at the donor/acceptor interface, 31,32 resulting in a voltagedependent photocurrent, a topic which will be discussed in Subsection "Photocurrent. "…”

Section: Photovoltaic Fundamentalsmentioning

confidence: 99%

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Solar cells utilizing small molecular weight organic semiconductors

Rand

Genoe

Heremans

et al. 2007

Progress in Photovoltaics

467

296

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In this review, we focus on the field of organic photovoltaic cells based on small molecular weight materials. In particular, we discuss the physical processes that lead to photocurrent generation in organic solar cells, as well as the various architectures employed to optimize device performance. These include the donor-acceptor heterojunction for efficient exciton dissociation, the exciton blocking layer, the mixed or bulk heterojunction, and the stacked or tandem cell. We show how the choice of materials with known energy levels and absorption spectra affect device performance, particularly the open-circuit voltage and short-circuit current density. We also discuss the typical materials and growth techniques used to fabricate devices, as well as the issue of device stability, all of which are critical for the commercialization of low-cost and high-performance organic solar cells.

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Section: Photovoltaic Fundamentalsmentioning

confidence: 99%

Section: Photovoltaic Fundamentalsmentioning

confidence: 99%

Solar cells utilizing small molecular weight organic semiconductors

Rand

Genoe

Heremans

et al. 2007

Progress in Photovoltaics

467

296

View full text Add to dashboard Cite

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“…The sporadic nature of the S curve, however, has prevented widespread discussion about it in the literature; we are aware of only a few papers that have investigated the origins of the S curve in polymer:fullerene BHJ solar cells. [11][12][13][14][15] In this Letter, we identify the cause of the S curve in P3HT:PCBM BHJ devices and provide a method to reproducibly eliminate the low fill factor through simple changes in the device architecture. Using the transient current technique known as photogeneration and charge extraction with a linearly increasing voltage ramp (photo-CELIV), 16 we first show that the S curve results from poor contact between the PCBM component of the BHJ and the cathode, which hinders electron extraction and leads to an imbalance in the rates at which holes and electrons are extracted from the active layer.…”

mentioning

confidence: 99%

Improving the Reproducibility of P3HT:PCBM Solar Cells by Controlling the PCBM/Cathode Interface

et al. 2009

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Plastic photovoltaic devices offer a real potential for making solar energy economically viable. Unfortunately, bulk heterojunction (BHJ) solar cells fabricated from blends of the commonly used materials poly(3-hexylthiophene), P3HT, and phenyl-C 61 -butyric acid methyl ester, PCBM, sometimes exhibit low efficiencies even when the procedures followed often produce solar cells with efficiencies exceeding 5%. In this Letter, we show that this irreproducibility is caused by subtleties in the film processing conditions that ultimately lead to poor electron extraction from the devices. For low-performing devices, photogeneration and charge extraction with a linearly increasing voltage ramp (photo-CELIV) measurements show an order-of-magnitude difference in the effective mobilities of the electrons and holes. Atomic force microscopy (AFM) experiments reveal that the top surface of these low-performing devices is nearly pure P3HT. We argue that small variations in the solvent evaporation kinetics during spin-coating of the BHJ active layer, which are difficult to control, cause PCBM to segregate toward the bottom of the P3HT film to different extents, explaining why electron extraction from the PCBM component of the BHJ is so difficult in poorly performing devices. Finally, we show that electron extraction can be greatly improved by spin-coating a thin PCBM layer on top of the BHJ before deposition of the cathode, allowing the reproducible fabrication of high-efficiency polymer solar cells.Organic photovoltaics based on bulk-heterojunction (BHJ) composites of conjugated polymers and fullerenes have shown rapid improvement in the past few years, 1,2 with power conversion efficiencies recently surpassing 6%.3 Although facile, solution-phase fabrication is one of the greatest advantages this class of solar cells has over its inorganic-based counterparts. The behavior of polymer/fullerene devices is sensitive to small variations in processing conditions; 4,5 for example, small changes in material blend ratios, 6 single-percent variations in the composition of the solvent used for spin-coating, 7,8 and changes in postfabrication treatments such as the time and/or temperature of thermal annealing 9 all can dramatically affect device performance. Perhaps even more troublesome, there is not always good reproducibility when different groups use the same processing recipe for producing polymer/fullerene thin-film photovoltaic devices, indicating that there are still processing parameters that we have not yet either correctly identified or properly learned to control in order to consistently optimize device performance.A prime example of this lack of reproducibility can be seen in BHJ devices fabricated from blends of the commonly used materials regioregular poly(3-hexylthiophene), P3HT, and phenyl-C 61 -butyric acid methyl ester, PCBM. BHJ solar cells fabricated from these materials can have power conversion efficiencies (PCEs) exceeding 5%, 9,10 but sometimes, cells fabricated with nominally identical processing conditions can ...

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“…These anomalous shapes have been interpreted in the past with low values of the surface recombination velocity S in the boundary relation J = −S (p A − p 0 ) [24,48], where p A is the hole density at the anode and p 0 its value at equilibrium (a similar relation can be written for the extraction of electrons at the cathode), or by the introduction of an insulator layer between the metal and organic layers that hinders the flow of the current.…”

Section: Adaptation Of the Boundary Condition Model For Oscs With Nonmentioning

confidence: 93%

“…Nevertheless, there exist large ranges of these variables in which the mobility can be assumed constant [48][49][50]. This assumption simplifies the complex numerical treatments as the ones found in OPV systems.…”

Section: Electrical Models For the Active Layer Of Oscsmentioning

confidence: 99%

Boundary condition model for the simulation of organic solar cells

López-Varo

Jiménez-Tejada

Marinov

et al. 2017

Organic Electronics

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Organic solar cells (OSCs) are promising photovoltaic devices to convert solar energy into electrical energy. Their many advantages such as lightweight, flexibility and low manufacturing costs are intrinsic to the organic/polymeric technology. However, because the performance of OSCs is still not competitive with inorganic solar cells, there is urgent need to improve the device performance using better designs, technologies and models. In this work, we focus on the developing an accurate physics-based model that relates the charge carrier density at the metal-organic boundaries with the current density in OSCs using our previous studies on singlecarrier and bipolar diodes. The model for the boundary condition of the charge carrier density at the interfaces of OSCs follows a power-law function with the current density, both in dark and under illumination, and simulated current-voltage characteristics are verified with experimental results. The numerical simulations of the current-voltage characteristics of OSCs consider wellestablished models for the main physical and optical processes that take place in the device: light absorption and generation of excitons, dissociation of excitons into free charge carriers, charge transport, recombination and injection-extraction of free carriers.Our analysis provides important insights on the influence of the metal-organic interfaces on the overall performance of OSCs. The model is also used to explain the anomalous S-shape current-voltage curves found in some experimental data.

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Efficiency limiting factors of organic bulk heterojunction solar cells identified by electrical impedance spectroscopy

Cited by 237 publications

References 8 publications

Solar cells utilizing small molecular weight organic semiconductors

Solar cells utilizing small molecular weight organic semiconductors

Improving the Reproducibility of P3HT:PCBM Solar Cells by Controlling the PCBM/Cathode Interface

Boundary condition model for the simulation of organic solar cells

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