On the Relation between the Open‐Circuit Voltage and Quasi‐Fermi Level Splitting in Efficient Perovskite Solar Cells

Caprioglio, Pietro; Stolterfoht, Martin; Wolff, Christian; Unold, Thomas; Rech, Bernd; Albrecht, Steve; Neher, Dieter

doi:10.1002/aenm.201901631

Cited by 320 publications

(398 citation statements)

References 42 publications

(75 reference statements)

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“…With PEAI surface treatment, the V OC increased by 25 mV from 1.086 to 1.111 V and with tBBAI by 41 mV to 1.125 V compared to the control sample, which is in good agreement with the trend seen in the Δ E F measurements. The offset of ≈40 mV between the stabilized V OC and Δ E F / q can be explained by energetic misalignment at both interfaces as has been shown by Caprioglio et al17 This demonstrates the beneficial effect of PEAI and tBBAI treatment in preventing interfacial charge carrier recombination. Furthermore, it shows that tBBAI is a significantly better interface passivation agent than PEAI with a 65% higher Δ V OC increase compared to the sample without interface passivation layer.…”

Section: Photovoltaic Parameters Of Champion Devices With and Withoutsupporting

confidence: 62%

Tailored Amphiphilic Molecular Mitigators for Stable Perovskite Solar Cells with 23.5% Efficiency

et al. 2020

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Passivation of interfacial defects serves as an effective means to realize highly efficient and stable perovskite solar cells (PSCs). However, most molecular modulators currently used to mitigate such defects form poorly conductive aggregates at the perovskite interface with the charge collection layer, impeding the extraction of photogenerated charge carriers. Here, a judiciously engineered passivator, 4‐tert‐butyl‐benzylammonium iodide (tBBAI), is introduced, whose bulky tert‐butyl groups prevent the unwanted aggregation by steric repulsion. It is found that simple surface treatment with tBBAI significantly accelerates the charge extraction from the perovskite into the spiro‐OMeTAD hole‐transporter, while retarding the nonradiative charge carrier recombination. This boosts the power conversion efficiency (PCE) of the PSC from ≈20% to 23.5% reducing the hysteresis to barely detectable levels. Importantly, the tBBAI treatment raises the fill factor from 0.75 to the very high value of 0.82, which concurs with a decrease in the ideality factor from 1.72 to 1.34, confirming the suppression of radiation‐less carrier recombination. The tert‐butyl group also provides a hydrophobic umbrella protecting the perovskite film from attack by ambient moisture. As a result, the PSCs show excellent operational stability retaining over 95% of their initial PCE after 500 h full‐sun illumination under maximum‐power‐point tracking under continuous simulated solar irradiation.

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Section: Photovoltaic Parameters Of Champion Devices With and Withoutsupporting

confidence: 62%

Tailored Amphiphilic Molecular Mitigators for Stable Perovskite Solar Cells with 23.5% Efficiency

et al. 2020

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“…However, if a considerable amount of interface states is present at the CTL/active layer interface in question, an increase in Δ E maj leads to a corresponding exponential increase of surface recombination at this interface as well (which outweighs the benefits due to the increased carrier density in the HTL), resulting in increased V oc losses. [ 23,35 ]…”

Section: Resultsmentioning

confidence: 99%

On the Question of the Need for a Built‐In Potential in Perovskite Solar Cells

Sandberg

Kurpiers

Stolterfoht

et al. 2020

Adv Materials Inter

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charge generation over a wide and tunable range, [4] but also exhibit high carrier mobilities and long diffusion lengths up to several microns. [5][6][7] In any light harvesting device, appropriate contacts are critical to efficiently collect the photogenerated charges and deliver them to the external circuit. The contacts are responsible for providing the built-in asymmetry needed to create a driving force for the extraction of photogenerated carriers; [8] this built-in asymmetry can either be established by kinetic selectivity (diffusion-controlled) or by an energetic mismatch (drift-controlled) between the electrodes.The generic thin-film solar cell is composed of an active layer, sandwiched between a hole-extracting anode contact and an electron-extracting cathode contact. Under illumination, charge carriers generated within the active layer will drift-diffuse to the contacts and be extracted by the builtin asymmetry, resulting in the production of a net photocurrent. In organic solar cells, characterized by low carrier mobilities and short diffusion lengths, a strong built-in electric field across the active layer is necessary to enhance the charge extraction rate and avoid recombination. [9][10][11] This field is induced by the built-in potential V bi (or contact potential), originating from the work function difference between the anode and cathode and is largely unscreened due to the relatively low dielectric constants of organic semiconductors. Conversely, in perovskite solar cells, exhibiting carrier diffusion lengths of several microns, photogenerated charges should, in the absence of electric fields, be able to effortlessly traverse active layers of 200-500 nm without recombining. Subsequently, provided that kinetic selectivity at the contacts can be ensured, [12] the charge collection is expected to be diffusion controlled, [8,13] and a consensus is emerging along these lines. Kinetic selectivity is established by employing separate charge transport layers (CTLs) in-between the electrodes and the active layer, resulting in either an n-i-p or p-i-n type device architectures, with a hole transport layer (HTL, p-layer) at the anode and an electron transport layer (ETL, n-layer) at the cathode. In the ideal case, these layers are able to conduct majority carriers, while simultaneously preventing the extraction of minority carriers, thus creating a preferred direction for a diffusion-driven charge collection. Within this framework of charge extraction requirements, there is still some conjecture as to the exact role of the in-built potential and hence the precise nature of the driving force responsible for charge extraction.Perovskite semiconductors as the active materials in efficient solar cells exhibit free carrier diffusion lengths on the order of microns at low illumination fluxes and many hundreds of nanometers under 1 sun conditions. These lengthscales are significantly larger than typical junction thicknesses, and thus the carrier transport and charge collection should be expected to be diffusion ...

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“…Note that, as almost any analytical relation, Equation is based on a number of assumptions that are further detailed in previous studies . An important assumption is that the solar cell presents an ideal diode behavior, with its V oc being equal to the quasi‐Fermi level splitting (QFLS) . However, the V oc of actual solar cells may not follow such an ideal diode behavior and present V oc s lower than that predicted from Equation , which presents an upper limit for the achievable V oc .…”

Section: Introductionmentioning

confidence: 99%

“…However, the V oc of actual solar cells may not follow such an ideal diode behavior and present V oc s lower than that predicted from Equation , which presents an upper limit for the achievable V oc . Some known reasons for such deviations from Equation are shunt resistance, carrier recombination not following the diode equation (Supporting Information S1), losses at contacts, or strong energy‐level offsets at interfaces . In sum, there are several nonidealities that may contribute to making the V oc of actual solar cells depart from V oc,rad .…”

Section: Introductionmentioning

confidence: 99%

Relation between Fluorescence Quantum Yield and Open‐Circuit Voltage in Complete Perovskite Solar Cells

et al. 2020

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Bringing the Voc of a perovskite solar cell toward its radiative value, corresponding to a 100% external fluorescence quantum yield (QY) of the cell, has been pursued to reach the highest performance photovoltaic devices. Therefore, much research has been focused on maximizing the QY of the active layer isolated from the rest of the cell layers. However, such quantity does not often correlate with the Voc following the ideal diode relation. Herein, the QYs of complete FA0.8MA0.2PbI3−yBry solar cells are reported, ranging from 0.1% to 3%, and compared with their Vocs, ranging from 1 to 1.13 V. By combining these measurements with electromagnetic simulations based on a full‐wavevector detailed balance and a fluorescence power‐loss model, it is demonstrated that a nonoptimal Voc in mixed‐cation lead halide perovskite cells is not only due to nonradiative photocarrier recombination at traps. In addition to the expected parasitic absorption of the emitted photons in the electrode layers, discrepancies appear between Voc and QY. These discrepancies are attributed to the rise of energy barriers, a side effect of trap removal. Indeed, although surface passivation may enhance the QY, its beneficial effect may be counterbalanced by the emergence of such barriers between active and charge‐transporting layers.

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On the Relation between the Open‐Circuit Voltage and Quasi‐Fermi Level Splitting in Efficient Perovskite Solar Cells

Cited by 320 publications

References 42 publications

Tailored Amphiphilic Molecular Mitigators for Stable Perovskite Solar Cells with 23.5% Efficiency

Tailored Amphiphilic Molecular Mitigators for Stable Perovskite Solar Cells with 23.5% Efficiency

On the Question of the Need for a Built‐In Potential in Perovskite Solar Cells

Relation between Fluorescence Quantum Yield and Open‐Circuit Voltage in Complete Perovskite Solar Cells

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