Unique Curve for the Radiative Photovoltage Deficit Caused by the Urbach Tail

Bisquert, Juan

doi:10.1021/acs.jpclett.1c01935

Cited by 10 publications

(9 citation statements)

References 33 publications

(62 reference statements)

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“…This means that the relation between voltage loss and E U is independent of the absorber material’s bandgap in the case of E U < kT . Furthermore, recently, Bisquert reported a universal relation to quantify the photovoltage loss: Δ V OC rad = ( kT / q ) ln(1 – E U / kT ), for E U < kT . For the absorber materials with more disorder ( E U > kT ), the band-filling effect becomes important in order to evaluate the radiative limits on the V OC and to examine realistic device performances.…”

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confidence: 99%

Life on the Urbach Edge

Ugur

Ledinský

Allen

et al. 2022

J. Phys. Chem. Lett.

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The Urbach energy is an expression of the static and dynamic disorder in a semiconductor and is directly accessible via optical characterization techniques. The strength of this metric is that it elegantly captures the optoelectronic performance potential of a semiconductor in a single number. For solar cells, the Urbach energy is found to be predictive of a material's minimal open-circuit-voltage deficit. Performance calculations considering the Urbach energy give more realistic power conversion efficiency limits than from classical Shockley−Queisser considerations. The Urbach energy is often also found to correlate well with the Stokes shift and (inversely) with the carrier mobility of a semiconductor. Here, we discuss key features, underlying physics, measurement techniques, and implications for device fabrication, underlining the utility of this metric.

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confidence: 99%

Life on the Urbach Edge

Ugur

Ledinský

Allen

et al. 2022

J. Phys. Chem. Lett.

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“…The Urbach tail appears due to lattice disorder with introducing an exponential distribution of energy states near the optical band edges, 65 which eventually leads to energy loss by nonradiative charge recombination with sub-bandgap absorption. 66 Consequently, the increasingly generated defects in the perovskite film on NiO X are likely responsible for the severely deteriorated FF whereas the compressive strain induced by pTPD substrate effectively suppresses the defect formation and, thus, results in the consistent absorbance onset. 67,68 Normal XRD analysis was conducted for the aged full devices after finishing the long-term performance tracking for 911 h. The resulted XRD patterns are shown in Figure 5 and Figure S8 for full spectra.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…Especially, the absorption onset of the perovskite film on NiO X gets much broader as it is being aged, revealing a pronounced tailing of the absorption band. The Urbach tail appears due to lattice disorder with introducing an exponential distribution of energy states near the optical band edges, which eventually leads to energy loss by nonradiative charge recombination with sub-bandgap absorption . Consequently, the increasingly generated defects in the perovskite film on NiO X are likely responsible for the severely deteriorated FF whereas the compressive strain induced by pTPD substrate effectively suppresses the defect formation and, thus, results in the consistent absorbance onset. , …”

Section: Resultsmentioning

confidence: 99%

Enhanced Phase Stability of Compressive Strain-Induced Perovskite Crystals

Lee

Kim

2022

ACS Appl. Mater. Interfaces

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Control of strain in perovskite crystals has been considered as an effective strategy to ensure the phase stability of perovskite films where a compressive strain is particularly preferred over a tensile strain due to a lowered Gibbs free energy by the unit cell contraction effect. Here we adapt the strategy of strain control into perovskite solar cells in which the compressive strain is applied by utilizing a thermal expansion difference between the perovskite film and an adjacent layer. Poly(4-butylphenyldiphenylamine), with a higher thermal expansion coefficient compared to that of perovskite, is employed as a substrate for perovskite crystal growth at 100 °C, followed by cooling to room temperature. The applied compressive strain at the interface, as a result of a greater contraction of the polymer compared to the perovskite film, is confirmed by grazing incidence X-ray diffraction showing a red peak shift with increasing secondary angle. The compressive strain-induced perovskite film shows relatively constant absorbance spectra as a function of time. In the meantime, the absorbance spectra of a film without strain control exhibit a gradual decay with developing an Urbach tail. Importantly, the effect of strain engineering is remarkably prominent in the long-term photovoltaic performance. The photocurrent drops by 41% over 911 h without controlling strain, which is significantly improved by employing compressive strain, showing only a 6% drop in photocurrent from a shelf-stability test without encapsulation. It is also noted that an S-shaped kink appears in the current–voltage curves since 579-h-long storage for the device without strain control, leading to unreliable and overestimated fill factor and conversion efficiency. On the other hand, a 16% increase in fill factor with a stable performance is derived over 911 h from the compressive strain-induced device.

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“…It was demonstrated that this property leads to a rapid rise of the JDOS that is concurrent with a shortening of disorder correlations, which implies a sharpening of the distribution of band edges, a reduction of the number of thermally induced tail‐states and, by extension, a sharper optical absorption profile. Since the related Urbach tail determines the losses of open‐circuit voltage in the radiative limit, [ 78 ] this finding is potentially relevant for the design of solar materials. We have also contrasted these findings to the case of PbTe, a similarly anharmonic material also exhibiting resonant bonding and a similar electronic structure, but lacking the dynamic structural flexibility of the perovskite lattice as expressed in transversality.…”

Section: Discussionmentioning

confidence: 99%

Transversal Halide Motion Intensifies Band‐To‐Band Transitions in Halide Perovskites

et al. 2022

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Despite their puzzling vibrational characteristics that include strong signatures of anharmonicity and thermal disorder already around room temperature, halide perovskites (HaPs) exhibit favorable optoelectronic properties for applications in photovoltaics and beyond. Whether these vibrational properties are advantageous or detrimental to their optoelectronic properties remains, however, an important open question. Here, this issue is addressed by investigation of the finite-temperature optoelectronic properties in the prototypical cubic CsPbBr 3 , using first-principles molecular dynamics based on density-functional theory. It is shown that the dynamic flexibility associated with HaPs enables the so-called transversality, which manifests as a preference for large halide displacements perpendicular to the Pb-Br-Pb bonding axis. The authors find that transversality is concurrent with vibrational anharmonicity and leads to a rapid rise in the joint density of states, which is favorable for photovoltaics since this implies sharp optical absorption profiles. These findings are contrasted to the case of PbTe, a material that shares several key properties with CsPbBr 3 but cannot exhibit any transversality and, hence, is found to exhibit much wider band-edge distributions. The authors conclude that the dynamic structural flexibility in HaPs and their unusual vibrational characteristics might not just be a mere coincidence, but play active roles in establishing their favorable optoelectronic properties.

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Unique Curve for the Radiative Photovoltage Deficit Caused by the Urbach Tail

Cited by 10 publications

References 33 publications

Life on the Urbach Edge

Life on the Urbach Edge

Enhanced Phase Stability of Compressive Strain-Induced Perovskite Crystals

Transversal Halide Motion Intensifies Band‐To‐Band Transitions in Halide Perovskites

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