Structural and optical properties of stoichiometric FAPbI3 films made from different fabrication methods.
(a) Excerpts from recorded X-ray diffraction (XRD) patterns focusing on the pseudocubic (100) peak of α-FAPbI3, the main peaks of the nonperovskite δ-FAPbI3 phase, (61) and the peak of PbI2 precursor residue. (62) Full XRD patterns are displayed in SI, Figure 1. (b) Absorption coefficient spectra (colored solid lines) and Elliott fits to the absorption onsets (dashed black lines) of FAPbI3 films fabricated via the “DMF-DMSO” (blue), “MACl route” (green), and “sequential” (yellow) solvent-based methods, recorded at 4, 160, and 295 K, respectively. The spectra recorded at different temperatures and for the different fabrication methods are vertically offset for visual clarity, with the black y-axis ticks marking the zero baseline for each successive curve. A direct comparison of the absorption coefficient spectra is presented in SI, Figure 7. (c) Peak features decoupled from the underlying, bulk-like absorption spectrum at 4, 160, and 295 K presented on the same scale. The spectra at different temperatures and for the different fabrication methods are vertically offset for visual clarity, with the black y-axis ticks marking the zero baseline for each successive curve. (d) Spectrally integrated area underneath the absorption peaks shown in (c), given as a percentage of the total area under the absorption coefficient spectrum, used to parametrize the occurrence of quantum confinement, and shown as a function of temperature for the differently fabricated FAPbI3 films. (The fabrication method legend in (a) also applies to (b), (c), and (d), and the temperature legend in (c) also applies to (b)).
We proceeded by probing how the three different fabrication approaches affect the occurrence of intrinsic quantum confinement in FAPbI3 films. For this purpose, we measured the absorption coefficient spectra at a range of different temperatures from 295 K down to 4 K (displayed in Figure 1b) in order to determine the fraction of the absorption spectrum that exhibits the peak features that have been associated with such confinement in the past. (50,51) While cryogenic temperatures are less relevant to photovoltaic operation, it has recently been shown that they enhance the sharpness and amplitudes of the absorption features, making them easier to discern and evaluate accurately. (50,51) To analyze the absorption coefficient spectra, we first fitted the onset with Elliott’s theory, which takes into account the excitonic contribution and Coulombic enhancement to the absorption spectrum (63,64) (see dashed lines in Figure 1b). These fits yielded parameters such as the optical bandgap and exciton binding energy (see SI, Figure 8) that are very similar in both value and temperature trends for the films made through the three fabrication routes, further highlighting that these films are compositionally similar. Similarly, room-temperature PL peak energies are found to be identical for all three types of FAPbI3 films (see SI, Figure 9). As a second step, to determine the contribution to the absorption spectrum arising from quantum confinement, we decoupled the above-bandgap absorption peak features from the underlying, smooth, bulk-like spectrum using the process previously reported. (50,51) We chose a phenomenological spline baseline connecting all the troughs of the features together and subtracted this baseline from the full absorption spectrum. The resulting decoupled peaks associated with intrinsic quantum confinement are shown in Figure 1c for different FAPbI3 film fabrication methods and temperatures. (50,51) As a numerical parameter characterizing the relative prominence of quantum confinement, we evaluated the area between the experimental absorption coefficient spectrum and the spline baseline connecting the troughs (i.e., the integral over the decoupled peak spectra such as those shown in Figure 1c) as a percentage of the overall area under the absorption spectrum. This “spectral area under the peaks” is shown in Figure 1d as a function of temperature for the FAPbI3 films processed by the three different methods.
Intriguingly, this analysis reveals that the fabrication method has a profound influence on the prominence of the absorption peak features, despite these all being stoichiometrically similar FAPbI3 films. While FAPbI3 films fabricated from the neat DMF-DMSO method exhibit very prominent absorption peak features, those features are over an order of magnitude lower in amplitude and hardly discernible for the films fabricated through the MACl and sequential deposition routes, even at very low temperatures down to 4 K (Figure 1c,d). These differences clearly point toward the crystallization process, the experienced strain in the system, and/or small inclusions of δ-phase or one of its polymorphs playing a significant role in the self-assembly of domains causing electronic confinement in the perovskite film. Such variations make sense in the context of quantum confinement in FAPbI3 having been proposed to be caused by inclusions of thin layers of δ-phase that act as electronic barrier to the α-phase, possibly to achieve periodic strain relief in the system. (50) Similarly, a range of different hexagonal polytypes have been shown to create peaked absorption features at various energies. (65) The presence of these peaked absorption features in FAPbI3 therefore appear to be a direct indicator of crystalline quality. The comparatively uncontrolled crystallization process associated with the neat DMF-DMSO fabrication method is known to result in poorer crystallographic quality of FAPbI3 films and an instability toward reversion into the δ-phase, (11,20,21,40,42) and these films exhibit strong absorption peak features. In contrast, the MACl additive provides intermediate-phase directed crystallization, which yields better control over growth dynamics, while the sequential deposition method nucleates α-phase growth, both of which lead to superior crystalline quality, and, as we show here, a significantly lower propensity toward formation of inferred quantum-confined domains. (55,66) We therefore demonstrate that thin-film nucleation and crystallization control can regulate and eliminate the quantum confinement experienced in FAPbI3.
The appearance of domains exhibiting electronic confinement has the potential to hinder charge-carrier extraction in solar cells. We thus proceeded by assessing the extent to which the presence of such absorption features affects the performance of photovoltaic devices incorporating FAPbI3 absorber layers fabricated using the neat DMF-DMSO, MACl, and sequential deposition routes. Devices were fabricated using a typical n-i-p configuration (FTO/SnO2/FAPbI3/PEAI/spiro-OMeTAD/Au). Here, the main difference in PV performance parameters should therefore be associated with the properties of each FAPbI3 photoabsorber layer. Accordingly, the steady-state short-circuit current density (JSC) of optimized devices can be treated as a parameter to gauge how efficient and unobstructed the charge-carrier motion is throughout the MHP layer. Full details of device fabrication and more detailed comparisons of the performance parameters for these devices are presented in section 7 in the SI. In addition, for each FAPbI3 film fabrication type, absorber layer thicknesses were optimized for best performance, yielding 340, 440, and 770 nm for the neat DMF-DMSO, MACl, and sequential deposition routes, respectively (see SEM images in SI, section 4, Figures 4–6). (58) While we acknowledge that generally, increasing the thickness of the perovskite layer will result in increased light absorption and hence, current generation, we note that the devices made with FAPbI3 films of similar thicknesses, processed with and without the MACl additive, still possessed a substantial difference in their steady-state JSC values (see SI, section 7). As such, we conclude that the thickness variations in the optimized devices do not affect our general conclusions.
A direct comparison of photovoltaic performance parameters for devices incorporating stoichiometric FAPbI3 films made with the three different fabrication methods is displayed in Figure 2 and SI, Figures 10–13. The PCEs and steady-state JSC values exhibit a notable improvement for FAPbI3 films fabricated through the MACl route and sequential deposition method compared with the neat DMF-DMSO method. We note that the MACl route and sequential deposition procedure have been shown to result in improved interfacial alignment with the employed transport layers and trap passivation (67−69) that are likely to be responsible for the slight increase in steady-state open-circuit voltage (VOC) of the corresponding devices we observe here (Figure 2b). However, such improvements would not be expected to also cause a significant enhancement to the charge-carrier extraction capabilities, and yet, devices incorporating films fabricated through the MACl and sequential deposition routes possess a substantially higher steady-state JSC than those made from the neat DMF-DMSO protocol. Instead, these improvements in steady-state JSC with the MACl route and sequential deposition approach appear to be directly correlated with the removal of the absorption peak features for these types of absorber layers we reported above. Such quantum confinement domains thus appear to induce significantly lower stabilsied JSC as a clear indication of poor charge-carrier extraction, consistent with the emergence of high-energy potential barriers obstructing transport across the film.
Structural and optical properties of stoichiometric FAPbI3 films made from different fabrication methods.
(a) Excerpts from recorded X-ray diffraction (XRD) patterns focusing on the pseudocubic (100) peak of α-FAPbI3, the main peaks of the nonperovskite δ-FAPbI3 phase, (61) and the peak of PbI2 precursor residue. (62) Full XRD patterns are displayed in SI, Figure 1. (b) Absorption coefficient spectra (colored solid lines) and Elliott fits to the absorption onsets (dashed black lines) of FAPbI3 films fabricated via the “DMF-DMSO” (blue), “MACl route” (green), and “sequential” (yellow) solvent-based methods, recorded at 4, 160, and 295 K, respectively. The spectra recorded at different temperatures and for the different fabrication methods are vertically offset for visual clarity, with the black y-axis ticks marking the zero baseline for each successive curve. A direct comparison of the absorption coefficient spectra is presented in SI, Figure 7. (c) Peak features decoupled from the underlying, bulk-like absorption spectrum at 4, 160, and 295 K presented on the same scale. The spectra at different temperatures and for the different fabrication methods are vertically offset for visual clarity, with the black y-axis ticks marking the zero baseline for each successive curve. (d) Spectrally integrated area underneath the absorption peaks shown in (c), given as a percentage of the total area under the absorption coefficient spectrum, used to parametrize the occurrence of quantum confinement, and shown as a function of temperature for the differently fabricated FAPbI3 films. (The fabrication method legend in (a) also applies to (b), (c), and (d), and the temperature legend in (c) also applies to (b)).
We proceeded by probing how the three different fabrication approaches affect the occurrence of intrinsic quantum confinement in FAPbI3 films. For this purpose, we measured the absorption coefficient spectra at a range of different temperatures from 295 K down to 4 K (displayed in Figure 1b) in order to determine the fraction of the absorption spectrum that exhibits the peak features that have been associated with such confinement in the past. (50,51) While cryogenic temperatures are less relevant to photovoltaic operation, it has recently been shown that they enhance the sharpness and amplitudes of the absorption features, making them easier to discern and evaluate accurately. (50,51) To analyze the absorption coefficient spectra, we first fitted the onset with Elliott’s theory, which takes into account the excitonic contribution and Coulombic enhancement to the absorption spectrum (63,64) (see dashed lines in Figure 1b). These fits yielded parameters such as the optical bandgap and exciton binding energy (see SI, Figure 8) that are very similar in both value and temperature trends for the films made through the three fabrication routes, further highlighting that these films are compositionally similar. Similarly, room-temperature PL peak energies are found to be identical for all three types of FAPbI3 films (see SI, Figure 9). As a second step, to determine the contribution to the absorption spectrum arising from quantum confinement, we decoupled the above-bandgap absorption peak features from the underlying, smooth, bulk-like spectrum using the process previously reported. (50,51) We chose a phenomenological spline baseline connecting all the troughs of the features together and subtracted this baseline from the full absorption spectrum. The resulting decoupled peaks associated with intrinsic quantum confinement are shown in Figure 1c for different FAPbI3 film fabrication methods and temperatures. (50,51) As a numerical parameter characterizing the relative prominence of quantum confinement, we evaluated the area between the experimental absorption coefficient spectrum and the spline baseline connecting the troughs (i.e., the integral over the decoupled peak spectra such as those shown in Figure 1c) as a percentage of the overall area under the absorption spectrum. This “spectral area under the peaks” is shown in Figure 1d as a function of temperature for the FAPbI3 films processed by the three different methods.
Intriguingly, this analysis reveals that the fabrication method has a profound influence on the prominence of the absorption peak features, despite these all being stoichiometrically similar FAPbI3 films. While FAPbI3 films fabricated from the neat DMF-DMSO method exhibit very prominent absorption peak features, those features are over an order of magnitude lower in amplitude and hardly discernible for the films fabricated through the MACl and sequential deposition routes, even at very low temperatures down to 4 K (Figure 1c,d). These differences clearly point toward the crystallization process, the experienced strain in the system, and/or small inclusions of δ-phase or one of its polymorphs playing a significant role in the self-assembly of domains causing electronic confinement in the perovskite film. Such variations make sense in the context of quantum confinement in FAPbI3 having been proposed to be caused by inclusions of thin layers of δ-phase that act as electronic barrier to the α-phase, possibly to achieve periodic strain relief in the system. (50) Similarly, a range of different hexagonal polytypes have been shown to create peaked absorption features at various energies. (65) The presence of these peaked absorption features in FAPbI3 therefore appear to be a direct indicator of crystalline quality. The comparatively uncontrolled crystallization process associated with the neat DMF-DMSO fabrication method is known to result in poorer crystallographic quality of FAPbI3 films and an instability toward reversion into the δ-phase, (11,20,21,40,42) and these films exhibit strong absorption peak features. In contrast, the MACl additive provides intermediate-phase directed crystallization, which yields better control over growth dynamics, while the sequential deposition method nucleates α-phase growth, both of which lead to superior crystalline quality, and, as we show here, a significantly lower propensity toward formation of inferred quantum-confined domains. (55,66) We therefore demonstrate that thin-film nucleation and crystallization control can regulate and eliminate the quantum confinement experienced in FAPbI3.
The appearance of domains exhibiting electronic confinement has the potential to hinder charge-carrier extraction in solar cells. We thus proceeded by assessing the extent to which the presence of such absorption features affects the performance of photovoltaic devices incorporating FAPbI3 absorber layers fabricated using the neat DMF-DMSO, MACl, and sequential deposition routes. Devices were fabricated using a typical n-i-p configuration (FTO/SnO2/FAPbI3/PEAI/spiro-OMeTAD/Au). Here, the main difference in PV performance parameters should therefore be associated with the properties of each FAPbI3 photoabsorber layer. Accordingly, the steady-state short-circuit current density (JSC) of optimized devices can be treated as a parameter to gauge how efficient and unobstructed the charge-carrier motion is throughout the MHP layer. Full details of device fabrication and more detailed comparisons of the performance parameters for these devices are presented in section 7 in the SI. In addition, for each FAPbI3 film fabrication type, absorber layer thicknesses were optimized for best performance, yielding 340, 440, and 770 nm for the neat DMF-DMSO, MACl, and sequential deposition routes, respectively (see SEM images in SI, section 4, Figures 4–6). (58) While we acknowledge that generally, increasing the thickness of the perovskite layer will result in increased light absorption and hence, current generation, we note that the devices made with FAPbI3 films of similar thicknesses, processed with and without the MACl additive, still possessed a substantial difference in their steady-state JSC values (see SI, section 7). As such, we conclude that the thickness variations in the optimized devices do not affect our general conclusions.
A direct comparison of photovoltaic performance parameters for devices incorporating stoichiometric FAPbI3 films made with the three different fabrication methods is displayed in Figure 2 and SI, Figures 10–13. The PCEs and steady-state JSC values exhibit a notable improvement for FAPbI3 films fabricated through the MACl route and sequential deposition method compared with the neat DMF-DMSO method. We note that the MACl route and sequential deposition procedure have been shown to result in improved interfacial alignment with the employed transport layers and trap passivation (67−69) that are likely to be responsible for the slight increase in steady-state open-circuit voltage (VOC) of the corresponding devices we observe here (Figure 2b). However, such improvements would not be expected to also cause a significant enhancement to the charge-carrier extraction capabilities, and yet, devices incorporating films fabricated through the MACl and sequential deposition routes possess a substantially higher steady-state JSC than those made from the neat DMF-DMSO protocol. Instead, these improvements in steady-state JSC with the MACl route and sequential deposition approach appear to be directly correlated with the removal of the absorption peak features for these types of absorber layers we reported above. Such quantum confinement domains thus appear to induce significantly lower stabilsied JSC as a clear indication of poor charge-carrier extraction, consistent with the emergence of high-energy potential barriers obstructing transport across the film.
Performance parameters for photovoltaic devices incorporating FAPbI3 films made from different fabrication methods.
Box and scatter plots showing the steady-state photovoltaic parameters for devices based on FAPbI3 absorber layers fabricated through the “DMF-DMSO” (blue), “MACl route” (green), and “sequential” (yellow) solvent-based methods. (a) Maximum power point tracked power conversion efficiency (ηmpp). (b) Steady-state open-circuit voltage (VOC). (c) Steady-state short-circuit current density JSC. The scatter points represent the different device pixels investigated. A comparison of the performance parameters for photovoltaic devices incorporating FAPbI3 films of different thicknesses fabricated via the “DMF-DMSO” route is displayed in SI, Figure 14.
Our analysis suggests that the quantum confinement present in FAPbI3-rich films heavily depends on the fabrication method and is detrimental to the performance of the associated photovoltaic device. To examine whether these findings hold more generally beyond the investigated fabrication methods, we conducted a meta-analysis across a wide range of literature reports. The meta-study was carried out by correlating reported PCE values of FAPbI3 solar cells with the presence or absence of quantum confinement, as determined from the analysis described and used above of the presence of peak features in the absorption spectra of corresponding films. Selection was effectively limited to devices whose photoactive layer was reported to be FAPbI3 three-dimensional perovskite thin films, i.e., we excluded any reports where additives or treatments were proven to be fully incorporated into the crystal structure such that they changed the stoichiometry. As a result, 244 unique publications were included in the meta-study, and a total of 825 device performance reports, providing statistically significant correlation data between PV performance and the presence of quantum confinement (QC). The PV performance data were either the batch-averages across many reported devices, or data corresponding to the champion cell if an average was not reported. Overall, the very large number of devices and published reports on the FAPbI3 composition (see also Figure 3j) is directly reflective of the historical and continued interest in this thermally stable composition. To assess the correlation between photovoltaic performance and the presence of quantum confinement, we divided the parameters associated with different devices into three unique categories. First, solar cells whose corresponding photoabsorber layer either possessed easily discernible above-bandgap absorption features or features above a certain minimum threshold were labeled “QC present” and amounted to 100 devices. Second, solar cells with no apparent visible features or features below the threshold were labeled “QC absent” and amounted to 302 devices. Finally, when absorption spectra were absent in the publication reporting the PV performance, or were too noisy to allow conclusion of presence or absence of features, this was labeled “QC data unavailable” and amounted to 423 devices. We note that only about half (48.7%) of the reported device performance parameters come with sufficient absorption data. While this still provides us with ample available data (402 device reports) to retrieve statistically significant correlations, it highlights that insufficient attention has been devoted to these absorption features and their relevance to photovoltaic device performance to date. Full details on the nuances of our meta-study can be found in SI, section 8.
Meta-analysis of photovoltaic device performance and presence of absorbance features for FAPbI3 thin films extracted from literature. (a) Reported PCE values of single-junction solar cells whose photoabsorber layers compromise stoichiometric bulk FAPbI3 thin films, plotted as a function of publication year, visualized for three categories of FAPbI3 films; films with above-bandgap “quantum confinement (QC)” absorption features present (labeled as “QC present”, blue), films for which absorption spectra were unavailable or of insufficient quality or spectral range (labeled as “QC data unavailable”, gray), and films for which absorption data showed no discernible peak features, or features below a certain low threshold (labeled as “QC absent”, orange); see SI, section 8, for full details. (b–d) The total time-integrated device count for each PCE range across the “QC present”, “QC data unavailable”, and “QC absent” categories, respectively. (e) The relative contribution of each category to the available device data for every PCE range, integrated over the years of publication. (f,g) Running average of PCE values recorded for all devices reported in literature, and for the best 10% of devices, respectively, for each category with an averaging window of one year. (h) The time evolution of the absorber film category associated with the highest PCE FAPbI3 devices. (i) The time evolution of the highest PCE device for each absorber film category. (j) Distribution of the number of unique publications with data on solar cell performance for FAPbI3 films as a function of publication year divided into the three absorber film categories. The color legend in (a) also applies to b–j).
The results from our meta-analysis are visualized in a range of different scatter plots in Figure 3 and SI, Figures 15–24. At first sight, it is apparent that PCE values of FAPbI3 solar cells follow a clear upward trajectory of continuous enhancement of PV properties over time, culminating in the highest certified PCE for a MHP single junction of 25.7% (3,17) for this composition (Figure 3a). Examining device performance according to the three categories of quantum confinement (present, absent, data unavailable), it also becomes clear that such QC effects have been present or absent since the first realization of working FAPbI3 PV devices, and that the simple running average of the PCE follows a similar upward trajectory (see Figure 3f). This unrelenting enhancement demonstrates the intense interest invested by the field in this composition and highlights the success of current research in tackling some major hurdles such as reducing defect concentration, improving crystallinity, interface quality, and charge extraction layers. However, more recently, noticeable differences have started to emerge as devices are breaking through the 20% PCE ceiling, reflective of a region where quantum confinement appears to become the decisive factor.
Such evidence for QC features in absorption restricting performance near the Shockley–Queisser limit can be seen in the meta-data in a number of forms. First, Figure 3a shows that since 2019, any meaningful progress in PCE values of devices in the “QC present” category (blue dots) has stalled, with a maximum of only 20.05% attained, compared with 25.06% for the “QC absent” and 25.18% for the “QC data unavailable” categories, suggesting that an intrinsic barrier exists for efficient charge-carrier extraction and PV performance for devices employing films possessing these peculiar QC features. The time-integrated PCE spread in Figure 3b,c,d clearly highlights an abundance of devices in the “QC absent” and “QC data unavailable” categories, with PCE values regularly exceeding the 20% mark, while devices in the “QC present” category very rarely approach that threshold. This is further illustrated in Figures 3h,i, which show that both the “QC absent” and “QC data unavailable” categories also dominate the champion performance since 2014. Finally, Figure 3g provides a temporal evolution of the running average of the best 10% of all devices within each of the three categories showing that even for such averages across the top end of the performance spectrum, the “QC absent” and “QC data unavailable” categories clearly outrun the performance of devices in the “QC present” category. Overall, these meta-data clearly reflect the stagnation in the PV enhancement of the devices in the “QC present” category and the futility of attempts to improve the PCE of a device beyond 20% without attempting to suppress these absorption peak features. Based on this established correlation, a necessary but insufficient condition to obtain devices with efficiencies exceeding ∼20% is to ensure the absence of any above-bandgap QC features in the absorption spectrum. We note that, in fact, most of the recent literature-reported attempts to alter the fabrication method of this promising composition with the aim of optimizing its photovoltaic performance have inadvertently been suppressing these peculiar absorption features. (38,47,57,70−79) However, the lack of comment on such features in literature reports, and an absence of absorption spectra in over half of the reports we examined, clearly show that little attention has to date been paid to this simple design and validation criterion. Accordingly, we propose that a first point for investigating a new fabrication protocol for FAPbI3 films to be incorporated into solar cells should be to discern the presence or absence of these peak features in the thin-film absorption spectrum. Such simple assessment can ascertain whether there is a tendency for the fabricated FAPbI3 films to include electronic barriers that are too thin to be detectable in XRD patterns, but that may still inhibit charge-carrier extraction and whose elimination will thus pave the way for high-performance solar cells.
In conclusion, our work has shown that to realize the highest performing solar cells based on the attractive FAPbI3 perovskite, quantum confinement needs to be fully eliminated from within the thin-film layer, most likely because it impedes charge-carrier transport and hence restrains the maximum attainable steady-state JSC values. We have conclusively demonstrated this finding through two approaches. First, we have examined a set of stoichiometric FAPbI3 films with varying degrees of quantum confinement induced by means of three different solution-based film fabrication processes. Those methods capable of reducing the strain, directing and controlling crystallization, and thus increasing perovskite crystalline quality strongly suppressed the quantum confinement features and clearly enhanced the solar cell performance parameters, in particular, obtainable steady-state JSC values. Second, we have performed a meta-analysis across 244 publications, analyzing 825 occurrences of PCE data from FAPbI3 solar cells in order to search for correlations with the presence or absence of the quantum confinement features evident from peaks in the matching absorption spectra. Our meta-study reveals that for FAPbI3 solar cells to break through the 20% PCE ceiling, quantum confinement effects should be absent from the film.
These conclusions clearly highlight simple checks for peaked absorption features as an easy step in the development of new thin-film fabrication procedures aimed at delivering efficient photovoltaic cells based on FAPbI3. We note that the presence or absence of phase impurities, such as the δ-phase, in XRD patterns is unlikely to be as conclusive because even FAPbI3 films with strong absorption peak features have at times been found to display no discernible δ-phase peak in XRD. (50) These electronic barriers may thus offer too few lattice planes to allow sufficiently sharp deflection of X-rays, leading to insufficiently sharp XRD peaks that cannot easily be detected. The alternative criterion we propose here, i.e., an examination of the absorption spectrum, is a simple and experimentally straightforward approach already accessible in the vast majority of research laboratories. This approach should therefore be an easy and powerful means to gauge the effectiveness of any newly developed treatment method for FAPbI3 thin-film absorber layers. Overall, our findings will accelerate power conversion efficiencies of the most commercially viable MHP on the last stretch toward the maximum theoretical attainable efficiencies for single-junction photovoltaic devices.
Corresponding Author
Laura M. Herz - Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom; Institute for Advanced Study, Technical University of Munich, Lichtenbergstrasse 2a, D-85748 Garching, Germany; Orcidhttps://orcid.org/0000-0001-9621-334X;
Karim A. Elmestekawy - Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom; Orcidhttps://orcid.org/0000-0002-7707-1611
Benjamin M. Gallant - Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom; Orcidhttps://orcid.org/0000-0001-7413-291X
Adam D. Wright - Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom; Orcidhttps://orcid.org/0000-0003-0721-7854
Philippe Holzhey - Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom; Orcidhttps://orcid.org/0000-0003-3688-1607
Nakita K. Noel - Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom; Orcidhttps://orcid.org/0000-0002-8570-479X
Michael B. Johnston - Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom; Orcidhttps://orcid.org/0000-0002-0301-8033
Henry J. Snaith - Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom; Orcidhttps://orcid.org/0000-0001-8511-790X
Notes
The authors declare the following competing financial interest(s): Henry Snaith is cofounder and CSO of Oxford PV ltd, a company commercializing perovskite PV technology.
References
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