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Tailoring of Electron Collecting Oxide NanoParticulate Layer for Flexible Perovskite Solar Cells Seong Sik Shin, Woon-Seok Yang, Eun Joo Yeom, Seon Joo Lee, Nam Joong Jeon, Young-Chang Joo, Jun Hong Noh, Sang Il Seok, and Ik Jae Park J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00295 • Publication Date (Web): 27 Apr 2016 Downloaded from http://pubs.acs.org on May 3, 2016

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The Journal of Physical Chemistry Letters

Tailoring of Electron Collecting Oxide Nano-Particulate Layer for Flexible Perovskite Solar Cells Seong Sik Shin1,2,§, Woon Seok Yang1,3,§, Eun Joo Yeom1,4, Seon Joo Lee1, Nam Joong Jeon1 Young-Chang Joo 2, Ik Jae Park2, Jun Hong Noh1,* and Sang Il Seok1,3,* 1

Division of Advanced Materials, Korea Research Institute of Chemical Technology, 4 Gajeong-Ro, Yuseong-Gu, Daejeon 305-600, Korea

2

Department of Materials Science and Engineering, Seoul National University, Seoul 151744, Korea 3

School of Energy and Chemical Engineering, Ulsan National Institute of Science and

Technology (UNIST), 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 689-798, Korea. 4

Department of Energy Science, 2066 Seoburo, Jangan-gu, Sungkyunkwan University, Suwon 440-746, Republic of Korea

ACS Paragon Plus Environment


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ABSTRACT: Low-temperature-processed perovskite solar cells (PSCs), especially those fabricated on flexible substrates, exhibit worse device performance than high-temperatureprocessed PSCs. One of main reasons for the inferior performance of low-temperatureprocessed PSCs is the loss of photo-generated electrons in the electron collection layer (ECL) or related interfaces, i.e., Indium Tin Oxide (ITO)/ECL and ECL/perovskite. Here, we report that tailoring of energy level and electron transporting ability in oxide ECL using Zn2SnO4 (ZSO) nanoparticles (NPs) and quantum dots (QDs) notably minimizes the loss of photogenerated electrons in the low-temperature fabricated flexible PSC. The proposed ECL with methylammonium lead halide (MAPb(I0.9Br0.1)3) leads to fabrication of significantly improved flexible PSCs with steady-state power conversion efficiency (PCE) of 16.0 % under air-mass 1.5 global (AM 1.5G) illumination of 100 mW cm–2 in intensity. These results provide an effective method for fabricating high performance, low temperature solutionprocessed flexible PSCs.


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Since 2009, hybrid inorganic/organic perovskite solar cells (PSCs) have received much attention for their potentially low fabrication cost and high power conversion efficiency (PCE).1,2 Recently, we have reported a certified PCE of PSCs exceeding 20 % for rigid devices.3 With the tremendous progress in PCEs over the last few years, the developemnt of flexible PSC has been a big challenge for the high-throughput production such as roll-to-roll (RtR) processes which can realize the great potential of PSCs.4-6 However, flexible PSCs fabricated using low temperature processes around 100 oC have exhibited lower device performance (~ 15 %) than rigid PSCs using extreme processing condition over 500 oC that allows to deposit high-quality oxide semiconducting electron collection layers (ECLs).7-9 For improving photovolatic performance in flexible PSCs processed around 100 oC, challenging ECL issues rasied by adopting a low temperautre process and a flexible substrate should be addressed. Although several ECL layers have been reported by vacuum processes at low temperature such as atomic layer deposition (ALD) and sputtering, solution processable ECL materials should be developed for processing consistency in RtR process with mass-production.4,9 In the operation of a PSC, the perovskite layer absorbs photon and generates excited electrons and holes. Subsequently, the excited photoelectrons reach the electron-collecting electrode (i.e., indium tin oxide called ITO) through the ECL with two relevant interfaces of perovskite/ECL and ITO/ECL. Therefore, the ECL in flexible PSCs should be designed such that it minimizes the electron transport resistance within ECL itself and the electron transfer barrier at two interfaces of the perovskite/ECL and ECL/ITO. Hence, the designed ECL should overcome inferior crystallinity and connection between grains in conventionally low-temperature processed oxide semiconductor layer such as TiO2 and ZnO ECLs.10-13 In addition, the energy level management of ECL should be considered to minimize Schottky barrier between ECL and transparent electrode in flexible substrate (i.e. ITO/ECL interface) and to maximize built-in potential between ECL and peroskite (i.e.



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perovskite/ECL interface). Hence, alternative oxides to TiO2, ZnO have been exploited to adjust electrical and opitical properties such as mobility, carrier concentration, band gap, band energy level, reflactive index considering low temperature process and transparent electrode on flexible substrate.14 However, it is difficult to find new oxide materials that satisfies all electrical and opitical requirments for flexible PSCs becasue a few n-type semiconducting oxide candidates are available to use in PSCs. Quantum dot (QD) is a representive example in solution processable nano-particulate layer applications at low-temperature to control material functionality by modulation of size instead of change in material composition.15 The quantum size effect has been already sucessfully applied to modulate band gap in calcogenites such as PbS, CdSe for optoelectric devices.16-18 Moreover, Sargent group have reported PbS based solar cells with quantum funnels using sequence of layers comprised of QDs selected to have different diameters for minimizing charge collecting loss.19 Therefore, to use semiconducting oxide QD might be a great strategy to design suitable ECLs for efficent flexible PSCs by delicately tunning electrical and optical properties. Here, we propose a general strategy using oxide QDs to design energy level-graded oxide ECL for efficient flexible PSCs. The developed ECL structure leads to natural electron flow from perovskite to ITO by tailoring energy level within ECL using quantum confined oxide QDs. Zinc stannate (Zn2SnO4, detonated by ZSO) was selected as a model semiconducting oxide for QDs due to its most outstanding semiconducting and optical properties for flexible PCSs.14 The energy level of ZSO nanoparticles (NPs) was successfully tuned by controlling their particle sizes, ZSO NPs under diameter around 10 nm exhibit quantum confinement effects, meaning those are ZSO QDs. Relatively large ZSO NPs are beneficial for electron transport within ZSO ECL and electron transfer at the ZSO/ITO interface owing to larger crystalline size and higher work function than ZSO QDs,


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representatively. On the contrary, ZSO QDs are favorable for leading to a large built in potential (Vbi) at the ZSO QDs/perovskite interface owing to their low work function.18 Because the decrease in loss of photo-generated electrons results in an increase of shortcircuit current density (Jsc) and fill factor (FF) as well as the increase in Vbi leads to high open-circuit voltage (Voc) in PSCs, ZSO QDs and NPs can be combined to design effective ECLs to simultaneously enhance Jsc, FF, and Voc. We found that the ECL designed by combining the features of ZSO QDs and NPs is highly beneficial for improving the overall performance of flexible PSCs. The ECL enabled the fabrication of a flexible PSC with a steady-state PCE of 16 % under one-sun illumination with high reproducibility. This is the highest PCE ever reported for flexible PSCs. Figure 1 shows a schematic illustration of the strategy using quantum size effect to fabricate efficient flexible PSCs.18,19 To effectively collect photo-generated electrons, the oxide ECL and related interfaces were controlled by consecutively depositing multi- layers of oxide NPs and QDs. At ITO/ZSO interface, highly crystalline ZSO NPs with a high work function on ITO can reduce the height of the Schottky barrier at the ITO/ZSO interface compared to the ECL with a low work function. Furthermore, their large size and high crystallinity can facilitate electron transport between ZSO nanoparticles, particularly in the case of using a low-temperature process. At ZSO/perovskite interface, the introduction of ZSO QDs with a low work function can lead to a large value of Vbi at the ZSO/perovskite interface, which improves the output voltage of the PSCs.18 The combination of two different ZSO layers can minimize the loss of photo-generated electrons that is caused by a lowtemperature-fabricated ECL and related interfaces, and effectively mitigate the problems associated with flexible PSCs. To demonstrate our strategy, first, ZSO NPs and QDs were synthesized using hydrazineassisted hydrothermal method from 120 to 200 °C.14 The detailed procedure is provided in



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the experimental section. Figure 2a compares the X-ray diffraction (XRD) patterns of powders synthesized at various temperatures (120 - 200 °C). All patterns closely match with those of the ZSO phase of the inverse spinel structure (JCPDS 24-1470). Furthermore, the secondary phase such as SnO2 and ZnO is not observed even at a low reaction temperature (120 oC). Our proposed synthetic method enables the formation of new intermediate complex, which can yield the pure Zn2SnO4 even in low temperature below 100 oC.14 As the reaction temperature rises from 120 to 200 °C, the full width of half maximum (FWHM) of the (311) diffraction peak at 2θ=34.3 ° decreases, indicating an increase in particle size. Figure 2b presents the particles size distribution and the average size of ZSO synthesized at various reaction temperature from 120 to 200 oC where the particle size was estimated from TEM images (Figure S1). At initial temperature, the synthesized particles show an average particle size of 5.7 nm with narrow size distribution. As the reaction temperature increases, the average size gradually increases: 140 oC: 9.3 nm, and 200 oC: 19.2 nm. To understand the relationship between the particle size and the energy level, the optical absorbance of synthesized ZSO particles is investigated using UV-Vis absorption measurement. As shown in Figure S2, the UV-Vis spectra have a clear blue shift with decreasing the size of the ZSO particles. To determine the band gap more exactly, the optical band gaps were estimated using the obtained UV-Vis spectra and Tauc’s formula for directband gap semiconductor, (ανh)2= A(hν-Eg),20 where α, ν, and Eg are the absorption coefficient, frequency, and band gap, respectively. The calculated band gaps (Figure 2c) increase from 3.79 to 3.99 eV with a decrease in particle size from 19.2 to 5.7 nm. Interestingly, the band gap energy of 3.99 eV at 5.7 nm and 3.85 eV at 9.3 nm is significantly larger than the previously reported band gap values (3.7-3.8 eV),20,21 which can be attributed to a quantum size effect.20 Consequently, ZSO QDs were successfully synthesized by the hydrazine-assisted hydrothermal method under 140 oC. Here, we denoted as QD-1, QD-2 and


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NP for the 5.7, 9.3 and 19.2 nm particles, respectively. Figure 2d presents the representative transmission electron microscopy (TEM) images of the synthesized ZSO QD-1. As shown in TEM image, the synthesized QDs are well dispersed in 2-methoxy ethanol without dispersing agent and have good crystallinity with lattice fringe spacing of 2.62 Å corresponding to the interlayer spacing of the (311) of Zn2SnO4 (inset in Figure 2d). Furthermore, the ZSO films composed of ZSO NPs or QDs (QD-1 and QD-2) were fabricated on ITO glass using continuous six-cycle spin-coating. As shown in Figure S3, the resultant ZSO films on ITO substrate have high transmittance above 80 %. The junction property of ZSO film fabricated with perovskite was evaluated to design the efficient ECLs in flexible PSCs. Capacitance-Volatage (C-V) measurement have been widely adopted to estimate built in potential (Vbi) at a n-p heterojunction in nano-particulate layered devices such as the TiO2 nanoparticle/perovskite and TiO2/PbS QDs layer solar cells.19,22,23 In the n-p heterojunction solar cells, the Vbi is a key factor for charge separation, charge transport, and charge collection, which is considered as the origin of the output voltage of solar cells.24,25,26 In PSCs, many groups have also observed the generation of the electrical field (i.e., Vbi) at the heterojunction between n or p-type material and perovskite interface.27,28. Generally, the Vbi corresponds to the difference in the work function (Fermi level) between the two sides (ECL and perovskite).28 Therefore, the variation of energy level for each material influences on junction property, namely Vbi. To understand the junction property at various sized ZSO/perovskite interface, capacitance-voltage (C-V) measurment for ZSO/perovskite/Au devices (inset in Figure 2e) was performed and their Mott-Schottky plots (C-2 vs. V) were represented in Figure 2e. Plateau of the Mott-Schottky plots below 0.4 V reveals that the devices are fully depleted. According to previous report, the depletion is dominant in perovskite layer because the carrier concentration of a metal oxide n-type layer is much higher than that of perovskite layer.28 Thererefore, the Vbi developed in the perovskite



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layer by the ZSO QD-1/perovskite, QD-2/perovskite and NP/perovskite junctions can be evaluated from the x-intercept of the linear regime in Mott-Schottky plot (Figure 2e). As shown in Figure 2f, the reduction of particle size increases the Vbi at ZSO/perovskite junction. Especially, the Vbi at the ZSO QD-1/perovskite junction is approximately 0.1 V higher than that at the ZSO NP/perovskite junction. Generally, a decrease in particle size causes an increase in the band gap (Figure 2c), i.e., a negative shift of the conduction band edge, which can accompany Fermi level shift to more negative potential. Therefore, a large Vbi can be obtained at the ZSO QD/perovskite junction. To confirm the innate characteristics of ZSO QD-1, QD-2, and NP on device properties, their device performance was evaluated (Figure S4 and Figure 3). For experimental convenience, a rigid substrate was used before using the flexible substrate. Figure S5 shows representative scanning electron microscopy (SEM) images of the device architecture for NPs film and QDs film. The deposited ZSO films present uniform and flat morphology with a thickness of 110±10 nm, which enables the formation of the dense and uniform perovskite film with a thickness of 380±10 nm using our solvent-engineering technique.29 Figure 3a shows the representative J-V curves for ZSO QD-1, QD-2 and NP based PSCs. The photovoltaic parameters are summarized in Table 1. The ZSO NP based PSC produces a higher PCE of 16.2 %, than the QD-1 and QD-2 based PSC showing PCE of 11.2 % and 14.0 %. With the increase in particle size (from 5.7

to 19.2 nm), the Jsc increased from 20.2

to 21.9 mA/cm2. In the case of the low-temperature process (below 100 oC), the particle size and crystallinity have a strong effect on electron collection because interconnectivity (the necking of NPs) between individual NPs is insufficient. Therefore, highly crystalline, relatively large NPs are advantageous for electron collection, especially for low-temperatureprocessed films, which is one of the major contributions to enhanced Jsc values.10 However, when ZSO QDs are employed as an ECL, the large number of grain boundaries between


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particles are created, which can disturb carrier transport across the film, resulting in the loss of photogenerated electrons and thus a low value of Jsc. Furthermore, the FF of the ZSO NP based PSC shows a significant increase up to 73.4 %, compared to the ZSO QD-1 and QD-2 based PSC. In order to investigate the effect of particle size on FF, impedance spectroscopy (IS) analysis was performed under 1 sun illumination. Figure 3b presents the Nyquist plots of ZSO based PSCs. The semicircle in the high frequency region results from the selective contact or interfaces of this contact, which contributes to the series resistance (Rseries).30 The diameter of the semicircle for the ZSO NP based PSC in high frequency regions significantly decreases, compared with QD-1 and QD-2 based PSC, which correlates with reduced value of the Rseries in the device.30 The reduction in the Rseries can be ascribed to improved electrical property of highly crystalline ZSO NPs in the ECL because all layers in the PSCs are identical except ZSO layer.13 In low-temperature processed ECL, QD based ECLs have a lot of interface without the formation of necking between particles which can hinder electrical property in ECL, resulting in the reduction of the Rseries. Furthermore, because the highly crystalline ZSO NPs have a higher work function than that of the ZSO QDs, they can reduce the formation of energy barriers at the ITO/ZSO interface, which can facilitate electron transfer from the ZSO to the ITO, reducing Rseries. As a result, high crystalline ZSO NP can behave a better electron collector than ZSO QDs, and thus induces the enhanced the FF.13,31,32 In contrast to Jsc and FF, ZSO QD-1 and QD-2 ECL has advantage for the Voc that exhibits higher value of 1.05 V and 1.03 V than 1.01 V in ZSO NPs ECL. As mentioned above, the QDs have a low work function compared to NPs, which can induce a large Vbi at ZSO/perovskite interface. A large Vbi can efficiently increase the separation of the photogenerated carriers and extend the depletion region, resulting in efficient suppression of back electron transfer from the ZSO to the perovskite, and therefore, contributing to the large



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output voltage of solar cells.22,23 Furthermore, the dark J-V curves (Figure 3c) reveal that the ZSO QD based PSC suppresses the dark current (i.e., the back-electron transfer), shifting its onset toward a higher forward bias. Figure 3d shows the diode ideality factor (m) and the dark saturation current (J0) of the dark curves in Figure 3c by fitting of the non-ideal diode equation, Jd = J0[exp(qV/mkT)-1] where q is the elementary charge, V is the applied voltage, k is the Boltzmann constant, T is the temperature. The QD-1 layer has the lowest value of J0 and the highest value of m, which correlates with the highest Voc value.33 However, much smaller difference (0.04V) in Voc values than the difference (0.10 V) in Vbi values bewteen QD-1 and NP devices is likely to be attributed to the loss of photogenerated electron by inferior electron transporting ability of QD layer which is originated from the large number of grain boundaries and poor crystallinity. Based on these results, it can be concluded that a proper design of ECL with the control of the particle size and its energy level can open up new possibilities for improving the performance of flexible PSCs. Based on the respective distinct characteristics of ZSO NPs and QDs in devices, we designed the energy level-graded ZSO ECLs (Figure 4a). First, to reduce the energy barrier at the ITO/ZSO interface, highly crystalline NPs with a high work function are deposited directly onto the ITO. The collection layer is based on highly crystalline NPs, because of their superior electron collection capabilities (minimum three-cycle coating was used). Second, to obtain a large Vbi value at the ZSO/perovskite junction, ZSO QDs with a low work function are deposited on the pre-deposited ZSO NP layer. The thickness of the two types of layers is controlled by their respective coating cycles, where 5-cycle-NP/1-cycle-QD-1, 4cycle-NP/2-cycle-QD-1, and 3-cycle-NP/3-cycle-QD-1 are denoted as (3), (2), and (1), respectively. From the designed ECLs, the PSCs was fabricated and their device performance was evaluated (Figure S6). Figure 4b shows the representative J-V curves for designed ZSO ECLs based PSCs and their corresponding photovoltaic parameters are summarized in Table


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1. Benefiting from the inserted ZSO QDs layer, the efficiency of device (3) increases to 17.1 %, which surpasses the performance of the NP-based-PSC. The Voc of device (3) is increased to 1.06 V, which is as high as that of QD-1 based device. The insertion of the QD layer results in a large value of Vbi at the ZSO/perovskite junction, which effectively suppresses the back electron reaction from ZSO to perovskite, resulting in an improved value of Voc. Interestingly, although the QD layer is inserted into the ECL, the high level of Jsc and

FF are maintained because a highly crystalline NP layer dominates the electron transport path in the designed ECL. Therefore, superior, stable electron collection with a large value of Vbi at the ZSO/perovskite junction can be obtained using the newly designed ZSO ECL. However, as the thickness of the QD layer in the ECL increases, i.e., in device (1) and (2), Jsc and FF significantly decrease with a relatively slight drop in Voc compared with device (3). When the QD layer dominates the electron transport path in the collection layer, the loss of photo-generated electrons significantly increases because of its high electrical resistance, which can cause poor device performance despite the increase in Voc due to a large value of

Vbi. To further improve the device performance and stability in ambient atmosphere, MAPb(I0.9Br0.1)3 is employed with designed ZSO ECL (3), as we previously reported.29,34 Figure S7 shows the J-V curve of the PSC consisting of ITO/designed-ZSO (1)/MAPb(I0.9Br0.1)3/PTAA. Employing the MAPb(I0.9Br0.1)3, the Jsc value is decreased to 21.24 mA/cm2 due to the broadened band gap, whereas the Voc increases from 1.06 to 1.11 V with a high FF over 0.74, resulting in a PCE of 17.6 % a steady-state PCE of 17 % (Figure S8). All of the solar cell parameters surpass the values of the planar TiO2/MAPb(I0.9Br0.1)3 PSCs.29 This phenomenon is ascribed to the favorable energy level alignment of the designed ZSO ECL, which is based on superior transmittance and electron collection ability. Based on the ECL with the best performance, we fabricated flexible PSCs. As shown in the J-V curve



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in Figure 4c, the relevant device parameters for the best performing cell are Jsc 20.4 mA/cm2,

Voc 1.1 V, FF 0.73, yielding a PCE of 16.5 %; hysteresis in the J-V curves was measured using reverse and forward scan (Figure S9 and Table S1). In spite of hysteresis in the J-V curves, however, the steady-state efficiency approaches the value measured by reverse scan, reaching 16.0 % (inset in Figure 4c). To the best of our knowledge, this is the highest efficiency that has been obtained for flexible PSCs to date. Interestingly, all photovoltaic parameters for Jsc, Voc, and FF exceed 20 mA/cm2, 1.1 V and 0.70, respectively, which are equivalent to that of glass substrate-based PSCs. Here, we see from other results that there is a large drop in the PCE, when a low temperature process using TiO2, ZnO, PCBM, and PEDOT:PSS charge-collection layers is applied to flexible PSCs.6,31 Additionally, we measured the work function for four commercial ITOs on glass and PEN substrate using photoelectron spectroscopy (PES). As shown in Figure 4d, the average work function of ITO/PEN (4.79 eV) is higher than that of ITO/glass (4.69 eV). The higher work function for ITO/PEN leads to higher Schottky barrier at the junction between ECL and ITO than ITO/glass, which is likely to lower FF in flexible PSCs.4,35 The designed ZSO ECL can contribute to reduce the Schottky barrier caused by a high work function. Therefore, rational design of the ECL leads to unprecedented flexible device performance of over 16 % based on the improvement of all photovoltaic parameters (Jsc, Voc, and FF). Moreover, because the designed ZSO ECL can be easily fabricated using a simple solution process without any dry processes or additional treatments in ambient air conditions, the high-performance flexible devices with average PCEs more than 15.0 % can be produced with a small deviation in the PCE (Figure S10). In summary, we have demonstrated performance improvement for flexible PSCs by employing a energy level-graded oxide ECL using oxide QDs. The ECL formed by


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sequential deposition of ZSO QDs and NPs on a flexible substrate at low temperature below 100 oC, exhibited effective electron collection in the ECL and related interfaces. The best performing cell, fabricated using CH3NH3(I0.9Br0.10)3, had a PCE of 16.5% with steady-state PCE of 16 % under standard conditions (AM 1.5 G, 100 mW cm−2), with a metal mask. This is, to the best of our knowledge, the highest reported efficiency for flexible PSCs. This result offers the basis for the development of low-temperature processed flexible PSCs and stimulates the low-cost mass production of PSCs, and shows availability of oxide QDs in optoelectric applications.

AUTHOR INFORMATION Corresponding Authors *[email protected]; [email protected] §

These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMETNS This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea under contract number. NRF-2007-00091 (Global Research Laboratory Program), NRF-2011-0031565 (Global Frontier R&D Program on Center for Multiscale Energy System), and NRF2015M1A2A2056542. This work was slso supported by a grant from the KRICT core program (KK1602-A01) and the KRICT-SKKU DRC program.



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Supporting Information Available: Detailed experimental procedures and additional Figures (Absorption and transmittance spectra, SEM and TEM images, J-V curves, Stabilized PCE, efficiency histogram).


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Table 1. Photovoltaic parameters of various ZSO ECLs-based PSCs Samples

Jsc (mA/cm2)

Voc (V)

FF

η (%)

QD-1 QD-2

20.2 21.3

1.05 1.03

0.54 0.64

11.2 14.0

NP

21.9

1.01

0.73

16.2

(1)

20.6

1.03

0.67

14.2

(2) (3)

21.1 21.8

1.04 1.06

0.71 0.74

15.6 17.1

Figure 1. Schematic illustration of strategy for the improving the collection of photogenerated electrons in PSCs.


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Figure 2. (a) XRD patterns, (b) Particle size distribution histogram, and (C) Band gap of ZSO NPs obtained at various temperature from 120 to 200 oC. (d) TEM image of the ZSO QD (5.7 nm). (e) Mott-Schottky analysis at 1 kHz of ZSO/perovskite heterojunction devices based on ZSO QD-1 (5.7 nm), QD-2 (9.3 nm), and NP (19.2 nm) layer (inset: Structure of ZSO/perovskite heterojunction device). (f) The built-in potential (Vbi) at the various-sized ZSO/perovskite junction.



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Figure 3. (a) J-V curves, (b) Nyquist plots, (c) dark J-V curves, and (d) m and J0 of varioussized ZSO based PSCs.


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Figure 4. (a) Schematic illustration of ZSO ECLs designed by incorporating ZSO QD-1 and NPs and (b) corresponding J-V curves. (c) J-V curve of flexible PSC based on designed ZSO ECL ((3) ECL) (inset: Stabilized PCE of the flexible PSC based on designed-ZSOECL/CH3NH3(I0.9Br0.1)3 measured close to the maximum power point (~0.916 V). (d) Average PES spectra for four ITOs on a PEN substrate.


Tailoring of Electron-Collecting Oxide ... - ACS Publications

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