Surface Band Bending Influences the Open-Circuit Voltage of

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Surface Band Bending Influences the OpenCircuit Voltage of Perovskite Solar Cells Hao Hu, Susanne T. Birkhold, Muhammad Sultan, Azhar Fakharuddin, Susanne Koch, and Lukas Schmidt-Mende ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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ACS Applied Energy Materials

Surface Band Bending Influences the Open-Circuit Voltage of Perovskite Solar Cells Hao Hu†, Susanne Birkhold†, Muhammad Sultan‡, Azhar Fakharuddin§, Susanne Koch†, and Lukas Schmidt-Mende†*

† Department

‡

of Physics, University of Konstanz, Konstanz 78457, Germany.

Nanoscience and Technology Department, National Centre for Physics, Quaid-I-Azam

University Campus, Islamabad 45320, Pakistan.

§

IMEC, Heverlee 3001, Belgium.

*E-mail: [email protected]

ACS Paragon Plus Environment

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KEYWORDS. VOC, annealing, band, bending, trap-assisted, recombination, surface.

ABSTRACT. With perovskite solar efficiencies exceeding 23%, more investigations are needed to understand and enhance the open-circuit voltage (VOC) for further efficiency gains. Here, by changing the perovskite film annealing condition, we achieve significant VOC variations in a p-i-n cell architecture. While the morphology is unaffected, we observe that the annealing conditions affect the surface stoichiometry. Subsequent variations in the VOC are identified to be caused by differences in trap-assisted recombination processes, as well as also largely by the suppression of charge collection due to surface band bending. These findings highlights the impact of surface band bending on VOC which can be permanently introduced by normal annealing treatments


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and impede the charge transport process. Our study suggests that the detailed states of the band bending in perovskite films should be carefully examined and designed to maximize the VOC and consecutively improve the performance of perovskite solar cells.

Metal halide perovskites have been a rising star in the solar cell community. In the past 10 years, perovskite based solar efficiencies have increased from initial 3.8% to 23.3%1-


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The striking development has been ascribed to the outstanding optoelectronic

properties of this material, including suitable bandgap,4-7 strong absorption coefficient,8-9 balanced charge carrier conductivity,10 low defect density,11-14 and high defect tolerances.15-16

Due to the excellent absorption and high defect tolerance of metal halide perovskites, perovskite solar cells have been reported with high photo-current densities very close to the theoretical maximal values based on the Shockley–Queisser limit.17-18 Also the fill factor (FF) which has reached values of around 80% in high performing solar cells is not expected to increase much further.19 Therefore, to further boost the efficiency towards higher values, additional focus on increasing the VOC is needed.20 Currently, the reported VOC values vary depending on the adopted solar cell architecture, but normally are still far away from the thermodynamic limit, which is reported to be 1.32 V for perovskite with a bandgap of 1.6 eV,21 indicating substantial room for efficiency enhancement.


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Based on the detailed balance theory, without any non-radiative recombination (trapassisted recombination, interfacial recombination, etc.), the VOC of a solar cell should reach its thermal radiative limit which only depends on the band gap of the absorber material. However, in reality the electron transporting material (ETM) and the hole transporting material (HTM) have strong influences on the VOC of the corresponding solar cells. Energy alignment between the conduction band of ETM/perovskites and the valence band of HTM/perovskites has been proposed.22-24 Based on the Anderson’ rule, an optimized band energy alignment may improve VOC by suppressing the interfacial recombination and at the same time maintaining the maximum built-in field (which is defined by the Fermi level difference of ETM and HTM), and in the end leads to a higher VOC.25 This model has inspired a lot of studies aiming on improving the band alignment to enhance the VOC of solar cells. For instance, Wang et al.26 and later Wu et al.27 used indene-C60-bisadduct (ICBA) to replace phenyl-C61-butyric acid methyl ester (PCBM) and obtained 0.2 V improvement in VOC, which is attributed to a raised lowest unoccupied molecular orbital (LUMO) level of the ETM. Polander et al.28 and Yan et al.29 used HTMs with different highest occupied molecular orbital (HOMO) levels and observed a


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correlation between the HOMO energy levels and the VOC of the solar cells. However, there are also studies reporting conflicting results. Belisle et al. reported evaporated HTMs with different HOMO levels, which had no significant influences on the VOC of the solar cells.30 Furthermore, Arora et al. found the VOC did not always correlate with the HOMO levels of the HTMs.31 Additionally, bromide-based perovskites have been reported to achieve VOC larger than the Fermi level differences of ETM and HTM.27, 32-33 These studies address the demand to further investigate factors which influence the VOC of perovskite solar cells.

In this study, we focus on the impact of the perovskite film properties on the VOC by investigating the role of different annealing conditions. By this approach, we can control the VOC by up to 100 mV in solar cells based on a PEDOT:PSS/CH3NH3PbI3 (MAPbI3) /C60 architecture. We identify a combination of trap-assisted recombination and charge collection suppression as the origin of these VOC variations. Our results highlight the importance of the control of surface properties by annealing conditions, which influence the VOC of perovskite solar devices through a combination of multiple mechanisms.


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In general, the perovskite films in the present study are deposited via a vacuumassisted annealing method, which has been reported previously.34-35 In this method, the perovskite films after spin-coating are transferred into a vacuum chamber on a hotplate to promote a quick crystallization process at 80 oC for 15 min. Afterwards, the films are annealed in three different conditions: under a Petri dish at 90 oC (referred as 90-in), or under N2 flow (no Petri dish) at 90 oC (90-out) or 100 oC (100-out), as shown in Figure 1. Introducing a Petri dish cover during the perovskite annealing process has been reported to greatly influence the crystallinity properties of perovskite films due to the influence of the solvent atmosphere.36 The scanning electron microscope (SEM) images of the resulting films are displayed in the Figure S1. All samples display similarly uniform and compact morphologies without signs of pin-holes. Independent of the annealing conditions, all three types of films are composed of closely packed grains in the range of tens to hundreds of nanometers. The X-ray diffraction (XRD) spectra (Figure S2) present similar features for all samples with dominating MAPbI3 (110) and (220) peaks implying a preferred orientation.37 The ultraviolet–visible (UV-Vis) absorption measurements (Figure S3) show similar


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absorbance spectra with a typical steep absorption onset for all samples suggesting a low Urbach energy.8 The X-ray photoelectron spectroscopy (XPS) was recorded to investigate the surface composition of the three types of perovskite films (Figure S4). After normalizing the Pb 4d5/2 signal of the 90-in, 90-out and 100-out samples, we observed that the relative intensities of the N 1s and I 3d5/2 signals display a consistent trend, namely the 90-out sample showed the strongest intensity of N 1s and I 3d5/2 signals while the 100-out showed the weakest intensity. This result presents a change in surface stoichiometry of the perovskite films due to the influences of the different annealing conditions. However, as the SEM, XRD and UV-Vis spectra of the different films present similar features, we conclude that these stoichiometric changes are on a small scale.


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Figure 1. Schematic illustration of the perovskite film annealing process. A home-made chamber allows quick pumping and venting operations on a hot plate. The pressure during vacuum annealing step is around 1.5 mbar. The solar cell architecture employed in this study is illustrated in Figure 2a. In brief, PEDOT:PSS is spin-coated on pre-cleaned ITO substrates as the HTM, and after the deposition of MAPbI3 films C60 is evaporated as the ETM. After evaporating 1 nm LiF blocking layer and 60 nm silver electrode, the device is completed and measured. Figure 2b displays the typical current density-voltage (J-V) scans for both forward and backward directions. The measurement is conducted from 0 V to 1 V with a scan rate of 0.1 V/s in a N2 atmosphere without pre-biasing under 1 sun. In general, all devices have the characteristics of an average short-circuit current density (~16-18 mA/cm2), a high FF (above 70%), a VOC lower than 1 V (between 0.75 V and 0.9 V) and negligible hysteresis behavior, which agree well with relevant reports on PDDOT:PSS based perovskite solar cells.38-39 The detailed solar cell parameters are listed in Table 1. The main variations between our differently treated samples lie in the VOC values, with 90-in sample reaching around 0.89 V, 90-out sample 0.81 V and 100-out sample 0.76 V


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respectively. A statistical distribution of the VOC of 90-in, 90-out and 100-out samples is given in Figure S5, showing a distinct trend. In Figure S6, the tracking of the power conversion efficiency, JSC and VOC are presented. It is worth mentioning that the VOC tracking curves of 90-out and 100-out samples show a slow improving trend when the cell was switched from short to open circuit condition(Figure S6c). Specifically, we observed the VOC increased from around 0.68 V to 0.78 V in the first 100 s of the tracking for the 90-out sample. The trend was similar for the 100-out sample while the VOC of the 90-in sample is stable. This phenomenon correlates with the VOC variations between the forward and backward scans in Figure 2b. The impedance spectra of the cells in dark show a higher recombination resistance for the 90-out sample and lower resistance for the 100-in sample, which is in accordance with the J-V behaviors (Figure S7).


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Figure 2. (a) Schematic illustration of the MAPbI3 solar cell structure. (b) Forward and backward J-V scans of 90-in, 90-out and 100-out samples.

Table 1. The typical solar cell parameters of the 90-in, 90-out and 100-out samples in this study.

Sample

90-in

VOC

JSC

FF

Efficiency

Forward

0.888 V

16.6 mA/cm2

76.4%

11.3 %

Backwar

0.888 V

16.4 mA/cm2

78.9%

11.5%

Forward

0.805 V

17.3 mA/cm2

75.1%

10.5%

Backwar

0.816 V

17.3 mA/cm2

75.3%

10.6%

d 90-out

d


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100-out

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Forward

0.747 V

16.5 mA/cm2

71.1%

8.8%

Backwar

0.761 V

16.8 mA/cm2

72.2%

9.2%

d

As in this study the device architecture remains identical, including the ETM and HTM, the VOC variations are ascribed to the changes of the MAPbI3 layer which further influence the charge carrier populations in the transporting layers under a certain illumination level. And the charge carrier population of the transporting layers under open circuit condition is determined by the dynamic interfacial charge injection and recombination process. For either interface (PEDOT:PSS/ MAPbI3 or C60/ MAPbI3), at VOC condition, the recombination current at the interface (Jrec) is equal to the injection current from the perovskite to the transporting material (Jinj):40

Jrec = Jinj

(1)


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A decrease in VOC is ascribed to the decrease of Jinj or/and increase of Jrec, as the density of states (DOS) together with the band structures of the ETM and HTM remain unchanged in our samples. Among others, the decrease of Jinj could be due to a decrease of the charge carriers generation rate, an increase of the recombination rate, or a decrease of the injection rate. The increase of Jrec could be due to an increased interfacial recombination rate. We suggest that the charge carrier generation profile is unlikely to vary as the absorption spectra of our samples are very similar (Figure S3). However, as revealed by XPS measurements, the observed changes in surface stoichiometry could possibly affect the recombination or charge collection process by influencing, for example, the trap state density39, 41 or the interfacial ion aggregation status.42-43 Note that in general only the recombination profiles could change the VOC of a cell, however, the variation of the charge collection process may change the charge carrier distribution across the cell, indirectly affect the recombination processes and influence the VOC. We further analyzed in more detail the mechanisms responsible for significantly influencing the VOC in our thin film solar cells.


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Light intensity dependences of the VOC and FF are investigated to gain insight into the recombination behavior in the solar cells (Figure 3). According to the simplified Shockley model, the VOC and JSC should follow the relation,44

∂VOC ∂ln JSC

=

SkBT q (2)

Where S is the ideality factor, kB is the Boltzmann’s constant, q is the elementary charge and T is temperature in Kelvin. Since JSC is directly proportional to the light intensity (I) (Figure S8a), VOC will increase with increasing I, characterized by a slope (S) of kBT/q when plotting against the natural logarithm of I. The value of S is typically indicative of the type of the dominant recombination mechanism in the devices: S=1 represents bimolecular recombination and S=2 represents trap-assisted recombination.45-46 It is worth mentioning that the bimolecular recombination is not limited to the radiative recombination. The interfacial recombination between the minor carriers and the adjacent transporting layer may also be bimolecular as reported for the PEDOT:PSS interface.39, 46-47 In this study, the S values extracted based on equation (2) are shown in


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Figure 3a. The slope S gradually increases from S=1 for 90-in, S=1.03 for 90-out to S=1.21 for 100-out sample, suggesting the trap-assisted recombination is gradually enhanced in our devices (the differential ideal factor S variations are presented in Figure S8b for reference). We see the reason for this enhancement of recombination in the slight stoichiometric change of the MAPbI3 films (Figure S4) due to the different annealing conditions, which leads to a different density of crystal defects and trapstates. According to the above discussion, the increase of trap-assisted recombination (no matter bulk or interfacial) will lead to the decrease of the VOC if other parameters remain unchanged. Another interesting feature in Figure 2a is the decreased light intensity dependence of the VOC at higher light intensities for 100-out sample . This feature has been discussed in both perovskite and organic solar devices, and the turning point is reported to locate at where the VOC approaches built-in field limits.46, 48 According to this explanation, we anticipate that the built-in electric field differs among our samples.


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Figure 3. Light intensity dependent measurement of (a) VOC and (b) FF. In Figure 3a, the S value is calculated in natural logarithm scale but presented with decimal scale for clarity. The values of S are marked for each sample. In Figure 3b, the dashed grey reference line at 75% is used to guide eyes.

The light intensity dependent FF measurements are displayed in Figure 3b. In a working solar cell with certain external load, the photo-generated charge carriers either recombine in the device or reach the external circuit. The FF qualitatively indicates the relative weight of these two pathways at the maximal power point. Higher FF implies


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more charge carriers could be injected into the external circuit while lower FF implies more charge carriers recombine inside the device. To compare the FF, we firstly checked the series resistance among the samples by the differential resistance extracted from the dark J-V curves (Figure S9). All the samples display a similar series resistance at their VOC points (~10 Ω/cm2). We note a clear trend for the FF dependency on the light intensity. From 0.0004 sun to 0.1 sun, the FF curves of all samples show an increase till stabilization trend. At this lower light intensity region, one main recombination loss is ascribed to trap-assisted recombination pathway,49 and the enhancement of the FF with increasing light intensity is then attributed to the diminished influence of the trap-assisted recombination with increasing carrier density. As the illumination intensity further increases, we observe the FF of all samples decreases, where the FF of the 90-in, 90-out and 100-out samples decreases 3.3%, 7.5% and 9.2% of their initial values from 0.1 to 1 sun respectively. The decrease of the FF at higher illumination intensity is due to the combination effect of increasing recombination losses as well as a largely fixed series resistance.50-53 As the studied samples have the similar series resistance (Figure S9), the different decline behavior of their FFs is ascribed to


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different growth trends of the recombination processes, which is bimolecular at this illumination level.49 We note that considering the used illumination intensity and cell’s VOC, the bimolecular recombination we refer here is unlikely to be the radiative recombination. We suggest the dominating recombination pathway which limits the VOC is the interfacial bimolecular recombination which may locate at the PEDOT:PSS/MAPbI3 interface.

These light intensity dependent VOC results suggest that the trap-assisted recombination is varied by the different annealing conditions and increases from 90-in, 90-out to 100-out samples, which is in good accordance with the observed decrease in VOC among our samples. However, a detailed look indicates that the increased trapassisted recombination is not the only influencing factor. We observe in Figure 2a that for 90-in and 90-out sample, the S values increase marginally from 1 to 1.03 while the VOC drops 80 mV. On the other hand, for 90-out and 100-out sample, the S values increase from 1.03 to 1.21 along with only 50 mV VOC drop. Additionally, the S value of PEDOT:PSS based perovskite solar cells has been reported to be close to 1.39


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Compared to a regular n-i-p architecture, the discrepancy of the lower VOC is suggested to arise from strong surface bimolecular recombination originated from poor hole selectivity of PEDOT:PSS layer.46 This explanation implies a weak influence of trapassisted recombination, as the interfacial bimolecular recombination would already be significant in the devices, especially under higher illumination intensities. Moreover, these light intensity dependencies of VOC and FF indicate differences in the built-in fields and bimolecular recombination behavior among the samples. The built-in field is directly related to the charge collection process, while the interfacial bimolecular recombination behavior could also be strongly affected by the charge collection dynamics. Therefore, these distinguished features suggest that the suppression of charge collection may also play a role in determining the solar cell’s VOC.


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Figure 4. KPFM images (the left column shows topography images and right column shows surface potential profiles) of (a) (b) 90-in, (c) (d) 90-out and (e) (f) 100-out MAPbI3 films.

As the ETM and HTM in this study remain the same, the variations of the built-in field can be assigned to the property of the different MAPbI3 film and its interface formation with the HTM/ETM , as reported in studies of hysteresis phenomenon.42-43 To investigate a change in the built-in fields upon different annealing conditions, we use


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Kelvin probe force microscope (KPFM) to investigate the spatial-resolved surface potential (Figure 4 and Table 2). The potential values have been calibrated using freshly cleaved highly oriented pyrolytic graphite (HOPG) as reference (Figure S10). The close correlation between topography and potential profile is clearly observed, and an apparent potential drop is found across 90-in, 90-out and 100-out sample (Table 2). This decrease in the surface potential indicates a downward shift of the MAPbI3 Fermi levels.

This downward shift could be due to an aggregation of surface charges or p-type doping of the MAPbI3 film. To further clarify the built-in field profile, we also need to ascertain if the shift localizes only at the surface or extends into the bulk. To sort these doubts, we further use photoelectron spectroscopy in the air (PESA) to determine the valence band position of these MAPbI3 films (Figure 5 and Table 2). The PESA characterization method is a surface sensitive technology, as only the top surface photoelectrons (several to hundreds of Å) could escape from the sample and get detected. In contrast, though the KPFM technology measures the surface potential, the result is also influenced by the


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properties of the bulk and the underlying layer, depending on the sample conductivity, doping level and so on.54 The determined valence band positions of the PESA measurements are summarized in Table 2, presenting different energy levels at the sample surfaces. We observe that the valence band positions are also lowered among the samples. Hence, the p-type doping effect seems unlikely as it would not lower the valence band position, and the charge aggregation is more likely to occur. As the differences in the values of the PESA measurement among the samples are comparable to those of the KPFM measurement (especially between the 90-in and 90-out sample), we infer that the change of the potential energy only locates at the top surface of the perovskite films. This result agrees with a picture of surface band bending where both valence band position and Fermi level are synchronously influenced. As we conclude that the band bending only locates at the surface, the bulk and bottom areas of the perovskite layers should not be affected, indicating the C60/ MAPbI3 interface is the influenced region. To make sure the PEDOT:PSS/MAPbI3 interface remains the same among the different annealing treatments, we performed the photoluminescence measurements on the PEDOT:PSS/MAPbI3 samples (Figure S11). The results show a similar carrier lifetime


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among the prepared samples, indicating the the charge extraction and recombination process at the PEDOT:PSS/MAPbI3 interface are the same among the differently treated films.

Figure 5. PESA measurements of 90-in, 90-out and 100-out samples.

Table 2. The surface topography root mean square (RMS) roughness, surface average and RMS potential and valence band position of the differently annealed MAPbI3 films. The RMS roughness, surface average and RMS potential data are collected by the KPFM, the valence band position is determined by the PESA.


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Average Sample

RMS roughness

potential

RMS potential

(Fermi level)

Valence band position

90-in

10.6 nm

-4.56 eV

41.2 mV

-5.34 eV

90-out

7.9 nm

-4.69 eV

25.4 mV

-5.46 eV

100-out

8.1 nm

-4.78 eV

26.5 mV

-5.49 eV

We notice that RMS potential is quite large in our samples compared to relevant reports,55-56 and a drastic fluctuation over 100 mV is seen across a line potential profile in 90-in sample (Figure S12). Moreover, the RMS potential decreases from 41.2 mV to 25.4 mV between 90-in and 90-out sample, suggesting the surface potential profile undergoes a significant change because of the annealing conditions. These phenomena indicate the variability of the surface potential which favors the formation of the surface band bending. The surface band bending model is in agreement with the hypothesis Belisle et al. proposed.30 They found the ionization potential (IP) of HTM only had marginal influence on the VOC of solar devices. They suggested that surface charge effect may be the explanation for the independence of VOC on contact work functions. Surface defect, surface dipoles or ionic accumulation were suggested to be possible


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charge origins.30 As the XPS measurements (Figure S4) indicate a stoichiometric change of surface compositions and the light intensity dependent VOC results suggest an enhancing trap-assisted recombination, we suggest that charged surface defects may arise from different annealing treatments.

Based on surface band bending, we propose a model to explain the correlation between the VOC and surface band bending (Figure S13). Briefly, the surface band bending screens the built-in field in the bulk. The charge transport in the bulk is then inhibited due to the decreased built-in field, which leads to the suppressed charge injection to the transporting layers. This would redistribute the charge carrier populations in the cell leading to a larger carrier density near the PEDOT:PSS interface. The model proposed here agrees well with the light intensity dependent VOC and FF results. At higher illumination intensities, when the bulk built-in field approaches zero (Figure S13f), the surface band banding model reasonably justifies the existence of low S value and strong recombination in Figure 3. The influence of perovskite band bending on VOC is also supported by other studies. For example, Xiao et al. reported the giant


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switchable photovoltaic effect where external biasing determined the cathode/anode direction.57 Zhang et al. reported a monotone VOC variation trend with the applied electric field during annealing.58 We want to address that the effect of the band bending on the VOC is dependent on the employed cell architecture. We found the studied different annealing treatments also significantly influence the behavior of solar cells with PTAA as the hole selective layer. However, the variations of the VOC are suppressed in the PTAA samples (Figure S14). As the band bending feature doesn’t change the recombination process directly, it only influences the recombination through redistributing charge carriers. It impact on the VOC relies on the interplay between the charge carrier distributions and the recombination profiles across the cell, and then varies with different cell architectures.

We have proposed two effects lead by the different annealing treatments which affect the VOC of the corresponding solar cells: trap-assisted recombination and surface band bending. The surface band bending would change the carrier distribution across the cell and affect the losses of different recombination pathways. On the other hand, the share


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of the trap-assisted recombination is relatively varied in the whole recombination losses among the different treated films. We find by replacing the PEDOT:PSS by PTAA, over 100 mV VOC enhancement can be readily achieved (Figure S14). Therefore, the VOC of the studied architecture is mainly limited by the interfacial recombination at the PEDOT:PSS/MAPbI3 interface46. As the influence of the trap-assisted recombination is relatively minor (which can also be seen in Figure 3a), the surface band bending is suggested to be the main reason which can modulate the recombination losses at the PEDOT:PSS interface by varying the carrier densities across the cell. As the ionic transport property of hybrid perovskite materials has been proposed to greatly influence the electric field distribution and charge injection at the interfaces,42, 59 it is of interest to examine if the surface band bending also correlates with the ionic movement in perovskite films. Hence, transient ionic current measurements were recorded (Figure S15). During this measurement, after the solar cell was biased at 0.9 V in dark, the dark short circuit current was recorded, which has been ascribed to the movement and redistribution of ions.60-62 The calculated density of the movable ions is 1.4 x 1017 cm-3 for 90-in, 4.6 x 1017 cm-3 for 90-out and 5.5 x 1017 cm-3 for 100-out


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sample, respectively. These increased densities of mobile ions suggest that the presence of surface band bending is accompanied by stronger ionic movements in the perovskite films.

In conclusion, we have presented that a controlled variation in the VOC’s of perovskite solar cells by up to 100 mV can be achieved by slight variations in the MAPbI3 film preparation process. While no significant changes in the morphology of the differently annealed perovskite films are observed, we identify the small stoichiometric changes at the film surface leading to increased trap-assisted recombination and suppressed charge collection due to the surface band bending. Considering that the reduction in FF at higher light intensities in this study is much more pronounced in comparison with reported silicon, organic and other perovskite solar cells,50-53 we suggest the suppression of charge collection to be a strongly influential effect. Perovskite material has been known for its ionic transport and consequent dynamic band bending.63 However, a persistent surface band bending introduced by the usual annealing treatments has not been reported yet. The presented significant impact on both trap-


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assisted recombination and charge collection by minor variations in the perovskite annealing procedure highlights the necessity to carefully control surface properties of metal halide perovskites to further enhance perovskite solar cell efficiencies.

ASSOCIATED CONTENT

Supporting Information. The supporting information contains the detailed experimental and characterization methods used in this study. It also provides supporting experimental results on the perovskite films and solar cells, including the SEM, XRD, UV-Vis, XPS, IS, KPFM, PL, J-V related measurements, etc. Their detailed descriptions are provided in the main text when they are introduced.

AUTHOR INFORMATION

[email protected] [email protected] [email protected] [email protected]


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[email protected] [email protected]

https://www.schmidt-mende.uni-konstanz.de/

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This research was funded by the China Scholarship Council. Additionally, the authors would like to acknowledge Christian Derricks of Konstanz University for the KPFM measurement support.

REFERENCES 1.

Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as

Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. 2.

Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.;

Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S. I. Iodide management in formamidinium-lead-halide– based perovskite layers for efficient solar cells. Science 2017, 356, 1376-1379. 3.

National Renewable Energy Laboratory (NREL). https://www.nrel.gov/pv/assets/pdfs/pv-

efficiencies-07-17-2018.pdf, 2018 (accessed October 2018).


30

Page 31 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.

Walsh, A. Principles of Chemical Bonding and Band Gap Engineering in Hybrid

Organic-Inorganic Halide Perovskites. J. Phys. Chem. C 2015, 119, 5755-5760. 5.

Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Chemical Management for

Colorful, Efficient, and Stable Inorganic–Organic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13, 1764-1769. 6.

Kulkarni, S. A.; Baikie, T.; Boix, P. P.; Yantara, N.; Mathews, N.; Mhaisalkar, S. Band-

gap tuning of lead halide perovskites using a sequential deposition process. J. Mater. Chem. A 2014, 2, 9221-9225. 7.

D’Innocenzo, V.; Srimath Kandada, A. R.; De Bastiani, M.; Gandini, M.; Petrozza, A.

Tuning the Light Emission Properties by Band Gap Engineering in Hybrid Lead Halide Perovskite. J. Am. Chem. Soc. 2014, 136, 17730-17733. 8.

De Wolf, S.; Holovsky, J.; Moon, S.-J.; Löper, P.; Niesen, B.; Ledinsky, M.; Haug, F.-J.;

Yum, J.-H.; Ballif, C. Organometallic Halide Perovskites: Sharp Optical Absorption Edge and its Relation to Photovoltaic Performance. J. Phys. Chem. Lett. 2014, 5, 1035–1039. 9.

Zhu, X.; Su, H.; Marcus, R. A.; Michel-Beyerle, M. E. Computed and Experimental

Absorption Spectra of the Perovskite CH3NH3PbI3. J. Phys. Chem. Lett. 2014, 5, 3061-3065. 10.

Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-hole

diffusion lengths > 175 µm in solution-grown CH3NH3PbI3 single crystals. Science 2015, 347, 967-970. 11.

Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland,

S.; Rothenberger, A.; Katsiev, K.; Losovyj, Y.; Zhang, X.; Dowben, P. A.; Mohammed, O. F.; Sargent, E. H.; Bakr, O. M. Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals. Science 2015, 347, 519-522.


31


12.

Page 32 of 40

Samiee, M.; Konduri, S.; Ganapathy, B.; Kottokkaran, R.; Abbas, H. A.; Kitahara, A.;

Joshi, P.; Zhang, L.; Noack, M.; Dalal, V. Defect density and dielectric constant in perovskite solar cells. Appl. Phys. Lett. 2014, 105, 153502. 13.

Yin, W.-J.; Shi, T.; Yan, Y. Unusual defect physics in CH3NH3PbI3 perovskite solar cell

absorber. Appl. Phys. Lett. 2014, 104, 063903. 14.

Duan, H.-S.; Zhou, H.; Chen, Q.; Sun, P.; Luo, S.; Song, T.-B.; Bob, B.; Yang, Y. The

identification and characterization of defect states in hybrid organic-inorganic perovskite photovoltaics. Phys. Chem. Chem. Phys. 2015, 17, 112-116. 15.

Meggiolaro, D.; Motti, S. G.; Mosconi, E.; Barker, A. J.; Ball, J.; Andrea Riccardo Perini,

C.; Deschler, F.; Petrozza, A.; De Angelis, F. Iodine chemistry determines the defect tolerance of lead-halide perovskites. Energy Environ. Sci. 2018, 11, 702-713. 16.

Steirer, K. X.; Schulz, P.; Teeter, G.; Stevanovic, V.; Yang, M.; Zhu, K.; Berry, J. J.

Defect Tolerance in Methylammonium Lead Triiodide Perovskite. ACS Energy Lett. 2016, 1, 360-366. 17.

Juan‐Pablo, C. B.; Miguel, A.; Gabriel, L.; Wolfgang, T.; Konrad, D.; Michael, S.;

Taisuke, M.; Jesper, J. T.; E., C. M.; Antonio, A.; Michael, G.; Hernán, M.; Anders, H. Unbroken Perovskite: Interplay of Morphology, Electro‐optical Properties, and Ionic Movement. Adv. Mater. 2016, 28, 5031-5037. 18.

Ball, J. M.; Stranks, S. D.; Horantner, M. T.; Huttner, S.; Zhang, W.; Crossland, E. J. W.;

Ramirez, I.; Riede, M.; Johnston, M. B.; Friend, R. H.; Snaith, H. J. Optical properties and limiting photocurrent of thin-film perovskite solar cells. Energy Environ. Sci. 2015, 8, 602-609.


32

Page 33 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19.

Tress, W. Perovskite Solar Cells on the Way to Their Radiative Efficiency Limit –

Insights Into a Success Story of High Open-Circuit Voltage and Low Recombination. Adv. Energy Mater. 2017, 7, 1602358. 20.

Correa-Baena, J.-P.; Tress, W.; Domanski, K.; Anaraki, E. H.; Turren-Cruz, S.-H.;

Roose, B.; Boix, P. P.; Gratzel, M.; Saliba, M.; Abate, A.; Hagfeldt, A. Identifying and suppressing interfacial recombination to achieve high open-circuit voltage in perovskite solar cells. Energy Environ. Sci. 2017, 10, 1207-1212. 21.

Wolfgang, T.; Nevena, M.; Olle, I.; K., N. M.; M., Z. S.; Michael, G. Predicting the Open

‐Circuit Voltage of CH3NH3PbI3 Perovskite Solar Cells Using Electroluminescence and Photovoltaic Quantum Efficiency Spectra: the Role of Radiative and Non‐Radiative Recombination. Adv. Energy Mater. 2015, 5, 1400812. 22.

Yajima, T.; Hikita, Y.; Minohara, M.; Bell, C.; Mundy, J. A.; Kourkoutis, L. F.; Muller,

D. A.; Kumigashira, H.; Oshima, M.; Hwang, H. Y. Controlling band alignments by artificial interface dipoles at perovskite heterointerfaces. Nat. Commun. 2015, 6, 6759. 23.

Schulz, P.; Whittaker-Brooks, L. L.; MacLeod, B. A.; Olson, D. C.; Loo, Y.-L.; Kahn, A.

Electronic Level Alignment in Inverted Organometal Perovskite Solar Cells. Advanced Materials Interfaces 2015, n/a-n/a. 24.

Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.;

Yang, Y. Interface engineering of highly efficient perovskite solar cells. Science 2014, 345, 542546. 25.

Fakharuddin, A.; Schmidt-Mende, L.; Garcia-Belmonte, G.; Jose, R.; Mora-Sero, I.

Interfaces in Perovskite Solar Cells. Adv. Energy Mater. 2017, 7, 1700623.


33


26.

Page 34 of 40

Wang, Q.; Shao, Y. C.; Dong, Q. F.; Xiao, Z. G.; Yuan, Y. B.; Huang, J. S. Large fill-

factor bilayer iodine perovskite solar cells fabricated by a low-temperature solution-process. Energy Environ. Sci. 2014, 7, 2359-2365. 27.

Wu, C.-G.; Chiang, C.-H.; Chang, S. H. A perovskite cell with a record-high-Voc of 1.61

V based on solvent annealed CH3NH3PbBr3/ICBA active layer. Nanoscale 2016, 8, 4077-4085. 28.

Polander, L. E.; Pahner, P.; Schwarze, M.; Saalfrank, M.; Koerner, C.; Leo, K. Hole-

transport material variation in fully vacuum deposited perovskite solar cells. APL Mater. 2014, 2, 081503. 29.

Yan, W.; Li, Y.; Ye, S.; Li, Y.; Rao, H.; Liu, Z.; Wang, S.; Bian, Z.; Huang, C.

Increasing open circuit voltage by adjusting work function of hole-transporting materials in perovskite solar cells. Nano Res. 2016, 9, 1600-1608. 30.

Belisle, R. A.; Jain, P.; Prasanna, R.; Leijtens, T.; McGehee, M. D. Minimal Effect of the

Hole-Transport Material Ionization Potential on the Open-Circuit Voltage of Perovskite Solar Cells. ACS Energy Lett. 2016, 1, 556-560. 31.

Abate, A.; Planells, M.; Hollman, D. J.; Barthi, V.; Chand, S.; Snaith, H. J.; Robertson,

N. Hole-transport materials with greatly-differing redox potentials give efficient TiO2[CH3NH3][PbX3] perovskite solar cells. Phys. Chem. Chem. Phys. 2015, 17, 2335-2338. 32.

Arora, N.; Orlandi, S.; Dar, M. I.; Aghazada, S.; Jacopin, G.; Cavazzini, M.; Mosconi, E.;

Gratia, P.; De Angelis, F.; Pozzi, G.; Graetzel, M.; Nazeeruddin, M. K. High Open-Circuit Voltage: Fabrication of Formamidinium Lead Bromide Perovskite Solar Cells Using Fluorene– Dithiophene Derivatives as Hole-Transporting Materials. ACS Energy Lett. 2016, 1, 107-112. 33.

Zhang, Y.; Liang, Y.; Wang, Y.; Guo, F.; Sun, L.; Xu, D. Planar FAPbBr3 Solar Cells

with Power Conversion Efficiency above 10%. ACS Energy Lett. 2018, 3, 1808-1814.


34

Page 35 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


34.

Hu, H.; Wong, K.; Kollek, T.; Hanusch, F.; Polarz, S.; Docampo, P.; Schmidt-Mende, L.

Highly Efficient Reproducible Perovskite Solar Cells Prepared by Low-Temperature Processing. Molecules 2016, 21, 542. 35.

Xie, F. X.; Zhang, D.; Su, H.; Ren, X.; Wong, K. S.; Grätzel, M.; Choy, W. C. H.

Vacuum-Assisted Thermal Annealing of CH3NH3PbI3 for Highly Stable and Efficient Perovskite Solar Cells. ACS Nano 2015, 9, 639-646. 36.

Numata, Y.; Kogo, A.; Udagawa, Y.; Kunugita, H.; Ema, K.; Sanehira, Y.; Miyasaka, T.

Controlled Crystal Grain Growth in Mixed Cation–Halide Perovskite by Evaporated Solvent Vapor Recycling Method for High Efficiency Solar Cells. ACS Appl. Mater. Interfaces 2017, 9, 18739-18747. 37.

Quarti, C.; Mosconi, E.; Ball, J. M.; D'Innocenzo, V.; Tao, C.; Pathak, S.; Snaith, H. J.;

Petrozza, A.; De Angelis, F. Structural and optical properties of methylammonium lead iodide across the tetragonal to cubic phase transition: implications for perovskite solar cells. Energy Environ. Sci. 2016, 9, 155-163. 38.

Xu, H.; Kai, W.; Chao, Y.; Tianyu, M.; Xiong, G. Efficient Perovskite Hybrid Solar Cells

by Highly Electrical Conductive PEDOT:PSS Hole Transport Layer. Adv. Energy Mater. 2016, 6, 1501773. 39.

Khadka, D. B.; Shirai, Y.; Yanagida, M.; Ryan, J. W.; Miyano, K. Exploring the effects

of interfacial carrier transport layers on device performance and optoelectronic properties of planar perovskite solar cells. J. Mater. Chem. C 2017, 5, 8819-8827. 40.

O'Regan, B. C.; Scully, S.; Mayer, A. C.; Palomares, E.; Durrant, J. The Effect of Al2O3

Barrier Layers in TiO2/Dye/CuSCN Photovoltaic Cells Explored by Recombination and DOS


35


Page 36 of 40

Characterization Using Transient Photovoltage Measurements. J. Phys. Chem. B 2005, 109, 4616-4623. 41.

M., W. C.; Fengshuo, Z.; Andreas, P.; Perdigón, T. L.; Norbert, K.; Dieter, N. Reduced

Interface‐Mediated Recombination for High Open‐Circuit Voltages in CH3NH3PbI3 Solar Cells. Adv. Mater. 2017, 29, 1700159. 42.

Belisle, R. A.; Nguyen, W. H.; Bowring, A. R.; Calado, P.; Li, X.; Irvine, S. J. C.;

McGehee, M. D.; Barnes, P. R. F.; O'Regan, B. C. Interpretation of inverted photocurrent transients in organic lead halide perovskite solar cells: proof of the field screening by mobile ions and determination of the space charge layer widths. Energy Environ. Sci. 2017, 10, 192-204. 43.

Richardson, G.; O'Kane, S. E. J.; Niemann, R. G.; Peltola, T. A.; Foster, J. M.; Cameron,

P. J.; Walker, A. B. Can slow-moving ions explain hysteresis in the current-voltage curves of perovskite solar cells? Energy Environ. Sci. 2016, 9, 1476-1485. 44.

Koster, L. J. A.; Mihailetchi, V. D.; Ramaker, R.; Blom, P. W. M. Light intensity

dependence of open-circuit voltage of polymer:fullerene solar cells. Appl. Phys. Lett. 2005, 86, 123509. 45.

Yang, D.; Zhou, X.; Yang, R.; Yang, Z.; Yu, W.; Wang, X.; Li, C.; Liu, S.; Chang, R. P.

H. Surface optimization to eliminate hysteresis for record efficiency planar perovskite solar cells. Energy Environ. Sci. 2016, 9, 3071-3078. 46.

Tress, W.; Yavari, M.; Domanski, K.; Yadav, P.; Niesen, B.; Correa Baena, J. P.;

Hagfeldt, A.; Graetzel, M. Interpretation and evolution of open-circuit voltage, recombination, ideality factor and subgap defect states during reversible light-soaking and irreversible degradation of perovskite solar cells. Energy Environ. Sci. 2018, 11, 151-165.


36

Page 37 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


47.

Serpetzoglou, E.; Konidakis, I.; Kakavelakis, G.; Maksudov, T.; Kymakis, E.; Stratakis,

E. Improved Carrier Transport in Perovskite Solar Cells Probed by Femtosecond Transient Absorption Spectroscopy. ACS Appl. Mater. Interfaces 2017, 9, 43910-43919. 48.

Solak, S.; Blom, P. W. M.; Wetzelaer, G. A. H. Effect of non-ohmic contacts on the light-

intensity dependence of the open-circuit voltage in organic solar cells. Appl. Phys. Lett. 2016, 109, 053302. 49.

Sherkar, T. S.; Momblona, C.; Gil-Escrig, L.; Ávila, J.; Sessolo, M.; Bolink, H. J.;

Koster, L. J. A. Recombination in Perovskite Solar Cells: Significance of Grain Boundaries, Interface Traps, and Defect Ions. ACS Energy Lett. 2017, 2, 1214-1222. 50.

Rühle, K.; Juhl, M. K.; Abbott, M. D.; Kasemann, M. Evaluating Crystalline Silicon

Solar Cells at Low Light Intensities Using Intensity-Dependent Analysis of I–V Parameters. IEEE J. Photovolt. 2015, 5, 926-931. 51.

Effect of leakage current and shunt resistance on the light intensity dependence of

organic solar cells. Appl. Phys. Lett. 2015, 106, 083301. 52.

Shuttle, C. G.; Hamilton, R.; O’Regan, B. C.; Nelson, J.; Durrant, J. R. Charge-density-

based analysis of the current–voltage response of polythiophene/fullerene photovoltaic devices. Proc. Natl. Acad. Sci. 2010, 107, 16448-16452. 53.

Liu, M.; Endo, M.; Shimazaki, A.; Wakamiya, A.; Tachibana, Y. Light Intensity

Dependence of Performance of Lead Halide Perovskite Solar Cells. J. Photopolym. Sci. Technol. 2017, 30, 577-582. 54.

Zohar, A.; Kulbak, M.; Levine, I.; Hodes, G.; Kahn, A.; Cahen, D. What Limits the

Open-Circuit Voltage of Bromide Perovskite-Based Solar Cells? ACS Energy Lett. 2019, 4, 1-7.


37


55.

Page 38 of 40

Yun, J. S.; Ho-Baillie, A.; Huang, S.; Woo, S. H.; Heo, Y.; Seidel, J.; Huang, F.; Cheng,

Y.-B.; Green, M. A. Benefit of Grain Boundaries in Organic–Inorganic Halide Planar Perovskite Solar Cells. J. Phys. Chem. Lett. 2015, 6, 875-880. 56.

Kim, D.; Kim, G. Y.; Ko, C.; Pae, S. R.; Lee, Y. S.; Gunawan, O.; Ogletree, D. F.; Jo,

W.; Shin, B. Effects of Postsynthesis Thermal Conditions on Methylammonium Lead Halide Perovskite: Band Bending at Grain Boundaries and Its Impacts on Solar Cell Performance. J. Phys. Chem. C 2016, 120, 21330-21335. 57.

Xiao, Z.; Yuan, Y.; Shao, Y.; Wang, Q.; Dong, Q.; Bi, C.; Sharma, P.; Gruverman, A.;

Huang, J. Giant switchable photovoltaic effect in organometal trihalide perovskite devices. Nat. Mater. 2014, 14, 193. 58.

Zhang, C.-C.; Wang, Z.-K.; Li, M.; Liu, Z.-Y.; Yang, J.-E.; Yang, Y.-G.; Gao, X.-Y.;

Ma, H. Electric-field assisted perovskite crystallization for high-performance solar cells. J. Mater. Chem. A 2018, 6, 1161-1170. 59.

Eames, C.; Frost, J. M.; Barnes, P. R. F.; O’Regan, B. C.; Walsh, A.; Islam, M. S. Ionic

transport in hybrid lead iodide perovskite solar cells. Nat. Commun. 2015, 6, 7497. 60.

Cheng, Y.; Li, H.-W.; Qing, J.; Yang, Q.-D.; Guan, Z.; Liu, C.; Cheung, S. H.; So, S. K.;

Lee, C.-S.; Tsang, S.-W. The detrimental effect of excess mobile ions in planar CH3NH3PbI3 perovskite solar cells. J. Mater. Chem. A 2016, 4, 12748-12755. 61.

Zhang, H.; Liang, C.; Zhao, Y.; Sun, M.; Liu, H.; Liang, J.; Li, D.; Zhang, F.; He, Z.

Dynamic interface charge governing the current-voltage hysteresis in perovskite solar cells. Phys. Chem. Chem. Phys. 2015, 17, 9613-9618.


38

Page 39 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


62.

Shao, Y.; Xiao, Z.; Bi, C.; Yuan, Y.; Huang, J. Origin and elimination of photocurrent

hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells. Nat. Commun. 2014, 5, 5784. 63.

Lopez-Varo, P.; Jiménez-Tejada, J. A.; García-Rosell, M.; Ravishankar, S.; Garcia-

Belmonte, G.; Bisquert, J.; Almora, O. Device Physics of Hybrid Perovskite Solar cells: Theory and Experiment. Adv. Energy Mater. 2018, 8, 1702772.


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