Dynamic study of the light soaking effect on perovskite solar cells by in-situ photoluminescence microscopy

Author’s Accepted Manuscript Dynamic study of the light soaking effect on perovskite solar cells by in-situ photoluminescence microscopy Xiaofan Deng, Xiaoming Wen, Jianghui Zheng, Trevor Young, Cho Fai Jonathan Lau, Jincheol Kim, Martin Green, Shujuan Huang, Anita HoBaillie

PII: DOI: Reference:

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S2211-2855(18)30094-6 https://doi.org/10.1016/j.nanoen.2018.02.024 NANOEN2511

To appear in: Nano Energy Received date: 11 October 2017 Revised date: 18 January 2018 Accepted date: 12 February 2018 Cite this article as: Xiaofan Deng, Xiaoming Wen, Jianghui Zheng, Trevor Young, Cho Fai Jonathan Lau, Jincheol Kim, Martin Green, Shujuan Huang and Anita Ho-Baillie, Dynamic study of the light soaking effect on perovskite solar cells by in-situ photoluminescence microscopy, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.02.024 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Dynamic study of the light soaking effect on perovskite solar cells by in-situ photoluminescence microscopy Xiaofan Denga, Xiaoming Wenb*, Jianghui Zhenga, Trevor Younga, Cho Fai Jonathan Laua, Jincheol Kima, Martin Greena, Shujuan Huanga, Anita Ho-Bailliea* a

Australian Centre for Advanced Photovoltaics, University of New South Wales, Sydney 2052, Australia. b

Centre for Micro-Photonics, Swinburne University of Technology, Hawthorn, 3122， Australia.

[email protected] (Xiaoming Wen) [email protected] (Anita Ho-Baillie)

Author Information *Corresponding Authors

Abstract Organic-inorganic halide perovskite solar cells (PSCs) have emerged as promising candidates for next generation solar cells due to the rapid increase in their power conversion efficiency. The instability of these cells under illumination, however, remains a major technical barrier for commercialization. In this work, by fabricating full perovskite cells (not test structures) that is compatible with in photoluminescence (PL), for the first time we have achieved in-situ monitoring of the localized charge carrier and ion dynamics in an operating perovskite solar cell under light soaking, using nanoscale resolved in-situ PL and time-

1

resolved PL (tr-PL) microscopy. By analyzing the dynamic PL lifetime and intensity under different light soaking conditions and its correlation with the shape of the voltage current curve, we explain the different scenarios of ion migration and accumulation at the interface and in the bulk that result in different hysteresis behaviors. Our results suggest that mobile positive

ions,

predominantly

iodide

vacancies

pre-accumulate

near

the

spiro-

MeOTAD/perovskite interface of as-fabricated devices, reducing charge-carrier separation and increasing recombination at that electrode. After light soaking for a short time at open circuit, these positive ions drift away from the interface under the altered electric field, improving device performance. After prolonged light soaking, however, negative ions, predominantly iodide interstitials drift to the spiro-MeOTAD/perovskite interface, significantly enhancing carrier recombination at that electrode. In contrast, light soaking had less effect at short-circuit because the electric field is invariant at short circuit. In addition to the light-soaked-induced ion movement under short circuit is by diffusion rather than by drift. This result in ionic redistribution and ion-recombination increases PL intensity uniformity across the device and resulting in relatively stable device performance. Our work reveals that the bias voltage during light soaking results in different dynamic processes, which can be either positive or negative. Analysis on the corresponding PL images revealed a difference in charge carrier and ion dynamics between grain interior and grain boundary during light soaking. The larger density of grain boundaries causes a faster ion migration rate in the regions with smaller grains. Therefore, it may be possible to reduce J-V hysteresis by producing larger grains. Our results provide novel insight into the effect of light soaking on ions and subsequent effect on carrier dynamics for better understanding of the operation of perovskite solar cells. Graphical abstract

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Keywords: Perovskite solar cell; Light soaking; Carrier dynamics; Ion migration; Photoluminescence microscopy

1. Introduction In the past few years, organic-inorganic halide perovskite solar cells (PSCs) have generated intense research interest due to their outstanding optical properties, large carrier mobility, long lifetime, low cost and low fabrication temperature [1, 2]. A remarkably high certified power conversion efficiency (PCE) of 22.1% has been achieved [3]. Nevertheless, PSCs are often unstable in the presence of humidity, external voltage bias and light illumination [4-11]. Instability due to exposure to light has been observed in current-voltage characteristics, in open-circuit photo-voltage decay measurements and by photoluminescence (PL) techniques such as steady state photoluminescence (ss-PL) and time-resolved photoluminescence (trPL). Several physical mechanisms have been proposed to explain the observed effects, which may be either beneficial or detrimental to the cell’s performance and may be either reversible or permanent. The first one is the reduction of trap-assisted recombination, which increases the carrier diffusion length during light soaking [12-16]. Zhao et al. proposed that traps inside perovskite films are essentially p-type defects that become filled by electrons during illumination [14]. Interfacial trap states may be passivated during illumination resulting in more efficient carrier transfer at the perovskite/carrier transport layer interface [12]. Mutual annihilation of iodide vacancies and interstitials may be accelerated during light soaking due to enhanced migration of these defects, thereby reducing trap densities [17, 18]. Other light

3

soaking effects that are benign to optoelectronic performance of PSCs were proposed such as a light-induced self-poling effect [19], charge screening and modulation of the band offsets [20]. On the other hand, negative effects of light illumination on PSCs such as the formation of light-activated trap states have also been suggested [21-23]. Domanski et al. proposed that ionic trap accumulation during light soaking could explain the reversible loss of performance observed [22]. Nie et .al. assigned the efficiency reduction under illumination to the formation of spatially localized deep charge states or small polarons [23]. Structural changes of the perovskite film during illumination may result in changes in the electronic properties such as favouring either electron or hole mobility [24, 25]. Zhao et al. suggested that the small irreversible component of the light induced degradation observed could be attributed to structural changes caused by illumination [14]. In summary, several different behaviours of PSCs under light soaking have been reported and several different mechanisms have been proposed to explain those effects, depending on the device structures, observation conditions and the nature of the hole and electron extracting contacts [4, 15, 20, 26]. However, a consistent explanation is still lacking. Although PL spectroscopy and microscopy have been used extensively to characterise carrier dynamics in perovskite thin films and perovskite test structures [13-15, 18, 27-29], it is difficult to apply these techniques to complete devices due to the high absorption and reflectivity of Au or Ag electrodes. Moreover, the very short working distance required for high resolution optical images limits their acquisition from the transparent (glass) side of the device [30]. Previously PL spectroscopy and PL imaging have been limited to the study of perovskite test structures only. Here we study the effect of light soaking on carrier lifetime within an operating perovskite solar cell via PL imaging with nanometre-scale resolution. This is achieved by fabricating perovskite solar cells compatible with PL spectroscopy and PL imaging. Different ion dynamic processes were observed during light soaking under

4

different working conditions. Current density-voltage (J-V) measurements that correlate with the results of PL studies are also presented. One observation was that the PCE of new devices increased as they operate. We propose that ionic pre-accumulation occurs in our devices when they are made, screening the built-in field and reducing the initial efficiency. After light soaking at open circuit (OC), the pre-accumulated ions are redistributed within tens of seconds, causing device performance to improve. After prolonged light soaking, however, mobile ions accumulated at the opposite electrode, increasing the surface recombination there. At short-circuit, slower and weaker light soaking effects were observed due to the reduced electric field across the perovskite layer. Localized PL evolution was analyzed to reveal the effect of grain size on ion dynamics during light illumination. Our observations of light soaking performed on operational solar cells provide new insights into the interaction between the dynamics of charge carriers and mobile ions and how they correlated to device performance.

2. Method 2.1 Sample preparation In order to measure the PL and tr-PL of fully operational solar cells under light soaking conditions, the special sample structure illustrated in Figure 1a was designed. Initially, a perovskite

solar

cell

with

the

TiO2/CH3NH3PbI3/spiro-MeOTAD

conventional was

structure

fabricated

using

of

glass/FTO/c-TiO2/mp-

processes

described

in

Supplementary Information (SI). Secondly, electrodes consisting of Au fingers 2 mm apart were deposited by shadow masked evaporation. Finally, a second, thinner Au layer was evaporated over and between the gold fingers. The purpose of the Au finger plus thin Au layer structure was to allow the excitation light and PL from the device to transmit through, while also providing a sufficiently low resistance that the solar cell could operate normally. The appropriate thickness of this layer is calculated in SI and is ~10 nm. Using this thin semi-

5

transparent electrode structure, PL/ tr-PL images were obtained with hundreds nanometre resolution. 2.2 Characterization The PL measurements were conducted on a confocal laser scanning microscopy system (MT200, Picoquant). The tr-PL was measured using time correlation single photon counting (TCSPC). As illustrated in Figure 1b, the PL and tr-PL images were acquired from the Au electrode side of the cell, through the area with thin Au using an objective lens with 60x / 1.4 NA. A 470 nm laser provided excitation at a pulse repetition rate of 20 MHz. At the same time, the sample was continuously illuminated (i.e. light soaked) on the glass side through a 525/50 nm bandpass filter (BP1). The intensity of the filtered light (hereafter referred to as the bias light) was 0.72 suns (72 mWcm-2, which was the highest intensity of our light source after filtering). The PL signal at ~760 nm [7] was detected through a 760/40 nm band-pass filter (BP2) which blocked light from the probe laser and bias light. Given the continuous and large area bias light, the bias light had only a minor effect on PL measurements. Also, the carrier densities generated by bias light (estimated to be 1014 to 1015 cm-3) is two orders of magnitude lower than those generated by the probe laser (~ 1016 to 1017 cm-3 (see SI for calculation)). In terms of the cumulative effect of the excitation by the probe laser beam, it was negligible due to the short (2 ms) dwell time on each location. Therefore, the PL signal was obtained independently of the bias light and could be used to monitor the effect of bias light on PSCs in real time. The PL signal is probed from the p-type side through the thin gold and spiro-MeOTAD layers with a penetration depth of ≈50 nm within the perovskite. In addition, due to the confocal nature of the measurement (detection of emission from the laser focus spot only when a confocal pinhole filter is used), PL signal from diffused carriers near the TiO2 interface will not be detected. Therefore, our discussion related to the interface is only limited to the spiro-MeOTAD/CH3NH3PbI3 interface in this work.

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The current density–voltage (J–V) measurements during light soaking were performed using a solar cell I–V testing system with 8x8 mm2 aperture. The light source was a green LED lamp with a wavelength of 520 – 535 nm, similar to that of the bias light used during PL measurements. The system was calibrated by a reference cell which was tested by an I–V testing system from Abet Technologies, Inc. (using a Class AAA solar simulator) under an illumination power of 100 mWcm-2. All measurements were undertaken at room temperature in ambient conditions. The temperature during light soaking was controlled within +/- 2 °C (see SI for more details). For the purpose of discussing the effect of light soaking on ion dynamics and their effect on carrier dynamics, we focus on the discussion and analysis of J-V curves scanned from Jsc to Voc (-0.1 V to 1.1 V, which will be referred to as “forward scan”. Reverse scan J-V curves show less effect of ions as a result of light soaking, see Supporting Information for the relevant discussion and Figure S6. For all J-V curves reported in this work, the scan rate was 0.2 V/s.

3. Results and Discussions 3.1 Light soaking effect at open-circuit (OC) In this work, we monitored the evolution of localized PL, tr-PL and J-V measurements caused by light soaking at open-circuit. To fit the tr-PL decay trace, to extract the lifetime components, a bi-exponential decay function The effective lifetime[31] was obtained as

(

)

(

) was used.

. Figure 2a shows how the PL

intensity and the effective lifetime changed during light soaking. Figure 2b shows the changes in J-V curves during light soaking and Figures 2c to 2e show the evolution of VOC, JSC and FF during light soaking. During light soaking at OC, the PL and J-V responses change in the timescale of seconds and minutes. We believe that the mechanism causing these variations is a process of ion migration and accumulation [7, 13, 32, 33]. Both PL and 7

J-V measurements show two stages of evolution during light soaking (Stage 1 and Stage 2) but with different response times, as shown in Figure 2. The shorter duration of stage 1 observed in J-V measurements (Fig 2c, 2d, and 2e) relative to PL measurements (Fig 2a) is attributed to the higher illumination intensity used: ~ 1 sun for J-V measurements and ~0.72 sun for PL measurements. The dynamic process can be characterized roughly into several stages; before illumination (Stage 0), during illumination (Stages 1 and 2) and after illumination (Stage 3). At stage 0 (without illumination), the PL intensity and effective lifetime were relatively constant. This indicates that illumination by the excitation laser had negligible light soaking effect on the sample and there is negligible noise in the data. After the bias light was turned on, the slow PL response and PV performance changed slowly over a period of several hundred seconds (see Stages 1 and 2 of Figure 2a). This slow response has been attributed to the migration of mobile ions[13, 32, 33], predominantly intrinsic ions such as iodide interstitials and vacancies, due to their low activation energy (hundreds of meV [17, 34-36]) and high mobility[7, 29] although extrinsic ions to a less extent such as those (e.g., Li+) from inter-layers [37] have also been suggested. The timescale is consistent with our previously reported study of ion migration in CH3NH3PbI3 perovskite which showed that mobile ions accumulate at the perovskite/electrode interface under an applied electric field within seconds to a few minutes [7, 10]. In the present work, we additionally propose that pre-accumulated ions exist near the interfaces between the perovskite and carrier transport layers, prior to illumination or application of any voltage bias. During device fabrication, as the perovskite layer and HTL (hole transport layer) equilibrate, a built-in electric field is formed initially due to the different work functions of these materials, as illustrated in the band diagram of Figure 3a. Subsequently, negative (e.g., I- interstitial) ions in the perovskite film drift under the influence of the built-in electric field.

8

The direction of the electric field towards the HTL pushes negative ions away from the HTL causing the positive (e.g. iodide vacancies ) ions to accumulate at that interface (Figure 3b) forming a narrow Debye layer (~ 5 to 10 nm width) [22, 34, 38]. This accumulation of positive ions can be described as ‘pre-accumulation’ because it occurs spontaneously as the ETL (electron transport layers)/perovskite/HTL system reaches complete electrochemical equilibrium, independently of illumination or applied bias. The band diagrams of the perovskite/HTL interface before and after pre-accumulation are shown in Figure 3. The effect of illumination on J-V performance was also investigated on identically prepared fresh sample. The J-V curves measured during light soaking process (by keeping the illumination on and the cells at OC between J-V scans) are plotted in Figure 2b and the evolutions of short-circuit current density (JSC), open-circuit voltage (VOC) and fill factor (FF) are shown in Figure 2c to 2e, respectively. J-V scans without continuous light soaking (where illumination is off between J-V scans) was also carried out. Results are shown in Figure S4, which agree with the trend observed for the Stage 0 PL results - repeatable results in the absence of light soaking. The existence of pre-accumulated ions explains the shape of J-V curve at V = 0 and t = 0 s (black curve in Figure 2b and inset of Figure 2b). The positive slope of the current densityvoltage curve at V = 0 is attributed to the decrease and subsequent removal of preaccumulated ions during the J-V scan. Prior to J-V scanning, the pre-accumulated positive charges in the Debye at the interface screen the built-in electric field in the perovskite bulk [10, 22, 33, 34] (cf. bands in the bulk between Figure 3(b) and Figure 3(a)) reducing the charge carrier separation and extraction efficiency[10, 33]. During the J-V scan, as photogenerated carriers are being extracted, the electric field at the interface is disturbed causing the pre-accumulated positive ions to diffuse away from the interface. This causes in increase in the built-in field in the perovskite bulk similar to what is shown in Figure 3a. This results

9

in more efficient carrier transportation. Therefore, the J-V curve exhibits a positive slope at V = 0. For our devices, ion migration may impact both carrier recombination and carrier extraction and hence affect the PL decay. The carrier recombination dynamics can be expressed as

, where the terms represent recombination

via defect trapping (Shockley-Read-Hall recombination), free electron-hole recombination and Auger recombination, respectively. Free electron-hole recombination rate is an intrinsic property of the material which is extraordinarily low relative to defect trapping and Auger recombination at this excitation level is negligible [41]. During Stage 1 (tens to hundreds of seconds), the PL measurements show enhancement of both the intensity and effective lifetime. At OC, carrier extraction is not expected as a limiting factor for PL lifetime. By considering that the bulk defect density in our high quality perovskite films is expected to be quite low compared to the interfacial trap density[42], we assign the enhanced PL intensity and lifetime can be attributed to reduction of the carrier recombination via traps at the HTL/perovskite interface. It is consistent to the J-V measurements which show a performance improvement in Stage 1. The mechanisms of the Stage 1 light soaking effect on PL and J-V measurements can be visualized with energy band diagrams. Figure 4 illustrates the band states throughout different stages of the light soaking process at open circuit. At the beginning of illumination, photo-generated carriers split the quasi-Fermi levels. As shown in Figure 4a, the accumulated ions with slow dynamics keep the band bending near the interface while the charge carriers (electron-holes) with fast dynamics establish an inversion electric field in the perovskite bulk to maintain zero net current across the device. The inversion field causes electron flow towards the HTL[43, 44]. More electrons therefore can be captured by the interface and recombined with holes in the HTL via surface states.[43] During the first stage of light

10

soaking at OC (Stage 1), the bulk electric field directed towards the ETL causes negative ions in the perovskite bulk to drift to the HTL/perovskite interface and neutralize the preaccumulated positive ions within tens of seconds. The band diagram at the end of Stage 1 is shown in Figure 4b. The increased built-in field enhances the carrier separation and reduces surface recombination, resulting in improved JSC and VOC [10, 43, 44] respectively as observed in Stage 1 results. In addition, we can observe that the positive slope at V = 0 disappears for J-V curves at t > 0 confirming the absence of positive ions at the interface as a result of light soaking at OC. With prolonged illumination (Stage 2), there is no more accumulated positive ions at the HTL/perovskite interface but negative ions (e.g. I-) started to be accumulated. This process corresponds to the previously reported light-induced self-poling[19]. With accumulation of negative ions, the energy band state at the end of Stage 2 is represented schematically in Figure 4c. The band bending resulting from the accumulation of negative ions at the HTL interface increases the built-in field in the bulk temporarily which improves carrier extraction [12, 19] causing an overshoot of JSC at V=0 (see t  90s curves in the inset of Figure 2b). During the J-V scan, accumulated negative ions migrate away from the HTL interface reducing the built-in field in the bulk and thus decreases the current density when the biasing voltage is small. As the sweeping voltage in the J-V scan becomes larger, negative ions move back towards the HTL interface which act as traps [22]. The conduction band bending at the HTL (see Figure 4c) also results in trapping of electrons at the HTL/perovskite interface, increasing surface recombination.[45] Both of these cause an increase in SRH recombination resulting in the reduction of PL intensity and lifetime in Stage 2 of light soaking as shown in Figure 2a. They are also responsible for the performance losses as shown in Figures 2b to 2e. Although the TiO2/perovskite interface is not assessed by the PL probe in this experiment, it is anticipated that opposite charge ions will follow the similar pre-accumulation, movement

11

and post-accumulation pattern at the opposite electrode at different stages of light soaking producing similar effect on J-V characteristics. After the light soaking process, we stored the sample in the dark and the PL degradation largely recovered after a few minutes, as shown in Stage 3 in Figure 2a. This means that the ion migration induced by light soaking is reversible, which is consistent with previous findings.[21, 22] To reveal the effect of microstructure on lifetime, successive images showing the spatial distribution of both PL intensity and PL lifetime were obtained on a small region (15 * 15 μm) as shown in Figures 5a to 5d. In these images, the local brightness indicates PL intensity and color indicates carrier lifetime. The original sample measured before light soaking shows a relatively uniform distribution (see Figure 5a). As light soaking progresses at OC, however, PL uniformity gradually decreased (Figures 5b-d). A number of bright spots appeared on the image after light soaking, as shown in Figures 5c and 5d. Figure 5e shows PL intensity distributions over the measured area before (blue curve, Stage 0) and after (red curve, Stage 2) light soaking. The two curves are fitted by Gaussian function. As can be seen, the full width at half maximum (FWHM) increased from 43.9 to 66.7 after light soaking for 10 minutes. The wider distribution indicates that the perovskite layer became less uniform. To understand this change, we track the PL evolution of two spots in the sample (spot A which is brighter and spot B which is dimer to start) (see Figure 5b). It is evident that contrast between the two spots increases with illumination. The localised PL intensities of the two spots were extracted plotted against light soaking time in Figure 5f. PL intensity of spot A increased initially in Stage 1 but PL intensity of spot B decreased in Stage 1. Given that spot A and spot B had the same PL intensity at t = 0, the most plausible reason for the different trend observed is the faster annihilation of pre-accumulated positive ions in spot B. Although PL intensity at spot A eventually decreases with longer light soaking, the rate of decrease is

12

always faster at spot B which also implies a faster accumulation of the negative ions at spot B in comparison to spot A. As shown in Figures 5c and 5d, spot A and other brighter spots have relatively regular shapes and uniform sizes. We extracted the line profiles of the PL intensity and determined that these bright spots are comparable in size to the largest grains in our perovskite films, as shown in Figure S5. Therefore, we can reasonably assign the brighter spots to large grains in the perovskite film and the dimmer spots/regions to clusters of small grains. The faster ion migration in the dimmer regions may be due to a larger density of grain boundaries (GB), as proposed by previous investigations of ion migration in PSCs induced by external voltage bias [32, 46, 47]. The mechanism can be ascribed to either the low activation energy and/or high ionic diffusivity from the relatively open structure at GBs; and/or the higher concentration of ions at GBs, which promotes faster migration [32, 46] and therefore greater J-V hysteresis [10, 32, 36, 48, 49]. These findings suggest that large grains with fewer GB can help to reduce the hysteresis of PSCs by slowing down ion migration. 3.2 Light soaking effect at short-circuit (SC) At SC, as extracted holes and electrons recombine instantly in the external circuit, the observed lifetime in our MAPbI3 cell depends on how quickly holes and electrons are extracted to the HTL and ETL, respectively. The effective lifetime at SC before light soaking is found to be ~2.2 ns (first 3 data points in Figure 6a) which is much shorter than that at OC (~10.7 ns, see first few data points in Figure 2a). Our value at SC is close to the reported hole extraction time from CH3NH3PbI3 to spiro-MeOTAD [50]. Figure 6 shows the effect of light soaking at SC on MAPbI3 PSC’s PL and J-V characteristics. Again, the illumination conditions are divided into the following stages: before illumination (Stage 0), during illumination (Stages 1 and 2) and after illumination (Stage 3). Similar to the case at OC, there was negligible variation in PL intensity or lifetime before illumination (Figure 6a) However, the variations in PL intensity and lifetime at SC are

13

smaller than 15% and 10%, respectively, while the PL intensity and lifetime variations at OC were over 40% and 25%, respectively (Figure 2a). The significant difference between the dynamic responses between SC vs OC conditions indicates that the applied voltage plays an important role in ion migration. In contrast to OC conditions, at SC, PL drops during Stage 1 of light soaking and the effective lifetime also decreased slightly (Figure 6a). The PL drop in Stage 1 is attributed to increasing hole extraction efficiency at SC compared to OC conditions. In addition, the ionic dynamics were slower at SC (longer in Stage 1) and the light soaking effect was weaker (reduced PL and J-V variation). This is because at SC, there is no biasing voltage, the movement of mobile ions is weak either by drifting caused by local electrostatic fields from surrounding charges or by diffusion caused by local concentration gradients. Under light illumination, holes being extracted by HTL disturb the electrical field at that interface and make the ionic vacancies having a higher chance to recombine with interstitials [17, 18] reducing the density of ionic traps (i.e. I-)[17, 18] and reducing the pre-accumulated positive ions at the interface as shown in Figure 7b. The increased built-in field in the bulk enhances the carrier separation and reduces surface recombination resulting in an improvement of J SC and VOC in Stage 1 (Figure 6c and 6d). The resultant faster extraction of holes by HTL is responsible for the PL drop in Stage 1. For the same reason, a positive slope in the J-V curve at V = 0 is observed again at t = 0 in Figure 6 (b) causing an inflated FF to be reported at t = 0 in Figure 6e. In Stage 2, the effect of light soaking on PL measurement was negligible and the device performance became relatively stable. This suggests that further illumination at SC does not significantly affect the carrier dynamics (Figure 7c) as opposed to the OC case. When the bias light was switched off (Stage 3), PL intensity took several minutes to recover to its initial value similar to the OC case.

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We then analyzed the corresponding PL images by tracking the PL response upon light soaking at SC in two different regions, Region 1 (red) and Region 2 (blue) shown in Figures 8a to 8d. The average PL intensity of each region was plotted as a function of soaking time in Figure 8f. The PL intensity of Region 2 was initially (Figures 6a and 6b) lower than that of Region 1 but they became similar after light soaking (Figure 6c and 6d). In Stage 2, the average PL intensity in these two regions converge (Figure 6f) and remained relatively stable due to the redistribution of ions. Figure 8e also shows the narrower spread of PL intensity over the measured area after light soaking (red curve – Stage 2). This is because under light soaking at SC, ions move by diffusion caused by light-induced concentration gradient rather than by drift. The redistribution of ions results in more uniform PL images.

4. Conclusion In summary, we performed dynamic monitoring on the localized charge carrier dynamics on complete perovskite solar cells. By fabricating PL-compatible perovskite cells, we are able to study the effects of light soaking on an operational perovskite solar cell using in-situ photoluminescence microscopy. We found that pre-accumulated positive ions (e.g. iodide vacancies) reduce charge carrier separation of the device. The concentration of preaccumulated ions can be minimized by light soaking under at either OC or SC. At OC, the annihilation of pre-accumulated ions occurs rapidly within tens of seconds. Initially, light soaking improves both JSC and VOC of the devices. After prolonged light soaking, however, negative ions drift to the spiro-MeOTAD/perovskite interface causing decreased performance. This is attributed to increased trapping by defects at the interface due to either increased ionic traps or capture of carriers at the interface. Localized PL analysis showed that ion migration is slower in large grains. At SC, rather than movement by drift, movement of ions is dominated by diffusion. This results in ionic redistribution and ion-recombination, which increases PL intensity uniformity across the device and resulting in relatively stable

15

device performance. Our work reveals that the bias voltage during light soaking results in different dynamic processes which can be either positive or negative. The findings also suggest that the light soaking effect pre-dominantly occurs at the perovskite/transport layer interface

due

to

ion

accumulation.

Therefore,

it

is

crucial

to

explore

the

architecture/fabrication dependent illumination induced dynamic processes for better understanding of the operation of perovskite solar cells in the future.

Supporting Information Additional details on sample fabrication, temperature variation during light soaking, thickness of post-evaporated gold layer, discussion of J-V curves by reverse scan during light soaking process, are available from the website free of charge.

Acknowledgement The Australian Centre for Advanced Photovoltaics (ACAP) encompasses the Australianbased activities of the Australia-US Institute for Advanced Photovoltaics (AUSIAPV) and is supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). This project is also supported by ARENA via the project 2014 RND075. We thank the Electron Microscopy Unit and the BioMedical Imaging Facility at UNSW for the SEM and fluorescence imaging supports.

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Figure 1: (a) Schematic diagrams of sample structure and metallization pattern; (b) Schematic diagram of PL characterization method.

Figure 2: PL and J-V measurement results for MAPbI3 solar cells light soaked at OC. (a) PL intensity and effective lifetime as a function of light soaking time showing four different stages which are before light soaking (Stage 0), during light soaking (Stages 1 and 2) and

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after light soaking (Stage 3); (b) J-V curves during light soaking. Evolution of (c) Voc, (d) Jsc, and (e) fill factor as a function of light soaking time.

Figure 3: Energy band diagrams at the HTL/perovskite interface that (a) and (b) are before pre-accumulation and after pre-accumulation, respectively, in the dark.

Figure 4: Energy band diagrams at the HTL/perovskite interface illuminated at OC. The diagrams show the band bending and ionic dynamics (a) at the beginning of light soaking, (b) at the end of 1st stage light soaking and (b) at the end of the 2nd stage of light soaking.

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Figure 5: PL images of an MAPbI3 PSC at OC (a) before, after (b) 156 s, (c) 390 s and (d) 624 s of light soaking. Spots A and B are marked by two red circles in the images. (e) The distribution of PL intensity over measured area before (blue) and after (red) light soaking and (d) Normalized PL intensity of the spot A (red) and spot B (blue) as a function of light soaking time.

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Figure 6: PL and J-V measurement results for a MAPbI3 PSC light soaked at SC. (a) PL intensity and effective lifetime as functions of light soaking progress showing four different stages which are before light soaking (Stage 0), during light soaking (Stage 1 and 2) and after light soaking (Stage 3); (b) J-V curves of the cell during light soaking process; (c) Voc, (d) Jsc, and (e) fill factor as functions of light soaking progress.

Figure 7: Energy band diagrams at the HTL/perovskite interface illuminated at SC. The diagrams show the band bending and ionic dynamics (a) at the beginning of light soaking, (b) at the end of 1st stage light soaking and (c) at the end of 2nd stage of light soaking.

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Figure 8: (a) PL images of an MAPbI3 PSC at SC (a) before, after (b) 156 s, (c) 312 s and (d) 624 s of light soaking. Region 1 and Region 2 are marked by a red square and a blue square, respectively for further analysis. (e) The PL intensity distributions over measured area before (blue) and after (red) light soaking. (d) Average PL intensity in Region 1 and 2 as a function of light soaking time.

Xiaofan Deng is a PhD candidate at the school of photovoltaic and renewable energy engineering, University of New South Wales (UNSW). Prior to starting the PhD program, he completed his Engineering Bachelor Degree in UNSW with first class honour. His research interests mainly focus on carrier and ion dynamics in perovskite solar cells.

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Xiaomign Wen is a senior research fellow at Centre for Micro-Photonics at Swinburne University of Technology. He received his PhD from Swinburne University of Technology in 2007. His expertise includes ultrafast time-resolved spectroscopy and microscopy. His research focuses on photogenerated carrier dynamics and photon-carrier interaction, in particular, in photovoltaic and photocatalytical materials and devices.

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Jianghui Zheng received his B.S. degree in Materials Science and Engineering from Tongji University in 2011 and Ph.D. degree in Photovoltaic Engineering from Xiamen University in 2017. He was also a joint PhD student at The University of New South Wales (UNSW) from 2016 to 2017. Currently, he is a research assistant in UNSW and his research interests mainly focus on high-efficiency large-area Perovskite/Silicon tandem solar cells.

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Dr Trevor Young is a Senior Research Fellow at the School of Photovoltaic and Renewable Energy Engineering at UNSW, Australia. Trevor received his Ph.D. in physical chemistry from the University of Queensland in 1987 before joining the UNSW team later that year. He was a foundation employee of Pacific Solar, CSG Solar and Suntech R&D Australia responsible for device fabrication and the development of contacting strategies. Trevor returned to UNSW in 2014 where he now works on perovskite cell fabrication and new solar cell materials.

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Cho Fai Jonathan Lau received his Bachelor's degree and is currently a Ph. D. at University of New South Wales. His research interest mainly focuses on perovskite solar cell.

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Jincheol Kim received his B.S. and M.S. in materials science and engineering from Seoul National University, Korea in 2008 and 2011, respectively. He is currently a Ph.D. student in school of photovoltaic and renewable energy engineering, UNSW. His research interests focus on the development of high performance perovskite solar cell and its stability.

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Martin Green is Scientia Professor at the University of New South Wales, Sydney and Director of the Australian Centre for Advanced Photovoltaics, involving several other Australian Universities and research groups. His group's contributions to photovoltaics are well known and include holding the record for silicon solar cell efficiency for 30 of the last 34 years, described as one of the “Top Ten” Milestones in the history of solar photovoltaics. Major international awards include the 1999 Australia Prize, the 2002 Right Livelihood Award, also known as the Alternative Nobel Prize the 2007 SolarWorld Einstein Award and, most recently, the 2016 Ian Wark Medal from the Australian Academy of Science.

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Shujuan Huang received her BE and ME in Electrical Engineering from Tsinghua University China and PhD in Material Engineering from Hiroshima University Japan. She joined the School of Photovoltaic and Renewable Energy Engineering at UNSW Sydney in 2006. Her research interests include nanomaterial synthesis using both chemical and physical methods, nanomaterial characterization, and Third generation and advanced concept photovoltaics.

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Dr. Anita Ho-Baillie is an Associate Professor at the School of Photovoltaics and Renewable Energy Engineering at the University of New South Wales. Her research interests on Si, thin film solar cells including perovskite solar cells span from design to fabrication, manufacturing costing and their integration for tandems (Si/III-V, Si/perovskite and perovskite/perovskite tandems), buildings and vehicles. She has been leading the perovskite solar research group at UNSW since 2013 and at the end of 2016 announced the energy conversion efficiency record for the largest certified monolithic perovskite solar cell.

Highlights 

Complete perovskite solar cell (not test structure) compatible with in-situ photoluminescence microscopy is fabricated for the first time allowing in-situ study of localized charge carrier dynamics during light soaking.



Dynamic PL intensity and lifetime under different light soaking conditions and its correlation with the shape of the voltage current curve explains the different scenarios of ion migration and accumulation, providing explanations of the different hysteresis behaviors caused by light soaking.



High resolution (nanoscale) photoluminescence microscopy reveal different charge carrier and ion dynamics between grain interior and grain boundary during light soaking highlighting the advantage of larger grains.

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