Performance and stability of hybrid organic-inorganic perovskite photovoltaics in outdoor illuminations conditions

In this study, we conducted real-world outdoor performance and stability tests on two different configurations of perovskite solar cells in the climate conditions of Bahir Dar, Ethiopia. Under outdoor illumination with an incident power of 69 mW cm−2, this device demonstrated a power conversion efficiency (PCE) of 15% for an active area of 0.1 cm2. The maximum power generated, measured at a solar irradiance of 72 mW/ cm2, and was 1.23 mW. The second perovskite device has planar p-i-n configuration. This device achieved a PCE of 18% without masking, but the PCE dropped to 11% when the device was covered with circular aperture mask. Maximum power tracking and stability measurements of the device were conducted. Maximum power generation occurred at operating voltages ranging from 0.8 V to 1.2 V for a device with an active area of 0.095 to 0.1cm2. After 30 days of environmental exposure, the device maintained more than 90% of its initial PCE.


Introduction
Perovskite solar cells (PSCs) represent one of the most promising emerging solar cell technologies due to their high-power conversion efficiencies (PCE).Recent research has demonstrated the PCEs exceeding 25% for single junction device architectures and above 33% for planar perovskite/silicon tandem device architectures [1][2][3][4][5][6][7].The performance of perovskite/silicon tandem surpasses the Shockley−Queisser (SQ) limit of conventional, single junction silicon photovoltaics (PVs), establishing PSCs as strong contenders for low-cost solar power generation systems [8].The performance of these devices primarily depends on factors such as the device architecture, composition of the absorber layer, morphology, and thickness of the perovskite film [9,10].Hybrid organic-inorganic halide perovskites have garnered significant attention owing to their excellent optoelectronic properties.These materials exhibit strong absorption coefficient, tunable direct band gap energy, long free charge diffusion lengths, high charge carrier mobilities (with long lifetimes), ambipolar transport of charges, low defect density, small Urbach energy, and weak exciton binding energy [5,8,10,11].
Perovskites have ABX 3 crystal structure where A is monovalent cation (usually Cs+, MA+, and FA+), B represents divalent cation (usually Pb 2+ or Sn 2+ ), and X represents halide anion (I − , Br − , Cl − ).The structural stability of the perovskite is determined by tolerance factor [12][13][14][15][16][17].The tolerance factor suggests that the variable radii within the thin film perovskite layer may induce morphology imperfections.Thus, the presence of defects into the perovskite layer (e.g. grain boundaries) expedites electronic charge carrier recombination and material degradation.To ensure the effectiveness of perovskite PV technology at an affordable cost, it is crucial to guarantee that PV modules possess long-term stability and high power conversion efficiency (PCE) comparable to conventional PV systems.Yet, PSCs have not attained the desired stability necessary for commercializing this technology [18,19].Perovskite devices or films degrade rapidly under environmental exposures such as high temperatures, illumination, moisture, and oxygen.However, encapsulating PSCs can help prevent moisture from entering the device, thus serving as one strategy to enhance stability [6,7].
Many studies that have undertaken on stability and performance tests on PSCs involve storing the device in ambient conditions.However, such testing conditions may not accurately simulate real PV module operations [7].There are very limited reports on outdoor testing of perovskite solar cells under outdoor conditions [7,[19][20][21][22][23]. Our review reports on outdoor measurements of perovskite solar cells shows that there are no reports from Africa climate conditions, characterized by elevated temperatures and strong solar radiation throughout a year.Therefore, testing perovskite solar cells under realistic outdoor environment [11] in Africa will enable us to assess their stability and provide pathways for commercialization and its market demand.Consequently, this work first conducted under controlled laboratory conditions at the University of Cambridge and then under outdoor conditions in Bahir Dar, Ethiopia.Perovskite solar cells are highly sensitive to environmental factors, and location specific studies helps to understand the extent to which device performance and stability can be affected by environmental factors.The climate conditions, including irradiance and temperature at the site significantly affect the performance and stability of perovskite solar cells.
In order to study the performance and stability of perovskite solar cells, it is essential to conduct J-V scans followed by maximum power point tracking under outdoor illumination and high temperature conditions [24].
In this work, we study the perfromance and stability of two types of perovskite devices.The first device has a configuration of FTO/TiO2/perovskite/ spiro-OMeTAD/Au with the perovskite has triple cation and mixed halide composition Cs  Researches conducted on similar composition of these devices reports that it has very good PCE and can with stand degradations since it often has small amount of phase impurities [12,25].We used this as baseline and change the composition to prepare the device and conduct outdoor testing to study performance (including the shading response) and stability.The following sections shows the experimental methods employed and the results obtained.

Experimental methods
The following characterization techniques had been employed to study the structural, optical, and electrical properties of the perovskite films/devices.

X ray diffraction (XRD)
The phase compositions, crystal structure, and crystal orientation were studied using powder XRD (XRD-7000 x-ray Diffractometer) with an x-ray tube at copper target with an accelerating voltage of 40 KV (wavelength 0.031 nm) and current of 30 mA.The XRD measurements provide spectral analysis of intensity as a function of diffraction angle 2θ (angle between the incident beam and diffracted beam).The scan was performed from 5 degree to 90 degree at scan speed of 3 deg min −1 in a continuous scan mode.During the measurement, smoothing and background subtraction were conducted.Baseline correction and further smoothing were also performed during the data analysis.

Absorbance
UV-VIS spectroscopy (using DR 6000) measurements were employed to study the absorbance of the deposited absorber layer/film.The wavelength was scanned from 300 nm to 800 nm.

Photoluminescence (PL)
Steady-state photoluminescence spectroscopy measurements were performed using a Cary Eclipse fluorescence spectrometer (Agilent) instrument to study the perovskite fluorescence emission.The setup utilizes a deuterium lamp light source at an excitation wavelength of 500 nm.Fluorescence emission wavelengths were scanned from 520 nm to 1000 nm at scan rate of 600 nm min −1 .

Solar cell parameters
All the current-voltage and stability measurements in this work are conducted under outdoor illumination in Bahir Dar, Ethiopia climatic conditions unless stated otherwise.
The solar cell parameters of the first devices were studied by current-voltage sweep characteristics using a Keithley 2400 source meter unit (SMU) with IV tracer software in a scan range of -1 V to 1.2 V.The first device was tested at a scan rate 300 mV s −1 while the second device was tested at a scan rate of 43 mV/s for an active area of 0.1 cm 2 without masking.Multipixel testing with the same scan range but at scan rate of 100 mV s −1 with active area of 0.095 cm 2 defined by a circular aperture mask, was also used to characterize the second device.The multiplexer measures four devices, each with seven pixels (devices pictures in supporting information figure 1) simultaneously.

Maximum power point tracking and stability
To estimate the maximum power generated from the PSCs, current-voltage measurements were conducted first.This result was used to determine the operating voltage at the maximum power point (MPP).Subsequently, DC (continuous) measurements were performed continuously at the previously determined operating voltage to track the current output over time.The measurements were taken from the instrument and displayed in realtime graphs.Initially, for the first batch of devices, two samples of the same composition and device architecture labelled as sample A and sample B were considered for testing.To track the maximum power point, the J-V (P-V extracted from it) measurements were carried out within every one hour interval to include peak radiation hours per day (peak ∼ 100 mW cm −2 ) (figure 1, supporting information figure 3).For the second batch of devices, the PCE has been measured at different times to observe the degradation effects, PCE versus time (figure 2, supporting information figure 6).This device was placed under outdoor illumination during measurement but stored under ambient room temperature after every day's measurement was completed.The maximum power  tracking and stability test measurements were conducted at the maximum power point with the previously defined active area using the Keithley 2400 for the first batches and using multipixel testing for the second batches.The PbIBr and CsFA phases exists at diffraction peaks (110) and (224) respectively.Additionally, there are small diffraction peaks at 24.5°, 30.26°, 34.9°, 37.5°, which indicate the formation of pure phase perovskite film [26][27][28][29].

Absorbance and band gap energy
The UV-Vis spectra measurement in figure 4 shows the occurrence of strong absorption peaks at wavelengths of 411 nm and 514 nm.These results indicate that the perovskite film strongly absorbs the ultraviolet and visible regimes of the solar spectrum, compared to the infrared region.
The band gap energy can be determined using the Tauc-plot method, which can be expressed for direct band gap transitions as Perovskites exhibit direct band gap transitions that follow the transition rules stated in equation (1).The intersection point of the extrapolation of the linear region of the graph of ( ) a n h 2 versus energy and the horizontal axis determines the band gap energy.Using this technique, the band gap energy of the perovskite layer is observed to be 1.69 eV.This result is consistent with our PL measurements and also agrees with others works [30,31].

Steady state PL measurement
Figure 5 shows the steady-state PL emission spectra of the double cation perovskite film as a function of (a) emission wavelength (b) and photon energy.The results indicate the presence of strong emission at 750 nm, suggesting that the band gap energy of the device is 1.65 eV at a margin of 1.654 eV with a margin error of 0.004 eV.Previous literature reports a band gap energy of 1.68 eV [32].The emission peak exhibits a Gaussian line shape with a full width at half maximum (FWHM) of 47.8 nm (corresponding to 25.94 eV resolution).The broadening of the peak suggests weak emission energy in the composition.This result aligns with the findings of Saliba et al [33].The measurement indicates light is absorbed by the perovskite molecules, exciting them to higher energy levels, and subsequently emitted after recombination, which is caused by defects and charge traps.The PL emission peak of this perovskite composition is blue shifted when the concentration of Cs increases [12], indicating a decrease in trap density [31] and an increase the spectral broadening of the line shape, as manifested by the FWHM.The PL spectra peak corresponds to the band-to-band transitions of excitonic states.The PL result is in agreement with other works [30,31].The first device (triple cation device) was encapsulated before XRD, absorbance, and PL measurements were conducted.The results of these measurements for the encapsulated devices are not suitable reporting here, as they already contain many layers apart from the perovskite layer.For the second batch (double cation) XRD, absorbance, and PL measurements were performed at the perovskite film stage.This facilitated the study of the structural and optoelectronic properties of double cation perovskite (when MA is totally replaced by other cations).However, the main purpose of this work is to study the outdoor performance and stability of the mesoporous n-i-p and for the planar p-i-n perovskite devices.

J-V sweep of the device under outdoor illuminations
The current-voltage characteristics of the encapsulated perovskite devices were measured in both forward and reverse scans.The sweeps were performed by scanning the voltage and measuring the current using source meter.The measurements were conducted from January to May around midday (13:47) for first device configuration, and from October to March of the following year around midday (13:47) for the second device configuration, to explore the performance and stability.Figure 6 show the JV-cruves of the first device  configuration in the forward and revese scans.The device exhibited a PCE of about 15%, a fill factor (FF) of 61.3%, an open circuit voltage (V oc ) of 1.0 V, and a short circuit current density (J sc ) of 17 mA cm −2 in the forward scan (from V sc to V oc ).In the reverse scan (from V oc to V sc ), it showed a a PCE of around 14%, FF = 57%, V oc = 0.97 V, and J sc = 17.12 mA cm −2 .
The second device configuration demonstrates a measured PCE of about 18.33%, FF = 59.8%,V oc = 1.09V, and J sc = 16.33 mA cm −2 in the forward scan, while.a PCE of about 11.79%, FF = 43.7%,V oc = 0.988 V, J sc = 15.84 mA cm −2 in the reverse scan when the devices were measured freshly under outdoor illumination.These devices have an average PCE of 15.1% and 17% respectively, under indoor controlled environment at AM 1.5 G radiation ∼100 mW cm −2 measured at University of Cambridge.The device performance under the two measurements conditions is almost the same.However, the device can show degradation when shipped from Cambridge to Bahir Dar for outdoor testing.Thus, we would have a lower PCE for the outdoor case due to degradations.This shows that the outdoor PCE of the device is slightly higher than the controlled indoor PCE (if the measurements were conducted simultaneously).The report on the second device still shows a difference of 1.33% (8% disparity with the controlled indoor test).The J-V curve indicates that the open circuit voltages of these devices are nearly 1 V and 1.1 V respectively without masking, and there is hysteresis in device (figure 7(a),) which is usually caused by the presence of mobile ions in the perovskite.
It has been observed that the parameters of photovoltaic solar cells change over time.The PCE of these devices often fluctuates under continuous exposure to the solar radiation.During the J-V measurements, changing the scan directions results in changes to solar cell parameters, and the sweep reveals a discrepancy between the forward and reverse scans (i.e.hysteresis).Observations on the hysteresis were made by scanning with 50pts, 100pts, and 200 pts.The scan rate decreases as the number of scanned points increases, and therefore, the discrepancy of the curve in the forward and reverse scan directions strongly depends on the scan rate [34].
When the measurement is performed at a high scan rate (50 pts), the discrepancy in the curves is not significant.This discrepancy confirms hysteresis behaviour of the device, which is caused by trap states and mobile ions in the perovskite, as well as the presence of defects or migration of ions, which limits the device performance by affecting the open circuit voltage [35].The device hysteresis is observed at lower photocurrent and slow scan rates compared to high photocurrent and fast scan rates.This observation is consistent with the dynamic changes in the polarization of perovskite absorber layer during J-V scans.It is associated with the differences in photocurrent, which strongly depends on the properties of contact layers [36].To study the behaviour of the devices under shading effects, they have been exposed to intermediate shading environment for seconds during J-V sweeps.It is observed that under shading, the device behaves similarly to under dark illumination (figure 7(b)).
Figures 7 (c) & (d) show the performance of the second device configuration with masking.This measurement was conducted on October of the following year of the previous measurements, during peak radiations hours, using multipixel testing.Under this measuring conditions, despite slight variations between the forward and reverse scans, a PCE 11.1%, a FF= 65%, V OC =1.17 V, and J SC =14.5 mA cm −2 are the observed solar cell parameters of the device.There is a difference in the solar cell parameters between the measurements conducted without mask and with a shadowing mask.The results show that masking prevents device edge effects caused by scattered light, overestimation of short circuit current density, underestimation of the open circuit voltage, and the fill factor of the device.It indicates that masking enables obtaining an accurate report of the device hysteresis.Therefore, it is desirable to use shadowing mask during J-V measurement of a solar cell under laboratory conditions where a solar simulator is used, as well as under outdoor measuring conditions [37][38][39].
In many studies, it is common to observe that the reverse scan PCE is higher than forward scan PCE.However, here we observe the opposite: the forward scan performance of the perovskite better than the reverse scan, indicating that the characteristics of the device change during the scanning process.One possible reason for this could be trap filling and light induced degradation.During the forward scan, the charge carriers occupy the trap states, enhancing the device performance by reducing the recombination losses due to trap states.These trap states may eventually be evacuated when the reverse scans are performed, increasing recombination, which can lower device performance.On the other hand, the device exhibits light-induced degradation when exposed illumination, which can alter the optoelectronic properties of the perovskite.Performing the forward scan after the device has stabilized under illumination can lead to better performance than the reverse scan.
The PCE perovskite solar cells heavily influenced by factors such as morphology, charge carrier dynamics at layer interfaces, and defect density.Grain boundaries, vacancies, and defects within the perovskite layer hinder charge carrier mobility, thereby lowering device efficiency.The perovskite composition determines absorption characteristics, which is crucial for PCE optimization.Both n-i-p and p-i-n device architectures, comprising electron and hole transport layers, absorber layers, and electrodes, can cause carrier recombination at interfaces, impeding transport.Charge carrier extraction relies on electrical contacts between transport layers and perovskite, indicating the significance interface dynamics for the PCE of the device.Defects and vacancies act as traps, limiting material electronic properties and reducing device PCE.Incorporating Cs improves perovskite film morphology, reduces grain boundaries, enhances charge mobility, and consequently increases device PCE [40].-i-n) device architecture of using multipixel testing.The multiplexer scans four devices with 32 pixels at a time.To exclude inactive devices and pixels, these result is selected from active pixels of devices that exhibit non-zero current and voltage under illuminations.These two results demonstrate the repeatability of the measurement and the dynamic nature of device response, including its variations in hysteric effects.

Maximum power point tracking and stability
Another technique used to characterize the performance of a solar cell is the maximum power point tracking.This involves measuring the current over time at fixed operating voltage of the device.The operating voltage of the solar cell is the voltage at which the PV module can produce maximum power, which can be extracted from J-V measurements of the device.The maximum power generated from a solar cell depends on the irradiance.In most cases, more power is generated from a solar cell at high incident radiation, which is strongest at midday (see supporting information figure 3).The stability of maximum power point tracking (MPPT) in PCE measurement is influenced by the climatic conditions where the outdoor testing is located [41].At particular irradiance and temperature, there is a fixed operating voltage on the J-V curve at which the power is maximum [42].For the perovskite device to generate maximum power output, it should operate at the corresponding peak of the P-V curve called maximum power point (MPP) (see figure 1).
Figure 8 shows current output tracked over time at fixed operating voltages of 0.56 V for the first device configuration.In indoor laboratory measurements, both the temperature and the irradiance can be controlled.However, outdoor measurements are conducted under continuously varying irradiance and temperature conditions, which can alter the current of the PSCs by affecting their band gap.An increase in incident irradiance leads to an increases in current and a decrease in voltage of the perovskite device [42].The device is exposed to spectral distribution, angle of incidence, intensity, and ambient temperature under realistic outdoor conditions.These parameters changes due to shading effects, which in turn affect the actual performance of the device [20,43].
Figure 2 shows the current output of the second device tracked over time at fixed operating voltage of 0.9 V.The requirement for longer time measurements to study solar cell parameters poses challenges due to degradation in PSCs.Although the short circuit current density depends on the device architecture, light soaking at open circuit followed by subsequent fast J-V scan rates is recommended to achieve reproducible measurement results.In other words, MPPT characterization shall be preceded by initial transient current density characterization of the PSCs [44].Thus, MPP tracking measurements are utilized to analyze the electrical performance of PSCs [45].These measurements can be extended to longer duration to investigate the long-time operation of the cell under maximum power operating conditions [45].The operation of PV systems predominantly depends on temperature and radiation, which respectively act as voltage and current sources for the PV system [46].
The causes of current oscillations in the current-time measurements are fluctuations in irradiance and temperature (see figure 2, supporting information figure 6).The presence of noise in the measurements typically corresponds to significant changes in oscillations.Thus, to reduce noise, we should decrease the oscillation, which can be achieved by tracking at faster rate and operating near the MPP with a small fixed step size [47].For solar cell life time assessment, long-term measurements are crucial.However, life time measurements in seconds, minutes, hours, and days are also valuable for studying the performance and stability of perovskite devices.Such measurements can provide standardized quantitative metrics for studying the performance and the rate of degradation over time.They can also used for simulation and prediction of how the device will behave under long-term outdoor exposure.We have conducted measurements ranging from seconds to months (see figures 8 & 2, supporting information figure 5).The ultimate goal is to study the long term stability of perovskite devices, which is not feasible throughout the entire device's lifetime.However, short-term measurements can also contribute significantly demonstrating the device's time evolution and informing research and development processes.

Conclusions
The absorbance and band gap energy a perovskite film can be determined using VU-VIS and steady state PL spectroscopy measurements.The p-i-n perovskite composition exhibits astrong fluorescence emission peak at 750 nm, corresponding to a band gap energy of 1.65 eV, in alignment with the previously studied literature.
The work primarily focuses on the outdoor performance and stability of the aforementioned perovskite devices in Bahir Dar, Ethiopia's climatic conditions, utilizing J-V sweep and maximum power tracking measurements.The first device demonstrates a power conversion efficiency (PCE) of 15%, a fill factor (FF) of 63%, V oc of 1.0 V, J sc of 17 mA cm −2 .The second device exhibits a PCE of 18%, a FF of 59%, V oc of 1.1 V, J sc of 16.33 mA cm −2 .Masking the device helps to clearly define its active area, preventing overestimation (as with J SC ) or underestimation (as with V OC ) of the solar cell parameters during measurement.
The important electrical performance parameters of solar cells change over time during outdoor, in situ measurements under continuous solar radiation.The device's PCE fluctuates under continuous solar illumination ambient temperature variations.Additionally, during the J-V measurements, altering the scan directions leads to changes in the solar cell parameters, resulting in a discrepancy between the forward and reverse scans.This observed hysteresis curve indicates the presence of trap states and mobile ions within the device.
The transient response of the device is influence by inherent defects within the solar cell.One potential approach to mitigate the performance degradation is by increasing the concentration of Cs in the perovskite, thereby reducing trap charge density and recombination rates in the absorber layer.The operation of PV systems primarily depends on radiation, which act as current source for the PV system.Fluctuations in temperature can alter the operation of the device, necessitating further study.

Figure 1 .
Figure 1.Power versus voltage for forward J-V scans of a perovskite device with the configuration FTO/TiO2/perovskite/ spiro-OMeTAD/Au.The comprises perovskite comprises triple cation and mixed halide composition Cs 0.05 (FA 0.83 MA 0.17 PbI 2.49 Br 0.51 ).The graph illustrates the peak power and operating voltage at one-hour intervals for (a) sample A and (b) sample B of the same device configuration.Measurements were conducted at different intensities (see supporting information figure 3 and table 1).

Figure 2 .
Figure 2. Stability test and maximum power tracking of a perovskite device with a planar inverted (p-i-n) device architecture consisting of indium tin oxide (ITO)/[2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz)/perovskite/C60/bathocuproine (BCP)/ Cu using and multipixel testing.The test was conducted on an active perovskite device, with each pixel subjected to J-V scanning at an operating voltage of 0.9 V.The different plots illustrate the stability of the same pixel over a duration of 3 h.The oscillations indicate current fluctuations caused by variation of the incident irradiance on the device.

Figure 3 .
Figure 3. XRD analysis of a perovskite film with a composition of a composition of Cs .0.25 FA .0.75 PbI 2.4 Br .0.6 for determination of phase from diffraction peaks.

Figure 4 .
Figure 4. Absorbance measurement using UV-Vis spectroscopy and determination of band gap energy using Tauc plot analysis.The measurements are conducted for two identical perovskite films with a composition of Cs .0.25 FA .0.75 PbI 2.4 Br .0.6 (labelled as samples A and B) to assess the repeatability of the measurement.

Figure 5 .
Figure 5. Determination of emission peaks and band gap energy of a perovskite film with a composition of Cs .0.25 FA .0.75 PbI 2.4 Br .0.6 using PL spectroscopy.(a)The PL Intensity versus wavelength and (b) the PL intensity versus photon energy are used to determine the emission peak wavelength and the band gap energy.

Figure 6 .
Figure 6.JV-curve of a mesoporous n-i-p perovskite device (a) in the forward scan (b) in the reverse scan.

Figure 7 .
Figure 7. Current density-voltage sweeps of a perovskite device with a planar inverted (p-i-n) device architecture of indium tin oxide (ITO)/[2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz)/perovskite/C60/bathocuproine (BCP)/Cu.(a) Tested using Keithley 2400 at incident intensity of 58 mW cm −2 under outdoor illumination in both forward and reverse directions.(b) Shading response of the planar perovskite device.(c & d) Current density-voltage scan sweeps of a perovskite device with a planar inverted(p-i-n) device architecture of using multipixel testing.The multiplexer scans four devices with 32 pixels at a time.To exclude inactive devices and pixels, these result is selected from active pixels of devices that exhibit non-zero current and voltage under illuminations.These two results demonstrate the repeatability of the measurement and the dynamic nature of device response, including its variations in hysteric effects.

Figure 8 .
Figure 8. Maximum power tracking of a perovskite device with configuration FTO/TiO2/perovskite/ spiro-OMeTAD/Au.The perovskiite possesses a triple cation and mixed halide composition.The measurements were conducted by tracking the current over time at a fixed operating voltage of 0.56 V for durations of (a) more than about 70 s and (b) 80 s.