Operational stability, low light performance, and long-lived transients in mixed-halide perovskite solar cells with a monolayer-based hole transport layer

doi:10.21203/rs.3.rs-1395435/v2

Due to their tunable bandgap and low manufacturing cost, metal halide perovskite solar cells are attractive for ambient/indoor light harvesting applications such as powering Internet of Things devices. In this work, we examined the viability of p-i-n perovskite solar cells fabricated with MeO-2PACz, a molecular monolayer hole transport layer, for ambient light harvesting applications. Two triple-cation mixed halide lead perovskite absorbers were compared, one with high bromide content (Br/I ratio 1:2, bandgap 1.72 eV) and the other with low bromide content (Br/I ratio 1:11, bandgap 1.57 eV). Both materials demonstrated good stability during 100 h maximum power operation under simulated sunlight, a cumulative light dose comparable to over two years of ambient use. After the test completed, however, the measured device efficiency was found to fall temporarily due to a transient loss of output current. This effect was more apparent in the wide bandgap device and was likely caused by light-induced halide segregation. The initial performance was re-established after a recovery period where the device was left in the dark. After recovery, both devices retained good performance under low light conditions. The wide bandgap device achieved a stable power conversion efficiency of 30.4% under a simulated ambient light source (835 lx white LED).

Energy Engineering

Materials Chemistry

Electronic Materials and Devices

Perovskite solar cells

self-assembled monolayers

bandgap

stability

ambient light harvesting

Solar cells based on metal halide perovskite materials have demonstrated exceptional efficiency (25.7%), approaching that of crystalline silicon.[1] The perovskite absorber layers have many favorable electro-optic properties, including high defect tolerance,[2] long charge carrier lifetimes and diffusion lengths,[3,4] and high absorption coefficients.[5] One particularly useful feature is the ability to tune the bandgap by substituting different ions in the perovskite crystal structure.[6,7] The bandgap of mixed composition perovskites can be optimized for use in tandem cells,[8–11] or cells designed to operate efficiently under artificial light sources.[12] In combination with the low manufacturing cost, the high potential efficiency of wide bandgap perovskite solar cells under artificial light makes them highly promising for powering Internet of Things devices such as PV-powered RFID tags.[13]

The bandgap of metal halide perovskites is commonly adjusted by varying the ratio of halide ions, usually iodide and bromide, occupying the X-site of the ABX₃ crystal structure.[14,15] The ratio of the A-site cations, typically a mix of cesium (Cs⁺), formamidinium (FA⁺) and methylammonium (MA⁺), is optimized to increase the stability and/or improve the electrical properties of the perovskite.[16,17] Increasing the bandgap via the halide ratio has limitations, however. As the Br/I ratio is increased, stability and performance are often compromised due to the tendency for the I⁻ and Br⁻ ions to segregate into distinct phases when the perovskite is illuminated.[18,19] This transient diffusion phenomenon – the halide distribution returns to a homogeneous mixture in the dark – is known as halide segregation.[20–23]

In this work, we focused on a p-i-n or “inverted” device structure employing a carbazole-based self-assembled monolayer (SAM) as a hole transport layer. Record efficiencies for both single junction and tandem solar cell have been recently achieved by coating the transparent oxide substrates with these molecular monolayers.[24–26] It has also been noted that perovskite layers deposited on SAM-treated substrates are resilient towards halide segregation.[24] As the suitability of this promising new device architecture for low light operation had not yet been confirmed, we wanted to explore the performance of the monolayer-based device stack with both wide and narrow gap absorbers. To this end, we prepared devices using mixed-halide perovskites with different bromide-to-iodide ratios. One material has a narrow bandgap optimal for natural sunlight, the other has a wide bandgap suitable for ambient light harvesting or tandem applications. Operational stability was assessed by tracking the maximum output power for 100 h under AM1.5G simulated sunlight, a total light flux equivalent to over two years operation under typical indoor lighting conditions. While the efficiency of the both the wide and narrow gap devices remained above the initial value over the course of the test, significant transient losses were observed after the test completed. These long-lived transients, which act to suppress the device responsivity under strong light, were more apparent in the wide bandgap perovskite with high bromide content. The initial performance was recovered after storing the devices in the dark. After recovery, both devices retained good performance under low light conditions. We confirmed that the wide bandgap device is stable and operates efficiently under simulated ambient light – a white LED with an illuminance of 835 lx – recording an average power conversion efficiency of 30.4% for 10 h. In addition to quantifying the stability and efficiency of the SAM-based device structure for ambient light harvesting, our results provide relevant insights into the influence of perovskite bandgap and composition on the operational stability, low light performance, and transient behavior of monolayer-based n-i-p perovskite solar cells.

2.1 Device fabrication and test conditions

For the low bromide content, or “low-Br”, material we selected our reference composition, Cs_0.05FA_0.80MA_0.15PbI_2.75Br_0.25.[27] The ratio of bromide to iodide is 1:11 (8.3% of the total halide content is bromide), and the bandgap of 1.57 eV (787 nm) is optimized for single junction performance under AM1.5G sunlight. For the high bromide content formulation, referred to hereafter as “high-Br”, we used Cs_0.05FA_0.80MA_0.15PbI₂Br. The bromide to iodide ratio is 1:2 (33% bromide), which leads to a wider bandgap of 1.72 eV (722 nm). This wide bandgap absorber is expected to perform well under ambient lighting such as white LEDs. The composition of the A-site is the same for both wide and narrow bandgap compounds. The hole extracting monolayer was prepared using a carbazole-based SAM molecule, [2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz, molecular structure shown in Fig. 1a), which was applied to indium tin oxide (ITO)/glass substrates by spin coating from 1 mM ethanol solution.[28] Perovskite solar cells were fabricated on the SAM-treated ITO glass substrates based on the processes elaborated in our previous reports[29–31] and outlined in the supporting information. Each device had an active (masked) area of 0.1 cm². The complete device structure, illustrated in Fig. 1b, is ITO/MeO-2PACz/perovskite/C₆₀/BCP/Ag. Cross-sectional SEM images (Fig. S1) confirm the perovskite layers in both devices to be about 600 nm thick, with comparable grain size and morphology. Devices were exposed briefly to air to apply solder to the electrode contacts before being stored in the dark in a nitrogen-filled glove box. All further tests and measurements were performed at ambient temperature under inert atmosphere.

2.2 Device performance

Stabilized output (300 s) and current–voltage (J–V) curves for representative low-Br and high-Br devices in their initial, as fabricated condition are shown in Fig. 1c. Under AM1.5G, the power conversion efficiency (PCE) for the low-Br device was 17.6%, while that of the high-Br device was 15.1%. The lower PCE of the high-Br device under broad-spectrum AM1.5G light source is expected considering the wider bandgap, as less light energy is absorbed. The difference in bandgap leads to the high Br device having a lower observed short circuit current (J_sc) and higher open circuit voltage (V_oc) compared to the low Br device.

After 100 h operation under AM1.5G the characteristics of both cells improved significantly as shown in Fig. 1d. The stabilized PCE of the low-Br device reached 19.1%, while the efficiency of the high-Br device increased to 16.1%. The open circuit voltages increased to 1.07 V and 1.16 V, respectively. The fill factor (FF) for the low-Br device remained slightly higher than that of the high-Br counterpart, at 0.79 and 0.76, respectively.

The output characteristics before and after 100 h operation are compiled in Table 1.

Table 1. Devices characteristics before and after 100 h operation under AM1.5G.

Cell	Scan Direction	J_sc (mA cm⁻²)	V_oc (V)	PCE^* (%)	FF	PCE^** (%)
Low-Br (Initial)	Forward	23.14	1.06	18.2	0.74	17.6
Low-Br (Initial)	Reverse	23.16	1.04	18.5	0.76	17.6
Low-Br (100 h)	Forward	22.19	1.08	19.0	0.79	19.1
Low-Br (100 h)	Reverse	22.13	1.08	19.0	0.79	19.1
High-Br (Initial)	Forward	19.22	1.14	15.0	0.69	15.1
High-Br (Initial)	Reverse	19.20	1.11	14.6	0.69	15.1
High-Br (100h)	Forward	18.34	1.17	16.2	0.76	16.1
High-Br (100h)	Reverse	18.30	1.17	16.1	0.75	16.1

* PCE calculated from the J–V scans

** Stabilized PCE (³300 s maximum power point tracking)

2.3 Operational stability

In this section we examine the evolution of the device properties during 100 h operation under AM1.5G. A maximum power point tracking (MPPT) algorithm maintained the devices at the maximum output power, p_m, pausing only for brief intervals every 20 minutes to record J–V curves. The progression of J_sc, V_oc, and FF was obtained from the J–V curves, while the PCE is calculated from the MPPT data. The results are compiled in Fig. 2, while representative J–V scans at 1, 10 and 100 h are shown in Fig. S2. V_oc and FF increased over the 100-h test interval for both the low-Br and high-Br devices, while J_sc fell slightly. The increase in voltage and fill factor offsets the decrease in output current such that the overall efficiency of both cells after 100 h MPPT remained largely unchanged from the initial value. The PCE initially increases before reaches a maximum at around 25 h. This “burn in” behavior has been previously noted as a characteristic of p-i-n perovskite solar cells.[32] In sharp contrast, considerable degradation was observed for both the low-Br and high-Br perovskite layers when used in typical p-i-n spiro-OMeTAD devices, confirming the superior operational stability of the monolayer HTL / n-i-p device stack. The 100 h MPPT data for the spiro-OMeTAD devices is given in Fig. S3.

2.4 Transient effects

Only minor changes to the J–V curves were observed throughout the MPPT experiments under continuous AM1.5G. We were therefore surprised to see a sharp drop in output current when, following the completion of the MPPT test, the devices were subsequently left under ambient room light for a short period (<1 h). This loss was transient, however, and the nominal performance was recovered after the device was left in the dark for a longer period (500 h). These transient effects were more significant for the high-Br device than the low-Br device. The details of the transient effect for the high-Br device are explored in Fig 3, while the corresponding observations for the low-Br device are shown in Fig. S4.

As illustrated by the J–V curves shown in Fig 3a, an acute, transient loss of device performance develops after the MPPT test finished and the device was left for an hour or so in the dark. J_sc fell from 18.3 mA cm⁻² to about 12 mA cm⁻², before returning to 18.7 mA cm⁻² after a 500-h recovery period in the dark. We noted, however, that the transient loss of output current became less significant when the device was probed with low intensity light. To help characterize this, it is instructive to examine J_sc normalized to the incident light power. This quantity is known as the responsivity of the device, (j is the total irradiance), and the responsivity is plotted against light intensity in Fig 3b. In the initial and recovered state, the responsivity remained essentially constant over the measured range of light intensities. In the transient state the responsivity fell with increasing light intensity from about 1/100 sun. Since at low light intensity below 1/100 sun the responsivity was unchanged from the initial and recovered values, the impact of the transient effects under ambient light is expected to be minimal.

External quantum efficiency (EQE) spectra, shown in Fig 3c, were taken to help clarify the transient changes. All three of the measured spectra – taken for the initial, transit and recovered states – show the same overall EQE of 80–85%. The device EQE is not reduced during the transient regime because the intensity of the monochromatic light source used for this experiment corresponds to only about 1/100 sun – the threshold below which, as noted above, the transient current losses cease to be significant. Overall, the spectral shape and intensity was unchanged. In the transient regime, however, the onset region temporarily developed a low energy shoulder which dissipated after the device was left for 500 h in the dark. The is more clearly seen in Fig 3d, where the negative differentials of the EQE vs. energy are plotted as a function of photon energy. Following the procedures developed by U. Rau et al. the bandgap is estimated from the peak centers determined from Gaussian peak fitting.[33] As listed in Table 2, the bandgap of the high-Br material shifts from 1.717 to 1.699 eV and back to 1.717 eV, while the peak width increases from 83 to 110 meV before returning to 85 meV. Interpolating the bandgap and bromide content of the two perovskite materials, the shift in bandgap of the high-Br device suggests a reduction in bromide content from 0.33% to 0.30%. In other words, the surface regions of the perovskite layer where light is most strongly absorbed became somewhat iodide rich. This is consistent with the previously reported segregation of the halide ions into Br-rich and I-rich subdomains under strong light.[22,23,34,35] Light-induced lattice expansion at the illuminated perovskite surface seems the likely driving force.[36–39] It’s noteworthy though that in the present instance the detrimental influence of the halide segregation on device performance is exhibited after exposure to AM1.5G rather than during it. While similar changes were observed for the low-Br device, the loss of output current was much smaller and no shift in the onset of the EQE spectrum was observed. These transients were similarly recovered after 500 h in the dark. The contrasting behavior for the high-Br and low-Br devices provides further evidence that the transient effect is related to halide segregation.

Irreversible changes in both devices were observed in the complex impedance measurements recorded under AM1.5G (Fig. S5 and Appendix A). The parallel resistance[40] falls over the course of the MPPT experiment and does not return to the initial state even after 500 h recovery. The effective shunt resistance remains high, however, and the impact on the measured device performance (as expressed by the J–V curves and MPPT under AM1.5G) is negligeable.

Table 2. Gaussian peak fitting results for the derivative of the external quantum efficiency, −d(EQE)/dE.

Condition	Low-Br		High-Br
Condition	Position (eV)	FWHM (eV)	Position (eV)	FWHM (eV)
Initial	1.574	0.068	1.717	0.083
After 100 h MPPT	1.576	0.070	1.699	0.110
Recovered	1.574	0.068	1.716	0.085

2.5 Light intensity dependence

Light intensity dependence tests and performance evaluation under ambient light were performed on cells previously stressed with 100 h MPPT, after full recovery from the transient effects described above. During the 100-h exposure to AM1.5G, the cells absorb a photon flux equivalent to over two years of continuous operation under ambient light. The results presented here are therefore indicative of how these devices, provided sufficient protection from the atmosphere, might perform after several years of operation in a typical Internet of Things application.

To arrive at a clearer picture of the performance of these stressed cells changes under different light intensities, simulated AM1.5G radiation was attenuated to the desired intensity using neutral density filters in combination with a variable aperture. (Calibration methods given in the supporting information.) This ensures the spectral composition remains constant, permitting direct comparison of the PCE data – something that would not have been possible if we had switched to a LED source for the low light measurements for example. Fig. 4a shows the PCE of the low-Br and high-Br devices as the AM1.5G source is attenuated from 1 sun to less than 1/1000 suns. The Shockley–Queisser (SQ) limit for the two materials is also indicated. (PCE is calculated from the average of forward and reverse J–V curves taken at each light intensity. Full datasets are given in Appendix B of the supporting information.) While the output from both devices decreases as the light intensity is reduced, the SQ limit also falls. At any given light intensity, both devices perform about equally well vs. their respective theoretical limits. This can be clearly seen in the plot of the cell output normalized to the SQ limit shown in Fig. S6.

2.6 Device characteristics under simulated ambient light

Having established that both materials retain the same performance relative to their respective SQ limits under attenuated AM1.5G, we proceeded to confirm the stable output of the widegap high-Br composition under simulated ambient indoor light. A 50 W white LED adjusted to an irradiance of 250 μW cm⁻² – illuminance 835 lx, roughly the brightness of a normal office environment – was used to simulate ambient light conditions. As shown in Fig. S7, the spectral intensity falls mostly in the range between 430–700 nm. The absence of any significant infrared component imparts an efficiency advantage to absorbers with wide bandgaps in the region of 1.7–1.9 eV, including the high-Br perovskite studied in this work. This theoretical advantage is quantified in Fig. S8, which compares the SQ limit for the AM1.5G solar spectrum with that of the white LED.

Fig. 4b shows the maximum power output of the high-Br device under the white LED. The cell was kept in the dark overnight before the measurements started. The average output of the high-Br cell during 10 h of operation was 76.2 μW cm⁻², which translates to a PCE of 30.4%.

The efficiency and operational stability of p-i-n perovskite solar cells using MeO-2PACz as a hole extracting monolayer was investigated under different lighting conditions to determine the suitability of the device stack for ambient light harvesting applications and to clarify the effect of the perovskite bromide-to-iodide ratio on device performance. Under inert atmosphere, we found that the p-i-n perovskite solar cells maintain high PCE during 100 h operation at maximum power under AM1.5G. The effect of the bromide-to-iodide ratio was examined under both AM1.5G and indoor lighting conditions. Two different mixed halide perovskite absorbers were trialed, a low-Br material (Br/I 1:11) with a bandgap of 1.57 eV and a high-Br formulation (Br/I 1:2) with a bandgap of 1.72 eV. Following 100 h MPPT and subsequent recovery from transients, the performance of both materials was comparable with respect to their respective SQ limits over a wide range of incident light intensity from less than 1/1000 suns to from 1 sun. Under white LED illumination at an irradiance of 250 μW cm⁻², the wide bandgap, high-Br perovskite material achieved a stable PCE of 30.4%. Despite the existence of long-lived transient effects relating to halide segregation, the wide bandgap perovskite absorber and monolayer-based p-i-n device structure are judged to be suitable for long term deployment in ambient light harvesting applications from the standpoint of both efficiency and operational stability.

Acknowledgements

This research was partially supported by the Ministry of Education, Culture, Sports, Science and Technology, Grant-in-Aid for Scientific Research (C), JSPS KAKENHI (grant number JP19K05666, JP19K23631 and JP20K22531), grant-in-Aid for Early-Career Scientists (21K14694), and by JST-COI (grant number JPMJCE 1307). Additional funding was provided by the New Energy and Industrial Technology Development Organization (NEDO), and research grants from Tokyo Ohka Foundation for the Promotion of Science and Technology, The Sumitomo Foundation, and the Mazda Foundation. The authors thank K. Aihara of Citizen Co for helpful discussions, and Y. Iwasaki for the performing the SEM measurements.

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MurdeyMeO2PACz4.3SI.pdf
SUPPORTING INFORMATION

Operational stability, low light performance, and long-lived transients in mixed-halide perovskite solar cells with a monolayer-based hole transport layer

Status:

Version 2

Abstract

Figures

Introduction

Results

2.1 Device fabrication and test conditions

2.2 Device performance

2.3 Operational stability

2.4 Transient effects

2.5 Light intensity dependence

2.6 Device characteristics under simulated ambient light

Conclusion

Declarations

Acknowledgements

References

Supplementary Files

Status:

Version 2