Stability and degradation in triple cation and methyl ammonium lead iodide perovskite solar cells mediated via Au and Ag electrodes

Abstract

Perovskite solar cells (PSCs), particularly based on the methyl ammonium lead iodide (MAPbI₃) formulation, have been of intense interest for the past decade within the photovoltaics (PV) community, given the stupendous rise in power conversion efficiencies (PCEs) attributed to these perovskite formulations, where PCEs have exceeded 25%. However, their long-term stability under operational conditions and environmental storage are still prime challenges to be overcome towards their commercialization. Although studies on the intrinsic perovskite absorber stability have been conducted previously, there are no clear mechanisms for the interaction of electrode-induced absorber degradation pathways, which is the focus of this study. In this report, we have conducted a comprehensive analysis on the impact of the electrode collector layer, specifically Ag and Au, on the degradation mechanism associated with the MAPbI₃ and a triple cation absorber, Cs_0.05FA_0.79MA_0.16PbI_2.45Br_0.55. Notably, Au-based PSCs for both absorbers in an n-i-p architecture showed superior PCE over Ag-based PSCs, where the optimized PCE of MAPbI₃ and triple cation-based PSCs was 15.39% and 18.21%, respectively. On the other hand, optimized PCE of MAPbI₃ and triple cation-based PSCs with Ag electrodes was 3.02% and 16.44%, respectively. In addition, the Ag-based PSCs showed a rapid decrease in PCE over Au-based PSCs through operational stability measurements. We hypothesize the mechanism of degradation, arising from the Ag interaction with the absorber through the formation of AgI in the PSCs, leads to corrosion of the perovskite absorber, as opposed to the benign AuI when Au electrodes are used in the solar cell stack. Additionally, novel use of photoluminescence spectroscopy (PL) here, allowed us to access key features of the perovskite absorber in situ, while it was in contact with the various layers within the n-i-p solar cell stack. A quenching in the PL peak in the case of Ag-contacted MAPbI₃ provided direct evidence of the Ag corrupting the optical properties of the absorber through the formation of AgI which our X-ray diffraction (XRD) results confirmed. This was supported by the fact that an emission peak was still present in the triple cation Ag-device. For the Au-contacted MAPbI₃ the presence of a well-defined PL peak, though attenuated from the triple cation Au-device, suggested the AuI does not quell the emission spectrum for either the triple cation or the MAPbI₃ absorber. The findings should aid in the understanding and design of new electrode materials with PSCs, which will help accelerate their introduction into the commercial sector in the future.

Figure 1

(a) Device architecture of n-i-p (FTO/compact-TIO₂/triple cation or MAPbI₃ perovskite/Spiro-OMeTAD/Au) PSC and, (b) optimized J–V Characteristics with Au-electrodes. (c) Device architecture of n-i-p (FTO/compact-TIO₂/triple caiton or MAPbI₃ perovskite/Spiro-OMeTAD/Ag) PSC and, (d) optimized J–V characteristics with Ag-electrodes.

Figure 2

Device characterization analysis of triple cation and MAPbI₃ PSCs with Au electrodes. For the triple cation PSC devices, shown are the: (a) time-dependent PCE; (b) J_SC–I characteristic; (c) V_OC–I characteristic; (d) MPPT measurements. For the MAPbI₃ PSC devices, shown are the: (e) time-dependent PCE; (f) J_SC–I characteristic; (g) V_OC–I characteristic; (h) MPPT measurements. The triple-cation PSC retained ~ 60% of its initial PCE values after 4 h of testing while the MAPbI₃ PSC degraded to ~ 10% of its initial value after the same duration.

Figure 3

Material characterization conducted on the triple cation and MAPbI₃ absorbers. (a–i) UV–Vis absorption spectra and steady-state PL spectra of the triple cation perovskite film on normalized scale; (ii) Tauc-plot for determination of the optical bandgap of the triple cation perovskite film, revealing an E_g ~ 1.63 eV. For the triple cation absorber, evolution of (b) absorbance spectra and (c) PL spectra with ambient exposure from day 1 to day 3. Insets in (b,c) represent the expanded scale spectra. For the MAPbI₃ absorber, evolution of (d) absorbance spectra and (e) PL spectra with ambient exposure from day 1 to day 3. Insets in (d,e) represent the expanded scale spectra. The ambient exposure conditions for the films were ~ 22 °C and ~ 50% RH.

Figure 4

SEM images of (a) fresh and (b) aged triple cation perovskite absorber on FTO/TiO₂. The exposure to ambient conditions included 10 days of storage in a petri dish/at 25 °C and RH ~ 30 to 40%. SEM images of (c) fresh and (d) aged MAPbI₃ perovskite absorber on FTO/TiO₂. The fresh and aged triple cation appeared to be more dense compared to the MAPbI₃ whose porosity increased with aging leading to potential avenues for nucleating degradation mechanisms.

Figure 5

XRD of aged PSCs stored in ambient condition over 10 days in a petri dish without encapsulation at 25 °C and RH ~ 30 to 40%. XRD spectra of: (a) triple cation with Au; (b) triple cation with Ag; (c) MAPbI₃ with Au; (d) MAPbI₃ with Ag. Asterisks in (b) and (d) indicate the diffractions at 2θ = 22.3°, 23.8°, and 39.2° corresponding to the (100), (002), and (111) peaks of β-AgI, respectively. These peaks indicate the rapid formation of AgI in Ag based PSCs, while in (a,c), the formation of AuI is almost negligible since the characteristic AuI diffraction peaks were absent from our spectra, as noted in the Supporting Information of Ref..

Figure 6

The PL spectra acquired using a 532 nm excitation laser, as it probed the absorber in situ within the solar cell stack in the n-i-p device architecture directly. (a) PL spectra of aged triple cation and MAPbI₃ based PSCs, where the cells were stored in ambient conditions for ~ 10 days in a petri dish without encapsulation at 25 °C and RH ~ 30 to 40% with Au electrodes. Inset represents peak shift between the Ag and Au based triple cation based perovskite. (b) PL spectra of aged triple cation and MAPbI₃ based PSCs with Ag electrodes. Inset represents the peak-shift between the Ag and Au-contacted MAPbI₃ within the solar cell stack. As shown in (a), PSCs with Au electrodes showed notable PL emission with a clearly visible peak maxima, which validates that both absorbers in Au-based PSCs are less vulnerable to degradation even with the formation of AuI. On the other hand, MAPbI₃ based PSCs in (b) showed a negligible PL peak over the same triple cation-based PSCs contacted with the Ag, which further supports the possible rapid degradation of MAPbI₃ based PSCs with Ag electrodes over the triple cation-based PSCs since the absorber in the former appears to be optically degraded through these in situ PL measurements to access the absorber layers for the degradation study mediated via the Ag and Au collector electrodes.