Decoupling the effects of defects on efficiency and stability through phosphonates in stable halide perovskite solar cells

Abstract

•Phosphonates interact strongly with halide perovskites through hydrogen bonds•Deep defect passivation and non-radiative recombination remain unaffected•Shallow point defect passivation and ion immobilization result in high device stability•Nearly 100% of the initial efficiency (21%) is retained after 1,000 h of illumination Shallow defects, which are considered benign in classical inorganic semiconductors, result unfavorably for ionic-electronic conductors such as halide perovskites, causing ion migration and poor operational stability when applied in a solar cell. We report that the organic molecule 3-phosphonopropionic acid (H3pp), added to the halide perovskite, passivates shallow point defects having minimal effect on deep-defect passivation and non-radiative recombination. This results in an unaffected overall optoelectronic performance while having a tremendous effect on the stability of the perovskite solar cell. The hydrogen binding modes benefits the immobilization of ions that leads to the exceptional operational stability. Our findings permit decoupling of the effects of defects on efficiency and stability to advance in the understanding of the relation between shallow and deep-defect passivation, non-radiative recombination, and PSC performance. Understanding defects is of paramount importance for the development of stable halide perovskite solar cells (PSCs). However, isolating their distinctive effects on device efficiency and stability is currently a challenge. We report that adding the organic molecule 3-phosphonopropionic acid (H3pp) to the halide perovskite results in unchanged overall optoelectronic performance while having a tremendous effect on device stability. We obtained PSCs with ∼21% efficiency that retain ∼100% of the initial efficiency after 1,000 h at the maximum power point under simulated AM1.5G illumination. The strong interaction between the perovskite and the H3pp molecule through two types of hydrogen bonds (H…I and O…H) leads to shallow point defect passivation that has a significant effect on device stability but not on the non-radiative recombination and device efficiency. We expect that our work will have important implications for the current understanding and advancement of operational PSCs. Understanding defects is of paramount importance for the development of stable halide perovskite solar cells (PSCs). However, isolating their distinctive effects on device efficiency and stability is currently a challenge. We report that adding the organic molecule 3-phosphonopropionic acid (H3pp) to the halide perovskite results in unchanged overall optoelectronic performance while having a tremendous effect on device stability. We obtained PSCs with ∼21% efficiency that retain ∼100% of the initial efficiency after 1,000 h at the maximum power point under simulated AM1.5G illumination. The strong interaction between the perovskite and the H3pp molecule through two types of hydrogen bonds (H…I and O…H) leads to shallow point defect passivation that has a significant effect on device stability but not on the non-radiative recombination and device efficiency. We expect that our work will have important implications for the current understanding and advancement of operational PSCs. Photovoltaic (PV) energy technologies are crucial to transform our society into a low-carbon economy, enabling the production of electricity from sunlight. Single-junction halide perovskite solar cells (PSCs) have already achieved a certified power conversion efficiency (PCE) of above 25%.1Kim H.S. Lee C.R. Im J.H. Lee K.B. Moehl T. Marchioro A. Moon S.J. Humphry-Baker R. Yum J.H. Moser J.E. et al.Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%.Sci. 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In this work, we study the addition of the 3-phosphonopropionic acid (H3pp) organic molecule to a HP absorber and investigate the effect that two different anchoring groups (phosphonate –PO(OH)2 and carboxylate –COOH) have on the performance and stability of PSCs. In order to ensure an accurate comparison between the different characterization methods carried out on materials, thin films, and complete devices, all our studies were carried out at room temperature. We fabricated complete solar cells with the configuration FTO/c-TiO2/m-TiO2/HP/Spiro-OMeTAD/Au based on the multication perovskite absorber with the nominal formula Rb0.05Cs0.05MA0.15FA0.75Pb1.05(I0.95Br0.05)3 (RbCsMAFA),31Kim H.S. Jang I.H. Ahn N. Choi M. Guerrero A. Bisquert J. Park N.G. Control of I–V hysteresis in CH3NH3PbI3 perovskite solar cell.J. Phys. Chem. Lett. 2015; 6: 4633-4639https://doi.org/10.1021/acs.jpclett.5b02273Crossref PubMed Scopus (375) Google Scholar with and without the H3pp molecule. The PSCs showed excellent and similar overall optoelectronic performance, with Voc of ∼1.1 V and PCE of ∼21%. Surprisingly, stability analyses of the PSC with H3pp showed negligible loss of the initial efficiency after 1,000 h under 1 sun and maximum power point (MPP) tracking, whereas the reference device (without H3pp) showed >20% loss under the same conditions. To understand the effect of the H3pp molecule on the optoelectronic properties of the perovskite and its stability, we employed contactless high-throughput characterizations, including time-resolved photoluminescence (TRPL) spectroscopy, photoluminescence quantum efficiency (PLQE) measurement, optical-pump THz-probe (OPTP) spectroscopy, and thermal admittance spectroscopy (TAS). First-principles density functional theory (DFT) calculations, supported by Fourier-transform infrared spectroscopy (FTIR) analyses, demonstrated the presence of two types of hydrogen bonds (H…I and O…H) between the –PO(OH)2 functional group and the perovskite. These strong binding modes occur through shallow point defects (e.g., FA and I vacancies)32Ball J.M. Petrozza A. Defects in perovskite-halides and their effects in solar cells.Nat. Energy. 2016; 1: 16149https://doi.org/10.1038/nenergy.2016.149Crossref Scopus (567) Google Scholar,33Yin W.J. Shi T. Yan Y. Unique properties of halide perovskites as possible origins of the superior solar cell performance.Adv. Mater. 2014; 26: 4653-4658https://doi.org/10.1002/adma.201306281Crossref PubMed Scopus (1353) Google Scholar that block or immobilize ions, resulting in remarkable device stability. 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A scanning electron microscopy (SEM) cross-sectional image of the HP:H3pp PSC is depicted in Figure 1A. The champion PSC fabricated with H3pp shows PCEs of 21.22% and 20.45% in reverse and forward scans, respectively (Figures 1B and S3), with a stabilized PCE of 21.1% (inset of Figure S3). The Jsc from the IV curves agrees with the integrated current density obtained from the incident photon-to-electron conversion efficiency (IPCE) analysis (Figure S4). The optimization of the fabrication process statistically yielded average PCEs of 20.2% ± 0.7% and 20.6% ± 0.4% for the reference and the H3pp:HP PSCs, respectively, and an average Voc of 1.12 ± 0.01 V for both samples (Figures 1C and 1D). Both types of PSCs also demonstrated similar values in their other PV parameters (current density [Jsc], fill factor [FF] and hysteresis index [HI]) (Figure S5). To evaluate the recombination in our PSCs, we carried out light intensity-dependent Voc analysis in complete devices. Analyses were conducted in devices with the configuration FTO/c-TiO2/m-TiO2/HP/Spiro-OMeTAD/Au, with and without the H3pp additive in the HP layer (Figure S6A). The diode ideality factor (nid) can be extracted from the relation Voc = Eg + (nidkT/q) ln(I/I0), where Eg is the band gap, k is the Boltzmann constant, T is the temperature, q is the elementary charge, and I0 is a reference intensity usually at 1 sun.41Tress W. Yavari M. Domanski K. Yadav P. Niesen B. Correa Baena J.P. Hagfeldt A. Graetzel M. Interpretation and evolution of open-circuit voltage, recombination, ideality factor and subgap defect states during reversible light-soaking and irreversible degradation of perovskite solar cells.Energy Environ. 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Transient photocurrent (TPC) and open-circuit voltage decay (OCVD) analyses also displayed similar carrier transport and recombination properties for the PSCs with and without the H3pp additive, consistent with the similar Jsc and Voc values obtained (Figures S6B and S6C). To understand whether the recombination behavior of the photoexcited carriers unequivocally comes from the perovskite films, TRPL analyses were carried out on HP thin films (deposited on glass) with and without the H3pp molecule. Figure 1E shows the TRPL decay of reference and optimized H3pp:HP (1:500 ratio) films measured at a low fluence (509 nm, 50 nJ/cm2 per pulse, equivalent to ∼1 sun). The monoexponential fits to the data result in characteristic lifetimes very similar for both samples of τ ≈ 230 ns.44Stranks S.D. Eperon G.E. Grancini G. Menelaou C. Alcocer M.J. Leijtens T. Herz L.M. Petrozza A. Snaith H.J. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber.Science. 2013; 342: 341-344https://doi.org/10.1126/science.1243982Crossref PubMed Scopus (7390) Google Scholar The charge carrier lifetime is strongly related to the defect/trap density of the material under study, especially the non-radiative recombination.42Yang D. Yang R. Wang K. Wu C. Zhu X. Feng J. Ren X. Fang G. Priya S. Liu S.F. High effic