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In this paper, the origin of main defects in perovskite solar cells (PSCs), their effect on photovoltaic performance, and operational stability are focused upon. Then, the corresponding passivation strategies are introduced. Finally, a brief summary is made and the thorny issues that need to be solved in the future development of PSCs are looked forward to.

Abstract

Defects are considered to be one of the most significant factors that compromise the power conversion efficiencies and long-term stability of perovskite solar cells. Therefore, it is urgent to have a profound understanding of their formation and influence mechanism, so as to take corresponding measures to suppress or even completely eliminate their adverse effects on device performance. Herein, the possible origins of the defects in metal halide perovskite films and their impacts on the device performance are analyzed, and then various methods to reduce defect density are introduced in detail. Starting from the internal and interfacial aspects of the metal halide perovskite films, several ways to improve device performance and long-term stability including additive engineering, surface passivation, and other physical treatments (annealing engineering), etc., are further elaborated. Finally, the further understanding of defects and the development trend of passivation strategies are prospected.

Inorganic CsPbI₃ perovskite is well-known for its resistance to organic cation substitution. Here, a specific organic cation of tetrabutylammonium (TBA⁺) with strong ionic binding to the Pb-I octahedral framework can effectively intercalate into CsPbI₃ and substitute the Cs⁺ cation. A post-synthesis TBAI passivation then leads to in situ formation of TBAPbI₃ layer to heal CsPbI₃ perovskite with lower defect density and enhanced stability.

Abstract

The in situ formation of reduced dimensional perovskite layer via post-synthesis ion exchange has been an effective way of passivating organic-inorganic hybrid perovskites. In contrast, cesium ions in Cs-based inorganic perovskite with strong ionic binding energy cannot exchange with those well-known organic cations to form reduced dimensional perovskite. Herein, we demonstrate that tetrabutylammonium (TBA⁺) cation can intercalate into CsPbI₃ to effectively substitute the Cs cation and to form one-dimensional (1D) TBAPbI₃ layer in the post-synthesis TBAI treatment. Such TBA cation intercalation leads to in situ formation of TBAPbI₃ protective layer to heal defects at the surface of inorganic CsPbI₃ perovskite. The TBAPbI₃-CsPbI₃ perovskite exhibited enhanced stability and lower defect density, and the corresponding perovskite solar cell devices achieved an improved efficiency up to 18.32 % compared to 15.85 % of the control one.

Publication date: 21 April 2021

Source: Joule, Volume 5, Issue 4

Author(s): Tianhao Wu, Xiao Liu, Xinhui Luo, Xuesong Lin, Danyu Cui, Yanbo Wang, Hiroshi Segawa, Yiqiang Zhang, Liyuan Han

A coadditive strategy using two chloride compounds in the precursor solution is elaborated to prepare methylammonium-free Cs_0.1MA_0.9PbI₃ layers with outstanding morphological, structural, electrical, and optical properties. It results in robust devices which are hysteresis-free and deliver a stabilized 20.02% power conversion efficiency.

Herein, the synthesis of methylammonium-free and bromide-free Cs _x FA_1−x PbI₃ (FA for formamidinium) perovskite (PVK) photovoltaic layers with intrinsic outstanding properties in terms of crystallinity, defect waiving/passivation, and ionic mobility blocking by using a coadditives approach is described. It consists in mixing two chloride additives in the PVK precursor solution: potassium chloride (KCl) and ammonium chloride (NH₄Cl). KCl favors the lead iodide (PbI₂) precursor solubilization and results in a purer PVK phase. NH₄Cl allows the control of the crystallization growth speed, leading to the formation of large crystal grains and well-crystallized layers. By the glow-discharge optical emission spectroscopy (GD-OES) technique, it is directly visualized that potassium incorporated in the whole film blocks the iodide mobility by defect passivation. Also, the reduction (or suppression) of iodide mobility and the reduction (or suppression) of the J–V curve hysteresis is clearly correlated. It is found that blocking the ionic mobility is insufficient to fully stabilize the halide perovskite material which also requires the crystallization monitoring carried out in parallel by the second additive. It resulted in Cs_0.1FA_0.9PbI₃ cells with a stabilized power conversion efficiency (PCE) of 20.02% and with superior stability.

The modification of charge alignment between hole transporting layer and perovskite film is done by addition of potassium acetate interfacial layer. This study demonstrates the importance of band alignment modification where the device short-circuit current of the devices is improved regardless of the device configuration or perovskite bandgap, thus proving its application as universal interfacial passivation material.

A passivation strategy for the perovskite/HTL interface is presented based on potassium acetate (K-Ac). Since K-Ac is soluble in both polar and nonpolar solvent, deposition of K-Ac on top and bottom of perovskite is possible. With this advantage, the universality of potassium interfacial passivation at the HTL/perovskite interface applied to various configurations with various ranges of perovskite bandgap is reported. Regarding the p–i–n configuration, various materials characterizations reveal that a potassium passivation layer underneath perovskite modifies perovskite orientations, resulting in better charge transport and film properties. Furthermore, the potassium passivation layer shifts the valence band position of the HTL upward, which results in a better extraction of charges (holes) across the HTL/perovskite interface, thus improving the short-circuit current density (J _sc). The modification of the band alignment at the HTL/perovskite by the potassium interfacial passivation layer is confirmed in n–i–p devices with both WBG and CBG perovskites. Compared to reference solar cells without a passivation layer, an increase in J _sc of approximately 1 mA cm⁻² is observed in all cases, resulting in power conversion efficiencies of 19.42%, 20.06%, and 21.57% for WBG p–i–n, CBG p–i–n and n–i–p solar cells, respectively, demonstrating the wide applicability of the passivation strategy.

Full dopant-free bifacial silicon solar cells utilizing a MoO₃/ITO/Ag hole contact and ZnO/LiF _x /Al electron contacts are first demonstrated. The electron contacts feature a full-area ZnO antireflective coating and a LiF _x /Al finger contact, allowing for bifacial application. The optimized cells with 60 nm ZnO and 50% metal contact fraction achieve an estimated output power density improvement of 0.7 mW cm⁻², compared with the monofacial reference cells.

Herein, challenges in the fabrication of full dopant-free bifacial silicon solar cells are discussed and efficient devices utilizing a MoO₃/ indium tin oxide (ITO)/Ag hole-selective contact and ZnO/LiF _x /Al electron-selective contacts with up to 79% short-circuit current bifaciality are demonstrated. The ZnO/LiF _x /Al rear electron contact features a full-area ZnO antireflective coating and a LiF _x /Al finger contact, allowing sunlight absorption from the back side, thus producing more overall power. The ZnO/LiF _x /Al electron contacts with a thinner ZnO layer and a larger contact fraction display a better selectivity and a lower resistance loss. When considering rear-side irradiance of 0.15 sun, the dopant-free bifacial solar cell with 60 nm ZnO and 50% LiF _x /Al metal contact fraction achieves a 3% estimated output power density improvement compared with its monofacial counterpart (21.0 mW cm⁻² compared to 20.3 mW cm⁻²) using the full-area back contact. Both the efficiency and bifaciality factor of this dopant-free device are still significantly lower than those of state-of-the-art devices relying on doped-silicon-based layers. The required improvement for this technology to become industry-relevant is discussed.

A wide-bandgap guest material is designed, synthesized, and used as the guest material for ternary polymer solar cells to improve the power conversion efficiency and reduce the energy loss.

The rational design of guest photovoltaic materials (the third components) for enhancing the power conversion efficiency (PCE) of ternary polymer solar cells (PSCs) is a challenge. In this work, a large-bandgap material ((5Z,5′Z)-5,5′-(((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(2,5-difluoro-4,1-phenylene))bis(methanylylidene))bis(3-ethyl-2-thioxothiazolidin-4-one, IBR-F) is designed, synthesized, and used as the guest material for ternary PSCs to improve the PCE and reduce the energy loss. IBR-F possesses a larger energy bandgap of 2.04 eV and a deeper highest occupied molecular orbital (HOMO) level compared to PM6, as well as exhibiting good miscibility with PM6. Thus, the HOMO level is effectively downshifted when blending PM6 with IBR-F, which is in favor of obtaining a higher open-circuit voltage (V _oc). Meanwhile, the incorporation of IBR-F into the PM6:Y6 blend can maintain the well-developed bulk-heterojunction morphological properties of the host blend without extra thermal annealing and improve the charge dissociation and collection efficiencies. As a result, the ternary PSC based on PM6:IBR-F:Y6 (1:0.2:1.2 w/w) demonstrates a higher V _oc of 0.887 V and a reduced energy loss of 0.55 eV compared to the binary device based on PM6:Y6 (1:1.2 w/w) with a V _oc of 0.840 V and energy loss of 0.59 eV, delivering an improved PCE of 17.15%.

An analytical solution of photoluminescence reabsorption (PLr) is used to determine the intrinsic radiative carrier recombination rate of metal-halide perovskite films. Simulation of its impact on the quasi-Fermi-level splitting (QFSL) reveals it is detrimental at high, but advantageous at low nonradiative recombination rates. Importantly, neglecting PLr results in overestimation of the effective nonradiative recombination rate in perovskite solar cells.

The precise quantification of the impact of photoluminescence reabsorption (PLr) in metal-halide perovskite solar cells (PSCs) remains challenging. Herein, the PLr effect is examined by combined time-resolved photoluminescence (TRPL) spectroscopy and time-resolved terahertz spectroscopy (TRTS) and a model is proposed that relates both, the PLr and nonradiative recombination rate (k _nr) to the quasi-Fermi-level splitting (QFLS). PLr is shown to be beneficial for QFLS when the nonradiative recombination rate (k _nr) is below a critical value of ≈7 × 10⁵ s⁻¹; at high k _nr PLr is detrimental to QFLS. By incorporating PLr into a two-diode model that allows extraction of the effective k _nr, the series resistance (r _s), and the shunt resistance (r _sh) in PSCs, it is found that neglecting PLr overestimates the effective k _nr, although it does not affect the value of r _s and r _sh. The findings herein provide insight into the impact of the PLr effect on metal-halide PSCs.

A series of halogen-free polymer donors is developed by the incorporation of the diketopyrrolopyrrole unit into the paradigm PBDB-T backbone. The optimal P75-based device shows the best power conversion efficiency of 10.28%. A slight addition of PC₇₁BM into the blend is found to further generate finer phase-separated domains and thus further increase the efficiency up to 12.20%.

High-performance polymer donors when paired with nonfullerene acceptors are mainly limited to flanking halogenated benzodithiophene (BDT)-based π-conjugated copolymers, which however involve complex synthetic procedures. Herein, a series of halogen-free polymer donors that link BDT moiety with two highly electron-deficient benzodithiophene-dione (BDD) and diketopyrrolopyrrole (DPP) units with various molar ratios is developed. Compared with the benchmark PBDB-T donor containing BDD unit, additional incorporation of a stronger electron-negative DPP unit markedly lowers frontier molecular orbital levels and extends optical absorption, potentially leading to simultaneously enhanced V _OC and J _SC in organic solar cells. A remarkable power conversion efficiency (PCE) of 10.28% is thus obtained in the optimal P75 (BDD : DPP = 3:1 mol%) and Y6 blend cells in comparison with the reference PBDB-T:Y6 (9.20%). A slight addition of PC₇₁BM into the blend is found to further generate finer phase-separated domains and thus increase the best efficiency up to 12.20%. The subtly critical roles of PC₇₁BM are determined by transient absorption measurements on both thin-film and in situ devices to be the prolonged free charge carrier lifetime and the shallow charge transfer states, which enhance J _SC and fill factor in the device, respectively.

Publication date: 21 April 2021

Source: Joule, Volume 5, Issue 4

Author(s): Di Wang, Haoran Liu, Yuhao Li, Guanqing Zhou, Lingling Zhan, Haiming Zhu, Xinhui Lu, Hongzheng Chen, Chang-Zhi Li

Publication date: 17 March 2021

Source: Joule, Volume 5, Issue 3

Author(s): Minjin Kim, In-woo Choi, Seung Ju Choi, Ji Won Song, Sung-In Mo, Jeong-Ho An, Yimhyun Jo, SeJin Ahn, Seoung Kyu Ahn, Gi-Hwan Kim, Dong Suk Kim

Despite the defect-tolerance of lead-halide perovskites, defects at the surface of colloidal nanocrystals and grain boundaries in thin films play a critical role in charge-carrier transport and nonradiative recombination, which lowers the photoluminescence quantum yields, device efficiency, and stability. This Review summarizes the defects, their influence on the optical and charge-carrier transport properties, and passivation strategies to mitigate the effects of defects.

Abstract

Lead-halide perovskites (LHPs), in the form of both colloidal nanocrystals (NCs) and thin films, have emerged over the past decade as leading candidates for next-generation, efficient light-emitting diodes (LEDs) and solar cells. Owing to their high photoluminescence quantum yields (PLQYs), LHPs efficiently convert injected charge carriers into light and vice versa. However, despite the defect-tolerance of LHPs, defects at the surface of colloidal NCs and grain boundaries in thin films play a critical role in charge-carrier transport and nonradiative recombination, which lowers the PLQYs, device efficiency, and stability. Therefore, understanding the defects that play a key role in limiting performance, and developing effective passivation routes are critical for achieving advances in performance. This Review presents the current understanding of defects in halide perovskites and their influence on the optical and charge-carrier transport properties. Passivation strategies toward improving the efficiencies of perovskite-based LEDs and solar cells are also discussed.

We constructed a series of noncovalently fused-ring electron acceptors (NFREAs) with S⋅⋅⋅O noncovalent intramolecular interactions. Combining the strategies of noncovalent conformational locks and π-extended end-group engineering, a record PCE of 14.53 % in labs and a certified PCE of 13.8 % for NFREAs based devices were achieved.

Abstract

Noncovalently fused-ring electron acceptors (NFREAs) have attracted much attention in recent years owing to their advantages of simple synthetic routes, high yields and low costs. However, the efficiencies of NFREAs based organic solar cells (OSCs) are still far behind those of fused-ring electron acceptors (FREAs). Herein, a series of NFREAs with S⋅⋅⋅O noncovalent intramolecular interactions were designed and synthesized with a two-step synthetic route. Upon introducing π-extended end-groups into the backbones, the electronic properties, charge transport, film morphology, and energy loss were precisely tuned by fine-tuning the degree of multi-fluorination. As a result, a record PCE of 14.53 % in labs and a certified PCE of 13.8 % for NFREAs based devices were obtained. This contribution demonstrated that combining the strategies of noncovalent conformational locks and π-extended end-group engineering is a simple and effective way to explore high-performance NFREAs.

The self-doping effect on the light stability of perovskite solar cells (PSCs) is systemically investigated through various opto-electrical characterizations. Although both PSCs with Pb-rich and Pb-deficient conditions exhibit similar initial performance, the Pb-rich PSC degrades relatively quickly under light illumination even without H₂O and O₂, resulting in the shift of the defect state associated with the formation of deep-level defects.

Abstract

Although there have been significant advances in the stability of perovskite solar cells through encapsulation techniques to remove extrinsic degradation factors, such as moisture and oxygen, irreversible photo-degradation originating from intrinsic defects is still challenging and remains elusive. Herein, the photo-aging mechanism due to intrinsic defects is investigated in nitrogen-filled conditions, excluding extrinsic degradation factors. Devices with similar power conversion efficiencies (PCE) of 21%, but with different Fermi levels in the perovskite films, via controlling the self-doping effect, have been investigated. Opto-electronic investigations and depth profiles of the elemental constituents show that after photo-aging, strain relaxation in the perovskite lattice and a Fermi level shift towards conduction band edge are observed, implying the formation of new defect states in Pb-rich devices. Furthermore, thermal admittance spectroscopy measurement of the devices suggests that the formation of the deep-traps in the perovskite leads to irreversible degradation. Thin-film solar cells that are relatively Pb-deficient (FA-rich) exhibit improved long-term stability, retaining over 90% of their initial PCE during 500 h of continuous 1-Sun illumination. This study suggests passivation of the Pb-I related antisite defects near the grain boundaries and the interface is crucial for the fabrication of solar cells with enhanced long-term stability.

Highly efficient and stable γ-CsPbI₃ perovskite solar cells (PSCs) can be obtained using a simple inorganic additive strategy by regulating the nucleation and crystallization process of the CsPbI₃ film. Improved grain boundaries and interfacial contact of the CsPbI₃ film lead to significant suppression in the non-radiative recombination, which shows better efficiency and remarkable stability in all-inorganic PSCs.

Abstract

All-inorganic perovskite cesium lead iodide (CsPbI₃) exhibits excellent prospects for commercial application as a light absorber in single-junction or tandem solar cells due to its outstanding thermal stability and proper bandgap. However, the device performance of CsPbI₃-based perovskite solar cells (PSCs) is still restricted by the unsatisfactory crystal quality and severe non-radiative recombination. Herein, inorganic additive ammonium halides are introduced into the precursor solution to regulate the nucleation and crystallization of the CsPbI₃ film by exploiting the atomic interaction between the ammonium group and the Pb–I framework. The grain boundaries and interfacial contact of the CsPbI₃ film have been improved, which leads to significant suppression in the non-radiative recombination and an enhancement in the charge transport ability. With these benefits, a high efficiency of 18.7% together with an extraordinarily high fill factor of 0.83–0.84 has been achieved, comparable to the highest records reported so far. Moreover, the cell exhibits ultra-high photoelectrical stability under continuous light illumination and high bias voltage with 96% of its initial power-conversion efficiency being sustained after 2000 h operation, even superior to the world-champion CsPbI₃ solar cell. The findings are promising for the development and application of all-inorganic PSCs using a simple inorganic additive strategy.

Ternary organic solar cells are fabricated, achieving a significant J _SC boost by virtue of an optimized crystalline feature with the formation of a eutectic mixture with better acceptor crystalline fibrils. The optimal morphology suppresses energetic disorder and recombination and increases charge transfer and transport, yielding a high efficiency of 17.84% with significant current boost.

Abstract

The intrinsic electronic properties of donor (D) and acceptor (A) materials in coupling with morphological features dictate the output in organic solar cells (OSCs). New physical properties of intimate eutectic mixing are used in nonfullerene-acceptor-based D–A₁–A₂ ternary blends to fine-tune the bulk heterojunction thin film morphology as well as their electronic properties. With enhanced thin film crystallinity and improved carrier transport, a significant J _SC amplification is achieved due to the formation of eutectic fibrillar lamellae and reduced defects state density. Material wise, aligned cascading energy levels with much larger driving force, and suppressed recombination channels confirm efficient charge transfer and transport, enabling an improved power conversion efficiency (PCE) of 17.84%. These results reveal the importance of utilizing specific material interactions to control the crystalline habit in blended films to form a well-suited morphology in guiding superior performances, which is of high demand in the next episode of OSC fabrication toward 20% PCE.