Nonradiative Carrier Recombination Research Articles

Defect passivation via incorporation of sodium thiocyanate for achieving high-performance MAPbI3-based perovskite solar cells with enhanced stability

The high density of defects in MAPbI3 perovskite films brings about severe carrier nonradiative recombination, which lowers the photovoltaic performance and the stability of MAPbI3-based perovskite solar cells (PSCs). To tackle this issue, we prepared stable and efficient MAPbI3-based PSCs with incorporation of sodium thiocyanate (NaSCN). NaSCN can increase grain size and improve crystallization quality of the perovskite films due to an Ostwald ripening effect of SCN- and surface passivation of Na+ which can effectively passivate the defect states and inhibit nonradiative recombination for an ultimate improvement of the device performance and stability. The excellent characteristics lead to a significant enhancement of the power conversion efficiency (PCE) from 18.1 % to 19.4 % both for MAPbI3-based and Cs0.06MA0.15FA0.79PbI2.5Br0.5-based PSCs, respectively. The unencapsulated NaSCN-treated PSCs also shows a improved stability after aging under a relative humidity of 40 ± 5 % for 15 days.

Recent Advances in Inverted Perovskite Solar Cells: Designing and Fabrication.

Inverted perovskite solar cells (PSCs) have been extensively studied by reason of their negligible hysteresis effect, easy fabrication, flexible PSCs and good stability. The certified photoelectric conversion efficiency (PCE) achieved 23.5% owing to the formed lead−sulfur (Pb−S) bonds through the surface sulfidation process of perovskite film, which gradually approaches the performance of traditional upright structure PSCs and indicates their industrial application potential. However, the fabricated devices are severely affected by moisture, high temperature and ultraviolet light due to the application of organic materials. Depending on nitrogen, cost of protection may increase, especially for the industrial production in the future. In addition, the inverted PSCs are found with a series of issues compared with the traditional upright PSCs, such as nonradiative recombination of carriers, inferior stability and costly charge transport materials. Thus, the development of inverted PSCs is systematically reviewed in this paper. The design and fabrication of charge transport materials and perovskite materials, enhancement strategies (e.g., interface modification and doping) and the development of all−inorganic inverted devices are discussed to present the indicator for development of efficient and stable inverted PSCs.

Enhancing the stability of planar perovskite solar cells by green and inexpensive cellulose acetate butyrate

Although the efficiency of organic–inorganic hybrid halide perovskite solar cells has been improved rapidly, the intrinsic instability of perovskite materials restricts their commercial application. Here, an eco-friendly and low-cost organic polymer, cellulose acetate butyrate (CAB), was introduced to the grain boundaries and surfaces of perovskite, resulting in a high-quality and low-defect perovskite film with a nearly tenfold improvement in carrier lifetime. More importantly, the CAB-treated perovskite films have a well-matched energy level with the charge transport layers, thus suppressing carrier nonradiative recombination and carrier accumulation. As a result, the optimized CAB-based device achieved a champion efficiency of 21.5% compared to the control device (18.2%). Since the ester group in CAB bonds with Pb in perovskite, and the H and O in the hydroxyl group bond with the I and organic cations in perovskite, respectively, it will contribute to superior stability under heat, high humidity, and light soaking conditions. After aging under 35% humidity (relative humidity, RH) for 3300 h, the optimized device can still maintain more than 90% of the initial efficiency; it can also retain more than 90% of the initial efficiency after aging at 65 °C, 65% RH, or light (AM 1.5G) for 500 h. This simple optimization strategy for perovskite stability could facilitate the commercial application of perovskite solar cells.

Bottom-up holistic carrier management strategy induced synergistically by multiple chemical bonds to minimize energy losses for efficient and stable perovskite solar cells

The defects from electron transport layer, perovskite layer and their interface would result in carrier nonradiative recombination losses. Poor buried interfacial contact is detrimental to charge extraction and device stability. Here, we report a bottom-up holistic carrier management strategy induced synergistically by multiple chemical bonds to minimize bulk and interfacial energy losses for high-performance perovskite photovoltaics. 4-trifluoromethyl-benzamidine hydrochloride (TBHCl) containing –CF3, amidine cation and Cl− is in advance incorporated into SnO2 colloid solution to realize bottom-up modification. The synergistic effect of multiple functional groups and multiple-bond-induced chemical interaction are revealed theoretically and experimentally. F and Cl− can passivate oxygen vacancy and/or undercoordinated Sn4+ defects by coordinating with Sn4+. The F can suppress cation migration and modulate crystallization via hydrogen bond with FA+, and can passivate lead defects by coordinating with Pb2+. The –NH2–CNH2+ and Cl− can passivate cation and anion vacancy defects through ionic bonds with perovskites, respectively. Through TBHCl modification, the suppression of agglomeration of SnO2 nanoparticles, bulk and interfacial defect passivation, and release of tensile strains of perovskite films are demonstrated, which resulted in a PCE enhancement from 21.28% to 23.40% and improved stability. With post-treatment, the efficiency is further improved to 23.63%.

2, 3, 4, 5, 6-Pentafluorophenylammonium bromide-based double-sided interface engineering for efficient planar heterojunction perovskite solar cells

In order to obtain high power conversion efficiency and high stability perovskite solar cells, many innovative optimization schemes have been proposed. Thereinto, the interface modification become an effective way to improve the performance of device. Here, A dual-interface passivation by 2, 3, 4, 5, 6-pentafluorophenylammonium bromide-based (5PFP-Br) is used to optimize the performance of device with the structure of ITO/SnO2/perovskite/PTAA/Ag. We design the 5PFP-Br layer at SnO2/perovskite and perovskite/PTAA interface respectively. The results show that the interface contact between perovskite and charge transport layer is improved obviously. At the same time, the density of carrier transport defect states in the device is reduced, thus reducing the non-radiative recombination of carriers. Meanwhile, the 5PFP-Br layer at perovskite/PTAA interface can effectively passivate Pb defects and reduce the number of grain boundaries in perovskite film, and increase the carrier transfer and collection efficiency. Finally, 5PFP-Br double-interface passivation can further optimize and improve the performance of device. The device based 5PFP-Br double-interface passivation presents an increased efficiency from 18.09% to 21.15%, where FF is raised from 73.30% to 80.04%. At the same time, unpackaged device can still be maintained to more than 90% efficiency after about 1200 h at 25 °C and 25%RH.

High grain boundary recombination velocity in polycrystalline metal halide perovskites.

Understanding carrier recombination processes in metal halide perovskites is fundamentally important to further improving the efficiency of perovskite solar cells, yet the accurate recombination velocity at grain boundaries (GBs) has not been determined. Here, we report the determination of carrier recombination velocities at GBs (SGB) of polycrystalline perovskites by mapping the transient photoluminescence pattern change induced by the nonradiative recombination of carriers at GBs. Charge recombination at GBs is revealed to be even stronger than at surfaces of unpassivated films, with average SGB reaching 2200 to 3300 cm/s. Regular surface treatments do not passivate GBs because of the absence of contact at GBs. We find a surface treatment using tributyl(methyl)phosphonium dimethyl phosphate that can penetrate into GBs by partially dissolving GBs and converting it into one-dimensional perovskites. It reduces the average SGB by four times, with the lowest SGB of 410 cm/s, which is comparable to surface recombination velocities after passivation.

Improved efficiency and stability of organic-inorganic hybrid perovskite solar cell via dithizone surface passivation effect

Passivating chemicals are recognized as an effective strategy to eliminate the defects inside and on the surface of the perovskite absorber. In this study, dithizone (DTZ) was explored as the passivation agent to modify the perovskite surface and improve the performance of the solar cells. DTZ contains the –NH, –SH, and benzene rings, where the –NH and –SH can interact with Pb 2+ while the benzene can stabilize the perovskite. It is demonstrated that DTZ could effectively promote the quality of the perovskite films by reducing the excess lead iodide and defects on the surface, and eventually suppressing the non-radiative carrier recombination. Furthermore, the introduction of the DTZ interface can optimize the band alignment to facilitate the hole collection. Resultantly, device parameters, including current density, open-circuit voltage, fill factor, and the power conversion efficiency have been enhanced substantially from 16.96% to 19.67%. Meanwhile, the shelf stability also exhibited a considerable promotion. • Dithizone is explored as an effective passivating chemicals for the perovskite surface. • The excess lead iodide and non-radiative carrier recombination are reduced after dithizone treatment. • The efficiency is enhanced from 16.96% up to 19.67%.

Strong room temperature exciton photoluminescence in electrochemically deposited Cu2O films

Strong room temperature exciton photoluminescence (PL) has been observed in copper (I) oxide films electrochemically deposited in a tartrate electrolyte. The PL intensity of these films is two orders of magnitude higher than that of films deposited from the classical lactate electrolyte. X-ray diffraction (XRD) and Raman spectroscopy analyses demonstrate that the films prepared using tartrate electrolyte are characterized by higher grain size, which reduces a non-radiative recombination of charge carriers. Better optical quality of the Cu2O films prepared using tartrate electrolyte is explained taking into account stronger tartrate-copper complexes, which results in lower density of grain boundaries in such films. Moreover, higher buffering capacity of the tartrate complex provides stability of pH value in the diffusion layer of the near-electrode space preventing defect formation. Our study demonstrates the promise of using Cu2O films deposited from tartrate solution for solar energy applications like photoelectric energy conversion, hydrogen production, and photocatalysis.

Replacing cesium with formamidinium cation in colloidal perovskite nanoplatelets: Synthetic alloying and post-synthetic cation exchange towards efficient blue emission

Blue-emitting perovskite materials have attracted remarkable attention in recent years owing to their potential in full-color display and lighting applications. Two-dimensional (2D) perovskite nanoplatelets (NPls) are considered as competitive blue emitters due to high carrier recombination probability and good spectrum stability compared with their mixed-halide counterparts. However, the 2D nature and high defect density of perovskite NPls generally result in nonradiative carrier recombination, making it problematic for achieving efficient blue emission. Herein, we reported a novel way to enhance the blue emission efficiency of perovskite NPls. We found that by using formamidinium cation (FA+) to replace the cesium cation (Cs+) during NPls synthesis, the photoluminescence (PL) of the NPls can be significantly enhanced. The PL quantum yield (PLQY) of NPls exhibited a monoclinic rising tendency with increased FA+ content, and the FAPbBr3 NPls finally show efficient deep blue emission at 433 nm with PLQY up to 70% as well as narrow emission bandwidth of only 12 nm. We further demonstrated that efficient blue emission can be realized not only by FA+ alloying but also by post-synthetic treatment of the as-synthesized CsPbBr3 NPls using FA+, where the FA+ was capable to replace the Cs+ in perovskite NPls through cation exchange. Resultantly, PLQY as high as 90% was achieved at a pure blue wavelength of 457 nm. Our work provides a promising strategy for achieving efficient and color-pure blue-emitting perovskite materials, which could potentially further promote their light-emitting applications.

Dually-passivated planar SnO2 based perovskite solar cells with ˃2,700h ambient stability: Facile fabrication, high performance and mechanism

In SnO2-based planar perovskite solar cells (SP–PSCs), stability issue derived by defects at grain boundaries (GBs) and surface in perovskite films has been one of the main reasons that limit its commercialization. Therefore, co-passivation of both defects is pivotal to maximize passivation effects for improving power conversion efficiency (PCE) and stability, which is rarely reported in SP-PSCs. Herein, two simple passivating agents including Pb(SCN)2 and choline chloride (ChCl) were selected to dually passivate GBs and surface defects in the perovskite films (ambient atmosphere), respectively. The influences of co-passivation on PCE and ambient stability of devices were intensively studied, and the related mechanisms were also elucidated in detail. The dually-passivated device obtained a champion PCE of 19.98%, and maintained about 94% of the initial efficiency after being stored for ˃2700h aging in the humid air. The reduction of GBs and surface defects not only weakens the non-radiative recombination of carriers, but also inhibits the penetration of moisture and oxygen, which ultimately greatly enhances the PCE and stability of the device. This paper provides support for the dual passivation of GBs and surface defects to ameliorate ambient stability of the SP-PSCs, and also enriches the selectivity of passivation agents on dual passivation.

Surface passivation of perovskite films by potassium bis(fluorosulfonyl)imide for efficient solar cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted much attention because of their high photoelectric conversion efficiency (PCE) and simple preparation process. At present, one of the major factors limiting the further improvement of PSCs’ performance is defects in perovskite active layers. Herein, we report an effective surface post-treatment method for perovskite active layers, which uses potassium bis(fluorosulfonyl)imide (KFSI) to passivate multiple defects, improve photovoltaic performance and reduce the hysteresis. It is demonstrated that not only S=O and S–N bonds but also charged K + and FSI − ions in KFSI contribute to the passivation of multiple defects in perovskite films via Lewis acid-base interaction and/or forming ionic bonds. After the optimized KFSI treatment, the trap density of the perovskite film decreased from 8.41 × 10 15 cm −3 to 3.97 × 10 15 cm −3 , which significantly suppressed trap-assisted nonradiative recombination of carriers in perovskite films and/or at perovskite/hole transport layer interface. The KFSI treated device achieved a PCE of ∼19.00% with less hysteresis, appreciably outperforming the control device (17.22%). At the same time, the performance reproducibility among the fabricated solar cells was remarkably improved. This work provides a facile way to passivate multiple defects in perovskite films and further enhance the performance of perovskite solar cells. • An effective passivator - potassium bis(fluorosulfonyl)imide (KFSI) for passivating multiple perovskite defects is reported. • Not only S=O and S–N bonds but also charged K + and FSI − ions in KFSI contribute to the passivation of multiple defects. • After the optimized KFSI treatment, the trap density decreased, which suppressed non-radiative recombination of carriers. • The KFSI treated device appreciably outperforms the control device in terms of efficiency, hysteresis and reproducibility.

3-Chloroperoxybenzoic acid doping spiroOMeTAD for improving the performance of perovskite solar cells

The perovskite solar cells (PSCs) is one of the fastest growing photovoltaic technology. However, the further improvement of performance and stability is the bottleneck issue of its large-scale manufacturing. The controllable oxidation of spiroOMeTAD is significant for high-performance PSCs. Here, a low-cost and facile strategy by doping 3-Chloroperoxybenzoic acid (m-CPBA) into spiroOMeTAD is demonstrated. The addition of m-CPBA accelerates the oxidation of spiroOMeTAD, produces more carriers, improves the hole conductivity, and reduces defects and carrier non-radiative recombination. On the other hand, the modification of m-CPBA adjusts the HOMO of hole transport layer (HTL), makes more match the energy array between HTL and perovskite layer, and reduces energy loss at interface. Consequently, the m-CPBA optimized PSCs achieves a power conversion efficiency (PCE) of 23.34% with an outstanding fill factor (FF) over 84%, along with excellent environmental stability.

Synergetic Effects of Zn Alloying and Defect Engineering on Improving the CdS Buffer Layer of Cu2ZnSnS4 Solar Cells.

The inferior electrical properties at the interface of the Cu2ZnSnS4/CdS (CZTS/CdS) heterojunction resulting in the severe loss of open-circuit voltage (Voc) highly restrict the photovoltaic efficiency of CZTS solar cell devices. Here, first-principles calculations show that the Zn-alloyed CdS buffer layer reverses the unfavorable cliff-like conduction band offset (CBO) of CZTS/CdS to the desirable spike-like CBO of CZTS/Zn0.25Cd0.75S, which suppresses carrier nonradiative recombination and blocks electron backflow. In addition, the weakened n-type conductivity of Zn0.25Cd0.75S can be enhanced by In, Ga, and Cl doping without the introduction of detrimental deep-level defects and severe band-tail states, which improves the Voc of CZTS solar cells by promoting strong band bending and large quasi-Fermi-level splitting at the absorber side of the CZTS/Zn0.25Cd0.75S heterojunction. This study finds that the synergetic effects of Zn alloying and defect engineering on the CdS buffer layer are promising for overcoming the long-standing issue of the Voc deficit in CZTS solar cells, and understanding the optimized interfacial electrical properties provides theoretical guidance for improving the efficiency of semiconductor devices.

Interfacial Passivation Engineering for Highly Efficient Perovskite Solar Cells with a Fill Factor over 83%

Charge carrier nonradiative recombination (NRR) caused by interface defects and nonoptimal energy level alignment is the primary factor restricting the performance improvement of perovskite solar cells (PSCs). Interfacial modification is a vital strategy to restrain NRR and enable high-performance PSCs. We report here two interfacial materials, PhI-TPA and BTZI-TPA, consisting of phthalimide and a 2,1,3-benzothiadiazole-5,6-dicarboxylicimide core, respectively. The difunctionalized BTZI-TPA with imide and thiadiazole shows higher hole mobility, better aligned energy levels, and stronger interaction with uncoordinated Pb2+ on the perovskite surface, suppressing NRR and carrier accumulation at the interface of perovskite/spiro-OMeTAD and yielding enhanced open-circuit voltage and fill factor. Consequently, the PSC based on BTZI-TPA delivers a high efficiency of 24.06% with an excellent fill factor of 83.10%, superior to that (21.47%) of the reference cell without an interfacial layer, and 21.45% efficiency for the device with a scaled-up area (1.00 cm2). These results underscore the potential of imide and thiadiazole groups in developing interfacial layers with strong passivation capability, effective charge transport property, and fine-tuned energetics for stable and efficient PSCs.

Polarized Molecule 4-(Aminomethyl) Benzonitrile Hydrochloride for Efficient and Stable Perovskite Solar Cells.

The rapid development of perovskite solar cells (PSCs) makes it one of the most competitive photovoltaic devices in the field of new energy. However, the suboptimal performance and poor stability caused by numerous defects are still the main factors limiting the development of PSCs. Herein, a polarized molecule additive of 4-(aminomethyl) benzonitrile hydrochloride (AMBNCl) is introduced into perovskite. Owing to its special polar electron density distribution, -C≡N group, -NH3+ terminal, and Cl- ions, the modification of AMBNCl can improve the quality of perovskite crystal growth, passivate the defects of Pb2+, adjust the energy level array between the perovskite layer and hole-transport layer, and alleviate the carrier nonradiative recombination. As a result, the AMBNCl-modified device achieves a champion efficiency of 23.52%. The unpacked device still maintained 91.2% of its original efficiency after storing in an air environment (RH ∼40%, 25 °C) for 50 days.

Phonon-Assisted Nonradiative Recombination Tuned by Organic Cations in Ruddlesden-Popper Hybrid Perovskites

Significant efforts have been devoted to further increasing the photoconversion efficiency of two-dimensional (2D) organic-inorganic halide perovskites, which are promising photovoltaic materials with superior stability but lower efficiency compared to their three-dimensional counterparts. One of the main factors that limit their photoconversion efficiency is the nonradiative recombination of photoexcited carriers. A widely used strategy for tuning the photoconversion efficiency of 2D perovskites is exchanging various organic cations. However, due to mixed effects contributed to by extrinsic factors, such as defect concentration and sample morphology, experiments alone are insufficient to explain the role of organic cations in phonon-assisted carrier recombination. With time-domain simulations based on first-principles nonadiabatic molecular dynamics, we investigate six prototypical 2D Ruddlesden-Popper perovskites to reveal the impact of organic cations on tuning the nonradiative recombination time through displacing inorganic ions and affecting electron-phonon coupling. The phonon-assisted band-to-band nonradiative recombination time is on the order of a few hundred nanoseconds and can be tuned up to 5 times by modifying organic cations. A distinct correlation between the pure-dephasing time and inorganic atom distortions induced by the motion of organic parts is revealed. Compared to the pure-dephasing time, nonadiabatic coupling (NAC) associated with the strength of electron-phonon coupling plays a more important role in determining the nonradiative carrier recombination time. Notably, fluorinated organic spacers increase the NAC between frontier electronic states and speed up the nonradiative recombination process.

Superfast crystalline powder synthetic strategy toward scale-up of perovskite solar cells

Commercialized perovskite solar cells (PSCs) are limited owing to the high cost of high-purity PbI2 (99.999%), raw material batch variance, and stoichiometric deviation. Here, we report a rapid antisolvent-assisted crystallization (RAC) method to speedily obtain high-grade perovskite powders from low-purity and inexpensive PbI2 (98%) raw materials. Without long-term stirring or employing any toxic antisolvent, RAC is a cost-efficient and environmentally friendly method with a high yield of over 92%. By precise characterization, the RAC method can reduce the impurities in raw materials that minimize the impurity-induced shallow-level trap density and hence the non-radiative recombination of carriers. As per the results, over 12% increase in power conversion efficiency (PCE) is obtained from low-grade raw materials, resulting in a champion PCE of 20.5% for small-area MAPbI3 PSCs and certified 18.35% for doctor-bladed 6.75-cm2 mini-module. With a reduction in material cost and a considerable increase in efficiency, RAC is a suitable alternative to the mass-produced, commercially available large-scale PSC.

Development of laser heterodyne photothermal displacement method for mapping carrier nonradiative recombination centers in semiconductors

The laser heterodyne photothermal displacement (LH-PD) method was used to characterize the nonradiative recombination centers of semiconductors, such as defects and deep-lying electronic levels. When a semiconductor surface is irradiated with a modulated continuous wave laser, the irradiated area is periodically heated and expanded owing to the nonradiative recombination of the photoexcited carriers. The LH-PD can measure an absolute value of surface displacement and its time variation at various excitation beam frequencies (fex). Si and GaAs substrate samples were used to confirm the usefulness of the proposed method. The obtained time variation of the surface displacement was well explained by theoretical calculations considering the carrier generation, diffusion, recombination, heat diffusion, and generated thermal strain. Because nonradiative carrier recombination generates local heat at defects in semiconductors, the LH-PD technique is useful for analyzing defect distributions. Additionally, measurements of intentional Fe-contaminated Si samples confirmed that this technique is suitable for defect mapping. Displacement mapping with changing fex suggests the potential to measure the distribution of nonradiative recombination centers in the sample depth direction.

Multifunctional Passivation Strategy of Cationic and Anionic Defects for Efficient and Stable Perovskite Solar Cells

The crystallinity of perovskite films is crucial for the performance and stability of perovskite solar cells (PSCs). Defects usually emerge in grain boundaries, leading to decomposition and non-radiative recombination of PSCs. Here, we present an effective additive engineering strategy to augment the long-term operation and stability of PSCs by doping a molecule with a symmetrical structure and bifunctional passivation, 2,5-thiophene dicarboxylic acid (TDCA), into the precursor solution. It is demonstrated that TDCA coordinates with the perovskite through hydrogen and Pb–O bonding, resulting in significantly enhanced crystallinity and defect passivation such as uncoordinated Pb2+ and I–. Meanwhile, the grain size is increased from 350 nm to about 650 nm for the perovskite, as well as the grain boundaries are reduced, which could inhibit the carrier non-radiative recombination loss. Consequently, the power conversion efficiency of champion PSCs is promoted from 19.31 to 22.78%. Furthermore, the enhanced crystallinity and defect passivation improve the wet-thermal stability of PSCs.

An all optical approach for comprehensive in-operando analysis of radiative and nonradiative recombination processes in GaAs double heterostructures

We demonstrate an all optical approach that can surprisingly offer the possibility of yielding much more information than one would expect, pertinent to the carrier recombination dynamics via both radiative and nonradiative processes when only one dominant deep defect level is present in a semiconductor material. By applying a band-defect state coupling model that explicitly treats the inter-band radiative recombination and Shockley–Read–Hall (SRH) recombination via the deep defect states on an equal footing for any defect center occupation fraction, and analyzing photoluminescence (PL) as a function of excitation density over a wide range of the excitation density (e.g., 5–6 orders in magnitude), in conjunction with Raman measurements of the LO-phonon plasmon (LOPP) coupled mode, nearly all of the key parameters relevant to the recombination processes can be obtained. They include internal quantum efficiency (IQE), minority and majority carrier density, inter-band radiative recombination rate (Wr), minority carrier nonradiative recombination rate (Wnr), defect center occupation fraction (f), defect center density (Nt), and minority and majority carrier capture cross-sections (σt and σtM). While some of this information is thought to be obtainable optically, such as IQE and the Wr/Wnr ratio, most of the other parameters are generally considered to be attainable only through electrical techniques, such as current-voltage (I-V) characteristics and deep level transient spectroscopy (DLTS). Following a procedure developed herein, this approach has been successfully applied to three GaAs double-heterostructures that exhibit two distinctly different nonradiative recombination characteristics. The method greatly enhances the usefulness of the simple PL technique to an unprecedented level, facilitating comprehensive material and device characterization without the need for any device processing.