In perovskite solar cells, the interfaces between the perovskite and charge-transporting layers contain high concentrations of defects (about 100 times that within the perovskite layer), specifically, deep-level defects, which substantially reduce the power conversion efficiency of the devices1–3. Recent efforts to reduce these interfacial defects have focused mainly on surface passivation4–6. However, passivating the perovskite surface that interfaces with the electron-transporting layer is difficult, because the surface-treatment agents on the electron-transporting layer may dissolve while coating the perovskite thin film. Alternatively, interfacial defects may not be a concern if a coherent interface could be formed between the electron-transporting and perovskite layers. Here we report the formation of an interlayer between a SnO2 electron-transporting layer and a halide perovskite light-absorbing layer, achieved by coupling Cl-bonded SnO2 with a Cl-containing perovskite precursor. This interlayer has atomically coherent features, which enhance charge extraction and transport from the perovskite layer, and fewer interfacial defects. The existence of such a coherent interlayer allowed us to fabricate perovskite solar cells with a power conversion efficiency of 25.8 per cent (certified 25.5 per cent)under standard illumination. Furthermore, unencapsulated devices maintained about 90 per cent of their initial efficiency even after continuous light exposure for 500 hours. Our findings provide guidelines for designing defect-minimizing interfaces between metal halide perovskites and electron-transporting layers. An atomically coherent interlayer between the electron-transporting and perovskite layers in perovskite solar cells enhances charge extraction and transport from the perovskite, enabling high power conversion efficiency.

Perovskite solar cells with atomically coherent interlayers on SnO2 electrodes.

Organic-inorganic hybrid perovskite materials are receiving increasing attention and becoming star materials on account of their unique and intriguing optical and electrical properties, such as high molar extinction coefficient, wide absorption spectrum, low excitonic binding energy, ambipolar carrier transport property, long carrier diffusion length, and high defects tolerance. Although a high power conversion efficiency (PCE) of up to 22.7% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley-Queisser limit efficiency (30.5%). Obviously, trap-assisted nonradiative (also called Shockley-Read-Hall, SRH) recombination in perovskite films and interface recombination should be mainly responsible for the above efficiency distance. Here, recent research advancements in suppressing bulk SRH recombination and interface recombination are systematically investigated. For reducing SRH recombination in the films, engineering perovskite composition, additives, dimensionality, grain orientation, nonstoichiometric approach, precursor solution, and post-treatment are explored. The focus herein is on the recombination at perovskite/electron-transporting material and perovskite/hole-transporting material interfaces in normal or inverted PSCs. Strategies for suppressing bulk and interface recombination are described. Additionally, the effect of trap-assisted nonradiative recombination on hysteresis and stability of PSCs is discussed. Finally, possible solutions and reasonable prospects for suppressing recombination losses are presented.

Causes and Solutions of Recombination in Perovskite Solar Cells

Even though the power conversion efficiency (PCE) of rigid perovskite solar cells is increased to 22.7%, the PCE of flexible perovskite solar cells (F-PSCs) is still lower. Here, a novel dimethyl sulfide (DS) additive is developed to effectively improve the performance of the F-PSCs. Fourier transform infrared spectroscopy reveals that the DS additive reacts with Pb2+ to form a chelated intermediate, which significantly slows down the crystallization rate, leading to large grain size and good crystallinity for the resultant perovskite film. In fact, the trap density of the perovskite film prepared using the DS additive is reduced by an order of magnitude compared to the one without it, demonstrating that the additive effectively retards transformation kinetics during the thin film formation process. As a result, the PCE of the flexible devices increases to 18.40%, with good mechanical tolerance, the highest reported so far for the F-PSCs. Meanwhile, the environmental stability of the F-PSCs significantly enhances by 1.72 times compared to the device without the additive, likely due to the large grain size that suppresses perovskite degradation at grain boundaries. The present strategy will help guide development of high efficiency F-PSCs for practical applications.

Record Efficiency Stable Flexible Perovskite Solar Cell Using Effective Additive Assistant Strategy

Chemical interaction at a heterojunction interface induced by an appropriate chemical linker is of crucial importance for high efficiency, hysteresis-less, and stable perovskite solar cells (PSCs). Effective interface engineering in PSCs is reported via a multifunctional chemical linker of 4-imidazoleacetic acid hydrochloride (ImAcHCl) that can provide a chemical bridge between SnO2 and perovskite through an ester bond with SnO2 via esterification reaction and an electrostatic interaction with perovskite via imidazolium cation in ImAcHCl and iodide anion in perovskite. In addition, the chloride anion in ImAcHCl plays a role in the improvement of crystallinity of perovskite film crystallinity. The introduction of ImAcHCl onto SnO2 realigns the positions of the conduction and valence bands upwards, reduces nonradiative recombination, and improves carrier life time. As a consequence, average power conversion efficiency (PCE) is increased from 18.60% ± 0.50% to 20.22% ± 0.34% before and after surface modification, respectively, which mainly results from an enhanced voltage from 1.084 ± 0.012 V to 1.143 ± 0.009 V. The best PCE of 21% is achieved by 0.1 mg mL-1 ImAcHCl treatment, along with negligible hysteresis. Moreover, an unencapsulated device with ImAcHCl-modified SnO2 shows much better thermal and moisture stability than unmodified SnO2 .

Multifunctional Chemical Linker Imidazoleacetic Acid Hydrochloride for 21% Efficient and Stable Planar Perovskite Solar Cells.

We report a highly effective interface engineering strategy for thermally stable perovskite solar cells (PSCs) by employing a zwitterion-modified SnO2 electron transport layer (ETL) and a dopant-free hole transport layer (HTL). A zwitterionic compound, 3-(1-pyridinio)-1-propanesulfonate, is used to modify the SnO2 ETL. The zwitterion, which forms interfacial dipoles, plays a few important roles: (1) it causes shifts in the work function of SnO2 resulting in more efficient charge extraction and an increase in the built-in potential. (2) It pulls electrons from perovskite layers to the ETL/perovskite interface, enhancing the electron transport ability. (3) Interfacial dipoles prevent back transfer of electrons from the ETL to the perovskite and suppress charge recombination. (4) Positively charged atoms in the zwitterion passivate Pb–I antisite defects, improving the stability of devices. With these desirable properties, the PSC with doped Spiro-OMeTAD obtained a power conversion efficiency of 21.43%. In addition, the PSC with the dopant-free HTL exhibited a record high efficiency of 20.5% among dopant-free polymeric HTLs using green solvents. The resulting PSCs without encapsulation showed excellent thermal stability. Accordingly, this work suggests that the use of a modified ETL and a dopant-free HTL is a promising strategy to overcome the thermal instability of planar-PSCs (P-PSCs).

Thermally stable, planar hybrid perovskite solar cells with high efficiency

If perovskite solar cells (PSCs) with high power conversion efficiencies (PCEs) are to be commercialized, they must achieve long-term stability, which is usually assessed with accelerated degradation tests. One of the persistent obstacles for PSCs has been successfully passing the damp-heat test (85°C and 85% relative humidity), which is the standard for verifying the stability of commercial photovoltaic (PV) modules. We fabricated damp heat–stable PSCs by tailoring the dimensional fragments of two-dimensional perovskite layers formed at room temperature with oleylammonium iodide molecules; these layers passivate the perovskite surface at the electron-selective contact. The resulting inverted PSCs deliver a 24.3% PCE and retain >95% of their initial value after >1000 hours at damp-heat test conditions, thereby meeting one of the critical industrial stability standards for PV modules. Description Stabilizing inverted solar cells Although inverted (p-i-n) perovskite solar cells (PSCs) have advantages in fabrication and scaling compared with n-i-p cells, their power conversion efficiencies (PCEs ) are usually lower. Azmi et al. show that by tailoring the number of octahedral inorganic sheets in two-dimensional perovskite (2DP) passivation layers for three-dimensional perovskite active layers, PCEs of more than 24% could be achieved (see the Perspective by Luther and Schelhas). The 2DP layers formed with oleylammonium iodide molecules at the electron-selective interface passivated trap states and suppressed ion migration. These PSCs retained more than 95% of their initial efficiency after 1000 hours of damp-heat testing (85°C and 85% relative humidity), which passes a key industrial stability standard. —PDS Tailored two-dimensional perovskite passivation layers enable efficient, damp-heat stable inverted perovskite solar cells.

/pdf/damp-heat-stable-perovskite-solar-cells-with-tailored-3ianatl1.pdf

Damp heat–stable perovskite solar cells with tailored-dimensionality 2D/3D heterojunctions

Herein, this study reports high-efficiency, low-temperature ZnO based planar perovskite solar cells (PSCs) with state-of-the-art performance. They are achieved via a strategy that combines dual-functional self-assembled monolayer (SAM) modification of ZnO electron accepting layers (EALs) with sequential deposition of perovskite active layers. The SAMs, constructed from newly synthesized molecules with high dipole moments, act both as excellent surface wetting control layers and as electric dipole layers for ZnO-EALs. The insertion of SAMs improves the quality of PbI2 layers and final perovskite layers during sequential deposition, while charge extraction is enhanced via electric dipole effects. Leveraged by SAM modification, our low-temperature ZnO based PSCs achieve an unprecedentedly high power conversion efficiency of 18.82% with a VOC of 1.13 V, a JSC of 21.72 mA cm−2, and a FF of 0.76. The strategy used in this study can be further developed to produce additional performance enhancements or fabrication temperature reductions.

High-Efficiency Low-Temperature ZnO Based Perovskite Solar Cells Based on Highly Polar, Nonwetting Self-Assembled Molecular Layers

High-performance dopant-free conjugated small molecule-based hole-transport materials for perovskite solar cells

High-efficiency colloidal quantum dot photovoltaic devices (CQDPVs) are achieved by improving the interfacial charge extraction via chemical modification of PbS-CQD/ZnO heterojunctions. Simple treatment of the heterojunctions using a chemical modifier, 1,2-ethanedithiol, effectively reduces the interband trap sites of the ZnO nanoparticles (ZnO-np) by passivation of the notorious intrinsic oxygen-deficient defects. As a result, the interfacial bimolecular recombinations between (i) trapped electrons in the ZnO-np layers and the holes in the CQD layers and (ii) accumulated electrons in the CQD layers and the holes in the CQD layers are suppressed. Consequently, the power conversion efficiency of the chemically modified CQDPVs reached a certified power conversion efficiency of 10.14% with decent air stability. Notably, the entire device fabrication process, including chemical modification, could be performed at room temperature under ambient atmosphere.

http://pubs.acs.org/doi/pdf/10.1021/acsenergylett.6b00070

Randi Azmi

Papers

Damp heat–stable perovskite solar cells with tailored-dimensionality 2D/3D heterojunctions

High-Efficiency Low-Temperature ZnO Based Perovskite Solar Cells Based on Highly Polar, Nonwetting Self-Assembled Molecular Layers

High-performance dopant-free conjugated small molecule-based hole-transport materials for perovskite solar cells

High-Efficiency Colloidal Quantum Dot Photovoltaic Devices Using Chemically Modified Heterojunctions

Simultaneous Improvement in Efficiency and Stability of Low-Temperature-Processed Perovskite Solar Cells by Interfacial Control