Dimethylammonium cation stabilizes all-inorganic perovskite solar cells

<正>Facing the poor environmental stability of traditional methylammonium or formamidinium-based lead halide perovskites,scientists turn their attention to inorganic lead halide perovskites (ILHPs) with narrow bandgaps,excellent thermal stability and reduced ion migration compared to their organic/inorganic counterparts^[1-4].Up to now,the PCEs for ILHP solar cells exceed 21%^[5].Especially,the preferred black ILHP (e.g.CsPbl₃) with the smallest bandgap of～1.7...     

p-Layer bandgap engineering for high efficiency thin film silicon solar cells

Spectroscopic ellipsometry (SE), high resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM) and optical transmittance measurements were used to study and establish a correlation between the open-circuit voltage (V_oc) of solar cells and the p-layer optical band gap (E_p). It is found that the ellipsometry measurement can be used as an inline non-destructive diagnostic tool for p-layer deposition in commercial operation. The analysis of ellipsometric spectra, together with the optical transmittance data, shows that the best p-layer appears to be very fine nanocrystallites with an E_p 1.95 eV. HRTEM measurements reveal that the best p-layer is composed of nanocrystallites ~9 nm in size. It is also found that the p-layer exhibits very good transmittance, as high as ~91.6% at ~650 nm. These results have guided us to achieve high V_oc value 1.03 V for thin film silicon based single junction solar cell.     

Designing novel thin film polycrystalline solar cells for high efficiency: sandwich CIGS and heterojunction perovskite

Heterojunction and sandwich architectures are two new-type structures with great potential for solar cells. Specifically, the heterojunction structure possesses the advantages of efficient charge separation but suffers from band offset and large interface recombination; the sandwich configuration is favorable for transferring carriers but requires complex fabrication process. Here, we have designed two thin-film polycrystalline solar cells with novel structures:sandwich CIGS and heterojunction perovskite, referring to the advantages of the architectures of sandwich perovskite (standard) and heterojunction CIGS (standard) solar cells, respectively. A reliable simulation software wxAMPS is used to investigate their inherent characteristics with variation of the thickness and doping density of absorber layer. The results reveal that sandwich CIGS solar cell is able to exhibit an optimized efficiency of 20.7%, which is much higher than the standard heterojunction CIGS structure (18.48%). The heterojunction perovskite solar cell can be more efficient employing thick and doped perovskite films (16.9%) than these typically utilizing thin and weak-doping/intrinsic perovskite films (9.6%). This concept of structure modulation proves to be useful and can be applicable for other solar cells.     

Progress in flexible perovskite solar cells with improved efficiency

Perovskite solar cell has emerged as a promising candidate in flexible electronics due to its high mechanical flexibil-ity,excellent optoelectronic properties,light weight and low cost.With the rapid development of the device structure and mater-ials processing,the flexible perovskite solar cells (FPSCs) deliver 21.1％ power conversion efficiency.This review introduces the latest developments in the efficiency and stability of FPSCs,including flexible substrates,carrier transport layers,perovskite films and electrodes.Some suggestions on how to further improve the efficiency,environmental and mechanical stability of FPSCs are provided.Specifically,we considered that to elevate the performance of FPSCs,it is crucial to substantially improve film quality of each functional layer,develop more boost encapsulation approach and explore flexible transparent electrodes with high conductivity,transmittance,low cost and expandable processability.     

Interface engineering in efficient vacuum deposited perovskite solar cells

We studied the effect of the charge transport layers in p-i-n perovskite solar cells using vacuum deposited methylammonium lead iodide thin-film absorbers. While solution-processed perovskite films are frequently deposited directly on PEDOT:PSS leading to good solar cell performances, in some cases even to very good V_oc values, we show that in devices employing vacuum deposited MAPbI₃ perovskites, the removal of the polyTPD electron blocker substantially reduces the photovoltaic behavior. This is indicative of rather different charge transport properties in the vacuum deposited MAPbI₃ perovskites compared to those prepared from solution. On the other hand, we investigated the use of ionic interlayers as a possible alternative to low work function electrodes, whose reactivity towards air and moisture compromises the device stability. Two different electron extraction materials were evaluated as interlayers between the fullerene electron transport layer and a silver electrode, in particular a perylenediimide derivative and a conjugated polyelectrolyte. By studying the photovoltaic response and the electroluminescence properties of planar diodes using the ionic films and comparing them with devices employing barium, we found that such ionic interlayers can successfully replace the use of reactive electrodes, since they facilitate the electron extraction while reducing the non-radiative recombination at the electron transport interface.     

Defect passivation with potassium trifluoroborate for efficient spray-coated perovskite solar cells in air

Defects as non-radiative recombination centers hinder the further efficiency improvements of perovskite solar cells (PSCs). Additive engineering has been demonstrated to be an effective method for defect passivation in perovskite films. Here, we employed (4-methoxyphenyl) potassium trifluoroborate (C₇H₇BF₃KO) with \begin{document}${{\rm{BF}}_3^-}$\end{document} and K⁺ functional groups to passivate spray-coated (FAPbI₃)_x(MAPbBr₃)_1–x perovskite and eliminate hysteresis. It is shown that the F of \begin{document}${{\rm{BF}}_3^-}$\end{document} can form hydrogen bonds with the H atom in the amino group of MA⁺/FA⁺ ions of perovskite, thus reducing the generation of MA⁺/FA⁺ vacancies and improving device efficiency. Meanwhile, K⁺ and reduced MA⁺/FA⁺ vacancies can inhibit ion migration, thereby eliminating hysteresis. With the aid of C₇H₇BF₃KO, we obtained hysteresis-free PSCs with the maximum efficiency of 19.5% by spray-coating in air. Our work demonstrates that additive engineering is promising to improve the performance of spray-coated PSCs.     

Copolymers based on thiazolothiazole-dithienosilole as hole-transporting materials for high efficient perovskite solar cells

The interlayers, including hole transporting layer (HTL) and electron transporting layer (ETL), segregating photoactive layer and the electrodes play an important role in charge extraction and transportation in perovskite solar cells (pero-SCs). Two novel copolymers, PDTSTTz and PDTSTTz-4, for the first time were applied as HTL in the n-i-p type pero-SCs, with the device structure of ITO/compact TiO₂/CH₃NH₃PbI_3-xCl_x/HTL/MoO₃/Ag. The highest occupied molecular orbitals (HOMO) levels of PDTSTTz and PDTSTTz-4 exhibit a suitable band alignment with the valence band edge of the perovskite. Both of them lead to improved device performances compared with reference pero-SCs based on P3HT as HTL. To further balance the charge extraction and the diffusion length of charge carriers, pristine C₆₀ was introduced at the cathode side of the pero-SCs, working together with TiO₂ as ETL. With insertion of both the HTL and ETL, the performance of pero-SCs was greatly enhanced. The optimized devices exhibited impressive PCEs of 14.4% and 15.8% for devices based on PDTSTTz and PDTSTTz-4. The improved performance is attributed to better light harvest ability, decreased interface resistance and faster decay time due to the introduction of the interlayers.     

Improved morphology and enhanced stability via solvent engineering for planar heterojunction perovskite solar cells

Solvent engineering technique for planar heterojunction perovskite solar cells is an efficient way to achieve uniformly controlled grain morphology for perovskite films. In this report, diethyl ether solvent engineering technique was used for Methyl ammonium lead triiodide (CH₃NH₃PbI₃) perovskite thin films for planar heterojunction solar cells which exhibited a PCE of 9.20%. Morphological improvements and enhanced grain sizes leads to enhanced absorption of CH₃NH₃PbI₃. Moreover solar cells have showed an excellent environmental stability of more than 100 days. This increase in efficiency is due to improved film morphology of perovskite layer after solvent treatment which has been revealed under UV–Vis spectroscopy, SEM images, X-ray diffraction and impedance spectroscopy.     

Anti-solvent engineering via potassium bromide additive for highly efficient and stable perovskite solar cells

Anti-solvent assisted crystallization is commonly employed method to achieve high-quality perovskites attributed to its great operability. Herein, we report an anti-solvent engineering approach via simply using potassium bromide (KBr) additive with commonly used chlorobenzene (CB) in triple-cation perovskite solar cells (PSCs). We show that the KBr additive in the CB anti-solvent not only increases the crystallinity and passivates the perovskite top surface defects, but also leads to suppressed nonradiative recombination and facilitates charge extraction at interfaces. Interestingly, due to the halide vacancies filling with K⁺ ions, hysteresis behavior in the treated perovskite layer was suppressed. Consequently, a champion power conversion efficiency (PCE) of 18.29% was yielded for anti-solvent engineering employing KBr (an 20% improvement in PCE compared to the CB-only anti-solvent device). Furthermore, the optimized device based on KBr demonstrated improved stability, maintaining 80% of its original efficiency after aging in an environment with a relative humidity of 30–50% for 1080 h. Our study reports the significant role of anti-solvent engineering in improving perovskite's quality for efficient PSCs and develops the potential for PSC commercialization.     

Inorganic perovskite/organic tandem solar cells with efficiency over 20%

The rapid development of perovskite solar cells is beyond our imagination.The power conversion efficiency(PCE)of organic-inorganic hybrid perovskite solar cells has reached 25.5%(https://www.nrel.gov/pv/cell-efficiency.html).However,the unsatisfactory stability of hybrid perovskites is an obstacle for their commercialization,which results from the volatile and hygroscopic organic cations[1].     

Improved efficiency and photo-stability of methylamine-free perovskite solar cells via cadmium doping

Although perovskite solar cells containing methylamine cation can show high power conversion efficiency, stability is a concern. Here, methylamine-free perovskite material Cs_xFA_1–xPbI₃ was synthesized by a one-step method. In addition, we incorporated smaller cadmium ions into mixed perovskite lattice to partially replace Pb ions to address the excessive internal strain in perovskite structure. We have found that the introduction of Cd can improve the crystallinity and the charge carrier lifetime of perovskite films. Consequently, a power conversion efficiency as high as 20.59% was achieved. More importantly, the devices retained 94% of their initial efficiency under 1200 h of continuous illumination.     

Alkali metal cation engineering in organic/inorganic hybrid perovskite solar cells

The past decade has witnessed the rapid advance in organic–inorganic hybrid perovskite solar cells(PSCs).Owing to unique optoelectronic properties of perovskites,the power conversion efficiency(PCE)of PSCs has jumped from 3.8%to25.5%[1–4].However,under the stimulus of illumination,moisture,oxygen and heat,perovskites exhibit unsatisfactory stability due to weak bonding among the components in these soft-lattice crystals[5–7].Doping and passivation engineering with alkali metal cations can enhance the intrinsic stability of perovskite materials.Here,the recent progress of alkali metal cations engineering is reviewed,and the impact on the crystallization,lattice structure,photovoltaic performance and stability is discussed.     

Low-bandgap Sn-Pb perovskite solar cells

Record power conversion efficiency(PCE)for organic-inor-ganic halide perovskite solar cells(PSCs)has been rapidly boos-ted from 3.8％to 25.5％,approaching the Shockley-Queisser(S-Q)limit for single-junction solar cells[1-3].Multi-junction tan-dem solar cells provide a feasible approach to break the effi-ciency limit for single-junction solar cells by maximizing the use of the solar spectrum and photon energy.All-perovskite tandem solar cells have the advantages of tunable bandgaps,solution processability,and flexibility[4-6].As long-wavelength-light absorbers,low-bandgap(Eg:～1.1-1.3 eV)perovskites play a vital role in making efficient all-perovskite tandem sol-ar cells[7,8].Generally,low-Eg perovskites are prepared by par-tially substituting lead(Pb2+)with tin(Sn2+)[9].However,mixed Sn-Pb perovskites usually suffer from short carrier life-times,high defect density,and easy oxidation of Sn2+[10].     

An ammonia modified PEDOT: PSS for interfacial engineering in inverted planar perovskite solar cells

Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) is one of the most widely used hole transport layers (HTL) in inverted perovskite solar cells (PSCs) due to its simple solution-processed ability, high transparency, and conductivity. However, PEDOT:PSS-based devices suffer a lower open-circuit voltage (V_oc) than devices with the conventional structure. To address this issue, we fabricated ammonia-modified PEDOT:PSS films by simply doping PEDOT:PSS solution with different ratio of ammonia. The acidity of PEDOT:PSS can be neutralized by the doped ammonia, which inhibits the ion-exchange reaction between PSS-H and CH₃NH₃I, thus retarding the reduction of the work function for PEDOT:PSS to some extent. As a result, a superior power conversion efficiency (PCE) of 15.5% was obtained for the device based on the ammonia-doped PEDOT:PSS HTL than that of the pristine PEDOT:PSS-based device. We ascribe the PCE enhancement to the increased V_oc and fill factor (FF), which is attributed not only to the better energy-level alignment between the ammonia-modified PEDOT:PSS film and perovskite layer but also to the increased grain size and crystallinity of perovskite film.     

Ultrathin perovskite based solar cells with the efficiency enhanced by charge transfer process

We report a new approach of improving the solar cells efficiency based on ultrathin perovskite films. We propose the addition of CuPc compound to perovskite active layer for enhanced charge generation and transfer process by charge transfer process between CuPc and perovskite. The performance of the devices with and without addition of CuPc was studied in respect to thickness of the active layer. The thickness was varied by the change of the spin coating speed in the range of 4000, 7000 and 10000 rpm, different concentration of CuPc also been studied. The process of charge carrier recombination, crystallinity and Raman characteristics of the obtained films was studied. The perovskite device with an active layer of MAPbI₃ mixed with CuPc spin coated with the speed of 10000 rpm with thickness of about 150 nm demonstrated the efficiency of 12.7%. The ultrathin mixed perovskite film (10000 rpm perovskite film of 15% CuPc) based device presents 33% thickness and 85% efficiency of common pure perovskite device (4000 rpm pure perovskite film).     

Tin dioxide buffer layer-assisted efficiency and stability of wide-bandgap inverted perovskite solar cells

Inverted perovskite solar cells (IPSCs) have attracted tremendous research interest in recent years due to their applications in perovskite/silicon tandem solar cells. However, further performance improvements and long-term stability issues are the main obstacles that deeply hinder the development of devices. Herein, we demonstrate a facile atomic layer deposition (ALD) processed tin dioxide (SnO₂) as an additional buffer layer for efficient and stable wide-bandgap IPSCs. The additional buffer layer increases the shunt resistance and reduces the reverse current saturation density, resulting in the enhancement of efficiency from 19.23% to 21.13%. The target device with a bandgap of 1.63 eV obtains open-circuit voltage of 1.19 V, short circuit current density of 21.86 mA/cm², and fill factor of 81.07%. More importantly, the compact and stable SnO₂ film invests the IPSCs with superhydrophobicity, thus significantly enhancing the moisture resistance. Eventually, the target device can maintain 90% of its initial efficiency after 600 h storage in ambient conditions with relative humidity of 20%–40% without encapsulation. The ALD-processed SnO₂ provides a promising way to boost the efficiency and stability of IPSCs, and a great potential for perovskite-based tandem solar cells in the near future.     

空气环境中湿度对钙钛矿太阳能电池微观形貌与光电效率的影响

有机无机卤化钙钛矿太阳能电池中钙钛矿吸收层对外界的湿度非常敏感,所以目前绝大多数钙钛矿太阳能电池的制备、封装都是在手套箱中完成的。从便于工业规模化角度,在全空气环境下,采用气相辅助溶液法制备了平板结构的钙钛矿太阳能电池,研究了外界湿度与钙钛矿膜形貌、电池效率之间的关系。通过比较研究发现:在全空气环境中,湿度从70%下降为20%时,电池的转换效率从0.95%增加到5.81%。分析认为湿度的降低,增加了钙钛矿膜的覆盖率以及减少膜的缺陷,改善了电池开路电压、短路电流等性能参数。     

面向高效单结GaInP太阳电池的介质复合纳米结构的仿真研究

以实现宽谱减反介质复合纳米结构表面的高 效单结GaInP太阳电池为目标,利用严格耦合波分析理论, 仿真研究了该电池表面的介质复合纳米结构对太阳电池宽谱减反、归一化吸收、最大化理想 效率的影响。该介质复 合纳米结构从上往下依次为SiO₂纳米锥、SiO₂介质层和SiNx介质层,通过对SiO₂纳米锥占空比、深宽比以及对SiO₂和SiNx介质层厚度等参数的系列仿真最终优化出适用于单结Ga InP电池的表面结构。结果表明:当SiO₂纳米锥底部 直径D=550nm、高度H=650 nm、SiN x介质层厚度为60 nm时电池具有最高的 最大化理想转换效率为28.58%。上述结果为后期实验以及该类电池 实现规模化生产奠定了基础。     

Self-assembled monolayers in perovskite solar cells

Perovskite solar cells (PSCs) have attracted great atten-tion due to excellent power conversion efficiency (PCE),low cost and simple solution processing.The certified PCE has reached 25.5％ from the initial efficiency of 3.8％,being com-parable to that of commercial crystalline silicon solar cells[1,2].The efficiency boosting is mainly ascribed to the excellent properties of halide perovskite materials,including suitable bandgaps,high absorption coefficient,long carrier diffusion length and high defect tolerance[3].Moreover,through the composition and interface engineering,the operational sta-bility of PSCs can exceed 1000 h under continuous illumina-tion[4].Therefore,PSCs show a great promise for commerciali-zation.