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 ℃, 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.

Modulating J-V hysteresis of planar perovskite solar cells and mini-modules via work function engineering

Commercialization of perovskite solar cells(PSCs) requires the development of high-efficiency devices with none current density-voltage(J-V) hysteresis. Here, electron transport layers(ETLs) with gradual change in work function(WF) are successfully fabricated and employed as an ideal model to investigate the energy barriers, charge transfer and recombination kinetics at ETL/perovskite interface. The energy barrier for electron injection existing at ETL/perovskite is directly assessed by surface photovoltage microscopy, and the results demonstrate the tunable barriers have significant impact on the J-V hysteresis and performance of PSCs. By work function engineering of ETL, PSCs exhibit PCEs over 21% with negligible hysteresis. These results provide a critical understanding of the origin reason for hysteresis effect in planar PSCs, and clear reveal that the J-V hysteresis can be effectively suppressed by carefully tuning the interface features in PSCs. By extending this strategy to a modified formamidinium-cesium-rubidium(FA-Cs-Rb) perovskite system, the PCEs are further boosted to 24.18%. Moreover, 5 cm × 5 cm perovskite mini-modules are also fabricated with an impressive efficiency of 20.07%, demonstrating compatibility and effectiveness of our strategy on upscaled devices.

Thermally Evaporated ZnSe for Efficient and Stable Regular/Inverted Perovskite Solar Cells by Enhanced Electron Extraction

Electron transport layers(ETLs)are crucial for achieving efficient and stable planar perovskite solar cells(PSCs).Reports on versatile inorganic ETLs using a simple film fabrication method and applicability for both low-cost planar regular and inverted PSCs with excellent efficiencies(>22%)and high stability are very limited.Herein,we employ a novel inorganic ZnSe as ETL for both regular and inverted PSCs to improve the efficiency and stability using a simple thermal evaporation method.The TiO_(2)-ZnSe-FAPbl_(3)heterojunction could be formed,resulting in an improved charge collection and a decreased carrier recombination further proved through theoretical calculations.The optimized regular PSCs based on TiO_(2)/ZnSe have achieved 23.25%efficiency with negligible hysteresis.In addition,the ZnSe ETL can also effectively replace the unstable bathocuproine(BCP)in inverted PSCs.Consequently,the ZnSe-based inverted device realizes a champion efficiency of 22.54%.Moreover,the regular device comprising the TiO_(2)/ZnSe layers retains 92%of its initial PCE after 10:00 h under 1 Sun continuous illumination and the inverted device comprising the C_(60)/ZnSe layers maintains over 85%of its initial PCE at 85℃for 10:00 h.This highlights one of the best results among universal ETLs in both regular and inverted perovskite photovoltaics.

Benzothiadiazole-based hole transport materials for high-efficiency dopant-free perovskite solar cells: Molecular planarity effect

A new benzothiadiazole-based D-A-D hole transport material(DTBT)has been designed and synthesized with a more planar structure by introducing of thiophene bridges.The results indicate a lower band gap and quite higher hole mobility for the DTBT.Furthermore,the enhancement in molecular planarity with simple thiophene unit increases the hole mobility of DTBT(8.77×10^-4cm^2 V^-1s^-1)by about 40%.And when DTBT is used as hole transport material in perovskite solar cells,the photoelectric conversion efficiency of the corresponding dopant-free devices is also significantly improved compared with that of the conventional BT model molecule without thiophene.In terms of device stability,DTBT-based devices show a favorable long-term stability,which keep 83%initial efficiency after 15 days.Therefore,the introducing of thiophene bridges in D-A-D typed HTMs can improve the molecular planarity effectively,thereby increasing the hole mobility and improving device performance.

Recent Progress in High-efficiency Planar-structure Perovskite Solar Cells

Lead halide perovskite owns charge diffusion length in micrometer range,which makes the planar-structure solar cells possible.The simple and lowtemperature process of planar devices makes it very promising.The power conversion efficiency of planar perovskite solar cells has increased from 1.8%to 23.7%in past several years,which can compete with the mesoporous structure counterpart.In this minireview,recent progress in high-efficiency planar perovskite solar cells will be summarized.

High isotropic dispiro structure hole transporting materials for planar perovskite solar cells

Two novel fluorene-based hole transporting materials (HTMs) were synthesized to be used in perovskite solar cells (PSCs). C102 was designed based on C101 by simply linking the two carb on-carbon single bonds to compose a "dispiro" structure. Their typically similar structures cause them sharing almost the same energy levels. However, their photovoltaic performances are quite different due to the small variations. The PSC that contained the "dispiro" structure, C102, reached a power conversion efficiency (PCE) of 17.4%, while the device contained C101, obtained a lower PCE of 15.5%. Electrochemical properties and Photovoltaic characterization of the two materials have been investigated to explain the result. It is shown that C102 has a stronger ability to transport holes and resist the charge recombination. Thus, the dispiro structure should be more appropriate being used as HTM in PSCs.

Improved interfacial property by small molecule ethanediamine for high performance inverted planar perovskite solar cells

We report a simple and effective method to realize desirable interfacial property for inverted planar perovskite solar cells(PSCs)by using small molecule ethanediamine for the construction of a novel polyelectrolyte hole transport material(P3CT-ED HTM).It is found that P3CT-ED can not only improve the hole transport property of P3CT-K but also improve the crystallinity of adjacent perovskite film.In addition,the introduction of ethanediamine into P3CT realigns the conduction and valence bands upwards,passivates surface defects and reduces nonradiative recombination.As a consequence,compared to P3CT-K hole transport layer(HTL)based devices,the average power conversion efficiency(PCE)is boosted from17.2% to 19.6% for the counterparts with P3CT-ED,with simultaneous enhancement in open circuit voltage and fill factor.The resultant device displays a champion PCE of 20.5% with negligible hysteresis.

Fluorine substitution position effects on spiro(fluorene-9,9’-xanthene) cored hole transporting materials for high-performance planar perovskite solar cells

Fluorine substitution in molecular design has become an effective strategy for improving the overall performance of organic photovoltaics.In this study,three low-cost small molecules of spiro-linked hole transporting materials(SFX-O-2 F,SFX-m-2 F,and SFX-p-2 F) endowed with two-armed t rip he ny la mine moieties were synthesized via tuning of the fluorine substitution position,and they were employed for use in highly efficient perovskite solar cells(PSCs).Despite the fluorine substitution position playing a negligible role in the optical and electrochemical properties of the resulting small molecules,the photovoltaic performance thereof was observed to vary significantly.The planar n-i-p PSCs based on SFX-m-2 F demonstrated superior performance(18.86%) when compared to that of the corresponding SFX-o-2 F(9.7%) and SFX-p-2 F(16.33%) under 100 mW cm^(-2) AM1.5 G solar illumination,which is competitive with the performance of the benchmark spiro-OMeTAD-based device(18.98%).Moreover,the SFX-m-2 Fbased PSCs were observed to be more stable than the spiro-OMeTAD-based devices under ambient conditions.The improved performance of SFX-m-2 F is primarily associated with improved morphology,more efficient hole transport,and extraction characteristics at the perovskite/HTM interface.This work demonstrated the application of fluorination engineering to the tuning of material film morphology and charge transfer properties,showing the promising potential of fluorinated SM-HTMs for the construction of low-cost,high-efficiency PSCs.

TiO_2 nanoparticle-based electron transport layer with improved wettability for efficient planar-heterojunction perovskite solar cell

The electron transport layer （ETL） plays an important role in planar heterojunction perovskite solar cell （PSCs）, by affecting the light-harvesting, electron injection and transportation processes, and especially the crystal- lization of perovskite absorber. In this work, we utilized a commercial TKD-TiO2 nanoparticle with a small diameter of 6 nm for the first time to prepare a compact ETL by spin coating. The packing of small-size particles endowed TKD-TiO2 ETL an appropriate surface-wettability, which is beneficial to the crystallization of perovskite deposited via solution-processed method. The uniform and high-transmittance TKD-TiO2 films were successfully incorporated into PSCs as ETLs. Further careful optimization of ETL thickness gave birth to a highest power conversion efficiency of 11.0%, which was much higher than that of PSC using an ETL with the same thickness made by spray pyrolysis. This TKD-TiO2 provided a universal solar material suitable for the further large-scale production of PSCs. The excellent morphology and the convenient preparation method of TKD-TiO2 film gave it an extensive application in photovoltaic devices.

Solution-processed perovskite solar cells

Perovskite solar cells(PSCs) have emerged as one of the most promising candidates for photovoltaic applications. Low-cost, low-temperature solution processes including coating and printing techniques makes PSCs promising for the greatly potential commercialization due to the scalability and compatibility with large-scale, roll-to-roll manufacturing processes. In this review, we focus on the solution deposition of charge transport layers and perovskite absorption layer in both mesoporous and planar structural PSC devices. Furthermore, the most recent design strategies via solution deposition are presented as well, which have been explored to enlarge the active area, enhance the crystallization and passivate the defects, leading to the performance improvement of PSC devices.

I/P interface modification for stable and efficient perovskite solar cells

Interface engineering has played an increasingly essential role in the development of perovskite solar cells(PSCs).Herein,we adopted an effective and simple one-step interface passivation method on a FA-based perovskite to fabricate efficient and stable planar PSCs.The surface defects are reduced by the perovskite interface passivation layer incorporated between the hole transport and perovskite absorber layers,and then non-radiative recombination is suppressed while interfacial carrier extraction is enhanced.The passivated planar PSCs demonstrates 20.83%power conversion efficiency(PCE),which is caused by the simultaneous enhancement of the fill factor and open-circuit voltage.In addition,the device also shows great ambient and thermal stability.It retains 94%of its original PCE after 1000 h under ambient air without encapsulation as well as90%of its initial efficiency after 400 h under continuous heating at 65°C with encapsulation.This research provides a strategy for the development of efficient and stable PSCs.

Synergistic passivation of MAPbI_(3) perovskite solar cells by compositional engineering using acetamidinium bromide additives

For the global commercialization of highly efficient and stable perovskite solar cells(PSCs),it is necessary to effectively suppress the formation of various defects acting as nonradiative recombination sources in perovskite light-harvesting materials.Interfacial defects between the charge-selective layer and the perovskite are easily formed in the solution process used to fabricate perovskite films.In addition,owing to the difference in thermal expansion coefficients between the substrate and the perovskite film,internal residual tensile stress inevitably occurs,resulting in increased nonradiative recombination.Herein,a simple compositional engineering scheme for realizing efficient and stable PSCs,which incorporates acetamidinium bromide(AABr)as an additive into the MAPbI_(3) lattice,is proposed.As an additive,AABr has been found to provide synergistic multiple passivation for both internal and interfacial defects.AABr was found to effectively release the tensile strain of the MAPbI_(3) film by forming a structure stabilized by NH-I hydrogen bonds,as evidenced by calculations based on density functional theory(DFT).Furthermore,the incorporated AABr additives created a charge carrier recombination barrier to enhance charge collection capability by reducing interfacial defects.Accordingly,a power conversion efficiency(PCE)of 20.18%was achieved using a planar device employing AABr-incorporated MAPbI_(3).This was substantially higher than the 18.32% PCE of a pristine MAPbI_(3)-based device.Notably,unencapsulated PSCs using AABr-incorporated MAPbI_(3) absorbers exhibited excellent long-term stability,maintaining>95% of initial PCE up to 1200 hours in ambient air.

Two-step processed efficient perovskite solar cells via improving perovskite/PTAA interface using solvent engineering in PbI2 precursor

The morphology and interface of perovskite film are very important for the performance of perovskite solar cells(PSCs).The quality of perovskite film,fabricated via two-step spin-coating process,is significantly influenced by the morphology and crystallinity of PbI2 film.With the addition of additive dimethyl sulfoxide(DMSO)into the PbI2 precursor,the roughness and trap-state density of perovskite film have been significantly reduced,leading to the excellent contact between perovskite layer and subsequent deposited carrier transport layer.Accordingly,the planar heterojunction PSCs with an architecture of ITO/SnO2/perovskite/PTAA/Ag show an efficiency up to 19.02%.Furthermore,PSCs exhibit promising stability in air with a humidity of ~45%,and retain 80% of initial efficiency after being exposed to air for 400 h without any encapsulation.

In-situ monitored chemical bath deposition of planar NiO x layer for inverted perovskite solar cell with enhanced efficiency

As a convenient,low-cost and up-scalable solution route,chemical bath deposition(CBD)has exhibited impressive advantages in fabricating electron transporting materials like SnO_(2),achieving record efficien-cies for regular n-i-p perovskite solar cells(PSCs).However,for the hysteresis-free and potentially more stable inverted p-i-n PSCs,CBD processing is rarely studied to improve the device performance.In this work,we first present a CBD planar NiO x film as the efficient hole transport layer for the inverted per-ovskite solar cells(IPSCs).The morphologies and semiconducting properties of the NiO x film can be ad-justed by varying the concentration of[Ni(H 2 O)x(NH 3)6-x]2+cation via in-situ monitoring of the CBD re-action process.The characterizations of ultraviolet photoelectron spectroscopy,transient absorption spec-troscopy,time-resolved photoluminescence suggest that the CBD planar NiO x film possesses enhanced conductivity and aligned energy band levels with perovskite,which benefits for the charge transport in the IPSCs.The devices based on planar NiO x at 50°C and low nickel precursor concentration achieved an enhanced efficiency from 16.14%to 18.17%.This work established an efficient CBD route to fabricate planar NiO x film for PSCs and paved the way for high performance PSCs with CBD-prepared hole transporting materials.

Preparation of CH_3NH_3PbI_3 thin films for solar cells via Vapor Transfer Method

Organometal halide perovskite based solar cells have emerged as one of the most promising candidates for low-cost and high-efficiency solar cell technologies. Here a Vapor Transfer Method （VTM） is used to fabricate high quality perovskite thin films in a balanced vacuum capsule. By adjusting the reaction tem- perature, CH_3NHl_3 saturated vapor which then reacts with Pbl_2 films can be controlled and the formation process of CH_3NH_3Pbl_3 perovskite films can be further influenced. Prepared perovskite films which ex- hibit pure phase, smooth surface and high crystallinity are assembled into planar heterojunction inverted solar cells. The whole fabrication process of solar cell devices is organic solution free. Finally, the cham- pion cell achieved power conversion efficiency （PCE） of 13.08% with negligible current-voltage hysteresis under fully open-air conditions. The photovoltaic performance could be further enhanced by optimizing perovskite composition and the device structure.

UV light absorbers executing synergistic effects of passivating defects and improving photostability for efficient perovskite photovoltaics

Metal halide perovskite-based solar cells(PSCs) have rapidly-increased power conversion efficiency(PCE)exceeding 25% but poor stability especially under ultraviolet(UV) light. Meanwhile, non-radiative recombination caused by diverse defects in perovskite absorbers and related interfaces is one of the major factors confining further development of PSCs. In this study, we systematically investigate the role of 2-(2-hydroxy-5-methylphenyl)benzotriazole(UVP) additive in perovskite layers. By adjusting the amount of doped UVP, the quality of perovskite absorbers is significantly improved with enlarged grains, longer lifetime and diffusion length of charge carriers. Furthermore, UVP not only reduces defects for less nonradiative recombination, but also matches energy level alignment for efficient interfacial charge extraction. X-ray photoelectron spectroscopy confirms that N-donor of UVP molecule coordinates with undercoordinated Pb^(2+) on the surface. Interestingly, UVP incorporated in PbI_(2) protects the perovskite by absorbing UV through the opening and closing of the chelating ring. Eventually, the UVP treated PSCs obtain a champion PCE of 22.46% with remarkably enhanced UV stability, retaining over 90% of initial PCE after 60 m W/cm^(2) strong UV irradiation for 9 h while the control maintaining only 74%. These results demonstrate a promising strategy fabricating passivated and UV-resistant perovskite materials simultaneously for efficient and stable perovskite photovoltaics.

Dopant-free hole transporting materials with supramolecular interactions and reverse diffusion for efficient and modular p-i-n perovskite solar cells

The rational design of dopant-free organic hole-transporting layer(HTL) materials is still a challenge for realizing high-efficient and stable p-i-n planar perovskite solar cells(pero-SCs). Here, we synthesized two π-conjugated small-molecule HTL materials through tailoring the backbone and conjugated side chain to carefully control molecular conformation. The resultant BDT-TPAs Th containing a planar fused benzo[1,2-b:4,5-b′]dithiophene(BDT) core and a conjugated thiophene side chain showed the planar conformation. X-ray crystallography showed a favorable stacking model in solid states under the parallel-displaced π-πand additional S-π weak-bond supramolecular interactions, thus achieving an obviously increased hole mobility without dopants.As an HTL material in p-i-n planar pero-SCs, the marginal solubility of BDT-TPA-s Th enabled inverse diffusion into the perovskite precursor solution for assisting the subsequent perovskite film growth and passivating the uncoordinated Pb2+ ion defects. As a result, the planar p-i-n pero-SCs exhibited a champion power conversion efficiency(PCE) of 20.5% and enhanced moisture stability. Importantly, the BDT-TPA-s Th HTL material also showed weak thickness-photovoltaic dependence, and the pero-SCs with blade-coated BDT-TPA-s Th as a HTL achieved a 15.30% PCE for the 1-cm2 modularized device. This HTL material design strategy is expected to pave the way toward high-performance, dopant-free and printing large-area planar p-i-n pero-SCs.

Enhancement of the efficiency and stability of planar p-i-n perovskite solar cells via incorporation of an amine-modified fullerene derivative as a cathode buffer layer

A methanol-soluble diamine-modified fullerene derivative(denoted as PCBDANI)was applied as an efficient cathode buffer layer(CBL)in planar p-i-n perovskite solar cells(pero-SCs)based on the CH_3NH_3PbI_(3-x)Cl_x absorber.The device with PCBDANI single CBL exhibited significantly improved performance with a power conversion efficiency(PCE)of 15.45%,which is approximately17%higher than that of the control device without the CBL.The dramatic improvement in PCE can be attributed to the formation of an interfacial dipole at the PCBM/Al interface originating from the amine functional group and the suppression of interfacial recombinationby the PCBDANI interlayer.To further improve the PCE of pero-SCs,PCBDANI/LiF double CBLs were introduced between PCBM and the top Al electrode.An impressive PCE of 15.71%was achieved,which is somewhat higher than that of the devices with LiF or PCBDANI single CBL.Besides the PCE,the long-term stability of the device with PCBDANI/LiF double CBLs is also superior to that of the device with LiF single CBL.

High-performance planar heterojunction perovskite solar cells： Preserving long charge carrier diffusion lengths and interracial engineering

We demonstrate that charge carrier diffusion lengths of two classes of perovskites, CH3NH3PbI3-xClx and CH3NH3PbI3, are both highly sensitive to film processing conditions and optimal processing procedures are critical to preserving the long carrier diffusion lengths of the perovskite films. This understanding, together with the improved cathode interface using bilayer-structured electron transporting interlayers of [6,6]-phenyl-C61-butyric acid methyl ester （PCBM）/ZnO, leads to the successful fabrication of highly efficient, stable and reproducible planar heterojunction CH3NH3PbI3-xCl2 solar cells with impressive power-conversion efficiencies （PCEs） up to 15.9%. A 1-square-centimeter device yielding a PCE of 12.3% has been realized, demonstrating that this simple planar structure is promising for large-area devices.

Rapid and Complete Conversion of CH3NH3PbI3 for Perovskite/C60 Planar-Heterojunction Solar Cells by Two-Step Deposition

In the present work,we proposed an improved two-step deposition method by optimizing the reaction temperature and the dipping time for the fabrication ofperovskite films.The perovskite film fabricated at 70 ℃ exhibits a full surface coverage and a smooth uniform crystal morphology with a particle size up to micrometer scale.The corresponding inverted perovskite solar cell with a structure of ITO/poly（3,4-ethylenedioxythiophene）：poly（styrene-sulfonate）（PEDOT：PSS）/CH3NH3PbI3/C60/2,9-dimethyl-4,7-diphenyl-l,l 0-phenanthroline （BCP）/Ag displayed a higher power conversion efficiency（PCE）of 13.6%than that of the device fabricated at 20 ℃ （8.06%）,as well as the high reproducibility.The small but meaningful modification for two-step deposition would provide an efficient and convenient way to optimize planar perovskite solar cells and facilitate the potential applications of perovskite solar cells more widely.