Solution-processed Perovskite Solar Cells Research Articles

Boosting the Performance of One-Step Solution-Processed Perovskite Solar Cells Using a Natural Monoterpene Alcohol as a Green Solvent Additive

The perovskite film is the core of a perovskite solar cell (PSC), and its quality is crucial for the performance of such devices. The morphology, crystallinity, and surface coverage of the perovski...

Small grains as recombination hot spots in perovskite solar cells

Summary Non-radiative recombination in the perovskite bulk and at its interfaces prohibits the photovoltaic performance from reaching the Shockley-Queisser limit. While interfacial recombination has been widely discussed and demonstrated, bulk recombination and especially the influence of grain boundaries remain under debate. Most studies explore the role of grain boundaries on perovskite films rather than devices, making it difficult to link the film properties with those of the devices. Here, we systematically investigate the effects of grain boundaries on the performance of perovskite solar cells by two different methods. By combining experimental characterization with theoretical device simulations, we find that the recombination at grain boundaries is diffusion limited and hence is inversely proportional to the grain area to the power of 3/2. Consequently, the prevalence of small grains—which act as recombination hot spots—across the perovskite active layer dictates the photovoltaic performance of the perovskite solar cells.

Improved Quantum Efficiency by Advanced Light Management in Nanotextured Solution-Processed Perovskite Solar Cells

Light management strategies can increase the efficiency of perovskite single-junction and tandem solar cells. In this study, we present perovskite solar cells deposited on different shallow nanotextures by spin-coating. A morphological and optoelectronic analysis demonstrates a high quality of the perovskite absorber, regardless of the substrate. Using both, a nanotexture and a sodium fluoride antireflective coating, enables us to improve the power conversion efficiency by 1.0%abs to 19.7%, when compared to its planar reference. A characterization of the optical performance of nanotextured perovskite solar cells and rigorous optical simulations reveal that the gain in efficiency can be largely attributed to reduced reflection losses and therefore increased absorption in the perovskite. Our nanotextured perovskite solar cells reach 93.6% of the attainable current density, marking the highest reported value for perovskite single-junctions and approaching those of established photovoltaic technologies. Our results demonstrate that nanotextures can be applied to solution-processed perovskite solar cells and pave the way to increased power conversion efficiencies by light management not only in perovskite single-junction but also perovskite–silicon tandem solar cells.

Water-assisted formation of highly conductive silver nanowire electrode for all solution-processed semi-transparent perovskite and organic solar cells

Transparent conductive electrode (TCE) is an essential part of modern optoelectronic devices. Silver nanowire (AgNW) is regarded as the most promising TCEs, owing to its balanced conductivity and transparency, and solution processability. The use of insulating polyvinyl pyrrolidone (PVP) surfactant limits the conductivity of the final AgNW networks. Herein, by introducing a small amount of deionized water into the AgNWs dispersion in isopropanol (IPA), the conductivity of the spray-coated AgNW electrode was significantly improved. Sheet resistance (Rs) of 27.0 Ω □−1 with transparency of 92% (at 550 nm) was obtained for the AgNW films spray coated from the AgNW ink with 20% water, which is much lower than the IPA-only AgNW film (120.9 Ω □−1 with similar transparency). Morphology analysis confirmed that water is able to wash PVP away from the AgNW surface and promote the formation of AgNW bundles, which increase the conductivity. The optimized AgNW ink was then used for perovskite and polymer solar cells. High power conversion efficiencies of 14.04% for perovskite solar cell and 6.44% for organic solar cells with averaged light transmittance of 21.7% and 33.12% are achieved, respectively, which are among the highest values for all solution-processed semi-transparent perovskite and polymer solar cells.

Controlling the Morphology and Interface of the Perovskite Layer for Scalable High-Efficiency Solar Cells Fabricated Using Green Solvents and Blade Coating in an Ambient Environment.

Low-cost and solution-processed perovskite solar cells have shown great potential for scaling-up mass production. In comparison with the spin coating process for fabricating devices with small areas, the blade coating process is a facile technique for preparing uniform films with large areas. High-efficiency perovskite solar cells have been reported using blade coating, but they were fabricated using the toxic solvent N,N-dimethylformide (DMF) in nitrogen. In this work, we present highly efficient blade-coated perovskite solar cells prepared using a green solvent mixture of γ-butyrolactone (GBL) and dimethyl sulfoxide (DMSO) in an ambient environment. By carefully controlling the interface, morphology, and crystallinity of perovskite films through composition variations and additives, a high power conversion efficiency of 17.02% is achieved in air with 42.4% reduction of standard deviation in performance. The findings in this work resolve the issues of scalability and solvent toxicity; thus, the mass production of perovskite solar cells becomes feasible.

Fully solution-processed perovskite solar cells fabricated by lamination process with silver nanoparticle film as top electrode

Perovskite solar cell is one of the most promising candidates for future photovoltaic market because of its high power conversion efficiency (>25%). However, most metal top electrodes are typically fabricated by a vacuum deposition method, which makes the fabrication expensive and unsuitable for commercial applications. In this paper, we present devices in which every layer was prepared using a solution process, with a laminated silver nanoparticle film serving as the top electrode. The silver nanoparticle film was produced by spin-coating the nanoparticle silver ink onto a poly(ethylene terephthalate) (PET) substrate followed by post-annealing at 150 °C for 5 min. Introduction of a thin layer of Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/D-sorbitol, plays an important role in improving the adherence of devices and electrical contact during lamination. Thereby, laminated perovskite solar cells with average power conversion efficiency (PCE) of 10.03% were achieved, almost of 90% of the PCE obtained for conventional devices (11.19%) with evaporated silver contact. The electrical and morphological properties of thermally annealed silver nanoparticle film were also investigated.

Thermal conductivity and diffusivity of triple-cation perovskite halide materials for solar cells

We report on the measurement of thermal conductivity and thermal diffusivity by a modulated thermoreflectance microscopy technique on a mixed-cation perovskite material [Cs0.05(formamidinium0.83methylammonium0.17)0.95Pb(I0.83Br0.17)3] widely applied for solution-processed perovskite solar cells. Such materials are supposed to present improved thermal stability compared to methylammonium-based single cation perovskites. Our measurements are performed on perovskite/TiO2/SnO2:F/SiO2 structures, with perovskite thicknesses ranging between 250 nm and 1000 nm. This configuration is the one of a real solar cell, with the same substrate and intermediate layers as of an operating device. We measured a thermal conductivity kper of 0.26 ± 0.03 W m−1 K−1 and a thermal diffusivity Dper of 3.5 × 10−7 ± 0.5 m2 s−1. The value for thermal conductivity is comparable to the one measured on single cation perovskites, which is generally in the 0.2–0.6 range. The value for thermal diffusivity has not been reported previously.

Correlating Hysteresis and Stability with Organic Cation Composition in the Two-Step Solution-Processed Perovskite Solar Cells.

The two-step solution-based process has demonstrated substantial success in fabricating high-efficiency perovskite solar cells in recent years. Despite the high performance, the underlying mechanisms that govern the formation of perovskite films and corresponding device performance are yet to be fully understood. Particularly, organic cation composition used in the two-step solution processing of mixed-cation lead halide perovskite solar cells plays a critical role in the perovskite film formation and the resultant device performance. However, little is understood about the impacts of organic cation composition on the current density-voltage (J-V) hysteretic behavior and stability of perovskite solar cells. To address this need, here, we study the effect of mixed organic cations, that is, the fraction of formamidinium (FA) and methylammonium (MA) contents, used for the two-step solution-processed perovskite thin films on solar cell performance, including efficiency, J-V hysteresis, and stability. In addition to the efficiency variations, we find that perovskite solar cells based on FA-rich and MA-rich stoichiometries show distinct characteristics in J-V hysteresis and stability. The origins of such a discrepancy are attributed to the thermodynamically driven conversion from lead iodide to perovskites, which is determined by the combination of organic cations. The perovskite solar cells based on the mixed cation FA0.6MA0.4PbI3 composition show a champion power conversion efficiency of over 21% and robust stability (retaining more than 90% of initial efficiency) under maximum power-point tracking in dry nitrogen for more than 500 h. Our work provides insights on understanding the formation of perovskite films in the two-step process, which may benefit further investigation on perovskite solar cells.

A new family of liquid and solid guanidine-based n-type dopants for solution-processed perovskite solar cells

A new family of n-type dopants for organic semiconductors is reported. Changing the alkyl chain of these triazabicyclodecene (TBD) dopants alters their physical properties, including thermal stability, and influences their doping efficiency.

A solution processed Ag-nanowires/C60 composite top electrode for efficient and translucent perovskite solar cells

All solution-processed semi-transparent perovskite solar cells (PSCs) with high efficiency were fabricated with Ag nanowire (AgNW) top electrode directly coated on the organic electron transport layer (ETL). A unique AgNW/ETL interface morphology was attained by the fabrication of the AgNW-C60 composite electrode through a sequential spin-coating method. The C60 filler spin-coated on the AgNW network engendered a high contact area and favourable energy level alignment at the interface between the ETL and AgNW. As a result, devices based on AgNW-C60 electrode far outperformed devices with pristine AgNW electrodes. By adjusting the thickness of the AgNW film, the AgNW-C60 devices attained a PCE of 11.02%. The incorporation of C60 into the AgNW electrode also led to remarkable stability improvement. The low-temperature solution-processed and semi-transparent AgNW-C60 top electrode is also applicable to other PSC structures with various interfacial layers, thus showing a promising path towards fully-printable, flexible and translucent photovoltaic devices.

High performance perovskites solar cells by hybrid perovskites co-crystallized with poly(ethylene oxide)

Hybrid perovskite materials have emerged as attractive alternatives for cost-effective solar cells in the past ten years. However, achieving hysteresis-free, stable and efficient solution-processed perovskites solar cells has remained as a significant fundamental challenge. In this study, we report a strategy that utilizes poly (ethylene oxide) to sequester the counter ions in the perovskite lattices to suppress the formation of point defects, reduce the migration of ions/vacancy and to facilitate crystal growth in a more thermodynamically preferred orientation. Systematical investigations indicate that poly (ethylene oxide) indeed form hydrogen bonds with perovskite, which reduces the formation of kinetically-driven point defects, minimize charge carrier recombination and sharpen the density of states distribution. As a result, un-encapsulated solution-processed perovskite solar cells exhibit stabilized power conversion efficiency with hysteresis-free characteristics and significantly improved ambient shelf- and thermal-stability at relative high humidity, in comparison to the reference devices that exhibit unstable power conversion efficiency with dramatically higher hysteresis factor and poorer device lifetime. Our studies demonstrate that development of hybrid perovskite materials co-crystallized with polymers is an efficient approach towards high performance perovskite solar cells.

Amide Additives Induced a Fermi Level Shift To Improve the Performance of Hole-Conductor-Free, Printable Mesoscopic Perovskite Solar Cells.

Solution-processable organic-inorganic perovskite solar cells have attracted much attention in the past few years. Energy level alignment is of great importance for improving the performance of perovskite solar cells because it strongly influences charge separation and recombination. In this report, we introduce three amide additives, namely, formamide, acetamide, and urea, into the MAPbI3 perovskite by mixing them directly in perovskite precursor solutions. The Fermi level of MAPbI3 shifts from -4.36 eV to -4.63, -4.65, and -4.61 eV, respectively, upon addition of these additives. The charge transfer between perovskite and mp-TiO2 is found to be promoted as determined via TRPL spectra, and recombination in the perovskite is suppressed. As a result, the built-in electric field (Vbi) of the printable, hole-conductor-free mesoscopic perovskite solar cells based on these perovskites with amide additives is enhanced and a peak power conversion efficiency of 15.57% is obtained.

A dopant-free twisted organic small-molecule hole transport material for inverted planar perovskite solar cells with enhanced efficiency and operational stability

Low-cost solution-processable inverted perovskite solar cells (PSCs) demonstrate great potential toward future photovoltaic market. Unfortunately, general hole transport materials (HTMs) within inverted structure make the performance and stability far uncompetitive compared to the normal structure. Interrogating the fundamentals of these materials, moderate charge carrier mobility and susceptible environmental stability of the undoped molecules are the main causes. Herein, a twisted molecule XY1 is developed as a potential robust dopant-free HTM for inverted PSCs. Compared with traditional poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), XY1 exhibits much higher hole transport ability and peculiar ultraviolet absorption. Hysteresis-free inverted PSCs based on XY1 exhibits the power conversion efficiency (PCE) of 18.78%, which is among the highest values for inverted PSCs based on dopant-free HTMs. After light soaking for 200 h, the original PCE of XY1-based device is still maintained at the 95% level, indicating the substantially improved operational stability. Besides, large-area (1.00 cm2) inverted PSC based on XY1 shows a competitive PCE of 17.82%.

Multiple Roles of Cobalt Pyrazol-Pyridine Complexes in High-Performing Perovskite Solar Cells.

Chemical doping is a ubiquitously applied strategy to improve the charge-transfer and conductivity characteristics of spiro-OMeTAD, a hole-transporting material (HTM) used widely in solution-processed perovskite solar cells (PSCs). Cobalt(III) complexes are commonly employed HTM dopants, whose major role is to oxidize spiro-OMeTAD to providep-doping for improved conductivity. The present work discloses additional, previously unknown important functions of cobalt complexes in the HTM films that influence the photovoltaic performance. Specifically, it is demonstrated that commercialp-dopant FK269 (bis(2,6-di(1H-pyrazol-1-yl)pyridine) cobalt(III) tris(bis(trifluoromethylsulfonyl)imide)) reduces the interfacial recombination and alleviates the decomposition of the perovskite layerunder the action oftert-butylpyridine and lithium bis(trifluoromethanesulfonyl)imide. These effects are demonstrated for 1 cm2 perovskite solar cells that achieve a stabilized power conversion efficiency of 19% under 1 sun irradiation.

Identifying inverted-hysteresis behavior of CH3NH3PbI3−xClx planar hybrid perovskite solar cells based on external bias precondition

Solution-processable hybrid perovskite solar cells offer potential in the photovoltaic field due to their low-cost fabrication and high efficiency. However, an undesirable current–voltage (J-V) hysteresis hampers the applications of perovskite solar cells. In particular, for the inverted device, the understandings for J-V hysteresis origination are not uniform, and the inverted hysteresis phenomenon has been further complicated the hysteresis behavior. In this report, an external bias precondition method is adopted to unveil the origin of the inverted hysteresis. The results indicate that the extents of inverted hysteresis are very much dependent on the bias direction of the precondition. To further unveil the effect of the precondition on inverted hysteresis, the microscopic J-V hysteresis was also observed by using conductive atomic force microscopy (c-AFM) measurements. The results indicate that ion migration and accumulation slowly built up at the grain boundaries of the perovskite film when repeating the scan using c-AFM. Furthermore, the transient characteristics based on capacity-frequency plots and open-circuit voltage decay further identify the presence of difference for the different bias preconditions on the device, because the different bias precondition could induce different directions of ions migration and accumulation. These observations are surprising; it can be further identified that the inverted hysteresis originates from the ionic migration and accumulation, and the grain boundary is as the channel of ionic migration and accumulation. Therefore, the grain boundary plays an important role on the hysteresis effect, and preparation of large-grain or single-crystal perovskite films is the way to reduce the hysteresis effect.

Enhancing photovoltaic performance of perovskite solar cells utilizing germanium nanoparticles

Morphological and crystalline control over hybrid organic-inorganic perovskite films is pivotal for efficient photovoltaic (PV) performance devices. Yet, this remains very challenging for solution processed perovskite solar cells (PVSCs), especially mesoscopic PVSCs, due to the complicated crystallization kinetics of hybrid semiconductor materials within dynamic spin-coating and post annealing. In this work, colloidal Ge nanoparticles (NPs) were added onto a mesoporous TiO2 (m-TiO2) electron transporting layer (ETL) to regulate perovskite crystal growth. Systematic investigation and optimization disclose that incorporation of an appropriate ratio of Ge NPs onto the m-TiO2 ETL can simultaneously increase the size of the CH3NH3PbI3 crystals, decrease the number of the grain boundaries and promote the interfacial properties of perovskite/m-TiO2. The related mechanisms are clarified through detailed morphology and crystal structure analyses. The electron mobility of the perovskite absorber, determined using the space charge limited current (SCLC) method, was increased by over 5 times when an optimized amount of Ge NPs were employed. Average power conversion efficiency (PCE) of 18.59% was achieved from 16 cells and the best PCE of 19.6% was attained via the addition of the optimized amount of Ge NPs. We study the fundamentals of optics and physics behind the PVSC device based on the high refractive index Ge NPs. This work offers an innovative scenario to enhance the performance of perovskite based optoelectronics by employing optically stable, chemically inert, low-cost and green semiconductor NPs.

(Invited) Designing Novel Organic Small Molecular As Electron Transport Layers for Planar Perovskite Solar Cells

Compared to the traditional-architecture perovskite photovoltaics (n-i-p type), which use metal oxide as electron transport layers(ETLs) and organic semiconducting materials as hole transport layers, the fabrication of metal-oxide-free, solution-processed inverted perovskite solar cells (PSCs) is more desired because of low-temperatures and all-solution-based applications in future commercial PSC modules. In a typical configuration of inverted PSCs, the widely used ETL compound is the fullerene-based phenyl-C61-butyric acid methyl ester (PCBM), which currently is the best organic ETL material. The cost of this compound is very high, and the morphology and electrical properties are very sensitive to experimental conditions. In this talk, I will talk the recent progress of new organic ETL materials in my group for the replacement of PCBM in inverted PSCs. We believe that easily-accessible simple n-type small molecules are promising ETL candidates to further propel inverted PSCs to practical applications. References Ning Wang, Kexiang Zhao, Tao Ding, Wenbo Liu, Ali Said Ahmed, Zongrui Wang, Miaomiao Tian, Xiao Wei Sun,* Qichun Zhang*, “Improving Interfacial Charge Recombination in Planar Heterojunction Perovskite Photovoltaics with Small Molecule as Electron Transport Layer”, Adv. Energy Mater. 2017, DOI: 10.1002/aenm.201700522.Pei-Yang Gu, Ning Wang, Chengyuan Wang, Yecheng Zhou, Guankui Long, Miaomiao Tian, Wangqiao Chen, Xiao Wei Sun,* Mercouri G. Kanatzidis*, Qichun Zhang,* “Pushing up the efficiency of planar perovskite solar cells to 18.2% with organic small molecular as electron transport layer”, J. Mater. Chem. A. 2017, 5, 7339 – 7344.Pei-Yang Gu, Ning Wang, Anyang Wu, Zilong Wang, Miaomiao Tian, Zhisheng Fu,* Xiao Wei Sun,* Qichun Zhang* “Azaacene Derivative as Promising Electron Transport Layer for Inverted Perovskite Solar Cells”, Chem. Asian J. 2016, 11(15), 2135–2138.

Open atmospheric processed perovskite solar cells using dopant-free, highly hydrophobic hole-transporting materials: Influence of thiophene and selenophene π-spacers on charge transport and recombination properties

We have synthesized three donor-π-acceptor polymers by crosslinking 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b'] and pyrrolo[3,4-c]pyrrole-1,3-dione with thiophene, thieno[3,2-b]thiophene, selenophene as π-spacer. These polymers have been integrated successfully as dopant-free hole-transporting materials in fully open atmosphere solution-processed lead halide perovskite solar cells. Due to the synergistic impact of linear and long alkyl chains with different π-spacers, these hole transporters retard interfacial charge recombination and afford rich solubility and morphology resulting long carrier lifetime with uniform film coverage. In addition, these hole transporters possess a deeper highest occupied molecular orbitals (−5.40, −5.37 and −5.42 eV) and suitable over potential with perovskite valence band (−5.43 eV), favorable for efficient hole extraction. Further, the large number of S-based heterocycles helps to strengthen the interaction of the perovskite and hole transporting layer interface and high hydrophobicity (∼104°) facilitate enhanced stability with insignificant hysteresis. As a result, the power conversion efficiency has reached 14% under AM1.5 G without any additives in fully open atmospheric conditions. Our findings highlight the better energy transfer capabilities of additive-free hole transporters as a promising candidate for developing stable PSCs in the open air.

Amine-Based Interfacial Engineering in Solution-Processed Organic and Perovskite Solar Cells.

Solution-processed organic solar cells (OSCs) and hybrid perovskite solar cells (PvSCs) generally require appropriate transparent electrode with a low work function, which improves the electron extraction, increases the built-in potential, and suppresses charge recombinations. Hence, interfacial modifiers between the cathode and the photoactive layer play a significant role in OSCs and PvSCs, as they provide suitable energy-level alignment, leading to desirable charge carrier selectivity and suppressing charge carrier recombinations at the interfaces. Here, we present a comprehensive study of the energy-level mapping between a transparent electrode and photoactive layers to enhance the electron-transport ability by introducing amine-based interfacial modifiers (ABIMs). Among the ABIMs, polyethylenimine ethoxylated (PEIE) incorporating inverted OSCs shows enhanced power conversion efficiencies (PCEs) from 0.32 to 9.83% due to large interfacial dipole moments, leading to a well-aligned energy level between the cathode and the photoactive layer. Furthermore, we explore the versatility of the PEIE ABIM by employing different photoactive layers with fullerene derivatives, a nonfullerene acceptor, and a perovskite layer. Promisingly, inverted nonfullerene OSCs and planar n-i-p PvSCs with PEIE ABIM show outstanding PCEs of 11.88 and 17.15%, respectively.

4-Tert butylpyridine induced MAPbI3 film quality enhancement for improving the photovoltaic performance of perovskite solar cells with two-step deposition route

As we known, high-efficiency of the solution-processed perovskite solar cells can be achieved by modulating the perovskite film quality. Here, the prevalent 4-tert-butylpyridine is firstly utilized as an additive of CH3NH3I solution for improving the CH3NH3PbI3 film quality fabricated with two-step deposition method. Due to the strong polarity and interaction between N atom on TBP and Pb atom on CH3NH3PbI3, this strategy leads to a substantial enhancement of the CH3NH3PbI3 film quality with larger grains, higher crystallinity and more complete coverage on the substrate than that of the pristine one. Thereby, the TBP-based CH3NH3PbI3 film simultaneously exhibits superior optical properties and surface hydrophobicity. By optimization, the perovskite solar cells fabricated with TBP addition at a volume ratio of 7/1000 (v/v, TBP/isopropanol) exhibits significant performance enhancement with a power conversion efficiency reaching 17.06%, while the pristine one merely obtains a lower value of 14.56%. More importantly, this type of cells also shows a desirable efficiency reproducibility and long-time stability in air atmosphere. In conclusion, our work demonstrates another meaningful application for the TBP additive and provides a facile and effective strategy for further improving the performance of perovskite solar cells.