Charge Extraction Layers Research Articles

Inverted Perovskite Solar Cells with Efficient Mixed‐Fullerene Derivative Charge Extraction Layers

AbstractThe short‐circuit current density of inverted perovskite solar cells is unsatisfactory due to the intrinsic relatively low electron mobility and electrical conductivity of PC61BM. To solve this problem, a solution‐processed mixed‐fullerene derivative electron transporting material by mixing PC61BM with C60 was developed to improve charge extraction capacity in fully low temperature processed devices. Incorporation of C60 into the PC61BM layer improves the mobility from 2.15×10−3 to 4.71×10−3 cm2⋅V−1⋅s−1 and the conductivity of electron transporting layers, thus enhancing the interfacial electron extraction capacity and reducing the electron transfer resistance. As a result, the average power conversion efficiency increases from 12.80% to 15.47% by 21% due to the significant increases in the average short‐circuit current density from 19.26 to 22.08 mA⋅cm−2 by 15%. Meanwhile, the optimal efficiency of 16.44% with rigid substrates and 12.93% with flexible substrates are obtained with almost no hysteresis. This work provides a novel and simple method to greatly improve the short‐circuit current density and hence the photovoltaic performance of inverted perovskite solar cells.

Realization of Intrinsically Stretchable Organic Solar Cells Enabled by Charge-Extraction Layer and Photoactive Material Engineering.

The rapid development of wearable electronic devices has prompted a strong demand to develop stretchable organic solar cells (OSCs) to serve as the advanced powering systems. However, to realize an intrinsically stretchable OSC is challenging because it requires all the constituent layers to possess certain elastic properties. It thus necessitates a combined engineering of charge-transporting layers and photoactive materials. Herein, we first describe a stretchable electron-extraction layer using a blend of poly[(9,9-bis(3'-( N, N-dimethylamino)propyl)-2,7-fluorene)- alt-2,7-(9,9-dioctylfluorene)] (PFN) and nitrile butadiene rubber (NBR, Nipol 1072). This hybrid PFN/NBR layer exhibits a much lower Derjaguin-Muller-Toporov modulus (0.45 GPa) than the value (1.25 GPa) of the pristine PFN and could withstand a high strain (60% strain) without showing any cracks. Moreover, besides enriching the stretchability of PFN, the terminal carboxyl groups of NBR can ionize PFN to promote its solution-processability in polar solvents and to ensure the interfacial dipole formation at the corresponding interface in the device, as evidenced by the Fourier transform infrared and ultraviolet photoelectron spectroscopy analyses. By further coupling the replacement of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) with nonfullerene acceptors owing to better mechanical stretchability in the photoactive layer, OSCs with improved intrinsically stretchability and performance were demonstrated. An all-polymer OSC can exhibit a power conversion efficiency of 2.82% after 10% stretching, surpassing the PCBM-based device that can only withstand 5% strain.

Direct Contact of Selective Charge Extraction Layers Enables High-Efficiency Molecular Photovoltaics

Summary Dye-sensitized solar cells (DSCs) are molecular photovoltaics that operate efficiently in direct solar and ambient light by employing dye-impregnated mesoscopic TiO 2 films with a redox electrolyte or hole conductor. Here, we report on an advanced DSC architecture, which achieves efficiencies of 13.1% under air mass 1.5 global, 100 mW cm −2 solar radiation, and power conversion efficiency of 32% under a standard Osram 930 Warm White fluorescent tube light at 1,000 lux intensity. The cell substantially benefits from the direct contact of the dye-impregnated TiO 2 film with the poly(3,4-ethylenedioxythiophene) (PEDOT) counter electrode acting as a hole collector. This reduces the diffusion path of redox mediator to merely the mesoporous TiO 2 film attenuating the Warburg resistance, which thereby boosts the photovoltaic performance. This architecture will not only accelerate the practical exploitation of DSCs, but also foster new types of light-harvesting devices using mesoscopic TiO 2 and PEDOT as electron and hole collection layers, respectively.

Highly Efficient and Stable Flexible Perovskite Solar Cells with Metal Oxides Nanoparticle Charge Extraction Layers.

In this study, the fabrication of highly efficient and durable flexible inverted perovskite solar cells (PSCs) is reported. Presynthesized, solution-derived NiOx and ZnO nanoparticles films are employed at room temperature as a hole transport layer (HTL) and electron transport layer (ETL), respectively. The triple cation perovskite films are produced in a single step and for the sake of comparison, ultrasmooth and pinhole-free absorbing layers are also fabricated using MAPbI3 perovskite. The triple cation perovskite cells exhibit champion power conversion efficiencies (PCEs) of 18.6% with high stabilized power conversion efficiency of 17.7% on rigid glass/indium tin oxide (ITO) substrates (comparing with 16.6% PCE with 16.1% stabilized output efficiency for the flexible polyethylene naphthalate (PEN)/thin film barrier/ITO substrates). More interestingly, the durability of flexible PSC under simulation of operative condition is proved. Over 85% of the maximum stabilized output efficiency is retained after 1000 h aging employing a thin MAPbI3 perovskite (over 90% after 500 h with a thick triple cation perovskite). This result is comparable to a similar state of the art rigid PSC and represents a breakthrough in the stability of flexible PSC using ETLs and HTLs compatible with roll to roll production speed, thanks to their room temperature processing.

Regulating the vertical phase distribution by fullerene-derivative in high performance ternary organic solar cells

The vertical phase distribution of components in bulk heterojunction is diversified in organic solar cells (OSCs). The electron donors (acceptors) can be accumulated (depleted) at the interface of active layer and charge extraction layer. The variation of vertical phase distribution significantly influences device performance because of its impact on the charge transport and charge recombination. In order to achieve favorable vertical phase distribution in OSCs based on poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene))-co-(1,3-di(5-thiophene-2-yl)–5,7-bis(2-ethylhexyl) benzo[1,2-c:4,5-c′]dithiophene-4,8-dione)] (PBDB-T):3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))−5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (ITIC), phenyl-C71-butyric-acid-methyl ester (PC71BM) was incorporated into the binary system to fabricate ternary OSCs. In the ternary blend, PC71BM can effectively regulate the phase distribution of PBDB-T and ITIC in vertical direction, which provides favorable vertical phase distribution for charge transport. Moreover, the addition of PC71BM can also effectively increase the π-π stacking coherence length of both donor and acceptor, which facilitates charge transport and reduces the bimolecular recombination. The addition of an appropriate quantity of PC71BM can obviously improve both fill factor and short-circuit current density of the OSC based on PBDB-T:ITIC while open-circuit voltage reduces only about 0.01 V, which indicates a rational low energy loss. Consequently, the ternary OSC exhibits a best PCE of 11.0% compared to the 9.6% PCE of the binary counterpart.

Crossing Up Charge Extraction Layers

Photovoltaics based on low-temperature processed thin films are a promising technology for inexpensive renewable energy. The efficiency of thin-film solar cells is greatly influenced by the design of the materials that extract charge from the light-absorbing layer. These charge extraction layers are additionally a critical determinant of device stability. In this issue of Joule, Zhu and colleagues have developed crosslinked molecular charge extraction layers that enhance the environmental, thermal, and photo-stability of thin-film perovskite photovoltaics.

Influence of doped charge transport layers on efficient perovskite solar cells

A generic solar cell structure using undoped charge extraction layer is presented, that lead to efficient and rather stable solar cells.

Functionalized Nickel Oxide Hole Contact Layers: Work Function versus Conductivity.

Nickel oxide (NiO) is a widely used material for efficient hole extraction in optoelectronic devices. However, its surface characteristics strongly depend on the processing history and exposure to adsorbates. To achieve controllability of the electronic and chemical properties of solution-processed nickel oxide (sNiO), we functionalize its surface with a self-assembled monolayer (SAM) of 4-cyanophenylphosphonic acid. A detailed analysis of infrared and photoelectron spectroscopy shows the chemisorption of the molecules with a nominal layer thickness of around one monolayer and gives an insight into the chemical composition of the SAM. Density functional theory calculations reveal the possible binding configurations. By the application of the SAM, we increase the sNiO work function by up to 0.8 eV. When incorporated in organic solar cells, the increase in work function and improved energy level alignment to the donor does not lead to a higher fill factor of these cells. Instead, we observe the formation of a transport barrier, which can be reduced by increasing the conductivity of the sNiO through doping with copper oxide. We conclude that the widespread assumption of maximizing the fill factor by only matching the work function of the oxide charge extraction layer with the energy levels in the active material is a too narrow approach. Successful implementation of interface modifiers is only possible with a sufficiently high charge carrier concentration in the oxide interlayer to support efficient charge transfer across the interface.

Charge Transport Limitations in Perovskite Solar Cells: The Effect of Charge Extraction Layers.

Understanding the charge transport characteristics and their limiting factors in organolead halide perovskites is of great importance for the development of competitive and economically advantageous photovoltaic systems derived from these materials. In the present work, we examine the charge carrier mobilities in CH3NH3PbI3 (MAPI) thin films obtained from a one-step synthesis procedure and in planar n-i-p devices based on these films. By performing time-of-flight measurements, we find mobilities around 6 cm2/V s for electrons and holes in MAPI thin films, whereas in working solar cells, the respective effective mobility values are reduced by 3 orders of magnitude. From complementary experiments on devices with varying thicknesses of electron and hole transport layers, we identify the charge extraction layers and the associated interfaces rather than the perovskite material itself as the major limiting factors of the charge carrier transport time in working devices.

High photovoltage in perovskite solar cells: New physical insights from the ultrafast transient absorption spectroscopy

To understand the cause of the high open circuit photovoltage (VOC) achieved by todays’ state of the art perovskite solar cells (PSCs), we examine formamidinium lead bromide CH(NH2)2PbBr3 films by ultrafast transient absorption spectroscopy (TAS). By using TiO2 and spiro-OMeTAD as charge extraction layers, the devices based on the CH(NH2)2PbBr3 films yield VOC as high as 1.5V ascertaining their high quality. TAS establish that the presence of charge extraction layers has very little influences on the nature of a negative band at 535nm corresponding to the bleaching of the absorption band edge and two positive bands in the CH(NH2)2PbBr3 films. Therefore, we contend that the VOC in PSC is predominantly determined by the quasi Fermi level splitting within the perovskite layer.

N-Doped Zwitterionic Fullerenes as Interlayers in Organic and Perovskite Photovoltaic Devices

The efficient operation of polymer- and perovskite-based photovoltaic devices depends on selective charge extraction layers that are placed between the active layer and electrodes. Herein, we demonstrate that integration of a tetra-n-butyl ammonium iodide-doped zwitterionic fulleropyrrolidine derivative, C60-SB, as a cathode modification interlayer significantly improves the photovoltaic device performance. Compared to the intrinsic (undoped) zwitterionic material, which is an efficient interlayer itself, the doped interlayers further improve average power conversion efficiencies from 8.37% to 9.68% in polymer-based devices and from 12.53% to 15.31% in perovskite-based devices. Ultraviolet photoelectron spectroscopy revealed that doping increases the interfacial dipole at the C60-SB/Ag interface, i.e., reduces the effective work function of the resultant composite cathode. This effect originates from the population of negative polaron states in C60-SB by extrinsic charges that prevent directional charge t...

Monitoring Thermal Annealing of Perovskite Solar Cells with In Situ Photoluminescence

Layer deposition of organometal halide perovskites for solar cells usually involves tedious experimentation to establish the optimum processing conditions. Important parameters are the time and temperature of thermal annealing. Here, it is demonstrated that in situ photoluminescence allows to determine the optimal annealing procedure without fabricating complete solar cells. A deposition method is used in which dense layers of perovskite crystals are formed within seconds in ambient air by hot casting a mixture of lead acetate, lead chloride, and methylammonium iodide. The as‐cast perovskite layers are highly luminescent because charge carriers are unable to reach the charge extraction layers that quench the photoluminescence. Thermal annealing enhances charge transport and quenches the photoluminescence, but deteriorates the photovoltaic performance via decomposition of the perovskite if applied for a too long time. It is demonstrated that the optimal annealing time coincides with the time required for the in situ measured photoluminescence intensity to reach its baseline value for annealing temperatures in the range of 80–100 °C. This results in efficient (>14%) perovskite solar cells and shows that in situ photoluminescence is a simple but powerful tool for in‐line quality monitoring of perovskite films.

Ionic liquid induced surface trap-state passivation for efficient perovskite hybrid solar cells

The highly crystalline methylammonium lead halides (MALHs) perovskite has large carrier diffusion length and thus minimal charge loss within the MALHs layers. But the charge extraction from the perovskite to the charge extraction layers can be significantly influenced by the interfacial contact. Since the fast-crystalized MALHs usually have a rough surface with numerous trap-states and crystal grain boundaries near the top surface, which will deteriorate the electrical coherence and physical contact with the top electron extraction layer (EEL) in the conventional device structure. In this study, we introduced an ionic liquid methyltrioctylammonium trifluoromethanesulfonate (MATS) to passivate the traps and boundaries at the interface of the i-n junction. The photoluminance results demonstrated an improved electron extraction upon the MATS surface treatment. And the impedance measurements also showed a reduced charge transfer resistance within the MATS treated device. Consequently, the conventional planar heterojunction solar cells based on the MALHs perovskite treated by MATS, showed an enhanced device efficiency of 16.10%. Moreover, after heating at elevated temperatures under 2 V forward bias and cooling down to room temperature, we found the MATS modified solar cell devices exhibit a further efficiency improvement to 17.51%.

Anticorrelation between Local Photoluminescence and Photocurrent Suggests Variability in Contact to Active Layer in Perovskite Solar Cells.

We use high-resolution, spatially resolved, laser beam induced current, confocal photoluminescence, and photoconductive atomic force microscopy (pcAFM) measurements to correlate local solar cell performance with spatially heterogeneous local material properties in methylammonium lead triiodide (CH3NH3PbI3) perovskite solar cells. We find that, for this material and device architecture, the photocurrent heterogeneity measured via pcAFM on devices missing a top selective contact with traditional Au-coated tips is significantly larger than the photocurrent heterogeneity observed in full devices with both electron- and hole-selective extraction layers, indicating that extraction barriers at the Au/perovskite interface are ameliorated by deposition of the organic charge extraction layer. Nevertheless, in completed, efficient device structures (PCE ≈ 16%) with state-of-the-art nickel oxide and [6,6]-phenyl-C61-butyric acid (PCBM) methyl ester contacts, we observe that the local photoluminescence (PL) is weakly anticorrelated with local photocurrent at both short-circuit and open-circuit conditions. We determine that the contact materials are fairly homogeneous; thus the heterogeneity stems from the perovskite itself. We suggest a cause for the anticorrelation as being related to local carrier extraction heterogeneity. However, we find that the contacts are still the dominating source of losses in these devices, which minimizes the impact of the material heterogeneity on device performance at present. These results suggest that further steps to prevent recombination losses at the interfaces are needed to help perovskite-based cells approach theoretical efficiency limits; only at this point will material heterogeneity become crucial.

Cross-Linkable Fullerene Derivatives for Solution-Processed n–i–p Perovskite Solar Cells

Hybrid perovskites form an extremely attractive class of materials for large scale, low-cost photovoltaic applications. Fullerene-based charge extraction layers have emerged as a viable n-type charge collection layer, and in “inverted” p–i–n device architectures the solar cells are approaching efficiencies of 20%. However, the regular n–i–p devices employing fullerenes still lag behind in performance. Here, we show that partial solubility of fullerene derivatives in the aprotic solvents used for the perovskites makes it challenging to retain integral films in multilayer solution processing. To overcome this issue we introduce cross-linkable fullerene derivatives as charge collection layers in n–i–p planar junction perovskite solar cells. The cross-linked fullerene layers are insolubilized and deliver improved performance in solar cells enabled by a controllable film thickness.

Light Manipulation in Organic Photovoltaics.

Organic photovoltaics (OPVs) hold great promise for next‐generation photovoltaics in renewable energy because of the potential to realize low‐cost mass production via large‐area roll‐to‐roll printing technologies on flexible substrates. To achieve high‐efficiency OPVs, one key issue is to overcome the insufficient photon absorption in organic photoactive layers, since their low carrier mobility limits the film thickness for minimized charge recombination loss. To solve the inherent trade‐off between photon absorption and charge transport in OPVs, the optical manipulation of light with novel micro/nano‐structures has become an increasingly popular strategy to boost the light harvesting efficiency. In this Review, we make an attempt to capture the recent advances in this area. A survey of light trapping schemes implemented to various functional components and interfaces in OPVs is given and discussed from the viewpoint of plasmonic and photonic resonances, addressing the external antireflection coatings, substrate geometry‐induced trapping, the role of electrode design in optical enhancement, as well as optically modifying charge extraction and photoactive layers.

Parasitic Absorption Reduction in Metal Oxide-Based Transparent Electrodes: Application in Perovskite Solar Cells.

Transition metal oxides (TMOs) are commonly used in a wide spectrum of device applications, thanks to their interesting electronic, photochromic, and electrochromic properties. Their environmental sensitivity, exploited for gas and chemical sensors, is however undesirable for application in optoelectronic devices, where TMOs are used as charge injection or extraction layers. In this work, we first study the coloration of molybdenum and tungsten oxide layers, induced by thermal annealing, Ar plasma exposure, or transparent conducting oxide overlayer deposition, typically used in solar cell fabrication. We then propose a discoloration method based on an oxidizing CO2 plasma treatment, which allows for a complete bleaching of colored TMO films and prevents any subsequent recoloration during following cell processing steps. Then, we show that tungsten oxide is intrinsically more resilient to damage induced by Ar plasma exposure as compared to the commonly used molybdenum oxide. Finally, we show that parasitic absorption in TMO-based transparent electrodes, as used for semitransparent perovskite solar cells, silicon heterojunction solar cells, or perovskite/silicon tandem solar cells, can be drastically reduced by replacing molybdenum oxide with tungsten oxide and by applying a CO2 plasma pretreatment prior to the transparent conductive oxide overlayer deposition.

Exploring the Limiting Open‐Circuit Voltage and the Voltage Loss Mechanism in Planar CH3NH3PbBr3 Perovskite Solar Cells

Perovskite solar cells based on CH3NH3PbBr3 with a band gap of 2.3 eV are attracting intense research interests due to their high open‐circuit voltage (Voc) potential, which is specifically relevant for the use in tandem configuration or spectral splitting. Many efforts have been performed to optimize the Voc of CH3NH3PbBr3 solar cells; however, the limiting Voc (namely, radiative Voc:Voc,rad) and the corresponding ΔVoc (the difference between Voc,rad and Voc) mechanism are still unknown. Here, the average Voc of 1.50 V with the maximum value of 1.53 V at room temperature is achieved for a CH3NH3PbBr3 solar cell. External quantum efficiency measurements with electroluminescence spectroscopy determine the Voc,rad of CH3NH3PbBr3 cells with 1.95 V and a ΔVoc of 0.45 V at 295 K. When the temperature declines from 295 to 200 K, the obtained Voc remains comparably stable in the vicinity of 1.5 V while the corresponding ΔVoc values show a more significant increase. Our findings suggest that the Voc of CH3NH3PbBr3 cells is primarily limited by the interface losses induced by the charge extraction layer rather than by bulk dominated recombination losses. These findings are important for developing strategies how to further enhance the Voc of CH3NH3PbBr3‐based solar cells.

Pyrite‐Based Bi‐Functional Layer for Long‐Term Stability and High‐Performance of Organo‐Lead Halide Perovskite Solar Cells

Organo‐lead halide perovskite solar cells (PSCs) have received great attention because of their optimized optical and electrical properties for solar cell applications. Recently, a dramatic increase in the photovoltaic performance of PSCs with organic hole transport materials (HTMs) has been reported. However, as of now, future commercialization can be hampered because the stability of PSCs with organic HTM has not been guaranteed for long periods under conventional working conditions, including moist conditions. Furthermore, conventional organic HTMs are normally expensive because material synthesis and purification are complicated. It is herein reported, for the first time, octadecylamine‐capped pyrite nanoparticles (ODA‐FeS2 NPs) as a bi‐functional layer (charge extraction layer and moisture‐proof layer) for organo‐lead halide PSCs. FeS2 is a promising candidate for the HTM of PSCs because of its high conductivity and suitable energy levels for hole extraction. A bi‐functional layer based on ODA‐FeS2 NPs shows excellent hole transport ability and moisture‐proof performance. Through this approach, the best‐performing device with ODA‐FeS2 NPs‐based bi‐functional layer shows a power conversion efficiency of 12.6% and maintains stable photovoltaic performance in 50% relative humidity for 1000 h. As a result, this study has the potential to break through the barriers for the commercialization of PSCs.

Achieving Ultrafast Hole Transfer at the Monolayer MoS2 and CH3NH3PbI3 Perovskite Interface by Defect Engineering.

The performance of a photovoltaic device is strongly dependent on the light harvesting properties of the absorber layer as well as the charge separation at the donor/acceptor interfaces. Atomically thin two-dimensional transition metal dichalcogenides (2-D TMDCs) exhibit strong light-matter interaction, large optical conductivity, and high electron mobility; thus they can be highly promising materials for next-generation ultrathin solar cells and optoelectronics. However, the short optical absorption path inherent in such atomically thin layers limits practical applications. A heterostructure geometry comprising 2-D TMDCs (e.g., MoS2) and a strongly absorbing material with long electron-hole diffusion lengths such as methylammonium lead halide perovskites (CH3NH3PbI3) may overcome this constraint to some extent, provided the charge transfer at the heterostructure interface is not hampered by their band offsets. Herein, we demonstrate that the intrinsic band offset at the CH3NH3PbI3/MoS2 interface can be overcome by creating sulfur vacancies in MoS2 using a mild plasma treatment; ultrafast hole transfer from CH3NH3PbI3 to MoS2 occurs within 320 fs with 83% efficiency following photoexcitation. Importantly, our work highlights the feasibility of applying defect-engineered 2-D TMDCs as charge-extraction layers in perovskite-based optoelectronic devices.