Chen Weijie

Publication date: Available online 22 July 2021

Source: Chem

Author(s): Ling Hong, Ziyi Ge

Further large-scale photovoltaic deployment mandates the reduction of scarce Ag, typically comprising the metallic contact of solar cells. Electrodeposited Cu contacts are demonstrated for the first time on single-junction FA_0.75Cs_0.25Pb(I_0.8Br_0.2)₃ perovskite solar cells (PSCs) using an atomic layer deposition (ALD) Al₂O₃ masking layer on ITO. The stable photoconversion efficiency after Cu²⁺ reduction confirms that PSCs can survive wet-chemical plating process.

Electroplated copper contacts on small-area single-junction perovskite solar cells (PSCs) using an atomic layer deposited (ALD) Al₂O₃ masking layer on ITO are demonstrated for the first time. The photoconversion efficiency of ≈11% after manufacturing the Cu contacts confirms that PSCs can survive the wet-chemical plating process. From the successful realization of plated contact fingers, the creation of an electrical contact between the Cu electrode and the ITO on the FA_0.75Cs_0.25Pb(I_0.8Br_0.2)₃ perovskite absorber is inferred. Furthermore, scanning electron microscopy (SEM) with energy-dispersive X-ray (EDX) analysis shows the formation of a compact interface between ITO and plated Cu. An additional plating approach, using self-passivated aluminum as mask, allows to produce well-defined 30 μm wide Cu contacts on the PSC. Such a plating process allows for plating a low-resistive Cu grid simultaneously on both sides of a perovskite silicon heterojunction tandem solar cell with TCO, independent of substrate size.

Co-evaporation methylammonium formamidinium lead iodide perovskites are investigated and different aspects of stability are addressed. The influence of the perovskite composition on the performance and the long-term stability of the resulting solar cells is studied. Monolithic fully textured perovskite/silicon tandem solar cells with co-evaporated perovskite absorber are realized. These tandem cells reach an efficiency of 24.6% and exhibit minimal reflection losses.

Abstract

Formamidinium iodide (FAI) based perovskite absorbers have been shown to be ideal candidates for highly efficient and operationally stable perovskite solar cells (PSC). A major challenge for formamidinium lead iodide (FAPbI₃) is to suppress the phase transition from the photoactive black phase into yellow nonperovskite δ-phase. Several approaches to stabilize the black phase have been developed for solution-based perovskites, whereas so far, vacuum-deposited FAPbI₃ has rarely been reported. This study demonstrates the preparation of FAPbI₃ by co-evaporation and discusses the influence of the subjacent hole transporting layer (HTL) on its phase stability. By using FAI excess in the evaporation process in combination with phosphonic acids groups from the HTL, the black perovskite phase is stabilized at room temperature. Further addition of 32–59% methylammonium iodide (MAI) during the co-evaporation process leads to good absorption properties and high PSC efficiencies of 20.4%. In addition, excellent stability is achieved for optimized MAI to FAI ratios, maintaining 100% of the initial PSC performance after 1000 h under constant operation. This highly stable perovskite composition enables the first monolithic fully textured perovskite/silicon tandem solar cells with co-evaporated perovskite absorbers. Due to the conformally covered pyramid texture, these tandem cells show minimal reflection losses and reach an efficiency of 24.6%.

Color-tunable white organic light-emitting diodes with hole-trapping thermally activated delayed-fluorescence-sensitized emissions realize significant Commission Internationale de l'Eclairage coordinates and correlated color temperature shifts from (0.40, 0.47) and 4088 K at 100 cd m⁻² to (0.27, 0.33) and 9269 K at 5000 cd m⁻², with a reported maximum external quantum efficiency of 30.7% and long lifetime of over 20 000 h at 80% of the initial luminance.

Abstract

White organic light-emitting diodes (WOLEDs) with high efficiencies and tunable colors attracts considerable interest from the industry and academia. Thermally activated delayed-fluorescence (TADF) emitters can revolutionize such WOLED devices; however, they still suffer from poor performances. In this study, an advanced double-emissive-layer device architecture capable of hole-trapping TADF-sensitized emissions is proposed to not only achieve a recombination zone shift for the tunable colors but also accelerate exciton emission dynamics for high efficiency and alleviated roll-off. The proof-of-concept WOLEDs exhibit significant shifts in their Commission Internationale de l'Eclairage (CIE) coordinates and correlated color temperatures from (0.40, 0.47) and 4088 K at 100 cd m⁻² to (0.27, 0.33) and 9269 K at 5000 cd m⁻². Additionally, the maximum external quantum efficiency (EQE) reaches 30.7% and remains >25% over a wide luminance range of 500–5000 cd m⁻², along with an extended LT80 of over 20 000 h at an initial luminance of 100 cd m⁻². This is the first time that all-fluorescent WOLEDs have been used to realize an EQE exceeding 30%, thereby establishing a new benchmark in this field.

A general and effective strategy is delivered to modulate the 2D/3D microstructure of tin-perovskite films by introduction of a 2D phase with the function of FPEABr, which induces high-orientation growth of 3D FASnI₃ by embracing the 3D grains at their surfaces and boundaries. That leads to a breakthrough of device performance of 14.81% in power conversion efficiency, along with 14.03% certified.

Abstract

As the most promising lead-free one, tin-halides based perovskite solar cells still suffer from the severe bulk-defect due to the easy oxidation of tin from divalent to tetravalent. Here, a general and effective strategy is delivered to modulate the microstructure of 2D/3D heterogeneous tin-perovskite absorber films by substituting FAI with FPEABr in FASnI₃. The introduction of 2D phase can induce highly oriented growth of 3D FASnI₃ and it is revealed in the optimal 2D/3D film that 2D phase embraces 3D grains and locates at the surfaces and grain boundaries. The FPEA⁺ based 2D tin-perovskite capping layer can offer a reducing atmosphere for vulnerable 3D FASnI₃ grains. The unique microstructure effectively suppresses the well-known oxidation from Sn²⁺ to Sn⁴⁺, as well as decreasing defect density, which leads to a remarkable enhanced device performance from 9.38% to 14.81% in conversion efficiency. The certified conversion efficiency of 14.03% announces a new record and moves a remarkable step from the last one (12.4%). Besides of this breakthrough, this work definitely paves a new way to fabricate high-quality tin-perovskite absorber film by constructing effective 2D/3D microstructures.

A thiadiazole-based polymer donor (PB2F) with an ultradeep highest occupied molecular orbital level and high electroluminescence is reported. In organic solar cell (OSC) devices, PB2F exhibits a power conversion efficiency (PCE) of 14.5% after blending with IT-4F, one of highest values among the IT-4F-based OSCs, and an outstanding PCE of 18.6% in the context of ternary OSC.

Abstract

Under the premise of ensuring favorable bulk heterojunction morphology in organic solar cells (OSCs), conjugated polymer donors with deep-lying highest occupied molecular orbital (HOMO) levels are highly important to improve power conversion efficiencies (PCEs) by reducing photovoltage loss. However, the development of such materials has lagged. Herein, a thiadiazole-based conjugated polymer, PB2F is reported, which has a very deep HOMO level of −5.64 eV, and high electroluminescence quantum efficiency of 3.9 × 10⁻³. In OSCs, the PB2F-based OSC gives an excellent PCE of 14.5% with an ultrahigh open-circuit voltage (V _OC) of 0.957 V by blending with an electron acceptor of IT-4F. More importantly, the PB2F as a third component is added into the PBDB-TF:BTP-eC9 blend to achieve an outstanding PCE of 18.6% (certified PCE 18.2%), which is one of the highest PCEs in OSCs.

Although organic photovoltaics have been established to be a promising candidate for indoor light recycling, standardized testing conditions to quantify their performance are still lacking. Therefore, a method to calculate the efficiency of solar cells on the basis of relative emission spectra, quantum efficiency and current-density-voltage measurements, which enables a fair ranking of champion solar cells, is proposed.

Abstract

With increasing efficiencies of non-fullerene acceptor-based organic solar cells, thin-film technology is becoming a promising candidate for indoor light harvesting applications. However, the lack of standardized comparison methods makes it difficult to quantify progress and to compare indoor performance. Herein, a simple method to calculate the efficiency of solar cells under any possible light source and illuminance with only using simple standard measurements (current–voltage curves and quantum efficiency) is presented. Thereby, equal evaluation conditions are ensured, so that indoor solar cells can be ranked and compared according to their efficiency. Efficiencies are shown to typically vary by ±20% when using different different light emitting diode spectra with color temperatures ranging from 2700 to 6500 K. Calculations based on a detailed balance model indicate that the optimal bandgap of the absorber material depends on the used light source and ranges between 1.75 and 2 eV. The approach is validated by comparison with literature data and many calculated efficiencies match well with experimental data obtained with a specific light source. However, some reported efficiencies cannot be reproduced with the model, which highlights the need to reassess low light measuring techniques. Furthermore, a script is provided for use by the community.

Publication date: 15 September 2021

Source: Joule, Volume 5, Issue 9

Author(s): Zhenyu Chen, Wei Song, Kuibao Yu, Jinfeng Ge, Jinsheng Zhang, Lin Xie, Ruixiang Peng, Ziyi Ge

Integrating a thin bulk-heterojunction layer composed of the typical poly (3-hexylthiophene-2,5-diyl) and [6,6]-phenyl methyl C61 butyric acid methyl ester (P3HT:PCBM) in a carbon-based all inorganic CsPbIBr₂ perovskite solar cell can effectively suppress interfacial energy loss and greatly improve the power conversion efficiency from 8,87% to 11.54%, which is also at the highest efficiency level of the reported counterparts.

The CsPbIBr₂ perovskite has obvious advantages in balancing the stability and efficiency in inorganic perovskite solar cells (PSCs). Its large bandgap of 2.08 eV, which leads to a narrow spectral absorption (<600 nm), is the key limit to yielding a high power conversion efficiency (PCE). Herein, it is demonstrated that by integrating a thin bulk-heterojunction (BHJ) layer (19 nm) composed of the typical poly (3-hexylthiophene-2,5-diyl) and [6,6]-phenyl methyl C61 butyric acid methyl ester (P3HT:PCBM) with CsPbIBr₂ perovskite, a carbon-based all-inorganic PSC achieves a much higher champion PCE (11.54%) than the original CsPbIBr₂ device (8.87%), and the value is also at the highest PCE level of all-inorganic CsPbIBr₂ PSCs. The integration of a thin BHJ layer brings an expanded light absorption range, better charge transfer dynamics, suppressed interfacial energy loss, and improved long-term stability. The unencapsulated CsPbIBr₂ PSC with an integrated BHJ layer shows excellent long-term stability in an ambient atmosphere with high relative humidity (RH ≈ 45%, T ≈ 25 °C). Therefore, the BHJ integration is an effective strategy on the road to industrialization of carbon-based all-inorganic PSCs with low cost, high efficiency, and excellent long-term stability.

A versatile co-evaporation approach to create perovskites layers with graded energy levels favorable for different device architectures is demonstrated. The p-i-n perovskite solar cells, incorporating co-evaporated MAPbI₃ with customized graded Fermi levels, achieve power conversion efficiency over 20% with different hole transporting layers and champion values of 20.6%, 19.1%, and 17.2% for 0.086, 1, and 1.96 cm² active areas, respectively.

Abstract

Recent progress of vapor-deposited perovskite solar cells (PSCs) has proved the feasibility of this deposition method in achieving promising photovoltaic devices. For the first time, it is probed the versatility of the co-evaporation process in creating perovskite layers customizable for different device architectures. A gradient of composition is created within the perovskite films by tuning the background chamber pressure during the growth process. This method leads to co-evaporated MAPbI₃ film with graded Fermi levels across the thickness. Here it is proved that this growth process is beneficial for p-i-n PSCs as it can guarantee a favorable energy alignment at the charge selective interfaces. Co-evaporated p-i-n PSCs, with different hole transporting layers, consistently achieve power conversion efficiency (PCE) over 20% with a champion value of 20.6%, one of the highest reported to date. The scaled-up p-i-n PSCs, with active areas of 1 and 1.96 cm², achieved the record PCEs of 19.1% and 17.2%, respectively, while the flexible PSCs reached a PCE of 19.3%. Unencapsulated PSCs demonstrate remarkable long-term stability, retaining ≈90% of their initial PCE when stored in ambient for 1000 h. These PSCs also preserve over 80% of their initial PCE after 500 h of thermal aging at 85 °C.

Current losses in perovskite solar cells (PSCs) are investigated using transient photoluminescence and charge extraction measurements. Mobile ions cause a substantial current and efficiency loss by accumulating at the perovskite/transport layer interfaces, which screens the internal electric field. This work elucidates the detrimental impact of mobile ions in PSCs and paves the path toward mitigating this key loss mechanism.

Abstract

Efficient mixed metal lead-tin halide perovskites are essential for the development of all-perovskite tandem solar cells, however they are currently limited by significant short-circuit current losses despite their near optimal bandgap (≈1.25 eV). Herein, the origin of these losses is investigated, using a combination of voltage dependent photoluminescence (PL) timeseries and various charge extraction measurements. It is demonstrated that the Pb/Sn-perovskite devices suffer from a reduction in the charge extraction efficiency within the first few seconds of operation, which leads to a loss in current and lower maximum power output. In addition, the emitted PL from the device rises on the exact same timescales due to the accumulation of electronic charges in the active layer. Using transient charge extraction measurements, it is shown that these observations cannot be explained by doping-induced electronic charges but by the movement of mobile ions toward the perovskite/transport layer interfaces, which inhibits charge extraction due to band flattening. Finally, these findings are generalized to lead-based perovskites, showing that the loss mechanism is universal. This elucidates the negative role mobile ions play in perovskite solar cells and paves a path toward understanding and mitigating a key loss mechanism.

A highly crystalline and wide bandgap electron acceptor, IDTT-M, is used to fabricate high efficiency ternary solar cells with PM6 and another narrow bandgap electron acceptor, Y6. Benefiting from efficient Förster resonance energy transfer, a significantly improved power conversion efficiency of up to 16.63% is achieved with simultaneously enhanced device characteristics.

Abstract

Introducing a third component into organic bulk heterojunction solar cells has become an effective strategy to improve photovoltaic performance. Meanwhile, the rapid development of non-fullerene acceptors (NFAs) has pushed the power conversion efficiency (PCE) of organic solar cells (OSCs) to a higher standard. Herein, a series of fullerene-free ternary solar cells are fabricated based on a wide bandgap acceptor, IDTT-M, together with a wide bandgap donor polymer PM6 and a narrow bandgap NFA Y6. Insights from the morphological and electronic characterizations reveal that IDTT-M has been incorporated into Y6 domains without disrupting its molecular packing and sacrificing its electron mobility and work synergistically with Y6 to regulate the packing pattern of PM6, leading to enhanced hole mobility and suppressed recombination. IDTT-M further functions as an energy-level mediator that increases open-circuit voltage (V _OC) in ternary devices. In addition, efficient Förster resonance energy transfer (FRET) between IDTT-M and Y6 provides a non-radiative pathway for facilitating exciton dissociation and charge collection. As a result, the optimized ternary device features a significantly improved PCE up to 16.63% with simultaneously enhanced short-circuit current (J _SC), V _OC, and fill factor (FF).

A hydrophobic ammonium salt, 4-(trifluoromethyl) benzylamine, is introduced to form a quasi-2D hybrid perovskite by a one-step spin-coating method. Due to the relatively low surface energy of fluorinated molecules, an upper gradient low-dimensional structure is formed spontaneously from top to bottom, and more stable devices are obtained with a power conversion efficiency of 17.07%.

Abstract

Highly efficient and stable quasi-2D hybrid perovskite solar cells (PSCs) using hydrophobic 4-(trifluoromethyl) benzylamine (4TFBZA) as the spacer cation are successfully demonstrated. It is found that the incorporation of hydrophobic 4TFBZA into MAPbI₃ can effectively induce a spontaneous upper gradient 2D (SUG-2D) structure, passivate the trap states, and restrain the ion motion. Meanwhile, the strong hydrogen bonding of F···HN between 4TFBZA ions and methylamine ions can effectively suppress the decomposition of perovskite, which gives the device a better thermal stability. Besides, due to the SUG-2D structure with hydrophobic 4TFBZA, the device also exhibits a better moisture stability. The SUG-2D-structure-based device exhibits a power conversion efficiency of 17.07% with a high open-circuit voltage of 1.10 V and a notable fill factor of 71%. This work provides a new strategy for constructing efficient and stable quasi-2D PSCs, and it is an inspiration for the packaging strategy of perovskites.

Publication date: November 2021

Source: Nano Energy, Volume 89, Part A

Author(s): Yi Long, Yeming Xian, Songyang Yuan, Kun Liu, Mingyuan Sun, Yang Guo, Naveed Ur Rahman, Jiandong Fan, Wenzhe Li

Lead-free double perovskites are considered as one of the most suitable alternatives to lead halide perovskites in view of their nontoxic character and long-term stability. The spray-coated double perovskite Cs₂AgBiBr₆ devices presented here show very promising photovoltaic performances, notably, impressively high values of open-circuit voltage, due to an improved Cs₂AgBiBr₆/Spiro-OMeTAD interface.

Lead-free Cs₂AgBiBr₆ double perovskite is considered a promising alternative photovoltaic absorber to the widely used lead halide perovskite due to its easy processability, high stability, and reduced toxicity. Herein, for the first time spray processing for the deposition of Cs₂AgBiBr₆ double perovskite thin films is reported. Microstructural (X-ray diffraction, scanning electron microscopy) and optoelectronic (absorbance, photoluminescence, photocurrent density versus applied voltage curves, electrochemical impedance spectroscopy) properties of spray-coated film are compared with the spin-coated benchmark. Incorporation of the spray-coated Cs₂AgBiBr₆ double perovskite thin films in solar cells leads to a 2.3% photoconversion efficiency with high open-circuit voltage of 1.09 V. This study highlights the suitability of ultrasonic spray deposition for the optimization of Cs₂AgBiBr₆ solar cells in terms of light absorption properties and charge transfer at the Cs₂AgBiBr₆/hole transporting layer interface.

A combined approach of simulation and experiment unravels the origin of very high external quantum efficiencies (EQEs up to 98% in the literature), which are frequently reported for highly efficient perovskite solar cells. The high refractive index of the perovskite and the thickness of the underlying transparent electrode are identified to mainly govern light in-coupling into the active layer.

With the emergence of highly efficient perovskite solar cells in both single- and multijunction architectures, there is an abundance of reports of extremely high external quantum efficiencies (EQE) up to 98%. Typically, the spectral maximum of the EQE is found in the range between 400 and 500 nm, which is even more surprising, as the transmittance of typically used indium tin oxide (ITO)/glass substrates does not exceed 90% in this wavelength range. Herein, the root cause of the high EQE values by a combination of experimental data and optical simulations is analyzed and explained. It is shown that the high refractive index of the perovskite absorber is strongly increasing the transmittance of incident light into the active perovskite layer, while the spectral distribution and ultimately the spectral position of the peak in the transmittance spectrum are strongly affected by the thickness and optical properties of the underlying transparent electrode.