Chen Weijie

Publication date: November 2021

Source: Nano Energy, Volume 89, Part B

Author(s): Yingchun Niu, Chen Tian, Jiajia Gao, Fan Fan, Yida Zhang, Yuanyuan Mi, Xiangcheng Ouyang, Lina Li, Jiapeng Li, Siyuan Chen, Yinping Liu, Hong-Liang Lu, Xuelin Zhao, Lifeng Yang, Huanxin Ju, Yingguo Yang, Chuan-Fan Ding, Meng Xu, Quan Xu

The series resistance is modulated by shortening the charge transport distance in the conductivity-limited electrodes through busbar and device geometry design and preparing a bilayer carbon electrode to improve the performance of large-area-printable mesoscopic perovskite solar cells. Further reducing series resistance is important for the development of efficient large-area perovskite solar modules.

Perovskite solar cells (PSCs) have achieved a certified power conversion efficiency (PCE) of 25.5% and show potential for low-cost photovoltaic applications. One key of pushing PSCs into industrialization is enlarging their areas. However, the PCE of larger-area PSCs is dramatically limited by the undesired increase in series resistance (R _S), which leads to obvious loss of fill factor (FF). Herein, R _S in fully printable mesoscopic PSCs is modulated with a carbon electrode by optimizing the device geometry, preparing busbars to collect currents from electrodes and improving the conductivity of the back electrode. A rectangular device shape and tin busbars on conductive substrate effectively reduce the R _S. Meanwhile, an additional hot-pressed highly conductive low-temperature carbon layer on the back carbon electrode also reduces the R _S. An enhanced PCE of 13.99% for 1 cm² PSCs by R _S modulation is obtained, whereas the control device exhibits a PCE of 7.74%. The PCE increase is due to the improvement of FF from 0.372 to 0.638 with reduced R _S from 27.13 to 9.66 Ω cm².

Nickel oxide (NiO _x ) nanoparticles with high crystallinity and good dispersibility by the polymer network micro-precipitation method is synthesized, and the colloidal solution of ionic liquid-assisted NiO _x NPs dispersion is used to fabricate high-quality NiO _x films. Ultimately, the 1.01 cm² perovskite devices with the optimized NiO _x layers achieve the champion power conversion efficiency of 20.91% and 19.17% on rigid and flexible substrates, respectively.

Abstract

As one of the most promising hole transport layers (HTLs), nickel oxide (NiO _x ) has received extensive attention due to its application in flexible large-area perovskite solar cells (PSCs). However, the poor interface contact caused by inherent easy-agglomeration phenomenon of NiO _x nanoparticles (NPs) is still the bottleneck for achieving high-performance devices. Herein, a general strategy to synthesize NiO _x NPs with high crystallinity and good dispersibility via the polymer network micro-precipitation method is reported. Promisingly, this approach realizes the flow-division of precipitant and the restraint of the NPs motion, thereby effectively alleviating the coagulation phenomenon caused by excessive local concentration and secondary movement adsorption. Furthermore, the addition of ionic liquid not only inhibits the secondary aggregation of NiO _x NPs during the dispersion process, but also significantly enhances the properties of the colloidal solution. Ultimately, the 1.01 cm² PSCs based on the optimized NiO _x HTLs achieve the champion power conversion efficiency of 20.91% and 19.17% on rigid and flexible substrates, respectively. Moreover, the reproducibility and stability of PSCs are also significantly improved, especially for flexible devices. Overall, this strategy provides the possibility for flexible, large-area fabrication of high-quality NiO _x HTLs to promote the development of stable and efficient perovskite devices.

Unique hetero-structured CsPbI₃/CaF₂ perovskite/fluoride nanocomposites are constructed for fabricating efficient and ultra-stable perovskite solar cells (PSCs). The PSC device based on CsPbI₃/CaF₂-deposited Cs_0.05FA_0.81MA_0.14PbI_2.55Br_0.45 thin-film yields a best power conversion efficiency (PCE) of 21.06% and can retain 85% of its original PCE after 1000 h of continuous operation at the maximum power point tracking under AM 1.5G illumination.

Abstract

Organic-inorganic hybrid perovskite solar cells (PSCs) have rapidly developed over the past decade and have achieved the latest certified power conversion efficiency (PCE) up to 25.5%. However, unsatisfactory long-term operational stability for these hybrid PSCs remains a huge obstacle to further development and commercialization. Herein, a unique hetero-structured CsPbI₃/CaF₂ perovskite/fluoride nanocomposites (PFNCs) is fabricated via a newly developed facile two-step hetero-epitaxial growth strategy to deliver efficient and ultra-stable PSCs. After being incorporated into the crystal lattice of α-phase CsPbI₃ perovskite, the cubic-phase CaF₂ in the resultant CsPbI₃/CaF₂ PFNCs can not only passivate the intrinsic defects of CsPbI₃ perovskite itself but also effectively suppress the notorious ion migration in hybrid perovskite Cs_0.05FA_0.81MA_0.14PbI_2.55Br_0.45 (CsFAMA) thin-films of PSCs. As such, the CsFAMA PSC devices based on CsPbI₃/CaF₂-deposited perovskite thin-film achieve a mean PCE of 20.45%, in sharp contrast to 19.33% of the control devices without deposition. Specifically, the CsPbI₃/CaF₂-deposited PSC retains 85% of its original PCE after 1000 h continuous operation at the maximum power point under AM 1.5G solar light, far better than those of the control and CsPbI₃-deposited PSCs with a device T ₈₅ lifetime of 315 and 125 h, respectively.

An alternative approach for photocurrent mapping of photovoltaic devices is introduced, which uses digital light processing (DLP) technology combined with compressed sensing theory. A high-power DLP projection system is developed for the experimental application. Through this new sampling and reconstruction procedure, a much higher resolution can be achieved toward megapixel current mapping, while the computational burden of previous methods is removed.

Photocurrent response mapping is a powerful imaging technique for assessing defects and losses in photovoltaic devices. However, it has not enjoyed widespread application because high-resolution measurements of large samples can last several hours, while weak signals from micrometer-sized laser spots require lock-in amplification. An alternative approach presented recently is the use of digital micromirror devices combined with compressed sensing theory. There are significant benefits when using such methods, such as signal amplification, undersampling options, and simplified measurement systems. Nevertheless, high computational requirements have limited the experimental application of this method to low-resolution outputs. Herein, the mathematical background and the experimental approach toward megapixel resolution, ultrafast compressed sensing current mapping are presented, overcoming previous computational and experimental barriers. A high-power digital light processing projection system is developed for the experimental application. Solutions to computational issues, sampling optimization, and measurement strategies are presented and the flexibility of the system regarding the sizes of photovoltaic devices that can be measured is demonstrated.

The origin for the burn-in of non-fullerene-based organic solar cells is investigated. It is revealed that burn-in is mainly due to the rapid increase in the interfacial resistance (R _int). The R _int is greatly improved by constructing a ternary photoactive layer through the sequential deposition of a polymer solution and a binary acceptor solution.

Non-fullerene acceptor (NFA)-based organic solar cells often exhibit significant cell degradation in power conversion efficiency (PCE) in the early stages of operation, called “burn-in.” Generally, to fabricate NFA-based solar cells, binary blend solution deposition (binary BSD) of a conjugated polymer and an NFA is utilized. Herein, the reasons for burn-in are investigated by aging organic photovoltaic cells with independent control of temperature and light. The results reveal that burn-in is mainly due to a rapid increase in the interfacial resistance (R _int) rather than photo-oxidation of the components or oxidation of the electrode. This R _int is effectively suppressed by constructing a ternary photoactive layer through the sequential deposition of a polymer solution and a binary acceptor solution consisting of an NFA and a fullerene acceptor (ternary sequential deposition [ternary SqD]). Under the illumination of 1 sun and thermal annealing at 80 °C for 500 h, the binary BSD exhibits a reduction in efficiency of 63% and 59%, respectively, whereas the ternary SqD demonstrates a reduction of only 32% and 35%, respectively. In addition, the ternary SqD improves the PCE on using fullerene acceptors to enhance light harvesting at short wavelengths.

Two new acceptor–donor–acceptor (A–D–A)-type small molecule donors SM-DTBDT and SM-BDT with dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT) and benzodithiophene (BDT) as the core D-units, respectively, are synthesized and their photovoltaic performance results indicate that tuning the center units of the donors can increase the efficiency and improve the stability of the all small-molecule organic solar cells.

Rational design and synthesis of new small-molecule donors are critically important to achieve highly efficient small-molecule organic solar cells (SM-OSCs) with desirable device stability. Herein, two new acceptor–donor–acceptor (A–D–A) structured small-molecule donor materials SM-DTBDT and SM-BDT with dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT) and benzodithiophene (BDT) as the core D-units are designed and synthesized, respectively. After thermal annealing treatment, the power conversion efficiencies (PCEs) of the SM-DTBDT- and SM-BDT-based devices with Y8 as acceptor material reach 12.45% and 10.68%, respectively. The main reason for the different device performance can be ascribed to the different crystallinity and morphology features of the small-molecule donor:Y8 blend films. Compared with the SM-BDT:Y8 blend films, the SM-DTBDT:Y8 blend films show a smoother surface, more uniform phase separation, mixed edge-on and face-on orientations, and enhanced crystallinity, resulting in the more efficient exciton dissociation and charge transport. In addition, it is observed that the SM-DTBDT-based devices also exhibit better stability. This work shows that fine-tuning the center units of small-molecule donors can not only increase the photovoltaic performance but can also be an effective method to improve the device stability.

Low-bandgap tin-containing halide perovskites have recently demonstrated impressive photovoltaic performances owing to their unique optoelectronic properties, which are yet to be fully explored. A perspective on the absorption, carrier generation, recombination, and transport phenomena in these materials is provided, underscoring an interesting interplay between doping, defects, and degradation that needs to be controlled for future optoelectronic applications.

Abstract

Riding on the coat tails of rapid developments in single-junction halide perovskite solar cells, all-perovskite multijunction solar cells have recently garnered significant attention, with the highest power-conversion efficiency already reaching 25.6%. Much of this progress has been fueled by the rapid rise in the photovoltaic performance of low-bandgap halide perovskite absorbers, materials, which, to date, have only been achievable by the partial or complete substitution of lead with tin. However, much room still exists to develop a more critical understanding of key material properties in these low-bandgap perovskites. Herein, the key optoelectronic properties of absorption, carrier generation, recombination, and transport in these tin-containing perovskites are discussed, showing that intrinsic doping distinctively impacts many of these properties, thereby rendering this class of halide perovskites unique within the family. Current understanding of the mechanisms that degrade optoelectronic performance in these materials and the corresponding devices are also summarized. These collective results highlight an important interplay between doping, defects, and degradation that will need to be controlled. Finally, the current gaps in understanding of these low-bandgap perovskites are outlined, thereby providing guidelines for further research, which will unlock their full potential for realizing a plethora of high-performance optoelectronic devices.

The efficiency potential of various solar cells is analyzed to develop high-efficiency solar cell modules for photovoltaic (PV)-powered vehicles. Analytical and practical data show that the III−V and Si 3-junction tandem solar cell modules with an efficiency of more than 30% have a potential driving range of 30 km day⁻¹ on average and more than 50 km day⁻¹ on a clear day.

Photovoltaic (PV)-powered vehicles are expected to play a critical role in a future carbon neutral society because it has been reported that the onboard PVs have great ability to reduce CO₂ emission from the transport sector. Although the demonstration car with a III−V-based solar cell module has shown the PV-powered driving range of 36.6 km day⁻¹ at solar irradiance of 6.2 kWh m⁻² day⁻¹, practical driving ranges of PV-powered vehicles are shown to be lower than estimated values due to some losses such as nonradiative recombination and resistance losses of solar cell modules under sunshine condition. This article presents analytical results for the effects of illumination intensity properties of various solar cell modules on the PV-powered driving range to develop highly efficient solar cell modules for vehicle-integrated applications. The analysis shows that improvements in shunt resistance and saturation current density of solar cell modules are necessary to improve illumination intensity properties of solar cell modules under low intensity sunshine condition. The calculations show that the III−V-based 3-junction solar cell modules with an efficiency of more than 30% have a potential PV-powered driving range of 30 km/day average and more than 50 km day⁻¹ on a clear day.

Interfacial nonradiative recombination limits the open-circuit voltage of perovskite solar cells. A buried interface passivation strategy is developed that can be used across metal oxide transport layers. Perovskite precursors containing large organic cations with high affinity for the substrate spontaneously form a 2D passivation layer on the underlying metal oxides, which reduces interfacial recombination by 72%.

Abstract

The open-circuit voltage (V _oc) of perovskite solar cells is limited by non-radiative recombination at perovskite/carrier transport layer (CTL) interfaces. 2D perovskite post-treatments offer a means to passivate the top interface; whereas, accessing and passivating the buried interface underneath the perovskite film requires new material synthesis strategies. It is posited that perovskite ink containing species that bind strongly to substrates can spontaneously form a passivating layer with the bottom CTL. The concept using organic spacer cations with rich NH₂ groups is implemented, where readily available hydrogens have large binding affinity to under-coordinated oxygens on the metal oxide substrate surface, inducing preferential crystallization of a thin 2D layer at the buried interface. The passivation effect of this 2D layer is examined using steady-state and time-resolved photoluminescence spectroscopy: the 2D interlayer suppresses non-radiative recombination at the buried perovskite/CTL interface, leading to a 72% reduction in surface recombination velocity. This strategy enables a 65 mV increase in V _oc for NiO _x based p–i–n devices, and a 100 mV increase in V _oc for SnO₂-based n–i–p devices. Inverted solar cells with 20.1% power conversion efficiency (PCE) for 1.70 eV and 22.9% PCE for 1.55 eV bandgap perovskites are demonstrated.

Publication date: November 2021

Source: Nano Energy, Volume 89, Part B

Author(s): Hailiang Wang, Huicong Liu, Zijing Dong, Tinglu Song, Weiping Li, Liqun Zhu, Yang Bai, Haining Chen

After optimization of small-area spin-coated solar cells based on a methylammonium-free perovskite absorber, a process for fabrication of larger-area cells with scalable methods is investigated. Perovskite solar cells are elaborated in which all solution-processable layers are deposited by blade coating. Ultimately, blade-coated perovskite modules with a stabilized efficiency of 12.6% are fabricated.

Solar cells based on hybrid organic/inorganic perovskites have shown an astonishing efficiency development in the past years, having peaked in power conversion efficiencies of >25% for small-area single-junction devices. To pave the way for future commercialization, however, high power conversion efficiencies also have to be demonstrated on areas multiple orders of magnitude larger. Herein, methylammonium-free perovskite photovoltaic modules with an active area of 66 cm² are presented. All functional layers processable from solution are deposited by blade coating without the use of an antisolvent, demonstrating the feasibility of this approach for large-area module fabrication. The coating process is analyzed in detail and a model based on the Landau–Levich problem is developed for the blade-coating setup. The perovskite crystallinity can be improved by the addition of lead(II) thiocyanate, which results in increased crystallite size as judged by Williamson–Hall's analysis of X-ray diffraction data and corresponding scanning electron microscopy images. The homogeneity of the final modules is investigated with dark lock-in thermography and electroluminescence imaging, indicating only few shunts in the module area. Modules are made up of 15 serially interconnected solar cells and reveal a stabilized efficiency of 12.6% under maximum power point tracking.

A robust hole transporting layer based on cobalt(II) acetate was developed by aqueous solution processing technology, low temperature thermal annealing and UV-ozone treatment, which enabled a record high-power conversion efficiency of 18.77 % of binary blend organic solar cells.

Abstract

A robust hole transporting layer (HTL), using the cost-effective Cobalt(II) acetate tetrahydrate (Co(OAc)₂⋅4 H₂O) as the precursor, was simply processed from its aqueous solution followed by thermal annealing (TA) and UV-ozone (UVO) treatments. The TA treatment induced the loss of crystal water followed by oxidization of Co(OAc)₂⋅4 H₂O precursor, which increased the work function. However, TA treatment differently realize a high work function and ideal morphology for charge extraction. The resulting problems could be circumvented easily by additional UVO treatment, which also enhanced the conductivity and lowered the resistance for charge transport. The optimal condition was found to be a low temperature TA (150 °C) followed by simple UVO, where the crystal water in Co(OAc)₂⋅4 H₂O was removed fully and the HTL surface was anchored by substantial hydroxy groups. Using PM6 as the polymer donor and L8-BO as the electron acceptor, a record high PCE of 18.77 % of the binary blend OSCs was achieved, higher than the common PEDOT:PSS-based solar cell devices (18.02 %).