Chen Weijie

Publication date: Available online 5 April 2022

Source: Chem

Author(s): Hsinhan Tsai

In this study, performance degradation factors of organic photovoltaics (OPVs) under indoor stress are unveiled. Morphological evolution and molecular packing variation are deeply investigated. Indoor stability enhancement is achieved with increasing photoactive components. The quaternary OPVs show great performance retention under viable indoor stresses.

Abstract

Despite recent improvements in their power-conversion efficiency (PCE), organic photovoltaics (OPVs) cannot yet be guaranteed stable in an indoor environment. In this study, the destabilizing effects of morphological evolution and molecular-ordering variation on photoactive layers containing two to four photoactive components are investigated under realistic indoor photothermal (>55 °C for 1000 h) and mechanical (10% strain and 1000 cycles) deformation conditions. Layers with more stable morphologies are obtained by increasing the number of photoactive components; consequently, the quaternary OPVs show the best PCE retention (over 90% and 82% of the initial values after the photothermal and mechanical stresses, respectively). The increase in entropy caused by the additional components in the quaternary blend leads to a more balanced molecular arrangement and excellent photothermal stability. Stronger intermolecular bonding and less variation of molecular ordering likewise occur in the quaternary OPVs, enhancing their mechanical endurance.

Homologous PbI₂ in situ passivation strategy is demonstrated to passivate defect at grain boundaries of black-phase FAPbI₃ film via methylamine chloride-assisted one-step deposition. The excess PbI₂ based self-passivation on the FAPbI₃ device not only enhances the power conversion efficiency from 14.87% to 22.13% but also leads to excellent durability in air, which is the highest PCE for FAPbI₃-based inverted PSCs reported to date.

Abstract

Formamidinium lead iodide (FAPbI₃) has endowed power conversion efficiencies (PCEs) up to 25.5% in regular-structured perovskite solar cells (PSCs) because of its optimal bandgap and enhanced thermal stability. However, the performance of FAPbI₃-based inverted-structured PSCs is unsatisfactory. Herein, four kinds of commonly used hole transport materials (HTMs) are selected, including PEDOT:PSS, PTAA, NiOx, and MeO-2PACz, to study their impact on the methylamine chloride (MACl)-assisted one-step deposition of FAPbI₃ films. It is found that MeO-2PACz is the optimal substrate for stabilizing black-phase FAPbI₃ and the corresponding inverted-structured PSCs show the best photovoltaic performance. Nonetheless, the PCE is restricted by low open-circuit voltage (V _OC) due to non-radiative recombination caused by MACl residues. Therefore, homologous PbI₂ in situ passivation is implemented to passivate defects at grain boundaries. The addition of excess PbI₂ in precursor solution remarkably decreases charge trap densities and elongates carrier lifetimes. As a result, the optimized device achieves an impressive PCE of 22.13%, which is the highest efficiency of FAPbI₃ based on inverted-structured PSCs. Moreover, the best device exhibits free hysteresis and excellent long-term stability, maintaining 92% of the initial PCEs after 800 h aging under ambient conditions.

A bottom-up infiltration method using HCOONH₄ as pre-buried additive in SnO₂ electron transport layer (ETL) enables a cross-layer defect manipulation throughout the SnO₂ ETL, perovskite layer, and their interface, along with a significantly reduced residual stress within perovskite film. As a result of the cross-layer treatment, a record power conversion efficiency of 22.37% (21.90% certified) is achieved on the optimized flexible perovskite solar cells.

Abstract

Halide perovskites have shown superior potentials in flexible photovoltaics due to their soft and high power-to-weight nature. However, interfacial residual stress and lattice mismatch due to the large deformation of flexible substrates have greatly limited the performance of flexible perovskite solar cells (F-PSCs). Here, ammonium formate (HCOONH₄) is used as a pre-buried additive in electron transport layer (ETL) to realize a bottom-up infiltration process for an in situ, integral modification of ETL, perovskite layer, and their interface. The HCOONH₄ treatment leads to an enhanced electron extraction in ETL, relaxed residual strain and micro-strain in perovskite film, along with reduced defect densities within these layers. As a result, a top power conversion efficiency of 22.37% and a certified 21.9% on F-PSCs are achieved, representing the highest performance reported so far. This work links the critical connection between multilayer mechanics/defect profiles of ETL-perovskite structure and device performance, thus providing meaningful scientific direction to further narrowing the efficiency gap between F-PSCs and rigid-substrate counterparts.

A crosslinked and compact membrane is constructed throughout the perovskite film by a self-assembly strategy based on cholesterol-based molecules. This enables durable passivation against harsh conditions and effective defects passivation from surface toward inner grain boundaries. The modified planar perovskite solar cells (PSCs) exhibit a 23.34 % champion efficiency and remain almost unchanged after heating at 85 °C for 500 h.

Abstract

Defect passivation via post-treatment of perovskite films is an effective method to fabricate high-performance perovskite solar cells (PSCs). However, the passivation durability is still an issue due to the weak and vulnerable bonding between passivating functional groups and perovskite defect sites. Here we propose a cholesterol derivative self-assembly strategy to construct crosslinked and compact membranes throughout perovskite films. These supramolecular membranes act as a robust protection layer against harsh operational conditions while providing effective passivation of defects from surface toward inner grain boundaries. The resultant PSCs exhibit a power conversion efficiency of 23.34 % with an impressive open-circuit voltage of 1.164 eV. The unencapsulated devices retain 92 % of their initial efficiencies after 1600 h of storage under ambient conditions, and remain almost unchanged after heating at 85 °C for 500 h in a nitrogen atmosphere, showing significantly improved stability.

Publication date: 15 June 2022

Source: Nano Energy, Volume 97

Author(s): Qinglian Zhu, Jingwei Xue, Guanyu Lu, Baojun Lin, Hafiz Bilal Naveed, Zhaozhao Bi, Guanghao Lu, Wei Ma

Addition of ferroelectric polymer polyvinylidene difluoride (PVDF) as additive into non-fullerene organic solar cells substantially enhances the built-in electric field. Promoted exciton separation, significantly accelerated charge transport, reduced charge recombination, as well as optimized film morphology were observed in the device, leading to a significantly improved device performance.

Abstract

Enhancing the built-in electric field to promote charge dynamitic process is of great significance to boost the performance of the non-fullerene organic solar cells (OSCs), which has rarely been concerned. In this work, we introduced a cheap ferroelectric polymer as an additive into the active layers of non-fullerene OSCs to improve the device performance. An additional and permanent electrical field was produced by the polarization of the ferroelectric dipoles, which can substantially enhance the built-in electric field. The promoted exciton separation, significantly accelerated charge transport, reduced the charge recombination, as well as the optimized film morphology were observed in the device, leading to a significantly improved performance of the PVDF-modified OSCs with various active layers, such as PM6 : Y6, PM6 : BTP-eC9, PM6 : IT-4F and PTB7-Th : Y6. Especially, a record efficiency of 17.72 % for PM6 : Y6-based OSC and an outstanding efficiency of 18.17 % for PM6 : BTP-eC9-based OSC were achieved.

Publication date: 15 June 2022

Source: Nano Energy, Volume 97

Author(s): Yu-Jin Kang, Seok-In Na

Energy harvesting by indoor photovoltaics is a very promising candidate to supply the energy for the demand of billions of internet-of-things devices. The indium-tin-oxide-free organic photovoltaic modules presented herein show great potential to fulfill these needs. Their large parallel resistance enables use cases at ultralow intensities of 50 lux, which is exceptional in this field.

The growing market of the internet-of-things (IoT) accelerates the development of indoor organic photovoltaics. Very high performances up to 29% are already achieved under illumination with white light-emitting diodes with intensities in the range of 500–1000 lux. However, topics relevant for mass production such as large area coating, use of nontoxic solvents, and inexpensive electrodes are hardly addressed thus far. Herein this work, an indium-tin-oxide-free device stack is presented, which shows very good performance with many novel absorber materials under both full sun light as well as indoor illumination. Furthermore, lab-scale cells are upscaled to small modules that can power typical IoT devices. This includes the transition from spin coating to slot-die coating as well as the use of green solvents. Further, as the parallel resistance (R _P) is crucial for the performance under low illumination intensities, a detailed analysis is carried out and it is found that a one-diode model with the R _P being determined from the dark current–voltage characteristics reproduces the behavior quite accurately. Efficiencies of 19.3% on cell level and above 17% on an 8.1 cm² module with eight interconnected cells are achieved under 500 lux cold white light-emitting diode illumination.

Oxidized poly(4-vinylpyridine) (O-P4VP) as an interfacial passivation layer of perovskite solar cells improves power conversion efficiencies to 21.11% and environmental stability. Based on metal–pyridine complexation, defect passivation of SnO₂ and increase in the grain size of perovskite are achieved simultaneously. The insoluble passivation layer in perovskite solutions also contributes to the fabrication of highly reproducible devices.

Interfacial engineering of perovskite solar cells (PSCs) has been a core process for enhancing their power conversion efficiencies (PCEs) and environmental stability. Particularly, polymeric passivation of a metal oxide–electron transport layer (ETL) not only leads to a reduction in its surface defects but also improves the morphology of perovskite. However, the dissolution of the passivation layers by dimethylformamide (DMF) in a perovskite solution can cause a significant drop in the PCE and reproducibility of devices. Herein, oxidized poly(4-vinylpyridine) (O-P4VP) is used as a bifunctional passivation layer. The O-P4VP layers are completely insoluble in DMF, which accompanies the highly reproducible deposition of perovskites without damaging the layers. Furthermore, based on metal–pyridine complexation, oxygen vacancies on the surface of the SnO₂ ETLs are passivated with the O-P4VP layers, and the perovskite grains become enlarged by the controlled nucleation and growth rate. The synergetic effects of interfacial passivation present deeper energy levels of the ETL and a prolonged photoluminescence lifetime of the perovskites. The resulting PCE jumps from 19.05% to 21.11% with respect to the pristine device, and the value is retained for 720 h under 1 sun illumination.

Selenium-incorporated organic conjugated materials show enhanced intermolecular interactions and reduced bandgap, which can not only improve the carrier mobility and passivate the traps on the perovskite surface for PVSCs, but also enhance the light-harvesting ability in NIR region and achieve a high J _sc for OSCs.

Organic conjugated materials play an extremely important role in the development of perovskite and organic solar cells (PVSCs and OSCs). Among different molecular design strategies, the introduction of selenium is an efficient way to optimize material properties and improve the performance of solar cells. The benefits can be attributed to the looser electron cloud delocalization and more polarizable nature of selenium, which help enable stronger intermolecular Se···Se interaction and extended conjugation length, leading to enhanced charge carrier mobility and redshifted absorption in selenium-incorporated organic conjugated materials. Herein, the design of the selenium-incorporated materials applied in the PVSCs and OSCs to provide a systematic study on the relationship between chemical structures, material properties, and device performance is focused on. The future direction for further development of selenium-incorporated organic conjugated materials is also provided.

Phenylethylammonium iodide (PEAI) treatment results in the formation of 2D PEA₂PbI₄ capping layer on 3D perovskite which passivates defects and increases hydrophobicity, but hinders electron collection. The ambient exposure downshifts energy levels facilitating charge collection, improving efficiency and stability. Energy-level alignment with PEAI treatment is also affected by hole transport layer (HTL)/perovskite interface modification, which facilitates favorable alignment with PEA₂PbI₄.

Perovskite solar cells (PSCs) are known to be sensitive to the exposure to ambient humidity, which typically results in the degradation and deterioration of performance, although positive effects of exposure to moisture have also been reported, due to recrystallization of the perovskite. Common approach to improve stability is to use 3D/2D perovskite active layer, where 2D capping layer is prepared by spin coating the bulky organic cation halide. Herein, it is shown that optimizing the exposure of the capping layer prepared by spin coating phenylethylammonium iodide (PEAI) to ambient atmosphere results in substantial improvement of the PSC performance. Furthermore, the initial effects of PEAI treatment are dependent on the NiO _x /perovskite interface, but in all cases except at very high humidity (80–85% RH) optimized exposure to ambient results in improved performance. The variations in device performance with PEAI treatment and ambient exposure can be attributed to defect passivation and changes in the charge extraction due to energy-level alignment at the interfaces. The best performing devices have passivation of NiO _x /perovskite interface and PEAI treatment of top surface followed by exposure to ambient atmosphere at RH of 40–45%, which results in the power conversion efficiency increase from 20.3% to 22.4%.

The pseudo-planar heterojunction strategy is an efficient strategy to improving average visible transmittance (AVT) and power conversion efficiency (PCE) values of semitransparent organic solar cells (ST-OSCs) simultaneously due to the reduction of the optical loss, and the semitransparent devices afford a highest efficiency of 14.62% with a considerable AVT of 20.42%.

Abstract

The existing conformation of the active layer is defective for employment of semitransparent organic solar cells (ST-OSCs) in solar windows. Herein, scalable solar windows are successfully printed by introducing a pseudo-planar heterojunction (PPHJ) structure. The PPHJ structure can effectively improve the average visible transmittance (AVT) value while boosting the power conversion efficiency (PCE) of semitransparent devices due to the reduced optical loss. The universality of the PPHJ structure in the preparation of ST-OSCs is proved. Furthermore, an inset of a superhydrophobic patterned soft insertion layer (PSIL) in the encapsulated window improves the waterproof performance without losing transparency. Accordingly, the semitransparent devices based on the 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6) system afford a maximal efficiency of 14.62%, with a considerable AVT of 20.42%, and the resultant solar windows achieve a stabilized efficiency of 13.34% with excellent waterproof performance. Moreover, the PCE of the unilateral broken solar windows retains 70.6% of the initial efficiency after being placed under simulated rainfall conditions for 1200 h at room temperature.

Publication date: 15 June 2022

Source: Nano Energy, Volume 97

Author(s): Yi-Ran Shi, Kai-Li Wang, Yan-Hui Lou, Ding-Bo Zhang, Chun-Hao Chen, Jing Chen, Yu-Xiang Ni, Senol Öz, Zhao-Kui Wang, Liang-Sheng Liao

Publication date: Available online 29 March 2022

Source: Chem

Author(s): Xiaoming Wen, Baohua Jia