Chen Weijie

Sn‐based perovskite solar cells (PSCs) with 6.33% power conversion efficiency are fabricated with an aperture area of 1 cm² by introducing a post‐deposition vapor annealing method. The fabricated Sn‐based PSCs show promising stability, both under dark and maximum power‐point tracking conditions.

Sn‐based perovskite solar cells (PSCs) are promising alternatives to replacing toxic Pb‐based PSCs, which have shown a rapid rise in photovoltaic applications in the past 1 year. However, the reported Sn‐based PSCs are often fabricated with a small aperture area (typically 0.02–0.1 cm²) because forming homogeneous pinhole‐free continuous films over a large surface area is still challenging. Herein, a post‐deposition vapor annealing (PDVA) process assisted by methylammonium chloride vapor is presented that enables the fabrication of stable, homogeneous pinhole‐free FASnI₃ perovskite absorber films with low crystal defects and low surface recombination over a relatively large area up to 1.02 cm². Inverted planar solar cells fabricated with a 1.02 cm² aperture area show a maximum power conversion efficiency of 6.33% with high reproducibility and stability. The shelf‐lifetime stability test shows that the PSCs retain 90% of their performance for more than 1000 h when stored in a N₂‐filled glove box and under dark conditions. The preliminary light‐soaking stability tests under continuous illumination and maximum power‐tracking conditions are relatively promising. This study marks an important step toward the up scaling of Sn‐based PSCs.

High‐performance perovskite solar cells with an average power conversion efficiency of 21.4% are achieved based on mixed 2D/3D perovskites with induced phenylethylammonium iodide and exhibit an ultrahigh fill factor (83.6%). The unencapsulated device exhibits enhanced operational stability under continuous simulated sunlight illumination and outstanding air stability after 1000 h of storage under ambient air conditions.

2D/3D perovskite heterostructures or composites are recognized as efficient strategies to improve the stability of perovskite solar cells. Herein, a novel solution process to develop 2D/3D perovskites with modulated diffusion passivation by introducing phenylethylammonium iodide (PEAI) and N,N‐dimethylformamide (DMF) additive, which could effectively enhance device performance and long‐term stability, is demonstrated. Compared with conventional devices, the device with PEAI and DMF solvent additive treatment exhibit enhanced charge transport, improved charge extraction, and suppressed nonradiative carrier recombination. The solar cells with an optimal 2D/3D perovskite passivation treatment exhibit an extremely high fill factor of 83.6% and an average power conversion efficiency of 21.4% (21.3% using integrated photocurrent from the incident photon‐to‐current efficiency spectra) based on the NiO _x hole transport layer. Furthermore, the unencapsulated device exhibits excellent stability under continuously simulated sunlight illumination and outstanding air stability after 1000 h of storage under ambient air conditions.

A novel method to synthesize an electron‐rich building block cyclopentadithienothiophene (CDTT) via a facile aromatic extension strategy is demonstrated and a promising nonfullerene small molecule acceptor (CDTTIC) is synthesized. The CDTTIC‐based as‐cast single‐junction organic solar cells exhibit efficiencies over 11% with an ultrahigh current density.

Abstract

A new method to synthesize an electron‐rich building block cyclopentadithienothiophene (9H‐thieno‐[3,2‐b]thieno[2″,3″:4′,5′]thieno[2′,3′:3,4]cyclopenta[1,2‐d]thiophene, CDTT) via a facile aromatic extension strategy is reported. By combining CDTT with 1,1‐dicyanomethylene‐3‐indanone endgroups, a promising nonfullerene small molecule acceptor (CDTTIC) is prepared. As‐cast, single‐junction nonfullerene organic solar cells based on PFBDB‐T: CDTTIC blends exhibit very high short‐circuit currents up to 26.2 mA cm⁻² in combination with power conversion efficiencies over 11% without any additional processing treatments. The high photocurrent results from the near‐infrared absorption of the CDTTIC acceptor and the well‐intermixed blend morphology of polymer donor PFBDB‐T and CDTTIC. This work demonstrates a useful fused ring extension strategy and promising solar cell results, indicating the great potential of the CDTT derivatives as electron‐rich building blocks for constructing high‐performance small molecule acceptors in organic solar cells.

1,2,4‐triazole is a stable and efficient aromatic compound with a triamine structure that can improve the bond strength and electronic properties of perovskite with reduced carrier traps. Proper alloying of 1,2,4‐triazole greatly stabilizes triple‐cation perovskite, allowing extremely high stability under 85 °C/85% relative humidity for 700 h and a high power conversion efficiency of 20.9% with spiro‐OMeTAD as a hole‐transporting material.

Abstract

Operational stability of perovskite solar cells has been a challenge from the beginning of perovskite research. In general, humidity and heat are the most well‐known degradation sources for perovskites, requiring ideal design of perovskite chemistry to withstand them. Although triple‐cation perovskite (Cs_0.05(FA_0.85MA_0.15)_0.95Pb(I_0.85Br_0.15)₃) has been already introduced as the stable perovskite material, the high reactivity of methylammonium and formamidinium in the cation sites demands further modification. Herein, 1,2,4‐triazole is suggested as an effective cation solute to improve the performance and stability of perovskite solar cells. 1,2,4‐Triazole is an aromatic cation with low dipole moment that is stable under humidity and heat. It also possesses three nitrogen atoms, forming additional hydrogen bonds in the lattice, stabilizing the material. In this study, the solar cell utilizing 1,2,4‐triazole alloying achieves a power conversion efficiency of 20.9% with superior stability under extreme condition (85 °C/85% of relative humidity (RH), encapsulated) for 700 h. The 1,2,4‐triazole‐alloyed perovskite exhibits reduced trap density and film roughness and enhanced carrier lifetime with electrical conductivity, suggesting an ideal perovskite structure for efficient and stable optoelectronic applications.

Eco‐compatible solvent‐processed organic photovoltaic cells with over 16% power conversion efficiency are achieved via modifying the flexible alkyl chains of BTP‐4F‐8. Combining with the polymer donor T1, over 14% power conversion efficiencies are obtained not only for using several kinds of greener solvents like o‐xylene, 1,2,4‐trimethylbenzene, and tetrahydrofuran but also for 1.07 cm² cells by the blade‐coating method.

Abstract

Recent advances in nonfullerene acceptors (NFAs) have enabled the rapid increase in power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells. However, this progress is achieved using highly toxic solvents, which are not suitable for the scalable large‐area processing method, becoming one of the biggest factors hindering the mass production and commercial applications of OPVs. Therefore, it is of great importance to get good eco‐compatible processability when designing efficient OPV materials. Here, to achieve high efficiency and good processability of the NFAs in eco‐compatible solvents, the flexible alkyl chains of the highly efficient NFA BTP‐4F‐8 (also known as Y6) are modified and BTP‐4F‐12 is synthesized. Combining with the polymer donor PBDB‐TF, BTP‐4F‐12 shows the best PCE of 16.4%. Importantly, when the polymer donor PBDB‐TF is replaced by T1 with better solubility, various eco‐compatible solvents can be applied to fabricate OPV cells. Finally, over 14% efficiency is obtained with tetrahydrofuran (THF) as the processing solvent for 1.07 cm² OPV cells by the blade‐coating method. These results indicate that the simple modification of the side chain can be used to tune the processability of active layer materials and thus make it more applicable for the mass production with environmentally benign solvents.

Publication date: October 2019

Source: Nano Energy, Volume 64

Author(s): Xiaolong Yang, Jun Xi, Yuanhui Sun, Yindi Zhang, Guijiang Zhou, Wai-Yeung Wong

Graphical abstract

The twisted organic small-molecule hole transport material XY1 exhibits advantages such as good film morphology, strong absorption in the UV range but high transmittance in the visible and near-infrared range, high hole mobility, and perfect energy level alignment with the perovskite layer. Consequently, both small- and large-area inverted perovskite solar cells based on the dopant-free XY1 show state-of-the-art photovoltaic performance.

Small‐molecule electrolyte (C6‐E‐OTs) hybridized ZnO layer is provided as the electron transporting layer. The device based on the blend of PTB7 and PC₇₁BM as the active layer shows an enhanced power conversion efficiency (PCE) from 7.6% based on ZnO to 8.8% using the C6‐E‐OTs hybridized ZnO layer. Hybridized ZnO layer process can overcome the limitation faced by the thickness tolerance of interlayer.

Abstract

Small‐molecule electrolyte (C6‐E‐OTs) hybridized ZnO layer is provided as the electron transporting layer. The device based on the blend of PTB7 and PC₇₁BM as the active layer shows an enhanced power conversion efficiency (PCE) from 7.6% based on ZnO to 8.8% using the C6‐E‐OTs hybridized ZnO layer. The device can be further improved by simultaneously using a C6‐E‐OTs hybridized ZnO layer and a 5 nm thick C6‐E‐OTs as the interlayer. The synergy effect of hybridization and interlayer enhanced the PCE of the device to 8.9%, which is a 17.1% increase in comparison with the device based on ZnO. The presence of C6‐E‐OTs hybridized ZnO and a 5 nm of C6‐E‐OTs as the interlayer in the device with PTB7‐Th as the donor significantly improves the PCE from 8.0% based on ZnO to 9.4%, resulting in a 17.5% enhancement. Main contribution for enhancing the PCE of the device is the improved J _sc, which results from the reduction of energy offset at the cathode interface. Thus, hybridized ZnO layer process can overcome the limitation faced by the thickness tolerance of interlayer.

Nickel oxide (NiO _x )‐based carrier‐selective contact Ag/ITO/NiO _x /n‐Si/LiF _x /Al solar cells are fabricated. The highest reported power conversion efficiency of ≈15.20% is achieved with a chemically grown SiO _x passivation layer on silicon in comparison with devices without SiO _x (efficiency ≈12.43%). Devices are analyzed systematically for performance enhancement with SiO _x and evidence for contact‐resistivity, minority‐carriers' lifetimes/diffusion‐lengths, recombination‐resistance, and density of interface‐defect‐states at the NiO _x /n‐Si interface is provided.

Carrier‐selective contact‐based silicon heterojunction solar cells are fabricated using nickel oxide (NiO _x ) as a hole‐selective layer by thermal evaporation. The highest power conversion efficiency of ≈15.20% with a chemically grown SiO _x interlayer is achieved from a Ag/ITO/NiO _x /n‐Si/LiF _x /Al cell structure in comparison with ≈12.43% without SiO _x . The cells without and with the SiO _x layer are analyzed by considering crucial parameters for conversion efficiency, such as minority carriers' diffusion lengths, lifetimes, recombination resistance, and density of interface defect states at the NiO _x /n‐Si junction, by studying the dark/light current density–voltage, quantum efficiency, impedance, and parallel conductance characteristics. Device analysis provides evidence for the cell's open‐circuit voltage and short‐circuit current enhancement with the SiO _x interlayer. This is due to an improvement in minority carrier lifetimes from ≈8.6 to ≈48.27 μs (photo‐conductance decay analysis), which is also estimated from ≈7.45 to ≈49.20 μs by impedance spectra analysis, increased minority carrier diffusion length from ≈171 to ≈952 μm, and decreased rear surface recombination velocity from ≈1106 to ≈170 cm s⁻¹ (quantum efficiency analysis). These investigations reveal that engineering the n‐Si/LiF _x interface by the SiO _x interlayer is more important than the NiO _x /n‐Si interface because of a thin unintentionally grown SiO _x layer during NiO _x evaporation simultaneously mediating silicon surface passivation.

Metal oxides (MO) with unique optoelectronic properties and outstanding stability are increasingly developed as effective electron transport layers (ETLs) for perovskite solar cells (PSCs). This Review focuses on the recent advances of MO ETLs from systematical synthesis to strategical optimization and provides feasible directions for future development of MO ETLs in higher‐performing PSCs.

Abstract

Organometallic mixed halide perovskite solar cells (PSCs) have emerged as a promising photovoltaic technology with increasingly improved device efficiency exceeding 24%. Charge transport layers, especially electron transport layers (ETLs), are verified to play a vital role in device performance and stability. Recently, metal oxides (MOs) have been widely studied as ETLs for high‐performance PSCs due to their excellent electronic properties, superb versatility, and great stability. This Review briefly discusses the development of PSCs' architecture and outlines the requirements for MO ETLs. Additionally, recent progress of MO ETLs from preparation to optimization for efficient PSCs is systematically summarized and highlighted to associate the versatility of MO ETLs with the performance of devices. Finally, a summary and prospectives for the future development of MO ETLs toward practical application of high‐performance PSCs are drawn.

A multipurpose interconnection layer for the fabrication of monolithic perovskite/silicon tandem solar cells with high power conversion efficiency is explored. The interconnection of independently processed silicon and perovskite subcells could be a simple add‐on lamination step, alleviating the common fabrication complexity of perovskite/silicon tandem devices.

Abstract

A multipurpose interconnection layer based on poly(3,4‐ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS), and d‐sorbitol for monolithic perovskite/silicon tandem solar cells is introduced. The interconnection of independently processed silicon and perovskite subcells is a simple add‐on lamination step, alleviating common fabrication complexities of tandem devices. It is demonstrated experimentally and theoretically that PEDOT:PSS is an ideal building block for manipulating the mechanical and electrical functionality of the charge recombination layer by controlling the microstructure on the nano‐ and mesoscale. It is elucidated that the optimal functionality of the recombination layer relies on a gradient in the d‐sorbitol dopant distribution that modulates the orientation of PEDOT across the PEDOT:PSS film. Using this modified PEDOT:PSS composite, a monolithic two‐terminal perovskite/silicon tandem solar cell with a steady‐state efficiency of 21.0%, a fill factor of 80.4%, and negligible open circuit voltage losses compared to single‐junction devices is shown. The versatility of this approach is further validated by presenting a laminated two‐terminal monolithic perovskite/organic tandem solar cell with 11.7% power conversion efficiency. It is envisioned that this lamination concept can be applied for the pairing of multiple photovoltaic and other thin film technologies, creating a universal platform that facilitates mass production of tandem devices with high efficiency.

Temperature‐dependent X‐ray diffraction, absorption and photoluminescence measurements on methylammonium lead iodide thin films with grain sizes ranging from the micrometer to the tens of nanometer scale reveal that the low‐temperature phase transition is increasingly suppressed with decreasing grain size. These results unveil the remarkable sensitivity of optoelectronic and structural properties to the local environment in perovskite thin films.

Abstract

Grain size in polycrystalline halide perovskite films is known to have an impact on the optoelectronic properties of the films, but its influence on their soft structural properties and phase transitions is unclear. Here, temperature‐dependent X‐ray diffraction, absorption, and macro‐ and micro‐photoluminescence measurements are used to investigate the tetragonal to orthorhombic phase transition in thin methylammonium lead iodide films with grain sizes ranging from the micrometer scale down to the tens of nanometer scale. It is shown that the phase transition nominally at ≈150 K is increasingly suppressed with decreasing grain size and, in the smallest grains, the first evidence of a phase transition is only seen at temperatures as low as ≈80 K. With decreasing grain size, an increasing magnitude of the hysteresis is also seen in the structural and optoelectronic properties when cooling to, and then upon heating from, 100 K. This work reveals the remarkable sensitivity of the optoelectronic, physical, and phase properties to the local environment of the perovskite structure, which will have large ramifications for phase and defect engineering in operating devices.

With the synthesis of two novel hole transport materials, the inverted planar perovskite solar cell achieves a high fill factor of 81.7%, with an efficiency exceeding 19%. More importantly, a highly possible correlation between the molecular packing, hole mobility, and device performance is revealed, which provides some insights for the rational design of hole transport materials.

Abstract

In this paper, two novel D‐π‐D hole‐transporting materials (HTM) are reported, abbreviated as BDT‐PTZ and BDT‐POZ, which consist of 4,8‐di(hexylthio)‐benzo[1,2‐b:4,5‐b′]dithiophene (BDT) as π‐conjugated linker, and N‐(6‐bromohexyl) phenothiazine (PTZ)/N‐(6‐bromohexyl) phenoxazine (POZ) as donor units. The above two HTMs are deployed in p‐i‐n perovskite solar cells (PSCs) as dopant‐free HT layers, exhibiting excellent power conversion efficiencies of 18.26% and 19.16%, respectively. Particularly, BDT‐POZ demonstrates a superior fill factor of 81.7%, which is consistent with its more efficient hole extraction and transport verified via steady‐state/transient fluorescence spectra and space‐charge‐limited current technique. Single‐crystal X‐ray diffraction characterization implies these two molecules present diverse packing tendencies, which may account for various interfacial hole‐transport ability in PSCs.

A temperature‐tuned antisolvent bathing method is introduced for fabricating highly oriented and large‐grain perovskite thin films. Using large‐area compatible cold antisolvent bathing, a high‐quality perovskite film is obtained with a reduced defect density and an enhanced charge‐carrier extraction capability, which achieves a champion power‐conversion efficiency of 18.50%.

Abstract

Scaling large‐area solar cells is in high demand for the commercialization of perovskite solar cells (PSCs) with a high power‐conversion efficiency (PCE). However, few roll‐to‐roll‐compatible deposition methods for the formation of highly oriented uniform perovskite films are reported. Herein, a facile cold antisolvent bathing approach compatible with large‐area fabrication is introduced. The wet precursor films are submerged in a cold antisolvent bath at 0 °C, and the retarded nucleation and growth kinetics allow highly oriented perovskite to be grown along the [110] and [220] directions, perpendicular to the substrate. The high degree of the preferred crystal orientation benefits the effective charge extraction and reduces the amount of inter‐ and intra‐grain defects inside the perovskite films, improving the PCE from 16.48% (ambient‐bathed solar cell) to 18.50% (cold‐bathed counterpart). The cold antisolvent bathing method is employed for the fabrication of large‐area (8 × 10 cm²) PSCs with uniform photovoltaic device parameters, thereby verifying the scale‐up capability of the method.

A new method of depressing E _loss for nonfullerene organic solar cells (OSCs) is reported, in which a small molecular material (NRM‐1) can be selectively dispersed into the acceptor phase in the PBDB‐T:IT‐4F‐based OSC, resulting in lower Elossrad and Elossnonrad and hence significant improvement in V _OC, and under an optimal feed ratio of NRM‐1, an enhanced efficiency can be gained.

Abstract

Reducing energy loss (E _loss) is of critical importance to improving the photovoltaic performance of organic solar cells (OSCs). Although nonradiative recombination (Elossnonrad) is investigated in quite a few works, the method for modulating Elossnonrad is seldom reported. Here, a new method of depressing E _loss is reported for nonfullerene OSCs. In addition to ternary‐blend bulk heterojunction (BHJ) solar cells, it is proved that a small molecular material (NRM‐1) can be selectively dispersed into the acceptor phase in the PBDB‐T:IT‐4F‐based OSC, resulting in lower Elossrad and Elossnonrad, and hence a significant improvement in the open‐circuit voltage (V _OC); under an optimal feed ratio of NRM‐1, an enhanced power conversion efficiency can also be gained. Moreover, the role of NRM‐1 in the method is illustrated and its applicability for several other representative OSCs is validated. This work paves a new pathway to reduce the E _loss for nonfullerene OSCs.

A major drawback of hybrid perovskites is the lack of long‐term stability. This is related to the degradation of organic cations. Light‐induced degradation of CH₃NH₃PbI₃ extends from ambient temperatures down to 5 K. Illumination of thin films and single crystals at T = 5 K causes the formation of localized states that can be annealed at T ≥ 15 K.

In a period of only a few years, the power conversion efficiency of organic–inorganic perovskite solar cells has surpassed a value of 24.2%. However, a major drawback is the lack of long‐term stability, which is partially related to the dissociation of organic cations under prolonged illumination. This degradation mechanism is not limited to ambient temperatures. At low temperatures (T = 5 K), illumination of methyl ammonium lead iodide (CH₃NH₃PbI₃) thin films with a photon energy of E _ph = 3.4 eV results in the formation of localized trap states located about 100 meV within the bandgap. These light‐induced defects are metastable, and annealing at T ≥ 15 K removes the localized states. Defect creation is not limited to polycrystalline perovskites but is also observed in single‐crystal CH₃NH₃PbI₃. The experimental data are discussed in terms of a two‐level model where the metastable state is separated from the annealed state by an energy barrier.

Herein, the fabrication of a high‐efficiency heterojunction solar cell with a tin sulfide (SnS) thin film formed on a nanostructured TiO₂ electrode by combining a solution process and rapid thermal annealing under Ar flow is reported. The secondary thermal treatment of the SnS thin film with SnCl₂ improves the efficiency by up to 5%.

Abstract

Tin sulfide (SnS) is one of the most promising solar cell materials, as it is abundant, environment friendly, available at low cost, and offers long‐term stability. However, the highest efficiency of the SnS solar cell reported so far remains at 4.36% even using the expensive atomic layer deposition process. This study reports on the fabrication of SnS solar cells by a solution process that employs rapid thermal treatment for few seconds under Ar gas flow after spin‐coating a precursor solution of SnCl₂ and thiourea dissolved in dimethylformamide onto a nanostructured thin TiO₂ electrode. The best‐performing cell exhibits power conversion efficiency (PCE) of 3.8% under 1 sun radiation conditions (AM1.5G). Moreover, secondary treatment using SnCl₂ results in a significant improvement of 4.8% in PCE, which is one of the highest efficiencies among SnS‐based solar cells, especially with TiO₂ electrodes. The thin film properties of SnS after SnCl₂ secondary treatment are analyzed using grazing‐incidence wide‐angle X‐ray scattering, and high‐resolution transmittance electron microscopy.

Stable and efficient perovskite solar cells (PSCs) are achieved via introducing PbPyA₂ as an additive. Benefiting from the strong interaction, incorporating PbPyA₂ can lower the defects, suppress ion migration and component volatilization of perovskite, resulting in great improvements in heat and humidity tolerance. More importantly, the resulting PSC maintains 93% of initial efficiency after maximum power point tracking for 540 h.

Abstract

Stability has become the main obstacle for the commercialization of perovskite solar cells (PSCs) despite the impressive power conversion efficiency (PCE). Poor crystallization and ion migration of perovskite are the major origins of its degradation under working condition. Here, high‐performance PSCs incorporated with pyridine‐2‐carboxylic lead salt (PbPyA₂) are fabricated. The pyridine and carboxyl groups on PbPyA₂ can not only control crystallization but also passivate grain boundaries (GBs), which result in the high‐quality perovskite film with larger grains and fewer defects. In addition, the strong interaction among the hydrophobic PbPyA₂ molecules and perovskite GBs acts as barriers to ion migration and component volatilization when exposed to external stresses. Consequently, superior optoelectronic perovskite films with improved thermal and moisture stability are obtained. The resulting device shows a champion efficiency of 19.96% with negligible hysteresis. Furthermore, thermal (90 °C) and moisture (RH 40–60%) stability are improved threefold, maintaining 80% of initial efficiency after aging for 480 h. More importantly, the doped device exhibits extraordinary improvement of operational stability and remains 93% of initial efficiency under maximum power point (MPP) tracking for 540 h.