Chen Weijie

A simple yet highly compatible interconnecting layer for organic tandem solar cell is presented. All double‐junction tandem devices with different active layers show high reproducibility and efficiencies in several laboratories. Among these tandem devices, an excellent PCE of 16.1% is achieved. In addition, most of the tandem devices achieve more than 40% enhancement from the single‐junction organic photovoltaic device.

Abstract

Tandem structure provides a practical way to realize high efficiency organic photovoltaic cells, it can be used to extend the wavelength coverage for light harvesting. The interconnecting layer (ICL) between subcells plays a critical role in the reproducibility and performance of tandem solar cells, yet the processability of the ICL has been a challenge. In this work the fabrication of highly reproducible and efficient tandem solar cells by employing a commercially available material, PEDOT:PSS HTL Solar (HSolar), as the hole transporting material used for the ICL is reported. Comparing with the conventional PEDOT:PSS Al 4083 (c‐PEDOT), HSolar offers a better wettability on the underlying nonfullerene photoactive layers, resulting in better charge extraction properties of the ICL. When FTAZ:IT‐M and PTB7‐Th:IEICO‐4F are used as the subcells, a power conversion efficiency (PCE) of 14.7% is achieved in the tandem solar cell. To validate the processability of these tandem solar cells, three other research groups have successfully fabricated tandem devices using the same recipe and the highest PCE obtained is 16.1%. With further development of donor polymers and device optimization, the device simulation results show that a PCE > 22% can be realized in tandem cells in the near future.

The desired α‐FAPbI₃ perovskite phase is stabilized by protecting it with a two‐dimensional (2D) IBA₂FAPb₂I₇ (IBA=iso‐butylammonium) overlayer, formed via stepwise annealing. The α‐FAPbI₃/IBA₂FAPb₂I₇‐based perovskite solar cell (PSC) reached a high power conversion efficiency (PCE) of close to 23 %. It showed excellent operational stability, retaining around 85 % of its initial efficiency under severe combined heat and light stress.

Abstract

As a result of their attractive optoelectronic properties, metal halide APbI₃ perovskites employing formamidinium (FA⁺) as the A cation are the focus of research. The superior chemical and thermal stability of FA⁺ cations makes α‐FAPbI₃ more suitable for solar‐cell applications than methylammonium lead iodide (MAPbI₃). However, its spontaneous conversion into the yellow non‐perovskite phase (δ‐FAPbI₃) under ambient conditions poses a serious challenge for practical applications. Herein, we report on the stabilization of the desired α‐FAPbI₃ perovskite phase by protecting it with a two‐dimensional (2D) IBA₂FAPb₂I₇ (IBA=iso‐butylammonium overlayer, formed via stepwise annealing. The α‐FAPbI₃/IBA₂FAPb₂I₇ based perovskite solar cell (PSC) reached a high power conversion efficiency (PCE) of close to 23 %. In addition, it showed excellent operational stability, retaining around 85 % of its initial efficiency under severe combined heat and light stress, that is, simultaneous exposure with maximum power tracking to full simulated sunlight at 80 °C over 500 h.

Interface energetics in 2D Ruddlesden–Popper perovskite solar cells are systematically investigated. The potential gradient across ligands that significantly decreases surface work function, promotes separation of the photogenerated charge carriers with electron transferring from perovskite crystal to ligand at the interface, suppressing the charge recombination and thus enhancing the open‐circuit voltage.

Abstract

2D Ruddlesden–Popper perovskites (RPPs) are emerging as potential challengers to their 3D counterpart due to superior stability and competitive efficiency. However, the fundamental questions on energetics of the 2D RPPs are not well understood. Here, the energetics at (PEA)₂(MA) _{n −1}Pb _n I_{3 n +1}/[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) interfaces with varying n values of 1, 3, 5, 40, and ∞ are systematically investigated. It is found that n–n junctions form at the 2D RPP interfaces (n = 3, 5, and 40), instead of p–n junctions in the pure 2D and 3D scenarios (n = 1 and ∞). The potential gradient across phenethylammonium iodide ligands that significantly decreases surface work function, promotes separation of the photogenerated charge carriers with electron transferring from perovskite crystal to ligand at the interface, reducing charge recombination, which contributes to the smallest energy loss and the highest open‐circuit voltage (V _oc) in the perovskite solar cells (PSCs) based on the 2D RPP (n = 5)/PCBM. The mechanism is further verified by inserting a thin 2D RPP capping layer between pure 3D perovskite and PCBM in PSCs, causing the V _oc to evidently increase by 94 mV. Capacitance–voltage measurements with Mott–Schottky analysis demonstrate that such V _oc improvement is attributed to the enhanced potential at the interface.

A highly‐transparent and true‐colored indoor light harvester should possess an absorbance, which is mismatched with the emission spectra of the light sources. Porphyrin‐based donor materials are potential candidates, and such a P2:PC71BM cell achieves a power conversion efficiencies (PCE) exceeding 10%, and an illuminance transparency ≈65%, while preserving the color perception of the light sources.

Abstract

Organic photovoltaic (OPV) cells are promising indoor light energy harvesters because organic materials absorb strongly in the visible range. An indoor photovoltaic (IPV) device is an effective tool for the remote off‐grid wireless charging. However, as the indoor light fluxes are much weaker than the 1‐Sun condition, high‐performance indoor cells should have large areas in order to generate appreciable energies. They would then appear as flat, but expansive and dark objects if deployed indoors. Their presence would then alter the indoor lighting environment and affect visual perceptions. This work addresses the lighting and perception issues of IPV cells in three ways. i) A concept is proposed such that a high‐efficiency, semitransparent indoor OPV cell should possess an absorbance which is mismatched with the emission spectra of the light sources. ii) It is demonstrated that bulk heterojunction (BHJ) OPV solar cells with porphyrin donors can serve as high‐transparency and high‐efficiency indoor light harvesters. iii) Quantitative assessment criteria are presented for the transparency and chromaticity of an indoor semitransparent OPV cell, demonstrating that porphyrin‐based P2:PC₇₁BM semitransparent BHJ cells can achieve a power conversion efficiency (PCE) exceeding 10%, and an illuminance transparency ~65%, while preserving the color perception of the light sources.

This study considers the critical functions of the interfacial layer in overcoming the various degradations processed at the interfaces of organic–inorganic hybrid perovskites photovoltaics (PePVs). The mechanisms can also be categorized according to the cause of the degradation. The stabilizing processes that attenuate the corresponding degrading process are studied with respect to organic or inorganic interfacial materials in PePVs.

Abstract

Organic–inorganic hybrid perovskites have excellent optical and electronic properties; exploitation of these traits has increased the power conversion efficiency of perovskite photovoltaics (PePVs) to 25.2%. However, perovskites are chemically unstable, and this deficit has critically impeded their commercialization. Device degradation occurs at the interfaces of PePVs with multiple degradation mechanisms: decomposition of organic cations in perovskites; generation of inorganic byproducts in perovskites; superoxide or trap sites at the interface of the charge‐transport layer; excess charge carriers in perovskites; interfacial migration between perovskites and electrodes. This review considers the critical functions of the interfacial materials to overcome the various degradation at the interfaces of the PePVs. The working mechanisms stabilizing the interface of PePVs are categorized: passivation from atmosphere; inactivation of defect states; migration‐blocking. Then, the outstanding interfacial layers made of organic materials (defect passivation, physical robustness, and chemical inactivation) and inorganic materials (chemically passivating metal oxide, physically passivating metal oxide, and low‐temperature processed inorganic materials) are reviewed according to the stabilizing mechanisms. In addition, the influences of inorganic interconnecting layers in tandem PePVs are reviewed, with respect of various effects of interfacial buffer materials at the interface with perovskites.

Asymmetric electron acceptors, by combining halogenated indandione and 3‐dicyanomethylene‐1‐indanone as two different conjugated terminals, are designed and synthesized. Such design enables reduced energy loss and boosts charge separation, thus leading to 16.37% binary organic photovoltaics (OPVs) and 17.43% ternary OPVs, which are among the best efficiencies for single‐junction OPVs.

Abstract

Low energy loss and efficient charge separation under small driving forces are the prerequisites for realizing high power conversion efficiency (PCE) in organic photovoltaics (OPVs). Here, a new molecular design of nonfullerene acceptors (NFAs) is proposed to address above two issues simultaneously by introducing asymmetric terminals. Two NFAs, BTP‐S1 and BTP‐S2, are constructed by introducing halogenated indandione (A₁) and 3‐dicyanomethylene‐1‐indanone (A₂) as two different conjugated terminals on the central fused core (D), wherein they share the same backbone as well‐known NFA Y6, but at different terminals. Such asymmetric NFAs with A₁‐D‐A₂ structure exhibit superior photovoltaic properties when blended with polymer donor PM6. Energy loss analysis reveals that asymmetric molecule BTP‐S2 with six chlorine atoms attached at the terminals enables the corresponding devices to give an outstanding electroluminescence quantum efficiency of 2.3 × 10⁻²%, one order of magnitude higher than devices based on symmetric Y6 (4.4 × 10⁻³%), thus significantly lowering the nonradiative loss and energy loss of the corresponding devices. Besides, asymmetric BTP‐S1 and BTP‐S2 with multiple halogen atoms at the terminals exhibit fast hole transfer to the donor PM6. As a result, OPVs based on the PM6:BTP‐S2 blend realize a PCE of 16.37%, higher than that (15.79%) of PM6:Y6‐based OPVs. A further optimization of the ternary blend (PM6:Y6:BTP‐S2) results in a best PCE of 17.43%, which is among the highest efficiencies for single‐junction OPVs. This work provides an effective approach to simultaneously lower the energy loss and promote the charge separation of OPVs by molecular design strategy.

A new spiro derivative, SPS‐4F, is designed and synthesized as a nonfullerene electron transport material in perovskite solar cells. An efficiency of 20.31% and high device stability are simultaneously achieved in the resultant devices. This work opens up opportunities to obtain a new family of spiro‐based electron transport materials and paves a way for realizing high‐performance devices with low cost.

Abstract

Electron transport materials (ETMs) play a significant role in perovskite solar cells (PSCs). However, conventional solution processable organic ETMs are mainly restricted to fullerene derivatives and it is challenging to obtain nonfullerene ETMs with satisfactory properties. In this work, a new organic semiconductor SPS‐4F is synthesized by utilizing the classical spiro[fluorine‐9′9‐thioxanthene] unit to construct a π‐extended core. Although spiro is normally used in hole transport materials, the new spiro derivative SPS‐4F is successfully used as an ETM in inverted PSCs with power conversion efficiency over 20%. In addition, SPS‐4F can strongly coordinate with MAPbI₃ perovskite and lead to efficient surface trap passivation. The resultant PSCs exhibit excellent stability in air because of the hydrophobic property of SPS‐4F. This work opens up opportunities to obtain a new family of ETMs based on spiro and paves a way to the fabrication of high‐performance PSCs with low cost.

Publication date: 17 June 2020

Source: Joule, Volume 4, Issue 6

Author(s): Zhenghui Luo, Ruijie Ma, Tao Liu, Jianwei Yu, Yiqun Xiao, Rui Sun, Guanshui Xie, Jun Yuan, Yuzhong Chen, Kai Chen, Gaoda Chai, Huiliang Sun, Jie Min, Jian Zhang, Yingping Zou, Chuluo Yang, Xinhui Lu, Feng Gao, He Yan

The incorporation of potassium can remarkably stabilize wide‐bandgap perovskites with a high Br content by the synergistic effect of the formation of 2D K₂PbI₄ at the grain boundaries and the interstitial occupancy in the perovskite lattices, which can effectively reduce the trap density and inhibit ion migration, thus suppressing the nonradiative recombination and photoinduced phase segregation.

Wide‐bandgap perovskites have great potential to enable high‐efficiency tandem photovoltaics by combining with the well‐established low‐bandgap absorbers. However, such wide‐bandgap perovskites are often necessarily constructed with a high Br content, and thus faced with issues of phase segregation–induced photoinstability and high defect density, severely hindering their photovoltaic performance. Herein, a remarkable boost of the stability and efficiency of wide‐bandgap perovskite solar cells (PSCs) is demonstrated by simply incorporating potassium ions. Experiments have shown the interstitial occupancy of potassium ions in the perovskite lattice and the formation of 2D K₂PbI₄ at the grain boundaries, both can reduce the trap density and inhibit ion migration, and thus suppress nonradiative recombination and photoinduced phase segregation. The average power conversion efficiency (PCE) of photovoltaic devices based on the perovskite with 40% Br is improved from 15.28% to 17.94%, among which the champion efficiency is 18.38% with an optimal 15% KI incorporation. Importantly, the champion open‐circuit voltage (V _oc) remains unchanged (≈1.25 V) even when the bandgap reduces from 1.80 to 1.75 eV due to KI doping, effectively reducing the V _oc deficit. In addition, the unencapsulated cells can sustain 94% of the initial PCE after 2000 h of storage in ambient atmosphere, affirming their outstanding stability.

A novel non‐conjugated polymer based on the polyethylene backbone, PVCz‐OMeTPA, with suitable energy levels, good hole mobility, as well as excellent film‐forming ability is developed as an efficient dopant‐free hole‐transporting material (HTMs) for inverted quasi‐2D perovskite solar cells (PSCs). Quasi‐2D PSCs using the dopant‐free PVCz‐OMeTPA as HTM exhibit an excellent power conversion efficiency of 17.22% and long‐term environmental stability.

Quasi‐2D perovskites with excellent stability have been recognized as an alternative to 3D counterparts for perovskite solar cells (PSCs). Although the power conversion efficiency (PCE) of quasi‐2D PSCs has increased over 18% by the compositional controlling and solvent engineering of perovskites, fewer studies have been conducted to exploit charge transport layers and investigate their interface relationships with quasi‐2D perovskites. To achieve high efficiency and good long‐term stability for quasi‐2D PSCs, hole‐transporting materials (HTMs) with matched energy levels and good chemical compatibility with quasi‐2D perovskites are explored and investigated. Herein, a novel non‐conjugated polymer based on polyethylene backbone, poly[3,6‐(4,4′‐dimethoxytriphenylamino)‐9‐vinyl‐9H‐carbazole] (PVCz‐OMeTPA), is easily synthesized and investigated as a promising dopant‐free HTM for quasi‐2D PSCs. Due to its more suitable energy levels, good hole mobility, as well as excellent film‐forming ability to assist the formation of high‐quality quasi‐2D perovskite films, the optimized p–i–n structured quasi‐2D PSCs based on PVCz‐OMeTPA exhibit the best PCE of 17.22%. The unencapsulated quasi‐2D PSCs based on PVCz‐OMeTPA maintain 82% of the initial efficiency after 1400 h under a relative humidity of ≈40% and sustain over 81% of the original efficiency after aging for 600 h upon 70 °C of continuous annealing.

High‐efficiency monolithic silicon‐based tandem solar cells require an adapted bandgap of the top cell. The perovskite composition FA_0.75Cs_0.25Pb(I_0.8Br_0.2)₃ has a theoretically optimal bandgap of 1.68 eV. Implementation in p–i–n tandem devices gives highest certified efficiency of 25.1%, whereas a substantial efficiency increase is observed over time. By eliminating remaining interfacial and reflection losses, >30% efficiency is feasible.

Monolithic perovskite silicon tandem solar cells can overcome the theoretical efficiency limit of silicon solar cells. This requires an optimum bandgap, high quantum efficiency, and high stability of the perovskite. Herein, a silicon heterojunction bottom cell is combined with a perovskite top cell, with an optimum bandgap of 1.68 eV in planar p–i–n tandem configuration. A methylammonium‐free FA_0.75Cs_0.25Pb(I_0.8Br_0.2)₃ perovskite with high Cs content is investigated for improved stability. A 10% molarity increase to 1.1 m of the perovskite precursor solution results in ≈75 nm thicker absorber layers and 0.7 mA cm⁻² higher short‐circuit current density. With the optimized absorber, tandem devices reach a high fill factor of 80% and up to 25.1% certified efficiency. The unencapsulated tandem device shows an efficiency improvement of 2.3% (absolute) over 5 months, showing the robustness of the absorber against degradation. Moreover, a photoluminescence quantum yield analysis reveals that with adapted charge transport materials and surface passivation, along with improved antireflection measures, the high bandgap perovskite absorber has the potential for 30% tandem efficiency in the near future.

The light absorption layer Cs_0.06FA_0.79MA_0.15Pb(I_0.85Br_0.15)₃ thin film is polished by femtosecond (fs) laser to make solar cells, achieving 1.6% power conversion efficiency (PCE) improvement compared with perovskite solar cells unpolished by fs laser. The ameliorated surface of perovskite films and reduced nonradiative recombination loss due to fs laser polishing are believed the keys for the PCE improvement.

Nonradiative recombination loss is a key process that determines the performance of perovskite solar cells, and how to control it is significant for the research and development of perovskites. Generally, traditional chemical modification/passivation methods are complicated and prone to secondary contamination. Herein, femtosecond (fs) laser polishing as a promising technique is demonstrated to ameliorate the surface of perovskite films, reduce nonradiative recombination loss, and improve solar cell performance. The high‐intensity fs laser pulses can remove around 20 nm‐thick perovskite top layer through an ionization process, help to decrease the grain boundary density, and enlarge the grain size of perovskite films after recrystallization. It is believed that fs laser polishing is a time‐effective and highly precise technique that is suitable for large‐scale device production, thus will trigger more applications in optoelectronics.

A straightforward approach is developed to decouple the degradation effects occurring at the interfaces between the lead halide absorber with a hole‐transport and electron‐transport layers in perovskite solar cells. The impact of the hole‐transport layer is shown to depend on its composition: materials such as nickel oxide aggressively interact with the perovskite, whereas organic polytriarylamine provides a stable interface.

There is growing evidence that the stability of perovskite solar cells (PSCs) is strongly dependent on the interface chemistry between the absorber films and adjacent charge‐transport layers, whereas the exact mechanistic pathways remain poorly understood. Herein, a straightforward approach is presented for decoupling the degradation effects induced by the top fullerene‐based electron transport layer (ETL) and various bottom hole‐transport layer (HTL) materials assembled in p‐i‐n PSCs. It is shown that chemical interaction of MAPbI₃ absorber with ETL comprised of the fullerene derivative most aggressively affects the device operational stability. However, washing away the degraded fullerene derivative and depositing fresh ETL leads to restoration of the initial photovoltaic performance when bottom perovskite/HTL interface is not degraded. Following this approach, it is possible to compare the photostability of stacks with various HTLs. It is shown that poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and NiO_x induce significant degradation of the adjacent perovskite layer under light exposure, whereas poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA) provides the most stable perovskite/HTL interface. A time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) analysis allows identification of chemical origins of the interactions between MAPbI₃ and HTLs. The proposed research methodology and the revealed degradation pathways should facilitate the development of efficient and stable PSCs.

Organic solar cells with unusual inverted structure (sequentially processed heterojunction) are fabricated by sequentially spin coating the acceptor layer FOIC1 after the donor layer PTB7‐Th, which yields better‐controlled vertical phase separation and improved efficiency compared with traditional bulk heterojunction devices.

A new fused‐ring electron acceptor FOIC1 is designed and synthesized. FOIC1 exhibits intense absorption in the range of 600–1000 nm, the highest occupied molecular orbital (HOMO)/the lowest unoccupied molecular orbital (LUMO) energy levels of −5.39/−3.99 eV, and electron mobility of 1.8 × 10⁻³ cm² V⁻¹ s⁻¹. Organic solar cells based on sequentially processed heterojunction (SHJ) with an unusual inverted structure are fabricated. Through sequentially spin‐coating polymer donor PTB7‐Th as the bottom layer and acceptor FOIC1 as the top layer, a better vertical phase distribution is formed in this SHJ compared with that in traditional bulk heterojunction (BHJ). In the upper‐half part, a more balanced donor/acceptor distribution is beneficial for exciton dissociation. At the bottom interface, more FOIC1 accumulation is beneficial for exciton generation and charge transport. Overall, the SHJ cells exhibit power conversion efficiency as high as 12.0%, higher than that of the BHJ counterpart (11.0%).

Radio frequency magnetron sputtered SnO₂ is used as an electron transport layer (ETL) for PbS quantum dot solar cells with an efficiency of 8.4%. Further to modify the SnO₂ surface, a thin sol–gel ZnO layer is spin‐coated on top of SnO₂ forming a SnO₂–ZnO double ETL. The best device with double ETL achieves an efficiency over 10%.

Herein, for the first time, it is successfully demonstrated that radio frequency (RF) magnetron sputtered SnO₂ can be a qualified alternative electron transport layer (ETL) for a high‐efficiency PbS quantum dot (QD) solar cell. The highest performing device using such a SnO₂ ETL obtains an efficiency of 8.4%, which is comparable to the sol–gel ZnO‐based one (8.8%). The excellent performance mainly results from the improved current density, which is attributed to the superior properties of the SnO₂ ETL, such as high electron mobility and excellent optical transmittance. However, it is also found that the sputtered SnO₂‐based devices show smaller voltage and fill factor due to the unsatisfied surface morphology and energy level alignment. By combining a thin (around 10 nm) sol–gel ZnO film on top of a sputtered SnO₂ film to form the double ETL, the best efficiency of 10.1% is obtained, which is the highest efficiency using SnO₂ ETL in a PbS QD solar cell. The work not only provides a new avenue to improve the efficiency of PbS QD solar cells but also offers the possibility to use an industry compatible sputtering technique for PbS QD solar cells.

Triplet materials are designed by introducing heavy atoms to enhance spin–orbit coupling or constructing donor and acceptor units with a twisted conformation to reduce ΔE _ST. However, the twisted materials have not been applied in solar cells due to weak absorption and low charge‐transport mobilities. Now two nonplanar acceptors with large π‐conjugated core were constructed that achieved over 15 % efficiency.

Abstract

Triplet acceptors have been developed to construct high‐performance organic solar cells (OSCs) as the long lifetime and diffusion range of triplet excitons may dissociate into free charges instead of net recombination when the energy levels of the lowest triplet state (T₁) are close to those of charge‐transfer states (³CT). The current triplet acceptors were designed by introducing heavy atoms to enhance the intersystem crossing, limiting their applications. Herein, two twisted acceptors without heavy atoms, analogues of Y6, constructed with large π‐conjugated core and D‐A structure, were confirmed to be triplet materials, leading to high‐performance OSCs. The mechanism of triplet excitons were investigated to show that the twisted and D‐A structures result in large spin–orbit coupling (SOC) and small energy gap between the singlet and triplet states, and thus efficient intersystem crossing. Moreover, the energy level of T₁ is close to ³CT, facilitating the split of triplet exciton to free charges.