Chen Weijie

Large energy loss has been a major obstacle for further efficiency improvement of inorganic perovskite solar cells. This review provides a basic understanding of energy loss and may inspire new designs or more impactful methods for further minimizing the energy loss of inorganic perovskite solar cells.

Abstract

Though the cesium‐based inorganic perovskite solar cells (IPSCs) have developed rapidly in recent two years, the power conversion efficiency (PCE) is still far away from the Shockley–Queisser limit due to the large open‐circuit voltage (V _oc) deficit, which results from the large energy loss (E _loss). Large E _loss has been a major obstacle for further efficiency improvement of IPSCs. In this review, the authors, for the first time, focus on investigating the E _loss of IPSCs and start from discussing the essence and origin of the E _loss. Then, the reported efficient methods for reducing the band tails and energy disorder are systematically summarized and reviewed, including crystallization optimization, defect passivation, and interface engineering. Finally, the authors offer an overall perspective on managing E _loss in IPSCs and point out the possible ways to reduce the E _loss and promote the efficiency. This review provides a basic understanding of E _loss and may inspire new designs or more impactful methods for further minimizing the E _loss of IPSCs.

Nature Energy, Published online: 23 September 2019; doi:10.1038/s41560-019-0466-3

Nature Chemistry, Published online: 23 September 2019; doi:10.1038/s41557-019-0332-8

CuGaO₂ nanoparticles modified with aminosilane are used as a hole‐transporting layer with CuSCN for perovskite solar cells. The enhanced power conversion efficiency and thermal stability compared to the cell using only CuSCN are correlated with the improved carrier extraction and reduced interfacial degradation by the inorganic nanoparticles.

Abstract

Herein, solution‐processible inorganic hole‐transport layer (HTL) of a perovskite solar cell that consists of CuGaO₂ nanoparticles and CuSCN, which leads to an improved device performance as well as long‐term stability, is reported. Uniform films of CuGaO₂ are prepared by first treating CuGaO₂ nanoparticles with aminosilane that leads to well‐dispersed CuGaO₂ solution, followed by spin‐coating of the suspension. Subsequent spin‐coating of CuSCN solution onto the CuGaO₂ forms a smooth HTL with excellent coverage and electrical conductivity. Comparing to the reference device with CuSCN HTL, the CuGaO₂/CuSCN device improves carrier extraction and reduces trap density by ≈40%, as measured by photoluminescence and capacitance analysis. Excellent thermal stability is also demonstrated: ≈80% of the initial efficiency of the perovskite solar cells with the CuGaO₂/CuSCN HTL is retained after 400 h under 85 °C/85% relative humidity environment.

An all‐layer‐inorganic perovskite solar cell (PSC) based on inorganic CsPbI₂Br perovskite absorber layer and tailored NiO hole‐transporting layer (HTL) is fabricated. The tailored NiO nanocrystalline films exhibit uniform, pinhole‐free morphologies, efficient charge‐extraction capabilities, and intrinsic chemical stability, which gives the whole photovoltaic device a high efficiency and much improved stability compared with PSCs based on the organic HTLs.

Cesium‐based inorganic perovskite solar cells (PSCs) have attracted great attention due to the superior thermal stability of the light absorbers. However, the reported devices normally contain organic charge‐transporting layers (CTLs), such as spiro‐OMeTAD, which is expensive and highly sensitive to ambient atmosphere and temperature. It is of great significance to develop inorganic CTLs with low cost and robust stability. To date, it is still a big challenge to achieve high‐quality inorganic CTL films via the solution process, especially for the hole‐transporting layer (HTL) in conventional n‐i‐p structures. Herein, tailored NiO nanocrystalline films as HTLs in an all‐layer‐inorganic CsPbI₂Br‐based PSCs are developed, which exhibit uniform, pinhole‐free morphologies and efficient charge‐extraction capabilities. Consequently, the as‐constructed all‐layer‐inorganic PSCs, with an optimal power conversion efficiency (PCE) of 15.14% and a stabilized power output of 14.82%, present robust long‐term thermal stability: retained 85% of their initial PCEs after a thermal treatment at 85 °C in the dark in a nitrogen atmosphere with encapsulation for 1000 h, greatly surpassing the performance of the PSCs based on the organic HTLs.

A small‐molecule acceptor (IDTS‐4F) is designed for a ternary approach, which enables the simultaneous increase in open‐circuit voltage and short‐circuit current density without sacrificing fill factor. The two acceptors form homogeneous acceptor phases, which synergize them with the increase in phase purity and crystallinity and the reduction in domain size, whereas the charge mobilities and recombinations are maintained.

Herein, an A–D–A‐type nonfullerene acceptor (named IDTS‐4F) with an alkyl thiophenyl side chain and dimethoxy thiophene bridging unit is reported. The use of an alkyl thiophenyl group is important, as the insertion of sulfur atoms can slightly downshift the highest occupied molecular orbital (HOMO) level of the molecule and allows IDTS‐4F to match with state‐of‐the‐art donor polymer PM6 (or PM7). Compared with conventional nonfullerene acceptors, IT‐4F, the IDTS‐4F molecule, has a smaller optical bandgap and higher lowest unoccupied molecular orbital (LUMO) level, which are beneficial to increase the V _oc and J _sc of the devices. Nonfullerene organic solar cell devices are fabricated using IDTS‐4F. Although the binary device based on IDTS‐4F exhibits a lower fill factor (FF, 70%), the ternary device by incorporating 0.2 of IDTS‐4F and 0.8 of IT‐4F (with PM6 as the donor polymer) can simultaneously achieve a higher V _oc and J _sc, while maintaining the high FF (77%) of IT‐4F based system. Morphology characterizations indicate the formation of homogeneous film morphology, the large increase in phase purity and crystallinity, and the reduction in domain size upon addition of crystalline IDTS‐4F, while the electron/hole mobilities and recombination losses of the IT‐4F system are both maintained.

Changing the alkyl chain position of a small‐molecule donor provides optimized conformation, improved phase aggregation, and enhanced photovoltaic properties. The strategy affords 12.3% efficiency single‐junction all‐small‐molecule organic solar cells (ASM OSCs) with reduced recombination and enhanced carrier lifetimes. The power conversion efficiency of 12.3% is higher than all reported single‐junction ASM OSCs.

Molecular stacking plays an important role in defining the active layer morphology in all‐small‐molecule organic solar cells (ASM OSCs). However, the precise control of donor/acceptor stacking to afford optimal phase separation remains challenging. Herein, the molecular stacking of a small‐molecule donor is tuned by changing the alky chain position to match a high‐performance small‐molecule nonfullerene acceptor (NFA), Y6. The alky chain engineering not only affects the planarity of the small‐molecule donor, but also the molecular aggregation and the active layer morphology, and thus the photovoltaic performance. Notably, single‐junction ASM OSCs with 12.3% power conversion efficiency (PCE) are achieved. The PCE of 12.3% is among the top efficiencies of single‐junction ASM OSCs reported in the literature to date. The results highlight the importance of fine‐tuning the molecular structure to achieve high‐performance ASM OSCs.

Through experimental and theoretical investigation, the synergetic effects of Cs and Br in assisting incorporation of guanidium (GA) in FAPbI₃ are revealed. It is found that GA incorporation enhances the bonding with surrounding halides and elevates the formation energy of halide vacancies, resulting in improved stability and photovoltaic performance.

Abstract

Recently, incorporating guanidium (GA) cations into organolead halide perovskites is shown to effectively improve the stability and performance of the solar cells. However, the underlying mechanisms that govern the GA incorporation have remained unclear. Here, FAPbI₃ is used as a basic framework to investigate experimentally and theoretically the role of cesium (Cs) and bromine (Br) substitutions in GA⁺ incorporation. It is found that simultaneous introduction of the small‐size Cs⁺ and Br^– in the FAPbI₃ lattice is critical to create sufficient space for the large GA⁺ and that the presence of the Cs⁺ prevents the formation of a GA‐contained low‐dimensional phase, which both assist GA⁺ incorporation. Upon entering the perovskite lattice, the GA⁺ can stabilize the lattice structure via forming strong hydrogen bonds with their neighboring halide ions. Such structure modification suppresses halide vacancy formation, thus leading to improved material properties. Compared to the GA‐free perovskite reference samples, the optimal system GA_0.05Cs_0.15FA_0.8Pb(I_0.85Br_0.15)₃ exhibits substantially improved thermal and photothermal stability, as well as increased photocarrier lifetime. Solar cells fabricated with the optimal material system show an excellent photovoltaic performance, with the champion device reaching a power conversion efficiency of 21.3% and an open circuit voltage of 1.229 V.

The role of DMAI in fabricating high quality CsPbI₃ inorganic perovskite thin films is demonstrated to be a volatile crystal growth additive rather than dopant. With optimal DMAI additive and PTACl passivation, a PTACl‐CsPbI₃ based champion photovoltaic device exhibits a record efficiency of 19.03 %.

Abstract

The controllable growth of CsPbI₃ perovskite thin films with desired crystal phase and morphology is crucial for the development of high efficiency inorganic perovskite solar cells (PSCs). The role of dimethylammonium iodide (DMAI) used in CsPbI₃ perovskite fabrication was carefully investigated. We demonstrated that the DMAI is an effective volatile additive to manipulate the crystallization process of CsPbI₃ inorganic perovskite films with different crystal phases and morphologies. The thermogravimetric analysis results indicated that the sublimation of DMAI is sensitive to moisture, and a proper atmosphere is helpful for the DMAI removal. The time‐of‐flight secondary ion mass spectrometry and nuclear magnetic resonance results confirmed that the DMAI additive would not alloy into the crystal lattice of CsPbI₃ perovskite. Moreover, the DMAI residues in CsPbI₃ perovskite can deteriorate the photovoltaic performance and stability. Finally, the PSCs based on phenyltrimethylammonium chloride passivated CsPbI₃ inorganic perovskite achieved a record champion efficiency up to 19.03 %.

An electron‐deficient unit containing B←N bonds, namely BNIDT, is developed to construct polymer acceptors for photovoltaic applications. Desirable optoelectronic properties such as broad absorption profiles, low‐lying energy levels, ambipolar charge transport properties, and strong electron‐affinity are found for these polymers. All‐polymer solar cells using these B←N embedded polymers as acceptor materials exhibit an enhanced efficiency of 8.78%.

Abstract

In the field of all‐polymer solar cells (all‐PSCs), all efficient polymer acceptors that exhibit efficiencies beyond 8% are based on either imide or dicyanoethylene. To boost the development of this promising solar cell type, creating novel electron‐deficient units to build high‐performance polymer acceptors is critical. A novel electron‐deficient unit containing B←N bonds, namely, BNIDT, is synthesized. Systematic investigation of BNIDT reveals desirable properties including good coplanarity, favorable single‐crystal structure, narrowed bandgap and downshifted energy levels, and extended absorption profiles. By copolymerizing BNIDT with thiophene and 3,4‐difluorothiophene, two novel conjugated polymers named BN‐T and BN‐2fT are developed, respectively. It is shown that these polymers possess wide absorption spectra covering 350–800 nm, low‐lying energy levels, and ambipolar film‐transistor characteristics. Using PBDB‐T as the donor and BN‐2fT as the acceptor, all‐PSCs afford an encouraging efficiency of 8.78%, which is the highest for all‐PSCs excluding the devices based on imide and dicyanoethylene‐type acceptors. Considering that the structure of BNIDT is totally different from these classical units, this work opens up a new class of electron‐deficient unit for constructing efficient polymer acceptors that can realize efficiencies beyond 8% for the first time.

Publication date: 20 November 2019

Source: Joule, Volume 3, Issue 11

Author(s): Julian S.W. Godding, Alexandra J. Ramadan, Yen-Hung Lin, Kelly Schutt, Henry J. Snaith, Bernard Wenger

Context & Scale

Summary

Graphical Abstract

A 2D metal–organic framework In‐Aipa‐derived N‐rich porous carbon material with rich pyridinic‐N and graphitic‐N is first introduced into the hole transport layers of perovskite solar cells as an auxiliary additive, contributing to the significantly improved power conversion efficiency from 16.47% to 18.51%, as well as the enhanced long‐term stability of over 85% efficiency retention under exposure to air for 720 h.

As the standard bidopants of hole transport layers (HTLs) in perovskite solar cells (PSCs), bis(trifluoromethane)sulfonimide lithium salt (Li‐TFSI) and 4‐tert‐butylpyridine not only induce adverse influence on the quality of thin films, but also seriously impair the long‐term stability of devices. Herein, a metal–organic framework‐derived 2D graphitic N‐rich porous carbon (NPC) is first introduced into the HTLs as an effective auxiliary additive. The introduction of NPC significantly reduces the aggregation of lithium salts and the formation of HTL defects, optimizing film quality for rapid hole extraction and migration. Furthermore, inherent porosity and hydrophobicity of NPCs are extremely beneficial to restrict the permeation of Li⁺ ions and anode metals, and prevent the moisture from eroding the HTLs and perovskite layers, enhancing the stability of PSCs. As expected, the PSCs with NPC realize a satisfactory fill factor of 0.76 and power conversion efficiency (PCE) of 18.51%, apparently higher than that of pristine devices (0.70% and 16.47%). In addition, over 85% of the initial PCE for optimized PSCs is maintained after 720 h of exposure to air. Obviously, an innovative strategy for highly efficient and long‐term stable PSC devices is provided.

The fabrication method of high‐quality (Cs) _x (FA)_1−x PbI₃ perovskite films by varying the amount of cesium chloride (CsCl) in the FAPbI₃ precursor solutions is demonstrated. The best photovoltaic performance with a power conversion efficiency of 19.20% is achieved for the device with 10 mol% excess of CsCl.

The quality of perovskite films plays a crucial role in improving the optoelectronic properties and performance of perovskite solar cells (PSCs). Herein, high‐quality Cs _x FA_1−x PbI₃ perovskite films with different compositions (x = 0, 5, 10, and 15) are achieved by controlling the amount of cesium chloride (CsCl) in the respective FAPbI₃ precursor solution. The effects of CsCl addition on the morphological and optoelectronic properties of the resulting perovskite films and on the performance of the corresponding devices are systematically studied. Introduction of CsCl into FAPbI₃ shows a great potential to stabilize the α‐FAPbI₃ perovskite phase by forming Cs _x FA_1−x PbI₃ films with improved morphology and carrier lifetimes. With an optimal 10 mol% CsCl additive, the average power conversion efficiency (PCE) is increased from 16.83 ± 0.30% for the reference FAPbI₃‐based PSCs to 18.87 ± 0.25% (with a steady‐state PCE of 18.89%). Moreover, the optimized device performance is more stable after 20 days than the controlled one under ≈40% humidity in air.

CsF is adopted to modify the PbI₂ seed for highly crystallized Cs‐doped perovskite film with very long carrier lifetime, and very high light, thermal and humidity stabilities. As a result, the planar perovskite solar cells based on the Cs‐doped film also show very good stability with negligible hysteresis, and display PCEs of over 21%.

Abstract

Adding a small amount of CsI into mixed cation‐halide perovskite film via a one‐step method has been demonstrated as an excellent strategy for high‐performance perovskite solar cells (PSCs). However, the one‐step method generally relies on an antisolvent washing process, which is hard to control and not suitable for fabricating large‐area devices. Here, CsF is employed and Cs is incorporated into perovskite film via a two‐step method. It is revealed that CsF can effectively diffuse into the PbI₂ seed film, and drastically enhances perovskite crystallization, leading to high‐quality Cs‐doped perovskite film with a very long photoluminescence carrier lifetime (1413 ns), remarkable light stability, thermal stability, and humidity stability. The fabricated PSCs show power conversion efficiency (PCE) of over 21%, and they are highly thermally stable: in the aging test at 60 °C for 300 h, 96% of the original PCE remains. The CsF incorporation process provides a new avenue for stable high‐performance PSCs.

Herein, acetate anion (Ac⁻) is used to partially replace I⁻ in the CsPbI₂Br framework. Ac⁻ doping changes the morphology, electronic properties, and band structure of the host CsPbI₂Br film. The obtained CsPbI_{2− x} Br(Ac) _x perovskite solar cells exhibit a power conversion efficiency of 15.56%, an open circuit voltage of 1.30 V, and great air stability.

Abstract

The Cs‐based inorganic perovskite solar cells (PSCs), such as CsPbI₂Br, have made a striking breakthrough with power conversion efficiency (PCE) over 16% and potential to be used as top cells for tandem devices. Herein, I⁻ is partially replaced with the acetate anion (Ac⁻) in the CsPbI₂Br framework, producing multiple benefits. The Ac⁻ doping can change the morphology, electronic properties, and band structure of the host CsPbI₂Br film. The obtained CsPbI_{2− x} Br(Ac) _x perovskite films present lower trap densities, longer carrier lifetimes, and fast charge transportation compared to the host CsPbI₂Br films. Interestingly, the CsPbI_{2− x} Br(Ac) _x PSCs exhibit a maximum PCE of 15.56% and an ultrahigh open circuit voltage (V _oc) of 1.30 V without sacrificing photocurrent. Notably, such a remarkable V _oc is among the highest values of the previously reported CsPbI₂Br PSCs, while the PCE far exceeds all of them. In addition, the obtained CsPbI_{2− x} Br(Ac) _x PSCs exhibit high reproducibility and good stability. The stable CsPbI_{2− x} Br(Ac) _x PSCs with high V _oc and PCE are desirable for tandem solar cell applications.

A UV‐inert ZnTiO₃ is demonstrated to be an electron selective layer in perovskite solar cells. ZnTiO₃ is a perovskite‐structured semiconductor with excellent chemical stability and poor photocatalysis. Planar perovskite solar cells based on ZnTiO₃ exhibit power conversion efficiency of 20.1% with improved photostability. The best device holds 90% of its initial efficiency after 100 h of ultraviolet soaking.

Abstract

Although planar‐structured perovskite solar cells (PSCs) have power conversion efficiencies exceeding 24%, the poor photostability, especially with ultraviolet irradiance (UV) severely limits commercial application. The most commonly‐used TiO₂ electron selective layer has a strong photocatalytic effect on perovskite/TiO₂ interface when TiO₂ is excited by UV light. Here a UV‐inert ZnTiO₃ is reported as the electron selective layer in planar PSCs. ZnTiO₃ is a perovskite‐structured semiconductor with excellent chemical stability and poor photocatalysis. Solar cells are fabricated with a structure of indium doped tin oxide (ITO)/ZnTiO₃/Cs_0.05FA_0.81MA_0.14PbI_2.55Br_0.45/Sprio‐MeOTAD/Au. The champion device exhibits a stabilized power conversion efficiency of 19.8% with improved photostability. The device holds 90% of its initial efficiency after 100 h of UV soaking (365 nm, 8 mW cm⁻²), compared with 55% for TiO₂‐based devices. This work provides a new class of electron selective materials with excellent UV stability in perovskite solar cell applications.

The passivation of the SnO₂ electron transport layer by fullerenes in metal halide perovskite solar cells is studied with X‐ray photoelectron spectroscopy depth profiling. Interfacial binding of fullerenes to the SnO₂ surface is essential for reproducible and effective passivation and improved solar cell performance.

Abstract

Interfaces between the photoactive and charge transport layers are crucial for the performance of perovskite solar cells. Surface passivation of SnO₂ as electron transport layer (ETL) by fullerene derivatives is known to improve the performance of n–i–p devices, yet organic passivation layers are susceptible to removal during perovskite deposition. Understanding the nature of the passivation is important for further optimization of SnO₂ ETLs. X‐ray photoelectron spectroscopy depth profiling is a convenient tool to monitor the fullerene concentration in passivation layers at a SnO₂ interface. Through a comparative study using [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) and [6,6]‐phenyl‐C₆₁‐butyric acid (PCBA) passivation layers, a direct correlation is established between the formation of interfacial chemical bonds and the retention of passivating fullerene molecules at the SnO₂ interface that effectively reduces the number of defects and enhances electron mobility. Devices with only a PCBA‐monolayer‐passivated SnO₂ ETL exhibit significantly improved performance and reproducibility, achieving an efficiency of 18.8%. Investigating thick and solvent‐resistant C₆₀ and PCBM‐dimer layers demonstrates that the charge transport in the ETL is only improved by chemisorption of the fullerene at the SnO₂ surface.