Chen Weijie

A simple perovskite solar cell architecture, which is based on dopant‐free electron and hole conductors and carbon back contact deposited at room temperature, is demonstrated. The resulting architecture leads to the fabrication of cheap and highly efficient perovskite solar cells exhibiting unprecedented long‐term operational and UV stability thus hold immense potential for large‐scale deployment.

Abstract

Today's perovskite solar cells (PSCs) mostly use components, such as organic hole conductors or noble metal back contacts, that are very expensive or cause degradation of their photovoltaic performance. For future large‐scale deployment of PSCs, these components need to be replaced with cost‐effective and robust ones that maintain high efficiency while ascertaining long‐term operational stability. Here, a simple and low‐cost PSC architecture employing dopant‐free TiO₂ and CuSCN as the electron and hole conductor, respectively, is introduced while a graphitic carbon layer deposited at room temperature serves as the back electrical contact. The resulting PSCs show efficiencies exceeding 18% under standard AM 1.5 solar illumination and retain ≈95% of their initial efficiencies for >2000 h at the maximum power point under full‐sun illumination at 60 °C. In addition, the CuSCN/carbon‐based PSCs exhibit remarkable stability under ultraviolet irradiance for >1000 h while under similar conditions, the standard spiro‐MeOTAD/Au based devices degrade severely.

A nonfullerene acceptor based active layer with high halogen contents is designed to fabricate efficient thick‐film organic solar cells. The conventional structure device using chlorinated acceptor F–2Cl and fluorinated donor PM6 exhibits a power conversion efficiency over 10% with an active layer thickness of 600 nm.

Abstract

Developing efficient organic solar cells (OSCs) with relatively thick active layer compatible with the roll to roll large area printing process is an inevitable requirement for the commercialization of this field. However, typical laboratory OSCs generally exhibit active layers with optimized thickness around 100 nm and very low thickness tolerance, which cannot be suitable for roll to roll process. In this work, high performance of thick‐film organic solar cells employing a nonfullerene acceptor F–2Cl and a polymer donor PM6 is demonstrated. High power conversion efficiencies (PCEs) of 13.80% in the inverted structure device and 12.83% in the conventional structure device are achieved under optimized conditions. PCE of 9.03% is obtained for the inverted device with active layer thickness of 500 nm. It is worth noting that the conventional structure device still maintains the PCE of over 10% when the film thickness of the active layer is 600 nm, which is the highest value for the NF‐OSCs with such a large active layer thickness. It is found that the performance difference between the thick active layer films based conventional and inverted devices is attributed to their different vertical phase separation in the active layers.

Recently, sequential deposition of donor and acceptor layers has been demonstrated to be an alternative method to fabricate highly efficient bulk‐heterojunction organic solar cells. A simple “needle” model to simulate its morphology indicates a different morphological requirement which rationalizes the high exciton dissociation efficiency.

Abstract

Bulk heterojunction (BHJ) nonfullerene organic solar cells prepared from sequentially deposited donor and acceptor layers (sq‐BHJ) have recently been shown to be highly efficient, environmentally friendly, and compatible with large area and roll‐to‐roll fabrication. However, the related photophysics at donor‐acceptor interface and the vertical heterogeneity of donor‐acceptor distribution, critical for exciton dissociation and device performance, have been largely unexplored. Herein, steady‐state and time‐resolved optical and electrical techniques are employed to characterize the interfacial trap states. Correlating with the luminescent efficiency of interfacial states and its nonradiative recombination, interfacial trap states are characterized to be about 40% more populated in the sq‐BHJ devices than the as‐cast BHJ (c‐BHJ), which probably limits the device voltage output. Cross‐sectional energy‐dispersive X‐ray spectroscopy and ultraviolet photoemission spectroscopy depth profiling directly visualize the donor–acceptor vertical stratification with a precision of 1–2 nm. From the proposed “needle” model, the high exciton dissociation efficiency is rationalized. This study highlights the promise of sequential deposition to fabricate efficient solar cells, and points toward improving the voltage output and overall device performance via eliminating interfacial trap states.

Operationally stable mixed‐cation‐halide perovskite solar cells are fabricated by halogen‐engineering concept via a Br‐rich seeding growth method. Bromine anions are effectively incorporated into the final perovskite film with larger grains and better vertical columnar alignment. Photovoltaic devices based on the film show a power conversion efficiency (PCE) of 21.5% and significantly enhanced operational stability for over 500 h.

Abstract

The performance of perovskite solar cells (PSCs) relies on the synthesis method and chemical composition of the perovskite materials. So far, PSCs that have adopted two‐step sequential deposited perovskite with the state‐of‐art composition (FAPbI₃)_{1− x} (MAPbBr₃) _x (x < 0.05) have achieved record power conversion efficiency (PCE), while their one‐step antisolvent dripping counterparts with typical composition Cs_0.05FA_0.81MA_0.14Pb(I_0.85Br_0.15)₃ with more bromine have exhibited much better long‐term operational stability. Thus, halogen engineering that aims to elevate bromine content in sequential deposited perovskite film would push operational stability of PSCs toward that of antisolvent dripping deposited perovskite materials. Here, a Br‐rich seeding growth method is devised and perovskite seed solution with high bromine content is introduced into a PbI₂ precursor, leading to bromine incorporation in the resulting perovskite film. Photovoltaic devices fabricated by Br‐rich seeding growth method exhibit a PCE of 21.5%, similar to 21.6% for PSCs having lower bromine content. Whereas, the operational stability of PSCs with higher bromine content is significantly enhanced, with over 80% of initial PCE retained after 500 h tracking at maximum power point under 1‐sun illumination. This work highlights the vital importance of halogen composition for the operational stability of PSCs, and introduces an effective way to incorporate bromine into mixed‐cation‐halide perovskite film via sequential deposition method.

Publication date: Available online 5 November 2019

Source: Nano Energy

Author(s): Nabonswende Aida Nadege Ouedraogo, Yichuan Chen, Yue Yue Xiao, Qi Meng, Chang Bao Han, Hui Yan, Yongzhe Zhang

Graphical abstract

Publication date: Available online 6 November 2019

Source: Nano Energy

Author(s): Ligang Xu, Mengyuan Qian, Chi Zhang, Wenzhen Lv, Jibiao Jin, Jinshi Zhang, Chao Zheng, Mingguang Li, Runfeng Chen, Wei Huang

Graphical abstract

An ideal materials combination based on the electron donor BSFTR and acceptor Y6 is selected to construct small‐molecule solar cells (SMSCs). By morphology optimization, an extraordinary power conversion efficiency of 13.69% with a remarkably low energy loss of 0.48 eV is achieved, which is beneficial from the matched photoelectric properties, the favorable blend morphology, and is the best binary SMSC performance reported so far.

Abstract

Compared with the quick development of polymer solar cells, achieving high‐efficiency small‐molecule solar cells (SMSCs) remains highly challenging, as they are limited by the lack of matched materials and morphology control to a great extent. Herein, two small molecules, BSFTR and Y6, which possess broad as well as matched absorption and energy levels, are applied in SMSCs. Morphology optimization with sequential solvent vapor and thermal annealing makes their blend films show proper crystallinity, balanced and high mobilities, and favorable phase separation, which is conducive for exciton dissociation, charge transport, and extraction. These contribute to a remarkable power conversion efficiency up to 13.69% with an open‐circuit voltage of 0.85 V, a high short‐circuit current of 23.16 mA cm⁻² and a fill factor of 69.66%, which is the highest value among binary SMSCs ever reported. This result indicates that a combination of materials with matched photoelectric properties and subtle morphology control is the inevitable route to high‐performance SMSCs.

Interfaces between the photoactive layer and electrodes play a critical role in ultimate device behaviors in organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs). Recent progress in interface modification for OSCs and PSCs aimed at improving interfacial charge extraction and mitigating surface recombination, and at enhancing trap passivation and device stability is presented.

Abstract

Organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs) are two promising photovoltaic techniques for next‐generation energy conversion devices. The rapid increase in the power conversion efficiency (PCE) in OSCs and PSCs has profited from synergetic progresses in rational material synthesis for photoactive layers, device processing, and interface engineering. Interface properties in these two types of devices play a critical role in dictating the processes of charge extraction, surface trap passivation, and interfacial recombination. Therefore, there have been great efforts directed to improving the solar cell performance and device stability in terms of interface modification. Here, recent progress in interfacial doping with biopolymers and ionic salts to modulate the cathode interface properties in OSCs is reviewed. For the anode interface modification, recent strategies of improving the surface properties in widely used PEDOT:PSS for narrowband OSCs or replacing it by novel organic conjugated materials will be touched upon. Several recent approaches are also in focus to deal with interfacial traps and surface passivation in emerging PSCs. Finally, the current challenges and possible directions for the efforts toward further boosts of PCEs and stability via interface engineering are discussed.

Herein, three polyfluorene copolymers (TFB, PFB, and PFO) are investigated as hole‐transport materials (HTMs) for the construction of inverted perovskite solar cells. The photovoltaic performance of the device is found to be closely correlated with the electronic properties of HTMs. The TFB‐based device exhibits the highest efficiency of 18.48% due to its high mobility and favored energy‐level alignment.

Inverted perovskite solar cells (PSCs) that can be entirely processed at low temperatures have attracted growing attention due to their cost‐effective production. Hole‐transport materials (HTMs) play an essential role in achieving efficient inverted PSCs, as they determine the effectiveness of charge extraction and recombination at interfaces. Herein, three polyfluorene copolymers (TFB, PFB, and PFO) are investigated as HTMs for construction of inverted PSCs. It is found that the photovoltaic performance of the solar cells is closely correlated with the electronic properties of the HTMs. Due to its high mobility along with the favored energy‐level alignment with perovskite, TFB shows superior charge extraction and suppressed interfacial recombination than PFB‐ and PFO‐based devices, which delivers a high efficiency of 18.48% with an open‐circuit voltage (V _OC) of up to 1.1 V. In contrast, the presence of a large energy barrier in the PFO‐based devices results in substantial losses in both V _OC and photocurrent. These results demonstrate that TFB can serve as a superior HTM for inverted PSCs. Moreover, it is anticipated that the performance of the three HTMs identified here might guide the molecular design of novel HTMs for the manufacture of highly efficient inverted PSCs.

The efficiencies of semitransparent perovskite device and four‐terminal perovskite/silicon multijunction/tandem solar cells rise to 18.3% and 27.0%, respectively. This is the highest recorded efficiency for semitransparent perovskite solar cells thus far. The high efficiencies originate from good transparency and high conductivity of the nanomesh‐structured gold top electrode.

Abstract

Multijunction/tandem solar cells have naturally attracted great attention because they are not subject to the Shockley–Queisser limit. Perovskite solar cells are ideal candidates for the top cell in multijunction/tandem devices due to the high power conversion efficiency (PCE) and relatively low voltage loss. Herein, sandwiched gold nanomesh between MoO₃ layers is designed as a transparent electrode. The large surface tension of MoO₃ effectively improves wettability for gold, resulting in Frank–van der Merwe growth to produce an ultrathin gold nanomesh layer, which guarantees not only excellent conductivity but also great optical transparency, which is particularly important for a multijunction/tandem solar cell. The top MoO₃ layer reduces the reflection at the gold layer to further increase light transmission. As a result, the semitransparent perovskite cell shows an 18.3% efficiency, the highest reported for this type of device. When the semitransparent perovskite device is mechanically stacked with a heterojunction silicon solar cell of 23.3% PCE, it yields a combined efficiency of 27.0%, higher than those of both the sub‐cells. This breakthrough in elevating the efficiency of semitransparent and multijunction/tandem devices can help to break the Shockley–Queisser limit.

Spiro‐OMeTAD [2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine) 9,9′‐spirobifluorene], the most used solid‐state hole conductor in perovskite solar cells (PSCs), is usually processed in solution, but requires dopants for efficient charge transport. Here, dopant‐free Spiro‐OMeTAD layers prepared by vacuum sublimation are reported. Temperature control of the samples during evaporation induces crystalline and microstructural changes. The implementation in PSCs demonstrates unprecedented superior stability with respect to solution‐processed devices.

Abstract

The main handicap still hindering the eventual exploitation of organometal halide perovskite‐based solar cells is their poor stability under prolonged illumination, ambient conditions, and increased temperatures. This article shows for the first time the vacuum processing of the most widely used solid‐state hole conductor (SSHC), i.e., the Spiro‐OMeTAD [2,2′,7,7′‐tetrakis (N,N‐di‐p‐methoxyphenyl‐amine) 9,9′‐spirobifluorene], and how its dopant‐free crystalline formation unprecedently improves perovskite solar cell (PSC) stability under continuous illumination by about two orders of magnitude with respect to the solution‐processed reference and after annealing in air up to 200 °C. It is demonstrated that the control over the temperature of the samples during the vacuum deposition enhances the crystallinity of the SSHC, obtaining a preferential orientation along the π–π stacking direction. These results may represent a milestone toward the full vacuum processing of hybrid organic halide PSCs as well as light‐emitting diodes, with promising impacts on the development of durable devices. The microstructure, purity, and crystallinity of the vacuum sublimated Spiro‐OMeTAD layers are fully elucidated by applying an unparalleled set of complementary characterization techniques, including scanning electron microscopy, X‐ray diffraction, grazing‐incidence small‐angle X‐ray scattering and grazing‐incidence wide‐angle X‐ray scattering, X‐ray photoelectron spectroscopy, and Rutherford backscattering spectroscopy.

A novel and stable 2D Ruddlesden–Popper‐type layered chalcogenide perovskite semiconductor Ca₃Sn₂S₇, with graphene‐like linear electronic dispersion, small carrier effective mass (0.04 m₀), ultrahigh carrier mobility (6.7 × 10⁴ cm² V⁻¹ s⁻¹), Fermi velocity (3 × 10⁵ m s⁻¹), and optical absorption coefficient (10⁵ cm⁻¹), is found. Particularly, its direct quasi‐particle bandgap of 0.5 eV realizes the dream of opening the graphene bandgap in a new way.

Abstract

Graphene, a star 2D material, has attracted much attention because of its unique properties including linear electronic dispersion, massless carriers, and ultrahigh carrier mobility (10⁴–10⁵ cm² V⁻¹ s⁻¹). However, its zero bandgap greatly impedes its application in the semiconductor industry. Opening the zero bandgap has become an unresolved worldwide problem. Here, a novel and stable 2D Ruddlesden–Popper‐type layered chalcogenide perovskite semiconductor Ca₃Sn₂S₇ is found based on first‐principles GW calculations, which exhibits excellent electronic, optical, and transport properties, as well as soft and isotropic mechanical characteristics. Surprisingly, it has a graphene‐like linear electronic dispersion, small carrier effective mass (0.04 m₀), ultrahigh room‐temperature carrier mobility (6.7 × 10⁴ cm² V⁻¹ s⁻¹), Fermi velocity (3 × 10⁵ m s⁻¹), and optical absorption coefficient (10⁵ cm⁻¹). Particularly, it has a direct quasi‐particle bandgap of 0.5 eV, which realizes the dream of opening the graphene bandgap in a new way. These results guarantee its application in infrared optoelectronic and high‐speed electronic devices.

Ionic conduction in the perovskite oxide La_0.9Sr_0.1Ga_0.95Mg_0.05O_{3– δ} (LSGM) is found to be strongly correlated with crystal structure. A structural design with simultaneously large unit‐cell volume and octahedral rotations for fast ionic conduction is proposed and realized in LSGM superlattice thin films, where the ionic conductivity is tuned with structure alone by a factor of ≈2.5 at 600 °C.

Abstract

Solid‐oxide fuel/electrolyzer cells are limited by a dearth of electrolyte materials with low ohmic loss and an incomplete understanding of the structure–property relationships that would enable the rational design of better materials. Here, using epitaxial thin‐film growth, synchrotron radiation, impedance spectroscopy, and density‐functional theory, the impact of structural parameters (i.e., unit‐cell volume and octahedral rotations) on ionic conductivity is delineated in La_0.9Sr_0.1Ga_0.95Mg_0.05O_{3– δ} . As compared to the zero‐strain state, compressive strain reduces the unit‐cell volume while maintaining large octahedral rotations, resulting in a strong reduction of ionic conductivity, while tensile strain increases the unit‐cell volume while quenching octahedral rotations, resulting in a negligible effect on the ionic conductivity. Calculations reveal that larger unit‐cell volumes and octahedral rotations decrease migration barriers and create low‐energy migration pathways, respectively. The desired combination of large unit‐cell volume and octahedral rotations is normally contraindicated, but through the creation of superlattice structures both expanded unit‐cell volume and large octahedral rotations are experimentally realized, which result in an enhancement of the ionic conductivity. All told, the potential to tune ionic conductivity with structure alone by a factor of ≈2.5 at around 600 °C is observed, which sheds new light on the rational design of ion‐conducting perovskite electrolytes.

Locked up: The steric PTA‐MA mixed‐cation 2D perovskite of PTAMAPbI₄ is demonstrated as an effective methylammonium (MA) cation locker to stabilize MAPbI₃ by steric effect of the phenyltrimethylammonium (PTA) cation. This MA cation locked MAPbI₃ based perovskite exhibited significantly enhanced stability and photovoltaic performance.

Abstract

The reduced dimension perovskite including 2D perovskites are one of the most promising strategies to stabilize lead halide perovskite. A mixed‐cation 2D perovskite based on a steric phenyltrimethylammonium (PTA) cation is presented. The PTA‐MA mixed‐cation 2D perovskite of PTAMAPbI₄ can be formed on the surface of MAPbI₃ (PTAI‐MAPbI₃) by controllable PTAI intercalation by either spin coating or soaking. The PTAMAPbI₄ capping layer can not only passivate PTAI‐MAPbI₃ perovskite but also act as MA⁺ locker to inhibit MAI extraction and significantly enhance the stability. The highly stable PTAI‐MAPbI₃ based perovskite solar cells exhibit a reproducible photovoltaic performance with a champion PCE of 21.16 %. Such unencapsulated devices retain 93 % of initial efficiency after 500 h continuous illumination. This steric mixed‐cation 2D perovskite as MA⁺ locker to stabilize the MAPbI₃ is a promising strategy to design stable and high‐performance hybrid lead halide perovskites.

Publication date: Available online 2 November 2019

Source: Nano Energy

Author(s): Ying Yang, Tian Chen, Dequn Pan, Jing Gao, Congtan Zhu, Feiyu Lin, Conghua Zhou, Qidong Tai, Si Xiao, Yongbo Yuan, Qilin Dai, Yibo Han, Xie Haipeng, Xueyi Guo

Graphical abstract

MAPbI₃/agarose photoactive composite serves as the humid stable light absorber for unencapsultated perovskite solar cells in air. Environmental stability for almost 2000 h are achieved. ∼46% enhancement in the light-to-electric efficiency are accomplished due to the passivation of agarose on perovskite and that MAPbI₃/agarose photoactive composite has potential in improving the operational stability of perovskite solar cells in humid air without glove box.

Hole‐transport material based on dibenzo[b,d]thiophene (DBTMT) is synthesized with low costs. A champion power conversion efficiency of the optimized p–i–n planar perovskite solar cells based on dopant‐free DBTMT reaches 21.12% with a high fill factor of 83.25%, due to good hole‐transport properties and the passivation effect of DBTMT.

N ²,N ²,N ⁸,N ⁸‐tetrakis(4‐(methylthio)phenyl)dibenzo[b,d]thiophene‐2,8‐diamine (DBTMT) is synthesized from three commercial monomers for application as a promising dopant‐free hole‐transport material (HTM) in perovskite solar cells (pero‐SCs). The intrinsic properties (optical properties and electronic energy levels) of DBTMT are investigated, proving that DBTMT is a suitable HTM for the planar p–i–n pero‐SCs. The champion power conversion efficiency (PCE) of the optimized pero‐SCs (with structure as ITO/pristine DBTMT/MAPbI₃/C₆₀/BCP/Ag) reaches 21.12% with a fill factor (FF) of 83.25%, which is among the highest PCEs and FFs reported for planar p–i–n pero‐SCs based on dopant‐free HTMs. The Fourier‐transform infrared spectroscopy, X‐ray diffraction, and X‐ray photoelectron spectroscopy spectra of MAPbI₃ and DBTMT–MAPbI₃ films demonstrate that there is an interaction between DBTMT and MAPbI₃ at the interface through the sulfur atoms in DBTMT to passivate the defects, which is corresponding to the higher FF and PCE of the corresponding device.

Niobium oxides are successfully prepared via a urea‐metal chloride route. Remarkably, the monoclinic NbO₂ counter electrode exhibits superior electrocatalytic activity and yields a high power conversion efficiency of 6.06% in dye‐sensitized solar cells, close to that of Pt counter electrodes (6.46%). The catalytic mechanism of the Nb‐based counter electrode is clarified in terms of its electronic structure and I adsorption using first‐principle calculations.

Two types of nanosized niobium oxides and their composites, pseudohexagonal Nb₂O₅ (TT‐Nb₂O₅), monoclinic NbO₂ (M‐NbO₂), and the coexistence of TT‐Nb₂O₅ and M‐NbO₂ (TT‐Nb₂O₅/M‐NbO₂), are successfully synthesized through the urea‐metal chloride route, and they exhibit excellent catalytic activity and photovoltaic performance in dye‐sensitized solar cells (DSSCs). First‐principles density function theory (DFT) calculations show that their catalytic activity is significantly influenced by their intrinsic electronic structures and properties. The lone‐pair 4d¹ electrons of Nb⁴⁺ in M‐NbO₂ enhance the Nb–I interaction and promote electron transfer from the M‐NbO₂ counter electrode (CE) to I, thus resulting in superior catalytic properties in M‐NbO₂‐based DSSCs. In addition, the adsorption energy of I on the M‐NbO₂ surface is in the optimal energy range of 0.3—1.2 eV, and the Fermi level of M‐NbO₂ is 0.6 eV, which is higher than the I₃ ⁻ reduction reaction potential, and I₃ ⁻ can be spontaneously reduced to 3I⁻. Herein, a general strategy for understanding the electronic structures and catalytic activities of transition metal compounds as CE catalysts for DSSCs is provided.