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This study reports the deposition of a TiO₂ electron transporting layer for perovskite solar cells by spray coating using a stable TiO₂ colloidal aqueous solution, which is synthesized via the self‐condensation of a titanium peroxide complex under hydrothermal conditions. Although the whole fabrication process for the cells is performed at 100 °C, 22.7% efficiency is achieved.

Abstract

TiO₂ is one of the most efficient and widely used materials for electron‐transporting layer (ETLs) in perovskite solar cells (PSCs). The formation of efficient TiO₂ layers is generally carried out at high temperature by baking at a temperature >400 °C or by vacuum deposition (e.g., atomic layer deposition and E‐beam). In this study, the preparation of a TiO₂ ETL for PSCs is reported with excellent properties at low temperatures based on the synthesis of a stable TiO₂ colloidal aqueous solution and spray coating. The prepared TiO₂ colloids are able to produce a dense and uniform ETL even if it is simply dried at 100 °C after spray coating. It is believed that this is owing to the peroxo functional group remaining on the surface of the TiO₂ colloids. The TiO₂ ETLs, combined with the TiO₂ underlayer formed by chemical bath deposition, and the sprayed TiO₂ colloids allowed the fabrication of PSCs with performance similar to those of PSCs produced by annealing at 450 °C with a TiO₂ paste. The PSCs fabricated entirely at 100 °C demonstrated power conversion efficiency of 22.7% in small cells, and 19.0% in mini‐modules.

Publication date: November 2020

Source: Nano Energy, Volume 77

Author(s): Naveen Harindu Hemasiri, Samrana Kazim, Shahzada Ahmad

Arising from the lubricant role of WS₂ nanoflakes between ETL and perovskite, the tensile strain in the perovskite film is released for a uniform perovskite with low defect density and high activation energy for ion migration. This significantly improves the device performance and stability.

Abstract

Perovskite lattice distortion induced by residual tensile strain from the thermal expansion mismatch between the electron‐transporting layer (ETL) and perovskite film causes a sluggish charge extraction and transfer dynamics in all‐inorganic CsPbBr₃ perovskite solar cells (PSCs) because of their higher crystallization temperatures and thermal expansion coefficients. Herein, the interfacial strain is released by fabricating a WS₂/CsPbBr₃ van der Waals heterostructure owing to their matched crystal lattice structure and the atomically smooth dangling bond‐free surface to act as a lubricant between ETL and CsPbBr₃ perovskite. Arising from the strain‐released interface and condensed perovskite lattice, the best device achieves an efficiency of 10.65 % with an ultrahigh open‐circuit voltage of 1.70 V and significantly improved stability under persistent light irradiation and humidity (80 %) attack over 120 days.

Two fully non‐fused acceptors are precisely designed and easily prepared. The side chain encapsulation can induce a planar molecular backbone conformation, endowing the acceptor with broad light absorption. Thermal annealing promotes molecular rearrangement to form J‐aggregates with even broader absorption and higher absorption coefficient. A PCE over 10 % is one of the highest PCE for fully non‐fused ring acceptors.

Abstract

Fused‐ring electron acceptors have made significant progress in recent years, while the development of fully non‐fused ring acceptors has been unsatisfactory. Here, two fully non‐fused ring acceptors, o‐4TBC‐2F and m‐4TBC‐2F, were designed and synthesized. By regulating the location of the hexyloxy chains, o‐4TBC‐2F formed planar backbones, while m‐4TBC‐2F displayed a twisted backbone. Additionally, the o‐4TBC‐2F film showed a markedly red‐shifted absorption after thermal annealing, which indicated the formation of J‐aggregates. For fabrication of organic solar cells (OSCs), PBDB‐T was used as a donor and blended with the two acceptors. The o‐4TBC‐2F‐based blend films displayed higher charge mobilities, lower energy loss and a higher power conversion efficiency (PCE). The optimized devices based on o‐4TBC‐2F gave a PCE of 10.26 %, which was much higher than those based on m‐4TBC‐2F at 2.63 %, and it is one of the highest reported PCE values for fully non‐fused ring electron acceptors.

Graphene‐based materials have demonstrated tremendous potentials in boosting the efficiency and stability of planar perovskite solar cells (PSCs). A comprehensive overview that focused on the specific applications of graphene in planar PSCs has never been published before. Herein the recent applications of graphene in planar PSCs are systematically discussed and concluding perspectives on current challenges and future developments are proposed.

Metal halide perovskite solar cells (PSCs) have recently become the most promising new‐generation solar cells, with a breathtaking growth of efficiency from 3.8% to 25.2% in just one decade. Scientists have abandoned the traditional high‐temperature‐processed mesoscopic layer of the initial mesoscopic PSCs in designing and manufacturing planar PSCs. This new feature endows planar PSCs with possibilities of low‐temperature processibility and large‐scale production. Nevertheless, the advancement of planar PSCs remains limited by two bottlenecks: charge loss and device degradation. To address these two issues, researchers have adopted graphene‐based materials, which demonstrate tremendous potentials due to their superb optical transparency, outstanding carrier mobility, remarkable electrical conductivity, and superior physicochemical stability. Defects inside films and at interfaces are regulated by graphene, thereby contributing to more efficient charge extraction and suppressed charge recombination. The graphene protective layer enhances the moisture and heat stability of planar PSCs, thereby extending the lifetime of devices. Herein, the typical synthesis methods of graphene and the recent applications of graphene in planar PSCs are summarized and discussed. Furthermore, concluding perspectives on current challenges and the future development of graphene in planar PSCs are proposed.

Herein, a bulk heterojunction structure hybridizing SnS QDs and CsPbBr₃ is designed as an absorber layer for solar cells. The prominent SnS QD–CsPbBr₃ structure improves the crystallinity of perovskite, enhances the charge transfer efficiency, and reduces the trap state density of the CsPbBr₃ film, thereby further boosting the photoelectric performance of perovskite solar cells.

Inorganic cesium lead halide perovskite solar cells (PSCs) have attracted great attention due to their remarkable thermal and moisture stability. However, the low photoelectric conversion efficiency (PCE) of inorganic PSCs due to the high trap state density, high carrier recombination rate, and poor carrier transport kinetics impedes their industrial applications. Herein, a remarkable bulk heterojunction structure coupling SnS quantum dots (QDs) with CsPbBr₃ is prepared for the first time via a facile spin‐coating process using a SnS‐QD‐dispersed toluene solution as an antisolvent. The introduction of SnS QDs provides extra crystallization sites and promotes the crystallization of perovskites along (100) and (200) faces. Meanwhile, the bulk heterojunction structure with matched energy structure can effectively reduce the trap state density of the perovskite film and improves the dynamic performance of carriers, suppressing the charge recombination rate and effectively boosting the PCE of solar cells. As a result, the optimal bulk heterojunction solar cell achieves a PCE of 8.01%, about 52% higher than that of the pristine solar cell. This work provides a low‐cost and facile strategy for preparing high‐performance bulk heterojunction PSCs and an effective method to boost the PCE by nontoxic QDs.

High‐performance thick‐film OSCs are essential for well matching the roll‐to‐roll (R2R) technology to realize its large‐area potential application. The critical factors and smart strategies on performance improvement of thick‐film OSCs are well summarized to inspire more fantastic ideas on achieving efficient thick‐film OSCs. Meanwhile, the challenges on achieving efficient thick‐film OSCs are outlined.

To date, the power conversion efficiency (PCE) of lab‐scale organic solar cells (OSCs) has exceeded 17%, which heralds the bright future for commercial applications of OSCs. High‐performance OSCs with thick active layers are essential for large‐scale production. First, the relatively thick active layers should be more compatible with the roll‐to‐roll (R2R) large‐area processing, which is conducive to forming uniform and defect‐free active layers in the process of high‐speed, mass production. Second, the thick active layers can absorb more incident light in their spectral range, which helps thick‐film OSCs to obtain relatively high short‐circuit current density (J _SC). So far, relatively little attention has been paid to thick‐film OSCs, and the PCE of thick‐film OSCs lags far behind its thin‐film analogues. Herein, the recent development of thick‐film OSCs is highlighted and the critical limit factors on the PCE of thick‐film OSCs are pointed out. Some strategies are highlighted to improve the efficiency of thick‐film OSCs. This review study will be helpful to the researchers engaging in the development of efficient thick‐film OSCs.

The drastic open‐circuit voltage drop at low temperatures in bulk heterojunction and bilayer organic solar cells is found to be dominated by the competition between the photocurrent and the parasitic leakage current. The leakage current should thus be carefully optimized in temperature dependent analysis as well as in practical applications.

While the efficiency of organic solar cells (OSCs) has increased considerably in recent years, there remains a significant gap between the experimental open‐circuit voltage (V _OC) and the theoretical limit. Understanding the origin of this energy loss is important for the future development of OSCs, with temperature‐dependent measurement of V _OC an important approach to help unlock the underlying physics. Interestingly, previous studies have observed a reduction in V _OC at low temperature that has been attributed by different studies to different phenomena. To resolve this issue, herein the temperature dependence of V _OC of various polymer‐based OSC systems covering a range of acceptor types (fullerene, polymer, and non‐fullerene small molecule) as well as device architectures (conventional, inverted, blend and bilayer) is studied. Across all systems studied, V _OC reduction at low temperatures is associated with high parasitic leakage current, providing a universal explanation for this phenomenon in OSCs. Moreover, it is shown that leakage current, which causes complexity in the analysis and raises reliability concerns in potential applications, can be suppressed by varying device architecture, providing an effective approach for analyzing the true temperature dependence of V _OC.

A novel perovskite‐compatible carbon electrode based on low polar alkane solvent decreases the defect at CsPbI₂Br/carbon interface and hinders moisture in the atmosphere. The champion device obtains a power conversion efficiency (PCE) of 13.16% and provides outstanding stability with a PCE maintaining 93% of the initial value after 1000 h under a humidity of 30–40% without additional encapsulation.

Carbon electrodes are a promising alternative to metal electrodes in the access of high‐stable and low‐cost perovskite solar cells (PSCs). However, polar components (including cyclohexanone, terpineol, etc.) in commercial carbon pastes for carbon electrodes usually corrode perovskite materials, thereby deteriorating the photovoltaic performance of the resulting solar cells. Therefore, the development of perovskite‐compatible carbon pastes and carbon electrodes is of great significance in obtaining high‐performance carbon‐based PSCs. Herein, carbon pastes based on low polar alkane solvents are developed for perovskite‐compatible carbon electrode (PCCE) in the construction of carbon‐based CsPbI₂Br PSCs. The optimized cells based on PCCE offer a champion efficiency of 13.16% (J _SC = 14.33 mA cm⁻², V _OC = 1.22 V, and fill factor (FF) = 0.75), which is remarkably higher than that of commercial carbon paste‐derived counterparts (11.51%). Even without encapsulation, CsPbI₂Br PSCs based on PCCE maintain over 93% of their initial efficiency in an air atmosphere with a humidity of 30–40% for over 1000 h.

Carbon nanotube electrode–laminated perovskite and n‐type tunnel oxide–passivated contact (TOPCon) silicon solar cells exhibit 24.42% efficiency when stacked in tandem. Both semitransparency and power conversion efficiency are important for top subcells of tandem solar cells. The carbon nanotube‐based perovskite solar cells demonstrate record high efficiency among the reported four‐terminal tandem solar cells while exhibiting good semitransparency.

Carbon nanotube electrode–laminated perovskite solar cells in combination with n‐type tunnel oxide–passivated contact silicon solar cells demonstrate a high power conversion efficiency (PCE) of 24.42% when stacked in tandem. This is compared with conventional indium tin oxide/MoO _x ‐deposited perovskite solar cells which give an efficiency of 22.35% when stacked in the same four‐terminal tandem system. Despite higher transmittance of the carbon nanotube electrode than that of the indium tin oxide/MoO _x in the infrared range, the carbon nanotube electrode‐laminated devices show lower transmittance in the same region due to the total internal reflection and scattering as evidenced by optical simulation. Yet, the exceptionally high PCE of the carbon nanotube electrode‐laminated semitransparent devices far exceeding than that of the indium tin oxide/MoO _x ‐deposited semitransparent top cell outweighs the effect of the optical transparency. Four types of silicon solar cells are compared as the bottom subcells, and the n‐type tunnel oxide‐passivated contact silicon solar cells are the best choice mainly due to their high absorption in the long‐wavelength region. The obtained 24.42% efficiency is one of the high PCEs among the reported four‐terminal perovskite–silicon solar cells, and this article is the first demonstration of the carbon nanotube electrode application in tandem solar cells.

The combination of two well‐defined conjugated small‐molecule (SM) donors FG3 and FG4 and Y6 as well‐known nonfullerene SM acceptors provides the fabrication of efficient ternary OSCs. This contribution shows an excellent power conversion efficiency (PCE) of 14.31% with a high fill factor (FF) and J _sc, in contrast with the binary counter parts.

An efficient organic solar cell (OSC) based on a ternary active layer consisting of two conjugated small‐molecule (SM) donors (FG3 and FG4) and a well‐known nonfullerene SM acceptor (Y6) is fabricated using a nonhalogenated solvent. An overall power conversion efficiency (PCE) of 14.31% is achieved, higher than that for the binary counterparts, i.e., 10.75% and 11.07% for FG3:Y6 and FG4:Y6, respectively. The short‐circuit current density (J _SC) of the ternary active layer organ is related to the broader absorption spectra when compared with the binary active layers. The open‐circuit voltage (V _OC) of the ternary active layer‐based OSCs falls between those of the OSCs based on FG3:Y6 and FG4:Y6, a situation that is consistent with the lowest unoccupied molecular orbital (LUMO) level of both SM donors (FG3 and FG4), and forms the alloy between the two donors. The overlap of the absorption spectra of FG4 with the photoluminescence of FG3 confirms the energy transfer from FG3 to FG4 and this leads to improvement in J _SC. The balanced charge transport, reduced charge recombination, and the fast charge extraction in the ternary active layer leads to the higher fill factor (FF) value. A combination of all of these effects affords a high PCE value.

A new PBTI2(30HD)‐FT terpolymer acceptor enables all‐polymer solar cells (all‐PSCs) with an ultrabroad donor:acceptor (D:A) ratio tolerance from 1:30 to 10:1 and better stability than other types of organic solar cells. PBTI2(30HD)‐FT can lead to well‐maintained interpenetrating network even at very low loading in blend films, yielding decent photovoltaic performance.

Achieving a broad donor:acceptor (D:A) composition tolerance in efficient organic solar cells (OSCs) is important for printing large‐area solar cell modules. Herein, all‐polymer solar cells (all‐PSCs) based on new terpolymer acceptors and a well‐known polymer donor PTB7‐Th are fabricated to explore the effect of D:A ratio on morphology and photovoltaic performances. The all‐PSCs show a promising power conversion efficiency (PCE) of 7.23% with an optimum D:A ratio of 1:2 and retain over 40% of its optimal PCE with ultrabroad D:A composition tolerance from 1:30 to 10:1. In addition, the all‐PSCs can maintain 90% of its original PCE after 400 h of storage despite such broad range of D:A ratio, which is much better than those of other types of OSCs and even better than the benchmark all‐polymer system with N2200 as the acceptor under the same condition. The results show the superiority of the all‐PSCs in terms of D:A ratio tolerance and performance stability, which should be conducive to practical applications of all‐PSCs.

High‐efficiency solution‐processed hybrid tandem photovoltaic devices, employing inorganic perovskite and organic bulk‐heterojunction as the photoactive layers, are demonstrated. A PCE of 18.04% in the hybrid tandem device is achieved, which is significantly higher than the comparable single‐junction devices, owing to a near‐optimal absorption spectral match.

Abstract

Although the power conversion efficiency (PCE) of inorganic perovskite‐based solar cells (PSCs) is considerably less than that of organic‐inorganic hybrid PSCs due to their wider bandgap, inorganic perovskites are great candidates for the front cell in tandem devices. Herein, the low‐temperature solution‐processed two‐terminal hybrid tandem solar cell devices based on spectrally matched inorganic perovskite and organic bulk heterojunction (BHJ) are demonstrated. By matching optical properties of front and back cells using CsPbI₂Br and PTB7‐Th:IEICO‐4F BHJ as the active materials, a remarkably enhanced stabilized PCE (18.04%) in the hybrid tandem device as compared to that of the single‐junction device (9.20% for CsPbI₂Br and 10.45% for PTB7‐Th:IEICO‐4F) is achieved. Notably, the PCE of the hybrid tandem device is thus far the highest PCE among the reported tandem devices based on perovskite and organic material. Moreover, the long‐term stability of inorganic perovskite devices under humid conditions is improved in the hybrid tandem device due to the hydrophobicity of the PTB7‐Th:IEICO‐4F back cell. In addition, the potential promise of this type of hybrid tandem device is calculated, where a PCE of as much as ≈28% is possible by improving the external quantum efficiency and reducing energy loss in the sub‐cells.

Introduction of multi‐cation hybrid halide perovskite quantum dots reduces ionic defects at the surface and grain boundaries via a solid‐state interdiffusion process. The enhanced moisture and photostability enable power conversion efficiency (PCE) exceeding 21% to be achieved with more than 90% of its initial PCE retained on exposure to continuous illumination of more than 550 h.

Abstract

Realization of reduced ionic (cationic and anionic) defects at the surface and grain boundaries (GBs) of perovskite films is vital to boost the power conversion efficiency of organic–inorganic halide perovskite (OIHP) solar cells. Although numerous strategies have been developed, effective passivation still remains a great challenge due to the complexity and diversity of these defects. Herein, a solid‐state interdiffusion process using multi‐cation hybrid halide perovskite quantum dots (QDs) is introduced as a strategy to heal the ionic defects at the surface and GBs. It is found that the solid‐state interdiffusion process leads to a reduction in OIHP shallow defects. In addition, Cs⁺ distribution in QDs greatly influences the effectiveness of ionic defect passivation with significant enhancement to all photovoltaic performance characteristics observed on treating the solar cells with Cs_0.05(MA_0.17FA_0.83)_0.95PbBr₃ (abbreviated as QDs‐Cs5). This enables power conversion efficiency (PCE) exceeding 21% to be achieved with more than 90% of its initial PCE retained on exposure to continuous illumination of more than 550 h.

The inclusion of a novel in situ polymerizable ionic liquid, 1,3‐bis(4‐vinylbenzyl)imidazolium chloride ([bvbim]Cl), allows perovskite films to be manufactured under humid environments, conferring improved materials quality, higher power conversion efficiency, and long‐term stability.

Abstract

Despite the excellent photovoltaic properties achieved by perovskite solar cells at the laboratory scale, hybrid perovskites decompose in the presence of air, especially at high temperatures and in humid environments. Consequently, high‐efficiency perovskites are usually prepared in dry/inert environments, which are expensive and less convenient for scale‐up purposes. Here, a new approach based on the inclusion of an in situ polymerizable ionic liquid, 1,3‐bis(4‐vinylbenzyl)imidazolium chloride ([bvbim]Cl), is presented, which allows perovskite films to be manufactured under humid environments, additionally leading to a material with improved quality and long‐term stability. The approach, which is transferrable to several perovskite formulations, allows efficiencies as high as 17% for MAPbI₃ processed in air % relative humidity (RH) ≥30 (from an initial 15%), and 19.92% for FAMAPbI₃ fabricated in %RH ≥50 (from an initial 17%), providing one of the best performances to date under similar conditions.

The development of the first high‐performance, printable CsPbI₃ solar cells via an ambient blade‐coating technique is reported. High‐quality CsPbI₃ films are grown via the introduction of a low concentration of the multifunctional molecular additive Zn(C₆F₅)₂. As a result, the additive‐treated perovskite solar cell delivers a power conversion efficiency (PCE) of 19%.

Abstract

All‐inorganic CsPbI₃ holds promise for efficient tandem solar cells, but reported fabrication techniques are not transferrable to scalable manufacturing methods. Herein, printable CsPbI₃ solar cells are reported, in which the charge transporting layers and photoactive layer are deposited by fast blade‐coating at a low temperature (≤100 °C) in ambient conditions. High‐quality CsPbI₃ films are grown via introducing a low concentration of the multifunctional molecular additive Zn(C₆F₅)₂, which reconciles the conflict between air‐flow‐assisted fast drying and low‐quality film including energy misalignment and trap formation. Material analysis reveals a preferential accumulation of the additive close to the perovskite/SnO₂ interface and strong chemisorption on the perovskite surface, which leads to the formation of energy gradients and suppressed trap formation within the perovskite film, as well as a 150 meV improvement of the energetic alignment at the perovskite/SnO₂ interface. The combined benefits translate into significant enhancement of the power conversion efficiency to 19% for printable solar cells. The devices without encapsulation degrade only by ≈2% after 700 h in air conditions.