Chen Weijie

Polystyrene is added into PBDB‐T:ITIC active layers of organic solar cells, leading to a power conversion efficiency enhancement of up to 16% relative to the control device. Other insulating polymers can also improve the performance of the organic solar cells for different levels dependent on the polymer‐side chain size. This work provides a guideline for the selection of polymer additives in organic solar cells.

A series of insulating polymers are used as additives in nonfullerene organic solar cells (OSCs) for the first time. A significant relative power conversion efficiency (PCE) enhancement of up to 16% is observed with an introduction of polystyrene for only 5.0 wt% into the active layer of OSCs. Other insulating polymers possessing linear nonconjugated backbones with different side chains are also incorporated into OSCs and the resultant PCE enhancement decreases with the decrease in the side chain size. Another important issue that is noted is the glass transition temperature of the polymer additive. When the glass transition temperature is higher than the thermal annealing temperature of the active layer, the polymer additive plays a negative effect on the device performance and the device efficiency decreases monotonically with the increase in addition amount. So the effect of the insulating polymer additives in nonfullerene OSCs can be attributed to the reconstruction of the active layer films, which increases the crystallinity, carrier mobility, and carrier lifetime of the organic semiconductors in the bulk heterojunction of the devices. This work provides a guideline for the selection of polymer additives in OSCs apart from the consideration on the optoelectronic property of the additives.

The introduction of F5PEA⁺ to partially replace PEA⁺ as the 2D perovskite passivation agent, with a strong noncovalent interaction between the two bulky cations and enhanced charge transport, is reported to improve the performance (from 19.58% to 21.10%) and stability of the corresponding wide‐bandgap perovskite solar cells.

The replacement of a small amount of organic cations with bulkier organic spacer cations in the perovskite precursor solution to form a 2D perovskite passivation agent (2D‐PPA) in 3D perovskite thin films has recently become a promising strategy for developing perovskite solar cells (PSCs) with long‐term stability and high efficiency. However, the long, bulky organic cations often form a barrier, hindering charge transport. Herein, for the first time, 2D‐PPA engineering based on wide‐bandgap (≈1.68 eV) perovskites are reported. Pentafluorophenethylammonium (F5PEA⁺) is introduced to partially replace phenylethylammonium (PEA⁺) as the 2D‐PPA, forming a strong noncovalent interaction between the two bulky cations. The charge transport across and within the planes of pure 2D perovskites, based on mixed ammoniums, increases by a factor of five and three compared with that of mono‐cation 2D perovskites, respectively. The perovskite films based on mixed‐ammonium (F5PEA⁺‐PEA⁺) 2D‐PPA exhibit similar surface morphology and crystal structure, but longer carrier lifetime, lower exciton binding energy, less trap density and higher conductivity, in comparison with those using mono‐cation (PEA⁺) 2D‐PPA. The performance of PSCs based on mixed‐cation 2D‐PPA is enhanced from 19.58% to 21.10% along with improved stability, which is the highest performance for reported wide‐bandgap PSCs.

Herein, polyethylene glycol (PEG) is used as an additive for the morphology control of lead‐reduced perovskite films. The power conversion efficiency of lead‐reduced perovskite solar cells with PEG additive improves from 13.7% to 16.1% without J –V hysteresis due to pinhole elimination of the perovskite film.

The organic–inorganic halide perovskite solar cells (PSCs) are rapidly developed in just a few years due to its high power conversion efficiency. However, it still faces some critical issues, one of which is the presence of toxic lead (Pb²⁺). Recent researches show that barium (Ba²⁺) can partially replace the Pb²⁺ in perovskite structure and achieve a promising device performance because of its adequate ionic radius. However, the optimal replacement amount of Ba²⁺ in perovskite is still limited. Herein, the methylammonium (MA)/formamidinium (FA) mixed‐cation perovskite is used as the active layer in PSCs and Pb²⁺ is partially substituted with Ba²⁺. Compared with the pure MA system, the best device efficiency can be achieved using higher Ba²⁺ replacement ratio. In addition, while introducing the appropriate polymer additive, the replacement ratio can be further increased without compromise of device efficiency. Using polyethylene glycol as polymer additive, 10.0 mol% Ba‐doped MA/FA mixed‐cation PSC with an efficiency of 16.1% can be realized. It is believed that this report provides an effective strategy to fabricate high‐performance lead‐reduced PSCs.

Nano‐TiO₂ is unprecedentedly used to load the commercial dye N719 forming N719@TiO₂ nanoparticles which promotes charge extraction and suppress shallow defects in the fully printable carbon‐based perovskite solar cells due to surface carboxyl groups comprising the ligands of the N719 dye. Accordingly, the optimal power conversion efficiency increases from 12.00% (control) to 13.95% (N719@TiO₂).

Shallow defects are one of the energy states that trap photoexcited electrons leading to charge recombination and limit the increase in the photocurrent of perovskite solar cells (PSCs). Due to the large perovskite thickness and uncontrollable crystallization processes, suppressing shallow defects, especially methylamine (MA) vacancies, has become a key challenge for fully printable PSCs. Herein, nano‐TiO₂ is unprecedentedly used to load the commercial dye N719, forming N719@TiO₂ nanoparticles, which crucially improves the passivation effect of MA vacancies on the surface of perovskite and charge extraction, by the unbounded carboxyl group of N719 as a shell on the surface of TiO₂. Meanwhile, the core TiO₂ serves as a centre to bind the dyes, assisting the perovskite crystallization and enhancing the passivation effect. It is found that the charge extraction increases to 1.8007 × 10⁻⁹ C for the devices based on N719@TiO₂ from 1.5507 × 10⁻⁹ C for the control group. Simultaneously, the short‐circuit current density (J _sc) is significantly enhanced to 23.58 mA cm⁻² in the device containing N719@TiO₂ over that of the control device (21.95 mA cm⁻²). This opens up a novel pathway to reduce shallow defects in PSCs via organic passivator with carboxyl anchoring group loaded on n‐type semiconductors (nano‐TiO₂).

Rubidium‐incorporated air‐stable Cs_{(1 −  x )}Rb_xPbI₂Br perovskite solar cells are fabricated through a surface passivation strategy. The resulting devices under optimal conditions yield an efficiency of over 15% with excellent long‐term thermal as well as light‐soaking stability in ambient atmosphere.

Inorganic CsPbI₂Br perovskite has gained growing attention due to its potential for improving device performance and stability. However, the notorious phase transition from the photoactive to photoinctive phase in ambient atmosphere hinders its further development. Herein, air‐stable rubidium (Rb)‐incorporated Cs_{(1 −  x )}Rb_xPbI₂Br perovskite with guanidinium bromide (GABr) post‐treatment is demonstrated. The incorporation of smaller monovalent Rb cation contributes to a contraction of the perovskite crystal, leading to an improvement in structure stability. In addition, GABr modification induces a 2D/3D heterostructure perovskite with high crystallinity, appropriate surface morphology, favorable electronic properties, and significantly reduced trap‐state density. Consequently, the fabricated perovskite solar cells deliver a power conversion efficiency (PCE) of 15.6%, which is much higher than the 12.9% reported for reference CsPbI₂Br‐based devices. Meanwhile, the significantly enhanced long‐term (88% of initial PCE after 60 days), thermal (76% of initial PCE after 30 days) as well as light soaking (90% of initial PCE after 300 min) stability in ambient atmosphere is demonstrated.

A nonhalogen organic salt sodium p‐toluenesulfonate (STS) is applied during the surface modification of perovskite films for the first time, yielding an obvious enhancement of power conversion efficiency from 18.70% to 20.05% for perovskite solar cells, which originates from the surface passivation of the perovskite film with reduced trap state densities and suppressed interfacial charge recombination.

Ionic defects at the surfaces of organolead halide perovskite films are detrimental to both the efficiency and stability of perovskite solar cells (PSCs). Herein, sodium p‐toluenesulfonate (STS) is applied during the surface modification of perovskite layer for the first time, leading to the efficient surface passivation of the perovskite film and consequently significant enhancements in both efficiency and stability of mixed‐cation PSC devices. Upon incorporating STS atop the perovskite layer, the power conversion efficiency of the Cs_0.05MA_0.12FA_0.83PbI_2.55Br_0.45 (abbreviated as CsMAFA) mesoporous‐structure mixed‐cation PSC devices improves from 18.70% to 20.05% with reduced hysteresis. The sulfonate (–SO₃ ⁻) anion of STS coordinates with the Pb²⁺ of CsMAFA perovskite, and the Na⁺ cation of STS electrostatically interacts with the anions (I⁻/Br⁻) of CsMAFA perovskite, resulting in the surface passivation of the CsMAFA perovskite film with reduced electron and hole trap state densities. In addition, STS modification induces an upshift of the valence band of perovskite, facilitating hole extraction from the perovskite layer to the hole transport layer with suppressed interfacial charge recombination. Moreover, such a trap state passivation of perovskite film leads to improvement of the ambient stability of PSC devices.

For the first time, the successful discrimination and spatial mapping of all three organic components in the active layer of a ternary organic solar cell are achieved with analytical transmission electron microscopy. Knowledge of this nanomorphology is key to understanding photophysical processes and is thus indispensable to further improve the device performance.

Elucidating the complex materials distribution in the active layers of ternary organic solar cells is one of the greatest challenges in the field of organic photovoltaics. Knowledge of the nanomorphology is key to understanding photophysical processes (e.g., charge separation, adjustment of the recombination mechanism, and suppression of the radiationless and energetic losses) and thus improving the device performance. Herein, for the first time, the successful discrimination and spatial mapping of the active layer components of a ternary organic solar cell are demonstrated using analytical transmission electron microscopy. The material distribution of all three organic components is successfully visualized by multimodal imaging using complementary electron energy loss signals. A complete picture of the morphological aspects can be gained by studying the lateral and cross‐sectional morphology as well as the morphology evolution as a function of the mixing ratio of the polymers. Finally, a correlation between the morphology, photophysical processes, and device performance of the ternary and the reference binary system is achieved, explaining the differences of the power conversion efficiency between the two systems.

By varying thermal annealing conditions, a thermally induced perovskite crystal control process of the wide‐bandgap perovskite films provides an opportunity to exploit both lead‐iodide passivation and perovskite orientation strategies with a fixed E _g of 1.73 eV. Based on this concept, the device efficiency is improved from 15.76% to 18.60% and the operational photostability is also enhanced without any encapsulation in ambient conditions.

Wide‐bandgap perovskite solar cells (WBG PSCs) have gained attention as promising tandem partners for silicon solar cells due to their complementary absorption, superb open‐circuit voltage, and an easy solution process. Recently, both their performance and stability have been improved by compositional engineering or defect passivation strategies, due to the modulation of perovskite crystal size and reduction of crystal defects. Herein, a report on the thermally induced phase control (TIPC) strategy is provided, which enables efficient and photostable WBG PSCs without compositional engineering by exploring a thermal annealing process window (100–175 °C and 3–60 min) of the WBG perovskite films. Within this window, a key annealing regime is found that produces preferred crystal orientations of lead iodide and the WBG perovskite, suppressing phase segregation and reducing charge recombination in the perovskites. The WBG PSCs (composition of FA_0.75MA_0.15Cs_0.1PbI₂Br and E _g of 1.73 eV) optimized by TIPC exhibit an excellent power conversion efficiency (PCE) of 18.60% and improved operational stability, maintaining >90% of the maximum PCE (during maximum power point tracking) without encapsulation after 12 h of operation (air mass 1.5 global irradiation in ambient air conditions) or after 500 h of operation (white light‐emitting diode irradiation (100 mW cm⁻²) in N₂ conditions).

An oxidized phenothiazine‐based (OPTZ) hole transporting material (HTM) synthesized from its neutral form (NPTZ) is used to accurately tune the concentration of radical cations in HTMs via its stoichiometric ratio. Using the optimized ratio of OPTZ as the dopant in the HTM, the hole transporting mobility is effectively enhanced, due to the intra‐and intermolecular charge transfer process, thus increasing the fill‐factor of perovskite solar cells.

In traditional n‐i‐p‐type perovskite solar cells (PSCs), most hole transporting materials (HTMs) rely on an uncontrolled oxidative process using Li salt and Co (III) complex to achieve sufficient hole mobilities. Herein, a stabilized oxidized phenothiazine‐based HTM (OPTZ) synthesized from its neutral form (NPTZ) through a photoredox reaction is demonstrated. This controllable and stable oxidation state is mainly derived from the planar structure and π conjugation of phenothiazine core in OPTZ. The energy gap between the singly occupied molecular orbital (SOMO) of OPTZ and highest occupied molecular orbital (HOMO) of NPTZ suitably promotes hole hopping in hole transporting layers. Using an optimized ratio of OPTZ as the dopant in NPTZ, the hole transporting mobility is effectively enhanced due to an intra‐ and intermolecular charge transfer process, resulting in an enhancement in the fill factor of the PSCs. Herein, a new strategy to obtain stabilized oxidized HTMs, which deliver significantly enhanced hole mobilities of HTMs in PSCs, is provided.

A new strategy is established to improve the performance of perovskite solar cells, which sheds more light on the currently proposed mechanism governing the action of moisture on the quality of perovskite film. Self‐passivated perovskite solar cells show an extraordinary V_OC of 1.17 V and the highest efficiency of 21.38%.

The performance of perovskite solar cells (PSCs) is known to be extremely sensitive to humidity in the preparation environment. However, the main mechanism by which the moisture influences the quality of the perovskite film and the device performance is not yet fully understood. Herein, a new strategy is established to obtain inverted PSCs with a remarkabll high V _OC by including a high‐humidity treatment and sufficient DMSO‐atmosphere annealing in the preparation process. It is found that the lattice distortion on the surface of perovskite grains caused by the high‐humidity treatment plays a key role in the self‐passivation of perovskite. Inverted (p‐i‐n) PSCs based on the self‐passivated perovskite films show effective suppression of nonradiative recombination, which increase the device V _OC to 1.17 V and achieve the highest efficiency of 21.38%. It is expected that the findings of this work shed more light on the currently proposed mechanism governing the action of moisture on the performance of the PSCs.

Herein, an antireflective cascaded SnO₂/TiO₂–Cl electron transport layer is devised to reduce the primary optical reflection of planar perovskite solar cells (PSCs) at the front side. A record‐high short‐circuit current density of 26.1 mA cm⁻² and a high power conversion efficiency of 22.9% are achieved in FAPbI₃‐based planar PSCs.

Planar perovskite solar cells (PSCs) hold promise for simple processing at low temperatures; however, they usually show lower short‐circuit current density (J _sc) than their mesoporous counterparts owing to their higher primary optical reflection losses at the front side. The antireflective nature of a mesoporous electron transport layer (ETL) enables a low optical reflection in the front surface of solar cells, which is challenging to realize in planar PSCs. Herein, an antireflective cascaded ETL structure using SnO₂/TiO₂–Cl bilayers is devised to reduce optical reflection losses and to improve electrical performance in planar PSCs. The antireflective cascaded ETL results in an enhanced J _sc of 25.4 mA cm⁻² in formamidinium lead triiodide based planar PSCs, compared with the control J _sc of 24.6 mA cm⁻² using single‐layered SnO₂ ETL. A record‐high J _sc of 26.1 mA cm⁻² is further achieved using an additional antireflective coating on the front glass side, leading to a power conversion efficiency of 22.9%.

The A‐site incorporation in the all‐inorganic cesium lead mixed halide (CsPbI₂Br) perovskite facilitates thermodynamic stability. The Rb cation‐incorporated Cs_1−x M _x PbI₂Br (M = Rb)‐based perovskite absorber layer processed by hot air method under ambient conditions with additives doped poly(3‐hexylthiophene‐2,5‐diyl) as a hole‐transporting layer produces a power conversion efficiency of more than 17%.

Due to its excellent thermal stability and high performance, inorganic cesium lead mixed halide (ABX₃, where A  = Cs, B = Pb, and X = I/Br) all‐inorganic perovskite solar cells (IPVSCs) have attracted much interest in optoelectronic applications. However, the film quality, enough absorption by desired film thickness, and nature of partial replacement of cations determine the stability of the CsPbI₂Br perovskite films. Herein, a hot air method is used to control the thickness and morphology of the CsPbI₂Br perovskite thin film, and the A‐site (herein, Cs⁺) cation is partially incorporated by rubidium (Rb⁺) cations for making the stable black phase under ambient conditions. The Rb cation‐incorporated Cs_1−x Rb _x PbI₂Br (x  = 0–0.03) perovskite thin films exhibit high crystallinity, uniform grains, extremely dense, and pinhole‐free morphology. The fabricated device with its Cs_0.99Rb_0.01PbI₂Br perovskite composition with poly(3‐hexylthiophene‐2,5‐diyl) as a hole‐transporting layer exhibits a power conversion efficiency (PCE) of 17.16%, which is much higher than that of CsPbI₂Br‐based IPVSCs. The fabricated Cs_0.99Rb_0.01PbI₂Br‐based IPVSC devices retain >90% of the initial efficiency over 120 h at 65 °C thermal stress, which is much higher than that of CsPbI₂Br samples.

Combined data of transient optical and electro‐optical experiments reveals the efficiency determining processes in all‐polymer solar cells and precisely quantifies their yields. For the test system presented here, field‐dependent charge separation limits the fill factor and thus the performance evident by comparing the experimentally measured current–voltage characteristics to those reproduced by drift‐diffusion simulations using the spectroscopically determined kinetic parameters.

All‐polymer solar cells lag behind the state‐of‐the‐art in small molecule nonfullerene acceptor (NFA) bulk heterojunction (BHJ) organic solar cells (OSCs) for reasons still unclear. Herein, the efficiency‐limiting processes in all‐polymer solar cells are investigated using blends of the common donor polymer PBDT‐TS1 with different acceptor polymers, namely P2TPD[2F]T and P2TPDBT[2F]T. Combining data from steady‐state optical spectroscopy and time‐resolved photoluminescence, transient absorption, and time‐delayed collection field experiments, provides not only a concise but also quantitative assessment of the losses due to limited photon absorption, geminate and nongeminate charge carrier recombination, field‐dependent charge generation, and inefficient carrier extraction. Although both systems exhibit a similar charge separation efficiency in the absence of external bias, charge separation is significantly enhanced in P2TPDBT[2F]T‐based blends when biased. Kinetic parameters obtained via pulsed laser spectroscopy are used to reproduce the experimentally measured device current–voltage (J –V ) characteristics and indicate that low fill factors originate either from nongeminate recombination competing with charge extraction, or from a pronounced field dependence of charge generation, depending on the acceptor polymer. The methodology presented here is generic and can be used to quantify the loss processes in BHJ OSCs including both all‐polymer and small molecule NFA systems.

An excellent‐performance and high‐CO‐selectivity N‐doped SnO _x @Sn/C electrocatalyst for CO₂ reduction is designed and synthesized. The modulation of surface electronic structure by introducing N element promotes electrical conductivity and catalytic selectivity. The optimized catalyst is used as a CO₂ reduction reaction (CO₂RR) electrode in a PV CO₂RR/OER system to achieve very durable and high‐efficiency solar‐to‐fuel conversion.

A plum pudding‐like Sn‐based electrocatalyst is synthesized by calcinating the precursor SnC₂O₄ on carbon black (CB) with polymeric carbon nitride (CN). This material exhibits a structure of a Sn metallic ball coated with a nitrogen‐doped SnO _x native layer (N‐doped SnO _x @Sn) embedded on a carbon matrix. The carbon matrix originates from the CB and the polymeric CN decomposition promotes electrical conductivity, leading to high electrochemical activity of the CN–Sn catalyst. The introduction of nitrogen that occupies the interstitial space of the surface SnO _x layer further enhances electron transport; furthermore, it provides an electron‐rich environment for oxygen because of its lower electronegativity, which is the fundamental cause of selectivity in the electrochemical reduction of CO₂ to CO. The maximum CO Faradaic efficiency over the optimal catalyst reaches 57.5%, with a high CO partial current density of 6.09 mA cm⁻² at −0.7 V versus reversible hydrogen electrode. This catalyst is further applied to construct a photovoltaic–electrocatalytic CO₂ reduction/oxygen evolution reaction device to stably convert CO₂ to chemicals over 6 h at a high solar‐to‐fuel efficiency of 11.3%. This work explores a strategy of rational modulation on surface electronic structure to obtain high‐performance electrocatalysts, inspiring the selectivity tuning in electrochemical CO₂ reduction via electronegativity difference of various elements.

A narrow‐bandgap polymer acceptor PF3‐DTCO is developed by increasing the conjugation of the acceptor unit from the five‐ring‐fused IDIC16 to the seven‐ring‐fused ITIC16 and enhancing the electron‐donating ability of the donor unit from carbon‐bridged DTC to carbon–oxygen‐bridged DTCO, and a high power conversion efficiency of 10.13% with a high J _sc of 15.75 mA cm⁻² in all‐polymer solar cells is achieved.

The limited light absorption capacity for most polymer acceptors hinders the improvement of the power conversion efficiency (PCE) of all‐polymer solar cells (all‐PSCs). Herein, by simultaneously increasing the conjugation of the acceptor unit and enhancing the electron‐donating ability of the donor unit, a novel narrow‐bandgap polymer acceptor PF3‐DTCO based on an A–D–A‐structured acceptor unit ITIC16 and a carbon–oxygen (C–O)‐bridged donor unit DTCO is developed. The extended conjugation of the acceptor units from IDIC16 to ITIC16 results in a red‐shifted absorption spectrum and improved absorption coefficient without significant reduction of the lowest unoccupied molecular orbital energy level. Moreover, in addition to further broadening the absorption spectrum by the enhanced intramolecular charge transfer effect, the introduction of C–O bridges into the donor unit improves the absorption coefficient and electron mobility, as well as optimizes the morphology and molecular order of active layers. As a result, the PF3‐DTCO achieves a higher PCE of 10.13% with a higher short‐circuit current density (J _sc) of 15.75 mA cm⁻² in all‐PSCs compared with its original polymer acceptor PF2‐DTC (PCE = 8.95% and J _sc = 13.82 mA cm⁻²). Herein, a promising method is provided to construct high‐performance polymer acceptors with excellent optical absorption for efficient all‐PSCs.

Herein, an all‐solution and all‐air processed organic solar cell is demonstrated. Optimized devices show a high power conversion efficiency of 11.12% along with promoted storage stability. The unique all‐solution and all‐air processing is highly desirable for inventing the cost‐effective printed organic photovoltaics.

Nonfullerene organic photovoltaics (OPVs) have achieved a breakthrough in pushing the efficiency beyond 15%. Although this sheds light on OPV commercialization, the high cost associated with the scalable device fabrications remains a giant challenge. Herein, a vacuum‐free, all‐solution and all‐air processed OPV is reported that yields 11.12% efficiency with a fill factor of 0.725, due to the usages of high‐merit polymeric electrodes and modified active blends. The design principle toward the high‐merit electrodes is to induce heavy acid doping into the matrices for a raised carrier concentration and mobility, make a large removal of insulating components in the whole matrices rather than surfaces, and restrain the formation of large‐domain aggregates. A unique layer‐by‐layer doping is developed to enable the polymeric electrodes with record‐high trade‐offs between optical transmittance and electrical conductivity. Moreover, solvent vapor annealing is proposed to boost device efficiency and it has the advantages of finely adjusting the active blend morphology and raising the electron mobility. The resulting devices are highly efficient and most (≈91%) of the initial efficiency are maintained in 30 day storage. This work indicates bright future for making cost‐effective all‐solution processed OPVs in air.

Publication date: September 2020

Source: Nano Energy, Volume 75

Author(s): Ignasi Burgués-Ceballos, Yongjie Wang, M. Zafer Akgul, Gerasimos Konstantatos

Publication date: August 2020

Source: Nano Energy, Volume 74

Author(s): Jungrak Choi, Donguk Kwon, Byeongsu Kim, Kyungnam Kang, Jimin Gu, Jihwan Jo, Kwangmin Na, Junseong Ahn, Dionisio Del Orbe, Kyuyoung Kim, Jaeho Park, Jongmin Shim, Jung-Yong Lee, Inkyu Park

FAPbI₃‐based perovskite materials are of interest for photovoltaics in view of their close‐to‐ideal bandgap; however, FAPbI₃‐based materials suffer from notorious phase transition from the photoactive black phase (α‐FAPbI₃) to nonperovskite yellow phase (δ‐FAPbI₃) under ambient conditions. This study reveals that 1‐hexyl‐3‐methylimidazolium iodide ionic liquid incorporation stabilizes the α‐FAPbI₃ phase and exhibits a promising efficiency exceeding 20% with excellent long‐term operational and shelf‐stability.

Abstract

Formamidinium lead triiodide (FAPbI₃)‐based perovskite materials are of interest for photovoltaics in view of their close‐to‐ideal bandgap, allowing absorption of photons over a broad solar spectrum. However, FAPbI₃‐based materials suffer from a notorious phase transition from the photoactive black phase (α‐FAPbI₃) to nonperovskite yellow phase (δ‐FAPbI₃) under ambient conditions. This transition dramatically reduces light absorbtion, thus, degrading the photovoltaic performance and stability of ensuring solar cells. In this study, 1‐hexyl‐3‐methylimidazolium iodide (HMII) ionic liquid (IL) is employed as an additive for the first time in FAPbI₃ perovskite to overcome the above‐mentioned issues. HMII incorporation facilitates the grain coarsening of FAPbI₃ crystal owing to its high‐polarity and high‐boiling point, which yields liquid domains between neighboring grains to reduce the activation energy of the grain‐boundary migration. As a result, the FAPbI₃ active layer exhibits micron‐sized grains with substantially suppressed parasitic traps with an Urbach energy reduced by 2 meV. Hence, the resulting perovskite solar cell achieves an efficiency of 20.6% with notable increase in open circuit voltage (V _OC) of 80 mV compared with HMII‐free cells (17.1%). More importantly, the HMII‐doped FAPbI₃‐based cells show a striking enhancement in shelf‐stability under high humidity and thermal stress, retaining >80% of their initial efficiencies at 60 ± 10% relative humidity and ≈95% at 65 °C.

A better way to polymer semiconductors : Distannylated electron‐deficient bithiophene imide (BTI‐Tin) leads to acceptor–acceptor type polymers, including both homopolymers and copolymers, with high molecular weights. When applied in organic electronic devices, the polymers synthesized from BTI‐Tin show greatly improved device performance over analogue polymers synthesized from the dibrominated monomers.

Abstract

A distannylated electron‐deficient bithiophene imide (BTI‐Tin) monomer was synthesized and polymerized with imide‐functionalized co‐units to afford homopolymer PBTI and copolymer P(BTI‐BTI2), both featuring an acceptor–acceptor backbone with high molecular weight. Both polymers exhibited excellent unipolar n‐type character in transistors with electron mobility up to 2.60 cm² V⁻¹ s⁻¹. When applied as acceptor materials in all‐polymer solar cells, PBTI and P(BTI‐BTI2) achieved high power‐conversion efficiency (PCE) of 6.67 % and 8.61 %, respectively. The PCE (6.67 %) of polymer PBTI, synthesized from the distannylated monomer, is much higher than that (0.14 %) of the same polymer PBTI*, synthesized from typical dibrominated monomer. The 8.61 % PCE of copolymer P(BTI‐BTI2) is also higher than those (<1 %) of homopolymers synthesized from dibrominated monomers. The results demonstrate the success of BTI‐Tin for accessing n‐type polymers with greatly improved device performance.

Publication date: September 2020

Source: Applied Materials Today, Volume 20

Author(s): Shuangshuang Liu, Li Guan, Tao Zhang, Xiu Gong, Xiaojuan Zhao, Qiang Sun, Xuxia Shai, Xiao Li Zhang, Xin Xiao, Yan Shen, Mingkui Wang

FAPbI₃‐based perovskite materials are of interest for photovoltaics in view of their close‐to‐ideal bandgap; however, FAPbI₃‐based materials suffer from notorious phase transition from the photoactive black phase (α‐FAPbI₃) to nonperovskite yellow phase (δ‐FAPbI₃) under ambient conditions. This study reveals that 1‐hexyl‐3‐methylimidazolium iodide ionic liquid incorporation stabilizes the α‐FAPbI₃ phase and exhibits a promising efficiency exceeding 20% with excellent long‐term operational and shelf‐stability.

Abstract

Formamidinium lead triiodide (FAPbI₃)‐based perovskite materials are of interest for photovoltaics in view of their close‐to‐ideal bandgap, allowing absorption of photons over a broad solar spectrum. However, FAPbI₃‐based materials suffer from a notorious phase transition from the photoactive black phase (α‐FAPbI₃) to nonperovskite yellow phase (δ‐FAPbI₃) under ambient conditions. This transition dramatically reduces light absorbtion, thus, degrading the photovoltaic performance and stability of ensuring solar cells. In this study, 1‐hexyl‐3‐methylimidazolium iodide (HMII) ionic liquid (IL) is employed as an additive for the first time in FAPbI₃ perovskite to overcome the above‐mentioned issues. HMII incorporation facilitates the grain coarsening of FAPbI₃ crystal owing to its high‐polarity and high‐boiling point, which yields liquid domains between neighboring grains to reduce the activation energy of the grain‐boundary migration. As a result, the FAPbI₃ active layer exhibits micron‐sized grains with substantially suppressed parasitic traps with an Urbach energy reduced by 2 meV. Hence, the resulting perovskite solar cell achieves an efficiency of 20.6% with notable increase in open circuit voltage (V _OC) of 80 mV compared with HMII‐free cells (17.1%). More importantly, the HMII‐doped FAPbI₃‐based cells show a striking enhancement in shelf‐stability under high humidity and thermal stress, retaining >80% of their initial efficiencies at 60 ± 10% relative humidity and ≈95% at 65 °C.

A novel material called phenylhydrazinium iodide (PHAI) is effective for defects minimization, surface passivation, and efficient charge transportation in hybrid perovskite solar cells. It plays multiple roles in controlled crystallization, stabilizing under‐coordinated ions, and as a self‐supported moisture barrier in perovskite films.

Abstract

In recent years, hybrid perovskite solar cells (HPSCs) have received considerable research attention due to their impressive photovoltaic performance and low‐temperature solution processing capability. However, there remain challenges related to defect passivation and enhancing the charge carrier dynamics of the perovskites, to further increase the power conversion efficiency of HPSCs. In this work, the use of a novel material, phenylhydrazinium iodide (PHAI), as an additive in MAPbI₃ perovskite for defect minimization and enhancement of the charge carrier dynamics of inverted HPSCs is reported. Incorporation of the PHAI in perovskite precursor solution facilitates controlled crystallization, higher carrier lifetime, as well as less recombination. In addition, PHAI additive treated HPSCs exhibit lower density of filled trap states (10¹⁰ cm⁻²) in perovskite grain boundaries, higher charge carrier mobility (≈11 × 10⁻⁴ cm² V⁻¹ s), and enhanced power conversion efficiency (≈18%) that corresponds to a ≈20% improvement in comparison to the pristine devices.

An effective strategy is demostrated to create a double barrier that not only blocks the invasion of the moisture but also takes advantage of the permeated moisture to increase the moisture durability of perovskite films, which results in an n–i–p perovskite solar cell with moisture stability over 115 days in a relative humidity of 70% and a champion efficiency up to 21.34%.

Abstract

The rapid growth in the device efficiency of perovskite solar cells (PSCs) has raised great demands for tackling their long‐term stability upon external environmental stimuli that restricts the commercialization of PSCs, in which the instability upon exposure to moisture has been one of the major obstacles. Herein, an effective way of building up double barriers for moisture degradation of the perovskite films is demonstrated by modifying them with rationally selected hydrolyzable hydrophobic molecules (1H,1H,2H,2H‐perfluorooctyl trichlorosilane, PFTS). The layer of oligomer derived from the hydrolyzed PFTS at the surface that increases the hydrophobicity of perovskite film could serve as an efficient wall preventing the moisture invasion. The long‐term exposure of the film upon moisture allows for the formation of a secondary wall that employs the hydrolyzation of PFTS at grain boundaries, favoring defects passivation to further improve the humidity stability. Such gradual hydrolyzation is encouragingly helpful for the enhancement of the open‐circuit voltage of the PSCs from the original 1.136 up to 1.205 V. The PSCs constructed with the double barriers demonstrate excellent stability upon moisture and improved thermal and light stabilities, as well as a champion power conversion efficiency up to 21.34%.

An anomalous, persistent, Stark effect from electroabsorption and nanosecond transient absorption measurement related to the presence of long‐lived screened electron–hole pairs in 2D perovskite is presented. A peculiarity in these materials, contrasting with large exciton binding energies, is presented, revealing a complex photophysical response and highlighting the role of cation's chemistry on the material's optical response.

Abstract

2D hybrid perovskites (2DP) are versatile materials, whose electronic and optical properties can be tuned through the nature of the organic cations (even when those are seemingly electronically inert). Here, it is demonstrated that fluorination of the organic ligands yields glassy 2DP materials featuring long‐lived correlated electron–hole pairs. Such states have a marked charge‐transfer character, as revealed by the persistent Stark effect in the form of a second derivative in electroabsorption. Modeling shows that electrostatic effects associated with fluorination, combined with the steric hindrance due to the bulky side groups, drive the formation of spatially dislocated charge pairs with reduced recombination rates. This work enriches and broadens the current knowledge of the photophysics of 2DP, which will hopefully guide synthesis efforts toward novel materials with improved functionalities.

The degradation of printed organic solar cells based on polymer PBDB‐T‐SF and small molecule IT‐4F is studied in operando for two different donor:acceptor ratios. Grazing incidence small angle X‐ray scattering, simultaneous current–voltage measurements and a theoretical model give insight into morphological changes during operation correlated with a decline of short‐circuit current.

Understanding the degradation mechanisms of printed bulk‐heterojunction (BHJ) organic solar cells during operation is essential to achieve long‐term stability and realize real‐world applications of organic photovoltaics. Herein, the degradation of printed organic solar cells based on the conjugated benzodithiophene polymer PBDB‐T‐SF and the nonfullerene small molecule acceptor IT‐4F with 0.25 vol% 1,8‐diiodooctane (DIO) solvent additive is studied in operando for two different donor:acceptor ratios. The inner nano‐morphology is analyzed with grazing incidence small angle X‐ray scattering (GISAXS), and current–voltage (I–V) characteristics are probed simultaneously. Irrespective of the mixing ratio, degradation occurs by the same degradation mechanism. A decrease in the short‐circuit current density (J _SC) is identified to be the determining factor for the decline of the power conversion efficiency. The decrease in J _SC is induced by a reduction of the relative interface area between the conjugated polymer and the small molecule acceptor in the BHJ structure, resembling the morphological degradation of the active layer.

Two isomeric crossconjugated polymer hole‐transporting materials (HTMs) are developed to demonstrate significantly distinct device power conversion efficiencies (PCEs) under the same device fabrication conditions, 11.1% PPE1 and 19.3% for PPE2 , which is found to be due to the improved quality of perovskite films made on top of PPE2 . More excitingly, the PPE2 ‐based perovskite solar cells (PVSCs) can further achieve a more impressive PCE of 21.3% through suitable surface passivation.

Abstract

Currently, there are only very few dopant‐free polymer hole‐transporting materials (HTMs) that can enable perovskite solar cells (PVSCs) to demonstrate a high power conversion efficiency (PCE) of greater than 20%. To address this need, a simple and efficient way is developed to synthesize novel crossconjugated polymers as high performance dopant‐free HTMs to endow PVSCs with a high PCE of 21.3%, which is among the highest values reported for single‐junction inverted PVSCs. More importantly, rational understanding of the reasons why two isomeric polymer HTMs (PPE1 and PPE2 ) with almost identical photophysical properties, hole‐transporting ability, and surface wettability deliver so distinctly different device performance under similar device fabrication conditions is manifested. PPE2 is found to improve the quality of perovskite films cast on top with larger grain sizes and more oriented crystallization. These results help unveil the new HTM design rules to influence the perovskite growth/crystallization for improving the performance of inverted PVSCs.

A multifunctional conjugated benzene ammonium halide is introduced to enhance phase purity, reduce trap‐state density, and suppress nonradiative charge recombination. Blade‐coated solar cells based on stabilized formamidinium‐dominant perovskite compositions deliver an impressive efficiency of 22.0% and an improved operational stability.

Abstract

Currently, blade‐coated perovskite solar cells (PSCs) with high power conversion efficiencies (PCEs), that is, greater than 20%, normally employ methylammonium lead tri‐iodide with a sub‐optimal bandgap. Alloyed perovskites with formamidinium (FA) cation have narrower bandgap and thus enhance device photocurrent. However, FA‐alloyed perovskites show low phase stability and high moisture sensitivity. Here, it is reported that incorporating 0.83 molar percent organic halide salts (OHs) into perovskite inks enables phase‐pure, highly crystalline FA‐alloyed perovskites with extraordinary optoelectronic properties. The OH molecules modulate the crystal growth, enhance the phase stability, passivate ionic defects at the surface and/or grain boundaries, and enhance the moisture stability of the perovskite film. A high efficiency of 22.0% under 1 sun illumination for blade‐coated PSCs is demonstrated with an open‐circuit voltage of 1.18 V, corresponding to a very small voltage deficit of 0.33 V, and significantly improved operational stability with 96% of the initial efficiency retained under one sun illumination for 500 h.