Ligang Yuan

In this work, a fully tin-based, mixed-organic-cation perovskite absorber (FA)_x(MA)_1−xSnI₃ (FA = NH₂CH = NH₂⁺, MA = CH₃NH₃⁺) for lead-free perovskite solar cells (PSCs) with inverted structure is presented. By optimizing the ratio of FA and MA cations, a maximum power conversion efficiency of 8.12% is achieved for the (FA)_0.75(MA)_0.25SnI₃-based device along with a high open-circuit voltage of 0.61 V, which originates from improved perovskite film morphology and inhibits recombination process in the device. The cation-mixing approach proves to be a facile method for the efficiency enhancement of tin-based PSCs.

For the first time, an efficiency of over 8% is achieved for tin-based perovskite solar cells along with a high open-circuit voltage of 0.61 V by utilizing (FA)_0.75(MA)_0.25SnI₃ as the absorber. The cation-mixing method is proven to effectively improve the morphology of tin-based perovskite films and reduce recombination process in the devices.

Large-scale high-quality perovskite thin films are crucial to produce high-performance perovskite solar cells. However, for perovskite films fabricated by solvent-rich processes, film uniformity can be prevented by convection during thermal evaporation of the solvent. Here, a scalable low-temperature soft-cover deposition (LT-SCD) method is presented, where the thermal convection-induced defects in perovskite films are eliminated through a strategy of surface tension relaxation. Compact, homogeneous, and convection-induced-defects-free perovskite films are obtained on an area of 12 cm², which enables a power conversion efficiency (PCE) of 15.5% on a solar cell with an area of 5 cm². This is the highest efficiency at this large cell area. A PCE of 15.3% is also obtained on a flexible perovskite solar cell deposited on the polyethylene terephthalate substrate owing to the advantage of presented low-temperature processing. Hence, the present LT-SCD technology provides a new non-spin-coating route to the deposition of large-area uniform perovskite films for both rigid and flexible perovskite devices.

For solvent-rich scalable processes, solvents in liquid films are thermally evaporated to form solid films. Due to the temperature difference during heating, a surface-tension difference in the perovskite precursor is established and drives convection to form film defects in cellular patterns. The cellular defects can be eliminated via soft-cover deposition through a strategy of surface tension relaxation.

Molecularly engineered novel dopant-free hole-transporting materials for perovskite solar cells (PSCs) combined with mixed-perovskite (FAPbI₃)_0.85(MAPbBr₃)_0.15 (MA: CH₃NH₃⁺, FA: NH=CHNH₃⁺) that exhibit an excellent power conversion efficiency of 18.9% under AM 1.5 conditions are investigated. The mobilities of FA-CN, and TPA-CN are determined to be 1.2 × 10⁻⁴ cm² V⁻¹ s⁻¹ and 1.1 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively. Exceptional stability up to 500 h is measured with the PSC based on FA-CN. Additionally, it is found that the maximum power output collected after 1300 h remained 65% of its initial value. This opens up new avenue for efficient and stable PSCs exploring new materials as alternatives to Spiro-OMeTAD.

Novel dopant-free hole-transporting materials for perovskite solar cells (PSCs), which exhibit an excellent power conversion efficiency of 18.9% under AM 1.5 conditions are investigated. The PSC based on FA-CN shows exceptional stability up to 500 h. The PCE collected during 1300 h is observed to remain at 65% of its initial value. This opens an avenue for efficient and stable PSCs exploring new materials.

Organic–inorganic hybrid perovskite materials exhibit a variety of physical properties. Pronounced coupling between phonon, organic cations, and the inorganic framework suggest that these materials exhibit strong light–matter interactions. The photoinduced strain of CH₃NH₃PbBr₃ is investigated using high-resolution and contactless in situ Raman spectroscopy. Under illumination, the material exhibits large blue shifts in its Raman spectra that indicate significant structural deformations (i.e., photostriction). From these shifts, the photostrictive coefficient of CH₃NH₃PbBr₃ is calculated as 2.08 × 10⁻⁸ m² W⁻¹ at room temperature under visible light illumination. The significant photostriction of CH₃NH₃PbBr₃ is attributed to a combination of the photovoltaic effect and translational symmetry loss of the molecular configuration via strong translation–rotation coupling. Unlike CH₃NH₃PbI₃, it is noted that the photostriction of CH₃NH₃PbBr₃ is extremely stable, demonstrating no signs of optical decay for at least 30 d. These results suggest the potential of CH₃NH₃PbBr₃ for applications in next-generation optical micro-electromechanical devices.

The photoinduced strain of CH₃NH₃PbBr₃ is investigated using high-resolution and contactless in situ Raman spectroscopy. Under illumination, the material exhibits large blue shifts in its Raman spectra that indicate significant structural deformations. The significant photostriction of CH₃NH₃PbBr₃ can be attributed to a combination of the photovoltaic effect and translational symmetry loss of the molecular configuration via strong translation–rotation coupling.

Significant efforts have lead to demonstrations of nonfullerene solar cells (NFSCs) with record power conversion efficiency up to ≈13% for polymer:small molecule blends and ≈9% for all-polymer blends. However, the control of morphology in NFSCs based on polymer blends is very challenging and a key obstacle to pushing this technology to eventual commercialization. The relations between phases at various length scales and photovoltaic parameters of all-polymer bulk-heterojunctions remain poorly understood and seldom explored. Here, precise control over a multilength scale morphology and photovoltaic performance are demonstrated by simply altering the concentration of a green solvent additive used in blade-coated films. Resonant soft X-ray scattering is used to elucidate the multiphasic morphology of these printed all-polymeric films and complements with the use of grazing incidence wide-angle X-ray scattering and in situ spectroscopic ellipsometry characterizations to correlate the morphology parameters at different length scales to the device performance metrics. Benefiting from the highest relative volume fraction of small domains, additive-free solar cells show the best device performance, strengthening the advantage of single benign solvent approach. This study also highlights the importance of high volume fraction of smallest domains in printed NFSCs and organic solar cells in general.

Precise control over the multilength scale morphology of printed all-polymer photovoltaic films by altering the concentration of a green additive is demonstrated by means of resonant soft X-ray scattering, in situ spectroscopic ellipsometry, and complementary methods. Additive-free nonfullerene devices show the best device performance due to the highest relative volume fraction of small domains and minimized length scale of large domains.

Perovskite solar cells typically use TiO₂ as charge extracting materials, which reduce the photostability of perovskite solar cells under illumination (including ultraviolet light). Simultaneously realizing the high efficiency and photostability, it is demonstrated that the rationally designed iron(III) oxide nanoisland electrodes consisting of discrete nanoislands in situ growth on the compact underlayer can be used as compatible and excellent electron extraction materials for perovskite solar cells. The uniquely designed iron(III) oxide electron extraction layer satisfies the good light transmittance and sufficient electron extraction ability, resulting in a promising power conversion efficiency of 18.2%. Most importantly, perovskite solar cells fabricated with iron(III) oxide show a significantly improved UV light and long-term operation stabilities compared with the widely used TiO₂-based electron extraction material, owing to the low photocatalytic activity of iron(III) oxide. This study highlights the potential of incorporating new charge extraction materials in achieving photostable and high efficiency perovskite photovoltaic devices.

A photostable and efficient perovskite solar cell is presented, employing the rationally designed iron(III) oxide nanoarchitecture consisting of discrete nanoislands in situ growth on the compact underlayer as electron extraction layer. Perovskite solar cells fabricated with iron(III) oxide nanoislands exhibit high power conversion efficiency (over 18%) and promising ultraviolet light and long-term operational stabilities.

Solution-processable small molecule (SM) donors are promising alternatives to their polymer counterparts in bulk-heterojunction (BHJ) solar cells. While SM donors with favorable spectral absorption, self-assembly patterns, optimum thin-film morphologies, and high carrier mobilities in optimized donor–acceptor blends are required to further BHJ device efficiencies, material structure governs each one of those attributes. As a result, the rational design of SM donors with gradually improved BHJ solar cell efficiencies must concurrently address: (i) bandgap tuning and optimization of spectral absorption (inherent to the SM main chain) and (ii) pendant-group substitution promoting structural order and mediating morphological effects. In this paper, the rational pendant-group substitution in benzo[1,2-b:4,5-b′]dithiophene–6,7-difluoroquinoxaline SMs is shown to be an effective approach to narrowing the optical gap (E_opt) of the SM donors (SM1 and SM2), without altering their propensity to order and form favorable thin-film BHJ morphologies with PC₇₁BM. Systematic device examinations show that power conversion efficiencies >8% and open-circuit voltages (V_OC) nearing 1 V can be achieved with the narrow-gap SM donor analog (SM2, E_opt = 1.6 eV) and that charge transport in optimized BHJ solar cells proceeds with minimal, nearly trap-free recombination. Detailed device simulations, light intensity dependence, and transient photocurrent analyses emphasize how carrier recombination impacts BHJ device performance upon optimization of active layer thickness and morphology.

Rational pendant-group substitutions in benzo[1,2-b:4,5-b′]dithiophene-6,7-difluoroquin-oxaline small molecule donor analogs yield power conversion efficiencies >8% and high open-circuit voltages nearing 1 V in bulk heterojunction (BHJ) solar cells with the fullerene acceptor PC₇₁BM. Charge transport in optimized BHJ solar cells proceeds with minimal, nearly trap-free recombination.

Among the various molecular designs developed for the synthesis of conjugated polymers and small molecules for optoelectronic applications, the donor: acceptor (D–A) approach is the most widely explored method over the past decades. Through the covalent linkage of electron-rich and electron-deficient units, a plethora of medium-low band gap materials has been developed and tested in organic photovoltaic devices. In particular, the quinoxaline aromatic structure and its derivatives are among the most studied electron deficient aromatic units used in D–A structures. Quinoxaline based materials are endowed with characteristics that are useful for large scale production in real world applications, such as easy synthetic procedures and excellent stability in air. Moreover, the use of quinoxaline based polymers/small molecules in bulk heterojunction (BHJ) devices led to power conversion efficiencies over 9%. Considering the potential of quinoxaline based materials, this review gathers together quinoxaline based polymers and small molecules reported in the literature during the last 5 years, summarizing and discussing the structure-properties relationships for this class of organic semiconductors, aiming to serve as a background and to promote efforts for the further development of new quinoxaline derivatives with improved and advanced properties for future applications.

This review summarizes the recent developments of quinoxaline-based polymers and small molecules for photovoltaic applications. The structure-properties relationships for this class of organic semiconductors are discussed by correlating the results reported in the literature in the last five years. The attractive properties and the compatibility with the massive production requirements anticipate a bright future for quinoxaline-based semiconductors in photovoltaics.

This paper investigates the impact of microstructure on the degradation rate of methylammonium lead triiodide (MAPbI₃) perovskite films upon exposure to light and oxygen. By comparing the oxygen induced degradation of perovskite films of different microstructure–fabricated using either a lead acetate trihydrate precursor or a solvent engineering technique–it is demonstrated that films with larger and more uniform grains and better electronic quality show a significantly reduced degradation compared to films with smaller, more irregular grains. The effect of degradation on the optical, compositional, and microstructural properties of the perovskite layers is characterized and it is demonstrated that oxygen induced degradation is initiated at the layer surface and grain boundaries. It is found that under illumination, irreversible degradation can occur at oxygen levels as low as 1%, suggesting that degradation can commence already during the device fabrication stage. Finally, this work establishes that improved thin-film microstructure, with large uniform grains and a low density of defects, is a prerequisite for enhanced stability necessary in order to make MAPbI₃ a promising long lived and low cost alternative for future photovoltaic applications.

This work elucidates the effect of MAPbI₃ film microstructure on the rate of oxygen induced photodegradation of perovskite photovoltaic devices. The degradation process is initiated at the layer surface and grain boundaries of the perovskite layer and progresses into the interior of the grains. As a result, films consisting of small irregular grains degrade much faster than those with large uniform grains.

2D halide perovskite materials have shown great advantages in terms of stability when applied in a photovoltaic device. However, the impediment of charge transport within the layered structure drags down the device performance. Here for the first time, a 3D–2D (MAPbI₃-PEA₂Pb₂I₄) graded perovskite interface is demonstrated with synergistic advantages. In addition to the significantly improved ambient stability, this graded combination modifies the interface energy level in such a way that reduces interface charge recombination, leading to an ultrahigh V_oc at 1.17 V, a record for NiO-based p-i-n photovoltaic devices. Moreover, benefiting from the graded structure induced continuously upshifts energy level, the photovoltaic device attains a high J_sc of 21.80 mA cm⁻² and a high fill factor of 0.78, resulting in an overall power conversion efficiency (PCE) of 19.89%. More importantly, it is showed that such a graded interface structure also suppresses ion migration in the device, accounting for its significantly enhanced thermal stability.

A designer cross-dimensional perovskite–perovskite (3D–2D) interface upshifts the energy level toward the surface, pronouncedly reduces the charge recombination in perovskite photovoltaics, leading to a record high V_OC at 1.17 V. Strongly enhanced ambient and thermal stability is also demonstrated.

In this study the thickness of the PTB7-Th:PC₇₁BM bulk heterojunction (BHJ) film and the PF3N-2TNDI electron transport layer (ETL) is systematically tuned to achieve polymer solar cells (PSCs) with optimized power conversion efficiency (PCE) of over 9% when an ultrathin BHJ of 50 nm is used. Optical modeling suggests that the high PCE is attributed to the optical spacer effect from the ETL, which not only maximizes the optical field within the BHJ film but also facilitates the formation of a more homogeneously distributed charge generation profile across the BHJ film. Experimentally it is further proved that the extra photocurrent produced at the PTB7-Th/PF3N-2TNDI interface also contributes to the improved performance. Taking advantage of this high performance thin film device structure, one step further is taken to fabricate semitransparent PSCs (ST-PSCs) by using an ultrathin transparent Ag cathode to replace the thick Ag mirror cathode, yielding a series of high performance ST-PSCs with PCEs over 6% and average visible transmittance between 20% and 30%. These ST-PSCs also possess remarkable transparency color perception and rendering properties, which are state-of-the-art and fulfill the performance criteria for potential use as power-generating windows in near future.

An efficient electron transport layer of PF3N-2TNDI is introduced to improve the performance of PTB7-Th:PC₇₁BM based semitransparent polymer solar cells (ST-PSCs). PF3N-2TNDI can facilitate extra photocurrent generation and promote formation of high quality ultrathin Ag transparent cathode. These combined effects eventually lead to a new performance record of 6% power conversion efficiency with the corresponding average visible transmittance of ≈30% for the polymer:fullerene based ST-PSCs, and remarkable transparency color perception and rendering properties are also realized.

A new 2D-conjugated medium bandgap donor–acceptor copolymer, J81, based on benzodifuran with trialkylsilyl thiophene side chains as donor unit and fluorobenzothiazole as acceptor, is synthesized and successfully used in nonfullerene polymer solar cells (PSCs) with low bandgap n-type organic semiconductor (n-OS) 3,9-bis(2-methylene- (3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4- hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]- dithiophene (ITIC) and m-ITIC as acceptor. J81 possesses a lower-lying highest occupied molecular orbital (HOMO) energy level of −5.43 eV and medium bandgap of 1.93 eV with complementary absorption in the visible–near infrared region with the n-OS acceptor. The PSCs based on J81:ITIC and J81:m-ITIC yield high power conversion efficiency of 10.60% and 11.05%, respectively, with high V _oc of 0.95–0.96 V benefit from the lower-lying HOMO energy level of J81 donor. The work indicates that J81 is another promising polymer donor for the nonfullerene PSCs.

By introducing trialkylsilyl-thienyl conjugated side chains to benzodifuran unit, a new medium bandgap polymer donor J81 is developed. The optimized nonfullerene polymer solar cells with J81 as donor show high power conversion efficiency of 11.05%.

Large area flexible electronics rely on organic or hybrid materials prone to degradation limiting the device lifetime. For many years, photo-oxidation has been thought to be one of the major degradation pathways. However, intense illumination may lead to a burn-in or a rapid decrease in performance for devices completely isolated from corrosive elements as oxygen or moisture. The experimental studies which are presented in here indicate that a plausible triggering for the burn-in is a spin flip after a UV photon absorption leading to the accumulation of electrostatic potential energy that initiates a rapid destruction of the nanomorpholgy in the fullerene phase of a polymer cell. To circumvent this and achieve highly stable and efficient devices, a robust nanocrystalline ordering is induced in the PCBM phase prior to UV illumination. In that event, PTB7-Th:PC₇₁BM cells are shown to exhibit T₈₀ lifetimes larger than 1.6 years under a continuous UV-filtered 1-sun illumination, equivalent to 7 years for sunlight harvesting at optimal orientation and 10 years for vertical applications.

In polymer cells, a spin flip at the donor/acceptor interface after the absorption of high energy photons leads to the accumulation of electrostatic potential energy initiating a rapid destruction of the fullerene nanomorpholgy. By inducing a robust nanocrystalline ordering in the fullerene phase, PTB7-Th cells with high efficiency (≈9%) and long lifetime (>7 and >10 years for optimal and vertical orientations, respectively) are fabricated.

The emerging field of tandem polymer solar cells (TPSCs) enables a feasible approach to deal with the fundamental losses associated with single-junction polymer solar cells (PSCs) and provides the opportunity to propel their overall performance. Additionally, the rational selection of appropriate subcell photoactive polymers with complementary absorption profiles and optimal thicknesses to achieve balanced photocurrent generation are very important issues which must be addressed in order to realize paramount device performance. Here, two side chain fluorinated wide-bandgap π-conjugated polymers P1 (2F) and P2 (4F) in TPSCs have been used. These π-conjugated polymers have high absorption coefficients and deep highest occupied molecular orbitals which lead to high open-circuit voltages (V_oc) of 0.91 and 1.00 V, respectively. Using these π-conjugated polymers, TPSCs have been successfully fabricated by combining P1 or P2 as front cells with PTB7-Th as back cells. The optimized TPSCs deliver outstanding power conversion efficiencies of 11.42 and 10.05%, with high V_oc's of 1.64 and 1.72 V, respectively. These results are analyzed by balance of charge mobilities, and optical and electrical modeling is combined to demonstrate simultaneous improvement in all photovoltaic parameters in TPSCs.

High performance of tandem polymer solar cells (TPSCs) with power conversion efficiencies (PCEs) up to 11.42 and 10.05% are realized by using fluorine-functionalized polymers. TPSCs with PCEs over 10% achieved open-circuit voltage (V_oc) of up to 1.72 V, which is among the highest V_oc observed in TPSCs to date. Furthermore, detailed analyses of TPSCs as well as guidelines for bottom cell design are demonstrated.

Organic–inorganic hybrid perovskite multijunction solar cells have immense potential to realize power conversion efficiencies (PCEs) beyond the Shockley–Queisser limit of single-junction solar cells; however, they are limited by large nonideal photovoltage loss (V _oc,loss) in small- and large-bandgap subcells. Here, an integrated approach is utilized to improve the V _oc of subcells with optimized bandgaps and fabricate perovskite–perovskite tandem solar cells with small V _oc,loss. A fullerene variant, Indene-C₆₀ bis-adduct, is used to achieve optimized interfacial contact in a small-bandgap (≈1.2 eV) subcell, which facilitates higher quasi-Fermi level splitting, reduces nonradiative recombination, alleviates hysteresis instabilities, and improves V _oc to 0.84 V. Compositional engineering of large-bandgap (≈1.8 eV) perovskite is employed to realize a subcell with a transparent top electrode and photostabilized V _oc of 1.22 V. The resultant monolithic perovskite–perovskite tandem solar cell shows a high V _oc of 1.98 V (approaching 80% of the theoretical limit) and a stabilized PCE of 18.5%. The significantly minimized nonideal V _oc,loss is better than state-of-the-art silicon–perovskite tandem solar cells, which highlights the prospects of using perovskite–perovskite tandems for solar-energy generation. It also unlocks opportunities for solar water splitting using hybrid perovskites with solar-to-hydrogen efficiencies beyond 15%.

High open-circuit voltage, V _oc (1.98 V) and power conversion efficiency, PCE (18.5%) is realized in an ideal bandgap-matched two-terminal perovskite–perovskite tandem solar cell via an integrated approach. A fullerene variant, Indene-C₆₀ bis-adduct is used to achieve optimized interfacial contact and alleviate hysteresis instabilities in the small-bandgap subcell. Compositional engineering is employed to realize more highly photostabilized V _oc in the large-bandgap subcell.

In organic solar cells continuous donor and acceptor networks are considered necessary for charge extraction, whereas discontinuous neat phases and molecularly mixed donor–acceptor phases are generally regarded as detrimental. However, the impact of different levels of domain continuity, purity, and donor–acceptor mixing on charge transport remains only semiquantitatively described. Here, cosublimed donor–acceptor mixtures, where the distance between the donor sites is varied in a controlled manner from homogeneously diluted donor sites to a continuous donor network are studied. Using transient measurements, spanning from sub-picoseconds to microseconds photogenerated charge motion is measured in complete photovoltaic devices, to show that even highly diluted donor sites (5.7%–10% molar) in a buckminsterfullerene matrix enable hole transport. Hopping between isolated donor sites can occur by long-range hole tunneling through several buckminsterfullerene molecules, over distances of up to ≈4 nm. Hence, these results question the relevance of “pristine” phases and whether a continuous interpenetrating donor–acceptor network is the ideal morphology for charge transport.

Transient measurements reveal that in organic solar cells a continuous donor network is not strictly necessary for hole transport. Hole hopping between isolated donor sites can occur by long-range hole tunneling through several buckminsterfullerene molecules (4 nm). This often disregarded mechanism questions the importance of pristine phases and whether a continuous donor–acceptor network is the ideal morphology for charge transport.

Recently, considerable progress is achieved in lab prototype perovskite solar cells (PSCs); however, the stability of outdoor applications of PSCs remains a challenge due to the high sensitivity of perovskite material under moist and ultraviolet (UV) light conditions. In this work, the UV photostability of PSC devices is improved by incorporating a photon downshifting layer—SrAl₂O₄: Eu²⁺, Dy³⁺ (SAED)—prepared using the pulsed laser deposition approach. Light-induced deep trap states in the photoactive layer are depressed, and UV light-induced device degradation is inhibited after the SAED modification. Optimized power conversion efficiency (PCE) of 17.8% is obtained through the enhanced light harvesting and reduced carrier recombination provided by SAED. More importantly, a solar energy storage effect due to the long-persistent luminescence of SAED is obtained after light illumination is turned off. The introduction of downconverting material with long-persistent luminescence in PSCs not only represents a new strategy to improve PCE and light stability by photoconversion from UV to visible light but also provides a new paradigm for solar energy storage.

A long persistent photon downshifting layer – SrAl₂O₄: Eu²⁺, Dy³⁺– is successfully incorporated into perovskite solar cells by the pulsed laser deposition approach to improve device performance. Optimized power conversion efficiency of 17.8% is obtained due to enhanced light harvesting and reduced carrier recombination. Furthermore, light-induced deep trap states are depressed and solar energy storage effect is obtained after illumination is turned off.

Developing novel materials that tolerate thickness variations of the active layer is critical to further enhance the efficiency of polymer solar cells and enable large-scale manufacturing. Presently, only a few polymers afford high efficiencies at active layer thickness exceeding 200 nm and molecular design guidelines for developing successful materials are lacking. It is thus highly desirable to identify structural factors that determine the performance of semiconducting conjugated polymers in thick-film polymer solar cells. Here, it is demonstrated that thiophene rings, introduced in the backbone of alternating donor–acceptor type conjugated polymers, enhance the fill factor and overall efficiency for thick (>200 nm) solar cells. For a series of fluorinated semiconducting polymers derived from electron-rich benzo[1,2-b:4,5-b′]dithiophene units and electron-deficient 5,6-difluorobenzo[2,1,3]thiazole units a steady increase of the fill factor and power conversion efficiency is found when introducing thiophene rings between the donor and acceptor units. The increased performance is a synergistic result of an enhanced hole mobility and a suppressed bimolecular charge recombination, which is attributed to more favorable polymer chain packing and finer phase separation.

Introducing additional thiophene rings in conjugated polymers increases the fill factor and power conversion efficiency of polymer:fullerene solar cells with thick active layers. This “thiophene ring effect” is a synergistic result of enhanced hole mobility and suppressed bimolecular charge recombination via the formation of more favorable polymer chain packing and finer phase separation.

Light management holds great promise of realizing high-performance perovskite solar cells by improving the sunlight absorption with lower recombination current and thus higher power conversion efficiency (PCE). Here, a convenient and scalable light trapping scheme is demonstrated by incorporating bioinspired moth-eye nanostructures into the metal back electrode via soft imprinting technique to enhance the light harvesting in organic–inorganic lead halide perovskite solar cells. Compared to the flat reference cell with a methylammonium lead halide perovskite (CH₃NH₃PbI_3−xCl_x) absorber, 14.3% of short-circuit current improvement is achieved for the patterned devices with moth-eye nanostructures, yielding an increased PCE up to 16.31% without sacrificing the open-circuit voltage and fill factor. The experimental and theoretical characterizations verify that the cell performance enhancement is mainly ascribed by the broadband polarization-insensitive light scattering and surface plasmonic effects due to the patterned metal back electrode. It is noteworthy that this light trapping strategy is fully compatible with solution-processed perovskite solar cells and opens up many opportunities toward the future photovoltaic applications.

A convenient and scalable light trapping scheme is demonstrated to enhance the light harvesting in organic–inorganic lead halide perovskite solar cells, which is realized by incorporating bioinspired moth-eye nanostructures into the metal back electrode via soft imprinting technique. The efficiency is enhanced to 16.3% due to self-enhanced absorption by broadband polarization-insensitive light scattering and surface plasmonic effect.

Organic–inorganic hybrid perovskite solar cells with mixed cations and mixed halides have achieved impressive power conversion efficiency of up to 22.1%. Phase segregation due to the mixed compositions has attracted wide concerns, and their nature and origin are still unclear. Some very useful analytical techniques are controversial in microstructural and chemical analyses due to electron beam-induced damage to the “soft” hybrid perovskite materials. In this study photoluminescence, cathodoluminescence, and transmission electron microscopy are used to study charge carrier recombination and retrieve crystallographic and compositional information for all-inorganic CsPbIBr₂ films on the nanoscale. It is found that under light and electron beam illumination, “iodide-rich” CsPbI_(1+x)Br_(2−x) phases form at grain boundaries as well as segregate as clusters inside the film. Phase segregation generates a high density of mobile ions moving along grain boundaries as ion migration “highways.” Finally, these mobile ions can pile up at the perovskite/TiO₂ interface resulting in formation of larger injection barriers, hampering electron extraction and leading to strong current density–voltage hysteresis in the polycrystalline perovskite solar cells. This explains why the planar CsPbIBr₂ solar cells exhibit significant hysteresis in efficiency measurements, showing an efficiency of up to 8.02% in the reverse scan and a reduced efficiency of 4.02% in the forward scan, and giving a stabilized efficiency of 6.07%.

Iodide-rich phase segregation near grain boundaries and the formation of iodide-rich “clusters” inside the film are observed in the CsPbIBr₂ perovskite thin films. The mobile ions generated by the phase segregation, moving alone grainboundaries and piling up at CsPbIBr₂/TiO₂ interface, can become charge injection barriers and exacerbate the current density–voltage hysteresis in inorganic CsPbIBr₂ solar cells.

Vertically oriented highly crystalline 2D layered (BA)₂(MA)_n−1Pb_nI_3n+1 (BA = CH₃(CH₂)₃NH₃, MA = CH₃NH₃, n = 3, 4) perovskite thin-films are fabricated with the aid of ammonium thiocyanate (NH₄SCN) additive through one-step spin-coating process. The humidity-stability of the film is certified by the almost unchanged X-ray diffraction patterns after exposed to humid atmosphere (H_r = 55 ± 5%) for 40 d. The photovoltaic devices with the structure of indium tin oxide(ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate)/(BA)₂(MA)_n−1Pb_nI_3n+1(n = 3,4)/[6,6]-phenyl-C₆₁-butyric acid methyl ester/Bathocuproine/Ag are fabricated. The devices based on (BA)₂(MA)₂Pb₃I₁₀ perovskite (n = 3) with the precursor composition of BAI:methylammonium iodide:PbI₂:NH₄SCN = 2:2:3:1 (by molar ratio) show an averaged power conversion efficiency (PCE) of 6.82%. In the case of (BA)₂(MA)₃Pb₄I₁₃ (n = 4), a higher PCE of 8.79% is achieved. Both of the unsealed devices perform unique stability with almost unchanged PCE during the period of storage in purified N₂ glove box. This work provides a simple and effective method to enhance the efficiency of the 2D perovskite solar cell.

A new route to realize the vertical-orientation growth of 2D layered (BA)₂(MA)_n−1Pb_nI_3n+ (n = 3, 4)-based perovskite with the aid of a NH₄SCN additive through a one-step spin-coating method is presented. The corresponding planar-structured solar cells present the averaged power conversion efficiency (PCE) of 6.82% (n = 3) and the best PCE of 8.79% (n = 4) with unique stability.

The combination of perovskite solar cells and quantum dot solar cells has significant potential due to the complementary nature of the two constituent materials. In this study, solar cells (SCs) with a hybrid CH₃NH₃PbI₃/SnS quantum dots (QDs) absorber layer are fabricated by a facile and universal in situ crystallization method, enabling easy embedding of the QDs in perovskite layer. Compared with SCs based on CH₃NH₃PbI₃, SCs using CH₃NH₃PbI₃/SnS QDs hybrid films as absorber achieves a 25% enhancement in efficiency, giving rise to an efficiency of 16.8%. The performance improvement can be attributed to the improved crystallinity of the absorber, enhanced photo-induced carriers' separation and transport within the absorber layer, and improved incident light utilization. The generality of the methods used in this work paves a universal pathway for preparing other perovskite/QDs hybrid materials and the synthesis of entire nontoxic perovskite/QDs hybrid structure.

A CH₃NH₃PbI₃/SnS quantum dots hybrid film is fabricated by a facile in situ crystallization process. This method takes advantage of complementarity of perovskite and quantum dots. Solar cells with hybrid absorber layer achieve 25% enhancement in efficiency than perovskite layer. A method for preparing perovskite/quantum dots hybrid materials is presented, exhibiting potential in design and preparation of other hybrid materials.

With regards to developing miniaturized coherent light sources, the temperature-insensitivity in gain spectrum and threshold is highly desirable. Quantum dots (QDs) are predicted to possess a temperature-insensitive threshold by virtue of the separated electronic states; however, it is never observed in colloidal QDs due to the poor thermal stability. Besides, for the classical II–VI QDs, the gain profile generally redshifts with increasing temperature, plaguing the device chromaticity. Herein, this paper addresses the above two issues simultaneously by embedding ligands-free CsPbBr₃ nanocrystals in a wider band gap Cs₄PbBr₆ matrix by solution-phase synthesis. The unique electronic structures of CsPbBr₃ nanocrystals enable temperature-insensitive gain spectrum while the lack of ligands and protection from Cs₄PbBr₆ matrix ensure the thermal stability and high temperature operation. Specifically, a color drift-free stimulated emission irrespective of temperature change (20–150 °C) upon two-photon pumping is presented and the characteristic temperature is determined to be as high as ≈260 K. The superior gain properties of the CsPbBr₃/Cs₄PbBr₆ perovskite nanocomposites are directly validated by a vertical cavity surface emitting laser operating at temperature as high as 100 °C. The results shed light on manipulating optical gain from the advantageous CsPbBr₃ nanocrystals and represent a significant step toward the temperature-insensitive frequency-upconverted lasers.

Temperature-insensitive optical gain is achieved by exploiting a novel lasing material composed of ligands-free CsPbBr₃ nanocrystals embedded in a wider band gap Cs₄PbBr₆ matrix based on low-temperature solution-phase synthesis. The unique electronic structures of CsPbBr₃ nanocrystals enable a temperature-insensitive gain profile while the lack of ligands and protection from Cs₄PbBr₆ matrix ensure the thermal stability and high temperature operation.