ZiQi Sun

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.

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.

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.

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.

In article number 1606656, Yiwang Chen, Thomas Riedl, and co-workers report perovskite solar cells based on an ITO-free bottom electrode. The impermeable SnO_x electron-extraction layer is a hole blocker and at the same time it protects the ultrathin silver bottom electrode against corrosion due to the halide-containing precursors of the perovskite.

Recent technological advances in conventional planar and microstructured solar cell architectures have significantly boosted the efficiencies of these devices near the corresponding theoretical values. Nanomaterials and nanostructures have promising potential to push the theoretical limits of solar cell efficiency even higher using the intrinsic advantages associated with these materials, including efficient photon management, rapid charge transfer, and short charge collection distances. However, at present the efficiency of nanostructured solar cells remains lower than that of conventional solar devices due to the accompanying losses associated with the employment of nanomaterials. The concurrent design of both optical and electrical components will presumably be an imperative route toward breaking the present-day limit of nanostructured solar cells. This review summarizes the losses in traditional solar cells, and then discusses recent advances in applications of nanotechnology to solar devices from both optical and electrical perspectives. Finally, a rule for nanostructured solar cells by concurrently engineering the optical and electrical design is devised. Following these guidelines should allow for exceeding the theoretical limit of solar cell efficiency soon.

Nanostructures produce unique optical and electronic properties, which have the potential to meet the goals of third-generation photovoltaic devices. However, most nanostructures bring accompanying optical or electrical losses to solar cells. Here, it is postulated that the concurrent design of both optical and electrical components will be an imperative route toward breaking the present-day limit of nanostructured solar cells.

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.

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 this work, a highly efficient parallel connected tandem solar cell utilizing a nonfullerene acceptor is demonstrated. Guided by optical simulation, each of the active layer thicknesses of subcells are tuned to maximize its light trapping without spending intense effort to match photocurrent. Interestingly, a strong optical microcavity with dual oscillation centers is formed in a back subcell, which further enhances light absorption. The parallel tandem device shows an improved photon-to-electron response over the range between 450 and 800 nm, and a high short-circuit current density (J _SC) of 17.92 mA cm⁻². In addition, the subcells show high fill factors due to reduced recombination loss under diluted light intensity. These merits enable an overall power conversion efficiency (PCE) of >10% for this tandem cell, which represents a ≈15% enhancement compared to the optimal single-junction device. Further application of the designed parallel tandem configuration to more efficient single-junction cells enable a PCE of >11%, which is the highest efficiency among all parallel connected organic solar cells (OSCs). This work stresses the importance of employing a parallel tandem configuration for achieving efficient light harvesting in nonfullerene-based OSCs. It provides a useful strategy for exploring the ultimate performance of organic solar cells.

High-efficiency nonfullerene solar cells are demonstrated with a parallel tandem structure. Compared to the single-junction cell, significantly improved power conversion efficiency is achieved owing to enhanced light trapping and reduced charge recombination with diluted light intensity distribution. The champion cell efficiency over 11% represents the highest among all reported organic parallel tandem cells.

In article number 1700749, Kai Xiao, Olga S. Ovchinnikova, and co-workers investigate a hybrid perovskite films by advanced band-excitation Kelvin probe force microscopy and molecular dynamic simulations. It is revealed that incorporation of PCBM or mobile Cl- ions into the grain boundaries of the film causes suppression or enhancement of ion immigration.

Over the past few years, hybrid halide perovskites have emerged as a highly promising class of materials for photovoltaic technology, and the power conversion efficiency of perovskite solar cells (PSCs) has accelerated at an unprecedented pace, reaching a record value of over 22%. In the context of PSC research, wide-bandgap semiconducting metal oxides have been extensively studied because of their exceptional performance for injection and extraction of photo-generated carriers. In this comprehensive review, we focus on the synthesis and applications of metal oxides as electron and hole transporters in efficient PSCs with both mesoporous and planar architectures. Metal oxides and their doped variants with proper energy band alignment with halide perovskites, in the form of nanostructured layers and compact thin films, can not only assist with charge transport but also improve the stability of PSCs under ambient conditions. Strategies for the implementation of metal oxides with tailored compositions and structures, and for the engineering of their interfaces with perovskites will be critical for the future development and commercialization of PSCs.

Hybrid perovskites are emerging as promising materials for low-cost photovoltaic technologies with high performance. Wide-bandgap metal oxides in the forms of nanostructures and compact thin films have been extensively applied as electron and hole transporters in perovskite solar cells. This review elucidates their crucial role in assisting perovskite solar cells to achieve optimal performance and stability.

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.

4-Tert-butylpyridine (tBP) is an important additive in triarylamine-based organic hole-transporting materials (HTMs) for improving the efficiency and steady-state performance of perovskite solar cells (PVSCs). However, the low boiling point of tBP (196 °C) significantly affects the long-term stability and device performance of PVSCs. Herein, the design and synthesis of a series of covalently linked Spiro[fluorene-9,9′-xanthene] (SFX)-based organic HTMs and pyridine derivatives to realize efficient and stable planar PVSCs are reported. One of the tailored HTMs, N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-3′,6′-bis(pyridin-4-ylmethoxy) spiro[fluorene-9,9′-xanthene]-2,7-diamine (XPP) with two para-position substituted pyridines that immobilized on the SFX core unit shows a high power conversion efficiency (PCE) of 17.2% in planar CH₃NH₃PbI₃-based PVSCs under 100 mW cm⁻² AM 1.5G solar illumination, which is much higher than the efficiency of 5.5% that using the well-known 2,2′,7,7′-tetrakis-(N,N-di-p-methoxy-phenyl-amine)9,9′-spirobifluorene (Spiro-OMeTAD) as HTM (without tBP) under the same condition. Most importantly, the pyridine-functionalized HTM-based PVSCs without tBP as additive show much better long-term stability than that of the state-of-the-art HTM Spiro-OMeTAD-based solar cells that containing tBP as additive. This is the first case that the tBP-free HTMs are demonstrated in PVSCs with high PCEs and good stability. It paves the way to develop highly efficient and stable tBP-free HTMs for PVSCs toward commercial applications.

A series of covalently linked organic hole-transporting materials (HTMs) and pyridine derivatives are developed for 4-tert-butylpyridine (tBP) free planar perovskite solar cells (PVSCs). One of the HTM-N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-3′,6′-bis(pyridin-4-ylmethoxy) spiro[fluorene-9,9′-xanthene]-2,7-diamine (termed XPP) shows a remarkable power conversion efficiency of 19.5% and excellent long-term stability in mix-ion planar PVSCs without tBP as additive.

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.

Semitransparent solar cells can provide not only efficient power-generation but also appealing images and show promising applications in building integrated photovoltaics, wearable electronics, photovoltaic vehicles and so forth in the future. Such devices have been successfully realized by incorporating transparent electrodes in new generation low-cost solar cells, including organic solar cells (OSCs), dye-sensitized solar cells (DSCs) and organometal halide perovskite solar cells (PSCs). In this review, the advances in the preparation of semitransparent OSCs, DSCs, and PSCs are summarized, focusing on the top transparent electrode materials and device designs, which are all crucial to the performance of these devices. Techniques for optimizing the efficiency, color and transparency of the devices are addressed in detail. Finally, a summary of the research field and an outlook into the future development in this area are provided.

Recent developments of semitransparent organic solar cells, dye-sensitized solar cells, and perovskite solar cells are reviewed with a focus on different device design, transparent top electrode materials, and the corresponding device fabrication techniques. Key issues related to the optimization of the efficiency, color, and transparency of the semitransparent photovoltaic devices are discussed in detail.