ZiQi Sun

Organic–inorganic halide perovskite has received extensive attention as a light harvester for next-generation low-cost and high-performance photovoltaics. Its superior optoelectronic properties are attractive among most thin film absorber materials, such as an extremely high absorption coefficient, optimal band gap, ambipolar carrier transport property, and high defects tolerance. However, it requires suitable electrodes and carrier transport materials to fulfill efficient photovoltaic process within an entire device. Thus, the interfaces along the device play a crucial role in determining device photovoltaic performance. Here, the progress of understanding interfaces in perovskite photovoltaics is reviewed from the perspective of processing chemistry and photophysics of carriers, which are the key parameters for the corresponding device photovoltaic behavior. This study is mainly focused on the relevant working mechanism, interface design fundamentals, and the resulting carrier dynamic control over the entire architecture. The study of the interfaces with appropriate materials design provides a fundamental understanding of the photocarrier behavior, including separation, transportation, and collection. The accumulative efforts will contribute to the construction of high-efficiency perovskite-based single junction and multijunction photovoltaic devices. It also affects other properties of perovskite solar cells, including J–V hysteresis phenomenon, and long-term stability. Suggestions with respect to required improvements and future research directions are provided based on the current field of available literature.

The progress of interfaces design in perovskite photovoltaics is reviewed, from the perspective of various aspects, including the relevant working mechanism, interface design fundamentals, devices long-term stability, as well as hysteresis phenomenon, etc., which are the key parameters for the corresponding materials understanding and device photovoltaic behavior.

Perovskite solar cells based on CH₃NH₃PbBr₃ with a band gap of 2.3 eV are attracting intense research interests due to their high open-circuit voltage (V_oc) potential, which is specifically relevant for the use in tandem configuration or spectral splitting. Many efforts have been performed to optimize the V_oc of CH₃NH₃PbBr₃ solar cells; however, the limiting V_oc (namely, radiative V_oc:V_oc,rad) and the corresponding ΔV_oc (the difference between V_oc,rad and V_oc) mechanism are still unknown. Here, the average V_oc of 1.50 V with the maximum value of 1.53 V at room temperature is achieved for a CH₃NH₃PbBr₃ solar cell. External quantum efficiency measurements with electroluminescence spectroscopy determine the V_oc,rad of CH₃NH₃PbBr₃ cells with 1.95 V and a ΔV_oc of 0.45 V at 295 K. When the temperature declines from 295 to 200 K, the obtained V_oc remains comparably stable in the vicinity of 1.5 V while the corresponding ΔV_oc values show a more significant increase. Our findings suggest that the V_oc of CH₃NH₃PbBr₃ cells is primarily limited by the interface losses induced by the charge extraction layer rather than by bulk dominated recombination losses. These findings are important for developing strategies how to further enhance the V_oc of CH₃NH₃PbBr₃-based solar cells.

CH₃NH₃PbBr₃ solar cells with the average open circuit voltage of 1.50 V are achieved. External quantum efficiency measurements and electroluminescence spectroscopy are employed to predict the limiting open circuit voltage and the corresponding voltage loss mechanism are clarified via temperature dependent measurements, beneficial for further open circuit voltage improvements of CH₃NH₃PbBr₃ solar cells.

In article number 1600228, Taiho Park, Sang Kyu Lee and co-workers achieve a high efficiency of 7.45% in a 77.8 cm² rigid module of organic photovoltaics. The delicate control of intermolecular interaction has an important role in achieving a well-interconnected morphology. The number of 2D-BDT (benzodithiophene) units is essential in creating a well-defined intermolecular interaction, and the desired interconnected bulk heterojunction structure.

On-fabrication solid-state N-doping of graphene using a fluorosurfactant (Zonyl)-added ZnO layer on a graphene surface is developed by Tae-Woo Lee and co-workers, in article number 1600172. The N-doping facilitates efficient electron transfer from the ZnO layer to the graphene cathode. Based on this, inverted organic solar cells with a power conversion efficiency of 7.5% are fabricated, which is 100% power conversion efficiency with respect to the ITO cathode.

Efficient quasi-2D-structure perovskite light-emitting diodes (4.90 cd A⁻¹) are demonstrated by mixing a 3D-structured perovskite material (methyl ammonium lead bromide) and a 2D-structured perovskite material (phenylethyl ammonium lead bromide), which can be ascribed to better film uniformity, enhanced exciton confinement, and reduced trap density.

In recent years, 2D layered materials have been considered as promising photon absorption channel media for next-generation phototransistors due to their atomic thickness, easily tailored single-crystal van der Waals heterostructures, ultrafast optoelectronic characteristics, and broadband photon absorption. However, the photosensitivity obtained from such devices, even under a large bias voltage, is still unsatisfactory until now. In this paper, high-sensitivity phototransistors based on WS₂ and MoS₂ are proposed, designed, and fabricated with gold nanoparticles (AuNPs) embedded in the gate dielectric. These AuNPs, located between the tunneling and blocking dielectric, are found to enable efficient electron trapping in order to strongly suppress dark current. Ultralow dark current (10⁻¹¹ A), high photoresponsivity (1090 A W⁻¹), and high detectivity (3.5 × 10¹¹ Jones) are obtained for the WS₂ devices under a low source/drain and a zero gate voltage at a wavelength of 520 nm. These results demonstrate that the floating-gate memory structure is an effective configuration to achieve high-performance 2D electronic/optoelectronic devices.

This study reports a novel float-gated memory structure phototransistor based on multilayer WS₂ with gold nanoparticles embedded in the gate dielectric. The device represents excellent photodetection capabilities demonstrating that the float-gated memory is an effective configuration to achieve high-performance 2D optoelectronic devices.

Polymer-fullerene packing in mixed regions of a bulk heterojunction solar cell is expected to play a major role in exciton-dissociation, charge-separation, and charge-recombination processes. Here, molecular dynamics simulations are combined with density functional theory calculations to examine the impact of nature and location of polymer side-chains on the polymer-fullerene packing in mixed regions. The focus is on poly-benzo[1,2-b:4,5-b′]dithiophene-thieno[3,4-c]pyrrole-4,6-dione (PBDTTPD) as electron-donating material and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as electron-accepting material. Three polymer side-chain patterns are considered: i) linear side-chains on both benzodithiophene (BDT) and thienopyrroledione (TPD) moieties; ii) two linear side-chains on BDT and a branched side-chain on TPD; and iii) two branched side-chains on BDT and a linear side-chain on TPD. Increasing the number of branched side-chains is found to decrease the polymer packing density and thereby to enhance PBDTTPD–PC₆₁ BM mixing. The nature and location of side-chains are found to play a determining role in the probability of finding PC₆₁BM molecules close to either BDT or TPD. The electronic couplings relevant for the exciton-dissociation and charge-recombination processes are also evaluated. Overall, the findings are consistent with the experimental evolution of the PBDTTPD–PC₆₁BM solar-cell performance as a function of side-chain patterns.

Polymer side-chains are expected to play a significant role in determining the polymer-fullerene packing in the mixed regions of bulk-heterojunction solar cells. The computational work, based on a combination of molecular dynamics simulations and density functional theory calculations, provides a detailed description of the impact that the nature and locations of the polymer side-chains have on the nanoscale polymer-fullerene packing.

Perovskite solar cells (PSCs) may offer huge potential in photovoltaic conversion, yet their practical applications face one major obstacle: their low stability, or quick degradation of their initial efficiencies. Here, a new design scheme is presented to enhance the PSC stability by using low-temperature hydrothermally grown hierarchical nano-SnO₂ electron transport layers (ETLs). The ETL contains a thin compact SnO₂ layer underneath a mesoporous layer of SnO₂ nanosheets. The mesoporous layer plays multiple roles of enhancing photon collection, preventing moisture penetration and improving the long-term stability. Through such simple approaches, PSCs with power conversion efficiencies of ≈13% can be readily obtained, with the highest efficiency to be 16.17%. A prototypical PSC preserves 90% of its initial efficiency even after storage in air at room temperature for 130 d without encapsulation. This study demonstrates that hierarchical SnO₂ is a potential ETL for fabricating low-cost and efficient PSCs with long-term stability.

Low-temperature hydrothermally grown hierarchical SnO₂ , a mesoporous layer of nanosheet arrays on a compact nanoparticle layer, is used as the electron transporting layer to enhance the long-term stability of perovskite solar cells. A mesoporous device preserves 90% of its initial efficiency, even after storage in air for 130 d without encapsulation.

Perovskite photovoltaics (PVs) have attracted attention because of their excellent power conversion efficiency (PCE). Critical issues related to large-area PV performance, reliability, and lifetime need to be addressed. Here, it is shown that doped metal oxides can provide ideal electron selectivity, improved reliability, and stability for perovskite PVs. This study reports p-i-n perovskite PVs with device areas ranging from 0.09 cm² to 0.5 cm² incorporating a thick aluminum-doped zinc oxide (AZO) electron selective contact with hysteresis-free PCE of over 13% and high fill factor values in the range of 80%. AZO provides suitable energy levels for carrier selectivity, neutralizes the presence of pinholes, and provides intimate interfaces. Devices using AZO exhibit an average PCE increase of over 20% compared with the devices without AZO and maintain the high PCE for the larger area devices reported. Furthermore, the device stability of p-i-n perovskite solar cells under the ISOS-D-1 is enhanced when AZO is used, and maintains 100% of the initial PCE for over 1000 h of exposure when AZO/Au is used as the top electrode. The results indicate the importance of doped metal oxides as carrier selective contacts to achieve reliable and high-performance long-lived large-area perovskite solar cells.

Doped metal oxides provide ideal electron selectivity, improved lifetime, and reliability for large-area perovskite-based photovoltaics. The proposed aluminum-doped zinc oxide electron selective contact provides suitable energy levels for carrier selectivity and stability, neutralizes the presence of pinholes, provides intimate interfaces, and maintains high power conversion efficiency for large-area solar cell devices.

While the extremes in organic photovoltaic bulk heterojunction morphology (finely mixed or large pure domains) are easily understood and known to be unfavorable, efficient devices often exhibit a complex multi-length scale, multi-phase morphology. The impact of such multiple length scales and their respective purities and volume fractions on device performance remains unclear. Here, the average spatial composition variations, i.e., volume-average purities, are quantified at multiple size scales to elucidate their effect on charge creation and recombination in a complex, multi-length scale polymer:fullerene system (PBDTTPD:PC₇₁BM). The apparent domain size as observed in TEM is not a causative parameter. Instead, a linear relationship is found between average purity at length scales <50 nm and device fill-factor. Our findings show that a high volume fraction of pure phases at the smallest length scales is required in multi-length scale systems to aid charge creation and diminish recombination in polymer:fullerene solar cells.

Charge creation and recombination improve with volume fraction of pure domains at the smallest lengths scale even in complex multilength scale morphology. Neither the existence of the multi-length scale morphology nor the length scale with the largest contrast controls performance.

High-performance polymer solar cells incorporating a low-temperature-processed aluminum-doped zinc oxide (AZO) cathode interlayer are constructed with power conversion efficiency (PCE) of 10.42% based on PTB7-Th:PC₇₁BM blends (insensitive to the AZO thickness). Moreover, flexible devices on poly(ethylene terephthalate)/indium tin oxide substrates with PCE of 8.93% are also obtained, and welldistributed efficiency and good device stability are demonstrated as well.

Over the past five years, a rapid progress in organometal-halide perovskite solar cells has greatly influenced emerging solar energy science and technology. In perovksite solar cells, the overlying hole transporting material (HTM) is critical for achieving high power conversion efficiencies (PCEs) and for protecting the air-sensitive perovskite active layer. This study reports the synthesis and implementation of a new polymeric HTM series based on semiconducting 4,8-dithien-2-yl-benzo[1,2-d;4,5-d′]bistriazole-alt-benzo[1,2-b:4,5-b′]dithiophenes (pBBTa-BDTs), yielding high PCEs and environmentally-stable perovskite cells. These intrinsic (dopant-free) HTMs achieve a stabilized PCE of 12.3% in simple planar heterojunction cells—the highest value to date for a polymeric intrinsic HTM. This high performance is attributed to efficient hole extraction/collection (the most efficient pBBTa-BDT is highly ordered and orients π-face-down on the perovskite surface) and balanced electron/hole transport. The smooth, conformal polymer coatings suppress aerobic perovskite film degradation, significantly enhancing the solar cell 85 °C/65% RH PCE stability versus typical molecular HTMs.

New in-chain donor–acceptor semiconducting copolymers are designed and synthesized as dopant-free perovskite solar cell hole transport materials. Combining the BDT donor and the BBTa acceptor building blocks yields pBBTa-BDT copolymers with strong interchain interactions, substantial quinoidal π-character, preferential π-face-on orientation, and therefore efficient hole extraction/collection and balanced electron/hole transport. Significant enhancement of solar cell performance and environmental stability are achieved.

An aqueous-solution-processed photoconductive cathode interlayer is developed, in which the photoinduced charge transfer brings multiple advantages such as increased conductivity and electron mobility, as well as reduced work function. Average power conversion efficiency over 10% is achieved even when the thickness of the cathode interlayer and active layer is up to 100 and 300 nm, respectively.

The strong ionic character endows all-inorganic CsPbX₃ (X = Cl, Br, I) perovskite nanocrystals (NCs) with different chemical features from classical Cd-based NCs, especially when considering their interaction with polar solvents and surfactants. This has aroused intensive interest, but is still short of comprehensive understanding. More significantly, above characteristic may be used to improve the quality of perovskite thin films, which is crucial for the carrier transport inside optoelectronic devices. Here, an interesting recyclable dissolution–recyrstallization phenomenon of all-inorganic pervoskite, as well as its application on room temperature (RT) self-healing of compact and smooth carrier channels in ambient atmosphere for high-performance PDs with high stability is reported. First, according to solubility equilibrium principle, the size of CsPbBr₃ crystals can be reversibly tuned in the range of 10 nm–1 μm through washing with polar solvent or stirring with assistance of surfactants at RT. Second, such phenomenon is applied for significant film quality improvement by forming a liquid circumstance within films, which can transport matter at surface and sharp parts into the gaps, healing themselves at RT. This strategy results in large-area, crack-free, low-roughness perovskite thin films. Obviously, such improvement facilitates transport and extraction of carriers in the channels of devices, which has been evidenced by the improvement of performances of the corresponding PDs at ambient condition.

An interesting surface chemical phenomenon of all-inorganic perovskite—recyclable dissolution and recrystallization—is reported, which is applied to build compact and smooth carrier channels for optoelectronic devices via self-healing under ambient condition. The advantages of this film treating strategy are convinced by the improved responsivity, external quantum efficiency, response speed, and stability of photodetectors.

The first visible-blind UV photodetector based on MAPbCl₃ integrated on a substrate exhibits excellent performance, with responsivities reaching 18 A W⁻¹ below 400 nm and imaging-compatible response times of 1 ms. This is achieved by using substrate-integrated single crystals, thus overcoming the severe limitations affecting thin films and offering a new application of efficient, solution-processed, visible-transparent perovskite optoelectronics.

A profound understanding of the photophysical processes at donor/acceptor interfaces is integral for the optimization of the photo conversion efficiency (PCE) of solar cell devices. Time-resolved laser spectroscopy and electron microscopic investigations provide the key information to accomplish high PCE in polymer-fullerene-based solar cells. This device optimization enhanced the PCE from 2% in IC₆₀BA-based to >9% in PC₇₁BM-based solar cells, as presented by Omar F. Mohammed and co-workers in article number 1502356.

A novel crosslinkable amino-functionalized conjugated polymer PFN-V is used as the cathode interlayer for high performance polymer solar cells, as reported by Lei Ying, Fei Huang, and co-workers in article number 1502563. The film can be rapidly crosslinked by UV-curing within 5 s in a nearly quantitative yield based on “thiol-ene” click reaction. This time- and labor-saving strategy may have practical applications in high throughput industrial production.

Developing cost-effective and efficient electrocatalysts for oxygen evolution reaction (OER) is of paramount importance for the storage of renewable energies. Perovskite oxides serve as attractive candidates given their structural and compositional flexibility in addition to high intrinsic catalytic activity. In a departure from the conventional doping approach utilizing metal elements only, here it is shown that non-metal element doping provides an another attractive avenue to optimize the structure stability and OER performance of perovskite oxides. This is exemplified by a novel tetragonal perovskite developed in this work, i.e., SrCo_0.95P_0.05O_{3– δ} (SCP) which features higher electrical conductivity and larger amount of O₂ ²⁻/O⁻ species relative to the non-doped parent SrCoO_{3– δ} (SC), and thus shows improved OER activity. Also, the performance of SCP compares favorably to that of well-developed perovskite oxides reported. More importantly, an unusual activation process with enhanced activity during accelerated durability test (ADT) is observed for SCP, whereas SC delivers deactivation for the OER. Such an activation phenomenon for SCP may be primarily attributed to the in situ formation of active A-site-deficient structure on the surface and the increased electrochemical surface area during ADT. The concept presented here bolsters the prospect to develop a viable alternative to precious metal-based catalysts.

Phosphorus-doped perovskite oxide SrCo_0.95P_0.05O_{3– δ} (SCP) is demonstrated for the first time as a high-efficient oxygen evolution reaction (OER) electrocatalyst in alkaline solution. The SCP exhibits enhanced OER activity and stability compared to parent SrCoO_{3– δ} (SC). More importantly, an activation process is observed for SCP during accelerated durability test, which primarily originates from the in situ formed Sr-deficient layer on its surface.

Organo-lead halide perovskite solar cells (PSCs) have received great attention because of their optimized optical and electrical properties for solar cell applications. Recently, a dramatic increase in the photovoltaic performance of PSCs with organic hole transport materials (HTMs) has been reported. However, as of now, future commercialization can be hampered because the stability of PSCs with organic HTM has not been guaranteed for long periods under conventional working conditions, including moist conditions. Furthermore, conventional organic HTMs are normally expensive because material synthesis and purification are complicated. It is herein reported, for the first time, octadecylamine-capped pyrite nanoparticles (ODA-FeS₂ NPs) as a bi-functional layer (charge extraction layer and moisture-proof layer) for organo-lead halide PSCs. FeS₂ is a promising candidate for the HTM of PSCs because of its high conductivity and suitable energy levels for hole extraction. A bi-functional layer based on ODA-FeS₂ NPs shows excellent hole transport ability and moisture-proof performance. Through this approach, the best-performing device with ODA-FeS₂ NPs-based bi-functional layer shows a power conversion efficiency of 12.6% and maintains stable photovoltaic performance in 50% relative humidity for 1000 h. As a result, this study has the potential to break through the barriers for the commercialization of PSCs.

A bi-functional layer based on hydrophobic ligand capped FeS₂ nanoparticles is an outstanding inorganic hole transporting materials (HTMs) for perovskite solar cells (PSCs). PSCs with FeS₂ bi-functional layer show high photovoltaic performance and improved long-term stability because HTMs based on FeS₂ have excellent hole transport ability and moisture-proof performance. Our approach will contribute to reliability improvement of PSCs.

Molecular orientation, with respect to donor/acceptor interface and electrodes, plays a critical role in determining the performance of all-polymer solar cells (all-PSCs), but is often difficult to rationally control. Here, an effective approach for tuning the molecular crystallinity and orientation of naphthalenediimide-bithiophene-based n-type polymers (P(NDI2HD-T2)) by controlling their number average molecular weights (M_n) is reported. A series of P(NDI2HD-T2) polymers with different M_n of 13.6 (P_L), 22.9 (P_M), and 49.9 kg mol⁻¹ (P_H) are prepared by changing the amount of end-capping agent (2-bromothiophene) during polymerization. Increasing the M_n values of P(NDI2HD-T2) polymers leads to a remarkable shift of dominant lamellar crystallite textures from edge-on (P_L) to face-on (P_H) as well as more than a twofold increase in the crystallinity. For example, the portion of face-on oriented crystallites is dramatically increased from 21.5% and 46.1%, to 78.6% for P_L, P_M, and P_H polymers. These different packing structures in terms of the molecular orientation greatly affect the charge dissociation efficiency at the donor/acceptor interface and thus the short-circuit current density of the all-PSCs. All-PSCs with PTB7-Th as electron donor and P_H as electron acceptor show the highest efficiency of 6.14%, outperforming those with P_M (5.08%) and P_L (4.29%).

The molecular orientation of a naphthalenediimide-bithiophene-based n-type polymer P(NDI2HD-T2) is effectively modulated by tuning the number average molecular weight (M_n). By increasing M_n values, the molecular orientation is shifted remarkably from edge-on to face-on with respect to the donor/acceptor interface and the electrodes. Thus, high M_n P(NDI2HD-T2) polymers produce enhanced charge generation and transport, resulting in highly efficient all-polymer solar cells.