Liuyanfeng

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.

The past two decades of vigorous interdisciplinary approaches has seen tremendous breakthroughs in both scientific and technological developments of bulk-heterojunction organic solar cells (OSCs) based on nanocomposites of π-conjugated organic semiconductors. Because of their unique functionalities, the OSC field is expected to enable innovative photovoltaic applications that can be difficult to achieve using traditional inorganic solar cells: OSCs are printable, portable, wearable, disposable, biocompatible, and attachable to curved surfaces. The ultimate objective of this field is to develop cost-effective, stable, and high-performance photovoltaic modules fabricated on large-area flexible plastic substrates via high-volume/throughput roll-to-roll printing processing and thus achieve the practical implementation of OSCs. Recently, intensive research efforts into the development of organic materials, processing techniques, interface engineering, and device architectures have led to a remarkable improvement in power conversion efficiencies, exceeding 11%, which has finally brought OSCs close to commercialization. Current research interests are expanding from academic to industrial viewpoints to improve device stability and compatibility with large-scale printing processes, which must be addressed to realize viable applications. Here, both academic and industrial issues are reviewed by highlighting historically monumental research results and recent state-of-the-art progress in OSCs. Moreover, perspectives on five core technologies that affect the realization of the practical use of OSCs are presented, including device efficiency, device stability, flexible and transparent electrodes, module designs, and printing techniques.

Bulk-heterojunction organic solar cells based on solution-processable organic semiconductors enable completely new functionalities of being printable, portable, wearable, biocompatible, and attachable to any curved surfaces. The recent major advances in device efficiency and stability, flexible transparent electrodes, module design, and printing technologies for their commercialization are reviewed. The existing challenges and perspectives for these five core technologies are discussed.

The alloy acceptor (indene-C₆₀ bis-adduct (ICBA)/[6,6]-phenyl-C₇₁-butyric acid-methyl-ester (PC₇₁BM)) is employed to replace the widely used fullerene acceptor (PC₇₁BM) in organic solar cells based on five different polymer donors, which exhibit a higher efficiency and much better device stability than the PC₇₁BM counterpart.

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.

This study has proposed to use a well-defined oligomer F4TBT4 to replace its analogue polymer as electron acceptor toward tuning the phase separation behavior and enhancing the photovoltaic performance of all-polymer solar cells. It has been disclosed that the oligomer acceptor favors to construct pure and large-scale phase separation in the polymer:oligomer blend film in contrast to the polymer:polymer blend film. This gets benefit from the well-defined structure and short rigid conformation of the oligomer that endows it aggregation capability and avoids possible entanglement with the polymer donor chains. The charge recombination is to some extent suppressed and charge extraction is also improved. Finally, the P3HT:F4TBT4 solar cells not only output a high V_OC above 1.2 V, but also achieve a power conversion efficiency of 4.12%, which is two times higher than the P3HT:PFTBT solar cells and is comparable to the P3HT:PCBM solar cells. The strategy of constructing optimum phase separation with oligomer to replace polymer opens up new prospect for the further improvement of the all-polymer solar cells.

A well-defined oligomer F4TBT4 is proposed to replace its polymer PFTBT as electron acceptor to fabricate fullerene-free polymer solar cells. The oligomer acceptor favors to construct pure and large-scale phase separation in polymer blend film due to decreased chain entanglement. The resulted P3HT:F4TBT4 solar cells not only output a high V_OC above 1.2 V, but also achieve a PCE of 4.12%.

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.

Recently, intensive studies on the role of water molecule in the formation of organic–inorganic perovskite film have been reported. However, not only the contradictive phenomena but also the complex processing technique has hindered the widespread use of water molecule in perovskite preparation. Here the hydration water is introduced into the precursors instead of water. By precisely controlling the content of hydration water, a smoother and more uniform perovskite film is obtained through a simple one-step spin coating method. The improvement of perovskite film quality leads to highly efficient planar perovskite solar cells. Summing up the device studies and the investigation of morphology, crystallization, and optical properties, the impact of water molecule in the formation of perovskite crystal and consequences of device performance is understood. Due to its universal adaptability and simplified process, precise control of hydration water is therefore of great utility to high quality perovskite films fabrication and large-scale production of this upcoming photovoltaic technology.

The content of hydration water is carefully controlled in precursor solution. The findings reveal that the hydration water has played an essential role in the formation of perovskite film. With an optimum amount of hydration water, a high quality perovskite film with better uniformity and smoother surface is formed. Based on it, an efficient perovskite solar cell is fabricated.

The interfacial microstructure in organic bulk heterojunction solar cells can dictate photovoltaic performance. By controlling the chemical interactions of bulk heterojunction components with specific surfaces, the electrical pathways at interfaces can be precisely varied to achieve suitable electronic properties across such interfaces. This is studied by J. J. Jasieniak and co-workers on page 3944.

High fatigue resistance, bistability, and drastic property changes among isomers allow efficient modulation of the current output of organic thin-film transistors (OTFTs) to be obtained by a photogating of the charge-injection mechanism.

A new fluorinated polythiophene (PT) derivative, PBDD-ff4T, is designed and synthesized. The PBDD-ff4T/PC₇₁BM-based device without any extra treatment exhibits a high efficiency of 9.2%, under the irradiation of AM 1.5G, 100 mW cm⁻², which is the highest power conversion efficiency reported for PT derivative-based polymer solar cells.

Three small molecules with different substituents on bithienyl-benzo[1,2-b:4,5-b′]dithiophene (BDTT) units, BDTT-TR (meta-alkyl side chain), BDTT-O-TR (meta-alkoxy), and BDTT-S-TR (meta-alkylthio), are designed and synthesized for systematically elucidating their structure–property relationship in solution-processed bulk heterojunction organic solar cells. Although all three molecules show similar molecular structures, thermal properties and optical band gaps, the introduction of meta-alkylthio-BDTT as the central unit in the molecular backbone substantially results in a higher absorption coefficient, slightly lower highest occupied molecular orbital level and significantly more efficient and balanced charge transport property. The bridging atom in the meta-position to the side chain is found to impact the microstructure formation which is a subtle but decisive way: carrier recombination is suppressed due to a more balanced carrier mobility and BDTT based devices with the meta-alkylthio side chain (BDTT-S-TR) show a higher power conversion efficiency (PCE of 9.20%) as compared to the meta-alkoxy (PCE of 7.44% for BDTT-TR) and meta-alkyl spacer (PCE of 6.50% for BDTT-O-TR). Density functional density calculations suggest only small variations in the torsion angle of the side chains, but the nature of the side chain linkage is further found to impact the thermal as well as the photostability of corresponding devices. The aim is to provide comprehensive insight into fine-tuning the structure–property interrelationship of the BDTT material class as a function of side chain engineering.

A comprehensive insight into fine-tuning the structure–property interrelationship (a trade-off beyond efficiency) of the bithienyl-benzo[1,2-b:4,5-b′]dithiophene material class as a function of side chain engineering is provided.