Chen Weijie

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.

Low-temperature solution processing opens a new window for the fabrication of oxide semiconductors due to its simple, low cost, and large-area uniformity. Herein, by using solution combustion synthesis (SCS), p-type Cu-doped NiO (Cu:NiO) thin films are fabricated at a temperature lower than 150 °C. The light doping of Cu substitutes the Ni site and disperses the valence band of the NiO matrix, leading to an enhanced p-type conductivity. Their integration into thin-film transistors (TFTs) demonstrates typical p-type semiconducting behavior. The optimized Cu_5%NiO TFT exhibits outstanding electrical performance with a hole mobility of 1.5 cm² V⁻¹ s⁻¹, a large on/off current ratio of ≈10⁴, and clear switching characteristics under dynamic measurements. The employment of a high-k ZrO₂ gate dielectric enables a low operating voltage (≤2 V) of the TFTs, which is critical for portable and battery-driven devices. The construction of a light-emitting-diode driving circuit demonstrates the high current control capability of the resultant TFTs. The achievement of the low-temperature-processed Cu:NiO thin films via SCS not only provides a feasible approach for low-cost flexible p-type oxide electronics but also represents a significant step toward the development of complementary metal–oxide semiconductor circuits.

A solution combustion synthesis is utilized to fabricate p-type oxide thin-film transistors (TFTs) at 150 °C. The doping of Cu into the NiO matrix can replace the Ni sites and enhance the p-type conductivity. The optimized Cu_5%NiO TFTs on both Si and ITO (indium tin oxide)/glass with ZrO₂ gate dielectrics exhibit an average hole mobility of >1 cm² V⁻¹ s⁻¹ and I_on/I_off of 10⁴.

Lead halide perovskites are intensively studied in past few years due to their potential applications in optoelectronic devices such as solar cells, photodetectors, light-emitting diodes (LED), and lasers. In addition to the rapid developments in material synthesis and device fabrication, it is also very interesting to postsynthetically control the optical properties with external irradiations. Here, the influences of very low energy (10–20 keV) electron beam of standard electron beam lithography are experimentally explored on the properties of lead halide perovskites. It is confirmed that the radiolysis process also happens and it can selectively change the photoluminescence, enabling the direct formation of nanolaser array, microsized light emitter array, and micropictures with an electron beam writer. Interestingly, it is found that discontinuous metallic lead layers are formed on the top and bottom surfaces of perovskite microplate during the radiolysis process, which can act as carrier conducting layers and significantly increase the photocurrent of perovskite photodetector by a factor of 217%. By using the electron beam with low energy to modify the perovskite, this method promises to shape the emission patterns for micro-LED with well-preserved optical properties and improves the photocurrent of photodetector.

A simple approach to tailor the optical properties of lead halide perovskite devices is presented. In contrast to conventional studies within transmission electron microscopy, it is shown for the first time that the MAPbBr₃ perovskites and their devices can be simply patterned and improved via electron-beam irradiation in an electron-beam writer or a scanning electron microscope with relatively low accelerating voltage.

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.

Cu(In,Ga)Se₂ solar cells sputtered from a single quaternary target without post-selenization yield efficiency of 14.1%, a record for using directly sputtering. The enhanced efficiency is achieved by sequential post-treatments of Na and K, which improve the absorber quality due to effective passivation. This is reported in article number 1602571 by Chih-Huang Lai and co-workers.

The relation of the thermoelectric figure of merit and the nanocomposite morphology is studied for thermoelectric thin films consisting of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) with different amounts of silicon nanoparticles (Si-NPs). An increase in the figure of merit of up to 150% is found for an Si-NP concentration of 0.5 wt% as compared to pristine PEDOT:PSS films. The improvement originates from a disruption in the molecular ordering and therefore reduced electrical conductivity, which leads to an increased Seebeck coefficient, while also reducing thermal conductivity for higher concentrations through phonon scattering. The thermal conductivity is measured with steady-state IR thermography on free-standing PEDOT:PSS/Si-NP composite films, enabling a full determination of the figure of merit. The morphology is investigated with grazing incidence resonant tender X-ray scattering (GIR-TeXS) around the sulfur K-absorption edge. Without need for extrinsic labeling, GIR-TeXS measurements have varying scattering contrast conditions for the components of the ternary system. By comparing the scattered intensities at different photon energies with the corresponding scattering contrast, the Si-NPs are found to be preferentially dispersed in the large and medium-sized PEDOT-rich domains. The changes in size for the PEDOT-rich domains as function of Si-NP concentration cause improvement of the thermoelectric properties of the films.

The morphology–function relationship of thermoelectric nanocomposite films from PEDOT:PSS with silicon nanoparticles is discussed. True in-plane thermal conductivity is measured via steady-state infrared thermography enabling calculation of figure of merits. An increase in the figure of merit is found for low nanoparticle concentrations and ascribed to morphological changes of the nanocomposite films as determined with grazing incidence resonant tender X-ray scattering.

This study reports two new, simple and cost-effective hole transporting materials for perovskite solar cells. These novel structures namely N⁴,N⁴,N⁴′″,N⁴′″-tetrakis(4-methoxyphenyl)-[1,1′:4′,1″:4″,1′″-quaterphenyl]-4,4′″-diamine (TPA-BP-TPA), and (E)-4′,4′″-(ethene-1,2-diyl)bis(N,N-bis(4-methoxyphenyl)-[1″,1′″-biphenyl]-4-amine) (TPA-BPV-TPA) are based on linear π-conjugated linkers and triphenylamine endcappers. These materials possess good solubility and appropriate highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. Upon testing them as hole transporting materials in perovskite solar cells, in particular, the device with TPA-BPV-TPA exhibits a higher power conversion efficiency (PCE) of 16.42%, which is almost equivalent to the PCE using the conventional expensive 2,2′,7,7′-tetrakis(N,N′-di-pmethoxyphenylamino)-9,9′-spirbiuorene (SPIRO-OMeTAD) compound under similar conditions. Additionally, the device stability measured using this newly developed low-cost compound retains almost 87% of the initial performance after 10 days compared to standard SPIRO-OMeTAD-based devices. From this outstanding outcome it is revealed that simple triphenylamine-based hole-transporting materials with various kinds of π-conjugated linkers can pave the way for developing a new generation of simple hole-transporting materials for low-cost perovskite solar cells.

Low-cost and straightforward to synthesize TPA-BPV-TPA is successfully used as the hole-transporting material layer in conventional perovskite solar cells. Its efficient performance, hysteresis, and stability are almost comparable with traditional SPIRO-OMeTAD. Thus, it is a promising alternative to high-cost SPIRO-OMeTAD.

Energy offset (E_DA) from a number of donor/acceptor heterojunctions is measured using ultraviolet photoemission spectroscopy. It is found that substrate work functions have little impact on the energy level alignments at donor/acceptor heterojunctions. Planar-heterojunction organic photovoltaic cells are made to test the relationship between energy offset and open-circuit voltage (V_OC). V_OC is found to increase linearly as a function of E_DA. The V_OC, however, takes a surprising turn at E_DA = 1.5 eV and starts to decrease as a function of donor–acceptor energy levels. To explain this experimental observation, a theoretical model to quantify the relationship between V_OC and E_DA is developed. The proposed model well explains the experimental data and, in particular, the reverse trend of V_OC on E_DA. By grouping several material constants into one variable, a simple universal plot that well describes the experimental data for both planar-heterojunction and bulk-heterojunction cells is generated.

Donor/acceptor energy offset (E_DA) and open circuit voltage (V_OC) for donor/fullerene (C₇₀) systems are concurrently measured by using ultraviolet photoemission spectroscopy and organic solar cells. V_OC is found to first increase and then decrease as a function of E_DA. Based on a two-step exciton dissociation process, a theoretical model is developed to quantify the observed relationship between V_OC and E_DA.

Here, a facial and scalable method for efficient exfoliation of bulk transition metal dichalcogenides (TMD) and graphite in aqueous solution with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to prepare single- and few-layer nanosheets is demonstrated. Importantly, these TMD nanosheets retain the single crystalline characteristic, which is essential for application in organic solar cells (OSCs). The hybrid PEDOT:PSS/WS₂ ink prepared by a simple centrifugation is directly integrated as a hole extraction layer for high-performance OSCs. Compared with PEDOT:PSS, the PEDOT:PSS/WS₂-based devices provide a remarkable power conversion efficiency due to the “island” morphology and benzoid–quinoid transition. This study not only demonstrates a novel method for preparing single- and few-layer TMD and graphene nanosheets but also paves a way for their applications without further complicated processing.

A novel PEDOT:PSS/2D nanosheets ink as hole extraction layer is used to fabricate high-performance organic solar cells. 2D nanosheets with single- and few-layer structure exist stably in the ink because of PEDOT:PSS exfoliation and functionalization. The enhanced power conversion efficiency arises from the “island” morphology and benzoid–quinoid transition upon addition of 2D nanosheets.

Organic solar cells that are free of burn-in, the commonly observed rapid performance loss under light, are presented. The solar cells are based on poly(3-hexylthiophene) (P3HT) with varying molecular weights and a nonfullerene acceptor (rhodanine-benzothiadiazole-coupled indacenodithiophene, IDTBR) and are fabricated in air. P3HT:IDTBR solar cells light-soaked over the course of 2000 h lose about 5% of power conversion efficiency (PCE), in stark contrast to [6,6]-Phenyl C61 butyric acid methyl ester (PCBM)-based solar cells whose PCE shows a burn-in that extends over several hundreds of hours and levels off at a loss of ≈34%. Replacing PCBM with IDTBR prevents short-circuit current losses due to fullerene dimerization and inhibits disorder-induced open-circuit voltage losses, indicating a very robust device operation that is insensitive to defect states. Small losses in fill factor over time are proposed to originate from polymer or interface defects. Finally, the combination of enhanced efficiency and stability in P3HT:IDTBR increases the lifetime energy yield by more than a factor of 10 when compared with the same type of devices using a fullerene-based acceptor instead.

Organic solar cells based on a nonfullerene acceptor are presented that are free of burn-in, the commonly observed rapid performance loss under light. The combination of enhanced efficiency and stability increases the lifetime energy yield by more than a factor of 10 when compared with the same type of devices using a fullerene-based acceptor instead.

Hybrid metal halide perovskites have become one of the hottest topics in optoelectronic materials research in recent years. Not only have they surpassed everyone's expectations and achieved similar performance as tried and true polycrystalline silicon photovoltaic devices, but they are also finding applications in a variety of different fields, including lighting. The main advantages of hybrid metal halide perovskites are simple processability, compatible with large-scale solution processing such as roll-to-roll printing, and abundance of ingredients, all coupled to materials properties reminiscent of GaAs. On the road to this remarkable success, a series of challenges have been overcome, while some still remain. In this review, some of these challenges and possible solutions are described. In particular, understanding of the perovskite crystallization process and how this knowledge can be harnessed to enable better performing devices, how to overcome reproducibility issues and mitigate hysteresis, and the long-term prospects of the technology in terms of stability and sustainability will all be discussed.

This review of perovskite solar cells discusses the current understanding of the perovskite crystallization process, and how this knowledge can be harnessed to enable better performing devices; how to overcome reproducibility issues and mitigate hysteresis; and the long-term prospects of perovskite solar cell technology in terms of stability, cost, and sustainability.

A comparison of the efficiency, stability, and photophysics of organic solar cells employing poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′″-di(2-octyldodecyl)-2,2′;5′,2″;5″,2′″-quaterthiophen-5,5′″-diyl)] (PffBT4T-2OD) as a donor polymer blended with either the nonfullerene acceptor EH-IDTBR or the fullerene derivative, [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) as electron acceptors is reported. Inverted PffBT4T-2OD:EH-IDTBR blend solar cell fabricated without any processing additive achieves power conversion efficiencies (PCEs) of 9.5 ± 0.2%. The devices exhibit a high open circuit voltage of 1.08 ± 0.01 V, attributed to the high lowest unoccupied molecular orbital (LUMO) level of EH-IDTBR. Photoluminescence quenching and transient absorption data are employed to elucidate the ultrafast kinetics and efficiencies of charge separation in both blends, with PffBT4T-2OD exciton diffusion kinetics within polymer domains, and geminate recombination losses following exciton separation being identified as key factors determining the efficiency of photocurrent generation. Remarkably, while encapsulated PffBT4T-2OD:PC₇₁BM solar cells show significant efficiency loss under simulated solar irradiation (“burn in” degradation) due to the trap-assisted recombination through increased photoinduced trap states, PffBT4T-2OD:EH-IDTBR solar cell shows negligible burn in efficiency loss. Furthermore, PffBT4T-2OD:EH-IDTBR solar cells are found to be substantially more stable under 85 °C thermal stress than PffBT4T-2OD:PC₇₁BM devices.

A high efficiency, burn-in-free nonfullerene-based PffBT4T-2OD:EH-IDTBR solar cell is reported, fabricated without processing additives. Transient absorption and optoelectronic analyses elucidate the causes of this high efficiency and stability, with the superior stability compared to PC₇₁BM devices being correlated with increased crystallinity and reduced photogeneration of trap states.