ZiQi Sun

Quantum cutting can realize the emission of multiple near-infrared photons for each ultraviolet/visible photon absorbed, and has potential to significantly improve the photoelectric conversion efficiency (PCE) of solar cells. However, due to the lack of an ideal downconversion material, it has merely served as a principle in the laboratory until now. Here, the fabrication of a novel type of quantum cutting material, CsPbCl_1.5Br_1.5:Yb³⁺, Ce³⁺ nanocrystals is presented. Benefiting from the larger absorption cross-section, weaker electron–phonon coupling, and higher inner luminescent quantum yield (146%), the doped perovskite nanocrystals are successfully explored as a downconverter of commercial silicon solar cells (SSCs). Noticeably, the PCE of the SSCs is improved from 18.1% to 21.5%, with a relative enhancement of 18.8%. This work exhibits a cheap, convenient, and effective way to enhance the PCE of SSCs, which may be commercially popularized in the future.

Cerium and ytterbium codoped halide perovskite quantum dots display an efficient near-infrared emission with inner luminescent quantum yield of 146%. The quantum dots are explored to enhance the performance of silicon solar cells with a relative enhancement of 18.8%.

An all-small-molecule ternary solar cell is achieved with a power conversion efficiency of 10.48% by incorporating phenyl-C₇₁-butyric-acid-methyl ester (PC₇₁BM) into a nonfullerene binary system. The addition of PC₇₁BM is found to modulate the film morphology by improving the domain purity and decreasing the domain size. This modulation facilitates charge generation and suppresses charge recombination, as manifested by the significantly enhanced short-circuit current density and fill factor. The results correlate the domain characteristics with the device performance and offer new insight from the perspective of morphology modulation for constructing efficient ternary devices.

An all-small-molecule ternary solar cell is achieved with a power conversion efficiency of 10.48% by incorporating the phenyl-C₇₁-butyric-acid-methyl ester (PC₇₁BM) into a nonfullerene small-molecule binary system. The addition of PC₇₁BM is found to modulate the film morphology by improving the domain purity and decreasing the domain size. This facilitates charge generation and suppresses charge recombination.

Optoelectronic devices based on hybrid perovskites have demonstrated outstanding performance within a few years of intense study. However, commercialization of these devices requires barriers to their development to be overcome, such as their chemical instability under operating conditions. To investigate this instability and its consequences, the electric field applied to single crystals of methylammonium lead bromide (CH₃NH₃PbBr₃) is varied, and changes are mapped in both their elemental composition and photoluminescence. Synchrotron-based nanoprobe X-ray fluorescence (nano-XRF) with 250 nm resolution reveals quasi-reversible field-assisted halide migration, with corresponding changes in photoluminescence. It is observed that higher local bromide concentration is correlated to superior optoelectronic performance in CH₃NH₃PbBr₃. A lower limit on the electromigration rate is calculated from these experiments and the motion is interpreted as vacancy-mediated migration based on nudged elastic band density functional theory (DFT) simulations. The XRF mapping data provide direct evidence of field-assisted ionic migration in a model hybrid-perovskite thin single crystal, while the link with photoluminescence proves that the halide stoichiometry plays a key role in the optoelectronic properties of the perovskite.

Bromide-ion migration is directly observed in a methylammonium lead bromide perovskite single crystal under bias using a synchrotron-based X-ray fluorescence nanoprobe. Photoluminescence mapping indicates that bromide-rich regions exhibit enhanced photoluminescence. The close correspondence between the local bromide concentration and photoluminescence in response to bias reveals the importance of non-stoichiometry in determining optoelectronic performance in halide perovskites.

An original set of experimental and modeling tools is used to quantify the yield of each of the physical processes leading to photocurrent generation in organic bulk heterojunction solar cells, enabling evaluation of materials and processing condition beyond the trivial comparison of device performances. Transient absorption spectroscopy, “the” technique to monitor all intermediate states over the entire relevant timescale, is combined with time-delayed collection field experiments, transfer matrix simulations, spectral deconvolution, and parametrization of the charge carrier recombination by a two-pool model, allowing quantification of densities of excitons and charges and extrapolation of their kinetics to device-relevant conditions. Photon absorption, charge transfer, charge separation, and charge extraction are all quantified for two recently developed wide-bandgap donor polymers: poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-difluorothiophene) (PBDT[2F]T) and its nonfluorinated counterpart poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-thiophene) (PBDT[2H]T) combined with PC₇₁BM in bulk heterojunctions. The product of these yields is shown to agree well with the devices' external quantum efficiency. This methodology elucidates in the specific case studied here the origin of improved photocurrents obtained when using PBDT[2F]T instead of PBDT[2H]T as well as upon using solvent additives. Furthermore, a higher charge transfer (CT)-state energy is shown to lead to significantly lower energy losses (resulting in higher V_OC) during charge generation compared to P3HT:PCBM.

The individual efficiencies and losses associated with each step of photocurrent generation in PBDT[2X]T:PC₇₁BM solar cells are determined using a combination of advanced transient spectroscopic, charge extraction, and steady-state spectroscopic techniques, aiding the development of a fuller understanding of the complex interplay between chemical structure, thin film processing conditions, and their impact on device performance.

Organometal halide perovskite solar cells (PeSCs) are regarded as promising photovoltaics due to their outstanding power conversion efficiencies (PCEs). However, even though their PCEs are achieved over 20%, their intrinsically poor stability is a big bottleneck for their practical uses. Here, a simple method is reported using phenyl-C₆₁-butyric acid methyl ester as a molecular additive to improve thermal stability of organometal halide perovskite crystals, which also improves the PCEs of the associated PeSCs. Moreover, by varying the grain size of perovskite crystals up to ≈150 µm, it is demonstrated that grain boundary plays a significant role in their thermal stability. Cells with smaller grain interface area (i.e., larger grain size) have higher thermal stability. The additive is located at grain boundaries and found to induce electron transfer reactions with halogens in the perovskite. The reaction products chemically passivate perovskite crystals and strongly bind halogen atoms at grain boundaries to their crystal lattice, preventing them from exiting from the crystal lattice, which improves thermal stability of perovskite crystals. This study offers a simple method for improving thermal stability of perovskite without any loss and opens up the possibility of the use of various molecular additives to achieve highly stable PeSCs.

Chemical passivation of organometal halide perovskite crystals with phenyl-C₆₁-butyric acid methyl ester (PCBM) is investigated. PCBM located at perovskite grain boundaries induces electron transport reactions with halogens in perovskite and chemically passivates the grain boundaries. It prevents halogens at the grain boundaries from exiting the crystal lattice and thereby results in improved thermal stability of organometal halide perovskite solar cells.

Organometal trihalide perovskites (OTPs) as a new subclass of perovskite materials have recently aroused increasing interest due to their numerous advantages of facile low-temperature processing, tunable bandgaps, diverse compositions, and superior charge transfer dynamics, which have been widely used in various applications. In particular, solar cells composed of these perovskites have made unprecedented progress in just a few years with maximum power conversion efficiency, evolving from 3.8 to 21.6%. In spite of such impressive achievement, a fundamental understanding of intrinsic optoelectronic and physiochemical properties is a key challenge impeding the development of the OTPs. This review article aims to provide a concise overview of the current status of OTPs research, highlighting the unique properties of OTPs, especially ferroelectric and piezoelectric properties, which are vital to photovoltaic and piezoelectric applications but still not adequately explained. Various material synthesis strategies of OTPs are surveyed, exhibiting that the OTPs architecture can serve as a promising and robust platform for opening new horizons in ferroelectric and piezoelectric researches. Several applications, including piezoelectric generators, solar cells, light-emitting diodes, lasers, photodetectors, and water-splitting cells, demonstrate the latent potentialities of OTPs.

Recent progress on organometal trihalide perovskites in terms of intrinsic properties, with a focus on ferroelectricity and piezoelectricity, synthesis strategies of thin films and bulk/nanocrystals, and various applications, is reviewed. The importance of an in-depth understanding on the underlying ferroelectricity and piezoelectricity is discussed, and a thorough investigation is the main factor necessary for facilitating the functionalization of these materials.

A kind of new fused-ring electron acceptor, IDT-OB, bearing asymmetric side chains, is synthesized for high-efficiency thick-film organic solar cells. The introduction of asymmetric side chains can increase the solubility of acceptor molecules, enable the acceptor molecules to pack closely in a dislocated way, and form favorable phase separation when blended with PBDB-T. As expected, PBDB-T:IDT-OB-based devices exhibit high and balanced hole and electron mobility and give a high power conversion efficiency (PCE) of 10.12%. More importantly, the IDT-OB-based devices are not very sensitive to the film thickness, a PCE of 9.17% can still be obtained even the thickness of active layer is up to 210 nm.

A new fused-ring electron acceptor (IDT-OB), bearing asymmetric side chains, is facilely synthesized for high-efficiency thick-film organic solar cells. The asymmetric structure makes it easier to form ideal phase separation when blended with a polymer donor. Power conversion efficiencies of 10.12% and 8.57% are obtained with active-layer thicknesses of 130 and 320 nm, respectively.