Ligang Yuan

Flexible and semitransparent organic solar cells (OSCs) have been regarded as the most promising photovoltaic devices for the application of OSCs in wearable energy resources and building-integrated photovoltaics. Therefore, the flexible and semitransparent OSCs have developed rapidly in recent years through the synergistic efforts in developing novel flexible bottom or top transparent electrodes, designing and synthesizing high performance photoactive layer and low temperature processed electrode buffer layer materials, and device architecture engineering. To date, the highest power conversion efficiencies have reached over 10% of the flexible OSCs and 7.7% with average visible transmittance of 37% for the semitransparent OSCs. Here, a comprehensive overview of recent research progresses and perspectives on the related materials and devices of the flexible and semitransparent OSCs is provided.

Flexible and semitransparent organic solar cells (OSCs) are regarded as the most promising photovoltaic devices for the application of OSCs in wearable energy resources and building-integrated photovoltaics. Here, a comprehensive overview of recent research progresses and perspectives on the related materials and devices of the flexible and semitransparent OSCs is provided.

High pressure (HP) can drive the direct sintering of nanoparticle assemblies for Ag/Au, CdSe/PbS nanocrystals (NCs). Instead of direct sintering for the conventional nanocrystals, this study experimentally observes for the first time high-pressure-induced comminution and recrystallization of organic–inorganic hybrid perovskite nanocrystals into highly luminescent nanoplates with a shorter carrier lifetime. Such novel pressure response is attributed to the unique structural nature of hybrid perovskites under high pressure: during the drastic cubic–orthorhombic structural transformation at ≈2 GPa, (301) the crystal plane fully occupied by organic molecules possesses a higher surface energy, triggering the comminution of nanocrystals into nanoslices along such crystal plane. Beyond bulk perovskites, in which pressure-induced modifications on crystal structures and functional properties will disappear after pressure release, the pressure-formed variants, i.e., large (≈100 nm) and thin (<10 nm) perovskite nanoplates, are retained and these exhibit simultaneous photoluminescence emission enhancing (a 15-fold enhancement in the photoluminescence) and carrier lifetime shortening (from ≈18.3 ± 0.8 to ≈7.6 ± 0.5 ns) after releasing of pressure from 11 GPa. This pressure-induced comminution of hybrid perovskite NCs and a subsequent amorphization–recrystallization treatment offer the possibilities of engineering the advanced hybrid perovskites with specific properties.

High pressure (up to tens of gigapascals), as a clean and powerful tool, can effectively alter crystal structures. It is experimentally investigated for the first time whether pressure can regulate the comminution and recrystallization of MAPbBr₃ nanocrystals. The initial nanocrystals slide into nanoslices during phase transformation to the orthorhombic polymorph, followed by recrystallization into nanoplates upon amorphization.

MXenes, a young family of 2D transition metal carbides/nitrides, show great potential in electrochemical energy storage applications. Herein, a high performance ultrathin flexible solid-state supercapacitor is demonstrated based on a Mo_1.33C MXene with vacancy ordering in an aligned layer structure MXene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) composite film posttreated with concentrated H₂SO₄. The flexible solid-state supercapacitor delivers a maximum capacitance of 568 F cm⁻³, an ultrahigh energy density of 33.2 mWh cm⁻³ and a power density of 19 470 mW cm⁻³. The Mo_1.33C MXene/PEDOT:PSS composite film shows a reduction in resistance upon H₂SO₄ treatment, a higher capacitance (1310 F cm⁻³) and improved rate capabilities than both pristine Mo_1.33C MXene and the nontreated Mo_1.33C/PEDOT:PSS composite films. The enhanced capacitance and stability are attributed to the synergistic effect of increased interlayer spacing between Mo_1.33C MXene layers due to insertion of conductive PEDOT, and surface redox processes of the PEDOT and the MXene.

A MXene-based solution processable flexible solid-state supercapacitor with high performance is developed from a MXene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) composite film. After posttreatment with concentrated H₂SO₄, the PEDOT nanofiber network is aligned between the MXene sheets, leading to highly improved flexibility and, most importantly, improved capacitances (1310 F cm⁻³), rate-capabilities, and stability.

The factors limiting the conductivity of fluorine-doped tin dioxide (FTO) produced via atmospheric pressure chemical vapor deposition are investigated. Modeling of the transport properties indicates that the measured Hall effect mobilities are far below the theoretical ionized impurity scattering limit. Significant compensation of donors by acceptors is present with a compensation ratio of 0.5, indicating that for every two donors there is approximately one acceptor. Hybrid density functional theory calculations of defect and impurity formation energies indicate the most probable acceptor-type defects. The fluorine interstitial defect has the lowest formation energy in the degenerate regime of FTO. Fluorine interstitials act as singly charged acceptors at the high Fermi levels corresponding to degenerately n-type films. X-ray photoemission spectroscopy of the fluorine impurities is consistent with the presence of substitutional F_O donors and interstitial F_i in a roughly 2:1 ratio in agreement with the compensation ratio indicated by the transport modeling. Quantitative analysis through Hall effect, X-ray photoemission spectroscopy, and calibrated secondary ion mass spectrometry further supports the presence of compensating fluorine-related defects.

Compensating acceptor defects dramatically reduce electronic performance of F:SnO₂ transparent conductor grown by chemical vapor deposition. Electron carrier mobilities are seen to be greatly diminished from the theoretically predicted optimum. Using hybrid density functional theory calculations and experimental methods, and analysis, the defect responsible for self-compensation in F:SnO₂ is determined to be the fluorine interstitial.

High crystallinity and compactness of the active layer is essential for metal-halide perovskite solar cells. Here, a simple pseudohalide-induced film retreatment technology is developed as the passivation for preformed perovskite film. It is found that the retreatment process yields a controllable decomposition-to-recrystallization evolution of the perovskite film. Corresponding, it remarkably enlarges the grain size of the film in all directions, as well as improving the crystallinity and hindering the trap density. Meanwhile, owing to an intermediate catalytic effect of the pseudohalide compound (NH₄SCN), no crystal orientation changing and no impurity introduction in the modified film. By integrating the modified perovskite film into the planar heterojunction solar cells, a champion power conversion efficiency of 19.44% with a stabilized output efficiency of 19.02% under 1 sun illumination is obtained, exhibiting a negligible current density–voltage hysteresis. Moreover, such a facile and low-temperature film retreatment approach guarantees the application in flexible devices, showing a best power conversion efficiency of 17.04%.

A facile and low-temperature pseudohalide-induced postprocessing technology is developed to improve the crystallinity and compactness of the perovskite active layer by integrating the modified perovskite film into the planar heterojunction solar cells, a best efficiency of 19.44%, with a negligible current density–voltage hysteresis. Meanwhile, successful application is obtained in flexible devices, showing a best power conversion efficiency of 17.04%.

For the commercial development of organic photovoltaics (OPVs), laboratory-scale OPV technology must be translated to large area modules. In particular, it is important to develop high-efficiency polymers that can form thick (>100 nm) bulk heterojunction (BHJ) films over large areas with optimal morphologies for charge generation and transport. Here, D₁-A-D₂-A random terpolymers composed of 2,2′-bithiophene with various proportions of 5,6-difluoro-4,7-bis(thiophen-2-yl)-2,1,3-benzothiadiazole and 5,6-difluoro-2,1,3-benzothiadiazole (FBT) are synthesized. It is found that incorporating small proportions of FBT into the polymer not only conserves the high crystallinity and favorable face-on orientation of the D-A copolymer FBT-Th4 but also improves the nanoscale phase separation of the BHJ film. Consequently, the random terpolymer PDT2fBT-BT10 exhibits a much improved solar cell efficiency of 10.31% when compared to that of the copolymer FBT-Th4 (8.62%). Moreover, due to this polymer's excellent processability and suppressed overaggregation, OPVs with 1 cm² active area based on 351 nm thick PDT2fBT-BT10 BHJs exhibit high photovoltaic performance of 9.42%, whereas rapid efficiency decreases arise for FBT-Th4-based OPVs for film thicknesses above 300 nm. It is demonstrated that this random terpolymer can be used in large area and thick BHJ OPVs, and guidelines for developing polymers that are suitable for large-scale printing technologies are presented.

D₁-A-D₂-A-type random terpolymers and organic photovoltaics (OPVs) are developed introducing that the resulting polymer achieves a high efficiency of 10.31%. Furthermore, reproducibility of 1 cm² OPVs shows a high efficiency up to 9.42% using thick active layers in the range of 250–380 nm.

Naphtho[1,2-b:5,6-b′]dithiophene is extended to a fused octacyclic building block, which is end capped by strong electron-withdrawing 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile to yield a fused-ring electron acceptor (IOIC2) for organic solar cells (OSCs). Relative to naphthalene-based IHIC2, naphthodithiophene-based IOIC2 with a larger π-conjugation and a stronger electron-donating core shows a higher lowest unoccupied molecular orbital energy level (IOIC2: −3.78 eV vs IHIC2: −3.86 eV), broader absorption with a smaller optical bandgap (IOIC2: 1.55 eV vs IHIC2: 1.66 eV), and a higher electron mobility (IOIC2: 1.0 × 10⁻³ cm² V⁻¹ s⁻¹ vs IHIC2: 5.0 × 10⁻⁴ cm² V⁻¹ s⁻¹). Thus, IOIC2-based OSCs show higher values in open-circuit voltage, short-circuit current density, fill factor, and thereby much higher power conversion efficiency (PCE) values than those of the IHIC2-based counterpart. In particular, as-cast OSCs based on FTAZ: IOIC2 yield PCEs of up to 11.2%, higher than that of the control devices based on FTAZ: IHIC2 (7.45%). Furthermore, by using 0.2% 1,8-diiodooctane as the processing additive, a PCE of 12.3% is achieved from the FTAZ:IOIC2-based devices, higher than that of the FTAZ:IHIC2-based devices (7.31%). These results indicate that incorporating extended conjugation into the electron-donating fused-ring units in nonfullerene acceptors is a promising strategy for designing high-performance electron acceptors.

A novel fused-ring electron acceptor (IOIC2) based on naphthodithiophene is designed and synthesized, and compared with a naphthalene-based counterpart (IHIC2). The IOIC2-based single-junction binary-blend organic solar cells exhibit efficiencies up to 12.3%, much higher than that of IHIC2 (7.45%).

In article number 1701305, Maria Antonietta Loi and co-workers show that the electron transport capability of the electron-extracting layer dominates the temperature dependence of the performance of hybrid perovskite solar cells. The authors further demonstrate efficient hybrid perovskite solar cells over a temperature range from 295 K to 160 K by n-doping the PCBM electron-extracting layer or using a fullerene derivative with intrinsically higher electron transport capability.

In a recent report, Lee et al. have proposed an “effective field model” for extrinsic doping to explain the electrical properties of Al-doped zinc oxide (ZnO) films grown by atomic layer deposition (ALD). They have introduced the doping model by considering the layered structure of the ALD-grown films as observed in the transmission electron microscopy measurements. However, in the present comment, we have demonstrated that the suggested doping model is misleading in which physically inconsistent assumptions are considered throughout. Herein, a reasonable interpretation of the electrical properties and doping mechanism of the ALD-grown films by taking into consideration the theoretical formulations of the disordered electronic system is suggested.

Mixed cation hybrid perovskites such as Cs_xFA_1−xPbI₃ are promising materials for solar cell applications, due to their excellent photoelectronic properties and improved stability. Although power conversion efficiencies (PCEs) as high as 18.16% have been reported, devices are mostly processed by the anti-solvent method, which is difficult for further scaling-up. Here, a method to fabricate Cs_xFA_1−xPbI₃ by performing Cs cation exchange on hybrid chemical vapor deposition grown FAPbI₃ with the Cs⁺ ratio adjustable from 0 to 24% is reported. The champion perovskite module based on Cs_0.07FA_0.93PbI₃ with an active area of 12.0 cm² shows a module PCE of 14.6% and PCE loss/area of 0.17% cm⁻², demonstrating the significant advantage of this method toward scaling-up. This in-depth study shows that when the perovskite films prepared by this method contain 6.6% Cs⁺ in bulk and 15.0% at the surface, that is, Cs_0.07FA_0.93PbI₃, solar cell devices show not only significantly increased PCEs but also substantially improved stability, due to favorable energy level alignment with TiO₂ electron transport layer and spiro-MeOTAD hole transport layer, increased grain size, and improved perovskite phase stability.

Cs-substituted mixed perovskite modules are prepared by a new developed large-area-compatible method combining hybrid chemical vapor deposition and cation exchange. Power conversion efficiency (PCE) as high as 14.6% is achieved, benefiting from the large-area film uniformity on macroscopic and microscopic scales. In-depth study shows that 7% Cs⁺ in Cs_xFA_1−xPbI₃ is the optimal ratio for achieving best device PCE and longest lifetime.