Ligang Yuan

Solution-processed metal nanowire networks have attracted substantial attention as clear transparent conductive electrodes (TCEs) to replace metal oxides for low-cost and flexible touch panels and displays. While targeting photovoltaic applications, TCEs are expected to be more hazy for enhancing light absorption in the active layer, but are still required to retain high transmittance and low sheet resistance. Balancing these properties (haze, transmittance, and conductivity) in TCEs to realize high performance but high haze simultaneously is a challenge because they are mutually influenced. Here, by precisely tailoring the diameter of thick–long silver nanowires using rapid radial electrochemical etching, high hazy flexible TCEs are fabricated with high figure of merit of up to 741 (4 Ω sq⁻¹ at 88.4% transmittance with haze of 13.3%), surpassing those of commercialized brittle hazy metal oxides and exhibiting superiority for photovoltaic applications. Laminating such TCEs onto the perovskite solar cells as top electrodes, the obtained semitransparent devices exhibit power efficiencies up to 16.03% and 11.12% when illuminated from the bottom and top sides, respectively, outperforming reported results based on similar device architecture. This study provides a simple strategy for flexible and hazy TCEs fabrication, which is compatible with mild solution-processed photovoltaic devices, especially those containing heat-sensitive or chemical-sensitive materials.

High-performance flexible hazy transparent electrodes based on silver nanowires network are constructed, and the high figure of merit (741) and haze (13.3%) can be achieved simultaneously by tailoring the nanowire diameters. Semitransparent perovskite solar cells using such hazy transparent electrodes as top electrodes exhibit power conversion efficiencies up to 16.03%.

Compositional grading has been widely exploited in highly efficient Cu(In,Ga)Se₂, CdTe, GaAs, quantum dot solar cells, and this strategy has the potential to improve the performance of emerging perovskite solar cells. However, realizing and maintaining compositionally graded perovskite absorber from solution processing is challenging. Moreover, the operational stability of graded perovskite solar cells under long-term heat/light soaking has not been demonstrated. In this study, a facile partial ion-exchange approach is reported to achieve compositionally graded perovskite absorber layers. Incorporating compositional grading improves charge collection and suppresses interface recombination, enabling to fabricate near-infrared-transparent perovskite solar cells with power conversion efficiency of 16.8% in substrate configuration, and demonstrate 22.7% tandem efficiency with 3.3% absolute gain when mechanically stacked on a Cu(In,Ga)Se₂ bottom cell. Non-encapsulated graded perovskite device retains over 93% of its initial efficiency after 1000 h operation at maximum power point at 60 °C under equivalent 1 sun illumination. The results open an avenue in exploring partial ion-exchange to design graded perovskite solar cells with improved efficiency and stability.

A compositionally graded perovskite absorber is designed by partial ion-exchange reaction to improve charge collection and suppress interfacial recombination, which leads to improved efficiency and operational stability in near-infrared-transparent planar perovskite solar cells. The non-encapsulated graded perovskite device retains over 93% of its initial efficiency after 1000 h operation at maximum power point at 60 °C under equivalent 1 sun illumination.

In this communication, novel and simplified structure Cu(In,Ga)Se₂ (CIGS) solar cells, which nominally consist of only a CIGS photoabsorber layer sandwiched between back and front contact layers but yet demonstrate high photovoltaic efficiencies, are reported. To realize this accomplishment, Si-doped CIGS films grown by the three-stage coevaporation method, B-doped ZnO transparent conductive oxide front contact layers deposited by chemical vapor deposition, and heat–light soaking treatments are used. Si-doping of CIGS films is found to modify the film surfaces and grain boundary properties and also affect the alkali metal distribution profiles in CIGS films. These effects are expected to contribute to improvements in buffer-free CIGS device performance. Heat–light soaking treatments, which are occasionally performed to improve conventional buffer-based CIGS device performance, are found to be also effective in enhancing buffer-free CIGS photovoltaic efficiencies. This result suggests that the mechanism behind the beneficial effects of heat–light soaking treatments originates from CIGS bulk issues and is independent of the buffer materials. Consequently, over 16.5% efficiencies, including an independently certified value, are demonstrated from completely buffer-free CIGS photovoltaic devices.

Buffer-free simplified structure chalcogenide thin film solar cells with high efficiencies over 16.5% are demonstrated using novel Cu(In,Ga)Se₂ (CIGS) photoabsorber layers grown with Si-doping. Heat–light soaking treatments boost device performance regardless of the use of buffer layers, implying that the light-induced metastable phenomena in CIGS devices originate from bulk CIGS rather than buffer layers or the CIGS/buffer interface.

State-of-the-art light-emitting diodes (LEDs) are made from high-purity alloys of III–V semiconductors, but high fabrication cost has limited their widespread use for large area solid-state lighting. Here, efficient and stable LEDs processed from solution with tunable color enabled by using phase-pure 2D Ruddlesden–Popper (RP) halide perovskites with a formula (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)_n−1Pb_nI_3n+1 are reported. By using vertically oriented thin films that facilitate efficient charge injection and transport, efficient electroluminescence with a radiance of 35 W Sr⁻¹ cm⁻² at 744 nm with an ultralow turn-on voltage of 1 V is obtained. Finally, operational stability tests suggest that phase purity is strongly correlated to stability. Phase-pure 2D perovskites exhibit >14 h of stable operation at peak operating conditions with no droop at current densities of several Amperes cm⁻² in comparison to mixtures of 2D/3D or 3D perovskites, which degrade within minutes.

Phase-pure Ruddlesden–Popper layered perovskites (RPLPs) are investigated for light-emitting diode application. This work demonstrates that RPLPs represent a promising candidate for optoeletronics and the crystal orientation of RPLPs plays a vital role in light-emitting diode devices, resulting in external quantum efficiency ≈1% with ultralow turn-on voltage.

Facile control over the morphology of phase pure tin monosulfide (SnS) thin films, a promising future absorber for thin film solar cells, is enabled by controlling the growth kinetics in vapor transport deposition of congruently evaporated SnS. The pressure during growth is found to be a key factor in modifying the final shape of the SnS grains. The optimized cube-like SnS shows p-type with the apparent carrier concentration of ≈10¹⁷ cm⁻³ with an optical bandgap of 1.32 eV. The dense and flat surface morphology of 1 µm thick SnS combined with the minimization of pinholes directly leads to improved diode quality and increased shunt resistance of the SnS/CdS heterojunction (cell area of 0.30 cm²). An open-circuit voltage of up to 0.3068 V is achieved, which is independently characterized at the Korea Institute of Energy Research (KIER). Detailed high-resolution transmission electron microscopy analysis confirms the absence of detrimental secondary phases such as Sn₂S₃ or SnS₂ in the SnS grains or at intergrain boundaries. The initial efficiency level of 98.5% is maintained even after six months of storage in air, and the final efficiency of the champion SnS/CdS cell, certified at the KIER, is 2.938% with an open-circuit voltage of 0.2912 V.

Facile control on morphology of phase pure tin monosulfide (SnS) thin films is enabled by controlling the growth kinetics in vapor transport deposition. Dense and flat surface morphology of cube-like orthorhombic SnS combined with the minimized pinholes directly leads to improved diode quality and increased shunt resistance of the SnS/CdS heterojunction, achieving the certified efficiency of 2.938%.

The use of perovskite materials as anion-based intercalation pseudocapacitor electrodes has received significant attention in recent years. Notably, these materials, characterized by high oxygen vacancy concentrations, do not require high surface areas to achieve a high energy storage capacity as a result of the bulk intercalation mechanism. This study reports that reduced PrBaMn₂O_6–δ (r-PBM), possessing a layered double perovskite structure, exhibits ultrahigh capacitance and functions as an excellent oxygen anion-intercalation-type electrode material for supercapacitors. Formation of the layered double perovskite structure, as facilitated by hydrogen treatment, is shown to significantly enhance the capacitance, with the resulting r-PBM material demonstrating a very high gravimetric capacitance of 1034.8 F g⁻¹ and an excellent volumetric capacitance of ≈2535.3 F cm⁻³ at a current density of 1 A g⁻¹. The resultant formation of a double perovskite crystal oxide with a specific layered structure leads to the r-PBM with a substantially higher oxygen diffusion rate and oxygen vacancy concentration. These superior characteristics show immense promise for their application as oxygen anion-intercalation-type electrodes in pseudocapacitors.

The formation of a double perovskite crystal oxide with a specific layered structure results in the reduced PrBaMn₂O_6-δ (r-PBM) with a substantially higher oxygen diffusion rate and oxygen vacancy concentration. These factors are highly beneficial to the oxygen vacancies as charge storage sites can be applied in the pseudocapacitors with an oxygen ion intercalation process.

Two bifunctional photovoltaic materials, named DRCN3TT and DRCN5TT with acceptor-donor-acceptor (A-D-A) type chemical structures containing thieno[3,2-b]thiophene as the central unit and 2-(1,1-dicyanomethylene)rhodanine as the end group, are designed and synthesized. As donors, DRCN5TT and DRCN3TT based devices with PC₇₁BM as acceptors show power conversion efficiencies (PCEs) of 7.03 and 3.85%, respectively. As acceptors, blended with the best donor materials such as P3HT or PDCBT, they behave as the acceptor and show PCEs of 3–4%.

A solid-state photoactive molecular mediator N(BAI)₃ is developed as an alternative to the commonly used solvent processing additives to achieve 11% enhancement of power conversion efficiency of prototypical poly(3-hexylthiophene-2,5-diyl) (P3HT):PC₆₀BM bulk heterojunction photovoltaic devices. Attributable to both the fine tuned phase morphology, and the additional optical absorption from the molecular additives.

A low-temperature processed nanoneedle/nanosheet rutile TiO₂ array film is fabricated as ETL of CH3NH3PbI3-based PSCs for the first time. The rutile TiO₂ array can act as a hole blocking layer and an electron transport layer to efficiently improve the photovoltaic performance and the stability of the PSCs.

Perovskite solar cells (PSCs) have gained a promising position during the past few years. However, as far as it goes, there is rare combination of the merits of metal–organic framework with PSCs. In this work, a 3D metal–organic framework, namely, [In₂(phen)₃Cl₆]·CH₃CN·2H₂O (In2) is first introduced into hole transport material of PSCs through band alignment engineering. By this facile strategy, the pinholes in the hole transport layer are effectively reduced, and the migration of Au into the entire PSC structure can be alleviated simultaneously. Meanwhile, In2 also plays a role in enhancing the light absorption of perovskite, which is due to: (1) the large particles of In2 acting as light scattering centers; (2) the emission wavelength of In2 is almost the same as the excitation wavelength of perovskite. Consequently, short-current density (J_sc), open circuit voltage (V_oc), and fill factor (FF) gain a significant increase from 19.53 to 21.03 mA cm⁻², 0.98 to 1.01 V, and 0.67 to 0.74, respectively. Thereby, the power conversion efficiency is remarkably enhanced from 12.8% to 15.8%. In the end, the stability of PSCs should also be improved.

The addition of [In₂(phen)₃Cl₆]·CH₃CN·2H₂O (In2) into the hole transport layer of perovskite solar cells (PSCs) through band alignment engineering is beneficial for charge transfer and restricts penetration of Au from back contact. Furthermore, the ultraviolet absorption, photoluminescence and light scattering properties of In2 can improve the light utilization of PSCs, leading to an increase in power conversion efficiency from 12.8% to 15.8%.

In this work, a new benzo[1,2-b:4,5-b′]dithiophene (BDT) building block containing alkylthio naphthyl as a side chain is designed and synthesized, and the resulting polymer, namely PBDTNS-BDD, shows a lower HOMO energy level than that of its alkoxyl naphthyl counterpart PBDTNO-BDD. An optimized photovoltaic device using PBDTNS-BDD as a donor exhibits power conversion efficiencies (PCE) of 8.70% and 9.28% with the fullerene derivative PC₇₁BM and the fullerene-free small molecule ITIC as acceptors, respectively. Surprisingly, ternary blend devices based on PBDTNS-BDD and two acceptors, namely PC₇₁BM and ITIC, shows a PCE of 11.21%, which is much higher than that of PBDTNO-BDD based ternary devices (7.85%) even under optimized conditions.

Ternary blend devices containing a new building block show much better power conversion efficiencies than their corresponding binary counterparts. These new building blocks contain alkylthio naphthyl as a side chain. Blending with two acceptors, PC₇₁BM and ITIC, results in PBDTNS-BDD devices with a power conversion efficiency of 11.21%, which is much higher than that of the PBDTNO-BDD (7.85%) analogue under optimized conditions.

The fabrication of a new type of solar cell encapsulation architecture comprising a periodic array of step-index waveguides is reported. The materials are fabricated through patterning with light in a photoreactive binary blend of crosslinking acrylate and urethane, wherein phase separation induces the spontaneous, directed formation of broadband, cylindrical waveguides. This microstructured material efficiently collects and transmits optical energy over a wide range of entry angles. Silicon solar cells comprising this encapsulation architecture show greater total external quantum efficiencies and enhanced wide-angle light capture and conversion. This is a rapid, straightforward, and scalable approach to process light-collecting structures, whereby significant increases in cell performance may be achieved.

Broadband waveguide array architectures are inscribed into polymer films as a new encapsulant material for solar cells. The architectures are grown in a binary-component, photocurable resin through light-induced self-writing, which elicits spontaneous formation of the core–cladding waveguide profile. Their light-collecting and light-guiding functions are inherited by the film, thereby enabling large-scale enhanced and wide-angle optical energy collection and conversion.

Most nonfullerene acceptors developed so far for high-performance organic solar cells (OSCs) are designed in planar molecular geometry containing a fused-ring core. In this work, a new nonfullerene acceptor of DF-PCIC is synthesized with an unfused-ring core containing two cyclopentadithiophene (CPDT) moieties and one 2,5-difluorobenzene (DFB) group. A nearly planar geometry is realized through the F···H noncovalent interaction between CPDT and DFB for DF-PCIC. After proper optimizations, the OSCs with DF-PCIC as the acceptor and the polymer PBDB-T as the donor yield the best power conversion efficiency (PCE) of 10.14% with a high fill factor of 0.72. To the best of our knowledge, this efficiency is among the highest values for the OSCs with nonfullerene acceptors owning unfused-ring cores. Furthermore, no obvious morphological changes are observed for the thermally treated PBDB-T:DF-PCIC blended films, and the relevant devices can keep ≈70% of the original PCEs upon thermal treatment at 180 °C for 12 h. This tolerance of such a high temperature for so long time is rarely reported for fullerene-free OSCs, which might be due to the unique unfused-ring core of DF-PCIC. Therefore, the work provides new idea for the design of new nonfullerene acceptors applicable in commercial OSCs in the future.

A new nonfullerene acceptor (DF-PCIC) is designed and synthesized by utilizing noncovalent interactions. Organic solar cells (OSCs) with DF-PCIC as the acceptor exhibit the best efficiency of 10.14% with a high fill factor of 0.72. More importantly, excellent morphological stability is achieved for DF-PCIC-based devices, which is meaningful for the future practical applications of OSCs.

Multijunction solar cells are designed to improve the overlap with the solar spectrum and to minimize losses due to thermalization. Aside from the optimum choice of photoactive materials for the respective sub-cells, a proper interconnect is essential. This study demonstrates a novel all-oxide interconnect based on the interface of the high-work-function (WF) metal oxide MoO_x and low-WF tin oxide (SnO_x). In contrast to typical p-/n-type tunnel junctions, both the oxides are n-type semiconductors with a WF of 5.2 and 4.2 eV, respectively. It is demonstrated that the electronic line-up at the interface of MoO_x and SnO_x comprises a large intrinsic interface dipole (≈0.8 eV), which is key to afford ideal alignment of the conduction band of MoO_x and SnO_x, without the requirement of an additional metal or organic dipole layer. The presented MoO_x/SnO_x interconnect allows for the ideal (loss-free) addition of the open circuit voltages of the two sub-cells.

A novel all-oxide recombination interconnect for organic tandem solar cells is reported. A large interface dipole between the high-work-function (WF) metal oxide MoO_x and low-WF tin oxide (SnO_x) affords ideal alignment of the conduction band of the two n-type metal oxides. The actual recombination of electrons with holes occurs at the interface of organic/MoO_x of the lower sub-cell.

Solar cells based on methylammonium lead triiodide (MAPbI₃) have shown remarkable progress in recent years and have demonstrated efficiencies greater than 20%. However, the long-term stability of MAPbI₃-based solar cells has yet to be achieved. Besides the well-known chemical and thermal instabilities, significant native ion migration in lead halide perovskites leads to current–voltage hysteresis and photoinduced phase segregation. Recently, it is further revealed that, despite having excellent chemical stability, the Au electrode can cause serious solar cell degradation due to Au diffusion into MAPbI₃. In addition to Au, many other metals have been used as electrodes in MAPbI₃ solar cells. However, how the external metal impurities introduced by electrodes affect the long-term stability of MAPbI₃ solar cells has rarely been studied. A comprehensive study of formation energetics and diffusion dynamics of a number of noble and transition metal impurities (Au, Ag, Cu, Cr, Mo, W, Co, Ni, Pd) in MAPbI₃ based on first-principles calculations is reported herein. The results uncover important general trends of impurity formation and diffusion in MAPbI₃ and provide useful guidance for identifying the optimal metal electrodes that do not introduce electrically active impurity defects in MAPbI₃ while having low resistivities and suitable work functions for carrier extraction.

Solar cell degradation has been the number one hurdle to the commercialization of the perovskite solar cells. The origin has been predominantly attributed to chemical and thermal instability of light-absorbing perovskite layers. A critically important, but largely hidden and underdiscussed, problem related to the solar cell degradation caused by metal electrode contamination is herein addressed.