Ligang Yuan

Dithienyldiketopyrrolopyrrole (DPP2T) and thieno[3,2-b]thiophene (TT) building blocks, enabling a large intermolecular overlap through π–π stacking, into an amorphous-like polymer composed of benzo(1,2-b:4,5-b′)dithiophene (BDT) and fluorinated thieno[3,4-b]thiophene (QTT), are introduced. Herein, through the variation of relative compositions of DPP2T-TT and BDT-QTT in the polymer backbone, the synthesis and characterization of a series of condensed random 2D-2A “quarterpolymers” with two reference alternating copolymers are reported. The best power conversion efficiency (PCE) of 9.45% is achieved for the optimum composition due to the synergistic effects such as improved photon absorption and reduced recombination loss, and optimized blend morphology via a change in the crystallinity and orientation of the blend films compared to the alternating copolymers. Moreover, by isolating higher molecular weight and narrower polydispersity fractions of the quarterpolymer via a marginal solvent-soaking technique, the PCE is further boosted to 10.30%, which is among the highest PCE reported to date for random polymer-based PSCs. Therefore, this simple 2D-2A strategy, reported for the first time, should be extended to numerous quaterpolymer systems, greatly accelerating random polymer systems toward further improving PSCs.

A set of quarterpolymers composed of dithienyldiketopyrrolopyrrole, thieno[3,2-b]thiophene, benzo(1,2-b:4,5-b′) dithiophene, and fluorinated thieno[3,4-b]thiophene building blocks is synthesized. This simple 2D-2A strategy obtains a high power conversion efficiency of 10.30% with the synergistic effects of four monomers such as improved charge transport, reduced recombination loss, and optimized blend morphology.

The thin-film photovoltaic absorber Cu₂ZnSn(S,Se)₄ (CZTSSe) holds considerable promise for large scale conversion of sunlight into electricity. CZTSSe is composed of Earth-abundant elements that exhibit low-toxicities, but improvements in device efficiency have been hampered by difficulties in increasing open circuit voltages (V_OC) due, at least in part, to disorder induced band tailing. We present a method to increase V_OC through direct modification of the back contact; our approach involves the separation of fully functioning devices from their Mo/glass substrate to reveal the back CZTSSe surface. Formation of a new back contact consisting of a thermally deposited high work function material (MoO₃), together with a higly reflective (Au) capping layer, creates an electrostatic field that drives electrons to the front p-n junction and leads to a decrease in electron-hole recombination. Model simulations indicating an increase in V_OC with decreasing absorber thickness are borne out by experiments with devices of varying thicknesses (0.7–2.0 μm). We report V_OC increases of up to 49 mV for a 1 μm thick absorber, with even greater increases up to 61 mV when the back CZTSSe surface is etched with bromine-methanol.

High performing kesterite photovoltaic devices show improved efficiency parameters after exfoliation and back contact engineering. The use of a high work function material (MoO₃) and a reflective back contact (Au) results in higher open circuit voltage (V_OC) and short circuit current (J_SC), which closely match model simulations.

The mixed perovskite (FAPbI₃)_1−x(MAPbBr₃)_x, prepared by directly mixing different perovskite components, suffers from phase competition and a low-crystallinity character, resulting in instability, despite the high efficiency. In this study, a dual ion exchange (DIE) method is developed by treating as-prepared FAPbI₃ with methylammonium brodide (MABr)/tert-butanol solution. The converted perovskite thin film shows an optimized absorption edge at 800 nm after reaction time control, and the high crystallinity can be preserved after MABr incorporation. More importantly, it is found that the threshold electrical field to initiate ion migration is greatly increased in DIE perovskite thin film because excess MABr on the surface can effectively heal structural defects located on grain boundaries during the ion exchange process. It contributes to the over-one-month moisture stability under ≈65% room humidity (RH) and greatly enhanced light stability for the bare perovskite film. As a result of preserved high crystallinity and simultaneous grain boundary passivation, the perovskite solar cells fabricated by the DIE method demonstrate reliable reproducibility with an average power conversion efficiency (PCE) of 17% and a maximum PCE of 18.1%, with negligible hysteresis.

A dual ion exchange (DIE) method is developed for mixed perovskite thin films by treating trigonal FAPbI₃ with MABr in tert-butanol. This DIE method can preserve the initial high crystallinity and passivate vacancies/defects at grain boundaries, leading to enhanced moisture and illumination stability and reduced ion migration. The solar cell device using the DIE method achieves the highest power conversion efficiency of 18.1%, with negligible hysteresis.

Engineering the chemical composition of organic and inorganic hybrid perovskite materials is one of the most feasible methods to boost the efficiency of perovskite solar cells with improved device stability. Among the diverse hybrid perovskite family of ABX₃, formamidinium (FA)-based mixed perovskite (e.g., FA_1−xCs_xPbI₃) possesses optimum bandgaps, superior optoelectronic property, as well as thermal- and photostability, which is proven to be the most promising candidate for advanced solar cell. Here, FA_0.9Cs_0.1PbI₃(Cl) is implemented as the light-harvesting layer in planar devices, whereas a low temperature, two-step solution deposition method is employed for the first time in this materials system. This paper comprehensively exploits the role of Cs⁺ in the FA_0.9Cs_0.1PbI₃(Cl) perovskite that affects the precursor chemistry, film nucleation and grain growth, and defect property via pre-intercalation of CsI in the inorganic framework. In addition, the resultant FA_0.9Cs_0.1PbI₃(Cl) films are demonstrated to exhibit an improved optoelectronic property with an elevated device power conversion efficiency (PCE) of 18.6%, as well as a stable phase with substantial enhancement in humidity and thermal stability, as compared to that of FAPbI₃(Cl). The present method is able to be further extended to a more complicated (FA,MA,Cs)PbX₃ material system by delivering a PCE of 19.8%.

A modified two-step processing approach based on CsI pre-intercalation in the inorganic framework is developed to fabricate an FA_1−xCs_xPbI₃(Cl) perovskite thin film and pertinent devices. Film growth (including the crystal orientation) and the defect property of FA_0.9Cs_0.1PbI₃(Cl) are thoroughly investigated via precursor chemistry.

Organic–inorganic hybrid perovskite materials are emerging as semiconductors with potential application in optoelectronic devices. In particular, perovskites are very promising for light-emitting devices (LEDs) due to their high color purity, low nonradiative recombination rates, and tunable bandgap. Here, using pure CH₃NH₃PbI₃ perovskite LEDs with an external quantum efficiency (EQE) of 5.9% as a platform, it is shown that electrical stress can influence device performance significantly, increasing the EQE from an initial 5.9% to as high as 7.4%. Consistent with the enhanced device performance, both the steady-state photoluminescence (PL) intensity and the time-resolved PL decay lifetime increase after electrical stress, indicating a reduction in nonradiative recombination in the perovskite film. By investigating the temperature-dependent characteristics of the perovskite LEDs and the cross-sectional elemental depth profile, it is proposed that trap reduction and resulting device-performance enhancement is due to local ionic motion of excess ions, likely excess mobile iodide, in the perovskite film that fills vacancies and reduces interstitial defects. On the other hand, it is found that overstressed LEDs show irreversibly degraded device performance, possibly because ions initially on the perovskite lattice are displaced during extended electrical stress and create defects such as vacancies.

High-efficiency CH₃NH₃PbI₃ perovskite light-emitting devices are demonstrated. The external quantum efficiency is boosted from 5.9% to 7.4% by subsequent electrical scans, which is related to excess ion motion reducing nonradiative decay channels, while overstressing the device will degrade device performance due to nonexcess ion migration.

Tandem organic solar cells (TOSCs), which integrate multiple organic photovoltaic layers with complementary absorption in series, have been proved to be a strong contender in organic photovoltaic depending on their advantages in harvesting a greater part of the solar spectrum and more efficient photon utilization than traditional single-junction organic solar cells. However, simultaneously improving open circuit voltage (V_oc) and short current density (J_sc) is a still particularly tricky issue for highly efficient TOSCs. In this work, by employing the low-bandgap nonfullerene acceptor, IEICO, into the rear cell to extend absorption, and meanwhile introducing PBDD4T-2F into the front cell for improving V_oc, an impressive efficiency of 12.8% has been achieved in well-designed TOSC. This result is also one of the highest efficiencies reported in state-of-the-art organic solar cells.

Simultaneously improving the open-circuit voltage (V_oc) and short current density (J_sc) is a particularly tricky issue for tandem organic solar cells (TOSCs). By employing the low-bandgap nonfullerene acceptor, IEICO, in the rear cell to extend absorption, and meanwhile introducing PBDD4T-2F into the front cell for improving V_oc, an impressive efficiency of 12.8% is achieved in TOSCs. This result is also one of the highest efficiencies reported in state-of-the-art organic solar cells.

The ease of processing hybrid organic–inorganic perovskite (HOIPs) films, belonging to a material class with composition ABX₃, from solution and at mild temperatures promises their use in deformable technologies, including flexible photovoltaic devices, sensors, and displays. To successfully apply these materials in deformable devices, knowledge of their mechanical response to dynamic strain is necessary. The authors elucidate the time- and rate-dependent mechanical properties of HOIPs and an inorganic perovskite (IP) single crystal by measuring nanoindentation creep and stress relaxation. The observation of pop-in events and slip bands on the surface of the indented crystals demonstrate dislocation-mediated plastic deformation. The magnitudes of creep and relaxation of both HOIPs and IPs are similar, negating prior hypothesis that the presence of organic A-site cations alters the mechanical response of these materials. Moreover, these samples exhibit a pronounced increase in creep, and stress relaxation as a function of indentation rate whose magnitudes reflect differences in the rates of nucleation and propagation of dislocations within the crystal structures of HOIPs and IP. This contribution provides understanding that is critical for designing perovskite devices capable of withstanding mechanical deformations.

Dynamic mechanical response of hybrid organic–inorganic and inorganic perovskite crystals suggests that the time-dependent mechanical properties of lead–halide perovskites are independent of the chemical character of the A-site cation. Moreover, significant viscoplastic behavior is revealed through creep and stress-relaxation measurements. These phenomena are interpreted as direct results of the crystal structures and how dislocations propagate within them.

Efficient wide-bandgap (WBG) perovskite solar cells are needed to boost the efficiency of silicon solar cells to beyond Schottky–Queisser limit, but they suffer from a larger open circuit voltage (V_OC) deficit than narrower bandgap ones. Here, it is shown that one major limitation of V_OC in WBG perovskite solar cells comes from the nonmatched energy levels of charge transport layers. Indene-C60 bisadduct (ICBA) with higher-lying lowest-unoccupied-molecular-orbital is needed for WBG perovskite solar cells, while its energy-disorder needs to be minimized before a larger V_OC can be observed. A simple method is applied to reduce the energy disorder by isolating isomer ICBA-tran3 from the as-synthesized ICBA-mixture. WBG perovskite solar cells with ICBA-tran3 show enhanced V_OC by 60 mV, reduced V_OC deficit of 0.5 V, and then a record stabilized power conversion efficiency of 18.5%. This work points out the importance of matching the charge transport layers in perovskite solar cells when the perovskites have a different composition and energy levels.

One major limitation of open-circuit voltage (V_OC) in wide-bandgap (WBG) perovskite solar cells comes from the nonmatched charge extraction contact. WBG perovskite solar cells with indene-C60 bisadduct-tran3 isomer with higher-lying lowest-unoccupied-molecular-orbital and reduced energy disorder show enhanced V_OC , and then a record stabilized power conversion efficiency of 18.5%.

A new polymer acceptor, naphthodiperylenetetraimide-vinylene (NDP-V), featuring a backbone of altenating naphthodiperylenetetraimide and vinylene units is designed and applied in all-polymer solar cells (all-PSCs). With this polymer acceptor, a new record power-conversion efficiencies (PCE) of 8.59% has been achieved for all-PSCs. The design principle of NDP-V is to reduce the conformational disorder in the backbone of a previously developed high-performance acceptor, PDI-V, a perylenediimide-vinylene polymer. The chemical modifications result in favorable changes to the molecular packing behaviors of the acceptor and improved morphology of the donor–acceptor (PTB7-Th:NDP-V) blend, which is evidenced by the enhanced hole and electron transport abilities of the active layer. Moreover, the stronger absorption of NDP-V in the shorter-wavelength range offers a better complement to the donor. All these factors contribute to a short-circuit current density (J _sc) of 17.07 mA cm⁻². With a fill factor (FF) of 0.67, an average PCE of 8.48% is obtained, representing the highest value thus far reported for all-PSCs.

A new polymer acceptor NDP-V is designed and applied in all-polymer solar cells (all-PSCs). With this polymer acceptor, a new record power conversion efficiency of 8.59% is achieved for all-PSCs, with an open-circuit voltage (V_oc) of 0.74 V, a short-circuit current density (J_sc) of 17.07 mA cm⁻², and a high fill factor (FF) of 0.67.

High-performance ternary organic solar cells are fabricated by using a wide-bandgap polymer donor (bithienyl-benzodithiophene-alt-fluorobenzotriazole copolymer, J52) and two well-miscible nonfullerene acceptors, methyl-modified nonfullerene acceptor (IT-M) and 2,2′-((2Z,2′Z)-((5,5′-(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydros-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IEICO). The two acceptors with complementary absorption spectra and similar lowest unoccupied molecular orbital levels show excellent compatibility in the blend due to their very similar chemical structures. Consequently, the obtained ternary organic solar cells (OSC) exhibits a high efficiency of 11.1%, with an enhanced short-circuit current density of 19.7 mA cm⁻² and a fill factor of 0.668. In this ternary system, broadened absorption, similar output voltages, and compatible morphology are achieved simultaneously, demonstrating a promising strategy to further improve the performance of ternary OSCs.

Ternary organic solar cells show over 11% power conversion efficiency by using two compatible nonfullerene acceptors with complementary absorption spectra, similar chemical structures, and similar lowest unoccupied molecular orbital levels. Broadened absorption, similar output voltages, and compatible morphology are achieved simultaneously, demonstrating a promising strategy to improve the performance of OSCs.