Ligang Yuan

In this work, a nonfullerene polymer solar cell (PSC) based on a wide bandgap polymer donor PM6 containing fluorinated thienyl benzodithiophene (BDT-2F) unit and a narrow bandgap small molecule acceptor 2,2′-((2Z,2′Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDIC) is developed. In addition to matched energy levels and complementary absorption spectrum with IDIC, PM6 possesses high crystallinity and strong π–π stacking alignment, which are favorable to charge carrier transport and hence suppress recombination in devices. As a result, the PM6:IDIC-based PSCs without extra treatments show an outstanding power conversion efficiency (PCE) of 11.9%, which is the record value for the as-cast PSC devices reported in the literature to date. Moreover, the device performances are insensitive to the active layer thickness (≈95–255 nm) and device area (0.20–0.81 cm²) with PCEs of over 11%. Besides, the PM6:IDIC-based flexible PSCs with a large device area of 1.25 cm² exhibit a high PCE of 6.54%. These results indicate that the PM6:IDIC blend is a promising candidate for future roll-to-roll mass manufacturing and practical application of highly efficient PSCs.

An efficient polymer solar cell (PSC) based on a polymer donor PM6 containing BDT-2F unit and an n-type organic semiconductor acceptor, IDIC, is developed. The power conversion efficiencies of PSCs without extra treatments reach up to 11.9% and are insensitive to the active layer thickness (95–225 nm) and device area (0.20–0.81 cm²), with values of over 11%.

Two novel wide-bandgap copolymers, PBDT-TDZ and PBDTS-TDZ, are developed based on 1,3,4-thiadiazole (TDZ) and benzo[1,2-b:4,5-b′]dithiophene (BDT) building blocks. These copolymers exhibit wide bandgaps over 2.07 eV and low-lying highest occupied molecular orbital (HOMO) levels below −5.35 eV, which match well with the typical low-bandgap acceptor of ITIC, resulting in a good complementary absorption from 300 to 900 nm and a low HOMO level offset (≤0.13 eV). Compared to PBDT-TDZ, PBDTS-TDZ with alkylthio side chains exhibits the stronger optical absorption, lower-lying HOMO level, and higher crystallinity. By using a single green solvent of o-xylene, PBDTS-TDZ:ITIC devices exhibit a large open-circuit voltage (V_oc) up to 1.10 eV and an extremely low energy loss (E_loss) of 0.48 eV. At the same time, the desirable high short-circuit current density (J_sc) of 17.78 mA cm⁻² and fill factor of 65.4% are also obtained, giving rise to a high power conversion efficiency (PCE) of 12.80% without any additive and post-treatment. When adopting a homotandem device architecture, the PCE is further improved to 13.35% (certified as 13.19%) with a much larger V_oc of 2.13 V, which is the best value for any type of homotandem organic solar cells reported so far.

Two novel 1,3,4-thiadiazole-based wide-bandgap copolymers, PBDT-TDZ and PBDTS-TDZ, are developed for efficient nonfullerene organic solar cells. The single-junction devices processed by a green solvent of o-xylene exhibit a high power conversion efficiency (PCE) of 12.80% with a low energy loss of 0.48 eV. The PCE is finally improved to 13.35% when using a homotandem device architecture.

The future commercialization of halide perovskite solar cells relies on improving their stability. There are several studies focused on understanding degradation under operating conditions in light, but little is known about the stability of these solar cells under reverse bias conditions. Reverse bias stability is important because shaded cells in a module are put into reverse bias by the illuminated cells. In this paper, a phenomenological study is presented of the reverse bias behavior of halide perovskite solar cells and it is shown that reverse bias can lead to a partially recoverable loss in efficiency, primarily caused by a decrease in V _OC. A general mechanism is proposed, supported by drift–diffusion simulations, to explain how these cells breakdown via tunneling caused by accumulated ionic defects and suggests that the reversible loss in efficiency may be due to an electrochemical reaction of these defects. Finally, the implications of these phenomena are discussed and how they can possibly be addressed is also discussed.

The stability of halide perovskite solar cells in reverse bias is investigated. The cells uniformly pass current across the device at breakdown voltages between –1 and −4 V. A partially recoverable decrease in open-circuit voltage is seen for cells held in reverse bias. Drift–diffusion modeling supports breakdown via tunneling, and the implications and some possible solutions are discussed.

Hybrid perovskites are on a trajectory toward realizing the most efficient single-junction, solution-processed photovoltaic devices. However, a critical issue is the limited understanding of the correlation between the degree of crystallinity and the emergent perovskite/hole (or electron) transport layer on device performance and photostability. Here, the controlled growth of hybrid perovskites on nickel oxide (NiO) is shown, resulting in the formation of thin films with enhanced crystallinity with characteristic peak width and splitting reminiscent of the tetragonal phase in single crystals. Photophysical and interface sensitive measurements reveal a reduced trap density at the perovskite/NiO interface in comparison with perovskites grown on poly(3,4-ethylene dioxy thiophene) polystyrene sulfonate. Photovoltaic cells exhibit a high open circuit voltage (1.12 V), indicating a near-ideal energy band alignment. Moreover, photostability of photovoltaic devices up to 10-Suns is observed, which is a direct result of the superior crystallinity of perovskite thin films on NiO. These results elucidate the critical role of the quality of the perovskite/hole transport layer interface in rendering high-performance and photostable optoelectronic devices.

Highly crystalline perovskite thin film can be grown on nickel oxide substrates evidenced by sharp X-ray diffraction pattern with characteristic tetragonal peak splitting observed only in single crystal. As a consequence, high-efficiency photovoltaic cells can be achieved with extended operation lifetime under constant illumination benefit by the high degree of crystallinity.

High-performance colored aesthetic semitransparent organic photovoltaics (OPVs) featuring a silver/indium tin oxide/silver (Ag/ITO/Ag) microcavity structure are prepared. By precisely controlling the thickness of the ITO layer, OPV devices exhibiting high transparency and a wide and high-purity color gamut are obtained: blue (B), green (G), yellow-green (YG), yellow (Y), orange (O), and red (R). The power conversion efficiencies (PCEs) of the G, YG, and Y color devices are greater than 8% (AM 1.5G irradiation, 100 mW cm⁻²) with maximum transmittances (T_MAX) of greater than 14.5%. An optimized PCE of 8.2% was obtained for the YG OPV [CIE 1931 coordinates: (0.364, 0.542)], with a value of T_MAX of 17.3% (at 561 nm). As far as it is known, this performance is the highest ever reported for a transparent colorful OPV. Such high transparency and desired transmitted colors, which can perspective see the clear images, suggest great potential for use in building-integrated photovoltaic applications.

High-performance colored aesthetic semitransparent organic photovoltaics (OPVs) featuring a silver/indium tin oxide/silver microcavity structure are demonstrated. Colored OPVs of high purity and a wide color gamut are obtained: blue, green, yellow-green, yellow, orange, and red. The highest power conversion efficiency was 8.2% for the yellow-green device, with CIE 1931 coordinates of (0.364, 0.542) and a transmittance of 17.3% at 561 nm.

The considerable improvement on the power conversion efficiency (PCE) for emerging nonfullerene polymer solar cells is still limited by considerable voltage losses that have become one of the most significant obstacles in further boosting desired photovoltaic performance. Here, a comprehensive study is reported to understand the impacts of charge transport, energetic disorder, and charge transfer states (CTS) on the losses in open-circuit voltage (V_oc) based on three high performing bulk heterojunction solar cells with the best PCE exceeding 11%. It is found that the champion poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)-benzo[1,2-c:4,5-c′]dithiophene-4,8-dione))] (PBDB-T):IT-M solar cell (PCE = 11.5%) is associated with the least disorder. The determined energetic disorder in part reconciles the difference in V_oc between the solar cells. A reduction is observed in the nonradiative losses (ΔV_nonrad) coupled with the increase of energy of CTS for the PBDB-T:IT-M device, which may be related to the improved balance in carrier mobilities, and partially can explain the gain in V_oc. The determined radiative limit for V_oc combined with the ΔV_nonrad generates an excellent agreement for the V_oc with the experimental values. The results suggest that minimizing the energetic disorder related to transport and CTS is critical for the mitigation of V_oc losses and improvements on the device performance.

Voltage losses and charge transport in three representative bulk heterojunction solar cells are investigated. By temperature-dependent open-circuit voltage (V_oc) analysis and photovoltaic electroluminescence spectroscopy, we find that the increased V_oc in the champion IT-M cell with an excellent balance in mobility is associated with reduced energetic disorder at the D/A interface and non-redative recombination losses at charge transfer states.

Recently, the influence of molecular weight (M_n) on the performance of polymer solar cells (PSCs) is widely investigated. However, the dependence of optimal thickness of active layer for PSCs on M_n is not reported yet, which is vital to the solution printing technology. In this work, the effect of M_n on the efficiency and especially optimal thickness of the active layer for PBTIBDTT-S-based PSCs is systematically studied. The device efficiency improves significantly as the M_n increases from 12 to 38 kDa, and a remarkable efficiency of 10.1% is achieved, which is among the top efficiencies of wide-bandgap polymer:fullerene PSCs. Furthermore, the optimal thickness of the active layer is also greatly increased from 62 to 210 nm with increased M_n. Therefore, a device employing a thick (>200 nm) active layer with power conversion efficiency exceeding 10% is achieved by manipulating M_n. This exciting result is attributed to both the improved crystallinity, thus hole mobility, and preferable polymer orientation, thus morphology of active layer. These findings, for the first time, highlight the significant impact of M_n on the optimal thickness of active layer for PSCs and provide a facile way to further improve the performance of PSCs employing a thick active layer.

As the molecular weight (M_n) of PBTIBDTT-S increases from 12 to 38 kDa, the efficiency and optimal thickness of the active layer are simultaneously improved from 6.99% to 10.11% and from 62 to 210 nm, respectively. This result demonstrates the importance of M_n in achieving highly efficient devices under a thick active layer.

Triplet materials have been employed to achieve high-performing organic solar cells (OSCs) by extending the exciton lifetime and diffusion distances, while the triplet non-fullerene acceptor materials have never been reported for bulk heterojunction OSCs. Herein, for the first time, three triplet molecular acceptors based on tellurophene with different degrees of ring fusing were designed and synthesized for OSCs. Significantly, these molecules have long exciton lifetime and diffusion lengths, leading to efficient power conversion efficiency (7.52 %), which is the highest value for tellurophene-based OSCs. The influence of the extent of ring fusing on molecular geometry and OSCs performance was investigated to show the power conversion efficiencies (PCEs) continuously increased along with increasing the extent of ring fusing.

Solar fusion: Three triplet molecular acceptors based on tellurophene with different degrees of ring fusing were prepared for organic solar cells (OSCs). These molecules have long exciton lifetime and diffusion lengths, leading to increasing power conversion efficiency (PCE) up to 7.52 %, which is the highest value for tellurophene-based OSCs, as the degree of ring-fusion increases.

Photoactive perovskite semiconductors are highly tunable, with numerous inorganic and organic cations readily incorporated to modify optoelectronic properties. However, despite the importance of device reliability and long service lifetimes, the effects of various cations on the mechanical properties of perovskites are largely overlooked. In this study, the cohesion energy of perovskites containing various cation combinations of methylammonium, formamidinium, cesium, butylammonium, and 5-aminovaleric acid is reported. A trade-off is observed between the mechanical integrity and the efficiency of perovskite devices. High efficiency devices exhibit decreased cohesion, which is attributed to reduced grain sizes with the inclusion of additional cations and PbI₂ additives. Microindentation hardness testing is performed to estimate the fracture toughness of single-crystal perovskite, and the results indicated perovskites are inherently fragile, even in the absence of grain boundaries and defects. The devices found to have the highest fracture energies are perovskites infiltrated into a porous TiO₂/ZrO₂/C triple layer, which provide extrinsic reinforcement and shielding for enhanced mechanical and chemical stability.

As reflected in the fragility of state-of-the-art perovskite solar cells, mechanical reliability has too long been an afterthought in their development. The aim of this work is to understand the effects of cation composition (combinations of methylammonium, formamidinium, cesium, butylammonium, and 5-aminovaleric acid) on perovskite mechanical integrity and determine design criteria to increase reliability toward the development of module-scale devices.

Side-chain engineering is an important strategy for optimizing photovoltaic properties of organic photovoltaic materials. In this work, the effect of alkylsilyl side-chain structure on the photovoltaic properties of medium bandgap conjugated polymer donors is studied by synthesizing four new polymers J70, J72, J73, and J74 on the basis of highly efficient polymer donor J71 by changing alkyl substituents of the alkylsilyl side chains of the polymers. And the photovoltaic properties of the five polymers are studied by fabricating polymer solar cells (PSCs) with the polymers as donor and an n-type organic semiconductor (n-OS) m-ITIC as acceptor. It is found that the shorter and linear alkylsilyl side chain could afford ordered molecular packing, stronger absorption coefficient, higher charge carrier mobility, thus results in higher J_sc and fill factor values in the corresponding PSCs. While the polymers with longer or branched alkyl substituents in the trialkylsilyl group show lower-lying highest occupied molecular orbital energy levels which leads to higher V_oc of the PSCs. The PSCs based on J70:m-ITIC and J71:m-ITIC achieve power conversion efficiency (PCE) of 11.62 and 12.05%, respectively, which are among the top values of the PSCs reported in the literatures so far.

Side-chain engineering is performed to optimize photovoltaic properties of the 2D-conjugated polymer donors. The polymer solar cells with m-ITIC as acceptor and J70 and J71 polymer donors with shorter and linear alkyl substituents in their alkylsilyl side chains achieve power conversion efficiency of 11.62% and 12.05%, respectively.

Nanostructures produce unique optical and electronic properties, which have the potential to meet the goals of third-generation photovoltaic devices. However, most nanostructures bring accompanying optical or electrical losses to solar cells. In article number 1602385, Jr-Hau He and Hsin-Ping Wang postulate that the concurrent design of both optical and electrical components will be an imperative route toward breaking the present-day limit of nanostructured solar cells.

Organic photovoltaic cells possess desirable practical characteristics, such as the potential for low-cost fabrication on flexible substrates, but they lag behind their inorganic counterparts in performance due in part to fundamental energy loss mechanisms, such as overcoming the charge transfer (CT) state binding energy when photogenerated charge is transferred across the donor/acceptor interface. However, recent work has suggested that crystalline interfaces can reduce this binding energy due to enhanced CT state delocalization. Solar cells based on rubrene and C₆₀ are investigated as an archetypal system because it allows the degree of crystallinity to be moldulated from a highly disordered to highly ordered system. Using a postdeposition annealing method to transform as-deposited amorphous rubrene thin films into ones that are highly crystalline, it is shown that the CT state of a highly crystalline rubrene/C₆₀ heterojunction undergoes extreme delocalization parallel to the interface leading to a band-like state that exhibits a linear Stark effect. This state parallels the direct charge formation of inorganic solar cells and reduces energetic losses by 220 meV compared with 12 other archetypal heterojunctions reported in the literature.

Highly crystalline organic solar cells are investigated, and reduced voltage loss attributed to a band-like charge transfer state is found.

The recent emergence of high-performance non-fullerene acceptors provides versatility of designing novel ternary blend organic solar cells (OSCs). They could be employed as host acceptor materials or as a third component to complement the absorption spectra with two other absorbers in the ternary blend, to enhance the light-harvesting capacity and thus to further improve the efficiencies of OSCs.

Organic photovoltaic (OPV) devices still work efficiently after sunset! To save energy and improve the sustainability of the many electronic systems, such as Internet of Thing (IoT), distributed harvesters of energy from the local environment could be particularly critical. Photovoltaic devices could be an energy source for such needs. Although traditional solar cells exhibit very high power conversion efficiencies (PCEs) under illumination at about 100 mW cm⁻² of solar irradiation, the efficiency drops significantly with the decreasing light intensity. Herein (article No. 201700174), OPV devices may provide a solution; their PCEs are much higher under indoor lighting conditions than they are under sunlight. For example, the device is capable of delivering a power output of 22.57 μW cm⁻², corresponding to a PCE of 13.76%, under illumination with indoor lighting conditions at 500 lux. These results might open up new directions for further improving the device performance of OPV devices for local energy harvesters under low-power lighting applications.

CH₃NH₃PbI₃ is used directly in precursor solution, together with PbCl₂ and CH₃NH₃I to prepare the CH₃NH₃PbI_3-xCl_x. Compared to the conventional precursor composed of PbI₂, PbCl₂, and CH₃NH₃I, the innovative precursor results in improved perovskite film morphology, enhanced charge transportation process and reduced hysteresis. An improved efficiency of 18.70% is achieved in planar perovskite solar cells.