ZiQi Sun

A series of simple alkylated triphenylamine molecules are designed and synthesized as hole transporting materials (HTMs) for perovskite solar cells. The effects of the alkyl chains on the photovoltaic performance are for the first time systematically investigated. A high efficiency of 17.33% is achieved by combining HTM X21 and the mixed-ion perovskites.

An efficiency of flexible perovskite solar cells (Pvs-SCs) of 16.09% is achieved, the highest value reported for flexible Pvs-SCs to date. The outstanding performance is attributed to the superior features of alternative electron-transport materials, such as antireflection, a suitable work function, high electron mobility, and a reduced trap-state density of the perovskite material.

Lead thiocyanate in the perovskite precursor can increase the grain size of a perovskite thin film and reduce the conductivity of the grain boundaries, leading to perovskite solar cells with reduced hysteresis and enhanced fill factor. A planar perovskite solar cell with grain boundary and interface passivation achieves a steady-state efficiency of 18.42%.

A scalable, hysteresis-free and planar architecture perovskite solar cell is presented, employing a flame spray synthesized low-temperature processed NiO (LT-NiO) as hole-transporting layer yielding efficiencies close to 18%. Importantly, it is found that LT-NiO boosts the limits of open-circuit voltages toward an impressive non-radiative voltage loss of 0.226 V only, whereas PEDOT:PSS suffers from significant large non-radiative recombination losses.

Organic solar cells based on two benzodithiophene-based polymers (PTB7 and PTB7-Th) processed at square centimeter-size under inert atmosphere and ambient air, respectively, are investigated. It is demonstrated that the performance of solar cells processed under inert atmosphere is not limited by the upscaling of photoactive layer and the interfacial layers. Thorough morphological and electrical characterizations of optimized layers and corresponding devices reveal that performance losses due to area enlargement are only caused by the sheet resistance of the transparent electrode reducing the efficiency from 9.3% of 7.8% for PTB7-Th in the condition that both photoactive layer and the interfacial layers are of high layer quality. Air processing of photoactive layer and the interfacial layers into centimeter-sized solar cells lead to additional, but only slight, losses (<10%) in all photovoltaic parameters, which can be addressed to changes in the electronic properties of both active layer and ZnO layers rather than changes in layer morphology. The demonstrated compatibility of polymer solar cells using solution-processed photoactive layer and interfacial layers with large area indicates that the introduction of a standard active area of 1 cm² for measuring efficiency of organic record solar cells is feasible. However electric standards for indium tin oxides (ITO) or alternative transparent electrodes need to be developed so that performance of new photovoltaic materials can be compared at square centimeter-size.

Increasing solar cells based on benzo-dithiophene-based polymers (PTB7 and PTB7-Th) to square-centimeter size leads to performance losses that are not caused by the area enlargement of the photoactive and interlayer, respectively, but are only related to the sheet resistance of the transparent electrode based on indium tin oxide. Air processing generates an additional but small loss in efficiency (<10%) due to changes of the electronic properties of each layer.

Highly efficient polymer solar cells with tandem structure are fabricated by using two excellent photovoltaic polymers and a highly transparent intermediate recombination layer. Power conversion efficiencies over 11% can be realized featured by a low-band-gap polymer with fine-tuned properties.

Despite recent dramatic enhancements in power conversion efficiencies (PCEs) resulting in values over 10%, the manufacturing of tandem organic solar cells (OSCs) via current printing technologies is subject to tremendous challenges. Existing complicated tandem structures consisting of six or more component layers have been a major obstacle that significantly increases the complexity of printing processes and substantially sacrifices the PCE for printed devices. Here, an innovative printing method is reported that simplifies the fabrication process of the tandem OSCs. By developing a new printing technique using a nanocomposites containing interfacial and photoactive materials, a simultaneously printed bilayer of consisting of interfacial and photoactive layers, achieved through vertical self-organization, is successfully demonstrated, resulting in tandem OSCs with only four printed layers. Moreover, by rigorously controlling the molecular weight of the interfacial materials, the self-assembly characteristics are improved and an efficient tandem OSC is yielded with a PCE of 9.1% achieved in printed layers.

Efficient and simplified tandem organic solar cells are demonstrated through a new self-assembly printing technique using a spontaneous vertical phase separation. A bilayer of interfacial and photoactive layers is simultaneously printed by improving vertical self-organization characteristics. A high tandem efficiency of 9.1% is obtained with a four-layer tandem structure.

Layer-by-layer (LL) processes, i.e., sequential deposition of different active layers, are widely used in the fabrication of organic solar cells (OSCs). Recently, LL vacuum deposition and LL solution processes have attracted considerable attention. LL processing presents some advantages over the blend method: a) donor and acceptor layers can be easily and independently controlled and optimized; b) the charge carriers dissociated from excitons at the donor–acceptor interface are confined to each phase, so bimolecular recombination losses can be reduced; c) bilayer geometries enable an easier way for understanding the physical processes taking place at the donor–acceptor interface; d) desired vertical phase separation for charge extraction can be obtained through changing the sequence of donor and acceptor deposition. This report summarizes the recent developments of LL processed OSCs. The remaining problems and challenges, and the key research direction in near future are discussed.

Layer-by-layer (LL) processing techniques exhibit some advantages over the traditional blend-casting technique in organic solar cells. The recent developments of LL vacuum-deposited and solution-processed solar cells are summarized.

The rapid degradation of organic photovoltaic (OPV) devices compared to conventional inorganic solar cells is one of the critical issues that have to be solved in order to make OPV a competitive commercial technology. The understanding of the fundamental mechanisms that reduce the power conversion efficiency (PCE) over time is beneficial for the design of new materials with enhanced stability. This paper focuses on bulk heterojunction organic solar cells based on thieno [3,4-b] thiophene-alt-benzodithiophene (PTB7) mixed with [6,6]-phenyl-C71-butyric acid methyl esther ([70]PCBM). In spite of being promising in terms of PCE, devices based on this blend are unstable and have a short lifetime. When exposed to light in inert atmosphere, the PCE drops by 15% in less than 1 h and by 35% in 8 h; this degradation is induced by the ultraviolet (UV) part of the spectrum. This paper analyzes the effect induced by UV light on the transport of charges in PTB7:[70]PCBM. Contrary to expectations, the electron transport shows evidence of trapping, while the transport of holes appears unaffected. Furthermore, it is proven that the loss of PCE is due to a reaction between PTB7 and [70]PCBM, while the intrinsic instability of the polymer plays a marginal role.

The effect of UV light on the charge transport in PTB7:[70]PCBM solar cells is investigated; while the hole transport is stable, a deterioration in the transport of electrons is found, related to the increased electron trapping. It is proven that efficiency losses of PTB7:[70]PCBM solar cells upon UV exposure are due to a reaction that involves both the donor and the acceptor.

Hybrid organic-inorganic perovskites have attracted considerable attention after promising developments in energy harvesting and other optoelectronic applications. However, further optimization will require a deeper understanding of the intrinsic photophysics of materials with relevant structural characteristics. Here, the dynamics of photoexcited charge carriers in large-area grain organic-inorganic perovskite thin films is investigated via confocal time-resolved photoluminescence spectroscopy. It is found that the bimolecular recombination of free charges is the dominant decay mechanism at excitation densities relevant for photovoltaic applications. Bimolecular coefficients are found to be on the order of 10⁻⁹ cm³ s⁻¹, comparable to typical direct-gap semiconductors, yet significantly smaller than theoretically expected. It is also demonstrated that there is no degradation in carrier transport in these thin films due to electronic impurities. Suppressed electron–hole recombination and transport that is not limited by deep level defects provide a microscopic model for the superior performance of large-area grain hybrid perovskites for photovoltaic applications.

The local dynamics of photoexcited charge carriers in “large-grain hybrid organic-inorganic perovskites” is presented. Under illumination conditions relevant for photovoltaics, a suppressed recombination of free carriers is observed where electronic impurities play a negligible role in the overall kinetics. This study provides a microscopic model for low defect large-grain hybrid perovskites where non-Langevin recombination results in superior photovoltaic performance.

Currently studied carbon nanotube-silicon (CNT-Si) solar cells are based on relatively small active areas (typically <0.15 cm²); increasing the active area generally leads to reduced power conversion efficiencies. This study reports CNT-Si solar cells with active areas of more than 2 cm² for single cells, yet still achieving cell efficiencies of about 10%, which is the first time for CNT-Si solar cells with an active area more than 1 cm² to reach the level for real applications. In this work, a controlled number of flattened highly conductive CNT strips is added, in simple arrangement, to form a CNT-Si solar cell with CNT strips in which the middle film makes heterojunctions with Si while the top strips act as self-similar top electrodes, like conventional metal grids. The CNT strips, directly condensed from as-grown CNT films, not only improve the CNT-Si junctions, but also enhance the conductivity of top electrodes without introducing contact barrier when the CNT strips are added onto the film. This property may facilitate the development of large-area high-performance CNT or graphene-Si solar cells.

High-efficiency large-area carbon nanotube-silicon (CNT-Si) solar cells with active areas of more than 2 cm² have been achieved. Controlled number of flattened, highly conductive CNT strips are added onto large-area CNT-Si solar cells as self-similar top electrodes, improving the efficiency to more than 10% with the assistance of TiO₂ antireflection layer and HNO₃ doping.

The rapid pace of development for hybrid perovskite photovoltaics has recently resulted in promising figures of merit being obtained with regard to device stability. Rather than relying upon expensive barrier materials, realizing market-competitive lifetimes is likely to require the development of intrinsically stable devices, and to this end accelerated aging tests can help identify degradation mechanisms that arise over the long term. Here, oxygen-induced degradation of archetypal perovskite solar cells under operation is observed, even in dry conditions. With prolonged aging, this process ultimately drives decomposition of the perovskite. It is deduced that this is related to charge build-up in the perovskite layer, and it is shown that by efficiently extracting charge this degradation can be mitigated. The results confirm the importance of high charge-extraction efficiency in maximizing the tolerance of perovskite solar cells to oxygen.

Key to the development of perovskite photovoltaics is the mitigation of long-term degradation mechanisms. When aging these solar cells in the presence of oxygen, two stages of degradation are evidenced that drive perovskite decomposition. This damage is coupled to the average density of charge within the perovskite, highlighting the need to maximize charge extraction efficiency when designing stable devices.

A dual-phase all-inorganic composite CsPbBr₃-CsPb₂Br₅ is developed and applied as the emitting layer in LEDs, which exhibited a maximum luminance of 3853 cd m^–2, with current density (CE) of ≈8.98 cd A^–1 and external quantum efficiency (EQE) of ≈2.21%, respectively. The parasite of secondary phase CsPb₂Br₅ nanoparticles on the cubic CsPbBr₃ nanocrystals could enhance the current efficiency by reducing diffusion length of excitons on one side, and decrease the trap density in the band gap on the other side. In addition, the introduction of CsPb₂Br₅ nanoparticles could increase the ionic conductivity by reducing the barrier against the electronic and ionic transport, and improve emission lifetime by decreasing nonradiative energy transfer to the trap states via controlling the trap density. The dual-phase all-inorganic CsPbBr₃-CsPb₂Br₅ composite nanocrystals present a new route of perovskite material for advanced light emission applications.

Dual-phase CsPbBr₃-CsPb₂Br₅ composites for all-inorganic perovskite light emitting diodes (LEDs) are fabricated, which exhibit significantly improved performance, representing a great increase in the CE and EQE, about 21- and 18-fold improvement than that of the best reported CsPbBr₃ LEDs. The dual-phase all-inorganic CsPbBr₃-CsPb₂Br₅ composite nanocrystals present a new route of perovskite material for advanced light emission applications.

On-fabrication solid-state N-doping of graphene is developed using a Zonyl-added ZnO layer on a graphene surface. Inverted organic solar cells based on the graphene cathode and the ZnO layer exhibit a power conversion efficiency of 7.5%—a high-record power conversion efficiency of single-junction organic solar cells with graphene electrodes. For the first time 100% power conversion efficiency with respect to the ITO cathode is achieved.