Chen Weijie

Publication date: June 2020

Source: Nano Energy, Volume 72

Author(s): Sora Oh, Nasir Khan, Seon-Mi Jin, Huyen Tran, Namsun Yoon, Chang Eun Song, Hang Ken Lee, Won Suk Shin, Jong-Cheol Lee, Sang-Jin Moon, Eunji Lee, Sang Kyu Lee

Self‐crystallized multifunctional 2D perovskite (M2P) is formed on top of a 3D perovskite light absorber. The M2P layer performs as a hole‐transfer facilitator and a surface‐trap passivator in perovskite solar cells (PSCs). PSCs using the developed 3D/2D perovskites achieve a power conversion efficiency of 20.79% with highly improved long‐term stability compared to devices without M2P.

Abstract

Recently, perovskite solar cells (PSC) with high power‐conversion efficiency (PCE) and long‐term stability have been achieved by employing 2D perovskite layers on 3D perovskite light absorbers. However, in‐depth studies on the material and the interface between the two perovskite layers are still required to understand the role of the 2D perovskite in PSCs. Self‐crystallization of 2D perovskite is successfully induced by deposition of benzyl ammonium iodide (BnAI) on top of a 3D perovskite light absorber. The self‐crystallized 2D perovskite can perform a multifunctional role in facilitating hole transfer, owing to its random crystalline orientation and passivating traps in the 3D perovskite. The use of the multifunctional 2D perovskite (M2P) leads to improvement in PCE and long‐term stability of PSCs both with and without organic hole transporting material (HTM), 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) compared to the devices without the M2P.

Light transmission is largely decoupled from harvesting by optically tailoring an organic cell architecture with 50% average visible transmission. In an outdoor measurement of vertically positioned devices, a 9.80% sunlight energy conversion into electricity during 1 day is demonstrated.

Abstract

For many years, it has been recognized that potential organic photovoltaic cells must be integrated into elements requiring high transparency. In most of such elements, sunlight is likely to be incident at large angles. Here it is demonstrated that light transmission can be largely decoupled from harvesting by optically tailoring an infrared shifted nonfullerene acceptor based organic cell architecture. A 9.67% power conversion efficiency at 50° incidence is achieved together with an average visual transmission above 50% at normal incidence. The deconstruction of a 1D nanophotonic structure is implemented to conclude that just two λ/4 thick layers are essential to reach, for a wide incidence angle range, a higher than 50% efficiency increase relative to the standard configuration reference. In an outdoor measurement of vertically positioned 50% visible transparent cells, it is demonstrated that 9.80% of sunlight energy can be converted into electricity during the course of 1 day.

Publication date: June 2020

Source: Nano Energy, Volume 72

Author(s): Govindasamy Sathiyan, Ali Asgher Syed, Cheng Chen, Cheng Wu, Li Tao, Xingdong Ding, Yawei Miao, Gongqiang Li, Ming Cheng, Liming Ding

Phenanthrene‐fused‐quinoxaline (PFQ) is demonstrated as an efficient auxiliary acceptor to realize long‐wavelength response, enhancing the photocurrent as well as power conversion efficiency (PCE) of copper‐electrolyte‐based dye‐sensitized solar cells. The resulting dye, termed HY64 , achieves an outstanding PCE of 12.5 %.

Abstract

Dye‐sensitized solar cells (DSSCs) based on Cu^II/I bipyridyl or phenanthroline complexes as redox shuttles have achieved very high open‐circuit voltages (V _OC, more than 1 V). However, their short‐circuit photocurrent density (J _SC) has remained modest. Increasing the J _SC is expected to extend the spectral response of sensitizers to the red or NIR region while maintaining efficient electron injection in the mesoscopic TiO₂ film and fast regeneration by the Cu^I complex. Herein, we report two new D‐A‐π‐A‐featured sensitizers termed HY63 and HY64 , which employ benzothiadiazole (BT) or phenanthrene‐fused‐quinoxaline (PFQ), respectively, as the auxiliary electron‐withdrawing acceptor moiety. Despite their very similar energy levels and absorption onsets, HY64 ‐based DSSCs outperform their HY63 counterparts, achieving a power conversion efficiency (PCE) of 12.5 %. PFQ is superior to BT in reducing charge recombination resulting in the near‐quantitative collection of photogenerated charge carriers.

Publication date: June 2020

Source: Nano Energy, Volume 72

Author(s): Chuanfei Wang, Fabrizio Moro, Shaofei Ni, Qilun Zhang, Guoxing Pan, Jinpeng Yang, Fapei Zhang, Irina A. Buyanova, Weimin M. Chen, Xianjie Liu, Mats Fahlman

Publication date: 18 March 2020

Source: Joule, Volume 4, Issue 3

Author(s): Hanjian Lai, Qiaoqiao Zhao, Ziyi Chen, Hui Chen, Pengjie Chao, Yulin Zhu, Yongwen Lang, Nan Zhen, Daize Mo, Yuanzhu Zhang, Feng He

Non‐radiative energy loss in organic photovoltaic cells can be achieved by tuning the charge‐transfer state ratio in the hybridized state with the local exciton state. The molecular electrostatic potential (ESP) is considered as a quantitative parameter. By adjusting the ESP difference between PBDB‐TF and BTP‐XF, non‐radiative energy loss is reduced from 0.23 to 0.15 eV.

Abstract

Decreasing the energy loss is one of the most feasible ways to improve the efficiencies of organic photovoltaic (OPV) cells. Recent studies have suggested that non‐radiative energy loss ( ) is the dominant factor that hinders further improvements in state‐of‐the‐art OPV cells. However, there is no rational molecular design strategy for OPV materials with suppressed . Herein, taking molecular surface electrostatic potential (ESP) as a quantitative parameter, we establish a general relationship between chemical structure and intermolecular interactions. The results reveal that increasing the ESP difference between donor and acceptor will enhance the intermolecular interaction. In the OPV cells, the enhanced intermolecular interaction will increase the charge‐transfer (CT) state ratio in its hybridization with the local exciton state to facilitate charge generation, but simultaneously result in a larger . These results suggest that finely tuning the ESP of OPV materials is a feasible method to further improve the efficiencies of OPV cells.

High‐performance all‐inorganic perovskite solar cells with efficiency exceeding 15% are achieved via short‐period deep‐ultraviolet (DUV) photoactivation process. The DUV treatment can efficiently decrease the work function, resulting in better band alignment. The unencapsulated device exhibits enhanced operational stability under continuous simulated sunlight illumination, thermal stability, and outstanding air stability after 30 days of storage under N₂ condition.

All‐inorganic perovskite CsPbI₂Br have been regarded as a promising candidate to tackle the thermal instability issue of organic–inorganic perovskite solar cells. However, the serious interfacial charge recombination and large voltage potential loss in cells circumscribe their performance and commercialization. Herein, a facile approach is demonstrated in which the SnO₂ electron transport layer is modified with short‐period deep‐ultraviolet (DUV) photoactivation process to decrease the work function and achieve better energy alignment with the conduction band of perovskites. Such modification triggers efficient charge transfer and reduces the charge recombination. Moreover, first‐principles calculation further demonstrates that DUV‐treated SnO₂ can strengthen the interface interaction, reduce the interface stress caused by lattice mismatch, induce more ordered perovskite structure, enlarge transfer charge from 0.71 to 2.33 e, gain larger built‐in field (from 0.74 to 2.09 eV), lower work function, and smaller conduction band offset. Thus, all‐inorganic CsPbI₂Br solar cells based on DUV‐treated SnO₂ exhibit a significant enhancement in power conversion efficiency, and the champion cell achieves an elevated efficiency of 15.1% with a superior V _oc of 1.22 V and better stability.

The built‐in electric field of a perovskite solar cell is reinforced by introducing electric dipole molecules, and the oriented charge transfer and collection are significantly improved. An efficiency of 21.5% is demonstrated and the average stability of NMFL device retains 95% PCE after storing over 2000 h under ambient conditions.

Abstract

Perovskite solar cells (PSCs) have received great attention due to their outstanding performance and their low processing costs. To boost their performance, one approach is to reinforce the built‐in electric field (BEF) to promote oriented carrier transport. The BEF is maximized by reinforcing the work function difference between cathode and anode (Δμ₁) and increasing the work function difference between lower and upper surfaces of perovskite film (Δμ₂) via introduction of electric dipole molecules, denoted as PTFCN and CF3BACl. The synergistic reinforcement of BEF improves charge transport and collection, and realizes markedly high photovoltaic performances with the best power conversion efficiency (PCE) up to 21.5%, a growth of 15.6% as compared to the control device, which is higher than the superposition of improvements achieved by either raising Δμ₁ or Δμ₂. Importantly, dual‐functional CF3BACl not only supplies dipole effect for tuning the surface potential of perovskite but offers hydrophobic trifluoride group toward the long‐term stable unencapsulated PSCs retaining more than 95% PCE after storing 2000 h under ambient conditions. This work demonstrates the synergistic effect of Δμ₁ and Δμ₂, providing an effective strategy for the further development of PSC in terms of photovoltaic conversion and stability.

A polymerization‐assisted grain growth strategy in the sequential deposition method of perovskite thin films is demonstrated by triggering a polymerization process during PbI₂ film annealing. This strategy effectively passivates undercoordinated lead ions, reduces defect density, and boosts power conversion efficiency up to 23.0%, together with a prolonged lifetime.

Abstract

Intrinsically, detrimental defects accumulating at the surface and grain boundaries limit both the performance and stability of perovskite solar cells. Small molecules and bulkier polymers with functional groups are utilized to passivate these ionic defects but usually suffer from volatility and precipitation issues, respectively. Here, starting from the addition of small monomers in the PbI₂ precursor, a polymerization‐assisted grain growth strategy is introduced in the sequential deposition method. With a polymerization process triggered during the PbI₂ film annealing, the bulkier polymers formed will be adhered to the grain boundaries, retaining the previously established interactions with PbI₂. After perovskite formation, the polymers anchored on the boundaries can effectively passivate undercoordinated lead ions and reduce the defect density. As a result, a champion power conversion efficiency (PCE) of 23.0% is obtained, together with a prolonged lifetime where 85.7% and 91.8% of the initial PCE remain after 504 h continuous illumination and 2208 h shelf storage, respectively.

By designing N‐functionalized asymmetrical acceptors N7IT and N8IT, the effects of nitrogen (N) atom on reducing nonradiative recombination loss (ΔE ₃) and dipole moment on morphology are revealed.

Abstract

Energy loss (E _loss) consisting of radiative recombination loss (ΔE ₁ and ΔE ₂) and nonradiative recombination loss (ΔE ₃) is considered as an important factor for organic solar cells (OSCs). Herein, two N‐functionalized asymmetrical small molecule acceptors (SMAs), namely N7IT and N8IT are designed and synthesized, to explore the effect of N on reducing E _loss with sulfur (S) as a comparison. N7IT‐based OSCs achieve not only a higher PCE (13.8%), but also a much lower E _loss (0.57 eV) than those of the analogue (a‐IT)‐based OSCs (PCE of 11.5% and E _loss of 0.72 eV), which are mainly attributed to N7IT's significantly enhanced charge carrier density (promoting J _SC) and largely suppressed nonradiative E _loss by over 0.1 eV (enhancing V _OC). In comparison, N8IT, with an extended π‐conjugated length, shows relatively lower photovoltaic performance than N7IT (but higher than a‐IT) due to the less favorable morphology caused by the excessively large dipole moment of the asymmetrical molecule. Finally, this work sheds light on the structure–property relationship of the N‐functionalization, particularly on its effects on reducing the E _loss, which could inspire the community to design and synthesize more N‐functionalized SMAs.