Chen Weijie

A series of wide bandgap donor polymers are designed and synthesized by incorporating a monothiophene functionalized with both a fluorine atom and an ester group. Fabricated from nonhalogenated solvent, power conversion efficiencies of 11.39% and 12.11% are achieved for binary and ternary nonfullerene solar cells, respectively.

Abstract

Significant progress has been made in nonfullerene small molecule acceptors (NF‐SMAs) that leads to a consistent increase of power conversion efficiency (PCE) of nonfullerene organic solar cells (NF‐OSCs). To achieve better compatibility with high‐performance NF‐SMAs, the direction of molecular design for donor polymers is toward wide bandgap (WBG), tailored properties, and preferentially ecofriendly processability for device fabrication. Here, a weak acceptor unit, methyl 2,5‐dibromo‐4‐fluorothiophene‐3‐carboxylate (FE‐T), is synthesized and copolymerized with benzo[1,2‐b:4,5‐b′]dithiophene (BDT) to afford a series of nonhalogenated solvent processable WBG polymers P1‐P3 with a distinct side chain on FE‐T. The incorporation of FE‐T leads to polymers with a deep highest occupied molecular orbital (HOMO) level of −5.60−5.70 eV, a complementary absorption to NF‐SMAs, and a planar molecular conformation. When combined with the narrow bandgap acceptor ITIC‐Th, the solar cell based on P1 with the shortest methyl chain on FE‐T achieves a PCE of 11.39% with a large V _oc of 1.01 V and a J _sc of 17.89 mA cm⁻². Moreover, a PCE of 12.11% is attained for ternary cells based on WBG P1, narrow bandgap PTB7‐Th, and acceptor IEICO‐4F. These results demonstrate that the new FE‐T is a highly promising acceptor unit to construct WBG polymers for efficient NF‐OSCs.

CsF is adopted to modify the PbI₂ seed for highly crystallized Cs‐doped perovskite film with very long carrier lifetime, and very high light, thermal and humidity stabilities. As a result, the planar perovskite solar cells based on the Cs‐doped film also show very good stability with negligible hysteresis, and display PCEs of over 21%.

Abstract

Adding a small amount of CsI into mixed cation‐halide perovskite film via a one‐step method has been demonstrated as an excellent strategy for high‐performance perovskite solar cells (PSCs). However, the one‐step method generally relies on an antisolvent washing process, which is hard to control and not suitable for fabricating large‐area devices. Here, CsF is employed and Cs is incorporated into perovskite film via a two‐step method. It is revealed that CsF can effectively diffuse into the PbI₂ seed film, and drastically enhances perovskite crystallization, leading to high‐quality Cs‐doped perovskite film with a very long photoluminescence carrier lifetime (1413 ns), remarkable light stability, thermal stability, and humidity stability. The fabricated PSCs show power conversion efficiency (PCE) of over 21%, and they are highly thermally stable: in the aging test at 60 °C for 300 h, 96% of the original PCE remains. The CsF incorporation process provides a new avenue for stable high‐performance PSCs.

Herein, acetate anion (Ac⁻) is used to partially replace I⁻ in the CsPbI₂Br framework. Ac⁻ doping changes the morphology, electronic properties, and band structure of the host CsPbI₂Br film. The obtained CsPbI_{2− x} Br(Ac) _x perovskite solar cells exhibit a power conversion efficiency of 15.56%, an open circuit voltage of 1.30 V, and great air stability.

Abstract

The Cs‐based inorganic perovskite solar cells (PSCs), such as CsPbI₂Br, have made a striking breakthrough with power conversion efficiency (PCE) over 16% and potential to be used as top cells for tandem devices. Herein, I⁻ is partially replaced with the acetate anion (Ac⁻) in the CsPbI₂Br framework, producing multiple benefits. The Ac⁻ doping can change the morphology, electronic properties, and band structure of the host CsPbI₂Br film. The obtained CsPbI_{2− x} Br(Ac) _x perovskite films present lower trap densities, longer carrier lifetimes, and fast charge transportation compared to the host CsPbI₂Br films. Interestingly, the CsPbI_{2− x} Br(Ac) _x PSCs exhibit a maximum PCE of 15.56% and an ultrahigh open circuit voltage (V _oc) of 1.30 V without sacrificing photocurrent. Notably, such a remarkable V _oc is among the highest values of the previously reported CsPbI₂Br PSCs, while the PCE far exceeds all of them. In addition, the obtained CsPbI_{2− x} Br(Ac) _x PSCs exhibit high reproducibility and good stability. The stable CsPbI_{2− x} Br(Ac) _x PSCs with high V _oc and PCE are desirable for tandem solar cell applications.

A UV‐inert ZnTiO₃ is demonstrated to be an electron selective layer in perovskite solar cells. ZnTiO₃ is a perovskite‐structured semiconductor with excellent chemical stability and poor photocatalysis. Planar perovskite solar cells based on ZnTiO₃ exhibit power conversion efficiency of 20.1% with improved photostability. The best device holds 90% of its initial efficiency after 100 h of ultraviolet soaking.

Abstract

Although planar‐structured perovskite solar cells (PSCs) have power conversion efficiencies exceeding 24%, the poor photostability, especially with ultraviolet irradiance (UV) severely limits commercial application. The most commonly‐used TiO₂ electron selective layer has a strong photocatalytic effect on perovskite/TiO₂ interface when TiO₂ is excited by UV light. Here a UV‐inert ZnTiO₃ is reported as the electron selective layer in planar PSCs. ZnTiO₃ is a perovskite‐structured semiconductor with excellent chemical stability and poor photocatalysis. Solar cells are fabricated with a structure of indium doped tin oxide (ITO)/ZnTiO₃/Cs_0.05FA_0.81MA_0.14PbI_2.55Br_0.45/Sprio‐MeOTAD/Au. The champion device exhibits a stabilized power conversion efficiency of 19.8% with improved photostability. The device holds 90% of its initial efficiency after 100 h of UV soaking (365 nm, 8 mW cm⁻²), compared with 55% for TiO₂‐based devices. This work provides a new class of electron selective materials with excellent UV stability in perovskite solar cell applications.

A dopant‐free polymeric hole transport material (HTM) is synthesized to fabricate perovskite solar cells. The carbonyl groups can passivate defects of under‐coordinated Pb atoms that exist in the surface of perovskite films. A PBT1‐C based device shows a power conversion efficiency of 19.06% with a fill factor of 81.22%, which is the highest value among the dopant‐free polymeric HTMs.

Abstract

Although perovskite solar cells (PVSCs) have achieved rapid progress in the past few years, most of the high‐performance device results are based on the doped small molecule hole‐transporting material (HTM), spiro‐OMeTAD, which affects their long‐term stability. In addition, some defects from under‐coordinated Pb atoms on the surface of perovskite films can also result in nonradiative recombination to affect device performance. To alleviate these problems, a dopant‐free HTM based on a donor‐acceptor polymer, PBT1‐C, synthesized from the copolymerization between the benzodithiophene and 1,3‐bis(4‐(2‐ethylhexyl)thiophen‐2‐yl)‐5,7‐bis(2‐alkyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione units is introduced. PBT1‐C not only possesses excellent hole mobility, but is also able to passivate the surface traps of the perovskite films. The derived PVSC shows a high power conversion efficiency of 19.06% with a very high fill factor of 81.22%, which is the highest reported for dopant‐free polymeric HTMs. The results from photoluminescence and trap density of states measurements validate that PBT1‐C can effectively passivate both surface and grain boundary traps of the perovskite.

Machine learning (ML) is used to predict the material bandgap and perovskite solar cell device performances. The findings from the ML model matches well with the trend in the solar cell theory derived from the “Shockley and Queisser limit.” Other findings, which are beneficial for the fabrication of high‐performing perovskite solar cells are also discussed.

Abstract

Perovskite solar cells (PSCs) have recently received considerable attention due to the high energy conversion efficiency achieved within a few years of their inception. However, a machine learning (ML) approach to guide the development of high‐performing PSCs is still lacking. In this paper ML is used to optimize material composition, develop design strategies, and predict the performance of PSCs. The ML models are developed using 333 data points selected from about 2000 peer reviewed publications. These models guide the design of new perovskite materials and the development of high‐performing solar cells. Based on ML guidance, new perovskite compositions are experimentally synthesized to test the practicability of the model. The ML model also shows its ability to predict underlying physical phenomena as well as the performance of PSCs. The PSC model matches well with the theoretical prediction by the Shockley and Queisser limit, which is almost impossible for a human to find from an ensemble of data points. Moreover, strategies for developing high‐performing PSCs with different bandgaps are also derived from the model. These findings show that ML is very promising not only for predicting the performance, but also for providing a deeper understanding of the physical phenomena associated with the PSCs.

An electron‐deficient unit containing B←N bonds, namely BNIDT, is developed to construct polymer acceptors for photovoltaic applications. Desirable optoelectronic properties such as broad absorption profiles, low‐lying energy levels, ambipolar charge transport properties, and strong electron‐affinity are found for these polymers. All‐polymer solar cells using these B←N embedded polymers as acceptor materials exhibit an enhanced efficiency of 8.78%.

Abstract

In the field of all‐polymer solar cells (all‐PSCs), all efficient polymer acceptors that exhibit efficiencies beyond 8% are based on either imide or dicyanoethylene. To boost the development of this promising solar cell type, creating novel electron‐deficient units to build high‐performance polymer acceptors is critical. A novel electron‐deficient unit containing B←N bonds, namely, BNIDT, is synthesized. Systematic investigation of BNIDT reveals desirable properties including good coplanarity, favorable single‐crystal structure, narrowed bandgap and downshifted energy levels, and extended absorption profiles. By copolymerizing BNIDT with thiophene and 3,4‐difluorothiophene, two novel conjugated polymers named BN‐T and BN‐2fT are developed, respectively. It is shown that these polymers possess wide absorption spectra covering 350–800 nm, low‐lying energy levels, and ambipolar film‐transistor characteristics. Using PBDB‐T as the donor and BN‐2fT as the acceptor, all‐PSCs afford an encouraging efficiency of 8.78%, which is the highest for all‐PSCs excluding the devices based on imide and dicyanoethylene‐type acceptors. Considering that the structure of BNIDT is totally different from these classical units, this work opens up a new class of electron‐deficient unit for constructing efficient polymer acceptors that can realize efficiencies beyond 8% for the first time.

The use of liquid exfoliated 2D WS₂ and MoS₂ as hole‐transporting layers (HTLs) in ultrahigh efficiency organic solar cells is reported. WS₂ yields cells with higher power conversion efficiency (PCE), fill‐factor, and short‐circuit current than MoS₂ and poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate). When WS₂ is introduced as HTL in PBDB‐T‐2F:Y6:PC₇₁BM organic solar cells, a maximum PCE value of 17% is achieved.

Abstract

The application of liquid‐exfoliated 2D transition metal disulfides (TMDs) as the hole transport layers (HTLs) in nonfullerene‐based organic solar cells is reported. It is shown that solution processing of few‐layer WS₂ or MoS₂ suspensions directly onto transparent indium tin oxide (ITO) electrodes changes their work function without the need for any further treatment. HTLs comprising WS₂ are found to exhibit higher uniformity on ITO than those of MoS₂ and consistently yield solar cells with superior power conversion efficiency (PCE), improved fill factor (FF), enhanced short‐circuit current (J _SC), and lower series resistance than devices based on poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) and MoS₂. Cells based on the ternary bulk‐heterojunction PBDB‐T‐2F:Y6:PC₇₁BM with WS₂ as the HTL exhibit the highest PCE of 17%, with an FF of 78%, open‐circuit voltage of 0.84 V, and a J _SC of 26 mA cm⁻². Analysis of the cells' optical and carrier recombination characteristics indicates that the enhanced performance is most likely attributed to a combination of favorable photonic structure and reduced bimolecular recombination losses in WS₂‐based cells. The achieved PCE is the highest reported to date for organic solar cells comprised of 2D charge transport interlayers and highlights the potential of TMDs as inexpensive HTLs for high‐efficiency organic photovoltaics.

Secondary amine, dimethylamine is intentionally included in MAPbI₃ perovskite to improve the rigidity and steric hindrance for water adsorption, giving rise to reduced defect density and enhanced hydrophobicity. Solar cells based on this perovskite structure demonstrate a record certified power conversion efficiency of 20.8% for NiO _x ‐based inverted perovskite solar cells and excellent operational stability under continuous light soaking.

Abstract

Large‐bandgap perovskites offer a route to improve the efficiency of energy capture in photovoltaics when employed in the front cell of perovskite–silicon tandems. Implementing perovskites as the front cell requires an inverted (p–i–n) architecture; this architecture is particularly effective at harnessing high‐energy photons and is compatible with ionic‐dopant‐free transport layers. Here, a power conversion efficiency of 21.6% is reported, the highest among inverted perovskite solar cells (PSCs). Only by introducing a secondary amine into the perovskite structure to form MA_{1− x} DMA _x PbI₃ (MA is methylamine and DMA is dimethylamine) are defect density and carrier recombination suppressed to enable record performance. It is also found that the controlled inclusion of DMA increases the hydrophobicity and stability of films in ambient operating conditions: encapsulated devices maintain over 80% of their efficiency following 800 h of operation at the maximum power point, 30 times longer than reported in the best prior inverted PSCs. The unencapsulated devices show record operational stability in ambient air among PSCs.

This review presents the present status and the future perspectives of Sn‐based perovskite solar cells. The strategies to find the breakthrough of highly efficient and robust Sn‐perovskite solar cells are discussed by focusing on current fabrication processes and defect physics scenario including compositional and dimensional engineering.

Sn‐based halide perovskites have attracted much interest due to their highly valuable electrical and optical properties. The promising optical and electrical properties of Sn‐based perovskites have enticed a lot of research to focus on developing the strategies and explore the in‐depth material characteristics. Sn‐halide perovskites exhibit apparent merits and demerits. The ideal electrical and optical properties are even better than that of Pb‐analogs, namely close‐to‐optimal bandgap, strong optical absorption, and good carrier mobilities. However, the present achievement of Sn‐halide perovskite solar cells is not satisfactory, which is commonly attributed to relatively low defect tolerance, fast crystallization, and oxidative instability. The efficiency of Sn‐based perovskites is far ahead, with a 9% power conversion efficiency (PCE), than the other (Ge, Bi, Sb, Cu, etc.) Pb‐free options but simultaneously lagging far behind Pb‐based analogs that have a 25.2% PCE. This review is aimed at presenting milestone works and revealing the pros and cons of Sn‐halide perovskites. In addition, the defect physics of Sn‐based perovskites is described. The improvement of open‐circuit voltage is a critical issue for Sn‐halide perovskites to compete with Pb‐based perovskites. The understanding of defect physics plays an instrumental role in designing strategies for efficient and robust Sn‐halide perovskite solar cells.

Publication date: December 2019

Source: Nano Energy, Volume 66

Author(s): Jiajia Wang, Hao Lin, Zilei Wang, Wenzhong Shen, Jichun Ye, Pingqi Gao

Graphical abstract

Publication date: December 2019

Source: Nano Energy, Volume 66

Author(s): Jigeon Kim, Bonkee Koo, Wook Hyun Kim, Jongmin Choi, Changsoon Choi, Sung Jun Lim, Jong-Soo Lee, Dae-Hwan Kim, Min Jae Ko, Younghoon Kim

Graphical abstract

We demonstrate that sodium acetate (NaOAc) directly generates short-chain OAc ions to exchange the long-chain oleate ligands of CsPbI₃ perovskite quantum dots (CsPbI₃-PQDs). NaOAc-based ligand exchange enables preservation of CsPbI₃-PQD size, minimization of surface trap states, and enhancement of electronic coupling in the resultant CsPbI₃-PQD solids. Consequently, NaOAc-treated CsPbI₃-PQD solar cells show improved device performance with 12.4% power conversion efficiency.

Nature Energy, Published online: 30 September 2019; doi:10.1038/s41560-019-0475-2

Nature Communications, Published online: 03 October 2019; doi:10.1038/s41467-019-12513-x

A fully automated spray‐coated technology for the commercial large‐scale solution‐based processing of colloidal CsPbI₃ quantum dot films is achieved. The solar cells based on such films show a high power conversion efficiency of 11.2%.

Abstract

A fully automated spray‐coated technology with ultrathin‐film purification is exploited for the commercial large‐scale solution‐based processing of colloidal inorganic perovskite CsPbI₃ quantum dot (QD) films toward solar cells. This process is in the air outside the glove box. To further improve the performance of QD solar cells, the short‐chain ligand of phenyltrimethylammonium bromide (PTABr) with a benzene group is introduced to partially substitute for the original long‐chain ligands of the colloidal QD surface (namely PTABr‐CsPbI₃). This process not only enhances the carrier charge mobility within the QD film due to shortening length between adjacent QDs, but also passivates the halide vacancy defects of QD by Br⁻ from PTABr. The colloidal QD solar cells show a power conversion efficiency (PCE) of 11.2% with an open voltage of 1.11 V, a short current density of 14.4 mA cm⁻², and a fill factor of 0.70. Due to the hydrophobic surface chemistry of the PTABr–CsPbI₃ film, the solar cell can maintain 80% of the initial PCE in ambient conditions for one month without any encapsulation. Such a low‐cost and efficient spray‐coating technology also offers an avenue to the film fabrication of colloidal nanocrystals for electronic devices.

The power conversion efficiency of N2200‐based all‐polymer solar cells (all‐PSCs) can be drastically enhanced from ≈1% to ≈11% by simply changing the solvent from chlorobenzene and 2‐methyltetrahydrofuran (Me‐THF). In‐depth investigations reveal that the preaggregation of donor (PTzBI) and acceptor (N2200) polymers in 2‐Me‐THF is the key to enable such high performance for N2200‐based all‐PSC device.

Herein, all‐polymer solar cells (all‐PSCs) are studied based on PTzBI:N2200 system processed from two different solvents, chlorobenzene (CB) and 2‐methyltetrahydrofuran (Me‐THF). It is found that the preaggregation of the donor and acceptor polymers in Me‐THF is the key factor that enables a drastic enhancement in cell efficiency from ≈1% (processed by CB) to ≈11% (processed by Me‐THF). When using CB as the solvent, both donor and acceptor polymers are well dissolved and mostly disaggregated. In contrast, the donor and acceptor polymers both exhibit strong aggregation in Me‐THF. As a result, the donor and acceptor blend films processed from Me‐THF exhibit pure domains with appropriate molecular packing structure, which leads to high charge mobilities (10⁻³–10⁻⁴ cm² V⁻¹ s⁻¹) and fill factors (FFs; 75%), whereas the blend films processed by CB suffer from highly miscible and impure domains, hence decreasing the charge mobilities by 1–2 orders of magnitude compared with those of the corresponding pure films. The current work reveals that the polymer preaggregation is the key reason enabling optimal morphology and high performance in N2200‐based all‐PSCs, and this strategy may be potentially applied in other systems to optimize the morphology and performance of all‐PSCs.

A spirobixanthene‐based dendrimer, DH1, is designed and synthesized. DH1 with the hyperbranched structure shows a large molecular size of up to 1.9 nm. The amorphous DH1 is the first dendrimer‐type HTM applied for MAPbI₃ perovskite solar cells, obtaining a power conversion efficiency of 17.13%. This work demonstrates that a quasiglobular dendrimer with a large molecular size is a promising design approach for excellent HTMs.

A dendrimer based on a spirobixanthene core, termed DH1, is designed and synthesized as a hole‐transporting material (HTM) for perovskite solar cells (PSCs). DH1 showing a hyperbranched structure with methoxydiphenylamine carbazole dendrons stretching outward along the para‐phenylene spacer acquires a large molecular size of up to 1.9 nm, which favors good thermal stability and amorphous property. The thus obtained DH1‐based pinhole‐free film as a hole‐transport layer results in a power conversion efficiency of 17.13% and reduced hysteresis behavior of MAPbI₃‐based planar PSCs. This work provides the first example of the use of dendrimer‐type HTM for PSC application, demonstrating a promising approach to design HTMs in a quasiglobular dendrimer with a large molecular size.

Here, a Ruddlesden–Popper 2D structure is introduced into γ‐CsPbI₃ film to stabilize the black phase via reducing dimensionality. The dimensionality of 2D, quasi 2D, and 3D structures is changed by adjusting the amount of phenylethylammonium iodide. Comprehensive consideration to obtain both the stability and high power conversion efficiency (PCE), quasi‐2D (n = 40) γ‐CsPbI₃ delivers a reproducible PCE of 13.65% with negligible hysteresis.

Abstract

The highest certified power conversion efficiency (PCE) of black phase based CsPbI₃ perovskite solar cells has exceeded 18%, and become a hotspot in recent progress. However, the black phase of CsPbI₃ rapidly transforms to yellow phase in ambient conditions due to its thermodynamic instability. Here, a Ruddlesden–Popper 2D structure is introduced into γ‐CsPbI₃ film to stabilize the black phase via reducing dimensionality. It is found that a judicious amount of phenylethylammonium iodide can adjust the dimensionality of γ‐CsPbI₃ film from 2D to quasi‐2D and 3D phase. Comprehensive consideration to obtain both the stability and high PCE, quasi‐2D (n = 40) γ‐CsPbI₃ delivers a reproducible PCE of 13.65% with negligible hysteresis. By utilizing femtosecond transient absorption and time‐resolved PL decay, similar carrier kinetics in n = 40 and ∞ samples are observed, meaning an efficient charge extraction. More importantly, when the device is placed at 80 °C in N₂ condition or in air with RH of 25–30%, its PCE keeps ≈88% and ≈89% of its initial PCE after 12 days, respectively. Such results are better than the 3D one (≈69% and ≈16%, respectively).

In article number https://doi.org/10.1002/smll.2019016501901650, Jia Liang, Jun Lou, and co‐workers synthesize lead‐free double perovskite Cs₂SnX₆ (X = Br, I) with a well‐defined vacancy‐ordered defect‐variant crystal structure via a facile hydrothermal method. The as‐obtained perovskite displays excellent stability against moisture, light, and high‐temperature.