Chen Weijie

Double‐sided 2D surface passivation of 3D perovskite film contributes to a remarkable device V _OC of 1.2 V, which is one of the highest open‐circuit voltages reported for perovskite cells with an optical bandgap of ≈1.6 eV. Discontinuous 2D perovskite films provide conductive pathways through these resistive layers, allowing for efficient charge transport between the 3D perovskite and charge transport layers.

Abstract

Defect‐mediated carrier recombination at the interfaces between perovskite and neighboring charge transport layers limits the efficiency of most state‐of‐the‐art perovskite solar cells. Passivation of interfacial defects is thus essential for attaining cell efficiencies close to the theoretical limit. In this work, a novel double‐sided passivation of 3D perovskite films is demonstrated with thin surface layers of bulky organic cation–based halide compound forming 2D layered perovskite. Highly efficient (22.77%) mixed‐dimensional perovskite devices with a remarkable open‐circuit voltage of 1.2 V are reported for a perovskite film having an optical bandgap of ≈1.6 eV. Using a combination of experimental and numerical analyses, it is shown that the double‐sided surface layers provide effective defect passivation at both the electron and hole transport layer interfaces, suppressing surface recombination on both sides of the active layer. Despite the semi‐insulating nature of the passivation layers, an increase in the fill factor of optimized cells is observed. The efficient carrier extraction is explained by incomplete surface coverage of the 2D perovskite layer, allowing charge transport through localized unpassivated regions, similar to tunnel‐oxide passivation layers used in silicon photovoltaics. Optimization of the defect passivation properties of these films has the potential to further increase cell efficiencies.

The enhancement in luminescence efficiency of halide perovskite films through a light passivation treatment is summarized, and the extreme sensitivity of the effect to experimental considerations is detailed to reconcile seemingly conflicting literature reports. The mechanisms are scrutinized and the prospects for these treatments to push the properties of polycrystalline films toward those of their single‐crystal counterparts are discussed.

Abstract

There has been a meteoric rise in the commercial potential of lead halide perovskite optoelectronic devices since photovoltaic cells and light‐emitting diodes based on these materials were first demonstrated. One key challenge common to each of these optoelectronic devices is the need to suppress nonradiative recombination, a process that limits the maximum achievable efficiency in photovoltaic cells and light‐emitting diodes. In this Progress Report, recent studies that seek to minimize this loss pathway in perovskites through a photobrightening treatment, whereby the luminescence efficiency is enhanced through a light illumination passivation process are examined. The sensitivity of this effect to various experimental considerations is examined, including atmosphere, photon energy, photon dose, and also the role of perovskite composition and morphology; under certain conditions there can even be photodarkening effects. Consideration of these factors is critical to resolve seemingly conflicting literature reports. Proposed mechanisms are scrutinized, revealing that there is now some consensus but further work is needed to identify the specific defects being passivated and elucidate universal mechanisms. Finally, the prospects for these treatments to minimize halide migration and push the properties of polycrystalline films towards those of their single‐crystal counterparts are discussed.

Two electron donor (D)–electron acceptor (A)‐type polymers PBDTT and PBTTT are developed as hole‐transporting materials for perovskite solar cells (PVSCs). Both polymers endow the PVSCs promising device performance. A power conversion efficiency of 20.28% is achieved from the devices with dopant‐free PBDTT. High device stability can be expected by employing these compact and hydrophobic polymeric hole‐transporting layers.

Abstract

The rich molecular design of electron donor (D)–acceptor (A) polymers offers many valuable clues to obtain high‐efficiency hole‐transporting materials (HTMs) for use in perovskite solar cells (PVSCs). The fused aromatic or heteroaromatic units can increase the conjugation of the polymer backbone to facilitate electron delocalization, which increases the rigidity of adjacent units to prevent rotational disorder and lower the reorganization energy, leading to improved carrier mobility and optimized film morphology. In this work, fused‐ring ladder‐type indacenodithiophene and indacenodithieno[3,2‐b]thiophene are used as D units, benzodithiophene‐4,8‐dione as the A unit, and thienothiophene as a π‐bridge to form the D–A polymers PBDTT and PBTTT, respectively. Both polymers exhibit favorable properties as HTMs including suitable energy levels, high hole mobility, and excellent film quality. Both dopant‐free HTMs endow n‐i‐p PVSCs with promising performance and stability. A maximum power conversion efficiency of 20.28% is achieved for PBDTT‐based devices, which is among the highest values reported to date.

The interplay of crystal structure and photophysics in the mixed cation, mixed halide perovskite (FAPbI₃)_0.85(MAPbBr₃)_0.15 is probed. It is found that changes in crystal structure, quantified by structural parameters such as lattice constant ratios and bond angles, influence optoelectronic properties in the film—the bandgap, Stokes shift, and charge carrier recombination rates all exhibit phase specificity.

Abstract

Mixed cation perovskites currently achieve very promising efficiency and operational stability when used as the active semiconductor in thin‐film photovoltaic devices. However, an in‐depth understanding of the structural and photophysical properties that drive this enhanced performance is still lacking. Here the prototypical mixed‐cation mixed‐halide perovskite (FAPbI₃)_0.85(MAPbBr₃)_0.15 is explored, and temperature‐dependent X‐ray diffraction measurements that are correlated with steady state and time‐resolved photoluminescence data are presented. The measurements indicate that this material adopts a pseudocubic perovskite α phase at room temperature, with a transition to a pseudotetragonal β phase occurring at ≈260 K. It is found that the temperature dependence of the radiative recombination rates correlates with temperature‐dependent changes in the structural configuration, and observed phase transitions also mark changes in the gradient of the optical bandgap. The work illustrates that temperature‐dependent changes in the perovskite crystal structure alter the charge carrier recombination processes and photoluminescence properties within such hybrid organic–inorganic materials. The findings have significant implications for photovoltaic performance at different operating temperatures, as well as providing new insight on the effect of alloying cations and halides on the phase behavior of hybrid perovskite materials.

Two organic small molecules are presented as dopant‐free hole transporting materials (HTMs) in inverted perovskite solar cells, namely, FB‐OMeTPA and FT‐OMeTPA. Due to the Pb–S interaction at interfaces between HTM and perovskites, devices based on FT‐OMeTPA deliver an impressive power conversion efficiency of 17.57%, which is considerably higher than that of devices with FB‐OMeTPA (14.35%).

One of the attractive ways to develop efficient and cost‐effective inverted perovskite solar cells (PVSCs) is through the use of dopant‐free hole transporting materials (HTMs) with facile synthesis and a lower price tag. Herein, two organic small molecules with a fluorene core are presented as dopant‐free HTMs in inverted PVSCs, namely, FB‐OMeTPA and FT‐OMeTPA. The two molecules are designed in such a way they differ by replacing one of the benzene rings (FB‐OMeTPA) with thiophene (FT‐OMeTPA), which leads to a significantly improved coplanarity as manifested in the redshift of the absorbance and a smaller bandgap energy. Density functional theory calculations show that FT‐OMeTPA has a strong Pb²⁺–S interaction at the FT‐OMeTPA/perovskite interface, allowing surface passivation and facilitating charge transfer across interfaces. As a result, the PVSCs based on FT‐OMeTPA exhibit a much higher hole mobility, power conversion efficiency, operational stability, and less hysteresis as compared with devices based on FB‐OMeTPA.

The fused‐ring acceptor IT‐M is added into an unfused‐core acceptor‐based binary blend of PBDB‐TF:HC‐PCIC. Notable fill factor enhancement and a broad compositional tolerance are achieved for the ternary solar cells. Thus, the power conversion efficiency is significantly improved from 11.14% for binary devices to 12.34% for ternary cells.

The ternary blend strategy has shown great potential to improve the photovoltaic performance of organic solar cells (OSCs). Usually, adopting two acceptors with similar chemical structures shows good compatibility but limited enhancement in performance, whereas adopting two acceptors with different chemical structures always has a compositional sensitivity issue. Herein, a highly efficient ternary OSC with an enhanced fill factor (FF) and a broad compositional tolerance is demonstrated by introducing the fused‐ring acceptor IT‐M to a binary blend based on an unfused‐core acceptor HC‐PCIC and polymer donor PBDB‐TF. Detailed studies on the optical, electrical, and morphological properties of ternary blends reveal the process of charge dynamics and work mechanisms in the ternary device. It is found that the addition of IT‐M into the PBDB‐TF:HC‐PCIC binary blend not only adapts to the parallel‐like model, but also optimizes the morphology and domain sizes in the ternary blend, resulting in a reduced trap‐assisted recombination and suppressed bimolecular recombination. Consequently, open‐circuit voltage (V _oc), short‐circuit current density (J _sc), and FF are synergistically enhanced, leading to an improved power conversion efficiency (PCE) of 12.34% with a high V _oc of 0.88 V, an increased J _sc of 18.69 mA cm⁻², and an enhanced FF of 73.82% for the ternary device with 5% IT‐M content. Moreover, the PCEs of ternary OSCs remain above 11% within an IT‐M ratio of 2.5–50%, exhibiting a broad compositional tolerance, which is rarely reported in fullerene‐free ternary OSCs.

Herein, amorphous‐Zn₂SnO₄ (am‐ZTO) is used to provide a large free energy difference (ΔG) to improve electron injection from perovskite to electron transport layers. In addition, the introduction of the am‐ZTO also leads to a dense physical contact between the am‐ZTO and the FTO substrate, leading to decreased leakage current. The optimized device exhibits a power conversion efficiency of 20.04%.

Charge extraction by electron transport layers (ETLs) plays a vital role in improving the performance of perovskite solar cells (PSCs). Here, PSCs with four different types of ETLs, such as SnO₂, amorphous‐Zn₂SnO₄ (am‐ZTO), am‐ZTO/SnO₂, and SnO₂/am‐ZTO, are successfully synthesized. The interface recombination behavior and the charge transport properties of the devices affected by four types of ETLs are systematically investigated. For dual am‐ZTO/SnO₂ ETLs, compact am‐ZTO ETL prepared by the pulsed laser deposition method provides a dense physical contact with FTO than the spin coating films, decreasing leakage current and improving charge collection at the interface of ETL/FTO. Moreover, dual am‐ZTO/SnO₂ ETLs lead to large free energy difference (ΔG), improving electron injection from perovskite to ETLs. One additional electron pathway from perovskite to am‐ZTO is formed, which can also improve electron injection efficiency. A power conversion efficiency of 20.04% and a stabilized efficiency of 19.17% are achieved for the device based on dual am‐ZTO/SnO₂ ETLs. Most importantly, the devices are fabricated at a low temperature of 150 °C, which offers a potential method for large‐scale production of PSCs, and paves the way for the development of flexible PSCs. It is believed that this work provides a strategy to design ETLs via controlling ΔG and interface contact to improve the performance of PSCs.

Herein, a facile strategy that can carry out double passivation to improve the performance of perovskite solar cells (PSCs) is demonstrated. By using the dilute halide salt PEABr solution to treat the perovskite film, PbI₂ can precipitate from the perovskite. Both PEABr and PbI₂ can passivate the perovskite film; double passivation improves the performance of PSCs significantly.

Material passivation is essential to enhance the quality of perovskite materials and boost the performance of perovskite solar cells (PSCs). However, most of the previous reports only paid attention to improving the quality of perovskite films by adopting single passivation. Here, a facile strategy that can carry out double passivation to improve the performance of PSCs is demonstrated. By using the dilute halide salt PEABr solution to treat the perovskite film, PbI₂ can precipitate from the perovskite. Both PEABr and PbI₂ can passivate the perovskite film, and by combining PEABr and PbI₂, the double passivation improves the performance of PSCs significantly. Very high short‐circuit current density of 24.30 mA cm⁻², open‐circuit voltage of 1.10 V, and fill factor of 79.75% are achieved which lead to a surprising efficiency of 21.32% for the passivated device. The improved efficiency is mainly according to the available surface passivation of the perovskite material, leading to repressed nonradiative recombination and unhindered charge collection. In addition, the passivated device exhibits better power conversion efficiency stability relative to the control device.

Recent progress in bandgap engineering strategies including the two main, widely used impurity and pressure as well as intermediate band, external electric field, and steric methods are reviewed comprehensively. Their underlying mechanism, achievements, and challenges are outlined. Additionally, future research directions are provided to realize direct and gap size continually tunable perovskites for further enhancing solar cell performance.

Metal halide perovskites are attractive for highly efficient solar cells. As most perovskites suffer large or indirect bandgap compared with the ideal bandgap range for single‐junction solar cells, bandgap engineering has received tremendous attention in terms of tailoring perovskite band structure, which plays a key role in light harvesting and conversion. In this Review, various reported bandgap engineering strategies are summarized. The recently widely used two main strategies including impurity and pressure as well as their underlying mechanisms are reviewed comprehensively. In addition, intermediate band and external electric field for bandgap engineering are also investigated. Moreover, future research directions are outlined to guide the further investigation.

An excellent antioxidant, tea polyphenol (TP), is introduced to a CsPb_0.5Sn_0.5I₂Br precursor solution to obtain high‐efficiency inorganic PSCs. TP can not only retard the oxidation of Sn²⁺ but also regulate the formation of Pb/Sn binary perovskite films, leading to a reduced density of defects. The corresponding device demonstrates power conversion efficiency as high as 8.10% with high stability.

Implementation of inorganic perovskite compounds and reduction toxicity of lead are important for developing sustainable and renewable photovoltaic power generation. The inorganic Pb/Sn binary metal halide perovskites offer a perfect opportunity for tuning optical bandgap and thus hold significant potential in emerging technologies such as solar cells. However, an easy oxidation of Sn²⁺ to Sn⁴⁺ has become one of the main issues for achieving efficient and stable Sn‐based perovskite solar cells (PSCs). Herein, an effective method for stabilizing CsPb_0.5Sn_0.5I₂Br is proposed to realize high‐efficiency PSCs via antioxidant tea polyphenol (TP). It is found that TP can not only slow down the oxidation of Sn²⁺ but also regulate perovskite film crystallization during the formation of perovskite films via coordination interaction, leading to a reduced density of defects and an enlarged open‐circuit voltage. The resultant perovskite solar cell using CsPb_0.5Sn_0.5I₂Br (TP) with an all‐inorganic mesoscopic framework of FTO/c‐TiO₂/m‐TiO₂/Al₂O₃/NiO/carbon achieves an impressive power conversion efficiency of 8.10% with high stability.

Organic amine cation, GA⁺ is intentionally incorporated in MA_0.7FA_0.3PbI₃ perovskite to stiffen the inorganic Pb–I lattice, restrain the formation of iodine vacancies defects, and reduce ion diffusion. Solar cells based on this component engineering and PFN‐Br interfacial strategy demonstrate an enhanced power conversion efficiency value over 21% for SnO₂‐based planar perovskite solar cells and excellent thermal stability.

Tin oxide (SnO₂) offers its advantages in widespread applications that require efficient carrier transport. However, the usages of SnO₂ in organic solar cells are hindered because of dangling bonds on the surface of SnO₂. Herein, PFN‐Br as an interfacial layer to tailor the work function of SnO₂ is adopted, making it an ideal candidate for interfacial material in organic electronics. Meanwhile, such an efficient SnO₂/PFN‐Br electron transport layer (ETL) can also be applied to perovskite devices and achieve competitive efficiency. To eliminate current–voltage hysteresis and improve poor thermodynamic stability of perovskite solar cells (PSCs), 5 mol% of guanidinium iodide (GAI) into the (MA) _x (FA)_1 − x PbI₃ precursor solution is incorporated, enabling the formation of triple‐cation perovskite films with excellent optoelectronic quality and stability. The combination of an SnO₂/PFN‐Br ETL and GAI doping strategy finally delivers power conversion efficiencies over 21% and negligible current–voltage hysteresis in planar PSCs. These improvements arise from the strong hydrogen bonding caused by the incorporation of GA⁺. It can stiffen the inorganic Pb–I lattice of the unit cell and restrain the formation of iodine vacancies defects. Moreover, the strong hydrogen bonding can immobilize iodide ion and thus enhance the thermal stability of the corresponding device.

Three perylene diimide tetramers annulated by different chalcogen atoms (O, S, and Se) are synthesized for efficient nonfullerene polymer solar cells. O‐fused PDI‐based device exhibits an outstanding power conversion efficiency of 8.904% with a high fill factor of 0.706, outcompeting its S‐ and Se‐annulated counterparts.

Three perylene diimide (PDI) tetramers annulated by oxygen (O), sulfur (S), and selenium (Se), named as SF‐4PDI‐O, SF‐4PDI‐S, and SF‐4PDI‐Se, are designed, synthesized, and paired with polymeric donor PDBT‐T1 to construct organic solar cells. The heteroatoms' effects on photoelectric properties, chemical geometry, charge transport, active‐layer morphology, and photovoltaic performance are investigated in detail. These PDI acceptors exhibit a similar absorption profile, whereas the highest occupied molecular orbitals and lowest unoccupied molecular orbitals are simultaneously upshifted when heteroatoms are altered from O and S to Se due to the gradually weakening electronegativity. Alongside PDBT‐T1, SF‐4PDI‐O achieves an outstanding power conversion efficiency of 8.904% with a high fill factor of 0.706, outcompeting its S‐annulated and Se‐annulated counterparts. The superiority of the PDBT‐T1:SF‐4PDI‐O system lies in its stronger crystallinity, more balanced hole and electron mobilities, and weaker bimolecular recombination, coupled with more efficient charge transfer and collection. These results shed light on the invention of high‐performance PDI acceptors by oxygen‐decorated methodology.

After the substitution of F atom on the benzothiadiazole (BT) acceptor unit of the polymer backbone, the power conversion efficiency of the polymer solar cell improves significantly due to the more balanced charge transport and small charge extraction time.

The fast evolution of the narrow bandgap nonfullerene acceptors requires new conjugated wide bandgap polymers for the use of nonfullerene polymer solar cells (PSCs). Herein, two new wide bandgap A1–D1–A2–D1 conjugated polymers with the same dithieno[2,3‐e:3′,2′‐g]isoindole‐7,9(8H)‐dione (DTID) acceptor (A1) and thiophene donor (D1) and different A2 acceptor units, i.e., benzothiadiazole (BT) and fluorinated benzothiadiazole (f‐BT) denoted as P113 and P114, are designed, and the effect of fluorination of the BT acceptor unit on the photovoltaic properties of PSCs using nonfullerene acceptors is investigated. It is found that the incorporation of fluorine atom into the BT acceptor unit increases the absorption coefficients and the relative dielectric constant. The increase in photoluminescence quenching, reduction in charge recombination loss, and improvement in the charge carrier life are observed for P114. All these factors result in the drastically improved power conversion efficiency of P114:ITIC‐m‐based PSC to 10.42% with a small energy loss of 0.56 eV as compared with the P113 counterpart (8.74% with an energy loss of 0.69 eV) under identical conditions. The low energy loss is beneficial to overcome the trade‐off between open‐circuit voltage and short‐circuit current.

An efficient solution‐processed interconnecting layer is designed for nonfullerene tandem solar cells by introducing hydrogen molybdenum bronze (H _x MoO₃). H _x MoO₃ circumvents wetting issues of aqueous PEDOT:PSS on the hydrophobic surface of active layers. Electrically, H _x MoO₃ films have high work function and improve hole collection. The H _x MoO₃ delivers high‐performance inverted single‐junction and tandem nonfullerene solar cells.

Tandem solar cells are attractive because they can break the Shockley–Queisser efficiency limit of single‐junction cells. However, it is still quite challenging to fabricate efficient nonfullerene tandem organic solar cells (OSCs) because of their complicated and vulnerable multilayers and interfaces. The interconnecting layer (ICL) between two subcells is the key part in efficient tandem solar cells, which should be designed properly based on the property of active layers. Herein, the incorporation of hydrogen molybdenum bronze (H _x MoO₃) between the bottom active layer and PEDOT:PSS/ZnO to form a new ICL is proposed. The contribution of the H _x MoO₃ is two fold: 1) it can well wet the hydrophobic active layer surface and form a uniform film. It provides a hydrophilic surface for the following deposition of PEDOT:PSS; 2) the H _x MoO₃ has a high work function of 5.4 eV that is higher than PEDOT:PSS (5.0 eV) and can extract holes more efficiently from the active layer with the deep highest occupied molecular orbital energy level. Nonfullerene tandem solar cells with a high fill factor of 76.0% and power conversion efficiency of 15.03% are obtained with this ICL.

The origin of the weak conductivity of Sb₂S₃ is unraveled. Improving the conductivity through direct extrinsic doping is intrinsically confined due to the comparable high concentration of intrinsic donor and acceptor. Interestingly, O doping fills the dominant donor V_S2 and makes the p‐type doping feasible. Therefore, a new strategy for overcoming the efficiency bottleneck of Sb₂S₃ solar cells is proposed.

The photovoltaic efficiency increase in Sb₂S₃‐based solar cells has stagnated for 5 years since the highest efficiency of 7.5% was achieved in 2014. One important bottleneck is the high electrical resistivity of Sb₂S₃. The first‐principle calculations reveal that the high‐resistivity results from the compensation between the intrinsic donor V_S and acceptors V_Sb, Sb_S, and S_Sb which have comparably high concentration (low formation energy). The compensation also limits the improvement of conductivity through direct extrinsic doping. Further calculations of O dopants show that O_S has low formation energy, so the dominant intrinsic donor V_S can be passivated by O and thus the p‐type doping limit imposed by V_S can be overcome. Meanwhile, other p‐type limiting and recombination‐center donor defects can be suppressed under the S‐rich condition, which explains why the highest efficiency is achieved in O‐doped Sb₂S₃ after sulfurization. Given the unexpected beneficial effects of O doping and sulfurization, a two‐step doping strategy is proposed for overcoming the efficiency bottleneck: 1) use O to passivate the V_S and S‐rich condition to suppress other detrimental defects, making p‐type doping feasible and minority carrier lifetime long; 2) introduce other p‐type dopants to increase hole carrier concentration.