Unencapsulated Perovskite Solar Cells Research Articles

Stable α-FAPbI3 via porous PbI2 for efficient perovskite solar cells.

Black-phase formamidinium lead iodide (FAPbI3), with a narrow bandgap and high thermal stability, has emerged as an in-demand material for highly efficient perovskite solar cells (PSCs). In a two-step sequential deposition, the PbI2 film plays an important role in the formation of a perovskite film with desirable qualities. This paper explores using N-methyl-2-pyrrolidone (NMP), a strong Lewis base, and N,N-dimethylformamide (DMF) as a mixed precursor solvent (DMF/NMP) of PbI2 and reports on preparing PbI2 films with a porous morphology by thermal treatment. Porous PbI2 films ensure the diffusion and sufficient reaction of the formamidinium iodide solution to form a smooth perovskite film. In addition, a dynamic spin coating method is also introduced to improve the uniformity of the perovskite film. Both methods yield a pure α-phase FAPbI3 film immediately in the unannealed state, which is necessary for the perovskite film to maintain phase stability. Finally, PSCs with a power conversion efficiency of 21.20% (0.13cm2) are fabricated and optimized. The unencapsulated PSCs retain 90% of the initial efficiency for 1000 hours in dry air and exhibit a good thermal stability when heated to 85 °C.

π-Conjugated Carbazole Cations Enable Wet-Stable Quasi-2D Perovskite Photovoltaics

Quasi-two-dimensional halide perovskites are commonly used in solar cells, as they are more stable than their three-dimensional analogues. Nevertheless, it is still challenging to meet the stability requirements under high-humidity conditions. Here, we design π-conjugated carbazole (CA) cations to increase the water resistance of perovskite. We control the crystallization kinetics by the anti-solvent strategy to locate the hydrophobic low-⟨n⟩-value phase on the surface of the perovskite film. The resulting CA2MA4Pb5I16 film does not decompose after being immersed in water for several minutes. We further regulate the vertical orientation of perovskite crystals by introducing NH4SCN additive, resulting in improved carrier transport dynamics. As a result, the optimized CA2MA4Pb5I16 device achieves a notable power conversion efficiency (PCE) of 18.23% and retains more than 85% of the original PCE after 2000 h under a relative humidity of 65% at 25 °C. This is one of the most stable reported unencapsulated perovskite solar cells in high-humidity environments.

Asymmetrically Substituted 10H,10'H-9,9'-Spirobi[acridine] Derivatives as Hole-Transporting Materials for Perovskite Solar Cells.

Hole-transporting materials (HTMs) based on the 10H, 10'H-9,9'-spirobi [acridine] core (BSA50 and BSA51) were synthesized, and their electronic properties were explored. Experimental and theoretical studies show that the presence of rigid 3,6-dimethoxy-9H-carbazole moieties in BSA 50 brings about improved hole mobility and higher work function compared to bis(4-methoxyphenyl)amine units in BSA51, which increase interfacial hole transportation from perovskite to HTM. As a result, perovskite solar cells (PSCs) based on BSA50 boost power conversion efficiency (PCE) to 22.65 %, and a PSC module using BSA50 HTM exhibits a PCE of 21.35 % (6.5×7 cm) with a Voc of 8.761 V and FF of 79.1 %. The unencapsulated PSCs exhibit superior stability to devices employing spiro-OMeTAD, retaining nearly 90 % of their initial efficiency after 1000 h operation output. This work demonstrates the high potential of molecularly engineered spirobi[acridine] derivatives as HTMs as replacements for spiro-OMeTAD.

Asymmetrically Substituted 10H,10′H‐9,9′‐Spirobi[acridine] Derivatives as Hole‐Transporting Materials for Perovskite Solar Cells

AbstractHole‐transporting materials (HTMs) based on the 10H, 10′H‐9,9′‐spirobi [acridine] core (BSA50andBSA51) were synthesized, and their electronic properties were explored. Experimental and theoretical studies show that the presence of rigid 3,6‐dimethoxy‐9H‐carbazole moieties inBSA 50brings about improved hole mobility and higher work function compared to bis(4‐methoxyphenyl)amine units inBSA51, which increase interfacial hole transportation from perovskite to HTM. As a result, perovskite solar cells (PSCs) based onBSA50boost power conversion efficiency (PCE) to 22.65 %, and a PSC module usingBSA50HTM exhibits a PCE of 21.35 % (6.5×7 cm) with aVocof 8.761 V and FF of 79.1 %. The unencapsulated PSCs exhibit superior stability to devices employing spiro‐OMeTAD, retaining nearly 90 % of their initial efficiency after 1000 h operation output. This work demonstrates the high potential of molecularly engineered spirobi[acridine] derivatives as HTMs as replacements for spiro‐OMeTAD.

Dual‐Site Synergistic Passivation for Highly Efficient and Stable Perovskite Solar Cells

AbstractDefect passivation has been recognized as an effective strategy to improve efficiency and stability of perovskite solar cells (PSCs). In this work, in‐depth theoretical calculations and experimental characterizations reveal the dual‐site synergistic passivation of 1H‐benzimidazole (BIZ), and the conjugated structure of the benzene ring tends to increase the interaction between BIZ and perovskite. High‐quality perovskite films are thus achieved, with increased grain size, reduced defect density, and suppressed ion migration. Simultaneously, the reduced work function and optimized band alignment promote carrier transport, reducing nonradiative recombination, and loss of open‐circuit voltages, as well as fill factor. Consequently, the target PSC devices achieve a champion power conversion efficiency (PCE) of 24.59%, and 20.49% for perovskite solar module (a designated area of 27.5 cm2). The unencapsulated PSC maintains 91.49% of original PCE after storing in air with an average relative humidity of 40% for 2400 h. Moreover, the device exhibits remarkable the long‐term operational stability, maintaining 90.47% of initial PCE after continuously operating at the maximum power point for 1000 h. This study not only provides insights into the synergistic passivation of BIZ but also provides a strategy for the application of BIZ derivatives in the photovoltaic field.

An improvement in un-Encapsulated perovskite solar cell’s environmental stability via introduction of an electrode interface layer

ABSTRACT Perovskite solar cells (PSCs) have shown power conversion efficiency (PCE) up to 25.5%,but they are still struggling with their poor environmental stability. We have improvedthe ecological stability of un-encapsulated PSCs via interface modification of the topmetal electrode. We used CH3NH3PbI3-xClx perovskite as the light absorber and Ag metal as the top electrode. The interfaceof Ag electrode was modified by introducing a thin layer of MoOx/Al before Ag. We observed that MoOx/Al interlayer before Ag imparts better stability to the devices than conventional Ag electrode only. We found that MoOx/Al interlayer impedes the diffusion of oxygen and metalelectrode intothe perovskite film and considerably halts the device degradation upon beingkept for about 350 h under laboratory ambient conditions (room temperature 25±2oC and humidity 40±5%). This work mainly highlights the role of interfaces in PSCs degradation, and will help to addressthe stability issues in PSCs.

Balancing Lattice Strain by Embedded Ionic Liquid for the Stabilization of Formamidinium-Based Perovskite Solar Cells

Formamidinium (FA)-based perovskites remained state-of-the-art in the field of perovskite solar cells (PSCs) owing to the exceptional absorption and carrier transport properties, while the transition from photoactive (α-) to photoinactive (δ-FAPbI3) phase is the impediment that causes performance degradation and thus limits the deployment of FA-based PSCs. The unfavorable phase transition originates from tensile strain in the FAPbI3 crystal lattice, which undergoes structural reorganization for lattice strain balancing. In this work, we found that the ionic liquid (IL) could be used as the strain coordinator to balance the lattice strain for stability improvement of FAPbI3 perovskite. We theoretically studied the electronic coupling between IL and FAPbI3 and unraveled the originality of the IL-induced compressive strain. The strain-relaxed α-FAPbI3 by IL showed robust stability against environmental factors, which can withstand ambient aging for 40 days without any phase transition or decomposition. Moreover, the strain-relaxed perovskite films showed a lower trap density and resulted in conversion efficiency improvement from 18.27 to 19.88%. Based on this novel strain engineering strategy, the unencapsulated PSCs maintained 90% of their initial efficiency under ambient-air aging for 50 days.

Passivation of positively charged cationic defects in perovskite with nitrogen-donor crown ether enabling efficient perovskite solar cells

Positively charged cationic defects are the main source of defects in metal-halide perovskite solar cells. They determine the quasi-Fermi level of electrons under illumination and contribute to non-radiative recombination loss, causing an open-circuit voltage deficit. In addition, they act as ion migration pathways for halide hopping, thus deteriorating long-term stability. Herein, a nitrogen-donor crown ether as a positive defect passivator is developed, which demonstrates a soft Lewis base nature, a large donor number and a higher formation constant with positively charged cationic defects. Density functional theory calculation indicates that the electron-donating nitrogen atom dramatically increases the electron density of oxygen atoms, leading to a strong affinity with positively charged cationic defects (Pb2+ and Cs+). The electron trap density in perovskite is significantly reduced by 27 %, resulting in an increased build-in potential. By adding a small amount of nitrogen-donor crown ether to the precursor solution, the perovskite solar cells achieve an efficiency of 24.07 % with an open-circuit voltage of 1.174 V and a fill factor of 82.15 %. Moreover, the unencapsulated perovskite solar cells show a T80 lifetime of 510 h under continuous operation (1 sun equivalent illumination, maximum power point tracking condition, dry N2 atmosphere), and enhanced moisture and heat stability.

High efficiency stable planar perovskite solar cells via heavy water additive

Although great advances have been made in low-cost manufacturing of air-processed perovskite solar cells (PSCs), the moisture degradation of halide perovskite films remains a key problem to be urgently solved. High-quality perovskite films with large grains and uniform coverage are essential to enhance the performance of PSCs with high efficiency and moisture stability. Herein, heavy water (D2O) is employed as an additive in PbI2 precursor to manipulate the average grain size and enhance surface coverage of perovskite film, resulting in improved humidity stability of PSCs under ambient conditions. It is found that the introduction of D2O effectively improves the quality of perovskite film and significantly reduces the grain boundaries for reduced carrier recombination. The optimal concentration of D2O can boost the in-plane growth of perovskite films and increase the average grain size of perovskite from 494 nm to 717 nm. Consequently, the PSCs based on the perovskite film with 1% D2O exhibits a superior PCE of 21.66% (0.055 cm2) in small area, which is 9.78% higher than that of the control device (19.73%). In addition, the large-area (1 cm2) PSCs with 1% D2O show an efficiency of 20.02%. The unencapsulated PSCs made with 1% D2O can remain 95% of its initial PCE after aging 60 days in ambient air under dark with 20–25% relative humidity.

Tert‑butyl peroxybenzoate-doped spiro-OMeTAD for perovskite solar cells with efficiency over 23%

Perovskite solar cells (PSCs) have made great strides as new generation photovoltaic technology. The hole transport materials (HTMs) play an indispensable part in high performing PSCs. Spiro-OMeTAD (Spiro) is the most frequently used HTM. However, pristine Spiro has low conductivity and poor hole mobility. Additives engineering is considered as an effective strategy to solve the dilemma. Here, an organic peroxide additive tert-butyl peroxybenzoate (TBPB) is introduced into Spiro. TBPB not only promotes the rapid oxidation of Spiro to generate more carriers, but also tunes the HOMO of Spiro to achieve proper energy level matching with perovskite layer. Consequently, the TBPB modified device achieves a power conversion efficiency (PCE) of 23.45% with high fill factor (FF) of 81.97%. Meanwhile, the modification of TBPB improves the hysteresis effect, reproducibility and environmental stability of the device. After 80 days of storage in ambient air (RH 26%), the TBPB-doped unencapsulated PSCs still maintained 88.8% of their initial efficiency.

Improving the performance and stability of large-area carbon-based perovskite solar cells using N, O co-doped biomass porous carbon

Recently, perovskite solar cells (PSCs) have attracted widespread attention due to their outstanding advantages. Hole-transport-material-free carbon-based PSCs (C-PSCs) are some of the most potential devices. However, intrinsically inferior contact at perovskite/carbon interfaces and low conductivity of carbon electrode are restricting the commercial application of C-PSCs. In this study, an N, O co-doped biomass porous composite carbon electrode based on KOH-activated soybean dregs carbon, conductive carbon black and polymethylmethacrylate—N-KSDC—is prepared via a spraying method to efficiently improve perovskite/carbon interface qualities as well as enhance the efficiency and stability of C-PSCs. The results showed that the best power conversion efficiencies of N-KSDC-based C-PSCs with an active area of 0.08 and 1 cm2 were 13.45% and 11.08%, respectively. In addition, unencapsulated PSCs retained 92% of their initial power conversion efficiencies under ambient conditions without intentionally controlling humidity for 62 days. In this study, we propose a feasible and effective strategy to improve the performance and stability of perovskite/carbon interfaces in C-PSCs.

Sequential vacuum-evaporated perovskite solar cells with more than 24% efficiency.

Vacuum evaporation is promising for the high-throughput fabrication of perovskite solar cells (PSCs) because of its solvent-free characteristic, precise control of film thickness, and compatibility with large-scale production. Nevertheless, the power conversion efficiency (PCE) of PSCs fabricated by vacuum evaporation lags behind that of solution-processed PSCs. Here, we report a Cl-containing alloy-mediated sequential vacuum evaporation approach to fabricate perovskite films. The presence of Cl in the alloy facilitates organic ammonium halide diffusion and the subsequent perovskite conversion reaction, leading to homogeneous pinhole-free perovskite films with few defects. The resulting PSCs yield a PCE of 24.42%, 23.44% (certified 22.6%), and 19.87% for 0.1, 1.0, and 14.4 square centimeters (mini-module, aperture area), respectively. The unencapsulated PSCs show good stability with negligible decline in performance after storage in dry air for more than 4000 hours. Our method provides a reproducible approach for scalable fabrication of large-area, high-efficiency PSCs and other perovskite-based optoelectronics.

Back-Contact Ionic Compound Engineering Boosting the Efficiency and Stability of Blade-Coated Perovskite Solar Cells.

Surface defect passivation, which plays a vital role in achieving high-efficiency perovskite solar cells (PSCs) in a spin-coating process, is rarely compatible with a printing process. Currently, printing PSCs with high efficiency remains a challenge, as only a few laboratories realized an efficiency of over 20%. In this work, zwitterionic compounds 2-hydroxyethyl trimethyl ammonium chloride (HETACl) and butyltrimethylammonium chloride (BTACl) were introduced, both of which can spontaneously adsorb on the surface perovskite and form an ultrathin passivation layer by a dip coating method. The complex formed by the strong interaction of HETACl with MAI on the surface of the perovskite film leads to the formation of a rough perovskite surface, which affects the enhancement of device performance. BTACl with a chemically inert side chain induces a weak interaction with the perovskite. It is demonstrated that BTACl not only passivates surface defects of the perovskite but also heals the grain boundaries and results in more uniform crystallizations. Finally, PSCs upon BTACl treatment were blade-coated in an ambient environment with a relative humidity of <50%, which produced a champion efficiency of 20.5% with negligible hysteresis, and the active area of the cell device was 0.095 cm2. After being stored in air for 30 days, unencapsulated PSCs treated with BTACl retained 95% of their initial efficiency, which is far superior to that of the control and those treated with HETACl.

Evaporated Undoped Spiro‐OMeTAD Enables Stable Perovskite Solar Cells Exceeding 20% Efficiency

AbstractThermal evaporation (TE) as a scalable and low‐cost technique for fabrication of organic hole transport materials (HTMs) typically produces low photovoltaic performance and poor device reproducibility in the application of perovskite solar cells (PSCs), and there is a clear need to understand the weaknesses of TE. Here, a versatile manufacturing technology, solvent‐annealing assisted thermal evaporation (SATE), enabling effective modulation of organic film morphology as well as optoelectronic properties, is introduced. The SATE method produces undoped spiro‐OMeTAD layers with high density, good film homogeneity, enhanced conductivity, and remarkable film stability, all of which are superior to that made by conventional TE. In addition, SATE films eliminate the dopant induced degradation mechanism and simultaneously improve the electrical conductivity of undoped HTMs. Significantly, the resulting devices yield a 36% enhancement of power conversion efficiency (PCE) from 14.68% (TE) to 20.02% (SATE), which is the highest reported PCE for evaporated HTMs in n–i–p PSCs. Moreover, unencapsulated PSC devices with SATE demonstrate an impressive environmental and thermal stability by maintaining 85% of initial performance after 2500 h in air with 30% humidity. The high efficiency with simultaneously improved stability demonstrates SATE can be generally applicable to controllable fabrication of organic thin film and reliable devices.

Azadipyrromethene Dye-Assisted Defect Passivation for Efficient and Stable Perovskite Solar Cells.

Organic-inorganic perovskite solar cells (PSCs) provide one of the most outstanding photovoltaic (PV) technologies, yet their efficiency, stability, and defect passivation engineering still remain challenging. We demonstrate the use of low-cost, eco-friendly, and multi-functional aza-dipyrromethene (Aza-DIPY) dye molecules to promote the power conversion efficiency (PCE) and the operating stability of PSC devices. The Aza-DIPY dye was meticulously synthesized and incorporated into PSC devices via a one-step solution processing approach. The pyrrole, benzene ring, and chlorine functional groups on the dye have intense interactions with perovskite to passivate surface defects and obtain high-quality perovskite absorbers, resulting in the lengthened carrier recombination time and enhanced fill factor of PSCs. Additionally, the hydrophobic phenyl and halogen functional groups on the Aza-DIPY perform as a protecting barrier against moisture and ameliorate the stability of PSCs. As a consequence, the PV performance of PSCs is considerably improved, with the average PCE increased from 16.71% to 19.71%, and the champion device with Aza-DIPY shows a PCE of 20.46%. The unencapsulated PSC devices with multi-functional molecular Aza-DIPY maintains 89.06% of their beginning PCEs after storage in ambient air (25-30 °C, 50-70% relative humidity) under dark conditions for 100 h, exhibiting a significantly enhanced ambient stability compared with the case of the reference cells without the dye. Furthermore, the Aza-DIPY-modified PSC devices exhibit strong and reversible photoresponses, with a high responsivity of 0.739 mA/W to near-infrared (NIR) laser beams. Our results highlight the potential of synthesizing multi-functional Aza-DIPY dyes-incorporated PSC devices with sensitive NIR/visible light responses, high PV efficiency, and stability.

Highly improved efficiency and stability of planar perovskite solar cells via bifunctional phytic acid dipotassium anchored SnO2 electron transport layer

The defects at the interface of SnO2 electron transport layers (ETLs) /perovskite layer hinder extraction and transfer of charge, which restrict the further improvement of the photoelectric conversion efficiency (PCE) of SnO2-based perovskite solar cells (PSCs). Herein, an effective bifunctional anchoring strategy is developed to passivate SnO2/perovskite interface defects by adequate chelate sites in polydentate phytic acid dipotassium (PAD) to anchor SnO2 colloids. It is confirmed that polydentate PAD anchoring can form new Sn-O-P bonds with uncoordinated Sn2+, which can efficiently passivate the SnO2 surface defects and alleviate the interfacial charge recombination between SnO2 and perovskite layer. The reduction of SnO2 surface defects enhances the electron transport efficiency and increases the conductivity of the PAD-SnO2 ETLs to 2.1 times that of SnO2 ETLs. Meanwhile, the anchoring group potassium ions (K+) in PAD can promote the in-plane growth of perovskite grains and increase the average grain size of perovskite from 560 nm to 850 nm. As a consequence, the PCE of the PAD-SnO2-based PSCs increased sharply to 21.61%, which is 10.7% higher than that of the PSCs made with SnO2 ETLs (19.52%). The unencapsulated PSCs deposited on PAD-SnO2 ETLs can remain 99.5% of their initial efficiency after 60 days under 25–30% humidity.

Unveiling Charge Carrier Recombination, Extraction, and Hot-Carrier Dynamics in Indium Incorporated Highly Efficient and Stable Perovskite Solar Cells.

Perovskite solar cells (PSCs) have been propelled into the limelight over the past decade due to the rapid‐growing power conversion efficiency (PCE). However, the internal defects and the interfacial energy level mismatch are detrimental to the device performance and stability. In this study, it is demonstrated that a small amount of indium (In3+) ions in mixed cation and halide perovskites can effectively passivate the defects, improve the energy‐level alignment, and reduce the exciton binding energy. Additionally, it is confirmed that In3+ ions can significantly elevate the initial carrier temperature, slow down the hot‐carrier cooling rate, and reduce the heat loss before carrier extraction. The device with 1.5% of incorporated In3+ achieves a PCE of 22.4% with a negligible hysteresis, which is significantly higher than that of undoped PSCs (20.3%). In addition, the unencapsulated PSCs achieve long‐term stability, which retain 85% of the original PCE after 3,000 h of aging in dry air. The obtained results demonstrate and promote the development of practical, highly efficient, and stable hot‐carrier‐enhanced PSCs.

Importance of tin (II) acetate additives in sequential deposited fabrication of Sn-Pb-based perovskite solar cells

Sn-Pb-based perovskite is a promising solar cell material for its low toxicity and narrow band gap. However, the poor crystallinity of Sn-containing perovskite films and the facile oxidation of Sn2+ are obstacles to the efficiency and stability of Sn-Pb-based perovskite solar cells (PSCs). Herein, we developed a new additive strategy of tin acetate (SnAc2) to fabricate high-quality (HC(NH2)2)n(CH3NH3)1-nPb0.7Sn0.3IxBr3-x films by a two-step method and simultaneously restrict the oxidation of Sn2+ to Sn4+ by coordination between Sn2+ and Ac-. Moreover, SnAc2 built an Sn-rich environment to alleviate the intrinsic Sn2+ vacancies. All these effectively reduced the defect state densities and carrier nonradiative recombinations in the device, resulting in improvements in the performance of PSCs. The PSCs with the SnAc2 additive exhibited a increase in average power conversion efficiency (PCE) from 15.60% to 16.69%. Furthermore, the phenethylammoniumiodide (PEAI) post-treatment was introduced to passivate the SnAc2-containing perovskite films. Finally, the optimal cell with a PCE of 17.93% was obtained, which was second-best compared to the reported Sn-Pb-based PSCs by the two-step methods. Furthermore, the producibility and stability of devices are also improved due to the inhibited oxidation of Sn2+ by SnAc2 additives and the protection of two-dimensional (2D) (PEA)2PbI4 perovskites, the unencapsulated PSC maintained 80.4% of its initial efficiency when stored in air after 24 days.

Prolonged Stability of Unencapsulated Perovskite Solar Cells Comprising Molybdenum Disulphide Hybrid Hole Transport Layer

Prolonged Stability of Unencapsulated Perovskite Solar Cells Comprising Molybdenum Disulphide Hybrid Hole Transport Layer

Molecular Engineering of Thienyl Functionalized Ullazines as Hole‐Transporting Materials for Perovskite Solar Cells

Organic hole‐transporting materials (HTMs) based on the Ullazine core yield so far only moderate power conversion efficiencies of up to 13.08% in perovskite solar cells (PSCs). Aiming to fabricate efficient and stable PSCs, novel Ullazine derivatives bearing thiophene units were designed and synthesized, allowing modulation of the electronic states of the HTMs and further providing defect passivation of the perovskite surface. Experimental and theoretical analysis show that thiophene units with ‐N(p‐MeOC6H4)2 groups improve the conductivity of Ullazine HTMs, boosting the efficiency of PSCs to 20.21%. This value is the highest reported to date for Ullazine‐based HTMs, and is close to the performance of Spiro‐OMeTAD. In addition, unencapsulated PSCs based on the champion Ullazine exhibit superior stability with respect to Spiro‐OMeTAD, retaining nearly 90% of the initial efficiency following 1000 h aging, which is ascribed to a combination of higher water repellency and passivation of defects on the perovskite surface. This work demonstrates the high potential of HTMs based on Ullazine core as substitutes to Spiro‐OMeTAD.