Photovoltaic Performance Stability Research Articles

Julolidine functionalized benzimidazoline‐doped fullerene derivatives for efficient and stable perovskite solar cells

AbstractFullerene derivatives are highly attractive materials in solar cells, organic thermoelectrics, and other devices. However, the intrinsic low electron mobility and electrical conductivity restrict their potential device performance, such as perovskite solar cells (PSCs). Herein, we successfully enhanced the electric properties and morphology of phenyl‐C61‐butyric acid methyl ester (PCBM) by n‐doping it with a benzimidazoline derivative, 9‐(1,3‐dimethyl‐2,3‐dihydro‐1H‐benzoimidazol‐2‐yl)‐julolidine (JLBI‐H) via a solution process. We found the n‐doping can not only improve the conductivity and optimize the band alignment but also enable the PCBM to have a constantly strong charge extraction ability in a wide temperature from 173 to 373 K, which guarantees a stable photovoltaic performance of the corresponding PSCs under a wide range of operating temperatures. With the JLBI‐H‐doped PCBM, we improved the efficiency from 17.9% to 19.8%, along with enhanced stability of the nonencapsulated devices following the aging protocol of ISOS‐D‐1.

Hydrophobic poly-TPD modified PEDOT PSS surface for improved and stable photovoltaic performance of MAPbI3 based p-i-n perovskite solar cells

Inferior morphology of perovskite films and suppressed hole extraction restricts the performance of perovskite solar cells (PSCs) with a PEDOT:PSS hole transporting layer (HTL). In this work, poly-TPD is used to modify the surface of PEDOT:PSS films in PSC. The presence of hydrophobic poly-TPD decreases the nucleation sites, and as a result, perovskite films with larger grains are obtained. Improved energy level alignment in the presence of poly-TPD results in enhanced hole extraction from the perovskite layer to the HTL. The improved morphology and charge extraction resulted in improved photovoltaic performance. The power conversion efficiency (PCE) of PSCs was increased from 13.6% to 16.1% with the incorporation of poly-TPD. Also, the shelf life of the PSCs has exhibited considerable improvement due to the presence of hydrophobic poly-TPD and fewer number of grain boundaries. After 66 days, the PSC with poly-TPD maintained 96% of its initial PCE, whereas the PCE of the control device degraded to 72% of its initial value.

Photovoltaic effect in paraelectric BiVO4 film

Photovoltaic effects of single-layer films, with simple structure and low cost, are generally found in ferroelectric films, but their applications in solar cell are restricted by the large band gap of ferroelectrics. Here, we demonstrate a large photovoltaic effect can be obtained in paraelectric single-layer BiVO4 (BVO) film based on the gradient distribution of charges. The circuit current density (JSC) and open circuit voltage (VOC) of the BVO film with best performance are 1.63 mA/cm2 and 0.38 V, respectively. The JSC of BVO film kept 86% of the maximum value after continuous operation for 16 h. The high thermal stability of BVO is proved by long time operation and stable photovoltaic performance at high temperature. This work not only proposes a novel built-in electric field mechanism, but also indicates that the single-layer paraelectrics can be used as the next generation photovoltaic materials.

Combined Experimental and Simulation Studies of Lithium and Cobalt‐Modified TiO2 and Their Impacts on the Performance and Stability of Perovskite Solar Cells (Adv. Mater. Interfaces 31/2022)

Photovoltaic Performance Stability The most fundamental properties of photovoltaic materials are impacted by the photoinduced charge separation behavior, which can be improved by modifying the underlying layer that the perovskite is prepared on top. In article number 2201632, Non Thongprong, Nopporn Rujisamphan, and colleagues investigate the impacts of using alkali salts on porous TiO2 from experimental and computational points of view to provide a better understanding of such surface passivation.

Combined Experimental and Simulation Studies of Lithium and Cobalt‐Modified TiO2 and Their Impacts on the Performance and Stability of Perovskite Solar Cells

AbstractPosttreatment of titanium oxide (TiO2) using lithium (Li) and cobalt (Co) precursors is widely adopted to modify the charge quenching property in perovskite solar cells (PSCs); however, the fundamental understanding of the effect of the modification layer on the material itself and, consequently, the photovoltaic performance stability is not complete. In this work, in situ X‐ray diffraction measurements show that the Li and Co ions can diffuse into TiO2 and consequently accelerate the rutile phase transformation. X‐ray photoelectron spectroscopy results reveal the appearance of a Ti3+ feature in both the Li‐ and Co‐treated samples, suggesting that the treatment ions are partially located at the subsurface/surface of the spin‐cast TiO2 layer. The Li‐treated TiO2 exhibits greatly upshifted conduction band edges, which benefits charge extraction properties and improves the average device parameters in a complete PSC. To complement the experiments, density functional theory calculations are performed. While Li treatment initially results in enhanced electronic properties, Li‐treated TiO2 tends to have more surface vacancies over time and is more susceptible to adsorption and accumulation of iodide ions compared to the Co‐treated sample, which is experimentally supported by surface photovoltage spectroscopy and time‐resolved photoluminescence results.

(Digital Presentation) Stable Organic Passivated Carbon Nanotube-Silicon Solar Cells with an Efficiency of 22%

The organic passivated CNT/Si solar cell is a new type of low-cost, high-efficiency solar cell, with challenges concerning the stability of the organic layer used for passivation. In this work, the stability of the organic layer is studied with respect to the internal and external (humidity) water content and additionally long-term stability for low moisture environments. It is found that the organic passivated CNT/Si complex interface is not stable, despite both the organic passivation layer and CNTs being stable on their own and is due to the CNTs providing an additional path for water molecules to the interface. With the use of a simple encapsulation, a record power conversion efficiency (PCE) of 22% is achieved and a stable photovoltaic performance is demonstrated. This work provides a new direction for the development of high-performance/lost-cost photovoltaics in the future and will stimulate the use of nanotubes materials for solar cells applications.

Regioregularity effects of p-type P3CT-Na polymers on inverted perovskite photovoltaic cells

X-ray diffraction patterns, Atomic-force microscopic images, water-droplet contact angle images, absorbance spectra, photoluminescence (PL) quenching experiments and nanosecond time-resolved PL curves were used to investigate the formation of CH3NH3PbI3 (MAPbI3) crystalline thin films on top of poly[3-(4-carboxybutyl)thiophene-2,5-diyl]-Na (P3CT-Na) with the different regioregularities. The experimental results show that the higher regioregularity P3TC-Na can result in the larger MAPbI3 grains along the (110) direction of tetragonal perovskite and thereby increasing the averaged power conversion efficiency (PCE) of the resultant perovskite photovoltaic cells from 14.13% to 16.81% mainly due to the reduced potential loss in the MAPbI3 crystalline thin film. Besides, a remarkable fill factor of 80.9% was achieved, which can be used to explain the stable photovoltaic performance of the encapsulated cells.

Improvement of interfacial contact for efficient PCBM/MAPbI3 planar heterojunction solar cells with a binary antisolvent mixture treatment

Atomic-force microscopic images, x-ray diffraction patterns, Urbach energies and photoluminescence quenching experiments show that the interfacial contact quality between the hydrophobic [6,6]-phenyl-C61-buttric acid methyl ester (PCBM) thin film and hydrophilic CH3NH3PbI3 (MAPbI3) thin film can be effectively improved by using a binary antisolvent mixture (toluene:dichloromethane or chlorobenzene:dichloromethane) in the anti-solvent mixture-mediated nucleation process, which increases the averaged power conversion efficiency of the resultant PEDOT:PSS (P3CT-Na) thin film based MAPbI3 solar cells from 13.18% (18.52%) to 13.80% (19.55%). Beside, the use of 10% dichloromethane (DCM) in the binary antisolvent mixture results in a nano-textured MAPbI3 thin film with multicrystalline micrometer-sized grains and thereby increasing the short-circuit current density and fill factor (FF) of the resultant solar cells. It is noted that a remarkable FF of 80.33% is achieved, which can be used to explain the stable photovoltaic performance without additional encapsulations.

Stable Organic Passivated Carbon Nanotube-Silicon Solar Cells with an Efficiency of 22.

The organic passivated carbon nanotube (CNT)/silicon (Si) solar cell is a new type of low‐cost, high‐efficiency solar cell, with challenges concerning the stability of the organic layer used for passivation. In this work, the stability of the organic layer is studied with respect to the internal and external (humidity) water content and additionally long‐term stability for low moisture environments. It is found that the organic passivated CNT/Si complex interface is not stable, despite both the organic passivation layer and CNTs being stable on their own and is due to the CNTs providing an additional path for water molecules to the interface. With the use of a simple encapsulation, a record power conversion efficiency of 22% is achieved and a stable photovoltaic performance is demonstrated. This work provides a new direction for the development of high‐performance/low‐cost photovoltaics in the future and will stimulate the use of nanotubes materials for solar cells applications.

Heterovalent Substitution in Mixed Halide Perovskite Quantum Dots for Improved and Stable Photovoltaic Performance

One of the strategies to improve the device performance of metal-halide perovskite solar cells is to alter the electronic and optical properties of the perovskite lattice by metal ion doping. In this work, aliovalent doping of silver at the lead site of CsPbBr1.5I1.5 quantum dots (QDs) shows striking prospects in improving the photovoltaic (PV) device performance. Lattice doping could be achieved only up to ∼3.5 atom % Ag+ with respect to Pb2+ which has significant impact on the electronic and optical properties of the QDs. The band gap could be reduced from 2.15 eV for pristine QDs to 2.12 eV with 3.5 atom % Ag-doping, beyond which midband gap states are created along with the formation of a secondary AgI phase with higher Ag+ substitution. Density functional theory (DFT) shows enhanced optical absorption coefficient and p-type character of the Ag-doped QDs. With the QDs having 3.5 atom % Ag, a significant ∼20% enhancement in photoconversion efficiency (PCE) is observed up to 9.67 ± 0.12% due to the reduction of surface and intrinsic defects, decrease in nonradiative recombination leading to an increase in carrier lifetime, and increase in charge transfer. Although in general all-inorganic perovskite (AIPSK) QDs are attractive because of their higher crystallinity, better optical property, and easy solution processability, the solution stability of AIPSK QDs is not the same as that in PV devices. After 3.5 atom % Ag-doping, our PV devices show extremely promising ambient stability for 575 h (24 days) with less than 5% drop in PCE in comparison to 12% drop for pristine QD devices.

Suppressing Co‐Crystallization of Halogenated Non‐Fullerene Acceptors for Thermally Stable Ternary Solar Cells

AbstractWhile photovoltaic blends based on non‐fullerene acceptors are touted for their thermal stability, this type of acceptor tends to crystallize, which can result in a gradual decrease in photovoltaic performance and affects the reproducibility of the devices. Two halogenated indacenodithienothiophene‐based acceptors that readily co‐crystallize upon mixing are studied, which indicates that the use of an acceptor mixture alone does not guarantee the formation of a disordered mixture. The addition of the donor polymer to the acceptor mixture readily suppresses the crystallization, which results in a fine‐grained ternary blend with nanometer‐sized domains that do not coarsen due to a high Tg ≈ 200 °C. As a result, annealing at temperatures of up to 170 °C does not markedly affect the photovoltaic performance of ternary devices, in contrast to binary devices that suffer from acceptor crystallization in the active layer. The results indicate that the ternary approach enables the use of high‐temperature processing protocols, which are needed for upscaling and high‐throughput fabrication of organic solar cells. Further, ternary devices display a stable photovoltaic performance at 130 °C for at least 205 h, which indicates that the use of acceptor mixtures allows to fabricate devices with excellent thermal stability.

Incorporated Guanidinium Expands the CH3NH3PbI3 Lattice and Enhances Photovoltaic Performance.

Guanidinium (GA) has been widely used as an additive in solar cells for enhanced performance. However, the size of the guanidinium cation is too large to be incorporated in the cage of the perovskite structure. Instead, GA forms a variety of structures with lead iodide, where its role in the perovskite crystal as well as solar cell devices is unclear. In this study, we demonstrate that GA can be incorporated into the structure of MAPbI3 as (GA)x(MA)1-xPbI3. From single-crystal X-ray crystallographic refinement, we observe lattice expansion and Pb-I bond elongation with GA incorporation similar to exerting "negative pressure", which weakens orbital overlap and widens the band gap from 1.49 to 1.53 eV. We find that the highest percentage of GA that can be incorporated into the 3D MAPbI3 structure is 5.26%, as confirmed by nuclear magnetic resonance. The alloyed (GA)x(MA)1-xPbI3 exhibits increased PL lifetimes from 154.4 to 266.3 ns after GA incorporation while the Voc of (GA)x(MA)1-xPbI3 devices enlarges from 1.05 to 1.11 V. High efficiencies in solar cell devices up to 20.38% with a Jsc of 23.55 mA cm-2, Voc of 1.11 V, and FF of 0.78 have been achieved, with stable photovoltaic performance for 900 h in air.

An all-solid-state lamellar-nanostructured polymer electrolyte in-situ-prepared from smectic liquid crystal for thermally stable dye-sensitized solar cells

A novel all-solid-state polymer electrolyte based on thiolate/disulfide redox couple was in-situ prepared from a lamellar-assembled liquid crystal precursor for dye-sensitized solar cells (DSSCs). The use of the thiolate/disulfide redox couple allowed the in-situ polymerization, which ensured both preservation of the lamellar nanostructure in electrolyte and sufficient interfacial contact in DSSC. Electrochemical analysis revealed that the lamellar-nanostructured electrolyte possessed charge transport ability and photovoltaic performance quite close to the liquid crystal electrolyte precursor. The resultant all-solid-state DSSCs showed a thermally stable photovoltaic performance with PCE higher than 2% from 35 to 90 °C. The method in using in-situ polymerization of nanostructured liquid crystals to prepare solid-state electrolytes shed a new light for efficient and stable all-solid-state DSSC preparation, especially in large-scale actual application.

Semi-transparent perovskite solar cells with a cross-linked hole transport layer

Semi-transparent perovskite solar cells (ST-PeSCs) have received great attention because of their excellent performance and promising application in areas such as tandem devices and building integrated photovoltaics (BIPVs). Critical across all these applications is achieving both high efficiency and stable photovoltaic performance of such devices. Realizing both of these properties simultaneously has not been possible using device architectures featuring the archetypal doped Spiro-OMeTAD as a hole transport layer (HTL). As such, in this work we explore the use of a solution-processed cross-linked HTL formed from N4,N4′-di(naphthalen-1-yl)-N4,N4′-bis(4-vinylphenyl)biphenyl-4,4′-diamine (VNPB) molecules as an alternative to the conventional Spiro-OMeTAD within an FTO/SnO2/C60-SAM/Perovskite/HTL/MoOx/ultra-thin gold/MoOx ST-PeSC device architecture. Through an optimized multi-step thermal treatment process that maximizes charge extraction and reduces recombination from these devices, we can achieve ST-PeSCs that exhibit record power conversion efficiencies for Spiro-OMeTAD-free devices with average visible transmittance values between 10 and 30%. These devices exhibit comparable efficiencies to their Spiro-OMeTAD counterparts, with the additional benefit that the use of the poly-VNPB as the HTL material provides significant improvements in long-term device stability under both continuous illumination and high humidity conditions.

Electronic Traps and Their Correlations to Perovskite Solar Cell Performance via Compositional and Thermal Annealing Controls.

Herein, underlying factors for enabling efficient and stable performance of perovskite solar cells are studied through nanostructural controls of organic-inorganic halide perovskites. Namely, MAPbI3, (FA0.83MA0.17)Pb(I0.83Br0.17)3, and (Cs0.10FA0.75MA0.15)Pb(I0.85Br0.15)3 perovskites (abbreviated as MA, FAMA, and CsFAMA, respectively) are examined with a grain growth control through thermal annealing. FAMA- and CsFAMA-based cells result in stable photovoltaic performance, while MA cells are sensitively dependent on the perovskite grain size dominated by annealing time. Micro-/nanoscopic features are comprehensively analyzed to unravel the origin that is directly correlated to the cell performance with the applications of electronic-trap characterizations such as photoconductive noise microscopy and capacitance analyses. It is revealed that CsFAMA has a lower trap density compared to MA and FAMA through the analyses of 1/ f noises and trapping/detrapping capacitances. Also, an open-circuit voltage ( Voc) change is correlated to the variation of trap states during the shelf-life test: FAMA and CsFAMA cells with the negligible change of Voc over weeks exhibit trap states shifting toward the band edge, although the power-conversion efficiencies are clearly reduced. The origins that critically affect the solar cell performance through the characterizations of shallow/deep traps with additional mobile defects in the perovskite and interfaces are discussed.

Nanostructured NiOx as hole transport material for low temperature processed stable perovskite solar cells

In this letter, we report on nanostructured NiOx with particle size ∼85 nm as an efficient hole transport material (HTM) for low temperature processed inverted planar perovskite solar cell (PSC) with power conversion efficiency of 15.64%. The homogenous NiOx HTM layer facilitated the growth of high quality CH3NH3PbI3 absorber with large crystal grains and the resulting PSC showed stable photovoltaic performance under continuous illumination.

Shift Happens. How Halide Ion Defects Influence Photoinduced Segregation in Mixed Halide Perovskites

Minimizing photoinduced segregation in mixed halide lead perovskites is important for achieving stable photovoltaic performance. The shift in the absorption and the rate of formation of iodide- and bromide-rich regions following visible excitation of mixed halide lead perovskites is found to strongly depend on the halide ion concentration. Slower formation and recovery rates observed in halide-deficient films indicate the involvement of defect sites in influencing halide phase segregation. At higher halide concentrations (in stoichiometric excess), segregation effects become less prominent, as evidenced by faster recovery kinetics. These results suggest that light-induced compositional segregation can be minimized in mixed halide perovskite films by using excess halide ions. The findings from this study further reflect the importance of halide ion post-treatment of perovskite films to improve their solar cell performance.

Stabilized Wide Bandgap Perovskite Solar Cells by Tin Substitution.

Wide bandgap MAPb(I1-yBry)3 perovskites show promising potential for application in tandem solar cells. However, unstable photovoltaic performance caused by phase segregation has been observed under illumination when y is above 0.2. Herein, we successfully demonstrate stabilization of the I/Br phase by partially replacing Pb2+ with Sn2+ and verify this stabilization with X-ray diffractometry and transient absorption spectroscopy. The resulting MAPb0.75Sn0.25(I1-yBry)3 perovskite solar cells show stable photovoltaic performance under continuous illumination. Among these cells, the one based on MAPb0.75Sn0.25(I0.4Br0.6)3 perovskite shows the highest efficiency of 12.59% with a bandgap of 1.73 eV, which make it a promising wide bandgap candidate for application in tandem solar cells. The engineering of internal bonding environment by partial Sn substitution is believed to be the main reason for making MAPb0.75Sn0.25(I1-yBry)3 perovskite less vulnerable to phase segregation during the photostriction under illumination. Therefore, this study establishes composition engineering of the metal site as a promising strategy to impart phase stability in hybrid perovskites under illumination.

Flexible Crystalline Silicon Solar Cell with Vertically Aligned Microwire Arrays

Much attention has been paid to thin crystalline silicon (c-Si) due to excellent flexible characteristics, stable performance, and possibility of high efficiency as a means for developing next generational flexible solar cells. Here, we confirm that the 50 µm thin c-Si with various length of microwire arrays (MWA) shows stable flexible characteristics regardless of MWA length. In our result, the fabricated flexible solar cells using 50 µm thin c-Si with 20 µm MWA exhibit power conversion efficiencies of up to 15.2% under AM 1.5G illumination with an open-circuit voltage of 576 mV and a short-circuit current density of 34.1 mAcm-2. Furthermore, to overcome the light reflection of flat-top-area of MWA, tapered MWA that can reduce surface reflection to around 1% is fabricated by simple wet-etching process. Applying the 20 µm tapered MWA on 50 µm thin c-Si, our flexible solar cell shows maximum efficiency of 16.8% and stable photovoltaic performance during bending tests up to 1000 times at the bending radius of 10 mm. Therefore, we believe that our proposed flexible solar cell can be a next-generation solar energy harvesting devices through high efficiency and stable flexibility.

Pyrite‐Based Bi‐Functional Layer for Long‐Term Stability and High‐Performance of Organo‐Lead Halide Perovskite Solar Cells

Organo‐lead halide perovskite solar cells (PSCs) have received great attention because of their optimized optical and electrical properties for solar cell applications. Recently, a dramatic increase in the photovoltaic performance of PSCs with organic hole transport materials (HTMs) has been reported. However, as of now, future commercialization can be hampered because the stability of PSCs with organic HTM has not been guaranteed for long periods under conventional working conditions, including moist conditions. Furthermore, conventional organic HTMs are normally expensive because material synthesis and purification are complicated. It is herein reported, for the first time, octadecylamine‐capped pyrite nanoparticles (ODA‐FeS2 NPs) as a bi‐functional layer (charge extraction layer and moisture‐proof layer) for organo‐lead halide PSCs. FeS2 is a promising candidate for the HTM of PSCs because of its high conductivity and suitable energy levels for hole extraction. A bi‐functional layer based on ODA‐FeS2 NPs shows excellent hole transport ability and moisture‐proof performance. Through this approach, the best‐performing device with ODA‐FeS2 NPs‐based bi‐functional layer shows a power conversion efficiency of 12.6% and maintains stable photovoltaic performance in 50% relative humidity for 1000 h. As a result, this study has the potential to break through the barriers for the commercialization of PSCs.