Chen Weijie

In polycrystalline perovskites, the nanoscale photoconduction measurements indicate that photocurrent collection along grain boundaries (GBs) is dependent on adjacent grains, exhibiting GB to GB heterogeneity. The photovoltage loss mainly originates from the shunting of GBs relative to the whole perovskite. This research reveals the heterogeneity of GBs and its influence on photovoltage, which is crucial for optimizing perovskite-based solar cells.

Abstract

In polycrystalline perovskites, grain boundaries (GBs) that isolate grains determine the optoelectronic properties of a semiconductor, and hence affect the photovoltaic performance of a solar cell. Photocurrent and photovoltage are affected by the microscopic structure of perovskites but are difficult to quantify on the intragrain length scale and are often treated as homogeneous within the photoactive layer. Here, the nanoscale through-film and lateral photoresponse of large-grained perovskite are studied by photoconductive atomic force microscopy. Photocurrent collection along GBs relies on the formation of adjacent grains, exhibiting GB to GB heterogeneity. Regarding to the spatially correlated heterogeneity, the photovoltage of grains deduced from the photoresponse curves at specific positions is larger than that of GBs by up to 0.4 V, suggesting that the photovoltage loss mainly originates from the shunting of GBs through the whole perovskite layer. These spatial heterogeneities are alleviated by depositing a capping layer onto the perovskite layer, highlighting the role of the inserted layer between the perovskite and electrode in real solar cells. This research reveals the heterogeneity of GBs and its influence on photovoltage that actually occurs in virtual solar cells, which is crucial for optimizing perovskite-based solar cells.

An aerosol-assisted crystallization method to prepare high-quality, pure α-FAPbI₃ films at only 100 °C without chemical additives is reported. Remarkably improved phase stability of the α-FAPbI₃ films and their applications in solar cells are demonstrated. The overriding mechanism of stabilizing α-FAPbI₃ is shown to be relaxation of residual tensile strains in the films.

Abstract

Formamidinium lead triiodide (FAPbI₃) is attractive for photovoltaic devices due to its optimal bandgap at around 1.45 eV and improved thermal stability compared with methylammonium-based perovskites. Crystallization of phase-pure α-FAPbI₃ conventionally requires high-temperature thermal annealing at 150 °C whilst the obtained α-FAPbI₃ is metastable at room temperature. Here, aerosol-assisted crystallization (AAC) is reported, which converts yellow δ-FAPbI₃ into black α-FAPbI₃ at only 100 °C using precursor solutions containing only lead iodide and formamidinium iodide with no chemical additives. The obtained α-FAPbI₃ exhibits remarkably enhanced stability compared to the 150 °C annealed counterparts, in combination with improvements in film crystallinity and photoluminescence yield. Using X-ray diffraction, X-ray scattering, and density functional theory simulation, it is identified that relaxation of residual tensile strains, achieved through the lower annealing temperature and post-crystallization crystal growth during AAC, is the key factor that facilitates the formation of phase-stable α-FAPbI₃. This overcomes the strain-induced lattice expansion that is known to cause the metastability of α-FAPbI₃. Accordingly, pure FAPbI₃ p–i–n solar cells are reported, facilitated by the low-temperature (≤100 °C) AAC processing, which demonstrates increases of both power conversion efficiency and operational stability compared to devices fabricated using 150 °C annealed films.

Polyacrylonitrile which has C≡N groups was introduced to passivate the uncoordinated lead cations in perovskite films. The coordination of C≡N with the lead cation was much stronger than that of the normally used C=O group. It could also reduce the I/Pb ratio at the film surface. The device efficiency was improved from 21.58 % to 23.71 %, with the open-circuit voltage enhanced from 1.12 V to 1.23 V.

Abstract

In solution-processed organic–inorganic halide perovskite films, halide-anion related defects including halide vacancies and interstitial defects can easily form at the surfaces and grain boundaries. The uncoordinated lead cations produce defect levels within the band gap, and the excess iodides disturb the interfacial carrier transport. Thus these defects lead to severe nonradiative recombination, hysteresis, and large energy loss in the device. Herein, polyacrylonitrile (PAN) was introduced to passivate the uncoordinated lead cations in the perovskite films. The coordinating ability of cyano group was found to be stronger than that of the normally used carbonyl groups, and the strong coordination could reduce the I/Pb ratio at the film surface. With the PAN perovskite film, the device efficiency improved from 21.58 % to 23.71 % and the open-circuit voltage from 1.12 V to 1.23 V, the ion migration activation energy increased, and operational stability improved.

Hydride vapor phase epitaxy (HPVE) is a promising growth technique for reducing the fabrication cost of III–V devices. The improvement in the solar cell performance by introducing AlInGaP back-surface field layers is reported. Consequently, 28.3% efficient InGaP/GaAs tandem solar cells, the highest conversion efficiency reported in cells grown via HPVE, is achieved.

Hydride vapor phase epitaxy (HVPE) is a III–V device fabrication technology that has received attention owing to its low production costs. The properties of passivation layers used to reduce surface and interface recombination losses in III–V materials considerably contribute to the performance of various devices. Herein, solar cells based on AlInGaP back-surface field (BSF) layers grown via HVPE using aluminum trichloride as the group-III precursor for Al deposition are presented. Although high-concentration Si contamination occurs in Al-containing layers grown using HVPE, AlInGaP with p-type conductivity can be grown by doping with high-concentration Zn. For InGaP single-junction solar cells, the short-circuit current density and open-circuit voltage are improved by introducing the AlInGaP BSF layer. Consequently, the InGaP single-junction solar cells measured under air mass 1.5 global solar spectrum illumination achieve a conversion efficiency of 17.1%. Furthermore, the progress in the development of tandem solar cells grown using HVPE is reported. By improving the performance of the InGaP top cells, InGaP/GaAs tandem cells are fabricated with a new record efficiency of 28.3% using the triple-chamber HVPE system.

Metal grids are reviewed based on their application in perovskite solar cells. The choices of materials, design, deposition technique, trade-off between electrical conductivity versus optical transmission, among other features, are discussed in conjunction with perovskite particularities. A patent analysis is also discussed, along with perspectives for the commercialization of large-area perovskite solar cells and modules.

Lab-scale perovskite solar cells have reached efficiencies as high as the best monocrystalline silicon cells, with expectations that their manufacturing costs could be lower than those of currently commercialized cells. As a result, efforts are now directed to the production of these cells. In this sense, the use of frontal metal grids in large-area perovskite cells and parallel-connected modules is imperative due to the low conductivity of the transparent conductive substrate usually employed in these devices. For the insertion of grids, a trade-off between reduced resistive losses and the increase in shadowed area must be achieved. Assessment of grid parameters is of utmost importance for the assembly of viable large-scale cells and modules. Furthermore, a direct transfer from grid parameters previously specified for other photovoltaic technologies might not be the best option for perovskite cells. Nonetheless, investigations of grids specifically designed for these cells are still scarce in the literature. Here, grid design, both in terms of metal composition and geometric factors, is discussed, including their development for tandem cells and flexible devices. To provide further insights into the maturity level of this technology, a patent analysis regarding grid designs for perovskite modules is also presented in this review.

A poly(methyl methacrylate) (PMMA) interlayer between the perovskite and CuPc improves the insufficient electron blocking due to the low LUMO energy level of CuPc. In addition, the PMMA layer strongly interacts with the perovskite, greatly reducing the defect density responsible for non-radiative recombination. Eventually, PMMA significantly improves the efficiency of PSCs fabricated using CuPc as a hole conductor.

Abstract

The use of inexpensive, highly efficient, and long-term stable hole-transporting layers (HTLs) while facilitating the fabrication process has become a critical issue for PSC commercialization. Among organic HTLs, copper phthalocyanine (CuPc) has been increasingly studied owing to its low cost and excellent thermal stability. Nevertheless, CuPc has a low energy level in the conduction band, resulting in low efficiency due to a poor electron barrier. In this study, an efficient and stable PSC is fabricated by combining CuPc with an ultrathin poly(methyl methacrylate) (PMMA) interlayer, which is deposited on a [(FAPbI₃)_0.95(MAPbBr₃)_0.05] absorption layer (here, FAPbI₃ and MAPbBr₃ denote formamidinium lead triiodide and methylammonium lead tribromide, respectively). PMMA in perovskite has been found to reduce perovskite surface defects and series resistance as well as the electronic barrier to HTL. The optimum concentration of PMMA allows for the fabrication of the PSC with a PCE of 21.3%, which is the highest PCE for PSCs featuring metal phthalocyanines as the HTL reported to date. The stability of the encapsulated PSC exceeds 80% after 760 h at 85 °C under 85% RH conditions.

Publication date: March 2022

Source: Nano Energy, Volume 93

Author(s): Jianhua Jing, Sheng Dong, Kai Zhang, Boming Xie, Jiabin Zhang, Yu Song, Fei Huang

Ternary organic solar cells are fabricated by rationally selecting and incorporating the perylenediimide (PDI)-based nonfullerene acceptor, PDI-diketopyrrolopyrrole-PDI, as the third component into a model solar cell and their 3D morphology is qualitatively and quantitatively explored at the nanoscale by developing the new use of advanced characterization tools in the field of photovoltaics.

Abstract

It is highly desired to develop advanced characterization techniques to explore the 3D nanoscale morphology of the complicated blend film of ternary organic solar cells (OSCs). Here, ternary OSCs are constructed by incorporating the nonfullerene acceptor perylenediimide (PDI)-diketopyrrolopyrrole (DPP)-PDI and their morphology is characterized in depth to understand the performance variation. In particular, photoinduced force microscopy (PiFM) coupled with infrared laser spectroscopy is conducted to qualitatively study the distribution of donor and acceptors in the blend film by chemical identification and to quantitatively probe the segmentation of domains and the domain size distribution after PDI-DPP-PDI acceptor incorporation by PiFM imaging and data processing. In addition, the energy-filtered transmission electron microscopy with energy loss spectra is utilized to visualize the nanoscale morphology of ultrathin cross-sections in the configuration of the real ternary device for the first time in the field of photovoltaics. These measurements allow to “view” the surface and cross-sectional morphology and provide strong evidence that the PDI-DPP-PDI acceptor can suppress the aggregation of the fullerene molecules and generate the homogenous morphology with a higher-level of the molecularly mixed phase, which can prevent the charge recombination and stabilize the morphology of photoactive layer.

The relationship between intermolecular electron–phonon coupling and trapping of electronic excitation behind kinetic aggregation of nonfullerene acceptors (NFAs) is revealed in multicomponent organic solar cells (OSCs). The synergistic regulation of donor and acceptor materials can prevent unfavorable aggregation of NFAs, maintaining stable intermolecular electron-phonon coupling to suppress the increase of trap depth and density for photocarrier trapping in quaternary OSCs.

Abstract

The kinetic aggregation of nonfullerene acceptors under nonequilibrium conditions can induce electron–phonon interaction roll-off and electronic band structure transition, which represents an important limitation for long-term operational stability of organic solar cells (OSCs). However, the fundamental underlying mechanisms have received limited attention. Herein, a photophysical correlation picture between intermolecular electron–phonon coupling and trapping of electronic excitation is proposed based on the different aggregation behaviors of BTP-eC9 in bulk-heterojunction and layer-by-layer processed multicomponent OSCs. Two separate factors rationalize their correlation mechanisms: 1) the local lattice and/or molecular deformation can be regarded as the results of BTP-eC9 aggregates in binary system under continuous heating, which brings about attenuated intermolecular electron–phonon coupling with intensified photocarrier trapping. 2) The higher density of trap states with more extended tails into the bandgap give rise to the formation of highly localized trapped polarons with a longer lifetime. The stabilized intermolecular electron–phonon coupling through synergistic regulation of donor and acceptor materials effectively suppresses unfavorable photocarrier trapping, delivering the improved device efficiency of 18.10% and enhanced thermal stability in quaternary OSCs. These results provide valuable property–function insights for further boosting photovoltaic stability in view of modulating intermolecular electron–phonon coupling.

Herein, a scalable coating approach for layer-by-layer deposition of the 3D/2D perovskite films with zero-wasting of the precursor materials is introduced, which achieves high efficiency and stability for large-area-cells (>19.5% with >1000 h T90 stability at 85 °C) and modules (>18.8% efficiency on 10 cm²) with >91% breakdown of the costs from spin coating to the blade–spin/blade coating approach.

Perovskite solar cells (PSCs) are a type of emerging thin-film photovoltaics, which show a promising potential to catch a major portion of the near future market. However, hazardous waste-generation and cost-management of the industrial production are two key issues which need to be addressed in the commercialization process. Herein, a universal coating strategy is introduced by combination of blade and spin coating to realize a zero-waste-generation of the perovskite precursor materials for layer-by-layer deposition of 3D/2D perovskite films on large-area substrates with high uniformity and reproducibility. This leads to eliminate 90% and 95% of the materials consumption compared to the conventional spin-coating for deposition of the 3D and 2D perovskite layers, respectively, without changing of the ink precursor. Large-area PSCs have been realized by blade–spin/blade coating approach reaching 19.55% photo-conversion efficiency (PCE) over 0.92 cm² masked area and >1000 h T90 thermal stability at 85 °C. Fabricated perovskite solar modules via blade–spin/blade deposition have been reached to 18.8% PCE over 10 cm² active-area. The economic analysis shows the potential for >90% decrease of the operational expenditure via break-down of the material costs from 1.99 € m⁻² in spin coating method to 0.18 € m⁻² in blade–spin/blade coating approach.

For the first time, a certified fill factor value of 79% is obtained for hole selective layer-free carbon-based perovskite solar devices. The influence of charge recombination and transport losses is assessed. An optimized saturated vapor crystallization in the mesoscopic stack allows for minimized losses, leading to a better interface formation between the perovskite and the adjacent interfaces.

Carbon-electrode-based perovskite solar cells (C-PSCs) are promising candidates for commercialization due to their high stability. However, the absence of a hole selective layer (HSL) often limits their performance, yielding low fill factors (FFs). Herein, a certified FF of 78.8% obtained through HSL-free C-PSCs is focused. This is found to approach the highest values reported for metal-electrode-based PSCs employing highly selective HSLs. The loss mechanisms affecting the FF in fully printed HSL-free C-PSCs are thus investigated. Methods commonly used to analyze and quantify the impact of recombination and transport losses on the FF of established photovoltage devices are assessed, and their applicability in C-PSCs is examined. In the improved devices, non-radiative recombination contributes to only 3%_abs loss with respect to the FF in the radiative limit, which is attributed to an optimal diode ideality factor approaching 1.0. Moreover, contributions of shunt resistive losses are determined to be negligible. Interfacial series resistance losses at the perovskite/carbon interface are identified as the main loss channel, highlighting the importance of the quality of the contact between the perovskite and the back-contact electrode.

A trifluoroethoxyl functionalized hole transport material, Spiro-4TFETAD, is designed and synthesized. Spiro-4TFETAD shows lower highest occupied molecular orbital level, improved hole mobility, conductivity, and hydrophobicity, as well as effectiveness in minimizing perovskite decomposition, compared to Spiro-OMeTAD. Spiro-4TFETAD-based perovskite solar cells exhibit a power conversion efficiency up to 21.11% with excellent stability, which are superior to those of Spiro-OMeTAD-based devices.

It is crucial to finely optimize the properties of hole transport materials (HTMs) to improve the performance and stability of perovskite solar cells (PSCs). Herein, a new spiro-based HTM (Spiro-4TFETAD) is developed by replacement of partial methoxy groups in Spiro-OMeTAD with trifluoroethoxy substituents. Spiro-4TFETAD has lower highest occupied molecular orbital level, higher thermal stability (T _g = 140 °C), hole mobility (2.04 × 10⁻⁴ cm²V⁻¹ s⁻¹), and better hydrophobicity with respect to Spiro-OMeTAD. The PSCs using Spiro-4TFETAD achieve a power conversion efficiency of 21.11% and excellent humidity resistance. It maintains an average 83% of their initial power conversion efficiency values even in high relative humidity of 60% without encapsulation and 82% of its initial performance after 100 h continuous illumination at the maximum power point. The superior performance underscores the promising potential of the trifluoroethoxyl molecular design in preparing new HTMs toward highly efficient and stable PSCs.

A room temperature solid-state ligand exchange method modified tantalum-doped tin oxide (Ta-SnO₂) with a functional ligand, enhancing the conductivity of Ta-SnO₂ and improving the band alignment with the electrode is reported. Furthermore, the efficiency of fullerene-free devices can achieve up to 15.48%. All-oxide devices can maintain 80% performance after 1000 and 540 h under damp heat and light soaking tests.

Although perovskite solar cells (PSCs) can be prepared with excellent optoelectronic properties and solution processability, the common use of [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as the electron-transporting layer (ETL), with its high cost and low stability, has hindered their commercialization. Metal oxides are inexpensive and highly chemically stable, making them potential alternatives to PCBM as ETLs, but requisite polar host solvents and high-temperature treatment limit the possibility of their direct deposition on perovskite layers. Herein, Ta-doped SnO₂ nanoparticles (NPs) are dispersed in a nonpolar solvent and they are also directly deposited to form a Ta-SnO₂ layer on a perovskite film. Then, room-temperature solid-state ligand exchange is applied to remove insulating molecules from the Ta-SnO₂ surface and thereby, enhance the band alignment between the Ta-SnO₂ layer and the Ag electrode. The highest power conversion efficiency of a PSC fabricated with Ta-SnO₂ as the ETL is 15.48%. In addition, the stability of the SnO₂-based devices toward damp heat and light soaking is superior to that of corresponding PCBM-based PSCs. Therefore, this effective strategy for incorporating metal oxides as ETLs appears to be an inexpensive method for manufacturing highly efficient PSCs with long-term stability.

A facile method is reported to passivate the SnO₂/perovskite interface and neutralize OH⁻ on the SnO₂ surface by modifying the SnO₂/pervoskite interface with phosphorus-containing Lewis acids. The power conversion efficiency of the optimal device is increased from 18.94% to 22.14%, along with negligible hysteresis. Meanwhile, the corresponding solar modules also achieved a champion efficiency of 15.69% with excellent stability under continuous illumination.

Expeditious charge transfer at the interfaces between photoactive and charge transport layers is critical in perovskite solar cells (PSCs). Defects on the surface of charge transport layers usually lead to the degradation of carrier mobility, resulting in low power conversion efficiency (PCE) and serious hysteresis. Herein, phosphorus-containing Lewis acids are applied to modify the SnO₂/perovskite interface and neutralize redundant OH⁻ on the SnO₂ surface. The interaction between the Lewis acids and SnO₂ significantly accelerate the electron transfer and greatly reduce the energy barrier at the SnO₂/perovskite interface, boosting the PCE of the PSCs. By modifying the SnO₂/pervoskite interface with diphenylphosphine oxide, the PCE of a small area device was increased from 18.94% to 22.14%, along with negligible hysteresis and improved stability. Moreover, the 5 × 5 cm² solar modules with an aperture area of 22.56 cm² achieved a best efficiency of 15.69%.

A successive surface treatment of alkali fluoride and organic ammonium salt for wide bandgap perovskite that synergistically passivates their surface defects is developed for highly efficient stacked tandem applications.

Abstract

Superior bandgap tunability enables solution-processed halide perovskite a promising candidate for multi-junction photovoltaics (PVs). Particularly, optically coupling wide-gap perovskite by stacking with commercially available PVs such as silicon and CIGS (also known as 4-terminal tandem) simplifies the technology transfer process, and further advances the commercialization potential of perovskite technology. However, compared with matured PV materials and the phase-pure FAPbI₃, wide-gap perovskite still suffers from huge voltage deficits. Here, the authors take advantage of the synergetic effect behind a sequential fluoride and organic ammonium salt surface passivation strategy to control non-radiative energy losses, and obtained a 17.7% efficiency in infrared-transparent wide-gap perovskite solar cells (21.1% for opaque device), and achieved efficiencies of over 25% when stacked with commercial Si and CIGS products with original PCEs of 18–20% under a 4-terminal working condition.

3-sulfopropyl methacrylate potassium salt (SPM) has a multifunctional effect on the crystallization and passivation of perovskite film. The devices passivated by SPM achieve comprehensive efficiency with ≈22% photovoltaic (PV) efficiency and 10.7% electroluminescent (EL) quantum efficiency (under an injection current of short-circuit photocurrent). The reciprocity between PV and EL is correlated.

Abstract

Integration of photovoltaic (PV) and electroluminescent (EL) functions and/or units in one device is attractive for new generation optoelectronic devices but it is challenging to achieve highly comprehensive efficiency. Herein, perovskite solar cells (PSCs) are fabricated, assisted by 3-sulfopropyl methacrylate potassium salt (SPM) additive to tackle this issue. SPMs not only induce large grain size during the film formation but also produce a secondary phase of 2D K₂PbI₄ to passivate the grain boundaries (GBs). In addition, its sulfonic acid group and potassium ion can coordinate to lead ion and fill the interstitial defects, respectively. Thus, SPM reduces the defective states and suppresses nonradiative recombination loss. As a result, planar PSC delivers a power conversion efficiency of ≈22%, with a maximum open-circuit voltage (V _oc) of 1.20 V. The V _oc is 94% of the radiative V _oc limit (1.28 V), higher than the control device (V _oc of 1.12 V). In addition, the reciprocity between PV and EL is also correlated to quantify the energy losses and understand the device physics. When operated as a light-emitting diode, the maximum EL external quantum efficiency (EQE_EL) is up to 12.2% (EQE_EL of 10.7% under an injection current of short-circuit photocurrent), thus leading to high-performance PV/EL dual functions.

PM6:AQx-2-based organic photovoltaics (OPV) featuring a lower temperature coefficient for power conversion efficiency (PCE) is utilized for PV–thermoelectric generator (TEG) integration. A lower dT _c for OPV–TEG integration than those of the traditional c-SiPV–TEG counterpart is observed, broadening application scenarios. Under AM 1.5G illumination, a 1-cm² OPV device integrated with a TEG delivers a record PCE of 18.2%.

Abstract

Photovoltaics (PVs, light to electricity) integrated with a thermoelectric generator (TEG, heat to electricity) has been viewed as a promising technique that can achieve high power gain and extend device lifetime, but which have rarely been applied in organic PVs. The critical temperature difference (dT _c) is a key parameter because only if the temperature difference (dT) is higher than dT _c a power gain can be realized, and dT _c is closely associated with device performance and application scenarios. By examining the performance of a simulated PV–TEG integrated device comprising a state-of-the-art organic solar cell and a commercial TEG at various dT _s, this dT _c is witnessed. An effective numerical simulation method is established to study the PV–TEG integration based on, the dT _c can be perfectly reproduced and corresponding optimal p–n leg density for the TEG module to generate the highest power output at a certain dT. Integrated organic PV–TEG (OPV–TEG) devices show a lower dT _c than c-SiPV–TEG devices, which, together with the lower temperature coefficient, are intrinsic advantages of OPVs for future applications. Under AM 1.5G illumination, the 1-cm² optimized OPV–TEG integrated device achieves a record energy conversion efficiency of 18.2%.