Chen Weijie

Publication date: 17 November 2021

Source: Joule, Volume 5, Issue 11

Author(s): Mahshid Ahmadi, Maxim Ziatdinov, Yuanyuan Zhou, Eric A. Lass, Sergei V. Kalinin

Terpolymer: Y6 system promotes significant breakthroughs in photovoltaic performance for both traditional opaque organic solar cells and semitransparent organic solar cells (ST-OSCs) based on all narrow bandgap (all-NBG) semiconductors. The terpolymer PCE10-BDT2F-0.8:Y6-based ST-OSCs achieve power conversion efficiencies (PCEs) of 12.00% and 10.85% with average visible transmittance (AVT) of 30.98% and 41.08%, respectively.

Abstract

Semitransparent organic solar cells (ST-OSCs) based on all narrow bandgap (all-NBG) semiconductors are attractive for building integration. Unfortunately, advanced NBG Y-series acceptors cannot well match with the NBG donors, resulting from their mismatched energy levels and poor compatibility. Herein, a facile terpolymer design strategy is adopted to improve the matching of Y6 with efficient NBG polymer donor PCE10. F or Cl atom functionalized benzodithiophene (BDT) are introduced into the PCE10 matrix to afford two series of terpolymers, namely PCE10-BDT2F and PCE10-BDT2Cl. Compared with PCE10, all terpolymers show deeper energy levels, higher extinction coefficients, enhanced face-on orientation, and better compatibility with Y6. Consequently, significant breakthroughs are achieved for both opaque and semitransparent devices. Particularly, a record power conversion efficiency (PCE) of 13.80% is achieved by PCE10-BDT2F:Y6-based device, nearly 40% higher than PCE10:Y6-based device. ST-OSCs also achieve impressive PCEs of 12.00% and 10.85% with average visible transmittance (AVT) of 30.98% and 41.08%, respectively, and both PCEs are the highest values with AVT over 30% and 40%. An outstanding light utilization efficiency (LUE) of 4.46% further demonstrates the successful balance of PCE and AVT. These results demonstrate that the design of NBG terpolymers is a facile and highly encouraging strategy for promoting breakthroughs in ST-OSCs.

Decay times of measured transient photoluminescence (TPL) and transient photovoltage (TPV) are both used to study recombination in halide perovskites but the decay times often differ substantially. Here, a consistent approach to interpret the decays is presented that allows the different parts of the optical and electrical transients to be correctly interpreted, and explains how they are related to material properties.

Abstract

Transient photoluminescence (TPL) and transient photovoltage (TPV) measurements are important and frequently applied methods to study recombination dynamics and charge-carrier lifetimes in the field of halide-perovskite photovoltaics. However, large-signal TPL and small-signal TPV decay times often correlate poorly and differ by orders of magnitude. In order to generate a quantitative understanding of the differences and similarities between the two methods, the impact of sample type (film vs device), large- versus small-signal analysis, and differences in detection mode (voltage vs. luminescence) are explained using analytical and numerical models compared with experimental data. The main solution to achieving a consistent framework that describes both methods is the calculation of a voltage or carrier density dependent decay time that can be interpreted in terms of a capacitive region, a region dominated by defect-assisted recombination and a region that is dominated by higher order recombination (radiative and Auger). It is experimentally shown that in the efficient methylammonium lead-iodide solar cells, effective monomolecular lifetimes ≈2 µs can be consistently measured with TPL and TPV. Furthermore, the shape of the decay time versus voltage or carrier density follows predictions derived from implicit and explicit solutions to differential equations.

Single-photon emitters are universally observed from all-inorganic perovskite films at the cryogenic temperature, owing to local thickness variations of the composing CsPbBr₃ microcrystals and the resulting low potential-energy regions. The discovery of such novel emitting species in a perovskite film, with the enriched structure–property relationship, will impart significant influences on the advancement of relevant optoelectronic devices and quantum-light sources.

Abstract

All-inorganic halide perovskites have drawn a lot of research attention very recently owing to their potential solution to the instability issue currently faced by the organic–inorganic counterparts. Meanwhile, the halide perovskites in a solid film are manifested as microscale morphologies whose functionalities are unavoidably affected by the interior or exterior presence of various nanoscale entities. Here all-inorganic solid films are fabricated with varying densities of single CsPbBr₃ microcrystals, showing that very sharp photoluminescence peaks can be universally observed at 4 K with the linewidths being as narrow as hundreds of μeV. The single-photon emission nature is confirmed for such a photoluminescence peak, whose intensity is completely quenched above ≈30 K to suggest its possible origin from a low potential-energy region of the single microcrystal. The discovery of such a novel emitting species in halide perovskites, with the enriched structure–property relationship, will surely impart significant influences on the advancement of relevant optoelectronic devices and quantum-light sources.

Light intensity analysis of photovoltaic parameters is introduced as a simple method, allowing understanding of the dominating mechanisms limiting the device performance in perovskite solar cells. The method is based on the drift-diffusion model and is aimed at helping in the explanation of parasitic losses from the trap-assisted recombination or ohmic losses in devices.

Abstract

The number of publications on perovskite solar cells (PSCs) continues to grow exponentially. Although the efficiency of PSCs has exceeded 25.5%, not every research laboratory can reproduce this result or even pass the border of 20%. Unfortunately, it is not always clear which dominating mechanism is responsible for the performance drop. Here, a simple method of light intensity analysis of the JV parameters is developed, allowing an understanding of what the mechanisms are that appear in the solar cell and limit device performance. The developed method is supported by the drift-diffusion model and is aimed at helping in the explanation of parasitic losses from the interface or bulk recombination, series resistance, or shunt resistance in the perovskite solar cell. This method can help not only point toward the dominating of bulk or interface recombination in the devices but also determine which interface is more defective. A detailed and stepwise guidance for such a type of light intensity analysis of JV parameters is provided. The proposed method and the conclusions of this study are supported by a series of case studies, showing the effectiveness of the proposed method on real examples.

A systematic understanding of 2D perovskite's carrier transport mechanism is critical for the development of high-performance 2D perovskite solar cells (PSCs). The recent advances on the carrier behavior of 2D Ruddlesden–Popper PSCs from the view of crystal structure, grain orientation, quantum-well width distribution, and device structure are summarized, and guidelines for successfully elevating carrier transports are provided.

Abstract

In recent years, 2D Ruddlesden–Popper (2DRP) perovskite materials have been explored as emerging semiconductor materials in solar cells owing to their excellent stability and structural diversity. Although 2DRP perovskites have achieved photovoltaic efficiencies exceeding 19%, their widespread use is hindered by their inferior charge-carrier transport properties in the presence of diverse organic spacer cations, compared to that of traditional 3D perovskites. Hence, a systematic understanding of the carrier transport mechanism in 2D perovskites is critical for the development of high-performance 2D perovskite solar cells (PSCs). Here, the recent advances in the carrier behavior of 2DRP PSCs are summarized, and guidelines for successfully enhancing carrier transport are provided. First, the composition and crystal structure of 2DRP perovskite materials that affect carrier transport are discussed. Then, the features of 2DRP perovskite films (phase separation, grain orientation, crystallinity kinetics, etc.), which are closely related to carrier transport, are evaluated. Next, the principal direction of carrier transport guiding the selection of the transport layer is revealed. Finally, an outlook is proposed and strategies for enhancing carrier transport in high-performance PSCs are rationalized.

A fully semitransparent large area module is optimally designed by investigation in three major directions. First is the effect of Europium in semitransparent perovskite film for improved stability. Second is a top transparent indium tin oxide contact engineered for delamination free and colorful asthetics for window applications. Third is the UV protection with energy harvesting by a downconverting material.

Abstract

Significant advancements in the perovskite solar cells/modules (PSCs/PSMs) toward better operational stability and large area scalability have recently been reported. However, semitransparent (ST), high efficiency, and large area PSMs are still not well explored and require attention to realize their application in building-integrated photovoltaics (BIPV). This work employs multiple synergistic strategies to improve the quality and stability of the ST perovskite film while ensuring high transparency. Europium ions, doped in the perovskite, are found to suppress the generation of detrimental species like elemental Pb and I, resulting in higher atmospheric stability. The effect of the top transparent contact is designed to obtain an average visible transparency (AVT) of >20% for full device and a green colored hue. Lastly, the lower current density due to the thinner ST absorber is enhanced by the application of a down-converting phosphor material which harvests low energy photons and inhibits UV-induced degradation. This multimodal approach renders a power conversion efficiency of 12% under dim light conditions and 9.5% under 1 sun illumination, respectively, on 21 cm² ST-PSM.

To enhance the photovoltaic performances of perovskite solar cells, an in-depth understanding of recombination processes in full devices is necessary. To gain this insight, transient opto-electronic measurements are applied, revealing that in full devices a superposition of first-, second-, and third-order recombination fully describes the recombination adequately, nonradiative first-order recombination dominating under solar illumination conditions.

Abstract

Ideally, the charge carrier lifetime in a solar cell is limited by the radiative free carrier recombination in the absorber which is a second-order process. Yet, real-life cells suffer from severe nonradiative recombination in the bulk of the absorber, at interfaces, or within other functional layers. Here, the dynamics of photogenerated charge carriers are probed directly in pin-type mixed halide perovskite solar cells with an efficiency >20%, using time-resolved optical absorption spectroscopy and optoelectronic techniques. The charge carrier dynamics in complete devices is fully consistent with a superposition of first-, second-, and third-order recombination processes, with no admixture of recombination pathways with non-integer order. Under solar illumination, recombination in the studied solar cells proceeds predominantly through nonradiative first-order recombination with a lifetime of 250 ns, which competes with second-order free charge recombination which is mostly if not entirely radiative. Results from the transient experiments are further employed to successfully explain the steady-state solar cell properties over a wide range of illumination intensities. It is concluded that improving carrier lifetimes to >3 µs will take perovskite devices into the radiative regime, where their performance will benefit from photon-recycling.

Nature, Published online: 20 October 2021; doi:10.1038/s41586-021-03964-8

Device engineering is an effective way to improve the photovoltaic performance of organic solar cells. Herein, PM6:BO-4F system is selected to prepare planar heterojunction (PHJ) devices. The PHJ devices are successfully fabricated by using green orthogonal solvents of o-xylene (O-XY) and tetrahydrofuran (THF), achieving an excellent power conversion efficiency (PCE) of 16%, which is the highest efficiency of the PHJ structure.

Abstract

Device engineering is an effective way to improve the photovoltaic performance of organic solar cells (OSCs). Currently, the widely used bulk heterojunction (BHJ) structure has problems such as material solubility limitations and the emerging pseudoplanar heterojunction (PPHJ) structure is also troubled by printing technology requirements. However, these issues can be solved by the reasonable application of traditional planar heterojunction (PHJ) structure. Herein, PM6:BO-4F system is selected to prepare PHJ devices by combining sequential spin-coating and orthogonal solvent strategy. In view of the good solubility of PM6 and BO-4F in commonly used high-boiling solvent chlorobenzene (CB) and green solvent tetrahydrofuran (THF), respectively, the PHJ devices are successfully prepared by using these two orthogonal solvents, achieving a power conversion efficiency (PCE) of 15.6%. On this basis, green nonhalogen reagent o-xylene (O-XY) is further used to process PM6. Due to the large polarity difference between O-XY and THF, all-green solvent-processed PHJ devices are successfully fabricated and obtain an astonishing PCE of 16%. As far as it is known, it is the highest efficiency for PHJ OSCs. The results prove the huge research potential of PHJ structure and point out new direction for solving OSC materials compatibility, long-term stability, and future commercial applications.

A surface reconstruction strategy is proposed to optimize the surface defect states and crystallization dynamics in an all-inorganic perovskite/organic two-terminal tandem solar cell, leading to efficient hole transport and charge recombination in the interconnecting layer. Finally, a power conversion efficiency of 21.04% and robust operational stability are obtained.

Abstract

The construction of monolithic two-terminal tandem solar cells (2T TSCs) offers the possibility of pursuing high power conversion efficiency (PCE) by overcoming the single-junction Shockley–Queisser limit in photovoltaics. However, little attention is paid to simultaneously improve the stability by utilizing the complementary properties of various photoactive layers. Here, beyond the stacked photoactive layers featuring complementary absorption, all-inorganic perovskite (CsPbI_1.8Br_1.2) is chosen as the photoactive layer of the front wide-bandgap subcell for its intrinsic high thermal stability and ultraviolet (UV)-filtering function to address the burn-in and UV degradation of organic rear subcells. To realize their monolithic integration, the charge recombination efficiency in the interconnecting layer (ICL) between the two types of subcells is tentatively improved by surface reconstruction of all-inorganic perovskite using trimethylammonium chloride. The repaired CsPbI_1.8Br_1.2 surface enables effective suppression of nonradiative recombination and facilitates hole transport, providing efficient charge recombination in the ICL in the 2T TSC. As a result, the all-inorganic perovskite/organic 2T TSC delivers a promising PCE of 21.04%, accompanied by an ultrahigh open-circuit voltage (V _oc) of 2.05 V, which is nearly equal to the superposition of the respective V _oc values of the subcells. More importantly, the 2T TSC simultaneously shows outstanding operational and UV stabilities.

Application of photoluminescence-based contactless series resistance imaging using nonuniform illumination is demonstrated on perovskite solar cells. The operating point of particular regions is controlled spatially via photoexcitation pattern manipulation across the device. The capability of this proposed contactless method to identify features with high and low absolute effective series resistance is validated qualitatively by comparison with other luminescence-based imaging techniques.

A contactless effective series resistance imaging method for large-area perovskite solar cells that is based on photoluminescence imaging with nonuniform illumination is introduced and demonstrated experimentally. The proposed technique is applicable to partially and fully processed perovskite solar cells if laterally conductive layers are present. The capability of the proposed contactless method to detect features with high effective series resistance is validated by comparison with various contacted mode luminescence imaging techniques. The method can reliably provide information regarding the severeness of the detected series resistance through photoexcitation pattern manipulation. Application of the method to subcells in monolithic tandem devices, without the need for electrical contacting the terminals, appears feasible.

The design strategy of thermally stable all-perovskite tandem solar cells is presented, where metal oxides are used for the charge transport layers and tunnel junction. The tandem devices retained 85% of their initial efficiency after thermal stressing at 85 °C for 2500 h. Achieving such remarkable thermal stability represents a crucial step toward commercial viability of all-perovskite tandem solar cells.

All-perovskite tandem solar cells offer a promising avenue to go beyond the efficiency limit of single-junction devices. Their efficiencies have been increasing rapidly in the past few years; however, their commercial viability is hindered by the instability under thermal stressing. Herein, comprehensive device design strategies are proposed to achieve thermally stable all-perovskite tandem solar cells while retaining the advantages of solution processing. Metal oxides, i.e., NiO _x and SnO₂, are used for the hole and electron transport layers in both wide bandgap and narrow subcells. The metal-based recombination layer is replaced with a stable and conductive indium tin oxide nanocrystals film to fabricate an all metal-oxide-based tunnel junction. Based on those design strategies, the encapsulated all-perovskite tandem solar cells retained 85% of their initial efficiency after stressing at 85 °C for 2500 h and maintained >80% of their initial performance after 900 h operation at the maximum power point and operating temperature of ≈65 °C. Achieving such thermal stability represents a crucial step toward commercial viability of all-perovskite tandem solar cells.

Herein, a promising power conversion efficiency of a simple additive-treated single-cell inverted perovskite solar cell fabricated under moderate humidity is further used in a novel mini-module architecture. This work opens possibilities of the next steps towards commercialization of the perovskite solar cells.

An additive to perovskite material can play a crucial role in the fabrication of perovskite solar cells (PSCs); the additive treatment investigated herein produces promising overall power conversion efficiency (PCE) enhancements. Currently, the fabrication of PSCs is mainly carried out under the dry air of a glove box to avoid moisture adsorption by methylammonium lead iodide perovskites; these tight restrictions in how PSCs are fabricated hinder the possibility of their commercialization. In this study, a technique that uses C₁₂F₄N₄ as an additive to the MAPbI₃ for the one-step deposition of perovskite layers under moderate humidity is presented; this approach is able to achieve high-quality perovskite films despite the adverse condition. The added C₁₂F₄N₄ bridges the gaps between the MAPbI₃ grain boundaries and significantly enhances all the photovoltaic parameters compared to using pristine methylammonium lead iodide; this method even eventually enhances the overall PCE. Both, single-cell and optimized five-cell integrated mini-module devices in moderately humid conditions are fabricated. This facile method presents new possibilities for scaling up PSCs production in humid environments.

Nicotinamide is incorporated into the PEDOT:PSS layer to regulate the energy level, modify the surface roughness of the transporting layer, and reduce the trap states. As a result, a champion device with a maximum power conversion efficiency of 8.28% and an indoor efficiency of 14.40% under 1000 lux, for tin perovskite photovoltaics, is realized.

Tin perovskites are generally considered as promising candidates to solve the issue of toxicity for lead perovskites. However, the open-circuit voltage (V _OC) of tin perovskite solar cells is usually lower than expected. Herein, the hole-transporting layer of PEDOT:PSS is treated with nicotinamide, and the tin perovskite active layer is deposited on the treated film to prepare a perovskite photovoltaic device. The fabricated device presented an increased V _OC from 0.63 to 0.82 V, yielding an improved power conversion efficiency (PCE) increased from 4.77% to 8.28%. More importantly, under the weak light (1000 lux) irradiation of a simulated indoor light source, the device presented a PCE as high as 14.40%, which is the highest indoor efficiency for tin-based perovskite photovoltaics reported so far. The promising indoor performance of tin perovskite photovoltaics paves a new way to fabricate eco-friendly indoor photovoltaics by using lead-free tin perovskites.

A facile ion-exchange strategy is applied to stabilize the surface outermost structure and the origination of FAPbI₃ perovskite surface disorder is unveiled. The replacement between metastable FA cation with stable acetamidine hydroiodide (AMH) cation reconstructs an energetic favorable surface, which suppresses the multirecombination paths, delivering a high open-circuit voltage value for the narrow bandgap FAPbI₃ perovskite solar cell.

Abstract

The ionic nature of organic trihalide perovskite leads to structural irregularity and energy disorder at the perovskite surface, which seriously affects the photovoltaic performance of perovskite solar cells. Here, the origin of the perovskite surface disorder is analyzed, and a facial ion-exchange strategy is designed to regulate the surface chemical environment. By the reconstruction of terminal irregular Pb–I bonds and random cations, the repaired surface is characteristic of the reduced band tail states, consequent to the suppression of the uplift of quasi-Fermi level splitting and photocarrier scattering. The optimized device gets a high open-circuit voltage and operational stability. These findings fully elaborate the underlying mechanism concerning perovskite surface problem, giving guidance on tailoring the energy disorder.