Chen Weijie

Herein, a review of edge isolation techniques is presented. Subsequently, novel wet-chemical approaches are introduced, which lower nitric acid consumption as well as production costs and increase efficiency. All investigations are performed on passivated emitter and rear or tunnel oxide-passivated contact solar cells. Cell results with efficiencies above 23% and total cost of ownership estimations are presented.

This work highlights present research and mass production results of wet-chemical solutions for industrial edge isolation of silicon solar cells, aiming for a reduction of nitric acid consumption and production costs as well as a simultaneous increase in efficiency. All processes are applied to either industrially passivated emitter and rear contact (PERC) or tunnel oxide-passivated contact (TOPCon) solar cells. Herein, a review of different edge isolation techniques in the history of silicon solar cell processing is presented. Subsequently, novel wet-chemical approaches are focused on, namely 1) HNO₃-reduced edge isolation (InOxSide Fusion), 2) HNO₃-free edge isolation (InOxSide Blue), and 3) batch cluster solution—a combination of an acidic inline and an alkaline batch tool for emitter edge isolation of PERC and TOPCon solar cells. For each of the approaches, cell results and total cost of ownership estimations are presented. Based on all findings, a comprehensive discussion between inline versus batch-cluster processing is presented. All investigations are performed on industrial equipment, wafer sizes, and a solar cell efficiency level of above 23%.

The recent progress on perovskite compositions, charge transfer materials, the interface modification layers, inert electrodes, and encapsulations for improving perovskite solar cells’ stability under 85% relative humidity/85 °C damp heat aging test and light and elevated temperature-induced degradation aging test is summarized. The differences and key difficulties in passing these two kinds of harsh stability tests are specifically highlighted.

Perovskite solar cells (PSCs) are promising candidates for further photovoltaic technology. However, the instability issue remains a major obstacle hampering their commercialization. Since perovskites are sensitive to external stressors including elevated temperature, humidity, and light, and the decomposition of perovskite under these combined stressors could largely aggravate and accelerate PSCs degradation, the stability aging test under combined stressors has been recognized as the harshest and the most important requirements for PSCs stability evaluation. Herein, the degradation mechanisms of PSCs at elevated temperature, humidity, and light illumination conditions are analyzed. Further, the recent progress on improving the stability of PSCs under combined stressors including 85% relative humidity/85 °C damp heat aging test and light and elevated temperature induced degradation aging test is summarized. The predictions for the further development of effective strategies for improving the stability of PSCs are provided at the end of this review.

This review provides a unique vision of the ecotoxicity and sustainability of emerging Pb halide perovskite solar cells and Pb chalcogenide quantum dot solar cells. The critical role of Pb, the causes of Pb leakage, the hazardous impact, Pb leakage reduction strategies, and Pb recycling strategies of end-of-life solar cells are systematically analyzed.

Emerging Pb-based photovoltaic (PV) technologies, including in particular solution processed halide perovskite solar cells (PSCs) and Pb chalcogenide quantum dot solar cells (QDSCs), are among the most promising next-generation PV technologies for a range of disruptive energy and electronic applications. However, the potential toxicity and leakage of hazardous Pb species have become one of the main barriers to their large-scale application. When solar cells are subject to physical damage or failure of encapsulation, rapid leakage of Pb may occur, which can be accelerated by exposure to external environmental weathering conditions such as rainfall and elevated temperature. Herein, an in-depth investigation on the essential role of Pb in PSCs and QDSCs, as well as common causes of Pb leakage, is undertaken. The hazardous effects of Pb toxicity on soil plants, bacteria, animals, and human cells are also evaluated. Recent progress in developing effective strategies for Pb leakage reduction, such as Pb-free or Pb-less perovskite materials, device architecture design, encapsulation absorbers for PSCs, and core–shell structure and ligand exchange method for QDSCs, in addition to Pb recycling strategies of end-of-life solar cells are summarized. This review provides quantitative insights into the future development of eco-friendly emerging Pb-based PV technologies.

By employing X-ray diffraction and piezoresponse force microscopy, a ferroelectric to nonferroelectric phase transition at room temperature when methylammonium is partially substituted by formamidinium and cesium is revealed. The structural study is correlated with solar cells around this phase transition to explore the impact of the microstructure on energy conversion.

After the discovery of ferroelectricity in archetypal methylammonium lead iodide (MAPbI₃), the discussion arose, if more advanced derivatives thereof are also ferroelectric and to what extent the polar domains mitigate detrimental charge carrier recombination in perovskite solar cells. Herein, the A-site cation methylammonium is gradually substituted with formamidinium and cesium. The domain contrast measured by piezoresponse force microscopy is correlated with the distortion of the crystal structure measured by X-ray diffraction. By Rietveld analysis, a detailed structural model together with texture information is determinded, which reveals an intriguing interplay between lead iodide and the perovskite phases. Finally, the bearing of ferroelectric domains in mixed-cation perovskites on the solar cell performance is discussed.

Carbon nanotube (CNT):SnO₂ in the top region within perovskite films not only forms a perovskite/CNT:SnO₂ heterojunction, leading to a rearrangement of the energy band structure and promoting the extraction and transport of photogenerated holes in the perovskite/hole transport layer, but also effectively passivates the defects of the perovskite film and significantly improves the efficiency and stability of the perovskite solar cell.

Compared with one-step-processed perovskite solar cells (PSCs), there are few reports on improving the power conversion efficiency (PCE) of two-step-processed PSCs by reducing the nonradiative recombination caused by defects through interface engineering. Herein, a new strategy is proposed, that is, by predepositing multifunctional inorganic SnO₂ quantum dots-modified single-walled carbon nanotubes (CNT:SnO₂) on PbI₂ film to form perovskite/CNT:SnO₂ heterojunction in the top region within two-step-processed perovskite films. The CNT:SnO₂ not only promotes the crystallization of perovskite and improves the quality of perovskite films, but also passivates perovskite defects, and effectively suppresses nonradiative recombination. Meanwhile, CNT:SnO₂ leads to the change of the Fermi energy level of the perovskite film, which optimizes the interface energy band arrangement and leads to an additional potential in the perovskite/CNT:SnO₂ heterojunction region, which further accelerate the charge separation and transport. In addition, the CNT:SnO₂ suppresses the migration of halogen anions in the perovskite film and improves the hydrophobicity of the perovskite film. Consequently, the PCE of CNT:SnO₂-based PSCs is significantly increased from 20.10% to 22.25%. They also exhibit improved stability; for instance, the unencapsulated cell maintains 75% of its original PCE even after 600 h of thermal aging at 85 °C.

A 4T perovskite/copper indium gallium selenium (CIGS) tandem with a power conversion efficiency (PCE) of 27.3% is demonstrated, achieved via systematic optimization of the optical properties of the top semitransparent perovskite solar cell. Transparent conductive oxides exhibiting low parasitic absorption are combined with optimal anti-reflection coatings. Finally, optical simulations are used to develop a detailed loss analysis outlining a path to approach 30% PCE for 4T perovskite/CIGS tandem devices.

Over the past decade, the impressive progress in power conversion efficiency (PCE) of organometallic halide perovskite solar cells (PSCs), coupled with their ready integration into tandem solar cells, has led them to approach PCEs of 30% for tandem solar cells with a silicon bottom subcell. However, the complementary technology of perovskite/copper indium gallium selenide (CIGS) tandem solar cells has been thus far unable to reach similar efficiency values. Herein, a further advance in the efficiency of 4T perovskite/CIGS tandems is demonstrated, increasing the PCE up to 27.3% via systematic optimization of the top semitransparent PSC. Improvements in light management through the optimization of anti-reflection coatings, coupled with the development of transparent conductive oxides that incur very low parasitic absorption are reported. It is revealed that both are crucial for maximizing efficiency and, by utilizing additional optical simulations, a detailed loss analysis that enables us to outline a path toward approaching 30% PCE for 4T perovskite/CIGS tandem devices is developed.

The copolymerization strategy with a pentacyclic aromatic lactam acceptor unit integration is used to improve the device processability in nonhalogen solvents, resulting in semitransparent organic solar cells (ST-OSCs) with a maximum power conversion efficiency of 14.6%, an average visible transmittance of 22.5%, and a plant growth factor of 26%, making them one of the best-performing ST-OSCs for photovoltaic greenhouse applications.

Semitransparent organic solar cells (ST-OSCs) offer unique features such as spectral tunability and see-through function, giving them great potential in photovoltaic (PV) agriculture. However, the combination of sufficient average visible transmittance (AVT) and high power conversion efficiency (PCE) with eco-friendly device fabrication has always been a key issue. Herein, a simple but effective strategy by incorporating pentacyclic aromatic lactam acceptor unit (TPTI) in copolymer donors for toluene processed high-efficiency ST-OSCs is performed. The comparisons between D18- and DEH-X-based ST-OSCs demonstrate the effect of TPTI inserting on the polymer main skeleton can not only lower the energy level, improve the processability in nonhalogen solvent, tune the ideal morphology for efficient charge dissociation, but also control a photon transport window suitable for plant absorption. Therefore, the resulting OSCs processed with toluene exhibit a PCE of 14.6% with an AVT of 22%, which represents one of the highest values for ST-OSCs made from nonhalogenated solvents. What's more, it is found that plant growth under ST-OSCs filtered light is comparable with that under natural light. Herein, a guide for developing high-performance ST-OSCs is provided and the prospect of ST-OSCs for green manufacturing PV greenhouse application is demonstrated.

A novel donor–acceptor–donor-type material BDTBT is synthesized to passivate interfacial defects between the perovskite and hole-transporting layer. The BDTBT interacts with uncoordinated Pb²⁺ via Pb–N/S interactions and improves charge transport properties. As a result, the modified perovskite device shows an improved power conversion efficiency of 20.42% than bare perovskite (17.18%).

Perovskite solar cells (PSCs) have exhibited a tremendous photovoltaic performance over the past few years. However, the ionic nature of perovskite and the solution-processable fabrication methods lead to various defects (vacancies, interstitials, and antisites) at the perovskite surface. Incorporating interfacial or surface passivation layers has proved to be crucial in passivating these defects. Herein, a novel donor–acceptor–donor (D–A–D)-based bidentate material, namely, BDTBT, consisting of benzothiadiazole (BDT) as the central acceptor unit and benzothiophene (BT) as a donor end cap unit, is synthesized. The various structural analyses reveal that N and S heteroatoms at BDTBT coordinate effectively to undercoordinated Pb²⁺ in perovskite via Pb–N/S bidentate interactions. As a result, the BDTBT-treated perovskite exhibits an improved power conversion efficiency (PCE) of 20.42% compared with the bare perovskite, having a PCE of 17.18%. The BDTBT incorporation provides favorable band alignment, increased hole transfer, and suppressed nonradiative recombination losses by reducing the surface defect states. In addition, there is significant increase in the device stability and moisture resistance owing to the hydrophobic nature of BDTBT. This study provides a simple and efficient route to obtain stable and highly efficient PSCs by incorporating small molecules as an additional interfacial layer.

Publication date: January 2023

Source: Journal of Energy Chemistry, Volume 76

Author(s): Jie Dou, Qizhen Song, Yue Ma, Hao Wang, Guizhou Yuan, Xueyuan Wei, Xiuxiu Niu, Sai Ma, Xiaoyan Yang, Jing Dou, Shaocheng Liu, Huanping Zhou, Cheng Zhu, Yihua Chen, Yujing Li, Yang Bai, Qi Chen

A ternary strategy is proposed toward highly efficient all-polymer solar cells (all-PSCs) by introducing end-capped PM6TPO. The champion device based on PM6:PM6TPO:PY-IT exhibits an impressive power conversion efficiency (PCE) of 17.0%, which is attributable to the optimal morphology with improved miscibility and reduced phase separation. It is worth mentioning this is one of the highest PCEs known to the authors for all-PSCs.

Abstract

Recently, all-polymer solar cells (all-PSCs) have received increasing attention and made tremendous progress. However, the power conversion efficiency (PCE) of all-PSCs still lags behind the polymer-donor-small-molecule-acceptor based organic solar cells, owing to the excessive phase separation with poor miscibility between polymer donor and acceptor. In this research, an “end-capped” ternary strategy is proposed by introducing PM6TPO as a third component to fabricate highly efficient all-PSCs. The PM6:PM6TPO:PY-IT based ternary devices exhibit impressive PCE of 17.0% with enhanced light absorption and optimal morphology, and the introduction of PM6TPO significantly reduces the phase separation. The ternary devices also exhibit improved stability, outstanding tolerance of active layer thickness, and high performance of 1 cm² unit cells. More importantly, the “end-capped” ternary strategy enables efficient and facile improvement of all-PSCs performance without additional selection and complicated synthesis for the third component.

Based on the low-temperature phase transition process guided by 2D perovskite templates formed with 3-chloropropylammonium chloride (Cl-PACl) as additives and the high-temperature volatilization of Cl-PACl, compositional-pure α-FAPbI₃ solar cells with low residual stress and high crystal orientation are fabricated.

Abstract

Transition of δ-phase formamidinium lead triiodide (δ-FAPbI₃) to pure α-phase FAPbI₃ (α-FAPbI₃) typically requires high processing temperature (150 °C), which often results in unavoidable residual stress. Besides, using methylammonium chloride (MACl) as additive in fabrication will cause MA residue in the film, compromising the compositional purity. Here, a stress-released and compositional-pure α-FAPbI₃ thin-film is fabricated using 3-chloropropylammonium chloride (Cl-PACl) by two-step annealing. The 2D template of n = 2 can preferentially form in perovskite with the introduction of Cl-PACl at a temperature as low as 80 °C. Such a 2D template can guide the free components to form ordered α-FAPbI₃ and promote the transition of the formed δ-FAPbI₃ to α-FAPbI₃ by reducing the phase transition energy. As a result, the obtained perovskite films via low-temperature phase-transition have a high degree of crystal orientation and reduced residual stress. More importantly, most of the Cl-PACl is volatilized during the subsequent high-temperature annealing process accompanied by the disintegration of the 2D templates. The residual trace of Cl-PA⁺ is mainly concentrated at the grain boundary near the perovskite surface layer, stabilizing α-FAPbI₃ and passivating defects. Perovskite solar cell based on pure α-FAPbI₃ achieves a power conversion efficiency of 23.03% with excellent phase stability and photo-stability.

A dopant-free polymeric hole-transport material (HTM) PFBTI is successfully developed for PVSCs. The suitable energy levels, high hole mobility, and excellent surface passivation effects endow PFBTI-based organic/inorganic hybrid PVSCs with a promising PCE of 23.1% and much-enhanced stability. PFBTI can be further used in inorganic PVSCs and perovskite/organic tandem solar cells and achieve high PCEs.

Abstract

The development of hole-transport materials (HTMs) with high mobility, long-term stability, and comprehensive passivation is significant for simultaneously improving the efficiency and stability of perovskite solar cells (PVSCs). Herein, two donor–acceptor (D–A) conjugated polymers PBTI and PFBTI with alternating benzodithiophene (BDT) and bithiophene imide (BTI) units are successfully developed with desirable hole mobilities due to the good planarity and extended conjugation of molecular backbone. Both copolymers can be employed as HTMs with suitable energy levels and efficient defect passivation. Shortening the alkyl chain of the BTI unit and introducing fluorine atoms on the BDT moiety effectively enhances hole mobility and hydrophobicity of the HTMs, leading to improved efficiency and stability of PVSCs. As a result, the organic–inorganic hybrid PVSCs with PFBTI as the HTM deliver a power conversion efficiency (PCE) of 23.1% with enhanced long-term operational and ambient stability, which is one of the best efficiencies reported for PVSCs with dopant-free polymeric HTMs to date. Moreover, PFBTI can be applied in inorganic PVSCs and perovskite/organic tandem solar cells, achieving a high PCE of 17.4% and 22.2%, respectively. These results illustrate the great potential of PFBTI as an efficient and widely applicable HTM for cost-effective and stable PVSCs.

Nonfullerene organic solar cells based on quasi-homojunction (QHJ) with extremely low donor contents (≤10 wt.%) are fabricated. A complete picture of the operation mechanisms of high-efficiency QHJ devices is illustrated, which is distinct from classical bulk heterojunction (BHJ) ones.

Abstract

In contrast to classical bulk heterojunction (BHJ) in organic solar cells (OSCs), the quasi-homojunction (QHJ) with extremely low donor content (≤10 wt.%) is unusual and generally yields much lower device efficiency. Here, representative polymer donors and nonfullerene acceptors are selected to fabricate QHJ OSCs, and a complete picture for the operation mechanisms of high-efficiency QHJ devices is illustrated. PTB7-Th:Y6 QHJ devices at donor:acceptor (D:A) ratios of 1:8 or 1:20 can achieve 95% or 64% of the efficiency obtained from its BHJ counterpart at the optimal D:A ratio of 1:1.2, respectively, whereas QHJ devices with other donors or acceptors suffer from rapid roll-off of efficiency when the donors are diluted. Through device physics and photophysics analyses, it is observed that a large portion of free charges can be intrinsically generated in the neat Y6 domains rather than at the D/A interface. Y6 also serves as an ambipolar transport channel, so that hole transport as also mainly through Y6 phase. The key role of PTB7-Th is primarily to reduce charge recombination, likely assisted by enhancing quadrupolar fields within Y6 itself, rather than the previously thought principal roles of light absorption, exciton splitting, and hole transport.

A sustainable dynamic passivation strategy by incorporating a photoisomeric molecule is put forward, which greatly enhances the photostability of perovskite photovoltaics. This dynamic strategy pays attention to and well matches the dynamics of defect generation during operation. The characteristics of the changeable molecular structure enable us to cope with defects updated in operation without introducing excess active sites.

Abstract

The generation of photoinduced defects and freely moving halogen ions is dynamically updated in real time. Accordingly, most reported strategies are static and short-term, which make their improvements in photostability very limited. Therefore, seeking new passivation strategies to match the dynamic characteristics of defect generation is very urgent. Without newly generated defects, a passivation molecule should exist in the configuration that would not become the initiation sites for defect generation. With newly generated defects, the passivation molecule should transfer into the other configuration that possesses the passivation sites. Herein, a classical photoisomeric molecule, spiropyran, is adopted, whose pre- and post-isomeric forms meet the requirements for two different configurations, to realize the state transition once the photoinduced defects appear during subsequent operation and dynamic capture for continuous renewal of defects. Consequently, spiropyrans work as light-triggered and self-healing sustainable passivation sites to realize continuous defect repair. The target devices retain 93% and 99% of their initial power conversion efficiencies after 456 h aging under ultraviolet illumination and 1200 h aging under full-spectrum illumination, respectively. This work provides a novel concept of sustainable passivation strategy to realize continuous defect-passivation and film-healing in perovskite photovoltaics.

Publication date: January 2023

Source: Journal of Energy Chemistry, Volume 76

Author(s): Bo Xiao, Yongxin Qian, Xin Li, Yang Tao, Zijun Yi, Qinghui Jiang, Yubo Luo, Junyou Yang

A solution-processed SnOCl ternary tin (II) alloy as hole-transport material (HTM) for Sn–Pb perovskite solar cells is reported, which results in efficiencies of 23.2% and 26.3% for Sn–Pb single-junction and all-perovskite tandem cells, respectively. Greatly enhanced light stabilities with T87 of >1200 h and 85 °C thermal stability with T85 of >1500 h are achieved by using the SnOCl HTM.

Abstract

Tin–lead (Sn–Pb) narrow-bandgap (NBG) perovskites show great potential in both single-junction and all-perovskite tandem solar cells. Sn–Pb perovskite solar cells (PSCs) are still limited by low charge collection efficiency and poor stability. Here, a ternary Sn (II) alloy of SnOCl is reported as the hole-transport material (HTM) with a work function of 4.95 eV for Sn–Pb PSCs. The solution-processed SnOCl layer has a texture structure that not only reduces the optical loss of the devices, but also changes grain growth of Sn–Pb perovskites and boosts the carrier diffusion length to 3.63 µm. The formation of small perovskite grains at the HTM/perovskite interface is suppressed. These result in an almost constant internal quantum efficiency (IQE) of 96 ± 2% across the absorption spectrum of Sn–Pb perovskites. The SnOCl HTM significantly enhances the stability of Sn–Pb PSCs with 87% of its initial efficiency retained after 1-sun illumination for 1200 h, and keeps 85% efficiency under 85 °C thermal stress for 1500 h. The hybrid HTM further improves the stabilized efficiencies of single-junction Sn–Pb PSCs and all-perovskite tandem solar cells to 23.2% and 25.9%, respectively. This discovery opens an avenue to the multicomponent metal alloys as HTM in PSCs.