Chen Weijie

Publication date: 20 March 2019

Source: Joule, Volume 3, Issue 3

Author(s): Ik Jae Park, Jae Hyun Park, Su Geun Ji, Min-Ah Park, Ju Hee Jang, Jin Young Kim

Context & Scale

Summary

Graphical Abstract

Publication date: 20 March 2019

Source: Joule, Volume 3, Issue 3

Author(s): Wei Li, Mengxue Chen, Jinlong Cai, Emma L.K. Spooner, Huijun Zhang, Robert S. Gurney, Dan Liu, Zuo Xiao, David G. Lidzey, Liming Ding, Tao Wang

Context & Scale

Summary

Graphical Abstract

Small‐molecule‐based ternary BHJ solar cells with the SM donor DR3, the SM acceptor ICC6, and the fullerene PC₇₁BM yield high power conversion efficiencies nearing 11% for active layer thicknesses >200 nm. With low geminate and nongeminate recombination, long carrier lifetimes, and high/balanced carrier mobilities, the ternary system maintains PCEs >8% over a wide range of active layer thicknesses within 200–500 nm.

Abstract

Solution‐processed small molecule (SM) solar cells have the prospect to outperform their polymer‐fullerene counterparts. Considering that both SM donors/acceptors absorb in visible spectral range, higher expected photocurrents should in principle translate into higher power conversion efficiencies (PCEs). However, limited bulk‐heterojunction (BHJ) charge carrier mobility (<10^‐4 cm² V^‐1 s^‐1) and carrier lifetimes (<1 µs) often impose active layer thickness constraints on BHJ devices (≈100 nm), limiting external quantum efficiencies (EQEs) and photocurrent, and making large‐scale processing techniques particularly challenging. In this report, it is shown that ternary BHJs composed of the SM donor DR3TBDTT (DR3), the SM acceptor ICC6 and the fullerene acceptor PC₇₁BM can be used to achieve SM‐based ternary BHJ solar cells with active layer thicknesses >200 nm and PCEs nearing 11%. The examinations show that these remarkable figures are the result of i) significantly improved electron mobility (8.2 × 10^‐4 cm² V^‐1 s^‐1), ii) longer carrier lifetimes (2.4 µs), and iii) reduced geminate recombination within BHJ active layers to which PC₇₁BM has been added as ternary component. Optically thick (up to ≈500 nm) devices are shown to maintain PCEs >8%, and optimized DR3:ICC6:PC₇₁BM solar cells demonstrate long‐term shelf stability (dark) for >1000 h, in 55% humidity air environment.

Here, an AI treatment is developed that provides a general method for optimizing the interfacial properties of inorganic perovskite solar cells, which leads to proper band edge bending, decreased surface defects, and a high‐quality quantum dots–modified layer. These changes prove effective at decreasing recombination loss and improving hole extraction efficiency. As a result, the FAI‐treated champion device achieves long‐term stabilized power conversion efficiencies above 14%.

Abstract

Recently, inorganic CsPbI₂Br perovskite is attracting ever‐increasing attention for its outstanding optoelectronic properties and ambient phase stability. Here, an efficient CsPbI₂Br perovskite solar cell (PSC) is developed by: 1) using a dimension‐grading heterojunction based on a quantum dots (QDs)/bulk film structure, and 2) post‐treatment of the CsPbI₂Br QDs/film with organic iodine salt to form an ultrathin iodine‐ion–enriched perovskite layer on the top of the perovskite film. It is found that the above procedures generate proper band edge bending for improved carrier collection, resulting in effectively decreased recombination loss and improved hole extraction efficiency. Meanwhile, the organic capping layer from the iodine salt also surrounds the QDs and tunes the surface chemistry for further improved charge transport at the interface. As a result, the champion device achieves long‐term stabilized power conversion efficiency beyond 14%.

Doping lead halide perovskites with lithium results in reduction of nonradiative loss and enhancement of photoluminescence. Spectroscopy studies indicate that lithium is incorporated into the lattice, rather than just in the surface region suggested in literature. The reduction of nonradiative recombination is attributed to filling of trap states by free electrons provided by n‐type Li doping.

Abstract

Adding alkali metal into lead halide perovskites has recently been demonstrated as an effective strategy for reducing nonradiative loss. However, the suggested role of the alkali metal is usually limited to surface passivation, and the semiconductor doping effect is rarely discussed. Here, the mechanism of lithium doping in the photocarrier recombination in solution‐processed methylammonium lead halide films is investigated by photoluminescence and photoelectron spectroscopies. It is demonstrated that lithium doping weakens the electron–phonon coupling and acts as donor in perovskites, which provide solid evidence that lithium enters the lattice rather than just in the surface region. The n‐type doping creates free electrons to fill the trap states in both the bulk and surface regions, leading to suppressed trapping of photocarriers and reduces nonradiative recombination.

A crystallization kinetics management “L‐I” process based on co‐regulation of Lewis adduct and the ion exchange process is developed to obtain high‐quality low‐dimensional Ruddlesden–Popper (LDRP) lead‐free perovskite films with large grain size and low trap density. The corresponding devices exhibit promising efficiency of 4.03% and improved stability of 94 d in nitrogen.

Abstract

Low‐dimensional Ruddlesden–Popper (LDRP) lead‐free perovskite has great potential due to its improved stability and oriented crystal growth, which is mainly attributed to the effective control of crystallization kinetics. However, the crystallization kinetics of LDRP lead‐free perovskite films are highly limited by Lewis theory. Here, the management of the crystallization kinetics of LDRP tin (Sn) perovskite films jointly controlled by Lewis adducts and the ion exchange process using a mixture of polar aprotic solvent dimethyl sulfoxide (DMSO) and ion liquid solvent methylammonium acetate (MAAc) (the process named as “L‐I”) is demonstrated. Homogeneous nucleated LDRP Sn perovskite films with average grain size close to 9 µm are achieved. Both low electron and hole defect density with a magnitude of 10¹⁶, high carrier mobility, and excellent electrical performance are obtained. As a result, the LDRP Sn perovskite solar cell (PSC) with power conversion efficiency (PCE) of 4.03% is achieved using a simple one‐step method without antisolvents, which is one of the best LDRP Sn PSCs. Most importantly, the PSC exhibits excellent stability with no degradation in PCE after 94 d in a nitrogen atmosphere owing to the high‐quality film and the inhibition of the oxidation of Sn²⁺.

The latest progress and issues toward scalable fabrication of perovskite solar cells are reviewed in an attempt to provide insights on the development of rational fabrication methods for large‐area perovskite films and solar modules.

Abstract

With the impressive record power conversion efficiency (PCE) of perovskite solar cells exceeding 23%, research focus now shifts onto issues closely related to commercialization. One of the critical hurdles is to minimize the cell‐to‐module PCE loss while the device is being developed on a large scale. Since a solution‐based spin‐coating process is limited to scalability, establishment of a scalable deposition process of perovskite layers is a prerequisite for large‐area perovskite solar modules. Herein, this paper reports on the recent progress of large‐area perovskite solar cells. A deeper understanding of the crystallization of perovskite films is indeed essential for large‐area perovskite film formation. Various large‐area coating methods are proposed including blade, slot‐die, evaporation, and post‐treatment, where blade‐coating and gas post‐treatment have so far demonstrated better PCEs for an area larger than 10 cm². However, PCE loss rate is estimated to be 1.4 × 10⁻²% cm⁻², which is 82 and 3.5 times higher than crystalline Si (1.7 × 10⁻⁴% cm⁻²) and thin film technologies (≈4 × 10⁻³% cm⁻²) respectively. Therefore, minimizing PCE loss upon scaling‐up is expected to lead to PCE over 20% in case of cell efficiency of >23%.

The advantage of methylammonium thiocyanate (MASCN) addition inside the two‐step deposition process in forming high quality FAPb_0.7Sn_0.3I₃ films is demonstrated. MASCN can tune morphology of the perovskite film and retard the oxidation of Sn²⁺. The power conversion efficiency of the device reaches 16.26%, and is almost independent of the storage time of the precursor solution in the glove box within 124 d.

Abstract

Regulation of the crystallization of perovskite films and avoiding the oxidation of Sn²⁺ during the deposition process are very important for achieving Sn/Pb binary perovskite solar cells (PVSCs) with high power conversion efficiency (PCE) and producibility. In this work, a high‐quality HC(NH₂)₂Pb_0.7Sn_0.3I₃ (FAPb_0.7Sn_0.3I₃) film deposited from the two‐step solution process by introducing methylammonium thiocyanate (MASCN) as a bifunctional additive into the precursor solution containing PbI₂ and SnI₂ is reported. MASCN can not only tune the morphology of the perovskite film but also stabilize the precursor solution via retarding the oxidation of Sn²⁺ through a strong coordination between SCN⁻ and Sn²⁺. The Sn/Pb binary inverted PVSCs based on FAPb_0.7Sn_0.3I₃ present a high fill factor of 0.79 and the best PCE of 16.26% in the case of 0.25 MASCN addition. The device fabrication producibility is also greatly improved due to the stabilized precursor solution with the aid of MASCN. The PCE of the device is almost independent of the storage time of the precursor solution within 124 d in the N₂‐filled glove box. These results indicate that the precursor engineering with multifunctionality additive is an effective approach toward highly efficient and producible PVSCs for future commercialization.

Here, urea treatment of hole transport layer (e.g., poly(3,4‐ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS)) is reported to effectively tune its morphology, conductivity, and work function for improving the efficiency and stability of inverted CH₃NH₃PbI₃ perovskite solar cells.

Abstract

Interface engineering is critical to the development of highly efficient perovskite solar cells. Here, urea treatment of hole transport layer (e.g., poly(3,4‐ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS)) is reported to effectively tune its morphology, conductivity, and work function for improving the efficiency and stability of inverted MAPbI₃ perovskite solar cells (PSCs). This treatment has significantly increased MAPbI₃ photovoltaic performance to 18.8% for the urea treated PEDOT:PSS PSCs from 14.4% for pristine PEDOT:PSS devices. The use of urea controls phase separation between PEDOT and PSS segments, leading to the formation of a unique fiber‐shaped PEDOT:PSS film morphology with well‐organized charge transport pathways for improved conductivity from 0.2 S cm⁻¹ for pristine PEDOT:PSS to 12.75 S cm⁻¹ for 5 wt% urea treated PEDOT:PSS. The urea‐treatment also addresses a general challenge associated with the acidic nature of PEDOT:PSS, leading to a much improved ambient stability of PSCs. In addition, the device hysteresis is significantly minimized by optimizing the urea content in the treatment.

Efficient carbon‐based hole conductor free planar heterojunction perovskite solar cells (PHJ‐PVSCs) fabricated in ambient air have been reported, achieving a champion power conversion efficiency of 13.52% and a best V _OC of 1.07 V. These PHJ‐PVSCs show superior reproducibility and stability. This work makes the preliminary exploration of cost‐efficient and stable C‐based hole transport material‐free PHJ‐PVSCs and paves the way to their further commercialization.

Organic‐inorganic hybrid metal halide perovskite solar cells have gained tremendous research interest in the past few years. Here, highly reproducible and efficient planar heterojunction perovskite solar cells (PHJ‐PVSCs) with good stability are fabricated using one‐step processed compact TiO₂ as electron‐selective layers (ESLs) and carbon as counter electrodes with no requirements of hole conductors. The fabrication process is fully conducted in ambient air with relatively high humidity (≥45%). Our PVSCs have a simplified planar architecture with only a compact TiO₂ ESL and carbon layer sandwiched with a methylammonium lead triiodide light absorber. Our best‐performing cell shows a good power conversion efficiency of 13.52% and superb illumination stability for 1200 s with no decrease under one sun. Our unencapsulated devices also show great thermal stability, retaining 90% of their initial efficiencies for 72 h under thermal stress at 80 °C in air. Notably, the open‐circuit voltage of our planar PVSCs are always over 1 V, and the best one is 1.07 V, which is the highest value ever reported for carbon‐based hole conductor free PVSCs. Our research demonstrates the feasibility of a fully ambient air fabrication process for carbon‐based PHJ‐PVSCs in simplified device architecture and paves the way to their further commercialization.