Cascade Type‐II 2D/3D Perovskite Heterojunctions for Enhanced Stability and Photovoltaic Efficiency

Abstract

The energy demand is rapidly increasing for the increased population and the improved life quality. [1] Disafforestation or mining will destroy the ecological balance and even bring some unknown diseases, which will kill thousands of people. Thus, an environmentally friendly method to obtain energy is necessary and urgent. Solar energy is a kind of clean and abundant energy, which can be harvested conveniently. Nowadays, silicon solar cells are dominant in the solar energy industry, but we need more cost-effective solar energy. [2] The 3D perovskite solar cell (PSC) is a low-cost photovoltaic research frontier for its roaring increase in power conversion efficiency (PCE) from 3.8% in 2009 to 25.2% in 2019 because the perovskite shows the excellent optoelectronic properties, including the large absorption coefficient, the small exciton binding energy, and the considerable carrier diffusion length. [3] However, the stability of PSC remains some distance ahead of the industrial application. [4] Therefore, the 2D perovskite can be introduced to protect the 3D perovskite, which not only keeps the high PCE but also improves the stability. Suitable alignment of the energy level in the heterojunction is vital to transfer charges and block the hole-electron recombination in the device, which can achieve better photovoltaic performance. [5] The 2D perovskite can form a wide-bandgap layer, [6] promote the hole extraction, hinder the electron transfer, and reduce the hole-electron recombination. [7] Moreover, the 2D perovskite is hydrophobic and stable under illumination, promoting the humidity and illumination stability of PSC. [8]

Review of Interface Passivation of Perovskite Layer

“…In addition, a hydrophobic 2D ultrathin perovskite layer can improve the humidity stability of the PSCs. It was reported that by using this method, a PSC maintained an initial efficiency of 80% for 350 h at 70–80% relative humidity [ 89 ]. Meanwhile, Wang et al utilized the amphiphilic structure of large molecule S-benzyl-L-cysteine and its unique solubility behavior.…”

Interfacial Engineering by In Situ Building of a 3D/2D Heterojunction for Inverted CsPbI2Br Solar Cells: Beyond Moisture Proof

“…Though the CBM of 2D perovskite is slightly lower than that (−4.3 eV) of PC 61 BM (see Figure a), , the electron transport may not be hindered due to the electron tunneling effect . Nevertheless, the hole transport toward ETL could be efficiently blocked due to the deeper VBM (−6.8 eV) of 2D perovskite with respect to that (−5.9 eV) of PC 61 BM, which might fundamentally account for the improved V OC . , Herein, the role of the 2D/3D structure is very different from those in regular (p–i–n) PSCs, where the as-formed 2D/3D structures promote the hole transfer and block the electron transfer. , …”

“…36,37 Herein, the role of the 2D/3D structure is very different from those in regular (p−i−n) PSCs, where the as-formed 2D/3D structures promote the hole transfer and block the electron transfer. 17,18 Moreover, to quantitatively estimate the trap densities in the CsPbI 2 Br perovskite film without and with the posttreatment, trap-filled limit (TFL) current model 46 was applied for the electron-only devices (EODs) with a configuration of ITO/ PC 61 BM/PVK/PC 61 BM/Ag. Dark J−V curves of EODs are provided in Figure 5b, showing an SCLC regime where the kink point voltage is determined as TFL voltage (V TFL ).…”

“…As a result, the 2D/3D structure in regular (n–i–p) PSCs may result in blocked hole transfer from the 3D perovskite absorber to the hole transport layer (HTL). To address this issue, many attempts have been made, such as reducing the 2D phase thickness and/or tailoring the organic cation. ,, In addition, very recent studies demonstrate that a well-designed 2D/3D structure could even efficiently promote the hole transfer due to the improved energy level alignment at the perovskite/HTL interface. , Thus, an ideal 2D/3D structure should not only combine the merits of 2D and 3D perovskites but also benefit the carrier transport behavior in PSCs.…”

“…8,15,16 well-designed 2D/3D structure could even efficiently promote the hole transfer due to the improved energy level alignment at the perovskite/HTL interface. 17,18 Thus, an ideal 2D/3D structure should not only combine the merits of 2D and 3D perovskites but also benefit the carrier transport behavior in PSCs.…”