Ligang Yuan

Rapid improvement in photoconversion efficiency (PCE) of solution processable organometallic hybrid halide based perovskite solar cells (PSCs) have taken the photovoltaic (PV) community with a surprise and has extended their application in other electronic devices such as light emitting diodes, photo detectors and batteries. Together with efforts to push the PCE of PSCs to record values >22% – now at par with that of crystalline silicon solar cells – origin of their PV action and underlying physical processes are also deeply investigated worldwide in diverse device configurations. A typical PSC consists of a perovskite film sandwiched between an electron and a hole selective contact thereby creating ESC/perovskite and perovskite/HSC interfaces, respectively. The selective contacts and their interfaces determine properties of perovskite layer and also control the performance, origin of PV action, open circuit voltage, device stability, and hysteresis in PSCs. Herein, we define ideal charge selective contacts, and provide an overview on how the choice of interfacing materials impacts charge accumulation, transport, transfer/recombination, band-alignment, and electrical stability in PSCs. We then discuss device related considerations such as morphology of the selective contacts (planar or mesoporous), energetics and electrical properties (insulating and conducting), and its chemical properties (organic vs inorganic). Finally, the outlook highlights key challenges and future directions for a commercially viable perovskite based PV technology.

The past few years marked a new era of organometallic halide hybrid perovskite efficient solar cell technology. To capitalize the potential of this new class of materials in solar cells, in particular, and in any electronic devices in general, an understanding of interfacial physical processes is crucial. Herein, a comprehensive analysis of the role of interfaces in determining the PV performance and long term operational stability of this PV technology is provided.

Perovskite solar cells (PSCs) use perovskites with an APbX₃ structure, where A is a monovalent cation and X is a halide such as Cl, Br, and/or I. Currently, the cations for high-efficiency PSCs are Rb, Cs, methylammonium (MA), and/or formamidinium (FA). Molecules larger than FA, such as ethylammonium (EA), guanidinium (GA), and imidazolium (IA), are usually incompatible with photoactive “black”-phase perovskites. Here, novel molecular descriptors for larger molecular cations are introduced using a “globularity factor”, i.e., the discrepancy of the molecular shape and an ideal sphere. These cationic radii differ significantly from previous reports, showing that especially ethylammonium (EA) is only slightly larger than FA. This makes EA a suitable candidate for multication 3D perovskites that have potential for unexpected and beneficial properties (suppressing halide segregation, stability). This approach is tested experimentally showing that surprisingly large quantities of EA get incorporated, in contrast to most previous reports where only small quantities of larger molecular cations can be tolerated as “additives”. MA/EA perovskites are characterized experimentally with a band gap ranging from 1.59 to 2.78 eV, demonstrating some of the most blue-shifted PSCs reported to date. Furthermore, one of the compositions, MA_0.5EA_0.5PbBr₃, shows an open circuit voltage of 1.58 V, which is the highest to date with a conventional PSC architecture.

Tolerance factors based on novel molecular descriptors are introduced and subsequently implemented experimentally in multication methylammonium/ethylammonium (EA) perovskite solar cells. It is shown that surprisingly large quantities of EA can be incorporated into the perovskite structure, which results in one of the highest reported open-circuit voltages for perovskite solar cells.

Enhancing interfacial charge transfer by the inner electric field is crucial for improving photovoltaic performance of heterojunction solar cells. Recent studies are focusing on how to utilize piezo-phototronic effect (strain-induced inner electric field) to modulate the interfacial charge transfer, whereas the preservation of solar cells from structure damage and performance decline under long-term strain becomes increasingly challenging. Here, without use of strain, a thermo-phototronic effect is presented to enhance the interfacial charge transfer in InP/ZnO nanorod heterojunction solar cells. Under a temperature gradient of 3.5 °C across the device, the output current and voltage of the solar cell under weak light illumination are enhanced by 27.3 and 76%, respectively. Moreover, the performance enhancement can be further regulated by applying different temperature gradients. This study serves as proof-of-principle for the thermo-phototronic effect and pushes forward the maximum utilization of solar energy by a one-circuit-based photovoltaic-thermoelectric system.

A one-circuit-based photovoltaic-thermoelectric system in terms of thermo-phototronic effect is designed to modulate the interfacial charge transfer for enhancing the output performance of heterojunction solar cells.

Under the presence of an externally applied electric field, hybrid and all-inorganic perovskite materials reveal both ionic and electronic conductivity. In article number 1602283, Pablo Docampo, Rubén D. Costa, and co-workers investigate this internal ion migration and rearrangement of different ionic species, namely bromide and methyl ammonium cations respectively, within perovskite nanoparticles in thin-film devices and find it to resemble the well-known signatures of the ionic motion in light-emitting electrochemical cells.

In article number 1602751, Sunho Jeong, Jooho Moon, and co-workers highlight recent progress on metal nanowire-electrode-integrated perovskite solar cells (PSCs). The reliance on vacuum-deposited electrode can be alleviated and the high-throughput production is easily achievable, owing to the suitability of both perovskite and metal nanowire electrodes toward solution-based processes. Also, semi-transparent and/or flexible PSCs is obtainable, since metal nanowire electrodes can be versatilely deposited regardless of the cell configurations.

The photovoltaic performance of core/shell quantum dots sensitized solar cells (QDSCs) can be significantly enhanced by optimizing the shell thickness, as demonstrated by Haiguang Zhao, Zhiming M. Wang, Federico Rosei, and co-workers in article number 1701468. By maintaining the core size of the quantum dot, the approach offers new opportunities to fabricate low cost, highly efficient, and long-term stable QDSCs.

The all-solution-processed switchable interconnecting layer (ICL) for both inverted and normal tandem organic solar cells (OSCs) is reported for the first time here. The fundamental challenges in the literature arise from mixing multiple functionalities into a single layer. For a widely used ICL composed of an electron transport layer (ETL)/a hole transport layer (HTL), ETL needs not only to efficiently extract electrons from an underneath photoactive layer, but also to fulfill optical, mechanical, chemical and electrical requirements to function as effective tunneling junction ICL with HTL atop. Taking on multiple functionalities for a single ETL makes ETL in ICL highly coupled and difficult to be replaced. This is also the case for HTL. Here, this study demonstrates an all-solution-processed switchable ICL, ETL/recombination layer (RL)/HTL and HTL/RL/ETL, for both normal and inverted tandem OSCs. In switchable ICL, ETL and HTL simply serve as carrier transport layers as they did in single OSCs. Electrical recombination, mechanical protection and chemical separation functionalities are realized by RL alone. This strategy shifts the views of ICL for tandem OSCs from conventionally complicated ETL/HTL tunneling junction ICL, where both ETL and HTL play several different roles, towards simplified ICL where ETL and HTL play a distinct decoupled role, advancing ICL for more adaptable tandem OSCs.

An all-solution-processed switchable interconnecting layer (ICL) for tandem organic solar cells (OSCs) is demonstrated the first time with hole transporting layer (HTL)/recombination layer (RL)/electron transporting layer (ETL) and its counterpart ETL/RL/HTL for inverted and normal structure configuration respectively. This three-layered switchable ICL controls the complexity of fabricating tandem OSCs to be as simple as single OSCs.