ZiQi Sun

Most of the high-performance all-polymer solar cells (all-PSCs) reported to date are based on polymer donor and polymer acceptor pairs with largely overlapped light absorption properties, which seriously limits the efficiency of all-PSCs. This study reports the development of a series of random copolymer donors possessing complementary light absorption with the naphthalenediimide-based polymer acceptor P(NDI2HD-T2) for highly efficient all-PSCs. By controlling the molar ratio of the electron-rich benzodithiophene (BDTT) and electron-deficient fluorinated-thienothiophene (TT-F) units, a series of polymer donors with BDTT:TT-F ratios of 1:1 (P1), 3:1 (P2), 5:1 (P3), and 7:1 (P4) are prepared. The synthetic control of polymer composition allows for precise tuning of the light absorption properties of these new polymer donors, enabling optimization of light absorption properties to complement those of the P(NDI2HD-T2) acceptor. Copolymer P1 is found to be the optimal polymer donor for the fullerene-based solar cells due to its high light absorption, whereas the highest power conversion efficiency of 6.81% is achieved for the all-PSCs with P3, which has the most complementary light absorption with P(NDI2HD-T2).

A series of poly(benzodithiophene-r-fluorinated-thienothiophene) [P(BDTT-r-TT-F)] random copolymers with tunable light absorption characteristics are developed by controlling the ratios of electron-rich BDTT and electron-deficient TT-F units. All-polymer solar cells (all-PSCs) fabricated from these polymer donors and the P(NDI2HD-T2) acceptor achieve efficiencies of up to 6.8% by optimizing the complementary light absorption of the polymer donor and acceptor.

Hybrid organic/inorganic perovskite solar cells are invigorating the photovoltaic community due to their remarkable properties and efficiency. However, many perovskite solar cells show an undesirable current–voltage (I–V) hysteresis in their forward and reverse voltage scans, working to the detriment of device characterization and performance. This hysteresis likely arises from slow ion migration in the bulk perovskite active layer to interfaces which may induce charge trapping. It is shown that interfacial chemistry between the perovskite and charge transport layer plays a critical role in ion transport and I–V hysteresis in perovskite-based devices. Specifically, phenylene vinylene polymers containing cationic, zwitterionic, or anionic pendent groups are utilized to fabricate charge transport layers with specific interfacial ionic functionalities. The interfacial-adsorbing boundary induced by the zwitterionic polymer in contact with the perovskite increases the local ion concentration, which is responsible for the observed I–V hysteresis. Moreover, the ion adsorbing properties of this interface are exploited for perovskite-based memristors. This fundamental study of I–V hysteresis in perovskite-based devices introduces a new mechanism for inducing memristor behavior by interfacial ion adsorption.

Ion migration at perovskite interfaces is investigated by varying the interface with cationic, anionic, and zwitterionic functionalities. The zwitterionic polymer interlayer generates an adsorbing boundary at the interface, increasing the local ion concentration, causing current–voltage (I–V) hysteresis in perovskite-based devices. This fundamental study of perovskite I–V hysteresis introduces a new mechanism for device memristor behavior by interfacial ion adsorption.

Fullerene derivatives, which possess extraordinary geometric shapes and high electron affinity, have attracted significant attention for thin film technologies. This study demonstrates an important photovoltaic application using carboxyl-functionalized carbon buckyballs, C60 pyrrolidine tris-acid (CPTA), to fabricate electron transport layers (ETLs) that replace traditional metal oxide-based ETLs in efficient and stable n-i-p-structured planar perovskite solar cells (PSCs). The uniform CPTA film is covalently anchored onto the surface of indium tin oxide (ITO), significantly suppressing hysteresis and enhancing the flexural strength in the CPTA-modified PSCs. Moreover, solution-processable CPTA-based ETLs also enable the fabrication of lightweight flexible PSCs. The maximum-performing device structures composed of ITO/CPTA/CH₃NH₃PbI₃/2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD)/Au yield power conversion efficiencies of more than 18% on glass substrates and up to 17% on flexible substrates. These results indicate that the CPTA layers provide new opportunities for solution-processed organic ETLs by substantially simplifying the procedure for fabricating PSCs for portable applications.

A low-temperature and solution-processed polar C60 pyrrolidine tris-acid (CPTA) electron transport layer (ETL) with conformal morphology, deposited on an indium tin oxide surface through an esterification step, is used to produce hysteresis-free, bendable, and durable perovskite solar cells. Our results suggest that CPTA is a promising candidate to replace metal oxides and shed light on employing these easily fabricated ETLs in other portable photovoltaic technologies.

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.

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.

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.