The past two decades of vigorous interdisciplinary approaches has seen tremendous breakthroughs in both scientific and technological developments of bulk-heterojunction organic solar cells (OSCs) based on nanocomposites of π-conjugated organic semiconductors. Because of their unique functionalities, the OSC field is expected to enable innovative photovoltaic applications that can be difficult to achieve using traditional inorganic solar cells: OSCs are printable, portable, wearable, disposable, biocompatible, and attachable to curved surfaces. The ultimate objective of this field is to develop cost-effective, stable, and high-performance photovoltaic modules fabricated on large-area flexible plastic substrates via high-volume/throughput roll-to-roll printing processing and thus achieve the practical implementation of OSCs. Recently, intensive research efforts into the development of organic materials, processing techniques, interface engineering, and device architectures have led to a remarkable improvement in power conversion efficiencies, exceeding 11%, which has finally brought OSCs close to commercialization. Current research interests are expanding from academic to industrial viewpoints to improve device stability and compatibility with large-scale printing processes, which must be addressed to realize viable applications. Here, both academic and industrial issues are reviewed by highlighting historically monumental research results and recent state-of-the-art progress in OSCs. Moreover, perspectives on five core technologies that affect the realization of the practical use of OSCs are presented, including device efficiency, device stability, flexible and transparent electrodes, module designs, and printing techniques.

Despite nearly two decades of research, the absence of ideal flexible and transparent electrodes has been the largest obstacle in realizing flexible and printable electronics for future technologies. Here we report the fabrication of ‘polymer-metal hybrid electrodes’ with high-performance properties, including a bending radius <1 mm, a visible-range transmittance>95% and a sheet resistance <10 Ω sq−1. These features arise from a surface modification of the plastic substrates using an amine-containing nonconjugated polyelectrolyte, which provides ideal metal-nucleation sites with a surface-density on the atomic scale, in combination with the successive deposition of a facile anti-reflective coating using a conducting polymer. The hybrid electrodes are fully functional as universal electrodes for high-end flexible electronic applications, such as polymer solar cells that exhibit a high power conversion efficiency of 10% and polymer light-emitting diodes that can outperform those based on transparent conducting oxides.

Polymer solar cells (PSCs) based on the bulk heterojunction (BHJ) blend of a conjugated polymer donor and a fullerene acceptor have gained considerable attention as a cost-effi cient, fl exible, and portable energy source. [1][2][3] Considering the practical aspects of its commercialization, the inverted device structure of PSCs (I-PSCs), consisting of a BHJ photoactive layer between indium tin oxide (ITO) as a bottom cathode and a high work function (WF) metal (Ag or Au) as a top anode, is an advantageous approach due to its superior long-term stability and printability. [4][5][6] In this type of inverted device structure, an additional interfacial layer between the active layer and ITO electrode must be introduced to establish the device concept. The high WF of the bottom ITO cathode (approximately 4.8 eV) hampers the formation of an ohmic contact with the lowest unoccupied molecular orbital (LUMO) level of fullerene and makes the creation of a high built-in fi eld used to break the electrical symmetry of the device diffi cult. Thus, many researchers explored a variety of interfacial materials to shift and modify the energy level of the ITO cathode. [4][5][6][7][8][9][10][11][12][13][14][15][16] Inorganic metal oxides (MOs), such as titanium oxide (TiO x ) or zinc oxide (ZnO), have been widely used as interfacial materials due to their solution processability (via a sol-gel precursor and nano-particle solutions) and their electron selective properties, which originate from their conduction band edges (typically at approximately 4 eV) that correspond well with the LUMO level of fullerene and the deep valence bands at approximately 8 eV. [8][9][10][11][12] However, I-PSCs using MOs typically require a high annealing temperature (over 200 ° C) to achieve a crystallinity of the MO that is suffi ciently high to yield a high charge carrier mobility, [ 8 , 9 ] and these devices require a crucial post-UV treatment to enhance the device performance (typically referred to as a 'light-soaking' problem). [10][11][12] Because the high-temperature annealing process is incompatible with typical printing processes that use fl exible plastic substrates and the post-UV treatment causes a harmful photo-oxidation in the conjugated polymers, these processes should be eliminated to promote the practical use of I-PSCs.To circumvent the inherent weaknesses of the MO interlayers, polymer-based interfacial materials, such as conjugated polyelectrolytes (CPEs) and nonconjugated polyethylene oxide (PEO), have also been employed. These polymer-based interlayers alter the WF of ITO by forming extremely thin interfacial dipoles (typically 1-2 nm) and facilitate charge transport into the ITO electrode. [17][18][19][20] Becuase cationic CPEs have charged ionic pendant groups in their structures, they can induce signifi cantly stronger dipole moments than the neutral PEO, resulting in a higher device performance than that of the I-PSC using PEO. [ 19 ] However, because CPEs typically require a delicate and complicated synthesis procedure, nonc...

Corrosive ionic defects in perovskite films degrade perovskite solar cells (PSCs) and long-term stable PSCs are realized by neutralizing the defects.