Organic and inorganic photovoltaic cells can be classified as excitonic solar cells and pn-junction solar cells, respectively. In this article, a fundamental difference in the operational mechanism between them is explained. The detailed balance limit of power conversion efficiency (PCE) was already established for pn-junction solar cells by Shockley and Queisser. Recent attempts to obtain theoretical limits of PCE for excitonic solar cells are reviewed.
This article focuses on the synthesis, design concepts and photovoltaic performance of fullerene-based electron acceptors used in solution processed bulk heterojunction organic solar cells. Excellent fullerene acceptors such as 56π-electron conjugated dihydromethanofullerene derivatives, ICBA, methano-indene-fullerenes, mix-PCBM are introduced from a synthetic chemistry viewpoint in addition to fullerene chemistry background being offered, which should benefit a wide range of organic solar cell research.
Organic solar cells have been intensively investigated as next-generation solar cells, because of their fascinating advantages of lightweight, low fabrication cost, resource-free, and flexibility. However, the fundamental aspects for improving the energy conversion efficiency (η) have been unclear satisfactorily. In this review, I report on the effects of molecular crystallinity/orientation of a donor film on the external quantum efficiency, which is one of the most important factors to determine the η, of Zinc-porphyrin/C60 hetero-junction solar cells.
This article introduces our recent studies on charge separation mechanism in organic solar cells. One of important questions on organic solar cells is how free electrons and holes are generated overcoming the Coulomb attraction at donor-acceptor interfaces. We carried out a combined electronic structure and quantum dynamical analysis that captures the elementary events from the exciton dissociation to the free carrier generation at polymer/fullerene donor-acceptor heterojunctions. Our calculations show that experimentally observed efficient charge separations can be explained by a combination of two effects: First, the delocalization of charges which substantially reduces the Coulomb barrier, and second, the vibronically hot nature of the charge transfer state which promotes charge dissociation beyond the barrier.
Time-resolved electron paramagnetic resonance (TREPR) spectroscopy has been utilized at T = 77 K to characterize geometries and electronic couplings (VCR) of transient charge-separated (CS) states in photoactive layers in organic thin film solar cells fabricated by spin-coating of mixed solutions of polyalkylthiophenes (RR-P3AT) and [6, 6]-C61-butyric acid methyl ester (PCBM). Electron-hole distance of the interfacial transient CS states has been revealed to be modulated by alkyl side-chain number in P3AT. This result is explained by a coupling of a hole dissociation to polymer librations by the side-chains. From an exponential decay of VCR with respect to the CS distance, an attenuation factor (βe) of the decay has been determined to be βe＝0.2 Å−1. Such a long-range tunneling feature is explained by generations of shallowly trapped, delocalized electron-hole pairs by the dissociations of the hole toward π-stacking directions at the organic photovoltaic interface.
Tandem-type solar cell is a multiple-layered solar cell to decrease thermal- and optical-energy losses. However, typical organic photovoltaics has been difficult to apply tandem photovoltaics because of the lack of organic photovoltaic cells with high open-circuit voltage (VOC) and/or near-infrared (NIR) photoelectric conversion. In this study, we constructed tandem-type organic photovoltaics using high VOC organometal halide perovskite solar cell and NIR dye-sensitized solar cell. Consequently, over 16% power conversion efficiency which is highest record among tandem-type organic photovoltaics has been accomplished.