Organic photovoltaic cells (OPVs) have attracted more and more attention due to its highly potential application to solve the energy crisis considering its advantages, such as low cost and ease of large area production. The power conversion efficiency (PCE) of OPVs has undergone a more than nine-fold increase from ~1.0% by Tang in 1986 to 9.2% in 2010 announced by Mitsubishi Chemical. The major challenges of obtaining high efficiency OPVs are the synthesis of new narrow band gap materials, controlling molecular arrangement, designing novel configuration cells for better photon harvesting in the active layer. In the article, we summarized the recent progress of novel narrow band gap photovoltaic materials and the effective methods to control the morphology of donor and acceptor in the blend films for high performance of OPVs. photovoltaic, narrow band gap materials, molecular arrangementCitation:

We studied the luminescent and photovoltaic properties of poly(9,9-dioctylfluorene-co-bithiophene) (F8T2) based on ITO/ PEDOT : PSS/F8T2/Bphen/LiF(0 or 1 nm)/Al and ITO/PEDOT : PSS/F8T2 : PCBM/Bphen/Al. A stable and bright yellow emission was obtained from polymer F8T2, and the electroluminescence power reached 45 W at a 15 V driving voltage. Polymer F8T2 shows a broad absorption band from 400 to 500 nm, and has a shorter absorption edge at about 560 nm compared to that of the typical electron donor P3HT (650 nm). The photoluminescence quenching of F8T2 occurs with only a small fraction of blended PCBM due to the effective exciton dissociation at the interface between F8T2 and PCBM. Polymer solar cells (PSCs) using F8T2 : PCBM as the active layer show a low power conversion efficiency ( The electron donor band gap and energy level alignment between donor and acceptor strongly influence the absorption, exciton dissociation and charge transfer, finally determining the PCE of PSCs [3][4][5]. Fullerene and its derivatives are considered to be the best electron acceptors so far, which is attributed to its ultrafast photo-induced charge transfer, high electron mobility, and better phase separation in the blend films [6,7]. The lowest unoccupied molecular orbit (LUMO) of typical donor material P3HT is about 0.5 eV higher than that of PCBM, which favors electron transfer from P3HT to PCBM. However, the relatively high-lying highest occupied molecular orbit (HOMO) limits the maximum open-circuit voltage (V oc ) and causes oxidation instability of the cells at ambient conditions [4]. To further improve the photovoltaic propriety of PSCs, effort should be