Sun is the largest carbon-neutral energy source that has not been fully utilized. Although there are solar cell devices based on inorganic semiconductor to efficiently harvest solar energy, the cost of these conventional devices is too high to be economically viable. This is the major motivation for the development of organic photovoltaic (OPV) materials and devices, which are envisioned to exhibit advantages such as low cost, flexibility, and abundant availability.[1] The past success in organic light-emitting diodes provides scientists with confidence that organic photovoltaic devices will be a vital alternate to the inorganic counterpart.At the heart of the OPV technology advantage is the easiness of the fabrication, which holds the promise of very low-cost manufacturing process. A simple, yet successful technique is the solution-processed bulk heterojunction (BHJ) solar cell composed of electron-donating semiconducting polymers and electron-withdrawing fullerides as active layers.[2] The composite active layer can be prepared as a large area in a single step by using techniques such as spin-coating, inkjet-printing, spraycoating, gravure-coating, roller-casting etc.[3] In the last fifteen years, a significant progress has been made on the improvement of the power-conversion efficiency (PCE) of polymer BHJ solar cells, and the achieved efficiencies have evolved from less than 1% in the poly(phenylene vinylene) (PPV) system in 1995,[2] to 4-5% in the poly(3-hexylthiphene) (P3HT) system in 2005, [4] to around 6%, as reported recently.[5] However, the efficiency of polymer solar cells is still significantly lower than their inorganic counterparts, such as silicon, CdTe and CIGS, which prevents practical applications in large scale.There are many factors limiting the performance of the BHJ solar cells.[6] Among them, the properties of materials of the active layer are the most determining factor in the overall performances of polymer solar cells. [7] Ideally, the polymers should have a broad absorption in the solar spectrum to ensure effective harvesting of the solar photons and a high chargecarriers mobility for charge transport. Further, suitable energy levels of the polymer are required that match those of the fullerides. The polymer should have a low-lying highest occupied molecular orbital (HOMO) energy level to provide a large open-circuit voltage (V oc ) and a suitable lowest unoccupied molecular orbital (LUMO) energy level to provide enough offset for charge separation. In addition, morphology of the active composite layer plays a very important role. It is imperative that a bicontinuous network with a domain width approximately twice that of the exciton diffusion length and a high donor/acceptor interfaces is formed, which favors the exciton dissociation and transport of the separated charges to the respective electrode. [8] Most of the polymers reported to date are far from ideal to fulfill all these requirements. [9] We have developed a series of novel semiconducting polymers based on alternating e...

Organic photovoltaic (OPV) technology has been developed and improved from a fancy concept with less than 1% power conversion efficiency (PCE) to over 10% PCE, particularly through the efforts in the last decade. The significant progress is the result of multidisciplinary research ranging from chemistry, material science, physics, and engineering. These efforts include the design and synthesis of novel compounds, understanding and controlling the film morphology, elucidating the device mechanisms, developing new device architectures, and improving large-scale manufacture. All of these achievements catalyzed the rapid growth of the OPV technology. This review article takes a retrospective look at the research and development of OPV, and focuses on recent advances of solution-processed materials and devices during the last decade, particular the polymer version of the materials and devices. The work in this field is exciting and OPV technology is a promising candidate for future thin film solar cells.

A method which enables the investigation of the buried interfaces without altering the properties of the polymer films is used to study vertical phase separation of spin‐coated poly(3‐hexylthiophene) (P3HT):fullerene derivative blends. X‐ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analysis reveals the P3HT enrichment at the free (air) surfaces and abundance of fullerene derivatives at the organic/substrate interfaces. The vertical phase separation is attributed to the surface energy difference of the components and their interactions with the substrates. This inhomogeneous distribution of the donor and acceptor components significantly affects photovoltaic device performance and makes the inverted device structure a promising choice.

The mixed solvent approach has been demonstrated as a promising method to modify nanomorphology in polymer solar cells. This work aims to understand the unique role of the additive in the mixture solvent and how the optimized nanoscale phase separation develops laterally and vertically during the non‐equilibrium spin‐coating process. We found the donor/acceptor components in the active layer can phase separate into an optimum morphology with the additive. Supported by AFM, TEM and XPS results, we proposed a model and identified relevant parameters for the additive such as solubility and vapor pressures. Other additives are discovered to show the ability to improve polymer solar cell performance as well.