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A new dyad of fullerene/disperse-red, denoted as PCBDR, strongly absorbs visible light in the range of 400–600 nm. PCBDR showed advantages over PCBM in several aspects such as enhanced visible-light absorption, improved solubility, and the possibility to facilitate cascaded electron transfer. P3HT:PCBDR bulk heterojunction (BHJ) solar cells, nevertheless, so far have not outperformed P3HT:PCBM BHJ solar cells under similar conditions. Among factors that affect the efficiency of P3HT:PCBDR BHJ solar cells, the suppression of the interchain interaction of P3HT in the P3HT:PCBDR blend played a major role, presumably due to better interfacial miscibility between P3HT and PCBDR than that in blends of P3HT:PCBM. In contrast, benzoporphyrin (BP), due to its unique crystallinity, morphology, and nonsolubility, afforded a better control of the morphology and the interface of the p/n junctions. As a consequence, the performance of solar cells with BP/PCBDR as the active layer was comparable to that of BP/PCBM solar...

The evaluation of the power conversion efficiency (PCE) of new materials for organic bulk heterojunction (BHJ) photovoltaics is difficult due to the large number of processing parameters possible. An efficient procedure to determine the optimum conditions for thermal treatment of polymer-based bulk heterojunction photovoltaic devices using in situ current-voltage measurements is presented. The performance of a new fullerene derivative, 1,9-dihydro-64,65-dihexyloxy-1,9-(methano[1,2] benzomethano)fullerene[60], in BHJ photovolatics with poly(3-hexylthiophene) (P3HT) was evaluated using this methodology. The device characteristics of BHJs obtained from the in situ method were found to be in good agreement with those from BHJs annealed using a conventional process. This fullerene has similar performance to 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methano fullerene in BHJs with P3HT after thermal annealing. For devices with thickness of 70 nm, the short circuit current was 6.24 mA/cm2 with a fill factor of 0.53 and open circuit voltage of 0.65 V. The changes in the current-voltage measurements during thermal annealing suggest that the ordering process in P3HT dominates the improvement in power conversion efficiency. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

A core phthalocyanine platform allows engineering of the solubility properties the band gap, shifting the maximum absorption toward the red. A simple method for increasing the efficiency of heterojunction solar cells uses a self-organized blend of phthalocyanine chromophores fabricated by solution processing.

Light and flexible photovoltaic devices based on organic materials are extensively studied as an alternative to expensive and fragile silicon-based solar cells. The efficiency of these devices is rapidly increasing with the most recent power conversion efficiency (PCE) of greater than 8% bringing them closer to commercial viability. Further improvements are needed and can be achieved by optimizing the ratio between donor and acceptor, modifying the electronic properties of the materials, and optimizing the morphology of the resulting bulk heterojunction (BHJ). A direct way to increase the efficiency is to lower the band gap of the donor material in order to absorb a greater fraction of the solar spectrum. This concept has been explored through the synthesis of a series of new low band gap polymers, which exhibit a decreased band gap as a result of lowering the lowest unoccupied molecular orbital (LUMO). An emerging challenge is the need for electronically compatible acceptors with sufficiently low LUMO levels so that charge separation is efficiently promoted. Optimum miscibility of a specific polymer and fullerene combination to create the optimum degree of phase separation is also a feature to take into account. Typically, the approach used to test new donor materials is to fabricate devices using PC61BM ([6,6]-phenyl-C61-butyric acid methyl ester), a well-studied benchmark acceptor. Only a limited number of other fullerene derivatives have been successfully employed. Although these fullerene derivatives bear different functional groups, they are related by the positioning of the substituents on carbons 1 and 2 of a six-membered ring. From literature studies it is apparent that even subtle modification of the nature and especially position of substituents can drastically alter the electronic properties of fullerene derivatives. For example, PC71BM has reduced symmetry that increases visible light absorption and has a positive effect on current generation in polymer BHJ cells. Here we report the synthesis of a series of novel fullerene derivatives functionalized through the “1,4” position and their use in organic photovoltaics (OPVs). Features that distinguish this class of fullerene derivatives are: 1) straightforward synthesis which includes versatility of functionalization with different substrates starting from the same material (fullerenol, Scheme 1); 2) tunable LUMO energy by appending electron-donating or electron-withdrawing groups; 3) lower symmetry which decreases the optical gap and produces an increased absorption in the visible (the extinction coefficient at 480 nm of a 1,4-adduct is approximately 8 times larger than that of PCBM) ; 4) tunable solubility which influences the morphology of the BHJ. Like PC71BM, 1,4addends have increased light absorption at ca. 500 nm (Figure 1) with the advantage that all the C60 derivatives