Polymer solar cells with enhanced open-circuit voltage and efficiency

Following the development of the bulk heterojunction1 structure, recent years have seen a dramatic improvement in the efficiency of polymer solar cells. Maximizing the open-circuit voltage in a low-bandgap polymer is one of the critical factors towards enabling high-efficiency solar cells. Study of the relation between open-circuit voltage and the energy levels of the donor/acceptor2 in bulk heterojunction polymer solar cells has stimulated interest in modifying the open-circuit voltage by tuning the energy levels of polymers3. Here, we show that the open-circuit voltage of polymer solar cells constructed based on the structure of a low-bandgap polymer, PBDTTT4, can be tuned, step by step, using different functional groups, to achieve values as high as 0.76 V. This increased open-circuit voltage combined with a high short-circuit current density results in a polymer solar cell with a power conversion efficiency as high as 6.77%, as certified by the National Renewable Energy Laboratory. Polymer solar cells (PSCs) have attracted much attention due to their potential in low-cost solar energy harvesting, as well as applications in flexible, light-weight, colourful and large-area devices. With the discovery of efficient photo-induced electron transfer from a conjugated polymer to fullerene1, the bulk heterojunction (BHJ) PSC has become one of the most successful device structures developed in the field to date. By simply blending polymers (electron donors) with fullerene (electron acceptor) in organic solvents, a selfassembling interpenetrating network can be obtained using various coating technologies ranging from laboratory-scale spin coating or spray coating to large-scale fabrication technologies such as inkjet printing5,6, doctor blading2, gravure7, slot-die coating8 and flexographic printing9. In the last few years, several effective methods have been developed to optimize the interpenetrating network formed by the electron donor and acceptor, including solvent annealing (or slow-growth)10, thermal annealing11–13 and morphology control using mixed solvent mixtures14 or additives15 in the solutions of donor/acceptor blends. Poly(3-hexylthiophene) (P3HT) in particular has been subject to increasing interest in the polymer research community, but significant progress has also been made in developing new active-layer polymer materials4,15–22. Since around 2008, the efficiency of PSCs has risen to 6% using new conjugated polymers as electron donors19. Although progress has been impressive, there is still much to do before the realization of practical applications of PSCs. Many factors need to be taken into account in efficiently converting sunlight into electricity. The absorption range, the photon–electron conversion rate and the carrier mobilities of the light-harvesting polymers are among the crucial parameters for achieving high-efficiency solar cells. Furthermore, fabricating largearea devices without significantly losing efficiency while maintaining long device lifetimes remains challenging23. In principle, the strategies used to improve BHJ solar cell efficiency include (i) reducing the bandgap of polymers so as to harvest more sunlight, which leads to higher short-circuit current density (Jsc) and (ii) lowering the highest occupied molecular orbital (HOMO) of the polymers, which increases the opencircuit voltage (Voc). With the rise in interest in using lowbandgap polymers to harvest more sunlight from longer wavelengths, much effort has been made recently in reducing the bandgap of polymers. Others4,15,21 have reported PSCs with power conversion efficiencies (PCE) of over 5% using different lowbandgap polymers. The extended absorption of sunlight at longer wavelengths directly reflects on the value of Jsc , and a current density of up to 16 mA cm22 has been achieved15. On the other hand, PSCs with high Voc have been realized by other groups19,20,22 by using polymers that absorb at shorter wavelengths. To push the PCE of PSCs towards the predicted theoretical limitation24, however, achieving both a high Jsc and a high Voc is critical, indeed essential. To match the energy level of the commonly used electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), both the HOMO and the lowest unoccupied molecular orbital (LUMO) of the polymer need to be considered while tuning the bandgap of polymers. It is known that the energy difference between the LUMOs of donor and acceptor should be larger than 0.3 eV for efficient charge separation24, which directly relates to the Jsc of solar cells. However, the Voc of PSCs is limited by the difference between the HOMO of the donor and the LUMO of the acceptor2.As a result, narrowing the bandgapof polymerswithout sacrificing efficient charge separation as well as high Voc becomes a major hurdle in achieving high-efficiency PSCs. In this work, we attempt to alter the HOMO of poly[4,8-bis-substituted-benzo [1,2-b:4,5-b0]dithiophene-2,6-diyl-alt-4-substituted-thieno[3, 4-b]thiophene-2,6-diyl] (PBDTTT)-derived polymers4 by adding different electron-withdrawing functional groups, step by step. We have shown that the addition of more than one electron-withdrawing group is effective in further lowering the HOMO of PBDTTT. As reported, PSCs based on the copolymer of benzo[1,2-b:4,5b0]dithiophene and thieno[3,4-b]thiophene (ref. 4) (hereafter referred to as PBDTTT–E, Fig. 1) can yield a Jsc greater than 15 mA cm22 with a Voc of 0.6 V. We chose this polymer system and tried to increase PSC performance by increasing the Voc through molecular design. Previous studies on thiophene-based polymers have shown that the alkyloxy chain has a much stronger electron-donating effect than an alkyl chain25. As a result, the HOMO of poly(3-alkoxythiophene) is higher than that of poly(3alkylthiophene). Based on this knowledge, we replaced the alkyloxy group on the carbonyl of the thieno[3,4-b]thiophene unit with an alkyl side chain (hereafter referred to as PBDTTT–C). Both