Polymer tandem solar cells with a PCE of 5.8% are demonstrated by employing a p–n junction as an interlayer between the two subcells. The role of the interlayer and several important issues of the tandem structure is addressed including optical optimization, interfacial engineering, and accurate efficiency characterization (see image). It is revealed that the interlayer acts as a metal/semiconductor contact as opposed to a traditional tunnel junction in inorganic tandem cells.

Polymer solar cells have evolved as a promising costeffective alternative to inorganic-based solar cells [1,2] due to their potential to be low-cost, light-weight, and flexible. Since the discovery of ultrafast photoinduced charge transfer from a conjugated polymer to fullerene molecules, followed by the introduction of the bulk heterojunction (BHJ) concept, [3] intensive research with potential materials has been carried out as future photovoltaic (PV) technology. [4][5][6][7][8] Two organic materials with distinct donor and acceptor properties are required to form a heterojunction in the bulk film, which is often achieved by solution processing. In such a case, the BHJ not only provides abundant donor/acceptor interfaces for charge separation, but also forms an interpenetrating network for charge transport. [8,9] Highly efficient polymer solar cells based on poly(3-hexylthiosphene) (P3HT) and [6,6]-phenyl C 61 butyric acid methyl ester (PC 61 BM) have been reported with power conversion efficiencies of 4-5%. [6,[10][11][12] The two most decisive parameters regarding polymer-solar-cell efficiencies are the open-circuit voltage (V oc ) and the short-circuit current (J sc ). J sc is mostly determined by the light absorption ability of the material, the charge-separation efficiency, and the high and balanced carrier mobilities. On the other hand, V oc is limited by the difference in the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor, where a small V oc (as compared to the photon energy) represents a smaller driving force for the PV process. For the P3HT:PC 61 BM system, the V oc is around 0.6 V, which significantly limits the overall device efficiency. An effective method to improve the V oc of polymer solar cells is to manipulate the HOMO level of the donor and/or LUMO level of the acceptor.[13] Until now, fullerene derivatives have proved to be one of the best and most commonly used electron acceptors. Fortunately, it is convenient to change the band gap and energy levels of the donor material by modifying the chemical structure to achieve a high V oc . [13,14] Amongst various polymers, poly{[2,7-(9-(20-ethylhexyl)-9-hexylfluorene])-alt- [5,50-(40, 70-di-2-thienyl-20,10,30-benzothiadiazole)]} (PFDTBT) has a deep HOMO level, which leads to a large V oc when blended with PC 61 BM. Svensson et al. [15] have reported polymer PV cells with a V oc of 1 V based on alternating copolymer PFDTBT blended with PC 61 BM. Moreover, Inganäs et al.[16] reported a systematic study of PV cells using four different fluorene copolymers by varying the length of the alkyl side chain and chemical structure, exhibiting power conversion efficiencies above 2-3%. Unfortunately, in their case, the low photocurrent becomes a major limiting factor in achieving higher efficiencies, suggesting low carrier mobilities.In this study, poly{[2,7-(9,9-bis-(2-ethylhexyl)-fluorene)]-alt-[5,5-(4,7-di-2 0 -thienyl-2,1,3-benzothiadiazole)]} (BisEH-PFDTBT) and poly{[2,7-(9,9-bis-(3,...

A kink is sometimes seen in the I-V curves for organic solar cells. In literature charge blocking has been speculated to be responsible for such kind of anomalous features. In this manuscript, we use poly(3-hexylthiophene):[6, 6]-phenyl-C61-butyric acid methyl ester as our model polymer system and investigate different device structures using ultraviolet photoelectron spectroscopy as our primary tool to investigate the reason for this S-shaped kink. We attribute this anomalous feature to the presence of strong interface dipoles. We further propose a model based on the standard set of Poisson equation, continuity equation, and current density equations including both drift and diffusion components.

With the aim of revealing the role of additives in polymer solar cells, different amounts of 1,8-octanedithiol (OT) were added to the poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend system in order to observe the transition between systems with and without additive. We found that highly efficient P3HT:PCBM networks can be formed in the very early stage of the spin-coating process when a small amount of additive was added. The carrier mobilities of this fast-grown network were found to be comparable with those processed with solvent annealing. As a result, short circuit current density (J sc) as high as ∼9 mA cm−2 can be obtained and the fill factor (FF) can reach ∼62%. The power conversion efficiency (PCE), however, is limited by the low open circuit voltage (V oc) of the devices processed with OT. According to the grazing incidence X-ray diffraction data, a much shorter interlayer spacing (d (100) ∼ 15.6−15.7 Å) compared with those processed with different methods is observed in the polymer blends processed with OT, which is likely the reason for the low V oc.