Effi cient bulk heterojunction (BHJ) solar cells are characterized by a large interface area between donor and acceptor materials that ensures effi cient photogenerated exciton dissociation into free charge. The optimal scale of the phase separation between these consistuents is that of the exciton diffusion length ( L D ), and the separated phases must be contiguous to allow for low-resistance charge transport pathways from the photosensitive region to the electrodes. [1][2][3][4][5][6] To realize such a BHJ nanostructure, techniques such as thermal [ 7 ] and solvent-vapor annealing [ 8 ] have been demonstrated. The most successful processing protocols affect the aggregation and morphology in a predictable and and controlled manner. In past work, we have shown that solution-processed squaraine (SQ), followed by vacuum thermally evaporated C 60 donor/acceptor solar cells can have power conversion effi ciencies of η p = 4.6 ± 0.1% when they are fabricated into a lamellar device that is subsequently annealed at high temperature (110 ° C). [ 9 ] It was found that the annealing roughens the SQ surface, thereby creating a highly folded BHJ interface with the C 60 and thus compensating for the very short (1.Although the L D of SQ is very small, this defi ciency is partially compensated by its high absorption coeffi cient compared to that of C 60 . This motivates the use of SQ:fullerene blends, whereby the ratio of materials strongly favors that of the fullerene to take advantage of its large L D and low absorption. In previous work this approach has been partially successful, with the highest external quantum effi ciencies ( EQE ) under low intensity illumination of SQ:PC 70 BM (1:6) blends approaching 50% across the visible spectrum. Unfortunately, devices fabricated using such blends exhibited exceptionally low fi ll factors ( FF ∼ 0.35) due to a large internal series resistance to charge extraction from the low density of SQ in the mixture. Hence, under standard simulated solar illumination conditions (100 mW/cm 2 , AM1.5G spectrum), the effi ciency was limited to only ∼ 3%. [ 10 ] In this work, we explore annealing of these SQ:PC 70 BM (1:6) blends in solvent vapor to create continuous crystalline (and hence low resistance) pathways for hole conduction through the rareifi ed SQ environment. We note that, while spin-casting of these mixtures provides a simple means to prepare homogeneous thin fi lms, rapid solvent evaporation does not allow for suffi cient molecular reorganization, which is needed to achieve an equilibrium, crystalline, and uniformly phase-separated mixture. [11][12][13][14] We fi nd that post-annealing through additional extended exposure of the blend to dichloromethane (DCM) can lead to a more optimized morphology that reduces series resistance, and hence increases the FF to 0.50 ± 0.01 and a power conversion effi ciency of η p = 5.2 ± 0.3% of the resulting cells under AM1.5G, 1 sun simulated solar emission (corrected for spectral mismatch). Indeed, our best cells measured reached effi ciencies of...

We demonstrate high open circuit voltage photovoltaic cells achieved by reducing the electron leakage current through the introduction of both organic and inorganic electron blocking layers between the donor layer and the anode contact. As an example, the blocking layers reduce the dark current in tin ͑II͒ phthalocyanine ͑SnPc͒ / C 60 solar cells with response across the visible and near infrared spectral region up to a wavelength of 1000 nm, is decreased by two orders of magnitude compared to cells lacking the layers, resulting in a doubling of the open circuit voltage. The structure: indium tin oxide/electron blocker/SnPc ͑100 Å͒ / C 60 ͑400 Å͒/bathocuproine ͑100 Å͒ / Al, has a power conversion efficiency of ͑2.1Ϯ 0.1͒% at 1 sun, standard AM1.5G solar illumination. This work demonstrates the importance of reducing dark current to achieve high organic thin film photovoltaic cell efficiencies.

The donor, 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) is used with the acceptor, [6,6]-phenyl C70 butyric acid methyl ester (PC70BM) to result in efficient, solution-processed, small-molecule bulk heterojunction photovoltaic cells. The distribution of the donor nanoparticles in the acceptor matrix as a function of relative concentrations results in a trade-off between exciton dissociation and hole mobility (and hence, cell series resistance). A bulk heterojunction solar cell consisting of an active region with a component ratio of SQ to PC70BM of 1:6 has a power conversion efficiency of 2.7 +/- 0.1% with a 8.85 +/- 0.22 mA/cm(2) short-circuit current density and an open-circuit voltage of 0.89 +/- 0.01 V obtained under simulated 1 sun (100 mW/cm(2)) air mass 1.5 global (AM1.5 G) solar illumination. This is a decrease from 3.3 +/- 0.3% at 0.2 sun intensity, and is less than that of a control planar heterojunction SQ/C60 cell with 4.1 +/- 0.2% at 1 sun, suggesting that the nanoparticle morphology introduces internal resistance into the solution-based thin film. The nanomorphology and hole mobility in the films is strongly dependent on the SQ-to-PC70BM ratio, increasing by greater than 2 orders of magnitude as the ratio increases from 28% to 100% SQ.

We report new derivatives of symmetric squaraine dyes with N,N-diarylanilino substituents that have high solubility and high absorptivity (ε = 0.71–4.1 × 105 M–1 cm–1) in the red solar spectral region (λmax = 645–694 nm) making them promising candidates for application in organic photovoltaics (OPVs). Unsymmetrical N,N-diisobutylanilino- and N,N-diphenylanilino(diphenylamino)squaraines have also been prepared that give blue-shifted absorption spectra (λmax = 529–535 nm) relative to their symmetric counterparts. Compared to bis(N,N-diisobutylanilino)squaraine, both symmetrical and unsymmetrical N,N-diarylanilino squaraines show markedly broader absorption bands in solution than their N,N-dialkylanilino squaraine counterparts: the full width at half-maximum (fwhm) for N,N-diarylanilino squaraines range from 1280–1980 cm–1, while the fwhm value for the N,N-diisobutylanilino squarine is only 630 cm–1. The absorption bands for thin films of N,N-diarylanilino squaraines broaden further to 2500–3300 cm–1. N,N-Diarylanilino squaraines are fluorescent, albeit with lower quantum yields than bis(N,N-diisobutylanilino)squaraine (ϕPL = 0.02–0.66 and 0.80, respectively). OPVs were prepared with solution processed squaraine layers using the following structure: ITO/squaraine (66–85 Å)/C60 (400 Å)/BCP (100 Å)/Al (1000 Å), BCP = bathocuproine. Devices using thin films of the bis(N,N-diarylanilino)squaraines as donor layers show improved performance relative to OPVs prepared with bis(N,N-dialkylanilino)squaraines, i.e. bis(N,N-diisobutylanilino)squaraine: open-circuit voltage V oc = 0.59 ± 0.05 V, short-circuit current J sc = 5.58 ± 0.16 mA/cm2, fill factor FF = 0.56 ± 0.03, and power conversion efficiency η = 1.8 ± 0.2% under 1 sun, AM1.5G simulated illumination, compared with bis(N,N-diphenylanilino)squaraine: V oc = 0.82 ± 0.02 V, J sc = 6.71 ± 0.10 mA/cm2, FF = 0.59 ± 0.01, and η = 3.2 ± 0.1%. Morphological studies of thin films suggest that the solubility of bis(N,N-diarylanilino)squaraines plays an important role in controlling the optoelectronic properties of the OPVs.

Spin-cast 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine (SQ) thin films only 62 A thick are converted from amorphous to polycrystalline via postannealing at elevated temperatures. The surface roughness of the SQ films increases by a factor of 2, while selected area electron diffraction spectra indicate an increase in the extent of postannealed film crystallinity. Dichloromethane solvent annealing is also demonstrated to increase the exciton diffusion length of SQ by a factor of 3 over thermally annealed SQ films as a result of further enhancement in crystalline order. We find that the roughened surface features have a length scale on the order of the exciton diffusion length. Hence, coating the donor SQ with the acceptor, C(60), results in a nearly optimum controlled bulk heterojunction solar cell structure. Optimized SQ/C(60) photovoltaic cells have a power conversion efficiency of eta(p) = 4.6 +/- 0.1% (correcting for solar mismatch) at 1 sun (AM1.5G) simulated solar intensity, and a corresponding peak external quantum efficiency of EQE = 43 +/- 1% even for the very thin SQ layers employed.