Guodan Wei

Open circuit voltage enhancement due to reduced dark current in small molecule photovoltaic cells

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 (II) phthalocyanine (SnPc)∕C60 solar cells with response across the visible and near infrared spectral region up to a wavelength of 1000nm, 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 (100A)∕C60 (400A)/bathocuproine (100A)∕Al, has a power conversion efficiency of (2.1±0.1)% at 1sun, standard AM1.5G solar illumination. This work demonstrates the importance of reducing dark current to achieve high organic thin film photovoltaic cell efficiencies.

Solution-Processed Squaraine Bulk Heterojunction Photovoltaic Cells

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.

Efficient, Ordered Bulk Heterojunction Nanocrystalline Solar Cells by Annealing of Ultrathin Squaraine Thin Films

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.

Small-Molecule Photovoltaics Based on Functionalized Squaraine Donor Blends

Two squaraine (SQ) donor molecules with different absorption bands are blended together for better coverage of the solar spectrum. The blend SQ device shows a significant improvement compared with single SQ donor devices. By applying a solvent annealing process and a compound buffer layer, a power-conversion efficiency of 5.9 ± 0.3% is achieved under 1 sun illumination.

Functionalized Squaraine Donors for Nanocrystalline Organic Photovoltaics

We study a family of functionalized squaraine (fSQ) donors for absorbing in the near-infrared (NIR) and green spectral regions. The NIR-absorbing materials are the symmetric molecules 2,4-bis[4-(N-phenyl-1-naphthylamino)-2,6-dihydroxyphenyl]squaraine (1-NPSQ), 2,4-bis[4-(N,N-diphenylamino)-2,6 dihydroxyphenyl]squaraine, and 2,4-bis[4-(N,N-dipropylamino)-2,6-dihydroxyphenyl]squaraine. The green light absorbing donors are asymmetric squaraines, namely, 2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl]squaraine and 2-[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]-4-diphenylamino]squaraine. Substitution of the arylamine groups enhances intermolecular packing, thereby increasing hole transport and the possibility of forming extended nanocrystalline junctions when annealed. Nanocrystalline solar cells based on fSQ and a C(60) acceptor have V(oc) = 1.0 V and fill factors 0.73 ± 0.01. Solar cells incorporating annealed 1-NPSQ films result in a power conversion efficiency of 5.7 ± 0.6% at 1 sun, AM1.5G illumination.

High efficiency organic photovoltaic cells based on a vapor deposited squaraine donor

2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) is used as a donor material in vapor deposited organic heterojunction photovoltaic cells. Devices with the structure indium tin oxide/SQ (x)∕C60 (400A)/bathocuproine (100A)∕Al (1000A), where x=65, 110, 150, and 200A were compared. Devices with x=65A exhibited a power conversion efficiency of 3.1% under 1sun, AM1.5G simulated solar irradiation, giving an open circuit voltage of 0.76±0.01V, a short circuit current of 7.01±0.05mA∕cm2, and a fill factor of 0.56±0.05. Thicker SQ films lead to lower short circuit currents and fill factors, giving conversion efficiencies in the range of 2.6% to 3.2%. The demonstration of sublimable SQ as a donor material opens up a family of compounds for use in small molecule based heterojunction photovoltaics.

Thermodynamic limits of quantum photovoltaic cell efficiency

The intermediate band solar cell has been proposed as an ultrahigh efficiency source of energy due to the possibility of absorption of two sequential sub-band-gap photons to excite charge from a quantum confined (e.g., quantum dot or well) region into the large band gap barrier region [A. Luque and A. Marti, Phys. Rev. Lett. 78, 5014 (1997)]. Unfortunately, high efficiencies using this structure have not yet been realized. Here, we analyze the fundamental limits to power generation in quantum solar cells. When a difference in quasi-Fermi energies between the barrier and the quantum well regions exists due to the presence of photogenerated charge, an upper efficiency limit of 44.5% is achievable due to single photon absorption only. This efficiency is significantly higher than the Shockley-Queisser limit of ∼31% for homojunction cells, but remains below that predicted for two photon excitation (>63%) previously predicted for quantum cells.

Inverted small molecule organic photovoltaic cells on reflective substrates

We demonstrate top-illuminated, inverted, small molecule photovoltaic cells grown on reflective substrates employing copper phthalocyanine as the donor and 3,4,9,10-perylenetetracarboxylic bis-benzimidazole as the acceptor, with a sputter-deposited transparent indium tin oxide top cathode and a metal anode, thereby reversing the conventional charge extraction properties of these contacts. The best device achieved a peak power conversion efficiency of 0.74±0.03%, reasonably consistent with the optical simulations under 1sun AM1.5G illumination giving 0.83±0.02%. This work suggests that inverted organic solar cells grown on reflective substrates have potential uses such as for power-generating coatings on opaque surfaces.

Electron conducting buffer layers in organic photovoltaics

It is common to incorporate a cathode-side buffer layer in organic photovoltaic devices (OPVs) to mitigate damage from the evaporation of metal onto the underlying acceptor layer (e.g. C60), which can lead to exciton quenching and/or a barrier to charge extraction. Additionally, these materials can act as both an optical spacer and an exciton blocking layer. One class of buffer layers consists of a wide bandgap material (e.g. bathocuproine), that transports carriers via damage-induced midgap states. A second class of buffers consists of a material such as tris(acetylacetonato) ruthenium(III) (Ru(acac)3), which has a small highest occupied molecular orbital (HOMO) energy. In that case, electrons from the C60 and holes from the cathode recombine at the C60/Ru(acac)3 interface. In this work, we introduce a third class of buffer that, due to alignment of its lowest unoccupied molecular orbital with that of the acceptor, allows for low resistance transport of electrons between the acceptor and cathode. By utilizing 3,4,9,10 perylenetetracarboxylic bisbenzimidazole (PTCBI) as a buffer layer, we show improved fill factor in squaraine/C60-based devices without a loss in open-circuit voltage or photocurrent, leading to a >20% increase in power conversion efficiency. Although limited exciton transfer occurs from C60 to PTCBI, the short exciton diffusion length of PTCBI, coupled with the lack of loss at the C60/PTCBI interface, suggests that PTCBI also blocks excitons from quenching at metal-induced defects that are present in the absence of a buffer layer.

Squaraine donors for high efficiency small molecule solar cells

It has been a challenge to achieve high efficiency organic photovoltaics (OPV) that absorb long wavelength solar radiation without incurring unacceptable reductions in open circuit voltage (Voc) or charge separation efficiency. Based on the parent structure of the 2, 4-bis[4-(N, N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ), we have increased Voc using a family of highly near-infrared absorbing SQs, achieving values as high as 0.94 V. These SQ donors are: 2, 4-bis[4-(N-Phenyl-1-naphthylamino)-2,6-dihydroxyphenyl] squaraine (1-NPSQ),2,4-bis[4-(N, N-diphenylamino)-2,6 dihydroxyphenyl] squaraine (DPSQ), 2,4-bis[4-(N, N-diphenylamino)-2,6-dihydroxyphenyl] asymmetric squaraine (DPASQ). The spin-cast SQ, 1-NPSQ, DPSQ and DPASQ donors are then coated with the acceptor C60 to form bulk heterojunction (BHJ) solar cells that take advantage of their exceptionally high absorption coefficient and nanocrystalline morphology to overcome the short diffusion length characteristic of these materials. Combined with a high short-circuit current density (Jsc=10.6 mA/cm2) and high fill factor (FF=0.64), the optimized 1-NPSQ/C60 photovoltaic cells with 1-NPSQ annealed at elevated temperature have a power conversion efficiency of ηp as high as 6.0% (correcting for solar mismatch) at 1 sun (AM 1.5G) simulated solar illumination, which to our knowledge is the highest efficiency reported to date for small molecule OPVs.