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Photoluminescence trapped inside a polymer film can be scattered out with a Bragg grating. The refractive index in the plane of the film and in the normal direction can be determined as a function of wavelength by measuring the scattering angles of the TE- and TM-waveguide modes. For the polymer poly[2-methoxy-5-(3′,7′-dimethyloctloxy)-paraphenylenevinylene], which is commonly used in light-emitting diodes, the refractive index in the plane of the film drops from 2.0 to 1.8 as the wavelength increases from 600 to 750 nm, while the refractive index in the normal direction is relatively constant at 1.57.

We have made photovoltaic cells by infiltrating the conjugated polymer regioregular poly(3-hexylthiophene) into films of mesoporous titania, which are self-assembled using a structure directing block copolymer. The mesoporous titania films were chosen because they have pores with a diameter slightly less than 10 nm, which is the exciton diffusion length in many conjugated polymers, and because they provide continuous pathways for electrons to travel to an electrode after electron transfer has occurred. The photovoltaic cells have an external quantum efficiency of 10% and a 1.5% power conversion efficiency under monochromatic 514 nm light. Experiments that vary the amount of polymer in the titania films suggest that the performance of the cells is limited by poor hole transport in the polymer.

Low thresholds (∼500 W/cm2) for amplified spontaneous emission(ASE) are reported in films of soluble poly(paraphenylene vinylene)-based conjugated polymer blends. Efficient Forster energy transfer from the absorbing host polymer to the emitting guest polymer is observed. Emission in the blends originates predominantly from the guest polymer. The large spectral shift between the absorption and emission wavelengths lowers the self-absorption losses and results in low ASE thresholds. Initial results show an enhancement in photoluminescencequantum efficiency of the blends.

We have fabricated photopumped distributed feedback lasers by spin-casting thin films of the semiconducting polymer poly(2-butyl, 5-(2′-ethyl-hexyl)-1,4-phenylenevinylene) over gratings in silicon oxide. The lasers have two modes that each have a linewidth of 0.2 nm. The lasing wavelength was tuned from 540 to 583 nm by adjusting the period of the gratings.

Gain narrowing and lasing from a soluble, highly photoluminescent conjugated polymer, poly(2-butyl, 5-(2′-ethyl-hexyl)-1,4-phenylene vinylene) (BuEH-PPV), are compared using two resonant structures: planar waveguides and microcavities. The gain narrowing and lasing thresholds are comparable, 0.05–0.1 μJ (10 ns pulse focused to ∼1.5 mm). Gain narrowing is not observed in films on indium tin oxide (ITO) unless a cladding layer is placed between the BuEH-PPV and ITO. Single-mode microcavity lasers are obtained when a cavity resonance occurs at the wavelength where the gain of the polymer is maximum.

With the advent of efficient high-bandgap metal-halide perovskite photovoltaics, an opportunity exists to make perovskite/silicon tandem solar cells. We fabricate a monolithic tandem by developing a silicon-based interband tunnel junction that facilitates majority-carrier charge recombination between the perovskite and silicon sub-cells. We demonstrate a 1 cm2 2-terminal monolithic perovskite/silicon multijunction solar cell with a VOC as high as 1.65 V. We achieve a stable 13.7% power conversion efficiency with the perovskite as the current-limiting sub-cell, and identify key challenges for this device architecture to reach efficiencies over 25%.

By transitioning to semicrystalline polymers, the performance of polymer-based solar cells has recently increased to over 5% [W. Ma et al., Adv. Fund. Mater. 15, 1665 (2005); G. Li et al., Nat. Mater. 4, 864 (2005); M. Reyes-Reyes et al., Org. Lett. 7, 5749 (2005); J. Y. Kim et al., Adv. Mater. (Weinheim, Ger.) 18, 572 (2005); J. Peet et al., Nat. Mater. 6, 497 (2007)]. Poly(2,5-bis(3-tetradecyllthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT) has caused recent excitement in the organic electronics community because of its high reported hole mobility (0.6cm2V−1s−1) that was measured in field effect transistors and its ability to form large crystals. In this letter, we investigate the potential of pBTTT as light absorber and hole transporter in a bulk heterojunction solar cell. We find that the highest efficiency of 2.3% is achieved by using a 1:4 blend of pBTTT and[6,6]-phenyl C61-butyric acid methyl ester. The hole mobility as measured by space charge limited current modeling was found to be 3.8×10−4cm2V−1s−1 in this blend.

Many thin-film solar cells make a compromise between achieving complete optical absorption using films that are thicker than the optical absorption length and achieving efficient conversion of the absorbed photons into photocurrent which is favored in thinner structures. We evaluate the performance of a V-shaped light trapping configuration that substantially increases the photocurrent generation efficiency for all angles of incidence and that is applicable to a broad class of low-cost thin-film solar cells. We experimentally demonstrate its effectiveness for small molecular weight and polymer organic solar cells. A 52% efficiency enhancement is obtained for a 170-nm-thick polymer cell.

The authors study exciton splitting at the interface of bilayer hybrid solar cells to better understand the physics controlling organic-inorganic device performance. Hydrogenated amorphous silicon (a-Si:H)∕poly(3-hexylthiophene) (P3HT) and a-Si:H∕poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) solar cells show photoresponse dominated by exciton production in the polymer. The a-Si:H∕P3HT devices are nearly as efficient as titania/P3HT cells. However, the a-Si:H∕MEH-PPV system has much lower photocurrent than a-Si:H∕P3HT, likely due to inefficient hole transfer back to the MEH-PPV after energy transfer from MEH-PPV to a-Si:H.

We fabricated flexible transparent conducting electrodes by printing films of single-walled carbon nanotube (SWNT) networks on plastic and have demonstrated their use as transparent electrodes for efficient, flexible polymer-fullerene bulk-heterojunction solar cells. The printing method produces relatively smooth, homogeneous films with a transmittance of 85% at 550nm and a sheet resistance (Rs) of 200Ω∕◻. Cells were fabricated on the SWNT/plastic anodes identically to a process optimized for ITO/glass. Efficiencies, 2.5% (AM1.5G), are close to ITO/glass and are affected primarily by Rs. Bending test comparisons with ITO/plastic show the SWNT/plastic electrodes to be far more flexible.