Shawn R. Scully

Polymer-based solar cells

A significant fraction of the cost of solar panels comes from the photoactive materials and sophisticated, energy-intensive processing technologies. Recently, it has been shown that the inorganic components can be replaced by semiconducting polymers capable of achieving reasonably high power conversion efficiencies. These polymers are inexpensive to synthesize and can be solution-processed in a roll-to-roll fashion with high throughput. Inherently poor polymer properties, such as low exciton diffusion lengths and low mobilities, can be overcome by nanoscale morphology. We discuss polymer-based solar cells, paying particular attention to device design and potential improvements.

Effects of molecular interface modification in hybrid organic-inorganic photovoltaic cells

We have systematically investigated the effects of surface modification of titania (TiO2) in hybrid TiO2∕regioregular poly(3-hexylthiophene) (P3HT) photovoltaic cells. By employing a series of para-substituted benzoic acids with varying dipoles and a series of multiply substituted benzene carboxylic acids, the energy offset at the TiO2∕polymer interface and thus the open-circuit voltage of devices can be tuned systematically by 0.25 V. Transient photovoltage measurements showed that the recombination kinetics was dominated by charge carrier concentration in these devices and were closely associated with the dark current. The saturated photocurrent of TiO2∕P3HT devices exhibited more than a twofold enhancement when molecular modifiers with large electron affinity were employed. The ability of modifiers to accept charge from polymers, as revealed in photoluminescence quenching measurement with blends of polymers, was shown to be correlated with the enhancement in device photocurrent. A planar geometry photo...

Effects of optical interference and energy transfer on exciton diffusion length measurements in organic semiconductors

Exciton diffusion is of great importance to the future design of high efficiency organic photovoltaics. Exciton diffusion studies require accurate experimental techniques. This paper addresses two important complications that can arise in exciton diffusion length measurements made by analyzing luminescence from thin films on quenching substrates: namely, the effects of optical interference and of energy transfer to the quencher. When there is modest contrast in the refractive indices of the quencher and organic material, as is the case for titania or C60 and most organic materials, interference effects can overwhelm the measurement, thereby making it impossible to accurately determine the diffusion length of excitons in the organic material. We show that this problem can be fully eliminated by using thin (<5nm) quencher films. The second complication that can occur is energy transfer to the quenching layer. We model the effect this has when fullerenes are used as quenchers. If energy transfer was ignored,...

An effective light trapping configuration for thin-film solar cells

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.

Incomplete Exciton Harvesting from Fullerenes in Bulk Heterojunction Solar Cells

We investigate the internal quantum efficiencies (IQEs) of high efficiency poly-3-hexylthiophene:[6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT:PCBM) solar cells and find them to be lower at wavelengths where the PCBM absorbs. Because the exciton diffusion length in PCBM is too small, excitons generated in PCBM decay before reaching the donor-acceptor interface. This result has implications for most state of the art organic solar cells, since all of the most efficient devices use fullerenes as electron acceptors.

Organic bulk heterojunction solar cells using poly(2,5-bis(3-tetradecyllthiophen-2-yl)thieno[3,2,-b]thiophene)

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...

Resonance energy transfer from organic chromophores to fullerene molecules

The mechanism of charge separation in polymeric bulk heterojunction photovoltaic cells is usually described as electron transfer from the absorbing polymer to an electron acceptor material such as (6,6)-phenyl C61 butyric acid methyl ester (PCBM). We consider the possibility of energy transfer to PCBM as another potential mechanism for charge separation. We demonstrate resonance energy transfer from a red-emitting organic chromophore (Nile red) to PCBM and measure a Forster radius of 3.1nm. Using standard Forster energy transfer theory, we calculate a Forster radius (R0) of around 2.7nm for this donor-acceptor pair in polystyrene. Nile red has a similar emission spectrum to commonly used conjugated polymers used in polymer/PCBM photovoltaic cells. We consider the implications of an energy transfer mechanism on the design requirements for future photovoltaic cells.

Exciton splitting and carrier transport across the amorphous-silicon/polymer solar cell interface

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.

Exciton harvesting, charge transfer, and charge-carrier transport in amorphous-silicon nanopillar/polymer hybrid solar cells

We report on the device physics of nanostructured amorphous-silicon (a-Si:H)/polymer hybrid solar cells. Using two different polymers, poly(3-hexylthiophene) (P3HT) and poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV), we study the exciton diffusion, charge transfer, and charge-carrier transport in bilayer and nanostructured a-Si:H/polymer systems. We find that strong energy transfer occurs in the a-Si:H/MEH-PPV system. However, inefficient hole transfer from the a-Si:H to the polymers renders negligible photocurrent contribution from the a-Si:H as well as very small currents in the a-Si:H/MEH-PPV devices. These results suggest that a-Si:H may be unsuitable for use in polymer-based hybrid cells. Nanosphere lithography and reactive ion etching were used to fabricate nanopillars in a-Si:H. The nanostructured a-Si:H/P3HT devices showed improved efficiency and almost perfect charge-carrier extraction under short-circuit conditions. By modeling these nanostructured devices, the loss mechan...

Physics and Materials Issues of Organic Photovoltaics

Organic materials hold promise for use in photovoltaic (PV) devices because of their potential to reduce the cost of electricity per kWh ultimately to levels below that of electricity produced by coal-fired power plants. Deposition of organics by techniques such as screen printing, doctor blading, inkjet printing, spray deposition, and thermal evaporation lends itself to incorporation in high-throughput low-cost roll-to-roll coating systems. These are low-temperature deposition techniques which allow the organics to be deposited on plastic substrates such that flexible devices can easily be made. In addition to the inherent economics of high-throughput manufacturing, lightweight and flexibility are qualities claimed to offer a simple way to reduce the price of PV panels by reducing installation costs. Flexible PVs also open niche markets like portable power generation and aesthetic-PV in building design. This chapter reviews the current state-of-the-art in making efficient organic and hybrid inorganic–organic PV devices. We discuss the basic physics of operation in a systematic way and also discuss current material limitations and identify areas that need improvement. Special emphasis is given to materials design and materials and device characterization. This chapter should serve as a guide to researchers in the field who plan to develop better material systems and optimize devices to push organic PV power conversion efficiencies above 10%.