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Research into polymeric photovoltaics is at a very early stage, but the results are encouraging. The best materials produced so far, using a polythiophene molecule as a hole-acceptor (to enhance the absorption of sunlight), have an energy conversion efficiency of 7% when irradiated with green light and about 2% in sunlight (4). Although these materials are currently much less efficient than their silicon counterparts, they do produce much higher open-circuit voltages. By using calcium anodes (which need to be capped to prevent oxidation in the atmosphere) and an ITO cathode, Grandstrom et al. obtained open-circuit voltages >2 V (4).

Silicon-based solar devices, on the other hand, have open-circuit voltages that are <1 V. The higher open-circuit voltages produced by the polymer-based devices mean that, compared with silicon cells, fewer polymer-based cells need to be cascaded together to obtain the same net output voltage. This may have significant advantages in certain low-power applications.

Device configuration

Schottky devices have been made from conjugated polymers by several researchers, including Marks et al. (5). These polymers include PA, polythiophene (PTh), poly(2-vinylpyridine) (P2VP), PVCZ, poly(p-phenylenevinylene) (PPV), PPy, and PAn. A common configuration is illustrated in Figure 6 (top). In general, ITO-coated glass is coated with the polymer, which in turn is coated with aluminum or another low work function metal such as magnesium or calcium.

Heterojunction and p–n-junction devices can be configured in a similar fashion to Schottky devices, except that the polymer layer now consists of bilayers of a p-type polymer and a n-type polymer (6). In addition, p–n junctions can be formed between n-type silicon and p-type conjugated polymers (7).

Photoelectrochemical cells have been made from PPy (8) and poly-(3-methylthiophene) (P3MTh) (9). We have used the configuration illustrated in Figure 6 (bottom). In this case, the polymer is coated onto the ITO-coated glass, and solid polymer electrolyte (SPE) is sandwiched between this polymer-coated electrode and a platinized ITO-coated glass counter- electrode. A liquid electrolyte can be used instead of the SPE.

The level of oxidation in the inherently conducting polymer has a dramatic effect on the photovoltaic efficiency. Highly oxidized materials are the most conducting, but they are less photoefficient (fewer excitons generated per photon absorbed). Fully reduced materials are highly resistive but the most photo efficient.

Progress to date

Three classes of conjugated polymers have attracted attention for use in photovoltaic devices in recent years:

Poly(p-phenylenevinylenes). So far, most success has been achieved by using photovoltaic devices containing PPVs. As early as 1994, Marks and co-workers described the fabrication of PPV-containing photodiodes with a structure similar to Figure 6 (top) (5). The PPV layer was obtained by spin coating the sulfonium salt precursor and then heating the polymer to 250 °C in vacuo. These devices were capable of generating open-circuit voltages of ~1.2 V when aluminum and magnesium electrodes were used or ~1.7 V when calcium electrodes were used. Quantum efficiencies of ~1% were obtained at low-light intensity (0.1 mW/cm2).

As discussed earlier, the overall efficiency of photovoltaic devices containing conjugated polymers is determined by the ability to generate excitons from incoming radiation, and then to separate these excitons at appropriate interfaces before they recombine. Given that typical exciton capture zones are limited to 10 nm or less, more efficient structures are needed. This need led several workers to the idea that interpenetrating networks of donor (electron donating–hole accepting) and acceptor (electron accepting–hole donating) polymers should give better results.

One approach involves the use of functionalized PPVs (3). The addition of cyano groups to a dialkoxy derivative of PPV (Figure 7, left) forms the CN–PPV (Figure 7, right), making it a better electron acceptor. Underivatized PPV is a good hole-transporting material. Using blends of MEH–PPV, a soluble PPV derivative, as a hole transporter and CN–PPV as an electron transporter results in quantum efficiencies of up to 6%.

More recently (4), even higher quantum efficiencies (up to 29%) with overall power conversion of ~2% (using a simulated solar spectrum) were obtained using a modified organic solvent soluble polythiophene (Figure 8, left) as the hole acceptor and a cyano derivative of PPV (MEH–CN–PPV) (Figure 8, right) as electron acceptor. Perylene is another electron acceptor that increases the quantum efficiency to 6% (10). An alternative approach uses C60 as the electron acceptor (11), giving a quantum efficiency of ~29% and an energy conversion efficiency of 2.9%

Polyanilines.
These are perhaps the most versatile of the ICPs with respect to processing options. Schottky barrier devices containing polyanilines have also been produced (12). They can be made using either chemical or electrochemical oxidation according to-oxidation reacion of aniline to polyaniline

The electrochemical method can be used to produce thin films directly on conductive substrates such as ITO. The chemical method can be used to produce a material with the de-doped emeraldine base (EB) form soluble in solvents such as 1-methyl-2-pyrrolidinone (NMP) and some doped forms. The materials are doped with appropriate surfactants such as dodecyl benzenesulfonic acid (DBSA), camphor sulfonic acid (CSA), or p-toluenesulfonic acid (pTS), all of which are soluble in common organic solvents.

The dopant has a marked effect. Interestingly, acid doping increased the engineering conversion efficiency from 0.04% (undoped) to 0.57% (poly[acrylic acid]-doped) to 0.88% (pTS-doped).

Polyaniline has been widely used in photoelectrochemical cells (13–15). Early researchers (13) investigated the photoelectrochemical reduction of chloral (CCl3CHO) to trichloro ethanol (CCl3CH2OH). Shen and Tian used PAn electrodes to induce the photoelectrochemical reduction of peroxidisulfate (14). Photocurrents generated at polyaniline are potential- and electrolyte-dependent (15, 16).

Polythiophenes. The photoelectrochemical properties of PThs have also been of interest for some time (7, 17–19). Their ability to electrodeposit regular structures with minimal impurities makes it possible to attain high photocurrents (17). The photocurrents that can be attained using PTh-based electrodes have been enhanced by using conjugated linkers to introduce electron acceptors to the PTh chain (18).

The electron-accepting NO2 group facilitates charge separation upon irradiation, resulting in sustained photocurrent. More recently, a photoelectrochemical cell that uses a solid polymer electrolyte based on poly(ethylene oxide) (PEO) was described. Quantum efficiencies of up to 0.6% could be achieved (9).

Semenikhin et al. have presented evidence that irradiation causes photoelectrochemical dedoping of poly(3-methylthiophene) or polybithiophene (19), and it is known (see later) that the level of doping influences the magnitude of the photovoltaic effect observed. There is at least one reference to the use of polythiophenes to create p–n-junction devices (20); however, no data on efficiency are given.

Future developments

Areas in which there is room for improvement are perhaps best identified by returning to the steps involved in the generation of the photovoltaic effect:

* exciton creation,
* exciton diffusion,
* electron transfer to charge transfer complex,
* electron–hole separation in built-in field,
* carrier transport to electrodes,
* electron transfer at electrodes, and
* electric power.

Yoshino et al. (21) have considered these steps and suggested that eta (energy conversion efficiency) can then be expressed as a product of five terms.
= Nexc Ne–hct Ne–hfree eVoc FF
• • • •
Nph Nexc Ne–hct hnu

= alpha(omega)• Phiex• phie–h• xien• FF