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Solar & Alternative Energy

Solar cells based on organic materials may provide low-cost power

The development of new polymers and organic electron acceptors for solar cells increases the commercial viability of these devices.
13 April 2007, SPIE Newsroom. DOI: 10.1117/2.1200702.0649

French physicist Edmond Becquerel discovered the photovoltaic (PV) effect in 1839. Attempts at commercialization did not begin until a century later; Bell Labs developed the first crystalline silicon PV device in 1954. Now silicon solar cells have achieved efficiencies of more than 24%, but their high cost and complicated technology limit further household applications. As such, researchers have begun to examine the potential of organic/polymer solar cells, which are competitive with inorganic PV technologies for several reasons. First, organic and polymer materials are inherently inexpensive and can have very high optical absorption coefficients (are highly effective at absorbing sunlight). Furthermore, these materials can be fabricated as flexible devices using well-established printing techniques in a roll-to-roll process. Organic/polymer solar cells with energy conversion efficiencies (ne) of ~5% have been reported.1 Since several companies and research institutions are focused on this field, ne of 8–10% is highly likely in the near future.

Like other solar cells, the performance of organic/polymer devices depends on absorbing as much light as possible, converting the photon energy into free electrons, removing the electrons, and minimizing resistance. Figure 1 illustrates the typical configuration of a bulk heterojunction photovoltaic device. To absorb more sunlight, the band gap (Eg) of the polymer must be as low as possible without sacrificing electron transfer. At present, substituted poly(p-phenylene vinylene)s (PPVs) and polythiophenes (PThs) are typically used as donors in polymer photovoltaic devices. The optical Eg of these conjugated polymers (Eg=2.0–2.2eV) is not optimized with respect to the solar emission, which has a maximum photon flux around 1.8eV. Furthermore, organic electron acceptors—other than the fullerene derivatives and perylene diimides in common use—are also needed. Our group has addressed these issues by synthesizing both small band gap polymer donors as well as novel organic acceptors.

Figure 1. The schematic structure of an organic/polymer bulk heterojunction photovoltaic device.

We synthesized the novel alternating conjugated copolymer PPV-BT (see Scheme 1) consisting of electron-rich dioctyloxyphenylene vinylene (PPV) and electron-deficient 2,1,3-benzothiadiazole (BT) units via a Pd-catalyzed Heck cross-coupling polycondensation. PPV-BT has a band gap of (EgEC=1.77eV, EgOPT=1.94eV), much closer to the sun's photon flux. Photovoltaic devices of indium tin oxide/ polyethylenedioxythiophene–poly(styrene sulfonic acid)/PPV-BT + [6,6]-phenyl C60 butric acid methyl ester (ITO/PEDOT-PSS/PPV-BT+PCBM) (1/4, w/w)/Ba/Al were fabricated. At 451nm, these exhibited an ne of 0.335% (see Figure 3) and external quantum efficiency (EQE) of 11.7%.

Scheme 1. Chemical structures of dioctyloxyphenylene vinylene–2,1,3-benzothiadiazole (PPV-BT), poly(phenylene ethynylene)–2,1,3-benzothiadiazole (PPE-BT) and [6,6]-phenyl C60 butric acid methyl ester (PCBM).

Figure 2. I-V characteristics of indium tin oxide (ITO)/ polyethylenedioxythiophene–poly(styrene sulfonic acid) (PEDOT-PSS)/PPV-BT + [6,6]-phenyl C60 butric acid methyl ester (PCBM) and ITO/PEDOT-PSS/PPE-BT + PCBM (1/4,w/w)/Ba/Al devices (A.M. 1.5, 78.2mW/cm2).

Figure 3. Current-voltage characteristics of ITO/PEDOT-PSS/PTBBQ+PCBM (1/4,w/w)/Ba/Al device (A.M. 1.5, 78.2mW/cm2).

We also synthesized the copolymer PPE-BT, a poly(phenylene ethynylene) (PPE) derivative that has alternating triple bonds and an EgEC of 1.94eV (see Scheme 1). PPV-BT and PPE-BT have similar structures except that the double bonds of PPV-BT are triple bonds in PPE-BT. As shown in Figure 2, a device based on PPV-BT is more than an order of magnitude more efficient than a device based on PPE-BT. This is probably because the former has a greater ability to remove electrons and a broader optical absorption range than the latter.2,3

Table 1. Photovoltaic parameters of cells based on poly(3-hexylthophene) (P3HT) alone and blends of P3HT with different acceptors (D/A weight ratio: 1:1, A.M. 1.5, 80mW/cm2).

Scheme 2. Chemical structures of poly(3-hexylthophene) (P3HT), diimide acceptor (PV), tetra-methyl perylene-3,4,9,10-tetracarboxylate (TMEP), tetra-hexyl perylene-3,4,9,10-tetracarboxylate (THEP) and tetra-cyclohexyl perylene-3,4,9,10-tetracarboxylate (TCHEP).

Figure 4. Optical polarizing micrographs of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and tetra-benzyl perylene-3,4,9,10-tetracarboxylate (TBEP) composite: (a) MEH-PPV+TBEP (1/4,w/w), room temperature; (b) MEH-PPV+TBEP (1/4,w/w), annealing (180° C, N2, 1h) and cool to room temperature.

Another small-bandgap polymer synthesized by our group is a poly(heteroarylene methines) derivative called PTBBQ (see Figure 3), which contains alternating aromatic and quinoid segments in the main chain. The EgOPT of PTBBQ is 1.88eV according to its absorption edge at 660nm. Figure 3 presents the I/V characteristics of a PTBBQ and PCBM device. The efficiency of the device is only 0.055%, lower than the MEH-PPV and PCBM devices, probably because the low molecular weight of PTBBQ resulted in rather poor film-forming properties.4

Similar to perylene diimides, perylenetetracarboxylate (see Scheme 2) has four electron-deficient ester groups, and the proximity of the p-orbitals of adjacent molecules indicates an ordered structure that makes it suitable for electron transport. We found that perylenetetracarboxylate can be used as an electron acceptor in organic photovoltaic devices. Table 1 presents the photovoltaic parameters of devices based on poly(3-hexylthophene) P3HT and perylenetetracarboxylates with different ester groups. The highest efficiency of a device based on tetra-methyl perylene-3,4,9,10-tetracarboxylate (TMEP) with P3HT is almost one order of magnitude higher than that of P3HT alone and six-fold higher than a diimide acceptor (PV) with P3HT composite device.5

Recently we synthesized another new perylenetetracarboxylate, tetra-benzyl perylene-3,4,9,10-tetracarboxylate (TBEP). We found that the annealing process resulted in the formation of a TBEP crystal network (see Figure 4). This property increases the EQE and ne of photovoltaic devices based on TBEP and the poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) composite by a factor of five as compared to non-annealing devices.6

In conclusion, the state of the art in polymer PVs is only just beginning to achieve the minimum level of performance necessary for broad market entry. Reaching 8–10% efficiency will require better designed organic PV cell architectures, but new conjugated polymers with smaller band gaps and higher carrier mobility, as well as new organic acceptors, are also needed. These innovations will be the key to plastic solar cells becoming a viable energy source for the 21st century.

This work was supported by the National Natural Science Foundation of China (Grant No. 20274039), the Zhejiang Natural Science Foundation (Grant No. Y106086), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

Shengli Lu
Institute for Organic Solar Energy Opto-Electronic Materials,
College of Biological and Chemical Engineering Zhejiang
University of Science and Technology
Hangzhou, China