**2. Criteria for an efficient BHJ solar cell polymer**

For a conjugated polymer to suit in organic photovoltaic bulk heterojunction solar cell, it should possess favorable physical and chemical properties in order to achieve reasonable device efficiency. Key words are: large absorption coefficient; low band gap; high charge mobility; favorable blend morphology; environmental stability; suitable HOMO/LUMO level and solubility.

Conjugated Polymers for Organic Solar Cells 455

Charge transport properties are critical parameters for efficient photovoltaic cells. Higher charge carrier mobility of the polymer increases the diffusion length of electrons and holes generated during photovoltaic process and at the same time reduces the photocurrent loss by recombination in the active layer, hence improving the charge transfer efficiency from the polymer donor to the PCBM acceptor (G. Li et al., 2005). This charge transport property of the photoactive layer is reflected by charge transporting behavior of both the donor polymer and the PCBM acceptor. The electron transport property of pure PCBM thin film has been reported in details and is known to be satisfactory for high photovoltaic performance (~10-3 cm2V-1s-1) (Mihailetchi et al., 2003). However, the mobility of the free charge carriers in thin polymer films is normally in the order of 10-3 to 10-11 cm2V-1s-1, which limits the PCE of many reported devices (Mihailetchi et al., 2006). Therefore, it is promising to increase the efficiency by improving the charge carrier property of the polymer part, since there is huge space to improve if we compare this average value with the mobility value of some novel

polymer organic field effect transistor materials (Ong et al., 2004; Fong et al., 2008).

The idea that morphology of the photoactive layer can greatly influence the device performance has been widely accepted and verified by literature reports (Arias, 2002; Peet et al., 2007). However, it is still a 'state-of-art' to control the morphology of specific polymer/PCBM blend. Even though several techniques (Shaheen et al., 2001) have been reported to effectively optimize the morphology of the active layer, precise prediction on the morphology can hardly been done. It is more based on trial-and-error philosophy and theory to explain the structure-morphology relationship is still in infancy. Nevertheless, several reliable and efficient methods have been developed in laboratories to improve the

The first strategy is to control the solvent evaporation process by altering the choice of solvent, concentration of the solution and the spinning rate (Zhang et al., 2006). The slow evaporation process assists in self-organization of the polymer chains into a more ordered structure, which results in an enhanced conjugation length and a bathochromic shift of the absorption spectrum to longer wavelength region. It is reported (Peet. et al., 2007) that chlorobenzene is superior to toluene or xylene as the solvent to dissolve polymer/PCBM blend during the film casting process. The PCBM molecule has a better solubility in chlorobenzene and therefore the tendency of PCBM molecule to form clusters is suppressed in chlorobenzene. The undesired clustering of PCBM molecules will decrease the charge carrier mobility of electrons because of the large hopping boundary between segregated

The second strategy is to apply thermal annealing after film casting process. This processing technique is also widely used for organic field effect transistor materials. The choice of annealing temperature and duration is essential to control the morphology. At controlled annealing condition, the polymer and PCBM in the blend network tend to diffuse and form better mixed network favorable for charge separation and diffusion in the photoactive layer

The air stability of the solar cell device, as it is important for the commercialization, has attracted more and more attention from many research groups worldwide. Even though

**2.4 Favorable blend morphology with fullerene derivatives** 

morphology as well as the performance of the solar cell devices.

**2.3 High charge carrier mobility** 

grains.

**2.5 Stability** 

(Hoppe & Sariciftci, 2006).

### **2.1 Large absorption coefficient**

For polymers used in solar cells, a large absorption coefficient in the film state is a prerequisite for a successful application since the preliminary physics regarding photovoltaic phenomenon is photon absorption. The acceptor component of the BHJ blend, usually PC60BM or PC70BM, absorbs inefficiently longer than 400 nm (Kim et al., 2007). It is thus the responsibility for the polymer to capture the photons above 400 nm. Means to increase the solar absorption of the photoactive layer include: 1) increasing the thickness of the photoactive layer; 2) increasing the absorption coefficient; and 3) matching the polymer absorption with the solar spectrum. The first strategy is rather limited due to the fact that the charge-carrier mobilities for polymeric semiconductors can be as low as 10-4cm2/Vs (Sariciftci, 2004). Series resistance of the device increases significantly upon increasing the photoactive layer thickness and this makes devices with thick active layer hardly operational. The short-circuit current (*J*sc) may drop as well because of the low mobility of charge carriers. With the limitation to further increase the thickness, large absorption coefficient (105 to 106) in the film state is preferred in order to achieve photocurrent >10 mA/cm2 (Sariciftci, 2004). By lowering the band gap, absorption of the polymer can be broadened to longer wavelength and photons of > 800nm can be captured as well.

### **2.2 Low band gap to absorb at long wavelength**

The solar irradiation spectrum at sea level is shown in Fig 1 (Wenham & Watt, 1994). The photon energy spreads from 300 nm to > 1000 nm. However, for a typical conjugated polymer with energy gap Eg~2.0 eV, it can only absorb photon with wavelength up to *ca.* 600 nm (blue line in Fig 1) and maximum 25% of the total solar energy. By increasing the absorption onset to 1000 nm (Eg=~1.2 eV) (red line in Fig 1), approximately 70 to 80% of the solar energy will be covered and theoretically speaking an increase of efficiency by a factor of two or three can be achieved. A controversy regarding low band gap polymer is that once a polymer absorbs at longer wavelength, there will be one absorption hollow at the shorter wavelength range, leading to a decreased incident photon to electron conversion efficiency at that range. One approach to address this issue is to fabricate a tandem solar cell with both large band gap polymer and narrow band gap polymer utilized simultaneously for solar photon capture (Kim et al., 2007).

Fig. 1. Reference solar irradiation spectrum of AM1.5 illumination (black line). Blue line: typical absorption spectrum of a large band gap polymer; Red line: typical absorption spectrum of a narrow band gap polymer.

### **2.3 High charge carrier mobility**

454 Solar Cells – New Aspects and Solutions

For polymers used in solar cells, a large absorption coefficient in the film state is a prerequisite for a successful application since the preliminary physics regarding photovoltaic phenomenon is photon absorption. The acceptor component of the BHJ blend, usually PC60BM or PC70BM, absorbs inefficiently longer than 400 nm (Kim et al., 2007). It is thus the responsibility for the polymer to capture the photons above 400 nm. Means to increase the solar absorption of the photoactive layer include: 1) increasing the thickness of the photoactive layer; 2) increasing the absorption coefficient; and 3) matching the polymer absorption with the solar spectrum. The first strategy is rather limited due to the fact that the charge-carrier mobilities for polymeric semiconductors can be as low as 10-4cm2/Vs (Sariciftci, 2004). Series resistance of the device increases significantly upon increasing the photoactive layer thickness and this makes devices with thick active layer hardly operational. The short-circuit current (*J*sc) may drop as well because of the low mobility of charge carriers. With the limitation to further increase the thickness, large absorption coefficient (105 to 106) in the film state is preferred in order to achieve photocurrent >10 mA/cm2 (Sariciftci, 2004). By lowering the band gap, absorption of the polymer can be

broadened to longer wavelength and photons of > 800nm can be captured as well.

The solar irradiation spectrum at sea level is shown in Fig 1 (Wenham & Watt, 1994). The photon energy spreads from 300 nm to > 1000 nm. However, for a typical conjugated polymer with energy gap Eg~2.0 eV, it can only absorb photon with wavelength up to *ca.* 600 nm (blue line in Fig 1) and maximum 25% of the total solar energy. By increasing the absorption onset to 1000 nm (Eg=~1.2 eV) (red line in Fig 1), approximately 70 to 80% of the solar energy will be covered and theoretically speaking an increase of efficiency by a factor of two or three can be achieved. A controversy regarding low band gap polymer is that once a polymer absorbs at longer wavelength, there will be one absorption hollow at the shorter wavelength range, leading to a decreased incident photon to electron conversion efficiency at that range. One approach to address this issue is to fabricate a tandem solar cell with both large band gap polymer and narrow band gap polymer utilized simultaneously for solar

Fig. 1. Reference solar irradiation spectrum of AM1.5 illumination (black line). Blue line: typical absorption spectrum of a large band gap polymer; Red line: typical absorption

**2.2 Low band gap to absorb at long wavelength** 

photon capture (Kim et al., 2007).

spectrum of a narrow band gap polymer.

**2.1 Large absorption coefficient** 

Charge transport properties are critical parameters for efficient photovoltaic cells. Higher charge carrier mobility of the polymer increases the diffusion length of electrons and holes generated during photovoltaic process and at the same time reduces the photocurrent loss by recombination in the active layer, hence improving the charge transfer efficiency from the polymer donor to the PCBM acceptor (G. Li et al., 2005). This charge transport property of the photoactive layer is reflected by charge transporting behavior of both the donor polymer and the PCBM acceptor. The electron transport property of pure PCBM thin film has been reported in details and is known to be satisfactory for high photovoltaic performance (~10-3 cm2V-1s-1) (Mihailetchi et al., 2003). However, the mobility of the free charge carriers in thin polymer films is normally in the order of 10-3 to 10-11 cm2V-1s-1, which limits the PCE of many reported devices (Mihailetchi et al., 2006). Therefore, it is promising to increase the efficiency by improving the charge carrier property of the polymer part, since there is huge space to improve if we compare this average value with the mobility value of some novel polymer organic field effect transistor materials (Ong et al., 2004; Fong et al., 2008).
