**3. Donor-acceptor materials**

Due to the decreased size of NCs down to the nanometer scale, quantum effects occur, thus a number of physical (e.g. mechanical, electrical, optical, etc.) properties change when compared to those of bulk materials. For example, the quantum confinement effect (Brus, 1984) can be observed once the diameter of the material is in the same magnitude as the wavelength of the electron wave function. Along with the decreasing size of NCs, the energy levels of NCs turn from continuous states to discrete ones, resulting in a widening of the band gap apparent as a blue shift in the absorption and photoluminescence (PL) spectra. In general, there are two distinct routes to produce NCs: by physical approaches where they can be fabricated by lithographic methods, ion implantation, and molecular beam deposition; or by chemical approaches where they are synthesized by colloidal chemistry in

Organic-Inorganic Hybrid Solar Cells: State of the Art, Challenges and Perspectives 99

Fig. 2. Up: Chemical structures of commonly used conjugated polymers as electron donors for bulk-heterojunction hybrid solar cells. Shown are Poly(3-hexylthiophene-2,5-diyl) (P3HT), Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene](MEH-PPV), and Poly[2,6-(4,4-bis-(2-ethylhexy)-4H-cyclopenta[2,1-b;3,4-b]-dithiophene)-alt-4,7-(2,1,3-

benzothiadiazole)](PCPDTBT). Down: Differently shaped semiconductor NCs as well as the

Fig. 3. Energy levels (in eV) of commonly used conjugated polymers as electron donors and

NCs as electron acceptors in bulk-heterojunction hybrid solar cells.

chemical structure of [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) as electron

acceptors.

solution. Colloidal synthetic methods are widely used and are promising for large batch production and commercial applications. The unique optical and electrical properties of colloidal semiconductor NCs have attracted numerous interests and have been explored in various applications like light-emitting diodes (LEDs) (Kietzke, 2007), fluorescent biological labeling (Bruchez et al., 1998), lasers (Kazes et al., 2002), and solar cells (Huynh et al., 2002).

Colloidal NCs synthesized in organic media are usually soluble in common organic solvents thus they can be mixed together with conjugated polymers which are soluble in the same solvents. With suitable band gap and energy levels, NCs can be incorporated into conjugated polymer blends to form so-called bulk-heterojunction hybrid solar cells (Borchert, 2010; Reiss et al., 2011; Xu & Qiao, 2011; Zhou, Eck et al., 2010). CdS, CdSe, CdTe, ZnO, SnO2, TiO2, Si, PbS, and PbSe NCs have been used so far as electron acceptors. In Table 2 different donor-acceptor combinations in 3rd generation solar cells are shown together with the respective highest achieved PCEs from laboratory devices.

Bulk-heterojunction hybrid solar cells are still lagging behind the fullerene derivative-based OPVs in respect of device performance. Nevertheless, they have the potential to achieve better performance while still maintaining the benefits such as potentially low-cost, thin and flexible, and easy to produce. By tuning the diameter of the NCs, their band gap as well as their energy levels can be varied due to the quantum size effect. Furthermore, quantum confinement leads to an enhancement of the absorption coefficient compared to that of the bulk materials (Alivisatos, 1996). As a result, in the NCs/polymer system, both components have the ability to absorb incident light, unlike the typical polymer/fullerene system where the fullerene contributes very little to the photocurrent generation (Diener & Alford, 1998; Kazaoui & Minami, 1997). In addition, NCs can provide stable elongated structures on the length scale of 2-100 nm with desirable exciton dissociation and charge transport properties (Huynh et al., 2002).


Table 2. Donor-acceptor combinations and best PCEs of 3rd generation solar cells.

Fig. 2 illustrates commonly used donor and acceptor materials in bulk-heterojunction hybrid solar cells. The conjugated polymers usually act as electron donors and semiconductor NCs with different shapes such as spherical quantum dots (QDs), nanorods (NRs) and tetrapods (TPs) as well as the C60 derivative PCBM as electron acceptor materials.

In Fig. 3 the energy levels (in eV) of commonly used conjugated polymers as donors and NCs as acceptors for bulk-heterojunction hybrid solar cells are summarized and compared. The Fermi levels of the electrodes and the energy levels of PCBM are shown as well. The variation of the values for the energy levels are deriving from different references and are due to different applied measurement methods for extracting the respective values of the lowest unoccupied molecular orbitals and highest occupied molecular orbitals (HOMO-LUMO) levels such as cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS). The data for the respective HOMO-LUMO levels have been extracted from various references which are given in a recent review article (Zhou, Eck et al., 2010).

solution. Colloidal synthetic methods are widely used and are promising for large batch production and commercial applications. The unique optical and electrical properties of colloidal semiconductor NCs have attracted numerous interests and have been explored in various applications like light-emitting diodes (LEDs) (Kietzke, 2007), fluorescent biological labeling (Bruchez et al., 1998), lasers (Kazes et al., 2002), and solar cells (Huynh et al., 2002). Colloidal NCs synthesized in organic media are usually soluble in common organic solvents thus they can be mixed together with conjugated polymers which are soluble in the same solvents. With suitable band gap and energy levels, NCs can be incorporated into conjugated polymer blends to form so-called bulk-heterojunction hybrid solar cells (Borchert, 2010; Reiss et al., 2011; Xu & Qiao, 2011; Zhou, Eck et al., 2010). CdS, CdSe, CdTe, ZnO, SnO2, TiO2, Si, PbS, and PbSe NCs have been used so far as electron acceptors. In Table 2 different donor-acceptor combinations in 3rd generation solar cells are shown together

Bulk-heterojunction hybrid solar cells are still lagging behind the fullerene derivative-based OPVs in respect of device performance. Nevertheless, they have the potential to achieve better performance while still maintaining the benefits such as potentially low-cost, thin and flexible, and easy to produce. By tuning the diameter of the NCs, their band gap as well as their energy levels can be varied due to the quantum size effect. Furthermore, quantum confinement leads to an enhancement of the absorption coefficient compared to that of the bulk materials (Alivisatos, 1996). As a result, in the NCs/polymer system, both components have the ability to absorb incident light, unlike the typical polymer/fullerene system where the fullerene contributes very little to the photocurrent generation (Diener & Alford, 1998; Kazaoui & Minami, 1997). In addition, NCs can provide stable elongated structures on the length scale of 2-100 nm with desirable exciton dissociation and charge transport properties

**Donor Acceptor PCE(%) Reference**  Polymer C60 derivative 8.3 (Konarka, 2010) Polymer CdSe Tetrapods 3.19 (Dayal et al., 2010) Polymer Polymer 2.0 (Frechet et al., 2009) Small molecule Small molecule 8.3 (Heliatek, 2010) Dye TiO2 11.2 (Han et al., 2006)

Table 2. Donor-acceptor combinations and best PCEs of 3rd generation solar cells.

(TPs) as well as the C60 derivative PCBM as electron acceptor materials.

Fig. 2 illustrates commonly used donor and acceptor materials in bulk-heterojunction hybrid solar cells. The conjugated polymers usually act as electron donors and semiconductor NCs with different shapes such as spherical quantum dots (QDs), nanorods (NRs) and tetrapods

In Fig. 3 the energy levels (in eV) of commonly used conjugated polymers as donors and NCs as acceptors for bulk-heterojunction hybrid solar cells are summarized and compared. The Fermi levels of the electrodes and the energy levels of PCBM are shown as well. The variation of the values for the energy levels are deriving from different references and are due to different applied measurement methods for extracting the respective values of the lowest unoccupied molecular orbitals and highest occupied molecular orbitals (HOMO-LUMO) levels such as cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS). The data for the respective HOMO-LUMO levels have been extracted from various references which are given in a recent review article

with the respective highest achieved PCEs from laboratory devices.

(Huynh et al., 2002).

(Zhou, Eck et al., 2010).

Fig. 2. Up: Chemical structures of commonly used conjugated polymers as electron donors for bulk-heterojunction hybrid solar cells. Shown are Poly(3-hexylthiophene-2,5-diyl) (P3HT), Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene](MEH-PPV), and Poly[2,6-(4,4-bis-(2-ethylhexy)-4H-cyclopenta[2,1-b;3,4-b]-dithiophene)-alt-4,7-(2,1,3 benzothiadiazole)](PCPDTBT). Down: Differently shaped semiconductor NCs as well as the chemical structure of [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) as electron acceptors.

Fig. 3. Energy levels (in eV) of commonly used conjugated polymers as electron donors and NCs as electron acceptors in bulk-heterojunction hybrid solar cells.

Organic-Inorganic Hybrid Solar Cells: State of the Art, Challenges and Perspectives 101

oriented more parallel to the electrode (Hindson et al., 2011). Devices based on pyridine treated CdSe TPs exhibited efficiencies up to 2.8% (Sun et al., 2005). Recently, by using the lower band gap polymer PCPDTBT, which can absorb a higher fraction of the solar emission, an efficiency of 3.19% was reported (Dayal et al., 2010). This value is up to date the

Elongated or branched NCs in principal can provide more extended and directed electrical conductive pathways, thus reducing the number of inter-particle hopping events for extracting electrons towards the electrode. However, device performance does not only benefit from the shape of the NCs, but also from their solubility and surface modification which influence significantly the charge transfer and carrier transport behavior. Despite the relatively high intrinsic conductivity within the individual NCs, the electron mobility through the NC network in hybrid solar cells is quite low, which could be mainly attributed to the electrical insulating organic ligands on the NC surface. Ginger *et al.* have investigated charge injection and charge transfer in thin films of spherical CdSe NCs covered with TOPO ligand sandwiched between two metal electrodes (Ginger & Greenham, 2000). Very low electron mobilities in the order of 10-5 cm2V-1s-1 were measured, whereas the electron mobility of bulk CdSe is in the order of 102 cm2V-1s-1 (Rode, 1970). In most cases, the ligands used for preventing aggregation during the growth of the NCs contain long alkyl chains, such as oleic acid (OA), trioctylphosphine oxide (TOPO) or hexadecylamine (HDA), form electrically insulating layers preventing an efficient charge transfer between NCs and polymer, as well as electron transport between the individual NCs (Greenham et al., 1996; Huynh et al., 2003). In order to overcome this problem, post-synthetic treatment on the NCs has been investigated extensively. Fig. 4 shows two general strategies of post-synthetic treatment on NCs for improving the performance of hybrid solar cells – ligand exchange from original long alkyl ligands to shorter molecules e.g. pyridine, and chemical surface treatment and washing for reducing the ligand shell. A combination of ligand shell reduction and ligand exchange afterwards might further improve the solar cell performance

highest efficiency for colloidal NCs based bulk-heterojunction hybrid solar cells.

by enhancing the electron transport in the interconnected NC network.

Fig. 4. Schematic illustration of two post-synthetic QD treatment strategies to enhance the PCEs in hybrid solar cells: ligand exchange (up) and reduction of the ligand surface of QDs by applying a washing procedure (middle). A combination of the two approaches might be

beneficial for further enhancing the performance of hybrid solar cells (down).

Energy levels of donor and acceptor materials should match for efficient charge separation at the D/A interface. PL spectroscopy is a simple and useful method to investigate if a material combination can be an appropriate D/A system (Greenham et al., 1996). Because pure polymers such as P3HT and MEH-PPV exhibit a strong PL behaviour, its PL intensity is quenched by the addition of NCs with matching energy levels. This is an indication that charge transfer occurs from polymer to NCs. However, the observation of PL quenching is not necessarily a proof of charge separation within the D/A system because Förster resonance energy transfer (FRET) could also happen from larger band gap materials to smaller band gap materials, leading to strong PL quenching as well (Greenham et al., 1996). Therefore, additional methods such as photoinduced absorption (PIA) spectroscopy and light-induced electron spin resonance (L-ESR) spectroscopy are used in order to exclude PL quenching due to FRET. A detailed review on these two methods has been recently published (Borchert, 2010).
