**2. Device structures and working principle**

Organic-inorganic hybrid solar cells are typically thin film devices consisting out of photoactive layer(s) between two electrodes of different work functions. High work function, conductive and transparent indium tin oxide (ITO) on a flexible plastic or glass substrate is often used as anode. The photoactive light absorbing thin film consists out of a conjugated polymer as organic part and an inorganic part out of e.g. semiconducting nanocrystals (NCs). A top metal electrode (e.g. Al, LiF/Al, Ca/Al) is vacuum deposited onto the photoactive layer finally. A schematic illustration of a typical device structure is shown in Fig. 1a. Generally there are two different structure types for photoactive layers - the bilayer structure (Fig. 1b) and the bulk heterojunction structure (Fig. 1c). The latter one is usually realized by just blending the donor and acceptor materials and depositing the blend on a substrate. In contrast to bulk inorganic semiconductors, photon absorption in organic semiconductor materials does not generate directly free charge carriers, but strongly bound electron-hole pairs so-called excitons (Gledhill et al., 2005). Since the exciton diffusion lengths in conjugated polymers are typically around 10-20 nm (Halls et al., 1996) the optimum distance of the exciton to the donor/acceptor (D/A) interface, where charge transfer can take place and excitons dissociate into free charge carriers, should be in the same length range. Therefore the bulk-heterojunction structure was introduced where the electron donor and acceptor materials are blended intimately together (Halls et al., 1995). The interfacial area is dramatically increased and the distance that excitons have to travel to reach the interface is reduced. After exciton dissociation into free charge carriers, holes and electrons are transported via polymer and NC percolation pathways towards the respective electrodes. Ideally, an interdigital donor acceptor configuration would be a perfect structure for efficient exciton dissociation and charge transport (Fig. 1d). In such a structure, the distance from exciton generation sites, either in the donor or the acceptor phase, to the D/A


al., (2011) --- 15.7%

al., 2001) --- 10.9% (Cunningham et

3-5% 3.86%

(Han et al., 2006) 5-9% 5,38% (Goldstein et

Table 1. Comparison of best and average PCE values of single solar cells and modules of

Organic-inorganic hybrid solar cells are typically thin film devices consisting out of photoactive layer(s) between two electrodes of different work functions. High work function, conductive and transparent indium tin oxide (ITO) on a flexible plastic or glass substrate is often used as anode. The photoactive light absorbing thin film consists out of a conjugated polymer as organic part and an inorganic part out of e.g. semiconducting nanocrystals (NCs). A top metal electrode (e.g. Al, LiF/Al, Ca/Al) is vacuum deposited onto the photoactive layer finally. A schematic illustration of a typical device structure is shown in Fig. 1a. Generally there are two different structure types for photoactive layers - the bilayer structure (Fig. 1b) and the bulk heterojunction structure (Fig. 1c). The latter one is usually realized by just blending the donor and acceptor materials and depositing the blend on a substrate. In contrast to bulk inorganic semiconductors, photon absorption in organic semiconductor materials does not generate directly free charge carriers, but strongly bound electron-hole pairs so-called excitons (Gledhill et al., 2005). Since the exciton diffusion lengths in conjugated polymers are typically around 10-20 nm (Halls et al., 1996) the optimum distance of the exciton to the donor/acceptor (D/A) interface, where charge transfer can take place and excitons dissociate into free charge carriers, should be in the same length range. Therefore the bulk-heterojunction structure was introduced where the electron donor and acceptor materials are blended intimately together (Halls et al., 1995). The interfacial area is dramatically increased and the distance that excitons have to travel to reach the interface is reduced. After exciton dissociation into free charge carriers, holes and electrons are transported via polymer and NC percolation pathways towards the respective electrodes. Ideally, an interdigital donor acceptor configuration would be a perfect structure for efficient exciton dissociation and charge transport (Fig. 1d). In such a structure, the distance from exciton generation sites, either in the donor or the acceptor phase, to the D/A

**PCEs Best module PCEs Average module** 

(MiaSolé, 2010) 10-14%

al., 2000) ~10%

al., 2009) ---

(Solarmer, 2009) 1-3%

22.9% (monocryst.) (Zhao et al., 1997) 17.55% (polycryst.) (Schott, 2010)

**PCEs** 

14-17.5% (monocryst.) 13-15% (polycryst.) 5-7% (amorphous)

**PV Technology Best cell PCEs Average cell** 

25.0% (monocryst.) (Zhao et al., 1998) 20.4% (polycryst.) (Schultz et al., 2004) 10.1% (amorphous) (Benagli et al., 2009)

8.3% (Konarka, 2010) 8.3% (Heliatek, 2010) 8.5% (Mitsubishi, 2011)

**2. Device structures and working principle** 

Si (bulk)

OPV (thin film)

different PV technologies.

CIGS (thin film) 20.3% Jackson et

CdTe (thin film) 16.7% (Wu X. et

DSSC 11.2%

interface would be in the range of the exciton diffusion length. After exciton dissociation, both holes and electrons will be transported within their pre-structured donor or acceptor phases along a direct percolation pathway to the respective electrodes. This interdigital structure can be realized by various nanostructuring approaches, which will be discussed in detail later in the section 6.2.2.

Fig. 1. Schematic illustration of typical device structures for hybrid solar cells.

In hybrid solar cells, photocurrent generation is a multistep process. Briefly, when a photon is absorbed by the absorbing material, electrons are exited from the valance band (VB) to the conduction band (CB) to form excitons. The excitons diffuse to the donor/acceptor interface where charge transfer can occur leading to the dissociation of the excitons into free electrons and holes. Driven by the internal electric field, these carriers are transported through the respective donor or acceptor material domains and are finally collected at the respective electrodes. To sum up, there are four main steps: photon absorption, exciton diffusion, charge separation as well as charge carrier transport and collection. The physics of organic/hybrid solar cells is reviewed in detail elsewhere (Greenham, 2008; Saunders & Turner, 2008).
