**4. CdSe NCs based hybrid solar cells**

CdSe NCs were the first NCs being incorporated into solar cells which still exhibit the highest PCEs compared to devices with NCs from other materials, and are still under extensive studies for utilization in hybrid solar cells. CdSe NCs have some advantages: they absorb at a useful spectral range for harvesting solar emission from 300 nm to 650 nm, they are good electron acceptors in combination with conjugated polymers, and the synthetic methods for their synthesis are well-established. The incorporation of CdSe spherical quantum dots into polymer for hybrid solar cells was firstly reported in 1996 (Greenham et al., 1996). At a high concentration of NCs of around 90% by weight (wt%), external quantum efficiencies (EQE) up to 10% were achieved, indicating an efficient exciton dissociation at the polymer/NCs interface. Although the phase separation, between the polymer and the NCs was observed to be in the range of 10-200 nm, the PCEs of devices were very low of about 0.1%. This was attributed to an inefficient electron transport between the individual NCs. After different shapes of NCs were synthetically available (Peng X. G. et al., 2000), different elongated CdSe structures were utilized in hybrid solar cells as electron acceptor materials.

Meanwhile numerous approaches were published regarding the synthesis of various morphologies and structures of CdSe NCs such as QDs, NRs and TPs and their application in hybrid solar cells. A significant advance was reported in 2002 (Huynh et al., 2002), when efficient hybrid solar cells based on elongated CdSe NRs and P3HT were obtained. Elongated NRs were used for providing elongated pathways for effective electron transport. Additionally, P3HT was used as donor material instead of MEH-PPV since it has a comparatively high hole mobility and absorbs at a longer wavelength range compared to PPV derivatives (Schilinsky et al., 2002). By increasing the NRs length, improved electron transport properties were demonstrated resulting in an improvement of the EQE. The optimized devices consisting out of 90wt% pyridine treated nanorods (7 nm in diameter and 60 nm in length) and P3HT exhibited an EQE over 54% and a PCE of 1.7%. Later on, 1,2,4 trichlorobenzene (TCB), which has a high boiling point, was used as solvent for P3HT instead of chlorobenzene. It was found that P3HT forms fibrilar morphology when TCB was used as solvent providing extended pathways for hole transport, which resulted in improved device efficiencies up to 2.6% (Sun & Greenham, 2006). Further improvement was achieved by using CdSe TPs, since TPs always have an extension perpendicular to the electrode for more efficient electron transport in comparison to NRs which are preferentially

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

CdSe NCs were the first NCs being incorporated into solar cells which still exhibit the highest PCEs compared to devices with NCs from other materials, and are still under extensive studies for utilization in hybrid solar cells. CdSe NCs have some advantages: they absorb at a useful spectral range for harvesting solar emission from 300 nm to 650 nm, they are good electron acceptors in combination with conjugated polymers, and the synthetic methods for their synthesis are well-established. The incorporation of CdSe spherical quantum dots into polymer for hybrid solar cells was firstly reported in 1996 (Greenham et al., 1996). At a high concentration of NCs of around 90% by weight (wt%), external quantum efficiencies (EQE) up to 10% were achieved, indicating an efficient exciton dissociation at the polymer/NCs interface. Although the phase separation, between the polymer and the NCs was observed to be in the range of 10-200 nm, the PCEs of devices were very low of about 0.1%. This was attributed to an inefficient electron transport between the individual NCs. After different shapes of NCs were synthetically available (Peng X. G. et al., 2000), different elongated CdSe structures were utilized in hybrid solar cells as electron acceptor materials. Meanwhile numerous approaches were published regarding the synthesis of various morphologies and structures of CdSe NCs such as QDs, NRs and TPs and their application in hybrid solar cells. A significant advance was reported in 2002 (Huynh et al., 2002), when efficient hybrid solar cells based on elongated CdSe NRs and P3HT were obtained. Elongated NRs were used for providing elongated pathways for effective electron transport. Additionally, P3HT was used as donor material instead of MEH-PPV since it has a comparatively high hole mobility and absorbs at a longer wavelength range compared to PPV derivatives (Schilinsky et al., 2002). By increasing the NRs length, improved electron transport properties were demonstrated resulting in an improvement of the EQE. The optimized devices consisting out of 90wt% pyridine treated nanorods (7 nm in diameter and 60 nm in length) and P3HT exhibited an EQE over 54% and a PCE of 1.7%. Later on, 1,2,4 trichlorobenzene (TCB), which has a high boiling point, was used as solvent for P3HT instead of chlorobenzene. It was found that P3HT forms fibrilar morphology when TCB was used as solvent providing extended pathways for hole transport, which resulted in improved device efficiencies up to 2.6% (Sun & Greenham, 2006). Further improvement was achieved by using CdSe TPs, since TPs always have an extension perpendicular to the electrode for more efficient electron transport in comparison to NRs which are preferentially

published (Borchert, 2010).

**4. CdSe NCs based hybrid solar cells** 

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

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 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).

Organic-Inorganic Hybrid Solar Cells: State of the Art, Challenges and Perspectives 103

Fig. 5. (a) J-V characteristic of a hybrid solar cell device containing 87 wt% CdSe QDs and P3HT as photoactive layer under AM1.5G illumination, exhibiting a PCE of 2.1% after spectral mismatch correction (Inset: Photograph of the hybrid solar cell device structure) [Zhou, Eck et al., 2010] – Reproduced by permission of The Royal Society of Chemistry. (b) Schematic illustration of the proposed QD sphere model: an outer insulating HDA ligand sphere is supposed to be responsible for the insulating organic layer in untreated QDs directly taken out of the synthesis matrix and is effectively reduced in size by methanol washing and additional acid treatment. Reprinted with permission from [Zhou, Riehle et al.,

Fig. 6. Comparison of J-V characteristics of the best devices fabricated based on HDA or

Other semiconductor NCs than CdSe were also used for hybrid solar cells. ZnO NCs have attracted a lot of attention because they are less toxic than other II-VI semiconductors and are relatively easy to synthesize in large quantities. Devices based on blends of MDMO-PPV and ZnO NCs at an optimized NC content (67 wt%) presented a PCE of 1.4% (Beek et al., 2004). By using P3HT as donor polymer which has a higher hole mobility together with an in-situ synthesis approach of ZnO directly in the polymer matrix, the efficiency was optimized up to 2% using a composite film containing 50 wt% ZnO NCs (Oosterhout et al., 2009). However, because of the relatively large band gap, the contribution to the absorption of light from ZnO NCs is very low. Another disadvantage is the low solubility of ZnO NCs in solvents which are commonly used for dissolving conjugated polymers (Beek et al., 2006).

TOP/OA ligand capped CdSe QDs and P3HT, exhibiting similar PCEs of 2.1%.

2010]. Copyright [2010], American Institute of Physics..

**5. Hybrid solar cells based on other NCs** 

Pyridine ligand exchange is the most commonly used and effective postsynthetic procedure so far, leading to the state-of-the-art efficiencies for hybrid solar cells (Huynh et al., 2002). Generally, as-synthesized NCs are washed by methanol several times and consequently refluxed in pure pyridine at the boiling point of pyridine under inert atmosphere overnight. This pyridine treatment is believed to replace the synthetic insulating ligand with shorter and more conductive pyridine molecules.

Treatments with other materials such as chloride (Owen et al., 2008), amine (Olson et al., 2009), and thiols (Aldakov et al., 2006; Sih & Wolf, 2007) were also investigated. Aldakov et al. systematically investigated CdSe NCs modified by various small ligand molecules with nuclear magnetic resonance (NMR), optical spectroscopy and electrochemistry, although their hybrid devices exhibited low efficiencies (Aldakov et al., 2006). Olson *et al.* reported on CdSe/P3HT blended devices exhibiting PCEs up to 1.77% when butylamine was used as a shorter capping ligand for the NCs (Olson et al., 2009). In an alternative approach, shortening of the insulating ligands by thermal decomposition was demonstrated and led to a relative improvement of the PCEs of the CdSe/P3HT-based solar cells (Seo et al., 2009).

However, NCs after ligand exchange with small molecules tend to aggregate and precipitate out of the organic solvent because long alky chain ligands are replaced (Huynh et al., 2002; Huynh et al., 2003), resulting in difficulties to obtain stable mixtures of NCs and polymer. Recently, a new strategy for post-synthetic treatment on spherical CdSe QDs was demonstrated (Zhou, Riehle et al., 2010), where the NCs were treated by a simple and fast hexanoic acid-assisted washing procedure. One advantage of avoiding the exchange of the synthesis capping ligands is that the QDs retain a good solubility after acid treatment, resulting in reproducible performance as well as allowing a high loading of the CdSe QDs in the blend, which is preferable for an efficient percolation network formation during the annealing step of the photoactive composite film. Devices with optimized ratios of QDs to P3HT exhibited reproducible PCEs up to 2.1% after spectral mismatch correction (Zhou, Eck et al., 2010) (Fig. 5a). This is the highest reported value for a CdSe QD / P3HT based hybrid solar cell so far. It is notable that the FF is relatively high up to 0.54, implying a good charge carrier transport capability in the devices. A simple reduced ligand sphere model was proposed to explain the possible reason for improved photovoltaic device efficiencies after acid treatment as shown in Fig. 5b (Zhou, Riehle et al., 2010). By the assistance of hexanoic acid this "immobilized" insulating spheres formed by HDA ligands are effectively reduced in size due to the salt formation of HDA. This organic salt is also much more easily dissolved in the supernatant solution than unprotonated HDA and can be separated easily from the QDs by subsequent centrifugation.

In addition, extended investigations on TOP/OA capped CdSe QDs suggested that the hexanoic acid treatment is also for this ligand system applicable for improving the device performance. Although these two kinds of QDs have different sizes (5.5 nm for HDAcapped QDs and 4.7 nm for TOP/OA capped QDs) which could result in different energy levels of QDs as well, after acid treatment both devices exhibit PCEs of 2.1% (Zhou et al., 2011) as shown in Fig. 6. Furthermore, using low band gap polymer PCPDTBT, optimized devices based on acid treated TOP/OA CdSe QDs were achieved and exhibited the highest efficiency of 2.7% for CdSe QD based devices so far (Zhou et al., 2011).

Pyridine ligand exchange is the most commonly used and effective postsynthetic procedure so far, leading to the state-of-the-art efficiencies for hybrid solar cells (Huynh et al., 2002). Generally, as-synthesized NCs are washed by methanol several times and consequently refluxed in pure pyridine at the boiling point of pyridine under inert atmosphere overnight. This pyridine treatment is believed to replace the synthetic insulating ligand with shorter

Treatments with other materials such as chloride (Owen et al., 2008), amine (Olson et al., 2009), and thiols (Aldakov et al., 2006; Sih & Wolf, 2007) were also investigated. Aldakov et al. systematically investigated CdSe NCs modified by various small ligand molecules with nuclear magnetic resonance (NMR), optical spectroscopy and electrochemistry, although their hybrid devices exhibited low efficiencies (Aldakov et al., 2006). Olson *et al.* reported on CdSe/P3HT blended devices exhibiting PCEs up to 1.77% when butylamine was used as a shorter capping ligand for the NCs (Olson et al., 2009). In an alternative approach, shortening of the insulating ligands by thermal decomposition was demonstrated and led to a relative improvement of the PCEs of the CdSe/P3HT-based

However, NCs after ligand exchange with small molecules tend to aggregate and precipitate out of the organic solvent because long alky chain ligands are replaced (Huynh et al., 2002; Huynh et al., 2003), resulting in difficulties to obtain stable mixtures of NCs and polymer. Recently, a new strategy for post-synthetic treatment on spherical CdSe QDs was demonstrated (Zhou, Riehle et al., 2010), where the NCs were treated by a simple and fast hexanoic acid-assisted washing procedure. One advantage of avoiding the exchange of the synthesis capping ligands is that the QDs retain a good solubility after acid treatment, resulting in reproducible performance as well as allowing a high loading of the CdSe QDs in the blend, which is preferable for an efficient percolation network formation during the annealing step of the photoactive composite film. Devices with optimized ratios of QDs to P3HT exhibited reproducible PCEs up to 2.1% after spectral mismatch correction (Zhou, Eck et al., 2010) (Fig. 5a). This is the highest reported value for a CdSe QD / P3HT based hybrid solar cell so far. It is notable that the FF is relatively high up to 0.54, implying a good charge carrier transport capability in the devices. A simple reduced ligand sphere model was proposed to explain the possible reason for improved photovoltaic device efficiencies after acid treatment as shown in Fig. 5b (Zhou, Riehle et al., 2010). By the assistance of hexanoic acid this "immobilized" insulating spheres formed by HDA ligands are effectively reduced in size due to the salt formation of HDA. This organic salt is also much more easily dissolved in the supernatant solution than unprotonated HDA and can be separated easily from the QDs by subsequent

In addition, extended investigations on TOP/OA capped CdSe QDs suggested that the hexanoic acid treatment is also for this ligand system applicable for improving the device performance. Although these two kinds of QDs have different sizes (5.5 nm for HDAcapped QDs and 4.7 nm for TOP/OA capped QDs) which could result in different energy levels of QDs as well, after acid treatment both devices exhibit PCEs of 2.1% (Zhou et al., 2011) as shown in Fig. 6. Furthermore, using low band gap polymer PCPDTBT, optimized devices based on acid treated TOP/OA CdSe QDs were achieved and exhibited the highest

efficiency of 2.7% for CdSe QD based devices so far (Zhou et al., 2011).

and more conductive pyridine molecules.

solar cells (Seo et al., 2009).

centrifugation.

Fig. 5. (a) J-V characteristic of a hybrid solar cell device containing 87 wt% CdSe QDs and P3HT as photoactive layer under AM1.5G illumination, exhibiting a PCE of 2.1% after spectral mismatch correction (Inset: Photograph of the hybrid solar cell device structure) [Zhou, Eck et al., 2010] – Reproduced by permission of The Royal Society of Chemistry. (b) Schematic illustration of the proposed QD sphere model: an outer insulating HDA ligand sphere is supposed to be responsible for the insulating organic layer in untreated QDs directly taken out of the synthesis matrix and is effectively reduced in size by methanol washing and additional acid treatment. Reprinted with permission from [Zhou, Riehle et al., 2010]. Copyright [2010], American Institute of Physics..

Fig. 6. Comparison of J-V characteristics of the best devices fabricated based on HDA or TOP/OA ligand capped CdSe QDs and P3HT, exhibiting similar PCEs of 2.1%.

### **5. Hybrid solar cells based on other NCs**

Other semiconductor NCs than CdSe were also used for hybrid solar cells. ZnO NCs have attracted a lot of attention because they are less toxic than other II-VI semiconductors and are relatively easy to synthesize in large quantities. Devices based on blends of MDMO-PPV and ZnO NCs at an optimized NC content (67 wt%) presented a PCE of 1.4% (Beek et al., 2004). By using P3HT as donor polymer which has a higher hole mobility together with an in-situ synthesis approach of ZnO directly in the polymer matrix, the efficiency was optimized up to 2% using a composite film containing 50 wt% ZnO NCs (Oosterhout et al., 2009). However, because of the relatively large band gap, the contribution to the absorption of light from ZnO NCs is very low. Another disadvantage is the low solubility of ZnO NCs in solvents which are commonly used for dissolving conjugated polymers (Beek et al., 2006).

Organic-Inorganic Hybrid Solar Cells: State of the Art, Challenges and Perspectives 105

**NC Shape Polymer PCE(%) Reference** 

**CdSe** TP PCPDTBT 3.19 (Dayal et al., 2010) **CdSe** TP OC1C10-PPV 2.8 (Sun et al., 2005) **CdSe** QD PCPDTBT 2.7 (Zhou et al., 2011) **CdSe** NR P3HT 2.65 (Wu & Zhang, 2010) **CdSe** NR P3HT 2.6 (Sun & Greenham, 2006) **CdSe** TP APFO-3 2.4 (Wang et al., 2006) **CdSe** Hyperbranched P3HT 2.2 (Gur et al., 2007)

**CdSe** QD P3HT 2.0 (Zhou, Riehle et al., 2010) **CdSe** QD P3HT 1.8 (Olson et al., 2009) **CdSe** NR P3HT 1.7 (Huynh et al., 2002) **ZnO** - P3HT 2.0 (Oosterhout et al., 2009) **ZnO** - P3HT 1.4 (Beek et al., 2004) **CdS** NR P3HT 2.9 (Liao et al., 2009) **CdTe** NR MEH-PPV 0.05 (Kumar & Nann, 2004) **CdTe** NR P3OT 1.06 (Kang et al., 2005) **PbS** QD MEH-PPV 0.7 (Gunes et al., 2007) **PbSe** QD P3HT 0.14 (Cui et al., 2006) **Si** QD P3HT 1.47 (Liu et al., 2010)

Table 3. Selected performance parameters of hybrid solar cells reported in literature based

Absorption of a large fraction of the incident photons is required for harvesting the maximum possible amount of the solar energy. Generally, incident photons are mainly absorbed by the donor polymer materials and partially also from the inorganic NCs. For example in blends containing 90 wt% CdSe nanoparticles in P3HT, about 60% of the total absorbed light energy can be attributed to P3HT due to its strong absorption coefficient (Dayal et al., 2010). Using P3HT as donor polymer, hybrid solar cells with spherical QDs, NRs, and hyperbranched CdSe NCs exhibited the best efficiencies of 2.0%(Zhou, Riehle et al., 2010), 2.6%(Sun & Greenham, 2006; Wu & Zhang, 2010), and 2.2%(Gur et al., 2007), respectively. However, due to the insufficient overlap between the P3HT absorption spectrum and the solar emission spectrum (Scharber et al., 2006), further improving of the

Assuming that all photons up to the band gap edge are absorbed and converted into electrons without any losses (i.e. external quantum efficiency (EQE) is constant 1), crystalline silicon with a band gap of 1.1 eV can absorb up to 64% of the photons under AM1.5 G illumination, with a theoretical achievable current density Jsc of about 45 mA/cm2. While in the case of P3HT having a band gap of 1.85 eV, only 27% photons can be absorbed, resulting in a maximal Jsc of 19 mA/cm2. By using a low band gap polymer with a band gap of e.g. about 1.4 eV, 48% photons can be absorbed leading to a maximum Jsc up to 32 mA/cm2 (Zhou, Eck et al., 2010). Nevertheless, lowering the band gap of photo-absorbing materials below a certain limit will lead to a decrease in device efficiency, because the energy of absorbed photons with a larger energy than the band gap will be wasted as the electrons

**6.1 Extension of the photon absorption and band gap engineering** 

PCE values seems to be difficult to obtain with this polymer system.

on colloidal NCs and conjugated polymers.

**6. Challenges and perspectives** 

and holes relax to the band edges.

This problem of processing ZnO NCs together with polymers to obtain well-defined morphologies limits up to now the further improvement of the solar cell performance of ZnO based hybrid solar cells.

Low band gap NCs such as CdTe, PbS, PbSe, CuInS2 and CuInSe2 NCs are promising acceptor materials due to their ability of absorbing light at longer wavelengths which may allow an additional fraction of the incident solar spectrum to be absorbed. For instance, CdTe NCs have a smaller band gap compared to CdSe NCs, while their synthesis routes are similar to CdSe NCs (Peng & Peng, 2001). However, suitable CdTe/polymer systems have not yet been found, and reported PCEs based on CdTe/MEH-PPV are quite below 0.1% (Kumar & Nann, 2004). A systematic investigation on hybrid solar cells based on MEH-PPV blended with CdSexTe1-x tetropods demonstrated a steady PCE decrease from 1.1% starting from CdSe to 0.003% with CdTe (Zhou et al., 2006). The reason of the dramatically decrease in efficiency could be attributed to the possibility that energy transfer rather than charge transfer could occur from the polymer to CdTe NCs in CdTe/Polymer blends, resulting in an insufficient generation of free charge carriers (van Beek et al., 2006; Zhou et al., 2006). However there is one work reporting over 1% efficiency using vertically aligned CdTe nanorods combined with poly(3-octylthiophene) (P3OT), indicating that CdTe NCs may be useful for hybrid solar cells when the energy levels are matching to the polymers (Kang et al., 2005). Further lowering of the NC band gap could be achieved by using semiconductors such as PbS or PbSe. Watt et al. have developed a novel surfactant-free synthetic route where PbS NCs were synthesized in situ within a MEH-PPV film (Watt et al., 2004; Watt et al., 2005). CuInS2 and CuInSe2 which have been successfully used in inorganic thin film solar cells are promising for hybrid solar cells as well. Although an early study performed by Arici et al. (Arici et al., 2003) showed very low efficiencies <0.1%, recent progress on colloidal synthesis methods for high quality CuInS2 (Panthani et al., 2008; Yue et al., 2010) might stimulate the development to more efficient photovoltaic devices. In general, using low band gap NCs as electron acceptors in polymer/NCs systems has been not successful yet, because energy transfer from polymer to low band gap NCs is the most likely outcome, resulting in inefficient exciton dissociation.

Recently it has been demonstrated that Si NCs are a promising acceptor material for hybrid solar cells due to the abundance of Si compounds, non-toxicity, and strong UV absorption. Hybrid solar cells based on blends of Si NCs and P3HT with a PCE above 1% have been reported (Liu et al., 2009). Si NCs were synthesized by radio frequency plasma via dissociation of silane, and the size can be tuned between 2 nm and 20 nm by changing chamber pressure, precursor flow rate, and radio frequency power. Devices made out of 50 wt% Si NCs, 3-5 nm in size, exhibited a PCE of 1.47% under AM1.5 G illumination which is a promising result (Liu et al., 2010).

The distribution of ligand-free NCs into the conjugated polymer matrix should be of great advantage for the resulting hybrid solar cells. This can be realized by an "in situ" synthesis approach of NCs directly in the polymer matrix. First attempts have been performed with a one pot synthesis of PbS in MEH-PPV by Watt et al. (Watt et al. 2005). Although the size distribution and concentration of synthesized NCs was not optimized, a PCE of 1.1 % was reached using this method. Liao et al. demonstrated successfully a direct synthesis of CdS nanorods in P3HT, leading to hybrid solar cells with PCEs up to 2.9% (Liao et al., 2009).

Table 3 summarized the selected performance parameters of hybrid solar cells based on colloidal NCs and conjugated polymers.


Table 3. Selected performance parameters of hybrid solar cells reported in literature based on colloidal NCs and conjugated polymers.
