**6. Challenges and perspectives**

104 Solar Cells – New Aspects and Solutions

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

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,

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

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

ZnO based hybrid solar cells.

resulting in inefficient exciton dissociation.

a promising result (Liu et al., 2010).

colloidal NCs and conjugated polymers.

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

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

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 and holes relax to the band edges.

Organic-Inorganic Hybrid Solar Cells: State of the Art, Challenges and Perspectives 107

that for a minimum energy offset of 0.3 eV between the donor and acceptor LUMO levels, PCEs of >10% are practical available for a donor polymer with an ideal optical band gap of ~1.4 eV (Riede et al., 2008). Recently, Xu et al. predicted the highest achievable cell efficiencies in polymer/NCs hybrid solar cells by considering the polymer band gaps and polymer LUMO energy levels (Xu & Qiao, 2011). Fig. 9 illustrates the 3D contour plots of polymer LUMO levels, polymer band gaps, and calculated device efficiencies for three representative inorganic NCs with CBs at ~4.2 eV (TiO2), ~4.4 eV (ZnO) and ~3.7 eV (CdSe). Assuming all of the photons are absorbed by the polymers and the Voc equals to the energy offset between the polymer HOMO and the NC LUMO, device efficiencies beyond 10% can

Fig. 8. 3D contour plots of polymer LUMO energy levels, polymer band gaps and cell efficiencies in a single junction solar cell structure with three representative inorganic semiconductor acceptors of (a) TiO2; (b) ZnO; and (c) CdSe. The conversion efficiencies of solar cells were calculated by assuming IPCE =65%, FF=60% under AM 1.5 with an incident light intensity of 100 mW cm2. [Xu & Qiao, 2011] – Reproduced by permission of The Royal Society of Chemistry. Another approach to increase the photon absorption in the active layer is to use light trapping structures as substrates or as electrodes. Light trapping can be used to overcome the problem of insufficient absorption in thin film solar cells in general (Rim et al., 2007). Nano- and microstructures of the photoactive film material can be utilized to enlarge the total pathway of incident light through the active layer. An early attempt for realizing a light trapping structure in organic solar cells was made by Roman et al. (Roman et al., 2000), who used a sub micrometer patterned grating created by lithography to mold the active layer in a way that it exhibits a cross sectional saw tooth characteristic on its surface. The subsequently deposited aluminum top electrode is then acting as a reflactive layer. Furthermore Niggemann et al. created buried nanoelectrodes (Niggemann et al., 2004) requiring an inverted cell design by using a structured aluminium coated electrode, and microprisms (Niggemann et al., 2008) as light trapping substrate for organic solar cells. So far light trapping was only applied on pure organic solar cells, but because of the similar

be achieved by using polymers with optimal band gaps and LUMO levels.

device structure these attempts can also be applied to hybrid solar cells.

**6.2 Enhancing of the charge carrier transport in hybrid solar cells** 

The film morphology plays a decisive role in the performance of a hybrid solar cell. Both the nano-phase separation and the charge extraction must be optimized for a highly efficient solar cell. For an optimal nano-phase separation the acceptor material must be homogeneously distributed in the blend. For optimizing the charge extraction towards the electrodes, continuous percolation pathways should exist for the charges to move towards

Most low band gap polymers are from the material classes of thiophene, fluorene, carbazole, and cylopentadithiophene based polymers, which are reviewed in detail in several articles (Kamat, 2008; Riede et al., 2008; Scharber et al., 2006). Among those low band gap polymers, PCPDTBT (chemical structure shown in Fig.2) with a band gap of ~1.4 eV and a relatively high hole mobility up to 1.5×10-2 cm2V-1s-1 (Morana et al., 2008) appears to be an excellent candidate as a photon-absorbing and electron donating material (Soci et al., 2007). OPVs based on PCPDTBT:PC70BM system achieved already efficiencies up to 5.5% (Peet et al., 2007) and 6.1%(Park et al., 2009). Recently, a bulk-heterojunction hybrid solar cell based on CdSe tetrapods and PCPDTBT was reported by Dayal et al.(Dayal et al., 2010) with an efficiency of 3.13%. Devices based on PCPDTBT and CdSe TPs, exhibited an EQE of >30% in a broad range from 350 nm to 800 nm, which is the absorption band of the polymer. It is notable that the devices reached very high Jsc values above 10 mA/cm2, indicating that the broad absorption ability of the photoactive hybrid film consequently contributes to the photocurrent. Zhou et al. reported on a direct comparison study of using PCPDTBT and P3HT as donor polymer for CdSe QDs based hybrid solar cells (Zhou et al., 2011). Fig. 7a shows the comparison of the best cells fabricated from blends of P3HT:CdSe and PCPDTBT:CdSe. The PCPDTBT based device showed a considerable enhancement of PCE to 2.7% compared to the P3HT based device mainly due to the increase of Jsc. Fig. 7b shows the EQE spectrum of photovoltaic devices comparing the two different polymers. The PCPDTBT based device showed a broader EQE spectrum from 300 nm to 850 nm, and considerable photocurrent contribution from the QDs was observed at 400 nm region where the QD absorption is strong. This implies that both components of the PCPDTBT:CdSe system contribute to the absorption of incident photons and to the photocurrent generation. The energy levels of donor and acceptor materials also play an important role determining the Voc and consequently device efficiency. The optimum LUMO offset between donor and acceptor has been investigated by many research groups. An offset energy of 0.3 eV was found to be sufficient for charge transfer (Brabec et al., 2002; Bredas et al., 2004). Therefore, fitting of the donor and acceptor energy levels as well as band gap engineering are desirable for eliminating energy losses during the charge transfer process. Scharber et al. demonstrated a relationship between PCEs of solar cells, band gaps, and the offsets between donor and acceptor LUMO levels of the donor materials.

Fig. 7. (a) J-V characteristics of the best solar cells fabricated from blends of P3HT:CdSe and PCPDTBT:CdSe with a PCEs of 2.1% and 2.7% respectively. (b) EQE spectra of the P3HT:CdSe and PCPDTBT:CdSe devices (c) Absorption spectra of CdSe QDs, P3HT, PCPDTBT in thin films, in comparison with the AM1.5G solar emission spectrum.

As a result, for PCEs of devices exceeding 10%, a donor band gap <1.74 eV and a LUMO level <-3.92 eV are required (Scharber et al., 2006). In addition, Dennler et al*.* demonstrated

Most low band gap polymers are from the material classes of thiophene, fluorene, carbazole, and cylopentadithiophene based polymers, which are reviewed in detail in several articles (Kamat, 2008; Riede et al., 2008; Scharber et al., 2006). Among those low band gap polymers, PCPDTBT (chemical structure shown in Fig.2) with a band gap of ~1.4 eV and a relatively high hole mobility up to 1.5×10-2 cm2V-1s-1 (Morana et al., 2008) appears to be an excellent candidate as a photon-absorbing and electron donating material (Soci et al., 2007). OPVs based on PCPDTBT:PC70BM system achieved already efficiencies up to 5.5% (Peet et al., 2007) and 6.1%(Park et al., 2009). Recently, a bulk-heterojunction hybrid solar cell based on CdSe tetrapods and PCPDTBT was reported by Dayal et al.(Dayal et al., 2010) with an efficiency of 3.13%. Devices based on PCPDTBT and CdSe TPs, exhibited an EQE of >30% in a broad range from 350 nm to 800 nm, which is the absorption band of the polymer. It is notable that the devices reached very high Jsc values above 10 mA/cm2, indicating that the broad absorption ability of the photoactive hybrid film consequently contributes to the photocurrent. Zhou et al. reported on a direct comparison study of using PCPDTBT and P3HT as donor polymer for CdSe QDs based hybrid solar cells (Zhou et al., 2011). Fig. 7a shows the comparison of the best cells fabricated from blends of P3HT:CdSe and PCPDTBT:CdSe. The PCPDTBT based device showed a considerable enhancement of PCE to 2.7% compared to the P3HT based device mainly due to the increase of Jsc. Fig. 7b shows the EQE spectrum of photovoltaic devices comparing the two different polymers. The PCPDTBT based device showed a broader EQE spectrum from 300 nm to 850 nm, and considerable photocurrent contribution from the QDs was observed at 400 nm region where the QD absorption is strong. This implies that both components of the PCPDTBT:CdSe system contribute to the absorption of incident photons and to the photocurrent generation. The energy levels of donor and acceptor materials also play an important role determining the Voc and consequently device efficiency. The optimum LUMO offset between donor and acceptor has been investigated by many research groups. An offset energy of 0.3 eV was found to be sufficient for charge transfer (Brabec et al., 2002; Bredas et al., 2004). Therefore, fitting of the donor and acceptor energy levels as well as band gap engineering are desirable for eliminating energy losses during the charge transfer process. Scharber et al. demonstrated a relationship between PCEs of solar cells, band gaps, and the offsets between

Fig. 7. (a) J-V characteristics of the best solar cells fabricated from blends of P3HT:CdSe and

As a result, for PCEs of devices exceeding 10%, a donor band gap <1.74 eV and a LUMO level <-3.92 eV are required (Scharber et al., 2006). In addition, Dennler et al*.* demonstrated

PCPDTBT:CdSe with a PCEs of 2.1% and 2.7% respectively. (b) EQE spectra of the P3HT:CdSe and PCPDTBT:CdSe devices (c) Absorption spectra of CdSe QDs, P3HT, PCPDTBT in thin films, in comparison with the AM1.5G solar emission spectrum.

donor and acceptor LUMO levels of the donor materials.

that for a minimum energy offset of 0.3 eV between the donor and acceptor LUMO levels, PCEs of >10% are practical available for a donor polymer with an ideal optical band gap of ~1.4 eV (Riede et al., 2008). Recently, Xu et al. predicted the highest achievable cell efficiencies in polymer/NCs hybrid solar cells by considering the polymer band gaps and polymer LUMO energy levels (Xu & Qiao, 2011). Fig. 9 illustrates the 3D contour plots of polymer LUMO levels, polymer band gaps, and calculated device efficiencies for three representative inorganic NCs with CBs at ~4.2 eV (TiO2), ~4.4 eV (ZnO) and ~3.7 eV (CdSe). Assuming all of the photons are absorbed by the polymers and the Voc equals to the energy offset between the polymer HOMO and the NC LUMO, device efficiencies beyond 10% can be achieved by using polymers with optimal band gaps and LUMO levels.

Fig. 8. 3D contour plots of polymer LUMO energy levels, polymer band gaps and cell efficiencies in a single junction solar cell structure with three representative inorganic semiconductor acceptors of (a) TiO2; (b) ZnO; and (c) CdSe. The conversion efficiencies of solar cells were calculated by assuming IPCE =65%, FF=60% under AM 1.5 with an incident light intensity of 100 mW cm2. [Xu & Qiao, 2011] – Reproduced by permission of The Royal Society of Chemistry.

Another approach to increase the photon absorption in the active layer is to use light trapping structures as substrates or as electrodes. Light trapping can be used to overcome the problem of insufficient absorption in thin film solar cells in general (Rim et al., 2007). Nano- and microstructures of the photoactive film material can be utilized to enlarge the total pathway of incident light through the active layer. An early attempt for realizing a light trapping structure in organic solar cells was made by Roman et al. (Roman et al., 2000), who used a sub micrometer patterned grating created by lithography to mold the active layer in a way that it exhibits a cross sectional saw tooth characteristic on its surface. The subsequently deposited aluminum top electrode is then acting as a reflactive layer. Furthermore Niggemann et al. created buried nanoelectrodes (Niggemann et al., 2004) requiring an inverted cell design by using a structured aluminium coated electrode, and microprisms (Niggemann et al., 2008) as light trapping substrate for organic solar cells. So far light trapping was only applied on pure organic solar cells, but because of the similar device structure these attempts can also be applied to hybrid solar cells.

#### **6.2 Enhancing of the charge carrier transport in hybrid solar cells**

The film morphology plays a decisive role in the performance of a hybrid solar cell. Both the nano-phase separation and the charge extraction must be optimized for a highly efficient solar cell. For an optimal nano-phase separation the acceptor material must be homogeneously distributed in the blend. For optimizing the charge extraction towards the electrodes, continuous percolation pathways should exist for the charges to move towards

Organic-Inorganic Hybrid Solar Cells: State of the Art, Challenges and Perspectives 109

polymer. The obtained visualization of the internal material distribution gives an important

In Fig. 10a a 3D visualization of a P3HT/ZnO thin hybrid film is shown (Oosterhout et al., 2009). The volume fraction of NCs present in the active area could be successfully extracted. Furthermore the fraction of NCs connected to the top electrode could be calculated, which was decreasing from 93% for a 57 nm thin film to 80% for a 167 nm thin film. This decrease could be correlated with the decrease of the IQE. Surprisingly despite of the better IQE, thinner films are showing a considerably coarser nanophase separation with only 60% of the fraction of the P3HT lying at a distance of 10 nm or less to the next acceptor, while this value

Fig.10b shows analyzed blends of OC1C10-PPV/CdSe by 3D TEM tomography (Hindson et al., 2011). It was demonstrated that the better performance of CdSe NR based devices is due to the higher connectivity between the NCs leading to a total fraction of ca. 90% NRs being connected to the top electrode while for QD based cells the fraction of connected NCs for the same weight ratio was found to be only 78%. It was additionally found that the alignment of

the NRs is mostly horizontal, since 82% are aligned within 10° of the x-y-plane.

(a) (b)

Morphology control on the nanoscale is a key issue to reduce the recombination of excitons. The optical absorption length within the donor material of the film is of about 100 nm (Peumans et al., 2003), while the generated excitons have a diffusion length of only 10 nm to 20 nm (Halls et al., 1996). Even if an exciton reaches the donor-acceptor interface before it recombines, the generated free charges must be extracted over continuous percolation pathways directly to the respective electrodes without being trapped or getting lost by

An interpenetrated donor acceptor structure on the nanoscale, as illustrated in Fig. 1d, would considerably improve the exciton diffusion, charge collection and charge transfer efficiency resulting in higher EQE value and so leading to a higher solar cell efficiency (Sagawa et al., 2010). Figure 1d is showing a conceptual design of an ideal structure of donor

Fig. 10. (a) 3D visualizations of the a hybrid P3HT/ZnO hybrid film. Reprinted by permission from Macmillan Publishers Ltd: [Nature Materials] (Oosterhout et al., 2009), copyright (2009).; (b) Distribution of NRs within a OC1C10-PPV/CdSe-NR hybrid film based on TEM tomography. Reprinted with permission from (Hindson et al., 2011). Copyright

**6.2.2 Morphology control by nanostructuring approaches** 

feedback for solar cell development.

was nearly 100% for the 167 nm thick film.

2011 American Chemical Society.

charge recombination.

the respective electrodes. For optimized hybrid films with suitable nano-phase separation the solvents used for the donor-acceptor blends must suit well for both the NCs and the polymer. In order to improve the dispersibility of the NCs inside the NC/polymer mixture, pyridine is added in a certain optimal concentration to the P3HT solvent chloroform. The experiment resulted in a reduction of the surface roughness of the hybrid film which was measured by AFM and in a higher EQE for hybrid devices (Huynh et al., 2003).

The crystallinity of the conjugated polymer is another important factor to consider for improving the hole extraction towards the anode of the solar cell. Therefore P3HT is a suitable material (Sharma et al., 2010). Greenham's group reported that using TCB as a solvent with a slow evaporation rate, in contrast to chloroform, is enhancing the self organization of the polymer and thereby the efficiency of a P3HT/CdSe (NR) solar cell (Sun & Greenham, 2006). Additionally during the the thermal treatment interfacial and access ligands (e.g. pyridine) are removed (Huynh et al., 2003). By treating the blend at temperatures of ca. 110°C it is reported that oxygen is removed from the P3HT (Olson et al., 2009). Erb et. al reported that the crystallinity of P3HT is improving significantly after thermal annealing which can be observed by the extension of the absorption spectra to longer wavelengths after thermal treatment of the polymer (Erb et al., 2005).

### **6.2.1 Visualization of the nanomorphology of thin hybrid films**

An AFM analysis of the active layer of the hybrid blend reveals information about the surface topography. Here the roughness is mostly regarded as indicator for the quality of the nanophase separation of NC and polymer phases. An AFM image of the surface of a CdSe/P3HT hybrid film is shown in Fig. 9a. In addition TEM can be used for the investigation of thin hybrid films. The two dimensional image delivers information about the distribution of donor and acceptor materials in the film (Fig. 9b). Hereby the quality of the mixing and the tendency of NC aggregation as well as nanophase separation can be observed. A relatively new approach for the analysis of the nanomorphology in hybrid solar cells is the use of 3D TEM tomography, where a series of TEM images are taken of the sample subsequently at different tilt angles.

Fig. 9. (a) AFM image of the surface of a spin coated CdSe/P3HT blend film, (b) TEM of a CdSe/P3HT thin film. The white areas represent the polymer phase and the dark areas the NC phase.

With the help of a computer software a three dimensional tomographic view of the donoracceptor blend can be achieved (Fig. 10). This method is especially well suited for hybrid solar cells because they exhibit a high contrast between the inorganic NCs and the organic polymer. The obtained visualization of the internal material distribution gives an important feedback for solar cell development.

In Fig. 10a a 3D visualization of a P3HT/ZnO thin hybrid film is shown (Oosterhout et al., 2009). The volume fraction of NCs present in the active area could be successfully extracted. Furthermore the fraction of NCs connected to the top electrode could be calculated, which was decreasing from 93% for a 57 nm thin film to 80% for a 167 nm thin film. This decrease could be correlated with the decrease of the IQE. Surprisingly despite of the better IQE, thinner films are showing a considerably coarser nanophase separation with only 60% of the fraction of the P3HT lying at a distance of 10 nm or less to the next acceptor, while this value was nearly 100% for the 167 nm thick film.

Fig.10b shows analyzed blends of OC1C10-PPV/CdSe by 3D TEM tomography (Hindson et al., 2011). It was demonstrated that the better performance of CdSe NR based devices is due to the higher connectivity between the NCs leading to a total fraction of ca. 90% NRs being connected to the top electrode while for QD based cells the fraction of connected NCs for the same weight ratio was found to be only 78%. It was additionally found that the alignment of the NRs is mostly horizontal, since 82% are aligned within 10° of the x-y-plane.

108 Solar Cells – New Aspects and Solutions

the respective electrodes. For optimized hybrid films with suitable nano-phase separation the solvents used for the donor-acceptor blends must suit well for both the NCs and the polymer. In order to improve the dispersibility of the NCs inside the NC/polymer mixture, pyridine is added in a certain optimal concentration to the P3HT solvent chloroform. The experiment resulted in a reduction of the surface roughness of the hybrid film which was

The crystallinity of the conjugated polymer is another important factor to consider for improving the hole extraction towards the anode of the solar cell. Therefore P3HT is a suitable material (Sharma et al., 2010). Greenham's group reported that using TCB as a solvent with a slow evaporation rate, in contrast to chloroform, is enhancing the self organization of the polymer and thereby the efficiency of a P3HT/CdSe (NR) solar cell (Sun & Greenham, 2006). Additionally during the the thermal treatment interfacial and access ligands (e.g. pyridine) are removed (Huynh et al., 2003). By treating the blend at temperatures of ca. 110°C it is reported that oxygen is removed from the P3HT (Olson et al., 2009). Erb et. al reported that the crystallinity of P3HT is improving significantly after thermal annealing which can be observed by the extension of the absorption spectra to

An AFM analysis of the active layer of the hybrid blend reveals information about the surface topography. Here the roughness is mostly regarded as indicator for the quality of the nanophase separation of NC and polymer phases. An AFM image of the surface of a CdSe/P3HT hybrid film is shown in Fig. 9a. In addition TEM can be used for the investigation of thin hybrid films. The two dimensional image delivers information about the distribution of donor and acceptor materials in the film (Fig. 9b). Hereby the quality of the mixing and the tendency of NC aggregation as well as nanophase separation can be observed. A relatively new approach for the analysis of the nanomorphology in hybrid solar cells is the use of 3D TEM tomography, where a series of TEM images are taken of the

(a) (b)

Fig. 9. (a) AFM image of the surface of a spin coated CdSe/P3HT blend film, (b) TEM of a CdSe/P3HT thin film. The white areas represent the polymer phase and the dark areas the

With the help of a computer software a three dimensional tomographic view of the donoracceptor blend can be achieved (Fig. 10). This method is especially well suited for hybrid solar cells because they exhibit a high contrast between the inorganic NCs and the organic

measured by AFM and in a higher EQE for hybrid devices (Huynh et al., 2003).

longer wavelengths after thermal treatment of the polymer (Erb et al., 2005).

**6.2.1 Visualization of the nanomorphology of thin hybrid films** 

sample subsequently at different tilt angles.

NC phase.

Fig. 10. (a) 3D visualizations of the a hybrid P3HT/ZnO hybrid film. Reprinted by permission from Macmillan Publishers Ltd: [Nature Materials] (Oosterhout et al., 2009), copyright (2009).; (b) Distribution of NRs within a OC1C10-PPV/CdSe-NR hybrid film based on TEM tomography. Reprinted with permission from (Hindson et al., 2011). Copyright 2011 American Chemical Society.

### **6.2.2 Morphology control by nanostructuring approaches**

Morphology control on the nanoscale is a key issue to reduce the recombination of excitons. The optical absorption length within the donor material of the film is of about 100 nm (Peumans et al., 2003), while the generated excitons have a diffusion length of only 10 nm to 20 nm (Halls et al., 1996). Even if an exciton reaches the donor-acceptor interface before it recombines, the generated free charges must be extracted over continuous percolation pathways directly to the respective electrodes without being trapped or getting lost by charge recombination.

An interpenetrated donor acceptor structure on the nanoscale, as illustrated in Fig. 1d, would considerably improve the exciton diffusion, charge collection and charge transfer efficiency resulting in higher EQE value and so leading to a higher solar cell efficiency (Sagawa et al., 2010). Figure 1d is showing a conceptual design of an ideal structure of donor

Organic-Inorganic Hybrid Solar Cells: State of the Art, Challenges and Perspectives 111

formed by spin coating of a TiO2 dispersion onto the AAO template. After sintering at 450°C for 1 h and the subsequent removal of the 300 nm thick AAO template by NaOH, the TiO2 nanopilllars were obtained. By covering the TiO2 structure with P3HT via spin coating and subsequent evaporation of Au contacts, hybrid solar cells were manufactured with a PCE of 0.512% in comparison to 0.12% for the bilayer structure of the same donor-acceptor material composition. By this method an inverted solar cell was created, using gold as top electrode. A drawback in this design is that donor and acceptor materials are in direct contact with the ITO substrate, where both, holes and electrons, could be extracted leading to additional recombination events at the ITO electrode which lowers the overall solar cell efficiency.

(a) (b)

Fig. 12. (a) Schematic illustration of an inverted TiO2/P3HT hybrid solar cell manufactured by Kuo et al. using an AAO template for formation of parallel aligned TiO2 nanopillars subsequently filled by P3HT; (b) Schematic illustration of the energy level diagram of the fabricated hybrid solar cell. Reprinted with permission from [Kuo et al., 2008]. Copyright

It was demonstrated that by filling of the AAO template with a conjugated polymer, aligned polymer nanopillars were obtained exhibiting an increased hole mobility due to an improved vertical alignment of the polymeric chains within the AAO template (Coakley et al., 2005). The hole mobility rose by a factor of 20 from 3x10-4 cm²V-1s-1 for a flat polymer layer in diode configuration to 6x10-3 cm²V-1s-1 for the aligned polymer inside the AAO pores. After the AAO template was removed the spacings between the obtained polymer pillars can in principle be filled with an acceptor material like e.g. NCs from a deposited dispersion. This leads to a nanostructured hybrid solar cell with an interdigital device

Since TiO2 is a semiconductor and could already be used as electron acceptor together with a conjugated donor polymer, the pores of a porous TiO2 film could be directly filled with a donor polymer to obtain a nanostructured bulk-heterojunction hybrid film. Recently Lim et al. demonstrated the successful infiltration of P3HT into TiO2 nanotubes of diameters of 60 nm to 80 nm. However, the diameters of the filled pores were above the desired diameters for an efficient charge extraction, so the reproducible and complete filling of the TiO2 nanotubes is still one of the main challenges to be solved before this nanostructuring

Another method which was successfully applied for the formation of a nanostructured bulkheterojunction organic solar cell is nanoimprint lithography (NIL). An AAO template was used as a mask for etching a Si substrate using a two-step inductively coupled plasma (ICP) etching process (Aryal et al., 2008). Thereby a silicon mold as shown in Fig. 13a is formed.

[2008], American Institute of Physics.

structure as illustrated in Fig. 1d.

method can be implemented into hybrid solar cells.

and acceptor phases within the heterojunction solar cell. Different nanostructuring approaches for hybrid heterojunction solar cells have been developed to implement such a device structure. A common method is the use of a porous template and the subsequent filling of the pores by a semiconducting material in order to fabricate vertically aligned nanopillars. One possibility to obtain porous templates is the anodic oxidation of Al to alumina, so-called Anodic Aluminum Oxidation (AAO) (Jessensky et al., 1998; Liu, P. A. et al., 2010). Here, vertical channels with diameters between 20 nm to 120 nm are formed by a first electrochemical oxidation and etching step, followed by a 2nd subsequent etching step for pore widening. The pores can be filled by different methods including simple pore filling, electrochemical deposition and vapor-liquid solid (VLS) growth processes. In principle the lengths, diameters and distances of the formed aligned nanopillars and nanowires can be controlled by the respective dimensions of the template and etching conditions. The height can be controlled by the thickness of the aluminium layer. In Fig. 11 a SEM image of an AAO template fabricated in our laboratory is shown. In a similar way the anodization of titanium films can lead to porous TiO2 films and structures. The fabrication of vertically aligned tubes with pore diameters between 10 nm (Chen et al., 2007) and 100 nm (Macák et al., 2005) are reported. The main technical relevant differences to the AAO template is that TiO2 itself is a semiconductor, while Al2O3 is an insulator, and that the pores in the TiO2 template are closed at the bottom towards the ITO and so the filled in semiconducting material is not in contact with the electrode.

Fig. 11. Left: Side view SEM image of a porous AAO membrane manufactured by anodic oxidation of aluminium at 40 V in the presence of 0.3 M oxalic acid; right: Side view SEM image of porous TiO2 nanotubes fabricated by anodization of titanium. (Macák et al., 2005). Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

One notable example for the integration of a nanostructuring method into solar cell device fabrication is the use of AAO templates for the deposition of CdS by a VLS process leading to aligned nanopillars. Subsequent chemical vapor deposition (CVD) of CdTe resulted into a nanostructured all inorganic solar cell with an impressive PCE of ca. 6% (Fan et al., 2009). A few attempts to use vertically aligned nanopillars to obtain nanostructured hybrid solar cells also exist (Kuo et al., 2008; Ravirajan et al., 2006). These approaches resulted so far in devices with significant lower efficiencies compared to state of the art hybrid solar cells without additional nanostructuring steps. One example for the utilization of an AAO template for a nanostructured hybrid solar cell was published by Kuo et al. (Kuo et al., 2008) and is schematically illustrated in Fig. 12a together with its energy level diagram (Fig. 12b). A direct comparison between a nanostructured bulk-heterojunction hybrid solar cell and a bilayer based hybrid solar cell was performed. First, free standing nanopillars of TiO2 were

and acceptor phases within the heterojunction solar cell. Different nanostructuring approaches for hybrid heterojunction solar cells have been developed to implement such a device structure. A common method is the use of a porous template and the subsequent filling of the pores by a semiconducting material in order to fabricate vertically aligned nanopillars. One possibility to obtain porous templates is the anodic oxidation of Al to alumina, so-called Anodic Aluminum Oxidation (AAO) (Jessensky et al., 1998; Liu, P. A. et al., 2010). Here, vertical channels with diameters between 20 nm to 120 nm are formed by a first electrochemical oxidation and etching step, followed by a 2nd subsequent etching step for pore widening. The pores can be filled by different methods including simple pore filling, electrochemical deposition and vapor-liquid solid (VLS) growth processes. In principle the lengths, diameters and distances of the formed aligned nanopillars and nanowires can be controlled by the respective dimensions of the template and etching conditions. The height can be controlled by the thickness of the aluminium layer. In Fig. 11 a SEM image of an AAO template fabricated in our laboratory is shown. In a similar way the anodization of titanium films can lead to porous TiO2 films and structures. The fabrication of vertically aligned tubes with pore diameters between 10 nm (Chen et al., 2007) and 100 nm (Macák et al., 2005) are reported. The main technical relevant differences to the AAO template is that TiO2 itself is a semiconductor, while Al2O3 is an insulator, and that the pores in the TiO2 template are closed at the bottom towards the ITO and so the filled in

semiconducting material is not in contact with the electrode.

Fig. 11. Left: Side view SEM image of a porous AAO membrane manufactured by anodic oxidation of aluminium at 40 V in the presence of 0.3 M oxalic acid; right: Side view SEM image of porous TiO2 nanotubes fabricated by anodization of titanium. (Macák et al., 2005).

One notable example for the integration of a nanostructuring method into solar cell device fabrication is the use of AAO templates for the deposition of CdS by a VLS process leading to aligned nanopillars. Subsequent chemical vapor deposition (CVD) of CdTe resulted into a nanostructured all inorganic solar cell with an impressive PCE of ca. 6% (Fan et al., 2009). A few attempts to use vertically aligned nanopillars to obtain nanostructured hybrid solar cells also exist (Kuo et al., 2008; Ravirajan et al., 2006). These approaches resulted so far in devices with significant lower efficiencies compared to state of the art hybrid solar cells without additional nanostructuring steps. One example for the utilization of an AAO template for a nanostructured hybrid solar cell was published by Kuo et al. (Kuo et al., 2008) and is schematically illustrated in Fig. 12a together with its energy level diagram (Fig. 12b). A direct comparison between a nanostructured bulk-heterojunction hybrid solar cell and a bilayer based hybrid solar cell was performed. First, free standing nanopillars of TiO2 were

Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

formed by spin coating of a TiO2 dispersion onto the AAO template. After sintering at 450°C for 1 h and the subsequent removal of the 300 nm thick AAO template by NaOH, the TiO2 nanopilllars were obtained. By covering the TiO2 structure with P3HT via spin coating and subsequent evaporation of Au contacts, hybrid solar cells were manufactured with a PCE of 0.512% in comparison to 0.12% for the bilayer structure of the same donor-acceptor material composition. By this method an inverted solar cell was created, using gold as top electrode. A drawback in this design is that donor and acceptor materials are in direct contact with the ITO substrate, where both, holes and electrons, could be extracted leading to additional recombination events at the ITO electrode which lowers the overall solar cell efficiency.

Fig. 12. (a) Schematic illustration of an inverted TiO2/P3HT hybrid solar cell manufactured by Kuo et al. using an AAO template for formation of parallel aligned TiO2 nanopillars subsequently filled by P3HT; (b) Schematic illustration of the energy level diagram of the fabricated hybrid solar cell. Reprinted with permission from [Kuo et al., 2008]. Copyright [2008], American Institute of Physics.

It was demonstrated that by filling of the AAO template with a conjugated polymer, aligned polymer nanopillars were obtained exhibiting an increased hole mobility due to an improved vertical alignment of the polymeric chains within the AAO template (Coakley et al., 2005). The hole mobility rose by a factor of 20 from 3x10-4 cm²V-1s-1 for a flat polymer layer in diode configuration to 6x10-3 cm²V-1s-1 for the aligned polymer inside the AAO pores. After the AAO template was removed the spacings between the obtained polymer pillars can in principle be filled with an acceptor material like e.g. NCs from a deposited dispersion. This leads to a nanostructured hybrid solar cell with an interdigital device structure as illustrated in Fig. 1d.

Since TiO2 is a semiconductor and could already be used as electron acceptor together with a conjugated donor polymer, the pores of a porous TiO2 film could be directly filled with a donor polymer to obtain a nanostructured bulk-heterojunction hybrid film. Recently Lim et al. demonstrated the successful infiltration of P3HT into TiO2 nanotubes of diameters of 60 nm to 80 nm. However, the diameters of the filled pores were above the desired diameters for an efficient charge extraction, so the reproducible and complete filling of the TiO2 nanotubes is still one of the main challenges to be solved before this nanostructuring method can be implemented into hybrid solar cells.

Another method which was successfully applied for the formation of a nanostructured bulkheterojunction organic solar cell is nanoimprint lithography (NIL). An AAO template was used as a mask for etching a Si substrate using a two-step inductively coupled plasma (ICP) etching process (Aryal et al., 2008). Thereby a silicon mold as shown in Fig. 13a is formed.

Organic-Inorganic Hybrid Solar Cells: State of the Art, Challenges and Perspectives 113

much smaller than in the case of DSSCs, OPV and hybrid solar cell technologies. Therefore the enhancement of the average module efficiencies of 3rd generation solar cells is one key issue to be addressed in order to extend this technology to wide range applications substituting traditional solar panels. In addition long-term stabilities of 3rd generation solar cells have to be improved tremendously to compete with existing PV technologies otherwise their utilization will be limited to small applications in devices with a limited lifetime such as e.g. disposable sensors and actuators. In case of hybrid solar cells the exploration of additional donor-acceptor materials is necessary, in order to replace toxic compounds by

Financial support from the German Federal Ministry of Education and Research (BMBF) within the project "NanoPolySol" under the contract No. 03X3517E as well as from the German Research Foundation (DFG) graduate school GRK 1322 "Micro Energy Harvesting"

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This mold is then used for creating NRs in a film of a conjugated polymer (e.g. regioregular P3HT). The created polymeric rods (Fig. 13c) show an increased crystallinity and preferential alignment of the polymer molecules in the vertical direction (Aryal et al., 2009) as well. The spacing between the polymer rods can then be filled with an acceptor material. After the evaporation of a top electrode the hybrid solar cell would be complete.

Kim et al. used NIL to create a nanostructured solar cell combining the molded polydithiophene derivative TDPDT with PCBM leading to a PCE of 0.8% compared to 0.25% of a bilayer structure (Kim et al., 2007).

Fig. 13. (a) Silicon mold created by ICP etching using a AAO template as mask (inset image: side view of the mold); (b) illustration of the molding process applied to P3HT; (c) molded parallel aligned P3HT nanopillars. Reprinted with permission from (Aryal et al., 2009). Copyright 2009 American Chemical Society.
