1 Introduction

Global environmental concerns and the increased consumption of fossil fuels to meet the world growing energy demand are opening up new opportunities for research on alternative energy sources. The sun represents the cleanest, the most abundant, and inexhaustible source of renewable energies, and solar cells are the promising means for tapping into this kind of limitless energy source [1, 2]. Among the different types of solar cells currently developed, those based on nanostructured materials are classified in three categories: organic cells based on molecules or semiconducting polymers sensitized with carbon nanomaterials [3–6]; inorganic cells based on inorganic semiconductor nanomaterials sensitized with another semiconductor [7–10]; and hybrid solar cells based on organic–inorganic nanostructures that combine the best properties of individual component in each system and thus achieve improved efficiency as the result of synergetic effects [11–17].

Inorganic quantum dot-sensitized solar cells (QDSSC) present advantages such as tunable band gaps to collect light from the visible to infrared spectrum [10, 18–21], high molar extinction coefficients [22], and multiple exciton generation [23–25]; all of which allow these cells to reach theoretical efficiencies as high as 44 % [26, 27]. So far, the power conversion efficiency achieved by these cells is 8 % [28]. Moreover, one of the most promising advantages of the nanostructured solar cells is the manipulability of factors, such as particles’ shape and size, which improves the conversion efficiency as a result of quantum confinement [29–31]. Another key factor in conversion efficiency enhancement is improved electron and hole mobility to avoid recombination processes and increase the charge transport in the solar cell [32]. QDSSCs based on cadmium chalcogenide (CdS, CdSe, or CdTe) exhibit high capacity for efficiently absorbing photons in the visible region, as well as a band gap above 1.4 eV, and hence their significant advances in research. CdS quantum dots (QDs) are reported to be relatively stable, and have shown a modest 2 % efficiency in QDSSCs [33–35].

Likewise, photovoltaic devices made from conductive polymers have the advantage of tunable energy levels by modification of their chemical structure [36]. Also, they offer flexibility with the possibility for integration into building materials and solar devices [37]. However, their performance is limited by low mobility of charge carriers. It has been pointed out that the combination of semiconductor nanoparticles such as CdSe, CdS, PbS, and SiO2 and polymers can improve the quantum efficiency of the solar cells [36, 38–40]. In trying to incorporate the inorganic nanomaterials into the polymers, several strategies have been followed, for example, ligand exchange, in situ synthesis of the QDs, and interconnected networks; still, these hybrid devices have low power conversion efficiencies [41]. An efficiency of 4.1 % was recently reported to have been obtained with a hybrid bulk heterojunction (BHJ) solar cell using CdS QDs capped with n-butylamine and blended with poly(3-hexylthiophene) (P3HT) nanowires [42]. Concerning the role of each material, discrepant results have been published by different research groups, but this is understandable given the complexity of a system that incorporates several polymers in a hybrid cell. P3HT, among the different varieties of poly-3-alkyl thiophenes, is the most widely used π-conjugated polymer for photovoltaic applications, followed by poly(3-octylthiophene) (P3OT) [43, 44].

In this work, the photoelectrochemical characteristics of hybrid QDSSC were studied using P3HT or P3OT organic polymer on a well-known TiO2/CdS/ZnS arrangement in order to recognize the role of these polymers in the performance of this kind of solar cells. Indeed, photoconversion efficiency improved substantially, to around 33 %, with the addition of the conductor polymers varying from 2.2 % in the reference cell (TiO2/CdS/ZnS) to 3 % in the configurations: TiO2/CdS/ZnS/P3HT and TiO2/CdS/ZnS/P3OT. This enhanced efficiency is a result of the increase of fill factor (FF) and open circuit voltage (V oc) associated with the reduction of transport resistance and recombination, as demonstrated by impedance measurements. The differences between the cells with P3HT and those with P3OT were a better coating of the TiO2/CdS/ZnS with P3HT, resulting in higher short circuit current (J sc), and obtaining higher V oc and FF with P3OT.

2 Experimental

In order to study the effects of the two polymers on the QD-sensitized TiO2, three samples were fabricated: (1) TiO2/CdS/ZnS, (2) TiO2/CdS/ZnS/P3HT, and (3) TiO2/CdS/ZnS/P3OT as follows.

2.1 Quantum dot-sensitized solar cells preparation

2.1.1 Materials

TiO2 paste WER2-O Reflector and DSL 18NR-T were obtained from DYESOL, titanium(IV) isopropoxide (97 %), acetylacetone (>99 %), cadmium acetate dihydrate (Cd(CH3COO)2∙2H2O), zinc acetate dihydrate (Zn(CH3COO)2∙2H2O), sodium hydroxide (NaOH), tetra-n-butylammonium hexafluorophosphate (TBAPF6), P3HT, and P3OT were obtained from Sigma-Aldrich. Sulfur (S) and sodium sulfide (Na2S∙9H2O) were obtained from KARAL. Fluorine-doped tin oxide glasses (FTO, TEC-15) were purchased from MTI.

2.1.2 TiO2 film Preparation

Fluorine-doped tin oxide glasses were cleaned with water, acetone, and ethanol in an ultrasonic bath for 15 min each before use. The photoelectrodes consist of three different TiO2 layers, stacked one on the top of the other. The first TiO2 film deposited was the compact layer. A solution of titanium(IV) isopropoxide (0.2 M) using acetylacetone/ethanol (V:V = 1:1) was deposited by spray pyrolysis over FTO and sintered at 450 °C for 30 min obtaining a 150-nm layer. The second layer was the transparent layer, prepared with TiO2 paste (DSL 18-NRT) deposited by Doctor Blade method obtaining an 8-μm-thick layer. The third TiO2 film (Wer2-O Reflector paste) was the scattering layer (opaque), consisting in a 9.5-µm layer obtained by Doctor Blade deposition method. These films were sintered for 30 min at 450 °C to obtain good electrical contact between nanoparticles.

2.1.3 Films’ sensitization

The TiO2 electrode films were sensitized by successive ionic layer adsorption and reaction (SILAR) method using 0.05 M Cd(CH3COO)2∙2H2O dissolved in ethanol and 0.05 M Na2S in methanol:water (V:V = 1:1) as Cd2+ and S2− sources, respectively. A single SILAR cycle consisted of 1-min dip-coating the TiO2 electrode into the cadmium acetate and subsequently into the sodium sulfide solutions, also for 1 min. Between successive dipping steps in precursor solutions, the electrodes were thoroughly rinsed by immersion in the corresponding solvent (ethanol and methanol/H2O, respectively) in order to remove the excess content of precursor. Seven SILAR cycles were performed to obtain a uniform coverage of the TiO2 NPs with CdS QDs. To enhance the photovoltaic performance, ZnS was deposited by SILAR to passivate CdS surface and to reduce the recombination of electrons from the TiO2 to the polysulfide electrolyte [45–47]. ZnS was obtained from 0.1 M Zn(CH3COO)2∙2H2O and 0.1 M Na2S both dissolved in water, using them as Zn2+ and S2− sources, respectively. The films were dipped for 1 min/dip in the solutions during two SILAR cycles. Finally, the P3HT and P3OT were deposited by the drop-casting method: this technique consists in spreading evenly a drop (10 μl) of P3HT 10−7 M (or P3OT) in toluene onto TiO2/CdS/ZnS films and drying for 10 min at room temperature.

2.1.4 Counter electrode fabrication and cell assembling

Cu2S counter electrodes were obtained by immersing a brass foil in HCl solution (38 vol%) at 90 °C for 1 h, and then they were sulfated by adding a drop (20 μl) of polysulfide electrolyte. The composition of the polysulfide electrolyte was 1.0 M Na2S, 1.0 M S, and 0.1 M NaOH, using deionized water. The solar cells were constructed by assembling the Cu2S counter electrode and sensitized TiO2 film electrode with a binder clip separated by a Scotch spacer. The polysulfide electrolyte was filled inside the cell.

2.2 Characterization

Field emission scanning electron microscopy (FE-SEM) images were obtained on a JEOL JSM-7800F microscope. The UV–Vis absorption spectra of P3HT and P3OT were measured by transmittance, and substrates were measured by diffuse reflectance in the range from 360 to 800 nm using an Agilent Technologies Cary Series UV–Vis–NIR spectrophotometer (Cary 5000) with an integrating sphere of 60 mm. The current density curves were measured using a Gamry reference 600 potentiostat/galvanostat. The samples were illuminated using an Oriel Sol 3A solar simulator while measuring current. The light intensity was adjusted employing a NREL-calibrated Si solar cell having KG-2 filter for one sun light intensity (100 mW cm−2). Incident photon-to-current conversion efficiency (IPCE) spectra were measured using a monochromator Newport model 74125. Electrochemical impedance spectroscopy (EIS) measurements were carried out by applying a small voltage perturbation (10 mV) at frequencies ranging from 100 kHz to 0.1 Hz for different forward bias voltages under dark conditions. The energy levels of the semiconductors were calculated also by EIS, with a three-electrode cell, using each material present in the photoanode as working electrode. Ag/AgCl and Pt wires were the reference and counter electrode, respectively, and a solution of 1 M Na2S was used as electrolyte. The fittings of the results were carried out using the ZView software. HOMO and LUMO levels of the polymers were calculated by cyclic voltammetry. The polymer films were deposited by the drop-casting method described above on FTO and were used as working electrode in 0.1 M TBAPF6 in acetonitrile electrolyte. The reference and counter electrode were the same.

3 Results

3.1 Morphological characterization

The morphology of the photoanode is shown in SEM images, (see Fig. 1). The TiO2 compact layer presented in Fig. 1a is formed of clusters around 150-nm size. Also, a homogeneous, highly porous TiO2 transparent layer composed of particles with an average size of 20 nm is shown in Fig. 1b. Figure 1c shows the TiO2 scattering layer. This film has a larger particle size (400 nm). Figure 1d demonstrates a good coverage of the CdS QDs over the TiO2 surface. The TiO2 film with CdS/ZnS QDs and P3HT is presented in Fig. 1e. Herein is observed that the particles agglomerate on the surface to form rods probably due to the P3HT deposited. In Fig. 1f, the P3OT deposited is observed as big clusters (of size 100 nm) wrapping the TiO2/CdS/ZnS particles.

Fig. 1
figure 1

SEM images of the morphology of each TiO2 films a TiO2 (compact layer), b TiO2 (transparent layer), c TiO2 (scattering layer); and the films d TiO2/CdS/ZnS, e TiO2/CdS/ZnS/P3HT, and f TiO2/CdS/ZnS/P3OT

Full size image

SEM image and EDS map of the cross-sectional photoanode TiO2/CdS/ZnS/P3HT are shown in Fig. 2. The FTO/glass substrate has a thickness of 150 nm as observed on the left side of the SEM image. Adjacent to the TiO2 compact layer is the FTO. Next are located the TiO2 active layer which has 8-μm thickness and the TiO2 scattering layer with 9.5-μm thickness. A homogeneous distribution of Cd, S and Zn on the TiO2 active layer is observed. Considering that the TiO2 active layer has smaller-sized nanoparticle than the TiO2 scattering layer, and that the CdS and ZnS QDs were added before the polymers, there is a bigger distribution of these QDs on the active layer. The largest distribution of the polymer was observed over the TiO2 scattering layer; this is related to the polymer not being able to penetrate the surface formed between the QDs and the TiO2 active layer, therefore it stays on the TiO2 scattering layer. The behavior of TiO2/CdS/ZnS/P3OT is very similar.

Fig. 2
figure 2

SEM image and EDS distribution of the cross sectional photoanode with the TiO2/CdS/ZnS/P3HT array

Full size image

3.2 Optical absorption

The absorption spectra of the cells and the polymers are shown in Fig. 3. The TiO2 film absorbs light on the short wavelength region centered at 350 nm, (Fig. 3a). The P3HT and the P3OT dispersed in toluene present a peak at 450 nm, however, P3HT has an absorption light up to 620 nm, while P3OT to 560 nm, see the inset figure. The TiO2/CdS/ZnS cell shows in Fig. 3b an absorption band centered at 430 nm. This light absorption corresponds to the CdS QDs [48, 49], while the absorption peak at 375 nm is related to the ZnS QDs [50, 51]. Moreover, by introducing the polymers to the cell the light absorption increases, for example the UV–Vis spectrum for the TiO2/CdS/ZnS/P3HT film shows two absorption peaks (Fig. 3c), one at 500 nm and another at 605 nm. Likewise, the TiO2/CdS/ZnS/P3OT film shows one absorption peak at 415 nm and a second peak at 605 nm, see Fig. 3d.

Fig. 3
figure 3

Absorption spectra of a TiO2, b TiO2/CdS/ZnS, c TiO2/CdS/ZnS/P3HT, and d TiO2/CdS/ZnS/P3OT arrays. The inset shows absorption spectra of (a) P3HT and (b) P3OT in toluene

Full size image

3.3 Photovoltaic characterization

The current density-voltage (J–V curves) are analyzed for the three configurations; TiO2/CdS/ZnS, TiO2/CdS/ZnS/P3HT and TiO2/CdS/ZnS/P3OT, (see Fig. 4). The photovoltaic parameters as J sc, V oc, FF, and efficiency (η) are summarized in Table 1. The FF and η were calculated using Eqs. 1 and 2.

Fig. 4
figure 4

J–V characteristic curves of the configurations a TiO2/CdS/ZnS, b TiO2/CdS/ZnS/P3HT, and c TiO2/CdS/ZnS/P3OT

Full size image
Table 1 Photocurrent, voltage, fill factor, and photoconversion efficiency values of the hybrid QDSSCs
Full size table
$${\text{FF}} = \frac{{P_{ \hbox{max} } }}{{J_{\text{sc}} V_{\text{oc}} }} \times 100,$$
(1)
$$\eta = \frac{{P_{ \hbox{max} } }}{{P_{\text{i}} }} \times 100 = {\text{FF}}\frac{{J_{\text{sc}} V_{\text{oc}} }}{{P_{\text{i}} }},$$
(2)

where P max is the maximum obtainable power and P i is the incident solar power. The J–V curves show the effect of P3HT or P3OT over CdS/ZnS QD-sensitized TiO2. The V oc increase from 0.453 V (TiO2/CdS/ZnS) to 0.573 V and 0.597 V adding P3HT and P3OT, respectively. However, the J sc of the TiO2/CdS/ZnS cell (9.8 mA cm−2) decreased to 8.3 mA cm−2 with the polymer P3OT addition. Another effect observed with the addition of P3HT and P3OT to the configuration TiO2/CdS/ZnS was the improvement of the FF. The configuration TiO2/CdS/ZnS has a FF of 50.5 % and an efficiency of 2.24 %. With the addition of the polymer P3HT to the cell, the FF increased to 53.6 %, and consequently, the η increased to 3.07 %. When P3OT was added, the FF increased even more than with the P3HT to 60.7 %, with no increase in efficiency (3 %). In spite of the variations in the V oc and J sc, the efficiencies for the TiO2/CdS/ZnS/P3OT and TiO2/CdS/ZnS/P3HT cells are the same due to the high FF of the TiO2/CdS/ZnS/P3OT array.

The IPCE characterization provides a measurement of the number of electrons that are photogenerated for each incident photon at a given wavelength (λ) and can be determined from the J sc, using the expression 3 [52].

$${\text{IPCE}} = \frac{{1240 \times J_{\text{sc}} }}{\lambda \times I} \times 100.$$
(3)

The IPCE curves of the TiO2/CdS/ZnS, TiO2/CdS/ZnS/P3HT and TiO2/CdS/ZnS/P3OT cells are shown in Fig. 5. The sample TiO2/CdS/ZnS has an external quantum efficiency (or IPCE) of 90 % at 480 nm. Adding the polymers decreases the IPCE at this wavelength. For TiO2/CdS/ZnS/P3HT, a decrease of the IPCE to 83 % is observed, and with the configuration TiO2/CdS/ZnS/P3OT the IPCE is 78 %, this reduction is evidenced by the decreased J sc. However, with the introduction of the polymers at longer wavelengths (up to 575 nm), the IPCE is higher.

Fig. 5
figure 5

IPCE spectra of the solar cells with the configuration: TiO2/CdS/ZnS, TiO2/CdS/ZnS/P3HT and TiO2/CdS/ZnS/P3OT

Full size image

Figure 6 shows a schematic band energy diagram for the configurations TiO2/CdS/ZnS/P3HT and TiO2/CdS/ZnS/P3OT cells. The conduction bands (CB) of TiO2, CdS, and ZnS QDs are −4.05, −3.87, and −3.28 eV, respectively, and they were calculated from the analysis of the Mott-Schottky curves as was pointed previously [51]. From EIS measurements of a three-electrode cell configuration, the capacitance was estimated, and with the intercept in the x-axis of the Mott-Schottky plot, the flatband potential was obtained [53, 54]. The highest occupied molecular orbital (HOMO) for P3HT is −5.19 eV, and the lowest unoccupied molecular orbital (LUMO) is −2.95 eV, and for P3OT, the corresponding values are −5.44 and −2.84 eV, respectively. The HOMO and LUMO were estimated from Eqs. 4 and 5 [55–57]. The onset of oxidation (E ox) were calculated from cyclic voltammetry curves, and E g is the optical band gap obtained from the derivatives of the absorption spectra. The band gaps were 3.2 eV for TiO2, 2.45 eV for CdS, 3.54 eV for ZnS QDs, 1.94 eV for P3HT and 2.3 eV for P3OT.

$${\text{HOMO}} = - e\left( {E_{\text{ox}} + 4.71} \right) {\text{eV,}}$$
(4)
$${\text{LUMO}} = E_{\text{g}} + 0.3 + {\text{HOMO}} {\text{eV }} .$$
(5)
Fig. 6
figure 6

Scheme illustrating energy levels of a TiO2/CdS/ZnS/P3HT and b TiO2/CdS/ZnS/P3OT arrays

Full size image

3.4 Electrochemical impedances

EIS measurements were carried out to explain the variation of the electrochemical parameters in terms of the internal physical mechanism when CdS QDs and P3HT or P3OT are combined. The transmission line model indicated in Fig. 7 was used to fit the impedance measurements [58, 59]. There, R s stand for the series resistance and is related to the FTO and the wires resistance, R t is the transport resistance, and is related to the resistance of TiO2 to the electron flux, R rec and C µ are the recombination resistance and the chemical capacitance, respectively. Z d is the Warburg element showing the Nernst diffusion of the electrolyte; R c and C c are the charge-transfer resistance and double-layer capacitance at the counter electrode.

Fig. 7
figure 7

Equivalent circuit employed in the QDSSC. R s and R t are the series resistance and transport resistance, respectively; Z d is the Warburg element; R c is the charge-transfer resistance; C c is the double-layer capacitance; R rec is the recombination resistance; and C μ is the chemical capacitance

Full size image

The impedance spectra of the sample TiO2/CdS/ZnS measured at different potentials under dark conditions are presented in Fig. 8a, while the impedance spectra for TiO2/CdS/ZnS, TiO2/CdS/ZnS/P3HT, and TiO2/CdS/ZnS/P3OT at 350 mV bias are shown in Fig. 8b. These spectra show the typical behavior of a large semicircle in the low-frequency region and a straight line in the high-frequency region [59, 60].

Fig. 8
figure 8

Impedance spectra of a the TiO2/CdS/ZnS cell at different potentials; and b the TiO2/CdS/ZnS, TiO2/CdS/ZnS/P3HT, and TiO2/CdS/ZnS/P3OT at 350 mV

Full size image

Figure 9 shows C µ, R rec, and R t of the TiO2/CdS/ZnS, TiO2/CdS/ZnS/P3HT, and TiO2/CdS/ZnS/P3OT solar cells extracted from the fitted EIS measurements. Cµ is plotted as a function of V F = V app−V series which is the voltage drop at the active electrode obtained as the applied voltage, V app, corrected by the voltage drop due to series resistance (V series) [58, 59]. The chemical capacitance does not change as shown in Fig. 9a. An increase in the recombination resistance was observed when P3HT or P3OT was used (Fig. 9b), while the transport resistance decreased, Fig. 9c.

Fig. 9
figure 9

Behaviors of the different parameters of the QDSSC obtained by EIS: a chemical capacitance, C μ; b recombination resistance, R rec; and c transport resistance, R t

Full size image

4 Discussion

4.1 Photoanode morphology

The TiO2 compact layer was deposited on the FTO surface before the TiO2 mesoporous layer to achieve higher photovoltage as it blocks the contact of the FTO surface and the electrolyte [27]. The TiO2 active layer, due to the small nanoparticles, has a large active area which helps to obtain high quantity of QDs deposited. The TiO2 scattering layer increases light scattering, and thus, light harvesting due to the large particles ensures adequate light trapping in the device, increasing the absorption path length of photons and optical confinement [58, 61]. As the QDSSCs’ performance depends on the film thickness, a balance among light scattering, QD loading, and recombination is necessary to optimize the performance. It has been reported that TiO2 scattering layers as thick as 11 μm still can scatter incident light within the film and return it to the surface, contributing to the improvement of the solar cells’ performance [62, 63].

SEM images confirm good coverage of CdS QDs deposited over TiO2 porous film using SILAR method. The morphology of the photoanode when polymers were deposited changed from nanorods with P3HT to disordered agglomerated spheres with P3OT. A similar effect was noted elsewhere [3]. The photoanode covered with P3HT was more uniform. The distribution of the polymer in the TiO2 layers indicates its good penetration, and as good interfacial contact is essential to maximize the interfacial areas and minimize the recombination of the charge carrier, it contributes to the enhancement of the fill factor [17].

The CdS QDs have light absorption in the visible region [35]. The ZnS QDs act as a barrier at the interface between the CdS QDs and the electrolyte, leading to the hindered recombination rate, but it does not contribute to light absorption [47, 50]. From the IPCE measurements, it can be observed that these polymers do not contribute to the light absorption considerably, as demonstrated by the decreased J sc. Nevertheless, it has been established that the increment of absorption in hybrid QDSSCs is associated to the light-trapping effect produced by increased incident photon scattering within the polymeric network [36], which could be related to the enhanced absorption observed in TiO2/CdS/ZnS when the polymers were incorporated (Fig. 3). Moreover, the polymers facilitate the hole transport and act as a potential barrier, as shown in the energy scheme, leading to a better electron–hole transport balance [32, 64].

4.2 Electrochemical performance

The unusual shape of the J–V curve (Fig. 4) for the TiO2/CdS/ZnS/P3HT and TiO2/CdS/ZnS/P3OT could be related with the hysteresis phenomenon. This behavior, observed in different kinds of solar cells, has been associated with trapping of charge carriers, ferroelectricity, ion migration, or ionic polarization [65–67]. According to the hybrid-QDSSC studied in this work, it is probable that the charge-trapping sites be present [68, 69]. If these traps could be filled under forward bias working conditions, then it results in a good p- and n-contacts decreasing the charge recombination, which would improve the J sc and FF [70, 71].

Moreover, the higher V oc observed with the addition of P3HT or P3OT in the TiO2-sensitized cells could be attributed to three processes: (1) an increase in the light absorption, (2) a shift in the TiO2 CB, and/or (3) a change in the recombination rate. An increase in the photoabsorption will result in an increase in J sc and produce, consequently, an increase of V oc. However, in this case, the J sc values are similar in both TiO2/CdS/ZnS/P3HT and TiO2/CdS/ZnS arrays, and hence, the increment of V oc does not occur for this process. For the second effect, a shift in the TiO2 CB produces a movement of the TiO2 electron at quasi-Fermi level which determines the V oc; then, an upward shift of the TiO2 CB will mean an increase in the V oc. Nevertheless, the same trend in the chemical capacitance is observed for the different configurations, Fig. 9a. For the third effect, the recombination rate can be also determined with EIS by analyzing the R rec. An increase of this parameter indicates a reduction in the electron recombination process from the TiO2 CB to the acceptor states, either in the electrolyte or the sensitizer [72]. This effect is observed in Fig. 9b where the R rec increases with the addition of P3HT or P3OT, thus decreasing the recombination of charge carrier in the system and increasing the V oc.

Another important parameter is the FF. Variations in this parameter can be due to the change in the transport resistance (R t) [59, 72, 73]. It has been previously reported that R t can vary depending on the sensitization process [59]. Analysis of the R t behavior in the V F convention (Fig. 9c), shows that R t decreases with the addition of the polymers indicating an increment in the electron transport, which results in the increase of FF observed in Table 1.

No significant difference was noted between the use of P3HT and P3OT in the QDSSC with regard to the R rec or R t. Some studies have demonstrated that the charge mobility is higher in P3HT than that in P3OT [43, 55, 74]. That, in addition to the more uniform layer obtained, could be the reason for the higher J sc on TiO2/CdS/ZnS/P3HT, but at the same time, it may create a larger protecting layer between the TiO2/CdS/ZnS array and the electrolyte, causing a lower FF compared with TiO2/CdS/ZnS/P3OT array.

Other issue related to the use of conducting polymers in solar cells is the material stability [75]. It has been proven that the poly3-alkylthiophenes under UV–Vis light irradiation and thermal aging conditions result in the formation of oxidized species [76]. Moreover, it was established that oxygen forms a charge transfer complex with poly3-alkylthiophenes, which could be beneficial for the conductivity. Although most of the solar cells based on conducting polymers are tested under inert conditions, it is important to consider the performance and its relations under ambient conditions and stability. In this work, the hybrid QDSSCs were prepared and analyzed under ambient conditions; however, the rigorous stability tests are beyond the scope of this work. Further efforts are being carried out to improve the solar cell performance and to analyze the stability of the hybrid QDSSCs.

5 Conclusions

QDSSCs based on CdS/ZnS QDs and P3HT P3OT were analyzed. Either of the polymers added to the reference cell TiO2/CdS/ZnS substantially increased V oc, FF, and η. The V oc values increased to 26 and 31 %, while the FF increased to 6 and 20 %, respectively, improving the conversion efficiency as a consequence. The polymer-coated photoanode enabled better protection between the QDs and the electrolyte, improving charge carrier transfer. Increased V oc and FF values are the result of the reduction in the recombination processes (increment in R rec) and the increment in the charge carrier’s transport (decrement in R t) obtained with the use of the polymers, as demonstrated by electrochemical impedance spectroscopy measurements. The difference between P3HT and P3OT offsets the improved J sc for TiO2/CdS/ZnS/P3HT and V oc for TiO2/CdS/ZnS/P3OT arrays; thus, the efficiency obtained was the same with either polymer. We can assume that poly-3-alkyl thiophenes and ZnS act similarly; however, the former enhances the solar cell performance.