Semiconductor quantum dots (QDs) have drawn great attention for application in a number of fields due to the optical properties of these materials (Chen et al., 2016; Liu et al., 2016, 2019a,b; Li et al., 2019). Now, nanoparticles prominently become a dye sensitized for the third-generation solar cells because of low-cost fabrication technology, high photostability, the controlled sizes (Peng and Peng, 2001), higher absorption coefficient (Beard Matthew, 2011), and the multiple exciton generation (Sargent, 2005). However, QDSSCs (quantum dot sensitized solar cells) have reached ~13% performance, which is lower than the theory limits (Jiao et al., 2017). Recently, plenty of QDs [CdS, CdSe, CdTe (Shen et al., 2015), and PbS (Jumabekov et al., 2014)] are widely applied in the QDSSCs because of their unique properties (Duan et al., 2014). It is noticeable that the CdS and CdSe QDs have attracted considerable interest due to their optical property stability (Lin et al., 2014), a higher conduction band than TiO₂ (Lee and Lo, 2009), low resistivity (Mendoza-Perez et al., 2009), and wide absorbed spectrum (Liji Sobhana et al., 2011). However, this result was still low compared with that of dye-sensitized solar cells (DSSCs). So, a CdS/CdSe system was widely investigated due to its wide absorption spectrum, the shift of the absorption peak toward in the visible region, and rising of the conduction band (CB) as the combined CdS and CdSe QDs compared with TiO₂ CB. However, the performance based on this system achieved 4% efficiency (Lee and Lo, 2009), and its performance was still lower than that of DSSCs due to much trapping and recombination at the TiO₂/QDs/electrolyte triple interfaces (Abdellah et al., 2014).

In recent times, metal ions doped into the QDs can be replaced by the single QDs and co-sensitized system to reduce achievable recombination (Hodes et al., 1987; Fang et al., 1997, 2011; Gratzel, 2001, 2003; William Yu et al., 2003; Shen and Lee, 2008; Fan et al., 2009; Gimenez et al., 2009; Zhuge et al., 2009; Gonzalez-Pedro et al., 2010; Schmid, 2014; Tan Phat et al., 2018) because it can improve the charge collection and transfer process. In addition, metal ions are famous for their lowest resistance and large mobility. For example, Tan Phat et al. recorded a performance of 4.22% as Cu²⁺ ion doped into CdSe QDs because its attractive optical and magnetic properties were more interesting than that of CdSe and PbS QDs (Tan Phat et al., 2018). Their improving properties can be archived by doping metal in QDs like those in Refs. (Hodes et al., 1987; Gratzel, 2001, 2003; Fan et al., 2009; Gimenez et al., 2009; Zhuge et al., 2009; Gonzalez-Pedro et al., 2010) to make contributions to the more absorption photons of photoelectrodes.

Herein, Mn²⁺ ions were doped on CdSe nanoparticles to study the optical and photovoltaic properties of the QDSSCs. We investigate how changing the thickness of Cd_1−xMn_xSe films affects efficiency performance. Besides, in order to explain this result, the experimental I–V curve was also used to determine the resistances at the interfaces and the resistances diffusion of the devices. This dynamic resistance can be compared with that of electrochemical impedance spectra.

Figures 2A–D are the FE-SEM and cross-section of TiO₂/CdS(3), TiO₂/CdS(3)/Cd_0.8Mn_0.2Se(3), and TiO₂/CdS(3)/Cd_0.8Mn_0.2Se(3) photoanodes with a Mn²⁺ concentration of 0.2 and a thickness of three layers, respectively. The porous TiO₂ nanoparticles look like a sphere, which can be seen obviously in the inset image with 65 nm of an average size. Every layer of TiO₂/CdS(3), TiO₂/CdS(3)/Cd_0.8Mn_0.2Se(3), and TiO₂/CdS(3)/Cd_0.8Mn_0.2Se(3) photoanodes was determined to be ~11.606, 11.750, and 12.056 μm from Figures 2B–D, respectively. Moreover, 0.5 μm in Figure 2C (0.563 μm in Figure 2D) and 11.006 μm are the thickness of FTO and the TiO₂/CdS(3) film without FTO. The energy peaks related to Ti and O elements in the TiO₂ film and Cd, Se, and S elements of CdS and CdSe nanocrystal were clearly found in the EDX spectra of TiO₂/Cd_1−xMn_xSe/CdSe photoanode. Si and C energy peaks had been originated from FTO and excessive organic solution remaining in the layer (since the electrodes were sintered in vacuum), respectively. Mn energy peaks came from the anion precursor solution. The EDX spectra confirmed that QDs had been assembled and crystallized on the TiO₂ layer (Figure 2E).

The optical properties of TiO₂/CdS/Cd_1−xMn_xSe photoelectrodes were investigated by UV-Vis spectra with different thicknesses (Supplementary Table 1). The red shift is more pronounced with the increase of SILAR cycles due to the growth and thickness of film and attributed to the size quantization effect. This indicates that the high absorption coefficient of CdSe:Mn²⁺ QDs is attributed to TiO₂ nanoparticles, which are extended to almost the whole visible region as corresponding to SILAR cycles from 1 to 3 (Figure 3A). However, a decline in overall absorption was observed when SILAR cycle is higher than 3. This can be attributed to the aggregation of CdSe:Mn²⁺ nanocrystal due to decreasing photocurrent and increase in dynamic resistances (Singh et al., 2008; Bhupendra et al., 2011; Cao et al., 2015; Muthalif et al., 2016). Furthermore, the Tauc plot and additional information on it are shown in Table 1, Supplementary Table 2 and Figure 3B. The bandgap of QDs decreased from 2.04 eV for Cd_0.8Mn_0.2Se (1) to 1.7 eV for Cd_0.8Mn_0.2Se (3) QDs. This result shows that there is a strong influence of the doped concentration and thickness on the energy band structure of the CdSe host material (Gopi et al., 2015).

Figure 4A exhibits an alignment energy of photoanode, which includes a dopant energy in the bandgap of CdSe QDs caused by the shift peak, an increasing absorption intensity (shown in Figure 3A), and the (αhν)² vs. (hν) curves (shown in Figure 3B). The results are also confirmed by the time-resolved photoluminescence spectrum in Figure 4B and the data in Supplementary Table 3 and Table 1. In a similar manner, the lifetimes of charges in the CB of CdSe nanoparticles were shorter than those of Cd_0.8Mn_0.2Se QDs. In particular, the lifetimes of charges increase from 198.1 to 206.5 ns when SILAR cycles changed from 1 to 3. The probability of charge transfer from Cd_0.8Mn_0.2Se to CdS and TiO₂ was facilitated as large lifetimes. However, a decline in the lifetimes was recorded with loading higher than three layers due to the aggregation of Cd_0.8Mn_0.2Se nanoparticles.

Herein, both the I–V model from Refs. (Thongpron and Kirtikara, 2006; Thanh et al., 2015) and our experimental I–V curves were used to calculate the external dynamic resistance (R_D) and the internal dynamic resistance (R_d), the series resistance (R_s), and the shunt resistance (R_SH) of cells. It is necessary and more important to obtain the reliable characterization in the QDSSCs when the dynamic parameters were determined. We can control and determine the amount of loss mechanism as accurately as possible to improve the efficiency performance in the next work (Sze and Ng, 1981).

The photo current density (I_ph) and open voltage circuit (V_OC) of a solar cell is given by

with α= qnkT.

R_D and R_d are the external dynamic resistance and internal dynamic resistance of the equivalent circuit of solar cells.

The shunt resistance (R_SH) was obtained:

where V_o is the initial voltage.

In order to determine the performance, we recorded the I–V curves of QDSSCs with the different layers of Cd_1−xMn_xSe QDs, which is shown in Figure 5. In comparison, It is obvious that the optimized thickness of Cd_1−xMn_xSe (3 layers) QDs made contributions to boost the efficiency of QDSSCs (~3.8%) (Supplementary Table 4). This result is suitable to that of UV-Vis, lifetime, and IES.

On the whole, our view is that resistances showed up as the increasing SILAR cycles of Cd_1−xMn_xSe films (Sze and Ng, 1981). The result agrees well with that of the I–V curve (3.8% of efficiency). Furthermore, R_SH was calculated from Equation 5, and it depended on the technology process. The values of R_SH are large, corresponding to a good QDSSC. Looking at Table 1, it reveals that the R_SH of CdS/Cd_1−xMn_xSe co-sensitized TiO₂ is the largest. This is also confirmed by the long lifetimes of charges with loading SILAR cycles more than 3. In brief, the dynamic resistances, saturated current intensity, lifetimes of charges, and bandgap depend on the thickness of TiO₂/CdS(3)/Cd_0.8Mn_0.2Se(3) with the highest efficiency of 3.8%.

Figure 6A gives information about the circuit, which corresponds to the QDSSCs. Figure 6B shows the experimental Nyquist plots of devices corresponding to the resistance at the surface of the polyelectrolyte/counter electrode (denoted as R_ct1) and the diffuse resistance in the TiO₂ film and TiO₂/QDs surface (denoted as R_ct2) (Veerathangam et al., 2017). The lifetime of excited electron (τ_n) is determined from Figure 4B, and the capacitance (c_μ) can be determined by cμ=τnRct2 and listed in Supplementary Table 5 and Table 2. As a rule, the Cd_0.8Mn_0.2Se (1), Cd_0.8Mn_0.2Se (2), Cd_0.8Mn_0.2Se (3), Cd_0.8Mn_0.2Se (4), and Cd_0.8Mn_0.2Se (5) photoelectrodes have significantly changed in the photovoltaic because layers played a role in the recombination process. It is obvious that the thicker the film is, the larger resistance becomes. From Table 2, the resistances of four to five layers are larger than the resistance of three layers, while the excited electrons' lifetime and capacitances are much lower (Omid et al., 2015). Above all, the performance increased because of a rise in CB of the Cd_1−xMn_xSe QDs and a shift of the absorption peak after doping (shown in Figure 4A).

Table 1 illustrates the value of dynamic resistances from one illuminated I–V curve and EIS with the same conditions. Looking at the graph, it is immediately obvious that they depend on the SILAR cycles of deposition of Cd_0.8Mn_0.2Se with the same rules. In this case, the results show that the R_D, R_d, R_ct1, and R_ct2 are the smallest with loading at three SILAR cycles of deposition, but the value of R_SH is the largest. We noted larger R_SH indicates a better quality of QDSSCs. The trend of the recombination resistance (R_ct2) of all devices can clearly be analyzed when the SILAR cycles of deposition are changed. The R_D, R_d, and R_ct2 are characterized by the dynamic processes, dynamic resistances, and resistance transfer at surfaces of TiO₂/QDs. With the smallest R_D, R_d, and R_ct2, the optimum energy conversion efficiency was obtained ~3.8% at three cycles of deposition (Supplementary Table 6). This is completely suitable with the results of UV-Vis and lifetimes.

To summarize, photoelectrodes such as TiO₂/CdS/Cd_1−xMn_xSe have successfully been prepared using SILAR. The thickness of the Cd_1−xMn_xSe film affected the optical and photovoltaic properties of QDSSCs. The J–V curves show that the conversion efficiency is improved due to the optimized thickness at three cycles and Cd_1−xMn_xSe QDs. In addition, this result is also confirmed by the shift of absorption toward to the visible region, increasing lifetimes, and reducing charge recombination at the polyelectrolyte/counter electrode, TiO₂/Cd_0.8Mn_0.2Se/polyelectrolyte interfaces, and diffusion resistance in TiO₂ films. As a result, QDSSCs exhibited a high conversion efficiency of 3.8%.

All datasets generated for this study are included in the article/Supplementary Material.

HT and DP conceived and planned the experiments and carried out the experiments, contributed to sample preparation, took the lead in writing the manuscript. They also performed the experiments about the structural materials and contributed to the analysis of the new results of the manuscript.