A high efficiency CdS/CdTe solar cell was designed with a reduced CdTe absorber-layer thickness and a distributed Bragg reflector (DBR) as an optical reflector and a ZnTe layer as back surface field (BSF) layer. Simulation results showed that with combination of DBR and BSF layers and 0.3 µm thick CdTe, the conversion efficiency was increased about 3.2% as compared with a reference cell (with 4 µm thick CdTe layer). It was also shown that the efficiency can be increased up to 6.02% by using a long carrier lifetime in the absorber layer. Under global AM 1.5G conditions, the proposed cell structure had an open-circuit voltage of 1.062 V, a short-circuit current density of 24.64 mA/cm2, and a fill factor of 81.3%, corresponding to a total area conversion efficiency of 21.02%.
© 2014 Optical Society of America
CdTe is a nearly perfect absorber material for thin film polycrystalline solar cells because its bandgap closely matches the peak of the solar spectrum and it has high absorption coefficient and good electronic properties. Fabrication of high-efficiency CdS/CdTe solar cells with an ultra-thin absorber layer is a challenge considering the cost factor of these cells and the rising price of Te. Most of today's CdTe solar cells utilize an absorber layer of about 2.5 to 8 µm thick. Thinner CdTe layers result in poorer cell performances due to shunting, incomplete photon absorption (deep penetration loss), interference between the main and the back contact junction . A ZnTe layer has been used as a BSF to repel the carriers at the CdTe/ZnTe structure and thus decrease the loss of carriers at the back contact . The cell’s performance has been optimized with consideration of the impact of variations in minority carrier lifetime and carrier density profile [3, 4].
The record cell efficiency of CdTe solar cells has increased by only 1.5% during the last 17 years [5–9]. Previous groups have reported a maximum cell efficiency of 16.5% for the CdTe solar cell [10–13]. However, First Solar company has recently reported a higher cell efficiency of 20.4%, and a module efficiency of 13.9% obtained under the lab conditions ; but no details about these cells are available.
In this paper a reference CdTe/CdS cell structure was considered as a reference cell  and the results obtained by the simulation procedure for the proposed cells were all compared with the data of this reference cell. Then, the thickness of CdTe was reduced in several steps to the extreme limit of 0.3 µm and a ZnTe layer was used as a BSF. In the next step, the effect of the BSF layer on the photovoltaic properties of CdTe/CdS solar cells was evaluated. Then, a suitable DBR with Si and SiO2 was inserted on the ZnO transparent back contact. By analyzing electrical characteristics of the DBR, the highest efficiency was obtained.
Finally, the carrier lifetime variation of the absorber layer was studied, which resulted in a considerable improvement in cell efficiency (as compared to the reference sample). All of the simulations were done using Silvaco software .
2. Simulation of the reference cell
In order to analyze ultra-thin and high-efficiency CdTe solar cells, the conventional CdTe reference cell was the starting point for these investigations. Schematic structure of the reference cell is shown in Fig. 1.
In this cell, the layers were composed of 500 nm SnO2, 100 nm CdS and 4 µm CdTe. Under AM 1.5G conditions, this cell had an open-circuit voltage of 870 mV, a short-circuit current density of 22.68 mA/cm2, a fill factor of 0.75, and a conversion efficiency of 15% . This cell was modeled and its simulated characteristics were obtained. The refractive index and extinction coefficient for different materials used in our simulation are shown in Figs. 2 and 3.
Table 1 compares the performance of the actual cell with the simulated data. The simulated results were slightly different, but still quite close to the actual data of the reference cell . In this work Shockley-Read-Hall (SRH) model was used to describe carrier recombination currents. Some of the most important parameters used in the simulations are shown in Table 2 [2–4].
Where ε is dielectric constant, μ mobility, n and p electron / hole density, Eg bandgap energy, NC and NV effective density of states, NDG and NAG Acceptor / Donor defect Concentration, EA and ED defect peak energy, WG distribution width, σ capture cross section.
3. Results and discussions
3.1 Investigation of ZnTe layer effect on ultra-thin CdS/CdTe cell performance
High-efficiency CdS/CdTe solar cells with ultra-thin (i.e. below 1 μm) absorber layers are highly desirable considering cost factor of these cells. Thicker absorber layers are generally used to avoid pinholes pinching through to the window layer, which may lead to shorting to the back contact. Obviously, the solar cell performance decreases due to local shunting, incomplete absorption (deep penetration loss), fully depleted CdTe layers, or interference between the main and the back-contact junction, when the CdTe layer thickness approaches a certain limit. It has been shown that by employing a comprehensive cleaning of the substrates; a very thin absorber layer (with only 0.25 μm of CdTe) without pinhole shorting can be achieved .
There are two strategies to increase the voltage of CdTe solar cells. One strategy is to increase the CdTe carrier density and lifetime that will study in section 3.3. The other is to add an electron reflector at the back contact of the CdS/CdTe cell. The use of an electron reflector is a strategy to improve the open‐circuit voltage of CdTe solar cells. In this work, ZnTe layer would act as a BSF to repel the carriers at the CdTe/ZnTe hetero structure and thus would decrease the loss of carriers at the back contact (due to the presence of the higher bandgap layer at the back) . Also, this layer reduces the carrier recombination at the back contact due to the barrier (i.e., conduction band discontinuity) for electrons at the interface between the CdTe and the ZnTe layers. Then, the electrons will be reflected at this interface and they will be collected with a higher probability at the CdS/CdTe hetero junction.
The initial step of the analysis was to decrease the CdTe absorber layer and add a ZnTe layer to reduce the back contact barrier height and back surface recombination loss of the ultra thin cell. The resulted structure is shown in Fig. 4. Figure 5 shows the calculated band diagram of the proposed cell with bias.
The electron reflector, which has a potential barrier (due to bandgap difference between the CdTe and the ZnTe layers) in the conduction band at the back surface of the proposed cell, will reflect minority carrier electrons away from the back surface and thus reduce the back‐surface recombination. It should be mentioned that it is not possible to optimize the ZnTe thickness to increase the optical reflection at the ZnTe/CdTe interface. Because the CdTe and ZnTe materials have optical indexes that are very close (the optical transmission is > 99%), but the second one is a very poor absorbing material for wavelength > 600 nm, so current is not so much affected.
Under AM 1.5G conditions, the cell had an open-circuit voltage of 980 mV, a short-circuit current density of 22.55 mA/cm2, a fill factor of 0.758 and a conversion efficiency of 16.43%.
3.2 The effect of the DBR on the performance of the ultrathin CdS/CdTe solar cell
Of the many studies that have attempted to improve light absorption in the absorber layer by increasing the optical path length, one method has utilized back reflectors in the bottom of cells to reflect the light back into the absorber layer. Although metallic back reflectors can also act as electrodes, metals suffer from intrinsic absorption losses incurred at the interface. This absorption can be reduced by adding an optical spacing layer [16, 17]. One-dimensional DBRs with a high reflectance can double the path length over a wide range of incident angles and wavelengths. It is because the optical absorption of the CdTe layer is very high even for wavelengths close to limit of the absorption (linked to the bandgap). A quick numerical application gives an absorption length of about La = 180nm around wavelength 800 nm (according to Fig. 2 and Fig. 3). For this reason, considering an efficient DBR, a 300 nm thick layer of CdTe corresponds to an equivalent optical path of 3 times La.
Dielectric DBRs have high transparency within a portion of the incident wavelengths , whereas metallic BRs suffer from a lack of light transparency. In addition to offering partial light transparency at visible wavelengths, DBR with ZnO can also act as a back contact. ZnO is a transparent oxide layer so it has no absorption. The broadband back reflectors can be achieved by connecting two DBR mirrors . In this work, a DBR consisting of three pairs of Si and SiO2 was placed on the backside of ZnO back contact of the CdS/CdTe solar cell. Figure 6 shows the DBR structure. Figures 7 and 8 illustrate the wavelength selectivity characteristics of the proposed DBR as a function of Si/SiO2 pairs.
The amount of light reflected back to the cell is related to the overlap between the spectral reflectance of DBR and the incident light spectrum. Not only, the wavelengths from 550nm to about 1µm are mainly reflected back into the CdTe layer by the DBR, but also the absorption in these wavelengths is almost zero. It is also possible to combine more DBRs to increase the efficiency of the cells because of the ZnTe absorption in the long wavelength region of the spectrum.
Figure 9 illustrates the proposed structure with the DBR on the back side of ZnO transparent contact. As mentioned above, the selected light can be achieved by adding another DBR to reflect most of the transmitted light (long wavelengths) back into the CdTe layer. In this work, we used three pairs of DBRs to obtain the high efficiency cell.
Figure 10 shows the J-V characteristics and power curve of the proposed cell structure with DBR and BSF layers. It shows a clear improvement in VOC and JSC of the cell compared to the reference cell.
The quantum efficiency (QE) of the cell was obtained in the wavelength range of 100~1100 nm for the above-mentioned structure. The JSC was increased from the value of 22.5 mA/cm2 (without DBR) to 24.6 mA/cm2 (with DBR). An efficiency of 18.2% was achieved for the cell, which is higher than that of the cells without the DBR structure (16.4%). Figure 11 shows the corresponding QE curves with and without the DBR structure.
3.3 Carrier lifetime in the CdTe layer
In the last set of the simulations, the basic parameters (JSC, VOC, FF and Efficiency) of the proposed cell were obtained as a function of carrier lifetime in the CdTe layer. The variations of minority-carrier lifetime in the model were obtained with variation of defect density. Increased defect density and purity of the CdTe alters the electron lifetime as well as the hole lifetime. As was expected, high defect density in the CdTe layer increases the recombination rate in the space-charge region (SCR) and limits the VOC. An equation for VOC is found by:
Where n is the ideally factor, k Boltzmann's constant, T absolute temperature, Jo dark current density, JSC short current density and q electrical charge. Jo is directly linked to the defect density and purity of the material (a higher minority lifetime corresponds to a purer or low defect density material). With this formula, if JSC saturates, VOC can be increased if Jo decreases. Obviously, as simulations predicted, longer carrier lifetimes resulted in a higher VOC. As a result, the conversion efficiency increases with increasing carrier lifetime. The simulation showed that if the carrier lifetime for electrons were taken as 10 ns, the efficiency as high as 21.02% could be obtained. The results are shown in Fig. 12.
Figure 10 shows J−V characteristics comparison between four structures under AM 1.5G illumination at 1 sun for the reference cell, the ultrathin cell with BSF layer, the ultrathin cell with DBR and BSF layers, and the ultrathin cell with DBR and BSF layers with longer carrier lifetimes in the CdTe layer. As this figure shows, the short- current density of the final ultrathin structure is higher than that of the reference cell (with 4 µm CdTe thickness), corresponding to an efficiency of 21.02% due to addition of DBR, BSF layer and long carrier lifetime in CdTe layer. The open-circuit voltage, short-circuit current, fill factor and conversion efficiency for different structures are given in Table 3.
In this work, an ultrathin high efficiency CdS/CdTe solar cell was designed with a reduced CdTe absorber layer thickness adding a DBR as an optical reflector, and a ZnTe layer as a BSF in the cell. The DBR structure was designed to increase quantum efficiency of the ultrathin CdS/CdTe solar cell. Use of DBR and BSF layers resulted in a conversion efficiency of 18.2%. These results showed a noticeable improvement over the 15% efficiency of the reference cell which had a 4 µm thick absorber layer. Subsequently, the effect of carrier lifetime in the CdTe layer on the performance of CdTe cell was investigated. The maximum conversion efficiency of 21.02% (VOC = 1.062 V, JSC = 24.64 mA/cm2, FF = 0.813) was obtained in the cell consisting of 0.45 µm DBR, 0.1 µm-ZnO, 0.1 µm-ZnTe BSF layer, 0.3 µm-CdTe layer with 10 ns carrier lifetime and 100 nm CdS layer.
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