Effect of TEA on characteristics of CdS/PbS thin film solar cells prepared by CBD

All chemicals were of analytical grade (Merck) and we used them as received without further purification. The total device structure was as follows: glass/ITO/CdS/PbS/Al (Fig. 1). For making the device, first the conducting glass/ITO commercial substrates with a sheet resistance of 25 Ω/□ were washed with soap in hot distilled water and rinsed many times in distilled water. Then, the substrates were cleaned ultrasonically by distilled water for 30 min and dried under nitrogen atmosphere.

Fig. 1

CdS thin films were prepared from a chemical bath containing 0.2 M CdCl₂, 1 M thiourea, 1 M triethanolamine (TEA) and 2 M ammonium hydroxide (NH₄OH). The temperature and pH value of the bath were 75 °C and 10 pH, respectively. In general, the prepared films were homogeneous, smooth, and adherent to the substrate.

PbS thin films were deposited on glass (for optical studies) and on glass/ITO/CdS structure by CBD at room temperature. The bath contained an aqueous solution of 0.5 M lead acetate, 1 M thiourea, 2 M sodium hydroxide and 1 M tri-ethanolamine. The pH value was adjusted to 12 in all baths. Although at the beginning the bath was colorless, its color changed to brown and then to black. The probable reaction for PbS thin film formation were reported in our previous work [15].

With the addition of TEA as the complexing agent, the metal complex is formed [15]: Pb2++TEA↔PbTEA2+$$\rm P{b^{2 + }} + TEA \leftrightarrow {\left[ {Pb\left( {TEA} \right)} \right]^{2 + }}$$(1)

The complexing agent helps to control the reaction rate. The formation of the metal complex reduces the concentration of free Pb²⁺ ions below the level required for the precipitation of solid phase Pb(OH)₂. The formation of nucleation centers through Pb(OH)₂ in the reaction bath is the first step in the growth of PbS film on the substrates.

After deposition, the samples were rinsed in deionized water. The non-adherent outer PbS layer was then removed with a cotton swab dipped in a dilute ammonium sulphate solution and again rinsed.

A 160 nm thick aluminum (Al) film was used as the bottom contact. It was deposited either by electron beam evaporation or thermal evaporation methods.

The structure of the films was analyzed by X-ray diffraction (XRD) using D8-advanced Bruker and CuKα radiation (λ = 1.5406 Å) equipment. A UV-Vis double-beam spectrophotometer (T80 UV-Vis, PG Instruments) was used to record the absorption spectra. The surface morphology of the deposited films was characterized using SEM (XL 30-Philips).

XRD spectra of the as-deposited CdS thin films shown in Fig. 2a have been used for evaluation of the crystallographic properties of the CdS thin films. The diffraction spectra were obtained by scanning in the 2θ range of 20° to 80°. The most attractive feature of CdS thin films prepared by CBD, from structural point of view, is its polymorphism. As a matter of fact, cubic and hexagonal crystallline structures in CdS films were generally reported by many authors. As seen, the obtained diffraction pattern shows a predominant peak at 2θ = 26.6° which can be assigned to the (111) plane of cubic CdS [17, 18]. As shown in Fig. 2a, the film shows also two diffraction peaks at 44.3° and 52.3° which are assigned to the (2 2 0) and (3 1 1) reflection planes in the CdS cubic structure. The broadening of the diffraction peaks indicates the nanocrystalline nature of the samples.

Fig. 2

Fig. 2b and Fig. 2c show the X-ray diffraction patterns of PbS films obtained from the bath with (PbS-A) and without (PbS-B) TEA. The PbS-A films have produced only peaks at 26.221°, 30.361°, 43.481°, and 51.241°, corresponding to the Miller planes (1 1 1), (2 0 0), (2 2 0), and (3 1 1), respectively (Fig. 2b). However, the patterns in Fig. 2c clearly show diffraction peaks at 26.411°, 30.501°, 43.501°, 51.471°, 53.861°, 69.331°, 71.361°, and 79.361°, which correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (3 3 1), (4 2 0), and (4 2 2) planes of the PbS phase with cubic structure. The intensity of the peaks in PbS-B films is higher than those of the PbS-A films, because the thickness of the PbS-B films is larger than that of the PbS-A films.

The influence of crystallite size on the electrical and optical behavior of the films is very important. The average crystallite size of CdS was calculated to be 11 nm using the Debye-Scherrer formula, D = Kλ/β_2θcosθ, where D is the crystallite size, λ is the wavelength of the X-ray radiation (1.5406 Å), K is usually taken as 0.9, θ is the Bragg angle, and β_2θ is the line width at halfmaximum height. The average crystallite sizes obtained for PbS-A and PbS-B sample were 25 nm and 250 nm, respectively. The crystallite size for PbS-B was greater than those for PbS-A. When TEA was added to the chemical bath, fewer ions were available for the film formation, so thinner films were formed, resulting in the formation of small grain sizes. To grow a film on a substrate using CBD, the first step is the formation of nucle-ation centers through the Pb(OH)₂ in the reaction bath and the second step is the growth stage. Nucleation is generally attributed to the formation of tiny seeds with well-defined crystallinity and a stable structure. In thinner films the number of tiny seeds for growing is low so the grain size is small.

Fig. 3 shows scanning electron microscopy (SEM) images of the deposited films. Fig. 3a indicates that CdS thin film is covered completely; no pinholes or cracks can be observed. Fig. 3b and Fig. 3c shows the SEM images of the PbS-A and PbS-B thin films, respectively. It can be seen from the SEM images that both groups form a continuous and compact polycrystalline films with well-defined grain boundaries. The grain sizes were estimated from SEM images to be 26 nm, 270 nm and 15 nm for PbS-A, PbS-B and CdS films, respectively. The crystallite sizes in the SEM images are greater than that obtained from the XRD patterns. Lager clusters, deposited on the surface of the film, clearly show an aggregate structure composed of small crystals or grains and SEM images show these clusters on the surface of the film. But in XRD method, the patterns were reflected from small crystals or grains.

Fig. 3

Fig. 4 shows the optical absorption of CdS sample at the wavelengths range of 300 nm to 1100 nm which shows a low absorption in visible region, thus making CdS a good candidate for a transparent window in solar cells. The inset in Fig. 4 presents the diagram normally used to determine the band-gap of films. The obtained band gap for CdS film is about 2.68 eV, which is higher than the CdS bulk band-gap value, due to quantum confinement effect in the nanocrystalline CdS films. The increased effective band gap causes nanocrystalline CdS films to be a more suitable window material in solar cell applications.

Fig. 4

Fig. 5 shows the transmittance and reflectance of CdS thin film from the near-ultraviolet (300 nm) to the near-infrared (1100 nm) region. The CdS film exhibits high transmittance and low reflectance in the visible region. The sharp increase of transmission starts at around 460 nm, in agreement with the band gap of approximately 2.6 eV; the average reflectance of 14 % is related to the transmission losses in the transparency region and is the result of comparatively high refractive index n (around 2.3) of the material. The reflection minimum at 700 nm and corresponding maximum in transmission (90 %) are observed.

Fig. 5

The transmission and reflection spectra of PbS films with different thicknesses (200 nm for PbS-A and 800 nm for PbS-B), deposited on glass, in the spectral range of 300 nm to 2600 nm, are shown in Fig. 6. In all cases, a clear interference pattern is seen. It shows that for the PbS-A and PbS-B films, the transmittance began to increase at 700 nm and 1200 nm, respectively (Fig. 6a). A decrease in transmission with increasing thickness of the films is also observed. The PbS-B film has lower transmission and reflection value than PbS-A in the visible range. These results indicate that the PbS-B film is a good absorber layer so, it can be considered as a potential material for solar cell applications due to the low transmittance in visible range. The determined energy gap for PbS thin films was found to be 1.47 eV and 0.64 for PbS-A and PbS-B, respectively (Fig. 7).

Fig. 8 shows the (J-V) characteristics of typical solar cells fabricated using CBD under illumination condition of 90 mW/cm². Two types of solar cells for different PbS absorber layers (prepared with and without TEA) have been analyzed. The one with 1 M TEA concentration is denoted as solar cell with PbS-A absorbing layer (SC/PbS-A) and the other without TEA is denoted as solar cell with PbS-B absorbing layer (SC/PbS-B). The solar cells structures have an open-circuit voltage (V_oc) of 285 mV for SC/PbSA and 275 mV for SC/PbS-B and a short-circuit current density (J_sc) of 9.41 mA/cm² for SC/PbSA and 12.24 mA/cm² for SC/PbS-B. The results reveal that without TEA, the open-circuit voltage decreases which may be due to the decreased energy band gap. The maximum voltage (V_max) and maximum current (J_max) are V_max = 150 mV and J_max = 7.74 mA/cm² for SC/PbS-A and V_max = 165 mV and J_max = 7.11 mA/cm² for SC/PbS-B. Resistive effects in solar cells reduce the efficiency of the solar cell by dissipating power in the resistances [17]. In the high field regime, the series resistance (RS) dominates and can be determined from the relationship as [18]:

Fig. 6

Fig. 7

In the low field regime, the shunt resistance (R_sh) dominates and can be determined as [18]: dIdVV=0=1Rsh$$\frac{{dI}}{{d{V_{V = 0}}}} = \frac{1}{{{R_{sh}}}}$$(3)

The (J-V) characteristics depend on the series and shunt resistances. Ideal solar cells have a zero series resistance and infinite shunt resistance. From equation 2 and equation 3, we obtained R_S =140 Q, for SC/PbS-A and R_S= 500 Ω for SC/PbS-B. We see that by adding TEA, the resistance decreases. The shunt resistance is R_sh =249 Ω for SC/PbS-A and R_sh = 5500 Ω for SC/PbS-B.

The fill factor (FF) was calculated from the following relationship [19]: FF=JM×VMJsc×Voc$$FF = \frac{{{J_M} \times {V_M}}}{{{J_{sc}} \times {V_{oc}}}}$$(4)

Fig. 8

The FF is 40 % for SC/PbS-A and 32 % for SC/PbS-B. This lower value of the FF may be a sign of slightly more efficient recombination of the electron hole pairs in the active layer.

The energy conversion efficiency η can be calculated using the equation [19]: η=FFJsc×VocPin$$\eta = FF\frac{{{J_{sc}} \times {V_{oc}}}}{{{P_{in}}}}$$(5)

The efficiency of the solar cells was found to be 1.21 % for SC/PbS-A and 1.31 % for SC/PbS-B, showing that adding TEA decreases the efficiency of the cells. It is believed that when we add TEA, the energy gap increases, leading to a decrease in electron hole pairs.