1 Introduction
Silicon-based solar cells have dominated the photovoltaic market and are approaching their theoretical limiting efficiency (29.4%) for single-junction [1]. To break through the Shockley–Queisser limit for single-junction devices, a tandem structure is expected to realize the power conversion efficiency (PCE) of more than 40% for multijunction solar cells [2]. Theoretically, a top cell with a bandgap of ~1.7 eV is preferred to match silicon (Si, 1.12 eV) bottom-cell and can achieve a high PCE of up to 45% in a tandem solar cell [3]. In a monolithic tandem structure, two PN junctions made of one wide-bandgap material and one narrow-bandgap material are series-connected to minimize the energy loss caused by thermal relaxation and produce higher PCE. Even many materials have been explored as the top cells in Si-based tandem devices, there are still various challenges in extensive commercial applications [4]. III-V semiconductors are not suitable for large-scale terrestrial photovoltaic commercial use owing to the high fabrication cost [5]. The bottleneck for perovskite top cells is that the stability has not met the requirement for practical application [6]. With a bandgap of 1.47 eV, the intrinsic cadmium telluride (CdTe) is incompatible with Si-based tandem solar cells [7]. Other than the advantages of good stability and low cost, cadmium selenide (CdSe) has an appropriate bandgap of ~1.7 eV and provides great potential in Si-based tandems.
With simple binary composition, CdSe has an appropriate bandgap of 1.72 eV [8] and high absorption coefficient in the visible light range [9]. Similar to other II-VI group compounds, it demonstrates excellent optoelectronic properties, which are widely used in light-emitting diodes (LED) [10] and photodetector [11]. However, CdSe has rarely been studied as a potential photovoltaic material in the past. One of the technical challenges is how to obtain highly crystallized CdSe films with low defect density. Furthermore, as an n-type semiconductor [12], finding a suitable p-type candidate is important to build a high-quality PN junction with CdSe thin film. Solving the above problems well, CdSe can be potentially used as a top subcell in tandem Si-based devices.
In a rapid thermal evaporation (RTE) system, the powdered source material is evaporated and deposited on the substrate located within 10 mm from the source. This design in the RTE system allows high deposition rates (~2 µm/min), effective use of source materials, high uniformity in thickness and composition of thin films. Therefore, RTE has been successfully used in synthesizing photovoltaic-grade thin films, such as Sb2Se3 [13], Sb2S3 [14], Bi2S3 [15], and GeSe [16]. Similar to those chalcogenides, CdSe has high saturated vapor pressure, which facilitates the deposition via the RTE method [17].
Here, we demonstrated the application of the RTE method to deposit CdSe thin films with large grain size and pure phase. The CdSe thin film exhibited a bandgap of 1.72 eV and good light response. Through the design of the device structure and optimization of CdSe film thickness, 1.88% conversion efficiency was achieved in CdSe thin-film solar cells. The RTE deposited CdSe thin-film solar cells here show great potential in Si-based tandem solar cell application.
2 Experimental section
2.1 CdSe solar cell fabrication
The fluorine doped tin oxide (SnO2:F, FTO) transparent conductive glasses were purchased (OPV Tech). The thickness, transmittance, and sheet resistance of the FTO layer are 500 nm,>85% (400–800 nm), and 8 Ω/sq, respectively. FTO was rinsed in the ultrasonic cleaning machine by the following solvent: detergent, acetone, isopropanol, ethyl alcohol, and deionized water in sequence. Titanium dioxide (TiO2) thin film was prepared using spray pyrolysis. The precursor solution used for spraying consisted of titanium diisopropoxide bis (acetylacetonate) (75 wt.% in isopropanol, Aladdin) and anhydrous ethanol (99.99% purity, Aladdin) in a ratio of 1:9. While spraying, the temperature and constant flow rate were 450°C and 12 mL/min, respectively. First, TiO2 was annealed on the hotplate at 450°C for 30 min in ambient air. Zinc oxide (ZnO) was prepared based on a previously reported solgel method [18]. 0.4-mol/L zinc acetate dihydrate and monoethanolamine (1:1 molar ratio) were dissolved in 2-methoxy ethanol solution and stirred at 60°C for 12 h. Then, the ZnO sol was dipped onto the FTO and spun at 3000 r/min for 30 s, following the annealing treatment at 500°C for 20 min. Cadmium sulfide (CdS) film was prepared via chemical bath deposition for 16 min on the substrate. Then, the CdS film was treated with 20 mg/mL cadmium chloride (CdCl2) absolute methanol solution via spin coating and annealed on the hotplate at 400°C for 6 min in ambient air. Rapid thermal evaporation (RTE, OTF-1200X-RTP, Hefei) was used for depositing CdSe films by evaporating CdSe powder (99.995% purity, Alpha). During the specific operation process: 0.5-g CdSe powder was evenly placed on the 5 cm × 5 cm × 0.1 cm quartz glass in the center of the RTE. The substrate was put 10 mm above the source. To maintain the low temperature of the substrate, a graphite block with a large heat capacity was placed on top of it. After pumping the pressure below 1 Pa, we first preheated the RTE system at 400°C for 900 s, then rapidly increased the temperature to 800°C at a ratio of 20°C/s and kept it for different periods (50, 100, and 150 s) to deposit the CdSe films.
PEDOT-4083 (Xi’an Polymer Light Technology Co.) with less than 0.5% TOYNOL Super wet-340 Surfactant (Tianjin SurfyChem T&D Co., Ltd., China) was deposited on the CdSe film by spin-coated method at 3000 r/min for 30 s. Subsequently, to remove the residual solvent, it was annealed at 150°C for 10 min on the hotplate. Copper iodide (CuI, 98% purity, Aladdin) was dissolved in diethyl sulfide (99% purity, Aladdin) with a concentration of 10 mg/mL. Then, the CuI precursor was spincoated at 3000 r/min for 30 s to deposit CuI film. Au electrodes (0.09 cm2) were deposited using a resistance evaporation thin-film deposition system (Beijing Technol Science Co., Ltd.) under a vacuum pressure of 5 × 10−3 Pa.
2.2 Materials characterizations
Scanning electron microscopy (SEM, FEI Nova NanoSEM450, without Pt coating) images were used to study the morphologies of CdSe films. The phase and orientation of CdSe films were determined using the X-ray diffraction (XRD) with Cu Kα radiation (Philips, X pert pro-MRD). UV–Vis spectrophotometer (PerkinElmer Instruments, Lambda 950 using integrating sphere) was conducted to determine the optical transmittance of CdSe thin-films. Photoluminescence (PL) spectra were measured using a confocal Raman microscope (LabRAM HR800, HORIBA, Ltd., France) with a 532-nm excitation wavelength at 100 multiples attenuation. The valence band and Fermi level of CdSe film were measured using ultraviolet photoelectron spectroscopy (UPS) (Specs UVLS, HeI excitation, 21.21 eV, referenced to the Fermi edge of argon etched gold).
2.3 Device characterizations
Current–voltage (I–V) curves were measured under the bias voltage varying from −1 to 1 V at 25℃ under dark or light (530 nm, 500 Ma, 5 mW/cm2) conditions. Under 1-V bias voltage, the photoelectric response of CdSe/gold (Au) was measured with a 530-nm light illumination and a 10-s signal circumference. The current-density–voltage (J–V) characterization was obtained using a source meter (Keithley 2400) under the dark conditions and simulated AM 1.5G (100 mW/cm2) illumination (Oriel, Model 9119, Newport). For external quantum efficiency (EQE) measurement, a light source generated using a 300 W xenon lamp of Newport (Oriel, 69911) was used and it was split into a specific wavelength using Newport oriel cornerstone TM 130 1/8 Monochromator (Oriel, model 74004).
3 Results and discussion
RTE is a similar thin-film deposition technology to the well-known closed-space sublimation (CSS) [19,20]. The main difference lies in the equipment design. RTE has a single heating source, whereas in CSS substrate and source are heated separately. It has the advantages of fast film formation, good film uniformity, high material utilization, and pure phase of the prepared materials. Theoretically, high-quality CdSe films can be prepared using RTE using the sublimation property of CdSe. The schematic diagram of RTE is shown in Fig. 1(a). The CdSe powder was uniformly scattered on the quartz glass, and the FTO substrate was placed above the CdSe powder (10-mm distance) with a graphite block on the top. The graphite block is used to minimize the variation of the substrate temperature during the evaporation. From the plot of saturation vapor pressure with temperature (Fig. 1(b)), it requires exceeding 1023 K and reach>1 Pa vapor pressure to evaporate CdSe. Based on the above analysis, the target evaporation temperature was set as 740 K to obtain enough CdSe vapor. The CdSe powder decomposes into element Se2 and Cd at high temperatures, as seen in Eq. (1).
At low temperatures, Cd and Se2 will react and form CdSe vapor [17]. This deposition mechanism makes it difficult for CdSe to introduce impurities in the deposition process [17]. Moreover, it is beneficial to the growth of CdSe grains, thus improving the crystallinity of CdSe films. The similar saturation vapor pressure of Cd and Se can enable an approximately stoichiometric ratio during the evaporation process. Eventually, the Cd and Se2 vapor combine to form CdSe and condense on the low-temperature FTO substrate.
The XRD (Fig. 1(c)) was performed to characterize the phase purity of the as-prepared CdSe films. Furthermore, the CdSe and remaining powders after RTE evaporation were tested as reference samples. The prominent XRD peaks (100), (002), (101), (110), (103), and (112) of the powder samples correspond to the hexagonal crystal structure (JCPDS: 00-008-0459) [21]. These demonstrate that CdSe evaporated congruently and no phase separation or selective evaporation occurred. As for the deposition of CdSe thin films, elemental Cd or Se peaks were not observed in the XRD pattern of CdSe films (Fig. 1(c)), even the vapor pressure of Se and Cd is much lower than CdSe. The X-ray photoelectron spectroscopy (XPS) was performed to characterize the chemical valency of the Cd and Se in the CdSe film (Fig. S1). The only Cd2+ and Se2− in the surface and bulk of CdSe indicate the no remain of elemental Cd or elemental Se. CdSe thin films show a single characteristic peak of (103), which is consistent with the hexagonal crystal structure. The single orientation, stable hexagonal crystal, and pure phase in the prepared CdSe thin films indicate high crystal quality.
The basic optoelectronic properties were characterized to further study the quality of CdSe thin films. The steep absorption edge in the absorption spectra suggests that CdSe is a direct bandgap semiconductor (Fig. 2(a)). From the Tauc plot [22], a direct bandgap of 1.72 eV was obtained by extrapolating the linear region to the x-axis (Fig. S2). The obtained value agrees well with the values found for single crystals (1.72 eV) [23]. Photoluminescence (PL) of CdSe was measured at 25°C. It shows a strong and narrow PL peak located around 713 nm (1.74 eV). The full-width half-maximum (FWHM) of PL peak is 23 nm (0.0185 eV). We also analyzed the PL spectrum more carefully, focusing on the wavelength>800 nm. The PL of CdSe overlaps with the background (Fig. S3), indicating fewer defects exist in the as-prepared CdSe films. Further, it indicates the absence of near band-edge defects and high quality of the films.
To determine the band position of the conduction band minimum and valence band in CdSe thin film, UPS characterization was further conducted (Fig. 2(b)). We found that the ionization potential and Fermi level of CdSe thin films were located at 6.07 and 4.47 eV, respectively. Combining the bandgap of 1.72 eV, the electron affinity is found to be at 4.35 eV. The Fermi level is located near the conduction band, implying the n-type nature of CdSe [24]. The n-type feature originates from the Se vacancy defects during the deposition process [25,26].
To study the photoresponse properties and test the suitability for optoelectronic applications, CdSe/Au was deposited on insulating glass substrates under the same conditions as in CdS/FTO substrate. The SEM and XRD (Fig. S4) show the similar quality of CdSe thin film on the CdS/FTO and glass substrates. A constant bias of 1 V was applied between electrodes, and a 530-nm wavelength LED was used as an excitation source. Under dark conditions, the current was approximately 10 nA, which indicated low conductivity (5 × 104Ω) of the CdSe thin films [27]. The specific calculation process is in the Electronic Supplementary Material. High resistivity is common for II-VI group materials without doping, such as CdTe [28] and CdS [29]. Under illumination, the conductivity increased significantly with an ON/OFF ratio of 304 (~5 mW/cm2) (Fig. 2(c)), which indicated that the CdSe film had an excellent optoelectronic performance. Furthermore, the current rise and fall durations were within 0.2 s (Figs. 2(d) and S5). The fast light response suggests a low density of deep defects and fewer intrinsic bulk defects in CdSe film [30]. Overall, the deposited CdSe thin film is of high quality and highly photosensitive, therefore it is suitable for working as an absorbing layer.
Fig.2 Optoelectronic properties of CdSe films. (a) Transmittance and PL spectra of CdSe thin film. (b) UPS spectra of CdSe thin film. The inset is the zoom-in image in the range of 0–2.5 eV. (c) I–V curves and (d) I–t curve of CdSe photo-conductance type detector under dark and 530 nm LED illumination |
To construct CdSe thin-film solar cells, suitable hole transport layers (HTLs) and electron transport layers (ETLs) need to be identified (Fig. 3(a)). Benefiting from the HTL and ETL, the photogenerated carriers can be efficiently separated and subsequently collected by the electrodes. The HTL should have a higher conduction band to block the migration of photogenerated electrons to the HTL and reduce recombination with holes at the CdSe/HTL interface. Similarly, the ETL needs to have a deeper valence band to block photogenerated holes (Fig. 3(b)). The CdSe is an n-type layer, then the p-type HTL is significant to build a PN junction. As shown in Fig. 3(c), we have made some attempts in HTL and CdS is used as an ETL. CuI is a typical wide-bandgap p-type material with high carrier concentration, and high mobility serving as an HTL in solar cells [31]. The CdSe solar cell with CuI demonstrates a high short-circuit current (JSC) exceeds 4 mA/cm2. Nevertheless, the open-circuit voltage (VOC) of the device is only 0.1 V, which might be caused by the interface defects. Polymers are considered to form good interfaces. The devices with PEDOT as HTL achieves a high VOC of 0.4 V. However, the high resistance of the PEDOT might not be favorable for carriers’ transport, resulting in a low JSC of 2.8 mA/cm2. Then, we designed a dual-hole collection layer (PEDOT/CuI). Consequently, the CdSe solar cell device exhibited a significant improvement in photogenerated carrier collection. Finally, we obtained an efficiency of 0.74% with high JSC (5.5 mA/cm2) and high VOC (0.45 V) (Fig. 3(c)).
Furthermore, the interfacial energy level mismatch in the FTO/CdS could limit the performance of CdSe solar cells, particularly the fill factor. Moreover, the CdS buffer layer might react during CdSe film deposition. It might lead to the direct contact of the CdSe film with the FTO electrode and result in leakage [32]. A thin TiO2 layer was introduced at the FTO/CdS interface as a barrier layer. The J–V curve of the device is shown in Fig. 3(d), and the VOC of the device is raised to 0.51 V. However, the high resistance of TiO2 might impede carrier transport to some extent and increase the resistance. Moreover, the conduction band energy level mismatch of TiO2 and CdSe leads to a kink phenomenon in the J–V curve. To address those issues, the TiO2 layer was replaced by the ZnO layer with lower resistance, leading to the enhancement of charge transfer and the reduction of interfacial recombination [33]. As a result, a high fill factor of 58%, a high VOC of 0.51 V, and a PCE of 1.45% were obtained in the device with ZnO/CdS as ETLs.
Fig.3 (a) Device structure schematic of CdSe thin-film solar cell. (b) Schematic diagram of energy band structure and charge transfer in CdSe solar cell. (c) J–V curves of CdSe solar cells with PEDOT, CuI, and PEDOT/CuI as HTL. (d) J–V curves of CdSe solar cells with TiO2/CdS and ZnO/CdS as ETL |
In addition to HTL and ETL, the thickness of CdSe directly affects its light absorption and the transport of photogenerated carriers. By adjusting the deposition time, we could control the thickness of the CdSe absorber layer. As the increasing deposition time from 50 to 150 s, the grain size of CdSe films increases from 560 to 2390 nm (Figs. 4(a)–4(c)), and the thickness of the increases from 500 to 2900 nm (Figs. 4(d)–4(f)). There is a good linear relationship between film thickness and deposition time (Fig. S6), which is more conducive to accurate control of the thickness of CdSe film. Furthermore, the SEM cross-sectional view shows that the CdSe grains also grow continuously upward from the substrate.
For brevity, we define the devices with CdSe thicknesses of 500, 1800, and 2900 nm as D500, D1800, and D2900, respectively. As shown in Fig. 5(a), the efficiency of CdSe thin-film solar cell decreased with the increase of the film thickness. A champion efficiency of 1.88% was obtained in the D500. The PCEs of D1800 and D2900 are 0.73% and 0.02%, respectively (Table 1). On one hand, the large absorption coefficient of CdSe was confirmed because film with hundreds of nanometers thickness demonstrated a good absorption. On the other hand, it illustrated that minority diffusion is difficult with the increase of absorption layer thickness, leading to a loss of JSC. The significant reduction in JSC decreases PCE. We measured the EQE to study the reason for the JSC reduction (Fig. 5(b)). All devices exhibit better long-wave EQE response, implying better carrier transport at the CdSe/HTL interface. The short-wavelength EQE response decreases significantly or even to 0 with increasing film thickness, because of the problematic carrier transport close to the ETL/CdSe interface.
To clarify possible reasons, we infer that the main PN junction is formed at the CdSe/HTL interface. Under the influence of the electric field, the carriers are separated rapidly and collected by the electrode. However, the photogenerated electrons and holes near the CdSe/ETL interface can only be collected by diffusion. Recombining is easy for electrons and holes photogenerated in the diffusion process. With the increase of absorption layer thickness, the photogenerated electrons and holes have a higher risk of being recombined. For a device with a thicker CdSe film, the photogenerated holes at the ETL/CdSe interface need to travel a longer diffusion distance and are easily recombined with electrons (Fig. 5(c)). For a device with a thinner CdSe film, the holes can quickly enter the space charge area and be effectively collected (Fig. 5(d)). Hence, in devices with thicker CdSe, the short-wavelength EQE is sharply reduced and the JSC is smaller.
Tab.1 Device performance of CdSe solar cell with different thicknesses |
| thickness/nm | VOC/V | JSC/(mA·cm−2) | FF/% | PCE/% |
|---|---|---|---|---|
| 500 | 0.501 | 6.45 | 58.1 | 1.88 |
| 1800 | 0.454 | 3.23 | 50.0 | 0.73 |
| 2900 | 0.222 | 0.17 | 53.1 | 0.02 |
4 Conclusion
In summary, the CdSe thin film was obtained by the RTE process with high quality, which is confirmed via the large grain size, single strong (103) peak, and high photoresponse. After designing a structure of FTO/ETLs/CdSe/HTLs/Au, we obtained a PCE of 1.88% in CdSe solar cell through the thickness optimization of CdSe. This is a preliminary exploration and demonstration of the feasibility of CdSe thin-film solar cells. Further effort is required to focus on solving the problem of carrier transport at the ETL/CdSe interface, to improve JSC, which will enhance the efficiency of CdSe solar cells to a new level. This work also paves the way for the in-depth study of CdSe thin-film solar cells and their application toward tandem solar cells.