Facile growth of a Sb₂Se₃ nanorod array induced by a MoSe₂ interlayer and its application in 3D p–n junction solar cells

Abstract

A uniformly grown Sb₂Se₃ nanorod array, with the introduction of a MoSe₂ interlayer, obtained by a co-evaporation process and its application in three-dimensional (3D) p–n junction high-efficiency Sb₂Se₃ solar cells were investigated in this study. The MoSe₂ interlayer played a crucial role as a seed layer for the preferential growth of Sb₂Se₃ crystals, which facilitated the formation of a Sb₂Se₃ nanorod array regardless of the process conditions. 3D p–n junction between the Sb₂Se₃ nanorod array and the CdS buffer layer improved the short-circuit current of Sb₂Se₃ solar cells due to improved carrier transportation from the Sb₂Se₃ absorber to the CdS buffer. The MoSe₂ interlayer also improved the contact quality between the Sb₂Se₃ nanorod array and the Mo substrate by forming a quasi-ohmic contact, which resulted in a higher open-circuit voltage due to a reduced contact barrier and series resistance in Sb₂Se₃ solar cells. The crystal growth rate of Sb₂Se₃ was controlled by the source evaporation rate and substrate temperature to tune the final nanostructure and crystalline orientation of the co-evaporated Sb₂Se₃ nanorods array. 3D p–n junction solar cells based on an ordered and (hk1) preferentially oriented Sb₂Se₃ nanorod array showed a power conversion efficiency of 5.637%. Therefore, by including a MoSe₂ interlayer, it is possible to achieve high-efficiency 3D p–n junction Sb₂Se₃ solar cells.

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Introduction

Antimony selenide (Sb₂Se₃) is one of the most promising semiconductor materials for thin-film solar cells because of its outstanding optoelectronic properties. Additionally, it is a non-toxic, relatively low-cost material with high absorption coefficient and low processing temperature.^1–6 Therefore, significant research has been conducted in the past decade to replace the conventional multinary compound semiconductors in thin-film solar cells for Sb₂Se₃.^7–11 However, the best power conversion efficiency (PCE) reported so far for Sb₂Se₃ solar cells is approximately 10%,¹² which is similar to those of thin-film solar cells made of conventional materials. In order to overcome the efficiency limitation of the Sb₂Se₃ solar cells, the application of three-dimensional (3D) nanostructured Sb₂Se₃ thin films for the formation of the 3D p–n junction solar cells could be one of the promising solutions.^13–15

Carrier transportation and p–n junction formation in Sb₂Se₃ solar cells are closely related to the Sb₂Se₃ nanostructure. Therefore, the latter is one of the critical factors determining the photovoltaic performance of Sb₂Se₃ solar cells. However, there is little study on the nanostructure control of the Sb₂Se₃ thin films for high efficiency solar cells. Li et al. improved the short-circuit current (J_SC) of a solar cell using a 1000 nm-high Sb₂Se₃–nanorod array with superior light-harvesting capacity and reduced optical reflection.¹⁶ In our previous work, we investigated the evolution of Sb₂Se₃ nanorods by controlling the process temperature of co-evaporation and proposed the photovoltaic performance enhancement of Sb₂Se₃ solar cells based on 3D p–n junction formation.¹⁸ However, the temperature window for the formation of Sb₂Se₃ nanorods was narrow and the orientation of the Sb₂Se₃ nanorods was random. From these previously reported studies, it was found that the application of 3D nanostructured Sb₂Se₃ absorber layers would be very useful to overcome of the performance limitation of Sb₂Se₃ solar cells based on the 3D p–n junction solar cell concept. However, to the best of the authors’ knowledge, there is no detailed investigation of the nanostructure evolution of Sb₂Se₃ thin films reported in the literature.

The objective of this work was to study the evolution of a nanostructured Sb₂Se₃ thin film fabricated using a MoSe₂ interlayer and the co-evaporation process. Additionally, a detailed investigation of the controlling factor to achieve fine-tuned Sb₂Se₃ nanorod arrays is presented in this work. In our work, we proposed the evolution of nanostructured Sb₂Se₃ thin films using a MoSe₂ interlayer and detailed investigation on the control factor of the co-evaporation process for the fine-tuning of the Sb₂Se₃ nanorod array. By introducing a MoSe₂ interlayer between the Mo substrate and Sb₂Se₃, it was possible to form uniformly ordered packed Sb₂Se₃ nanorod formation using co-evaporation of Sb₂Se₃ and Se sources. The MoSe₂ interlayer has locally distributed dangling bonds, which play the role of nucleation seed for Sb₂Se₃ crystalline growth perpendicular to the Mo substrate. Because of the seeding effect of the MoSe₂ interlayer, Sb₂Se₃ thin films deposited on the MoSe₂ interlayer dominantly showed a uniformly ordered packed Sb₂Se₃ nanorod array. In addition, the formation of a quasi-ohmic contact between Sb₂Se₃ and MoSe₂ was another improving point of open-circuit voltage (V_OC) of Sb₂Se₃ solar cells compared with Sb₂Se₃ solar cells without the MoSe₂ interlayer.

The nanostructure of the Sb₂Se₃ nanorod arrays on the MoSe₂ interlayer was also sensitively affected by the conditions of the co-evaporation processes, such as the evaporation rate of sources and substrate temperature. The growth behavior of Sb₂Se₃ crystals during the co-evaporation process was closely related with the final nanostructure of Sb₂Se₃ nanorod array, which was the main factor that determines the short-circuit current (J_SC) of Sb₂Se₃ solar cells. The crystalline orientation of co-evaporated Sb₂Se₃ thin films, which is a crucial factor for high efficiency Sb₂Se₃ solar cells, was also affected by the substrate temperature.^18–25 After chemical bath deposition (CBD) of n-type CdS buffer layers on the Sb₂Se₃ nanorod array, the 3D p–n junction between Sb₂Se₃ and CdS was formed and the 3D p–n junction was effective for the enhancement of J_SC. By introducing the MoSe₂ interlayer, the growth of a uniformly ordered Sb₂Se₃ nanorod array and the quasi-ohmic contact between Sb₂Se₃ and MoSe₂ were possible and 3D p–n junction Sb₂Se₃ solar cells with 5.637% efficiency was achieved.

Experimental

MoSe₂ interlayer and Sb₂Se₃ thin-film preparation

The structure of the studied Sb₂Se₃ thin films consisted of a soda-lime glass (SLG) substrate and a 600 nm-thick Mo layer as the back-contact layer. Before deposition, the substrate was sequentially cleaned in an ultrasonic bath using acetone, methanol, and deionized water. The Mo layer was deposited on the SLG substrate via direct current (DC) magnetron sputtering using a Mo target with a purity of 99.99%.

To form the MoSe₂ interlayer on Mo substrates, the latter were treated in a tube furnace in a Se atmosphere. Vacuum (10⁻³ Torr) was applied to the furnace to remove moisture and impurities. Then, Ar gas was introduced into the furnace, allowing the pressure inside the furnace to reach the atmospheric pressure. The furnace was heated to 430 °C with Se pellets inside for 10 min and then annealed for 10 min. The MoSe₂ layer obtained was approximately 100 nm-thick.

The Sb₂Se₃ thin films were prepared on bare Mo substrates or MoSe₂/Mo substrates using a co-evaporator system using Sb₂Se₃ and Se sources. The substrate temperature was controlled in the range of 250–290 °C. The substrate was heated at 0.53 °C s⁻¹ from room temperature (∼25 °C) to the desired temperature. The evaporation rate of Sb₂Se₃ and Se sources varied from 50–200 Å s⁻¹. Sb₂Se₃ was deposited to obtain thin films (thickness: approximately 500 nm). After deposition, the samples were allowed to cool to room temperature naturally.

Solar cell fabrication

Chemical bath deposition (CBD) was used to cover the Sb₂Se₃ thin films obtained with a 50 nm-thick n-type cadmium sulfide (CdS) buffer layer, and radiofrequency (RF) sputtering was used to sequentially deposit a 50 nm-thick intrinsic ZnO window layer and a 300 nm-thick Al-doped ZnO (AZO) A transparent conducting oxide (TCO) layer was deposited on top of the buffer layer to fabricate the solar cells. Thermal evaporation was then used to deposit a 1 μm-thick Al collection grid on top of the device.

Characterization of Sb₂Se₃ solar cells

The Sb₂Se₃ thin films were analyzed using various techniques to determine the influence of substrate temperature on the chemical composition and morphology of the films, and consequently, on the photovoltaic characteristics of the Sb₂Se₃ solar cells. The surface morphology and chemical composition of the Sb₂Se₃ thin films were observed using field emission scanning electron microscopy (FE-SEM, Hitachi, SU8020) and transmission electron microscopy (FE-TEM, Hitachi, HF-3300) with a focused ion beam (FIB) system (Hitachi, NB 5000). The crystalline structure of the Sb₂Se₃ thin films was determined using X-ray diffraction (XRD) analysis (Panalytical, Empyrean). The photovoltaic characteristics of the solar cells were characterized using a source meter (Keithley, 2400) unit and a solar simulator (Newport, 94022A) to simulate 1.5AM solar irradiation.

Results and discussion

To obtain a controlled Sb₂Se₃ crystalline structure in Sb₂Se₃ solar cells, an MoSe₂ interlayer between the Sb₂Se₃ absorber and the Mo substrate was adopted. Additionally, the MoSe₂ interlayer between Sb₂Se₃ and the Mo substrate played an important role in the growth control of the Sb₂Se₃ crystalline structure. To investigate the role and effect of the MoSe₂ interlayer on Sb₂Se₃ crystal growth, Sb₂Se₃ thin films were deposited on bare Mo substrates and MoSe₂/Mo substrates using the co-evaporation process at different source evaporation rates (50–200 Å s⁻¹). The source evaporation rate is closely related to the Sb₂Se₃ crystal growth mechanism. Fig. 1 shows the cross-sectional SEM images of Sb₂Se₃ thin films with different substrates and at different evaporation rates.

Fig. 1 Top and cross-sectional SEM images of co-evaporated Sb₂Se₃ thin films on bare Mo substrates or MoSe₂/Mo substrates at 290 °C and different source evaporation rates: (a), 50 Å s⁻¹; (b), 100 Å s⁻¹; and (c), 200 Å s⁻¹ on bare Mo substrates; and (d), 50 Å s⁻¹; (e), 100 Å s⁻¹; and (f), 200 Å s⁻¹ on MoSe₂/Mo substrates.

In the case of co-evaporated Sb₂Se₃ thin films on bare Mo substrates, thin films deposited at a slow source evaporation rate (50 Å s⁻¹) showed a small and randomly oriented Sb₂Se₃ nanorod structure (Fig. 1a). However, as the source evaporation rate increased to 100 Å s⁻¹, Sb₂Se₃ thin films showed a densely packed columnar structure with some long Sb₂Se₃ nanorods (Fig. 1b). At a source evaporation rate of 200 Å s⁻¹, a flat Sb₂Se₃ thin-film structure comprising densely packed grains was observed (Fig. 1c). Generally, the nucleation rate is closely related to the evaporation rate.²⁶ Thus, a easily formed continuous film under a high evaporation rate was expected. For film materials having an anisotropic crystal structure, such as Sb₂Se₃, rod-shaped film formation is highly probable when the evaporation rate is low. Under these conditions, the impingement rate of adatoms on the substrate decreases and their diffusion length may increase, causing the formation of a non-continuous film. Based on these results, authors believe that controlling the evaporation rate is critical to developing the Sb₂Se₃ rod array for 3D p–n junction solar cells.

Contrary to the co-evaporated Sb₂Se₃ thin films on Mo substrates, Sb₂Se₃ thin films prepared on MoSe₂/Mo substrates showed a different nanostructure. All Sb₂Se₃ thin films on MoSe₂/Mo substrates showed non-flat thin-film structures, regardless of the source evaporation rate. At an evaporation rate of 50 Å s⁻¹, thin films showed a vertically oriented narrow Sb₂Se₃ nanorod array of length 500 nm (Fig. 1d). The diameter and length of the Sb₂Se₃ nanorods increased with increasing evaporation rate from 50 to 100 Å s⁻¹ (Fig. 1e). At an evaporation rate of 200 Å s⁻¹, the thin-film showed ordered arrays of thick Sb₂Se₃ nanorods with a diameter of 100 nm (Fig. 1f). Sb₂Se₃ thin films deposited on MoSe₂ substrates showed a Sb₂Se₃ nanorod array structure regardless of the source evaporation rate. Additionally, the Sb₂Se₃ nanorod array nanostructure was tuned by varying the source evaporation rate. The formation of Sb₂Se₃ nanorod array on the MoSe₂ interlayer might be attributed to different Sb₂Se₃ crystal growth mechanisms (Fig. 2). Moreover, the MoSe₂ interlayer has unique anisotropic structural properties. For example, the (00l) plane of MoSe₂ has no surface dangling bond, making the nucleation process difficult while facilitating the diffusion and desorption of adatoms on the (00l) plane of MoSe₂. However, the nucleation process of Sb₂Se₃ could be easily formed on a plane having a high-dangling bond density, such as the one perpendicular to the (00l) plane. Therefore, the MoSe₂ interlayer was expected to provide a selective nucleation site, facilitating the formation of the Sb₂Se₃ rod array on the MoSe₂ interlayer. To investigate in detail the interface between Sb₂Se₃ and MoSe₂, a high-angle annular detector dark-field scanning transmission electron microscopy (HADDF-STEM) image of the Sb₂Se₃/MoSe₂/Mo sample was observed (Fig. S3, ESI†). The MoSe₂ showed a two-dimensional crystal structure with weak van der Waals bonding forces and without dangling bonds in the c-plane of MoSe₂. Overall, the layer structure was oriented perpendicular to the Mo substrate. Interestingly, in our case, the density of the MoSe₂ interlayer bunch seemed relatively low. Thus, the orientation of the flexible bunch of the MoSe₂ layer was not perfectly aligned. The exposed-MoSe₂ layer on the surface consisted of a c-plane without dangling bonds and another plane with dangling bonds.

Fig. 2 Schematic diagram of the selective nucleation and growth mechanism of Sb₂Se₃ on the randomly grown MoSe₂ interlayer; (a) low surface energy plane, (b) high surface energy plane, and (c) growth of Sb₂Se₃ nanorods on MoSe₂ interlayer.

To confirm the effect of the substrate type on the morphology of the co-evaporated Sb₂Se₃ thin films, we also deposited Sb₂Se₃ thin films on indium tin oxide (ITO) substrates using the same co-evaporation process. Unlike Mo substrates, all the co-evaporated Sb₂Se₃ thin films on ITO substrates showed a similar dense thin-film morphology, regardless of the source evaporation rate (Fig. S1, ESI†) and substrate temperature (Fig. S2, ESI†). These results show that the type of substrate had a significant effect on the formation of Sb₂Se₃ nanorods by the co-evaporation process.

To evaluate the photovoltaic characteristics of the co-evaporated Sb₂Se₃ thin films on different Mo and MoSe₂/Mo substrates, three different Sb₂Se₃ solar cells with a planar device structure of SLG/Mo/MoSe₂/Sb₂Se₃/CdS/i-ZnO/AZO/Al were fabricated. Fig. 3 shows the cross-sectional TEM and energy dispersive X-ray spectroscopy (EDS) mapping images of the Sb₂Se₃ solar cells fabricated at different source evaporation rates and with different substrates. The Sb₂Se₃ solar cells without the MoSe₂ interlayer showed a planar structure (Fig. 3a and b), due to the flat Sb₂Se₃ thin-film structure shown in Fig. 1c. The CdS buffer, i-ZnO window, and AZO TCO layers were uniformly deposited on the flat Sb₂Se₃ absorber layer. However, the Sb₂Se₃ solar cells prepared on MoSe₂/Mo substrates showed completely different nanostructures than those using the Mo substrate. The Sb₂Se₃ absorber layer showed a complex nanorod array, and the CdS buffer layer was deposited on the Sb₂Se₃ nanorods conformally. The ZnO and AZO layers deposited on the rough CdS/Sb₂Se₃ interface showed irregular nanostructures. At the bottom of the Sb₂Se₃ absorber layer, a uniformly formed MoSe₂ interlayer was confirmed. Moreover, the source evaporation rate showed a considerable effect on the nanostructure of the Sb₂Se₃ absorber layer. The Sb₂Se₃ solar cells prepared using a 200 Å s⁻¹ source evaporation rate on MoSe₂/Mo substrates showed an irregular and agglomerated Sb₂Se₃ grain structure (Fig. 3c and d). However, when the evaporation rate decreased to 100 Å s⁻¹, the Sb₂Se₃ absorber layer showed a more ordered and regular nanorod array (Fig. 3e and f). These different Sb₂Se₃ nanorod array structures observed according to the source evaporation rate were consistent with the SEM images (Fig. 1e and f).

Fig. 3 Cross-sectional TEM and EDS mapping images of Sb₂Se₃ solar cells fabricated with different source evaporation rates and at a temperature of 290 °C. (a) and (b), 200 Å s⁻¹ on a Mo substrate; (c) and (d), 200 Å s⁻¹ on a MoSe₂/Mo substrate; and (e) and (f) 100 Å s⁻¹ on a MoSe₂/Mo substrate.

Table 1 lists the photovoltaic properties of the Sb₂Se₃ solar cells with and without the MoSe₂ interlayer. Despite the planar and uniform device structure of the Sb₂Se₃ solar cells without the MoSe₂ interlayer, the solar cells with the MoSe₂ interlayer showed a superior photovoltaic performance. The higher open-circuit voltage (V_OC) showed by the Sb₂Se₃ solar cells with the MoSe₂ interlayer might be attributed to the quasi-ohmic contact formation between Sb₂Se₃ and MoSe₂. The quasi-ohmic contact formation of the MoSe₂ interlayer is well known for copper indium gallium diselenide (CIGS) solar cells.^27–31 This quasi-ohmic contact could reduce the contact barrier and series resistance of Sb₂Se₃ solar cell devices. The higher J_SC could be related to the 3D p–n junction formation between the Sb₂Se₃ absorber and CdS buffer layers. Due to the 3D p–n junction structure, photo-generated carriers can easily escape into the CdS buffer layer. Thus, nanostructured Sb₂Se₃ solar cells show a higher J_SC than flat thin-film Sb₂Se₃ solar cells. This superior photovoltaic performance was consistent with the results reported by Park et al. and was also attributed to the 3D p–n junction formation between the Sb₂Se₃ and CdS buffer layers.¹⁷ Between the two Sb₂Se₃ solar cells with a MoSe₂ interlayer, the one fabricated at an evaporation rate of 100 Å s⁻¹ showed higher J_SC and fill factor (FF) than that at 200 Å s⁻¹, which might be attributed to the more regular and ordered 3D p–n junction formation between Sb₂Se₃ and CdS.

Table 1 Photovoltaic characteristics of the Sb₂Se₃ solar cells using MoSe₂/Mo substrates at 290 °C. Open-circuit voltage (V_OC), short-circuit current (J_SC), fill factor (FF), power conversion efficiency (PCE), and series and shunt resistances (R_s, R_sh)

Substrate	Evaporation rate [Å s^−1]	V OC [V]	J SC [mA cm^−2]	FF [%]	Eff [%]	R s [Ω cm^2]	R sh [Ω cm^2]
Mo	200	0.20297	8.4189	27.2369	0.465	18.105	26.703
MoSe2/Mo	200	0.43951	21.1832	36.0716	3.358	9.847	51.383
MoSe2/Mo	100	0.44999	23.6606	40.2727	4.288	8.488	70.319

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A previous report showed that high substrate temperatures induce the evolution of Sb₂Se₃ nanorod arrays, demonstrating the close relationship between the nanostructure of co-evaporated Sb₂Se₃ thin films and substrate temperature.¹⁷ Therefore, we have tried to tune the nanostructure of co-evaporated Sb₂Se₃ thin films by controlling the substrate temperature at a source evaporation rate of 100 Å s⁻¹ on MoSe₂/Mo substrates. We also prepared co-evaporated Sb₂Se₃ thin films on bare Mo at 315 °C, which is the requirement for Sb₂Se₃ nanorod array formation on bare Mo substrates. Fig. 4 shows the top and cross-sectional SEM images of co-evaporated Sb₂Se₃ thin films at different substrate temperatures on bare Mo or MoSe₂/Mo.

Fig. 4 Top and cross-sectional SEM images of Sb₂Se₃ thin films at a source evaporation rate of 100 Å s⁻¹ and different deposition temperatures: (a), 250; (b), 260; (c), 270; and (d), 290 °C using MoSe₂/Mo substrates; and (e), 315 °C using a bare Mo substrate.

The co-evaporated Sb₂Se₃ thin films on the MoSe₂/Mo substrate did not show the abrupt morphology change, from the flat thin-film structure to a nanorod array, observed in our previous report on Mo substrates.¹⁷ Because the MoSe₂ interlayer already easily induces the formation of the Sb₂Se₃ nanorod array, the substrate temperature showed little effect on the formation of the Sb₂Se₃ nanorod array. However, the detailed nanostructure of the Sb₂Se₃ nanorods was affected by the substrate temperature. As the substrate temperature decreased from 290 to 250 °C, the shape regularity and the degree of ordering of the Sb₂Se₃ nanorods improved. As the substrate temperature decreases, the Sb₂Se₃ nuclei density increases, suppressing the rapid growth of Sb₂Se₃ nanorods. Therefore, the growth of Sb₂Se₃ nanorods could be controlled effectively to form an ordered Sb₂Se₃ nanorod array by adjusting the substrate temperature on MoSe₂/Mo. Moreover, using the MoSe₂ interlayer, it was possible to control the nanostructure of co-evaporated Sb₂Se₃ thin films by varying both the evaporation rate and substrate temperature. Co-evaporated Sb₂Se₃ thin films on a bare Mo substrate at 315 °C also showed a similar Sb₂Se₃ nanorod structure to that on MoSe₂/Mo substrates, which is consistent with the results from our previous work.¹⁷ The bare Mo substrate required a higher substrate temperature to form a similar Sb₂Se₃ nanorod array to the one obtained using MoSe₂/Mo substrates and the co-evaporation process.

The crystalline orientation of Sb₂Se₃ thin films is well known to be crucial for determining the photovoltaic properties of Sb₂Se₃ solar cells due to the intrinsic one-dimensional ribbon structure of Sb₂Se₃.^32–34 Therefore, the crystalline structure of co-evaporated Sb₂Se₃ thin films on MoSe₂/Mo substrate at different substrate temperatures was analyzed by XRD analysis. All the co-evaporated Sb₂Se₃ thin films showed characteristic XRD peaks of Sb₂Se₃. However, the relative intensity of specific peaks corresponding to different crystalline orientations was different for each substrate temperature. Particularly, at 260 °C where the (020) and (120) peaks were not observed, and the intensity of the (211) and (221) peaks increased (Fig. 5a). To further investigate the crystalline orientation of the co-evaporated Sb₂Se₃ thin films, their texture coefficient was calculated.^35–37 At 260 °C the texture coefficient of the (211) peak was remarkably higher than that at other substrate temperatures (Fig. 5b). Additionally, all other Sb₂Se₃ thin films showed relatively higher texture coefficients of (hk0) than those at 260 °C. The (hk1) preferential orientation of the Sb₂Se₃ thin film at a substrate temperature of 260 °C could help in improving the photovoltaic performance of Sb₂Se₃ solar cells.

Fig. 5 XRD peaks (a) and texture coefficient (b) of co-evaporated Sb₂Se₃ thin films at different substrate temperatures and at a source evaporation rate of 100 Å s⁻¹.

Sb₂Se₃ solar cells were fabricated at different substrate temperatures to investigate the effect of substrate temperature of co-evaporated Sb₂Se₃ thin films on their photovoltaic performance. Fig. 6 shows the I–V curves of the Sb₂Se₃ solar cells at a source evaporation rate of 100 Å s⁻¹ and different substrate temperatures, and Table 2 lists the photovoltaic properties obtained for the Sb₂Se₃ solar cells. All the Sb₂Se₃ solar cells using the MoSe₂ interlayer showed a V_OC > 0.4 V, regardless of the substrate temperature. Because the MoSe₂ interlayer already improved the V_OC, the structural change in the Sb₂Se₃ nanorods had little effect on the V_OC. The Sb₂Se₃ solar cell using a bare Mo substrate showed a slightly lower V_OC (0.37 V) than that with MoSe₂/Mo substrates. These results show that the introduction of the MoSe₂ interlayer significantly improves the V_OC of Sb₂Se₃ solar cells. The Sb₂Se₃ solar cell prepared at a substrate temperature of 260 °C showed the highest J_SC. This might be attributed to the (hk1) preferential orientation of the Sb₂Se₃ absorber layer. Moreover, the ordered and regular Sb₂Se₃ nanorod array could help increase the area of the 3D p–n junction between Sb₂Se₃ and CdS, improving the J_SC of the Sb₂Se₃ solar cells. By the way, it showed comparable J_SC (25.138 mA cm⁻²) to those of the Sb₂Se₃ solar cell with MoSe₂/Mo substrates because of the densely packed Sb₂Se₃ nanorod array. Interestingly, Sb₂Se₃ solar cells using a MoSe₂/Mo substrate at 250, 270 and 290 °C showed a relatively smaller J_SC than bare Mo substrate. The nanostructure changes of the co-evaporated Sb₂Se₃ nanorod array by temperature is remarkable (Fig. 4), which might be closely related to the J_SC and FF of Sb₂Se₃ solar cells. At 250 °C, the width of Sb₂Se₃ nanorods is smaller than that at 260 °C, which is disadvantageous for sufficient light absorption. The length uniformity of Sb₂Se₃ nanorods at 270 and 290 °C is inferior compared to that at 260 °C. The irregular nanostructure of the Sb₂Se₃ nanorods induces insufficient light absorption and photo-generated carrier transportation. The thinner rods (250 °C) and increased empty space (270 °C) could deteriorate the FF by increasing the shunt paths too.

Fig. 6 I–V curves of Sb₂Se₃ solar cells at a source evaporation rate of 100 Å s⁻¹ and different substrate temperatures: (a), 250; (b), 260; (c), 270; and (d), 290 °C using MoSe₂/Mo and bare Mo substrates.

Table 2 Photovoltaic characteristics of the Sb₂Se₃ solar cells using MoSe₂/Mo and bare Mo substrates at different temperatures (T). Open-circuit voltage (V_OC), short-circuit current (J_SC), fill factor (FF), power conversion efficiency (PCE), and series and shunt resistances (R_s, R_sh)

Substrate	Temperature [°C]	V OC [V]	J SC [mA cm^−2]	FF [%]	Eff [%]	R s [Ω cm^2]	R sh [Ω cm^2]
MoSe2	250	0.40	22.167	43.99	3.886	5.48	67.59
MoSe2	260	0.43	27.430	47.35	5.637	5.89	118.26
MoSe2	270	0.42	23.834	44.47	4.464	6.04	80.89
MoSe2	290	0.45	23.661	40.27	4.288	8.49	70.32
Mo	315	0.37	25.138	47.11	4.412	5.69	112.25

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To investigate the correlation between the 3D p–n junction structure and the photovoltaic properties of Sb₂Se₃ solar cells, TEM analysis of the best Sb₂Se₃ solar cell, with the evaporation rate of 100 Å s⁻¹ and the substrate temperature of 260 °C, was performed. Fig. 7 shows the TEM-EDS mapping images of the Sb₂Se₃ solar cell device with a nanorod array with a PCE of 5.637% (Table 2). The Sb₂Se₃ solar cell using a MoSe₂/Mo substrate showed a 1.225% higher PCE than the one using a bare Mo substrate, which might be attributed to the improved V_OC. A densely packed, uniformly distributed, Sb₂Se₃ nanorod array and the 3D p–n junction between CdS and Sb₂Se₃ nanorods were observed. These properties are advantageous for the enhancement of J_SC. Additionally, the uniform MoSe₂ interlayer between the Mo substrate and the Sb₂Se₃ nanorod array is beneficial for the formation of a quasi-ohmic contact between the layers.

Fig. 7 STEM-EDS mapping images of Sb₂Se₃ nanorod array solar cell with PCE of 5.637%.

The results presented in this work reveal that the MoSe₂ interlayer between a Mo substrate and a Sb₂Se₃ absorber layer improves the performance of Sb₂Se₃ solar cells in two relevant ways. First, the contact quality between Sb₂Se₃ and Mo layers improved due to quasi-ohmic contact formation, which was confirmed by the higher V_OC values of Sb₂Se₃ solar cells using MoSe₂/Mo substrates compared to those using a bare Mo substrate; second, a facile formation of the Sb₂Se₃ nanorod array and a lower substrate temperature were achieved using the co-evaporation process, which was followed by the 3D p–n junction Sb₂Se₃ solar cell fabrication. Moreover, the MoSe₂ interlayer played a key role in improving the photovoltaic performance of the Sb₂Se₃ nanorod array solar cell by increasing both V_OC and J_SC. The quasi-ohmic contact caused by the MoSe₂ interlayer effectively improved the contact quality between Sb₂Se₃ layers and Mo substrates. Moreover, the MoSe₂ interlayer also played a critical role as a seed layer for the uniform growth of the Sb₂Se₃ nanorod array.

The Sb₂Se₃ solar cells prepared by co-evaporation at a source evaporation rate of 100 Å s⁻¹ and substrate temperature of 260 °C on a MoSe₂/Mo substrate were analyzed by an external certified laboratory to verify our PCE laboratory measurements. The certified J–V curve (Fig. 8a) and external quantum efficiency data (Fig. 8b) showed similar results (PCE of 5.484%) to our lab-scale photovoltaic measurements. We calculated integrated J_SC from EQE data and compared with the J_SC of J–V curve. The J_SC from integrated EQE (25.89 mA cm⁻²) is similar to the J_SC of the J–V curve (26.05 mA cm⁻²).

Fig. 8 Certified J–V curve (a) and external quantum efficiency (EQE) data (b) of Sb₂Se₃ solar cells at a substrate temperature of 260 °C and a source evaporation rate of 100 Å s⁻¹ using a MoSe₂/Mo substrate (Korea Institute of Energy Research, KIER).

Conclusions

This work investigated the role of the MoSe₂ interlayer on the growth of the Sb₂Se₃ nanorod array and the contact quality between Sb₂Se₃ thin films and Mo substrates. By introducing a MoSe₂ interlayer between Sb₂Se₃ layers and Mo substrates, uniformly ordered densely packed Sb₂Se₃ nanorods were successfully prepared. The MoSe₂ interlayer, with locally distributed high surface energy planes, played a crucial role as a seed layer for the preferential growth of Sb₂Se₃ crystals perpendicular to the substrate, which facilitated the formation of a Sb₂Se₃ nanorod array on the MoSe₂ interlayer. The latter also significantly improved the photovoltaic performance of Sb₂Se₃ solar cell devices by forming a quasi-ohmic contact between Sb₂Se₃ absorber layers and Mo substrates, resulting in higher V_OC and J_SC. The nanostructure and crystalline orientation of the Sb₂Se₃ nanorods array were affected by the substrate temperature and the source evaporation rate during the co-evaporation process. 3D p–n junction solar cells based on an ordered and (hk1) preferentially oriented Sb₂Se₃ nanorods array showed a PCE of 5.637%.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the DGIST R & D Programs of the Ministry of Science and ICT, Republic of Korea (21-CoE-ET-01, 21-ET-08). We thank Mr Eun and Mr Cheon at the Center for Core Research Facilities (CCRF) of DGIST for FIB sampling and STEM measurements.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ma00804h

‡ These authors contributed equally to this paper.

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