10001077_Optical-loss analysis of ZnSp-Si heterojunction solar cell with WO3 as hole-selective contact

1 Optical-loss analysis of ZnS/p-Si heterojunction solar cell with WO3 as hole-selective contact Kaifu Qiua,b, Qi Xiea,b, Depeng Qiub, Lun Caia,b, Weiliang Wub, Wenjie Linb, Zhirong Yaoa,b, Bin Aib, Zongcun Liang a,b,c, and Hui Shen a,b,c * a School of Physics, Sun Yat-Sen University, Guangzhou, 510275, PR China b Institute for Solar Energy Systems, Guangdong Provincial Key Laboratory of Photovoltaic Technology, Sun Yat-sen University, Guangzhou, 510004, PR China c School of Physics and State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat- Sen University, Guangzhou, Guangdong Province, PR China ABSTRACT A novel dopant-free ZnS/p-Si heterojunction solar cell with WO3 thin films as hole-selective contact was fabricated using thermal evaporation method. The obtained maximum power conversion efficiency (PCE) of 10.94% is the highest recorded value for ZnS/p-Si heterojunction solar cells, to the best of our knowledge. The transfer line matrix (TLM) measurements indicate that the contact between WO3 thin films and p-Si is ohmic behavior, with a contact resistivity (ρc) of 12.7 m·cm2. The forming mechanism of the ohmic contact behavior between WO3 thin films and p-Si was explained from the aspect of energy band. Optical-loss analysis based on the ZnS/p-Si heterojunction solar cell was carried out for the first time. The results reveal that shading loss, NIR parasitic absorption, and base collection loss occupy the main optical loss pathways. Based on the Optical-loss analysis, several optimization strategies are proposed. Keywords: dopant free, ZnS thin films, WO3 thin films, hole-selective contact, silicon-based heterojunction, optical-loss analysis 1. Introduction In recent years, dopant-free Si-based heterojunction solar cells have occupied the research frontier in solar cells field. The concept of the dopant-free asymmetric hetero-contacts (DASH) solar cell was first proposed in 2016 by James Bullock et al.[1]. MoO3 thin films and LiFx thin films are used as hole- selective contact layer and electron-selective contact layer for hole and electron extraction, respectively. The efficiency of DASH solar cell reaches 20.6% [2]. Unlike HIT solar cells, which utilize toxic gas- precursors as dopant[3], the DASH solar cells are free of doping and can be fabricated at room temperature, which indicates the characteristic of energy-saving. In addition, transition-metal oxides (TMOs, e.g. V2O5 [4], MoO3 [5], WO3 [6]) possess large bandgaps, which results in lower parasitic optical absorption compared to HIT cells. The work function of TMOs is usually very high [6], which can be used for ohmic contacts with p-Si [7]. The contact resistance between TMOs with p-Si turn out to be sufficiently low, and the reverse saturation current density is not far from boron-diffused state of the art technology (J0~100 fA/cm2) [8]. Furthermore, TMOs can be easily fabricated at room temperature with thermal evaporation method. However, seldom of the DASH solar cells are based on p type silicon substrate, in which low work function materials (LWFs, such as TiO2, MgF2 and Cs2CO3. [9-11]) act as emitter layer while TMOs as back surface field layer. Most of the LWFs should be coated with an Al over layer to obtain 2 the low work function [12-14]. However, Al will lead to high parasitic absorption and greatly decrease the Jsc. It’s an extreme challenge to enable heterojunction to p-Si. Little work on the p-Si based DASH solar cells had been reported. James bullock [1] reported a basic p-type DASH cells without a-Si:H(i) passivating interlayers with efficiency of 11.2%, whose Jsc is just 33.4 mA/cm2. ZnS/p-Si heterojunction solar cells have attracted increasing attention [15-17], because ZnS possess good electrical properties and a low absorption coefficient in the visible range of optical spectrum [18]. Besides, ZnS is nontoxic, stable and easy to be prepared, which makes it suitable as solar cell material [19]. What’s more, ZnS/p-Si heterojunction solar cells do not need the assistance of Al over layer. Y.J.Hsiao[15] reported the AZO/CBD-ZnS/planar p-Si heterojunction solar cells for the first time and investigated the thickness influence of ZnS on the device performance. The device achieved the efficiency of 2.72%. Liang- Wen Ji[16] studied the influence of annealing temperature on the PCE of the AZO/CBD-ZnS/textured p-Si solar cells and PCE of 3.66% was achieved. Previous work [17] of our group reported the AZO/ZnS/textured p-Si heterojunction solar cells fabricated by thermal evaporation for the first time and achieved the published highest PCE of 8.83%. The existing ZnS/p-Si heterojunction solar cells mostly use post-annealed Al-electrodes to form an ohmic contact with p-Si. The post-annealing process at 500℃ for 30 mins in N2 ambient requires relatively high temperatures and is energy consuming. Besides, the high temperature process will introduce lattice damage in Si, which is detrimental to the device performance. In this study, WO3 thin films were prepared by means of thermal evaporation at room temperature to form an ohmic contact with p-Si in ZnS/p-Si heterojunction solar cells. The formation mechanism of the ohmic contact is explained in detail. A optical-loss analysis of ZnS/p-Si heterojunction solar cells was also carried out. Furthermore, we present several ideas for further optimization, which are based on the optical-loss analysis. 2. Experimental procedure P-type (100) oriented CZ silicon wafers (3 ·cm, 180 μm) were subjected to double-facial texturing in alkali solution, then followed by standard RCA processing and hydrofluoric acid (10 vol%) treatment to remove the native oxide. ZnS particles (99.99% purity) were placed in a molybdenum boat for evaporation. Prior to the deposition, the chamber was evacuated to a pressure of 6×10-6 Torr. ZnS thin films with thickness of 13 nm were deposited on the front side of the p-Si wafer via thermal evaporation at room temperature. The ITO layer and Ag electrode were deposited via RF sputtering and thermal evaporation, respectively. WO3 powder (99.99% purity) was placed in a tungsten boat for evaporation. Then, WO3 thin films with thickness of 10 nm were deposited onto the rear side of the p- Si wafer using thermal evaporation at room temperature. The deposition rate was about 0.2 Å/s. X-ray photoelectron spectroscopy (XPS, ESCALab250) was used to study the composition of the ZnS and WO3 thin films. Transmission electron microscopy (TEM, FEI Tecnai G2 F30) was used to study the high-resolution cross-sectional graph of the device. The Ultraviolet Photoelectron Spectroscopy (UPS, VG Scienta R4000) was used to measure the work function and valence band level with the monochromated He І radiation (21.22 eV) under a sample bias of -5 V. The J-V characteristic was investigated using a NewPort system under standard conditions (STC: AM 1.5G, 100 mW/cm2, 25℃), and recorded with a Keithley 2400 digital source meter. External quantum efficiency (EQE) measurements were performed on a QEX10 quantum-efficiency measurement system (PV Measurements, Inc.). The value of ρc for WO3/p-Siwas extracted using the TLM method[20]. The optical properties were obtained using an ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrophotometer (U4100, HITACHI). 3 3. Results and discussion 3.1 Preparation and characterization of the ZnS/p-Si heterojunction solar cells ZnS and WO3 thin films were deposited using thermal evaporation without intentional doping or exposure to a precursor gas. Fig.1 shows the XPS spectrum for the ZnS and WO3 thin films, respectively. The atomic concentrations of ZnS thin films were Zn=55.55% and S=44.45%. They were calculated based on the XPS spectrum. The atomic ratio between Zn and S is 1.25:1, which indicates the presence of Zn interstitials or S vacancies in as-deposited ZnS thin films. The Zn interstitials or S vacancies act as donor-like states and introduce the donor-like level in the forbidden band of ZnS, thus ZnS turns out to be weak n type. The atomic concentrations of WO3 thin films were W=22.4% and O=77.6%. The atom ratio between O and W is 3.46:1, which is similar to the result presented by Luis G. Gerling[4]. Fig. 1 The XPS spectrum of (a) the Zn atom, (b) the S atom, (c) the W atom, (d) the O atom. Fig.2 shows the UPS spectra of ZnS and WO3 thin films. Fig. 2 The secondary electron cut-off spectrum of a) ZnS and, b) WO3; The valence band spectrum of c) ZnS and, d) WO3 4 The work function of ZnS and WO3 can be determined to be 4.40 eV and 5.18 eV, respectively. As will be presented in the next section, the high work function of WO3 plays a crucial role in enabling Ohmic contact between Ag electrode and p-Si substrate. The valence band of ZnS and WO3 are determined to be 3.17 eV and 3.00 eV. The optical bandgap can be obtained from the transmittance spectra of ZnS and WO3, then the energy band of ZnS and WO3 can be obtained (shown in Fig. 1S). A schematic of the device is shown in Fig.3 (a). The Si area is 2 2 cm2. ZnS thin films about × 13nm was deposited on the front side of the p-Si wafer, followed by the ITO layer and the front Ag electrode deposition via sputtering and thermal evaporation, respectively. The thickness of front Ag electrode is 1μm for lower contact resistivity. ITO layer act as TCO and single anti-reflection layer, whose thickness is optimized for maximum anti-reflection. Subsequently, WO3 thin films about 10 nm was deposited on the rear side of the p-Si wafer, which was immediately followed by rear Ag electrode deposition. Hence, there was no need to break the vacuum. All of the depositions were carried out at room temperature. Fig.3 (b) and (c) show the high-resolution TEM cross-section image for ITO/ZnS/p-Si and p- Si/WO3/Ag. The images confirm the amorphous characteristic of ZnS thin films and the diffusion of Ag atoms into WO3 thin films. Since the as-deposited ZnS thin films exhibited amorphous characteristic, there is no need to consider the problem of lattice mismatching between ZnS and p-Si. The SiO2 layer was formed by the natural oxidation of Si after RCA cleaning and before transferring it to the vacuum chamber. The native oxide layer usually has a very loose structure with defects that are detrimental for surface passivation[21]. The thickness of SiO2 was larger than 2 nm, which is hard for tunneling, so there should be pin holes in the loose SiO2 and the carriers transport through the pin holes. Fig.3 (a) Schematic diagram of ZnS/p-Si heterojunction solar cells; (b) and (c) The TEM cross-sections of the ITO/ZnS/p-Si and p-Si/WO3/Ag Fig.4 (a) and (b) show the PCE and quantum efficiency (QE) spectra of the prepared ZnS/p-Si heterojunction solar cell. The cell structure and electrical parameters of conventional c-Si solar cells for comparison can be found in supplementary materials. The J-V characteristic of the device was measured under STC. The device with efficiency of 10.94% exhibits open-circuit voltage (Voc) of 0.525 V, Jsc of 33.75 mA/cm2, and fill factor (FF) of 61.73%. The efficiency is the highest reported to date, to our knowledge for ZnS/p-Si heterojunction solar cells. Rs of 2.14 ·cm2 and Rsh of 10546.6 ·cm2 were deduced from the single diode fitting. The large Rs should be responsible to the low FF. Fig.4 (b) shows the reflectance spectra of the device without metallization and QE spectra for the device as well as for conventional monocrystalline Si solar cells. The mutation of EQE spectra in around 570 nm was due to the instrumental error, which would result in smaller JSC-QE. The average reflectance (R) of the device without metallization in the range of 300-1100 nm is 5.3%, which is much higher than the R of the conventional mono-Si solar cells (~2.5%) and shows the insufficient anti-reflection effect of ITO single layer. The EQE for the long wavelength range is much smaller compared with the conventional mono-Si solar cells, which may be due to the 5 relatively poor internal back reflectance (IBR) of the fabricated device. Besides, insufficient back surface field (BSF) passivation also results in the severe recombination in back surface, thus the poor spectra response in long wavelength range. The poor EQE for the short wavelength range should be stem from parasitic absorption by the ITO layer, which can be seen from the difference between IQE and EQE spectra. The as- deposited ITO layer exhibited the carrier concentration (ne) of 1.13E21 cm-3 and mobility of 6.1 cm2/(V·s), with sheet resistance of 60.5 /□. Free carrier absorption (FCA) is high if ne exceed above 1E20 cm-3[22]. Furthermore, the absence of an a-Si:H (i) passivation layer is likely responsible for the lower Voc and FF since the a-Si:H (i) passivation layer is critical for the performance of heterojunction solar cells[23]. A detailed power-loss analysis of the device is presented in later section. Fig.4 (a) The illuminated J-V characteristic. (b) The reflectance and QE spectra of the device and regular monocrystalline Si solar cells 3.2 Contact characteristic of WO3/p-Si Fig.5 (a) shows the ohmic contact formation mechanism of the WO3 thin films and p-Si with respect to the energy band. The Ohmic I-V behavior is resulted from the holes accumulated layer at the Si surface, which is induced by WO3 thin films due to the high work function. The energy band of p-Si would bend up to balance the Fermi energy when p-Si contacts with the WO3 thin films. Thus, holes accumulate in the hollow as shown in Fig.5 (a). When the concentration of holes is sufficiently high, an accumulation layer forms for the holes, and the probability for tunneling increases. In other words, holes can tunnel through the SiO2 layer and come across with electrons in WO3 thin films, then recombine with each other. In this way, the hole current transforms into an electron current. In addition, the accumulated layer would act as the p+-Si layer and passivate the surface of p-Si via field-passivation; therefore, it can enhance the Voc. The higher work function of WO3 thin films is, the better field-passivation effect would be. Fig.5 (b) shows the of the WO3 𝜌𝑐 thin films and p-Si obtained via the TLM method. The inserted graphs on the upper left and bottom right are the schematic diagram of the structure for TLM test and fitting result, respectively. It can be seen that the contact exhibited Ohmic behavior allowing accurate extractions of . of 12.7 𝜌𝑐𝜌𝑐 mΩ·cm2 was deduced from the fitting result, which indicates the excellent contact between the WO3 thin films and p-Si. 6 Fig.5 The ohmic contact formation mechanism, and of the WO3 thin films and p-Si𝜌𝑐 3.3 Optical-loss analysis of ZnS/p-Si heterojunction solar cells The EQE spectrum of the solar cells, the transmittance (Tcell) and the reflectance spectra (Rcell) of the solar cells without metalization were measured, - shown in Fig.6 (a). Here, ROPAL represents the reflectance spectra of the device without metalization,