Amorphous hydrogenated silicon microstructure analysis and effect of plasmons on solar cell performance

Abstract

Hydrogenated amorphous silicon (a?Si:H), a well?known material for photovoltaics, thin?film transistors, MEMS, etc., is most extensively deposited using the PECVD technique, which is well studied and is primarily used in the industrial manufacturing of a?Si:H–based devices. The microstructure and its related material properties (structure–property–process relationship) are well studied and established for PECVD. At present, sputtering of a?Si:H is confined to R&D laboratories. Therefore, any initiative toward industrializing sputtering for a?Si:H deposition is highly valuable. Sputtering can be performed in three different target?power modes: DC, pulsed DC (PDC), and RF. It is therefore essential to understand which of these modes yields the best?quality a?Si:H film. There is much to uncover regarding the structure–property–process relationships in these three power modes. A good understanding of these correlations is necessary for improving device performance and paving the way for industrial adoption of sputtering for a?Si:H deposition. We initiated the study of structure–process–property relationships by analyzing and investigating the microstructure of a?Si:H deposited by DC, PDC, and RF sputtering using Fourier Transform Infrared (FTIR) spectroscopy. The microstructure was analyzed in terms of: (i) microstructure factor, R* (ii) hydrogen concentration, C_H (iii) configuration of hydrogen bonding, Si–H? (iv) vacancy and void incorporation (v) density, ?_{a?Si:H}. The effect of substrate temperature (T_s) on these microstructural properties was also studied. The lowest value of R* achieved at 250°C is 0.029 in DC?deposited films. At 500°C, thin films with R* = 0 were obtained. Hydrogen incorporation was found to be highest for RF?deposited films, followed by PDC and DC films. The variation in microstructure originates primarily from the different target?power modes. Since DC, PDC, and RF modes have significantly different ion and electron energy distributions, a detailed plasma analysis was carried out using a Langmuir probe. The resulting plasma parameters-ion density (N_i), electron density (N_e), electron temperature (T_e), plasma potential (V_p), and floating potential (V_f)-were correlated with the microstructure of a?Si:H films. N_i and T_e were highest in RF plasma, followed by PDC and then DC plasma. Electrons with two characteristic energy groups were observed in all plasmas. The formation of the a?Si:H network also differed across the three sputtering modes. The Continuous Random Network (CRN) model, which describes amorphous networks, was used to analyze the structure. DC?deposited films follow the CRN model up to C_H = 8 at.% H, whereas PDC and RF films deviate from the model even at low hydrogen concentrations. Differences in microstructure are directly reflected in mechanical properties such as hardness and internal stress. Since device fabrication requires acceptable stress levels, understanding stress behavior in films deposited with each power mode is important. The variation of hardness and stress with C_H is presented and correlated with microstructure. With increasing C_H, the stress transitions from compressive to tensile at approximately 13, 18, and 23 at.% H for DC, PDC, and RF films, respectively. DC?deposited films exhibited the highest hardness, followed by PDC and RF films. For all power modes, hardness was maximum at ~10 at.% H. Optical and photoconductivity characteristics were studied, and films deposited under optimum conditions were chosen for device fabrication. It was found that E_04 (optical band?edge parameter) follows the order: E04RF>E04PDC>E04DCE_{04}^{\mathrm{RF}} > E_{04}^{\mathrm{PDC}} > E_{04}^{\mathrm{DC}} E04RF?>E04PDC?>E04DC? for equal C_H values. We attempted solar?cell fabrication using the p?i?n structure. High?quality absorber layers were deposited using DC sputtering under optimum conditions identified earlier. Finally, we studied the effect of incorporating Ag nanoparticles in a?Si:H on solar?cell performance. It is well known that when sunlight interacts with metal nanoparticles, surface plasmons are excited, which have applications in spectroscopy, photovoltaics, and related fields. We incorporated Ag nanoparticles into the absorber layer to improve spectral response in weakly absorbing regions. The performance was evaluated using current–voltage characteristics and external quantum efficiency (EQE). Solar cells with and without Ag nanoparticles were compared, and the impact on device performance was analyzed in detail.