Investigation of textured c-Si wafers for application in silicon heterojunction solar cells

The hydrogenated amorphous/crystalline silicon (a-Si:H/c-Si) heterojunction (SHJ) solar cell is considered a great competitor of the most commercialized homojunction c-Si solar cells while it combines the high performance and the low cost potential of c-Si and a-Si technologies, respectively. Furthermore, the implementation of thin intrinsic (i) a-Si:H film between the c-Si wafer and the p/n doped a-Si:H layers has increased even more the high efficiency potential of the SHJ solar cell due to the excellent surface passivation of c-Si. By applying a randomly textured wafer instead of a double-side polished, optical enhancement is achieved resulting in significant reflection reduction and high short-circuit current densities (JSC). However, texturing-induced defects lead to an a-Si:H/c-Si interface with higher recombination which limits the open circuit voltage (VOC) of the SHJ device after using the same cleaning treatment as for the flat wafer. Thus, a one-to-one transfer of process parameters from flat to textured c-Si substrate is not necessarily appropriate and a different wet-chemical treatment needs to be developed. Random pyramidal texturing is commonly achieved by chemical anisotropic etching in potassium hydroxide (KOH) or sodium hydroxide (NaOH) and isopropyl alcohol (IPA). Since an a-Si:H/c-Si interface of high quality is of great importance, etching solution based on tetramethyl ammonium hydroxide (TMAH) of low concentration due to its high cost, is used as an alternative for avoiding contamination from K+ and Na+ metal ions. Afterwards, investigation for the development of an appropriate wet-chemical treatment of the textured substrate prior to a-Si:H layer deposition is realized. This thesis reports a texturing recipe consisting of low concentration TMAH and IPA which provides a pyramidal morphology with an average reflectance (R350nm-1100nm) of 11.8% at the wavelength range of 350 nm-1100 nm. The reproducibility and homogeneity of the textured surface are confirmed by using scanning electron microscopy (SEM) along with reflectance measurements. Subsequently, a wet-chemical treatment of the pyramidal surface involving multiple oxidation steps in 69% nitric acid (HNO3) solution at 110oC is developed and leads to high a-Si:H/c-Si interface lifetime measurements. Finally, the textured substrate and the newly developed treatment prior to the a-Si:H layer growth result in the fabrication of textured SHJ devices with efficiencies above 18% due to the significant improvement of the VOC, highlighting the efficiency of 19.5% mainly thanks to the considerable achievement of a VOC of 680 mV which is similar to the VOC measured on SHJ solar cells with flat substrate.Photovoltaic materials and devicesElectrical Engineering, Mathematics and Computer Scienc