Session A03: Optics and Biophysics

Binary hole transport materials blending to linearly tune HOMO for high efficiency and stable perovskite solar cells

To maximize the photovoltaic performance of perovskite solar cells (PVSCs）by developing new hole-transport layer (HTL) materials, the precise tuning of their energy levels especially the highest occupied molecular orbital (HOMO) is highly desirable. Here, a simple binary strategy for the first time is proposed to acquire ideal HOMO level by optimizing the composition of binary blend HTLs including CZ-TA (HOMO = -5.170 eV) and CZ-STA (HOMO = -5.333 eV). By adding 10 wt% CZ-STA, the binary HTM (HOMO = -5.199 eV) based perovskite solar cells achieve a maximum power conversion efficiency of 19.85% (18.32% for CZ-TA). The introducing of S atom in CZ-STA not only downshifts HOMO level but also forms stronger Pb-S interaction with perovskites than Pb-O in CZ-TA, leading to better device performance and reduced hysteresis. Importantly, the un-encapsulated PVSCs using CZ-TA:CZ-STA (90:10, w/w) binary HTL exhibit good environment stability in ambient air, maintaining over 82% of their initial efficiency after 60 days’ storage with a relative humidity around 50%. Therefore, this strategy provides new insights on HTL development to push forward the progress of the emerging PVSCs   

Optical Simulation of Si Wafer Solar Cell Reflectance and Benefit for Temperature Control

Si wafer solar cell makes a ~90% of the market share with the fabrication process impacting the infrared optical response of the complete devices. The major optical losses in a silicon wafer solar cell are the parasitic absorption of sunlight within the solar cell at the rear metal back contact and reflection at long wavelength region which may heat the devices and reduces the operating efficiency. The temperature of a solar module can be reduced by reducing/rejecting unwanted heat and parasitic absorption. To reduce heating, it may be desirable to make devices more reflective in longer wavelength (infrared) and with less parasitic absorption in the near-infrared and visible spectral range. We will discuss optical simulation of aluminum back surface field (Al-BSF) and passivated emitter rear contact (PERC) solar cells. We use commercial Ray Tracing software to simulate the total reflectance of devices. This approach, focuses on simulating reflectance, is serve as input for the model used in reducing operating temperature for solar cells.   

Applications of Mapping Spectroscopic Ellipsometry in Photovoltaics: Correlations between Window Layer Thickness and Device Performance in CdSe/CdTe and CdS/CdSe/CdTe Solar Cells Prepared by Magnetron Sputtering

CdTe photovoltaic technology has traditionally used CdS as an n-type window layer in a CdS/CdTe heterojunction. The lattice mismatch between CdS and CdTe, the lack of collection from CdS, and its 2.4 eV bandgap, however, limit the short circuit current (J_SC) and open circuit voltage (V_OC) of the solar cell.  Recently, CdSe has been recognized as a promising component layer in CdTe solar cells. J_SC for CdTe devices incorporating CdSe shows an enhancement due to extended collection over both long and short wavelength ranges. For CdSe/CdTe and CdS/CdSe/CdTe solar cells, analysis of the effect of the CdSe and CdS/CdSe layer thicknesses on the CdTe solar cell parameters is critical for the purpose of optimizing the cell performance. Sets of CdTe devices were prepared by magnetron sputtering on TEC^TM15/HRT type glass with different CdS and CdSe layer thicknesses. Spectroscopic ellipsometry has been applied to generate maps for each single layer and also for the complete PV stack at different stages of processing. In this work, non-uniformity of the deposition processes leads to mapped distributions of thicknesses about the nominal values and enables accurate optimization of device performance through correlations of device parameters with the CdSe and CdS/CdSe bilayer thicknesses.