Light Trapping In Thin Film Silicon Solar Cells By Micro/nano-optic Structures

Thin film solar cells?TFSCs?have great potential in reducing the cost of photovoltaic module and simplifying the production technics.In addition,a thining active-material can ehance open circuit voltage and fill factor,so as to improve the electric performances of cell device effectively.The limitation factor for TFSCs from large-scale application is the insufficient absorption of active materials,which resulted in a low photo-generated current and an shame efficiency.Optical structure with optimized design can regulate the propagation of light at nano-or micro-scale.With such a regulation,optical path and dewell time of light in cell device are enhanced,thereby enhancing the spectral respose of solar cell and the corresponding efficiency.In this paper,we focus on the improvement of spectral respose of thin film silicon solar cell by employing different nano/micro optical strutures.The main contents and conclusions of the paper are as follows:Firstly,the mechanisms of action for sub-wavelengths nano-bionic tructure and Ag nano particles?Ag_NPs?on incident light were studied by employing rigorous coupled-wave analysis?RCWA?and finite-difference time-domain?FDTD?method,and the corresponding solar cells with light trapping structures were optimized.The results are as follows:Moth eye structures?MES?,which based on nano-biomimetics,can efficiently breakthrough the optical difference between air and Si materials,leading photons from outside into Si active materials,thereby enhancing the absorption and efficiency of solar cells.The propagation characteristics of incident light depend on the cross section shape,height,and period of the MES.With an optimized MES?K=0.3,P=600nm,H=700 nm?,the efficiency of c-Si with 2?m thickness absorbing layer efficiency reached to 14.6%.As a comparison,the efficiency for flat and ARC structures were8.1%and 11.3%in the case of equivalent thickness,respectively.When the thickness increases to 5?m,cell efficiency of MES reached to 17.5%.In addition,moth eye structures have superior angle response compared to the solar cells with flat structures;The spectral response of a-Si:H solar cell with Ag nano particles?Ag_NPs?can significantly enhance due to the effect of surface plasmon resonance.The surrounding materials and feature sizes are the key factors for plasmon resonance.The result from simulation found that the resonant frequency of Ag_NPs appears red-shift with the refractive index of surrounding materials.For the air surrounding,formant appears at360 nm,while for the glass,ITO,and a-Si surroundings,formant transition to 400,540,730 nm,respectively.Resonance turns from dipole to high-order mode with the feature size Ag_NPs.Based on these phenomenons,two surrounding materials,a-Si and ITO,were considered.For the case of Ag NPs placed in a-Si,optimized parameters are pitch P=200 nm,height H=135 nm,diameter D=70 nm.For the case of Ag NPs placed in ITO,optimized parameters are P=150 nm,D=20 nm.The photocurrents are increased by 28.9%and 17.6%for the two optimized structures compared to flat structures.Secondly,an random texture interface model was constructed aimed at the interfaces optical losses of a-Si:H and a-Si:H/?c-Si:H,and the regulative role of intermediate layers to interfaces optical properties were studied by using optical simulation.TCO/a-Si and a-Si/?c-Si are two important interfaces in TFSCs,the former affect the total absorption while the latter determine the photocurrent distribution between the two sub-cells.FDTD simulation results show that TiO₂optical insert layer enhanced light transmission of TCO/a-Si interface,thereby enhancing total absorption of TFSCs.While the SiOx intermediate reflect layer can regulate the 600-900 nm light distribution at a-Si/?c-Si interface.With an optimized interface structures?TCO/TiO₂/a-Si,TiO₂=40 nm;a-Si/SiOx/?c-Si,SiOx=50 nm;crater-shape?,photocurrent for a-Si:H and a-Si:H/?c-Si:H are enhanced by 6.0%and7.9%,respectively,as well as the current for the subcells of a-Si:H/uc-Si:H reached to a delicated matching.Finally,hemispherical pit arrays?HPAs?with high light trapping were fabricated on glass substrates by using the photolithography and wet etching technique.The optical losses of reflection and escape are suppressed via double bounces and total reflection due to HPAs structures.Moreover,the interaction between HPAs structure and wave is independent of the wavelengths and angle of incident light,thereby a broadband and large angle spectral response could be realized.Measurements show that the optical losses from the surfaces of a-Si:H and a-Si:H/?c-Si:H with 10?m HPAs are decreased from 18%and 10%to 2.5%and 1.5%,respectively,as well as an apparent improvement in the corresponding quantum efficiency.The efficiency of the two type solar cells are enhanced by 4.7%and 5.5%,respectively,as confirmed by the I-V curves.In addition,the HPAs structure can be replicated on the other substrates precisely,and exhibits hydrophobic property,which make it great potential for industrial application.