Single and multi-junction thin film silicon solar cells for flexible photovoltaics Thèse présentée à la Faculté des Sciences Institut de Microtechnique Université de Neuchâtel Pour l’obtention du grade de docteur ès sciences Par Thomas Söderström Acceptée sur proposition du jury : Prof. C. Ballif, directeur de thèse Dr. F.-J. Haug, rapporteur Dr. V. Terrazzoni-Daudrix, rapporteur Dr. W. Soppe, rapporteur Dr. D. Fischer, rapporteur Prof. H. Stoekli-Evans, rapporteur Soutenue le 20 mars 2009 Université de Neuchâtel 2009 iii Mots clés Cellules solaires en couches minces, silicium amorphe, silicium microcristallin, cellule micromorph tandem, piégeage de la lumière, cracks, substrats flexibles Keywords Solar cells, thin film silicon, amorphous, microcrystalline, micromorph tandem cells, light trapping, cracks, flexible substrates Abstract This thesis investigates amorphous (a-Si:H) and microcrystalline ( µc-Si:H) solar cells deposited by very high frequency plasma enhanced chemical vapor deposition (VHF- PECVD) in the substrate (n-i-p) configuration. It focuses on processes that allow the use of non transparent and flexible substrates such as plastic foil with T g < 180°C like poly- ethylene-naphtalate (PEN). In the first part of the work, we concentrate on the light trapping properties of a variety of device configurations. One original test structure consists of n-i-p solar cells deposited directly on glass covered with low pressure chemical vapor deposition (LP-CVD) ZnO. For this device, silver is deposited below the LP-CVD ZnO or white paint is applied at the back of the glass as back reflector. This avoids the parasitic plasmonic absorptions in the back reflectors, which are observed for conventional rough metallic back contacts. Furthermore, the size and morphology of the LP-CVD ZnO are varied. The relation between the substrate morphology and the short circuit current density (J sc ) is experimentally explored. As a result, the J sc can be increased by 23% for a-Si:H and 28% for µc-Si:H solar cells compared to the case of flat substrate and the role of the size and shape can be clearly separated. We also explore the optical behavior of single and multi- junction devices prepared with different back and front contacts. The back contact consists either of a 2D periodic grid with moderate slope, or of LP-CVD ZnO with random pyramids of various sizes. The front contacts are either a 70 nm thick, nominally flat ITO or a rough 2 µm thick LP-CVD ZnO. We observe that, for a-Si:H, the cell performance is critically dependent on the combination of thin flat or thick rough front TCOs and the back contact. Indeed, for a-Si:H, a thick LP-CVD ZnO front contact provides more light trapping on the 2D periodic substrate. The J sc relatively increases by 7 % with LP-CVD ZnO compared to ITO. Then, we study the influence of the thick and thin TCOs in conjunction with thick absorbers like triple junction or µc-Si:H solar cells. Because of the different nature of the optical systems, thick (> 1 µm) against thin (<0.3 µm) absorber layer, the antireflection effect of ITO becomes more effective and the structure with the flat TCO provides as much light trapping as the rough LP-CVD ZnO. Finally, the conformality of the layers is investigated and guidelines are given to understand the effectiveness of the light trapping in devices deposited on periodic gratings. iii In the second part, we quantitatively describe the effect of continually varying the substrate morphology on open-circuit voltage (V oc ) and fill factor (FF) for the device in the n-i-p configuration. Transmission electron microscope (TEM) observations show that V shape morphology creates nano-cracks and reduces the V oc and FF of the solar cells. Hence, we investigate cell designs and processes that avoid V oc and FF losses. For a-Si:H solar cells, we introduce an amorphous silicon carbide n-layer (n-SiC), a buffer layer at the n/i interface, and show that the new cell design yields high V oc and FF on both flat and textured substrates, contrary to the usual microcrystalline silicon n-doped layer. Finally, the beneficial effect of our optical and electrical findings is used to fabricate a- Si:H solar cell with an initial efficiency of 8.8 % and stabilized efficiency of 7% on plastic foil. We find that for our reduced temperature processes windows, the light- induced degradation of a-Si:H solar cells depends strongly on the thickness of the absorber layer. Indeed, the relative efficiency degradation is reduced from 27% to 17% for 400 nm and 200 thick cells, respectively. This degradation can be further lowered to 15 % in a-Si/a-Si tandem structure, and still using a total 300 nm thick absorber layer. For µc-Si:H solar cells, we introduce a buffer layer with a higher amorphous fraction between the n-doped and intrinsic layer. Our study reveals that the buffer layer limits the formation of voids and porous areas (nano-cracks), which promotes oxygen diffusion in the µc-Si:H material. Therefore, this layer mitigates the V oc and FF losses which enhances the performance of the µc-Si:H solar cell. By applying our findings, we make µc-Si:H solar cells with an efficiency of 8.7% on plastic foil for an only 1.2 µm absorber layer thickness. The micromorph solar cell (stack of amorphous and microcrystalline cells) concept is the key for achieving high efficiency stabilized thin film silicon solar cells. We present results with and without an intermediate reflector. In particular, we introduce an original device structure that allows a better control of the layer growth and of the light in- coupling into the two sub-cell components. It is based on an asymmetric intermediate reflector (AIR), which increases the effective thickness of the a-Si:H by a factor of more than three. Hence, the a-Si:H thickness reduction diminishes the light-induced degradation, and micromorph tandem cells with 11.2 % initial and 9.8% stabilized 2 efficiencies (1000h, 50°C, 100mW/cm ) are achieved on plastic foil. The stabilized J sc of the n-i-p tandem solar cells is close to 12 mA/cm 2, which offers the possibility for the low Tg flexible substrate technology to compete with state of the art stabilized thin film silicon devices. Based on the results obtained here, the use of ITO front contact, a further optimisation of the ITO/p and p-i interfaces, should allow it to be possible to exceed 12.5 % stabilized efficiency on low Tg plastic substrate for micromorph tandems cells. iv Abbreviations AM1.5g Air mass 1.5 global (standard solar spectrum on the earth) a-Si:H Amorphous silicon AIR Asymmetric intermediate reflector Ag Silver Al Aluminum AFM Atomic force microscopy BL Buffer layer c-Si Crystalline silicon CH 4 Methane CR Raman crystallinity Dilution H2/ SiH 4 EQE External quantum efficiency Eg Energy band gap η Efficiency FTPS Fourier transform photocurrent spectroscopy FF Fill factor FIB Focused ion beam Ge Germanium H2 Hydrogen ITO Indium tin oxide IR Intermediate reflector JV J(V) or current voltage curve Jsc Short circuit current density k Extinction coefficient LP-CVD Low pressure chemical vapor deposition LID Light induced degradation λn Light wavelength in media with refractive index n µc-Si:H Microcrystalline silicon n Refractive index n-µc Microcrystalline n-layer n-SiC Amorphous silicon carbide n-layer RF Radio frequency PECVD Plasma enhanced chemical vapor deposition PH 3 Phosphine SiH 4 Silane SIR Symmetric intermediate reflector SOIR Silicon oxide intermediate reflector SIMS Secondary ion mass spectroscopy SC Silane concentration: SiH 4/ (H 2+SiH 4) SEM Scanning electron microscope TCO Transparent conductive oxyde v TZIR Textured zinc oxide intermediate reflector Tg Glass transition temperature TMB Trimethylboron TEM Transmission electron microscope Voc open circuit voltage VHF Very high frequency VIM Variable illumination measurement ZnO Zinc oxide ZIR Zinc oxide intermediate reflector vi TABLE OF CONTENTS Abstract .....................................................................................................iii Abbreviations ............................................................................................. v Chapter 1: Introduction ........................................................................ 1 1.1 Introduction ....................................................................................................... 1 1.1.1 Energy Issues .......................................................................................................................... 1 1.1.2 Photovoltaics energy (PV) ...................................................................................................... 2 1.1.3 Thin film silicon technology................................................................................................... 4 1.2 Thin Film Materials .......................................................................................... 6 1.2.1 Amorphous silicon (a-Si:H).................................................................................................... 6 1.2.2 Microcrystalline silicon ( µc-Si:H) .......................................................................................... 8 1.3 Solar cells ......................................................................................................... 10 1.3.1 Solar cells in the substrate configuration (n-i-p solar cells).................................................. 10 1.3.2 Light trapping.......................................................................................................................