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Tensile Stress-Dependant Fracture Behavior and Its Influences on Photovoltaic Characteristics in Flexible PbS/CdS Thin Film Solar Cells Seung Min Lee, Deuk Ho Yeon, Bhaskar Chandra Mohanty, and Yong Soo Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/am507301c • Publication Date (Web): 09 Feb 2015 Downloaded from http://pubs.acs.org on February 15, 2015
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ACS Applied Materials & Interfaces
Tensile Stress-Dependant Fracture Behavior and Its Influences on Photovoltaic Characteristics in Flexible PbS/CdS Thin Film Solar Cells Seung Min Lee1†, Deuk Ho Yeon1†, Bhaskar Chandra Mohanty2 and Yong Soo Cho1* 1
Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea 2
School of Physics & Materials Science, Thapar University, Patiala, 147004, India.
ABSTRACT. Tensile stress-dependant fracture behavior of flexible PbS/CdS heterojunction thin film solar cells on indium tin oxide-coated polyethylene terephthalate (PET) substrates is investigated in terms of the variations of fracture parameters with applied strains and their influences on photovoltaic properties. The PbS absorber layer that exhibits only mechanical cracks within the applied strain range of ~0.67 to 1.33% is prepared by chemical bath deposition at different temperatures of 50, 70 and 90 oC. The PbS thin films prepared at 50 oC demonstrate better mechanical resistance against the applied bending strain with the highest crack initiating bending strain of ~1.14% and the lowest saturated crack density of 0.036 µm-1. Photovoltaic properties of the cells depend on the deposition temperature and the level of applied tensile stress. The values of short circuit current density and fill factor are dramatically reduced above a certain level of applied strain while open circuit voltage is nearly maintained. The dependency of photovoltaic properties on the progress of fractures is understood as related to the reduced fracture energy and toughness, which is limitedly controllable by microstructural features of the absorber layer.
KEYWORDS. lead sulfide, chemical bath deposition, flexible solar cell, fracture behavior
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Introduction Photovoltaic cells on flexible polymer substrates have been investigated over the last decade because of their great potential for extended applications including as a mobile energy source. Particularly, the choice of absorber layer for the flexible substrate is very limited since the absorber layer needs to be processed below the softening point of the polymeric substrates. Several organic absorber materials, such as poly(3-hexylthiophene) (P3HT) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), may be suitable for the flexible applications, but the practical utilization is very limited due to their compatibility issue with polymer substrates and their poor intrinsic long-term stability.1,2 On the other hand, typical inorganic absorbers for thin film solar cells, including Cu(In,Ga)Se2 and CdTe, have been actively studied for the flexible applications.3-5 The selection of substrates for the inorganic absorber is critical since it determines maximum processing temperature for the synthesis and densification of the absorber materials. Only highly thermal resistive substrates including metal foils have been considered as an ideal choice for the inorganic absorbers demanding at least 500 oC for densification. To cover the broader range of applications potentially favoring lower softening temperature-polymer substrates including polyethylene terephthalate (PET), polyethersulfone (PES) and polyethylene naphthalate (PEN), however, nonconventional absorber-driven cell structures that are processable below 100 oC are preferred. Lead sulfide (PbS) has drawn considerable interest as one of the rare p-type absorbers for thin film solar cells due to its high absorption coefficient of ~105 cm-1 in the visible light, and adjustable band gap toward a higher value (>1.3 eV) suitable for better band alignment in typical cell structures.6-8 There has been no report concerning the deposition of PbS thin films on any flexible polymer substrate even though polycrystalline PbS can be easily formed at temperature below 100 oC by the known chemical bath deposition (CBD) process. There are
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several reports on the PbS/CdS heterojunction thin film photovoltaic cells fabricated by CBD on a rigid glass substrate.9-11 In the meantime, there have been extensive studies on mechanical or fracture behavior of thin films deposited onto polymer substrates for a variety of flexible electronic devices.12-14 Cracks or macro-defects developed on the surface of thin films as a result of externallyapplied stress are regarded as the reason for substantial degradation of the devices' functional performance.15 In the field of flexible thin film solar cells, surprisingly, there have been no systematic studies about the fracture behavior under tensile stress even though there is a rare report proposing that the photovoltaic performance of Cu(In,Ga)Se2 cells can be critically affected by flexibility stress and mechanical cycling.5 In this work, PbS/CdS thin film solar cells fabricated at
