(Invited) Reliability Improvement and Remaining Issues on Photovoltaic Cells and Modules

Abstract

Worldwide rapid growth of photovoltaics requires not only high-efficiency but also high-reliability and long-lifetime photovoltaic cells and modules. Reliability of photovoltaic modules is determined by module materials such as encapsulant, backsheet, etc. and, of course, also by photovoltaic cells themselves. There are some complicated phenomena concerning degradation of photovoltaic properties. Those physical and chemical degradation phenomena should be microscopically analyzed in detail in order to clarify degradation mechanism and also realize highly reliable photovoltaic cells and modules. Although such degradation phenomena are complicated and often related each other, degradation mechanism is classified into only three categories; 1) less incident light into solar cells, 2) less collection of photogenerated carriers, and 3) less photovoltaic ability itself. Less incident light often originates from soiling of cover glass; however, in some cases originates from browning of encapsulant or reduction of transparent conductive oxide electrode. Less correction of carriers originates from damage of electrodes on the cell or damage of interconnector ribbons between the cells. Less photovoltaic ability originates from surface or bulk recombination or shunting. These three categories are schematically shown in the attached figure. There are various degradation factors, for example, thermal or mechanical stress, high temperature, high humidity, UV irradiation, and potential difference. In particular, hygrothermal stress, UV irradiation, potential difference, and also their combination are considerable factors. For example, UV irradiation accelerates degradation by hygrothermal stress [1], on the other hand, delays degradation by potential difference [2].Both development of indoor acceleration test method and estimation of acceleration factor are also important for predicting the lifetime of photovoltaic modules. As mentioned above, although UV light irradiation much influences degradation phenomena, test method with UV light irradiation is not popular. The reason is that it is quite difficult to keep the module temperature under irradiation during test duration and also to realize uniform irradiation for large-size photovoltaic modules over 1.5 m. However, using acetic acid concentration in the encapsulant as the mediator between outdoor exposure and indoor acceleration test, it was elucidated that damp heat test at 85°C and 85% relative humidity for 4000 h corresponds to outdoor exposure at Japan for 30 years [3].Based on degradation mechanism clarified, highly reliable photovoltaic cells and modules should be developed. It is clarified that the most important key material is electrode of solar cells. Even when photocarriers are generated, if damage or degradation occurs in the electrodes, such carriers are not collected and photovoltaic performance is lowered. Problems in photocarrier collection occupy majority of origins for degradation observed outdoors. Degradation by potential difference has also attracted much attention in the last 10 years especially in mega-watt scale photovoltaic plant with high system voltage. Encapsulant materials and anti-reflection coating of solar cells are key materials for suppressing such kinds of degradation.There are some remaining issues on photovoltaic cells and modules for improving reliability and increasing lifetime. First, interaction between module materials and cells should be completely clarified. Photovoltaic modules are composed of various materials including polymers, metals, ceramics, and, of course, semiconductors. Complicated reactions should occur both at the interface and in the bulk of those materials by many stressors such as high temperature, high humidity, thermal cycle, UV irradiation, and potential difference. Many observations and knowledges have been obtained so far, however, we do not reach the complete understanding, and essential point is that materials with less change and less interaction with other materials should be employed. Second, acceleration test results should be always analyzed considering outdoor exposure results. Especially, climate conditions such as desert, tropical zone, etc. for outdoor exposure also should be employed for acceleration test conditions. Photovoltaic cells and modules appropriate to specific climate conditions should be developed. Third, high efficiency photovoltaic cells and modules should be developed always considering high reliability and long lifetime. Those are inseparable issues for development of photovoltaic cells and modules. Especially, reliability issues on high-efficiency perovskite cells and modules should be intensively studied and urgently solved for realizing high-efficiency and high-stable perovskite/silicon tandem solar cells and modules.The author is grateful to Professors Keisuke Ohdaira, Sachiko Jonai, Yasuaki Ishikawa, Yasushi Sobajima, Fumitaka Ohashi, Kentaro Iwami, Dr. Seira Yamaguchi, Mr. Yasushi Tachibana, and Ms. Yukiko Hara for their collaboration and fruitful discussion. This study was in part supported by the New Energy and Industrial Technology Development Organization.