Recent portable electronic devices have strict requirements regarding size, weight, and power. The power needed directly impacts the size of the required batteries, and larger batteries compromise the sizes and weights of portable devices. In order to design successful portable devices that maximize power and minimize size, devices should be operated in lowpower-consuming or self-powered mode. There has been increasing demand for highly effi cient portable energy harvesters due to the development and mass consumption of portable electronic devices. Photovoltaic, thermoelectric, and piezoelectric energy scavengers have become strong candidates for portable energy harvesters in future self-powered portable electronic device applications. [1][2][3][4][5][6] Among these, piezoelectric energy harvesters that can convert mechanical energy into electrical energy have recently attracted attention. Compared to time-constrained photovoltaics, piezoelectric energy harvesters are not limited in time and space because mechanical energy sources such as body movement, heartbeat, blood fl ow, wind, tide, low-frequency seismic vibrations, and sound from the surrounding environment are prevalent. [7][8][9][10][11][12][13][14] Commercially available polymer materials such as polyethylene terephthalate (PET), polyethersulfone (PES), and polyethylene naphthalate (PEN) have emerged as promising substrate materials for a wide range of fl exible electronics applications. Despite the enormous progress that has recently been achieved in the optimization of physical and mechanical properties of polymer substrates, large difference in coefficients of thermal expansion (CTE) between inorganic fi lms and polymer substrates cause drastic increases in thermally induced stresses on polymer substrates with increasing temperature. Recently, various devices such as thin-fi lm transistors, lithium-ion batteries, active-matrix organic light-emitting displays, and sensors have been fabricated on cellulose paper substrates. [15][16][17][18][19][20] Cellulose is promising as it is thermally stable