The improved photoresponse of the substrate-free InGaN solar cells with a bottom reflector

In this study, the laser lift-off (LLO) technique is used to fabricate the substrate-free InGaN thin-film solar cells (TF-SCs). The epitaxial structures consist of 8-pair In_0.23Ga_0.77N/GaN multiple-quantum-well (MQW), which is sandwiched between the p- and n-GaN to construct the conventional p-i (intrinsic layer)-n solar cells. After the LLO process to remove the sapphire substrate, a silver layer with reflectivity of 97% was deposited onto the bottom of cell to act as the metallic reflector. Under forward bias operation, the fabricated TF-SCs exhibit a low operating voltage of 3.34 V along with an ideality factor of 6 V. Both of which are comparable to their counterparts that reveals the electrical property of device does not damage after the LLO process. On the other hand, it is observed that the spectral response of the InGaN solar cells changes significantly as the incident wavelength below 510 nm. This wavelength corresponds to the energy transition (~ 2.43 eV) between the first electron and hole energy level of the In_0.23Ga_0.77N/GaN well. Since part of the unabsorbed photons can be re-reflected by the bottom reflector, the TF-SCs show the enhanced photoresponse as compared to the normal solar cells with the sapphire substrate. Finally, the conversion efficiency of the TF-SC is 1.6 times larger than the normal InGaN solar cell under one sun AM 1.5G spectrum.