UC San Diego

Intermediate band solar cells (IBSC) have been proposed as a promising candidate to exceed the Shockley-Queisser limit of photovoltaic cells and they have caught a lot of attention in the research community. Besides the quantum dot system, highly mismatch alloys (HMAs) such as dilute nitrides, which undergo conduction band splitting during alloying, have been proven as ideal platforms to achieve IBSCs. Dilute nitride GaNAs has been made into IBSC device, but the band spacing in GaNAs cannot meet the limiting efficiency requisite for a 3-band IBSC (63 %). By adding P, GaNAsP then falls into the IBSC ultimate efficiency development road-map. This thesis is devoted to the study on the growth of high-quality GaNAsP and the fabrication and measurement of GaNAsP based IBSCs. The thesis is divided into two major parts. In the first part, the rationale and the theoretical framework of the GaNAsP material system is laid out. The material compositions that lead to high efficiency is simulated and discussed. GaNAsP material growth by gas-source molecular beam epitaxy is studied. An in situ and ex situ composition control is demonstrated and the metamorphic buffer is characterized by X-ray reciprocal space mapping. Optical properties as well as annealing technique to improve them are investigated. The IB characteristic of GaNAsP is demonstrated by photoreflectance and photoabsorption. In the second part, the general IBSC design principles on GaNAsP is discussed, followed by the bandgap engineering for GaNAsP IBSC device. Particular attentions are paid to important issues, such as Si doping efficiency in dilute nitrides and IB electron blocking, which are the keys of IBSC design. An iteration on optimizing the IBSC deice structure has been run and the measurement and evaluations on these IBSC devices are presented. GaNAsP IBSCs with an open circuit voltage as high as 1.2 V and a fill factor of 0.66 have been obtained, which are the highest reported to date for HMA based IBSC