Self-Consistent Drift-Diffusion Analysis of Intermediate Band Solar Cell (IBSC): Effect of Energetic Position of IB on Conversion Efficiency Katsuhisa Yoshida 1,2 , Yoshitaka Okada 1,2 , and Nobuyuki Sano 3 1 Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, JAPAN 2 Recearch Center for Advanced Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, JAPAN 3 Insititute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, JAPAN ABSTRACT The intermediate band solar cell (IBSC) has been intensively investigated both experimentally and theoretically. Numerical analyses based on the detailed balance method are performed to search for the best suitable candidates of material combination for IBSC and the operation conditions. Analytical treatment of drift- diffusion equations has also been reported under limited approximations. However, to study the device characteristics, self-consistent treatments of both the carrier continuity equations and the Poisson equation are required. In this work, we report on the dependence of conversion efficiency on energetic position of IB and on the concentration by using 1-D self-consistent drift- diffusion simulator which we developed for GaAs based solar cell with InAs quantum dots. The dependence of the efficiency on energetic position of IB above the midgap of GaAs was calculated for 1, 10, 100 and 1000 suns conditions with and without doping in IB region. The optimal IB position shifted to lower energies with increase of concentration in the case of intrinsic IB region. While, in the case of doped IB region, the optimal IB position was almost fixed at 0.95eV. If, however, the IB was set in the middle of the energy gap of GaAs, efficiencies showed lower values with higher concentrations. This is because, according to our present model, very few photons contribute to the optical transition (generation) in CB-IB and most photons are absorbed in VB-IB transitions such that there is a large mismatch in the generation-recombination rates in these transition paths. INTRODUCTION The intermediate band solar cell (IBSC) [1] has been intensively investigated both experimentally [2] and theoretically. IBSC utilizes photons, which have lower energies than the bandgap energy of host material and do not contribute to carrier generation in single-junction solar cells, by two-step photogeneration via IB. Thus, optimal combination of host material bandgap and intermediate band gives about 63% conversion efficiency under full concentration. Experimentally, quantum dot based structure [2] is one of the candidates for realizing the IBSC operation principle. Numerical analyses based on the detailed balance method have been performed to search for the best suitable candidates of material combination for IBSC and the operation conditions [1]. Analytical treatments of drift-diffusion equation are also reported under limited approximations [3,4]. Unfortunately, the self- consistency of both the carrier continuity equations and the Poisson equation is seldom achieved. From the viewpoint of a realistic analysis, a self-consistent treatment would be of most importance. Recently, Lin et al. [5] have reported IBSC device operations based on the self- consistent drift-diffusion method by extending the approach proposed by Schmeits et al. [6]. In this work, we report on the dependence of conversion efficiency on energetic position of IB by using 1-D self-consistent drift- diffusion simulator developed appropriate for GaAs based solar cell with InAs quantum dots (QDs), and on the concentration. In the following section, treatments of the Self- Consistent Drift-Diffusion Method with IB Balance Equation and modeling of optical generation rates via IB are presented. By using our method, optimal combinations of GaAs bandgap and energy position of IB under 1, 10, 100 and 1000 concentrated the sun light were investigated. NUMERICAL METHOD The self-consistent drift-diffusion method is usually adopted to analyze the device characteristics. This method is based on the basic device equations in semiconductors, namely, the Poisson equation and carrier continuity equations for electrons in conduction band (CB) and holes in valence band (VB). In the case of IBSC, there Figure 2 Calculated solar cell structure. x0 is the end of p emitter layer, LI is the length of IB region and L is total length. Figure 1 (a) Illustration of energy states EC, EI and EV and of quasi Fermi states C, I and V, (b) Relations of absorption specta used in the simulation, respectively. 978-1-4244-5892-9/10/$26.00 ©2010 IEEE 1 978-1-4244-5892-9/10/$26.00 ©2010 IEEE 000071