Preparation And Properties Of Silicon Based Supersaturated Vanadium Doped Intermediate Band Photovoltaic Materials

Intermediate band solar cell（IBSC）is a hot area of research in third-generation solar cells.It is envisioned that by introducing an intermediate energy band between the conduction band and valence band of a semiconductor material,the channels for photon leap are expanded from one to three,thus achieving the absorption of lower energy photons.The theoretical limit efficiency of single interstitial band IBSC under concentrated light conditions can reach 63.2%,which breaks the theoretical efficiency limit of conventional single junction solar cells.However,the current methods for the preparation of intermediate band materials with impurities suffer from tedious processes,non-uniform impurity distribution and low activation rate,and difficulty in batch preparation,which limit the practical application of intermediate band solar cells.In this paper,the ion implantation combined with rapid thermal annealing method is used to prepare a silicon based intermediate band PV material with supersaturated doping of vanadium（V）ions.The preparation technique is relatively simple,and the implanted impurities can be effectively activated and uniformly distributed,which is more suitable for batch preparation.Preliminary results were obtained by testing the properties of the samples such as minority carrier lifetime,square resistance,ellipsometric spectra and absorption spectra.The main results are as follows.1.The n-type<111>crystal-oriented single-crystal silicon surface was implanted with V ions exceeding its solid solution degree,and the sample was subjected to rapid thermal annealing.The SIMS test results of the samples showed that the vanadium ion concentration exceeded 5.9×10¹⁹cm^-3within a thin layer of60-110 nm on the surface of the samples,which reached the concentration conditions for the formation of intermediate bands in silicon materials.2.The Si:V samples annealed once were characterized by relevant tests from both lattice structure and optoelectronic properties.The results show that after annealing,the lattice structure and surface morphology of the ion-implanted samples are effectively repaired,and the carrier concentration and mobility are enhanced.Compared with the unimplanted sample,the near-infrared absorption of the Si:V material is significantly enhanced,and the minority carrier lifetime is enhanced with increasing doping concentration,with an effective intermediate band formed at about0.43 e V below the conduction band edge of Si.The comparison of the properties of the samples under different annealing conditions reveals that the lattice repair and impurity activation effect of RTP on the samples is highly dependent on the annealing temperature and time,and the annealing temperature set to 800°C and time set to 8 to 12 s has the best effect on the lattice repair.3.The secondary annealing focused more on the electrical activation of the impurities in the implanted silicon than the primary annealing.Comparative tests showed that the sample with the secondary annealing temperature set at 1100°C had the best activation of impurities.Compared with the primary annealed sample,the high temperature and short time secondary annealing has a better effect on the lattice repair,and the carrier concentration can be increased up to 2-3 times that of the primary annealed sample,with a significant increase in the electrical activation rate.The minority carrier lifetime is also improved and more uniformly distributed.The secondary annealing also promotes the optical absorption of the intermediate band materials.The absorption at 1300 nm of Si:V samples with implantation doses of1×10¹⁶cm^-2and 5×10¹⁶cm^-2increases from 8.64%and 19.56%to 14.24%and29.38%,respectively,after secondary annealing,which effectively enhancing the activation rate of impurities in the primary annealed samples..4.Simple solar cells were prepared using Si:V intermediate band material as the absorber layer,and their external quantum efficiency and current-voltage characteristics were tested.The results show that the quantum efficiency of the second annealed sample is improved in the near-infrared band;the open-circuit voltage of the device decreases by less than one-tenth after the second annealing,while the short-circuit current density increases by more than one-third,which significantly increases the short-circuit current of the cell without basically changing the open-circuit voltage,thus improving the cell conversion efficiency.From the above experimental results,it is concluded that the intermediate band material can be prepared by ion implantation combined with rapid thermal annealing of single crystal silicon doped with supersaturated V ions.This paper provides a new way of preparation for silicon-based impurity intermediate band materials in terms of impurity selection and preparation process,and provides an experimental basis and reference for relevant research.