Iron Pyrite / a super absorber for PV solar energy

降低材料成本和提高太阳能电池的效率是全球关注的一个正在进行的研究领域。目前，晶体硅电池占光伏市场的90%。虽然转换效率水平被认为很高，但晶体硅的生产成本明显高于薄膜材料。然而，薄膜PV的电流转换效率明显低于晶体硅，因此降低了一些成本优势。薄膜太阳能电池的发展有相当大的空间，可以通过改进材料和器件结构来提高电池效率。一个改进将导致更好的转换效率是增加光子吸收到电池，接近结。作为光伏器件吸收层的一个很有前途的候选者是黄铁矿（FeS2）。FeS2的高吸收系数与其在导带中的高态密度是一致的。相对较小的带隙允许可见光和红外波长被吸收，直接和间接带隙跃迁的结合有助于其高吸收系数。然而，这还没有变成一个有效的光伏设备。黄铁矿有潜力成为一种非常重要的材料，用于大规模制造薄膜光伏组件，其中元素成分非常丰富，并且与所需的吸收材料数量少得多（厚度为100纳米或更少）结合，使其成为一种非常可持续和低成本的材料。该可行性提案将利用最近在PV Supergen项目上MOCVD CdTe光伏器件的成功，研究有前途的MOCVD方法来沉积非常薄的FeS2薄膜，并将其引入新的p-i-n结构。这种结构将利用超吸收特性在n型CdS和p型CdTe:As层之间夹心一个非常薄的薄膜吸收器。其目的是证明可以用FeS2制造高效率的PV器件，其中光产生的载流子通过电场中的漂移而不是扩散来收集，从而减少载流子损耗。作为可行性的一部分，它旨在通过用更可持续的材料取代cd和CdTe的n型和p型层来确定未来的发展。

Reducing the materials costs and improving efficiencies for solar cells is an ongoing research area that has global interest. Currently, crystalline silicon cells account for 90% of the PV market. Whilst conversion efficiency levels are considered high, the production costs for crystalline silicon are significantly higher than for thin film materials. However, the current conversion efficiency for thin film PV is significantly less than for crystalline silicon hence reducing some of the cost advantage. There is considerable scope for thin film solar cell development, where improvements in cell efficiency can be made with improved materials and device structures. One improvement that will lead to better conversion efficiency is to increase the photon absorption into the cell, close to the junction. A promising candidate for application as an absorbing layer in a photovoltaic device is iron pyrite (FeS2). The high absorption coefficient of FeS2 is consistent with its high density of states in the conduction band. The relatively small band gap allows for visible and infrared wavelengths to be absorbed, and the combination of direct and indirect band gap transitions contribute to its high absorption coefficient. However, this has not yet been turned into an efficient PV device. Iron pyrite has the potential to become a very important material for very large scale manufacture of thin film PV modules where the elemental constituents are very abundant and combines with the much smaller amounts of absorber material needed (thickness of 100 nm or less) making this a very sustainable and low cost material.This feasibility proposal will investigate the promising MOCVD route to deposit very thin films of FeS2 and introduce these into a novel p-i-n structure, taking advantage of the recent success with the MOCVD CdTe PV devices on the PV Supergen project. This structure will take advantage of the super-absorption characteristics to sandwich a very thin film absorber between n-type CdS and p-type CdTe:As layers. The purpose will be to show that high efficiency PV devices can be made from FeS2 where the photo-generated carriers are collected by drift in the electric field rather than by diffusion, thus reducing carrier loss. As part of the feasibility it is intended to scope future developments by replacing the CdS and CdTe n-type and p-type layers with more sustainable materials.