How 1,8-diiodooctane (DIO) enhances performance of polymer solar cells based on polymer HXS-1 and fullerene [6,6]-phenyl C(71)-butyric acid methyl ester (PC(71)BM) from 3.6% to 5.4% is scrutinized with several techniques by comparing devices or blend films spin-coated from dichlorobenzene (DCB) to those from DCB/DIO (97.5:2.5 v/v). Morphology of blend films is examined with atomic force microscopy (AFM), transmission electron microscopy (TEM) and electron tomography (3-D TEM), respectively. Charge generation and recombination is studied with photoluminescence, and charge transport with field effect transistors. The morphology with domain size in 10-20 nm and vertical elongated clusters formed in DIO system is supposed to facilitate charge transport and minimize charge carrier recombination, which are the main reasons for enhancing power conversion efficiency (PCE) from 3.6% (without DIO) to 5.4% (with DIO). Furthermore, a two year inspection shows no significant impact of DIO on the shelf-stability of the solar cells. No visible degradation in the second year indicates that the morphology of the active layers in the devices is relatively stable after initial relaxation in the first year. ; Original Publication: Weiwei Li, Yi Zhou, Viktor Andersson, Mattias Andersson, Yi Thomann, Clemens Veit, Kristofer Tvingstedt, Ruiping Qin, Zhishan Bo, Olle Inganäs, Uli Wuerfel and Fengling Zhang, The Effect of additive on performance and shelf-stability of HSX-1/PCBM photovoltaic devices, 2011, Organic electronics, (12), 9, 1544-1551. http://dx.doi.org/10.1016/j.orgel.2011.05.028 Copyright: Elsevier Science B.V., Amsterdam. http://www.elsevier.com/