Printed　electronics　is　a　rapidly　developing　field　of　research　which　covers　any　electronic　devices　or　circuits　that　can　be　processed　using　direct　printing　techniques.　Among　those　printing　techniques,　inkjet　printing　is　a　technique　of　increasing　interest　for　organic　field-effect　transistors　(FETs)　due　to　its　fully　data　driven　and　direct　patterning.　In　this　work,　the　morphology　of　semi-conducting　polymer/insulating　polymer　blends　from　inkjet　printing　and　their　FET　properties　have　been　investigated.　We　attempted　to　optimize　the　morphology　of　the　blends　by　the　addition　of　a　co-solvent　to　the　blend　solution　prior　to　film　deposition.　By　varying　the　boiling　temperature　of　the　co-solvent,　blend　films　are　fabricated　with　varying　domain　purity　and　different　degree　of　semi-conducting　polymer　ordering.　The　morphologies　of　all　the　as-cast　samples　from　inkjet　printing　and　subsequently　thermally　annealed　samples　are　characterized　by　grazing　incidence　wide　angle　x-ray　scattering　and　small　angle　neutron　scattering.　The　results　indicate　that　the　sample　where　a　low　boiling　temperature　co-solvent　is　used　exhibits　a　lower　degree　of　semi-conducting　polymer　ordering　and　less　pure　domains,　resulting　in　a　decrease　of　hole　mobility.　The　morphologies　that　are　formed　when　high　boiling　temperature　co-solvent　is　used,　however,　give　a　higher　degree　of　semi-conducting　polymer　ordering　along　with　higher　domain　purity,　significantly　improving　hole　mobility　up　to　1.44　cm2　V−1　s−1　at　VDS　=　40　V.　More　importantly,　with　thermal　annealing,　all　the　samples　exhibit　similar　semi-conducting　polymer　ordering　and　domain　sizes　while　the　domain　purity　significantly　varies.　This　work　is　a　unique　example　that　demonstrates　the　importance　of　domain　purity　in　the　optimization　of　morphology　and　FET　performance,　which　is　previous　unavailable.　It　also　provides　a　novel　process　that　can　efficiently　control　the　morphology　of　semi-conducting　polymer/insulating　polymer　mixtures　during　deposition　to　maximize　FET　performance　from　inkjet　printing.　©　2016　Wiley　Periodicals,　Inc.　J.　Polym.　Sci.,　Part　B:　Polym.　Phys.　2016,　54,　1760–1766.　©　2016　Wiley　Periodicals,　Inc.