semiconducting materials 
 have long played a pivotal role in the 
 development and advancement of organic electronic applications such 
 as organic light - emitting diodes , organic field - effect transistors , 
 and organic solar cells .
more recently , semiconducting polymers have made 
 their entry into the new field of organic bioelectronics , which broadly 
 encompasses any application that couples a relevant function of organic 
 electronic materials with a targeted biological event . in this context ,
recent endeavors have seen organic electronic materials 
 utilized for example in biologically relevant ion sensing and ion pumps , and as transducers of neural 
 activity .
in contrast to inorganic materials , organics 
 can easily be modified for biocompatibility , while there is already 
 a comprehensive understanding of structure  property relations 
 from the well - established organic field - effect transistor ( ofet ) community . despite this large body of work relating to the transduction of 
 electronic signals in organic semiconductors , high - performing bioelectronic 
 devices such as organic electrochemical transistors ( oects )
are predominantly 
 fabricated with the commercially available blend of poly(3,4-ethylenedioxythiophene ) 
 and polystyrenesulfonate ( pedot : pss ) . from a synthetic 
 point of view
, it is very challenging to modify pedot : pss , which makes 
 it difficult to control important device parameters such as operational 
 voltage .
the intrinsically conducting nature of the pedot : pss blend 
 furthermore means that the oect must be operated in depletion mode 
 rather than accumulation mode , giving rise to a device that is on 
 at no external bias and shows inferior on / off ratios at low bias , 
 which is often desired in biological applications .
therefore , we have 
 recently focused on developing new accumulation mode oect materials 
 with an underlying motivation of addressing the current lack of understanding 
 relating to the chemical design of semiconducting oect materials . 

our aim is to help establish molecular design criteria and to elucidate 
 important structure  property relations for the synthesis of 
 high - performing semiconducting polymers for oect applications . in 
 this context , we present here a series of five thiophene- and benzodithiophene - based 
 polymers and show how our rational molecular design strategy affords 
 a top - performing polymer that , in an accumulation mode oect configuration , 
 for the first time , outperforms state - of - the - art pedot : pss - based depletion 
 mode devices with peak transconductances above 20 ms , peak currents 
 in the ma regime , on / off ratios above 10 , and excellent 
 switching times below 1 ms .
since oects are mainly operating 
 as transducers , the figure of 
 merit is the transconductance ( gm ) defined 
 as did / dvg ; transconductance therefore describes the peak performance of an 
 oect in transducing a small change in effective gate voltage ( vg ) into a large modulation in current running 
 through the transistor channel ( drain current , i d ) . in the saturation regime ,
the transconductance is furthermore 
 proportional to the charge carrier mobility (  ) , the capacitance 
 per unit volume ( c * ) , and the active channel area 
 defined by the channel width ( w ) , length ( l ) , and thickness ( d ) as displayed in eq 1 , where vt is the threshold voltage.1 therefore , to optimize 
 oect performance from a molecular design 
 point of view , charge and ion transport must be optimized in a balanced 
 fashion to maximize the product c*. to address 
 this , we have investigated a series of thiophene - based polymers with 
 ethylene glycol side chains as further outlined below .
whereas one 
 can simplistically associate the polythiophene backbone with the charge 
 transport and the ethylene glycol side chains with the ion transport , 
 there is ample literature evidencing that the nature , positioning , 
 and density of the solubilizing side chains play a huge role in the 
 charge transport properties of the semiconducting material . with this 
 caveat in mind ,
we have aimed to explore synthetic design criteria 
 in a systematic fashion in order to optimize both the charge transport 
 and the ion transport and more importantly the product of the two 
 ( c * ) .
for efficient ion transport 
 in the solid state , triethylene glycol ( teg ) chains were grafted onto 
 benzo[1,2-b:4,5-b]dithiophene 
 ( bdt ) and 2,2-bithiophene ( 2 t ) , and the resultant monomers 
 were either homopolymerized , or copolymerized with thiophene or bithiophene . 

these comonomers were chosen due to their electron - rich conjugated 
 systems , and the fact that they could facilitate high degrees of backbone 
 coplanarity , thus achieving low operational voltages and good charge 
 transport properties .
the polymers are designed with 
 the aim of investigating a number 
 of important structure  property relations including the following : 
 ( 1 ) backbone conformation is modulated as illustrated for example 
 by the polymers gbdt , gbdt - t , and gbdt-2 t . while bdt ensures a linear connectivity along the 
 polymer backbone , the thiophene and bithiophene comonomers introduce 
 various degrees of backbone curvature as illustrated in figure 1 .
this is likely to influence 
 important parameters such as side chain orientation , polymer solubility , 
 packing , and , consequently , charge transport .
( 2 ) teg chain density 
 is modulated along the polymer backbone as illustrated for example 
 by the polymer series discussed above or by gbdt-2 t versus gbdt - g2 t .
this will again have an effect on polymer solubility 
 and packing , but also importantly on the polymers ionization 
 potentials ( because the oxygen is directly grafted onto the thiophene 
 ring ) and their abilities to promote ion penetration / transport in 
 the solid state .
( 3 ) structure of teg - functionalized monomer through 
 comparison of gbdt - t and g2t - t to investigate 
 the role of the -conjugated backbone and its charge transport 
 properties without making significant changes to the backbone conformation 
 or the side chain density .
energy - minimized structures for gbdt - t ( top ) , gbdt-2 t ( middle ) , and gbdt - g2 t ( bottom ) 
 illustrating 
 important molecular design criteria employed in this study .
benzo[1,2-b:4,5-b]dithiophene 
 ( bdt ) and 2,2-bithiophene ( 2 t ) were functionalized with methoxy - terminated 
 triethylene glycol ( teg ) chains according to literature protocols 
 to afford the two teg - functionalized comonomers . utilizing conventional stille - type cross - coupling polymerization 
 reactions , these building blocks allowed for the synthesis of the 
 bdt homopolymer ( gbdt ) and the two thiophene copolymers 
 ( gbdt - t and g2t - t ) , while the teg - functionalized 
 bdt monomer could furthermore be copolymerized with either unsubstituted 
 2 t or teg - functionalized 2 t to afford gbdt-2 t and gbdt - g2 t , respectively .
molecular weight analysis was 
 carried out by size exclusion chromatography in n , n - dimethylformamide or chlorobenzene as summarized 
 in table s1 .
good degrees of polymerization 
 were observed for gbdt - g2 t and g2t - t , with 
 number - average molecular weights of 22 and 63 kda , respectively , while 
 the very limited solubility of the three other polymers resulted in 
 inconclusive molecular weight analysis .
vis spectroscopy 
 on thin films of the polymers ( figure 2 , table s1 ) revealed a gradual 
 red - shift in absorption maximum 
 when going from gbdt ( 533 nm ) to gbdt - t ( 549 
 nm ) and gbdt-2 t ( 562 nm ) with all three polymers showing 
 strong absorption features from both the 00 and the 01 
 optical transitions .
the absorption profiles of gbdt - g2 t and g2t - t were further red - shifted with max values of 585 and 650 nm , respectively , while these spectra 
 were less resolved in terms of vibronic features .
optical band gaps 
 were determined from the onsets of absorption and ranged from 2.18 
 ev for gbdt to 1.73 ev for g2t - t as further 
 detailed in table s1 . the observed red - shift 
 and narrowing of band gap
when increasing the thiophene content relative 
 to benzodithiophene in the polymer is likely attributed to the stronger 
 quinoidal contribution from the thiophene - rich polymers resulting 
 in a higher degree of backbone coplanarity .
additional backbone planarization 
 of the g2t - based polymers can also be envisioned due to attractive 
 intramolecular s  o interactions .
we furthermore note that density 
 functional theory ( dft ) calculations support the observed trend in 
 optical properties ( si , table s2 ) . uv  vis 
 spectra of thin spin - cast films of the polymers .
oxidative electrochemical properties were investigated by 
 cyclic 
 voltammetry ( cv ) on thin polymer films on indium tin oxide ( ito ) coated 
 glass substrates using a conventional three - electrode setup in acetonitrile 
 solution .
the onsets of oxidation were determined for all polymers , 
 while all polymers except gbdt also showed well - defined 
 first half - wave potentials as summarized in table 1 ( voltammograms in si , figure s3 ) .
ionization potentials ( ips ) were estimated from 
 the onsets of oxidation ; gbdt , gbdt - t , and gbdt-2 t all have ips around 4.84.9 ev ( slightly higher 
 than what was measured for poly(3-hexylthiophene ) ( 4.78 ev ) under 
 identical conditions ) , whereas the electron - rich all - thiophene polymer g2t - t and the fully tegylated gbdt - g2 t have significantly 
 lower ips around 4.4 ev as expected ( figure 3 ) .
dft calculations of the highest occupied 
 molecular orbital ( homo ) energy levels are in good agreement with 
 the experimental trend discussed above ( table s2 ) . from the cv data it is evident that our molecular design 
 strategy provides good control over the onset of oxidation which
importantly , 
 we also note qualitatively that the current density and thus the capacitance 
 are significantly higher for g2t - t than for the four 
 other polymers ( figure s4 ) .
cyclic voltammetry of polymer 
 thin films on ito coated glass substrates in acetonitrile with 0.1 
 m tetrabutylammonium hexafluorophosphate as the supporting electrolyte .
ionization 
 potentials ( bottom ) and electron affinities ( top , estimated 
 from ionization potential and optical band gap ) for the polymers . to investigate the electrochemical 
 properties under more oect relevant 
 conditions , the oxidative properties were also measured in aqueous 
 solution .
the resulting cyclic voltammograms are depicted in figure 4 and in the si ( figure s5 ) .
generally , the same trends were observed 
 within the polymer series when comparing the onsets of oxidation in 
 acetonitrile and water although the onsets in water were 30140 
 mv lower than those in acetonitrile , which is likely due to the smaller 
 anion ( chloride vs hexafluorophosphate ) and better solvated
teg side 
 chains in water than in acetonitrile and hence easier ion penetration . 

this hypothesis is supported by the fact that the difference in onset 
 is largest for gbdt - g2 t , which has the highest teg side 
 chain density .
thus , it appears that , for this system , the cv characteristics 
 are a convolution of not only the energetics of the polymer , but also 
 the energetics associated with ion penetration ( injection ) and transport . 
 in conjunction with the aqueous cv experiment ,
the electrochromic 
 properties of the polymers were also studied by means of spectroelectrochemistry . 
 the data for the three polymers included in figure 4 illustrate the different electrochromic 
 properties observed for the polymer series .
barely any electrochromic 
 response was observed for the homopolymer gbdt ( figure 4 , top panel ) when 
 sweeping the potential from 0.2 to 0.6 v versus ag / ag , whereas gbdt - t and gbdt-2 t ( figure 4 , middle panel , and figure s5 , respectively ) both showed partial 
 extinction of the  * absorption band with the 
 simultaneous gradual appearance of a very broad absorption feature 
 around 750 nm . the new absorption feature is likely from the oxidized 
 polymer ( polaron ) and appears stable when subjected to consecutive 
 cycling within the potential range 0.20.7 v ( figure 4 , right panel ) .
the 
 more electron - rich polymers gbdt - g2 t and g2t - t ( figure s5 and figure 4 , bottom panel , respectively ) both show a 
 more complete oxidation of the bulk polymer film evident from the 
 disappearance of the  * absorption band with 
 a concurrent appearance of a broad polaron absorption band around 
 850 nm .
again , the two polymers are stable to several oxidative cv 
 cycles with no sign of significant current or absorbance loss as evident 
 from figure 4 ( right 
 panel ) . from a structural point of view
, it is clear that the two 
 low - ip polymers containing the tegylated bithiophene unit show superior 
 electrochromic properties ( judged from the complete bleaching of the 
  * transition and the stable cycling between 
 colored and bleached state ) compared to the three other tegylated 
 bdt polymers .
the applied potentials are kept low deliberately since 
 these materials are investigated for oects that should be operational 
 in aqueous media and hence a narrow electrochemical potential window . 
 moreover , with application of higher potentials , gbdt , gbdt - t , and gbdt-2 t degrade rapidly . 

thus , the spectroelectrochemical properties reported here represent 
 the highest degree of doping that can be obtained under conditions 
 relevant for oect operation and without polymer degradation taking 
 place ; in other words , increasing the bias further does not result 
 in further doping but merely in degradation of the active material .
cyclic 
 voltammograms in water with 0.1 m sodium chloride as the 
 supporting electrolyte ( left panel ) and corresponding uv 
vis 
 absorption spectra ( middle panel ) with color coding indicating the 
 applied potential during recording of the uv  vis spectrum . 

right panel shows the switching behaviors with the two wavelengths 
 used for monitoring the optical switching indicated in the corresponding 
 uv  vis spectra by the green and red lines .
grazing incidence wide - angle x - ray scattering ( giwaxs ) was 
 employed 
 to study the structural organization of thin spin - cast films of the 
 polymers on si substrates ( table 2 , figure 5 , and figure s10 ) .
polymers gbdt and gbdt - t were found to orient preferably in a face - on 
 fashion with -stacking distances of 3.72  , whereas gbdt-2 t showed a predominant edge - on orientation with nearly 
 identical -stacking distances of 3.71  .
the lamellar 
 stacking distances gradually decrease from roughly 20 to 14  
 when increasing the unsubstituted thiophene content within this series 
 due to the decrease in side chain density .
this observation could 
 be explained by a less extended and more disordered side chain orientation . 

the variation of structural order and orientation on a substrate with 
 varying comonomer ( gbdt vs gbdt - t vs gbdt-2 t ) is well - known from the literature . for the polymers discussed herein
, the explanation most 
 likely relates to -stacked lamellar platelets forming in solution , 
 thereby favoring an edge - on orientation ( platelets orient face - on 
 to the substrate , with the polymer backbones correspondingly oriented 
 edge - on ) as seen for the least soluble polymer , gbdt-2 t .
going from gbdt-2 t to gbdt - g2 t , the side chain density is increased significantly 
 along the polymer backbone , which not only brings the lamellar stacking 
 distance back to approximately 20  as seen for gbdt , but also increases the -stacking distance to 3.84  . 

the increased side chain density furthermore causes a shift in preferential 
 orientation from edge - on to face - on for gbdt - g2 t in agreement 
 with the trend noted above .
polymer g2t - t compares well 
 with gbdt - t in having a lamellar repeat distance around 
 17  , whereas the -stacking distance is markedly decreased 
 to 3.59  . in resemblance with gbdt-2 t , g2t - t orients predominantly edge - on relative to the substrate , which should 
 be favorable for horizontal charge transport as required in a conventional 
 oect configuration . upon further comparison ,
it is 
 evident that the high side chain density of gbdt - g2 t , 
 with teg chains on each comonomer unit , is detrimental to effective 
 -stacking whereas g2t - t has much stronger  
 interactions indicative of good interchain charge transport .
vertical ( top ) 
 and horizontal ( bottom ) line - cuts of the two - dimensional 
 giwaxs spectra for gbdt - g2 t and g2t - t ; qz and qxy are the perpendicular and parallel wave 
 vector transfers with respect to substrate surface . organic field - effect transistors ( ofets ) were 
 successfully fabricated 
 for the high - ip polymers as further evidenced in the si ( figure s8 ) with extracted hole mobilities of 
 1  10 cm/(v s ) for both gbdt and gbdt - t and 9  10 cm/(v s ) for gbdt-2 t .
the nearly 1 order 
 of magnitude higher mobility of the latter is in good agreement with 
 the preferential edge - on orientation relative to the substrate , which 
 should favor in - plane charge transport as noted above .
difficulties with fabricating and testing stable ofet devices with gbdt - g2 t and g2t - t , due to unintentional doping , 
 were circumvented by extracting charge carrier mobilities from the 
 oects affording hole mobilities of 0.01 and 0.28 cm/(v 
 s ) , respectively . here , we note that the significantly 
 tighter -stacking of g2t - t compared to gbdt - g2 t , as well as the edge - on packing observed , results in a considerably 
 higher charge carrier mobility , as expected .
furthermore , from a structural 
 point of view , it appears that the higher backbone flexibility of 
 the thiophene - based polymer , g2t - t , improves film formation 
 and morphology , which also aid charge transport compared to the more 
 rigid bdt - containing polymer . because the hole mobilities of gbdt , gbdt - t , and gbdt-2 t are extracted 
 differently from those of gbdt - g2 t and g2t - t , we are not able to compare directly the charge transport properties 
 between the two sets of materials .
organic electrochemical transistors 
 were initially fabricated with 
 thin spin - cast active layers with comparable thicknesses of 82110 
 nm in order to ensure a fair comparison of the five polymers since 
 device performance scales with wd / l , where w and l are the channel width and length , 
 respectively , and d is the active layer thickness .
channel dimensions ( w  l , in m ) of 100  10 , 10  10 , and 50  
 50 were tested .
full details on oect fabrication and operation can 
 be found in the si . as detailed in table 3 , gbdt and gbdt - t
show the poorest performances with transconductances 
 around 0.10.5 s , which is corroborated nicely by the 
 poor electrochromic activity and the low charge carrier mobilities . 

polymer gbdt-2 t shows a transconductance around 0.06 
 ms and a similar 100-fold improvement in peak current when compared 
 to gbdt and gbdt - t .
this improvement is 
 supported by the improved electrochromic properties as well as the 
 better hole mobility of gbdt-2 t , which can be ascribed 
 to its edge - on orientation .
it should be noted that these polymers 
 do not show a peak transconductance , as is commonly observed for pedot : pss , 
 within the stable operation window .
for this reason , we quote the 
 highest attainable reproducible transconductance value which occurs 
 at a gate bias of 0.6 v. this suggests that these materials 
 could perform better if they were able to operate in a stable manner 
 at higher gate biases .
a further and quite significant improvement 
 in device performance of 1 and 2 orders of magnitude , respectively , 
 is seen for gbdt - g2 t and g2t - t .
we note 
 again that a good correlation is observed with the much improved electrochromic 
 properties compared to the other polymers , while it also appears that 
 the strong  interactions and the edge - on orientation 
 of g2t - t result in superior oect performance with a peak 
 transconductance ( gm ) of 
 nearly 8 ms at a gate voltage ( vg ) of 
 0.6 v , a peak drain current ( i d ) of 3.3 ma ( at vg = 0.7 v ) , and a good on / off ratio above 10 . 

the nearly 20-fold increase in peak transconductance when going from gbdt - g2 t to g2t - t at identical device dimensions 
 is in good agreement with the 28-fold improvement in charge carrier 
 mobility as discussed above .
we show below ( figure 6 ) , when considering multiple devices and 
 dimensions , that the improvement in device performance of g2t - t compared to gbdt - g2 t is actually closer to 100-fold , 
 which indicates that not only hole transport but also ion transport 
 are greatly improved .
accumulation 
 mode p - type oect 
 operation ; all devices are spin - cast devices with active areas of 
 100 m  10 m ( w  l ) , and all transfer characteristics and transconductances 
 are measured at a drain voltage of 0.4 v. peak transconductances as a function of active 
 layer dimensionality 
 ( wd / l ) for all polymers and including 
 pedot : pss - based devices with ethylene glycol dispersion additive ( data 
 from ref ( 12 ) ) .
focusing on the top - performing 
 material , g2t - t , we 
 subsequently turned our attention to oects with drop - cast active layers 
 in order to fabricate thicker devices .
a drastic improvement in device 
 performance was observed when compared to spin - cast devices , already 
 achieving a high peak transconductance of 15 ms , a peak current of 
 6.6 ma , and an on / off ratio of 4  10 for 
 a drop - cast device of similar thickness ( 110 nm ) to that of the spin - cast 
 device . with a 160 nm active layer thickness
, the peak transconductance 
 had increased to 21 ms with a peak current of 7.7 ma and an 
 on / off ratio of 2.6  10 . to date , this is the highest 
 reported peak transconductance for an oect in accumulation mode , while it also outperforms the archetypical pedot - based depletion 
 mode devices reported in the literature thus far . given the expected scaling of gm with 
 channel thickness
, the 40% increase in transconductance from 
 the 110 to 160 nm device is well - explained . to unequivocally demonstrate the superior performance of g2t - t in oects relative to that of pedot - based devices , a 
 thorough study of the relationship between peak transconductance and 
 dimensionality ( in the form of wd / l )
was carried out as summarized in figure 6 . from this , it is clear that g2t - t - based devices display higher transconductances than pedot : pss - based 
 devices at all tested active layer dimensions .
we also note that the 
 trend outlined in table 3 and discussed above comparing the five polymers in this study is 
 valid across all tested dimensions .
it is moreover evident from figure 6 that the peak transconductance 
 for each of these materials can be varied 12 orders of magnitude 
 by varying the active layer dimensions , as previously reported . with the change in device preparation 
 from spin- to drop - casting 
 and the significantly increased active layer thicknesses , we found 
 it relevant to reinvestigate the structural order in the solid state . 

specular x - ray diffraction ( xrd ) data on thick drop - cast films was 
 generally in good agreement with the giwaxs data on thinner spin - cast 
 films as further detailed in the si ( figure s9 ) .
output and transfer characteristics along with the related 
 peak 
 transconductances for optimized oects with gbdt - g2 t and g2t - t are displayed in figure 7 .
the peak current is obtained at slightly less negative 
 potential for g2t - t relative to gbdt - g2 t , while a more significant shift is seen for the peak transconductance , 
 which is recorded at a gate potential around 0.6 v for gbdt - g2 t and 0.2 v for g2t - t . the lower 
 turn - on voltage of g2t - t - based devices relative to gbdt - g2 t
an excellent 
 turn - on voltage around 0 v is found for g2t - t independent 
 of wd / l , while gbdt - g2 t devices turn on around 0.3 v and the higher ip polymers 
 all turn on from 0.5 to 0.7 v as one would expect 
 from the differences in oxidation potentials ( table 1 ) measured both in organic and aqueous media . 

all the polymers show minimal variations in turn - on voltage over 23 
 orders of magnitude of wd / l , emphasizing 
 how this device dimensionality parameter can be used to control the 
 peak transconductance without adversely affecting the turn - on voltages .
output 
 characteristics ( left ) , transfer characteristics and related 
 transconductances ( middle ) , and transient characteristics ( right ) 
 for optimized drop - cast oects using gbdt - g2 t ( top , 400 
 nm thick device ) and g2t - t ( bottom , 162 nm thick device ) 
 measured in 0.1 m nacl aqueous solution ; devices have active areas 
 of 100 m  10 m ( w  l ) , and transfer characteristics and transconductances are 
 measured at a drain voltage of 0.4 v. oect turn - on voltages as a function of active layer dimensionality 
 ( wd / l ) for all polymers . another important aspect of oects and their application 
 in biological 
 environments is the transient behavior .
as depicted in figure 7 ( right panel ) , we have investigated 
 this aspect by repeated cycling of the oects between their on and 
 off states using a square - wave applied gate potential .
gbdt - g2 t is switched from 0.2 to 0.4 v with excellent stability as 
 evident from the constant peak current of roughly 2.5 a 
 obtained for each cycle .
similarly , g2t - t also shows 
 excellent stability when switched from 0.3 to 0.2 v with a 
 peak current around 1 ma ; a faster rise in peak current is 
 observed upon switching from 0.3 to 0.8 v ( figure s7 ) .
we find no discernible variation in the transient 
 stability with device dimensions as evidenced for several g2t - t - based oects in the si ( figure s7 ) .
notably , 
 these excellent switching properties are achieved with 2 s pulses 
 of on and off gate voltages .
the time constant (  ) for the temporal 
 response of a spin - coated g2t - t - based device ( d = 103 nm ) was 0.64 ms for a 10 m  10 m 
 active area , while a larger active area of 100 m  10 
 m as expected afforded a slightly slower response of 1.4 ms . 
 in comparison with the -values of the solid state accumulation 
 ( 40192 ms ) and depletion ( 300360 ms ) mode devices 
 having the highest peak transconductances previously reported in the 
 literature , the -values for g2t - t - based devices are 23 orders of magnitude lower . 

the lowest -values for accumulation mode devices have been 
 reported by inal and colleagues , who showed , with a polyelectrolyte - based 
 oect , that  could be decreased from 38 to 0.4 ms by active 
 layer casting with ethylene glycol cosolvent .
our results clearly indicate that equally fast temporal responses 
 can be achieved through judicious synthetic design , optimizing not 
 only the conjugated polymer backbone for efficient charge transport 
 but also the solubilizing side chains for efficient swellability and 
 ion transport .
this approach circumvents the need for solvent additives 
 and thus allows for a simpler device processing protocol . in 
 conclusion
, we have prepared a series of semiconducting thiophene - based 
 polymers with triethylene glycol side chains with the aim of simultaneously 
 optimizing both charge and ion transport in the bulk polymer film 
 in order to improve accumulation mode oect performance .
our molecular 
 design strategy entailed the copolymerization of two different tegylated 
 monomers with each other or with unsubstituted thiophene and bithiophene . 

this allowed us to access a series of polymers with various degrees 
 of teg side chain density and orientation , various backbone curvatures , 
 and ionization potentials ranging from 4.4 to 4.9 ev .
cyclic voltammetry 
 and in particular spectroelectrochemistry clearly indicated that polymers 
 containing the tegylated bithiophene unit were more electrochemically 
 stable than polymers with the tegylated benzodithiophene unit .
the 
 copolymer containing both units , gbdt - g2 t , showed better 
 stability than the other bdt - based polymers indicating that the intrinsic 
 stability issues with the bdt unit could be reduced by incorporating 
 the tegylated bithiophene .
this very electron - rich unit causes a significant 
 lowering of the ionization potential of approximately 0.40.5 
 ev ( both when it replaces tegylated bdt and unsubstituted bithiophene ) , 
 which is likely an underlying reason for the improved electrochemical 
 stability .
moreover , this shift brings the oxidative process into 
 an electrochemical potential window compatible with stable aqueous 
 operation . structural characterization by xrd and giwaxs confirmed 
 the semicrystalline 
 properties of these polymers , while it was also evident that the choice 
 of comonomer could be used to control the polymer backbone orientation 
 relative to the substrate .
this was emphasized by the fact that both 
 the tegylated and the unsubstituted bithiophene unit promoted a favorable 
 edge - on orientation .
a high degree of side chain density was furthermore 
 found to unfavorably increase the -stacking distance in the 
 solid state .
the latter point nicely illustrates the balance one must 
 strike when optimizing side chain density for swellability and ion 
 transport , while simultaneously maintaining a high degree of structural 
 order in the solid state for efficient charge transport .
the conclusions 
 from the structural characterization were corroborated by the charge 
 carrier mobilities of the polymers ; the edge - on oriented materials 
 generally showed higher hole mobilities , and stronger  
 interactions were likewise found to promote better charge carrier 
 mobilities as expected .
oect fabrication and analysis confirmed that 
 the polymers with the better electrochromic properties , better structural 
 organization , and higher hole mobilities also showed significantly 
 higher device performance .
after device optimization , the highest 
 reported peak transconductances of above 20 ms were achieved for g2t - t with excellent turn - on voltages around 0 v , high on / off 
 ratios around 10 , and fast temporal responses in the millisecond 
 regime with high operational stability at peak transconductance .
this molecular design strategy has elucidated a number of important 
 structure  property relationships for the continued development 
 of high - performing accumulation mode organic electrochemical transistors , 
 and it is evident that the tegylated bithiophene unit ( g2 t ) is a very promising building block in this context .
it is furthermore 
 evident from this study that cyclic voltammetry and in particular 
 spectroelectrochemistry are excellent screening tools for the development 
 of new oect materials .
 the organic electrochemical 
 transistor ( oect ) , capable of transducing 
 small ionic fluxes into electronic signals in an aqueous environment , 
 is an ideal device to utilize in bioelectronic applications . 
 currently , 
 most oects are fabricated with commercially available conducting poly(3,4-ethylenedioxythiophene ) 
 ( pedot)-based suspensions and are therefore operated in depletion 
 mode . here , we present a series of semiconducting polymers designed 
 to elucidate important structure  property guidelines required 
 for accumulation mode oect operation . 
 we discuss key aspects relating 
 to oect performance such as ion and hole transport , electrochromic 
 properties , operational voltage , and stability . 
 the demonstration 
 of our molecular design strategy is the fabrication of accumulation 
 mode oects that clearly outperform state - of - the - art pedot - based devices , 
 and show stability under aqueous operation without the need for formulation 
 additives and cross - linkers . 
Introduction
Results and Discussion