document: antireflection ( ar ) technology plays an important role in the fabrication of high - efficiency solar cells by increasing light coupling into the active region of devices . 
   generally , a relatively low - light reflectivity can be obtained for crystalline silicon solar cells by combining surface texturization with sixny - thin - film deposition . however , in order to further reduce the light reflectivity for the improvement of solar cell efficiency , a variety of nanostructures with size comparable to the wavelength of m : most of the utilized solar spectrum have been proposed , such as photonic crystals , porous structures , plasm : monics [ 5 - 8 ] , etc . 
   the subwavelength structures can be nanowires , nanocones , frustum nanorods , biomimetic nanostructures [ 13 - 16 ] , dielectric nanoparticles   etc . 
   all these nanostructures should be based on the wide bandgap materials which , themselves , can not absorb sunlight . 
   the zno material has a wide bandgap , which has been popularly used in the fields of photocatalysis   and optoelectronic nanodevices . for photovoltaic application , the zno nanostructures as antireflective layers could be fabricated by various approaches like pulsed laser deposition , chemical vapor deposition , vapor  liquid - solid growth , hydrothermal method , and electrochemical deposition technique . 
   am : mong them , a seeding and growth two - step process in zinc salt and amide mixed solutions seems quite acceptable for photovoltaics due to its low cost and good potential for scale - up . 
   experiments have shown that zno nanorod arrays can exhibit better antireflective properties than other oxide compounds . 
   a weighted global reflectance of 6.6% has been achieved in the broadband range from 400 to 1,200  nm . 
   however , m : most work on zno nanorod arrays as antireflective layers is performed on the naked silicon wafer by now . the application of zno nanostructures in the practical silicon solar cells is seldom reported . in this work , 
   we have grown zno nanowhiskers ( nws ) with different lengths by the seeding and growth two - step process , and meanwhile have integrated the zno nw technique into the fabrication of czochralski ( cz ) silicon solar cells based on the current standard celling technology . the density and the length of zno nws 
   have been optimized for the improvement of solar cell performances by controlling the time of growth . 
   the result shows that the efficiency of cz silicon solar cell can be improved by a value of 3% relatively using the zno nws as the ar layer in practice . 
   the starting samples are p - type cz silicon solar cells fabricated by a standard process including surface texturization , phosphorus diffusion , sixny film deposition , screen printing , and metallization . 
   the seed layer of zno was first deposited on the front surface of solar cells by a magnetron sputtering system . 
   the deposition temperature is 150c , and the seed layer thickness is about 10  nm . after protecting the parts without the zno seed film with waterproof tapes , 
   the samples were immersed in the solution of zinc nitrate hexahydrate ( zn(no3)26h2o ) and ( ch2)6  n4 mixture for 30 to 60  min for the growth of zno nw . 
   the zno nw growth temperature is 90c . for characterization , a scanning electron microscope ( sem ; hitachi s-4800 , hitachi , ltd . 
   , chiyoda , tokyo , japan ) was used to observe the zno nws on the solar cells . 
   the reflection spectra of the samples were measured by a uv  vis spectrophotometer ( hitachi u-4100 ) in the wavelength range of 300 to 1,100  nm . 
   the external quantum efficiencies ( eqes ) of the final solar cells were obtained from the spectral responses . 
   the current  voltage characteristics of the cells under the dark and illumination circumstances were measured . for illumination , 
   one sun ( 100  mw / cm ) with am 1.5 global spectrum was used . 
   figure  1 shows the sem micrographs of a silicon solar cell with the zno nws after a two - step growth process . 
   it can be seen that the zno nws are uniformly fabricated on the surface of silicon solar cell . generally , the fabrication of zno nws based on aqueous solution consists of nucleation   and growth . 
   the main chemical reactions through the whole process can be described as follows : ( 1)znno326h2ozn2++  2no3+6h2o ( 2)ch26n4 + 6h2o6hcho+4nh3   sem images of top ( a ) and cross - sectional ( b ) views of zno nw - coated solar cell surface . 
   the decomposition of hexamethylenetetramine produces amm : monia that can supply the oh needed for the zno nw formation reaction and acts as a ph buffer to regulate the ph value of the solution . 
   the ph value of the solution should be 6 or 7 for the whole reaction process . during the process 
   , the zno nanocrystals existing in the seed layer serve as the nucleation centers where the growth of zno nws takes place . due to its growth habit , 
   c - axis is the m : most preferred growth orientation , which leads to the formation of zno nws . 
   figure  2 shows the sem micrographs of zno nws on the solar cells after growing at different times . 
   figure  2a , d shows the plan view and its cross - sections of zno nws after growing 30  min , respectively . 
   one can see that the tapered zno nws with the length of approximately 100  nm are obtained on the pyramid - like textured surface of solar cell via this method . with the growth time increasing , the density of zno nws increases ( see figure  2b , c , e , f ) . 
   meanwhile , the length of zno nws becomes larger , e.g. , approximately 200  nm after 45  min and approximately 300  nm after 60  min . 
   sem images of zno nws grown under different times and their cross - section view . 
   ( a ) and ( d ) 30  min , ( b ) and ( e ) 45  min , and ( c ) and ( f ) 60  min . 
   figure  3 shows the reflectance spectra of the solar cells without or with the zno nw coating . it can be seen that , compared with the solar cell without zno nw coating ( reference ) , all the solar cells with zno nw coating exhibit lower reflectance than in the whole wavelength range . 
   the reflectivity of zno nw - coated solar cells decreases with an increase of zno nw growth time . 
   the solar cell with zno nws grown for 45  min has the lowest reflectivity . 
   however , when the zno nw growth time reaches to 60  min , the reflectivity of coated solar cell becomes higher in the wavelength range of 500 to 1,100  nm . 
   since the density and length of zno nws increase with an increase of growing time , it is suspected that the zno nws grown for 60  min could have formed something like 
   this finally results in a higher reflection of zno nw grown for 60  min . 
   nevertheless , these results clarify that the zno nw - coated solar cells have better antireflection effect than the reference without zno nw coating . 
   the decrease of reflectivity in the long wavelength range mainly results from the effect of zno nw trapping light by the multiple scattering of the incident light . 
   meanwhile , their gradually and continuously changing refractive index profile from the air to the substrate is also beneficial for its antireflection property . 
   the drastic decrease of reflectance in the low - wavelength range should be mainly attributed to the absorption of light by zno nws , themselves , which hardly makes any contribution to the improvement of the solar cell performance . after a prolonged growing time 
   , e.g. , 60  min , the light absorption of long zno nws becomes even stronger , and therefore , their contribution to the solar cell efficiency is further smaller . 
   the reflectance ( r ) spectra for the solar cell with zno nw grown under different times . 
   figure  4 shows the eqes of the solar cells with zno nws grown for different times . 
   it can be seen that the solar cells with zno nws grown for 30 and 45  min generally have higher eqes than the referenced one in the whole spectral region . 
   it signifies that zno nws on cz silicon solar cells can effectively improve the response of light spectrum . for the solar cell with 
   zno nws grown for 60  min , the eqes in the short - wavelength range are smaller than that of the referenced due to the stronger light absorption of long zno nws . 
   table  1 lists the detailed parameters of the solar cells coated with zno nws grown for different times . 
   it can be seen that even though the open - circuit voltages for all the solar cells keep the same , the short - circuit current density of the solar cells coated by zno nws are higher . 
   this finally results in that the efficiency of the solar cells coated by zno nws grown for 30 and 45  min are improved by a value of 0.1% and 3.0% , respectively , in comparison with the referenced cell . 
   figure  5 examples the current density as a function of bias voltage for the solar cells with zno nws grown for 45  min . 
   it reveals that zno nw - coated solar cells have higher short - circuit current density than the referenced one , while the open - circuit voltages keep constant . 
   the improvement of short - circuit current density is obviously a result of zno nws suppressing the light reflectivity . in the case of zno nws 
   grown for 60  min , the fill factor of the cell has a low fill factor and , therefore , a low efficiency . 
   this deterioration of the cell performance should be related to the long - time immersion of the solar cell in the 90c aqueous solution during the zno nw growth process . 
   the external quantum efficiencies ( eqes ) of the solar cells with zno nws grown for different times . 
   performance parameters of c - si solar cells with and without zno nws ff fill factor , jsc short - circuit current density , voc , open - circuit voltage 
   . the current density - voltage curves of the solar cells with 45-min grown zno nws . under a standard am 1.5 illumination . 
   in summary , around 100 to 300  nm zno nws are grown on the surface of czochralski si solar cells based on a low cost aqueous solution method . 
   zno nws grown for different times , such as 30 , 45 , and 60  min , can reduce the reflectivity of the solar cells effectively ( r  <  3% ) in a large spectral region and improve the external quantum efficiency in the corresponding range . 
   furtherm : more , zno nws with 45-min growth time can improve the efficiency of the solar cells by 3% , corresponding to their increment of short - circuit current density and fill factor . 
   finally , long - time immersion of solar cells in aqueous solution during the zno nw growth process can deteriorate the solar cell performance drastically . 
    
    
   deren yang and xuegong yu are both professors and doctorate degree holders in the state key laboratory of silicon materials , department of materials science & engineering , zhejiang university . 
   dong wang , dong lei , and genhu li are masters degree holder candidates of the same university . 
   this work is supported by the national natural science foundation of china ( grant nos . 
   60906002 and 50832006 ) , national key technology r&d program ( 2011bae03b13 ) , key laboratory of optoelectronic materials chemistry and physics , cas ( 2008dp173016 ) , and innovation team project of zhejiang province ( 2009r50005 ) .
summary: an efficient antireflection coating is critical for the improvement of silicon solar cell performance via increased light coupling . here 
   , we have grown well - aligned zno nanowhisker ( nw ) arrays on czochralski silicon solar cells by a seeding - growth two - step process . it is found that the zno nws have a great effect on the macroscopic antireflection effect and , therefore , improves the solar cell performance . 
   the zno nw array - coated solar cells display a broadband reflection suppression from 500 to 1,100  nm , and the minimum reflectance smaller than 3% can easily be achieved . by optimizing the time of zno nw growth 
   , it has been confirmed that an increase of 3% relatively in the solar cell efficiency can be obtained . 
   these results are quite interesting for the application of zno nanostructure in the fabrication of high - efficiency silicon solar cells .