Regulating The Ratio Of Element In Absorbed Layer Improve The Solar Cell Open Voltage And Fill Factor

As an alternative to the well-known Cu?In,Ga??S,Se?₂?CIGS?material,the direct band gap Cu₂ZnSn?S,Se?₄?CZTSSe?has attracted more considerable attention in thin film solar cells,due to its low toxicity,abundance of composition elements and the potential commercial value.Benefiting from the similar crystalline structure to CIGS,kesterite CZTSSe solar cell has high absorption confficient for photons from visible to near-IR spectral range and could achieve the theoretical efficiency of up to 32.2%.The main fabricate methods of CZTSSe absorbed layer include nonvaccum method and vaccum method,the nonvaccum method has the advantage of low cost and simple to operate.The champion performance of CZTSSe solar cells is 12.6%obtained from hydrazine solution method by Mitzi's group.However the hydrazine solution is highly toxic and explosive.So compare with the vaccum mothed,the non-hydrazine solvent method attract more attention.Compared with the CIGS thin film solar cell which the champion performance have reached 22.6%,the CZTSSe solar cell production by non-hydrazine solvent still have some problems to be solved:?1?the molecular precursor solution can not avoid introduce unpurity elements,such carbon,oxygen,selenide incomplete thin film;?2?the high voltage(V_oc)deficits result in low open-circuit voltage of the solar cell;?3?Given the significant increase of the band gap and lattice defect concentration with the increase of S/?S+Se?ratio,however,lead to the decrease of the fill factor?FF?and the increase of the series resistance?Rs?.In order to solve those problems,we optimized the selenide condition,the ratio of S/?S+Se?and the doping concentration of Indium,eventually achieved the hightest conversion efficiency of 8.47%.So the work of this paper mainly includes the three parts:?1?Though investigating the precursor solution,the precursor film and the selenide condition, finally we obtain the smooth,high-quality selenide CZTSSe absorber layer under the high temperature of550 ^oC.Eventually the photoelectric conversion efficiency of 5.12%was obtained.?2?We adjust the ratio of the S/?S+Se?in the precursor solution,to increase the band gap and open circuit voltage of the solar cell.In the experiment we dissolve elemental sulfur and selenium in the precursor solution,to adjusting the band gap of precursor film range from 1.20 eV to 1.47 eV.And the open circuit voltage(V_oc)improved from 320 mV to 422 mV.However the fill fector?FF?significant decreased when X=1(S_1-xSe_x)and lead the photoelectric conversion efficiency present the change trend of decrease after increase first.When X=0.5,that is,the ratio of S and Se of 1:1,the hightest power convention efficiency of 6.53%was achieved.?3?We use solution method to directly doping indium into the absorber layer of hybrid buffer structured CZTSSe solar cells.The doping content can be precisely controlled here rather than only relying on diffusion from In₂S₃ emitter.Mott-Schottky plots and resistivity test indicate that the carrier concentration and charge transport process are monotonically enhanced with increasing In contentration,leading to the decrease of series resistance and benefiting to the improvement of FF.While the substitution of Sn by In could form In_Sn-shallow defects and become recombination center.The high concentration defects and the overflow carrier concentration induced increase of shunt conductance(G_sh)and larger recombination for the 12 at.%In content device.Due to the performance losses at high doping content,FF becomes the dominant parameter and the 6 at.%In content device finally contributes the highest cell efficiency of 8.47%.This work inspires us that the charge transport process in absorber could be precisely controlled via the substitution of Sn by In,and this precise regulation is more favorable for fill factor enhancement of efficient CZTSSe devices.