Performance Enhancement Of Cu?In,Ga?Se₂ Solar Cells By Incorporating KF And Grading Band-Gap

Copper Indium Gallium Selenium?Cu?In,Ga?Se₂,CIGS?thin film solar cells have developed dramatically in 2010s,and achieved the excellent power conversion efficiency of 23.35%based on rigid substrate?Soda-lime glass,SLG?,efficiency of over 20%based on flexible substrate?polymide or metal foil?.In the last decade,the fabulous improvement of CIGS solar cells was attributed to the the powerful method,which is alkali elements incorporation in CIGS thin film.Thank to the alkali doping,the major increase of CIGS solar cells benefited from it and then boosted CIGS solar cells into a new level.In this paper,two main topics have been researched.Based on our optimized“three steps co-evaporation”method,the effect of annealing time of so called post doping treatment of potassium fluoride?KF-PDT?on thin film and cells has been investigated.Then,high and low Ga concentration samples,which are also named as different Ga gradient samples,have been compared after K passivation.The major result are as follows:?1?The performance of CIGS solar cells is sensitive to the annealing time,which shows a trend that open circuit voltage(V_OC),short circuit current density(J_SC),and conversion efficiency???increase and then decrease over the time,without fill factor?FF?that is almost unchanged.The best efficiency is 17.15%at 5 minutes annealing in this annealing time series.By means of other characterization test,diode test?Dark I-V?,capacity-voltage?C-V?test,time resolved phtotoluminescence spectrum?TRPL?test and external quantum efficiency?EQE?test,we find that quality factor?A?and reverse saturation current density?J₀?decrease and then enhance over the annealing time;carrier concentration,minor carrier lifetime and long wave response showed the reverse trend.Thin film characterization showed that potassium has no effect on crystallization orientation and Raman peak shift.Seldom changing of surface elements ratio verified by XPS,which means the valid effect is from grain boundary.After optimization,sample with conversion efficiency of 18.80%has been achieved.?2?After K incorporation,performance of high Ga concentration sample has been improved,nevertheless,still lower than the low Ga concentration sample by comparison.On the morphology and crystallinity,Ga's adverse effect is not eliminated.Blue shift of Raman peak and large angle shift of?112?orientation are due to high Ga concentration.In light of cells characterization,we found the parameters that have positive correlation with Ga concentration are open circuit voltage,carrier concentration,series resistance,and minor carrier lifetime under low temperature.Short current density,reverse saturation current density,fill factor,barrier width and long wave response are decreased due to high Ga concentration.Through the Ga concentration optimization,efficiency of 20.88%has been achieved.