太陽能電池的工作原理(li) 太陽能電池的構造介紹
以前,從電(dian)的(de)(de)角度來(lai)看,我們(men)所用的(de)(de)硅都是(shi)中性的(de)(de)。多(duo)余(yu)的(de)(de)電(dian)子(zi)(zi)被(bei)磷中多(duo)余(yu)的(de)(de)質子(zi)(zi)所中和。缺(que)失(shi)電(dian)子(zi)(zi)(空(kong)穴(xue))由(you)硼中缺(que)失(shi)質子(zi)(zi)所中和。當空(kong)穴(xue)和電(dian)子(zi)(zi)在N型(xing)硅和P型(xing)硅的(de)(de)交(jiao)界處(chu)混合時(shi),中性就(jiu)被(bei)破壞了。所有自由(you)電(dian)子(zi)(zi)會(hui)填(tian)充所有空(kong)穴(xue)嗎(ma)?不會(hui)。如果是(shi)這樣,那么(me)整個(ge)準(zhun)備工(gong)作就(jiu)沒有什(shen)么(me)意義了。不過(guo),在交(jiao)界處(chu),它們(men)確(que)實會(hui)混合形成一(yi)道屏障,使得(de)N側(ce)的(de)(de)電(dian)子(zi)(zi)越來(lai)越難以抵達P側(ce)。最終會(hui)達到平衡狀態,這樣我們(men)就(jiu)有了一(yi)個(ge)將兩側(ce)分開的(de)(de)電(dian)場。
這個(ge)電場相當于一個(ge)二極(ji)管(guan),允許(甚(shen)至推動)電子從P側(ce)流向N側(ce),而不(bu)是相反。它(ta)就像一座山(shan)——電子可以輕(qing)松地滑下山(shan)頭(到達N側(ce)),卻不(bu)能(neng)向上攀升(到達P側(ce))。
這樣,我(wo)們(men)就得到了一(yi)個(ge)作用相當于二極管的電(dian)場,其(qi)中的電(dian)子(zi)只能向一(yi)個(ge)方向運(yun)動(dong)。讓(rang)我(wo)們(men)來看一(yi)下在太陽光照射電(dian)池時(shi)會發(fa)生什么。
當光以光子(zi)的形式撞擊太陽能電池時,其(qi)能量會使電子(zi)空穴對(dui)釋放(fang)出來。
每個(ge)攜帶足夠能量的(de)(de)(de)光子(zi)(zi)通(tong)常會(hui)正(zheng)好釋放一(yi)(yi)個(ge)電(dian)(dian)(dian)(dian)(dian)子(zi)(zi),從而(er)產(chan)生一(yi)(yi)個(ge)自由(you)的(de)(de)(de)空穴(xue)(xue)。如(ru)果(guo)這(zhe)發生在(zai)離電(dian)(dian)(dian)(dian)(dian)場(chang)足夠近的(de)(de)(de)位置,或者(zhe)自由(you)電(dian)(dian)(dian)(dian)(dian)子(zi)(zi)和自由(you)空穴(xue)(xue)正(zheng)好在(zai)它的(de)(de)(de)影響(xiang)范圍之內,則電(dian)(dian)(dian)(dian)(dian)場(chang)會(hui)將(jiang)電(dian)(dian)(dian)(dian)(dian)子(zi)(zi)送到N側(ce),將(jiang)空穴(xue)(xue)送到P側(ce)。這(zhe)會(hui)導(dao)致電(dian)(dian)(dian)(dian)(dian)中性進一(yi)(yi)步被破(po)壞,如(ru)果(guo)我們提供一(yi)(yi)個(ge)外部(bu)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)通(tong)路,則電(dian)(dian)(dian)(dian)(dian)子(zi)(zi)會(hui)經(jing)過該(gai)通(tong)路,流(liu)(liu)(liu)向(xiang)它們的(de)(de)(de)原(yuan)始側(ce)(P側(ce)),在(zai)那里與電(dian)(dian)(dian)(dian)(dian)場(chang)發送的(de)(de)(de)空穴(xue)(xue)合并(bing),并(bing)在(zai)流(liu)(liu)(liu)動的(de)(de)(de)過程中做功。電(dian)(dian)(dian)(dian)(dian)子(zi)(zi)流(liu)(liu)(liu)動提供電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu),電(dian)(dian)(dian)(dian)(dian)池的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)場(chang)產(chan)生電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)。有了電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)和電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya),我們就有了功率,它是二(er)者(zhe)的(de)(de)(de)乘積(ji)。
我們的(de)光伏(fu)電池可以吸收多少(shao)太(tai)陽(yang)光的(de)能(neng)量(liang)?遺憾的(de)是,此處(chu)介紹的(de)簡易(yi)電池對(dui)太(tai)陽(yang)光能(neng)量(liang)的(de)吸收率(lv)至多為(wei)25%左右,通常(chang)的(de)吸收率(lv)是15%或更低。為(wei)什么(me)吸收率(lv)會這么(me)低?
可(ke)見光只是電磁頻譜的一(yi)部(bu)分(fen)。電磁輻射不(bu)是單(dan)頻的——它由一(yi)系列不(bu)同波長(進而產(chan)生的一(yi)系列能級)組成。(有(you)關電磁頻譜的詳(xiang)細(xi)介紹,請參閱狹義(yi)相對論基本原理。)
光(guang)可(ke)分為(wei)不同(tong)波長(chang),我們可(ke)以通過(guo)彩虹看出這一點。由于射(she)到電(dian)(dian)(dian)池(chi)的(de)(de)光(guang)的(de)(de)光(guang)子(zi)(zi)(zi)能(neng)(neng)(neng)(neng)量(liang)范圍很(hen)廣(guang),因此有(you)些光(guang)子(zi)(zi)(zi)沒有(you)足夠(gou)的(de)(de)能(neng)(neng)(neng)(neng)量(liang)來形成電(dian)(dian)(dian)子(zi)(zi)(zi)空(kong)穴對。它們只是(shi)穿(chuan)過(guo)電(dian)(dian)(dian)池(chi),就像電(dian)(dian)(dian)池(chi)是(shi)透(tou)明的(de)(de)一樣。但其他一些光(guang)子(zi)(zi)(zi)的(de)(de)能(neng)(neng)(neng)(neng)量(liang)卻很(hen)強。只有(you)達到一定的(de)(de)能(neng)(neng)(neng)(neng)量(liang) -- 單位為(wei)電(dian)(dian)(dian)子(zi)(zi)(zi)伏特(eV),由電(dian)(dian)(dian)池(chi)材料(liao)(對于晶(jing)體(ti)硅(gui),約為(wei)1.1eV)決定——才能(neng)(neng)(neng)(neng)使(shi)電(dian)(dian)(dian)子(zi)(zi)(zi)逸出。我們將這個能(neng)(neng)(neng)(neng)量(liang)值稱為(wei)材料(liao)的(de)(de)帶隙能(neng)(neng)(neng)(neng)量(liang)。如果光(guang)子(zi)(zi)(zi)的(de)(de)能(neng)(neng)(neng)(neng)量(liang)比所需的(de)(de)能(neng)(neng)(neng)(neng)量(liang)多(duo),則多(duo)余的(de)(de)能(neng)(neng)(neng)(neng)量(liang)會損(sun)失掉(除非光(guang)子(zi)(zi)(zi)的(de)(de)能(neng)(neng)(neng)(neng)量(liang)是(shi)所需能(neng)(neng)(neng)(neng)量(liang)的(de)(de)兩倍,并且可(ke)以創建多(duo)組電(dian)(dian)(dian)子(zi)(zi)(zi)空(kong)穴對,但這種(zhong)效應(ying)并不重要)。僅這兩種(zhong)效應(ying)就會造成電(dian)(dian)(dian)池(chi)中(zhong)70%左(zuo)右的(de)(de)輻射(she)能(neng)(neng)(neng)(neng)損(sun)失。
為(wei)何我們不選擇一種帶(dai)隙(xi)很(hen)低的(de)材料(liao),以(yi)便利用更多的(de)光子(zi)?遺憾的(de)是,帶(dai)隙(xi)還決定了(le)電(dian)(dian)場強度(電(dian)(dian)壓),如果帶(dai)隙(xi)過(guo)低,那(nei)么(me)在增大電(dian)(dian)流(liu)(通(tong)過(guo)吸收更多電(dian)(dian)子(zi))的(de)同時,也(ye)會損失一定的(de)電(dian)(dian)壓。請記住,功(gong)率是電(dian)(dian)壓和(he)電(dian)(dian)流(liu)的(de)乘積。最優帶(dai)隙(xi)能量必須能平衡(heng)這兩種效應,對于(yu)由單(dan)一材料(liao)制成的(de)電(dian)(dian)池,這個(ge)值約為(wei)1.4電(dian)(dian)子(zi)伏特。
我們(men)還有(you)(you)其(qi)他能(neng)量損(sun)失(shi)。電(dian)(dian)(dian)(dian)子(zi)(zi)(zi)(zi)必須通過外部(bu)電(dian)(dian)(dian)(dian)路從電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)的(de)(de)(de)(de)一側(ce)流(liu)到另一側(ce)。我們(men)可以在(zai)(zai)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)底(di)部(bu)鍍上一層(ceng)金(jin)屬(shu),以保證良好的(de)(de)(de)(de)導(dao)電(dian)(dian)(dian)(dian)性。但如果(guo)我們(men)將電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)頂(ding)部(bu)完全(quan)鍍上金(jin)屬(shu),光子(zi)(zi)(zi)(zi)將無(wu)法穿過不透光導(dao)體(ti)(ti),這樣就會喪(sang)失(shi)所有(you)(you)電(dian)(dian)(dian)(dian)流(liu)(在(zai)(zai)某些(xie)(xie)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)中(zhong),只(zhi)有(you)(you)上表面而(er)(er)(er)非所有(you)(you)位置(zhi)(zhi)使用了透明導(dao)體(ti)(ti))。如果(guo)我們(men)只(zhi)在(zai)(zai)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)的(de)(de)(de)(de)兩側(ce)設置(zhi)(zhi)觸(chu)點(dian),則(ze)電(dian)(dian)(dian)(dian)子(zi)(zi)(zi)(zi)需要經過很長(chang)一段(duan)距(ju)離(li)(對(dui)于(yu)電(dian)(dian)(dian)(dian)子(zi)(zi)(zi)(zi)而(er)(er)(er)言)才能(neng)抵達接觸(chu)點(dian)。要知(zhi)道,硅是(shi)半導(dao)體(ti)(ti),它(ta)傳輸電(dian)(dian)(dian)(dian)流(liu)的(de)(de)(de)(de)性能(neng)沒有(you)(you)金(jin)屬(shu)那么好。它(ta)的(de)(de)(de)(de)內部(bu)電(dian)(dian)(dian)(dian)阻(zu)(zu)(稱為串聯電(dian)(dian)(dian)(dian)阻(zu)(zu))相當高(gao)(gao),而(er)(er)(er)高(gao)(gao)電(dian)(dian)(dian)(dian)阻(zu)(zu)意味著高(gao)(gao)損(sun)耗。為了最大(da)限度(du)地降(jiang)低(di)這些(xie)(xie)損(sun)耗,電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)上覆(fu)有(you)(you)金(jin)屬(shu)接觸(chu)網(wang),它(ta)可縮短(duan)電(dian)(dian)(dian)(dian)子(zi)(zi)(zi)(zi)移動的(de)(de)(de)(de)距(ju)離(li),同時只(zhi)覆(fu)蓋電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)表面的(de)(de)(de)(de)一小部(bu)分。即使是(shi)這樣,有(you)(you)些(xie)(xie)光子(zi)(zi)(zi)(zi)也會被(bei)網(wang)格(ge)阻(zu)(zu)止,網(wang)格(ge)不能(neng)太(tai)小,否則(ze)它(ta)自(zi)身的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)阻(zu)(zu)就會過高(gao)(gao)。
在(zai)(zai)實際使用(yong)電(dian)池(chi)(chi)之前,還要執行其(qi)他幾個(ge)步驟。硅是一種有光澤的材料,這(zhe)意味著它的反射(she)(she)性能很好。被反射(she)(she)的光子不能被電(dian)池(chi)(chi)利用(yong)。出于這(zhe)個(ge)原因,在(zai)(zai)電(dian)池(chi)(chi)頂(ding)部采用(yong)抗反射(she)(she)涂層,可(ke)將反射(she)(she)損失降低到5%以下(xia)。
最后(hou)(hou)一步是(shi)安裝玻璃(li)蓋板,用(yong)來將電池(chi)(chi)與元(yuan)件(jian)分開,以(yi)保護電池(chi)(chi)。光伏(fu)模塊(kuai)(kuai)由(you)多(duo)塊(kuai)(kuai)電池(chi)(chi)(通常是(shi)36塊(kuai)(kuai))串聯(lian)和(he)并聯(lian)而成,以(yi)提供(gong)可用(yong)的電壓和(he)電流等級,這些電池(chi)(chi)放在(zai)一個(ge)堅固(gu)的框(kuang)架(jia)中,后(hou)(hou)部分別(bie)引出正極(ji)端子和(he)負(fu)極(ji)端子,并用(yong)玻璃(li)蓋板封上。
單晶硅(gui)(gui)并非(fei)光(guang)伏電池中使用的(de)(de)(de)(de)(de)唯一材(cai)(cai)料(liao)(liao)(liao)。電池材(cai)(cai)料(liao)(liao)(liao)中還(huan)采(cai)用了(le)多(duo)(duo)晶硅(gui)(gui),盡(jin)管這(zhe)樣(yang)生(sheng)產出來的(de)(de)(de)(de)(de)電池不如單晶硅(gui)(gui)電池的(de)(de)(de)(de)(de)效率(lv)高,但可以(yi)降低(di)(di)成(cheng)本。此外,還(huan)采(cai)用了(le)沒有(you)晶體(ti)結構的(de)(de)(de)(de)(de)非(fei)晶硅(gui)(gui),這(zhe)樣(yang)做同(tong)(tong)樣(yang)是為了(le)降低(di)(di)成(cheng)本。使用的(de)(de)(de)(de)(de)其他材(cai)(cai)料(liao)(liao)(liao)還(huan)包括砷化鎵、硒化銦銅和碲化鎘。由于不同(tong)(tong)材(cai)(cai)料(liao)(liao)(liao)的(de)(de)(de)(de)(de)帶(dai)(dai)隙(xi)不同(tong)(tong),因此它們似乎針對(dui)不同(tong)(tong)的(de)(de)(de)(de)(de)波(bo)長或(huo)(huo)不同(tong)(tong)能(neng)量的(de)(de)(de)(de)(de)光(guang)子(zi)進行了(le)“調諧”。一種提(ti)高效率(lv)的(de)(de)(de)(de)(de)方(fang)法(fa)是使用兩層(ceng)或(huo)(huo)者多(duo)(duo)層(ceng)具(ju)有(you)不同(tong)(tong)帶(dai)(dai)隙(xi)的(de)(de)(de)(de)(de)不同(tong)(tong)材(cai)(cai)料(liao)(liao)(liao)。帶(dai)(dai)隙(xi)較(jiao)(jiao)高的(de)(de)(de)(de)(de)材(cai)(cai)料(liao)(liao)(liao)放在表面,吸收較(jiao)(jiao)高能(neng)量的(de)(de)(de)(de)(de)光(guang)子(zi);而帶(dai)(dai)隙(xi)較(jiao)(jiao)低(di)(di)的(de)(de)(de)(de)(de)材(cai)(cai)料(liao)(liao)(liao)放在下方(fang),吸收較(jiao)(jiao)低(di)(di)能(neng)量的(de)(de)(de)(de)(de)光(guang)子(zi)。這(zhe)項技術(shu)可大大提(ti)高效率(lv)。這(zhe)樣(yang)的(de)(de)(de)(de)(de)電池稱為多(duo)(duo)接面電池,它們可以(yi)有(you)多(duo)(duo)個(ge)電場。