太(tai)陽能電池的工作原理 太(tai)陽能電池的構造(zao)介紹

以(yi)前，從電的(de)(de)角(jiao)度來(lai)看(kan)，我(wo)們所用的(de)(de)硅(gui)(gui)都是中(zhong)性的(de)(de)。多余(yu)的(de)(de)電子被磷中(zhong)多余(yu)的(de)(de)質子所中(zhong)和(he)。缺失電子（空穴）由(you)(you)硼(peng)中(zhong)缺失質子所中(zhong)和(he)。當(dang)空穴和(he)電子在(zai)(zai)N型(xing)硅(gui)(gui)和(he)P型(xing)硅(gui)(gui)的(de)(de)交界處(chu)混合時，中(zhong)性就(jiu)被破(po)壞了(le)。所有(you)自由(you)(you)電子會(hui)填充所有(you)空穴嗎？不會(hui)。如(ru)果(guo)是這(zhe)(zhe)樣，那(nei)么整個(ge)準備工作就(jiu)沒(mei)有(you)什么意義了(le)。不過(guo)，在(zai)(zai)交界處(chu)，它們確實會(hui)混合形(xing)成一(yi)道屏障(zhang)，使(shi)得N側(ce)的(de)(de)電子越來(lai)越難(nan)以(yi)抵(di)達P側(ce)。最終會(hui)達到平衡狀(zhuang)態，這(zhe)(zhe)樣我(wo)們就(jiu)有(you)了(le)一(yi)個(ge)將兩側(ce)分開的(de)(de)電場。

這個(ge)電場相當于一個(ge)二極管，允許（甚至(zhi)推(tui)動(dong)）電子從P側流向(xiang)N側，而(er)不是相反。它(ta)就像(xiang)一座山(shan)——電子可以輕(qing)松(song)地滑下山(shan)頭（到達(da)N側），卻不能向(xiang)上攀升（到達(da)P側）。

這樣(yang)，我(wo)們就得到了(le)一(yi)個作用相當于二(er)極(ji)管(guan)的電(dian)場，其中的電(dian)子只能(neng)向(xiang)一(yi)個方向(xiang)運動。讓我(wo)們來(lai)看一(yi)下(xia)在(zai)太陽(yang)光照射電(dian)池時會發生什么。

當光以光子(zi)的形式撞擊太陽(yang)能電池時，其(qi)能量會使電子(zi)空(kong)穴對(dui)釋放出來。

每個攜帶足夠(gou)能量的光子通(tong)常會(hui)(hui)正好釋放一(yi)(yi)個電(dian)(dian)(dian)子，從而(er)產(chan)生一(yi)(yi)個自由的空(kong)穴(xue)。如果(guo)(guo)這發(fa)生在(zai)(zai)離電(dian)(dian)(dian)場足夠(gou)近的位置，或者自由電(dian)(dian)(dian)子和(he)自由空(kong)穴(xue)正好在(zai)(zai)它(ta)(ta)的影(ying)響(xiang)范圍之(zhi)內，則(ze)電(dian)(dian)(dian)場會(hui)(hui)將電(dian)(dian)(dian)子送到(dao)N側(ce)(ce)，將空(kong)穴(xue)送到(dao)P側(ce)(ce)。這會(hui)(hui)導(dao)致電(dian)(dian)(dian)中性(xing)進(jin)一(yi)(yi)步被破壞，如果(guo)(guo)我(wo)們提(ti)供一(yi)(yi)個外部電(dian)(dian)(dian)流(liu)(liu)通(tong)路，則(ze)電(dian)(dian)(dian)子會(hui)(hui)經過該通(tong)路，流(liu)(liu)向它(ta)(ta)們的原始側(ce)(ce)（P側(ce)(ce)），在(zai)(zai)那(nei)里與電(dian)(dian)(dian)場發(fa)送的空(kong)穴(xue)合(he)并，并在(zai)(zai)流(liu)(liu)動的過程中做功。電(dian)(dian)(dian)子流(liu)(liu)動提(ti)供電(dian)(dian)(dian)流(liu)(liu)，電(dian)(dian)(dian)池的電(dian)(dian)(dian)場產(chan)生電(dian)(dian)(dian)壓(ya)。有了(le)電(dian)(dian)(dian)流(liu)(liu)和(he)電(dian)(dian)(dian)壓(ya)，我(wo)們就有了(le)功率(lv)，它(ta)(ta)是二者的乘積。

我(wo)們的(de)光伏電池可(ke)以(yi)吸(xi)收多(duo)(duo)少太陽光的(de)能量(liang)？遺憾的(de)是(shi)，此(ci)處介紹(shao)的(de)簡易電池對(dui)太陽光能量(liang)的(de)吸(xi)收率至多(duo)(duo)為(wei)25%左(zuo)右，通(tong)常的(de)吸(xi)收率是(shi)15%或更低(di)。為(wei)什(shen)么吸(xi)收率會這么低(di)？

可見光只是(shi)電磁頻譜的(de)一(yi)部分。電磁輻(fu)射不是(shi)單頻的(de)——它由一(yi)系(xi)列不同波長(chang)（進而(er)產生的(de)一(yi)系(xi)列能級(ji)）組成。（有(you)關(guan)電磁頻譜的(de)詳細介(jie)紹，請參閱狹義相(xiang)對論(lun)基本原(yuan)理。）

光(guang)(guang)(guang)(guang)可分為(wei)(wei)(wei)不(bu)同波長，我們可以通過(guo)彩虹(hong)看(kan)出這一點(dian)。由于(yu)射到(dao)電(dian)(dian)(dian)池的(de)(de)(de)(de)光(guang)(guang)(guang)(guang)的(de)(de)(de)(de)光(guang)(guang)(guang)(guang)子(zi)(zi)能(neng)(neng)(neng)量(liang)(liang)(liang)(liang)范圍很廣(guang)，因(yin)此有(you)(you)些光(guang)(guang)(guang)(guang)子(zi)(zi)沒有(you)(you)足(zu)夠(gou)的(de)(de)(de)(de)能(neng)(neng)(neng)量(liang)(liang)(liang)(liang)來形(xing)成電(dian)(dian)(dian)子(zi)(zi)空(kong)穴對(dui)。它們只(zhi)是(shi)穿過(guo)電(dian)(dian)(dian)池，就(jiu)像電(dian)(dian)(dian)池是(shi)透明的(de)(de)(de)(de)一樣。但其(qi)他(ta)一些光(guang)(guang)(guang)(guang)子(zi)(zi)的(de)(de)(de)(de)能(neng)(neng)(neng)量(liang)(liang)(liang)(liang)卻很強(qiang)。只(zhi)有(you)(you)達(da)到(dao)一定(ding)的(de)(de)(de)(de)能(neng)(neng)(neng)量(liang)(liang)(liang)(liang) -- 單位為(wei)(wei)(wei)電(dian)(dian)(dian)子(zi)(zi)伏(fu)特(te)（eV），由電(dian)(dian)(dian)池材料（對(dui)于(yu)晶體硅，約(yue)為(wei)(wei)(wei)1.1eV）決定(ding)——才能(neng)(neng)(neng)使電(dian)(dian)(dian)子(zi)(zi)逸出。我們將這個能(neng)(neng)(neng)量(liang)(liang)(liang)(liang)值稱為(wei)(wei)(wei)材料的(de)(de)(de)(de)帶(dai)隙能(neng)(neng)(neng)量(liang)(liang)(liang)(liang)。如果光(guang)(guang)(guang)(guang)子(zi)(zi)的(de)(de)(de)(de)能(neng)(neng)(neng)量(liang)(liang)(liang)(liang)比所(suo)需的(de)(de)(de)(de)能(neng)(neng)(neng)量(liang)(liang)(liang)(liang)多，則多余的(de)(de)(de)(de)能(neng)(neng)(neng)量(liang)(liang)(liang)(liang)會損失掉（除非(fei)光(guang)(guang)(guang)(guang)子(zi)(zi)的(de)(de)(de)(de)能(neng)(neng)(neng)量(liang)(liang)(liang)(liang)是(shi)所(suo)需能(neng)(neng)(neng)量(liang)(liang)(liang)(liang)的(de)(de)(de)(de)兩(liang)倍，并且可以創建多組電(dian)(dian)(dian)子(zi)(zi)空(kong)穴對(dui)，但這種(zhong)(zhong)效應(ying)并不(bu)重要）。僅這兩(liang)種(zhong)(zhong)效應(ying)就(jiu)會造成電(dian)(dian)(dian)池中70%左右的(de)(de)(de)(de)輻射能(neng)(neng)(neng)損失。

為何(he)我們(men)不選擇一種帶(dai)隙很低(di)的(de)材(cai)料，以(yi)便(bian)利用更多的(de)光子(zi)(zi)？遺憾的(de)是，帶(dai)隙還決定了電(dian)(dian)場強度（電(dian)(dian)壓(ya)），如(ru)果帶(dai)隙過低(di)，那么在增大電(dian)(dian)流（通(tong)過吸收更多電(dian)(dian)子(zi)(zi)）的(de)同時，也會損失一定的(de)電(dian)(dian)壓(ya)。請(qing)記住，功率(lv)是電(dian)(dian)壓(ya)和電(dian)(dian)流的(de)乘積(ji)。最優帶(dai)隙能量必須能平衡這(zhe)兩(liang)種效應，對于(yu)由單一材(cai)料制(zhi)成的(de)電(dian)(dian)池(chi)，這(zhe)個值約為1.4電(dian)(dian)子(zi)(zi)伏(fu)特。

我(wo)們(men)還有其他能(neng)量損失。電(dian)(dian)(dian)(dian)(dian)(dian)(dian)子(zi)必須(xu)通(tong)過外部(bu)(bu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路從電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池的(de)(de)一(yi)(yi)側流(liu)到另(ling)一(yi)(yi)側。我(wo)們(men)可以(yi)在電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池底部(bu)(bu)鍍(du)上一(yi)(yi)層金屬，以(yi)保證(zheng)良好的(de)(de)導(dao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)性(xing)。但如(ru)果我(wo)們(men)將(jiang)(jiang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池頂部(bu)(bu)完全鍍(du)上金屬，光(guang)子(zi)將(jiang)(jiang)無法(fa)穿過不透光(guang)導(dao)體(ti)，這樣就會喪(sang)失所(suo)有電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)（在某些電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池中，只有上表(biao)面(mian)而非所(suo)有位(wei)置使用了透明導(dao)體(ti)）。如(ru)果我(wo)們(men)只在電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池的(de)(de)兩側設置觸點，則電(dian)(dian)(dian)(dian)(dian)(dian)(dian)子(zi)需要(yao)(yao)經過很長一(yi)(yi)段(duan)距離(li)（對于電(dian)(dian)(dian)(dian)(dian)(dian)(dian)子(zi)而言）才能(neng)抵達接(jie)觸點。要(yao)(yao)知道，硅是半導(dao)體(ti)，它傳(chuan)輸電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)的(de)(de)性(xing)能(neng)沒有金屬那么好。它的(de)(de)內(nei)部(bu)(bu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻（稱(cheng)為串聯電(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻）相當高，而高電(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻意味著高損耗。為了最(zui)大限度地降低(di)這些損耗，電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池上覆有金屬接(jie)觸網(wang)(wang)，它可縮(suo)短電(dian)(dian)(dian)(dian)(dian)(dian)(dian)子(zi)移動(dong)的(de)(de)距離(li)，同時只覆蓋(gai)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池表(biao)面(mian)的(de)(de)一(yi)(yi)小部(bu)(bu)分(fen)。即使是這樣，有些光(guang)子(zi)也(ye)會被網(wang)(wang)格阻止，網(wang)(wang)格不能(neng)太(tai)小，否則它自身的(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻就會過高。

在實際使用(yong)電池之前，還要執行其他幾個(ge)(ge)步驟。硅是一種有光(guang)澤的(de)材料，這意味著它(ta)的(de)反射性能(neng)很好。被反射的(de)光(guang)子不能(neng)被電池利用(yong)。出(chu)于這個(ge)(ge)原因，在電池頂部采用(yong)抗反射涂層，可將(jiang)反射損失降低到5%以下。

最后一步是(shi)安裝玻(bo)璃蓋(gai)板，用來(lai)將電池(chi)(chi)與元件分開，以保護(hu)電池(chi)(chi)。光伏模塊由多塊電池(chi)(chi)（通常是(shi)36塊）串聯和(he)并聯而成，以提(ti)供可用的(de)電壓和(he)電流等(deng)級，這些電池(chi)(chi)放在一個堅固(gu)的(de)框架中(zhong)，后部(bu)分別引出(chu)正極端子和(he)負(fu)極端子，并用玻(bo)璃蓋(gai)板封上。

單(dan)晶硅(gui)并非光(guang)(guang)伏電(dian)(dian)(dian)池中使(shi)用的(de)(de)(de)(de)唯一(yi)材(cai)料(liao)(liao)。電(dian)(dian)(dian)池材(cai)料(liao)(liao)中還采用了多(duo)晶硅(gui)，盡管(guan)這(zhe)(zhe)樣生產出來的(de)(de)(de)(de)電(dian)(dian)(dian)池不(bu)(bu)如(ru)單(dan)晶硅(gui)電(dian)(dian)(dian)池的(de)(de)(de)(de)效(xiao)率(lv)高(gao)，但可以降低(di)成(cheng)本。此(ci)外，還采用了沒(mei)有晶體結(jie)構的(de)(de)(de)(de)非晶硅(gui)，這(zhe)(zhe)樣做同(tong)(tong)樣是為了降低(di)成(cheng)本。使(shi)用的(de)(de)(de)(de)其他(ta)材(cai)料(liao)(liao)還包括砷化鎵、硒(xi)化銦銅和碲化鎘。由于不(bu)(bu)同(tong)(tong)材(cai)料(liao)(liao)的(de)(de)(de)(de)帶隙不(bu)(bu)同(tong)(tong)，因此(ci)它(ta)們(men)似乎針對不(bu)(bu)同(tong)(tong)的(de)(de)(de)(de)波長或(huo)不(bu)(bu)同(tong)(tong)能(neng)量(liang)的(de)(de)(de)(de)光(guang)(guang)子進行了“調諧(xie)”。一(yi)種提高(gao)效(xiao)率(lv)的(de)(de)(de)(de)方法(fa)是使(shi)用兩(liang)層或(huo)者多(duo)層具(ju)有不(bu)(bu)同(tong)(tong)帶隙的(de)(de)(de)(de)不(bu)(bu)同(tong)(tong)材(cai)料(liao)(liao)。帶隙較高(gao)的(de)(de)(de)(de)材(cai)料(liao)(liao)放(fang)在(zai)表面，吸收較高(gao)能(neng)量(liang)的(de)(de)(de)(de)光(guang)(guang)子；而帶隙較低(di)的(de)(de)(de)(de)材(cai)料(liao)(liao)放(fang)在(zai)下方，吸收較低(di)能(neng)量(liang)的(de)(de)(de)(de)光(guang)(guang)子。這(zhe)(zhe)項技術可大大提高(gao)效(xiao)率(lv)。這(zhe)(zhe)樣的(de)(de)(de)(de)電(dian)(dian)(dian)池稱為多(duo)接(jie)面電(dian)(dian)(dian)池，它(ta)們(men)可以有多(duo)個電(dian)(dian)(dian)場。