TW202143497A - 背接觸式太陽電池 - Google Patents

背接觸式太陽電池 Download PDF

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TW202143497A
TW202143497A TW110114988A TW110114988A TW202143497A TW 202143497 A TW202143497 A TW 202143497A TW 110114988 A TW110114988 A TW 110114988A TW 110114988 A TW110114988 A TW 110114988A TW 202143497 A TW202143497 A TW 202143497A
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艾利克 霍夫曼
尤爾根 維納爾
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Abstract

本發明涉及一種背接觸式太陽電池，包括半導體基板，特別是矽晶圓，該半導體基板包括正側及背側，其中該太陽電池在該背側上包括第一極性之若干電極及第二極性之若干電極，其中在該第一極性之該等電極下方設有穿隧層及重摻雜的矽層，且該第二極性之該等電極直接與該半導體基板電接觸及機械接觸。

Description

背接觸式太陽電池
本發明係有關於一種太陽電池以及一種製造太陽電池之方法。
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眾所周知，太陽電池用作將光轉換為電能之光電元件。將吸收光時在半導體基板中產生的電荷載流子對在發射極區與基極區之間的接面上分離，此發射極區具有(用於產生第一極性之)第一摻雜類型，例如n型或p型，此基極區具有(用於產生相反極性之)相反摻雜類型。可透過接觸發射極區之發射極觸點及接觸基極區之基極觸點將以上述方式產生並分開的電荷載流子對輸往外部電路。
已知一極性的觸點佈置在正側，相反極性的觸點佈置在背側之太陽電池。將朝向太陽之一側稱為正側，背離太陽之一側稱為背側。為將因正側上的觸點之遮擋所造成的損耗最小化，研發出背接觸式太陽電池，其中該二觸點類型，即發射極觸點及基極觸點皆佈置在半導體基板之背側。
背接觸式太陽電池在提高效率方面具有較大的潛力，因為太陽電池之正側上無任何遮擋，因此，入射輻射會產生更多的自由電荷載流子。該二極性之電極並排佈置在太陽電池之背側上。因此，所生成的電荷載流子亦必須橫向地在太陽電池中流動。為使得該橫向電流所引起之電阻損耗最小化，以及為防止自由電荷載流子在到達電極前複合，該二極性之電極應儘可能靠近彼此。電極視具體極性與p型矽或n型矽連接，因此，pn接面亦儘可能地緊靠在一起。例如可藉由雷射輻射來實現pn接面，即解析度低於500 µm之精細的梳狀結構。在該製程中，脈衝雷射束透過在不同的時間及位置局部地熔化表面來使得兩個不同的摻雜劑，如硼及磷，進入矽，並且視具體摻雜劑產生p型或n型重摻雜。如此精細的結構能夠實現較低的內部串聯電阻以及高達η= 24%之效率η。更高的效率大體受基極中以及重摻雜的接觸及非接觸表面上之複合機構的限制。基極中之複合與晶圓品質相關，且在太陽電池之進一步的製造製程中很少受影響。在例如藉由非晶氫化矽之表面鈍化良好的情況下，重摻雜的n型及p型表面上之複合在非接觸區域內受奧格複合之限制，此奧格複合隨矽中之摻雜劑濃度之增大而增加。在接觸區域內，矽與金屬接觸，從而導致高度的界面複合。在太陽電池製程中，在矽中之摻雜劑儘可能少的情況下，非接觸表面上之奧格複合能夠減少，而金屬/矽觸點上之界面複合可透過儘可能減小接觸面積而減少。但簡單地減少摻雜劑以及減小接觸面積會導致串聯電阻增大，從而成為效率之限制因素。
有鑒於此，應用鈍化或選擇性觸點。其中，金屬電極並非與結晶基極直接電連接，而是被一較薄的穿隧氧化層隔開，此穿隧氧化層在使得矽表面鈍化的同時又如此之薄，使得電子可視具體極性穿過氧化物自半導體隧穿至電極或自電極進入半導體。為激勵電子隧穿，必須在穿隧氧化層上設置一電場。可透過重摻雜的n型或p型矽來在穿隧氧化層上生成電場。穿隧氧化層上方之矽摻雜導致穿隧氧化層下方之矽基中發生能帶彎曲，因此，毋須再對矽基進行更重的摻雜。在重摻雜的n型矽上僅有電子穿過穿隧氧化物，亦稱為電子流，而在重摻雜的p型矽上，僅出現所謂的空穴流：電子自重摻雜的p型矽進入矽基。金屬電極本身僅在穿隧氧化物上方與重摻雜的n型或p型矽存在電接觸及機械接觸。視具體摻雜，重摻雜矽區之選擇性以及穿隧氧化物使得幾乎僅有一種類型的電荷載流子傳輸至金屬/矽接觸面，並且將界面複合最小化。在此所提出之結構進一步減少矽基朝向穿隧氧化物之表面上的奧格複合，因為在該界面上毋須針對pn接面或歐姆觸點進行重摻雜。迄今為止，具有鈍化觸點之背接觸式太陽電池所達到的最高效率為η = 26.7%。但迄今為止，此類電池之製造較為複雜，因為該二以不同方式摻雜的選擇性觸點僅能以各種複雜的遮蔽與結構化步驟來進行施覆。在此過程中，必須保證遮蔽/結構化之高精度及高解析度。選擇性觸點之間距不可過大，且不應超過自由電荷載流子之擴散長度，同樣不應導致內部串聯電阻因基極中之橫向電流而增大。
透過本發明之太陽電池及本發明之製造此種太陽電池之方法克服該等缺點。
針對一種背接觸式太陽電池，包括半導體基板，特別是矽晶圓，該半導體基板包括朝向太陽之正側及背側，其中該太陽電池在該背側上包括第一極性之電極及第二極性之電極，本發明提出，在該第一極性之電極下方設有穿隧層及重摻雜的矽層，且該第二極性之電極直接與該半導體基板電接觸及機械接觸。
根據一種實施方式，該第二極性之電極在重摻雜的基極區內與該半導體基板接觸。需要重摻雜來實現較小的接觸電阻。
根據一種實施方式，非接觸的輕摻雜區域將重摻雜的矽層與基極區隔開。該非接觸的鈍化區域內之摻雜有利地輕於接觸的重摻雜的基極區。
其他實施方式係有關於一種製造根據前述實施方式之背接觸式太陽電池之方法。背側上之精細解析結構之製造係以無遮罩的方式實施。透過局部的雷射輻射來實現該製造。下面對該方法進行描述。
太陽電池之半導體基板，特別是矽晶圓，包括特別是經拋光或組織化之背側以及特別是組織化之正側。在該方法之第一步驟中，在該正側之表面上及/或該背側之表面上施覆一特別是包括氧化矽之穿隧層。穿隧層之厚度較佳最大為約四奈米。例如以熱製程或濕式化學製程或透過沈積來製造穿隧層。
根據一種實施方式，該方法還包括一用於將特別是整面的第一極性之重摻雜的矽層沈積在背側上之穿隧層上的步驟。例如可藉由電漿輔助化學氣相沈積(PECVD)、常壓化學氣相沈積(APCVD)、低壓化學氣相沈積(LPCVD)或陰極濺鍍來沈積第一極性之重摻雜的矽層。第一極性之重摻雜的矽層具有約100奈米至300奈米之厚度。
替代地，亦可在兩個步驟中沈積第一極性之重摻雜的矽層。根據一種實施方式，第一極性之重摻雜的矽層之沈積包括：沈積非摻雜的矽，隨後，引入摻雜劑。例如藉由離子植入、爐內擴散或雷射擴散來引入摻雜劑。摻雜劑例如指硼、鋁或鎵。
根據一種實施方式，該方法包括用於自正側移除穿隧層，特別是二氧化矽及/或第一極性之重摻雜的矽層之步驟。可整面地或局部地將穿隧層及/或第一極性之重摻雜的矽層蝕刻去除。
根據一種實施方式，該方法包括用於將前驅體層施覆在背側上及/或正側上之第一極性的重摻雜的矽層上之步驟，該前驅體層包括第二極性之摻雜劑。若第一極性的重摻雜的矽層係指p型矽層，則在第二極性中用於摻雜矽之摻雜劑係指磷。為施覆前驅體層，例如可實施爐內擴散製程，其中磷矽玻璃在背側及正側上之重摻雜的p型矽層上生長。爐內擴散製程較佳可如此地實施，使得在爐內擴散製程後，磷矽玻璃中含有大量的磷。在爐內擴散期間，磷擴散入太陽電池之正側，並且將正側之表面摻雜成n型。在背側上，磷同樣擴散入重摻雜的p型矽層。其中，如此地選擇爐內擴散之參數，使得磷的摻雜劑濃度不超過p型矽層中之硼的摻雜劑濃度。亦可藉由其他方法，例如PECVD，來施覆前驅體層，例如磷矽玻璃。
根據一種實施方式，在爐內擴散期間，前驅體層中之第二摻雜劑的一部分擴散入重摻雜的矽層。爐內擴散製程較佳如此地實施，使得第二摻雜劑之擴散的摻雜劑濃度不超過重摻雜的矽層中之第一摻雜劑的摻雜劑濃度。藉此，防止爐內擴散期間第一摻雜劑之過度補償。若第一極性的重摻雜的矽層係指p型矽層，且第二極性之摻雜劑係指磷，則重摻雜的p型矽層仍保持p型摻雜。而仍處於前驅體層中之第二摻雜劑的摻雜劑量高於重摻雜的矽層中之第一摻雜劑的摻雜劑量。
根據一種實施方式，該方法包括用於對背側進行雷射輻射，特別是用於產生第二極性之局部重摻雜的基極區之步驟。若先前施覆之第二極性之摻雜劑係指磷，則磷矽玻璃用作雷射輻射期間產生局部重摻雜的n型基極區之摻雜源。雷射輻射之能量使得磷矽玻璃、重摻雜的p型矽層、穿隧層及背側之表面在選中的區域內局部地熔化。磷矽玻璃及重摻雜的p型矽層中之摻雜劑擴散入背側之表面上的矽熔體。在雷射輻射衰減後，熔體冷卻下來並且凝固。該二摻雜劑皆保留在矽中，其中可能導致一或該二摻雜劑富集在表面上。由於第二極性之摻雜劑在前驅體層中之摻雜劑量更大，凝固的矽表面中之第二摻雜劑的濃度明顯高於重摻雜的p型矽層中之第一摻雜劑的濃度，因此，因過度補償而在選中的區域內導致局部較重的n型摻雜。透過選擇適宜的雷射參數，亦可在選中的區域內局部選擇性地產生摻雜程度不同的n型矽區段。
根據一種實施方式，該方法包括用於選擇性地移除第一極性之重摻雜的矽層及/或前驅體層之步驟。透過移除不同極性之鄰接區域內的層或層疊堆，使得該二以不同方式摻雜的區域之接觸中斷，並且可以防止在接觸區域內發生高度的複合或短路。例如可透過雷射輻射、濕式化學蝕刻或該二製程之組合來選擇性地移除第一極性之重摻雜的矽層。例如可透過濕式化學蝕刻來移除前驅體層，特別是磷矽玻璃。若第一極性的重摻雜的矽層係指p型矽層，且第二極性之摻雜劑係指磷，則可透過p型矽層之數奈米的回蝕將具有較高磷含量的矽層之一部分移除，並且透過磷擴散限制p型矽層之層電阻的增大。
根據一種實施方式，該方法包括用於將鈍化層施覆在該正側及/或該背側之表面上之步驟。鈍化層例如包括熱生長二氧化矽、氮化矽、氧化鋁或兩個或兩個以上的介電質層所形成之層疊堆。其中，背側上之鈍化層的厚度、折射率及組成不同於正側上之鈍化層的厚度、折射率及組成。有利地，鈍化層之厚度如此地得到最佳化，從而減少正側上之反射並且增加背側上之反射。
根據一種實施方式，該方法包括用於將電極施覆在該太陽電池之背側上之步驟。例如可藉由網板印刷、蒸鍍、濺鍍、或者電流沈積一或多個金屬或其他導電層來施覆電極。電極例如可包括銀漿、銀/鋁漿、鋁漿，或者純鋁、銅、錫、鈀、銀、鈦、鎳，或者上述金屬之層疊堆或合金，或者其他導電層，特別是導電聚合物或氧化物，或者此類層與金屬之組合。兩個極性之電極的組成及沈積製程可互不相同。電極特別是可在網板印刷後之高溫步驟中局部地穿過鈍化層，並且視電極之具體極性與重摻雜的p型矽層或重摻雜的基極區接觸。
根據一種實施方式，該方法包括用於選擇性地移除該鈍化層之步驟。可在沈積電極前例如透過雷射輻射來局部地移除鈍化層。
圖1示出太陽電池10之局部，其具有半導體基板12特別是矽晶圓、背側14及朝向太陽之正側16。矽晶圓12可為n型或p型摻雜。結合n型摻雜之矽晶圓12，即「基極」，對太陽電池10進行例示性說明。
在背側14上設有重摻雜的多晶p型矽層20。該矽層形成背側14上之第一極性。在重摻雜的p型矽層20之區域內，特別是包含二氧化矽18之穿隧層18使得矽晶圓12之表面鈍化。
太陽電池10在背側14上包括第一極性之電極34及第二極性之電極36。在第一極性之電極34下方設有穿隧層18及重摻雜的p型矽層20。第二極性之電極36直接與半導體基板12電接觸及機械接觸。在所示實施方式中，第二極性之電極36在重摻雜的基極區26內與半導體基板12接觸。非接觸的邊緣區域28將重摻雜的p型矽層20與重摻雜的n型基極區26隔開。
此外，結合圖2a至2h對太陽電池10之製造製程進行說明。圖2a至2h繪示出具有單一類型的選擇性觸點之太陽電池10的無遮蔽製造之製程流程。作為起始材料，具有正側14及背側16之矽晶圓12可為n型或p型摻雜。結合n型摻雜之晶圓，即「基極」，對製程流程進行說明。
圖2a示出具有經拋光之背側14及組織化之正側16的矽晶圓12。在該二表面上，或較佳僅在背側14上，例如以熱製程或濕式化學製程或透過沈積來製造穿隧層18，例如二氧化矽，其厚度較佳最大為約四奈米。
在該方法之下一步驟中，將特別是整面的第一極性之重摻雜的矽層20沈積在背側14上之穿隧層18上。此外，假定p型摻雜為第一極性。圖2b示出，將重摻雜的p型矽層20整面沈積背側14上之穿隧層18上。例如可藉由電漿輔助化學氣相沈積(PECVD)、常壓化學氣相沈積(APCVD)、低壓化學氣相沈積(LPCVD)或陰極濺鍍來沈積p型矽層20。重摻雜的p型矽層20具有約100奈米至300奈米之厚度。
替代地，亦可在兩個步驟中沈積重摻雜的p型矽層20。在此情形下，p型矽層20之沈積包括：沈積非摻雜的矽，隨後，引入摻雜劑。例如藉由離子植入、爐內擴散或雷射擴散來引入摻雜劑。摻雜劑例如指硼、鋁或鎵。
若正側16上設有穿隧層18及/或特別是無意沈積的重摻雜的p型矽層20，則在該方法之下一步驟中將該穿隧層18及/或該重摻雜的p型矽層20自正側16移除。可整面地或局部地將穿隧層18及/或重摻雜的p型矽層20蝕刻去除。此點在圖2c中示出。
在該方法之如圖2d所示的下一步驟中，將前驅體層22施覆在背側14及/或正側16上之第一極性的重摻雜的矽層20上，該前驅體層包括摻雜劑，特別是磷。根據所示實施方式，第二極性之摻雜劑係指磷。為施覆前驅體層22，例如實施爐內擴散製程或PECVD沈積，其中磷矽玻璃22在背側14及正側16上之重摻雜的p型矽層20上生長。在爐內擴散期間，磷矽玻璃22中之磷擴散入太陽電池10之正側16，並且將正側16之表面摻雜成n型。在背側14上，磷同樣擴散入重摻雜的p型矽層20。爐內擴散製程較佳如此地實施，使得在爐內擴散製程後，磷矽玻璃22中含有大量的磷。
有利地，如此選擇摻雜劑在重摻雜的p型矽層20及前驅體層22中之濃度，使得在在爐內擴散後，在重摻雜的p型矽層20中，一方面第一摻雜劑之摻雜劑濃度高於第二摻雜劑之摻雜劑濃度，但另一方面，前驅體層中之第二摻雜劑之摻雜劑量高於重摻雜的p型矽層20中之第一摻雜劑之摻雜劑量。藉此，防止爐內擴散期間第一摻雜劑之過度補償。亦即，重摻雜的p型矽層仍保持p型摻雜。但就雷射輻射之下一步驟而言，藉由該方式，在磷矽玻璃22中存在足夠的摻雜劑，用來在雷射輻射期間過度補償第一極性之摻雜劑。
根據一種實施方式，該方法包括用於對背側14進行雷射輻射，特別是用於產生局部重摻雜的n型基極區26之步驟。圖2e示出以雷射局部輻射背側14後之太陽電池10。磷矽玻璃22用作雷射輻射期間在局部重摻雜的n型基極區26內產生局部重摻雜的n型基極表面之摻雜源。雷射輻射之能量使得磷矽玻璃22、重摻雜的p型矽層20、穿隧層(即二氧化矽18)及背側14之表面局部地熔化，從而產生局部重摻雜的n型基極區26。磷矽玻璃22及重摻雜的p型矽層20中之摻雜劑擴散入背側14之表面上的矽熔體。在雷射輻射衰減後，熔體冷卻下來並且凝固。該二摻雜劑皆保留在矽中，其中可能導致一或該二摻雜劑富集在表面上。由於第二極性之摻雜劑在磷矽玻璃22中之摻雜劑量更大，凝固的矽表面中之第二摻雜劑的濃度明顯高於重摻雜的p型矽層20中之第一摻雜劑的濃度，因此，因過度補償而導致局部重摻雜的n型基極區26內之局部的n型重摻雜。透過選擇適宜的雷射參數，亦可在局部重摻雜的n型基極區26內局部選擇性地產生摻雜程度不同的n型矽區段。
在具有兩個極性的選擇性觸點之背接觸式太陽中，若該二以不同方式摻雜的矽層相接觸，則可能發生高度的複合。因此，該方法包括用於選擇性地移除第一極性之重摻雜的矽層20及/或前驅體層22之步驟。
圖2f示出將局部重摻雜的n型基極區26之邊緣區域28內的重摻雜的p型矽層20移除後之太陽電池10。例如可透過雷射輻射、濕式化學蝕刻或該二製程之組合來移除邊緣區域28內的p型矽層20。此外，可應用濕式化學製程來將磷矽玻璃22自重摻雜的p型矽層以及太陽電池之正側16移除。重摻雜的p型矽層20之數奈米的回蝕可將具有較高磷含量的矽層移除，並且透過磷擴散限制重摻雜的p型矽層20之層電阻的增大。
圖2g示出將鈍化層30、32施覆在正側16及/或背側14之表面上後的太陽電池10。鈍化層30、32例如包括熱生長二氧化矽、氮化矽、氧化鋁或兩個或兩個以上的介電質層所形成之層疊堆。其中，背側14上之鈍化層30的厚度、折射率及組成不同於正側16上之鈍化層32的厚度、折射率及組成。有利地，鈍化層30、32之厚度、組成及折射指數如此地得到最佳化，從而減少正側16上之反射並且增加背側14上之反射。
圖2h示出將電極34、36施覆在太陽電池10之背側14上後的太陽電池10。例如可藉由網板印刷、蒸鍍、濺鍍、或者電流沈積一或多個金屬或其他導電層來施覆電極34、36。電極34、36例如可包括銀漿、銀/鋁漿、鋁漿，或者純鋁、銅、錫、鈀、銀、鈦、鎳，或者上述金屬之層疊堆或合金，或者其他導電層，特別是導電聚合物或氧化物，或者此類層與金屬之組合。兩個極性之電極34、36的組成及沈積製程可互不相同。電極34、36特別是可在網板印刷後之高溫步驟中局部地穿過鈍化層，並且視電極之具體極性與重摻雜的p型矽層20或局部重摻雜的n型基極區26接觸。
可選地，可在施覆電極34、36前例如透過雷射輻射選擇性地移除鈍化層，使得電極與p型矽層20或局部重摻雜的n型基極區26直接接觸。
10:太陽電池 12:半導體基板；矽晶圓 14:背側 16:正側 18:穿隧層；氧化矽 20,26:矽層 22:前驅體層；磷矽玻璃 24:摻雜劑 28:邊緣區域 30,32:鈍化層 34,36:電極
下文將藉由附圖所示實施例對本發明之特徵、用途及優點作進一步說明。凡在說明中述及或在附圖中示出之單項特徵或特徵組合，不論申請專利範圍對其如何歸總或如何回溯引用，亦不論說明書對其如何表述，附圖如何示之，皆屬發明項目。 其中：
圖1本發明之太陽電池之局部的示意圖，以及 圖2a至2h示出製造太陽電池之方法的不同步驟中之圖1所示太陽電池。
12:半導體基板；矽晶圓
14:背側
16:正側
18:穿隧層；氧化矽
20,26:矽層
24:摻雜劑
28:邊緣區域
30,32:鈍化層
34,36:電極

Claims (14)

  1. 一種背接觸式太陽電池，包括半導體基板，特別是矽晶圓，該半導體基板包括正側及背側，其中該太陽電池在該背側上包括第一極性之若干電極及第二極性之若干電極，其特徵在於，在該第一極性之該等電極下方設有穿隧層及重摻雜的矽層，且該第二極性之該等電極直接與該半導體基板電接觸及機械接觸。
  2. 如請求項1之太陽電池，其特徵在於，該第二極性之該等電極在重摻雜的基極區內與該半導體基板接觸。
  3. 如請求項2之太陽電池，其特徵在於，非接觸的輕摻雜區域將該重摻雜的矽層與該基極區隔開。
  4. 一種製造如請求項1至3中任一項之背接觸式太陽電池之方法，其中太陽電池之半導體基板包括特別是經拋光或組織化之背側以及特別是組織化之正側，其特徵在於，在該正側之表面上及/或該背側之表面上施覆一特別是包括二氧化矽之穿隧層。
  5. 如請求項4之方法，其特徵在於，該方法包括用於將特別是整面的第一極性之重摻雜的矽層沈積在該背側上之該穿隧層上的步驟。
  6. 如請求項5之方法，其特徵在於，該第一極性之重摻雜的矽層之該沈積包括：沈積非摻雜的矽，隨後，引入摻雜劑。
  7. 如請求項4至6中任一項之方法，其特徵在於，該方法包括用於自該正側移除該穿隧層，特別是氧化矽及/或該第一極性之重摻雜的矽層之步驟。
  8. 如請求項4至7中任一項之方法，其特徵在於，該方法包括用於將前驅體層施覆在該背側上及/或該正側上之該第一極性的重摻雜的矽層上之步驟，該前驅體層包括摻雜劑，特別是磷。
  9. 如請求項8之方法，其特徵在於，在該第一極性之重摻雜的矽層及該前驅體層中之該摻雜劑的濃度係經選擇，使得該前驅體層中之第二極性的該摻雜劑之摻雜劑量高於該第一極性之重摻雜的矽層中之第一極性的該摻雜劑之摻雜劑量。
  10. 如請求項4至9中任一項之方法，其特徵在於，該方法包括用於對該背側進行雷射輻射，特別是用於產生局部重摻雜的基極區之步驟。
  11. 如請求項4至10中任一項之方法，其特徵在於，該方法包括用於選擇性地移除該第一極性之重摻雜的矽層及/或該前驅體層之步驟。
  12. 如請求項4至11中任一項之方法，其特徵在於，該方法包括用於將鈍化層施覆在該正側及/或該背側之表面上之步驟。
  13. 如請求項4至12中任一項之方法，其特徵在於，該方法包括用於將若干電極施覆在該太陽電池之該背側上之步驟。
  14. 如請求項4至13中任一項之方法，其特徵在於，該方法包括用於選擇性地移除該鈍化層之步驟。
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